Composition and process parameter dependence of yield strength in laser powder bed fusion alloys

Abstract

Understanding the factors influencing yield strengthening in alloys processed by laser powder bed fusion (LPBF) is critical in designing new formulations, and for predicting the optimum parameters for their processing. In this work, a relationship between the heat input and strengthening and softening mechanisms is proposed for a titanium, nickel and stainless steel alloy (Ti-6Al-4V, IN718 and 316L, respectively). Maximum strength is obtained with increasing heat input in 316L stainless steel; whereas IN718 and Ti-6Al-4V require low heat inputs. The results demonstrate that yield strength can be described in terms of the normalised enthalpy. The variation in the yield strength of LPBFed alloys depends prominently on dislocation multiplication/annihilation at certain processing temperatures and thermal straining, which are alloy dependent; as well as on dislocation strengthening and heat dissipation during cooling, which are process dependent. These dependencies are modelled via well-known metallurgical approaches. The relative contribution of various strengthening mechanisms is revealed. The findings of this work can be used as a metric for the prediction and further improvement of yield strength based on the choice of LPBF process parameters and chemical composition.