Powder Metallurgy-Module III

Powder Metallurgy
Powder Metallurgy,
Powder Production methods,
Powder characteristics,
Micromachining
ME 010 803 PRODUCTION ENGINEERING
Module III

Overview
 The Characterization of Engineering Powders
 Production of Metallic Powders
 Using them to make finished/semi-finished products.
 Conventional Pressing and Sintering
 Alternative Pressing and Sintering Techniques
 Materials and Products for PM
 Design Considerations in Powder Metallurgy
2

3
Modern powder
metallurgy dates
only back to the
early 1800
Powder Metallurgy (PM)
Metal processing technology in which parts
are produced from metallic powders
 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

Introduction
• Earliest use of iron powder dates back to 3000 BC.
Egyptians used it for making tools
• Modern era of P/M began when W lamp filaments were
developed by Edison
• Components can be made from pure metals, alloys, or
mixture of metallic and non-metallic powders
• Commonly used materials are iron, copper, aluminium,
nickel, titanium, brass, bronze, steels and refractory metals
• Used widely for manufacturing gears, cams, bushings,
cutting tools, piston rings, connecting rods, impellers etc.
4

Powder Metallurgy
. . . 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.
5

History of Powder Metallurgy
 IRON Metallurgy >
 How did Man make iron in 3000 BC?
 Did he have furnaces to melt iron air blasts, and
 The reduced material, which would then be spongy,
[DRI ], used to be hammered to a solid or to a near
solid mass.
 Example: The IRON PILLER at Delhi
 Quite unlikely, then how ???
6

History of P/M
 Going further back in Time . . .
 The art of pottery, (terracotta), was known to the pre-
historic man (Upper Paleolithic period, around
30,000 years ago)!
 Dough for making bread is also a powder material,
bound together by water and the inherent starch in it.
Baked bread, in all its variety, is perhaps one of the
first few types of processed food man ate.
 (Roti is a form of bread.)
7

Renaissance of P/M
 The modern renaissance of powder metallurgy began
in the early part of last century, when technologists
tried to replace the carbon filament in the Edison
lamp.
 The commercially successful method was the one
developed by William Coolidge. He described it in
1910, and got a patent for it in 1913.
 This method is still being used for manufacturing
filaments.
8

 The Wars and the post-war era brought about huge
leaps in science, technology and engineering.
 New methods of melting and casting were perfected,
thereby slowly changing the metallurgy of refractory
materials.
 P/M techniques have thereafter been used only when
their special properties were needed. 9
Renaissance of P/M

Powder Metallurgy
 An important point that comes out :
The entire material need not be melted to fuse it.
 The working temperature is well below the melting
point of the major constituent, making it a very
suitable method to work with refractory materials,
such as: W, Mo, Ta, Nb, oxides, carbides, etc.
 It began with Platinum technology about 4 centuries
ago… in those days, Platinum, [mp = 1774°C], was
"refractory", and could not be melted.
10

Metal processing technology in which parts are produced
from metallic powders.
Usually during PM production, the powder is compressed
(pressed) into the desired shape and then heated
(sintered) to bond the particles into a hard, rigid mass.
− Pressing is accomplished in a press-type machine using
punch-and-die tooling designed specifically for the part to
be manufactured
− Sintering is performed at a temperature below the
melting point of the metal
Powder Metallurgy
11

 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….
12
Why Powder Metallurgy is
Important?

 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.
13
Why Powder Metallurgy is
Important?

