Post-building successive treatments using surface laser remelting and ultrasonic nanocrystalline surface modification for rough surface built via directed energy deposition

In this study, surface laser melting (SLR) and ultrasonic nanocrystalline surface modification (UNSM) were applied as post-building treatments to modify the surface of additively manufactured parts via directed energy deposition (DED). The DED building of STS316L powder was conducted on a substrate, and the deposited surface was irradiated with a laser of the DED equipment to perform SLR on the surface. Subsequently, the remelted surface was re-treated with UNSM, and the changes in each step were examined. Via the experiment, the as-built surface roughness and waviness of 12.5-mu m Ra and 8.22-mu m Wa, were improved to 2.3 mu m and 2.5 mu m, respectively, by the successive treatments of SLR and UNSM (denoted by SLRed-UNSMed). From the perspective of grain refinement by UNSM treatment, performing SLR with a small overlap followed by UNSM was determined to be effective in maximizing the effect of UNSM. The SEM and EBSD analysis results indicated that grain refinement occurred in a deeper region in SLRed-UNSMed than in the sample (denoted by UNSMed) that was treated solely with UNSM and no SLR. In addition, according to the pole figure analysis, the primary surface modification and grain reorientation in the (100) direction by SLR allowed the UNSM energy to be transferred more evenly and deeper. Meanwhile, high hardness and compressive residual stress were observed in SLRedUNSMed, and a nano-indentation test confirmed an increase in the effective depth of UNSM relative to that of UNSMed. Based on this study, an SLR treatment followed by UNSM can be expected to achieve surface modification and improve the mechanical properties of additively manufactured products.