Microstructural characterization and mechanical behavior of ultrasonic impact peened and laser shock peened AISI 316L stainless steel

The effects of ultrasonic impact peening (UIP) and laser shock peening (LSP) on 316L stainless steel were compared in terms of surface morphologies, microstructural evolutions and mechanical properties. The grain refinement mechanisms by mechanical and laser shock wave were subsequently analyzed. Experimental results showed that both UIP and LSP produced micro-grooves with the same depth (similar to 48 mu m) at the surface of 316L. The nano-grain size induced by double UIP treatment (10-90 nm) was much smaller than that by triple LSP treatment (>70 nm) because the impact numbers and total impact energy of UIP were much higher. The mechanical twinning was almost complete absence in the sample by UIP. On the contrary, the mechanical twinning was frequently observed in samples by LSP. The magnitude of peak pressure determined the transition from dislocation-dominated mechanism (similar to 680 MPa for UIP) to twinning-dominated mechanism (similar to 2200 MPa for LSP). The resultant dislocation cell size by UIP was much smaller than that by LSP due to the difference of dislocation density caused by different shock wave speed and impact numbers. Additionally, the compressive residual stress on the surface by UIP was higher than that by LSP in both measuring direction. Furthermore, both grain refinement and high dislocation density induced by UIP contributed to a significant increase in the hardness (similar to 433 HV) and yield strength (similar to 447 MPa). By contrast, the LSP induced mechanical twins which can act as dislocation blockers significantly improved the yield strength (similar to 423 MPa).