Relationship between microstructure and mechanical properties of friction stir processed AISI 316L steel produced by selective laser melting

Abstract

In this work, AISI 316L steel produced through selective laser melting (SLMed 316L) was successfully modified through friction stir processing (FSP) technology. The microstructure, microhardness, nano-hardness and tensile properties of SLMed 316L were studied and the results were discussed. The effects of microstructure evolution, including grain size, grain orientation and dislocations, on both macroscopic and microscopic mechanical behavior of the material were examined. The relationship between grain orientation and nano-hardness was corrected for the Berkovich nanoindenter. It was demonstrated that FSP could effectively eliminate the holes and cracks of SLMed 316L, leading to refined, homogenous and dense microstructure. During FSP, SLMed 316L sustained both continuous and discontinuous dynamic recrystallization. Subsequently to FSP, the grain size was refined from 6.6 to 0.9 μm, total dislocation density decreased from 2.43 × 1015 to 1.04 × 1015 m−2, while geometrically necessary dislocation density increased from 2.16 × 1014 to 8.25 × 1014 m−2. The average microhardness and nano-hardness increased from 272 to 218 HV and 3.61 to 4.18 GPa, respectively. The yield and tensile strengths increased by 29% and 18%, respectively, while elongation was maintained. The main factors affecting the strength of SLMed 316L was grain size, followed by grain orientation and dislocation density.