Processing-microstructure-property relationship for AM metals and the effect of thermal properties

Models of phase and microstructure formation during laser powder bed fusion require laser-metal interactions, heat transfer, and rapid solidification theories to be applied simultaneously. Processing parameters such as laser settings, powder spreading, and build atmospheres control the temperature-time profile at any location in the powder bed and printed part and, when combined with conservation and constitutive equations, allow for their predictions. A wide array of thermophysical and optical properties enter the physics-based descriptions of powder bed heating, melting, and solidification. Material properties such as surface tension are sensitive to alloy chemistries and atmosphere conditions and represent theoretical, computational, and experimental research fields in their own right. Melt pools solidify against a solid of identical composition, thereby largely eliminating the energy barrier to nucleate the solidifying phase or phases. Common approaches to predicting the phases that form during solidification assume that under steady-state conditions, those phases and microstructures that are stable at the highest temperatures are formed first. Theoretical models exist to predict the solid-liquid interface velocities and temperatures for plane front, dendritic, and, to a limited extent, for the cellular growth morphologies and interface velocities encountered in additive manufacturing. It is therefore possible to predict phases and growth morphologies that develop during melt-pool solidification. Without applying postprocessing techniques such as machining or hot-isostatic pressing, the remnant porosity and surface roughness as well as sub-surface defects affect mechanical properties. Processing-microstructure-property linkages for metal additive manufacturing are on a firm footage from a viewpoint of theoretical models, but quality material data remain a challenge and a bottleneck for successful model predictions.