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i
Microstructural Evolution and Tensile Properties of Direct
Metal Laser Sintered (DMLS) CoCrMo and Direct Metal
Laser Deposited (DMLD) FSX-414 Cobalt base superalloys
A Dissertation
Submitted in Partial Fulfillment of the
Requirements for the degree of
Master of Engineering
In
Materials Engineering
By
Kaustubh Krishna Bawane
In collaboration with
GE India Technology Centre Pvt. Ltd., Bangalore, India
Under the guidance of
Prof. Dipankar Banerjee (IISc, Bangalore)
Dr. Dheepa Srinivasan (GE Power)
Department of Materials Engineering
Indian Institute of Science
Bangalore – 560012, India
June 2016
ii
ABSTRACT
Direct metal laser sintering (DMLS) and Direct Metal Laser Deposition (DMLD) are emerging
additive manufacturing or 3D printing technologies based on slicing a solid model into multiple
layers and building part layer by layer. Therefore parts with intricate shapes and cavities can be
built without need of dedicated tools and machining unlike conventional methods. This study
comprises microstructural characterization and tensile properties of DMLS CoCrMo and DMLD
FSX-414 cobalt based superalloys.
DMLS CoCrMo was investigated for microstructures and tensile properties in the as
printed and after heat treatments. As printed DMLS CoCrMo showed columnar dendritic
microstructure with the column width of 0.6 to 1 µm. STEM-EDS analysis showed Mo and Si
enrichment in the interdendritic region. Solution treatment at 1150oC showed fully equiaxed
grain structure due to breakdown elongated grains from as printed samples following Rayleigh
like instability. Solution treated samples also showed some remnants of the previous
interdendritic region. Extensive precipitation was observed along the grain boundaries as well as
inside grains after ageing treatment at 980oC. SEM-EDS mapping showed Mo and Si enrichment
in the precipitates with composition very similar to those observed in the interdendritic region.
Solution treatment resulted in decrease in room temperature tensile strength from 1378 MPa to
1114 MPa and increase in ductility from 5.7 to 15%, which was attributed to increase in grain
size from 0.6-1 µm (column width) in as printed to ~40 µm (grain size) in solution treated
samples. Room temperature tensile strength had dropped marginally to 982 MPa after ageing
treatment, implying grain size as major factor in determining strength over precipitation.
Considerable drop in ductility to 5.3% was reported after ageing treatment due to extensive
precipitation along grain boundaries. High temperature tensile properties were studied for
solution treated and aged specimens. Both of them showed considerable drop in tensile strength
and increase in ductility due to thermally activated mechanisms.
As deposited DMLD FSX-414 showed columnar dendritic structure with (Cr21W2)C6
precipitates in the interdendritic region (column width: 9-12 µm). DMLD FSX-414 was
subjected to three different solution treatment temperatures, viz. 1150oC, 1200
oC, 1250
oC, etc. in
order to evaluate the thermal stability of the alloy. Equiaxed microstructure with remnants of
interdendritic precipitates was observed after 1250oC treatment due to breakdown of as deposited
elongated grains following Rayleigh like instability. Both solution treatment at 1150oC and
ageing treatment at 980oC showed same columnar dendritic microstructure. Room temperature
tensile properties showed only marginal drop in tensile strength after solution (1150oC) and
ageing (980oC) heat treatment, which was attributed to negligible change in respective
microstructures. Solution (1150oC) and aged (980
oC) DMLD FSX-414 showed higher tensile
strength than Solution (1150oC) and aged (980
oC) Cast FSX-414 which was attributed to their
respective secondary dendrite arm spacing (4-6 µm for DMLD FSX-414 and 70 µm for Cast
FSX-414). All samples showed fully ductile fractures.
The results suggest possible applications of these techniques in the field of gas turbine
repair technology.
iii
ACKNOWLEDGEMENT
First of all, I would like to express my sincere gratitude to my supervisors Prof. Dipankar
Banerjee, IISc Bangalore and Dr. Dheepa Srinivasan for their patient, support and guidance
throughout this project. It is because of their valuable insights, suggestions that I could
successfully complete my dissertation. I express my deep sense of indebtedness to both my
supervisors for giving me this wonderful opportunity to work in GE. I feel privileged to be a part
of this GE-IISc collaboration. I also thank Prof. Abhik Chaudhury for his valuable inputs on my
project work.
I am thankful to Dr. Krishnamurthy Anand and Dr. Sundar Amancherla for allowing me
to use facilities at GE to carry out experiments. I also extend my gratitude to Prof. T.
Abinandanan, Chairman, Department of Materials Engineering and all other faculty members for
allowing me to use experimental facilities at IISc.
I would like to thank Mr. N Raghunandan and Mr. CA Jagadish from Intech DMLS,
Bangalore for providing DMLS CoCrMo parts and Dr. Bhaskar Dutta, DM3D Technologies,
Auburn, USA for providing DMLD FSX-414 parts.
I would like to express my sincere gratitude to Dr. Joydeep Pal and Mr. Dayananda
Narayana for their help in various aspects on my project. I extend my unlimited thanks to Mr.
Vinay Kunnathully for his help in TEM characterization. I am thankful to Mr. Hariharan S. for
his help in carrying out vacuum heat treatment. I would like to thank Mr. Shivanandappa Meti
and Mr. Lakshmikanth S. for assisting me in my work. My special thanks to Dr Amuthan Ramar
and Mr Prasanna for help with twin-jet polishing.
I wish to acknowledge the cooperation and assistance of technical and non-technical staff
of the Department of Materials Engineering, IISc for my project. My special thanks to M. S.
Sasidhara for his help in tensile testing.
I am grateful to Mr. Hariharan S, Dr. Prasad Raghupathruni, Ms Shalaka Shinde, Mr.
Subramahnyam Adabala, Mr. Aravind Prasanth, Mr. Joel Bhagyanath and all other GE
colleagues for constant support and encouragement and making my working experience very
comfortable.
My sincere thanks to all my labmates and classmates for their support and time to time
help during my stay at IISc.
I would like to extend my sincere thanks to my parents for their patience, love and
constant support.
iv
TABLE OF CONTENTS
ABSTRACT .................................................................................................................................... ii
ACKNOWLEDGEMENT ............................................................................................................. iii
LIST OF FIGURES ...................................................................................................................... vii
LIST OF TABLES ........................................................................................................................ xii
LIST OF ACRONYMS ............................................................................................................... xiii
1. INTRODUCTION ............................................................................................................... 14
1.1 Additive Manufacturing ............................................................................................. 14
1.2 The Land based turbine and Co alloys ....................................................................... 16
1.3 Metallurgy of Cobalt base alloys ................................................................................ 18
1.4 Direct Metal Laser Sintering (DMLS) ........................................................................ 20
1.4.1 Process Parameters of DMLS ....................................................................................... 21
1.4.2 Microstructural defects in DMLS processing ............................................................... 21
1.4.3 Microstructural evolution during DMLS processing .................................................... 22
1.4.4 Tensile properties of DMLS processed parts ............................................................... 23
1.5 Direct Metal Laser Deposition (DMLD) .................................................................... 24
1.6 Motivation and Objectives .......................................................................................... 25
2. MATERIALS AND EXPERIMENTAL PROCEDURE..................................................... 26
2.1 Materials and Heat treatment ......................................................................................... 26
2.2 Powder characterization ................................................................................................. 27
2.3 Processing conditions..................................................................................................... 27
2.4 Chemical Analysis – Inductively Coupled Plasma (ICP) .............................................. 28
2.5 Surface roughness .......................................................................................................... 28
2.6 X-ray Tomography........................................................................................................ 29
2.7 Metallographic procedure .............................................................................................. 29
2.8 Porosity area fraction ..................................................................................................... 30
2.9 Optical and Scanning Electron Microscopy .................................................................. 31
v
2.10 Transmission Electron Microscopy ............................................................................. 32
2.11 Electron Backscattered Diffraction (EBSD) ................................................................ 32
2.12 X-ray Diffraction (XRD) ............................................................................................. 32
2.13 X-ray Diffraction Sin2 ψ technique for residual stress measurement .......................... 33
2.14 Microhardness .............................................................................................................. 34
2.15 Tensile testing .............................................................................................................. 35
3. RESULTS - PART A: DIRECT METAL LASER SINTERING OF CoCrMo .................. 37
3.1 Powder characterization ................................................................................................. 37
3.2 Chemical analysis .......................................................................................................... 38
3.3 Surface Roughness ......................................................................................................... 38
3.4 X-ray Microtomography of as printed DMLS CoCrMo ................................................ 39
3.5 Porosity .......................................................................................................................... 40
3.6 Microstructural Characterization of as printed DMLS CoCrMo ................................... 41
3.6.1 Optical microscopy ....................................................................................................... 41
3.6.2 Scanning Electron Microscopy ..................................................................................... 42
3.6.3 TEM Micrographs of As printed DMLS CoCrMo ....................................................... 44
3.7 Microstructural characterization of Solution heat treated DMLS CoCrMo .................. 47
3.8 Microstructural characterization of Sol HT + Aged DMLS CoCrMo ........................... 49
3.9 X-ray Diffraction ........................................................................................................... 51
3.10 Electron Backscattered Diffraction (EBSD) – Inverse Pole Figure maps ................... 53
3.11 X-ray Diffraction Sin2 ψ technique for residual stress measurements ......................... 54
3.12 Hardness ....................................................................................................................... 54
3.13 Tensile properties ......................................................................................................... 55
4. RESULTS - PART B: DIRECT METAL LASER DEPOSITION OF FSX-414 ................ 60
4.1 Powder characterization ................................................................................................. 60
4.2 Chemical analysis .......................................................................................................... 61
4.3 X-ray Microtomography of as deposited DMLD FSX-414 ........................................... 61
4.4 Porosity .......................................................................................................................... 62
4.5 Microstructural Characterization of as deposited DMLD FSX-414 .............................. 62
4.5.1 Optical microscopy ....................................................................................................... 62
vi
4.5.2 Scanning Electron Microscopy ..................................................................................... 64
4.6 Microstructural characterization of Solution heat treated DMLD FSX-414. ................ 67
4.6.1 Optical microscopy ....................................................................................................... 67
4.6.2 Scanning Electron Microscopy ..................................................................................... 67
4.7 Microstructural characterization of Sol HT 1150oC + Aged DMLD FSX-414. ............ 70
4.8 Microstructural characterization of Sol HT+Aged Cast FSX-414. ............................... 72
4.9 X-ray Diffraction ........................................................................................................... 74
4.10 Hardness ....................................................................................................................... 75
4.11 Tensile properties ......................................................................................................... 77
5. DISCUSSION ...................................................................................................................... 81
5.1 Microstructural evolution in Direct Metal Laser Sintered CoCrMo .............................. 81
5.1.1 Porosity and microcracks .............................................................................................. 81
5.1.2 Macrostructure in as printed DMLS CoCrMo .............................................................. 81
5.1.3 Microstructure in DMLS CoCrMo ............................................................................... 82
5.2 Tensile properties of Direct Metal Laser Sintered CoCrMo .......................................... 86
5.3 Microstructural evolution in Direct Metal Laser Deposited FSX-414 .......................... 88
5.4 Tensile properties of Direct Metal Laser Deposited FSX-414 ...................................... 89
5.5 Comparison between DMLS CoCrMo and DMLD FSX-414 ....................................... 90
6. CONCLUSIONS AND FUTURE WORK .......................................................................... 92
6.1 Conclusions .................................................................................................................... 92
6.2 Future work .................................................................................................................... 93
BIBLIOGRAPHY ......................................................................................................................... 94
vii
LIST OF FIGURES
Figure 1.1 (a) Polymeric AM part showing kind of intricate designs that can be built; b) AM Fuel
Nozzle used in General Electric’s LEAP Jet Aircraft engine.
Figure 1.2 AM applications timeline of past, present and potential future applications [2]
Figure 1.3 Classification of Additive manufacturing (AM) techniques [3]
Figure 1.4 Land based gas turbine showing its three different sections namely, compressor,
combustor and hot gas path (Image source: gesol.com).
Figure 1.5 Equilibrium phase diagrams of (a) Co-Cr, (b) Co-Mo, (c) Co-Si systems [8]
Figure 1.6 Schematic showing Direct Metal Laser Sintering (DMLS) technology (Image source:
Custompart.net)
Figure 1.7 Relationship between DMLS process parameters and resulting properties [11].
Figure 1.8 Microsections of the Ti-6Al-4V specimens parallel to the building direction for
different beam powers: (a) 90 W, (b) 120 W, (c) 180 W [15].
Figure 1.9 Schematic diagram of generation of melt pools [21].
Figure 1.10 Schematic drawing showing Direct Metal Laser Deposition technology (Courtesy:
DM3D Technology) [3].
Figure 2.1 (a) Flat Direct Metal Laser Sintered (DMLS) CoCrMo coupon with cooling holes, (b)
Cylindrical DMLS CoCrMo for making tensile specimens, (c) Direct Metal Laser Deposited
(DMLD) FSX-414 on a cast nozzle
Figure 2.2 Procedure for determining hole roughness using optical microscopy
Figure 2.3 Nomenclature of various sections of DMLS CoCrMo and DMD FSX-414 component.
Figure 2.4 Method of % porosity evaluation on transverse section of DMLS component.
Figure 2.5 Method of % porosity evaluation on transverse section of DMLD FSX-414
component.
Figure 2.6 Method used for measuring residual stress on as printed DMLS CoCrMo part.
Figure 2.7 (a) DMLS CoCrMo and (b) DMLD FSX-414 components showing different locations
for taking hardness readings.
Figure 3.1 SE images showing size and morphology of as received CoCrMo powder, at (a) low
magnification (b) high magnification.
Figure 3.2 Particle size distribution of CoCrMo powder
Figure 3.3 Representative micrographs showing topography of (a) surface and (b) hole of as
printed DMLS CoCrMo.
Figure 3.4 2D X-ray Microtomography images of as printed DMLS CoCrMo with 0ᵒ tilt
(Voltage: 200 kV, Current: 500 µA).
Figure 3.5 2D X-ray Microtomography images of as printed DMLS CoCrMo with 30ᵒ tilt
(Voltage-200 kV, Current-500 µA).
viii
Figure 3.6 Porosity distribution on transverse section of as printed DMLS CoCrMo along the
build direction and corresponding unetched microstructure showing porosity at locations A, B, C
respectively.
Figure 3.7 Optical micrographs of transverse, front planar and base sections of as printed DMLS
CoCrMo, etched with 5% HCl - electrolytic – 6V.
Figure 3.8 Optical micrographs of transverse section of as printed DMLS CoCrMo at (a) low
magnification showing irregular pores at the interlayer boundaries and (b) high magnification
showing microcracks (etched with 5% HCl, electrolytic-6V)
Figure 3.9 SE image of as printed DMLS CoCrMo at (a) low magnification showing columnar
microstructure and domains (grains) and (b) high magnification one to one matching along melt
pool boundary (Etched with 5% HCl, electrolytic-6V).
Figure 3.10 BSE images of as printed DMLS CoCrMo at (a) low magnification, and (b) high
magnification showing bright contrast in the interdendritic region, etched with 5% HCl
(electrolytic-6V)
Figure 3.11 (a,b) High magnification BSE images of unetched as printed DMLS CoCrMo
showing interdendritic precipitates and ε-HCP (arrow) cutting across the columns.
Figure 3.12 TEM bright field image of as printed DMLS CoCrMo sample showing ε-HCP
phases in γ-FCC CoCrMo matrix.
Figure 3.13 High resolution-(HR) TEM image of as printed DMLS CoCrMo and its
corresponding FFT pattern showing existence of HCP phase
Figure 3.14 FFT pattern of entire HRTEM image from Figure 13 showing spots for both FCC
and HCP and their orientation relationship
Figure 3.15 High Angle Annular Dark Field (HAADF) STEM images of as printed DMLS
CoCrMo specimen showing elongated bright precipitates and globular black precipitates in the
interdendritic region.
