Modeling the Microstructure-Property Relationships in Electron Beam Additively Manufactured Ti-6Al-4V

The electron beam additive manufacturing (EBAM) process is being used to produce large parts which do not have a strategic structural application, but which mimic many features found in specific components. To support an informed qualification approach, it is necessary to integrate modeling, experimentation, and data. Until recently, there has not been a predictive model for yield strength in Ti-6Al-4V produced via directed energy deposition processes. The alloy Ti-6Al-4V results in complex two-phase microstructures that are rich with microstructural features that span across length scales, and which are interconnected across length scales in ways that strongly influence the mechanical properties. Constitutive equations have been developed to predict the mechanical behavior of Ti-6Al-4V using a hybrid artificial neural network/genetic algorithm approach for wrought Ti-6Al-4V structures. These equations have served as the basis for this work. As will be shown, it has been necessary to adjust these equations to account for microstructural variables that are not present in forged products. Specifically, the strong influence of texture on the mechanical properties has been determined, and incorporated into the models. In addition, as there are multiple heat-treatment strategies that might be adopted for additively manufactured materials ranging from simple stress-relief anneals to extended high temperature hot-isostatic pressing to β-processing to produce microstructures nominally equivalent to wrought β-processed Ti-6Al-4V, a secondary goal has been to produce a single equation that is insensitive to heat-treatment, and captures the systematic variability in properties through microstructure alone. A single expression for the yield strength of Ti-6Al-4V has been developed that can predict the yield strength for additively manufactured Ti-6Al-4V subjected to multiple post-deposition heat-treatments. The equation, while slightly different than the expression previously developed for wrought structures, is functionally identical. This material is based upon work supported by the Defense Advanced Research Projects Agency under Contract No. HR0011-12-C-0035.