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.