POWDER COMPACTION

Powder Technology
MM 6521
Dr. Debasis Chaira

1
CONTENTS

WHAT IS COMPACTION?

COMPRESSIBILITY AND
COMPACTIBILITY

INFLUENCING FACTORS

TYPES OF COMPACTION

COMPACTION TOOLS

CHARACTERIZATION OF
COMPACTS

2
 COMPACTION

Compaction is defined as filling of powder in a die to produce compact of required

shape under pressure.
Three steps are involved in compaction process
1. Die filling
2. Compaction (packing of powder)
3. Ejection of compacts Upper Punch

Die
Powder compact
Lower Punch

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 COMPACTION

Concept of packing density

• The success of the compaction process depends on the packing efficiency of powders.

• The packing density is related with atomic packing factor (APF).

𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑎𝑡𝑜𝑚𝑠
APF =
𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑢𝑛𝑖𝑡 𝑐𝑒𝑙𝑙

Crystal structure APF

Simple cubic 0.52
Body centered cubic 0.68
Face centered cubic 0.74
Hexagonal closed packed 0.74

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 COMPACTION
Concept of packing density

 Packing density is influenced by powder shape and size

 For monosize powder with spherical shape fraction of packing density : 0.60-0.64
 Bimodal powder mixture shows enhanced packing density
 To achieve fraction of packing density of 0.86 in bimodal mixture 26.6 wt.% should
be of finer particles.

X. Chateau, Particle packing and the rheology of concrete, in Understanding the

Rheology of Concrete., N. Roussel (Ed.) Woodhead Publishing Limited, 2012, 117-143.
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 COMPACTION
STEPS IN COMPACTION

•Rearrangement of particles (filling of large pores)

1

•Increase in pressure generate new particle contact

2 improves packing

• Increase number of particle contact and contact area

3

• Elastic deformation
4

• Plastic deformation assisted contact enlargement

5

• Contact deformation, collapsing of interparticle voids

6

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 COMPACTION
STEPS IN COMPACTION

Density
localized
deformation

Bulk compression
Homogeneous
deformation

rearrangement
Apparent
density
Compaction pressure

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 COMPACTION
STEPS IN COMPACTION IN A NUTSHELL

1. Rearrangement
2. Elastic deformation
3. Plastic deformation
4. (A) Fragmentation (Brittle)
(B) Strain hardening (ductile)
5. Bulk deformation

R.M. German, Powder Metallurgy Science, 2nd Ed. Metal powder Industries Federation,
1994

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 COMPACTION
STEPS IN COMPACTION IN A NUTSHELL

Particle rearrangement Elastic deformation Plastic deformation

R.M. German, Powder Metallurgy Science, 2nd Ed. Metal powder Industries Federation,
1994
9
 COMPACTION
STEPS IN COMPACTION IN A NUTSHELL

1
• Loose powders (low co-ordination number)

• Increased pressure large pore filling Increase in

2 co-ordination number
• Reduction in porosity due to increase in no of
3
particle/particle contact and contact area (elastic
deformation)
• Increased pressure  Increase in particle contact
4 area deformation Strain hardening (plastic
deformation)

5 • Agglomeration of particle/particle contact 

Strength enhancement of compact

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11
 COMPACTION

Compressibility and Compactibility

Compressibility : Densification ability of powder under pressure. Usually

indicated by compression ratio.

𝐆𝐫𝐞𝐞𝐧 𝐝𝐞𝐧𝐬𝐢𝐭𝐲
Compression ratio : Click to add text
𝐀𝐩𝐩𝐚𝐫𝐞𝐧𝐭 𝐝𝐞𝐧𝐬𝐢𝐭𝐲

Compactibility : Ability of compact to retain the shape without any distortion

during ejection from die and handling.

