POWDER METALLURGY

MODULE 5A

LEARNING OBJECTIVES Introduction

Advantages/Limitations

PM Products

PM Materials/Process Description

PM Steps

Powder manufacturing

Mixing/blending

Conventional pressing / compaction

Conventional sintering

Alternate methods to compact and sintering

Tape casting , isostatic pressing, powder extrusion , powder rolling ,

injection molding

Secondary operations in PM

Economic and design guide lines

Comparison with other manufacturing process

INTRODUCTION

Powder metallurgy, or PM, is a process for forming metal parts by

heating compacted metal powders to just below their melting points.

Although the process has existed for more than 100 years, over the

past quarter century it has become widely recognized as a superior

way of producing high-quality parts for a variety of important

applications.

This success is due to the advantages the process offers over other

metal forming technologies such as forging and metal casting,

advantages in material utilization, shape complexity, near-net-shape

dimensional control, among others.

POWDER METALLURGY PICTORIAL DESCRIPTION

Metal

Powder

Metal Product

POWDER METALLURGY ADVANTAGES

PM parts can be fabricated to final or near-net shape, thereby eliminating orreducing scrap metal, machining and assembly operation.

High melting point metals and composite materials can be produced.

PM is useful in making parts that have complex shapes or difficult to machine.

Permits a wide variety of alloy systems.

Provides materials which may be heat-treated for increased strength orincreased wear resistance.

Provides controlled porosity for self-lubrication or filtration.

Long term reliability through close control of dimensions and physicalproperties.

High production rate.

PM LIMITATIONS

Porosity originates as the spaces between powder particles i.e. low

elongation.

High cost of powder material.

Less strong parts than wrought ones.

Relatively high die cost.

High material cost.

Design Limitations

The mechanical properties of P/M materials are degraded by the

presence of pores.

EXAMPLES OF POWDER METAL PRODUCTS

Gears

Cams

Cranks

Bearings

Roller bearing cages

Housings

Light bulb filaments

Sprinkler mechanisms

PM PRODUCTS

PM PRODUCTS

PM PRODUCTS

PM PRODUCTS

PM PRODUCTS

POWDER METAL MATERIALS

Elemental

A pure metal, most commonly iron, aluminum or copper

Pre alloyed

An alloy of the required composition, most commonly copper

alloys, stainless steel or high-speed steel

PM BASIC PROCESS DESCRIPTION

The component powders are mixed, together with lubricant, until a

homogeneous mix is obtained. The mix is then loaded into a die and

compacted under pressure, after which the compact is sintered.

An exception is the process for making filter elements from spherical

bronze powder where no pressure is used; the powder being simply

placed in a suitably shaped mould in which it is sintered. This

process is known as loose powder sintering

POWDER METALLURGY PROCESS PICTORIAL

DESCRIPTION

BASIC STEPS IN POWDER METALLURGY

Powder Production

Blending or Mixing

Powder Consolidation

Sintering

Finishing Operation

Lets examine these steps in detail in succeeding slides

FLOW CHART PM

Outline of processes and operations involved in making powder-metallurgy parts.

POWDER PRODUCTION

POWDER PRODUCTION

It involves the production of a fine metallic powder.

Several techniques have been developed which permit large

production rates of powdered particles, often with considerable

control over the size ranges of the final grain population.

There are four main processes used in powder production

Solid-state reduction

Atomization

Electrolysis

Chemical.

However, atomization is widely used for powder production.

SOLID STATE REDUCTION

This has been for long the most widely used method for the

production of iron powder. Selected ore is crushed, mixed with

carbon, and passed through a continuous furnace where reaction

takes place leaving a cake of sponge iron which is then further

treated by crushing, separation of non-metallic material, and sieve to

produce powder.

Since no refining operation is involved, the purity of the powder is

dependent on that of the raw materials. The irregular sponge-like

particles are soft, and readily compressible, and give compacts of

good green strength.

Refractory metals are normally made by hydrogen reduction of

oxides, and the same process can be used for copper.

PARTICLE SIZE REDUCTION

ATOMIZATION

In this process molten metal is broken up into small droplets and

rapidly frozen before the drops come into contact with each other or

with a solid surface.

The principal method is to disintegrate a thin stream of molten metal

by subjecting it to the impact of high energy jets of gas or liquid.

Air, nitrogen and argon are commonly used gases, and water is the

liquid most widely used

Contd

By varying the several parameters: design and configurations of the

jets, pressure and volume of the atomizing fluid, thickness of the

stream of metal etc. - it is possible to control the particle size

distribution over a wide range.

