Machined helical springs from NiTiHf shape memory alloy

Abstract

Machined helical springs were fabricated from rod stock using a Ni-rich Ni50.3Ti29.7Hf20 (at. %) high-temperature shape memory alloy. Springs with three different spring rates of 35, 87.5, and 175.1 N mm−1 were designed, and the rectangular cross-section spring formulas were used to approximate the deflection and shear stresses. Springs were then subjected to isothermal testing at room temperature and constant-force thermal cycling experiments at varying loads. Actuation displacements on the order of 19 mm were obtained for starting flexures of 20 mm in length, resulting in nearly 95% total actuation extension. Thermal cycling under loads ranging from 0 to ∼500 N (stresses of 0 to ∼500 MPa) showed higher actuation displacements for springs with the lowest spring rates compared to the stiffer, higher spring rate counterpart. With increasing applied load, actuation displacements were still increasing even at stresses near 500 MPa (no maxima reached), indicating more actuation potential at higher loads. Two-way shape memory effect was assessed at 0 load and found to be very minimal (less than 3 mm) due to lack of training.