New clean binder system developed for titanium Metal Injection Moulding

The use of the Metal Injection Moulding process to produce titanium components has been extensively studied in recent years due to the benefits that MIM offers as a net-shaping process for small, intricate shaped components. These benefits include high material utilisation and lower production costs compared with conventional processes such as CNC machining and casting. However, despite the many advantages that the MIM process offers, its adoption for mass production of Ti components has been limited. One reason is said to be the challenge of controlling the interstitial elements such as carbon and oxygen in green Ti-MIM compacts during the debinding stage of the MIM process. This is especially the case for binder systems used in Ti-MIM where commercial backbone polymer binders such as polypropylene (PP), polyethylene (PE), and poly(methyl methacrylate) (PMMA), have a decomposition temperature above 400°C, and it is well-known that at above 260°C, binders may introduce impurities during the thermal debinding of the titanium parts.

A group of researchers at the University of Auckland and University of Waikato in New Zealand and the Kunming University of Science and Technology in China, have undertaken work to develop a low decomposition temperature and environmentally friendly binder system for Ti-MIM. The new binder system is based on polypropylene carbonate (PPC) and polyoxymethylene (POM), which is said to offer lower decomposition temperatures during thermal debinding with effective mitigation of impurities such as O₂ and C. The results of this research were published in a paper entitled: ‘Formulating titanium powder feedstocks for metal injection moulding from a clean binder system’ by K Lim, M D Hayat, K D Jena, Z Yuan , L Li, and P Cao, Powder Technology Vol 433, 2024, 119214, 7 pp.

The authors reported using a commercial purity gas atomised Ti powder with mean particle size D50 = 45 μm. The maximum interstitial contents in the Ti powder are: 0.122% O₂, 0.003% C and 0.008% N (in wt.%). The newly developed binder system presented in the paper is composed of polyethylene glycol (PEG) as the primary component removed at the solvent debinding stage, with polypropylene carbonate (PPC) and polyoxymethylene (POM) as the major and minor backbone components which are removed during thermal debinding, and stearic acid (SA) as a surfactant. The compositions of the binders studied are listed in Table 1. Ti powder loading in the feedstock was 63 vol.%.

The injection moulding parameters were: injection pressure 19 MPa, holding pressure 16 MPa, injection time 5 s, and injection temperature and mould temperature were 170°C and 30°C, respectively. All feedstocks were said to exhibit pseudoplastic behaviour, indicating rotation and re-arranging of polymeric binders and metallic particles within the feedstock along the flow direction. This facilitates inter-particle motion, making the feedstock well suited for MIM.

The authors stated that PEG was incorporated into the PPC/POM feedstock to compensate for the poor shape retention of the PPC/POM binder system by allowing the formation of a network of pore channels within the compact structure during the initial water debinding stage. This porous network then facilitates the efficient removal of residual PPC/POM binder during subsequent thermal debinding.

The results showed that increasing the PEG content leads to a higher binder removal rate, while adding POM enhances shape retention during thermal debinding and PCC, as the predominant backbone binder, enhances the adhesion between the titanium powder and the binder system. The optimal PEG/PPC/POM binder system composition was identified as 40/48/10% w/w, respectively, offering a higher PEG removal rate at a shorter debinding time and improved shape retention. However, all feedstocks tested were successfully injection moulded without any defects.

The injection moulded Ti green parts were first subjected to water debinding at 30°C for different intervals up to 21 h to remove the majority of PEG. This was followed by thermal debinding of the PPC/POM under flowing argon gas. The authors stated that the advantages of using PPC and POM are their low to moderate decomposition temperatures and clean decomposition behaviour. PPC and POM degrade entirely in a single step at 275°C and 370°C, respectively, which is below 400°C. Microstructural observations in the debound samples after thermal debinding (Fig. 1 (d-f)) showed the complete combustion of the PPC/POM binder, resulting in binder-free titanium compacts.

The authors concluded that the environmentally friendly binder system based on PEG/PPC/POM also resulted in Ti-MIM samples with admissible impurity levels, with oxygen content of 0.27 wt% and carbon content of 0.05 wt%. These levels comply with the stringent requirements outlined in ASTM F2989 for Grade 3 titanium products.

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