Figure 3.16 Optical micrographs of Transverse, front planar and base sections of Sol HT DMLS
CoCrMo showing fully equiaxed grains on all sides, etched with 5% HCl, electrolytic – 6V
Figure 3.17 (a) Optical micrograph of Sol HT DMLS CoCrMo showing equiaxed microstructure
with average grain size of 44 µm and (b) corresponding grain size distribution.
Figure 3.18 (a) BSE image of unetched Sol HT DMLS CoCrMo showing twins and equiaxed
microstructure, (b) corresponding EDS spectrum.
Figure 3.19(a,b) High magnification BSE images of unetched Sol HT DMLS CoCrMo showing
remnants of previous interdendritic precipitates inside grains as well as precipitates along grain
boundaries.
Figure 3.20 Optical micrographs of Sol HT+Aged DMLS CoCrMo at (a)200x and (b)1000x
showing equiaxed grain structure and some precipitation (arrow), etched with 5% HCl –
electrolytic, 6V.
Figure 3.21(a,b) BSE images of Sol HT+Aged DMLS CoCrMo samples showing bright and dark
precipitates inside the grains as well as along the grain boundaries.
ix
Figure 3.22 (a) High magnification BSE image of Sol HT+Aged DMLS CoCrMo sample
showing bright and dark precipitate, and (b) corresponding EDS elemental mapping.
Figure 3.23 X-ray Diffractions patterns of (a) CoCrMo Powder, (b) as printed (c) Sol HT , (d)
Sol HT+aged, DMLS CoCrMo, all showing peaks for both γ-FCC and ε-HCP Cobalt phases
(Target: Cr-Kα -2.29 Aº)
Figure 3.24 EBSD IPF Maps of (a) As printed, (b) Solution heat treated DMLS CoCrMo
Figure 3.25 (a) Residual stress distribution on the surface of as printed CoCrMo, (b) Variation of
residual stress along the build direction
Figure 3.26 Hardness profile along build direction (a) as printed, (b) Sol HT, (c) Sol HT+aged,
DMLS CoCrMo.
Figure 3.27 Room temperature and high temperature tensile properties of as printed, Sol HT, Sol
HT+aged DMLS CoCrMo, (a) % Y.S., (b) UTS, (c) Ductility.
Figure 3.28 Engineering stress-Engineering strain curve of as printed DMLS CoCrMo tested at
room temperature
Figure 3.29 Engineering stress-Engineering strain curves of Sol HT DMLS CoCrMo tested at (a)
room temperature, (b) 925 oC
Figure 3.30 Engineering stress-Engineering strain curves of Sol HT+Aged DMLS CoCrMo
tested at (a) room temperature, (b) 925 oC
Figure 3.31 Fractographs of as printed DMLS CoCrMo tensile sample showing cracks.
Figure 3.32 Fractographs of Sol HT DMLS CoCrMo tensile samples showing mixed brittle and
ductile type failures.
Figure 3.33 Fractographs of Sol HT+Aged DMLS CoCrMo tensile samples showing
intergranular fracture in room both room temperature and high temperature tests.
Figure 4.1 SE images showing morphology of FSX-414 powder in the as received condition at
(a) low magnification and, (b) high magnification .
Figure 4.2 Particle size distribution of FSX-414 powder.
Figure 4.3 2D X-ray Microtomography images of as deposited DMLD FSX-414 samples with (a)
0o tilt and (b) 15
o tilt (Voltage: 200 kV, Current: 500 µA)
Figure 4.4 Porosity distribution along transverse section of as deposited DMLD FSX-414 and
Cast FSX-414, and corresponding unetched microstructure showing porosity at locations A, B, C
respectively.
Figure 4.5 Optical micrographs of the unetched as deposited DMLD FSX-414 showing
solidification cracks in (a) DMLD Part and (b) DMLD and Cast FSX-414 joint.
Figure 4.6 (a) Optical micrograph of as deposited DMLD FSX-414 showing dendritic
microstructure. (b) as deposited DMLD FSX-414 and Cast FSX-414 joint showing dendritic
growth direction relative to cast FSX-414 substrate, etched with 5% HCl, electrolytic-6V.
Figure 4.7 Stitched optical micrograph of as deposited DMLD FSX-414 showing dendritic
microstructure and domains/bundles of dendrites with same orientation.
x
Figure 4.8 SEM micrographs of etched as deposited DMLD FSX-414 showing (a) primary
dendrites growing on the substrate cast FSX-414 and (b) domain boundary; etched with 5% HCl,
electrolytic – 6V
Figure 4.9 (a,b) BSE images of unetched as deposited DMLD FSX-414 showing columnar
structure with elongated bright and globular dark phases in the interdendritic region.
Figure 4.10(a) High magnification BSE image of as deposited DMLD FSX-414 showing bright
and dark precipitate, and (b) corresponding EDS elemental mapping.
Figure 4.11(a,b) Optical micrographs of Sol HT-1150oC DMLD FSX-414 showing fully
dendritic structure, etched with 5% HCl, electrolytic-6V.
Figure 4.12(a,b) Optical micrographs of Sol HT-1200oC DMLD FSX-414 showing dendritic
structure with the indication of the grain boundary, etched with 5% HCl, electrolytic-6V.
Figure 4.13(a,b) Optical micrographs of Sol HT-1250oC DMLD FSX-414 showing complete
breakdown of dendritic structure.
Figure 4.14(a,b) BSE images of Sol HT-1150oC DMLD FSX-414 showing interdendritic
precipitates.
Figure 4.15(a,b) BSE images of Sol HT-1200oC DMLD FSX-414 showing interdendritic
precipitates.
Figure 4.16(a,b) BSE images of Sol HT-1250oC DMLD FSX-414 showing remnants
interdendritic precipitates.
Figure 4.17 (a) High magnification BSE image of Sol HT-1150oC DMLD FSX-414 showing
bright and dark precipitate, and (b) corresponding EDS elemental mapping.
Figure 4.18(a,b) Optical micrographs of Sol HT-1150oC+aged DMLD FSX-414 showing
dendritic microstructure, etched with 5% HCl, electrolytic – 6V.
Figure 4.19(a,b) BSE images of Sol HT-1150oC+aged DMLD FSX-414 samples showing bright
and dark precipitates in the interdendritic regions and ε-HCP bands crossing across the column.
Figure 4.20 (a) High magnification BSE image of Sol HT-1150oC+aged DMLD FSX-414
sample showing bright and dark precipitate, and (b) corresponding EDS elemental mapping.
Figure 4.21 Optical micrographs of Sol HT-1150oC+aged Cast FSX-414 at (a) low magnification
showing coarse dendritic structure, (b) high magnification showing interdendritic precipitates,
etched with 5% HCl – electrolytic, 6V.
Figure 4.22(a,b) BSE images of Sol HT-1150oC+aged Cast FSX-414 samples showing eutectic
phases in the interdendritic region.
Figure 4.23 (a,b) High magnification BSE image of Sol HT-1150oC+aged Cast FSX-414 sample
showing interdendritic precipitates and (b) corresponding EDS elemental mapping.
Figure 4.24 X-ray Diffractions patterns of (a) as deposited, (b) Sol HT-1150oC (c) Sol HT-
1150oC+aged, DMLD FSX-414, showing peaks for both γ-FCC and ε-HCP Cobalt phases
(Target: Cr-Kα -2.29 Aº)
Figure 4.25 Hardness profile along build direction as deposited, Sol HT-1150oC, Sol HT-
1150oC+aged, DMLD FSX-414 and Cast FSX-414.
Figure 4.26 Hardness comparison between DMLD and Cast FSX-414.
xi
Figure 4.27 Variation in hardness with different solution heat treatment temperatures.
Figure 4.28 Room temperature tensile properties of As deposited, Sol HT+Aged DMLD and
Cast FSX-414, (a) 0.2% Y.S. and UTS, (b) % Elongation (ductility).
Figure 4.29 Fractographs of as deposited DMLD FSX-414 tensile sample showing (a) cracks at
low magnification and (b) dimples at high magnification.
Figure 4.30 Fractographs of Sol HT-1150oC+aged DMLD FSX-414 tensile samples showing (a)
curved facets at low magnification and (b) dimples at high magnification
Figure 4.31(a,b) Fractographs of Sol HT-1150oC+aged Cast FSX-414 tensile samples showing
dimples and big voids.
Figure 4.32 Engineering stress - Engineering strain curve for as deposited DMLD FSX-414.
Figure 4.33 Engineering stress - Engineering strain curve for Sol HT-1150oC+aged DMLD FSX-
414.
Figure 4.34 Engineering stress - Engineering strain curve for Sol HT-1150oC+aged Cast FSX-
414
Figure 5.1 Schematic representation of microstructural evolution in DMLS CoCrMo.
Figure 5.2 Schematic representation of athermal ε-HCP growing on γ-FCC Cobalt
Figure 5.3 Isothermal section of CoCrMo ternary diagram at 1200 0C.
Figure 5.4 Isothermal section of CoCrMo ternary diagram at 924oC.
Figure 5.5 Schematic representation of microstructural changes during solution and ageing heat
treatments.
Figure 5.6 Schematic representation of engineering stress vs. engineering strain curve for as
printed and solution treated DMLS CoCrMo (room temperature)
xii
LIST OF TABLES
Table 1.1 Comparison Between Various Additive Manufacturing Technologies [3]
Table 1.2 General Properties of Elemental Cobalt [5]
Table 1.3 Effect of notable alloying elements in Cobalt base alloys.
Table 1.4 Tensile properties of As printed DMLS CoCrMo-EOS Materials Data Sheet [30]
Table 2.1 Nominal chemical compositions of CoCrMo and FSX-414 alloys
Table 2.2 Heat treatment conditions for both DMLS CoCrMo and DMLD FSX-414
Table 2.3 Process parameters for Direct Metal Laser Sintering (DMLS) of CoCrMo and Direct
Metal Laser Deposited (DMLD) FSX-414
Table 2.4 Polishing steps followed for DMLS CoCrMo and DMLD FSX-414
Table 2.5 X-ray diffraction parameters
Table 2.6 Parameters for X-ray residual stress measurements
Table 2.7 Specimen geometries and testing conditions used for tensile testing
Table 3.1 Composition of CoCrMo powder analyzed using EDS
Table 3.2 Chemical composition of Direct Metal Laser Sintered (DMLS) CoCrMo and
corresponding nominal composition
Table 3.3 Surface roughness of as printed DMLS CoCrMo coupon
Table 3.4 Chemical analysis of various phases in as printed DMLS CoCrMo specimen using
TEM-EDS.
Table 3.5 EDS composition of solution heat treated specimen
Table 3.6 Composition of various phases in Sol HT+Aged DMLS CoCrMo
Table 3.7 Lattice parameters of FCC and HCP phases in various CoCrMo samples
Table 3.8 %Phase fraction of HCP phase in various CoCrMo samples
Table 4.1 Composition of FSX-414 powder analyzed using EDS
Table 4.2 Chemical composition of Direct Metal Laser Deposited (DMLD) FSX-414 and
corresponding nominal composition
Table 4.3 Chemical composition of various phases in As deposited DMLD FSX-414
Table 4.4 Chemical Composition of bright precipitates in DMLD FSX-414 samples solution
treated at various temperatures (all in weight %)
Table 4.5 Composition of various phases in Sol HT 1150oC + Aged DMLD FSX-414
Table 4.6 Composition of various phases in Sol HT 1150oC + Aged Cast FSX-414
Table 5.1 Size and morphology of grains in As printed, Sol HT, Sol HT+Aged DMLS CoCrMo
Table 5.2 Size and morphology of precipitates observed in As printed, Sol HT and Sol HT+Aged
DMLS CoCrMo
Table 5.3 Comparison between DMLS CoCrMo and DMLD FSX-414
xiii
LIST OF ACRONYMS
AM Additive Manufacturing
LENS Laser Engineered Net Shaping
DMLS Direct Metal Laser Sintering
DMLD Direct Metal Laser Deposition
SLM Selective Laser Melting
PBF Powder Bed Fusion
DED Directed Energy Deposition
SEM Scanning Electron Microscopy
TEM Transmission Electron Microscopy
HAADF High Angle Annular Dark Field
FFT Fast Fourier Transform
EDS Energy Dispersive X-ray Spectroscopy
Sol HT Solution heat treated at 1150oC
Sol HT+Aged Solution heat treated at 1150oC and aged at 980
oC
14
1. INTRODUCTION
1.1 Additive Manufacturing
Additive manufacturing (AM) or 3D printing technology is gaining lot of popularity in
various fields, right from electronics, biomedical to structural engineering components and
construction industry. American Society for Testing and Materials (ASTM – F2792) defines
additive manufacturing as ‘the process of joining materials to make objects from 3D model
data, usually layer upon layer, as opposed to subtractive manufacturing methodologies.’
Almost all AM techniques involve design of finished product using Computer Aided Design
(CAD), slicing this solid model into 2-dimensional layers and building the part layer by layer.
The materials used can be polymer, metal, ceramic, concrete or even biological tissues.
Figure 1.1 (a) Polymeric AM part showing kind of intricate designs that can be built; b) AM
Fuel Nozzle used in General Electric’s LEAP Jet Aircraft engine.
Additive Manufacturing is relatively recent manufacturing technology and has its roots in the
development of stereolithography technique used for polymer based materials in the 1980’s.
3D printing of metallic materials started around early 2000’s, when Optomec first
commercialized its Laser Engineering Net shaping (LENS) metal powder system based on
technology developed by Sandia National Labs.[1] Extensive research and numerous
processes have since been introduced to improve quality of products and efficiency of
process. Figure 1.2 gives timeline of past, present and potential future AM development and
applications.
15
Figure 1.2 AM applications timeline of past, present and potential future applications [2]
For years, constraints in fabrication methods have been primary obstacle for designers.
Advent of additive manufacturing has provided more flexibility in designs, and adding
complex features in product has now been possible without adversely affecting cost,
production rate or quality. Moreover, designers have the key to success of AM, as they can
come up with more and more sophisticated designs which were earlier limited by
conventional manufacturing.
AM has proven highly profitable in cases of low volume production, parts with highly
intricate features and where changes in designs are frequent. Excellent efficiency, low cost,
energy savings, low wastage and customizability are main advantages of AM. However, for
higher volume of production AM is considerably slower and also high initial investments,
discontinuous process cycle, and limited build size are the major problem in getting
companies to use AM.[2]
According to ASTM, Additive Manufacturing (AM) techniques for metallic materials can be
classified into two categories: Directed Energy Deposition (DED) and Powder based fusion
16
(PBF) as shown in Figure 1.3. Directed energy deposition involves injecting material into the
weld pool while Powder based fusion technology involves scanning layer of powder on the
build platform with a heat source.[3] Table 1.1 shows comparison between various aspects of
three different AM processes.
This work deals with additive manufacturing of CoCrMo and FSX-414 Cobalt base
superalloys for gas turbine applications.
Figure 1.3 Classification of Additive manufacturing (AM) techniques [3]
1.2 The Land based turbine and Co alloys
Typical land based gas turbine can be divided into three sections, viz. compressor, combustor
and turbine as shown in Figure 1.4. In combustor fuel is burnt with the help of compressed air
from compressor and thus it is the hottest section. In turbine or hot gas path, there is an
assembly of nozzles and rotors. Due to specific aerodynamic shape of nozzles, hot gas from
combustor flows in a particular way that drives the rotor next to it efficiently. Due to
proximity to combustor and the hot gases coming from it, the hot gas path component
experiences high temperatures. Hot gas path components were made of high temperature
resistant cobalt base superalloy such as FSX-414.
Additive Manufacturing
Directed Energy Deposition (DED)
Direct Metal Deposition (DMD)
Laser Engineered Net Shaping (LENS)
Direct Manufacturing (Electron beam and metal wire)
Powder Bed Fusion (PBF)
Direct Metal Laser Sintering (DMLS)
Selective Laser Melting (SLM)
Electron Beam Melting (EBM)
17
Table 1.1 Comparison Between Various Additive Manufacturing Technologies [3]
Figure 1.4 Land based gas turbine showing its three different sections namely, compressor,
combustor and hot gas path (Image source: gesol.com).
