Microstructural responses and strengthening mechanism in different zones of laser additive repaired Ti17 titanium alloy via laser shock peening

Laser additive manufacturing has been employed to repair titanium alloy aero blades and blisks due to its low heat input and narrow heat-affected zone (HAZ). However, laser additive repaired (LAR) specimens exhibit weak tensile and fatigue properties due to coarse microstructure and tensile residual stresses. In this study, laser shock peening (LSP) was applied to the LARed Ti17 titanium samples. The present study systematically compared the microstructural responses to LSP between the LDZ and WSZ. Results showed that the highest dislocation density was found in the wrought substrate zone (WSZ) with a value of 19.01 × 10¹⁴ m⁻², accompanied by stacking faults (SFs) distributed within grain boundaries of α phase. Unlike the dislocation proliferation observed in the WSZ, the LSP-treated laser deposited zone (LDZ) exhibits a distinct deformation mechanism: obstruction of dislocation glide triggers phase transformation from hexagonal-close-packed Ti (HCP–Ti) to face-centered-cubic Ti (FCC–Ti), with extensive twinning within the resultant FCC-Ti accommodating additional plastic strain. The orientation relationship between HCP-Ti and FCC-Ti was (0002)_HCP//(1‾ 11‾)_FCC and [2 1‾1‾ 0]_HCP//[1‾1‾ 0]_FCC. Interactions between dislocations, twins and SFs fragmented the coarse microstructure into refined structures. Besides, a high-pressure laser shock wave induced compressive residual stress on the surface. Consequently, the synergistic contributions from both the LDZ and WSZ, which included grain refinement and induced compressive residual stress, resulted in an extension of fatigue life.