Metal Injection Moulding of Ti-6Al-4V with Yttrium Addition

Abstract

For highly loaded parts, such as medical implants as well as for engineering application, high strength and good ductility are indispensable, but also the fatigue resistance plays an important role. For both, quasi static and fatigue properties, a fine microstructure is essential. Furthermore, especially for titanium alloys the control of interstitial impurities like oxygen and nitrogen is very important. In a former study additions of several oxides of rare earth elements were tested with respect to their effect on the microstructure of MIM processed Ti-6Al-4V. Yttrium oxide has shown the strongest effect on the colony size of Ti-6Al-4V.In this study elementary yttrium powder was added to Ti-6Al-4V. During sintering, the yttrium scavenges oxygen out of the titanium matrix, increasing the ductility. The yttrium oxide, which is formed, leads to a colony refinement and therefore to a higher strength.For preliminary tests cylindrical samples consisting of Ti-6Al-4V powder blended with coarse yttrium powder and a wax and polyethylene-based binder were manufactured by uniaxial pressing and sintering. The addition of yttrium led to a slight decrease of the colony size: specimens sintered at a high temperature of 1400 °C show a stronger dependency of yttrium content on the colony size than those sintered at 1300 °C.Tensile test specimens were produced by MIM using gas atomized Ti-6Al-4V-powder with additions of yttrium powder between 0.1 and 0.5 wt.-%. Sintering took place at temperatures between 1300 °C and 1400 °C with two different dwell times (2 and 4 hours). Tensile tests were conducted in air at room temperature. The microstructures were observed by SEM and light optical microscope. Oxygen and nitrogen contents were analysed by a melt extraction technique.