Study of Residual Stresses in Selective Laser Melting

Additive Manufacturing (AM) is a valid and economically viable technique to produce fully functional, geometrically complex parts. One such AM technique, Selective Laser Melting (SLM), uses a high power laser to selectively melt layers of metal powder. Selective Laser Melting can be compared to a repetitive welding process, stacking thousands of welds next to and on top of each other to produce a 3D geometry. The difference lies in the fact that the purpose of welds is to join two or more separate geometries into one, while in SLM, the welds itself are the geometry. Moreover, the process takes place on a much finer scale compared to commercial welding processes, with melt pool dimensions in the order of 0.1mm³. The localized melt pool and its heat affected zone are small compared to the thermal heat sink effect of the base plate and previously consolidated layers, creating extremely large and directional thermal gradients. In turn, these gradients lead to the buildup of residual stresses which have an effect on the mechanical performance and cause deformation, as well as micro- and macrocracks. Stress is built up locally by the thermal shrinkage of the solidified melt pool, and is larger in the direction of a scan track than perpendicular to it. The shrinkage is impeded in the horizontal direction by the solid material below, causing horizontal tensile stresses at the top surface of SLM produced parts. These horizontal stresses exert a pulling force which would cause the part to curl up if it were not anchored to the base plate. Because the part is anchored, curl up is avoided but vertical tensile stresses are introduced at the side surfaces. Compressive stress in all directions exists in the center of the part. Different possibilities to induce lower residual stresses were explored in this work, at various stages throughout the SLM process chain. The effects of thermal and other material properties are difficult to isolate from the influence of process parameters during the process, or material specific problems such as ductile to brittle transitions, solidification cracking, and low temperature allotropic transformations. Overall, using process parameters that increase the heat input reduces macroscopic residual stresses. This includes using thin layers, slow scan speeds, high laser powers and most importantly, base plate preheating. In addition, modified alloy compositions introduce benefits of tailored mechanical properties and, to a certain extent, a tailored microstructure. Finally, the impact of residual stress on the mechanical behavior was highlighted by the overlap of 2D maps of the residual stress with the observed crack growth behavior of Ti6Al4V compact tension specimens produced by SLM. Base plate preheating, as well as tailored alloy compositions may provide the largest gains. Combining all the favorable procedures described in this work will lower residual stresses, which enables production of bigger parts or processing of new materials.nrpages: 10status: publishe