Class30 Powder Metalurgy

7/31/2019 Class30 Powder Metalurgy

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Powder Metallurgy

First used in 1900s to produce tungsten

filaments for light bulbs.

Net-Shape forming

Typical Products:

Gears, cams

Filters, oil impregnated bearings


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Advantages

Availability of a wide range of compositions

Net- or near-net-shape technique

Use materials which are otherwise difficult to

process

Limitations

Size and complexity limitations

High cost of powder metals compared to other

raw materials

High cost of tooling and equipment for small

production runs


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Production Sequence

Powder production

Blending

Compaction

Sintering

Finishing


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Methods of Powder Production

Atomization

Produces a liquid-metal stream by injecting moltenmetal through small orifice. The stream is broken up

by jets of inert gas, air, or water.

Reduction

Uses gases (hydrogen and CO) to remove oxygen frommetal oxides.

Electrolytic deposition

Utilizes aqueous solutions or fused salts. Producespurest form of metal powder.


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Carbonyls

Are formed by letting iron or nickel react withCO. The reaction products are thendecomposed to iron and nickel.

Comminution

Mechanical comminution involves crushing,milling in a ball mill.

Mechanical alloying

Powders of two or more pure metals are mixedin a ball mill. This process forms alloypowders


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Blending

The ideal mix is one in which all the particles ofeach material are distributed uniformly

Powders of different metals and other materialsmay be mixed in order to impart special physicaland mechanical properties

Lubricants may be mixed with the powders to

improve their flow characteristics. Hazards: Over-mixing may wear particles or

work-harden them. High surface area to volumeratiosusceptible to oxidation; and may explode!


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Compaction

Blended powders are pressed into shapes in

dies.

Pressure distribution:


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044

22

dxDdpp

Dp

Dxxxx

04 xxDdp

xx kp

04

D

dxkpdp xx

D

kdx

p

dp

x

x 4

Dkx

x epp/4

0

Balancing the vertical forces:

which simplifies to

introduce k (interparticle friction)

or

Integrating and using boundary conditions:


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Equipment

Required pressure ranges from 70 MPa (foraluminum) to 800 MPa (high density iron)

Isostatic Pressing

Cold isostatic pressing (CIP)the powder is placed ina flexible rubber mold. The assembly is thenpressurized hydrostatically in a chamber, usually withwater. This results in a pressure of 400 MPa.

Hot isostatic pressing (HIP)the container is usually

made of a high melting point sheet metal, and thepressurizing medium is inert gas or vitreous fluid.Pressure is 100 MPa at 1100 C. Results in 100%density.


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Other compacting processes

(see section 11.3.4) Forging

Rolling

Extrusion

Injection Molding

Pressureless compaction Ceramic molds


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Sintering

The process whereby compressed metal powder isheated in a controlled atmosphere furnace to atemperature below its melting point, but highenough to allow bonding of the particles.

Sintered density depends on its green densityand sintering conditions (temperature, time andfurnace atmosphere).

Sintering temperatures are generally within 70 to90% of the melting point of the metal or alloy.

Times range from 10 minutes for iron and copper

to 8 hours for tungsten and tantalum


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Sintering mechanisms are complex and depend on

the composition of metal particles as well as

processing parameters. As temperature increases

two adjacent particles begin to form a bond by

diffusion (solid-state bonding).

If two adjacent particles are of different metals,alloying can take place at the interface of two

particles. One of the particles may have a lower

melting point than the other. In that case, one

particle may melt and surround the particle thathas not melted (liquid-phase sintering).


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Finishing

Repressing

Additional compacting operations, performed underhigh pressure in presses (coining, sizing)

Impregnation

Utilizes inherent porosity of PM components byimpregnating them with a fluid (oil)

Infiltration

A slug of lower melting point metal is placed againstthe sintered part, the assembly is heated to melt slug.By capillary action, the liquid slug fills the pores of thesintered part.


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Design Considerations

Shape of compact must be kept as simple and

uniform as possible. Sharp changes in contour,

thin sections, etc. should be avoided. Provisions must be made for ejection of the green

compact from the die without damaging the

compact.

Parts should be produced with the widest

tolerances.


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Class30 Powder Metalurgy