Optimization of mechanical properties of additive manufactured IN 718 parts combining LPBF and in-situ high-speed milling

Abstract

Mechanical properties of laser powder bed fusion (LPBF) processed components are sensitively affected by the inferior surface quality, typical for this additive manufacturing technique for metal parts. Using a promising hybrid additive manufacturing approach, combining LPBF and in-situ high-speed milling, the surface quality can be strongly improved within a single process cycle. The milling process is directly integrated into the additive manufacturing chamber, machining the 3D-built surfaces while interrupting the LPBF-process. As a result, surface defects and surface near porosities are greatly reduced.Even though Inconel 718 is difficult to machine, as well as the milling process within the powder bed poses different challenges on this promising approach, a surface roughness of Ra < 1µm is generally achievable, using this particular hybrid additive manufacturing approach.We present a comparative study of the mechanical properties of conventional LPBF-and hybrid-build components with particular focus on the static and dynamic load behaviour, in turn allowing to evaluate the effects of finished surfaces on these mechanical properties. In addition, the influence of different heat treatments on the static load behaviour as well as the fatigue behaviour is investigated. Different heat-treatments are performed, improving the density of the components, and leading to a higher maximum load behaviour. Furthermore, the fatigue behaviour is modified, as the material properties can be changed by virtue of the development of different material phases during the heat treatment processes. Comparing the sole LPBF-and hybrid-built components for the as-built and the different heat-treated states, an improvement of the maximum load capacity as well as of the dynamic load behaviour can be observed for the hybrid-built specimens.