Comparative study on the influence of subsequent thermal cycling on microstructure and mechanical properties of selective laser melted 316L stainless steel

摘要

Subsequent thermal cycling (STC), as the unique thermophysical process of selective laser melting (SLM), affects the microstructure, residual stress and mechanical properties of SLMed parts. In this study, the influence of STC was comparatively studied by characterizing micro-structure and nanohardness in the single-, three- and five-layer scanned tracks. Results show that microstructure transforms from finer cellular into coarser columnar grains as number of layers increases, and the cell spacing of columnar grains becomes larger as the number of STC increases. Moreover, microstructure in the surface regions of scanned tracks is predominately fine cellular grains, which differ with those in the core regions. EMPA analysis showed that the content of Cr decreases and Ni increases as the number of STC increases; moreover, the repeated STC is able to reduce the solidification velocity and cooling rate within the molten pool, which can promote the uniform distribution of Ni and Cr and alleviate the inter-grain segregation as consequence. Nanohardness at first layers of the three scanned tracks are 4.15, 3.64 and 2.92 GPa. While they are 3.34,2.75 and 2.92 GPa at the top, middle and bottom regions of the five-layer scanned track, indicating that nanohardness decreases as the number of STC and layers increases. All the above illustrate that the STC has significant influences on the microstructure and mechanical properties of SLMed parts.