The mechanisms of thermal engineered laser shock peening for enhanced fatigue performance

Thermal engineered laser shock peening (LSP) is a technique combining warm laser shock peening (WLSP) with subsequent post-shock tempering treatment to optimize the surface strength and fatigue performance of metallic materials. This technique integrates the advantages of LSP, dynamic strain aging (DSA), dynamic precipitation (DP) and post-shock tempering to obtain optimized microstructures for extending fatigue life, such as nanoprecipitates and highly dense dislocations. In this work, AISI 4140 steel is used to evaluate the thermal engineered LSP process. The resulting microstructures as well as mechanical properties are studied under various processing conditions. The mechanism underlying the improvements in fatigue performance is investigated. It is found that the extended fatigue life is mainly caused by the enhanced cyclic stability of compressive residual stress as well as surface strength. This improved material stability and reliability are attributed to the enhanced dislocation pinning effect corresponding to the number density, size and space distribution of nanoprecipitates, which could be tailored by manipulating the WLSP processing conditions and by post-shock tempering. The effects of the precipitate parameters on the precipitation kinetics as well as on the dislocation pinning strength are discussed.