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Shaping and compaction of powder materials

SHAPING AND COMPACTION OF POWDER MATERIALS

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Methods for compacting of powder materials can be divided to two basic groups:
1. Compaction with the aid of static or dynamic compacting pressure at room or elevated
temperature – compacting by single-action or multi-action static compacting pressure in a die
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with an upper and bottom punch for high-volume manufacturing of parts of various types:
forging, isostatic pressing, extrusion pressing, rolling, hot pressing e.g. for draw tools from
cemented carbides.
2. Pressureless compaction – free pouring (e.g. a manufacture of filters), jolting, vibration
compacting e.g. for fuel elements for nuclear technology and the ceramic casting method.
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In Table 6.1, basic differences in behaviour of powder particles during their pressure forming and
pressureless forming are shown.
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Table 6.1 Basic differences in behaviour of powder particles during pressure compacting and
pressureless compaction.

Pressureless compaction (loose poured Pressure-assisted compaction (deformed

particles) particles)
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Particles are placed more closely by applying

Particles are freely placed by gravitational
pressure to overcome interparticle adhesion and
forces
friction
Surface contact of particles, whereas a size
Particles maintain their shape depends particularly on compacting pressure –
metal contacts
Even during cold pressing a local temperature
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Temperature distribution between particles is increase occurs on places of particle contacts

equilibrium (as a result of interparticle friction). Some
diffusion processes may occur as well.
A crystal lattice of particles includes original Due to the plastic deformation an increase of
imperfections. dislocation density occurs inside particles.
Density increases and depends on a deformation
Density is low
ability of the material, particle morphology and
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Shaping and compaction of powder materials

compacting pressure.
A shape of pores depends on the particle A shape of pores depends on the pressing
morphology method and the applied pressure
Most of the pores are open, closed pores are
Pores are open (interconnected) – open porosity
existing already, too.

Cold forming involves all methods by which compacts can be made which have a demanded

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geometrical shape with such dimensions, so that after sintering they have final dimensions and such a
consistency (green strength), that it is possible to handle them (e.g. to take-off from a pressing tool, to
place into a sintering furnace etc.). The necessary “green” strength can be achieved by compacting, i.e.

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reducing porosity and increasing particular interparticle adhesion. Pressing is used most frequently for
this purpose.

6.1 Fundamentals of pressing processes

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When pouring powder, particles become arranged by acting of gravitational forces. Poured
powders include bridges and cavities which can be reduced by jolting and vibrations.
By using slurry, a tighter arrangement can be reached due to reduction of interparticle friction.
After drying-up of “cast” shape compacts, they can be handled freely. When pressing in tools, which
is prevailing in the industrial scale, a shape body is subjected to a complicated mechanical loading.
The applied compacting pressure leads to the compaction of powder. However, the pressure
A
distribution in a compact is not uniform, which is given by the fact that a part of the incoming energy
is consumed to overcome the friction between particles and mould walls and among particles
themselves.

6.1.1 Processes occurring during compaction of powder materials by compacting

pressure
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If stress and density gradients caused by friction on the die walls are not taken into account,
the achieved compact density is only a compacting pressure function. The following processes run in a
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compact with an increasing pressure (Fig. 6.1): 1. Arrangement of particles occurs, formed clusters
(bridges) are removed and cavities are filled. 2. A contact between particles increases as a result of the
plastic deformation and a particle surface is levelled. An oxidation layer is torn and a formation of the
agglomerate occurs by mechanical interlocking of particles. Particles are strengthened. 3. Further on,
when pressure increases, an increase of a contact surface between particles and further strengthening
of particles occur, adhesion between particles increases and particles, which have run out of their
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plasticity, disintegrate.
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Fig. 6.1 Processes running in a compact with increasing pressure.

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Shaping and compaction of powder materials

According to the quality of the powder, which can consist of particles of different sizes or
various chemical composition, these processes can run simultaneously and they occur more or less
depending on the powder properties. A density and porosity change influences other measurable
values which can be used for a description of properties of compacts, in particular for formable
materials. For example, this is hardness which depends on a degree of strengthening, a powder size, a
degree of oxidation etc., or electrical conductivity which increases with decreasing porosity at first,
but an increasing hardness slows down this rise.
During die pressing the conditions are strongly affected by friction on the die walls, which

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causes uneven distribution of material density in a compact. During single-action pressing, the highest
compacting pressure is on edges of a pressing tool, the lowest on the bottom edge of a compact.
During double-action pressing, i.e. when using a movable upper and bottom punch, compacts of a

