Investigation and validation of the flow stress equation and damage model parameters of an electron beam melted Ti6Al4V alloy with a martensitic phase

Abstract

The Johnson and Cook flow stress and damage model parameters of an electron beam melt (EBM)-Ti64 alloy composed of α' (martensite) and α+β and an extruded-annealed conventional Ti64 alloy were determined experimentally. The validities of the determined flow stress equations and damage model parameters were then verified by the numerical simulations of the compression tests on the Body Centered Cubic lattices produced using the same EBM parameters with the solid EBM samples. In addition, a compression flow stress equation was extracted from the small-size test specimens (1 and 2 mm diameter) taken directly from the struts of the as-built lattices. The microscopic observations, XRD analyses and hardness tests confirmed the presence of α′ phase in the EBM solid samples and in the struts of the BCC lattices, which reduced the ductility of the EBM solid specimens and struts compared to the conventional Ti64. Furthermore, the partially melt particles on the surfaces of the struts acted as the stress concentration sides for micro-cracking; hence, the compression flow stresses of the struts were found to be significantly lower than those of the as-built EBM solid specimens. The flow stress equation derived from the struts predicted more accurately the compression behavior of the lattices. The compression tests and models showed that early damage formation in the lattices was noted to decrease the initial peak and post-peak stresses. As with the experiments, the initial damage occurred in the models with the separation of the nodes at the lattice cell surface edges. This resulted in an abrupt reduction in the stresses after the peak stress. The numerical lattices without damage showed a localized lattice deformation at the mid-sections and the stress increased continuously as a function of normal strain.