Topic 4.pdf

Topic 4 - Powder Metal Process

Introduction
 Powder metallurgy (PM), in which metal powders are compacted into desired and often complex
shapes and sintered (heated without melting) to form a solid piece.
 First was used in Egypt in about 3000 B.C. to make iron tools.
 Early 1900s, one of the first uses of PM was making the tungsten filaments for incandescent light
bulbs.
 A wide range of parts and components are made by PM techniques;
o Balls for ballpoint pens;
o Automotive components (about 70% of the PM market)
 Piston rings, connecting rods, brake pads, gears, cams, and bushings
o Tool steels, tungsten carbides, and cermet as tool and die materials
o Graphite brushes impregnated with copper for electric motors
o Magnetic materials
o Metal filters and oil-impregnated bearings with controlled porosity
o Metal foams
o Surgical implants

Introduction
 The most commonly used metals in PM:
 iron, copper, aluminum, tin, nickel,
titanium, and the refractory metals.
 Pre-alloyed powders
o are used for parts made of brass,
bronze, steels, and stainless steels.
 The sources for metals
o are generally bulk metals and alloys,
ores, salts, and other compounds.
 Basic PM Process consists 5
steps:
i. Po wd er P ro d u c tio n
ii. B len d in g
iii. Co m p ac tin g
iv. Sinterin g
v. F in is h ing
Powder Production
Blending
Compaction
Sintering
Finishing Operations
Additives (lubricant or Binder)
Atomization
Chemical Reduction
Electrolytic deposition
Comminution
Mechanical alloying
Cold
Hot
Pressing
Isostatic pressing
Combustion synthesis
Rolling
Extrusion
Injection molding
Isostatic Pressing
Atmosphere
Vacuum
Coining
Forging
Machining
Heat treating
Impregnation
Infiltration
Plating
B a s i c P M P r o c e s s

Introduction
 Elimination or reduction of machining
• Accuracy of dimension
• Fine product
 High production rates
• All steps in PM process are simple and readily
automated.
• Low labor cost
 Production of complex shape
• Quite complex shape can be produced (combination
gears, cam)
 Possibility of wide variations in compositions
• Can be readily produced for parts of very high purity.
 Availability of wide variations in properties
• Magnetic, wear and others properties can be
designed to match the need of specific application.
 Reduction or elimination of scrap
• No material is wasted
 Inferior strength properties
• May be limited use when high stresses are involved.
• Need to select the appropriate material for a particular
use.
 Relatively high die cost (tooling and equipment)
• Die must be made strong and massive.
o High pressure and severe abrasion involve in the
process.
 High material cost
• On a unit weight basis, powder metals are considerably
more expensive than wrought or cast stock.
 Design Limitations
• Simply not feasible for many shapes.
• Parts ejected from the die.
• Thin sections are difficult.
 Property variations produced by variations in
density.
• non -uniform product density resulting in non -uniform
shape compaction. (affecting the property)
Advantages Disadvantages

Production of Metal Powders
Atomization
Chemical Reduction
Mechanical Comminution (Milling and Crushing)
Mechanical alloying
Methods:
The properties of PM products highly dependent upon the characteristics of the material (metal)
powder that are used.
Imparting distinct properties and
characteristics to the resulting
powder hence to the final product.
Important Properties and characteristic:
o Chemistry and purity
o Particle size
o Size distribution
o Particle shape
o Surface texture of particle

Atomization
Methods of metal powder production
by atomization:
o Gas atomization
o Water atomization
o Centrifugal atomization with a spanning disk or
cup
o Atomization with a consumable electrode

Atomization
 Involves a liquid-metal stream produced by injecting
molten metal through a small orifice.
 The stream is broken up by jets of inert gas or air or
water, known as gas or water atomization,
respectively.
 The size and shape of the particles formed depend
on the temperature of the molten metal, rate of
flow, nozzle size, and jet characteristics.
 The use of water results in a slurry of metal powder
and liquid at the bottom of the atomization
chamber.
 Although the powders must be dried before they
can be used, the water allows for more rapid cooling
of the particles and thus higher production rates.
 Gas atomization usually results in more spherical
particles.
Water atomization
Gas atomization

Atomization
 In centrifugal atomization, the molten-metal
stream drops onto a rapidly rotating disk or cup;
the centrifugal forces break up the stream and
generate particles.
 In a variation of this method, a consumable
electrode is rotated rapidly (about 15,000
rev/min) in a helium-filled chamber.
 The centrifugal force breaks up the molten tip of
the electrode into metal particles.
Atomization with a
consumable electrode
Centrifugal atomization with a
spanning disk or cup

Chemical Reduction
 The reduction (removal of oxygen) of metal oxides
o Uses gases as reducing agents.
o Hydrogen and carbon monoxide
 By this means, very fine metallic oxides are reduced to
the metallic state.
 The powders produced
o Spongy and porous,
o Uniformly sized spherical or angular shapes.

