The production and preparation of cemented carbide includes three main processes of mixture preparation, forming and sintering. In order to make the alloy have the necessary strength and hardness, the bonding metal is added to the carbide in the form of fine powder, and other components are added if necessary. Ball mill or rod mill these materials for a certain period of time to make a homogeneous mixture with a certain particle size. Grinding is usually carried out in organic solutions such as alcohol and ethane. In addition to the mixture used for hot pressing, a forming agent (such as paraffin) must be added in the mixing process. The slurries obtained by grinding can be transferred to organic liquids by different drying methods, such as spray drying.
Cold pressing is the most common forming method, which is the process of confining the mixture into a compact with a certain density, strength, shape and size. Using a mold to make the final product shape, considering the size reduction during sintering, the size of the mold should be larger than the size of the final product. Ensuring the uniformity of the density of each part of the green compact, without delamination, cracks, uncompacted and other phenomena, is the key to restricting the forming technology. Two-way restriction or multi-directional restriction is often used in production.
Relative to cold pressing, another forming method is hot pressing. It is to load the mixed powder into a graphite mold and limit the forming at the sintering temperature. This method is generally used for large or some special products. Another way to produce large, special-shaped products is isostatic confinement. The powder is filled into a closed soft mold and then into the liquid in a closed pressure vessel. The powder is properly confined by the soft mold jacket by allowing the pressure rise in the liquid. Because the pressure acting on the powder is the same in all directions, the density of each part of the compact is uniform. Thermoplastic soft resin is the main material for making soft molds at present. The composition of the plasticizer can be adjusted to suit the manufacture of molds for different purposes.
Kneading and forming is suitable for the production of long and complex shape products of equal cross-section, such as pipes, rods and ribbons of cemented carbide, etc. The plasticizer has an important influence on the kneading process, which is characterized by a large increase and good adhesion. Plasticizers generally consist of solvents, organic binders and surfactants. It is an important part of kneading quality control to evenly blend the plasticizer into the mixing material. In the kneading process, in order to fully exhaust, it is best to carry out on a press with a vacuum system. The kneading pressure depends on the function of the plasticized powder. Poor kneading often causes defects such as cracks and poor density, and special attention should be paid to the quality of the kneaded blank.
The sintering of cemented carbide is liquid phase sintering, that is, it is carried out under the condition that the binder phase is in the liquid phase. The compacts are heated to 1350°C to 1600°C in a vacuum furnace. The linear shortening rate of the compact during sintering is about 18%, and the volume shortening is about 50%. The exact value of the shortening depends on the particle size of the powder and the composition of the alloy.
The sintering of cemented carbide is a complex physical and chemical process. This process includes plasticizer removal, degassing, solid phase sintering, liquid phase sintering, alloying, densification, dissolution separation and other processes. The compact forms a product with a certain chemical composition, organization, function and size under specific sintering conditions. These process conditions vary greatly according to different sintering equipment.
