Abstract
Multi-pass depositions of titanium by a gas metal arc based wire arc additive manufacturing (GMA-WAAM) process to create thin-walled structures (less than 5 mm wall thickness) is considered challenging due to the (i) instability in arc ignition and arcing, (ii) out-of-axis transfer of droplets from the consumable due to arc wandering, (iii) spatter formation and (iv) uneven widening of the deposited beads. These shortcomings are primarily caused by the cathode spot instability, experienced at the electrode or melt pool primarily due to the selective oxidation of titanium.
In this work, attempts were made to improve the cathode spot stability by selectively changing the surface conditions of the consumable wire to produce thin-walled titanium structures with acceptable dimensional accuracy. The arc ignition characteristics, arc stability and metal transfer behaviour were analysed using high-speed camera image analysis under straight and reverse polarity. Current-voltage waveforms were suitably modified to achieve stable free flight pulsing drop transfer and controlled dip short-circuiting transfer of commercially pure (CP) titanium consumables in bare and surface-treated conditions. The surface treated feedstock wire contained an oxidised surface and was found to exhibit stable arc ignition and droplet transfer characteristics in both polarities. Due to the improvement in arc stability while using surface-treated feedstock wire, reduction in arc wandering lead to straightening of the beads with acceptable geometrical homogeneity during thin-walled deposition. On contrary, use of bare wire as feedstock resulted in unstable arcing with irregular deposition and spatter formation. For a given arc energy (heat input), straight polarity lead to wider beads than for reverse polarity in both bare and surface-treated feedstock wires. Based on the correlation between current-voltage waveforms and metal transfer characteristics, optimum deposition procedures were identified to manufacture thin-walled titanium components by GMA-WAAM process using appropriate feedstock wire.