Development of PEG based binders for metal injection moulding with special focus on titanium

Abstract

Metal Injection Moulding (MIM) combines the benefit of polymer injection moulding with powder metallurgy, and involves the moulding of a metal/alloy powder and polymeric binder system mixture using conventional plastic injection moulding. Thus, the resulting products have the subsequent strength of metal or alloy with the shape complexity of conventional injection moulding. In a nutshell, MIM is a cost-effective and high production rate fabrication technique that provides an excellent alternate for metal and alloys, which are otherwise expensive and hard to machine. Although titanium (Ti) and its alloys MIM (Ti-MIM) technology fits the former description, it has not seen significant breakthroughs as these materials can easily be contaminated by the binders during the debinding and sintering stages. The lack of industrial confidence in the Ti-MIM is likely due to the limited understanding of binder chemistry. As the same binder systems are often used for different powders in MIM, a poor binder may still lead to acceptable properties for less reactive metals/alloys, but for Ti-MIM it can be detrimental to the final mechanical properties. Water soluble binder systems are garnering increasing attention due to their environmentally friendly nature. Among these, binder systems based on polyethylene glycol (PEG) are most popular due to its commercial availability, high solubility in water and non-toxicity. However, PEG based binder systems are less commonly found in the MIM industry. Therefore, such systems have been given special attention in this thesis in order to fully integrate them into the current MIM industry. In this research, extensive study of water soluble PEG based binder systems is carried out. Rheological analysis, scanning electron microscopy (SEM), differential scanning calorimetry (DSC), thermal analysis and mechanical tests have been extensively used to evaluate binder systems. This study provides a useful guideline towards the Ti-MIM practice. The main findings are summarised below. (1) It is noted that the effect of PEG molecular weight on the rheological properties of the feedstock, as well as its water debinding behaviour, has not been well investigated. In this work, four molecular weights of PEG, i.e. 1500, 4000, 10,000 and 20,000 g/mol were selected and the PEG/polymethyl methacrylate (PMMA) based feedstocks were formulated with titanium metal powder. A systematic investigation of the rheological properties of the prepared feedstocks, and the study of the effects of shear rate and temperature on viscosity was reported. The water debinding behaviour of each feedstock was also investigated in terms of its debinding temperature and the PEG molecular weight. (2) Considering the role of the binder in metal injection moulding (MIM) process, it is important to have a good interaction between the metal powder and binder system in order to produce defect free green parts. Surfactants are often employed in the MIM process to enhance or create this interaction. To study the effect of different surfactants on the metal-and-binder interaction that has not been reported before, three different surfactants – stearic acid (SA), peanut oil and castor oil – were chosen to prepare PEG/PMMA based Ti feedstocks. It was found that castor oil as a surfactant results in excellent overall properties of the feedstock by enhancing metal-and-binder interaction. The reason for this higher interaction is due to the increased polar ester functional groups in castor oil that latch onto the metal powder surface via dipole-dipole attraction forces, thereby resulting in a better powder dispersion and a higher powder-binder interaction. This is a significant finding that will improve the mouldability of MIM feedstocks. (3) Irregular hydride-dehydride (HDH) Ti powders have poor rheological properties and a high initial content of impurities, therefore giving way to spherical powders, which provide better control of the moulding operation and good mechanical properties, particularly elongation. However, spherical powders are expensive and increase the cost of the final product. For components where elongation is not the main requirement, HDH irregular powder can be utilized. However, solids loading are kept low so as to accommodate poor rheological properties. To facilitate the use of irregular Ti-HDH powder in MIM in order to improve rheological properties, a new binder system was developed. The rheological behaviour of the feedstock based on the newly developed binder system, is presented and compared with the existing binder system. Rheological properties analysis showed that by using this new binder system, a higher solids loading can be employed. (4) Due to environmentally friendly nature, water soluble binder systems have received much attention in recent years. PEG and PMMA binder system is one such example and has been widely reported in the literature. In this thesis, a comprehensive investigation of the PEG/PMMA binder system was carried out. Feedstocks were made using a stainless steel 17-4PH powder and subsequent conventional and micro injection moulding (μMIM) processes were carried out. DSC and fracture surface analysis of moulded samples were performed for complete evaluation. It was found that despite great potential there are certain drawbacks associated with this binder system. The main problem was the formation of shrinkage voids during solidification. It is proposed that this binder system is more suitable for μMIM process. (5) Efforts were made to increase the workability of the PEG/PMMA binder system for conventional metal injection moulding (MIM) by adding a crystallization inhibitor, polyvinylpyrrolidone (PVP). After complete evaluation of moulded samples, it was found that by incorporating PVP into PEG/PMMA binder system, void free high quality MIM components can be produced whilst maintaining the clean nature of PEG/PMMA binder system. (6) To minimize impurity uptake during the thermal debinding process, a low decomposition temperature binder system was developed. PEG was used as the major component, while polymer Q was used as the backbone polymer. The decomposition temperature of polymer Q is 100 °C lower than that of the PMMA: a significant achievement in Ti-MIM. Thermal debinding and rheological properties analysis further demonstrated excellent results. On the other hand, solvent debinding properties were poor. The solvent debinding properties can be improved by incorporating little amount of PMMA into the binder system. In summary, water soluble binder systems had been investigated in this study with a special focus on a low decomposition temperature binder system for the Ti-MIM. However, it was difficult, if not impossible, to obtain excellent mechanical properties of Ti-MIM samples due to different processing constraints. Nonetheless, the binders developed in this study have great potential and can be used in the industry. This project also opens further questions for future investigations.