3_ Powder Metallurgy (1_5hr-24 Slides)

8/13/2019 3_ Powder Metallurgy (1_5hr-24 Slides)

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Sample Parts made by P/M



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• Powder metallurgy is the name given to a processwherein fine metal powders are blended (mixed),

pressed into a desired shape (compacted), and

then heated (sintered) in a controlled atmosphere

at a temperature below the melting point of themajor constituent for sufficient time to bond the

contacting surfaces of the particles and establish

desired properties.

• 1930’s carbide tool materials

• 1960’s automobile parts

• 1980’s aircraft engine turbine parts

Introduction



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Some important p roperties o f P/M are as fo l lows.

● Used in mass p roduc t ion o f small , int r icate

parts of high p recis ion ,

● No o r l i t t le material is wasted,

● Usual ly no machining is requ ired,

● Sem isk i l led labor is su ff ic ient,

● Some un ique propert ies, such as contro l led

degrees of poro si ty or bu i l t -in lub r ication(made by impregnation process) can be

obtained.

Introduction



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Controlled Degree of Porosity

Before sintering After sintering

Powder particles of an organic material

Organic mater ia l part ic les burn and d isappear during sinter ing.

At their locat ions small empty volum es are left pores .

By con tro l l ing the sizes o f the organic mater ia l part ic les, and by

con tro l l ing their percentage in the pow der mix , i t becomes

pos sib le to have con tro l led degree of poro si ty .



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The most commonly used methods for powder

manu factu re are:

1. Melt atom ization ,

2. Reduct ion of ox ides or ores,

3. Electro lyt ic depos i t ion from so lut ions or fused

salts.

Lesser used processes are:

1. Pulverizat ion or gr ind ing ,2. Thermal decompos i t ion of hydr ides or carbony ls,

3. Condensat ion of metal vapo rs.

Powder Manufacture



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Powder Manufacture

Melt Atomization Rotating Electrode Method



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Before compact ing, powder is m ixed and blended to,

1. Obtain un iform part ic le size dis tr ibut ion,

2. Mix powders of dif ferent materials,

3. Coat powder part ic les w ith lub ricants.

● Can be done wet or dry.

● Water or so lvent can be used for better m ixing.

Dust ing is reduced, exp losion hazards are lessened.

● Lubricants such as g raphi te or s tear ic acid are used toimp rove f low character ist ics, to reduc e die wear.

● Obtaining a uni form m ix aids in the pressing operat ion

and helps to assu re uni form i ty throughout a run.

Powder Blending



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Loose powder is compressed and dens i f ied into a

shape known as a green com pact . Compact ing is

an impo rtant s tep, since,

1. Powder is formed into the desired shape,

2. It determ ines the dens i ty of the product,

3. It determ ines the un iform ity of the dens i ty.

Compacting (Pressing)



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● Flow characterist ics o f the powder is also very

impo rtant, since it determ ines the f low ing abi l i ty of

the powder under pressure, which af fects the

speed o f pressing and the f inal dens i ty.

● The streng th of a P/M produc t depends on thechem ical composi t ion of the powder and the f inal

densi ty.

The greater the densi ty , the higher the streng th is .

● Pure metal and non-metal powders can be m ixed.

● Pre-alloyed powders can be used.

● Precoated powders can be used.

Powder Properties



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● The powder does no t f low l ike a liquid, but

develops an opposing force in the opposi te

di rect ion o f the appl ied force by means of fr ict ion

between the part ic les themselves and the die

surfaces.

● General ly, i t is no t possib le to transm it uni form

pressures, and obtain un i form densi ty throughout

the compact .

● Maximum pressure occurs near the plunger anddecreases w ith the distance from the plunger.

Maximum densi ty also occ urs near the plunger.




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● The thickness /w idth ratio should be kept below 2

whenever poss ible.

● Dies used for com pact ing are made from hardened

too l s teel.

Their su rfaces are highly pol ished.

They should be heavy and stron g enough tow i thstand h igh pressing p ressures.




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Powder Rol l ing Powder Extrusion

Other Compacting Techniques



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● The compacts are subjected to elevated

temperatures in a contro l led atmosphere.

● Sinter ing s tr ips contaminants from the surfaces of

the powder part ic les, perm it t ing d i f fus ion bon ding

to occur and resu l ts in a sing le piece of mater ial.● Sinter ing is carr ied out below the melt ing po int o f

the major cons t i tuent .

● Sinter ing atmospheres mus t be con tro l led

carefu l ly to p revent oxidat ion and combust ion.

Sintering



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Sintering



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Secondary Operations

For many app l icat ion s, P/M parts are ready fo r use as

they come from the sinter ing oven.

However, many produc ts require one or more

secondary operat ions to p rovide enhanced

precision or special character ist ics.● Repressing, coin ing or sizing

● P/M Forg ing ● Impregnat ion ● Inf i l t rat ion



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Repressing, coining or sizing

A second pressing operat ion, known as repressing,

coin ing or s izing, may be used to restore

dimens ional p recis ion.

The part is p laced in a die and subjected to pressures

equal to o r greater than the ini t ial press ingpressure.

A small amount of plast ic f low takes place, resul t ing

in a very uni form produc t wi th respect to size and

sharpness of detail .



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P/M Forging

If mass ive metal deformat ion o ccurs in the second

press ing , the operat ion is known as P/M forging.

Here P/M is used to produce preforms fo r forg ing s.

Scrap during forging

Preform

made by

P/M



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Impregnation - Infiltration

Impregnation refers to the forcing of o i l or o ther liquid

into the po res b y ei ther immersing the part in a

bath and apply ing p ressure or by a combinat ion o f

vacuum -pressure process .

(e.g. Used to make oil-impregnated o r selflubr icat ing bearings.)

In in f i l t ration , a mo lten metal of a lower melt ing poin t

than the major const i tuent is forced into theproduct under pressure or absorbed by capi l lary

act ion. Streng th of the produc t is inc reased.



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- In general , the streng th propert ies of P/M products

are infer ior to pu re wrought or cast metals. This is

also true for al loys .

- As larger pressures are appl ied and secondary

operat ions such as co in ing o r P/M forging areemployed to p rov ide greater dens i ty, the strength

propert ies of P/M produc ts inc rease and be more

nearly equal those of wrought mater ials.

(w roug ht: Beaten out o r shaped with the hammer or oth er tools .)

Properties of P/M Products



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1. Porous products , su ch as bearings , f i l ters, and

pressu re or f low regu lators .

2. Produ cts of complex shapes that wou ld require

con siderable machin ing when made by other

pro cesses. (e.g. Small gears, paw ls, cams, small

act ivat ing levers.)

3. Produ cts made from materials that are very dif f icu lt

to machine . (e.g. Tungs ten carb ide cu tt ing too ls.)

4. Products where the com bined proper t ies of twometals, or of m etals and nonmetals are desired. (e.g.

Motor generator bru shes, electr ical contacts,

bearings.)

Classification of P/M Products

3_ Powder Metallurgy (1_5hr-24 Slides)

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