When to use PM
• Competitive with processes such as
casting, forging, and machining.
• Used when melting point is too high (W,
Mo).
• reaction occurs at melting (Zr).
• too hard to machine.
• very large quantity.
• Near 70% of the P/M part production is
for automotive applications.
• Good dimensional accuracy.
• Controllable porosity.
• Size range from tiny balls for ball-point
pens to parts weighing 100 lb. Most are
around 5 lb.
14

P/M Applications
 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
15

Hi-Tech Applications of P/M
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 . . .
THESE COMPONENTS ARE
USED IN AIR & SPACE
CRAFTS, HEAVY
MACHINERY, COMPUTERS,
AUTOMOBILES, etc…
16

Powder Metallurgy Merits
 The main constituent need not be melted
 The product is porous – [note: the porosity can be
controlled]
 Controlled porosity for self lubrication or filtration
uses
 Constituents that do not mix can be used to make
composites, each constituent retaining its individual
property
17

Powder Metallurgy Merits
 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 18

Advantages of P/M
 Virtually unlimited choice of alloys, composites, and
associated properties
– Refractory materials are popular by this process
 Can be very economical at large run sizes (100,000
parts)
 Long term reliability through close control of
dimensions and physical properties
 Wide latitude of shape and design
 Very good material utilization
19

20
Limitations and Disadvantages
 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.

Powder Metallurgy Disadvantages
 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
21

Powder Metallurgy Products
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
22

23
Materials and Products for PM
• Raw materials for PM are more expensive than for
other metalworking because of the additional energy
required to reduce the metal to powder form.
• Accordingly, PM is competitive only in a certain range
of applications.
• What are the materials and products that seem most
suited to powder metallurgy?

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

25A collection of powder metallurgy parts.
PM Parts

Connecting Rods:
Forged on left; P/M on right
26
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

28
PM Work Materials
 Largest tonnage of metals are alloys of iron,
steel, and aluminum
 Other PM metals include copper, nickel, and
refractory metals such as molybdenum and
tungsten
 Metallic carbides such as tungsten carbide are
often included within the scope of powder
metallurgy

29
Engineering Powders
A powder can be defined as a finely divided
particulate solid
Engineering powders include metals and ceramics
Geometric features of engineering powders:
– Particle size and distribution
– Particle shape and internal structure
– Surface area

30
Chemistry and Surface Films
• Metallic powders are classified as either
– Elemental - consisting of a pure metal
– Pre-alloyed - each particle is an alloy
• Possible surface films include oxides, silica,
adsorbed organic materials, and moisture
– As a general rule, these films must be removed prior to
shape processing

31
PM Materials – Elemental
Powders
 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

32
PM Materials – Pre Alloyed Powders
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

Powder Characterization
Different Shapes and
Sizes
Sizes and shapes are
important in blending and
compaction.
Often a mixed size is
beneficial.
33

34
Figure: Several of the possible (ideal) particle shapes in
powder metallurgy.
Particle Shapes in PM

35
Inter-particle Friction and
Powder Flow
Friction between particles
affects ability of a powder to
flow readily and pack tightly
A common test of inter-particle
friction is the angle of repose,
which is the angle formed by a
pile of powders as they are
poured from a narrow funnel.
Figure: Inter-particle friction as indicated by the angle of repose of a
pile of powders poured from a narrow funnel.
Larger angles indicate greater inter-particle friction.

36
Let’s check!
Smaller particle sizes show steeper angles or
larger particle sizes?!
Smaller particle sizes generally show greater friction
and steeper angles!
Finer
particles

37
Let’s check!
Which shape has the lowest inter-particle friction?
Spherical shapes have the
lowest inter-particle friction!
Little friction between
spherical particles!
As shape deviates from
spherical, friction between
particles tends to increase

38
Observations
 Easier flow of particles correlates with lower inter-
particle friction.
 Lubricants are often added to powders to reduce inter-
particle friction and facilitate flow during pressing.
 Smaller particle sizes generally show greater friction
and steeper angles.
 Spherical shapes have the lowest inter-particle friction.
 As shape deviates from spherical, friction between
particles tends to increase.

40
Measuring Particle Size
Most common method uses screens of different mesh sizes
Mesh count
It refers to the number of openings per linear inch of screen
– A mesh count of 200 means there are 200 openings per linear inch
– Since the mesh is square, the count is the same in both directions,
and the total number of openings per square inch is 2002
= 40,000
– Higher mesh count = smaller particle size
Figure: Screen mesh for
sorting particle sizes.

