Abstract:
The recent acceptance and standardized certification of powder metallurgy titanium for commercial aircraft engineering applications marks a milestone in titanium powder metallurgy. However, there are still many issues to be addressed for the wider development of powder metallurgy of titanium such as in-depth identification of impurities arising from raw materials and sintering atmosphere, development of novel sintering methods and new alloy design. Since limited works currently focus on the study of residual magnesium chloride impurities originating from the mainstream Ti sponge production process — Kroll process, the starting point of this thesis is to identify the chloride impurities in titanium powders. Jigsaw-like agglomerates were observed on the particle surface of Kroll-processed hydrogenated-dehydrogenated (HDH) commercially pure (c. p.) titanium powders and further detailed surface chemical analysis revealed that the magnesium and chlorine mainly existed as Mg(OH)Cl and titanium chloride respectively. HDH c.p. titanium and pre-alloyed Ti-6Al-4V powders were sintered in a graphite furnace with a potential to generate low oxygen pressure. Spherical hollow Ti(CNO) particles were observed on the surfaces of sintered Ti samples, but not present on the sintered surface of pre-alloyed Ti-6Al-4V, which is considered as a Cl free powder. The formation of spherical hollow particles is considered to result from the reaction between the gaseous carbon and gaseous titanium atoms decomposed from titanium chlorides. Therefore the presence of chloride impurities in HDH powders is detrimental to the final sintering densification and a model is proposed to describe the evolution of magnesium chlorides in titanium powders during sintering period. With respect to sintering in the graphite furnace, a contaminated scale was formed on the surface identified as titanium oxycarbonitride (Ti(CNO)) while cross sections were composed of only single phase alpha titanium. Corresponding tensile mechanical properties were strongly dependent on the distance of the position from the surface mainly because of the impurity gradient. There was almost no ductility for tensile slices manufactured from the area close to the surface, while an average tensile elongation of 5 % - 7 % was obtained with the highest value being 10.2% for slices taken from the interior. A model is used to calculate the thickness of the contaminated layer on the surface based on Fick’s second law using the solution for semi-infinite solids. In order to remove the contamination arising from the furace atmosphere, different contamination-reduction solutions were employed. Experimental results indicated that both densification and mechanical properties can be improved by the application of contamination reduction methods. Ultrafine HDH titanium powders with a median particle size of 8.84 m blended with Al-V master powders were sintered with an addition of low-cost iron element. Spherical micropores with diameter <15 m were formed during sintering with ultrafine titanium powder even at the lowest sintering temperature (1100 ºC) for Ti-6Al-4V. It was also observed that the microporosity was reduced and improved microstructural and compositional homogenization was observed with increasing additions of iron, which is due to fast diffusion of iron in titanium and thereby enhancing densification. However, the mechanical properties were low because of high oxygen content in as-received titanium powders, inhomogeneous microstructure and macroporosity due to high evaporation of aluminium in master powders. The evaporation process is modelled using the Miedema model and Langmuir equation.