The particle shape is determined largely by the rate of solidification

and varies from spherical, if a low heat capacity gas is employed, to

highly irregular if water is used. In principle the technique is

applicable to all metals that can be melted, and is commercially

used for the production of iron, copper, including tool steels, alloy

steels, brass, bronze and the low-melting-point metals, such as

aluminum, tin, lead, zinc, cadmium.

The readily oxidizable metals, for example chromium-bearing alloys,

are being atomized on an increasing scale by means of inert gas,

specially argon

GAS ATOMIZATION PICTORIAL DESCRIPTION

PARTICLE SHAPE

DUE USE OF GAS

WATER ATOMIZATION PICTORIAL DESCRIPTION

PARTICLE SHAPE DUE USE OF WATER

CENTRIFUGAL ATOMIZATION

There are basically two types of centrifugal atomization

processes:

In one a cup of molten metal is rotated on a vertical axis at a speed sufficient to throw off

droplets of molten metal, or a stream of metal is allowed to fall on a rotating disc or cone;

In the other a bar of the metal is rotated at high speed and the free end is progressively

melted e.g. by an electron beam or plasma arc

ELECTROLYSIS POWDER PRODUCTION

By choosing suitable conditions - composition and strength of the

electrolyte, temperature, current density, etc., many metals can be

deposited in a spongy or powdery state.

Extensive further processing - washing, drying, reducing, annealing

and crushing may be required.

Copper is the main metal to be produced in this way but chromium

and manganese powders are also produced, by electrolysis. In

these cases, however, a dense and normally brittle deposit is

formed and requires to be crushed to powder.

Electrolytic iron was at one time produced on a substantial scale but

it has been largely superseded by powders made by less costly

processes. Very high purity and high density are two distinguishing

features

ELECTROLYTIC CELL OPERATION PICTORIAL

DESCRIPTION

POWDER CHARACTERISTICS

The further processing and the final results achieved in the sintered

part are influenced by the characteristics of the powder:

particle size,

size distribution,

particle shape,

structure

and surface condition.

A very important parameter is the apparent density (AD) of the

powder, i.e. the mass of a given volume, since this strongly

influences the strength of the compact obtained on pressing. The

AD is a function of particle shape and the degree of porosity of the

particles

PARTICLE SHAPE

PARTICLE SIZE/CLASSIFICATION

The process of separating particles by size is called classification

PARTICLE SIZE

Micrograph of screened powder particles, showing that particles may

be longer than the mesh is wide

PARTICLE SIZE

void

smaller, more numerous voids

voids filled by smaller particles, small voids

remain

Mixing particles of different sizes allows decreased porosity and a

higher packing ratio

PARTICLE SIZE MEASUREMENT TECHNIQUES

Particle size is measured by screening

In addition to screen analysis one can use:

Sedimentation measuring the rate that particles settle in a fluid

Microscopic analysis using a scanning electron microscope

Optical particles blocking a beam of light that is sensed by a photocell

Suspending particles in a liquid & detecting particle size and

distribution

TRADE OFF BETWEEN POWDER CHARACTERISTICS

The choice of powder characteristics are normally based on

compromise, since many of the factors are in direct opposition to

each other:

An increase in the irregularity and porous texture of the powder

grain, i.e. decrease in apparent density, increases the reduction in

volume that occurs on pressing and thus the degree of cold-welding,

which, in turn, gives greater green strength to the compact

Additionally the greater reduction in volume necessary to give the

required green density may require greater pressure and

consequently larger presses and stronger dies. The ease and

efficiency of packing the powder in the die depends to a large extent

on a wide particle size distribution.

Contd

The ease and efficiency of packing the powder in the die depends to

a large extent on a wide particle size distribution so that the voids

created between large particles can be progressively filled with

those of smaller size.

Fine particle sizes tend to leave smaller pores which are easily

closed during sintering.

An excess of fines, however, reduces flow properties with the results

already detailed above

METAL POWDER SHAPE SUMMARY

BLENDING OR MIXING

BLENDING OR MIXING

Blending a coarser fraction with a finer fraction ensures that theinterstices between large particles will be filled out.

Powders of different metals and other materials may be mixed inorder to impart special physical and mechanical properties throughmetallic alloying.

Lubricants may be mixed to improve the powders flowcharacteristics.

Binders such as wax or thermoplastic polymers are added toimprove green strength.

Sintering aids are added to accelerate densification on heating.

BLENDING AND MIXING

Blending

Combining powders of the same material but possibly different

particle sizes

Mixing

Combining powders of different materials

BLENDING /MIXING DEVICES PICTORIAL

DESCRIPTION

BLENDING POWDERS PICTORIAL DESCRIPTION

Some common equipment geometries for mixing or blending powders. (a) cylindrical, (b) rotating cube, (c) double cone, and (d) twin shell.

