The Wikipedia page on sintering defines it as
Sintering or frittage is the process of compacting and forming a solid mass of material by heat or pressure without melting it to the point of liquefaction.
Now, how do I understand how to not melt it up to the point of liquefaction? A detailed answer to this is most welcome. Obviously the temperature will be beyond the melting temperature, but what about pressure?
Here is how it works.
At temperatures above about 1/3 to 1/2 the melt temperature (in degrees absolute), solid state diffusion in metals is kinetically enabled. Metal atoms in their crystalline lattice networks start hopping between adjacent positions and if some positions are under more compressive stresses than others, there will be a net migration of atoms away from those points; for tensile stresses, there will be in-migration. In addition, because the atoms at grain boundaries are more highly stressed than their neighbors, diffusive transport will be similarly concentrated and particularly vigorous at and across those interfaces.
Now note that a green pressing (a chunk of metal powder that has been strongly compressed together into a desired shape prior to being sintered in an oven) consists of a network of deformed spherical particles of metal which have been mechanically squished together; it has very high stress concentrations present at all those squeezed-together joints. When you put the green pressing into a sintering oven, the strain energy present there furnishes part of the activation energy for diffusion and the grains diffusively weld themselves together into an almost solid mass at a temperature far below the melt temperature.
As this occurs, the interstitial gaps between the squished spheroids shrink and the whole part shrinks in volume by as much as 20% to 30% and the resulting sintered metal part can reach 95% to 99% of full density.
One of the advantages of the sintering process is avoiding macro segregations. The common foundry process can result in differences in chemical compositions between core and shell for example, due to differences in the rate of cooling along the solidification. It is particularly critical for high alloyed materials.
In the sintering process, the components of the alloy can be first mixed in powder form, assuring a good homogeneity. They are then cold pressed close to the desired form. Here comes the second advantage: even hard alloys as tungsten carbide + cobalt can be easily machined after this step, (if some binder product is added to give enough mechanical resistance). It is now close to the final form but scaled. That is: the relation between length, width and height is correct, but all of them are greater, to the porosities.
Finally, the temperature is raised so that part of the alloy with lower melting point is allowed to locally melt, closing micro porosities and assuring metallurgical bonding. The material shrinks to its final dimensions in the process. Some manufactures also uses hydrostatic pressure besides the temperature to help to close the porosities.