𝐂𝐨𝐦𝐩𝐫𝐞𝐬𝐬𝐢𝐯𝐞 𝐬𝐭𝐫𝐞𝐧𝐠𝐭𝐡 𝐨𝐟 𝐠𝐫𝐞𝐞𝐧 𝐜𝐨𝐦𝐩𝐚𝐜𝐭

Compactibility factor :
𝐂𝐨𝐦𝐩𝐚𝐜𝐭𝐢𝐨𝐧 𝐩𝐫𝐞𝐬𝐬𝐮𝐫𝐞

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 COMPACTION

Factors affecting Compressibility

1. Particle shape
2. Particle size and size distribution
3. Compaction pressure
4. Compaction temperature
5. Compact dimension (thickness to diameter ratio)
6. Nature of powder (ductile/brittle), plasticity of particles.
7. Hardness of metal/alloy powder

 Compactibility is directly influenced by apparent density of powder.

 Any factors which changes the apparent density of powder will also
effect the compactibility.

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 COMPACTION

Enhanced Compressibility
1. Spherical shape particles
2. Bimodal particle size distribution (better
mechanical interlocking)
3. Increased compaction pressure (enhanced
deformation)
4. Lower thickness to diameter ratio
5. Lower hardness

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 Fundamentals of compaction

Consider a cylindrical compact of diameter D and height H. Assume a thin section of height
dH. The pressure at the top of the element is P and at the bottom is P b . Using force balance
Σ𝐹 = 0 = 𝐴 (𝑃𝑏 -P)+u𝐹𝑛
Where 𝐹𝑛 is the normal force and u is the coefficient of friction between powder and the die
wall. A is cross sectional area.

Normal force 𝐹𝑛 =𝜋𝑧𝑃𝐷𝑑𝐻

Friction force 𝐹𝑓 =𝜋𝑢𝑧𝑃𝐷𝑑𝐻
Z- Ratio of radial to axial stress
The pressure difference between the top and bottom of
the powder element dP as
𝐹
𝑑𝑃 = 𝑃 − 𝑃𝑏 = − 𝑓ൗ𝐴= −4𝑢𝑧𝑃𝑑𝐻Τ𝐷
Integrating the pressure term with respect to compact
height gives the pressure at any position x below the
punch as follows:
4𝑢𝑧𝑥
−( )
𝑃𝑥 = 𝑃𝑒 𝐷
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 Contd.

• The average stress is dependent on both (H/D)

ratio, axial to radial (z) pressure distribution and
die wall friction (u).
• High average stresses are attained in short
compact with large dia., and lubricated die wall.
• Decrement in load transmission with increase in compact thickness
• Pressure ratio decreases with increase in uzH/D. The wall friction contributes to a
decreased pressure with depth.
• The pressure decay depends on the compact H/D ratio.
• With a decreasing D, the pressure decreases more rapidly with depth (H).
• For homogeneous compaction, small H/D16ratio are desirable.
 • The H/D ratio is important to create uniform
compact properties.
• When H/D> 5, die compaction is unsuccessful.
• A low H/D is preferred for compaction.

• Powder lubrication raises z while lowering u. Die wall friction dominates at low
lubricant content, pressing homogeneity is improved with small quantities of admixed
lubricant.
• An increase in H/D ratio results in greater17density gradients and lower green density.
• At pressure of approximately 300 MPa, the behaviour shifts, indicating a change in the
mechanism controlling compaction from yielding to work hardening. Cu is lowest strength
material and shows the lowest porosity at all compaction pressures. Stainless steel is a
prealloyed powder that exhibits the highest strength and most rapid work hardening, so it
exhibits the highest porosity.
• Below 300 MPa, material yield strength dominates. At high compaction pressure (>300
MPa), compaction is governed by work hardening behaviour of the material.
• The higher the strength of the powder the more
18
difficulty expected in compaction.
 • Alloying a powder decreases the
compressibility. Carbon is a potent
strengthener and has a strong negative
effect on compressibility. Cr has a lower
negative impact on compressibility.
• A high particle hardness hinders
compaction. The higher the hardness, the
lower the green density at any compaction
pressure. For a hard material, both fracture
and deformation are expected.

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 COMPACTION
Effect of compact dimension

 Increasing the compact dimension (H/D) increase the density gradient and lowers
compact density (green density).

 Sintering shrinkage α 1/green density.

