Laser peening of 420 martensitic stainless steel using ultrashort laser pulses

摘要

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 nm)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.