EPFL

SINGLE CRYSTALSURFACE ENGINEERING

W. KurzC. BezençonS. MokademJ.-D. Wagnière

M. HoebelM. Konter

Swiss Federal Instituteof Technology

Lausanne (EPFL)

ALSTOM PowerBaden

J.-M. Drezet CALCOM Lausanne

Physical Metallurgy -

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SX - SURFACE ENGINEERINGSX - SURFACE ENGINEERING

• What is SX surface engineering ?• Why ?• How does it work ?

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THE CONCEPT OFTHE CONCEPT OFSX - SURFACE ENGINEERINGSX - SURFACE ENGINEERING

• SX superalloys are generally used for HPT turbine blades.

• Mechanical properties � when

1. Cr (oxidation) �2. C, B, Zr (grain boundaries) �

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THE CONCEPT OFTHE CONCEPT OFSX - SURFACE ENGINEERING - ISX - SURFACE ENGINEERING - I

COATINGCOATING

Low Cr superalloys require protection againstoxidation and corrosion :

• Present day technique: plasma sprayCr rich polycrystalline NiCrAlY coating.

• Ni has high crystal (E) anisotropy �incompatibility stresses in thermal cycling � premature cracking.

SOLUTION: SX COATING ON SX BLADE

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E-MODULUS ANISOTROPY OF Ni

E[100] ~ 125 [GPa]E[111] ~ 310 [GPa]

<100>

Phase � - Siebörger et al.

[001]

[111]

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LASER CLADDING PROCESS

Substrate

Clad

Molten pool

LaserBeam Injection

Nozzle

Vb

P, Db

To

��y m .

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SX - Substrate

[001

]

Equiaxed grains

Non-epitaxialcolumnar grains

Defects toavoid

SX - Substrate

[001

]

SX - DepositColumnar

Ideal caseEpitaxialgrowth

MICROSTRUCTURES / DEFECTS

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EPITAXY

Ensure at the surface• metallic contact with the substrate• similar crystal structure of primary

phases

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Substrate

Liquid

z

x

Mushy zoneSolid

Remelted zone

Vb

Deposit G,VS

LASER CLADDING PROCESS

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���primary phase���primary phase

EPITAXYMCrAlY with

low Al high Al

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Epitaxy

ColumnarDendriticGrowth

Equiaxed

CMSX-4

MCrAlY

SINGLE CRYSTAL LASER CLADDING OF

MCrAlY ON CMSX 4

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DENDRITE BRANCH SELECTION

[100]

[001]

[010][001]

[010][100]

LaserRemelting

Vb

Dendrites growalong

<100> direction

(direction closest toheat flux is selected)

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COLUMNAR-EQUIAXED TRANSITIONCET

Avoid nucleation and growth in the constitutionally undercooled region ahead of the columnar dendritic zone

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Process

Alloy

Constitutional Undercooling

• TemperatureGradient

• SolidificationVelocity

G

V

• GrowthUndercooling

• Nucleation SiteDensity

• NucleationUndercooling

�Tc

N0

�Tn

VT*

TL

�Tc

T equilibrium

z

T

G

T actual

N0-1/3

COLUMNAR TO EQUIAXED TRANSITION

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Effect of G and V

High G

z

TG1 G2

�Tc

V1

V2

z

T

�Tc Low V

COLUMNAR TO EQUIAXED TRANSITION

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3/10�

�� NGTc

3/10

/10 )( �

���� NGVaT n

nn

TNaVG )( 0

30 ����

n

c

n

nTNa

VG

���

���

�

�

134 03 0

�

�

COLUMNAR TO EQUIAXED TRANSITION

nc VaTT /1

0 )( ����

, for a columnar structure KVGn

�

z

T

N0-1/3

�TcG

R

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10-5

10-4

10-3

10-2

103 104 105 106 107Solid

ifica

tion

Vel

ocity

[m/s

]

Temperature Gradient [K/m]

Planar Front

DendriticEquiaxed

ColumnarDendritic

N0

KVGn

�

MICROSTRUCTURE SELECTION MAP

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PROCESSING PARAMETERS

Calculation of

• temperature gradient, G• interface velocity, V

as a function of position in the melt pool

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Solid fractionTemperature

Mesh

calcom -

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Temperature Gradient

Rosenthal Solution of the Heat-flux equation

G

TL TL-�T Vb

Temperature gradientM

elt p

ool d

epth

SOLIDIFICATION CONDITIONS

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�

Viso

Vb

Vb

�cos�� biso VVIsotherm velocity :

Solidification velocitySOLIDIFICATION CONDITIONS

Velocity [mm/s]

Viso

Vb0

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�cos�� biso VVIsotherm velocity :

Dendrite velocity :�cos

isohkl

VV �

[001]

[100]

�

Viso

V001

V100

Vb

Velocity [mm/s]

V001

V100Viso

Solidification velocitySOLIDIFICATION CONDITIONS

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10-5

10-4

10-3

10-2

103 104 105 106 107Solid

ifica

tion

Vel

ocity

[m/s

]

Temperature Gradient [K/m]

T0=1000°C T

0=20°C

SOLIDIFICATION CONDITIONS

bottom

surface50%

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COMBINATION OFPARAMETERS CHARACTERISTIC FOR

MICROSTRUCTURES &

PROCESSING

MICROSTRUCTURE-PROCESSING MAP

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PROCESSING-MICROSTRUCTURE MAP

10-5

10-4

10-3

10-2

103 104 105 106 107Solid

ifica

tion

Vel

ocity

[m/s

]

Temperature Gradient [K/m]

DendriticEquiaxed

ColumnarDendritic

T0=1000°C T

0=20°C

PlanarFront

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T0 = 20°C

0° 60°

0° 60°

T0 = 1000°C

LASER REMELTING

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Orientation analysis (EBSD)

5°

[100] Pole Figure

1 mm

0°

60°300 �m

LASER CLADDING MCrAlY

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MCrAlY - 0,3 mm

Transverse Section

SX CLADDING OF MECH. TESTSPECIMEN

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<100>

[100] [110]

MICROSTRUCTURE OF TESTSPECIMEN

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REPAIR ENGINEERINGREPAIR ENGINEERING

High price of SX components, typically 5 k€ per blade, asks for life time extension techniques

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THE CONCEPT OFTHE CONCEPT OFSX - SURFACE ENGINEERING - IISX - SURFACE ENGINEERING - II

REPAIR ENGINEERINGREPAIR ENGINEERING

Low C, B, Zr superalloys form (at high T)mechanically strong crystals - but arevery week at grain boundaries:

• avoid formation of g.b.

SOLUTION: SX REPAIR OF SX BLADES

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Deposit Molten pool

Laser Beam Injection

Nozzle P, Db

To

��z

Substrate

m .

Vb

LASER METAL FORMING

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Platform crack due to thermo-mechanical fatigue

After cutting/polishing

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AFTER LASER METAL FORMING

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deposit

substrate

Microstructure Analysis

8 laser traces

LASER METAL FORMING

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CONCLUSIONS

• Epitaxial Cladding,• Epitaxial Laser Forming,• (also Epitaxial Welding),can be achieved on SX components. Itrequires solidification theory for a closecontrol of•macroscopic heat flux (epitaxy) and•microstructure development to control CET.

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CONCLUSIONS

Today there is industrial activity to transfer the concepts into production for stationary gas turbines and aircraft engines.

Ideal ground for application of long term university research to industrial problems.