Effect of plasma gas composition on deposition characteristics in plasma arc directed energy deposition of Ti-6Al-4 V

The increasing demand for high-efficiency and high-quality wire-based arc directed energy deposition processes presents a significant challenge to the plasma arc, which is greatly influenced by plasma gas composition. This paper introduced mixed gases of helium (He) and argon (Ar) into the plasma gas for the deposition of Ti-6Al-4 V. The effect of plasma gas composition on the deposition characteristics concerning arc electrical characteristics, keyhole formation, metal transfer, bead geometry, and surface waviness were investigated. Results demonstrate that the arc voltage and heat input exhibited a linearly increasing tendency with the increase of He in the plasma gas, attributed to a higher ionization potential and thermal conductivity. The threshold current value for keyhole formation was effectively increased by adding He to the plasma gas. The keyhole formation was completely suppressed as the He content was higher than 50% for the range of parameters tested. The metal transfer mode changed from surface tension transfer to droplet surface tension transfer and to free space droplet transfer, as the He content in the plasma gas increased. For the bead geometry, the bead height, aspect ratio, and contact angle showed a decreasing trend, while the bead width presented an increasing trend. The relative waviness and absolute waviness of the deposited single-pass multi-layer walls reduced remarkably with a higher He content. The results lay a solid foundation for an improved plasma arc DED process with high efficiency and good forming quality through plasma gas composition control.