Additive manufacturing of Nickel-based superalloy: optimization of surface roughness using integrated high-speed milling

Hybrid additive manufacturing techniques capitalise the advantages of additive manufacturing, while, at the same time, circumvent its disadvantages by using in-situ combined subtractive processes. A particularly promising hybrid additive manufacturing approach unites Laser Powder Bed Fusion (LPBF) with in-situ high-speed milling. Here, the advantages of LPBF such as, e.g., design of freedom and choice of a variety of qualified metal powders can be used, shaping complex as well as custom-designed 3D metal parts. The in-situ high-speed milling process, in turn, yields finished surfaces with superior surface roughness and guarantees high geometrical accuracy without the need of any further post processing. While the typical process sequence of this hybrid approach interrupts the LPBF process every 3-10 layers by milling the accessible surface regions of interest, different challenges arise, as this milling process is directly integrated into the powder bed of the LPBF chamber. Wear characteristics of the milling tools increase as a consequence of the surrounding powder, getting reinforced by the elevated temperatures of the thus far printed parts. As being a super-alloy, Inconel 718 is a promising and well-studied candidate for LPBF additive manufacturing, exhibiting high strength and good mechanical properties. However, it is also well known to be a difficult to machine material. This contribution evaluates the process parameters of this innovative hybrid combination with particular focus on optimizing the surface quality. By varying the high-speed milling process parameters infeed, z-pitch, feed rate and rotational velocity of the milling cutter, smooth surfaces with a roughness of Ra < 1 mu m are accomplished. In addition, the wear characteristics of the milling cutter are investigated, characterizing the flank wear as well as the abrasion of the cutters coating. As the flank wear gets increased by the surrounding powder during the milling process, a specific milling path suction is installed, extracting the powder at the surfaces of the components ahead of the milling path. Advantages of this approach are quantified in a comparative study by the analysis of the milling cutters lifetime.