Effect of Laser Shock Peening on Residual Stress and Mechanical behaviour of Aluminium alloy AA2219 Friction Stir Weld

Abstract

The Aluminium alloy AA2219 is a precipitation-hardenable wrought alloy with copper as the major alloying element. Large-volume propellant tanks of space launch vehicles are manufactured by joining AA2219 alloy through Friction Stir Welding (FSW). Laser shock peening (LSP) is one of the most promising surface modification techniques to improve the performance of weld joints. During the LSP process, a high-energy laser beam impacts the surface of the specimen and generates ionised 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 vaporised, resulting in the generation of an ionised plasma. In this process, rapidly expanding plasma is entrapped between the specimen and transparent layer, generating a high surface pressure and propagating into the sample as a shock wave. When the peak pressure exceeds the dynamic yield strength of the material, plastic deformation takes place in the specimen which introduces compressive stress. An improvement in the performance of the FSW joint directly improves the payload capability of aerospace vehicles. Residual stress is an important design input for fracture assessment of pressure vessels. Hence, an accurate evaluation of residual stress is essential for the optimal design of pressure vessels, and it is also necessary to reduce or mitigate residual stress to improve structural margins. The main objective of the present study is to evaluate the residual stress of the AA2219 FSW joint and its mitigation through LSP. Another aim is to investigate the effect of LSP on the global tensile behaviour at various temperatures, microhardness, tensile behaviour of various zones of FSW, stress corrosion cracking behaviour, microstructure, and surface roughness of AA2219 T87 FSW joints using various experimental and characterisation techniques. The surface and through-thickness residual stresses were investigated in this study. In the as-welded condition, tensile residual stress exists in the weld region with a peak value of +123.5 MPa in the thermo-mechanically affected zone (TMAZ). LSP significantly affected all regions of the weld and reduced the tensile residual stress to compressive stress. A proportional increase in the compressive residual stress is observed because of the increase in the number of layers of the LSP. Longitudinal residual stress is non-uniform through thickness as well as across the weld. The peak tensile residual stress is +160 MPa at the center of the weld at mid-thickness and the LSP process led to a 55% reduction. AA2219 T87 FSW exhibits Yield Strength (YS) of 197 MPa and Ultimate Tensile Strength (UTS) of 348 MPa at ambient temperature. The LSP process increased the YS of the FSW joint by 7 – 14%. A similar increase is observed at cryogenic temperatures. The increase in the YS is due to the strain-hardening effect induced by the LSP. Repeated layers of peening led to a progressive increase in YS at all studied temperatures. The responses of different zones of the FSW to tensile loading and LSP were investigated using the digital image correlation technique. The weld nugget showed increase of 7%, 8%, and 16% in YS with single, three, and six layers of LSP, respectively and similar increase was observed in the TMAZ region. However, the HAZ did not exhibit a significant increase in the YS. The LSP process led to an increase in microhardness of 7–20%. Single-layer peening affected < 0.5 mm depth, whereas three and six layers of peening influenced a depth of 1.0 mm and more than 2 mm, respectively. Transmission Electron Microscopic study of the LSP specimens confirmed an increase in the dislocation density, which caused an increase in YS and microhardness. The LSP process increased the surface roughness in all the regions of the 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 joints was conducted using various experimental and characterisation techniques, and the advantages of LSP were brought out. The LSP of the AA2219 T87 FSW joint led to a reduction in the tensile residual stress at both the surface and through-thickness levels. The LSP led to an increase in YS at all investigated temperatures. Repeated layers of LSP led to a progressive increase in YS. This study also quantified the improvement in the YS of various zones of AA2219 FSW due to LSP. An increase in microhardness was observed on the surface and through-thickness levels owing to the LSP process. An increase in the number of LSP layers led to a proportional increase in hardness. LSP led to an increase in the dislocation density owing to a single layer of LSP, and a further increase was noticed owing to repeated layers of LSP, which is the cause of the above-mentioned advantages. The outcome of this study is useful for improving the margin of safety and reducing the inert mass of aerospace structures and pressure vessels.