Working distance passive stability in laser directed energy deposition additive manufacturing

In Laser Directed Energy Deposition (L-DED), closed loop control systems can be used to enhance system reliability; however, modulating controlled parameters can have unintended secondary morphological and microstructural effects. To enable development of control systems more sensitive to the complicated interplay between powder flow, thermal transfer, and long-term stability in the machine, the L-DED process, in an open loop configuration, was studied both experimentally and theoretically. A fully physics based semi-analytical model was created that incorporates descriptions of the powder spray pattern, laser attenuation through the powder cloud, and a thermal equilibrium model to predict melt dimensions. The model was validated against an experimental matrix of 258 single track deposition experiments with stainless steel 316 L. It was found that the powder flow field causes working distance (WD) to converge to an equilibrium value, and that this equilibrium position is strongly influenced by many effects, such as thermal energy accumulation in the part and powder flow dispersion. Several metrics to quantify the stability of this equilibrium working distance are proposed and discussed. (C) 2018 Elsevier Ltd.