Introduction
Powder metallurgy sintering technology is an important part of powder metallurgy technology, which has been widely used in various areas. It is often used to produce net shaped components with higher performance and complex structure from powdery raw materials. Sintering is a process in which metallic particles are subjected to a temperature and pressure that is sufficient to cause softening and welding. During the process, there is a rearrangement or a redistribution of the particles and thus a necessarily size change.
Principle of Powder Metallurgy Sintering
The powder metallurgy sintering process fundamentally involves an initial state and a final state, with the transition taking place between them. The initial state consists of fine particles such as metal, alloy, and ceramic powder usually with a complex and variable particle size, shape and chemistry. Heat applied to this powder causes the particles to soften through a process of plastic or viscous flow.
At some point during the cooling back along the heating/cooling cycle, a critical transition temperature is reached at which the force of particle cohesion is greater than the force of particle repulsion. This is the transition from fluid flow to the transition from the liquid state to the solid state and occurs at what is termed the sintering temperature. Small surface irregularities fill to form a single homogenous material.
The mechanical properties, density and dimensional integrity of the sintered product are derived from the choice of powder size, morphology shape and chemistry, the degree of compaction, sintering temperature and the sintering atmosphere.
Factors Affecting Sintering
There are three primary factors that affect the sintering of powder metallurgy: temperature, atmosphere and time.
Temperature: The temperature employed dictates the rate of sintering, the amount of sintering, the amount of shrinkage, the particle size distribution and the microstructure of sintered product. The sintering temperature should be about 10-25% above the melting point of the material being sintered.
Atmosphere: The atmosphere in which sintering takes place influences the rate of sintering, the microstructure of the resulting product, and the type and amount of intermetallic compounds and oxides formed. Atmospheres used in sintering are either inert and reducing.
Time: In a general term, time is the rate of sintering. As time increases, sintering increases, as well as the amount of shrinkage and chemical reaction taking place between the particles. Thus, a balance between time and temperature is required in order to achieve the desired sintering results.
Conclusion
The powder metallurgy sintering process involves a critical transition temperature that is reached when the force of particle cohesion is greater than that of repulsion. A proper balance between time, temperature, atmosphere and material composition are necessary for successful sintering and for the production of strong parts with acceptable structural integrity.