MESH SIZES
-100/+200 mesh: negative means particle goes through, positive means particle does
not go through. Thus, this mesh means particle sizes between 75 and 150 microns.
42

47
Particle Density Measures
True density
density of the true volume of the material
– The density of the material if the powders were
melted into a solid mass
Bulk density
density of the powders in the loose state after
pouring
Which one is smaller?!
Because of pores between particles, bulk density is
less than true density.

48
Packing Factor
Typical values for loose powders range between 0.5 and 0.7
Bulk density
true density
Packing factor =
• If powders of various sizes are present, smaller powders
will fit into spaces between larger ones, thus higher
packing factor
• Packing can be increased by vibrating the powders,
causing them to settle more tightly
• Pressure applied during compaction greatly increases
packing of powders through rearrangement and
deformation of particles
How can we increase the bulk density?

49
Porosity
Ratio of volume of the pores (empty spaces) in the
powder to the bulk volume
• In principle
Porosity + Packing factor = 1.0
• The issue is complicated by possible existence of closed
pores in some of the particles
• If internal pore volumes are included in above porosity,
then equation is exact

Powder Metallurgy Process
 Powder production
 Blending or mixing
 Powder compaction
 Sintering
 Finishing Operations
50

51
Usual PM production sequence
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

Powder Metallurgy Processing
POWDER
PROCESSING
PROPERTIES
Powder fabrication
Size and shape characterization
Microstructure (eg. dendrite size)
Chemical homogeneity, and ppt. size
Compaction
Sintering
Forging/Hot pressing
Density, Porosity
Ductility, Strength
Conductivity
Other functional properties 52

Powder
metallurgy
(P/M) consists
of several steps.
53
Powder Metallurgy Process

55
Production of Metallic Powders
 In general, producers of metallic powders are not the same
companies as those that make PM parts
 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

56
High velocity gas stream flows through expansion nozzle,
siphoning molten metal from below and spraying it into
container.
Figure: (a) gas atomization method
Gas Atomization Method

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.
57
Gas Atomization Method

DETAILS OF DROPLET
FORMATION
60

Electrode Centrifugation
A consumable electrode is rotated
rapidly in a helium-filled chamber.
The rotating electrode is melted by
an arc using a tungsten cathode or
plasma torch
The centrifugal force breaks up the
molten tip of the electrode into
metal particles.
The powder is formed from melt
electrode and is solidified in a
vacuum or inert gas environment.
62

OXIDE REDUCTION PROCESS FOR
METAL POWDER FABRICATION
63

• Reduce metal oxides with H2/CO
• Powders are spongy and porous and they have
uniformly sized spherical or angular shapes
64
OXIDE REDUCTION

CARBONYL PROCESS
• React high purity Fe or Ni with CO to form gaseous
carbonyls
• Carbonyl decomposes to Fe and Ni
• Small, dense, uniformly spherical powders of high
purity.
66

Electrolytic Process for Powder
Production
67

ELECTROLYSIS PROCESS
• Metal powder deposits at the cathode from aqueous
solution.
• Powders are among the purest available.
68

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
70

Mechanical Comminution to Obtain
Fine Particles
Figure: Methods of mechanical comminution to obtain fine particles:
(a) roll crushing, (b) ball mill, and (c) hammer milling.
71

Mechanical Alloying
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.
72

Particle Shapes in Metal Powders
Figure: Particle shapes in metal powders, and the processes by which they
are produced. Iron powders are produced by many of these processes.
73

74
Conventional Press and Sinter
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.

75
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

76
Blending and Mixing of Powders
• 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.
For successful results in compaction and
sintering, the starting powders must be
homogenized (powders should be blended
and mixed).

