Influence of Support Structures on the Microstructure and Mechanical Properties of Case Hardening Steel in Laser Powder Bed Fusion

Abstract

Additive manufacturing, especially laser powder bed fusion (LPBF), enables the manufacturing of complex and lightweight parts. LPBF is a technology with high geometric degrees of freedom, but there are production restrictions, e.g., with regard to overhangs. One strategy to produce these components despite the restrictions is the use of support structures. After the LPBF process the support structures are removed by hand or through machining before general post-processing, e.g., manufacturing of functional surfaces or heat-treatments are concluded. However, support structures change the local process conditions during LPBF depending on the parts geometry (no support structures vs. use of support structures). The influence is mainly caused by the changed heat dissipation of the support structures compared to dense part material. In this contribution, the influence of support structures on the resulting microstructure and the mechanical properties are determined for the case hardening steel 16MnCr5. Case hardening steels are used for gearings and pinions for wide applications in the aerospace industry, e.g., engine gear boxes for helicopters or UAVs (unmanned aerial vehicles). It can be seen that the height of the applied support structures has great influence on the microstructure. Mechanical properties are determined through notched bar impact testing showing an increase in toughness in the as-built state with increasing support structure height. The findings correspond with the increasing content of ferrite seen in the microstructure. The effect of subsequent heat treatments (stress relief annealing and case hardening) on the mechanical properties is also investigated. Furthermore, generalized operating principles for the influence of support structures and the occurring insitu heat treatment in LPBF are presented and described based on the results shown.