 Lower density  greater shrinkage during sintering  less dimensional precession

Other parameters

 Die wall friction (µ) minimizes the effectiveness of compaction enhances density
variation throughout the compact.
 Addition of lubricant is highly effective in this regard.
 Selection of lubricant has to be proper as impurity can form during
sintering process.

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 COMPACTION
Compaction pressure
Materials Range of Compaction pressure (MPa)
Aluminium 70-275
Brass 400-700
Bronze 200-275
Iron 350-800
Iron-Copper (2%) 600-720
Tungsten 70-140
Alumina 100-140
Hard metals 150-400
Magnetic ceramics 110-165

Ductile powder produces more steady compact as compared to brittle material.

P. C. Angelo, R. Subramaaniam, Powder metallurgy: science, technology and Applications, PHI
Learning Pvt. Ltd., 2008 21
 • Class 1: Single level, simple
type; they can be compacted
PM Part Classification using single action dies with
motion of only one punch.
• Class 2: parts are single level,
with compaction pressure
applied from both directions.
Large height to diameter
ratios and more involved
tooling.
• Class 3: Parts have two levels
and pressure is applied from
both directions.
• Class 4: Shapes are most
difficult to press. These are
multiple level components
which are pressurized from
both upper and lower
punches. The tooling usually
has several independent
motions built in to ensure
adequate powder fill and
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pressurization to each
individual levels.
Tooling Concerns
Assume, green density- G, A-
apparent density, H0-fill height,
H- compacted height
𝜌 𝐻
𝜌𝐺 = 𝐴 0ൗ𝐻
Compacted height 𝐻 = 𝐻0 − ∆𝐻
Where H is height change
𝜌 𝐻
𝜌𝐺 = 𝐴 0ൗ(𝐻 − ∆𝐻)
0

• If lower punch is a single piece,

the punch motions are the same
and the values of H are the
same for both thick and thin
sections. The pressed density is
higher for thin section with the
smaller initial height.
• If the lower punch is split, then
the right and left portions can
move with different relative
displacements and that allows
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pressing to a uniform final
density.
 Tool Design

• Proper design and specification of the compaction tools provides long life and proper

functioning.

• Tool steels are appropriate for shorter production runs. Cemented carbide tools are

used for high volume production.

• The powder shrinkage and swelling due to sintering and elastic recovery on ejection

must be incorporated into the tooling dimension.

• The press size, motions, part complexity and required surface finish influence the

tooling design.
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 • Several independent
motions may be required
to ensure uniform
multiple level parts. The
lower tool assembly
consists of a core rod to
form the through hole,
and both outer and inner
lower punches. The
upper punch has a hole
to accommodate the
core rod movement.
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 COMPACTION
TYPES OF COMPACTION

1. Conventional die compaction

2. Warm compaction
3. Cold Isostatic compaction
4. Powder roll compaction
5. Powder extrusion
6. Powder Injection molding

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 CONVENTIONAL DIE COMPACTION
Single action and Double action compaction

Density gradient
Single action Double action

 Uneven density gradient  Less density gradient

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 Die Compaction

Steps followed in Conventional Die

Die-Punch assembly in Compaction
Conventional Die Compaction
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 WARM COMPACTION

 Developed in mid-1990’s.
 Heated tools and powders are employed to improve the green density and
strength.
 Powder heating is normally carried out by microwave or slotted heat exchanger.
 Temperature for powder heating: 130 ºC and for die and punch 150 ºC.
 Turbine hubs are fabricated by warm compaction.

Luo et al., J. Mater. Process. Tech. 214 (2014) 660-666.

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 WARM COMPACTION

 More finer, spherical, uniform distribution of porosity

 Better dimensional precision
 Enhanced fatigue endurance limit in certain ferrous alloys
 Lesser compaction pressure to achieve higher green density.
 25% higher cost than conventional compaction powder metallurgical process,
40% less than forging and 10% less than infiltration process.
 The concerns of die filling, agglomeration, particle flowability during warm
compaction need adequate attention.
 Optimum use of lubrication can be effective in this regard.
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 COLD ISOSTATIC COMPACTION

 Flexible mold (natural/synthetic rubber) is filled with powder with isostatic

pressure.
 Oil and water based medium.
 Powder filled mold is kept in pressure compartment.
 Normally pressure is provided less than 350 MPa.

