Processing condition dependency of increased layer thickness on surface quality during electron beam powder bed fusion

The energy beam and powder layer interaction influences the dynamic melt behavior and determines the surface quality in electron beam powder bed fusion (PBF-EB). It is generally believed that increasing the powder layer thickness favors production efficiency but is contradictory to improving the forming quality. How variations in the powder layer thickness affect the interaction between the electron beam and the powder bed, which influences the melt behavior and resultant surface quality, has not been well understood. In this study, cylindrical specimens with increased nominal layer thicknesses from 80 to 140 μm were prepared using PBF-EB. The study verified the processing feasibility of ensuring the forming quality under a high layer thickness. Within the processing regime of this study, a relatively large powder layer thickness expanded the processing window. According to the thermophysical-property analysis of the powder bed, the emissivity and thermal conductivity exhibited upward and downward trends, respectively, with increased powder layer thickness. The increased thickness reduced the fusion efficiency, restricting the height difference within the overall sample surface caused by overheating. The numerical simulation clarified the dependence of the layer thickness-effect on the processing conditions. The proportion of incomplete melted powder in the electron beam irradiating area increased at a high scan speed. Subsequently, the hindering effect on heat absorption and transfer caused by the powder layer and its increased thickness was fully manifested. That is, the evolution trend of melt behavior and surface morphology resulting from increased layer thickness is remarkable at high scan speeds.