Compressor
Combustor
Hot gas path
18
1.3 Metallurgy of Cobalt base alloys
Cobalt has been in service of mankind for last 5000 years, first used by early Egyptians as
blue pigment for glaze. Cobalt ranks 33rd
in abundance.[4] Cobalt has both metallurgical and
non-metallurgical uses. Non-metallurgical uses involves paint pigments, radioactive source,
batteries, varnishes, inks, magnetic recording media, ground coats for porcelain enamels, and
catalysts for chemical and petroleum industries.[5] Metallurgical uses of cobalt exploit its
properties such as high temperature strength, biocompatibility, high wear and corrosion
resistance, magnetic properties, low expansion coefficient etc. It is widely used in gas turbine
nozzles, jet engine blades and vanes and hardfacing wear resistant applications.[6] General
properties of elemental cobalt are listed in Table 1.2.
Table 1.2 General Properties of Elemental Cobalt [5]
Density 8.85 g/cc
Melting Point 1493 °C
Curie Temperature 1127 °C
Coefficient of Thermal Expansion 13.8 μm/m.K
Thermal Conductivity 69 W/m.K
Electrical resistivity 7.8 μΩ.cm
Elastic modulus 211 GPa
0.2% Yield strength 305 to 345 MPa
Tensile strength 800 to 875 MPa
Elongation 15 to 30%
Cobalt has HCP crystal structure at room temperature (ϵ-Cobalt) and shows allotropic
transformation to FCC structure (α-Cobalt) at 417 °C temperature. Alloying elements are
categorized into two types, those that stabilize HCP structure such as molybdenum, tungsten,
chromium and silicon etc. These elements increase the transformation temperature and
decrease the stacking fault energy in FCC Co. Thus for CoCrMo alloy, allotropic
transformation temperature increases to near 970 °C owing to the high percentage of HCP
stabilizing elements.[7] Other type of elements such as carbon, iron and nickel stabilize FCC
structure and has opposite effects on transformation temperature and stacking fault energy in
FCC Co. Even though equilibrium phase diagrams (Figure 1.5) shows stable HCP phase at
room temperature, the transformation from FCC to HCP is extremely sluggish for pure
cobalt. Thus ϵ or α+ϵ phases are rarely observed in pure cobalt under normal cooling
conditions and metastable α (FCC) is more common.[5] Table 1.3 shows effect of various
alloying elements on cobalt base alloys.
19
Figure 1.5 Equilibrium phase diagrams of (a) Co-Cr, (b) Co-Mo, (c) Co-Si systems [8]
Table 1.3 Effect of notable alloying elements in Cobalt base alloys.
Element Property
C It produces strengthening by formation of various carbides of type M7C3, M23C6, M6C,
MC etc.[9] Carbide distribution, morphology, type, amount depends on composition,
heat treatment and cooling rates. Use of high carbon alloys usually limits
manufacturing process to hot working only, for cold working carbon content must be
less than 0.15%. Excess carbon in cobalt tends to decrease ductility.[6]
Cr For resistance to oxidizing and sulfidizing environments chromium is preferred
alloying element. It improves hot corrosion resistance and also acts as solid solution
strengthener for cobalt. Chromium also forms carbides of type M7C3 and M23C6. These
carbides are effective in pinning dislocations and thereby improving strength. Being a
HCP stabilizer chromium decreases stacking fault energy thereby making cross-slip
and glide of dislocations even more difficult.[6] But when chromium is added in excess
(>58%), undesirable TCP (Topologically Closed Packed) phases can form and
degrades properties.[9]
(a)
(c)
(b)
20
Mo, W Both are excellent solid solution strengtheners by virtue of their large atomic size and
are generally added in Co-Cr alloys.[5] They also give strengthening effect by
producing intermetallic like Co3M and carbides like M6C. When present in small
amount they substitute for chromium in M23C6 carbides.[10] Molybdenum has been
found to enhance wear resistance and corrosion resistance of cobalt-base alloys
Ni It stabilizes FCC phase at room temperature and also inhibits stacking fault formation
in FCC cobalt. It produces strengthening by formation of intermetallic compound
Ni3Ti. It improves forgeability of cobalt-base alloys. But when added in excess it
lowers the corrosion resistance.[9]
Ta, Nb,
Ti
It produces strengthening due to formation of intermetallic compound Co3M and MC
type carbides. It also produces solid solution strengthening effect.[6]
B, Zr They produce strengthening by effect on grain boundaries and precipitate formation.
They also increases stress rupture strength of alloy. Zirconium forms MC type carbides
and boron promotes formation of borides with other alloying elements.[9]
1.4 Direct Metal Laser Sintering (DMLS)
Figure 1.6 Schematic showing Direct Metal Laser Sintering (DMLS) technology (Image
source: Custompart.net)
DMLS is an AM technique that uses Yb (Ytterbium) Fiber laser (200 or 400 W) to sinter
metal powder particles (Figure 1.6). It was developed by EOS, Munich, Germany. DMLS has
capabilities to produce small batches of dimensionally accurate and structurally sound
metallic parts. Steps involved in this process are mentioned below [3]:
3D CAD model of the component is prepared and digitally sliced into 2D layer model.
Substrate is fixed on a build platform.
21
Build chamber is filled with inert gas and oxygen content is reduced to the desired level
to avoid oxidation.
Scraper is used to transfer thin layer of powder from supply cylinder to the substrate.
Laser beam scans powder bed following the CAD data of the component.
This process is repeated for the next layers until component is completely built.
1.4.1 Process Parameters of DMLS
Process parameters of DMLS play an important role in determining quality of the final
component. Figure 1.7 shows relationship between process parameters and their influence on
the resulting properties.
Figure 1.7 Relationship between DMLS process parameters and resulting properties [11].
1.4.2 Microstructural defects in DMLS processing
Major microstructural defects observed in additive manufactured parts are lack of fusion,
porosity, part distortion, microcracks and delamination [12]. Among these, the most common
defects in DMLS processed parts are mainly porosity and microcracks [13]–[17]. Zhou et al
[17] studied the 3D morphology of defects in selective laser melted CoCrMo alloy using
synchrotron based micro-CT. Two types of defects were observed viz., (i) defects in single
powder layer and (ii) defects in multiple powder layers. Defects were observed to have
complex 3D shape anisotropy. These defects were attributed to melt pool
dynamics/oscillations and melt track instabilities. Baurei et al [15] systematically studied
22
defect generation mechanism using the numerical modelling in EBM (Electron Beam
Melting) processed Ti-6Al-4V parts. It has been reported that small faults in the molten layer
can expand into large channel like defects over multiple layers as shown in Figure 1.8a.
These types of pores were eliminated by process control methods such as increasing laser
power (Figure 1.8). Small near spherical pores were also observed as shown in Figure 1.8c.
Formation of these small pores was attributed to entrapped gas within gas atomized
powders[14], [15] and bubbles from metallic evaporation due to the high power laser beam
[17].
Microcracks were observed in various DMLS processed alloys [13], [16]. Cracks
formation is attributed mainly to the solidification shrinkage in upper molten layer which is
restricted by cooler substrate or earlier layers [13]. Quian et al’s [16] work on selective laser
melted CoCrMo showed that in spite of the microcracks and other defects, mechanical
properties are still better than its cast counterpart.
Figure 1.8 Microsections of the Ti-6Al-4V specimens parallel to the building direction for
different beam powers: (a) 90 W, (b) 120 W, (c) 180 W [15].
1.4.3 Microstructural evolution during DMLS processing
Considerable work has been carried out to understand microstructural evolution during
DMLS processing of various alloys. It was observed that many DMLS / SLM processed
materials such as titanium, cobalt and nickel based superalloys shows similar macrostructure
consisting of a series of melt pools stacked over each [13], [16], [18]–[21]. Yan et al [21]
described the macrostructural features using the schematic as shown in Figure 1.9. The
Gaussian energy distribution of the laser beam is the main cause of the arc shaped melt pool
in the structure. The greatest intensity at the center of the beam produces deep melt pools. In
order to accomplish good bonding between layers and high densification, the generated melt
pool overlaps with previous layer as well as neighboring scan tracks.
23
Figure 1.9 Schematic diagram of generation of melt pools [21].
Many researchers working on DMLS/SLM of various nickel based and cobalt based
alloys observed cellular / columnar microstructure with the segregation of few alloying
elements towards cell / column boundaries[18], [20], [22]–[27]. Few researchers have carried
out in depth analysis of the microstructural features of as printed DMLS/SLM CoCrMo.
Quian et al [16] studied the microstructures and mechanical properties of the SLM processed
biomedical CoCrMo alloy. The presence of fine (~1 µm) cellular subgrains was observed
inside much larger single crystal grains. These single crystal grains were basically clusters of
these fine cellular grains which grew coherently along one crystallographic orientation.
Molybdenum enrichment was observed at the intercellular boundaries. Takaichi et al [27]
also observed columnar structure with ~2.7 µm diameter for SLM processed CoCrMo alloy.
Needle like precipitates enriched with Chromium and Molybdenum were observed at the
interdendritic boundaries. Takaichi thought needle like precipitate could be the σ phase based
on the ternary phase diagram of Co-Cr-Mo system. Both the researchers also observed the
presence ε (HCP) martensite phase in their XRD patterns.
Barucca et al [18] studied microstructural evolution in as printed DMLS CoCrMoW
biomedical alloy. Columnar structure was observed with the diameter ranging from 300-400
nm. TEM analysis revealed that the columnar structure is mainly due to aggregation of
athermal ε-HCP martensite phase. Small quantity of metal carbide of type M23C6 was also
observed. Mengucci et al [23] carried out detailed TEM analysis of columnar structure in
DMLS CoCrMoW alloy. Elongated precipitates with HCP structure and composition
resembling Co3(Mo,W)2Si were observed at the column boundaries. STEM-EDS analysis
also confirmed the presence of small dark spherical Si-rich inclusions close to these
precipitates. Such Si-rich inclusions were also observed in the microstructures of as cast
biocompatible CoCrMo alloy in the study by Giacchi et al. [28].
1.4.4 Tensile properties of DMLS processed parts
Room temperature tensile properties of as printed DMLS CoCrMo are reported in the
literature. Table 1.4 shows the average tensile properties of the as printed DMLS CoCrMo
specimens tested along build direction (vertical, Z-axis) and perpendicular to the build
direction (horizontal, XY plane). Considerable mechanical anisotropy as shown in Table 1.4
24
was also observed by Takaichi et al [27] for SLM CoCrMo and Vilaro et al [29] for Nimonic
263 nickel based superalloy. Tensile properties were also observed to vary with the
processing parameters [27].
Mengucci et al [23] observed mixed areas of ductile failure as well as quasi cleavage
facets for fracture surfaces of as printed DMLS CoCrMoW tensile specimens (room
temperature). High UTS and hardness was attributed to intricate network of ε-HCP martensite
phase in the γ-FCC matrix. Quian et al [16] also observed similar cleavage facets (brittle) at
low magnification and some dimples (ductile) at high magnification on as printed SLM
CoCrMo fracture surfaces.
Table 1.4 Tensile properties of As printed DMLS CoCrMo-EOS Materials Data Sheet
[30]
Property Vertical direction (Z) Horizontal direction (XY)
0.2% Yield strength 800±100 MPa 1060±100 MPa
UTS 1200±150 MPa 1350±100 MPa
% Elongation at break 24±4 11±3
Modulus of elasticity 190±20 GPa 200±20 GPa
Most of the existing literature for DMLS processed CoCrMo alloy deals with its
biomedical applications. The suitability of DMLS processed CoCrMo alloy for high
temperature structural applications has not been tested so far. Also the effect of solution and
ageing heat treatment on microstructure and mechanical properties at RT and high
temperature is not reported in literature.
1.5 Direct Metal Laser Deposition (DMLD)
DMLD is an AM technique which works by injecting powder into the melt pool created with
laser rather than sintering a powder bed [3]. DMLD process can operate with local shielding
and doesn’t require inert gas chamber for less reactive metals such as Nickel and Cobalt
alloys. Figure 1.10 shows schematic representation of DMLD process. Steps involved in
DMLD process are given below [3]:
1. Substrate or existing block is placed on the work table.
2. The process nozzle with concentric laser beam is focused on the surface to create melt
pool.
3. Coaxial nozzle is used to feed powder into the melt pool
4. Process nozzle moves at a constant speed and follows a predetermined tool path created
using CAD data.
5. Melt pool solidifies when nozzle moves away forming a layer of solidified metal.
25
6. The process is repeated and part is built layer by layer.
Figure 1.10 Schematic drawing showing Direct Metal Laser Deposition technology
(Courtesy: DM3D Technology) [3].
1.6 Motivation and Objectives
This thesis presents the use of two popular additive manufacturing techniques, i.e., Direct
Metal Laser Sintering (DMLS) on CoCrMo and Direct Metal Laser Deposition (DMLD) on
FSX-414 for the possible use in repair of hot gas path components in industrial turbines. The
focus of this work is on microstructural characterization, room temperature and high
temperature tensile properties of AM parts with the view to evaluate suitability of this
technique for the current high temperature application.
Objectives of the thesis are as follows:
Study the microstructural evolution and tensile properties of DMLS processed CoCrMo
Study the effect of solution and ageing heat treatments on microstructures and tensile
properties of the DMLS CoCrMo alloys.
Study of microstructural evolution and tensile properties of DMLD processed FSX-414
Study of solution and ageing heat treatment response on microstructure and room
temperature tensile properties of DMLD processed FSX-414.
26
2. MATERIALS AND EXPERIMENTAL
PROCEDURE
2.1 Materials and Heat treatment
The material used for this study comprised two Cobalt based alloys viz., CoCrMo and
FSX-414, whose nominal composition is listed in Table 2.1. The CoCrMo alloy was
processed with direct metal laser sintering (DMLS), a powder bed fusion technique, at Intech
DMLS, Bangalore. This part will henceforth be referred to as DMLS CoCrMo through the
rest of this report. The FSX-414 alloy was processed with direct metal laser deposition
(DMLD), a directed energy deposition technique, at DM3D Technologies, USA and thus will
be referred to as DMLD FSX-414 in this thesis.
Both DMLS CoCrMo and DMLD FSX-414 parts were subjected to solution heat
treatment in vacuum furnace at 1050 ᵒC for 4 hours followed by aging heat treatment at 980
ᵒC for 4 hours. Heat treatment conditions (Table 2.2) were chosen in order to mimic the heat
treatment of the actual nozzle. As-printed, solution heat treated and aged specimens from
both DMLS CoCrMo and DMLD FSX-414 were cut using abrasive wheel cutter and wire
Electrical Discharge Machining (EDM) for detailed microstructural and mechanical
characterization.
Table 2.1 Nominal chemical compositions of CoCrMo and FSX-414 alloys
Elements Co Cr Mo Ni W Mn Si C Fe
CoCrMo Bal. 28.7 7 - - 0.9 0.9 0.1 -
FSX-414 Bal. 29.8 - 10.6 7 0.9 06 0.2 1
Table 2.2 Heat treatment conditions for both DMLS CoCrMo and DMLD FSX-414
Heat treatment Conditions
Solution heat treatment 1050ᵒC in vacuum furnace for 4 hours followed by
argon fan quench
Solution heat treatment + Ageing 980ᵒC in vacuum furnace for 4 hours followed by
furnace cool
27
2.2 Powder characterization
Particle size distribution (PSD) analysis was performed on both CoCrMo and FSX-414
powders using ‘Mastersizer 2000E’ laser diffraction based powder size analyzer. SEM-EDS
analysis was performed on Zeiss EVO18 Scanning Electron Microscope along with Oxford
link energy dispersive spectroscopy (EDS) to characterize morphology and composition of
the powders. Preliminary phase identification was carried out using X-ray diffraction on the
powders using a Rigaku Miniflex600 (Cr Kα – 2.29 Aᵒ wavelength).
