Elsevier

Surface and Coatings Technology

Volume 347, 15 August 2018, Pages 123-135
Surface and Coatings Technology

Effect of alloy temper on surface modification of aluminium 2624 by laser shock peening

https://doi.org/10.1016/j.surfcoat.2018.04.069Get rights and content

Highlights

  • Al 2624 alloy temper influenced the microstructural response of laser shock peening.

  • Difference in unpeened yield strength between the T351 & T39 affected both hardness and residual stress response.

  • The maximum compressive residual stresses of ∼ −350 MPa were induced at 3-18-4.

  • ‘Overpeening’ or ‘underpeening’ resulted in strong surface-effects.

  • LSP caused a maximum of 25% & 14% hardness increase in the T351 & T39 respectively.

Abstract

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.

Introduction

Laser shock peening (LSP) is a surface processing technology that offers life extension of metallic structural components for aerospace, automotive, and power generation industries, among others, by inducing deep compressive residual stresses that improve strength, hardness, fatigue life and corrosion resistance of the material [1,2]. Fatigue life improvement via LSP is largely dependent on the residual stress generated, which in turn is dependent upon the peening parameters. A number of researchers have reported the effect of LSP on the mechanical properties, residual stress, and the resulting life improvement of various aluminium alloys. The peak compressive residual stresses for Al 2024 T3 were reported as around −150 MPa at a power density of 3 GW/cm2 [3], and about −180 and −300 MPa at 1 and 4 GW/cm2, respectively [4]. Sano et al. [5] also reported peak residual stress of −300 MPa and hardness of about 2.4 GPa for the same alloy, although a different laser system was used.

Al 2624 is a newly developed alloy (to replace Al 2024) that has improved fracture toughness and damage tolerance compared to Al 2024. At present, developing an LSP-based fatigue design for enhanced structural integrity relies heavily on trial-and-error without a detailed understanding of the correlation between the plastic deformation and the consequent hardening and generation of residual stress. It is therefore costly and time-consuming. Although efforts have been made previously to understand the effect of laser treated area on the residual stress and fatigue [6], a systematic study on the effect of single vs. multiple peen overlaps at different peening intensities, and their effects on the elastoplastic response is still lacking.

The goals of the current research are to quantify the relationships between peening conditions, induced residual stresses, hardness, and material state for aluminium 2624. We investigate the effect of peening intensity and the number of impacts on the hardness and residual stress in Al 2624 alloy in the T351 and the T39 heat treatment conditions. Two heat treatment conditions were selected to study the effects of yield strength and hardening capacity while maintaining nominally identical elastic properties.

Section snippets

Materials

Al 2624 alloy was supplied by Alcoa (now Arconic) in two heat treatment conditions (T351 & T39). T351 alloy is solution heat-treated, stress-relieved by stretching, and naturally-aged; T39 is cold-worked and naturally-aged after solution heat treatment. The materials were received as plates with a thickness of 25 mm. The test coupons for residual stress measurement were 70 × 70 × 12.5 mm3 (see Fig. 1). The specimens were extracted using wire electro-discharge machining (EDM). Since a smooth

Surface profile

The surface deformation following laser peening was measured using a Bruker ContourGT interferometer microscope. The measurement accuracy was estimated to be about 0.3 μm. Fig. 7 shows the 2D surface profiles of specimens peened with condition 3-18-7 (using the standard terminology for laser peening: a-b-c, where a = power density in GW/cm2; b = pulse duration in ns; c = number of impacts) for T351 and T39. The laser spot size was 5 × 5 mm2. The average depth of the peened spot is 57 μm for

Discussion

The correlation between residual stress generation and hardening for both heat treatment conditions is shown in Fig. 18. From these plots, the following observations can be made:

  • 1.

    From Fig. 18, there seems to be an optimum peening condition that yields the best combination of compressive residual stress with strength, with ‘underpeening or ‘overpeening’ giving poorer outcomes. For example, peening with 6 GW/cm2 results in almost 50% reduction in the surface compressive residual stresses as

Conclusions

We have undertaken a comprehensive study of the effect of laser shock peening on two different tempers of aluminium alloy 2624. The two tempers, T351 and T39, have different yield and ultimate tensile strengths, and different hardening characteristics post-yield.

  • 1.

    Laser shock peening induces compressive residual stresses at the surface for both tempers, with the magnitude of stress increasing with power density and number of shocks up to a saturation point which is approximately 3 GW/cm2 for four

Acknowledgements

This research study was sponsored by the Air Force Office of Scientific Research, Air Force Material Command, USAF, under grant number FA8655-12-1-2084, and the Air Force Research Laboratory's Aerospace Vehicles Directorate. The U.S. Government is authorized to reproduce and distribute reprints for Government purpose notwithstanding any copyright notation thereon. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the

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