Abstract:
Metal injection moulding is a low cost technique used to produce complex, near net shape components for titanium through a binder system that acts as a temporary vehicle for the shaping of the feedstock. Water soluble binders are favoured due to their non-toxic, environmentally-friendly characteristic during debinding. Polyethylene glycol (PEG) is a frequent primary binder component in the design of water soluble binder systems, along with polymethyl methacrylate (PMMA) as the backbone binder component. However, PEG tends to crystallise in the presence of PMMA, producing mechanically degraded and porous sintered metal components. This thesis studies the potential use of polyvinylpyrrolidone (PVP) as a crystallisation inhibitor in the PEG/PMMA binder system. In this present study, the PEG/PMMA/PVP binder-based feedstocks were prepared via a metal injection moulding route with a 67% solid volume loading. The effects of PVP incorporation into the PEG/PMMA binder were studied in detail: at 15 wt/wt, 20 wt/wt and 25 wt/wt %PVP of PEG feedstock mixtures. The effect of varying amounts of PVP on the feedstock homogeneity and rheological behaviour; as well as debinding, thermal degradation and mechanical properties were investigated and the results were compared with the pure PEG/PMMA binder-based feedstock. Microstructure characterisation, porosity, hardness and density analyses, as well as impurity testing following the tensile fracture of the sintered component was characterised using the optical microscope, ESEM, the Archimedes principle, macro-hardness tester and LECO impurity analyser. Results show that the feedstock with the most optimal mechanical properties is the 20 wt/wt% PVP of PEG binder mixture, which achieved an average elongation of 9.5% and average final density of 98%, thereby satisfactory to ASTM standards. Increasing amounts of PVP inhibits PEG crystallisation, however beyond 20 wt/wt% PVP of PEG, the relatively low amount of PEG in the binder mixture leads to feedstock separation, sub-standard tensile mechanical properties, as well as increased porosity and impurity values. The results of the modified binder developed in this study provides useful information on the behaviour of PVP as a PEG crystallisation inhibitor for titanium metal injection moulding, which is essential in developing sintered components of optimal mechanical properties. Keywords: Titanium, Metal injection moulding, PEG, PMMA, PVP, Crystallization inhibitor, Elongation, Density, Tensile, Porosity, Impurity, Homogeneity, Debinding.