Study on Low-cost Alternatives for Synthesising Powder Metallurgy Titanium and Titanium Alloys

The aim of this work was to carry out research about cost effective ways for synthesising powder metallurgy Ti and Ti alloys. This was done: using low cost Ti powders produced by the hydrogenation dehydrogenation (HDH) method; using induction heating, instead of the more traditionally used electrically heated vacuum furnace, for sintering and in the thermomechanical processing of powders; using low cost alloying elements such as Fe and stainless steel powders and investigating the possibility of using low amounts of alloying elements. The first main stream investigation in this work focussed on a study of induction sintering of Ti and pre-alloyed Ti6Al4V powders. This initial work included the following investigations: the effect of powder compact density on induction heating rates, the levels of porosity and the tensile properties of sintered samples; and the effect of induction sintering process parameters, such as temperature and time on the sintered densities, porosity distribution, microstructure, tensile properties and fracture characteristics of Ti and pre-alloyed Ti6Al4V powders. The second main study was on the application of induction sintering for consolidating pre-alloyed Ti6Al4V powders by thermomechanical processing via open die forging. The aim was to reduce processing time by eliminating the need for lengthy vacuum sintering and additional reheating required before final processing. The effect of the induction sintering parameters on the deformation behaviour, microstructure, mechanical properties and fracture behaviour were studied. Furthermore, the effect of a recrystallization annealing heat treatment on the tensile properties and microstructure was also analysed. The third study was focused on the development of low cost powder metallurgy Ti alloys with mechanical properties similar to those of the most commonly used wrought Ti6Al4V alloy. To achieve this aim an approach using Ti HDH powder with a high oxygen content of 0.25wt% was used along with low cost alloying elements such as Fe and stainless steel powders. The maximum alloying element content was limited to 5% and a blended elemental approach for making the alloys was employed. Induction sintering and vacuum sintering prior to thermomechanical processing by open die forging and extrusion were used for consolidation. The alloy compositions studied were Ti3Al2V, Ti5Fe with two different particle sizes of Fe powder and Ti5SS (addition of 316 stainless steel powder). A blended elemental Ti6Al4V alloy was also studied for comparison. The relationship between microstructure, mechanical properties and fracture behaviour was studied. The effect of recrystallization annealing after forging and extrusion was investigated for some of the alloys on its effect on microstructure and tensile properties. The results showed that induction sintering significantly reduces the sintering time. Induction sintering for four to ten minutes produces Ti and Ti alloy with closed porosity structures as well as densities and mechanical properties comparable with those found in vacuum sintered Ti based material. The use of induction sintering in direct open die forging of prealloyed Ti6Al4V powders with a high oxygen content of 0.5wt% resulted in material with a solid structure and tensile properties, such as YS and UTS, significantly higher than those given by wrought Ti6Al4V alloy. The ductility was affected by the high oxygen content and was generally lower than that of the wrought material. Thermomechanical processing of the low cost alloys Ti5Fe, Ti5SS and Ti3Al2V with oxygen contents between 0.33wt% and 0.45wt% resulted in material with an excellent combination of tensile strength and ductility, comparable to the values found in wrought Ti6Al4V alloy. A post forging or extrusion recrystallization annealing heat treatment played a significant role in producing microstructures with fine α+β lamellae, which was a controlling factor for achieving good ductility in Ti alloys with a high oxygen content