Surface Engineering of Additively Manufactured Shape Memory Alloy Actuators To Enable Internal Liquid Metal Energy Circuits

Abstract

Shape memory alloys (SMAs) are capable of producing some of the highest actuation stresses of any active material; however, large monolithic SMA actuators are currently limited to low cyclic actuation frequencies due to their associated high thermal masses coupled with innate low thermal diffusivities of the material. An SMA actuator system with an increased actuation frequency could position such devices to exceed the highest power density option among all types of actuator de-vices broadening their implementation to new applications. To achieve this goal, room-temperature liquid metal (gallium-indium) has been integrated internally into an additively manufactured NiTi actuator to act as a multifunctional induction heating and forced fluid convection cooling system. Proper implementation of such a liquid metal energy circuit requires a surface engineering approach to electrically isolate as well as to thermally integrate the liquid metal/SMA system.

Laser powder bed fusion (LPBF) is a type of additive manufacturing approach that consists of a bed of metallic powder which is melted by a laser, line-by-line and layer-by-layer, to produce 3D part. Due to this additive construction, there is special consideration of the produced surfaces from overhanging geometries and internal features which are highly irregular and rough. Complex internal channels produced by LPBF require a procedure that can effectively homogenize these surface; this has been demonstrated using a simple chemical route using a relatively safe sodium fluoride solution, allowing for the application of subsequent processes in a controlled manner. A systematic investigation into the anodization of the NiTi SMA for the purpose of electrical insulation has been studied to provide mechanically robust and electrically insulative coatings. With such prior enabling surface modifications, the actuation of internal channel liquid metal SMA actuators are presented. Liquid metal-enabled induction heating and closed loop forced fluid convective cooling has been achieved for twelve cycles with tensile bar geometries consisting of a through-hole internal channel. Such demonstrations indicate that higher actuation frequencies of large-scale SMA actuators are achievable.