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Practical MetallurgyMEC 172

Introduction

Powder Metallurgy is one of the four methods of forming metal: casting molten metal plastic deformation of hot or cold metal machining

PM was developed by the auto industry- still the greatest user

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Advantages of PM

Good surface finish can be obtained

Suited for high volume of production of small parts

Almost any combination of alloys can be used to produce high temperature components

Close dimensional tolerances

Eliminates/Minimizes machiningXII 3

Disadvantages of PM

Compared with solid material:

Lower resistance to corrosionPoorer plastic properties

Reduced impact strength Less ductility Reduced elongation

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Products made of PM

Cutting tools such as tungsten carbide inserts and cerments

Precision parts such as cams gears

Antifriction materials bearings that are self lubricating

High strength magnets

Jet engine turbine bladesXII 5

Steps to manufacture PM parts1. Blending – mixing metal powders with alloys

and lubricants until blended

2. Compacting – powders are fed into a precision die and pressed at 10 to 60 tons/in2 The parts ejected are call briquette or green

compact

3. Sintering – heating the green part in a controlled atmosphere just under the melting point to bond the compressed powders

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Powder Metallurgy Presses The most common is a simple die

compacting

Advanced process such as isostatic [pg. 245] pressing formed without die or

traditional press

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Simple Die Compacting

Usually very Small parts

Compacted to 20 to 30 tons/in2

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Cold and Hot Compaction Metal powder separated into two divisions:

Cold and Hot

Isopressing For hollow hemispheres or cones Chamber is pressurized to 50 to 60,000 psi with

liquid or gas Pressure is equal on all sides

Cold Isostatic pressing Powder is loaded into molds of rubber or plastics Placed under pressure at room temperature

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Metal Powders

Range ins size from .0001in to .002in diameter

Produced by several methods: Mechanical Milling Chemical Reduction Shotting Atomization

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Sintering

Solid phase sintering at 1.5 to 2 hours at 60 to 80 percent of melting point

Liquid phase is carried out at the melting point of one of the alloy metals Diffusion Densification Recrystallization & Grain Growth

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Advanced Processes

Powder Forging

Metal powder injection molding similar to plastic injection molding

Powder extrusion

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Powder Metallurgy Products & Uses Pre-lubricated bearings

Food and textile industries

Small motors lubricated for life – Oilite bearings

Aluminum based anti friction materials

Creep resistant alloys made from tungsten molybdeum and tantalum

Cutting tools: i.e. cobalt

Metallic brake shoes XII 13