Pros
 Even density of compacts.
 Enhanced green density (5-15%) than die compaction at same pressure.
 Enhanced green strength.
 Low tooling and machining cost
 Complex shape can be fabricated with high
31 density.
 COLD ISOSTATIC COMPACTION
• Isostatic pressing is more useful
for making large, homogeneous
compacts.
Cons • More efficiently reaches higher
1. Less dimensional precision than die compaction. densities at given pressures
2. Less smooth surface. compared to die compaction and
3. Less rate of production. density gradients are smaller.
4. lower life of flexible moulds • It allows for more shape
complexity but sacrifices
dimensional control and pressing
speed.
• A tube geometry would be
difficult to press (without density
gradient) by die compaction
because of the large dimensional
ratio.
• With isostatic compaction, the
fluid provides uniform pressures
Wet Bag Cold isostatic compaction and consequently, uniform
density.
In the wet bag technique, the filled and sealed mold is immersed into a fluid chamber which
is pressurized by an external hydraulic system. After pressing, the wet bag is removed from
32
the chamber and the compact extracted from the mold.
 COLD ISOSTATIC COMPACTION
WET BAG VS DRY BAG
In wet bag powder is filled in the flexible mold which is placed in liquid in a pressure
Container. It is used for large size product with small and large quantity.

In dry bag system a flexible membrane is used and separates the fluid from mold.
The process is more cleaner with rapid production.
Material Pressure (MPa)
Aluminium 8-20
Iron 45-60
Stainless steel 45-60
Copper 20-40
Lead 20-30
Tungsten carbide 20-30
Dry bag cold isostatic compaction
The dry bag approach is much more rapid because the bag is built
directly into the pressure cavity. Both powder filling and ejection
33
are performed without removing the bag assembly.
 POWDER ROLL COMPACTION

 Also known as roll compacting

 Powder is feed into hopper into rolling mill to produce compacted green
strip.
 Arrangement of roll : Vertical or horizontal.
 Roll diameter : 50-150 times of strip thickness
 Strip thickness can be increased by roughening rolls.

Influencing factors
1. Particle shape (irregular)- to obtain green strip of maximum strength
to withstand handling during rolling process
2. Compressibility (80- 85% of theoretical density of green strip): good
compressibility for sufficient interlocking of particles to obtain
adequate green strength of green strip
3. Particle size: the thickness of the finished strip and particle
segregation restricts the max. particle size that can be tolerated.
4. Flowability: (non sticking, non cohesive powders)
5. Surface oxidation 34
 POWDER ROLL COMPACTION

 Three distinct zones

1. Free zone: blended powder in the hopper is transported freely downward
under gravity
2. Feed zone: Here powder is being dragged by the roll surface into the mill bite,
but not attained coherence.
3. Compaction zone: Close to the roll nip, where the powder becomes coherent, the
density changes rapidly and air has to be expelled.
35
 After fabricating compacted strip it is forwarded to sintering.
 Calculation of rolled strip thickness

D-diameter of rolls
- gripping angle Compression ratio, 𝐶 =
ℎ𝑜
ℎ𝑔

ℎ𝑜 ℎ𝑜 − ℎ𝑔
𝐶−1= −1 =
ℎ𝑔 ℎ𝑔
𝑥
𝑐𝑜𝑠𝛼 =
𝐷ൗ
2
𝐷
𝑥 2 −𝑥
1 − 𝑐𝑜𝑠𝛼 = 1 − 𝐷Τ = 𝐷/2
2
𝐷
𝐷 1 − 𝑐𝑜𝑠𝛼 = 2 − 𝑥 = ℎ𝑜 − ℎ𝑔
2
ℎ𝑜 = 𝐷 1 − 𝑐𝑜𝑠𝛼 + ℎ𝑔

