Microstructural and mechanical evaluation of post-processed SS 316L manufactured by laser-based powder bed fusion

Abstract

Post-processing is one of the main ways to improve mechanical and microstructural characteristics of stainless steel 316 L fabricated by the laser-based powder bed fusion (LPBF) process. In this study, optimized LPBF parameters were used to manufacture SS316L bars. For the post-processing, two main heat treatment strategies have been used, quenching and tempering, with various heating and dwelling conditions. While micro-CT scanning was used to identify the porosity inside the as-built specimen, the microstructures of both as-built and heat-treated specimens were additionally investigated by optical microscopy and scanning electron microscopy (SEM). The tensile test's wrought specimens were obtained at various strain rates of 0.1, 0.01, and 0.001 s−1. A two-dimensional (2D) digital image correlation (DIC) technique and fractography analysis were used to understand the tensile behavior further. The results show that the as-built specimen density level was in the range of 99.993–99.997%, with only extremely small pockets of pores present. The microstructure results show that temperature distribution is the most important factor in the formation of columnar grains (CG). The columnar-shaped grains formed from the edge of the melt pool (MP) in the direction of the laser motion path. The resulting dimensions and form of the cellular structures are presented. The crystal orientation of the specimens was also studied with electron backscatter diffraction (EBSD). The result shows that the fraction of directional grains <001> is relatively small due to a scan rotation and the scanning strategies adopted during the LPBF process. With heating at 1050 °C with a dwell time of 40 min, followed by quenching in cold water, smaller grain sizes were obtained, meaning longer grain boundaries and major impediments to dislocation motion, leading to better mechanical properties and fracture characteristics over wrought specimens. The results of EBSD and SEM were also correlated with the 2D DIC test results.