Post-processing treatment of Wire Arc Additive Manufactured NiTi shape memory alloy using laser shock peening process: a study on tensile behavior and fractography analysis

This study explores the synergistic effect of Wire Arc Additive Manufactured (WAAM) NiTi shape memory alloys (SMAs) subjected to laser shock peening (LSP) on the surface morphology, microstructure, tensile properties, and fracture behavior, with a prime focus on enhancing material performance for advanced engineering applications. NiTi walls were fabricated using the WAAM technique and subjected to laser shock peening (LSP) with various intensities. The microstructural study of the as-manufactured (AM) NiTi alloy displayed a non-uniform distribution of phases, including retained austenite and martensite, which was refined with successive LSP treatments. The application of LSP treatment resulted in surface plastic deformation and enhanced carbon diffusion with Ni-enriched surface. Phase transition behavior was assessed using differential scanning calorimetry (DSC) analysis. For martensitic and austenitic transformations, the AM samples showed wide temperature ranges. Transformation temperatures gradually decreased with successive LSP treatments, suggesting that the austenitic phase had stabilized; this is mainly due to the microstructural refinement brought on by LSP, demonstrating how LSP can be used to customize phase transformation properties for improved functional performance. Microtensile testing demonstrated that LSP had a substantial positive impact on both the yield strength (YS) and ultimate tensile strength (UTS) of the NiTi alloy. The LSP-3 (10 GW/cm2) treated sample exhibited a maximum UTS of 426.57 MPa and a toughness of 17.90 MJ/mm3. Post-SEM fractographic study of the AM NiTi samples displayed a brittle fracture mode, which was characterized by hard cleavage facets. The use of LSP treatment resulted in the development of a more uniform surface characterized by transgranular fracture properties and ductile tearing, which suggests an enhancement in toughness and strength. In addition, the application of LSP resulted in a considerable decrease in porosity in the samples. The samples treated with LSP-3 had the lowest porosity measuring at 4.10%. These findings highlight the potential of LSP as a transformational post-processing technique for improving the mechanical characteristics of WAAM-fabricated NiTi SMAs.