𝐷(1−𝑐𝑜𝑠𝛼) ℎ𝑜−ℎ𝑔
= .ℎ𝑔 =ℎ𝑔 = strip thickness
36 𝐶−1 ℎ𝑜 −ℎ𝑔
POWDER ROLL COMPACTION
Nip angle α is related as :
𝒙
cos α =
𝑫/𝟐
𝒉
Hf= 𝒐 , C: Compression ratio
𝑪
 Due to slipping tendency actual gripping angle is
lesser than calculated friction angle α.
 Larger diameter rolls are very effective in the
context.
 Trapped air need to be properly released.
 Density variation of the strip can be controlled by
regulating the powder flow characteristics, roll
speed.
 It is necessary to use very much larger diameter rolls
than the required for producing similar strip from
solid material.
 Roll diameter: 50-150 times the strip thickness.
 The maximum strip thickness can be increased by
increasing the “µ” i.e. by roughening the roll surface.
During rolling, the surface gets polished.
 Air entrapped
37
in the powder should be properly
released.
 After green strip formation, the next operation is sintering, in
which the strip shrinks in all 3 dimensions depending on process
parameters and material composition.
 For all applications except porous strip, the sintered strip is
rerolled.
 To obtain completely dense strip, additional cold rolling and
annealing steps are incorporated.
Applications: Ni and Co alloys strip for electronic and magnetic
applications, porous SS strip for filters, Ni strip for electrodes and
Al alloy strip for bearing applications.

38
 POWDER EXTRUSION
 Mostly used for fabricating structures like tube and rod (high aspect ratio).
 Mixture and binder is loaded in a vacuum tight container, degassed and extruded
hot to produce structure of high aspect ratio.
 Binder is removed prior to sintering.
 Extrusion are of two types : cold extrusion, hot extrusion.
 In cold extrusion the process is identical as mentioned above.
 In hot extrusion the pre compacted powder is placed in extrusion die and heated to
the desired temperature followed by ejection from the die.

Container 39
 POWDER EXTRUSION
𝑨𝑶
 Extrusion Ratio : , Ao = cross sectional area of preform
𝑨𝒇
Af = cross sectional area of final extruded product

Influencing factors for final product

1. Extrusion type
2. Extrusion ratio
3. Ram speed
4. Extrusion temperature: generally performed at temp. over two-thirds of
absolute melting temp.
5. Lubrication

 For hot extrusion process high extrusion ratio and more enhanced densification
are observed.

Common anomalies in extrusion

1. Inadequate strength and stiffness 4. No uniform flow of powder
2. Cracks
3. Surface craters and blisters: forming continuous or sporadic bulges during
extrusion due to entrapped of air, solid or liquid contaminant
Use: employed to consolidate rapidly solidified
40 powders, composites and oxide
dispersion strengthened alloys
 POWDER INJECTION MOULDING (PIM)

Salient Features
 Cheap process
 Used for fabricating complex shape such as SiC turbo chargers, radial
rotors, hard disk drives, stainless steel gear wheels.
• Mixing of powder and binder (wax,
1 polymers, oil, lubricants)

• Mixing and granulation

2

• Injection molding
3

• Solvent+ thermal debinding

4

• Sintering
5
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 POWDER INJECTION MOULDING

• Depending on the input powder PIM is divided into metal injection

molding (metal powder), ceramic injection molding (ceramic powder).
• During injection molding the powder is heated to soften and pushed into
the mold for compacting and cooling.
Several influencing factors on its success
1. Particle size and size distribution (effects the viscosity, powder flow)
powder size < 20 µm is desired.
2. Initial powder composition
3. Particle shape (desired spherical shape)
4. Injection temperature
5. Injection speed
6. Mold filling time

Points to remember
 Shrinkage is isotopic as compared to compacted and sintered products.
 Small, rounded, isolated pores influences much less than compacted and
sintered products. 42
 • The process starts with mixing selected
powders and binders.
• The binders are thermoplastic mixtures
of waxes, polymers, oils, lubricants and
surfactants.
• The powder-binder mixture is granulated
and injection molded into the desired
shape.
• The polymer imparts viscous flow
characteristics to the mixture to aid
forming, die filling and uniform mixing.
• After molding, the binder is removed,
and the remaining powder structure
sintered.
• The product may then be further
Schematic diagram of powder injection 43
densified, heat treated or machined.
molding
Die Compaction (Press)

