Energy-Efficient Ultrasonic Shot Peening as Post-treatment for SS316L Fabricated by Laser Powder Bed Fusion

In the present work, an experimental study was carried out aiming at optimization of ultrasonic shot peening (USP) process to enhance the surface integrity of stainless steel 316 produced by selective laser melting (SLM). Here a series of USP experiments were carried out based on response surface design to incorporate effect of ultrasonic power, processing time, ball diameters and gap distance on hardness and surface roughness. Multi-objective optimization of process using desirability approach function was performed to obtain a process window including a trade-off between energy consumption, hardness and surface roughness. After obtaining the optimal solutions, a series of surface integrity examination tests such as microstructure evaluation, surface morphology, hardness and residual stress distributions as well as corrosion resistance were conducted on optimal samples to understand whether energy consumption influence the surface integrity of post-processed samples. Obtained results from analysis of variance revealed that among the processing factor ball diameter and ultrasonic power have greatest influence on hardness and surface roughness, respectively. The optimization results showed that under the energy consumption of 180 kJ that is one-third of maximum energy, by selection of ball diameter of 6 mm and gap distance of 30 mm, the foregoing surface integrity aspects reach their desired value, and no further improvements are observed at higher energy consumption. Under such condition, thanks to surface severe plastic deformation, 70% reduction in surface roughness, 90% enhancement in surface hardness, 176% decrease in the tensile residual stress and 90% improvement in corrosion resistance indices of USPed sample compared to as-received additively manufactured material were attained.