Powder metallurgy is a metalworking process for forming precision metal components from metal powders by compacting them in a die.
The following are the basic steps of powder metallurgy:
- Power production: Numerous processes can produce metallic powders: grinding, electrodeposition, comminution, atomization, chemical reduction, etc. In atomization, a liquid metal stream is produced by injecting molten metal through a small orifice and the steam is broken up by jets of inert gas, air or water. The powder is also produced by reduction of metal oxides using hydrogen or carbon monoxide, as reducing agents. Mechanical comminution involves crushing, milling in a ball mill, or grinding brittle or less ductile metals into small particles.
- Powder Mixing: The process of mixing includes mixing various metal powders are thoroughly intermingled. This is carried out in batch mixers. The temperature during mixing affects the friction between powder particles. With increasing temperature, the friction coefficient between most materials increases and the flow of powders is impaired.
- Compacting: A controlled amount of the mixed powder is introduced into a precision die, and then it is pressed or compacted at room temperature and pressure in the range 100 Mpa to 1000 Mpa. In doing so, the loose powder is consolidated and densified into a shaped model. The model is called green compact. As it comes out of the die, the compact has the size and shape of the finished product. The strength of the compact is just sufficient for in-process handling and transportation to the sintering furnace.
- Sintering: Sintering involves heating of the green compact in a protective atmosphere furnace to a suitable temperature below the melting point of the metal. Typical sintering atmospheres are endothermic gas, exothermic gas, dissociated ammonia, hydrogen, and nitrogen. Sintering is responsible for producing physical and mechanical properties by developing a metallurgical bond among the powder particles. It also serves to remove the lubricant from the powder, prevents oxidation, and controls carbon content in part.
Porosity is a unique and inherent characteristic of powder metallurgy that can be exploited to create special products by filling the available pore space with oils, polymers, or metals. This is categorized as impregnation and infiltration. Impregnation is the process in which oil or other fluid is permeated into the pores of a sintered part, for example in oil-impregnated bearings, gears, and similar components. Infiltration is an operation in which the pores of the powder metallurgy part are filled with molten metal using capillary action.
Some prominent powder metallurgy products are as follows:
- Filters: Powder metallurgy filters have greater strength and shock resistance than ceramic filters. Fiber metal filters, having porosity up to 95% and more, are used for filtering air and fluids.
- Cutting Tools and Dies: Cemented carbide cutting tool inserts are produced from tungsten carbide powder mixed with a cobalt binder.
- Machinery Parts: Gears, bushes, and bearings, sprockets, rotors are made from metal powders mixed with sufficient graphite to give the product desired carbon content.
- Bearing and Bushes: Bearing and bushes to be used with rotating parts are made from copper powder mixed with graphite.
- Magnets: Small magnets produced from different compositions of powders of iron, aluminum, nickel, and cobalt have shown excellent performance, far superior to that cast.