We have investigated the hardening response, residual stress generation and microstructural changes in aluminium alloy 2624 owing to laser shock peening. The alloy was studied in two heat treatment conditions, T351 and T39, that have 20% difference in yield strength: hence the effects of laser power density and multiple peen impacts on materials with nominally identical physical properties but with different hardening responses has been studied. Hardness was characterised by nanoindentation, and residual stresses were measured by incremental hole drilling. The magnitude and the depth of the peak compressive residual stresses increase with increasing power densities as well as the number of laser impacts, before reaching a saturation point above which loss of surface compression occurs. Maximum compressive residual stresses were around −350 MPa, and maximum hardness increase was around 22%. The treatment has a noticeable effect in changing the microstructures of the T351 temper while the T39 remained almost unchanged.

1. Suraiya Zabeen, Kristina Langer, Mike Fitzpatrick
Correlation between Residual Stress
and hardness response generated by
Laser Shock Peening in Al-2624
6/1/2020

2. Presentation Layout
• Aim
• Materials
• Experimental Method
• Results
• Conclusions
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6/1/2020Grenoble, France

3. Objective
• To determine the effect of laser peening
parameters – on the induced hardness and
residual stress.
• To determine the cyclic stress-strain behaviour.
• To establish a correlation between measured
hardness and residual stress generated by LSP.
• to develop a predictive model for hardening and
consequent residual stress generation.
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6/1/2020Grenoble, France

4. Characterization Techniques
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1.Laser Confocal Microscopy for peened surface profile
measurement
2.Cyclic Stress Strain test
3.Residual Stress Measurement
a) Laboratory X-ray Diffraction
b) Contour Method
c) Synchrotron X-ray Diffraction
d) Incremental Hole Drilling
4. Hardness characterization using Nanoindentation
technique

5. Materials
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Al-2624 T351 and T39 heat treatment conditions
T39T351
Stress Strain Curve Heinimenn et
al.
Heat
Treatment
Elastic
Modulus
/ GPa
Yield
Strength
/MPa
Ultimate
Tensile
Strength
/MPa
E/σ
y
Elon
gatio
n /%
Strain
Hardening
Exponent
T39 70 460 550 152 14 0.07
T351 70 360 535 194 20 0.11
T351 alloy was solution heat
treated, stress-relieved by
stretching, and naturally aged
T39 alloy was only cold
worked and naturally aged
after solution heat treatment.

6. Peen Matrix
6/1/2020 Grenoble, France
24 test coupons has been peened by the
Metal Improvement Company, Earby, UK.
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Material
P.D.-Pulse
duration-#
Layers
(GW/cm2-ns-
#)
Spot Size
/ mm
Al-2624
T351
Al-2624
T39
1-18-1
8.51-18-2
1-18-4
1-18-7
3-18-1
53-18-2
3-18-4
3-18-7
6-18-1
3.56-18-2
6-18-4
6-18-7
Single Laser
Spots
Specimen Dimensions: 70 × 70 × 12.5 mm3

7. Synchrotron X-ray Diffraction: Id-31, ESRF
6/1/2020
Slit size: vertical 0.35mm Horizontal 0.75mm • E=62 keV
• λ =0.2 Å
• CrystalPlane:
(311)
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8. • Semi-destructive Technique.
• 2 mm diameter hole/ 1mm deep
stress.
• Stress is calculated from the relaxed
strain measured by the strain gauge.
E
Incremental Hole Drilling
6/1/2020 Grenoble, France
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CEA 13 062UL 120 EA 13 062RE 120

9. Contour Method
• Contour Method is a 3-Step RS measurement
technique that relies on Buckner’s principle.
6/1/2020
Schematic representation of the
contour method principle,
(a) The component with significant
residual stress is CUT into two
halves.
(b) RS is relaxed and the contour of the
newly created surface deviate from
planarity which is measure by CMM
(c) Deformed contour is pushed into
plane surface using FEM and stress
is calculated by the post-processing
of the result.
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10. Nanoindentation Technique
Hardness = Force/Area
Area = f (Contact Depth)
Oliver and Pharr Method
(Oliver and Pharr, 1992)
Load Displacement Sensing
technique used for mechanical
properties:
• Elastic Modulus
• Yield Strength
• Strain Hardening Exponent
• Hardness
Berkovich
Indenter
0
50
100
150
200
250
0 1000 2000 3000
Load
Displacement (nm)
Load Displacement Curve

11. Indentation Procedure
• 20 and 50 mN
Load were used
• Two Lines
were created
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12. Surface line profiles by Laser Confocal Microscopy
Effect of Laser Power Density on the surface
deformation
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The magnitude of the depression into the surface increases
with the laser energy.
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Distance (mm)
Depth(micron)

13. Results: Cyclic Stress-Strain Behaviour
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Progressive Hardening for T351
Rapid Hardening for T39
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14. 2D Residual Stress Contour plots for Al-2624 T351,
6-18-7
• Peening Condition: 6-18-7 ( 3.5 mm Spot Size)
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20 mm
3.5 mm

15. Depth-resolved Residual Strain Profiles: Id-31
Comparison between
1-18-1 and 1-18-7
6/1/2020
(Laser Energy-
Laser Pulse Duration-
No of Layers)
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1 GW/cm2
6 GW/cm2
The magnitude of the
compressive RS induced
by LSP increases with
the number of layer and
laser energy.
T351 T39
Comparison between
6-18-1 and 6-18-7

16. Residual Stress By ICHD
1 GW/cm2
1 2 4 7
3 GW/cm2 6 GW/cm2

17. Nanoindentation Hardness Response
Effect of Power Density
After 1 Shock
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T351 T39
After 7 Shocks

18. Hardness Response by Nanoindentation
Effect of number of Layers
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T351 T39
3 GW/cm2
6 GW/cm2

19. Conclusion
• Hardness as well as residual stresses increase with
increasing number of shocks at lower power densities (1
GW/cm2), reach saturation at 3 GW/cm2. A hardness
increase of 10% is evident when the power density was
doubled.
• T39 alloy that had higher strength and lower hardening
capabilities than T351 alloy actually showed cyclic
softening at 6 GW/cm2.
• The optimum process parameters for this alloy are
identified as 3 GW/cm2 –18 ns - 4 shocks.
• A lower surface residual stress (possibly due to reverse
yielding), and maximum CRS is observed at 6 GW/cm2
after 7 shocks.
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20. Acknowledgements
• Air Force Office of Scientific Research, Air force
Material Command, USAF.
• Dr. Markus Heinimann at Alcoa Inc.
• Dr. Andy Fitch at ESRF, Grenoble, France
• Mr. Pete Ledgard and Mr. Stan Hiller at The Open
University, UK.
• Dr. Philip Whitehead at Stresscraft, UK.
• Lloyd’s Register Foundation.
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Questions???