Reducing residual stress by selective large-area diode surface heating during laser powder bed fusion additive manufacturing

High residual stresses are typical in additively manufactured metals and can reach levels as high as the yield strength, leading to distortions and even cracks. Here, an in situ method for controlling residual stress during laser powder bed fusion additive manufacturing was demonstrated. By illuminating the surface of a build with homogeneously intense, shaped light from a set of laser diodes, the thermal history was controlled thereby reducing the residual stress in as-built parts. 316L stainless steel bridge-shaped parts were built to characterize the effect of in situ annealing on the residual stress. A reduction in the overall residual stress value of up to 90% was realized without altering the as-built grain structure (no grain growth). Some annealing effects on the cellular-dendritic solidification structure (patterns of higher solute content) occurred in areas that experienced prolonged exposure to elevated temperature. A comparison of the in situ process to conventional post-build annealing demonstrated equivalent stress reduction compared to rule-of-thumb thermal treatments. Use of this method could reduce or remove the need for post processing to remove residual stresses.