The effect of processing parameters and material properties on residual forces induced in Laser Powder Bed Fusion (L-PBF)

During the L-PBF process stresses are developed due to the large temperature gradients induced by the laser beam and the fast cooling rates which occur while the part is being built. In this study, the development of forces induced layer by layer from the cycles of thermal gradient during L-PBF are measured in-situ using a Force Transducer Device (FTD) for several processing parameters, including laser power and scanning strategy in various metal powders. Three scan strategies were studied: Stripe, Meander and Chessboard, and the results showed that Meander developed the lowest residual forces. All of the processing parameters had a strong influence on the development of residual strains in L-PBF; and, it was found that the laser power had relatively little effect on the residual forces above a minimum threshold and that above a critical value the point distance had little effect on the residual forces. Four materials were investigated: Ti-6Al-4V, Inconel 625, Inconel 718 and maraging steel. Maraging steel exhibited much lower residual forces than the other materials, which could be due to the low temperature phase transformation in this material. Evidence for this mechanism in the maraging steel was provided by changing the inert atmosphere in the L-PBF chamber. In addition, changing the inert gas atmosphere from argon to nitrogen for the maraging steel samples had an effect on the development of forces which changed the direction of the part deformation when it was released at the end of the build process.