Shape Memory NiTiHf Machined Helical Springs: Balancing Displacement and Force Output for Actuation

NiTiHf shape memory alloys are of high interest to the scientific community due to their high strength, excellent stability, and high transformation temperature. While many geometrical forms can be manufactured using NiTiHf, traditional springs are difficult to manufacture due to the material’s limited workability to wire form. Machined springs offer a great alternative to achieve a spring form using NiTiHf. Machined springs were fabricated from Ni_50.3Ti_29.7Hf₂₀ (at.%) at spring rates of 87.6, 175.1, and 262.7 N mm⁻¹. To manufacture the springs, a solid rod was cored out and a helix was machined into the tube. For each spring rate, two spring types were made: single–start and dual–start. The springs were subjected to constant-force thermal cycling tests, isothermal tests, and constant displacement thermal cycling tests. Computational modeling was used to determine the stress profiles within the springs. The NiTiHf springs showed very good stability, with less than 0.5 mm of change between the nineteenth and twentieth cycles. The machined springs were capable of actuation displacements up to 15.5 mm, over 50% extension from the original flexure length, superelastic stresses greater than 2.4 GPa, and tensile blocking forces ranging from 750 to more than 2000 N.