Processing and property assessment of NiTi and NiTiCu shape memory actuator springs

Among the multifarious engineering applications of NiTi shape memory alloys (SMAs), their use in actuator applications stands out. In actuator applications, where the one-way effect (1WE) of NiTi SMAs is exploited, SM components are often applied as helical coil springs. Ingots are generally used as starting materials for the production of springs. But before SM actuator springs can be manufactured, the processing of appropriate wires from NiTi ingots poses a challenge because cold and hot working of NiTi SMAs strongly affect microstructure, and it is well known that the functional properties of NiTi SMAs are strongly dependent on their microstructure. The objective of the present paper is therefore to produce binary Ni50Ti50 and ternary Ni40Ti50Cu10 SMA actuator springs, starting from ingots produced by vacuum induction melting. From these ingots springs are produced using swaging, rolling, wire drawing and a shape-constraining procedure in combination with appropriate heat treatments. The evolution of microstructure during processing is characterized and the mechanical properties of the wires prior to spring-making are documented. The mechanical and functional characteristics of the wires are investigated in the stress-strain-temperature space. Finally, functional fatigue testing of actuator springs is briefly described and preliminary results for NiTi and NiTiCu actuator springs are reported. Untersuchungen zur Herstellung und zu den Eigenschaften von NiTi- und NiTiCu-Aktorfedern NiTi-Formgedachtnislegierungen (FGL) zeichnen sich durch eine hohe Attraktivitat fur verschiedene Aktorik-Anwendungen aus. Dabei werden FGL haufig in Form von zylindrischen Federn verwendet, wobei der Einwegeffekt genutzt wird. Die Herstellung von solchen Aktor-Federn ist jedoch keinesfalls trivial. Sowohl die Herstellung von geeignetem Drahtmaterial als auch die Formgebungsbehandlung stellen in gewisser Weise eine Herausforderung dar. Die funktionellen Eigenschaften von NiTi-FGL hangen sehr stark von mikrostrukturellen Randbedingungen ab, und erforderliche thermomechanische Behandlungen sind mit weit reichenden mikrostrukturellen Veranderungen verbunden. Das Ziel dieser Arbeit war, eine Prozesskette zur Herstellung von Ni50Ti50 und Ni40Ti50Cu10-Aktorfedern ausgehend vom Gussmaterial zu erarbeiten und metallkundlich zu charakterisieren. Aus den Gussblocken wurden durch Rundkneten, Walzen, Drahtziehen und speziellen Formgebungsbehandlungen Aktorfedern hergestellt. Dabei wurden die mikrostrukturellen und funktionellen Anderungen der beiden FGL charakterisiert. Zum Schluss wurden die mechanischen und funktionellen Eigenschaften der verschiedenen Draht-Materialien und der fertigen Federn untersucht. Dabei wurde ein besonderes Augenmerk auf die funktionelle Ermudung der Aktorfedern gelegt.

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