The effects of the confining medium and protective layer during femtosecond laser shock peening

Nanosecond laser shock peening is an important material strengthening technique, but its application is limited by the requirement of the confining medium and protective coating. These limitations can be potentially overcome by femtosecond laser shock peening. This article presents a study on the effects of the confining medium and protective coating on femtosecond laser shock peening of 304 stainless steel. The surface hardness can be increased by 45.5% by peening directly in air without any confining medium and coating. The surface quality is also maintained at a good condition. Numerical simulation by a hydrodynamic model reveals that femtosecond laser shock peening can induce extremely strong shock waves (hundreds of GPa) directly in air, which is much stronger than those by nanosecond laser peening (~10 GPa). Surprisingly different from nanosecond laser peening, it is found that by adding the confining medium and protective layer, the peening effect is significantly weakened. It is unveiled that the super high intensity of the femtosecond laser causes strong ionization of the confining medium (water), which shields 98% of the laser energy from deposition into the sample and weakens the peening effect. The enhancement depth by femtosecond laser peening is found to be less than 100 µm, which is the reason that the peening effect is weakened when a 100 µm thick coating is used. This study shows that femtosecond laser peening works the best directly in air without any confining medium and coating, which significantly broadens its application where high flexibility and precision are required.