 Utilizes either aqueous solutions or fused salts.
 The powders produced are among the purest
available.
 Direct deposition of a loosely adhering powdery
or spongy deposit that can easily be disintegrated
mechanically into fine particles (e.g. Cu and Ag)
 Deposition of a dense, smooth, brittle layer of
refined metal that can be ground into powder
(e.g. Fe and Mn).

Mechanical Comminution
 Mechanical comminution (pulverization)
 Involves , milling in a ball mill, or grinding brittle or
less ductile metals into small particles.
o Brittle metals - the particles produced have angular shapes
o Ductile metals - they are flaky and not particularly suitable
for powder metallurgy applications
 A ball mill is a machine with a rotating hollow cylinder,
partly filled with steel or white cast iron balls.
o The powder or particles placed into a ball mill are
impacted by the balls as the cylinder is rotated, or its
contents may be agitated.
 This action has two effects:
o The particles periodically fracture, resulting in smaller
particles
o The shape of the particles is affected.
Roll crushing
Ball Milling
Hammer Milling/Grinding

Mechanical alloying
 In mechanical alloying, powders of two or more pure metals are mixed in a ball mill.
 Under the impact of the hard balls, the powders fracture and bond together by diffusion,
entrapping the second phase and forming alloy powders.
 The dispersed phase can result in strengthening of the particles, or can impart special electrical or
magnetic properties to the powder.

Characteristics of Metal Powder
i. Powder particle size and distribution
ii. Particle shape and microstructure
iii.Chemical Composition
iv.Flow characteristic

Characteristics of Metal Powder i. Powder particle size and distribution
o Particle size is generally controlled by screening, that is, by passing
the metal powder through screens (sieves) of various mesh sizes.
o The screens are stacked vertically, with the mesh size becoming finer
as the powder flows downward through the screens.
o The larger the mesh size, the smaller is the opening in the screen.
o A mesh size of 30, for example, has an opening of 600 µm, size 100
has 150 µm, and size 400 has 38 µm.
o This method is similar to the numbering of abrasive grains, the larger
the number, the smaller is the size of the abrasive particle.
o The particle size influences the control of porosity, compressibility
and amount of shrinkage.
o Particle size distribution influences the packing of powder and its
behaviour during moulding and sintering.

Characteristics of Metal Powder ii. Particle shape and microstructure
o A major influence on processing
characteristics, particle shape usually is
described in terms of aspect ratio or
shape factor (SF).
o Aspect ratio is the ratio of the largest
dimension to the smallest dimension of
the particle, and ranges from unity, for a
spherical particle, to about 10, for
flakelike or needle like particles.
o Shape factor (shape index) is a measure of
the ratio of the surface area of the
particle to its volume. Normalized by
reference to a spherical particle of
equivalent volume
o Thus, the SF for a flake is higher than that
for a sphere.
1D 2D
3D

Characteristics of Metal Powder iii. Chemical Composition
o The chemical composition of powders is the outstanding characteristic.
o It usually reveals the type and percentage of impurity and determines the particle
hardness and compressibility.
o The term impurity refers to some elements or compounds which has an undesirable effect.
o Impurities influence not only the mechanical properties of the powder compacts, but also
their chemical electrical and magnetic properties.
o It may also exert a decisive effect on pressing, sintering and other post sintering operation
which are essential for the production of finished product from powders.
o The chemical composition of a powder is determined by the well established standard
techniques of chemical analysis.
o Oxygen content is determined either by wet analysis or by loss of weight in hydrogen.
o Some oxides may not be reduced at all or there may be error due to incomplete reduction
of oxides, therefore it is desirable for both processing and optimum properties of the final
product to have a low oxygen content.

Characteristics of Metal Powder iv. Flow characteristic
o The flow rate is a very important – characteristic of powders which measures, the ability of a
powders to be transferred.
o It is defined as the rate at which a metal powder will flow under gravity from a container through
an orifice, both having the specific shape and finish.
o The powder filling of die must be rapid and uniform without bidge formation for obtaining a
rapid rate of production consistent compacts and economy.
o On the other hand poor flow properties of the powder result in slow and uneconomical feeding
of the cavity and the possibility during pressing of uneven filling of the die cavity.
o It is affected only by particle size, size distribution and shape, but also by absorbed air or gas,
moisture, lubricant, coefficient of inter particle friction etc. In general line or dendritic, irregular,
coarse and spherical powders have poor, reduced, good and maximum flow rates respectively.
o Flow rate increases with decreased particle irregularly and increased particle size, specific
gravity, and apparent density.
o It can also be increased by tapping or vibrating.

