Microstructure and residual stress modulation of Ti6Al4V for improving fatigue performance by combining laser shock peening and low temperature annealing

Laser shock peening (LSP) is an advanced surface enhancement technology that significantly improves the fatigue life of titanium alloys. However, the high density dislocations induced by LSP can be released under cyclic loading, and the uneven distribution of residual compressive stress may negatively impact fatigue performance. In this study, a low-temperature annealing process was developed to regulate the microstructure and residual stress after LSP treatment, and applied to titanium alloy materials. The average fatigue life of untreated, LSP, and LSP + annealing (LSPA) samples were approximately 6780, 10890, and 14970 cycles. Compared with the untreated specimens, the fatigue lives of the LSP and LSPA samples increased by 60.6% and 120.8%, respectively. The results showed that the low temperature annealing treatment optimized the microstructure after LSP through atomic thermal activation, dislocation slip reconstruction, and the formation of equilibrium grain boundaries, thereby reducing the density of mobile dislocations and the system potential energy. The evolution of the microstructure during annealing improves the residual stress distribution, eliminated stress concentration, reduced subsurface residual tensile stress from 118 MPa in the LSP state to 95 MPa after annealing, and significantly enhanced the stability of residual compressive stress, while also reducing the fatigue crack growth rate. These findings are applicable to a range of alloys and provide a novel approach to improving the stability of LSP, further enhancing the fatigue life of materials.