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A Review on the Effect of Powder Oxidation, Internal
Porosity and Crystallographic Texture on the Charpy
Impact Energy of Ti-6Al-4V Specimen Fabricated using
Electron Beam Melting and Hot-isostatic Pressing Post
Processing
Abhinav Maurya Department of Manufacturing Process
and Automation Engineering
Netaji Subhas University of Technology
New Delhi, India
Akshay Dabas Department of Manufacturing Process
and Automation Engineering
Netaji Subhas University of Technology
New Delhi, India
Andriya Narasimhulu Department of Manufacturing Process
and Automation Engineering
Netaji Subhas University of Technology
New Delhi
Abstract — Electron beam melting (EBM) is an innovative
additive manufacturing process in which metal powder is
completely melted by a concentrated beam of electrons. In this
paper, the effect of powder oxidation, internal porosity and
crystallographic texture on the charpy impact energy of Ti-
6Al-4V specimen fabricated using electron beam melting and
hot-isostatic pressing post processing was reviewed. It was
observed that the charpy impact energy dramatically
decreases in a smooth but rapid fashion due to excessive
powder oxidation and has a deleterious effect due to the
internal porosity present in EBM Ti-6Al-4V specimen.
Crystallographic texture was also found to influence Charpy
absorbed energy. However, HIP post-processing significantly
increases the impact toughness for specimens in each case.
Keywords — Additive Manufacturing, Electron Beam Melting,
Hot-isostatic pressing, internal porosity, powder oxidation,
Crystallographic Texture
I. INTRODUCTION
Electron beam melting (EBM) is an innovative additive
manufacturing (AM) process in which metal powder or
filament is completely melted by a concentrated beam of
electrons. Production in a vacuum chamber ensures that
oxidation will not deteriorate highly reactive materials like
titanium. Vacuum production is also required so electrons do
not collide with gas molecules. Among other additive
manufacturing (AM) technologies, metallic powder bed fusion
electron beam melt (EBM) has recently gained considerable
attention in the medical, automotive, and aerospace
communities for the fabrication of production components
directly from 3D CAD files bringing enhanced design freedom,
minimal material waste, and little post-processing (1).
The electron beam technology began with the experiments by
physicists Hittorf and Crookes, who first tried to generate
cathode rays in gases (1869) and to melt metals (1879). The
heat created by electrons colliding had damaging effect and
attempts were made to inhibit this by means of cooling. In 1906,
physicist Marcello von Pirani first made use of this effect by
building a piece of apparatus for melting tantalum powder and
other metals using electron beams. In 1948, Dr. H.C. Karl-
Heinz Steigerwald built the first electron beam processing
machine in 1952. The initial development work was done in
collaboration with Chalmers University of Technology in
Gothenburg. In 1993, a patent was filed describing the principle
of melting electrically conductive powder, layer by layer, with
an electric beam, for manufacturing three-dimensional bodies.
In 1997 Arcam AB was founded and the company continued
the development on its own, with the objective to further
develop and commercialize the fundamental idea behind the
patent.
The EBM process involves spreading a layer of pre-alloyed
metallic powder in the evacuated build space, selectively
melting regions in the layer with an electron beam, spreading
another layer of powder and repeating the process until a three-
dimensional solid metal part is contained within the powder
cake (1). A tungsten filament in the electron beam gun is
superheated to create a cloud of electrons that accelerate to
approximately one-half the speed of light. A magnetic field
focuses the beam to the desired diameter. A second magnetic
field directs the beam of electrons to the desired spot on the
print bed.
Once a component or prototype has been printed, the build
envelope is removed and the build platform and attached object
are removed from the loose powder. Powder clinging to the
object or remaining in internal cavities is blown or blasted
away. Post-processing methods, including hot isostatic pressing
(HIP), heat treatment in inert gas or vacuum heat treatment may
be employed to release residual stresses and improve
mechanical properties.
Today, the potential of electron beam melting technology is
recognized and is used to print components used in aerospace,
automotive, military, petrochemical and medical applications.