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 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.
77

Blending
To make a homogeneous mass with uniform distribution of
particle size and composition.
– Powders made by different processes have different sizes and
shapes
– Mixing powders of different metals/materials
– Add lubricants (<5%), such as graphite and stearic acid, to
improve the flow characteristics and compressibility of mixtures.
Combining is generally carried out in
– Air or inert gases to avoid oxidation
– Liquids for better mixing, elimination of dusts and reduced
explosion hazards
Hazards
– Metal powders, because of high surface area to volume ratio are
explosive, particularly Al, Mg, Ti, Zr, Th
78

Bowl Geometries for Blending Powders
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.
79
Some common equipment geometries used
for blending powders
(a) Cylindrical, (b) rotating cube, (c) double
cone, (d) twin shell

80
Compaction
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.

Compaction
• 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:
81

Compaction
82
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.

Compacting is usually performed at room temperature.
Pressures range from 10 tons per square inch (tons/in2
) (138
MPa) to 60 tons/in2
(827 MPa), or more.
83
Compacting

Compacting
• 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.
84

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.
85

Compaction Sequence
Figure: Typical compaction sequence for a single-level part, showing the
functions of the feed shoe, die core rod, and upper and lower punches.
Loose powder is shaded; compacted powder is solid black.
86

Additional Considerations During
Compacting
When the pressure is applied by only one punch, the maximum
density occurs right below the punch surface and decreases
away from the punch.
For complex shapes, multiple punches should be used.
Compaction with a single moving
punch, showing the resultant non
uniform density (shaded), highest
where particle movement is the greatest.
Density distribution obtained with a double-
acting press and two moving punches. Note the
increased uniformity than in a single punch.
Thicker parts can be effectively compacted.
87

Friction problem in cold compaction
The effectiveness of pressing with a single-acting punch is
limited. Wall friction opposes compaction. The pressure tapers
off rapidly and density diminishes away from the punch.
Floating container and two counteracting punches help
alleviate the problem.
88

• Smaller particles provide greater strength mainly due to
reduction in porosity
• Size distribution of particles is very important. For same size
particles minimum porosity of 24% will always be there
– Box filled with tennis balls will always have open space
between balls
– Introduction of finer particles will fill voids and result in ↑
density. 89

Because of friction between (i) the metal particles and (ii)
between the punches and the die, the density within the
compact may vary considerably. Density variation can be
minimized by proper punch and die design
Density variation in compacting metal powders in various dies:
(a) and (c) single-action press; (b) and (d) double-action press.
Note in (d) the greater uniformity of density from pressing with two punches
with separate movements when compared with (c).
(e) Pressure contours in compacted copper powder in a single-action press.
90

Increased compaction pressure
– Provides better packing of particles and leads to ↓
porosity
– ↑ localized deformation allowing new contacts to be
formed between particles
Effects of Compaction
91

• At higher pressures, the green density approaches density of
the bulk metal
• Pressed density greater than 90% of the bulk density is
difficult to obtain
• Compaction pressure used depends on desired density.
Effects of Compaction
92

Complex Compacting
• 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
93

(a)Compaction of metal powder to form bushing
(b)Typical tool and die set for compacting spur gear 94

95
Sintering
Heat treatment to bond the metallic particles, thereby
increasing strength and hardness.
Usually carried out at between 70% and 90% of the
metal's melting point (absolute scale)
– Generally agreed among researchers that the primary
driving force for sintering is reduction of surface energy
– Part shrinkage occurs during sintering due to pore size
reduction

Sintering
 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.
96

97
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.
Sintering Sequence
 Parts are heated to 0.7~0.9 Tm.
 Transforms compacted mechanical bonds to much
stronger metallic bonds.

