On the influence of heat treatment in suppressing the layer orientation effect in finishing of electron beam melted Ti6Al4V

The main issue in the finishing of the electron beam melted (EBM) parts is their directional properties that lead to different surface roughness values for different part orientations for the same machining parameters. In this study, EBM Ti6Al4V parts were subjected to heat treatment to suppress the effect of the part/layers’ orientations during the finish milling process. Three different heat treatments were employed to the as-fabricated EBM Ti6Al4V parts. The results of the heat treatments showed that the EBM Ti6Al4V parts heated at 600°C for 3 h, and then air-cooled exhibited the same fine lamellar microstructure and microhardness as that of the as-fabricated EBM parts. The as-fabricated and heat-treated EBM Ti6Al4V parts were subjected to the milling operation by considering the three possible parts/layers orientation with respect to the tool feed directions. The milling results showed that the as-fabricated parts showed up to a 27% difference in surface roughness for different part orientations. In contrast, the heat-treated parts (600°C@3h) showed uniform surface roughness for the three part orientations with a variation of 8%. Similarly, considerable differences were observed in the machined surface microhardness of the as-fabricated EBM parts (18%) as compared to the heat-treated parts) 2%). The heat-treated parts showed more uniform and superior surface morphology across all the part orientations. This is because of the more uniform microstructure, less porosity, higher consolidation of the EBM layers, and elimination of the columnar grains in the case of the heat-treated parts as compared to the as-fabricated EBM parts.