Adaptive Process Planning for Additive/Subtractive Hybrid Manufacturing of Overhang Features

Additive/subtractive hybrid manufacturing (ASHM) combines the selective laser melting (SLM) with precision milling for a better surface finish as well as geometric and dimensional accuracies of a part. In the SLM-based ASHM, the inherent characteristic of the powder bed determines that the AM and milling process are both three-axis. To produce a complex part with overhang features, alternations between SLM and milling are needed. The alternating sequence is of significance in the overall process: a large slab height of continuous SLM may cause tool/part collision; while a small slab height could reduce the fabrication efficiency. In addition, the remelting of the material during SLM causes the distortion of substantial machined surface, and thus deteriorates the surface finish of a part. However, there have little published reports on process planning for three-axis SLM-based ASHM with the consideration of alternation planning and surface finish improvement. This paper proposed a new adaptive process planning method, in which maximum slab heights of continuous SLM were calculated adaptively, and the shielding height was also employed to protect the machined surface. The overhang angles at different locations of a typical complex part were calculated by a triangular facet method. A collision detection method to justify the interference of the tool and as-built part was proposed, and the maximum slab heights were then determined. ASHM experiment was conducted to fabricate small samples with different overhang angles, and the shielding height was determined based on the surface quality. A propeller with overhang features was manufactured using this new planning method. The result shows that the proposed method improved the fabrication efficiency, reduced the number of process alternations as well as improved the surface finish.