Electrochemical machining of variable wire diameter structure on smart micro SMA coil

Micro Shape Memory Alloy (SMA) coil is often used as a temperature control and drive element, and the length of it can be changed in response to changes in temperature or stress. Because of its fast response and small size, it is widely applied in various fields, such as micro-channel heat exchangers, micro -driver and temperature control valve. Aiming at the manufacturing of variable wire diameter structure on micro SMA coil, this paper presents a new method, i.e. mobile ring cathode electrochemical machining, which fabricates smart SMA coils with a specific law of wire diameter change. Firstly, analyzed the corrosion mechanism of SMA coil, and studied the influence of anode voltage variation and passivation effect induced by resistance on machining uniformity. After that, established an equivalent circuit model of SMA coil electrochemical machining, from which obtained the theoretical basis for regulating the law of wire diameter change by adjusting the ring cathode movement velocity (V) and applied voltage (UR), and successfully manufactured SMA coils with linear, convex and concave wire diameter change with nonlinear deformation ability, of which the linear slope is -0.98 x 10-3 and the linearity is 0.992. The processed SMA coils with variable wire diameter show good section roundness Ro (maximum section roundness error 0.03) and surface roughness Ra (average surface roughness 0.95 mu m), and the change in phase transition temperature before and after processing is only +/- 2 degrees C. Applying SMA coil in micro-channel heat exchangers effectively alleviated local overheating caused by random hot spots. Specifically, it increased the temperature uniformity of the channel and the overall heat transfer performance by 18.3 % and 9.5 % respectively. Therefore, the manufacturing method presented in this study can accurately control the gradient rate and consistency of the wire diameter change of SMA coils, and is expected to be extensively applied in smart devices and systems in need of complex SMA structures.