Sintering
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. 98

• Green compact obtained after compaction is brittle and
low in strength
• Green compacts are heated in a controlled-atmosphere
furnace to allow packed metal powders to bond together
Carried out in three stages:
• First stage: Temperature is slowly increased so that all
volatile materials in the green compact that would
interfere with good bonding is removed
– Rapid heating in this stage may entrap gases and produce
high internal pressure which may fracture the compact.
Sintering – Three Stages
99

Promotes solid-state bonding by diffusion. Diffusion is time-
temperature sensitive. Needs sufficient time.
Promotes vapor-phase transport. As material is heated very
close to MP, metal atoms will be released in the vapor phase
from the particles. Vapor phase resolidifies at the interface.
Sintering: High temperature stage
100

Third stage:
Sintered product is cooled in a controlled atmosphere.
– Prevents oxidation and thermal shock
Gases commonly used for sintering:
H2, N2, inert gases or vacuum
Sintering: 3rd
stage
101

102
Figure: (a) Typical heat treatment cycle in sintering; and (b) schematic
cross section of a continuous sintering furnace.
Sintering Cycle and Furnace

Sintering Time, Temperature, and
Indicated Properties
103

Sintering Time and Temperature
for Metals
104

sintered
sintered
ρ
ρgreen
greenV
V
shrinkageVolume ==
3/1
sintered






=
ρ
ρgreen
shrinkageLinear
105
Sintering

Finishing
• 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.
106

Secondary Operations
 Most powder metallurgy products are ready to use
after the sintering process.
 Some products may use secondary operation to
provide enhanced precision, improved properties, or
special characteristics.
 Distortion may occur during non uniform cool-down
so the product may be repressed, coined, or sized to
improve dimensional precision.
107

Secondary Operations
• If massive metal deformation takes place in the second
pressing, the operation is known as P/M forging
– Increases density and adds precision
• Infiltration and impregnation- oil or other liquid is forced
into the porous network to offer lubrication over an
extended product lifetime
• Metal infiltration fills in pores with other alloying elements
that can improve properties
• P/M products can also be subjected to the conventional
finishing operations: heat treatment, machining, and
surface treatments
108

109
Densification and Sizing
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

110
Impregnation and Infiltration
 Porosity is a unique and inherent characteristic of
PM technology
 It can be exploited to create special products by
filling the available pore space with oils, polymers,
or metals
Two categories:
1. Impregnation
2. Infiltration

111
Impregnation
The term used when oil or other fluid is permeated
into the pores of a sintered PM part
Common products are oil impregnated bearings,‑
gears, and similar components
Alternative application is when parts are impregnated with
polymer resins that seep into the pore spaces in liquid form
and then solidify to create a pressure tight part

112
Infiltration
Operation in which the pores of the PM part are filled
with a molten metal.
The melting point of the filler metal must be below that of
the PM part.
TM (filler) < TM (Part)
Involves heating the filler metal in contact with the
sintered component so capillary action draws the filler
into the pores.
– Resulting structure is relatively nonporous, and the
infiltrated part has a more uniform density, as well as
improved toughness and strength.

113
• 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
Alternative Pressing and Sintering
Techniques

Isostatic Pressing
114
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.

Special Process: Hot compaction
• Advantages can be gained by combining consolidation and
sintering,
• High pressure is applied at the sintering temperature to
bring the particles together and thus accelerate sintering.
• Methods include
– Hot pressing
– Spark sintering
– Hot isostatic pressing (HIP)
– Hot rolling and extrusion
– Hot forging of powder preform
– Spray deposition
115

Hot - Iso static Pressing
• 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).
116

Hot-Isostatic Pressing
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
117

Hot Iso static Pressing (HIP)
Steps in HIP
Schematic illustration of hot isostatic pressing. The pressure and
temperature variation versus time are shown in the diagram.
118

Combined Stages
Simultaneous compaction + sintering
Produces compacts with almost 100% density.
Good metallurgical bonding between particles and good
mechanical strength.
Container: High MP sheet metal
Container subjected to elevated temperature and a very high vacuum
to remove air and moisture from the powder.
Pressurizing medium: Inert gas
Operating conditions -100 MPa at 1100 C
Uses
– Superalloy components for aerospace industries
– Final densification step for WC cutting tools and PM tool steels.
119

