Selective laser melting of 316L stainless steel: homogeneity of thermal history, microstructure, and mechanical properties

Abstract

Manufacturers using additive manufacturing to build parts need to know how the complex thermal histories imparted by rapid heating and cooling affect the homogeneity of the microstructure and mechanical properties within their parts. To identify the possible differences for a part built via powder bed fusion, a simulation incorporating superlayers was used to predict the thermal history for a part built by a selective laser melting process. The simulation captured effects of the increasing distance from the substrate as well as the increasing cross-sectional area for the conical geometry of the part, which were meant to exacerbate thermal history discrepancies between layers. The largest thermal gradient difference between layers was 3.7%, and the largest cooling rate difference was about 10%. Grain size and microhardness measurements on the built parts were used to determine whether the microstructure and mechanical properties changed as a function of distance from the substrate and the increasing cross-sectional area. Differences were found to be negligible because the microhardness and grain size were uniform throughout the part in the build direction. The time (8 s) required to recoat the next layer provided enough cooling time to give similar enough thermal histories to fabricate parts with consistent grain size and microhardness in the build direction.