2.3 Processing conditions
Table 2.3 enlists process parameters used for both Direct Metal Laser Sintering (DMLS) of
CoCrMo and Direct Metal Laser Deposition (DMLD) of FSX-414 alloy. Flat DMLS
CoCrMo coupons (125 mm×43 mm×10 mm) were printed with holes in order to monitor
roughness of both flat as well as curved surface (within holes). Cylindrical DMLS CoCrMo
coupons (length-95 mm, dia-15 mm) were also made to prepare tensile specimens. In case of
DMLD, FSX-414 powder was deposited directly on the investment cast FSX-414 nozzle.
Figure 2.1 shows representative macro photographs of as printed DMLS CoCrMo and
DMLD FSX-414 parts.
Table 2.3 Process parameters for Direct Metal Laser Sintering (DMLS) of CoCrMo and
Direct Metal Laser Deposited (DMLD) FSX-414
Source INTECH-DMLS DM3D Technologies
Material CoCrMo FSX-414
Laser power 290 W 1000 W
Laser beam diameter 80 µm 2 mm
Powder feeding rate - 15 g/min
Layer thickness 40 µm ~ 400 µm
Hatch distance 110 µm -
Scanning speed 950 mm/s 8.3 mm/s
Powder size 10 to 50 µm 35-95 µm
Powder source EOS Praxair
Time for printing
Mini Coupons (24 in one run)
48-50 hours 80-90 mins
28
Figure 2.1 (a) Flat Direct Metal Laser Sintered (DMLS) CoCrMo coupon with cooling holes,
(b) Cylindrical DMLS CoCrMo for making tensile specimens, (c) Direct Metal Laser
Deposited (DMLD) FSX-414 on a cast nozzle
2.4 Chemical Analysis – Inductively Coupled Plasma (ICP)
The chemical composition of the DMLS CoCrMo, DMLD FSX-414 and cast FSX-414
nozzle was measured using an Inductively Coupled Plasma (ICP) technique.
2.5 Surface roughness
The first step in the characterization of any additively manufactured coupon is the part
surface roughness which was measured using a Zeiss Surfcom 1800D surface profilometer.
The Ra roughness parameter which is arithmetic average of heights of each point on the
surface was chosen for the measurements.
𝑅𝑎 = 1
𝑛 ∑|𝑦𝑖|
𝑛
𝑖=1
n = No. of points, y = height at particular point
(a)
(c)
(b)
Buil
d d
irec
tion
Buil
d d
irec
tion
DMLD FSX-414
Buil
d d
irec
tion
125 mm×43 mm×10 mm
Length-95 mm, Dia-15 mm
130 mm×25 mm×10 mm
Cast FSX-414 nozzle
29
Roughness of cooling holes:
Measurement of roughness of cooling holes was not possible using profilometer. Thus
section perpendicular to cooling holes was cut, mounted and subjected to metallographic
preparation for investigation using optical microscope as shown in Figure 2.2.
Figure 2.2 Procedure for determining hole roughness using optical microscopy
2.6 X-ray Tomography
X-ray Tomography was performed using GE Phoenix v-tome-xs machine with a 240 kV/320
W microfocus tube (resolution – 7 to 10µm). 3D Computed Tomography was not possible
because of dimensional constraints of the part. Hence, 2D X-ray images were taken at
different angles to determine the presence of voids and cracks in DMLS CoCrMo and DMLD
FSX-414 samples which could seriously affect mechanical behavior of the printed
component.
2.7 Metallographic procedure
Various cross sections of the part shown in Figure 2.3 were cut and hot mounted using
phenolic resin in Buehler SimpliMet 3000 machine. Samples were polished using Struers
automatic polishing machine (Tegramin 25). Table 2.4 enlists sequence of polishing steps
followed. All the samples were subjected to ultrasonic cleaning in a water-detergent solution
to remove colloidal silica particles entrapped in voids.
Cooling holes
Cut section
30
Figure 2.3 Nomenclature of various sections of DMLS CoCrMo and DMD FSX-414
component.
Table 2.4 Polishing steps followed for DMLS CoCrMo and DMLD FSX-414
Abrasive type Suspension Time (min) Force applied (N)
MD Piano 220 Water 3:00 50
Allegro Diamond – 9 µm 5:00 40
Dac Diamond – 6 µm 3:00 30
Mol Diamond – 3 µm 2:00 30
Nap Diamond – 1 µm 2:00 20
Chem Colloidal Silica 5:00 50
2.8 Porosity area fraction
Porosity was evaluated on transverse section of DMLS component using Nikon Optical
Microscope (Eclipse MA200) and Clemex Image analysis software. Stage pattern was created
on the image using Clemex software. Sample stage was made to navigate automatically on
each block of the pattern and calculate porosity area fraction by autofocus and auto-gray scale
methods using a macro code in Clemex software. Porosity distribution was plotted on
transverse section of coupon along the build direction. Figure 2.4 & 2.5 shows method of %
porosity evaluation for DMLS and DMLD components respectively.
Buil
d d
irec
tion
Transverse section
Base
Front planar
Longitudinal
direction
31
Figure 2.4 Method of % porosity evaluation on transverse section of DMLS component.
Figure 2.5 Method of % porosity evaluation on transverse section of DMLD FSX-414
component.
2.9 Optical and Scanning Electron Microscopy
Microstructural investigation was carried out on the Nikon Eclipse Optical microscope.
Samples were etched using 5% HCl (Electrolytic, 6V) for 10 seconds. Optical micrographs of
as-printed as well as heat treated DMLS CoCrMo and DMLD FSX-414 parts were taken at
various magnifications. Grain size measurement and distribution was carried out using
ImageJ software.
Detailed microstructural characterization was performed using scanning electron
microscopy (SEM). Zeiss SIGMA (Field Emission) microscope was used at an accelerating
voltage of 20 kV and working distance of 8.5 mm. BSE images were taken on the unetched
specimens to get qualitative information on extent of elemental segregation. An Oxford –
LINK system EDS (Energy dispersive spectroscopy) attached to the microscope was used for
getting compositions of samples and various phases within it. EDS mapping was also
performed to systematically identify various phases present in the sample.
0 40 mm
0 40 mm 60 mm
Stage Pattern
Build direction
DMLD FSX-414 Cast FSX-414
Build direction
32
2.10 Transmission Electron Microscopy
Transmission Electron Microscopy was carried out to identify composition and crystal
structure of fine precipitates and phases in the samples. TEM samples were prepared by first
cutting thin section (around 300 µm) using ‘Buehler Isomet Low speed saw’ and then
mechanically polishing on SiC paper ranging from 1200 grit to 4000 grit. Mechanical
polishing was done till sample reaches thickness of 80 µm. Streurs Twin jet electropolishing
machine was used to make electron transparent hole in the sample. FEI Technai F30 was used
for generating TEM bright field, HAADF and high resolution images. EDAX energy
dispersive spectroscopy attached to TEM was used to get compositions of phases present.
2.11 Electron Backscattered Diffraction (EBSD)
Electron Backscattered Diffraction (EBSD) analysis was performed to determine texture
evolution during 3D printing as well as texture changes after the heat treatment. EBSD was
carried out on both DMLS CoCrMo and DMLD FSX-414 samples using HKL EBSD link
system attached to Zeiss EVO18 Scanning Electron Microscope with step width of 1µm.
Samples for EBSD were prepared using normal metallographic procedure followed by
additional fine polishing in colloidal silica in Beuhler Vibromet polishing machine. EBSD
orientation maps were generated using TSL OIM analysis software.
2.12 X-ray Diffraction (XRD)
The Rigaku Miniflex 600 X-ray diffractometer with Cu Kα target (wavelength-1.54 Aᵒ) and
Ni-filter was used for identifying phases in samples. XRD analysis was done using PDXL
software using ICDD database. XRD parameters are listed in Table 2.5.
Table 2.5 X-ray diffraction parameters
Parameter Values
Starting angle (deg) 10
Finishing angle (deg) 120
Step size (deg) 0.005
Speed (deg/min) 0.5
Wavelength (Å) 1.54
Voltage (kV) 40
Current (mA) 15
Slit width (mm) 10
33
2.13 X-ray Diffraction Sin2 ψ technique for residual stress measurement
Residual stress along both the build and longitudinal direction was measured using a Rigaku
Automate II Micro-area X-ray residual stress. Surface was electropolished in order to reduce
roughness and to remove the impurities. In this study, residual stress was measured at various
locations on the electropolished surface of as printed DMLS CoCrMo as shown in the Figure
2.6. Sample stage was made to navigate automatically on selected grid to calculate residual
stress at every intersection. 2D residual stress distribution along the build direction was
plotted in Origin8.5 software.
Residual stress measurement was carried out using (220) diffraction peak of FCC
Cobalt with a Chromium Kα source (wavelength-2.29 Aᵒ). Table 2.6 enlists various
parameters used for measurement of residual stress.
Table 2.6 Parameters for X-ray residual stress measurements
Parameters Values
X-ray Source Cr - kα (λ = 2.29 Aᵒ)
Generator settings 40 kV, 40 mA
Diffraction peak (220) at 130ᵒ
Scanning range 127ᵒ to 133ᵒ
Step width 0.1ᵒ
Counting time 30 s
Collimator size 1 mm
ψ – angles 0, 10, 15, 20, 25, 30, 35, 40, 45, 50
Stress constant -720.28 MPa/deg
Young’s modulus 241 GPa
Poisson’s ratio 0.3
Absorption coefficient 2389 1/cm
Kα2 elimination ratio 0.5
34
Figure 2.6 Method used for measuring residual stress on as printed DMLS CoCrMo part.
2.14 Microhardness
The Vickers hardness was carried out using Shimadzu Micro Hardness tester with 300 gm
load and 10 sec dwell time. Standard formula for Vickers hardness is given below:
HV = 1.854
𝑑2 𝑃
Where, P is force applied in kgf and d is the mean diagonal of the indentation.
Hardness readings were taken on the transverse sections of DMLS CoCrMo coupon at the
three different locations viz. 10, 50 and 90 % spans (Figure 2.7a). Hardness profile was
plotted along build direction for all these three locations. While for DMLD FSX-414
hardness readings were taken on only 10% span section as shown in Figure 2.7b.
Electropolished surface
Buil
d d
irec
tion
30 m
m
10 mm
DMLS CoCrMo
Longitudinal direction
35
Figure 2.7 (a) DMLS CoCrMo and (b) DMLD FSX-414 components showing different
locations for taking hardness readings.
2.15 Tensile testing
Both room temperature and high temperature (925ᵒC) tensile tests were carried out on DMLS
CoCrMo, DMLD FSX-414 and Cast FSX-414 samples. Varied specimen geometries were
used owing to the material constraint as shown Table 2.7.
Table 2.7 Specimen geometries and testing conditions used for tensile testing
Room temperature High temperature
DMLS
CoCrMo
Flat tensile specimen (ASTM standard) Round tensile specimen (ASTM
standard)
Gauge length 1” Gauge length 1”
Strain rate upto 2% 0.005 in/in/min Strain rate upto
2%
0.005 in/in/min
Strain rate after 2% 0.05 in/min Strain rate after
2%
0.05 in/min
DMLD
FSX-414
Micro-tensile specimen
90% span
50% span
10% span
Buil
d d
irec
tion
10% span
Cast
FSX-414
DMLD
FSX-414
36
t=0.5mm
-
Gauge length 6 mm
Strain rate 0.006 mm/s
Cast
FSX-414
Micro-tensile specimen
(same as above)
- Gauge length 6 mm
Strain rate 0.006 mm/s
37
3. RESULTS - PART A: DIRECT METAL
LASER SINTERING OF CoCrMo
This chapter presents outcome of the experimental work carried out to characterize DMLS
processed CoCrMo and DMLD processed FSX-414 in order to evaluate the suitability of
these processes for possible gas turbine applications. The results are described in two parts:
DMLS of CoCrMo and DMLD of FSX-414. First part describes microstructural evolution in
as-printed DMLS CoCrMo, effect of solution treatment and ageing on microstructure and
both room temperature and high temperature tensile properties. Second part describes
microstructural evolution in as deposited DMLD FSX-414, effect of solution treatment and
ageing on microstructure and room temperature tensile properties.
3.1 Powder characterization
CoCrMo powder was analyzed for the morphology, composition and particle size. SEM
micrograph of powder in Figure 3.1 reveals a spherical morphology with particle size in the
range of 5-45 µm. Table 3.1 shows the composition of CoCrMo powder analyzed using EDS.
Particle size was calculated using Mastersizer 2000E laser diffraction based powder size
analyzer. Particle size distribution in Figure 3.2 shows the powder size is normally distributed
with average size of 24 µm.
Figure 3.1 SE images showing size and morphology of as received CoCrMo powder, at (a)
low magnification (b) high magnification.
Table 3.1 Composition of CoCrMo powder analyzed using EDS
Elements Co Cr Mo Mn Si
Wt % 62.81 28.34 6.97 0.75 0.77
At% 60.7 30.9 4.12 1.66 0.84
(a) (b)
~ 24 µm
38
Figure 3.2 Particle size distribution of CoCrMo powder
3.2 Chemical analysis
Table 3.2 gives chemical composition of as printed DMLS CoCrMo analyzed using
Inductively Coupled Plasma (ICP). The measured composition is roughly similar to the
nominal CoCrMo composition except for iron and carbon which are little bit higher.
Table 3.2 Chemical composition of Direct Metal Laser Sintered (DMLS) CoCrMo and
corresponding nominal composition
Elements Co Cr Mo Ni Mn Si C Fe
CoCrMo (wt%) 62.5 27.66 6.42 0.1 0.75 0.56 0.18 1.12
Nominal
CoCrMo[30]
(wt%)
60-65 26-30 5-7 0.1 1.0 1.0 0.16 0.75
3.3 Surface Roughness
Table 3.3 shows surface roughness readings in Ra parameter measured along the build and
longitudinal direction. The surface roughness ranges between 3-5 µm, Ra along the vertical
direction and 1-5 µm along the horizontal direction. Figure 3.3 shows representative optical
micrographs of transverse sections of surface and hole. Micrograph of hole (Figure 3.3b)
shows very smooth surfaces even on the curved surface.
39
Table 3.3 Surface roughness of as printed DMLS CoCrMo coupon
Roughness (µm,
Ra)
Reading 1
(µm, Ra)
Reading 2
(µm, Ra)
Reading 3
(µm, Ra)
Reading 4
(µm, Ra)
Average
(µm, Ra)
Vertical
direction 4.34 4.08 3.97 3.81
3.59 ± 1.04
Horizontal
direction 1.00 4.54 3.43 3.53
Figure 3.3 Representative micrographs showing topography of (a) surface and (b) hole of as
printed DMLS CoCrMo.
3.4 X-ray Microtomography of as printed DMLS CoCrMo
2D X-ray microtomography images at 0ᵒ and 30ᵒ sample tilt are shown in Figure 3.4 and 3.5
respectively. X-ray images do not show any indication of significant voids or cracks with the
size big enough to get resolved with X-ray microtome (resolution: 10-15 µm in the as printed
samples. Black and white contrast in both 0ᵒ and 30ᵒ tilt image is due to the holes (Figure
3.4) present in the samples. Two different tilts were chosen because some features can be
invisible in one of the image if they are parallel to the X-ray beam.
(a) (b)
40
Figure 3.4 2D X-ray Microtomography images of as printed DMLS CoCrMo with 0ᵒ tilt
(Voltage: 200 kV, Current: 500 µA).
Figure 3.5 2D X-ray Microtomography images of as printed DMLS CoCrMo with 30ᵒ tilt
(Voltage-200 kV, Current-500 µA).
3.5 Porosity
Figure 3.6 shows porosity distribution along the transverse section of the as printed DMLS
CoCrMo coupon. The average porosity is around 0.043±0.04% and also porosity does not
vary much along the build direction. Unetched micrograph in different locations viz, A, B
and C show pores has fine, spherical morphology. Some pores with irregular morphology can
41
be seen in location C. Interlayer porosity which is commonly observed in the various DMLS
processed alloys is absent in the given sample. The processed parameters have been
optimized to enable a dense component.