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double height can be pressed. The pressure distribution is better than during the single-action pressing,
but it is also influenced by friction on the pressing mould walls.
Inhomogeneous pressure distribution in the vertical and horizontal direction causes a
disintegration of a compact to horizontal layers of a “plate” shape while pushed-off from the die.
According to some authors, during pressing parabolic areas are formed in a compact, which do not

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contain a tangential component of stress. On these areas, there is no shear motion of particles, and not
even occurrence of cold welded joints.
Consider a cylindrical compact of a diameter D and height l, where its element dl is analysed
(Fig. 6.2). The pressure in the upper part of the element p and the pressure passing through an element
pb differs by a normal (perpendicular) force acting against friction. Force equilibrium along the
compression axis can be expressed mathematically as:
A
F = 0 = A·(pb - p) + ·Fn
where Ff = µ·Fn; A = D2/4
The normal force can be expressed with the aid of an applied press and a constant of proportionality z,
which changes with a compact density:
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Fn = ·z·p·D·dl
The friction force Ff can be calculated directly from the normal force and the friction coefficient µ as:
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Ff = ·µ·z·p·D·dl
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D

Fig. 6.2 Relation of the applied compacting pressure to its distribution in the compact volume.

Combining the above mentioned relations, the difference in pressures dp of the upper and
004

Shaping and compaction of powder materials

bottom part of the element can be expressed as:

dp = p – pb = -Ff / A = -4 ·z·p·dl/D
Through integration of the above mentioned relation regarding the height of the compact, the
following relation of the pressure px in a point x under the punch can be obtained:
px = p·exp(-4 ·z·x/D)
This equation is applicable for the single-action pressing. The relation implies that the pressure

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decreases with an increasing distance from the punch in the depth under the punch.
The double-action pressing has a profile of the simultaneous compacting pressure profile both
from the upper and from the bottom punch. For this type of compression the obtained relation px is

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valid, however, the distance x expresses the distance to the nearest punch. The result is a more
homogenous pressure distribution in the compact. In such a case the pressure distribution depends on
the compact height to diameter ratio, which should be as low as possible. The single-action
compression is usually limited to a simple geometry of a compact.

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6.1.2 Activation effect of pressing
When shapes and sizes of particles and their contact areas are changing during pressing, lattice
and structure imperfections occur (e.g. dislocations the number of which is proportional to the
increasing compacting pressure and pressing speed). These increase the activity structure of powder,
which has been formed during its manufacture, and thus also free energy of the dispersive substance
and during the follow-up sintering lead to an increase of compaction intensity. Phenomena
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contributing to the sintering process activation are influenced by pressing conditions. These are in
particular as follows:
 a change in a particle specific area by further disintegration;
 an enlargement of a particle contacting area as a result of a failure of surface oxidation layers;
 an increase of the number of lattice imperfections.
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These phenomena belong among the main ones, which have a crucial influence on a sintering
process and thus on the final product properties.
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6.2 Pressure forming

Die pressing finds the best practical use of the mentioned powder material forming processes.

6.2.1 Die pressing

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Properties of compacts depend on the pressing process to a great extent. In Fig. 6.3, basic
principles of the most widely used processes of die pressing are shown. The double-action pressing is
preferred, as it ensures more favourable density distribution and complex density of a compact.
Further, it enables to eject a compact off a die faster.
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Fig. 6.3 Methods of die pressing: a) single-action pressing; b) double-action pressing; c) floating die
pressing [1].
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Shaping and compaction of powder materials

During this pressing method the powder poured into a die occupies a height H (loose powder
height) and at the upper and bottom punch acting simultaneously it is compressed to a height h. In Fig.
6.4, the particular cycles of the pressing process are shown – 1. Filling a die with powder; 2. Pressing
of the powder; 3. Ejection and take-off of a compact.

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Fig. 6.4 Particular cycles of the process of die pressing [2].

