Influence of scanning length and energy input on residual stress reduction in metal additive manufacturing: Numerical and experimental studies

An excessive residual stress is one of the primary challenges for additive manufacturing of metal products. It often leads to part's distortion, powder recoating blade failure, and cracking. The present study investigated the influence of process parameters on residual stress formation. A mesoscale model was developed to examine thermal and mechanical responses in a single layer. A numerical-based process window was constructed to show the correlation among process parameters, surface temperature, and residual stress. It was found that residual stress minimization could be achieved by using short scan vectors as it resulted in higher surface temperature and smaller temperature gradient at solidification front. To verify this hypothesis, bridge structures and cuboid samples were additively made from Ti-6Al-4V under various scanning lengths. Bridge structures were removed from a substrate, where the bending curvature was used to indicate stress magnitude. Moreover, residual stress in cuboid samples was measured using X-ray diffraction. Apparently, smaller residual stress was detected when a shorter scanning length was used for both cases. The optimal process condition for the present study was when laser power, scanning velocity, and scanning length were 200 W, 1000 mm/s, and 1 mm, respectively. It was found that under this condition, residual stress could be reduced by almost two-fold from a typically seen value. In addition to experiments, a multiscale numerical model was developed to predict distortion and estimate residual stress in entire parts. A good agreement between experimental and numerical results was observed. The part scale simulation not only further confirmed the effect of scanning lengths, but also illustrated how mesoscale results can be integrated into the macroscale model to enable an accurate multiscale prediction.