By improving access to emerging high-growth submarkets,
electron beam melting technology offers a competitive edge to
progressive enterprises. In many applications, designers enjoy
unprecedented design flexibility. It produces parts with
properties similar to wrought parts and better than those of cast
parts. For many applications, EBM is a cost-effective process
that reduces inventory requirements and with build rates of
almost four times those of other AM technologies. The electron
beam melting process reduces residual stresses in a variety of
International Journal of Engineering Research & Technology (IJERT)
ISSN: 2278-0181http://www.ijert.org
IJERTV8IS080024(This work is licensed under a Creative Commons Attribution 4.0 International License.)
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ways. It is possible to control residual stress during the
preparation of CAD data, during printing and in post-
processing. During printing, residual stress is reduced by
preheating the print bed and by the heating of the material
before it is struck by the electron beam. To a degree, lower
residual stress is also a function of the process’ high build
temperatures and slower cool-down rates compared to laser-
based AM processes.
Electron beam melting requires the use of pure, unadulterated
metals. Any changes in powder properties such as powder
oxidation, internal porosity and crystallographic texture leads
to changes in material property and hence product properties.
Many researchers have investigated the effects of EBM
processing parameters on material properties, with the aim of
identifying the optimal parameters for near-fully-dense parts
[11-14]. The EBM process has many parameters that can be
altered by the user, but the EBM manufacturer provides
recommended settings for Ti-6Al-4V.
II. LITERATURE REVIEW
In this paper, the effect of powder oxidation, internal porosity
and crystallographic texture on the charpy impact energy of Ti-
6Al-4V specimen fabricated using electron beam melting and
hot-isostatic pressing post processing was reviewed. It was
observed that the charpy impact energy dramatically decreases
in a smooth but rapid fashion due to excessive powder oxidation
and has a deleterious effect due to the internal porosity present
in EBM Ti-6Al-4V specimen. Crystallographic texture was also
found to influence Charpy absorbed energy. However, HIP
post-processing significantly increases the impact toughness for
specimens in each case.
A. Effect of powder oxidation
Powder quality in additive manufacturing (AM) electron beam
melt (EBM) is crucial in determination of material properties.
The effect of powder oxidation on the Charpy impact energy of
Ti-6Al-4V specimen manufactured using EBM was studied by
considering oxygen content significantly higher than 0.2 %
mass fraction and precisely controlling the amount of oxygen
as a function of time and temperature [3].
Powder samples were prepared by artificially oxidising powder
in air at elevated temperature and then mixing it with low
oxygen content powder. Using this method, four batches of
powders were used (A) virgin powder (B) five-times reused
powder (C) marginally oxidized powder, made up of a mixture
of five-times reused powder mixed with artificially oxidized
powder and (D) highly oxidized powder (Table 1). These
powder samples were then used to fabricate Charpy specimen
with the EBM process in three distinct orientations. Further,
half of the blanks were post-processed using a standard hot-
isostatic pressing (HIP) procedure common for Ti-6Al-4V
castings and EBM produced components and then machining
them into Charpy specimens and testing at room temperature.
The comparison of the absorbed energies and fracture
appearances was done.
TABLE 1: POWDER LOT DESCRIPTIONS AND POWDER OXYGEN
CONTENTS [1] Identifier Powder Mixture
Description
Powder Oxygen Content (% wt)
(± 2.5 %.)
Before build After build
A 100% Virgin Powder 0.11 0.123
B 1005 5x Reused
powder
0.142 0.157
C Mixture of 96.5 % 5x
reused and 3.5 % aged at 650 °C for 4 h
0.340 0.292
D Mixture of 88.8 % 5x
reused and 11.2 % aged at 650 °C for 4 h
0.525 0.455
Fig. 1: SEM images and cross-sections of (a, b) virgin and (c, d) highly oxidized (650 °C for 4 h in air) powder. (b, d) [1]
for all test conditions. The HIP process is typically performed
at α+β annealing temperatures, and thus results in a coarsened
microstructure compared to the as-built condition.
