Laser peening of 420 martensitic stainless steel using ultrashort laser pulses
Niroj Maharjan, Zhenyuan Lin, Dennise Tanoko Ardi, Lingfei Ji, Minghui Hong
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.