Cemented carbide is the most widely used class of high-speed machining tool materials. These materials are produced by powder metallurgy and consist of hard carbide (usually tungsten carbide WC) particles and a softer metal bond. At present, there are hundreds of WC-based cemented carbides with different compositions, most of them use cobalt (Co) as a binder, nickel (Ni) and chromium (Cr) are also commonly used binder elements, and other some alloying elements.
Tungsten carbide powder is obtained by carburizing tungsten (W) powder. The characteristics of tungsten carbide powder (especially its particle size) mainly depend on the particle size of raw tungsten powder and the temperature and time of carburizing. Chemical control is also critical and the carbon content must be kept constant (close to the stoichiometric value of 6.13% by weight). In order to control the particle size of the powder through the subsequent process, a small amount of vanadium and/or chromium can be added before carburizing. Different downstream process conditions and different final processing uses require specific combinations of tungsten carbide particle size, carbon content, vanadium content and chromium content, and through changes in these combinations, various tungsten carbide powders can be produced. For example, ATI Alldyne, a manufacturer of tungsten carbide powder, produces a total of 23 standard grades of tungsten carbide powder, and the variety of tungsten carbide powder customized according to user requirements can reach more than 5 times that of standard grades of tungsten carbide powder.
When mixing and grinding tungsten carbide powder and metal binder to produce a certain grade of cemented carbide powder, various combinations can be used. The most commonly used cobalt content is 3% - 25% (by weight), while nickel and chromium need to be added when it is necessary to enhance the corrosion resistance of the tool. In addition, the metal bond can be further improved by adding other alloy components. For example, adding ruthenium to WC-Co cemented carbide can significantly improve its toughness without reducing its hardness. Increasing the content of binder can also improve the toughness of cemented carbide, but it will reduce its hardness.
Reducing the size of tungsten carbide particles can increase the hardness of the material, but the tungsten carbide particle size must remain constant during the sintering process. During sintering, tungsten carbide particles combine and grow through a process of dissolution and precipitation. In the actual sintering process, in order to form a fully dense material, the metal binder must become liquid (called liquid phase sintering). The growth rate of tungsten carbide particles can be controlled by adding other transition metal carbides, including vanadium carbide (VC), chromium carbide (Cr3C2), titanium carbide (TiC), tantalum carbide (TaC) and niobium carbide (NbC). These metal carbides are usually added when tungsten carbide powder is mixed and milled with a metal bond, although vanadium and chromium carbides can also form when carburizing tungsten carbide powder.
The processing conditions of mixing and milling tungsten carbide powder and metal bond are also crucial process parameters. The two most commonly used milling techniques are ball milling and attrition. Both processes result in a homogeneous mix of milled powders and a reduction in particle size. In order to make the pressed workpiece have sufficient strength, keep the shape of the workpiece, and enable the operator or manipulator to pick up the workpiece for operation, it is usually necessary to add an organic binder during grinding. The chemical composition of this binder can affect the density and strength of the pressed workpiece. In order to facilitate the operation, it is best to add a high-strength binder, but this will result in a lower compaction density and may produce hard lumps, causing defects in the final product.
After milling is complete, the powder is usually spray-dried to produce free-flowing agglomerates held together by an organic binder. By adjusting the composition of the organic binder, the fluidity and filling density of these briquettes can be tailored as desired. The particle size distribution of the agglomerate can also be further tailored by screening out coarser or finer particles to ensure good flowability when loaded into the mold cavity.
