What happens in powder metallurgy?
1. Derivation of powder metallurgy
Particles of the powder are mostly sized 10~20um (1um=10-6m). Three common production methods are:
(1) Atomization method
By this method, melted metal is force into a nozzle, and then blew by compressed air, water, or inert gas into extremely fine metal particles. Metal powder produced in this way is mostly in sphere or teardrop shape.

(2) Reduction method
Reducing gas such as hydrogen and CO reduces metal compound, normally oxide, to porous friable blocks. The blocks are then ground to make powder which has particles with irregular shapes.

(3) Electrolysis method
This method is very similar to plating method but uses very powerful electric current. The powder is made by firstly scratching the metal powder depositing on the negative electrode and heat treating to soften it. Such powder is purer and has irregular branch-like shapes.

2. Common production process of parts:
(1) Uniformly mixed powder has the following effect:
1. Uniformly mix particles of different sizes.
2. Uniformly mix particles of different powder.
3. Lubricant can be added to reduce friction force during compression.

(2) Compression forming
Through compression, friable powder can be forced into expected sizes and shapes in the molds. The molds are normally made of tool steel or wolfram carbide. The compression force required is about 5~120 tons/inch2 (700~17,000kgs/cm2). Compression has the following effects:
1. Increase density and reduce interspace between particles.
2. Deform the particles, crush the oxide on their surface to increase contact surface between particles, and so accelerate sintering.

(3) Sinter
During sintering, the compressive part is placed under protective or reducing gas (or vacuum) and heated with high temperature to combine the particles. Combination force derives from reduction of total surface area of the particles. The combination is formed by solid diffusion effect between atoms. Generally, sintering temperature is about 80% of melting point of the major element in the compressive part. Sintering aims to eliminate the barrier between particles and increase density so as to form the property of the products. Figure 6 is a type of sintering furnace.

(4) Further process
Many powder metallurgy parts works after sintering. But some need better properties such as surface smoothness, dimensional precision, and strength. Sintered parts can be referred to further processes such as alignment, compression, cutting, penetration, and heat treatment. For example, to increase strength of the sintered workpiece, copper liquid is penetrated into pores of the workpiece through capillarity, so strength of the workpiece increases by 70%~100%.

3. Property and application of powder metallurgy parts
Physical property of powder metallurgy parts is related to their density. Generally, tensile strength of high density powder metallurgy parts is about 75% of forged parts of the same metal. Since pores exist in the parts, their ductility is worse than that of forged parts. Because of this, powder metallurgy used to produce parts with special property or parts receiving low stress.