Effects of Thermally Assisted Warm Laser Shock Processing on the Microstructure and Fatigue Property of IN718 Superalloy

The effects of laser shock processing (LSP) and warm laser shock processing (WLSP) on the microstructure of surface hardening layer and high-cycle fatigue performance at room temperature and high temperature (600 °C) of IN718 alloy were investigated. It has been revealed that the grain refined hardening layer with greater residual compression stresses, higher fraction of coincidence site lattice (CSL) boundaries and dislocation densities was formed in WLSP-treated alloy than in LSP-treated alloys. Moreover, microtwins included γ″ phase/high density dislocation complex was found in the surface of WLSP-treated alloy. These characters caused the significant enhancement of the medium value fatigue strength of WLSP-treated alloy at room temperature and elevated temperature. Apparently, the microtwins included γ″ phase/high density dislocation complex formed in the surface hardening layer of LSP-treated alloy has more complicated steric structure and more stable at elevated temperature than γ″ phase/low density dislocation complex formed in LSP-treated alloy, leading to the slow recovery process. Therefore, the surface hardening layer in the WLSP-treated alloy remained more ideal strengthening effect under high-cycle fatigue at elevated temperature than that in LSP-treated alloy. This resulted in the much longer fatigue crack initiation incubation and longer high-cycle life of WLSP-treated IN718 alloy under cycling load at 600 ℃. This discovery provides a new cognition of fatigue resistance by WLSP treatment of precipitation strengthening superalloy.