Metal Sintering Process ...Powder Metal Forming

Introduction

The fabrication of metal shapes from small particles of metal by sintering is an oldtechnology of over 1000 years. The sintering of metal and ceramics on a largescale is however a much more recent development. Sintering is mainly used for cermets i.e components made for a combinationof metal and ceramics

Sintered normally involves three processes :

Production of the powder
Shaping of the component from the powder
Stengthening of the component by heat treatment

Advantages of Sintering

Sintering has a number of advantages.

The parts produced have an excellent surface finish, and good dimensional accuracy.
The porosity inherent in sintered components is useful for specialised application such as filters and bearings
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 process results in brittle components which often require further heat treatmentbefore use. The tooling costs are relatively high and so large batch quantities are requiredfor economic production runs compared to other processes.Production of Powders

The powder production methods are by either chemical or mechanical processes.

The chemical route includes for reduction, precipitation, chemical reaction, and electolysis. 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 whichis crushed into a powder. Large quanties of iron powders are produced using the carbon route

A typical precipitation method includes preparation of a carbonyl vapour by passing carbon monoxideat 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 thetank or as a spongy mass on the electrode.

Powders can be produced by disintegration of molten metal by a jet of gas or water athigh 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
Cold Pressing
Hot Pressing

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 removedfrom 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 pressuresare fairly low from about 80 N. mm^-2 for soft copper based alloys to 80N.mm^-2 for steels.The pressures used are sufficient to produce cold welding of the powder granules. The processimparts 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 recrystallisation temperatureof the metal during the pressing process. The resulting components have a high density and accuratedimensions. The process is best completed in a reducing atmosphere or a vacuum to eliminatethe risk of oxidisation of the metal. If the temperature needed is below 1000 ^oC then metaldies can be used otherwise graphite or ceramic materials should be used. The hotpressing process is normally carried out a pressures below 30 N mm^-2Sintering Systems

Prior to sintering the components they are heated to a temperature sufficient to evaporateany volatile components.

Sintering is generally based on heating the green component to a temperatureabove the melting point of at least one of the powder constituents. The process has to be completed in a reducing atmosphere or in a vacuum furnace. The sintering results in an improvement in strength, electrical conductivity, and ductility. The process also results in dimensional changes.

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