Characterization of Tool Surface Properties Generated by Directed Energy Deposition and Subsequent Ball Burnishing

By directed energy deposition (DED), a flexible design of cooling channels in forming tools, e.g., hot stamping, with a variety of sizes and a high positioning flexibility compared to machining processes is possible. The subsequent ball burnishing of the tool surfaces, in combination with a variation of the DED process parameters, enables control of the tool surface properties and friction behavior. Parameters such as the ball-burnishing pressure or the path overlapping in the DED process are investigated to quantify their effects on roughness, hardness, friction, residual stresses, and heat transfer coefficient of generic tool surfaces. The friction coefficient at elevated temperatures depends strongly on the surface roughness of the tool steel surfaces generated by DED and ball burnishing. The latter process improves the surface integrity: the roughness peaks are leveled by up to 75%, and the hardness and the residual stresses are enhanced by up to 20% and 70%, respectively. However, the roughness of the tool surfaces is determined mainly by the path overlapping of the welded beads in the DED process. Despite the higher surface roughness, the heat transfer coefficient is in the range of conventionally manufactured tool surfaces of up to 2700 W/m²K for contact pressures up to 40 MPa. First hot stamping experiments demonstrate that the tools manufactured by the novel process combination are able to manufacture 22MnB5 hat profiles with an increased and more homogeneous hardness, as well as more homogeneous thickness distribution, compared to conventionally manufactured tools.