Anisotropic tensile properties and failure mechanism of laser metal deposited Ti–5Al–5Mo–5V–3Cr alloy before and after sub-transus heat-treatment

Abstract

The tensile behaviour and failure mechanism of Ti-5553 (Ti–5Al–5Mo–5V–3Cr) components manufactured with Laser Metal Deposition (LMD) on forged and heat-treated Ti-5553 substrate were analysed. Two Ti-5553 blocks have been manufactured by LMD on substrate of the same alloy. One block has been analysed in the as-deposited condition, the other following a solution treatment and ageing (STA) heat-treatment. Tensile specimens were extracted parallel and perpendicular to the build direction to analyse the influence of anisotropic β grains on the tensile properties at room temperature. As-deposited material is characterised with long columnar β grains aligned with the build direction and no α-phase resolvable with Scanning Electron Microscopy (SEM). Tensile strength in the as-deposited condition is low, but ductility is good. A homogeneous microhardness distribution was noted throughout the build direction of the as-deposited material. The heat-treated block was characterised with the presence of grain boundary α, and both primary as well as secondary α laths. Tensile strength of this condition has improved, but the ductility varies significantly with respect to the tensile loading direction. A homogenous microhardness distribution was noted throughout the heat-treated deposited material and substrate. The fracture surface and microstructure in the plane parallel to the loading direction from fractured samples was analysed using SEM to identify critical locations for strain induced porosity and crack propagation. Failure mechanisms for both the as-deposited and heat-treated conditions have been proposed. In the as-deposited condition, failure occurs through microvoid nucleation and coalescence along slip bands and along the β/β grain boundaries. Slip bands of multiple slip systems frequently form at 45° to the loading direction and correspond to slip systems of the highest Schmid factors. In the heat-treated condition, failure occurs predominantly through easy crack propagation along the GBα/β interfaces. When the loading direction was parallel to the columnar β grain boundaries, the ductility was improved compared to specimens containing columnar β grain boundaries perpendicular to the loading direction.