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

Yoshimitsu Okazaki

doi:10.3390/ma5081439

Abstract

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 (ΔT) from β-transus temperature (Tβ) for the volume fraction of the α- and β-phases were almost the same for both Ti-15Zr-4Nb-4Ta and Ti-6Al-4V alloys. In the α-β-forged Ti-15Zr-4Nb-4Ta alloy, a fine granular α-phase structure containing a fine granular β-phase at grain boundaries of an equiaxed α-phase was observed. The Ti-15Zr-4Nb-4Ta alloy billet forged at Tβ-(30 to 50) °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 (σmax) markedly increased with decreasing testing temperature below Tβ. The reduction in area (R.A.) value slowly decreased with decreasing testing temperature below Tβ. The temperature dependences of σmax for the Ti-15Zr-4Nb-4Ta and Ti-6Al-4V alloys show the same tendency and might be caused by the temperature difference (ΔT) from Tβ. 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 (ΔT) between Tβ 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 α-β-type Ti alloy.

Highlights

Orthopaedic implants require biomechanical and biochemical compatibilities as well as biological safety
Recent studies of Ti alloys have been carried out, assuming their long-term use in the human body and possible long-term health problems associated with the release of toxic ions and fatigue fracture [1,2,3,4,5,6]
Adding a small amount of alternative metal elements improves the quality of Ti alloys

Summary

Introduction

Orthopaedic implants require biomechanical and biochemical compatibilities as well as biological safety. Titanium (Ti) materials have been widely used to replace failed hard tissues, namely, bone screws, bone plates, compression hip screws, intramedullary fixations, short femoral nails, artificial hip joints, artificial knee joints, spinal instruments, and dental implants. Recent studies of Ti alloys have been carried out, assuming their long-term use in the human body and possible long-term health problems associated with the release of toxic ions and fatigue fracture [1,2,3,4,5,6]. The relationship between the cytocompatibility and polarization resistance of various pure metals has previously been summarized [7,8,9,10]. Among the 70 metals, only Ti and Zr show excellent cytocompatibility to both soft-tissue-derived mouse fibroblast cells and bone-derived mouse osteoblast cells [8]. Adding a small amount of alternative metal elements improves the quality of Ti alloys.

Methods

Results

Conclusion