NiTi shape memory elements for smart micro-actuation: laser processing, chemical etching and functional characterization

Shape Memory Alloys (SMAs) are smart materials that can be considered good candidates for the activation of micro-devices for the automotive, aerospace, biomedical and mechanical systems. In this context, an experimental study on the thermo-mechanical behavior of NiTi SMA elements is proposed; the effect of the production steps on the final functional properties is presented, too. The basic geometry of these micro-elements is the well-known snake like one, that is generally used to guarantee relevant strokes and reasonable forces. The thermo-mechanical performances of the SMA elements have been also compared with the straight linear geometry. The production route, selected for this work for allowing high productivity and good repeatability for industrial applications, consists in cold rolling for the realization of thin NiTi stripes (140 micron thickness) and in laser microcutting for the patterning of the different geometries. The main results on the study of laser microcutting were carried out and discussed by means of Design of Experiments approach. Then, chemical post-processing has been adopted for removing the thermal damage that comes from the previous laser machining; as consequent, reduction of the element dimensions was also reached. Results show how the post-processing can enhance the SMA functional behavior, in terms of stress/strain response as well as of recovery strain.