2. Metal processing technology in which parts
are produced from metallic powders
2
Modern powder
metallurgy dates
only back to the
early 1800 PM parts can be mass produced to net shape
or near net shape, eliminating or reducing the
need for subsequent machining
PM process wastes very little material ~ 97% of starting powders
are converted to product
PM parts can be made with a specified level of porosity, to
produce porous metal parts
Examples: filters, oil-impregnated bearings and gears
Certain metals that are difficult to fabricate by
other methods can be shaped by PM
Tungsten filaments for lamp bulbs are made by PM
3. . . . is a forming technique
Essentially, Powder Metallurgy (PM) is an art &
science of producing metal or metallic powders, and
using them to make finished or semi-finished products.
Particulate technology is probably the oldest forming
technique known to man.
There are archeological evidences to prove that the
ancient man knew something about it.
3
4. PM parts can be mass produced to net shape or near
net shape, eliminating or reducing the need for
subsequent machining.
PM process wastes very little material - about 97% of
the starting powders are converted to product.
PM parts can be made with a specified level of
porosity, to produce porous metal parts.
Examples: filters, oil-impregnated bearings, gears….
4
Why Powder Metallurgy is
Important?
5. Certain metals that are difficult to fabricate by other
methods can be shaped by powder metallurgy.
Example: Tungsten filaments for incandescent lamp bulbs
are made by PM
Certain alloy combinations and cermets made by PM
cannot be produced in other ways.
PM compares favorably to most casting processes in
dimensional control.
PM production methods can be automated for economical
production.
5
Why Powder Metallurgy is
Important?
6. Electrical Contact materials
Heavy-duty Friction materials
Self-Lubricating Porous bearings
P/M filters
Carbide, Alumina, Diamond cutting tools
Structural parts
P/M magnets
Cermets
and more, such as high tech applications
6
7. Anti-friction products
Friction products
Filters
Electrical Contacts
Sliding Electrical Contacts
Very Hard Magnets
Very Soft Magnets
Refractory Material Products
Hard and Wear Resistant Tools
Ferrous & Non-ferrous Structural parts etc . . .
7
THESE COMPONENTS ARE
USED IN AIR & SPACE
CRAFTS, HEAVY
MACHINERY, COMPUTERS,
AUTOMOBILES, etc…
8. Near Nett Shape is possible, thereby reducing the
post-production costs, therefore:
Precision parts can be produced
The production can be fully automated, therefore,
Mass production is possible
Production rate is high
Over-head costs are low
Break even point is not too large
Material loss is small
Control can be exercised at every stage
8
9. High tooling and equipment costs.
Metallic powders are expensive.
Problems in storing and handling metal powders.
◦ Degradation over time, fire hazards with certain metals
Limitations on part geometry because metal powders
do not readily flow laterally in the die during pressing.
Variations in density throughout part may be a
problem, especially for complex geometries.
9
10. Porous !! Not always desired.
Large components cannot be produced on a large
scale.
Some shapes are difficult to be produced by the
conventional p/m route.
Whatever, the merits are so many that P/M, as a
forming technique, is gaining popularity
10
11. 1. Porous or permeable products such as bearings, filters, and
pressure or flow regulators
2. Products of complex shapes that would require considerable
machining when made by other processes
3. Products made from materials that are difficult to machine or
materials with high melting points
4. Products where the combined properties of two or more
metals are desired
5. Products where the P/M process produces clearly superior
properties
6. Products where the P/M process offers economic advantage
11
12. Gears, bearings, sprockets, fasteners, electrical
contacts, cutting tools, and various machinery parts
When produced in large quantities, gears and bearings
are ideal for PM because:
− The geometry is defined in two dimensions
− There is a need for porosity in the part to serve as a
reservoir for lubricant
PM Products
14. Connecting Rods:
Forged on left; P/M on right
14
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
15. A pure metal in particulate form
Applications where high purity is important
Common elemental powders:
– Iron
– Aluminum
– Copper
Elemental powders can be mixed with other metal
powders to produce alloys that are difficult to
formulate by conventional methods.
– Example: tool steels
15
16. Each particle is an alloy comprised of the desired
chemical composition
Used for alloys that cannot be formulated by mixing
elemental powders
Common pre-alloyed powders:
– Stainless steels
– Certain copper alloys
– High speed steel
16
18. Figure: Several of the possible (ideal) particle shapes in powder
metallurgy.
18
Particle Shapes in PM
19. Powder production
Blending or mixing
Powder compaction
Sintering
Finishing Operations
19
20. 20
Blending and mixing (Rotating drums,
blade and screw mixers)
Pressing - powders are compressed into
desired shape to produce green compact
Accomplished in press using punch-and-die
tooling designed for the part
Sintering – green compacts are heated to
bond the particles into a hard, rigid mass.