Press

Hydraulic Mechanical Rotary

Image courtesy : LPR Global Image courtesy : IEEE

44
GlobalSpec
 Die Compaction (Press)
Hydraulic press :
• Hydraulic press produces working force through the application of fluid pressure on a
piston by means of piston or valves.
• Less economical, slow in production, quick release of pressure in each stroke
leads to incorporate stress on pipe joints, valves, seals.
• Drawing speed and load can be controlled, Continual pressure application
is possible.
• Complex shapes.
Mechanical press :
• Highly economical and faster in production as compared to hydraulic press.
• Deliver full pressure at the bottom of the stroke.
• Simple shapes.
• In mechanical press, a flywheel stores energy, which is then released and transferred by
any one mechanisms (eccentric, crank, knuckle, joint, toggle etc.)
Rotary Press
• Number of tool sets (rotating) are used.
• Controlling is possible during the operation.
• Punches move within the die bore during the operation.
• Production rate is high.
• A rotary press is a mechanically operated machine, which uses a number of identical sets
of tools to produce parts at high production45rate.
 Die Compaction (Die-punch)

Die Requirements
1. High hardness
2. High wear resistant
3. Low co-efficient of friction

Punch Requirements
1. High toughness
2. High fatigue strength
3. High surface hardness
4. High wear resistant
5. Low co-efficient of friction

46
 Die Compaction (Die-punch)

Die-Punch Materials

1. Tool steel (AISI D2: %C- 1.4-1.6%, Mn, Si- 0.60%, Co: 1.00%, Cr-11-13%
Mo – 0.7-1.2%, V: 1.10%, P, S- 0.03%, Ni – 0.30%, Cu-0.25%)
Hardness : 60-62 HRC).
AISI M2, AISI A2, SAE6150

2. 88% Tungsten Carbide + 12% Co/Cemented carbide

(extremely hard, wear resistant, enhanced fracture toughness).

3. Coated steel (Chrome/chromium nitride, TiN, TiCN, TiAlN coating): To

prevent punch and die wear and minimize the powder sticking on punch and
die surface.

47
 Die Compaction (Die-punch)

Influencing factors for tool design

1. Fill : powder fill into the die opening before compaction. Requirements:
a) Appreciable speed, (b) even density, (c) less cohesion
Application of core rod (up and down lifting to avoid air entrapment)

𝑯𝒆𝒊𝒈𝒉𝒕 𝒐𝒇 𝒇𝒊𝒍𝒍𝒊𝒏𝒈 𝒄𝒂𝒑𝒂𝒄𝒊𝒕𝒚 𝑮𝒓𝒆𝒆𝒏 𝒅𝒆𝒏𝒔𝒊𝒕𝒚

=
𝒄𝒐𝒎𝒑𝒂𝒄𝒕 𝒉𝒆𝒊𝒈𝒉𝒕 𝑨𝒑𝒑𝒂𝒓𝒆𝒏𝒕 𝒅𝒆𝒏𝒔𝒊𝒕𝒚

2. Flow : Powder flow should be fast enough, to lower the frictional effect
and powder welding, lubricant is used.

3. Compaction pressure : Limit of compaction pressure is influenced by tensile

strength of tool material.

4. Change in product dimension.

48
 Die Compaction (Lubrication) Good or Bad?

General requirement of lubrication

1. Reduce die wall friction

2. Reduce inter-particle friction
3. Reduce ejection force

Commonly used lubricants:

1. Zinc stearate (Zn-stearate)
2. Ethylene bis stearamide (EBS)
3. Composite lubricant (GS-lube, Kenolube, Metallub)

 What should we opt for?

a) powder lubrication or (b) die wall lubrication

49
 Die Compaction (Lubrication) Good or Bad?

 Powder lubrication is required in case powder particle possess poor

flowability.

