Modelling alpha case formation and embrittlement for Ti-6Al-4V produced by additive manufacturing and subjected to thermomechanical post-processing

Abstract

Additive Manufacturing enables cost-effective production of complex geometries. Notched tensile properties of Ti-6Al4V alloy are here numerically studied considering the Selective Laser Melting (SLM) method and different postprocessing conditions. The formation of an oxygen-enriched brittle layer or an “alpha case” in Ti-6Al-4V is also reproduced for non-inert atmospheres. Exploiting other Finite Element frameworks for environmentally assisted cracking, e.g. hydrogen embrittlement models, the local reduction of fracture energy as a function of oxygen concentration is implemented in a Phase Field model for crack nucleation and propagation. Different notched tensile specimens are modelled in Comsol Multiphysics. The analysis is sequential: oxygen uptake and diffusion are simulated for different temperatures and dwell times, reproducing typical SLM and HIP processes and different partial pressures of oxygen. In a second step, mechanical tensile testing is simulated, and the damage Phase Field scheme is solved considering a linear reduction of fracture toughness as a function of oxygen concentration. The effects of the characteristic length value and of plastic behaviour are evaluated. Results show that the present framework, after parameter calibration, is able to predict the influence of thermomechanical post-processing on notch fracture.