Powder metallurgy is a process which involves converting powder into a solid object. Usually, the powder is a type of metal, such as iron, and the object to be made is usually shaped with a die or cast. To form and fabricate metal objects through powder metallurgy, extremely high pressures and high temperatures are required. Sintering, a heating process, and metal injection molding are commonly used techniques in powder metallurgy.
The first step in powder metallurgy is converting raw material to a powder form. Crushing, grinding, and using chemical reactions are common ways to produce powder. Atomization is a technique in which the material is melted into a molten liquid and forced through a small nozzle or tube at high velocity. This causes the liquid to separate into individual droplets as it exits the tube. The droplets are collected and allowed to harden, resulting in fine, grain-sized particles.
Once the powder is produced, it is compacted in a die with high pressure. Usually, the powder is poured into the die, and a press plate is lowered to compress it. The resulting object is ejected from the die. Typically, the pressure ranges from 80 to 1,000 psi.
Another method for compressing the powder is called isostatic powder compacting. This technique utilizes a flexible mold that is situated within a pressure mold. The powder is placed in the flexible mold, while a machine delivers pressurized liquid or gas to the pressure mold. The pressure ranges from 15,000 psi to 40,000 psi. This method doesn't require lubricants when the powder is compacted, but standard die compacting does. Afterward, the object is strengthened by sintering.
Sintering is a heating process that binds individual metal powder particles together. It usually takes place in a sintering furnace, where the temperature is set just below the melting point of the metal. This method is commonly used in the manufacturing of ceramics. The process was patented by A. G. Bloxam in 1906, but there is evidence that the practice of fusing metal powders into solid objects has its origins in ancient times.
Some methods combine the powder compaction and the sintering process, such as hot isostatic pressing. The process typically involves the use of pressurized argon gas and temperatures between 900°F (480°C) and 2250°F (1230°C). The powder is simultaneously compacted and sintered, saving time and material. This method also produces thinner walls and larger parts when compared to standard powder compression methods.