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Measurement and Modeling of Residual Stress in Ti-6Al-4V Linear Friction Welds
Daira Legzdina/Vincent Chung
Clement Buhr/ Paul Colegrove
TWI Conference, Cambridge UK
March 19, 2015
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Page 1
Outline
• Motivation
• Test specimens
• Measurement methods
– Neutron diffraction
– Contour method
– Comparison
• Residual stress modeling
– presented by Clement Buhr /Cranfield University
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Motivation
• Titanium alloys are popular for aerospace applications
– Structural members
– Turbine engine bladed disks/rotors
• Advanced joining methods allow for more efficient manufacture
– Linear friction welding (LFW)
• Linear friction welding
– Two surfaces forced into contact
– Repeated linear rubbing
– Advantageous for titanium because carried out in air
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Test specimen
• Test specimen construction
– Two blocks of Ti-6Al-4V
– Join using LFW
– Remove flash
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Measurement details
• Goal: quantify residual stress in weld region
• Two measurement methods:
– Neutron diffraction measurements along line
• Measurement performed at Los Almos National Lab SMARTS beam line – 2 mm x 2 mm x 2 mm gage volume
– Contour measurement over plane
• Measurement performed by Hill Engineering
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Neutron diffraction results
• Summary of residual stress measured using
neutron diffraction
– High magnitude tensile stress near LFW plane
• Largest in long direction
– Decreases to near-zero at approximately 5 mm
Neutron diffraction
-400
-200
0
200
400
600
800
1000
-10 -5 0 5 10 15 20 25 30
Distance from LFW center (mm)
Resid
ual
Str
ess (
MP
a) Szz
Syy
Sxx
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Contour method overview- Hill Engineering
• Contour method can generate a 2D map of residual stress normal to a plane
• Contour method steps (illustrated for 2D body)
– Part contains unknown RS (a)
– Cut part in two: stress release deformation (b)
– Measure deformation of cut surfaces
– Apply reverse of average deformation to finite element model of body (c)
– Map of RS normal to surface determined
Cut measure FEM residual stress
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Contour method results
• Summary of results from contour method measurement
– 2D map of long-direction stress
– High magnitude tensile stress near LFW joint
– Near-zero stress elsewhere
yy-component
-400
-200
0
200
400
600
800
1000
-20 -15 -10 -5 0 5 10 15 20
Distance from LFW center (mm)
Resid
ual
Str
ess (
MP
a)
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Comparison
• Neutron diffraction
– Single line, 3 stress components
• Contour method
– 2D map, single stress component
• Compare region of overlap
– Similar peak magnitude (800 MPa vs 750 MPa)
• Contour method is slightly lower
– Similar peak width
yy-component
-400
-200
0
200
400
600
800
1000
-20 -15 -10 -5 0 5 10 15 20
Distance from LFW center (mm)
Resid
ual
Str
ess (
MP
a)
Neutron Diffraction
Contour
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Effect of thermal stress relief
• Additional measurement on different (but similar) specimen
• Post weld thermal stress relief
• Compare with non-stress relieved
– Significant reduction in tensile magnitude (750 MPa vs 100 MPa)
yy-component
(contour method)
-400
-200
0
200
400
600
800
1000
-20 -15 -10 -5 0 5 10 15 20
Distance from LFW center (mm)
Resid
ual
Str
ess (
MP
a)
Thermal stress relief
As weldedAs welded
Thermal stress relief
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PhD Project Outline
“Residual stress prediction for 3D linear friction welded coupons”
Experiments :
Time consuming
Expensive Modelling
Parametric studies difficult
Modelling complementary to experiments
Solution
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State of Art
Very few publications on modelling RS for LFW components
Existing models :
2-D - Long solution times
Model the oscillations - 3-D model hard to achieve
Hypothesis:
Oscillations are not critical in predicting RS; it is the thermal
profile during cool-down which drives the RS
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ABAQUS finite element package was used
2-D model considered
Material used: Ti-6Al-4V
Sequentially coupled thermal stress analysis was used:
Transient analysis to model the heating and cooling stages and get the evolution of the thermal profile
-Heat flux applied at the weld interface during
the heating stage then deactivated for the cool down
Stress analysis to investigate the presence of RS in
the welded structure
Note: Due to symmetric conditions, only a quarter
of the coupon was actually modelled
Modelling Approach
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Thermal Analysis: Validation
12
0
40
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Stress Analysis
Boundary conditions:
Symmetry is applied on two edges to account for the whole coupon
Thermal profile of the cool down applied
(duration 100s) then the coupon is brought
to the temperature of 20°C (duration 100s)
Plane-strain elements used
X
-Sym
m
Y-Symm Page 15
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Stress Analysis
t=0s t=10s
+883 +788 +693 +598 +502 +407 +312 +217 +122 +26 -69 -164 -259 t=200s
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Stress Analysis
Dimensions: 10x20x30mm Dimensions: 20x40x120mm (MPa)
R. Turner (2011) et al “The Magnitude and Origin of Residual Stress in Ti-6Al-4V Linear Friction Welds: An Investigation by
Validated Numerical Modeling” Abaqus Page 17
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Stress Analysis
Δx=5.4mm
R. Turner (2011)
Abaqus
1
2
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Conclusion & Further Work
Initial work indicates that it is feasible to predict residual stresses without:
A fully coupled model
Modelling the oscillations
Improvement of the model : Heat flux distribution, 3-D model
Study of the influence of the flash on the residual stress distribution
Modelling the heat treatment stress-relief process
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