B. Effect of internal porosity and crystallographic texture
The effects of internal porosity and crystallographic texture on
Charpy absorbed energy (over temperature range of -196 to 600
°C) were determined by two heat treatment conditions (As-
Built and Hot Isostatically Pressed (HIPed)) on Vertical and
Horizontal specimen orientations [4]. The specimens were
fabricated using an Arcam1 A1 EBM machine at the standard
Arcam A1 build theme for Ti-6Al-4V (accelerating voltage 60
kV, layer thickness 50 μm, speed factor 35, software version
3.2.132) and the standard Arcam Ti-6Al-4V gas atomized
powder (particle size range approximately 40–100 μm, average
approximately 70 μm). Charpy build volumes were built 5mm
above the build plate and connected to the build plate using
standard thin wafer supports. It is important to note that Chaput,
et al. [5] did not use supports and instead built parts directly
attached to the build plate.
Total twenty specimen for each orientation (Horizontal,
Vertical) were fabricated of which half were randomly assigned
for HIPing, and other half remained As-Built leading to four
testing conditions (As-Built/Horizontal, As-Built/Vertical,
International Journal of Engineering Research & Technology (IJERT)
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HIPed/Horizontal, and HIPed/Vertical). The standard Ti-6Al-
4V HIP cycle was used (2 h at 900 °C and 100 MPa in argon,
with 12 °C/min heating and cooling rates). Charpy tests were
performed at temperatures ranging from -196–600 °C (±3°C).
Wrought Ti-6Al-4V (mill-annealed condition) was also Charpy
tested to provide a non-AM comparison for Charpy properties
due to its different microstructure. Chemistry was measured for
wrought Ti-6Al-4V, As-Built EBM Ti-6Al-4V, and HIPed
EBM Ti- Al-4V and then compared to ASTM F2924 [6].
Internal porosity was non-destructively evaluated for As-Built
and HIPed material using a laboratory x-ray micro-computed
tomography (CT) instrument. Texture was measured using
electron backscatter diffraction (EBSD) on a scanning electron
microscope (SEM) operated at 20 keV. Samples were mounted
and polished with diamond slurry to 1 μm and then vibratory
polished with 0.05 μm colloidal silica for up to 8 h. Both β and
α texture were measured directly at 30 nm step size for 13 fields
of view per testing condition.
No pores were observed (1 μm voxel size) in the HIPed
condition and the pore size distribution for the as-built
condition agreed well with previous work on the same material
with pores up to 10 μm diameter in HIPed material (calculated
relative density as 99.8% dense) [7]. All observed porosity was
approximately spherical, indicating it is of the gas porosity, and
not the lack of fusion, variety [8-10].
Representative fracture surfaces for all conditions are shown in
Figure 3, displaying two main features of interest, internal pores
and ridges. Internal pores (Figure 3f and black arrows) were
observed on all As-Built fracture surfaces but not on HIPed
fracture surfaces. Ridges (Figure 3e and white arrows) were
observed on all Horizontal fracture surfaces but not on Vertical
fracture surfaces which ran perpendicular to the layers (X-Y
plane) and parallel to the build direction (Z).
Significant coarsening was observed due to HIPing (Figure 4)
whilst, α lath thicknesses did not change appreciably as a
function of distance from the bottom of the build volume. The
As-Built/Vertical testing condition had a majority of 001 β
orientation, but it was far from an exclusive 001 α fibre texture.
The Burgers relationship was found to hold for α orientation
with respect to β. It is apparent from Figure 5 that prior-β grains
are elongated in the build (Z) direction. In larger area texture
maps, no appreciable differences were perceived between the
between HIPed/Horizontal (Figure 5a, Figure 5c) and
HIPed/Vertical (Figure 5b, Figure 5d) conditions when
compared. However, considerable differences were observed
between the two planes (X-Y and XZ) due to the anisotropic
morphology of the elongated prior-β grains in the z-direction. It
has recently been shown that prior-β grain boundaries impede
dislocation motion as long as the two adjacent prior-β grains are
of different texture orientation [19].
III. RESULTS
A. Effect of Powder Oxidation
For a given oxygen content, no differences were observed in the
overall roughness and appearance between the samples tested
in different orientations. For the five-times reused and
marginally oxidized samples, the fracture surface features had
characteristics of both the virgin and highly oxidized samples,
with more brittle-type features in the artificially oxidized
samples and more ductile-type features in the five-times reused
samples. However, voids were always detected on the fracture
surfaces of the non-HIP specimens but were not observed on
the HIP surfaces, irrespective of their oxidation levels and
orientations. This was found to be true for the X-ray Computed
Tomography (CT) scan results of HIP and non-HIP specimens.
