MICROSTRUCTURE OF SLM MANUFACTURED 316L AND 420 GRADES STAINLESS STEEL

Selective laser melting (SLM) is an additive manufacturing process, involving track-by-track powder material melting on the previous fabricated layer. Because of the complete remelting, SLM objects have a cast microstructure and as a result of high cooling rates the microstructure itself is fine. During manufacturing, each next track heats up already solidified materials beneath the surface. Therefore, inner layers are subjected to multiple heating-cooling cycles. In this investigation, microstructural characterization of SLM parts made of AISI 316L and 420 stainless steel grades were performed in order to understand the influence of the SLM process parameters on the final steel microstructure. It was shown that rapid solidification after track melting resulted in the formation of colonies grown epitaxially from the fusion boundary surface. Each colony consists of very fine cells with submicron cell spacing, coherent and grown in the same direction. In AISI 316L steel, this structure remains unchanged since austenite is stable and is not transformed at cooling down to room temperature. In AISI 420 steel, martensite is formed with rapid cooling and undergoes in-situ heat-treatment at further thermal cycling. The effect of thermal cycling on the final microstructure of the built part, i.e. the depth of the heat-affected zone of each next single track, is dependent on the material properties and the SLM process parameters. In the case of AISI 420 martensitic stainless steel, thermal cycling initiated a partitioning heat treatment process, which resulted in high amounts of austenite in the microstructure. The in-situ heat treatment conditions in the solidified inner parts were numerically simulated using a time-dependent “Heat transfer in solids” Comsol module, and a comparison of modeled isotherms with experimentally observed microstructures showed a good agreement between simulation and experiment. Since the model demonstrated a good agreement with experimental results, it was used to evaluate the influence of laser power and laser scanning speed on the in-situ heat treatment. This gives useful background to predict the microstructure of SLM products at manufacturing.