Sintering By Robert Hamilton

Introduction Sintering is a method for making objects from powder, by heating the material in a sintering furnace below its melting point (solid state sintering) until its particles adhere to each other. Sintering is traditionally used for manufacturing ceramic objects, and has also found uses in such fields as powder metallurgy.

Production of Powders The powder production methods are by either chemical or mechanical processes The chemical route includes for reduction, precipitation and electrolysis. The mechanical route includes atomization and disintegration The reduction method of produces powders from oxides of metals with high melting points. The most common reducing media are hydrogen and carbon. The reduced particles sinter together into a spongy mass which is crushed into a powder. A typical precipitation method includes preparation of a carbonyl vapour by passing carbon monoxide at a high temperature over the heated metal and producing powder by precipitation of the resulting vapour. This is an expensive process. Electrolysis of a salt of a metal results in metal being deposited as a sludge on the bottom of the tank or as a spongy mass on the electrode. Powders can be produced by disintegration of molten metal by a jet of gas or water at high pressure (Atomization). The main methods of producing powder are the atomization process and the reduction process

Shaping Methods Three main methods are using for production of the sintered shapes from the powders: Unpressured Forming When highly porous components are required, they can be produced by loose sintering. The powder is poured or vibrated into a mould, which is heated to the sintering temperature. The sintered parts shrink on cooling and shapes are only processed which can be removed from the mould on completion of the process. Other methods of unpressured forming include slip and slurry casting. Cold Pressing The powder is formed into the required shape by mechanical or hydraulic pressure. The pressures are fairly low from about 80N.mm2 for soft copper based alloys and steels. The pressures used are sufficient to produce cold welding of the powder granules. The process imparts sufficient strength to hold the powder together allowing the parts to be handled for the sintering process. Other methods of cold pressing include isostatic pressing and explosive forming. Hot Pressing This process involve heating the powder to a temperature above the re-crystallisation temperature of the metal during the pressing process. The resulting components have a high density and accurate dimensions. The process is best completed in a reducing atmosphere or a vacuum to eliminate the risk of oxidisation of the metal. The hot pressing process is normally carried out a pressures below 30N.mm2

Advantages The parts produced have an excellent surface finish, and good dimensional accuracy. Refractory materials which are impossible to shape using other methods can be fabricated by sintering with metals of lower melting points A wide range of parts with special electrical and magnetic properties can be produced. The sintering results in an improvement in strength, electrical conductivity, and ductility

Disadvantages Temperatures have to be recorded the hole time It enables changes in the linear dimensions The mould cost thousands of pounds to produce.

Rare earth magnets