Enabling High Power Additively Manufactured SMA Actuators with Internal Liquid Metal Channels

Large monolithic shape memory alloy (SMA) actuators are currently limited to applications with low cyclic actuation frequency due to their high thermal masses coupled with low thermal diffusivities. Previously, surface-embedded gallium-based liquid metal coils have been successfully able to actuate additively manufactured NiTi SMAs via induction heating [1]. Now, these liquid metal surface channels have been moved from the surface to the interior of the SMA itself, enabling better performance and reliability, to ultimately obtain high power density actuators with large-scale SMAs. This has been achieved with two types of actuators: a 75 mm long cantilever beam and a 55 mm long tensile bar. The cantilever beam has a liquid metal channel in the shape of a planar spiral parallel to the neutral axis to enable AC induction heating at 30 W, while the tensile beam has a straight channel along the tensile direction to enable DC Joule heating at 60 A. Rapid cooling is achieved via flowing the same liquid metal from a low-temperature reservoir within a closed loop system. Further, the ability to produce sufficient internal channels was enabled by laser powder-bed fusion additive manufacturing and the chemical reduction of internal roughness with Multi-Etch®, a safe hydrofluoric acid-free etchant for many metals including the resilient titanium. A small study on Multi-Etch® reducing surface roughness within internal NiTi SMA features will be discussed.

[1] Hartl et al., Shape Memory and Superelasticity, Vol. 3, No. 4, pp. 457-466, 2017. https://doi.org/10.1007/s40830-017-0137-9