Effects of direct metal deposition combined with intermediate and final milling on part distortion

Combining direct metal deposition and milling in one machine promises the additive fabrication of complex parts with a high surface quality and dimensional accuracy. However, residual stress induced by the additive process can impair the final part shape after finishing. Undercuts and inaccessible areas are particularly prone to distortion since they require intermediate milling steps during buildup. Herein, strategies to reduce residual stress by process optimisation are discussed and demonstrated. The effects of intermediate and final milling on dimensional accuracy are analysed for the fabrication of a distortion-critical beam from stainless steel. 3D scans reveal that additive buildup on a semi-finished part causes local warpage of milled surfaces, resulting in deviations in length that are by factor 10 higher than the milling accuracy. Global distortion of the substrate plate is significant, but the milling sequence itself has finally no considerable influence.