Development of water-soluble PEG-based binder systems for Ti Metal Injection Moulding

Abstract

Titanium (Ti) has attracted wide attention for decades since it exhibits outstanding properties for various applications. However, due to expensive raw Ti and costly processing, its applications are still limited – only in those applications where the performance can justify the costs. Therefore, Metal Injection Moulding (MIM) has received increasing attention in recent decades to prepare Ti products as a cost-effective and competitive processing route. Nevertheless, impurity control is still a great challenge to Ti Metal Injection Moulding (Ti- MIM). Binder system design and the corresponding debinding techniques are crucial for the impurity control of Ti-MIM. Hence, developing new binder systems specifically for Ti-MIM is of great importance. Water-soluble binder systems are environmentally friendly. Much attention has been paid to binder systems based on polyethylene glycol (PEG) due to the commercial availability, high solubility in water and non-toxicity. Although PEG-based binder systems have been reported in the literature for years, their applications in the Ti-MIM industry are rarely seen. Furthermore, few studies on specific interactions among the particular binder system components and the effects of such interaction on Ti-MIM feedstocks are available. Therefore, this work aims to develop an easy-to-decompose PEG-based binder system for Ti- MIM and evaluate polymeric interactions between different binder components to promote the applications of water-soluble binder systems in the Ti-MIM industry. In this project, an extensive study of water-soluble PEG-based binder systems was carried out. Rheological analysis, thermal analysis (thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC), mechanical tests, impurity measurements (oxygen (O), carbon (C) and nitrogen (N)), and microstructural analysis (optical microscopy and scanning electron microscopy (SEM)) have been extensively used to evaluate binder systems. Importantly, the macromolecular interactions between different binder components were investigated with Fourier-transform infrared (FTIR) spectroscopy. Furthermore, an in situ polarised optical microscope was employed to observe the PEG crystallisation process and reveal the mechanism of void formation in the PEG/polymethyl methacrylate (PMMA) binder systems and how to eliminate these voids. This study may benefit the development of environment-friendly and clean binder systems suitable for Ti-MIM. The main findings are summarised below: 1) The biodegradable polymer, polypropylene carbonate (PPC), was proven a promising backbone polymer for the PEG-based binder systems. Although PPC cannot act as the backbone component alone, incorporating a small amount of PMMA into the PEG/PPC binder system can effectively improve the green strength and provide adequate dimensional stability for the subsequent thermal processing. Therefore, a PEG/PPC/PMMA binder system was proposed and investigated for Ti-MIM. In this binder system, the development of H-bonding interactions through the C=O group from PMMA and PPC in the ternary binder blend resulted in enhanced green strength and improved thermal stability. Ti feedstock made from this binder system obtained good rheological properties and feedstock homogeneity. Furthermore, the resulting sintered samples showed low impurity (O, C and N) contents and good mechanical properties fulfilling ASTM F-2989 Grade 3 requirements. 2) The promising PEG/PPC/PMMA binder system was further improved by the adding a small amount of polyvinyl acetate (PVAc). The small amount of PVAc effectively improved the rheological properties and homogeneity of Ti feedstock and the green strength of the injection moulded samples. The resulting sintered samples made from this improved binder system showed excellent mechanical properties (98% of relative density, 549 MPa of UTS and 14.7% of elongation rate) and low impurity (O, C and N) contents. The optimised binder system D (76wt% PEG + 17wt% PPC + 3wt% PMMA + 2wt% stearic acid (SA) + 2wt% PVAc) provided good mechanical properties while maintaining the environment-friendly and clean nature of the PEG/PPC based binder system. 3) PVAc addition was proven a simple yet effective way to solve the issue of voids formation in MIM green samples associated with the PEG/PMMA binder systems. The direct evidence of the void formation of the PEG/PMMA binder system was found for the first time using in situ polarizing optical microscopy. The mechanisms of the PEG/PMMA binder system void formation and the PEG/PMMA/PVAc binder system void elimination were investigated. The voids were successfully eliminated by incorporating PVAc into the binder system. PVAc addition into the PEG/PMMA binder system leads to complex interactions between the three components. The three components interact in a way that the voids created due to PEG/PMMA interactions are filled up by molecular chains of PVAc. Titanium feedstock made from this novel binder system had good rheological properties, which resulted in a defect-free injection moulding process. The green samples had excellent shape retention during the subsequent water debinding process. As a result, the sintered articles showed good mechanical properties. The designed binder system maintains the environmentfriendly nature of the PEG-based systems without compromising quality. In short, PPC might be a promising option for Ti-MIM due to its low decomposition temperature and no decomposition residues. Although PPC cannot act as a backbone component alone, with a small amount of PMMA and PVAc, a promising easy-to-decompose and clean PEG/PPC based binder system was developed. Furthermore, PVAc addition was effective to solve the issue of voids formation in MIM green samples associated with the PEG/PMMA binder system. Hence, another easy-to-decompose and feasible PEG/PMMA binder system was developed with the incorporation of PVAc.