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Metal Sintering Process ...Powder Metal Forming

Introduction

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

Sintered normally involves three processes :

  1. Production of the powder
  2. Shaping of the component from the powder
  3. Stengthening of the component by heat treatment
Advantages of Sintering

Sintering has a number of advantages.

  1. The parts produced have an excellent surface finish, and good dimensional accuracy.
  2. The porosity inherent in sintered components is useful for specialised application such as filters and bearings
  3. Refractory materials which are impossible to shape using other methods can be fabricated by sintering with metals of lower melting points
  4. 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 treatment before use. The tooling costs are relatively high and so large batch quantities are required for 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 which is 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 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

  1. Unpressured Forming
  2. Cold Pressing
  3. 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 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 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 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 recrystallisation 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.   If the temperature needed is below 1000 o C then metal dies can be used otherwise graphite or ceramic materials should be used.   The hot pressing process is normally carried out a pressures below 30 N mm-2

Sintering Systems

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

Sintering is generally based on heating the green component to a temperature above 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.  



Links Providing information on Metal Sintering
  1. Review on Metal Powders .. Malvern.co.uk - Useful Information on Metal powders
  2. Process Encyclopaedia..U.of Cambridge website- Useful information
  3. Library of Manufacturing;.. A complete document providing detailed info on all primary forming processes


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Last Updated 15/01/2013