Tungsten Carbide and Metal Injection Molding

?What is tungsten carbide?

Tungsten carbide is one of the most popular materials used in modern industry. Tungsten carbide also referred to as hard metal, cemented carbide, and tungsten alloy, is made by powder metallurgy. It exhibits an excellent combination of high strength, and wear resistance in making hard metal the preferred material for wear parts and cutting tools in machining since decades ago. Combined with tungsten and carbon, tungsten carbide has more performances, such as high strength, corrosion resistance, and chemical stability. And it has a wide application nowadays, and it can be made into different products, such as tungsten carbide strips, tungsten carbide rods, tungsten carbide buttons, tungsten carbide burrs, and so on, for different applications.

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What is metal injection molding?

Metal injection molding is a newly developed technology to form metals, and alloys into the desired shape. Metal injection molding is a combination of conventional plastic injection molding and powder metallurgy. This process consists of four main steps. They are mixing, injection molding, debinding, and sintering. During the mixing process, the metal powder is mixed with a binder at a selected volume ratio to form a homogenous feedstock. The binder is the key component in metal injection molding that supplies the metal powder's flow ability and formability necessary for molding. The attained feedstock from the mixing step is molded to produce a green compact, and the binders hold particles together. During the debinding steps, the green compact is processed by partially removing the binder component to produce the brown compact. Finally, the sintering process is performed to give the required mechanical properties for the sintered product also known as the sintered body. Thus, the development and improvement of binders result in faster debinding procedures, cost reduction, and fewer environmental defects.

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The window of economic advantage in metal injection molded parts lies in complexity and volume for small-size parts. MIM materials are comparable to metal formed by competing methods, and final products are used in a broad range of industrial, commercial, medical, dental, firearms, aerospace, and automotive applications. Increased costs for traditional manufacturing methods inherent to part complexity, such as internal/external threads, miniaturization, or identity marking, typically do not increase the cost in a MIM operation due to the flexibility of injection molding.

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Sintering Process

Liquid phase sintering

Liquid phase sintering (LPS) is applied to alloys and composites that melt over a range of temperatures. In the typical situation, the solid grains are soluble in the liquid. This solubility causes the liquid to wet the solid, providing a capillary force that pulls the grains together. At the same time, the high temperature softens the solid, further assisting densification. High – diffusion rates are associated with liquids, giving fast sintering or lower sintering temperatures.?

The LPS is ideal for densifying hard materials that cannot be fabricated using other manufacturing approaches. The WC-Co system is a prime example, where the eutectic at 1310 oC enables the bonding of micrometer size WC grains into a dense component, such as a drill or cutting insert. However, the common form of LPS is persistent LPS, where at the sintering temperature the solid is soluble in the liquid. On cooling, the liquid solidifies to produce a composite microstructure with tailored properties.?

After LPS, the microstructure consists of the solid grain with a solidified liquid network, and?

possibly residual pores are retained for lubrication, frangibility, or filtration attributes. Thus, liquid phase sintered microstructures exist in several variants with differences in the amount, size, shape, and distribution of the phases. Accordingly, substantial performance differences result, especially in properties such as hardness, strength, and elastic modulus. This is especially true for the WC – Co cemented carbides. The study of LPS focuses on linking composition, processing, and properties, with recent attention to improved dimensional precision. The glue between these factors is in the microstructure. A homogenous green structure greatly improves the LPS response. The most effective is the placement of the liquid phase on the interface between the solid grains.

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Microwave sintering

Microwave sintering (MS), as a novel technology, has been employed in powder metallurgy for the last few years. It has several advantages, including volumetric heating, non-thermal effect, and selective heating over the conventional method. These typical characteristics are beneficial to prepare materials such as accelerated heating rate, shortened processing cycle, high energy efficiency, and environmentally friendly. The microstructures can be improved greatly in terms of fine grain size, and uniform cobalt distribution for WC-Co alloys, which can enhance the mechanical properties.?

In conventional sintering processing, the vacuum was employed to avoid the atmosphere?

effect on carbon content in WC – Co alloy. Nevertheless, vacuum condition is difficult to achieve in microwave furnaces because of arc discharge at higher temperatures (≥ 1400 oC). Therefore, the atmosphere such as Nitrogen, N2, Argon, Ar, and Hydrogen, H2 are widely employed in microwave sintering. It can be seen that the sintering atmosphere has an important effect on the microstructures. The carbon content in cemented carbide plays an important role in the morphologies and microstructures. Moreover, the carbon activity in the microwave furnace chamber is expected to be extremely sensitive because of many factors such as moisture level, purity of the protective atmosphere, oxygen content in raw materials, etc.

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