A Rate- and Temperature-Dependent Thermomechanical Internal State Variable Model of the Directed Energy Deposition Process

Residual stress and distortion in additively manufactured parts contribute to build failures and reduced fatigue life, which hinders process qualification and part certification in industry. Predictive models can be used to better understand thermally induced residual stresses and distortion. This study investigated the influence of two mechanical models on simulated residual stress and distortion for the directed energy deposition additive manufacturing process by comparing the thermomechanical effects of two different scanning strategies on a Ti-6Al-4V thin wall. The Evolving Microstructural Model of Inelasticity (EMMI) and an elastic–perfectly plastic (EPP) model were chosen due to their inherent differences and use in manufacturing simulations in the literature. EMMI is a physically based strain-rate- and temperature-dependent dislocation mechanics-based internal state variable plasticity model while the EPP model is a phenomenological temperature-dependent yield strength model that does not account for hardening or thermal softening. Simulation results show significant differences in the predicted stress evolution and the as-built stress contours despite similar maximum von Mises stress. In particular, EMMI demonstrated more realistic stress evolution from cyclic thermal history and large thermal gradients while EPP showed discontinuities and larger oscillations in the evolution of multiple components in the stress tensor.