Ph.D.(Engg) : Effect of Laser Shock Peening on Residual Stress and Mechanical behaviour of Aluminium alloy AA2219 Friction Stir Weld

Aluminium alloy AA2219 is a precipitation hardenable wrought alloy with copper as a major alloying element. Large-volume propellant tanks of space launch vehicles are manufactured by joining AA2219 aluminium alloy through Friction Stir Welding (FSW) and it is designed optimally to improve the payload capability.  An increase in the strength of the FSW joint results in payload improvement of space launch vehicles. Residual stress is one of the crucial parameters for the design of pressure vessels, and it is also necessary to mitigate or reduce the same to improve structural margins. The main challenge is understanding the cause of residual stress, its evaluation, and mitigation due to the FSW process. Laser shock peening (LSP) is one of the most promising surface modification techniques to improve the performance of weld joints. In the LSP process, a high-energy laser beam impacts the surface of the specimen and generates ionized plasma by evaporating a thin ablative layer on the specimen. When a high-energy laser pulse passes through the transparent layer and hits the sample, the thin ablative layer is vaporized and continues to absorb the laser energy resulting in the generation of ionized plasma. Rapidly expanding plasma is entrapped between the specimen and the transparent layer, generating high surface pressure and propagating into the sample as a shock wave. When the peak pressure exceeds the material’s yield strength, plastic deformation occurs in the specimen.

The present work aims to investigate the impact of LSP on residual stress, microhardness, global tensile behaviour, tensile behaviour of various zones (local tensile behaviour), stress corrosion cracking behaviour and surface roughness of AA2219 T87 FSW. Surface and through-thickness residual stress were investigated in this work. In as-welded conditions, tensile residual stress exists in the weld region with a peak value of +123.5 MPa in the Thermo-Mechanically Affected Zone (TMAZ). LSP has significantly affected all the regions of the weld and reduced tensile residual stress to compressive. Longitudinal residual stress is non-uniform through thickness as well as across the weld. Peak tensile residual stress is +160 MPa at the centre of the weld in mid-thickness, and the LSP process led to a 55% reduction.

AA2219 T87 FSW exhibits a yield strength of 197 MPa and an ultimate tensile strength of 348 MPa at ambient temperature. The LSP process increased the yield strength of the FSW joint by 7 – 14%. A similar increase is seen in cryogenic temperatures also. The increase in the yield strength is due to the strain-hardening effect induced by LSP. The response of different zones of FSW to tensile lading and LSP was investigated using the digital image correlation technique. LSP led to an increase in YS in Weld Nugget and TMAZ. However, HAZ does not exhibit a significant increase in YS. The LSP process led to an increase in microhardness of 7 – 20%. Single-layer peening has affected < 0.5 mm depth, whereas three and six layers of peening have influenced a depth of 1.0 mm and more than 2 mm, respectively. Metallographic study of LSP specimen confirms an increase in dislocation density, which is the cause for the increase in YS and microhardness.  The LSP process has increased surface roughness in all regions of FSW, and the increase is substantial in the weld nugget and TMAZ regions. The LSP process has not affected stress corrosion cracking resistance, irrespective of the number of layers of peening.

In summary, a systematic investigation of the effect of LSP on AA2219 T87 FSW joint is carried out using various experimental and characterization techniques and the benefits of LSP are clearly brought out. LSP of AA2219 FSW reduces tensile residual stress and increases YS. This study has also quantified the improvement in YS of various zones of AA2219 FSW due to the LSP. An increase in microhardness was also noticed due to LSP. In addition, resistance to stress corrosion cracking is not compromised due to LSP. This research outcome will be useful in improving the structural safety margin or reducing the inert mass of aerospace structures and pressure vessels.