Powder Metallurgy

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POWDER METALLURGY
MODULE 5A

LEARNING OBJECTIVES







Introduction
Advantages/Limitations
PM Products
PM Materials/Process Description
PM Steps







Alternate methods to compact and sintering






Powder manufacturing
Mixing/blending
Conventional pressing / compaction
Conventional 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 or
reducing 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 or
increased wear resistance.



Provides controlled porosity for self-lubrication or filtration.



Long term reliability through close control of dimensions and physical
properties.



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
Mixing particles of different sizes allows decreased porosity and a
higher packing ratio

void

smaller, more numerous voids

voids filled by smaller particles, small voids
remain

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 the
interstices between large particles 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
characteristics.



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



Sintering aids are added to accelerate densification on heating.

be

mixed

to

improve

the

powders’

flow

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 - 1250°C.



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 - 500°C, 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 powder’s temperature is first raised
until 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 a
tube.The powder is enclosed in a flexible container around a solid
core rod.Pressure is applied iso-statically to the assembly inside a
high-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. copperbased 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.

Metal filters

OIL IMPREGNATED PRODUCTS

Oil-impregnated Porous Bronze Bearings

nic.sav.sk

www.ondrives.com

www.hd-bearing.com

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

SIDE HOLES AND UNDERCUTS



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

CHAMFERS AND CORNER RADII



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.

PM COMPARISON WITH OTHER MANUFACTURING
PROCESS

POWDER METALLURGY: CONNECTING RODS

www.dps-performance.com

Forged on left; P/M on right

POWDERED METAL TRANSMISSION GEAR

www.chipm.com

 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

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
THANK YOU

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