Influencing mechanisms of melt behavior on metal vapor characteristic and columnar grain formation in wire-arc directed energy deposition of titanium alloy

The metal vapor characteristic and columnar grain formation, influenced by the melt behavior in wire-arc directed energy deposition (WA-DED) of titanium alloy, have great effects on the tungsten electrode contamination and component performance. In this study, a spectroscopic system was adopted to measure the arc temperature and metal vapor distributions, and a multi-physical wire-melt pool model was developed to investigate the wire metal transfer and melt pool convection in WA-DED of titanium alloy. The microstructures of the fabricated components were studied. The influencing mechanisms of melt behavior on the metal vapor characteristic and columnar grain formation, and the tungsten electrode contamination mechanism were revealed. The experimental and numerical results showed that a stable liquid bridge transfer mode was achieved during the WA-DED process. Compared with the one-layer case, the molten metal flow patterns in the multi-layer case were unchanged, but the higher melt temperature caused stronger Ti evaporation; then more Ti vapor entered the arc plasma and absorbed its energy, causing a higher concentration of Ti+ ion in the whole arc region, and thus the arc temperature was decreased. The Ti+ ion can be transported to the negative electrode surface by the cataphoresis, dominating the electrode contamination. Low heat input and high arc force were beneficial to the columnar to equiaxed grain transition.