Alloy and microstructure design for additive manufacturing

There is a need for theoretical and numerical approaches to describe microstructureproperty relationships in metal additive manufacturing (AM). Such relationships are particularly unclear as a function of alloy composition. In this work, a variety of computational models have been developed to optimise the chemical composition, process parameters and mechanical properties of alloys for laser powder bed fusion (LPBF). A computational framework combining genetic algorithms and the calculation of phase diagrams (CALPHAD) methodology is developed to design new alloys with minimum susceptibility to solidification cracking. A methodology to control process parameters to prevent porosity and defect formation during LPBF is also presented. The models are validated using 316L stainless steel (SS), one of the most accepted grades by industry. Metallurgical models are presented to describe the yield strengthening mechanisms during LPBF processing of IN718, Ti-6Al-4V, and 316L SS. It has been shown that dislocation multiplication/annihilation processes such as dynamic recovery and recrystallization are paramount in controlling yield strength of LPBFed alloys. The numerical models have been complemented by detailed advanced microstructural characterisation methods such as electron backscattered diffraction (EBSD), and transmission electron microscopy (TEM). A classical Zener-Hollomon and a modified thermostatistical approach are used to interpret microstructural evolution phenomena during LPBF of 316L SS under various building conditions, improving the understanding of yield strength control in such LPBFed alloys. Furthermore, the important role of low-angle grain boundaries (LAGBs) in strengthening of LPBFed 316L SS has been revealed quantitatively. A unified Hall-Petch relationship is proposed, for the first time, to relate the yield strength of LPBFed 316L SS to the subgrain size (the size of the grains with LAGBs). An alloy and process design framework is presented to maximise yield strength of austenitic SSs through grain boundary engineering. The contribution of various deformation mechanisms such as dislocation hardening, twinning-induced plasticity, dynamic recrystallization, as well as dislocation types in the strain hardening behaviour of LPBFed 316L SS is modelled