• Metal powder placed in
a flexible rubber mold
• Assembly pressurized
hydrostatically by water
(400 – 1000 MPa)
• Typical: Automotive
cylinder liners →
120
Cold Isostatic Pressing

Cold Isostatic Pressing
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
121

Liquid Phase Sintering
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.
122

123
Powder Injection Molding
Metal injection molding
 Pellets made of powders and binder
 Heated to molding temperature and
injected into a mold
 Can create complex designs

Metal Injection Molding (MIM)
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.
124

Metal Injection Molding (MIM)/
Powder Injection Molding (PIM)
• 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
125

MIM
Table: Comparison of conventional powder metallurgy and metal
injection molding
126
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%

Powder Rolling
Figure 17.17 Schematic illustration of powder rolling. 128

Powder Rolling
Figure: One method of producing continuous sheet products
from powdered feed stock.
129

 Slip: Suspension of colloidal (small particles that do not
settle) in an immiscible liquid (generally water).
 Slip is poured in a porous mold made of plaster of paris.
Air entrapment can be a major problem.
 After mold has absorbed some water, it is inverted and the
remaining suspension poured out.
 The top of the part is then trimmed, the mold opened, and
the part removed.
 Application: Large and complex parts such as plumbing
ware, art objects and dinnerware.
130
Slip-Casting

(i) Slip is first poured into an absorbent mould
(ii)a layer of clay forms as the mould surface absorbs water
(iii)when the shell is of suitable thickness excess slip is poured away
(iv)the resultant casting
Slip-Casting
131

Other Techniques to Produce
High-Density P/M Products
 High-temperature metal deformation processes can
be used to produce high density P/M parts
 Ceracon process- a heated preform is surrounded by
hot granular material, transmitting uniform
pressure.
 Spray forming- inert gases propel molten droplets
onto a mold.
132

Spray Deposition
Figure: Spray deposition (Osprey Process) in which molten metal is
sprayed over a rotating mandrel to produce seamless tubing and pipe
133

Properties of P/M Products
The properties of P/M products depend on multiple variables
– Type and size of powder
– Amount and type of lubricant
– Pressing pressure
– Sintering temperature and time
– Finishing treatments
Mechanical properties are dependent on density
Products should be designed (and materials selected) so that
the final properties will be achieved with the anticipated
final porosity.
134

• The Metal Powder Industries Federation (MPIF)
defines four classes of powder metallurgy part
designs, by level of difficulty in conventional
pressing.
• Useful because it indicates some of the limitations on
shape that can be achieved with conventional PM
processing.
136
PM Parts Classification System

Classes of P M Equipment
• The complexity of the part dictates the complexity of
equipment
• Equipment has been grouped into four classes.
Figure: Sample geometries of the four basic classes of press-and-sinter powder
metallurgy parts. Note the increased pressing complexity that would be
required as class increases.
137

Classes of Powder Metallurgy
Equipment
Table: Features that define the various classes of
press-and –sinter P/M parts
Class Levels Press Actions
1 1 Single
2 1 Double
3 2 Double
4 more than 2 Double or multiple
138

• 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 would be difficult if not
impossible to produce by other means
139
Design Guidelines for PM Parts - I

The part geometry must permit ejection from die after
pressing
− This generally means that part must have vertical or near-
vertical sides, although steps are allowed
− Design features such as undercuts and holes on the part sides
must be avoided
− Vertical undercuts and holes are permissible because they do
not interfere with ejection
− Vertical holes can be of cross-sectional shapes other than
round without significant difficulty
140
Design Guidelines for PM Parts - II

• Screw threads cannot be fabricated by PM; if required,
they must be machined into the part.
• Chamfers and corner radii are possible by PM
pressing, but problems arise in punch rigidity when
angles are too acute.
• Wall thickness should be a minimum of 1.5 mm
(0.060in) between holes or a hole and outside wall.
• Minimum recommended hole diameter is 1.5 mm
(0.060in).
141
Design Guidelines for PM Parts - III

