For sintering to be effective, the powder particles must be in intimate contact. For this reason, sintering is typically performed on compacted or molded powder and not on loose powder. While most powder parts are sintered during their manufacture, the densification stage of sintering may not be utilized, and in these cases the primary use of sintering is to achieve metallurgical bonding of particles. Die pressed parts typically fall into this category because dimensional control is of primary importance.

Process routings that rely on sintering specifically to achieve high densities usually fall into three categories. A very fine particle size is used so that bulk diffusion paths are minimized at the last stage of sintering, and there is a high initial surface area to drive sintering in its early stages. This is one reason for the use of ultrafine powders in injection molding where sintering is the primary mechanism for development of high property levels. A liquid phase, either permanent or transient, is present during sintering so that particle rearrangement is promoted. A high temperature is used so that local melting may accompany sintering.

Liquid Phase Sintering. The use of blended powders allows the combination of a low melting powder with a higher melting powder so that a liquid is present during sintering. This liquid can aid powder rearrangement and densification of a green compact. Cermets are typically produced by this technique. Nearly pore-free microstructures may be achieved by this technique.

Transient Liquid Phase Sintering. A subclass of liquid phase sintering is transient liquid phase sintering. Here, a blend of powders contains a lower melting point phase, as above. Only in this case, the liquid alloys with the solid phase which remains solid, and the liquid is present only for a brief period. This process is not used as the primary densification method, but it is used to achieve another increment in density. For example, it may be used in high alloy steel or tool steel production to increase the density of a green compact from 80% to >95% of full density.

High-Temperature Sintering. If the temperature of the green compact is raised to levels very close to the solidus temperature of the equilibrium diagram, small pools of liquid can form due to microsegregation of alloying elements. This small amount of liquid accelerates densification during sintering. Although this represents a somewhat extreme use of high temperature sintering, this technique is useful for a wide range of alloys, and it is used especially for tool steels and other high alloy steel components. The article "High-Temperature Sintering of Ferrous Powder Metallurgy Components" in this Volume contains more information on high-temperature sintering.

Reactive Sintering. By taking advantage of the flexibility of powder metallurgy, a blend of powders may be compacted and then sintered so that during sintering a self propagating reaction occurs (see "Combustion Synthesis of Advanced Materials" in this Volume). This reaction, if controlled properly, can densify the compact rapidly while forming the new alloy. If not controlled, the component may revert to powder!

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