Post-processing of the Inconel 718 alloy parts fabricated by selective laser melting: Effects of mechanical surface treatments on surface topography, porosity, hardness and residual stress

The turbine blade test parts were manufactured by the selective laser melting (SLM) process using a nickel-based pre-alloyed Inconel (IN) 718 powder. Various mechanical post-processing techniques, such as barrel finishing (BF), shot peening (SP), ultrasonic shot peening (USP), and ultrasonic impact treatment (UIT), were applied to improve the surface layer properties of the SLM-built specimens. Effects of mechanical surface treatments on surface topography, porosity, hardness, and residual stress were studied. In comparison with the SLM-built state the surface roughness (Sa = 5.27 mu m) of the post-processed specimens were respectively decreased by 20.6%, 26.2%, and 57.4% after the BF, USP, and UIT processes except for the SP-treated ones. The Sz parameter was reduced in all treated SLM-built specimens except for the SP-treated ones. The surface microhardness of the SLM-built specimen (similar to 390 HV0.025) was increased after the BF (by 14.2%), LISP (by 23.8%), UIT (by 50%), and SP (by 66.5%) processes. The deepest hardened layers were formed after the UIT and SP processes. Residual porosity of the SLM-built specimen was decreased by 23.1%, 40.6%, 55%, and 84% after the BF, SP, LISP and UIT processes, respectively. The UIT process formed a densified subsurface layer of significantly reduced porosity (0.118%). All mechanical surface treatments successfully transformed the tensile residual stresses generated in SLM-built specimen into the compressive residual stresses (-201.4...510.7 MPa). The thickness of hardened, densified and compressed near-surface layers ranges from similar to 80 mu m after BF to similar to 140 mu m after USP, and similar to 180 mu m after SP and UIT processes, which correlates to the accumulated energy and deformation extent of the treated surface. The effect of the accumulated energy on the outcomes of the applied surface treatments is also addressed.