Review of current challenges in the implementation of WAAM for Ti-6Al-4 V alloys

Wire arc additive manufacturing (WAAM) is a direct energy deposition additive manufacturing technique. In this, the system comprises numerous variables that make a difference to the shape, morphology and mechanical behaviours of the WAAM processed Ti–6Al–4 V bead. The travel speed, wire feed speed, heat input and shielding gas flow rate are the most influential variables among them that can be found in the literature. WAAM is a material deposition technique that uses multiple thermal cycles or repeated heat treatments during the entire process. As a consequence of which, an uneven dissemination of microstructure, anisotropy in mechanical properties, high residual stresses and distortion of parts are some of the challenges during the implementation of WAAM on Ti–6Al–4 V. With the ongoing need to reduce the cost of manufacturing Ti–6Al–4 V parts, WAAM system should overcome these challenges to get the good part dimensions and qualities of the manufactured Ti–6Al–4V alloy components. This review presents an overview of the current and common issues that the WAAM system encounters while fabricating Ti–6Al–4 V parts. In addition, an attempt is made to discuss the existing possible solutions to these issues. The review begins by introducing the effect of different process parameters like travel speed, wire feed speed, heat input and shielding gas flow rate on the geometry of the deposited materials. In addition, the morphology and mechanical properties of the WAAM-manufactured Ti-6Al-4 V alloy components have been discussed, followed by the influence of different post-thermal treatments which try to optimise the microstructure and enhance the mechanical properties of the deposited materials. Moreover, distortions and residual stresses which are associated with the WAAM systems and the methods to minimise them are discussed.