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6.2.1.1 Powder behaviour during pressing
Powder behaviour when compressed can be evaluated from various points of view. To
determine dimensions of pressing tools, a ratio of a loose powder height H to a compact height h is
important. In order not to extend a pressing path, the height H has to be as short as possible, which
assumes an adequate powder apparent density. The H:h ratio should not exceed a value of 3.
A
A degree of compaction at pressing is usually characterized by relative density which is
defined as a ratio of a density of a porous compact to an adequate compact material density. The
relation of relative density to compacting pressure for selected metal powders is given in Fig. 6. 5.
In principle, high density of compacts is required. It ensures a satisfactory strength and
manipulability of a compact and its low shrinkage during the subsequent sintering, so that demanded
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dimensions of a product can be achieved more easily. However, the enhancement of density of a
compact is limited by economical and constructional factors. The density enhancement requires higher
compacting pressures, resulting in faster wear of pressing tools and an increase of dimensions of
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pressing machines. Therefore in a case of an unsatisfactory density, other enhancement methods are to
be used, e.g. coining. This means a re-pressing of a compact in order to achieve accurate dimensions
and to improve properties, while enhancing the relative density substantially at the same time.
Regarding to pressing and sintering, contending demands are often posed on powders. For
good sintering, fine powders with a large specific surface are demanded. However, these materials
have the high loose powder height H and thus the pressing process is made more difficult. A similar
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situation is for mechanically “activated” powders which are often strengthened as much that their
deformation is not possible at room temperature. Then, these compacts exhibit low relative density
after pressing. To enhance the density, such powders must be processed in a different way.
In principle, the very pressing can be performed in two ways:
 Pressing by a stable compacting pressure – performed on hydraulic presses which have to be
set to a required maximum compacting pressure. Compacts may show certain differences in
height as a result of different properties of the initial powder material. Pressing force calculation:
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F = S·p
where F – pressing force (N), S – total compressed area (m2), p - compacting pressure (N.m-2).
 Pressing to a constant height – performed on mechanical presses with a constantly set stroke of
a punch. Differences in the initial powder quality result in density changes of obtained compacts.
Determination of the loose powder height:
HV = HS·q
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Shaping and compaction of powder materials

where Hv – loose powder height (mm), Hs – height of a compressed part (mm), q – filling coefficient
(q = k/s); k – density of a compact (kg.m-3), s –apparent density of powder material (kg.m-3).

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Fig. 6.5 Relation of the relative density to the compacting pressure for selected metal powders: 1 – Al,
2 – electrolytic Cu, 3 – sponge Fe, 4 – electrolytic Fe, 5 – carbonyl Fe, 6 – H2-reduced W [3].

6.2.1.2 Manufacture of complicated compacts

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Certain limitations need to be respected in manufacture with regard to forming of compacts.
These include above all high sensitivity to notches. When die pressing, a compact height to diameter
ratio should not exceed a value of 2, max. 3. Further, thin walls and sharp transitions of a surface are
to be reduced – see Fig. 6.6.
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mm
r. A
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Fig. 6.6 Structural design of shaped parts of sintered Fe [4].

A proper design and specifications of pressing tools ensure their long-time service life and
proper functioning. For a low-volume manufacture, pressing tools of tool steels are usually used, for a
high-volume manufacture - cemented carbides. When designing a pressing tool dimension, one has to
consider shrinkage, or swelling, of a compact as a result of sintering and
“relaxation” (elastic
007

Shaping and compaction of powder materials

recovery) after ejection of a compact from the die. Pressures applied during pressing are limited by a
tool shape and a type of material. Moreover, a pressing tool design is influenced by the pressure
intensity, a movement, a shape complexity and a demanded surface finishing. Compressed powder
shapes are usually complicated and consisting of several levels or thicknesses in a pressing direction,
therefore the successful pressing of complicated shapes demands an independent control of tool
movements – Fig. 6.7.

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Fig. 6.7 Ways of crack origination during pressing as a result of an improperly designed pressing tool:
a) different dilatations of two bottom punches; b) ejection of a compact in a sharp corner of a die [1].
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When forming parts with an unequal height, it is necessary to use several different punches
which are connected. Their movement is controlled, so that particles can only move along the
direction of the pressing. Separation of as-compressed sections from the rest of the compact by shear
must be avoided. This leads to an origination of cracks which do not reach up to the surface and
cannot be eliminated during sintering (Fig. 6.8).
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Fig. 6.8 Typical errors during the pressing: a) crack origination during the pressing as a result of
improper filling (1 – the crack does not reach to the surface, 2 – the area of failure is smooth); b) crack
origination during the ejection (3 – the crack protruding and visible, 4 – the fracture area is rough) [3].

6.2.1.3 Presses
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Productivity of pressing procedures depends particularly on efficiency of used presses. In

modern aggregates, the entire pressing process is software-controlled (die filling, pressing, ejection of
a compact, etc.) In principle, two types of presses are used:
 Mechanical presses – up to a pressing force of 1000 kN; they allow a manufacture of a high
number of compacts per unit of time. A number of strokes ranges between 20 – 100 per minute
(Fig. 6.9a).
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Shaping and compaction of powder materials

 Hydraulic presses – above a pressing force of 1000 kN; the pressing force is well-controllable.
However, the pressing speed is low and they allow 10 – 15 strokes per minute (Fig. 6.9b).

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a) b)
Fig. 6.9 Hydraulic (a) and mechanical (b) press made by DORST company.

