Martensitic transformation in temporally shaped femtosecond laser shock peening 304 steel

Laser shock peening (LSP) plays an important role in simultaneously strengthening and toughening materials for many engineering applications, such as key structures of aircraft and ship. This work focused on the application of temporally shaped femtosecond (fs) laser in shock peening 304 stainless steel. In our experiment, a single-fs laser pulse was divided by Michelson interferometer to generate two subpulses with a time delay ranging from 5 to 50 picoseconds (ps), and then the double-pulse fs laser was focused by an objective and hit the stainless steel surface. We revealed the mechanism of fs-LSP theoretically by a one-dimensional microscale heat transfer and thermoelastic model and experimentally by the time-resolved shadowgraph, X-ray diffraction, and election backscattering diffraction. When the first subpulse hit the material, plasma gradually formed and then the plasma electron density and temperature decreased. More energy deposited into the material because of the higher laser transmission caused by the decreased plasma electron density. The lower energy transfer rate caused by the decreased plasma electron temperature, along with the lower stress propagation rate caused by the plasma expansion, hindered the energy and stress transfer into the material. Herein, the optimal mechanical response achieved at the double-pulse delay of 20 ps was beneficial to martensitic transformation, and it ultimately induced more than a 20% improvement of hardness. Our method provided insight for structural transformation and surface strengthen by controlling the electron dynamics, which holds significance not only for strengthening 304 steel, but also for other extensive material systems.