What is the basic process of powder metallurgy?

The powder metallurgy production process involves five key steps.

1. Powder Production:

This process involves converting raw materials into metal powders. Common methods for powder production include oxide reduction, atomization, electrolysis, rotating electrode, and mechanical milling.

2. Mixing:

Mixing is the process of combining the required powders in specific proportions to form a homogeneous mixture, which is then used to create powder compacts. There are three main mixing methods: dry mixing, semi-dry mixing, and wet mixing. During the premixing of powders, segregation can occur due to the different densities of the particles. Binders can effectively prevent segregation, ensure even distribution of particles, fill voids, and enhance the strength of the compact.

3. Forming:

Forming involves placing the uniformly mixed powders into a mold and pressing them to create a compact with a defined shape, size, and density. Common forming methods include cold pressing and hot pressing.

4. Sintering:

Sintering is the process of heating the compact to facilitate diffusion, welding, and recrystallization among the particles, leading to strong bonding, reduced porosity, and increased density. This results in the formation of a "crystalline bond," which imparts the desired physical and mechanical properties to the material.

5. Post-Processing:

Sintered powder metallurgy products can be used directly, but when higher performance is required, additional post-processing may be necessary. Common post-processing methods include:

1. Sizing: This involves placing the sintered parts into a sizing die, similar in structure to the original mold, and applying pressure to improve dimensional accuracy and reduce surface roughness, correcting any minor deformations caused during sintering.

2. Oil Impregnation: The process of immersing parts in hot oil at 100–200°C or vacuum-impregnating oil into the pores of the powder parts. Oil-impregnated parts exhibit improved wear resistance and rust prevention.

3. Steam Treatment: Treating iron-based parts in steam at 500–600°C forms a hard, dense oxide layer on the surface, enhancing wear resistance and rust prevention.

4. Sulfurization: Parts are placed in a molten sulfur bath at 120°C, removed after several minutes, and then heated to 720°C under a hydrogen atmosphere. This forms sulfides in the surface pores, significantly improving the anti-friction properties and machinability of the parts.