Papers by Author: Peng Cao

Abstract: Impurity control remains to be a challenge to titanium metal injection moulding (Ti-MIM). Much attention has been paid to polyethylene glycol (PEG) based binder systems due to the eco-friendly and water-soluble feature of PEG. In this study, a new easy-to-debind PEG/polypropylene carbonate (PPC)-based binder system (76% PEG+17% PPC+3% polymethyl methacrylate (PMMA)+2% stearic acid (SA)+2% polyvinyl acetate (PVAc)) was developed. The rheological properties of the feedstocks prepared with the binder system in different proportions were assessed. Debinding behaviours of the moulded samples and impurity contents of oxygen (O), carbon (C), and nitrogen (N) of the thermal debound specimens were investigated as well.

Abstract: Titanium alloys have been widely used for medical implants due to their good biocompatibility and excellent corrosion resistance. 60NiTi, an intermetallic nickel-titanium alloy containing approximately 60 wt.% Ni and 40 wt.% Ti, is a promising material for medical components such as implants and prostheses. 60NiTi is hard with good biocompatibility, highly corrosion resistant and has relatively low stiffness. In this study, conventional press-and -sinter method was employed to produce porous 60NiTi parts suitable for general bone replacement applications such as spinal and cranial inserts. The effect of solution treatment in a non-protected furnace and water quenching on the mechanical and microstructural properties of 60NiTi were investigated. It was found that this procedure produces a hard integral ceramic layer, a complex mixture of nickel and titanium oxide compounds, on the surface and around the pores of 60NiTi parts. Results showed that this heat treatment procedure causes the embrittlement of the parts due to an increase in oxide content. However, the produced ceramic surface can also enhance the resistance to corrosion, which is beneficial from a biocompatibility point of view.

Abstract: NiTi is characterized as a shape memory alloy that has found interesting applications from aerospace to biomedical engineering. The use of NiTi in biomedical applications is due to its excellent biocompatibility, shape memory and pseudoelastic properties. These properties make NiTi an excellent candidate for many functional designs in biomedical fields. However, difficulties in manufacturing and processing of this alloy are significant hindrance to widespread applications. Advances in additive manufacturing (AM) such as selective laser and electron beam techniques have provided opportunities in manufacturing complex shaped NiTi parts. In this research paper, we demonstrate manufacturing of NiTi parts using a selective electron beam melting (SEBM) technique. Complete evaluation of physical, chemical and mechanical properties was carried out to determine the suitability of SEBM process. Differential scanning calorimeter (DSC), X-ray diffraction (XRD), and metallographic analyses were employed for the thermal and structural characterizations. The obtained results suggest that it is imperative to, and challenging to control the additive manufacturing process in order to obtain the desired microstructures and avoid unwanted texture. An exhaustive heat treatment of the samples after SEBM process might also be necessary.

Abstract: Metal matrix composites (MMCs) are the new generation materials that combine both the metallic properties (ductility and toughness) and ceramic characteristics (high strength and modulus), leading to higher strength in shear and compression, at higher service temperatures. Titanium matrix composites possess light weight, high strength and good corrosion resistance and are used as structural materials in automobiles and aerospace industries. In the present study, in situ Ti-TiB composites were fabricated by reinforcing (2, 5, 10 and 20 wt. %) TiB₂ powder (mean size <10 microns) into titanium powder (mean particle size ~26.58 μm) and subsequently consolidated by vacuum sintering at 1300 °C for 3 h. X-ray diffraction, scanning electron microscopy (SEM) and density measurements were carried out to characterize the prepared composites. The results showed that all compositions led to high density composites, and the hardness of the composites increased with an increase in the amount of reinforcement. The mechanism of vacuum sintering is yet to be understood in the consolidation of composites and the detailed evolution of microstructure needs to be analysed.

