Exploring Miniaturized HVOF Systems for the Deposition of Ti-6Al-4V

Abstract

AbstractThe exceptional properties of Ti-6Al-4V of high strength, lightweight, corrosion resistance and machinability make it one of the most widely used alloys in in the aerospace industry. Significant efforts are underway to establish powder bed additive manufacturing technologies for Ti-6Al-4V. There are also increasing attempts to use thermal and cold spray to build near-net-shaped parts with buildup rates orders of magnitude higher than powder bed. Thermal spraying, such as HVOF, can oxidize and degrade the alloy due to the high processing temperature. Lowering the flame temperature through inert gas addition in full-size HVOF systems is a possible approach to retain solid-state deposition of the feedstock particles, thereby limiting oxidation and detrimental α-case formation, while providing sufficient heat input for particle softening and plastic deformation at impact. Novel miniaturized HVOF systems, with spray jets of only a few millimeters in width, may further offer the possibility to improve the spatial resolution of the buildup for near-net shape forming. The process parameter range for solid-state deposition of Ti-6A-4V using the liquid-fuelled TAFA Model 825 JPid and the novel hydrogen-fuelled Spraywerx ID-NOVA MK-6 with the addition of nitrogen is shown. Buildups at over 80% deposition efficiency generally yield as-sprayed porosities below 3% and hardness above 200 HV100gf. Attainable microstructures and oxygen content as a function of spray parameters are delineated. Recrystallization and beta-annealing of selected samples lower the residual porosity and can create equiaxed α and intergranular ß-phases. Ultimate tensile strengths of up to 1100 MPa were attained; however, the residual oxygen content of above 0.7% was found to limit β-phase formation, which contributes to a limited elongation to failure.