Investigation into microstructural and mechanical properties of a Ti-6Al-4V hybrid manufactured component

Abstract

Hybrid manufacturing, as a combination of investment cast and additive manufacturing, is investigated in this study. Hybrid manufacturing acts as a potential manufacturing process for Ti6Al4V components. The project investigates the effect of different surface preparations, cooling mediums and high-temperature applications, on the diffusion zone of a hybrid manufactured component. The microstructures and mechanical properties obtained from the various testing conditions are compared to wrought, investment cast and additive manufactured properties. The microstructures and tensile fractures were characterized using light optical microscopy, stereo microscopy, microhardness and Scanning Electron Microscopy (SEM) to investigate the microstructural morphology and the structural hardness variation. Six different surface preparations were applied to Ti6Al4V investment cast rods prior to additive manufacturing. Smooth surfaces led to better diffusion as opposed to a rougher surface. Plain just-cut surface preparation was concluded as the most suitable surface preparation technique. Hybrid manufacturing revealed three different regions; investment cast region, diffusion zone region and the additive manufactured region. The microstructures of the investment cast and additive manufactured region compared well with the microstructures of investment cast and additive manufacturing specimens, respectively. The diffusion zone resulted in acicular α’ martensitic morphology. The fractures of hybrid manufactured specimens with sufficient bonding fractured in the investment cast region and showed similar tensile properties to the investment cast specimens. Additive manufactured specimens proved to have higher yield strength, ultimate tensile strength, hardness and strain hardening exponent, compared to the investment cast, wrought and hybrid manufactured specimens. A temperature of 1050 °C was used for 2 hours to solution-treat hybrid manufactured specimens, followed by different cooling mediums; air cooling, water quenching and furnace cooling. It was found that the hardness of the water quenched specimen indicated a more homogeneous structure across the different regions of the hybrid manufactured specimen while also producing a higher hardness compared to the non-heat treated hybrid manufactured specimen. The investigation of high-temperature applications was done by furnace tensile testing hybrid manufactured specimens at 400 °C, 600 °C and 800 °C. The investigation concluded that the yield strength, ultimate tensile strength and strain hardening exponent decrease with an increase in the furnace tensile test temperature. The ductility was found to increase with temperature.