Figure 3.6 Porosity distribution on transverse section of as printed DMLS CoCrMo along the
build direction and corresponding unetched microstructure showing porosity at locations A,
B, C respectively.
3.6 Microstructural Characterization of as printed DMLS CoCrMo
3.6.1 Optical microscopy
Figure 3.7 shows optical micrographs of transverse, front planar and base sections of the as
printed DMLS CoCrMo. Size of this pool is very small around ~120 µm wide and ~60 µm
measured using ImageJ software. Transverse section macrostructure in Figure 3.7 clearly
shows solidified melt pools stacked layer by layer. Solidified scan paths can be seen in the
base macrostructure. It can be observed that the angle between two scan paths is 67ᵒ as
shown in Figure 3.7. The optical micrographs of transverse section in Figure 3.8a show
location of irregular pores as close to the melt pool boundary. High magnification optical
micrograph in Figure 3.8b clearly shows microcracks. It can be seen that microcracks are
almost perpendicular to the melt pool boundary.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 5 10 15 20 25 30 35 40
% P
oro
sity
are
a fr
acti
on
Distance from the base
A B
C
A B C
42
Figure 3.7 Optical micrographs of transverse, front planar and base sections of as printed
DMLS CoCrMo, etched with 5% HCl - electrolytic – 6V.
3.6.2 Scanning Electron Microscopy
Figure 3.9 shows SEM micrographs of as printed DMLS CoCrMo samples. Columnar
structure is evident inside the solidified melt pool. Melt pool boundaries can be seen clearly.
Figure 3.9b shows a one to one matching of columns across the melt pool grain boundaries.
Thus columns growing inside melt pool during solidification seem to be adopting orientation
of columns in earlier layers. Similarly oriented columns forms domains inside the melt pool
as can be seen in Figure 3.9a.
~60
µm
43
Figure 3.8 Optical micrographs of transverse section of as printed DMLS CoCrMo at (a) low
magnification showing irregular pores at the interlayer boundaries and (b) high magnification
showing microcracks (etched with 5% HCl, electrolytic-6V)
Figure 3.10 shows BSE images of etched as printed DMLS CoCrMo sample. The
primary dendrite arm spacing (PDAS) is around 600 nm to 1000 nm and shows no branching
into secondary dendritic arms throughout the sample. Unetched BSE images in the Figure
3.11 show the presence of elongated bright precipitates in the interdendritic regions. This
bright contrast in the interdendritic region indicates possible elemental segregation along the
column width. Dark globular phases can be observed sometimes in conjunction with bright
precipitates and sometimes isolated in the matrix. Elongated bright precipitates have width of
around 30-70 nm and globular dark phases have diameter of around 20-70 nm measured
using ImageJ software. In order to further discern location, size, morphology and
composition of various phases, transmission electron microscopy was carried out.
Figure 3.9 SE image of as printed DMLS CoCrMo at (a) low magnification showing
columnar microstructure and domains (grains) and (b) high magnification one to one
matching along melt pool boundary (Etched with 5% HCl, electrolytic-6V).
(b) (a)
Melt pool boundary
(b) (a)
44
Figure 3.10 BSE images of as printed DMLS CoCrMo at (a) low magnification, and (b) high
magnification showing bright contrast in the interdendritic region, etched with 5% HCl
(electrolytic-6V)
Figure 3.11 (a,b) High magnification BSE images of unetched as printed DMLS CoCrMo
showing interdendritic precipitates and ε-HCP (arrow) cutting across the columns.
3.6.3 TEM Micrographs of As printed DMLS CoCrMo
Transmission electron microscopy was carried out on the as printed DMLS CoCrMo sample
to find out location, size and morphology of various inter-dendritic and intra-dendritic phases.
Figure 3.12 shows TEM bright field image of intra-dendritic region of the sample at [011]
zone axis. Dark plates like phases are in the 70.52o angle. Fringe contrast can also be
observed. High resolution (HR) TEM image and corresponding Fast Fourier Transform (FFT)
pattern in Figure 3.13 clearly shows the presence of fully coherent alternate bands of ε-HCP
and γ-FCC phase in the region of dark plate phase. The width of the ε-HCP band is around
9.2 nm. Figure 3.14 shows FFT pattern of entire HRTEM image (Figure 3.13). This pattern
confirms (111) γ || (0001)ε, [011]γ || [1210]ε , i.e., Shoji-Nishiyama orientation relationship
between γ-FCC and ε-HCP phases.
(a) (b)
(a) (b)
(111)
45
High Angle-Annular Dark Field (HAADF) STEM images in Figure 3.15(a,b) shows
precipitates with elongated and globular morphology in the interdendritic region similar to
the BSE image. Bright and dark contrast of elongated and globular precipitates respectively
clearly indicates possible elemental segregation in the interdendritic region. TEM-EDS
analysis was carried out in order to get chemical compositions of these precipitates. Table 3.4
enlists chemical composition of bright, dark precipitates and matrix in both weight% and
atomic% units. It can be observed that interdendritic bright precipitates are Mo and Si rich,
while globular dark precipitates and Si rich.
Figure 3.12 TEM bright field image of as printed DMLS CoCrMo sample showing ε-HCP
phases in γ-FCC CoCrMo matrix.
Figure 3.13 High resolution-(HR) TEM image of as printed DMLS CoCrMo and its
corresponding FFT pattern showing existence of HCP phase
70.52ᵒ
30.1ᵒ
46
Figure 3.14 FFT pattern of entire HRTEM image from Figure 13 showing spots for both FCC
and HCP and their orientation relationship
Figure 3.15 High Angle Annular Dark Field (HAADF) STEM images of as printed DMLS
CoCrMo specimen showing elongated bright precipitates and globular black precipitates in
the interdendritic region.
(a) (a)
47
Table 3.4 Chemical analysis of various phases in as printed DMLS CoCrMo specimen
using TEM-EDS.
Elements Co Cr Mo Mn Si
Bright
precipitate
Wt% 44.2 27.4 22.3 2.7 3.3
At% 44.7 31.4 13.0 3.0 7.0
Dark
precipitate
Wt% 55.1 28.4 7.6 3.0 5.8
At% 51.2 30.0 4.4 3.1 11.3
Matrix Wt% 60.4 26.4 7.4 3.0 1.8
At% 58.4 29.1 4.5 3.9 3.7
3.7 Microstructural characterization of Solution heat treated DMLS
CoCrMo
Figure 3.16 shows optical micrographs of transverse, front planar and base section of solution
heat treated DMLS CoCrMo. The microstructure shows fully equiaxed grains on all sections
in contrast to the as printed DMLS CoCrMo. The average grain size is around 44 µm. Figure
3.17 shows the optical micrograph of solution treated DMLS CoCrMo and its corresponding
grain size distribution. Grain size ranges between 5 to 100 µm. The dendritic structure and
melt pool boundaries in as printed sample has homogenized completely after heat treatment
and grain size shows no correlation with either powder size or melt pool size.
Figure 3.16 Optical micrographs of Transverse, front planar and base sections of Sol HT
DMLS CoCrMo showing equiaxed grains on all sides, etched with 5% HCl, electrolytic – 6V
48
Figure 3.17 (a) Optical micrograph of Sol HT DMLS CoCrMo showing equiaxed
microstructure with average grain size of 44 µm and (b) corresponding grain size distribution.
BSE images of the solution treated samples show in Figure 3.18a shows twinning in the
equiaxed microstructure. Dark and bright contrast is due to orientation difference between the
grains. A representative EDS spectrum in Figure 3.18b and corresponding chemical
composition in Table 3.5 shows similar chemistry as that of nominal CoCrMo alloy. High
magnification BSE images of unetched solution treated samples in Figure 3.19 shows the
elliptical shaped bright precipitates on the grain boundaries having contrast very similar to
what observed for elongated bright precipitates in as printed DMLS CoCrMo (Figure 3.11).
Also some bright contrast can be seen inside the grains as well (Figure 3.19b). Dark globular
phases can also be observed and are very similar to that observed in as printed DMLS
CoCrMo in Figure 3.11.
Figure 3.18 (a) BSE image of unetched Sol HT DMLS CoCrMo showing twins and equiaxed
microstructure, (b) corresponding EDS spectrum.
0
5
10
15
20
25
30
5 20 35 50 65 80 95
No. of
gra
ins
Grain size (µm)
Grain size distribution (a) (b)
(a) (b)
49
Figure 3.19(a,b) High magnification BSE images of unetched Sol HT DMLS CoCrMo
showing remnants of previous interdendritic precipitates inside grains as well as precipitates
along grain boundaries.
Table 3.5 EDS composition of solution heat treated specimen
Elements Co Cr Mo Si Mn C
Wt% 62.83 28.23 6.95 0.82 0.81 0.37
At% 60.70 30.91 4.12 1.67 0.84 1.75
3.8 Microstructural characterization of Sol HT + Aged DMLS CoCrMo
In order to evaluate the high temperature stability, the solution treated CoCrMo was given an
ageing heat treatment. Figure 3.20(a,b) shows optical micrographs of Sol HT+aged DMLS
CoCrMo sample. Microstructure shows equiaxed grains with similar grain size as that of
solution heat treated samples. Precipitation of various phases inside grains and along the
grain boundaries can be seen after aging (Figure 3.20b). SEM-EDS analysis was carried out
to discern size, morphology and composition of the precipitates. Figure 3.21 shows BSE
images of the aged samples clearly showing various bright and dark precipitates. Dark
precipitates have globular morphology everywhere similar to what observed in as printed and
solution heat treated specimens. Most of the intragranular bright precipitates have plate
morphology while some are globular as well. Almost all the intergranular bright precipitates
have either globular or elliptical morphology similar to the intergranular precipitates in
solution treated samples (Figure 3.19). It can be seen that precipitates cover almost the entire
grain boundary. Table 3.6 gives chemical composition of the matrix and both bright and dark
precipitates. Composition wise there is no difference between plate shaped and globular
bright precipitates. Chemical composition of precipitates in Table 3.6 shows that though both
(a) (b)
50
the bright precipitates are rich in Molybdenum and Silicon. Dark phase is rich in Silicon and
Manganese. EDS mapping of high magnification BSE image in Figure 3.22 confirms the
Molybdenum and Silicon enrichment in the bright precipitates.
Figure 3.20 Optical micrographs of Sol HT+Aged DMLS CoCrMo at (a)200x and (b)1000x
showing equiaxed grain structure and some precipitation (arrow), etched with 5% HCl –
electrolytic, 6V.
Figure 3.21(a,b) BSE images of Sol HT+Aged DMLS CoCrMo samples showing bright and
dark precipitates inside the grains as well as along the grain boundaries.
Table 3.6 Composition of various phases in Sol HT+Aged DMLS CoCrMo
Elements Co Cr Mo Mn Si C
Bright
precipitate
Wt% 51.2 27.5 17.3 1.5 2.6 -
At% 51.1 31.2 10.7 1.6 5.5 -
(a) (b)
(a) (b)
51
Figure 3.22 (a) High magnification BSE image of Sol HT+Aged DMLS CoCrMo sample
showing bright and dark precipitate, and (b) corresponding EDS elemental mapping.
3.9 X-ray Diffraction
Phase identification was carried out using XRD, as shown in Figure 3.23. X-ray diffraction
peak identification was done using two ICDD cards, one for FCC-Co and other for HCP Co.
XRD patterns of all four samples viz, powder, as printed and solution heat treated and Sol
HT+aged DMLS CoCrMo, etc. shows peaks for both the γ-FCC and ε-HCP Cobalt phases.
Dark
precipitate
Wt% 47.6 38.4 10.9 2.0 1.2 -
At% 46.5 42.5 6.5 2.1 2.4 -
Matrix Wt% 63.0 28.4 6.8 0.6 0.8 0.3
At% 60.9 31.1 4.07 0.6 1.6 1.7
(a)
(b)
52
Table 3.7 shows lattice parameters of both γ-FCC and ε-HCP phases in all samples. %Phase
fraction of ε-HCP phase in all the samples is shown in the Table 3.8. % HCP was calculated
using Sage and Guillad [31] equation:
Figure 3.23 X-ray Diffractions patterns of (a) CoCrMo Powder, (b) as printed (c) Sol HT , (d)
Sol HT+aged, DMLS CoCrMo, all showing peaks for both γ-FCC and ε-HCP Cobalt phases
(Target: Cr-Kα -2.29 Aº)
Table 3.7 Lattice parameters of FCC and HCP phases in various CoCrMo samples
Specimen Lattice parameter (Ao)
γ-FCC ε-HCP
CoCrMo powder a= 3.5794 a= 2.5310, c= 4.7773
As printed DMLS CoCrMo a= 3.5787 a= 2.5305, c= 4.7742
Sol HT DMLS CoCrMo a= 3.5846 a= 2.5347, c= 4.7867
0
50
100
150
200
250
300
350
400
10 30 50 70 90 110 130 150
Inte
nsi
ty (
arbit
rary
unit
s)
2Ɵ
As printed
Sol HT
Sol HT+Aged
CoCrMo Powder
(a)
(b)
(c)
(d)
* *
* * *
*
*
2
2
2
2
2
2
2
2
1
1
1
1
1, 2
1, 2
1, 2
1, 2
1, 2
1, 2
1, 2
1, 2
*
1 – γ(FCC) Co
ICDD: 00-015-806
2 – ε(HCP) Co
ICDD: 00-005-726
*Unidentified
53
Sol HT + Aged DMLS CoCrMo a= 3.5743 a= 2.5274, c= 4.7493
Pure Cobalt [Source:
periodictable.com]
a= 3.5447 a= 2.5071, c= 4.0695
Table 3.8 %Phase fraction of HCP phase in various CoCrMo samples
Specimen % Phase fraction of ε-HCP phase
CoCrMo powder 45.1
As printed DMLS CoCrMo 19.7
Sol HT DMLS CoCrMo 13.6
Sol HT+Aged DMLS CoCrMo 28.6
3.10 Electron Backscattered Diffraction (EBSD) – Inverse Pole Figure
maps
Figure 3.24 shows EBSD IPF maps of as printed and solution treated DMLS CoCrMo. Large
elongated grains can be observed in the as printed EBSD pattern. Heat treated pattern shows
equiaxed grains with twins. No preferred orientation can be observed in both as printed and
heat treated microstructures.
Figure 3.24 EBSD IPF Maps of (a) As printed, (b) Solution heat treated DMLS CoCrMo
(a) As printed DMLS CoCrMo (b) Solution heat treated DMLS CoCrMo
54
3.11 X-ray Diffraction Sin2 ψ technique for residual stress measurements
Residual stress analysis was carried out on the electropolished surface of as printed DMLS
CoCrMo. Figure 3.25a shows the distribution of residual stress on the surface of the sample.
Residual stress ranged between 550 MPa to 950 MPa. It can be seen that residual stress
decrease gradually along the build direction (Figure 3.25b).
Figure 3.25 (a) Residual stress distribution on the surface of as printed CoCrMo, (b)
Variation of residual stress along the build direction
3.12 Hardness
Hardness profiles were plotted against build direction for transverse sections from different
spans as can be seen in Figure 3.26. Hardness value of as printed sample are between 420-
460 HV and after solution heat treatment it decreases to 340-380 HV. Hardness almost
remains constant after ageing treatment.
450
600
750
900
1050
2 4 6 8 10 12 14 16 18 20
Res
idual
str
ess
(MP
a)
Distance from base (mm)
Build direction
(a)
(b)
55
Figure 3.26 Hardness profile along build direction (a) as printed, (b) Sol HT, (c) Sol
HT+aged, DMLS CoCrMo.
