Cold Bending a Ni-Ti Shape Memory Alloy Wire

Abstract

Because of being in possession of shape memory effect and superelasticity, Ni-Ti shape memory alloys have earned more intense gaze on the next generation applications. Conventionally, Ni-Ti shape memory alloys are manufactured by hot forming and constraint aging, which need a capital-intensive investment. To have a cost benefit getting rid of plenty of die sets, this study is aimed to form Ni-Ti shape memory alloys at room temperature and to age them at elevated temperature without any die sets. In this study, starting with solution treatments at various temperatures, which served as annealing process, Ni-rich Ni-Ti shape memory alloy wires were bent by V-shaped punches in different curvatures at room temperature. Subsequently, the wires were aged at different temperatures to have shape memory effect. As a result, springback was found after withdrawing the bending punch and further after the aging treatment as well. A higher solution treatment temperature or a smaller bending radius leads to a smaller springback, while a higher aging treatment temperature made a larger springback. This springback may be compensated by bending the wires in further larger curvatures to keep the shape accuracy as designed. To explore the shape memory effect, a reverse bending test was performed. It shows that all bent wires after aging had a shape recovery rate above 96.3% on average.

Deep Drawing of a Ni-Ti Shape Memory Alloy Sheet

“…A springback caused by superelastic deformation does almost not exist and the formed shape is extremely clear. The above-mentioned formed parts were placed in a furnace treated at 300°C for one hour as recommended in [9], and water quenched thereafter. This precipitation or aging served as a shape memory treatment.…”

“…These two hurdles result in the application of Ni-Ti shape memory alloy being limited to the tube or wire as raw material. Fann and Huang [9] have developed a cold forming process for Ni-Ti shape memory alloy wires to eliminate the use of expensive die sets, not only in the forming, but also in the constraint aging process at elevated temperature, which can help to extend the application of Ni-Ti shape memory alloys to the sheet forming process. However, the stress state appearing in sheet metal forming is a plane stress state, which is different to the wire bending process.…”