Blending Process
 Powders made by different processes will have different sizes
and shapes and must be well mixed-impart special physical &
mechanical properties & characteristic.
 Blending can obtain uniformity from part to part.
 Lubricants can be mixed with the powders to improve their flow
characteristics (reduce friction, improve flow & die life).
(a) to (d) Some common bowl geometries for mixing
or blending powders.
A mixer suitable for blending metal powders. Since
metal powders are abrasive, mixers rely on the
rotation or tumbling of enclosed geometries as
opposed to using aggressive agitators.
Rotating Drum
Rotating Double Cone

Compacting Process
 Blended metal powders are pressed
together into various shape of die.
 The powder must flow easily into the die.
 In compaction, size distribution is
important that:
o They should not be all the same size.
o Should be a mixture of large and
small particle
 The higher the density; the higher the
strength.
 The density of the metal powders
depends on the pressure applied.
 The pressed powder parts called Green
compact
o which is low density and strength
o very fragile, and can easily crumble or
become damaged
Compaction of metal powder to form a bushing.
Typical tool and die set for compacting a spur gear.
Source: Courtesy of Metal Powder Industries Federation.

Sintering Process
 Green compacts are heated in a furnace to a temperature below melting point.
 Improves the strength of the material.
 Proper furnace control is important for optimum properties.
 Particles start forming a bond by diffusion.
 Vapor phase transport
o Heated very close to melting temperature allows metal atoms to release to the vapor phase.
 Sintering temperatures are generally within 70-90% of the melting point of the metal or alloy.
 Sintering times range from a minimum of about 10 min for iron and copper alloys to as much as
eight hours for tungsten and tantalum.
 Continuous-sintering furnaces, which are used for most production, have three chambers:
i. Burn-off chamber, for volatilizing the lubricants in the green compact, in order to improve
bond strength and prevent cracking.
ii. High-temperature chamber, for sintering
iii. Cooling chamber

Sintering Process
Illustration of two mechanisms for sintering
metal powders:
(a) Solid-state material transport
(b) Vapor-phase material transport.
R = particle radius,
r = neck radius, and
p = neck-profile radius.

Sintering Process
Sintering on a microscopic scale : Sintering at between 70% and 90% of the
metal's melting point (absolute scale)
(1) Particle bonding is initiated at contact points;
(2) Contact points grow into "necks";
(3) The pores between particles are reduced in size;
(4) Grain boundaries develop between particles in place of the necked regions

Sintering Process
Schematic illustration of liquid phase sintering using a mixture of two powders.
a) Green compact of a higher melting point base metal and
lower temperature additive;
b) Liquid melting, wetting and reprecipitation on surfaces;
c) Fully sintered solid material.

Finishing Operations
 To improve the properties of sintered P/M products several additional operations may be used:
o Coining and sizing
 compaction operations
• Pressing a sintered part to improve dimensional accuracy (sizing)
• Press working operation on a sintered part to press details into its surface (coining)
o Impact forging – cold or hot forging may be used
o Parts may be impregnated with a fluid to reduce the porosity
o Infiltration
 metal infiltrates the pores of a sintered part to produce a stronger part and produces a
pore free part.
o Other finishing operations:
a. Heat treating
b. Machining
c. Grinding
d. Plating

Consideration on Parts design for PM
 Simple and uniform shape as possible.
 Provision for ejection without damaging the
 green compact.
 Made with the widest acceptable tolerances to
maximize tool life.
 Walls should not be less than 1.5 mm thick; walls with
length-to-thickness ratios above 8:1 are difficult to
press.
 A true radius cannot be pressed; instead use a
chamfer.

Consideration on Parts design for PM
Examples of P/M parts showing poor and good designs.
Note that sharp radii and re-entry corners should be avoided
and that threads and transverse holes have to be produced
separately by additional machining operations.
Source: Courtesy of Metal Powder Industries Federation.
PP from Dr Mas Ayu Bt Hassan, Faculty of Mechanical Engineering, UMP

Costs Advantages for PM
 Zero or minimal scrap
 High production rates
 Avoids high machining cost needed for holes, gear teeth,
etc.
 Extremely good surface finish
 Very close tolerance without a machining operation
 Assembly of two or more parts (by injection molding) can
made one piece.

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