Machining of Stent-Like Geometries In Thin NiTi-Sheets Using Waterjet Cutting

High-pressure waterjet cutting features superior cutting quality, flexibility and cost effectiveness, as compared to traditional thermal cutting methods like laser cutting. Sequential machining is minimized and it is an environmentally friendly process, because no gases or vapours are produced. Machined parts are practically free of burrs and no heat affected zones (HAS) are formed. Nearly all kinds of materials can be cut economically up to a thickness of 100 mm. As a result, waterjet cutting is an established alternative cutting method for the machining of thick sheets. However, laser cutting and electrical discharge machining (EDM) are the most commonly used fabrication processes of NiTi shape memory alloys (SMA). In the present study binary, pseudoelastic NiTi-sheets (380 x 80 mm, thickness: 0.1 mm) with an oxide free surface were purchased from Memry GmbH (Wheil am Rein, Germany). From these sheets, structures were cut out using a waterjet, which enabled us to find parameters, such as cutting speed, water pressure, etc., for the processing of NiTi. The structure size was sequentially reduced to that of commercially available stent geometries. Differential scanning calorimetry (DSC) was performed to determine the phase transformation temperatures in the as-received condition, after solution annealing and following the cutting process. The influence of waterjet cutting on the surface roughness of NiTi was examined using confocal laser scanning microscopy (CLSM). The microstructure was analysed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) before and after cutting. EDX-analysis was used to chemically characterize secondary phases.