On the Effects of Hot Forging and Hot Rolling on the Microstructural Development and Mechanical Response of a Biocompatible Ti Alloy

Zr, Nb, and Ta as alloying elements for Ti alloys are important for attaining superior corrosion resistance and biocompatibility in the long term. However, note that the addition of excess Nb and Ta to Ti alloys leads to higher manufacturing cost. To develop low-cost manufacturing processes, the effects of hot-forging and continuous-hot-rolling conditions on the microstructure, mechanical properties, hot forgeability, and fatigue strength of Ti-15Zr-4Nb-4Ta alloy were investigated. The temperature dependences with a temperature difference (Delta T) from beta-transus temperature (T beta) for the volume fraction of the alpha -and beta-phases were almost the same for both Ti-15Zr-4Nb-4Ta and Ti-6Al-4V alloys. In the alpha-beta-forged Ti-15Zr-4Nb-4Ta alloy, a fine granular alpha-phase structure containing a fine granular beta-phase at grain boundaries of an equiaxed alpha-phase was observed. The Ti-15Zr-4Nb-4Ta alloy billet forged at T beta-(30 to 50) degrees C exhibited high strength and excellent ductility. The effects of forging ratio on mechanical strength and ductility were small at a forging ratio of more than 3. The maximum strength (sigma(max)) markedly increased with decreasing testing temperature below T beta. The reduction in area (R. A.) value slowly decreased with decreasing testing temperature below T beta. The temperature dependences of sigma(max) for the Ti-15Zr-4Nb-4Ta and Ti-6Al-4V alloys show the same tendency and might be caused by the temperature difference (Delta T) from T beta. It was clarified that Ti-15Zr-4Nb-4Ta alloy could be manufactured using the same manufacturing process as for previously approved Ti-6Al-4V alloy, taking into account the difference (Delta T) between T beta and heat treatment temperature. Also, the manufacturing equivalency of Ti-15Zr-4Nb-4Ta alloy to obtain marketing approval of implants was established. Thus, it was concluded that continuous hot rolling is useful for manufacturing alpha-beta-type Ti alloy.