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Powder injection moulding of NiTi shape memory materials
Technical Paper: PIM International, Vol.2 No. 1 March 2008, pages 59-62, 1470 words
University of Applied Sciences Western Switzerland, Design & Materials Unit, CH-1950 Sion, Switzerland
Abstract
NiTi parts have been produced by a new powder injection moulding route using a feedstock consisting of elemental TiH2 and Ni powder blended with an organic binder. The microstructure has been characterised by scanning electron microscopy coupled with energy-dispersive X-ray analysis. The sintered parts are porous except a dense surface layer. The bulk is NiTi with some Ti-rich domains. The surface layer is mostly NiTi with a dispersion of Ni-rich phases and some oxides and carbides. The presence of a martensitic transformation in the temperature range above room temperature has been shown by differential scanning calorimetry. Shape memory effects of more than 4% have been achieved.
Introduction
NiTi alloys are used in actuators and medical devices because of their unique shape-memory properties [1,2]. More widespread application of NiTi is still limited by their poor machinability and high processing cost. Injection moulding would provide near-net shape manufacturing, minimising the need for machining. So far only few investigations of NiTi powder injection moulding have been performed [3-6]. Those using elemental powders for feedstock are even more limited [7]. This last route is however interesting from an economic point of view since it avoids the delicate production of the pre-alloyed powder.
The purpose of the present paper is to examine the feasibility of manufacturing NiTi by powder injection moulding using elemental powder for feedstock. For the first time, titanium hydride (TiH2) powder is used for the fabrication of NiTi parts by injection moulding. Titanium hydride is interesting since it is less reactive and more easily available as fine powder with the required purity [8,9].
Further sections of this article include:
- Experimental
- Results and discussion
- Acknowledgments
- References
Figures and Tables:Fig. 1 Scanning electron microscopy images of starting powders a) TiH2 b) Ni Fig. 2 Tensile test specimen (dimensions in mm) Fig. 3 Temperature program for thermal debinding and sintering Fig. 4 Schematics of the device for measurement of shape memory effects and typical result for a shape memory material Fig. 5 NiTi green and sintered parts Fig. 6 Optical micrograph of NiTi sintered part (a) bulk (b) surface layer Fig. 7 SEM micrograph of sintered NiTi specimen cross-section (a) general view (b) bulk (c) surface layer microstructure Fig. 8 Differential scanning calorimetry of sintered NiTi Fig. 9 Tensile strain vs. temperature of sintered NiTi under various applied stresses |
