Consolidation Principles

This article discusses the important changes in density, microstructure, and mechanical properties that occur when metal powder compacts in the as-pressed condition (green compacts) are sintered. These changes are described phenomenologically and from the point of view of the driving forces and the material transport mechanisms that are important in the modeling and characterization of changes in density, microstructure, and mechanical properties from sintering green compacts. The mechanisms of homogenization during sintering and of activated and liquid phase sintering also are examined. In addition, process modeling is discussed at the end of this article.

Changes in the dimensions and density of compacts are important considerations in sintering. Changes in density during sintering are primarily due to changes in phases, in alloying, or in volume (dimensions), but changes in mass, which may be caused by the loss of lubricant added during compacting, volatilization of a component such as zinc from brass, and the reduction of oxide skins on the powder particles during sintering should also be considered.

In studies of the fundamentals of sintering, dimensional changes are determined as the difference in dimensions between green and sintered compacts, parallel and perpendicular to the direction of pressing, and are expressed in percent of the green dimensions. From a technological standpoint, the dimensional change during sintering of greatest importance is the difference between the dimensions of the die and the sintered compact in the radial direction, that is, the direction perpendicular to that of pressing.

Another approach to studying shrinkage and densification during sintering is to determine the dimensional changes with a dilatometer. A compact is heated at a constant rate to a given sintering temperature and then cooled at this rate from the maximum temperature to room temperature. For example, a dilatometer curve is shown in Fig. 1. This curve represents the dimensional changes in the axial direction of the same type of copper compacts as Fig. 2. Compacts were pressed at a pressure of 138 MPa (20,000 psi) and were heated and cooled at a uniform rate of 3.9 °C/min (7 °F/min). This type of experiment is particularly well suited for studies of the rate of densification as a function of temperature.


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