FEEDSTOCK FLOW CHARACTERIZATION AND PROCESSING OF POROUS NITI BY METAL INJECTION MOULDING (MIM)

Porous NiTi alloy with a nearly three-dimensionally interconnected pore structure has been successfully fabricated by a transient liquid phase sintering following the metal injection moulding (MIM) process, using a mixture of Ni and Ti elemental powders. The elemental Ni and Ti powders mixture was mixed with a binder system, comprised mainly polyethyleneglycol (PEG) in an alternative technique using a speed mixer, principally incorporating a dual asymmetric centrifuge (DAC). The powder-binder mixture was then characterized using a capillary rheometer at various temperatures and shear rates. It was found that the feedstock exhibited pseudo-plastic behaviour, which is favourable for the MIM process. A temperature range of 120 degrees C -130 degrees C was considered as the optimum operating condition for the injection moulding processing. The parts were moulded into cylindrical shapes, leached in warm water (60 degrees C for 10 hours), thermally debound in argon and subsequently sintered in a vacuum furnace at four different temperatures ranging from 950 degrees C to 1250 degrees C. All samples underwent expansion in both diameter and height after water leaching and sintering. The XRD results showed that increasing the sintering temperature resulted in a major fraction of the B-2 NiTi phase due to phase homogenization and subsequently decreased the amount of secondary phases such as NiTi2 and Ni3Ti. Besides that, the formation of the transient liquid phase during sintering enabled major fraction of pores to be developed with porosity and average size of 39 - 45% and 100 - 120 mu m, respectively. The porous parts produced have a great potential to be used as an implant in biomedical applications.