Figure 6 and figure 7 show the comparison between Charpy
absorbed impact energies and Hardness Rockwell C values
from the ends of the Charpy specimens (respectively) for the
Fig. 2: SEM images of Charpy specimen fracture surfaces made from virgin powder in the (a, d) X-Y, (b, e) X-Z, and (c, f) Z-X orientations with (a, b, c)
HIP or (d, e, f) non-HIP post-processing (arrow/arrow point indicates the build direction Z) [1]
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Fig. 4: EBSD images showing representative direct β texture measurements (bright green) using 30 nm step size for a) As-Built and b) HIPed conditions
[2]
Fig. 5: EBSD larger area α texture maps for a) HIPed/Horizontal X-Y plane, b) HIPed/Vertical X-Y plane, c) HIPed/Horizontal X-Z plane, and d) HIPed/
Vertical X-Z plane [2]
four oxygen levels including the effects of the three specimen
orientations and non-HIP versus HIP post-processing. Figure 8
shows the charpy absorbed impact energy as a function of
consolidated material oxygen content, including the effects of
the three specimen orientations and non-HIP versus HIP post-
processing
It was perceived from the figures 6-8 that the Charpy absorbed
impact energies decreased dramatically with increasing oxygen
content in case of HIP and that HIP post-processing always
Fig. 3: SEM images of fracture surfaces of a) As-Built/Horizontal, b) As-Built/Vertical, c) HIPed/Horizontal, and d) HIPed/Vertical [2]
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Fig. 6: Charpy absorbed impact energies for the four oxygen levels including
the effects of the three specimen orientations and non-HIP versus HIP
post-processing [1] (Error bars represent ± 1 standard deviation)
Fig. 7: Hardness Rockwell C values from the ends of the Charpy specimens
for the four oxygen levels including the effects of the three specimen
orientations and non-HIP versus HIP post-processing [1] (Error bars represent
± 1 standard deviation)
improved the impact energy compared to as-built samples. The
orientation effects were found to be much less discernible for
the highly oxidized specimens and the Z-X orientation was
found to be the toughest. Additionally, the instrumented striker
force-time histories indicated relative differences in ductility
between the specimens, where the specimens with the lowest
oxygen contents, the most brittle behavior by the highly
oxidized specimens exhibited the most ductile behavior.
It was observed that HIP post-processing always improved the
impact energy (as expected) compared to as-built samples and
that reduces the effects of orientation in the most severely
oxidized samples due to the dropped impact energies. In
addition, HIP post-processing removes the typical micro-voids
encountered and improves the ductility of EBM Ti-6Al-4V
while maintaining the strength at acceptable levels [15-16].
B. Effect of internal porosity and crystallographic texture
The results suggest that internal porosity has a deleterious effect
on Charpy absorbed energy, which has been shown previously
for other material systems too [17]. X-ray CT measurements
showed internal porosity in the As-Built condition (99.8%
dense), but not in the HIPed condition which was supported by
the fractography (Figure 3 through evidence of pores on the
fracture surfaces of As-Built Charpy specimens. In Charpy
results (Figure 10), HIPed conditions have higher absorbed
energy compared to As-Built conditions and was attributed to
the effect of internal porosity. α lath thicknesses as a function
of distance from bottom of build volume are shown in Fig. 9.
The observed coarsening of α laths due to HIPing (Figure 4,
Figure 9) also contribute to this trend as it is known that coarser
Ti-6Al-4V microstructures have higher Charpy absorbed
energy [18].