BLENDER PICTORIAL DESCRIPTION

POWDER MIXING MACHINE

MIXING/BLENDING MACHINE PICTORIAL

DESCRIPTION

POWDER

CONSOLIDATION/COMPACTION

POWDER CONSOLIDATION/COMPACTION

In the typical powder pressing process a powder compaction press is

employed with tools and dies.

A die cavity that is closed on one end (vertical die, bottom end closed

by a punch tool) is filled with powder.

The powder is then compacted into a shape and then ejected from

the die cavity. Various components can be formed with the powder

compaction process.

The compaction step requires the part to be removable from the die

in the vertical direction with no cross movements of the tool

members.

The pressing process bonds the powder particles together only

through mechanical clamping and cold welding.

The pressed part thus formed, known as a green compact.

CONVENTIONAL PRESSING IN PM PICTORIAL

DESCRIPTION

Pressing in PM: (1)

filling die cavity with

powder by automatic

feeder; (2) initial and (3)

final positions of upper

and lower punches

during pressing, (4) part

ejection.

COMPACTION PICTORIAL DESCRIPTION

High pressure is applied to squeeze the powder into the desired

shape

COMPACTING CYCLE PICTORIAL DESCRIPTION

EXAMPLE OF A POWDER PRESS

COMPACTION PRESS PICTORIAL DESCRIPTION

Uses 100-300 ton press

Selection of the press

depends on the part and

the configuration of the part

MN (825 ton) mechanical press for compacting metal powder.

COMPACTION SUMMARY

Application of high pressure to the powders to form them into the

required shape

Conventional compaction method is pressing, in which opposing

punches squeeze the powders contained in a die

The work part after pressing is called a green compact, the word

green meaning not yet fully processed

The green strength of the part when pressed is adequate for

handling but far less than after sintering

SINTERING

SINTERING

Sintering is a heat treatment wherein the pressed parts gain strength.

The most common sintering temperature range for iron-based alloys

is 1100 - 1250C.

The time at temperature varies between 10 and 60 minutes,

depending on the application.

The most common type of furnaces is the mesh belt furnace.

Components are placed on a tray, or directly on the mesh belt, which

transports them through the furnace.

An atmosphere, which prevents oxidation, is necessary in the

sintering furnace.

CONTD

A sintering operation consists of de-waxing, sintering and cooling steps.

In the de-waxing zone of the furnace, the lubricant is burned off.

In the cooling zone of the sintering furnace, the parts are cooled under

protective atmosphere in order to not oxidise in contact with air.

The cooling speed, especially in the range 850 - 500C, also affects the

mechanical properties, due to phase transformations in the material.

The main mechanisms of sintering are surface and volume diffusion.

SINTERING PARTICLE BONDING PICTORIAL

DESCRIPTION Heats the powder below the melting point to allow solid-state

diffusion and bond the particles together

SINTERING PARTICLE BONDING PICTORIAL

DESCRIPTION

Diagram of particles in sintering, showing the possible movements of atoms

LIQUID-PHASE SINTERING

The presence of a liquid phase significantly increases the rate of

sintering. Thus this process is commonly used in industry for both

metal and ceramic alloys (e.g., cemented carbide cutting tools).

Substantially full densities can be obtained through good wetting of

the liquid on the solid particles, thus eliminating porosity.

In this multistage process, the powders temperature is first raiseduntil the melting of one of the components. During this stage, solid

state sintering is already initiated. Subsequently, in the presence of

the liquid phase, densification occurs through rearrangements (due

to capillary forces), solution re-precipitation (i.e., grain growth), and

final solid-state sintering.

LIQUID PHASE SINTERING PICTORIAL DESCRIPTION

SINTERING TEMPERATURES AND TIME FOR DIFFERENT

METALS

SINTERING PRODUCTION LINES PICTORIAL

DESCRIPTION

PICS OF SINTERING PRODUCTION LINES

SINTERING FURNACE

SINTERING STRENGTH RELATED TO DENSITY

Strength of sintered structures as related to density, showing that the strength is higher when the density is higher (less

residual porosity)

OTHER PRESSING AND SINTERING

METHODS FOR METALLIC POWDER

ALTERNATIVES TO PRESSING AND SINTERING

Conventional press and sinter sequence is the most widely used

shaping technology in powder metallurgy

Additional methods for processing PM parts include:

Slip Casting

Cold Isostatic Pressing

Hot Isostatic Pressing

Powder Extrusion

Injection Molding

Powder Rolling

SLIP CASTING

Green compacts of tungsten, molybdenum, are made by this

process.

