Directed-energy deposition of Ti-6Al-4V alloy using fresh and recycled feedstock powders under reactive atmosphere

Abstract

This research studies the directed-energy deposition of Ti-6Al-4V (wt%) in an argon atmosphere containing 9–9500 ppm oxygen (from air), using both fresh and recycled feedstock powders. For fresh powder builds, when exposed to a 3500 ppm oxygen-containing atmosphere, a yield strength (YS) of 1061 ± 0.6 MPa and an elongation-to-fracture (ETF) of 10.5 ± 1.6 % are obtained. The oxygen pickup in the build deposit tends to plateau at 1000 ppm beyond 3500 ppm oxygen exposure. Conversely, the nitrogen pickup shows an increasing trend, reaching 1000 ppm for the air exposure range investigated. Recycled powder builds are stronger but less ductile than equivalent fresh powder builds made under comparable atmospheres. A higher degree of interstitial element pickup with no apparent saturation is identified for builds made from recycled powders, due to their smaller average particle size and a correspondingly larger specific surface area. The resulting microstructure for all samples made with air exposure comprises full lamellar α + β, formed through the in-situ decomposition of martensite at a temperature close to 600 °C. This conclusion is supported by the β volume fraction at 600°C as predicted by Thermo-Calc agreeing with image analysis data. The α lath thickness is found to increase with air exposure level. The increased β-transus temperature and martensite start temperature, and the accelerated diffusion-driven phase transformation due to the presence of elevated interstitial element contents accounts for the α lath thickening behaviour. YS and ultimate tensile strength (UTS) are found to increase linearly with oxygen equivalent. Solid solution strengthening from nitrogen and oxygen pickup is responsible for this increase. YS and UTS models based on oxygen equivalent are proposed. Nitrogen and oxygen in the builds dictate the ETF, with nitrogen content being the more potent factor.