POWDER METALLURGY :BASIC STEPSADVANTAGES AND LIMITATIONS

Powder metallurgy(P/M) has been defined as the art and science of producing metal powders and making semifinished and finished objects fro individual ,mixed or alloyed powders with or without addition of nonmetallic constituents.The main steps in P/M component production are

1]POWDER PRODUCTION

2]COMPACTION

3]SINTERING

4]SECONDARY AND OTHER FINISHING OPERATIONS

POWDER PRODUCTIONSThe raw material for P/M components is the powder . These powders are engineered materials on the sense are manufactured to precise specifications to facilitate subsequent processing. The powders used in P/M can be pure elements, elemental blends or prealloyed powders. .The choice of starting material is influenced by the product type and to a lesser extent by the fabrication process to be used. Several production methods are available for making powders . Most common method is the atomization ,which produces nearly 80%of the total volume of powders .The method produces powders of both ferrous and non ferrous metals. specialty powders like stainless steels ,superalloy as well as titanium alloy powders are also made by this technique .Reduction of compounds is another widely used technique used for the productions of iron ,copper as well as tungsten and molybdenum. Electrolysis is yet another important powder production method used widely for making copper,iron and silver powders.COMPACTIONCompaction of powder mixtures is generally carried out using dies machined to close toledances. Dies are made typically from die steels or cemented carbides.Equipment used for compaction includes mechanical or hydraulic presses ,the, latter being more common due to their safe operation and flexibility .Die design is important and must ensure easy ejection of compact.The powder type and its characterstics influence the compaction pressure .The basic purpose of compaction is to produce a green compact with sufficient strength to withstand further handling operations.The pressed part ,usually called the green compact is the taken for sintering Consolidation of powders may also be carried out at high temperature .Hot extrusion, hot pressing and hot Isostatic Pressing (HIP) are examples of this category .These methods are used for critical metallic and ceramic components requiring near theoretical density.SINTERING

Sintering of the green compact is carried out in a furnace under a controlled atmosphere to bond the particles metallurgically .Sintering is carried out at temperatures about 70% of the absolute melting point of the material .Bonding occurs by diffusion of atoms ,giving integrity to the compact . In other words ,Sintering serves to consolidate the mechanically bonded powders into a coherent body having the desired service properties .Shrinkage occurs during sintering resulting densification of the part .The densification enables significant improvements in the physical and mechanical properties of the part .Sintering can be carried out in a variety of furnaces...Mesh belt furnaces are the most common type and are used type to 1200 C .Walking beam and pusher type furnaces can be used for still higher temperatures .Batch type furnaces include bell type or box type furnaces .Sintering is normally done under a protective atmosphere .Most commonly used atmospheres include endothermic and dissociated ammonia .Nitrogen is also widely used because of its lower cost.SECONDARY AND OTHER FINISHING OPERATIONS

A sintered part may be subjected to one or more secondary or finishing operations .These are used to ensure close dimensional tolerances ,improve surface finish ,increase density and corrosion resistance .Typical operations include repressing .grinding ,and planting. Sometimes resintering is also done after repressing.

ADVANTAGES:

PROCESS ADVANTAGES

Powder metallurgy offers the following important process advantages

I]Eliminations or minimizes machining(little or no scrap)

ii]Efficient materials utilization- above 95% material utilization

iii]Enables close dimensional tolerancesnear net shapes possible

iv]Produces good surface finish

v]Provides opinion for heat treatment .for increasing strength or enhanced wear resistance and plating for improving corrosion resistance.

Vi]Facilities manufacture of complex shapes , which would be impractical with other metal working process.Vii] Suited to moderate to high volume component production requirement.

Viii]Components can be produced cost as compared to many other processes i.e cost effective

ix] Components of hard materials which are difficult to machine can be readily manufactured,e.g tungsten wires for incandescent lamps.

x]It is possible to produce components in pure form ,purity of the starting materials can be preserved throughout the process,a requirement for any critical applications.xi]Energy efficientxi]Environment friendly

COMMERCIAL ADVANTAGES /SPECIAL CHRACTERSTICS OF POWER METALLURGY PARTS:

Important commercial advantages ofP/M include

I]Ferrous and non ferrous powder metallurgy parts can be oil impregnated to function as self lubricating bearings,Similarly parts can be resin impregnated to seal interconnected porosity to improve density or they can be infiltrated with a lower melting point metal for greater strength and shock resistance ,and for making electrical contactsII]Parts can be heat treated and plated iof required P/M parts are also amenable to processing by conventional metal forming process like rolling and forging

III]Cost effective production of simple and complex parts ,very close to final dimensions at productions rates that can range from a few hundreds to several thousands parts per hour

IV}Offers long term performances reliability in critical applications.

LIMITATIONS OF POWDER METALLURGY

In industrial practices, Powder Metallurgy is justified only when it allows a cost reduction .In such a case .Two different situations can arise i) the same part can be made by an alternatives route namely,casting,forging,machining with similar chracterstics in which case cost comparisoncan be easily carried out.Depending on the cost saving the procedure can be identified and ii) in cases where the same product cannot be produced by o ther techniques the cost comparison is difficult to make .However at present there are certain limitation in adopting PowderMetallurgy route due to part size and shape limitations dimensional precision ,mechanical properties,alloy clealiness and volume production

Size LimitationsFor die pressed parts,the cost of tooling and of the presses required increase rapidly with size.Cost efficiency in die pressing can be obtained only for small parts weighing 20to100gram and produced in large numbers .P/M therefore is not recommended for large parts .The other major limitations is on the component design .P/M components cannot accommodate certain features such as re entrant angles or radial holes in vertically pressed parts by fixed by the die pressing.Mechanical Properties

The presence of porosity introduces structural hetergemeity in die pressed mechanical parts and hence the mechanical properties such as ultimate tensile strength ,and elongation and inferior to wrought material .Impurity content of powders is another factor which affects the properties of sintered parts,

Psychological

In most cases POWDER METALLURGY is a new production route and hence if the parts currently being used are satisfactory the user is reluctant to change over the Powder Metallurgy route.