S. A. M. Tofail, J. Butler, P. Tiernan, A. A. Gandhi, University of Limerick, Limerick, Ireland
Spark plasma sintering (SPS) is a relatively new cost-effective processing route where powder is compacted and consolidated by passing high current, low voltage spark pulsed DC current directly through the powder compact and graphite die under uniaxial pressure. A very high heating and cooling rate is possible which ensures the powders are consolidated and densified without coarsening. This fast processing also limits diffusion mechanisms, making possible the formation and the stabilization of microstructures which are not attainable by other common powder metallurgy routes such as Hot Isostatic Pressing. Hence, SPS is ideal for producing new materials without having to go down the more expensive ingot melting route. For this study the shape memory alloy Nitinol is spark plasma sintered and extruded to wire. Equaiatomic NiTi powders were sintered into small billets with less than 2 % porosity, put into a protective sleeve and isothermally extruded into 3 mm diameter wire. Characterisation by Differential Scanning Calorimetry (DSC), Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), Energy Dispersive X-Ray (EDX), microhardness and tensile testing showed that the extruded wire has similar properties to commercially produced NiTi wire with all remaining pores being submicron. The results show that SPS followed by extrusion is a quick and efficient method of obtaining a commercial grade NiTi material. This result opens the possibility of developing new NiTi alloy materials in SPS promoting a cost effective research route. SPS on an industrial scale is forecast to be a cost effective processing route in the near future, so research work with SPS is currently becoming more prevalent.
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