What are the different manufacturing processes and how do they differ?
1 Base materials
For the manufacture of tungsten carbide, the granulate (hard material and binder) and the pressing agent are mixed wet in ball mills. The pressing agent is used to hold the compacts together. A pourable powder is then created via spray drying.
2 Shaping
There are four basic methods for shaping: isostatic pressing, uniaxial pressing, injection molding and extrusion.
Shaping
There are four basic methods for shaping: isostatic pressing, uniaxial pressing, injection molding and extrusion.
Isostatic pressing
Cold isostatic pressing is suitable for simple shapes and/or larger geometries. The powder is placed in a rubber mold, sealed tightly and then pressure of around 2,000 bar is applied from all sides in a liquid.
Extrusion
Extrusion is used for simpler, mostly symmetrical components. For this process, it’s not a powder but rather a viscous mass that is required as the base material. In this process, the granulate is mixed and kneaded with thermoplastic plasticizers (wax) or organic polymers. The mass is then pressed through a mold die under high pressure using a screw press or hydraulic press.
Injection molding
Injection molding of hard metal is mainly used for complicated components in large quantities. Since a viscous mass is also required for injection molding, the preparation of the base material is the same as for extrusion. The mass is then pressed into the injection molding tool under high pressure. After shaping, the state of the component is referred to as a “green body”, whereby the component is still relatively soft and can be further processed via conventional machining such as milling, turning or drilling.
3 Sintering
Sintering takes place after shaping and is usually carried out in a vacuum furnace. During this process, the components go through a defined temperature program. After sintering and post-compacting, the hard metal component has reached its full hardness and further processing must be carried out via hard machining.
The temperature program: first, the temperature is slowly increased to 500°C to burn off the plasticizers, followed by a further slow increase to remove the carbon monoxide. With a further increase in temperature to 1,350–1,500°C, the melting process takes place, whereby the binder (cobalt or nickel) wets the tungsten carbide crystallites. Now the shrinking process of up to 25% begins. As soon as a cohesive melt is achieved, slow cooling can begin.
4 Hot isostatic pressing (HIP)
Most of the pores in the component can be removed during sintering. However, in order to further increase the density and thus the material quality, an overpressure can be applied during sintering using argon gas and the densification process can be improved. This process is called hot isostatic pressing.
5 Hard machining
When it comes to components made of tungsten carbide, there are often strict requirements for dimensional accuracy and surface quality. Since the components shrink by around 25% during the sintering process, precise dimensional accuracy cannot be maintained, and the surface quality is inconsistent as a result of the sintering process. It is therefore often necessary to machine the functional surfaces of the component in the hard state.
Due to the high hardness of the tungsten carbide, only a few manufacturing processes can be used economically for hard machining, such as grinding, honing, lapping, spark erosion and laser machining. During grinding and honing, material is removed using a bound diamond fragment without a defined cutting edge. There are many different types of grinding, such as cylindrical, face, shape and centerless grinding. In contrast to grinding, honing involves working with more pressure at a lower speed. During lapping, a loose diamond particle is mixed with oil and a – usually small – amount of material is removed through pressure over a longer period of time. This allows very precise tolerances to be maintained, however.