Residual stress analysis of in situ surface layer heating effects on laser powder bed fusion of 316L stainless steel

Abstract

Fabricating parts using laser powder bed fusion (LPBF) is of growing interest to many fields, ranging from medical to aerospace, but this process is often plagued with residual stresses that can reach magnitudes as high as the yield strength of the material. Previous work has demonstrated the ability to reduce residual stress during LPBF by over 90% using an in situ annealing method that makes use of large area, shaped light illumination from a set of laser diodes. In this work, an in-depth analysis of the effectiveness of this in situ residual stress reduction technique is presented. A custom LPBF system was used to fabricate 316L stainless steel parts, and the stresses of these parts were analyzed using the contour method and neutron diffraction on various planes within the samples. These spatial measurements revealed stress reductions near the edges and base of the samples in each of the three measured orthogonal stress directions, in addition to an overall reduction in stress owing to in situ application of laser diode heating. The experimental results were found to be in excellent agreement with numerical thermomechanical simulations that captured the effects of various processing parameters. Furthermore, in cases where the annealing was only performed once every 5 layers, the residual stress was similarly reduced, which indicates that further optimization might be achieved to limit additional processing time during the builds while still relieving equivalent amounts of stress.