Effect of multiple laser peening on microstructural, fatigue and fretting-wear behaviour of austenitic stainless steel

The present work investigates the influence of laser peening without coating (LPwC) on the fatigue and fretting wear behaviour of AISI 304 alloy subjected to one and five peening passes peened at 9 GW cm−2. Laser peened samples were characterised for microstructure, residual stress, phase evolution and hardness. After LPwC, the samples induced maximum compressive residual stress of −380 and −581 MPa for one and five peening passes at 50 μm from the surface. Microhardness was increased to 31 % and 71 % for one and five pass samples at 50 μm depth. The microstructure of laser peening samples was characterised using a transmission electron microscope (TEM) and electron backscattered diffraction (EBSD). Dislocation density was increased after peening with twin-twin interaction observed in both the peening cases and martensite induced at the junction of the twin-twin in five-time peened samples. Low angle grain fraction increased from 29 % (unpeened) to 41 % (one pass) and 63 % (five passes), and grain refinement was observed, where the average grain size reduced from 22.28 μm (unpeened) to 18.36 μm (one pass) and 9.51 μm (five passes) for LPwC samples. Fretting wear behaviour at various loading (5, 10 and 20 N) displayed significant improvement in wear resistance of LPwC samples, as evident from lower wear volume loss. Fatigue testing performed at R = 0.1 exhibited an improvement in fatigue life of 1.4 and 2.4 times for one and five pass samples compared to unpeened samples at a stress amplitude of 450 MPa. Fatigue fractography revealed a decrease in striation spacing for LPwC samples (979 nm/cycle and 565 nm/cycle for one- and five-time peening) with a delay in crack-propagation rate.