3.13 Tensile properties
Figure 3.27 shows 0.2% yield strength, UTS and ductility data for room temperature and high
temperature (925ºC) tensile tests. As printed samples were tested only at room temperature
and shows highest yield strength and UTS as compared to Sol HT and Sol HT+Aged
samples. Sol HT and Sol HT+Aged samples were tested at both room temperature and at
925oC. For room temperature tensile tests, Sol HT samples shows very high ductility as
compared to as printed or Sol HT+Aged specimens. For high temperature tests, ductility of
Sol HT samples is marginally higher than Sol HT+aged samples. 0.2% Y.S. and UTS values
of the Sol HT and Sol HT +Aged samples are almost similar for both room temperature and
high temperature testing. Sol HT samples shows Sol HT+aged sample shows around 70-80%
decrease in both 0.2% yield strength and UTS, and 300% increase in ductility when tested at
925 ºC as compared to room temperature tests. Figures 3.28, 3.29, 3.30 show engineering
300
340
380
420
460
0 10 20 30 40
Har
dnes
s (H
V)
Distance from base (mm)
10% span
50% span
90% span
300
340
380
420
460
0 10 20 30 40
Har
dnes
s (H
V)
Distance from base (mm)
10% span
50 % span
90 % span
300
340
380
420
460
0 10 20 30 40
Har
dn
ess
(HV
)
Distance from base (mm)
Build direction
(b) Solution heat treated
Build direction Build direction
(c) Sol heat treated
+ aged DMLS
10 % span
(a) As printed
56
stress vs. engineering strain curves for as printed, Sol HT and Sol HT+aged DMLS CoCrMo
respectively. Figure 3.31 shows fracture surface of as printed DMLS CoCrMo tested at room
temperature. Cracks and transgranular facets can be observed. Fractographs of room
temperature tested Sol HT DMLS CoCrMo is shown Figure 32(a,b). Cracks can be observed
in low magnification (Figure 32a). High magnification fractographs in Figure 32b shows fine
dimples. Figure 32c,d shows fractographs of Sol HT samples tested at high temperature. Here
also high magnifaction image shows fine dimples. Figure 33a,b shows fractographs of room
temperature tested Sol HT+Aged DMLS CoCrMo. Cracks and facets of grains can be
observed. Fractographs of high temperature tested Sol HT+aged samples shows intergranular
cracks at low magnification and dimples at high magnification (Figure 33c,d).
Figure 3.27 Room temperature and high temperature tensile properties of as printed, Sol HT,
Sol HT+aged DMLS CoCrMo, (a) % Y.S., (b) UTS, (c) Ductility.
0
300
600
900
1200
1500
As printed Sol HT Sol HT +
Aged
0.2
% Y
.S. (M
Pa)
0
300
600
900
1200
1500
As printed Sol HT Sol HT +
Aged
UT
S (
MP
a)
0
5
10
15
20
25
30
As printed Sol HTSol HT + Aged
% E
longat
ion
0.2% Yield strength
Ductility
Ultimate Tensile Strength
(a) (b)
(c)
57
Figure 3.28 Engineering stress-Engineering strain curve of as printed DMLS CoCrMo tested
at room temperature
Figure 3.29 Engineering stress-Engineering strain curves of Sol HT DMLS CoCrMo tested at
(a) room temperature, (b) 925oC
Figure 3.30 Engineering stress-Engineering strain curves of Sol HT+Aged DMLS CoCrMo
tested at (a) room temperature, (b) 925oC
0
250
500
750
1000
1250
1500
0 2 4 6 8
Str
ess
(MP
a)
Strain %
0
200
400
600
800
1000
1200
0 20 40
Str
ess
(MP
a)
Strain %
0
50
100
150
200
250
0 1 2
Str
ess
(MP
a)
Strain %
0
200
400
600
800
1000
0 0.5 1 1.5 2
Str
ess
(MP
a)
Strain %
0
30
60
90
120
150
180
0 0.5 1 1.5 2
Str
ess
(MP
a)
Strain %
(a)
(a)
(b)
(b)
58
Figure 3.31 Fractographs of as printed DMLS CoCrMo tensile sample showing cracks.
Figure 3.32 Fractographs of Sol HT DMLS CoCrMo tensile samples showing mixed brittle
and ductile type failures.
(a) (b)
(a)
(c) (d)
(b)
Room
tem
per
ature
925ºC
R
oom
tem
per
ature
59
Figure 3.33 Fractographs of Sol HT+Aged DMLS CoCrMo tensile samples showing
intergranular fracture in room both room temperature and high temperature tests.
(a) (b)
(c) (d)
Room
tem
per
ature
925ºC
60
4.RESULTS - PART B: DIRECT METAL
LASER DEPOSITION OF FSX-414
4.1 Powder characterization
FSX-414 powder was analyzed for the morphology, composition and particle size. SEM
micrograph of powder in Figure 4.1 reveals spherical morphology with particle size in the
range of 35-95 µm. Table 4.1 shows composition of FSX-414 powder analyzed using EDS.
Particle size was calculated using laser diffraction based Mastersizer 2000E analyzer. Particle
size distribution in Figure 4.2 shows powder is normally distributed with average size of
around 60 µm.
Figure 4.1 SE images showing morphology of FSX-414 powder in the as received condition
at (a) low magnification and, (b) high magnification .
Table 4.1 Composition of FSX-414 powder analyzed using EDS
Elements Co Cr Ni W Mn Si
Wt % 50 29.3 10.7 7.1 1.7 1.2
Std dev. 0.3 0.2 0.2 0.3 0.1 0.1
(a) (b)
61
Figure 4.2 Particle size distribution of FSX-414 powder.
4.2 Chemical analysis
Table 4.2 gives chemical composition of as printed DMLD FSX-414 analyzed using
Inductively Coupled Plasma (ICP). Measured composition is roughly similar to the nominal
FSX-414 composition except for tungsten which is little bit higher.
Table 4.2 Chemical composition of Direct Metal Laser Deposited (DMLD) FSX-414 and
corresponding nominal composition
Elements Co Cr Ni W Mn Si C Fe
FSX-414 (wt%) 48.9 29.73 10.30 9.90 - - 0.18 0.8
Nominal FSX-
414[30] (wt%)
Bal. 28.5-
30.5
9.5-
11.5
6.5-7.5 0.4-1.0 0.5-1.0 0.2-0.3 1.0
4.3 X-ray Microtomography of as deposited DMLD FSX-414
2D X-ray microtomography images at 0ᵒ and 15ᵒ sample tilt are shown in Figure 4.3. X-ray
images do not show any indication of significant voids or cracks with size big enough to be
resolved with X-ray Microtome (Resolution ~ 10-15 µm) in the as printed samples. Two
different tilts were chosen because some features can be invisible in one of the image if they
are parallel to the X-ray beam.
62
Figure 4.3 2D X-ray Microtomography images of as deposited DMLD FSX-414 samples
with (a) 0o tilt and (b) 15
o tilt (Voltage: 200 kV, Current: 500 µA)
4.4 Porosity
Figure 4.4 shows porosity distribution along the transverse section of the as deposited DMLD
FSX-414. The average porosity in the DMLD part and Cast FSX-414 part is around 0.24 ±
0.06% and 0.4 ± 0.09% respectively. Porosity is not varying much along the build direction.
Unetched micrographs (all 100×) in different locations viz, A, B and C in Figure 4.4 shows
some irregular pores with entrapped unmelted particles as well as spherical pores in the as
deposited DMLD FSX-414 (locations A,B). Pores in cast substrate part (location C) are fine
and densely populated while pores in DMLD part are relatively coarse and sparsely
populated.
4.5 Microstructural Characterization of as deposited DMLD FSX-414
4.5.1 Optical microscopy
The optical micrograph of unetched DMLD FSX-414 in Figure 4.5 clearly shows cracks in
the DMLD part as well as in the DMLD-Cast FSX-414 joint. Optical micrographs of the
etched section of DMLD and DMLD-Cast FSX-414 joint in Figure 4.6a & b show fine
dendritic microstructure. Primary dendrite arm spacing (PDAS) is around 8-12 µm while
secondary dendrite arm spacing (SDAS) is around 5-6 µm. Dendritic columns in Figure 4.6b
are almost perpendicular to the substrate. Layer thickness is around 400 µm as shown in the
stitched optical micrograph in Figure 4.7. Domains or bundles of dendrites with same
orientation can be observed in Figure 4.7.
(a) (b)
63
Figure 4.4 Porosity distribution along transverse section of as deposited DMLD FSX-414 and
Cast FSX-414, and corresponding unetched microstructure showing porosity at locations A,
B, C respectively.
Figure 4.5 Optical micrographs of the unetched as deposited DMLD FSX-414 showing
solidification cracks in (a) DMLD Part and (b) DMLD and Cast FSX-414 joint.
0
0.2
0.4
0.6
0 10 20 30 40 50 60
% P
oro
sity
Distance from Trailing Edge (mm)
Cast FSX-414
A
B
C
A B C
Build direction
DMLD FSX-414
64
Figure 4.6 (a) Optical micrograph of as deposited DMLD FSX-414 showing dendritic
microstructure. (b) as deposited DMLD FSX-414 and Cast FSX-414 joint showing dendritic
growth direction relative to cast FSX-414 substrate, etched with 5% HCl, electrolytic-6V.
Figure 4.7 Stitched optical micrograph of as deposited DMLD FSX-414 showing dendritic
microstructure and domains/bundles of dendrites with same orientation.
4.5.2 Scanning Electron Microscopy
SEM micrographs in Figure 4.8a show primary dendrites growing on the cast FSX-414
substrates. Boundary between domains i.e. bundles of dendrites with same orientation is
evident in the Figure 4.8b.
(a) (b)
DMLD FSX-414
Cast FSX-414
Layer
thic
kn
ess
~ 4
00 µ
m
Domain 1
Domain 2 Domain 3
65
Figure 4.9a,b shows BSE images of the unetched DMLD FSX-414. The interdendritic
region is in light contrast indicating segregation of high atomic number elements. Fine
globular dark phases can be observed in the interdendritic region often surrounded by a
region of light contrast, as well as well as within the dendrites. A fine network of plate like
precipitates is present within the dendrites. The high magnification micrograph of the
interdendritic region in Figure 4.10a shows the interdendritic region consists of a 2-phase
mixture with phases in light contrast in a matrix of darker contrast. EDS mapping in Figure
4.10b shows the segregation of chromium, tungsten and carbon to the interdentritic region.
The dark globular precipitate is rich in Silicon, Manganese and Oxygen. Table 4.3 enlists
chemical composition of interdendritic region and dark phase as well as matrix measured
using EDS.
Figure 4.8 SEM micrographs of etched as deposited DMLD FSX-414 showing (a) primary
dendrites growing on the substrate cast FSX-414 and (b) domain boundary; etched with 5%
HCl, electrolytic – 6V
Figure 4.9 (a,b) BSE images of unetched as deposited DMLD FSX-414 showing columnar
structure with elongated bright and globular dark phases in the interdendritic region.
(a) (b)
(a) (b)
66
Figure 4.10(a) High magnification BSE image of as deposited DMLD FSX-414 showing
bright and dark precipitate, and (b) corresponding EDS elemental mapping.
Table 4.3 Chemical composition of various phases in As deposited DMLD FSX-414
Elements Co Cr Ni W Mn Si O
Bright Wt% 17.0 59.6 3.0 18.3 1.9 0.2
Dark Wt% 36.5 34.2 7.2 9.1 3.6 3.5 5.8
Matrix Wt% 51.4 28.5 10.9 7.1 1.3 0.8 -
(a)
(b)
67
4.6 Microstructural characterization of Solution heat treated DMLD FSX-
414.
4.6.1 Optical microscopy
Solution heat treatment was done at three different temperature namely, 1150oC, 1200
oC, and
1250oC. Figure 4.11 shows optical micrographs of solution heat treated DMLD FSX-414. A
fully dendritic structure is retained after heat treatment at 1150oC (Figure 4.11a). The
microstructure of the 1200oC treated sample in Figure 4.12 shows dendritic structure with the
breakdown of the interdendritic regions. Some indication of the grain boundaries can be seen
as well. Microstructure of samples solution treated at 1250oC in Figure 4.13 shows complete
breakdown of the dendritic structure. There are some small equiaxed grains in combination
with large elongated grains. Figure 4.13b highlights the curvature in the grain boundaries of
the two elongated grains. Figure 4.13a shows the remnants of the interdendritic regions
(arrow).
4.6.2 Scanning Electron Microscopy
Figure 4.14 shows BSE images of the unetched DMLD FSX-414 sample solution treated at
1150oC. The continuity of the interdendritic region has broken down. The dark contrast
within the grains points to a remnant of microsegeration. BSE image of 1200oC treated
sample in Figure 4.15 indicates that the former interdendritic regions have broken down into
secondnd phases in light contrast. BSE image of 1250oC treated samples in Figure 4.16
shows only globular bright precipitates. A fine equiaxed network of bright and dark regions
bound by veins of light contrast appears within the grains in both the 1200oC and 1250
oC heat
treated samples. Globular dark precipitates can be observed in all 1150oC, 1200
oC and
1250oC treated samples and does not show any perceivable change in their volume fraction
and size.
Table 4.4 enlists composition of bright precipitates for all solution heat treatment
temperatures. The composition is roughly similar for all treatments. It can be observed that
the precipitates are changing their morphology from elongated to globular following a
systematic breakdown as a response to increase in the solution treatment temperature. These
precipitates are mainly rich in chromium, tungsten and carbon and the composition is roughly
similar for all treatments. EDS mapping of 1150oC treated specimen in Figure 4.17b confirms
the enrichment of Cr, W and C in the bright precipitates. Figure 4.17b also shows that
globular dark precipitates are rich in Mn, Si and O similar to as deposited samples. The high
magnification BSD image in Figure 4.17a also shows the presence of a network of plate
shaped precipitates.
68
Figure 4.11(a,b) Optical micrographs of Sol HT-1150
oC DMLD FSX-414 showing fully
dendritic structure, etched with 5% HCl, electrolytic-6V.
Figure 4.12(a,b) Optical micrographs of Sol HT-1200
oC DMLD FSX-414 showing dendritic
structure with the indication of the grain boundary, etched with 5% HCl, electrolytic-6V.
Figure 4.13(a,b) Optical micrographs of Sol HT-1250
oC DMLD FSX-414 showing complete
breakdown of dendritic structure.
(a) (b)
(a) (b)
(a) (b)
69
Figure 4.14(a,b) BSE images of Sol HT-1150
oC DMLD FSX-414 showing interdendritic
precipitates.
Figure 4.15(a,b) BSE images of Sol HT-1200oC DMLD FSX-414 showing interdendritic
precipitates.
Figure 4.16(a,b) BSE images of Sol HT-1250oC DMLD FSX-414 showing remnants
interdendritic precipitates.
(a) (b)
(a) (b)
(a) (b)
70
Figure 4.17 (a) High magnification BSE image of Sol HT-1150
oC DMLD FSX-414 showing
bright and dark precipitate, and (b) corresponding EDS elemental mapping.
Table 4.4 Chemical Composition of bright precipitates in DMLD FSX-414 samples
solution treated at various temperatures (all in weight %)
Sol HT Co Cr Ni W Mn Si
1150oC 15.0 61.2 2.7 18.5 2.2 0.4
1200oC 14.5 64 2.2 17 2.1 0.1
1250oC 15.3 63.8 2.0 16.7 2.0 0.2
4.7 Microstructural characterization of Sol HT 1150oC + Aged DMLD
FSX-414.
Ageing treatment (980oC) was given to solution treated (1150
oC) DMLD FSX-414 samples
to evaluate its high temperature stability. Optical micrographs in Figure 4.18 shows dendritic
microstructure similar to solution heat treated samples. Figure 4.19 shows BSE images of
(a)
(b)
71
unetched aged DMLD specimens. The inderdendritic regions are in bright contrast and
globular dark precipitates can be seen. High magnification BSE image in Figure 4.19b also
shows intersecting bands of ε-HCP martensite (as confirmed later by XRD). EDS mapping in
Figure 4.20b shows Cr, W and C enrichment in the elongated bright precipitates while Mn, Si
and O enrichment in dark globular precipitates. Precipitates were similar in composition and
morphology to that of as deposited and solution treated ones and ageing treatment has not
resulted in any new phase. Table 4.5 shows composition of precipitates measured using EDS.
Figure 4.18(a,b) Optical micrographs of Sol HT-1150
oC+aged DMLD FSX-414 showing
dendritic microstructure, etched with 5% HCl, electrolytic – 6V.