A slurry mixture with metal powder is made.

Plaster of Paris is poured.

As mold is porous so the liquid drains off leaving a solid layer of

material on the surface.

For hollow objects, upon drying green compacts are sintered.

SLIP CASTING PICTORIAL DESCRIPTION

ISOSTATIC PRESSING

High pressures are used during compacting.

Isostatic pressing means the pressure exerting medium is a gas.

Hydrostatic pressing refers to the pressure exerting medium

containing liquid.

In Isostatic pressing, the powder is sealed in an elastic mould and

exerted to the hydrostatic pressure of a liquid pressure medium.

Two types of Isostatic molding are there

(A) Cold Isostatic Pressing

(B) Hot Isostatic Pressing

COLD ISOSTATIC PRESSING

CIP is a process in which powder materials is compressed in a

temperature region where high temperature deformation mechanics

like dislocation or diffusion creep can be neglected.

It is the most important compaction method in powder metallurgy.

It is conducted at room temperature..

Metal powder is placed in a rubber mold.

It is then pressurized hydrostatically in a chamber with pressure up

to 400 MPa & then sintered.

CONTD.

There are two types of cold Isostatic pressing

(A) Wet Bag

(B) Dry Bag

WET BAG

In the wet bag method the mold is removed and refilled after each

pressure cycle.

This method is suitable for compaction of large and complicated

parts.

DRY BAG

In this method the mold is an integral part of the vessel.

The dry bag method is suitable for compaction of simpler and smaller

parts.

COLD ISOSTATIC PRESSING PICTORIAL DESCRIPTION

Schematic diagram, of cold isostatic, as applied to forming atube.The powder is enclosed in a flexible container around a solidcore rod.Pressure is applied iso-statically to the assembly inside ahigh-pressure chamber.

COLD ISOSTATIC PRESSING PICTORIAL DESCRIPTION

HOT ISOSTATIC PRESSING

HIP involves Isostatic pressing conducted at increased temperature.

As a pressure medium a gas (Nitrogen or Argon) is used.

The work pressures, which are applied in the hot Isostatic pressing

method, are commonly b/w 100 MPa to 300 MPa.

HIP combines pressing and sintering, causing consolidation of powder

particles, healing voids and pores.

The part shrinks and densifies, forming sound high strength structure.

The method may be used without a mold.

In this case the part is first compacted by cold Isostatic pressing

method, and then it is sintered in order to close the interconnecting

porosity.

HOT ISOSTATIC PRESSING PICTORIAL DESCRIPTION

The sintered (but still porous) part is then pressed Isostatically at

high temperature without any can (mold).

HOT ISOSTATIC PRESSING PICTORIAL DESCRIPTION

Schematic illustration of hot isostatic pressing. The pressure and

temperature variation vs.time are shown in the diagram

PICTURE OF AN ISOSTATIC PRESS

ISOSTATIC PRESSING SUMMARY

Uses pressurized fluid to compress the powder equally in all directions

Cold Isostatic Pressing

Compaction performed at room temperature

Hot Isostatic Pressing

Performed at high temperatures and pressures

PM MANUFACTURING SUMMARY USING HOT

ISOSTATIC PROCESS

POWDER EXTRUSION

POWDER EXTRUSION PICTORIAL DESCRIPTION -I

Powders are placed in vacuum tight sheet can, heated and extruded

with container

POWDER EXTRUSION PICTORIAL DESCRIPTION-II

The powder can be extruded within a container or after being formed

into billets using conventional compaction and sintering

POWDER ROLLING

POWDER ROLLING PICTORIAL DESCRIPTION

Powder is compressed in a rolling mill to form a strip

METAL INJECTION MOLDING

The processing technology comprises the following stages:

Mixing the fine metallic powder with 30% - 40% of a binder low melt polymer.

Injection of the warm powder with molten binder into the mold by means of

the screw.

Removal of the part from the mold after cooling down of the mixture.

De-binding removal of the binder. There are two de-binding methods:

solvent debinding the binder is dissolved by a solvent or by water;

thermal debinding the binder is heated above the volatilization temperature.

Sintering the green compact

METAL INJECTION MOLDING

The powder is mixed with a binder and molded, and the binder is

removed before sintering

FINISHING OPERATIONS

Finishing operations include:

Machining

Heat Treatment

Calibration

Infiltration

Oil Impregnation

Sizing and Coining

Joining

MACHINING

Wherever possible final machining operations are avoided to reduce

costs.

However there are features, such as re-entrant angles and cross holes,

that cannot be developed in the pressed component and must be

produced by machining, usually after final sintering.