 Mixed lubricant in powder reduces green strength problem in

handling green compacts.

 Lubricants in powders leads to inhibit mass transport during sintering,

give rise to porosity in sintered product.

 Less precision in sintered product.

 Die wall lubrication is more effective in compaction as compared to

powder lubrication. The problem of poor flowability powders can be
counteracted by size enhancement by selecting proper powder fabrication
method.
50
 Cracks in Powder Compaction

 Cracks due to inadequate taper of tool.

 Trapped air in powder.

 Large density gradient, unregulated ejection time.

 Incorrect lubricant addition.

 Improper tool surface finish.

 Inadequate dwell time applied to the compact after applying maximum

compaction pressure.

 Powder contamination/ powder agglomeration specific to nanopowders.

Par Jonsen, Fracture and Stress in Powder Compacts, Doctoral thesis, Lulea
University of Technology, 2006 51
 Density Measurement of Green Compact

 Archimedes' principle is the most acceptable method for measuring the

density of a green compact.

𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑠𝑎𝑚𝑝𝑙𝑒 𝑖𝑛 𝑎𝑖𝑟 (𝑔)

𝐺𝑟𝑒𝑒𝑛 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 = x density of water (g/cm3)
𝑆𝑜𝑎𝑘𝑒𝑑 𝑤𝑒𝑖𝑔ℎ𝑡 −𝑠𝑢𝑠𝑝𝑒𝑛𝑑𝑒𝑑 𝑤𝑒𝑖𝑔ℎ𝑡 (𝑔)
at ambient

𝑮𝒓𝒆𝒆𝒏 𝒅𝒆𝒏𝒔𝒊𝒕𝒚
% Densification : × 𝟏𝟎𝟎
𝑻𝒉𝒆𝒓𝒐𝒓𝒊𝒕𝒊𝒄𝒂𝒍 𝑫𝒆𝒏𝒔𝒊𝒕𝒚

% porosity = (100-% densification).

* Theoretical density can be calculated from rule of mixture.

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 Density Measurement of Green Compact

 Archimedes' principle is the most acceptable method for measuring the

density of a green compact.

𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑠𝑎𝑚𝑝𝑙𝑒 𝑖𝑛 𝑎𝑖𝑟 (𝑔)

𝐺𝑟𝑒𝑒𝑛 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 = x density of water (g/cm3)
𝑆𝑜𝑎𝑘𝑒𝑑 𝑤𝑒𝑖𝑔ℎ𝑡 −𝑠𝑢𝑠𝑝𝑒𝑛𝑑𝑒𝑑 𝑤𝑒𝑖𝑔ℎ𝑡 (𝑔)
at ambient

𝑮𝒓𝒆𝒆𝒏 𝒅𝒆𝒏𝒔𝒊𝒕𝒚
% Densification : × 𝟏𝟎𝟎
𝑻𝒉𝒆𝒓𝒐𝒓𝒊𝒕𝒊𝒄𝒂𝒍 𝑫𝒆𝒏𝒔𝒊𝒕𝒚

% porosity = (100-% densification).

• Theoretical density can be calculated from rule of mixture.

• Water should be purest form.

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 SUMMARY

 Compaction is production of pellet of required dimension by applying pressure to

the powder with help of punch.

 In general, powder shape, size, density, nature of powder (ductile/brittle) effects

the compaction process.

 For successful compaction spherical, bigger size particle is advantageous. Bimodal

particle size distribution is also useful in this context.

 Selection of proper compaction load, die wall lubrication can improve the density
of compact, reduce the density variation and shape distortion of compact.

 Proper finishing, heat treatment of compaction tool, storage of powder, vibrating

the nanopowders before compaction, applying external pressure during ejection
can reduce the cracking issues in compacts.

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 Problems
• Three powders are die pressed using a pressure of 440 MPa. One of the three is -
325 mesh Ti, one is -325 mesh stainless steel and one is -100 mesh Cu. Which
powder gives highest green strength? Why?

• For a given powder, the density achieved at any pressure is higher for isostatic
compaction than for uniaxial die compaction. Why?

55