Optimized Tensile and Fatigue Properties of Ti-6Al-4V Processed By Metal Injection Moulding

Metal Injection Moulding (MIM) of titanium alloys has evolved from laboratory state to a serious production technique during the last decade. Today it is applied commercially and ASTM standards for medical products exist. However, even if some excellent products are produced by MIM from titanium powders, it is not a standard method, yet, and there are still concerns against the usage of MIM of titanium alloys for the production of highly loaded or fatigue loaded components. In this paper it is shown, that by means of MIM Ti-6Al-4V powders can be processed to components with mechanical properties equivalent to standard wrought material. Experimental results from specimens made by MIM from gas-atomised Ti-6Al-4V powders reveal the dependence of porosity and interstitial content on tensile properties and show that oxygen amounts of 0.3 wt% and more are beneficial in terms of gain in strength without loss in ductility. Thus, a UTS value of 900 MPa in connection with a plastic elongation of 14% can be achieved, despite of 3.5% residual porosity. By closing the pores by an additional hot isostatic pressing process even the demands from the Grade 5 standard for the wrought material are fulfilled. In contrast to static load, a comprehensive study of the factors determining the fatigue properties reveals, that in this case a small grain size is more important than low porosity. In order to form a fine microstruture a slightly modified Ti-6Al-4V was produced by adding elemental boron powder to the feedstock. The resulting Ti-6Al-4V-0.5B alloy shows a grain size of only 18 µm and a 10⁷ endurance limit of 640 MPa, being equivalent to standard wrought material and matching also aerospace requirements. The tensile properties match the Grade 23 specification. This result shows the potency of MIM specific titanium alloys for optimising specific properties.