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
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
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.
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
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 OperationFLOW CHART PM
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.
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
.
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
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
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 SIZE/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
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
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 be mixed to improve the powders’ flow characteristics.
Binders such as wax or thermoplastic polymers are added to improve green strength.
BLENDING AND MIXING
Blending
Combining powders of the same material but possibly different particle sizes
Mixing
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.
MIXING/BLENDING MACHINE PICTORIAL
DESCRIPTION
POWDER
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.
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
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 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.
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
.
SINTERING TEMPERATURES AND TIME FOR DIFFERENT METALS
SINTERING PRODUCTION LINES PICTORIAL
DESCRIPTION
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
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.
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.
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
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
PM MANUFACTURING SUMMARY USING HOT
ISOSTATIC PROCESS
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 PICTORIAL DESCRIPTION
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.
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
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 Bearings
nic.sav.sk
www.hd-bearing.com
www.ondrives.com
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.
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
Part features to be avoided in PM: side holes and (b) side undercuts since part ejection is impossible.
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
Forged on left; P/M on right
POWDER METALLURGY: CONNECTING RODS
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
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
