Microstructure model for Ti-6Al-4V used in simulation of additive manufacturing

Abstract: The microstructure and the mechanical properties of titanium alloys are highly dependent on the temperature history experienced by the material. The developed microstructure model accounts for thermal driving forces and is applicable for general temperature histories. It has been applied to study wire feed additive manufacturing processes that induce repetitive heating and cooling cycles. The microstructure model adopts internal state variables to represent the microstructure through microstructure constituents’ fractions. This makes it possible to apply the model efficiently for large computational models of general thermo-mechanical processes. The model is calibrated and validated versus literature data. It is applied to Gas Tungsten Arc Welding -also known as Tungsten Inert Gas welding- wire feed additive manufacturing processes. Four quantities are calculated in the model: the volume fraction of α phase, assumed interpreted to be Widmanstätten α, grain boundary α, and martensite α. The phase transformations are modelled based on diffusional theory described by a Johnson-Mehl-Avrami-Kolmogorov formulation, except for diffusionless α martensite formation where the Koistinen-Marburger equation is used. A parabolic growth rate equation is used also for the α to β transformation upon heating. An added variable, grain size indicator of assumed Widmanstätten α, has also been tested through the implementation of a simple Arrhenius law after parameter calibration. The coupling with physically based constitutive model gives first steps towards a more comprehensive and predictive model of the properties that evolve during processing.