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Powder Metallurgy

- PM -

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Powder Metallurgy (P/M)

Competitive with other processes suchas casting, forging, and machining.

Used when melting point is too high (W, Mo).

reaction occurs at melting (Zr). to hard to machine. very large quantity.

Near 70% of the P/M part productionis for automotive applications.

Good dimensional accuracy. Controllable porosity. Size range from tiny balls for ball-point

pens to parts weighing 100 lb. Mostare around 5 lb.

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Basic Steps In Powder Metallurgy

Powder Production

Blending or Mixing Powder Consolidation

Sintering

Finishing

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Preparation of the mixture of powders

Determination of the apparent

density of the mixture

Determination of the fluidity

of the mixture

Analyze of

sieving

Pressing

Sintering

Post sintering operations

Deburring, washing, drying Lubrication Calibration Washing Sorting

Final control

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ENGINE: valve Guide, valve

seat, piston, gear and chainwheels for transmission, gear

for water pump, injectionpump parts.

Parts for the installationof air conditioning Parts for shockabsorber

TRANSMISSION: synchronizing rings and hubs,hubs for wheels, turbine pump, sensor rings

ABS sensorrings

Toothed gears foroil pump

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1. Powder Production

(a) (b) (c)

(a) Water or gas atomization; (b) Centrifugal atomization; (c) Rotating electrode

Many methods:

Chemical - reducing metal oxides;

Mechanically - milling of waste from cutting;

Atomization - the dominant process.

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Characterization of Powders

Size of powders 0.1 m 1 mm

Sieve size quoted as mesh number

Particle D = 15/mesh number (mm)325 mesh45 m

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2. Blending or Mixing

Blending a coarser fraction with a finer fraction ensures

that the interst ices between large partic les will be filled out.

Powders of different metals and other materials may be

mixed in order to impart special physical and mechanical

properties through metallic alloying. Lubricants may be mixed to improve the powders flow

characteristics (MoS2 or Zn Stearat).

Binders such as wax or thermoplastic polymers are added

to improve green strength.

Sintering aids are added to accelerate densification onheating.

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10Double conical homogenizer

loading

cap bearings

shaft

support

conveyance

reducer

double conical drum

chain transmission

engine

tipper trailer

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3. Powder Consolidation

Die pressing

Cold compaction with 100 900MPa to produce a Green body.

Die pressing

Cold isostatic pressing

Rolling

Gravity

Injection Molding - small, complex

parts.

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Friction problem in cold compaction

The effectiveness of pressing with a single-acting punch is

limited. Wall friction opposes compaction.

The pressure tapers off rapidly and density diminishes away

from the punch.

Floating container and two counteracting punches helpalleviate the problem.

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4. Sintering

Parts are heated to 0.7~0.9 Tm.

Transforms compacted mechanical

bonds to much stronger metallic

bonds.

Shrinkage always occurs:

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Sintering Temperature and Time for

Various Metals

Material Temperature [C] Time [Min]

Copper, brass, and bronze

Iron and iron-graphite

NickelStainless steels

Alnico alloys

(for permanent magnets)

Ferrites

Tungsten carbide

Molybdenum

Tungsten

Tantalum

760900

10001150

1000115011001290

12001300

12001500

14301500

2050

2350

2400

1045

845

30453060

120150

10600

2030

120

480

480

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5. Finishing

The porosity of a fully sintered part is still significant (4-15%).

Density is often kept intentionally low to preserve interconnected

porosity for bearings, filters, acoustic barriers, and battery

electrodes.

To improve properties, finishing processes are needed:

Cold re-pressing, re-sintering, and heat treatment.

Impregnation with heated oil .

Infiltration with metal (e.g. Cu for ferrous parts).

Machining to tighter tolerance.

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Special Process: Hot compaction

Advantages can be gained by combining consolidation and

sintering

High pressure is applied at the sintering temperature to bring

the particles together and thus accelerate sintering.

Methods include

Hot pressing

Spark sintering

Hot isostatic pressing (HIP)

Hot rolling and extrusion

Hot forging of powder pre-form

Spray deposition

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Cold Isostatic Pressing

Presare izostatic la rece

Hot Isostatic Pressing

Presare izostatic la cald

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Rolling Extrusion

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Design Aspects

(a) Length to thickness ratio limited to 2-4; (b) Steps limited to avoid density variation; (c) Radiiprovided to extend die life, sleeves greater than 1 mm, through hole greater than 5 mm; (d) Feather-

edged punches with flat face; (e) Internal cavity requires a draft; (f) Sharp corner should be avoided;(g) Large wall thickness difference should be avoided; (h) Wall thickness should be larger than 1 mm.

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Advantages and Disadvantages of P/M

Virtually unlimited choice of alloys, composites, andassociated properties.

Refractory materials are popular by this process.

Controlled porosity for self lubrication or filtration uses.

Can be very economical at large run sizes (100 000 parts).

Long term reliability through close control of dimensions

and physical properties.

Very good material util ization.

Limited part s ize and complexity. High cost of powder material.

High cost of tool ing.

Less strong parts than wrought ones.

Less well known process.

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Economical qualitative comparisons of different technological processes

Nr.crt.

Technologicalprocess

The cost oftools

The unitcost

Processingpossibilities

Freedom indesigning

Series o ffabrication

1. Fabrication of powders medium low large mediummedium

large

2. Mechanical processing low high high high low

3. Precision stamping high low medium low - medium high

4. Stamping medium- high very low medium low mass

5. Forging high medium medim insignificant high

6. Casting in sand molds low medium high medium low - medium

7. Precision casting medium high high large low - medium

8. Casting in metallic mold high low limited high high

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Dicionar

English Romn

Abrasive wheels Roi abrazive

Bush Buc

Core Miez

Die MatriDiffusion Difuzie

Feed shoe Alimentator

Filter Filtru

Formed product Pies format

Green compact Compact crud

Homogenization Omogenizare

Lower punch Poanson inferior

Lubricant Lubrifiant

Neck Gt

English Romn

Particles bond Contactul granulelor

Pore size Mrimea porilor

Powder Pulbere

Powder extrusion Extrudarea pulberilorPowder rolling Laminarea pulberilor

Pressing Presare

Raw powder Pulbere grob

Shrinkage Contracie

Sintered product Produs sinterizat

Sintering Sinterizare

Upper punch Poanson superior