Figure 4.19(a,b) BSE images of Sol HT-1150
oC+aged DMLD FSX-414 samples showing
bright and dark precipitates in the interdendritic regions and ε-HCP bands crossing across the
column.
(a) (b)
(a) (b)
72
Figure 4.20 (a) High magnification BSE image of Sol HT-1150
oC+aged DMLD FSX-414
sample showing bright and dark precipitate, and (b) corresponding EDS elemental mapping.
Table 4.5 Composition of various phases in Sol HT 1150oC + Aged DMLD FSX-414
4.8 Microstructural characterization of Sol HT+Aged Cast FSX-414.
Optical micrographs of Sol HT 1150oC + Aged Cast FSX-414 samples Figure 4.21 shows
very coarse dendritic structure with secondary dendrite arm spacing (SDAS) of around
around 70 µm. BSE images were taken in order to identify interdendritic phases as shown in
Figure 4.22. Figure 4.22 shows that the interdendritic region has typical irregular eutectic
type morphology and globular dark phases distributed all over the sample. EDS mapping in
Elements Co Cr Ni W Mn Si O
Bright Wt% 12.5 64.2 1.9 19.4 2.0 0.1 -
Dark Wt% 12.4 41.8 2.7 3.2 19.4 1.1 19.2
Matrix Wt% 48.5 27.7 10.2 6.3 1.4 0.9 -
(a)
(b)
73
Figure 4.23b confirms segregation of chromium, tungsten and carbon in the interdendritc
bright precipitates, while globular dark phases are enriched in silicon, manganese and
oxygen. Table 4.6 enlists the composition of the phases present. Except the size, there is no
difference between the precipitates observed in DMLD FSX-414 and Cast FSX-414, both of
which have roughly similar composition.
Figure 4.21 Optical micrographs of Sol HT-1150
oC+aged Cast FSX-414 at (a) low
magnification showing coarse dendritic structure, (b) high magnification showing
interdendritic precipitates, etched with 5% HCl – electrolytic, 6V.
Figure 4.22(a,b) BSE images of Sol HT-1150
oC+aged Cast FSX-414 samples showing
eutectic phases in the interdendritic region.
(a) (a) (b)
(a) (b)
74
Figure 4.23 (a,b) High magnification BSE image of Sol HT-1150oC+aged Cast FSX-414
sample showing interdendritic precipitates and (b) corresponding EDS elemental mapping
.
Table 4.6 Composition of various phases in Sol HT 1150oC + Aged Cast FSX-414
4.9 X-ray Diffraction
Phase identification was carried out using XRD. X-ray diffraction peak identification was
done using two ICDD cards, one for FCC-Co and other for HCP Co. XRD patterns of As
deposited, Sol HT-1150oC and Sol HT-1150
oC+aged DMLD FSX-414, etc. shows peaks for
both the γ-FCC and ε-HCP Cobalt phases as shown in Figure 4.24.
Elements Co Cr Ni W Mn Si O
Bright Wt% 12.8 64.8 2.2 18 2.2 - -
Dark Wt% 40 29.2 8.5 0.9 2.8 8.2 10.5
Matrix Wt% 50.6 29.1 10.7 7.1 1.4 0.8 -
(a)
(b)
75
Figure 4.24 X-ray Diffractions patterns of (a) as deposited, (b) Sol HT-1150
oC (c) Sol HT-
1150oC+aged, DMLD FSX-414, showing peaks for both γ-FCC and ε-HCP Cobalt phases
(Target: Cr-Kα -2.29 Aº)
4.10 Hardness
Hardness profiles were plotted against build direction for transverse sections from 10% span
as can be seen in Figure 4.25. Hardness value of as deposited sample is between 310- 330 HV
and after solution heat treatment at 1150oC, it decreases to 340-380 HV. Hardness roughly
remains same after ageing treatment. Hardness values of DMLD part are 30-50 HV higher
than the cast FSX-414 for all the treatments as shown in Figure 4.26. Figure 4.27 shows
variation in hardness with increasing solution heat treatment temperature. The hardness
decreases with increasing solution treatment temperature. Hardness value of Solution HT-
1250oC is 100 HV lower than that of as deposited sample.
0
50
100
150
200
250
300
10 30 50 70 90 110 130 150
Inte
nsi
ty (
arb.
unit
s)
2θ
As deposited
Sol HT-1150 C
Sol HT+Aged
(a)
(b)
(c)
*
*
2
2
2
2
2
1
1
1, 2
1, 2
1, 2
1, 2
1, 2
1, 2
1 – γ(FCC) Co
ICDD: 00-015-806
2 – ε(HCP) Co
ICDD: 00-005-726
*Unidentified
76
Figure 4.25 Hardness profile along build direction as deposited, Sol HT-1150
oC, Sol HT-
1150oC+aged, DMLD FSX-414 and Cast FSX-414.
Figure 4.26 Hardness comparison between DMLD and Cast FSX-414.
250
270
290
310
330
350
370
390
0 10 20 30 40 50 60
Har
dnes
s (H
V)
Distance from Trailing Edge (mm)
As Deposited
Sol HT
Sol HT + Aged
0
50
100
150
200
250
300
350
400
450
As deposited Sol HT 1150 C Sol HT 1150 C +
Aged
Har
dnes
s (H
V)
DMLD Cast FSX-414
Build direction
DMLD FSX-414 Cast FSX-414
77
Figure 4.27 Variation in hardness with different solution heat treatment temperatures.
4.11 Tensile properties
Figure 4.28 shows 0.2% yield strength, UTS for room temperature tensile tests. As deposited
samples shows highest yield strength and UTS. Marginal drop in UTS and 0.2% Y.S. can be
seen after solution and ageing treatment on as deposited DMLD FSX-414. Figure 4.29 shows
fracture surfaces of the as deposited samples. Some curved facets and cracks can be seen at
lower magnification (Figure 4.29a). Fine dimples can be observed in the higher magnification
fractographs (Figure 4.29b). Similar curved facets at low magnification and dimples at high
magnification can be observed in fractographs of solution treated and aged samples (Figure
4.30). Both as deposited and Sol HT-1150oC+aged DMLD samples show higher 0.2% Y.S.
and UTS than that of cast FSX-414 sample as shown in Figure 4.28. Fractographs of Cast
FSX-414 in Figure 4.31 shows mainly dimples and some big voids. Figure 4.32, 4.33, 4.34
shows engineering stress-engineering strain curves for as deposited, Sol HT-1150oC+aged
DMLD FSX-414, Sol HT-1150oC+aged Cast FSX-414,
0
100
200
300
400
As Deposited HT - 1150 C HT - 1200 C HT - 1250 C
Har
dn
ess
(HV
)
0
200
400
600
800
As deposited Sol HT 1150 C+Aged
DMLD FSX-414
Sol HT 1150 C+Aged
Cast FSX-414
Str
enght
(MP
a)
0.2% Y.S. UTS(a)
78
Figure 4.28 Room temperature tensile properties of As deposited, Sol HT+Aged DMLD and
Cast FSX-414, (a) 0.2% Y.S. and UTS, (b) % Elongation (ductility).
Figure 4.29 Fractographs of as deposited DMLD FSX-414 tensile sample showing (a) cracks
at low magnification and (b) dimples at high magnification.
Figure 4.30 Fractographs of Sol HT-1150
oC+aged DMLD FSX-414 tensile samples showing
(a) curved facets at low magnification and (b) dimples at high magnification
0
10
20
30
40
As Deposited Sol HT + Aged DMLD
FSX-414
Sol HT+Aged Cast
FSX-414
Du
ctil
ity
(a) (b)
(a) (a) (b)
(b)
79
Figure 4.31(a,b) Fractographs of Sol HT-1150oC+aged Cast FSX-414 tensile samples
showing dimples and big voids.
Figure 4.32 Engineering stress - Engineering strain curve for as deposited DMLD FSX-414.
Figure 4.33 Engineering stress - Engineering strain curve for Sol HT-1150
oC+aged DMLD
FSX-414.
0
200
400
600
800
1000
0 10 20 30 40
Str
ess
(MP
a)
Strain %
0
200
400
600
800
1000
0 10 20 30 40
Str
ess
(MP
a)
Strain %
(a) (a) (b)
80
Figure 4.34 Engineering stress - Engineering strain curve for Sol HT-1150
oC+aged Cast
FSX-414
0
200
400
600
800
0 5 10 15 20 25 30
Str
ess
(MP
a)
Strain %
81
5. DISCUSSION
This chapter presents an analysis and discussion of the results of DMLS CoCrMo and DMLD
FSX-414. The results are discussed in five sections: first two sections describe
microstructural evolution and tensile properties of DMLS CoCrMo, later two sections include
microstructural evolution and tensile properties of DMLD FSX-414. The final section
presents the comparison between solidification behavior of DMLS and DMLD processes.
5.1 Microstructural evolution in Direct Metal Laser Sintered CoCrMo
5.1.1 Porosity and microcracks
Porosity and microcracks can be observed in as printed DMLS CoCrMo samples as examined
in this thesis (Figure 3.6 & 3.8). Most of pores are fine and spherical, while some have
irregular morphology. Fine and spherical pores can form due to entrapped gas within gas
atomized powders and bubbles[14], [15] from metallic evaporation due to high power laser
beam [17]. Location of irregular pores is mainly at the interlayer boundaries as shown in
Figure 3.8. Thus irregular can be due to incomplete remelting of the previous layer. However
the extent of porosity in sample is not very high and processing parameters have been
optimized properly to enable a dense component. Microcracks are almost perpendicular to the
melt pool boundary. Solidification shrinkage in upper molten layer which is restricted by
cooler substrate or previous layers is the reason behind formation of microcracks [13]
5.1.2 Macrostructure in as printed DMLS CoCrMo
The DMLS process is very similar to the laser welding process. The molten metal pool is
created when the laser beam with around 80 µm diameter hits the thin CoCrMo powder layer
with thickness of 40 µm. Since size of this pool is very small which is around 120 µm wide
and 60 µm deep as shown in Figure 3.7, it solidifies very rapidly owing to the high rate of
heat extraction from the cooler substrate / previous layers. The laser beam has maximum
intensity in the center which gradually decreases towards the edge of beam due to its
Gaussian energy distribution. This is the main reason for the formation of arc shaped melt
pools [21]. The laser beam creates a heat distribution profile with maximum temperature at
the center, which can melt the powder in area more than its size. Thus width of the melt pool
is higher than the laser beam diameter. Subsequently, laser beam scans the powder layer in a
predefined path and process is repeated for next layers. Melt pool boundaries as well as scan
paths can be seen transverse and base microstructures respectively in Figure 3.7. Schematic
representation of microstructural evolution during DMLS processing is shown in Figure 5.1.
Remelting of previous layers as well as adjacent melt tracks is important in order to achieve
good bonding between them. Depth of melt pool (~60 µm) is higher than the powder layer
thickness (40 µm). Microstructure of front planar section in Figure 3.7 shows more width.
82
Since the section taken for microstructure may not be perpendicular to the melt tracks, melt
apparent pool widths in the metallographic sections can be larger than actual widths as seen
in Figure 5.1.
The laser scanning direction is rotated by 67o
for every new powder layer. This can be
observed in the base microstructure in Figure 3.7. Changing scan direction by 67o decreases
the residual stress and porosity in the sample [17].
Figure 5.1 Schematic representation of microstructural evolution in DMLS CoCrMo.
5.1.3 Microstructure in DMLS CoCrMo
As printed DMLS CoCrMo show columnar microstructure with molybdenum and silicon
enrichment in the interdendritic regions. Meacock et al [32], Quian et al [16], Takaichi et al
[27] observed similar molybdenum and silicon rich phases in the interdendritic regions in
their studies on DMLS / SLM CoCrMo. Silicon rich dark globular phase in DMLS CoCrMo
(Figure 3.11) were also observed by Mengucci et al [23] for DMLS CoCrMoW and Giacchi
et al [33] for Cast CoCrMo. Identification of elongated bright precipitate in the interdendritic
regions requires extensive TEM characterization and will be studied in greater detail in
future. Mengucci et al[23] reported similar microstructural features with molybdenum and
silicon enriched interdendritic precipitate for DMLS CoCrMoW alloy. The precipitate was
identified as Co3(Mo,W)2Si phase with HCP crystal structure. Owing to the similar
composition used in this study except the tungsten content, the most probable phase in the
interdendritic region can be Co3Mo2Si.
Remelting of
previous layer and
adjacent melt tracks
CoCrMo
powder layer Large width of
melt pool due
to inclined melt
track.
83
TEM analysis of as printed DMLS CoCrMo reveals presence of ε-HCP martensite in
the form of plates as shown in Figure 3.12 & 3.13. XRD analysis also shows 19.7% HCP in
as printed sample (Table 3.8). Presence of athermal ε-HCP martensite phase and its formation
mechanism in CoCrMo alloy is widely reported in literature [7], [19], [23], [27], [31], [32],
[34]–[37]. ε-HCP phases grow on 111 planes of γ-FCC matrix. Since 111 planes have
70.71o
angle for [011] zone axis, ε-HCP phases can be observed with same angle as shown in
Figure 3.12. Fringe contrast in Figure 3.12 should be due the ε-HCP phases inclined to the
surface. Schematic representation of HCP phase growing on the 111 plane of the FCC is
shown in Figure 5.2. In pure cobalt FCC to HCP transformation is around 417oC. But due to
HCP stabilizer elements like Cr, Mo, Si, etc, transformation temperature can increase.
Because of sluggish kinetic of FCC-HCP transformation, samples contains majority of
metastable γ-FCC phase.
Figure 5.2 Schematic representation of athermal ε-HCP growing on γ-FCC Cobalt
The microstructure of the solution heat treated sample is fully equiaxed as shown in Figure
3.16. Table 5.1 shows size and morphology of grains in as printed, solution treated and aged
DMLS CoCrMo. Grains (or domains) in as printed sample are elongated. Solution heat
treatment can cause perturbations in the grain boundaries following Rayleigh instability
criteria, which finally results in breakdown of grain into several equiaxed grains. Rayleigh
instability is based on the principle in which elongated objects breaks into spheres in order to
decrease the overall surface energy. This process is aided by diffusion at high temperature
treatment. Grain sizes in Table 5.1 support this mechanism, where average width in as printed
grains is approximately similar to the grain size of solution treated condition. Figure 5.5
shows a schematic representation of equiaxed grain formation.
Table 5.2 shows size, location and morphology of various precipitates in the as
printed, Sol HT and Sol HT+Aged DMLS CoCrMo. The bright precipitate in solution treated
sample (Figure 3.19) is due to the remnants of the previous interdendritic region (Figure 3.11)
Zone Axis
= [011]
(11
1)
Pla
ne
x
y
z
(0001) Basal plane
84
indicating incomplete homogenization. Few globular bright precipitates can also be observed
along the grain boundaries. Size of these precipitates is around 235 nm, which is higher than
the width of interdendrtic precipitates from as printed structure (~80 nm) as shown in Table
5.2. Globular dark precipitates in solution treated samples must be the silicon rich inclusions
from the as printed samples. Microstructure of solution treated and aged samples shows
bright and dark precipitates covering almost entire grain boundary. BSE images do not show
any evidence of the interdendritic segregation (Figure 3.21). Thus ageing seems to have
dissolved these regions completely. The precipitates along the grain boundary are very
similar to those observed in solution heat treated sample except they are little coarser (Table
5.2). Ageing treatment has nucleated more bright precipitates along the grain boundary
(globular) as well as inside grains (plate-like). Plate like bright precipitates can be observed
predominantly along the twin boundaries and ε-HCP plates as shown in Figure 3.21. Both
grain boundary bright precipitate and plate like precipitate inside grains are molybdenum and
silicon rich as shown in EDS mapping in Figure 3.23. Figure 5.3 shows isothermal section of
the CoCrMo ternary phase diagram at 1200oC. Current alloy composition is exactly at the
boundary between γ and σ (Co9Mo15) phase. Thus it is expected that samples will have
certain amount of σ phase solution treatment at 1150 oC. Isothermal section of CoCrMo
ternary phase diagram at 924oC also shows presence of ε and σ phase. XRD analysis shows
increase in volume fraction of ε-HCP percentage as after ageing at 980oC as compared to
solution treated DMLS CoCrMo as shown in Table 3.8. Molybdenum and silicon rich
precipitates in Sol HT+Aged DMLS CoCrMo samples show very similar composition as that
of interdendritic precipitates. Thus, though ternary diagram predict the presence of σ-phase in
Sol HT and Sol HT+Aged samples, silicon might have shifted the equilibria towards the same
precipitate as that of interdendritic precipitate (probably Co3Mo2Si). The proper identification
of precipitates in as printed and heat treated DMLS CoCrMo requires extensive TEM
characterization which will be studied in greater detail in future. Figure 5.5 shows schematic
representation of microstructural evolution after solution and ageing heat treatments.
