Corrosion resistance of 3D printed SS316L post-processed by ultrasonic shot peening at optimum energy level

The layer-upon-layer nature of production by additive manufacturing (AM) made surface treatment as mandatory post-processing for increasing the surface integrity and subsequent tribological properties. In the present work an experimental study was carried out to understand how the ultrasonic shot peening (USP) enhances the corrosion rate, surface compressive residual stress, surface roughness and hardness of stainless steel 316 manufactured by selective laser melting (SLM). In order to do so, series of USP experiments carried out based on full factorial design taking into account the effects of ultrasonic power, processing time, and ball diameters on foresaid responses. Multi-objective optimization of process using desirability approach function was performed to obtain a process window including trade-off between energy consumption and sample's quality characteristics. The optimization results revealed that it is not possible to have minimum energy and the best product's quality at same time. In other word, to fabricate samples with best quality characteristics, more energy needs to be consumed. The optimum sample was selected through the trade-off graph based on the transition point where after this point the energy consumption dramatically increases but no further improvement in product's quality characteristics are attained. By identifying the optimum sample, series of surface integrity examination tests were carried out to show the efficiency of USP on enhancement the surface integrity and corrosion properties of AM material. It was found that the sample processed by USP at optimum parameter setting by ultrasonic power of 60%, process time of 15 min, and ball diameter of 6 mm is an optimum process setting that meets the condition described for transition point and causes 120% improvement in corrosion rate.