The results also suggested that crystallographic texture has an
effect on Charpy absorbed energy. For the HIPed condition, the
Vertical orientation exhibited higher absorbed energy
compared to the Horizontal orientation but for the As-Built
condition, there appeared to be no difference (Figure 10). Even
Fig. 8: Charpy absorbed impact energy as a function of consolidated material oxygen content, including the effects of the three specimen orientations and
non-H IP versus HIP post-processing [1]
International Journal of Engineering Research & Technology (IJERT)
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Fig. 9: α lath thickness as a function of distance from the bottom of the build
volume [2]
Fig. 10: Charpy absorbed energy for all four EBM Ti-6Al-4V test conditions as well as Wrought Ti-6Al-4V (mill annealed) [2]
though the crack pathway for the Vertical orientation crosses
more prior-β grain boundaries than the crack pathway for the
Horizontal orientation, the latter was found to be tougher for the
As-Built conditions. It is hypothesized this is because of the
differing texture (i.e., predominantly 110 β for Horizontal and
predominantly 001 β for Vertical). The fact that the
predominantly 001 β texture has lower toughness compared to
the predominantly 110 β texture [20] led to the negligible
difference in Charpy absorbed energy for As-Built/Horizontal
and As-Built/Vertical test conditions.
Figure 10 shows the charpy absorbed energy for all four EBM
Ti-6Al-4V test conditions as well as Wrought Ti-6Al-4V (mill
annealed). The results suggest that HIPed conditions have
higher absorbed energy than As-Built conditions (Figure 10).
For the HIPed condition, the Vertical orientation appear to have
higher absorbed energy than the Horizontal orientation.
However, this apparent trend did not seem to exist for the As-
Built condition. All of these observed differences appear to be
larger at higher temperatures. HIPed EBM Ti-6Al-4V compares
well with Wrought Ti-6Al-4V, despite differences in
microstructure (mill annealed) and chemistry (e.g. oxygen
content).
IV. CONCLUSION
It was found that excessive powder oxidation (oxygen mass
fraction above 0.25 % and up to 0.46 %) dramatically decreases
the impact energy, about seven times at the room temperature.
As the powder oxygen mass fraction increased from 0.11 % to
0.53 %, Charpy impact energy of Ti-6Al-4V decreased in a
smooth but rapid fashion (depending on orientation and post-
processing). In addition, HIP post-processing significantly
increases the impact toughness, especially for specimens with
lower or normal oxygen content. The specimen orientation
effect was found to be more significant for low oxidation levels.
HIP post-processing increased toughness of the alloy in all
directions, especially for low and medium oxygen content. Both
the HIP post-processing and orientation effects on toughness
largely disappeared at the 0.5 % mass fraction level.
Results suggest that internal porosity has a deleterious effect on
the Charpy absorbed energy of EBM Ti-6Al-4V. This was
evident as HIPed material displayed higher Charpy absorbed
energy over a range of temperatures (-196 to 600 °C) compared
to As-Built material, and spherical internal porosity was found
(x-ray CT, fractography) in the As-Built condition (99.8%
dense) but not after HIPing. Crystallographic texture was also
found to influence Charpy absorbed energy along with the
anisotropic grain morphology (i.e., prior-β grains elongated in
the build (Z) direction). It was observed that for similar texture,
crack pathways that cross more prior-β grain boundaries lead to
higher Charpy absorbed energy. However, differing textures
were found to negate the prior-β grain boundary strengthening
effect in some cases, emphasizing the influence of texture and
variations in texture on Charpy absorbed energy. For the four
testing conditions (As-Built/Vertical, As-Built/Horizontal,
HIPed/Vertical, and HIPed/Horizontal), types of texture varied
from predominantly 001 β to predominantly 110 β, but none
matched the most commonly reported texture i.e. 001 β-fibre.
V. FUTURE SCOPE
Apart from the many advantages of EBM, it has many
challenges which are all related to powder used (as powder is
the raw material for EBM). So far, researchers have
qualitatively reviewed the (limited) parameters related to
powder i.e. powder oxidation, internal porosity and
crystallographic texture and their effect on the charpy impact
energy only. Quantification of the respective contributions of
porosity and coarsening haven’t been done till now. In addition,
a deeper understanding of the influences of texture processing
is necessary and needs to be addressed in future work.
International Journal of Engineering Research & Technology (IJERT)
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International Journal of Engineering Research & Technology (IJERT)
ISSN: 2278-0181http://www.ijert.org
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