The production of Ti-6Al-4V components via powder metallurgy routes is looked upon as an efficient production method that reduces wastage, but leaves finished products with high interstitial oxygen concentrations that do not meet industrial standards. The ability to control the interstitial oxygen concentration in Ti-6Al-4V powder metallurgy would improve the viability of near net shape processing for the production of industrial components. One process that has demonstrated the ability to remove oxygen from titanium alloys is calciothermic reduction, which is a reduction process originally developed to reduce titanium dioxide to commercial purity titanium using a molten flux of calcium and calcium chloride. The aim of this thesis is to examine whether calciothermic reduction can be used to control the interstitial concentration of oxygen and nitrogen in powder metallurgy Ti-6Al-4V and understand the reaction mechanisms that enable this process to work. By understanding these mechanisms, the process can then be optimised to improve the properties of powder metallurgy Ti-6Al-4V components, and provide a basis to extend this to other alloy systems.
Calciothermic reduction was demonstrated to be effective at reducing the interstitial oxygen concentration in powder metallurgy Ti-6Al-4V to acceptable industrial standards (< 2,000 wt .ppm). The optimisation of the process required the balance of thermodynamics and kinetics to be controlled; thermodynamics was important to ensuring the reaction would begin, with the kinetics becoming more important during the reduction process because the removal of interstitial oxygen concentration relied upon a diffusion based mechanism.
Evaluation of the mechanism that underpins the removal of oxygen via calciothermic reduction, was assessed using a FIB-SIMS based technique. This method of analysis was developed during this research and demonstrated to be effective at quantifying interstitial oxygen concentrations in titanium alloy, which was used to confirm the formation of oxygen concentration gradients from titanium alloy bulk to the surface during calciothermic reduction.
Further investigation of the reduction process indicated that calciothermic reduction could facilitate the nitriding of Ti-6Al-4V in a sealed air environment, forming a wear resistant surface layer in a novel process referred to as “Calciothermic Assisted Immersion Nitriding” (CAIN). The nitriding process produced a consistent TiCxNyOZ surface layer where the chemical composition of the layer developed in a three stage reaction involving the inward diffusion of interstitial carbon and nitrogen, whilst oxygen was removed from the surface. This surface layer improved the tribological properties of the Ti-6Al-4V samples by changing the wear mechanism from adhesive to abrasive, which resulted in an increased wear resistance, which was comparable to a commercial produced, physical vapour deposition TiCN coating.Open Acces
