Hot Forming a V-Shaped Ni-Ti Shape Memory Alloy Wire

Abstract

Ni-Ti shape memory alloys have shape memory effect, that if they are deformed from martensitic phase state at a lower temperature, they will recover their original shape by heating them to austenitic phase state. To have them for an application using this shape memory effect, usually they undergo a constraint aging after plastic deformation. That is, they are fixed with tool set and together heat treated in a furnace after they are formed at room temperature. However a large load is needed to form them at room temperature. Thus, this study is aimed to lower the forming load by combining the forming and aging process together in a furnace at high temperature. In this study, a Ni-Ti shape memory alloy wire having a diameter of 0.63 mm is bent in a heated chamber at 450°C, 500°C, 550°C, and 600°C, respectively, by a V-shaped punch of 2 mm in radius to an angle of 60°, then held along with the die set at its dead center in the chamber for maximum one hour long, and then quenched in the water. All of the bent wires have the shape memory effect. That is, the wires recover their bent geometry once they are unbent at about 4°C and heated again at about 100°C. The experiment results showed that the bent wires can have the geometry accuracy as desired because of stress relaxation found in the process, which depends on the process temperature and duration. As a result, the higher the process temperature is and the longer the duration is, the better the accuracy of the formed wires is.