Effect of laser shock peening on elevated temperature residual stress, microstructure and fatigue behavior of ATI 718Plus alloy

Laser Shock Peening (LSP) is a mechanical surface treatment that induces large compressive residual stresses and microstructural changes in the material by using repetitive shocks from laser pulses. In this study, we investigate the use of LSP to improve the fatigue life of ATI 718 Plus (718Plus) at high temperature of 650°C. LSP led to severe surface plastic deformation, which, in turn, led to a high magnitude of surface compressive residual stresses and changes in the near-surface microstructure which caused high surface hardening. This change in the near-surface microstructure was in the form of high dislocation density forming dislocation entanglements and slip bands and formation of near-surface nanoscale sub-grains/crystallites that remained stable at elevated temperatures. In addition, LSP retained ∼−470MPa residual stress (68% of its initial residual stress) even after 140h exposure to 650°C. The retained residual stresses and the stable microstructure from the LSP increased the yield strength by ∼14% (∼140MPa) and endurance limit by ∼10% (∼90MPa) in corresponding tests at 650°C. This improvement in fatigue life was attributed to near-surface microstructure, hardening and high compressive residual stress. The estimated crack growth rates were 72% lower for LSP-treated 718Plus as compared with untreated material. The thermal-mechanical residual stress relaxation indicates the effectiveness of LSP in improving the fatigue life of 718Plus at 650°C.