Performed at temperatures below the
melting point of the metal
22. Any metal can be made into powder form
Three principal methods by which metallic powders are
commercially produced
1. Atomization (by gas, water, also centrifugal one)
2. Chemical
3. Electrolytic
In addition, mechanical methods are occasionally used to
reduce powder sizes
22
23. High velocity gas stream flows through expansion nozzle,
siphoning molten metal from below and spraying it into
container.
23
Figure: (a) gas atomization method
Gas Atomization Method
24. 24
Produces a liquid-metal stream by injecting molten
metal through a small orifice.
Stream is broken by jets of inert gas, air, or water.
The size of the particle formed depends on the
temperature of the metal, metal flow rate through
the orifice, nozzle size and jet characteristics.
Gas Atomization Method
31. 31
Comminution
Crushing
Milling in a ball mill
Powder produced
– Brittle: Angular
– Ductile: flaky and not particularly suitable for P/M
operations
Mechanical Alloying
Powders of two or more metals are mixed in a ball mill
Under the impact of hard balls, powders fracture and
join together by diffusion
32. 32
Figure Mechanical alloying of nickel particles with dispersed smaller
particles. As nickel particles are flattened between the two balls, the
second smaller phase impresses into the nickel surface and eventually is
dispersed throughout the particle due to successive flattening, fracture,
and welding events.
33. 33
Figure: Particle shapes in metal powders, and the processes by which they
are produced. Iron powders are produced by many of these processes.
34. After metallic powders have been produced, the
conventional PM sequence consists of:
1. Blending and mixing of powders
2. Compaction - pressing into desired shape.
3. Sintering - heating to temperature below melting point to
cause solid-state bonding of particles and strengthening of part.
In addition, secondary operations are sometimes
performed to improve dimensional accuracy, increase
density, and for other reasons.
34
35. Figure: Conventional
powder metallurgy
production sequence:
(1) blending,
(2) compacting,
shows the operation and/or
work part during the
sequence.
(3) Sintering;
shows the condition of the
particles while sintering
35
36. • Blending - powders of same chemistry but possibly different
particle sizes are intermingled
– Different particle sizes are often blended to reduce porosity
• Mixing - powders of different chemistries are combined .
PM technology allows mixing various metals into alloys that
would be difficult or impossible to produce by other means.
36
For successful results in compaction and
sintering, the starting powders must be
homogenized (powders should be blended
and mixed).
37. • 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 powder’s flow
characteristics.
• Binders such as wax or thermoplastic polymers are added to
improve green strength.
• Sintering aids are added to accelerate densification on
heating.
37
38. 38
Figure: (e) 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.
Some common equipment geometries used
for blending powders
(a) Cylindrical, (b) rotating cube, (c) double
cone, (d) twin shell
39. 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.
39
40. Press powder into the desired shape and size in dies using
a hydraulic or mechanical press
Pressed powder is known as “green compact”
Stages of metal powder compaction:
40
41. 41
Powders do not flow like liquid, they simply compress until
an equal and opposing force is created.
– This opposing force is created from a combination of
(1) resistance by the bottom punch and
(2) friction between the particles and die surface
Compacting consolidates and densifies the component for
transportation to the sintering furnace.
Compacting consists of automatically feeding a controlled
amount of mixed powder into a precision die, after which it is
compacted.
42. Loose powder is compacted and densified into a shape,
known as green compact.
Most compacting is done with mechanical presses and rigid
tools.
◦ Hydraulic and pneumatic presses are also used.
42
43. 43
Figure: (Left) Typical press for the compacting of metal powders. A removable
die set (right) allows the machine to be producing parts with one die set while
another is being fitted to produce a second product.
44. If an extremely complex shape is desired, the powder
may be encapsulated in a flexible mold, which is then
immersed in a pressurized gas or liquid
◦ Process is known as isostatic compaction
In warm compaction, the powder is heated prior to
pressing
The amount of lubricant can be increased in the powder
to reduce friction
Because particles tend to be abrasive, tool wear is a
concern in powder forming
44
45. Parts are heated to ~80% of melting temperature.
Transforms compacted mechanical bonds to much
stronger metal bonds.
Many parts are done at this stage. Some will require
additional processing.
45
46. Figure: Sintering on a microscopic scale: (1) particle bonding is initiated
at contact points; (2) contact points grow into "necks"; (3) the pores
between particles are reduced in size; and (4) grain boundaries develop
between particles in place of the necked regions.
