Heat treatment, also called annealing, influences the functional properties of NiTi nearly as much as alloying. Treatment processes vary from simple temperate/time soaks to complex, multi-stage sequences. While complex processes can yield useful performance improvements, they are difficult to design and often proprietary to manufacturers, therefore not widely used by researchers. This work attempts to demystify the heat treatment process and make high quality NiTi actuators more available to researchers. By designing a constantly fed heat treatment oven, purpose-built for NiTi wire treatment, a wide gamut of heat treatment sequences were performed and the resulting wire properties tested. The initial material is ‘as-drawn’ 0.25 mm diameter shape memory wire. An experiment was designed using the full range of the new equipment, annealing the wire up to temperatures ranging from 300 to 800 C, for 5 to 120 minutes, with up to 3 stages per sequence and initial and final ramps to/from room temperature. In addition to applying consistent heat for a controlled amount of time, the new machine also provides controlled tension to the wire, allowing for thermal expansion to be neglected in the resulting trained shape. This results in a much more consistent and controllable end product when compared to conventional annealing methods. Experimental results show a wide range of NiTi actuator behavior can be achieved and heat treatment processes selected from the experimental ranges tested. Optimal heat treatments determined by experiment yield NiTi actuators with performance characteristics comparable to those of several manufacturer’s proprietary actuator-wire. 


Machine Design for Multiscale Nitinol Annealment Process and End Product Performance Analysis

Division / Sponsors: 
Aerospace Division

Paper Proceedings: https://asmedigitalcollection.asme.org/SMASIS/SMASIS2020/volume/84027

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