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

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