Shape memory alloys are capable of producing some of the highest actuation stresses (~300 MPa) of any active material. However, large monolithic SMA actuators, which can induce the tremendous forces required in some applications, 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 increase in the effective thermal diffusivity of an SMA actuator system will result in an increase in actuation frequency; accordingly, this would lead to the ability to perform more cycles over a given time interval and subsequently yield an overall higher actuator power density (energy density with time). This current work presents ongoing research in the design, manufacturing, enabling surface engineering (such as chemical etching and anodization), and testing of internal channel additively manufactured SMA actuator designs, including a tensile bar variation and planned testing of an optimized cantilever beam.

Advanced micromachining processes like laser micromachining, electric discharge machining (EDM) and milling are key processes when fabricating Nitinol medical devices. Unfortunately, each machining process alters the thermomechanical properties of Nitinol - especially around the processing zone. To judge how much this affects the functionality of Nitinol devices, precise knowledge about the micromachining processes applied is crucial. Performance of a medical device from a manufacturer point of view is governed by its geometry. Attainable geometries are linked to the respective machining technology. Lastly the process itself might be limited concerning surface roughness, contour accuracy, and aspect ratio. Ecological aspects include the achievable material removal rate (MRR, volume per time) and necessary post processes. In this work, the authors report on recent developments in the field of micromachining Nitinol, especially in which way the respective technology affects the properties and the design of medical components. A comparative analysis of micromachining technologies is presented.

Shape memory alloys (SMA) stand out due to the ability that they can be deformed and then return to their initial shape by heating. This process results in a high actuation energy density. All SMA manufacturing methods, regardless of which matrix material is used, result in heat input into the SMA, which could induce a phase transformation and therefore disable the actuator function. In this regard, options to increase the transition temperatures of NiTi alloys above the processing temperatures are fundamental. In this study, the influence of the pre-tension on the transformation temperatures of SMA wires was investigated as a way to prevent phase transformation due to heat impact during the production of SMA-polymer-actuators. Applying a pre-tension of 400 MPa to the NiTi wire, the austenite start temperature could be increased by a factor of 1.9 whereas it could be increased by a factor of 2.2 with a pre-tension of 550 MPa. Therefore, after preloading the wires, a phase transformation should not be induced when the wire contacts the polymer droplets. However, the shift of the phase transformation has to be further investigated.

In this work, the authors introduce an approach in additive manufacturing that enables control of the crystallographic texture through controlling the build orientation in the laser powder bed fusion (LPBF) method. The LPBF parameters play a key role in tailoring the microstructure of the as-fabricated parts. The proposed approach provides the capability of altering/enhancing the properties of the as-printed NiTi shape memory alloys by controlling the texture. The anisotropy may not be preferred for applications with complex and multi-axial loading regimes; however, the approach can be suitable for application with the main loading direction such as torque tube actuators.