On the effect of rapid annealing on the microstructure and mechanical behavior of additively manufactured stainless steel by Laser Powder Bed Fusion

This work presents an investigation on the effect of rapid annealing on the microstructure evolution and mechanical performance of stainless steel 316L (SS316L) fabricated by Laser Powder Bed Fusion. In this process, it is well documented that the intensive thermal gradient in the heat flux direction leaves remarkable residual stress and promotes strong texture along the building direction (BD). In the current research, to swiftly reduce residual stresses, a short-term heat treatment at 1300 °C for different holding times of 10–120 s was designed and studied. Microstructural observation through the optical microscopy and scanning electron microscopy, as well as the X-ray diffraction analysis, revealed that rapid annealing longer than 30 s resulted in the gradual annihilation of the cell walls which are indeed the sub-grain boundaries (SGBs). In addition, electron backscatter diffraction outputs demonstrated that rapid annealing significantly reduced the low angle grain boundaries and attenuated the texture developed along the BD. Moreover, the analysis of grains size and shape illustrated that annealing up to 30 s recovered the structure. In contrast, longer annealing resulted in increasing the aspect ratio of grains along the direction with maximum residual strain (i.e. BD) due to activation of the strain induced grain boundary migration (SIGBM) mechanism. After the rapid annealing, the mechanical performance of the samples also indicated that annealing up to 30 s would not reduce the compressive and tensile strength more than 6%. In comparison, longer annealing resulted in 24% reduction in mechanical properties along with coarsening of dimples in the fracture surface of the samples.