Laser peening of 420 martensitic stainless steel using ultrashort laser pulses

Abstract

Abstract Laser shock peening is an established method used to increase resistance of a surface to cracking and fatigue damage by inducing deep compressive residual stresses. Most of the current laser shock peening system utilizes a nanosecond pulse laser with very slow repetition rate (about 5-10 Hz), high pulse energy and a confining medium to constrain the plasma. On the other hand, ultrashort pulse lasers generally have a higher peak power density and operate at higher repetition rate than nanosecond pulse laser. Therefore, there is an opportunity to employ ultrashort pulse lasers to peen the surface in a fast and efficient way. However, limited studies have been performed to investigate the peening capability of ultrashort laser pulses. In this study, femtosecond (fs) pulse laser is used to peen a 420 martensitic steel surface under different coverage. The results show that fs laser can induce peening effect; however, peened depths are much smaller (around 20-30 μm) compared to high energy nanosecond pulse laser peening (up to 1 mm). A maximum compressive stresses of about -80 MPa was recorded at 981% coverage. Increase in coverage produced stress relaxation and did not increase the depth of influence. It was found that the state of the residual stresses depends on four main factors – intensity of ablation-induced shock wave, thermal effect of laser beam, phase transformation of the steel and surface mechanisms such as presence of nano-ripples and oxidation. Further experiments are ongoing to achieve higher magnitude of compressive residual stresses and higher depth of influence.