Zugl.: Bochum, Univ., Diss., 2009

Die Geschichte der magnetischen Formgedächtnislegierungen (MFGL) ist im Kontext der Materialwissenschaften noch recht jung. In den letzten 30 Jahren sind dabei viele Erkenntnisse in der Forschung gewonnen worden. Der industrielle Durchbruch wiederum ist mit diesem Material noch nicht gelungen. Die existierenden Publikationen zur Auslegung von magnetischen Formgedächtnis- (MFG)-Aktoren sind in dieser Arbeit daher aufbereitet und systematisiert worden. Dabei stellt insbesondere die thermische Auslegung eine Wissenslücke dar, die entsprechende Grundlagenuntersuchungen erfordert. Aufbauend auf dieser Wissensbasis ist eine spezifische Entwurfsmethodik für MFGL entwickelt worden. Die Dimensionierung erfolgt zunächst mit analytischen Gleichungen, um eine zeit- und kosteneffiziente Vorgehensweise für Entwicklungsingenieure zu gewährleisten. Finite-Elemente-Methode-(FEM)-Software wird erst im zweiten Schritt zur Prüfung oder Optimierung eingesetzt. Außerdem kann mittels dieser Methodik auf unwirtschaftliche Iterationsschleifen und „trial and error“-Ansätze verzichtet werden. Abschließend ist die Methodik anhand eines Anwendungsszenarios verifiziert worden. Die abgeleiteten Anforderungen konnten dabei ohne Nachbesserungen oder weitere Iterationsschleifen erfüllt werden, so dass die Methodik eine strukturierte und erfolgreiche Entwicklung unterstützt hat.:1 Einleitung 2 Stand der Wissenschaft und Technik 3 Grundlagenuntersuchungen 4 Entwurfsmethodik für MFG-Aktoren 5 Zusammenfassung und Ausblick

A novel, environmentally friendly, fast, and flexible polishing process for Nitinol parts is presented in this study. Nitinol samples with both superelastic and shape memory properties at room temperature were investigated. The chemical contamination and surface roughness of superelastic Nitinol plates were examined before and after plasma electrolytic polishing. The shift in phase transformation temperature and tensile strength before and after the polishing process were analysed using Nitinol wire with shape memory properties. The obtained experimental results were compared to the data obtained on reference samples examined in the as-received condition. It was found that plasma electrolytic polishing, when the right process parameters are applied, is capable of delivering Nitinol parts with extremely high surface quality. Moreover, it was experimentally proven that plasma electrolytic polishing does not have a negative impact on functionality or mechanical properties of polished parts.

Additive manufacturing of Nitinol is a promising field, as it can circumvent the challenges associated with its conventional production processes and unlock unique advantages. However, the accompanying surface features such as powder adhesions, spatters, ballings, or oxide discolorations are undesirable in engineering applications and therefore must be removed. Plasma electrolytic polishing (PeP) might prove to be a suitable finishing process for this purpose, but the effects of post-processing on the mechanical and functional material properties of additively manufactured Nitinol are still largely unresearched. This study seeks to address this issue. The changes on and in the part caused by PeP with processing times between 2 and 20 min are investigated using Nitinol compression springs manufactured by Laser Beam Melting. As a benchmark for the scanning electron microscope images, the differential scanning calorimetry (DSC) measurements, and the mechanical load test cycles, conventionally fabricated Nitinol springs of identical geometry with a medical grade polished surface are used. After 5 min of PeP, a glossy surface free of powder adhesion is achieved, which is increasingly levelled by further polishing. The shape memory properties of the material are retained without a shift in the transformation temperatures being detectable. The decreasing spring rate is primarily attributable to a reduction in the effective wire diameter. Consequently, PeP has proven to be an applicable and effective post-processing method for additively manufactured Nitinol.

Pseudoelastic NiTi shape memory alloys exhibit different stress–strain curves and modes of deformation in tension vs. compression. We have recently shown that under a combination of compression and shear, heterogeneous deformation can occur. In the present study, we use digital image correlation to systematically analyze how characteristic features of the nominally uniaxial engineering stress–strain curves (particularly the martensite nucleation peak and the plateau length) are affected by extensometer parameters in tension, compression, and the novel load case of shear-compression. By post-experimental analysis of full surface strain field data, the effect of the placement of various virtual extensometers at different locations (with respect to the nucleation site of martensite bands or inhomogeneously deforming regions) and with different gauge lengths is documented. By positioning an extensometer directly on the region corresponding to the nucleating martensite band, we, for the first time, directly record the strain-softening nature of the material—a specific softening behavior that is, for instance, important for the modeling community. Our results show that the stress–strain curves, which are often used as a basis for constitutive modeling, are affected considerably by the choice of extensometer, particularly under tensile loading, that leads to a distinct mode of localized deformation/transformation. Under compression-shear loading, inhomogeneous deformation (without lateral growth of martensite bands) is observed. The effects of extensometer gauge length are thus less pronounced than in tension, yet systematic—they are rationalized by considering the relative impact of differently deforming regions.