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.