Table 5.1 Size and morphology of grains in As printed, Sol HT, Sol HT+Aged DMLS
CoCrMo
Treatment Grain morphology Grain / Domain size (µm)
Range Average value
As printed Elongated Width 10-70 µm ~39 µm
Length 40-290 µm ~104 µm
Sol HT Equiaxed 5-90 µm ~44 µm
Sol HT+Aged Equiaxed 5-90 µm ~43 µm
85
Table 5.2 Size and morphology of precipitates observed in As printed, Sol HT and Sol
HT+Aged DMLS CoCrMo
Treatment Morphology
and contrast
Location Precipitate size
Range Average
As printed Elongated
bright
Interdendritic
region
Width (nm) 50-105 nm ~ 80 nm
Globular dark Everywhere Diameter
(nm)
15-100 nm ~35 nm
Sol HT Globular /
Elliptical
bright
Grain boundary Diameter
(nm)
100-400
nm
~235
nm
Globular dark Everywhere Diameter
(nm)
40-160 nm ~86 nm
Sol
HT+Aged
Globular /
Elliptical
bright
Predominantly
Along grain
boundary, Some
inside grains
Diameter
(nm)
175-725
nm
~390
nm
Plate bright Inside grains –
along twin
boundaries and ε-
plates
Width 70-120 nm ~95 nm
Length 1000-3000
nm
~1480
nm
Globular dark Everywhere Diameter
(nm)
26-325 nm ~160
nm
Figure 5.3 Isothermal section of CoCrMo ternary diagram at 1200
0C.
86
Figure 5.4 Isothermal section of CoCrMo ternary diagram at 924
oC.
Figure 5.5 Schematic representation of microstructural changes during solution and ageing
heat treatments.
5.2 Tensile properties of Direct Metal Laser Sintered CoCrMo
Strengthening can take place due to work hardening, grain boundary strengthening (hall petch
effect), solid solution strengthening and precipitate strengthening. The equation 1 gives the
relation for the yield strength of the material.
As printed Sol HT Sol HT + Aged
87
σ = σo + σgs + σss + σpp
where, σo = Y.S. of a single crystal
σgs = increase in strength due to hall petch effect
σss -= increase in strength due to solid solution stregntheing
σpp = increase in strength due to precipitate strengtheining
As printed samples shows highest 0.2% Y.S. It can be observed that it decreases considerably
after solution heat treatment and remains unaffected after ageing. σss will be same for as
printed, solution and ageing treated samples due to same chemical composition. Due to the
difference in grain size, i.e. ~800 nm (column width) for as printed and ~40 µm (equiaxed
grain size) for solution heat treated, σgs is possibly the dominant factor for high strength in as
printed DMLS CoCrMo as compared to heat treated. After ageing there is almost no change
in the grain size. However extensive precipitation along the grain boundaries as well as inside
grains does not seem to play any role in increasing the strength of the alloy. Thus σpp has
negligible contribution towards overall yield strength.
Fractographs of as printed samples showing facets and cracks indicate cleavage type
fracture. Thus as printed samples have a brittle failure mode at room temperature. Low
ductility values of as printed samples confirm the same. Solution treated samples shows
relatively higher ductility for room temperature tensile tests. Figure 5.6 shows schematic
representation of engineering stress vs. engineering strain curve for room temperature tested
as printed and solution treated DMLS CoCrMo. Very high UTS of as printed sample as
compared to cleavage stress of CoCrMo must be the reason for its brittle failure mode.
Figure 5.6 Schematic representation of engineering stress vs. engineering strain curve for as
printed and solution treated DMLS CoCrMo (room temperature)
As printed
Sol HT
Cleavage stress
Very low ductility Relatively higher ductility
Strain %
Str
ess
(MP
a)
88
The presence of intergranular cracks and dimples in the fractographs of room
temperature tested samples reveal mixed ductile and brittle failure modes. For room
temperature tests, Sol HT+Aged samples shows considerably low ductility values than
solution heat treated samples. Fractographs shows cracks and reveals mainly intergranular
brittle fracture. This can be attributed to the intergranular precipitate phases, which might
have paved easy way for the propagation of intergranular cracks.
Low 0.2% Y.S. and UTS at high temperature tests is mainly because thermally
activated mechanisms assists deformation and decrease strength of the material [38]. In many
FCC materials UTS is more temperature dependent than 0.2% Y.S [38]. Figure 3.27 shows
UTS is dropping more rapidly than 0.2% when tested at high temperature for both Sol HT
and Sol HT+Aged samples. This phenomenon along with decrease in strain hardening
exponent with increasing temperature [38] causes flattening of Stress-strain curve when
tested at higher temperature (Figure 3.29 & 3.30). Both strength (0.2% Y.S., UTS) and
ductility is almost similar for high temperature tested Sol HT and Sol HT+Aged samples. The
mixed ductile and brittle mode can be observed in the fractographs of both high temperature
tested Sol HT and Sol HT+Aged samples (Figure 3.32 & 3.33). The grain boundary
precipitates in Sol HT+Aged samples does not harm the high temperature tensile properties
especially ductility unlike room temperature tests.
5.3 Microstructural evolution in Direct Metal Laser Deposited FSX-414
Microcracks and porosity can be observed in Figure 4.5. Microcracks are due to solidification
shrinkage which is restricted by the cooler substrate. Most of the pores are spherical, thus
might have originated from gas entrapped within gas atomized powders or due to metallic
vapors caused by high power laser beam same as what is observed for DMLS CoCrMo.
Microstructure of the as deposited samples shows very fine dendritic structure.
Dendritic columns in Figure 4.6b are almost perpendicular to the substrate. Melt pool cools
rapidly owing to its very small size relative to the substrate. The rapid heat extraction from
the substrate during solidification causes columnar dendrites to grow in a direction opposite
to that of heat flux. This phenomenon is also observed by Dinda et al [39], Bi e al [40] and
Hussein et al [41] for DMLD on Nickel based superalloys. Epitaxial growth can be observed
in Figure 4.7 (domain 3). Partially remelted grains from the previous layers acts as pre-nuclei
for the directional columns and thus leading to epitaxial growth [39].
Effect of varying solution heat treatment can be seen in Figures 4.11, 4.12 & 4.13.
Dendritic structure finally breaks down into equiaxed at 1250oC. Highlighted grains (no. 1
and 2) in Figure 4.13 shows curved boundaries for some elongated grains. Thus Rayleigh
instability criterion is acting on these boundaries to make them curved and eventually meet to
form equiaxed dendrites. Grain no. 1&2 in Figure 4.13 are actually in the process of
breakdown and just require extra thermodynamic driving force. Thus the mechanism of
breakdown is similar to what is observed for DMLS CoCrMo as shown in Figure 5.3. BSE
images for all three solution treatment temperature in Figures 4.14, 4.15 & 4.16 shows
89
systematic breakdown of the interdendritic precipitates following Rayleigh instability
criterion. Solution treatment at 1250oC does not dissolve the interdendritic precipitates fully,
and remnants can be seen in a globular form as shown in Figure 4.16. Hardness values
decreases considerably with increasing solution treatment. (Figure 4.27) This can be
attributed to lower volume fraction of precipitates due to partial dissolution and breakdown of
the dendritic structures.
BSE images and corresponding EDS mapping of as deposited samples in Figure 4.9 &
4.10 show Cr, W and C rich elongated bright precipitates in the interdendritic region. EDS
mapping of Sol HT+Aged Cast FSX-414 in Figure 4.23 also shows Cr, W and C rich
precipitates in the interdendritic region. Interdendritic phases in cast FSX-414 were identified
previously by Foster et al [42] and Mezzedimi et al [43] as Cr21W2C6 (M23C6 type).
Chemical composition (atomic %) of bright phase in both as deposited (Table 4.3) and Sol
HT+Aged cast FSX-414 (Table 4.6) samples corresponds to the presence of Cr21W2C6
precipitates.
Microstructure of Solution treated (1150oC) shows dendritic structure. EDS mapping
(Figure 4.17) and composition (Table 4.4) confirms the presence of Cr21W2C6 precipitates.
Solution treatment at higher temperatures (1200oC, 1250
oC) show similar composition of the
precipitates (Table 4.4) but with different morphology due to the breakdown. Microstructure
and corresponding EDS mapping of Sol HT-1150oC+Aged samples in Figure 4.20 also shows
same elongated Cr21W2C6 precipitates in the interdendritic region. No new precipitate can be
observed. Thus both solution treatment and ageing has almost no perceivable effect on the
microstructures. It has been reported that commercial heat treatment cycles for cast FSX-414
does not bring about complete carbide precipitation and more types of carbide precipitates
during service at high temperature [44].
Globular dark phases can be observed in all treatments of DMLD FSX-414 and Cast
FSX-414. EDS mapping and chemical composition suggests Si, Mn and O enrichment in
these phases (Figures 4.10, 4.17, 4.20, 4.23). These particles can be Si/Mn oxide inclusions
which form during solidification.
5.4 Tensile properties of Direct Metal Laser Deposited FSX-414
As deposited samples shows highest 0.2% Y.S. and UTS. Fractographs of as deposited in
Figure 4.29 samples show curved facets which seems to be following domain boundaries and
cracks at low magnification and dimples at high magnification reveal ductile type fracture.
Similar fractographs were observed for Sol HT+Aged samples (Figure 4.30) thus also show
fully ductile fracture. 0.2% Y.S and UTS of the Sol HT+Aged DMLD FSX-414 is only
marginally lower than that of as deposited owing to the similar microstructure as shown in
Figure 4.28a. Higher strength of the Sol HT-1150oC+aged DMLD FSX-414 as compared to
Sol HT 1150oC +Aged Cast FSX-414 is attributed to difference in grain size, i.e., 4-5 µm for
Sol HT-1150oC+aged DMLD FSX-414 and 70 µm for Sol HT 1150
oC +Aged Cast FSX-414.
Fractographs of the Sol HT 1150oC +Aged Cast FSX-414 samples in Figure 4.31 shows
dimpled rupture indicating fully ductile failure mode. All samples, as deposited and Sol HT
1150oC + Aged DMLD FSX-414 and Sol HT 1150
oC + Aged Cast FSX-414 shows
considerable ductility as shown in Figure 4.28b.
90
5.5 Comparison between DMLS CoCrMo and DMLD FSX-414
Table 5.3 shows comparison between various microstructural and mechanical properties
aspects of DMLS CoCrMo, DMLD FSX-414 and cast FSX-414. Though both CoCrMo and
FSX-414 are two cobalt based superalloys, very similar in composition except their Mo and
W contents respectively. It can be observed that ε-HCP phase is present in both DMLS
CoCrMo and DMLD FSX-414 in varying quantities in metastable γ-FCC matrix. It can be
seen that the difference of cooling rates between DMLS (106 oC/s) and DMLD (10
3 oC/s) is
reflected in the primary dendritic arm spacing of respective solidification microstructures
(DMLS: 0.6-1.0 µm, DMLD: 9-12 µm).
Table 5.3 Comparison between DMLS CoCrMo and DMLD FSX-414
DMLS CoCrMo DMLD FSX-414 Cast FSX-
414
Cooling rate 106 oC/s [45] 10
3 oC/s [26] -
As
printed/deposite
d microstructure
Columnar dendritic
PDAS = 0.6-1.0 µm
Columnar dendritic
PDAS = 9-12 µm
Interdendritic
region Rich in Mo and Si (Cr21W2)C6 (Cr21W2)C6
ε-HCP Present Present -
Solution treated
(1150oC)
microstructure
Equiaxed grains with size ~
44 µm, Few remnants of
interdendritic precipitates
1150oC
Columnar
dendritic
PDAS = 9-12
µm
-
1200oC
Partially
dissolved
interdendritic
region
-
1250oC
Equiaxed grains
with remnants
of interdendritic
precipitates
-
Solution treated
(1150oC) + Aged
(980oC)
Equiaxed grains with size ~
44 µm, Extensive
precipitation of Mo and Si
rich precipitates along
grains boundaries as well
as inside grains.
Columnar dendritic
PDAS = 9-12 µm
SDAS = 4-6 µm
Dendritic
SDAS = 70
µm
91
DMLS CoCrMo DMLD FSX-414 Cast FSX-
414
As
printe
d
Sol
HT
Sol
HT+Age
d
As
deposited
Sol
HT
1150 oC
Sol HT
1150 oC +
Aged
Sol HT
1150 oC +
Aged
Hardness (HV) 467 365 346 353.2 323.9 330 306.5
Tensile strength
(MPa) 1378.9 1114 982.5 887.7 - 805.7 729.9
Ductility (%
Elongation) 5.7% 15% 5.3%
92
6. CONCLUSIONS AND FUTURE WORK
This thesis presents detailed microstructural characterization and tensile properties of direct
metal laser sintered (DMLS) CoCrMo and direct metal laser deposited (DMLD) FSX-414.
The conclusions drawn from studies are as follows:
6.1 Conclusions
Part A: Direct Metal Laser Sintering (DMLS) of CoCrMo
Microstructure inside the melt pool shows very fine columnar structure with the Mo and
Si segregation in the interdendritic regions.
Columns with same orientation which can grow across the melt pool boundaries form
elongated domains / grains.
TEM studies reveals presence of athermal ε-HCP martensite which forms due to rapid
solidification.
Fully equiaxed grain structure after solution heat treatment is due to breakdown of the
elongated grains from the as printed samples following Rayleigh like instability.
Precipitates can be observed after ageing heat treatment both along the grain boundaries
as well as inside grains.
Hardness and tensile strength decreases after the solution heat treatment, which can be
attributed to increase in grain size (Hall-Petch effect).
Hardness and tensile strength remains same after ageing treatment, thus precipitates in the
ageing treatment seems to be not playing any role in increasing the strength and grain size
is dominant factor in determining strength.
Intergranular failure seen with very low ductility after ageing can be attributed to
extensive precipitates along the grain boundaries.
Part B: Direct Metal Laser Deposition (DMLD) of FSX-414
As deposited structure shows fine columnar dendritic structure with (Cr21W2)C6
segregation at the interdendritic regions.
Columns with same orientation which can grow across the layers form elongated
domains/grains.
Dendritic structure breaks down during solution treatment at 1250oC temperature
following Rayleigh like instability.
With increase in solution heat treatment, hardness decreases from 353.2 HV for as
deposited samples to 258 HV for 1250oC treated samples.
Precipitates also breakdown following Rayleigh like instability.
No change in microstructure was observed after solution and ageing treatments.
93
Hardness and tensile properties decreases only marginally due to Sol HT 1150oC +
Ageing treatment can be attributed to almost no change in microstructure.
Hardness and tensile strength of both as deposited and Sol HT 1150oC + Aged DMLD
FSX-414 is higher than that of Sol HT 1150oC + Aged Cast FSX-414 can be attributed to
the grain size (SDAS).
Both as deposited, Sol HT 1150oC + Aged DMLD FSX-414 samples shows significant
ductility.
6.2 Future work
TEM analysis in order to identify various precipitates in as printed, solution treated and
aged DMLS CoCrMo.
High temperature tensile testing of DMLD FSX-414 samples.
Creep and fatigue studies on both DMLS CoCrMo and DMLD FSX-414 in order to
evaluate its suitability in high temperature gas turbine applications.
94
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