6.6 Dimensional Changes

Changes in dimensions resulting from sintering represent an important field in powder metallurgy, especially with respect to large scale production of parts with small dimensional tolerance. The fundamental process of sintering leads to a reduction in volume because of pore shrinkage and elimination. Following factors in this regard may be considered:

Entrapped Gases: The expansion of gas in closed porosity has been postulated as producing compact expansion.

Chemical Reactions: Hydrogen is a common component of sintering atmospheres and can often diffuse through the metal to isolated portions of the compact where it reacts with oxygen to form water vapour. The pressure of the water vapour can lead to expansion of the entire mass. It is also possible to have reactions that lead to the loss of some element from the sinter mass to the atmosphere, such as volatilizing, and result in a shrinkage of the material.

Alloying: Alloying that may take place between two or more elemental powders very often leads to compact expansion. This effect which is due to the formation of a solid solution is often offset by shrinkage of the original porosity. Dimensional changes may also occur in a binary system where the rate of diffusion of each metal into the other is different.

Shape Changes: Green parts invariably contain variations in green density. Such variations can lead to substantial changes in shape because of the strong dependence of sintering, especially shrinkage, on green density. Low green density regions will...