46
Sintering Sequence
Parts are heated to 0.7~0.9 Tm.
Transforms compacted mechanical bonds to much
stronger metallic bonds.
47. In the sintering operation, the pressed-powder compacts are
heated in a controlled atmosphere to right below the
melting point.
Three stages of sintering
Burn-off (purge)- combusts any air and removes lubricants or
binders that would interfere with good bonding
High-temperature- desired solid-state diffusion and bonding occurs
Cooling period- lowers the temperature of the products in a
controlled atmosphere.
All three stages must be conducted in oxygen-free conditions
of a vacuum or protective atmosphere.
47
48. Figure: (a) Typical heat treatment cycle in sintering; and (b)
schematic cross section of a continuous sintering furnace.
48
Sintering Cycle and Furnace
51. The porosity of a fully sintered part is still significant (4-
15%).
Density is often kept intentionally low to preserve
interconnected porosity for bearings, filters, acoustic
barriers, and battery electrodes.
However, to improve properties, finishing processes are
needed:
◦ Cold restriking, resintering, and heat treatment.
◦ Impregnation of heated oil.
◦ Infiltration with metal (e.g., Cu for ferrous parts).
◦ Machining to tighter tolerance.
51
52. Secondary operations are performed to increase density,
improve accuracy, or accomplish additional shaping of
the sintered part.
• Repressing - pressing sintered part in a closed die to increase
density and improve properties
• Sizing - pressing a sintered part to improve dimensional
accuracy
• Coining - pressworking operation on a sintered part to press
details into its surface
• Machining - creates geometric features that cannot be achieved
by pressing, such as threads, side holes, and other details
52
53. Conventional press and sinter sequence is the most
widely used shaping technology in powder metallurgy.
Additional methods for processing PM parts include:
◦ Isostatic pressing
◦ Powder injection molding
◦ Powder rolling, extrusion and forging
◦ Combined pressing and sintering
◦ Liquid phase sintering
53
Alternative Pressing and Sintering
Techniques
54. 54
Cold isostatic pressing is performed at room temperature
with liquid as the pressure medium. Hot isostatic pressing
is performed at elevated temperature with gas as the
pressure medium.
55. Hot-isostatic pressing (HIP) combines powder compaction
and sintering into a single operation
◦ Gas-pressure squeezing at high temperatures
Heated powders may need to be protected from harmful
environments.
Products emerge at full density with uniform, isotropic
properties.
Near-net shapes are possible.
The process is attractive for reactive or brittle materials,
such as beryllium (Be), uranium (U), zirconium (Zr), and
titanium (Ti).
55
56. HIP is used to
Densify existing parts
Heal internal porosity in casting
Seal internal cracks in a variety of products
Improve strength, toughness, fatigue resistance, and
creep life.
HIP is relative long, expensive and unattractive for
high-volume production
56
57. Metal powder placed in
a flexible rubber mold
Assembly pressurized
hydrostatically by water
(400 – 1000 MPa)
Typical: Automotive
cylinder liners →
57
58. 58
Figure: Schematic diagram of cold isostatic pressing, as applied to
forming a tube. The powder is enclosed in a flexible container around a
solid-core rod. Pressure is applied isostatically to the assembly inside a
high-pressure chamber
59. During sintering, a liquid phase from the lower MP
component, may exist.
Alloying may take place at the particle-particle interface.
Molten component may surround the particle that has not
melted.
High compact density can be quickly attained.
Important variables:
◦ Nature of alloy, molten component/particle wetting,
capillary action of the liquid.
59
60. 60
Metal injection molding
Pellets made of powders and binder
Heated to molding temperature and
injected into a mold
Can create complex designs
61. 61
Figure: Flow chart of MIM process
used to produce small, intricate shaped
parts from metal powder.
Figure: Metal injection
molding is ideal for producing
small, complex parts.
62. Ultra-fine spherical-shaped metal, ceramic, or carbide
powders are combined with a thermoplastic or wax.
◦ Becomes the feedstock for the injection process
The material is heated to a paste like consistency and
injected into a heated mold cavity.
After cooling and ejection, the binder material is
removed.
◦ Most expensive step in MIM and PIM
62
63. Table: Comparison of conventional powder metallurgy and metal
injection molding
63
Feature P/M MIM
Particle size 20-250 mm <20 mm
Particle response Deforms plastically Un deformed
Porosity (% nonmetal) 10 – 20% 30 - 40%
Amount of binder/Lubricant 0.5 – 2% 30 – 40%
Homogeneity of green part Non homogeneous Homogeneous
Final sintered density <92% > 96%
