Improvement of Fatigue Strength of Injection Molded Ti-6Al-4V Alloys and Elucidation of Controlling Factors in Fatigue Strength

Abstract

Since Ti alloys are difficult-to-machine materials, the application of powder metallurgy such as metal injection molding (MIM) is desired. Although injection molded Ti-6Al-4V alloys showed sufficient tensile properties, their fatigue strength was significantly lower than that of the wrought material. In this study, various approaches were applied to improve the fatigue strength through MIM processing parameters such as raw material powders, addition of 4th and 5th elements such as Mo and B, sintering techniques, and heat treatment conditions. Eventually, we obtained the fatigue strength as same as wrought materials. We also investigated the factors, which control the fatigue strength of injection molded Ti alloys. Regarding the relationship between the pore size and grain size on the fatigue strength, it was found that the crack initiation occurred from the largest pore when the ratio of the pore size to the grain size was large, and simultaneously the fatigue strength was significantly decreased.