Design Aspects
142
(a) Length to thickness ratio limited to 2-4;
(b) Steps limited to avoid density variation;
(c) Radii provided to extend die life, sleeves greater than 1 mm,
through hole greater than 5 mm;
(d) Feather-edged punches with flat face;

Design Aspects
(e) Internal cavity requires a draft;
(f) Sharp corner should be avoided;
(g) Large wall thickness difference should be avoided;
(h) Wall thickness should be larger than 1 mm.
143

Design Considerations for P/M
• The shape of the compact must be kept as simple and
uniform as possible.
• Provision must be made for ejection of the green compact
without damaging the compact.
• P/M parts should be made with the widest acceptable
tolerances to maximize tool life.
• Part walls should not be less than 1.5 mm thick; thinner
walls can be achieved on small parts; walls with length-to-
thickness ratios above 8:1 are difficult to press.
144

• Steps in parts can be produced if they are simple and their
size doesn’t exceed 15% of the overall part length.
• Letters can be pressed if oriented perpendicular to the
pressing direction. Raised letters are more susceptible to
damage in the green stage and prevent stacking.
• Flanges or overhangs can be produced by a step in the die.
• A true radius cannot be pressed; instead use a chamfer.
• Dimensional tolerances are on the order of ±0.05 to 0.1
mm. Tolerances improve significantly with additional
operations such as sizing, machining and grinding.
145
Design Considerations for P/M

Die Design for Powder-Metal
Compaction
Figure: Die geometry and design features for powder-metal compaction.146

Poor and Good Designs of P/M Parts
Figure: Examples of P/M parts showing poor and good designs.
Note that sharp radii and reentry corners should be avoided and that threads and
transverse holes have to be produced separately by additional machining operations.147

Design Features for Use with
Unsupported Flanges or Grooves
Figure: (a) Design features for use with unsupported flanges.
(b) Design features for use with grooves.
148

Die Design for P/M
• Thin walls and projections create fragile tooling.
• Holes in pressing direction can be round, square, D-
shaped, keyed, splined or any straight-through shape.
• Draft is generally not required.
• Generous radii and fillets are desirable to extend tool life.
• Chamfers, rather the radii, are necessary on part edges to
prevent burring.
• Flats are necessary on chamfers to eliminate feather-edges
on tools, which break easily.
149

Design of Powder Metallurgy Parts
Basic rules for the design of P/M parts
– Shape of the part must permit ejection from die
– Powder should not be required to flow into small
cavities
– The shape of the part should permit the construction of
strong tooling
– The thickness of the part should be within the range for
which P/M parts can be adequately compacted
– The part should be designed with as few changes in
section thickness as possible
150

Basic Rules for P/M Parts
• Parts can be designed to take advantage of the fact that
certain forms and properties can be produced by P/M that
are impossible, impractical, or uneconomical by any
other method
• The design should be consistent with available equipment
• Consideration should be made for product tolerances
• Design should consider and compensate for dimensional
changes that will occur after pressing
151

Figure: Examples of poor and good design features for powder metallurgy
products. Recommendations are based on ease of pressing, design of tooling,
uniformity of properties, and ultimate performance. 152

Financial Considerations
Typical size part for automation is 1” cube
– Larger parts may require special machines (larger surface
area, same pressure equals larger forces involved)
153
Die design
must withstand 700 MPa, requiring
specialty designs.
Can be very automated
 1500 parts per hour not uncommon
for average size part
 60,000 parts per hour achievable for
small, low complexity parts in a
rolling press.

P/M Summarizing:
Powder Metallurgy is sought when -
a) It is impossible to form the metal or material by any other
technique
b) When p/m gives unique properties which can be put to good
use
c) When the p/m route is economical.
There may be over-lapping of these three points.
154

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