Abstract: As the world is moving towards green manufacturing, there is an increasing demand for the use of clean and environmentally friendly binder systems in metal injection moulding (MIM) industry. One example of these developed binders is polyethylene glycol (PEG) - polymethyl methacrylate (PMMA) based system. We have systematically evaluated and optimized this binder system, and reported some interesting new results. In this article, we reported the effect of PEG molecular weight on rheological properties of the feedstock and its water debinding behaviour. We also investigated the effects of different surfactants on MIM feedstock rheological and mechanical properties, and identified a potential surfactant that enhances compatibility between the binder components and metal powders. Furthermore, we reported an interesting problem – ‘voids formation’, which is associated with PEG crystallization. To minimize this void formation a crystallization inhibitor is incorporated in the PEG/PMMA system, thereby eliminating the void formation while maintaining the clean nature of this system. This paper is concluded with some new thoughts with regard to binder design.

Abstract: In this study, hydroxyapatite (HA) coatings on Ti6Al4V substrate were deposited using an ion beam sputtering technique. Owing to its medical applications, the crystalline phases present in the HA must be controlled. This study investigated the effect of post-deposition heat treatment at different temperatures and evaluated the microstructure of the HA coatings and their behaviours in simulated body fluid (SBF). The post-deposition treatment of the as-deposited samples was carried out in an air-circulated furnace at a temperature between 300 ⁰C and 600 ⁰C. The XRD patterns reveal that the minimum temperature to transform the HA coating from amorphous to crystalline phase is 400 ⁰C. A higher temperature at 600 ⁰C leads to a growth of the crystalline HA phases. Fourier transform infrared spectroscopy (FTIR) measurements show the existence of hydroxyl and PO-bonds in all coatings and the amounts varied with temperature. Atomic Force Microscopy (AFM) study suggests that the nanostructured crystalline HA starts to grow at 400 ⁰C and becomes more obvious at a higher temperature of 600 ⁰C. The simulated body fluid (SBF) test reveals that better apatite formation with post deposition heat treatment at 600 ⁰C would potentially enhance the formation of new bone (osseointegration).

Abstract: The development of grain refinement technologies began in the 1930s in response to the need to improve the mechanical properties of as-cast components. Commercial grain refining technologies were developed by industrial and experimental trials often with good success including the production of effective master alloys. In parallel, researchers developed theories to explain the mechanisms of refinement in order to improve the efficiency of refiners and develop new better performing grain refining master alloys. This research continues today. Here we briefly present the history of these developments. It is shown that many developments in our understanding were based on assumptions arising from experimental and industrial observations and the prevailing solidification theories of the time.

Abstract: It has been two decades since the first report on titanium metal injection moulding (Ti-MIM) was published in 1988. The top secret for MIM practitioners and feedstock suppliers is the binder, which largely determines the success of Ti-MIM processes. As a consequence of its high affinity to oxygen and carbon, Ti injection moulding requires a more stringent control of contamination and therefore a different design strategy must be used for Ti-MIM binders, as compared to that for other metals injection moulding. This paper first discusses key requirements for Ti-MIM binder and then provides some case studies to discuss several major binders systems, followed by an outlook of the future binder development.

Abstract: This paper reports the synthesis of porous titanium with a nominal composition of Ti-6 wt%Al-4wt%V through a press-and-sinter process. Blended elemental (BE) mixtures of Ti and master alloy Al-40V powders were uniaxially pressed and sintered in vacuum. Porosity of the sintered samples was determined in the range of 23vol. % to 37vol. % by the Archimedes method. Tensile strengths were found to range from 73 to 147MPa and Young’s moduli of the sintered samples varied from 3.4GPa to 13GPa. Both tensile strength and elastic modulus decreased with increasing porosity. Electrochemical assessment of the sintered porous samples showed deteriorated corrosion resistance, as compared to 95% dense Ti-6Al-4V prepared by sintering pre-alloyed powder. The challenge of using blended elemental powder sintering to fabricate porous Ti-6Al-4V alloys is discussed.

Abstract: A water soluble binder system is used to prepare Ti-6Al-4V and NiTi pre-alloyed powder feedstock. The binder dissolution and transport kinetics through the porous powder skeleton are studied for various powder morphologies and powder loadings from 60 to 69.5 vol.%. The binder removal behaviours are evaluated with different debinding time intervals. The focus of this work is to investigate the influences of shaping pressure, specimen thickness and water bath temperature on the binder extraction behaviour.