Evolution of cyclic thermal stress in selective laser melting of 316L stainless steel: a realistic numerical study with experimental verification

Abstract

It is well known that high thermal stresses in the formed parts during selective laser melting (SLM) heavily affect the part quality and performance. In this study, the mechanism of thermal stress formation due to temperature gradients in SLM of 316L stainless steel is investigated. The uniqueness of this study lies in the fact that a realistic multi-track and multi-layer SLM process is modeled and simulated that adopts the identical conditions of the actual part building on a commercial SLM system, and the final residual stresses are compared between the simulation and the experimental measurements on the actual parts. The results indicate that both temperature and stress evolution of material points exhibit cyclic patterns during the SLM process, and the changes with regard to time are dramatic thanks to the rapid heating and cooling of localized areas. However, the rapid temperature decrease is actually accompanied by stress increase in the material points, and vice versa. Under the laser power of 160 W and scanning speed of 500 mm/s, a melting pool is formed and rapidly expanded to the adjacent pre-solidified layer, and the diffusion depth is about 200 μm. Also, unlike the overall attenuation of temperature as the SLM build process progresses, the magnitude of stress oscillation actually increases. Furthermore, during the rapid solidification, the core of melting pool is subject to tensile stress. The tensile stress could be partially mitigated by the laser heating of subsequent layers, but it cannot be completely removed. As a result, the accumulation effect of multiple layers leads to significant tensile stress, in particular, in the bottom layers that are closer to the base plate. The effective stress in the bottom layer attached to the base plate reaches as high as 680 MPa based on the settings of SLM process. Finally, the experiment measurement on residual stress overall agrees with the simulation results. Both show that the residual stress ranges from 100 to 350 MPa for the layers 1.4 mm above the base plate, and that the tensile stress tends to increase along the direction towards the base plate.