Influence of Thermal Exposure on the Microstructure and Fatigue Performance of Laser Shock Peened Ti6Al4V Alloy

In the present study, the influence of thermal exposure on the microstructural evolution and fatigue performance of LSP-treated Ti6Al4V alloy was systematically investigated in order to demonstrate the probable variation for the LSP-strengthened Ti6Al4V alloy components in temperature-rise environment, particularly for those components that had to be strengthened wholly. For this research target, the double-side surfaces were strengthened, while the side walls retained their original state, and a relatively high-temperature environment with 600 °C/1 h was applied on the LSP-treated samples to accelerate the influence of thermal environment. The results reveal that LSP reconstructs the superficial layer by refining coarse grains and merging ultrafine grains into larger ones while introducing high-density dislocations and {10–12} twins. These microstructural changes reduce the grain size difference and enhance the resistance to crack initiation and propagation, leading to a fatigue life 3.33 times that of the as-received (AR) state. After thermal exposure, the outermost nanograins and twins are preserved. Meanwhile, the difference in grain size is further decreased, accompanying the relatively rational distribution of different-sizes grains, which further decreases the fatigue striation width in the superficial layer. However, the matrix and adjacent regions undergo significant microstructural degradation, including grain coarsening, dislocation recovery and unsuitable dislocation redistribution, which promote multiple crack initiation sites and decrease the fatigue life to only 10.5% higher than the AR state. These findings provide valuable insights into the service reliability of LSP-strengthened titanium alloy components operating under thermal–mechanical coupling conditions.