In some cases, where the fully sintered material is too strong to

machine economically, the part is pre-sintered to give some strength,

machined and then fully sintered to fully develop the properties.

Where possible the material composition is altered to enhance its

machine ability.

HEAT TREATMENT

Powder metallurgy components are usually heat treated, to develop

the desired mechanical properties.

However, it is important to remember that there is interconnected

porosity in the components and that any gaseous process could well

affect the core of the material as well as the external surface.

The usual processes of carburizing, nitro-carburizing, carbo-nitriding,

etc can be carried out to provide hardened surfaces.

Heat treatment induces considerable corrosion resistance, increased

hardness, increased resistance to compressive strength, and

improved wear resistance.

CALIBRATION

During calibration the sintered component is re-pressed in a

calibration tool similar to the pressing tool at pressures of 60 to 80

kN/cm2.

This improves the mechanical properties through strain hardening, in

addition to the dimensional accuracy and surface quality.

Especially softer materials of sintering class C can be improved

significantly through calibration.

INFILTRATION

Infiltration is a secondary process step used to either improve

strength or seal parts and make them gas- or liquid-tight. e.g. copper-

based alloys infiltrate ferrous parts, usually during the sintering

phase.

Infiltration makes the components impermeable and there is some

increase in mechanical properties, but at expense of dimensional

accuracy.

Infiltration simplifies some heat treatments.

For instance, it is easier to obtain a defined case depth without

interconnected porosity.

OIL IMPREGNATION

Sintered parts achieve greater protection against corrosion by being

impregnated by oil or other non-metallic material.

Self-lubricating bearings are manufactured by impregnating porous

sintered bearings with lubricants and these bearings can only be

produced by powder metallurgy.

Through oil impregnation, used on PM self-lubricating bearing

components, components can absorb 12% 30% oil by volume.

Oil impregnation can also be performed on PM components to

improve machine ability or to prepare the surface for plating.

OIL IMPREGNATED PRODUCTS

Oil-impregnated Porous Bronze Bearingsnic.sav.sk

www.hd-bearing.com

www.ondrives.com

Metal filters

VACUUM OIL IMPREGNATION

SIZING AND COINING

Sizing and coining are additional press operations after sintering.

The main objective is to improve the dimensional accuracy, but the

surface finish is also normally improved.

Quite moderate pressures are normally required for sizing, since only

a slight plastic deformation is necessary.

Coining has a double purpose.

Not only is dimensional accuracy improved, but the use of higher

pressures also increases the density of the part.

Normally, a press tool specific to the task of sizing or coining is used.

FINISHING OPERATION SUMMARY

PM PROCESS SUMMARY

PRODUCTION/ECONOMIC GUIDELINES FOR PM

Economics usually require large quantities to justify cost of

equipment and special tooling

Minimum quantities of 10,000 units are suggested

PM is unique in its capability to fabricate parts with a controlled level

of porosity

Porosities up to 50% are possible

PM can be used to make parts out of unusual metals and

alloys - materials that are difficult if not impossible to produce by

other means

DESIGN GUIDELINES FOR PM PARTS -

Part geometry must permit ejection from die

Part must have vertical or near vertical sides, although steps are

allowed

Design features like holes and undercuts on part sides must be

avoided

Vertical undercuts and holes are permissible because they do not

interfere with ejection

Vertical holes can have cross-sectional shapes other than round

without significant difficulty

Part features to be avoided in PM: side holes and (b) side undercuts since part ejection is impossible.

SIDE HOLES AND UNDERCUTS

Chamfers and corner radii are accomplished but certain rules should be observed: (a) avoid acute angles; (b) larger angles preferred for punch rigidity; (c) inside radius is desirable; (d) avoid full outside corner radius because punch is fragile at edge; (e) problem solved by combining radius and chamfer.

CHAMFERS AND CORNER RADII

PM COMPARISON WITH OTHER MANUFACTURING

PROCESS

Forged on left; P/M on right

POWDER METALLURGY: CONNECTING RODS

www.dps-performance.com

POWDERED METAL TRANSMISSION GEAR

Warm compaction method with 1650-ton press Teeth are molded net shape: No machining UTS = 155,000 psi 30% cost savings over the original forged part

www.chipm.com

POWDERED METAL TURBINE BLADE-DISK 1 PIECE!

PM SUMMARY

ASSIGNMENT

Q1. what is the commercial importance of PM?

Q2. What do you understand by the term mesh count?

Q3. What do you understand by open pores and closed pores in

metallic powder?

Q4. what is meant by the term green compact

Q5 why we need a control atmosphere furnace in sintering

NEXT LECTURE CERAMICS AND

GLASS FORMING

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