Processing-microstructure relationships in ferrous alloys via mixed powder laser powder bed fusion

Abstract

Three different approaches to control the phase formation in powder bed fusion - laser beam (PBF-LB) printing using a powder mixture of duplex stainless steel (DSS) 2507 and austenitic stainless steel 316 L were examined: (i) in situ solid state reheating, (ii) utilization of an island scan strategy, and (iii) switching process parameters between layers of a build. Analytical modeling was used to determine the time-temperature profiles during each of these processes, and was combined with thermodynamic simulations through Thermo-Calc to predict and explain the phase formation measured experimentally. It was found that the reheating from additional layers was critical to the nucleation of austenite, even with the increased Ni content in powder mixtures. This was evident in the island scan strategy prints where the corners of the islands showed significantly less austenite (∼45%) as compared to the centers of the islands (∼96%), as well as in the multi-layer samples where alternating laser power for fixed chemical composition of powder mixture resulted in varying content from ∼75% (lower power) to ∼90% (higher power). Additionally, in situ reheating using a lower power laser was capable of minimally increasing the austenite content layer-by-layer, by raising the temperature of the layer above the nucleation temperature of austenite. These results showcase the capability of PBF-LB to manipulate phase structure in ways not possible through traditional manufacturing techniques, as well as the ability of Thermo-Calc to accurately predict the trends observed that can be applied to other alloy systems.