Design and Characterization of Self-Biasing NiTi Spring Actuator

Abstract

Spring actuators are essential components in vehicles as they are used for climate control, braking ventilation, engine control and fuel management. However, commercially available spring actuators are heavy and expensive and need to be replaced by lower-weight SMA spring actuators. Potential applications of the particularly promising NiTi shape memory alloys (SMA) in automotive spring actuators is currently limited due to low awareness. Recently, laser processing has been used to make processing monolithic NiTi wires more flexible than ever by locally changing the wire composition to achieve multiple functional properties. Using laser-processing to add multiple memories in a monolithic NiTi spring wire would allow for the fabrication of a novel self-biasing spring actuator which would lead to lower equipment costs, reduced weight and enhanced flexibility. To obtain a successful self-biasing spring, processing and spring fabrication techniques for NiTi wires need to be studied.

The first part of the study involved investigating the effect of incrementing the number of laser pulses per spot and post-processing heat treatment temperatures on the composition of Ni-49.2 at.% Ti SMA wires. This was achieved by performing Differential Scanning Calorimetry (DSC) analysis and observing the shift in phase transformation temperatures. Results revealed that increasing the number of laser pulses increased the transformation temperatures up to a point at which the samples become Ti-rich and there is negligible effect on composition. Heat treatment study showed that heat treatment temperature had a significant impact on the transformation temperature peaks resulting in distinct peaks for base material and laser-processed samples. Mechanical properties of different heat-treated samples were examined using tensile test plots to determine the optimal heat treatment temperature for shape-setting springs.

In the second part of the study, the effect of spring geometry on spring force and the effect of incorporating multiple memory in a spring actuator was studied. A unique spring-shape setting fixture was used to fabricate springs with consistent spring pitch and diameter. Results showed that increasing the spring pitch and decreasing the mean coil diameter increased the spring force, which conforms to the established spring force equation. Incorporating multiple memories in a monolithic spring enabled thermally-induced self-biasing spring actuation due to various austenitic transformation temperatures triggering unique functional properties.