6.2.1.4 Hot pressing

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This is pressing while heating the powder, the precompressed piece or the pressing tool. In this
pressing arrangement, forming and sintering processes take place at the same time. Therefore this
procedure is sometimes called the pressure sintering.
Hot pressing is applicable for powder materials in bulk, compacts or pre-sintered compacts.
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Tools for pressing up to temperatures of 600 °C are made of heat-resistant steels. Above this
temperature, ceramics and graphite are used (up to temperatures of 3000 °C). Examples of various
arrangements of hot pressing are shown in Fig. 6.10.
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Fig. 6.10 Schematic depiction of the hot pressing equipment: a) indirect heating through a heating tube
or a heating spiral; b) direct heating by passing current through punches; c) direct heating of a die; d)
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induction heating of a die or a compact [4].

Temperatures above 800 °C are achieved by direct heating of a tool or a compact. In order to
avoid powder oxidation, the pressing equipment is placed into a casing with a protective atmosphere
inside. The hot pressing technology is used for difficult to press powders and difficult to sinter
powders - e.g. dispersion-hardened powders, cermets etc. Another application is a manufacture of
compacts of cemented carbides with low porosity.
A low productivity and high wear of pressing dies resulted in seeking possibilities for
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Shaping and compaction of powder materials

achieving demanded effects. For cemented carbides, isostatic additional compacting at high
temperatures is applied.

6.2.2 Extrusion
This is a procedure of metal powder compaction or semi-products, manufactured by pre-
pressing, or possibly by pre-sintering, in a continuous strand. The process can be carried out at a
normal or elevated temperature (sintering temperature). In the case of extrusion pressing at room
temperature, a mixture of powder and a plastificator is pressed through a nozzle into rods or tubular

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forms. A plastificator is an additive, usually an organic substance (e.g. wax, methylcellulose, synthetic
resin, etc.), which is added to metal powders in order to form a plastic pasty material suitable for

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forming. A principle scheme of the direct and undirect way of extrusion is shown in Fig. 6.11.
The extrusion technology is used for all materials which can be processed using powder
metallurgy procedures. These are cemented carbides, high-temperature materials with high hardness
and especially technical ceramics. A pressing mixture contains 20 – 30 % of a plastificator which has
to be removed prior to the actual sintering. The compacting pressure is performed using e.g. a piston

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or a screw. Fine powders require higher pressures than for coarse powders extrusion, because more
joints occur, which are fractured and welded repeatedly during the process. A schematic depiction of a
powder material extrusion machine is shown in Fig. 6.12.
A
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Fig. 6.11 Extrusion methods: a) undirect extrusion; b) direct extrusion [5].
mm
r. A

Fig. 6.12 Schematic depiction of the powder material extrusion machine [6].
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Powder extrusion at higher temperatures is similar to compact material processing. An initial

material is in a form of pre-compacted or pre-sintered material. To avoid oxidation and contamination
from extrusion process lubricants, the material can be canned to a container of a mild steel plate. This
method allows reaching high density for such materials, which have oxide coatings by reason of high
affinity for oxygen, therefore they are difficult to sinter - for instance Al, Mg, Ti, high-speed steels,
etc. Extrusion pressing of sintered materials, such as high-speed steels or superalloys, where
almost
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Shaping and compaction of powder materials

porousless structure can be achieved, or of sintered materials dispersion-hardened by oxides, is very

effective. The used cans oxidize during the process, so they are removed from the compact by
stripping, or possibly by chemical etching.

6.2.3 Powder rolling

Rolling processes can be basically divided according to the direction of rolling and material
feeding – see Fig. 6. 13.

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Fig. 6.13 Powder rolling arrangement: a) horizontal arrangement with a screw powder feed, b) vertical
powder feed (bimetal), c) vertical powder feed (trimetal preparation) [7].
A
Processes occurring during powder rolling are shown in Fig. 6.14. In a region 1, there is a non-
compacted loose powder material. When passing into the second zone, characterized by an angle ,
the compacting process induced by friction between contra-rotating rolls and powder particles starts to
be performed. The applied forces reach the maximum values in the narrowest point between the rolls.
In contrast to rolling of compact materials, the acting pressure increases slowly at first.
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mm
r. A

Fig. 6.14 Powder in the rolling gap: region 1 – non-compacted powder, region 2 – zone of powder
compaction [3].
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Only at the end of the compaction zone the pressure increases as much that the running-out
strip has a satisfactory strength. A relation between a thickness ho and h1 can be expressed as follows:
ho 2 R 1- cos α
= +1
h1 h1
If neglecting a difference in a speed of powder inlet and strip running-out, a relation applies for the
powder density in the zone 1 (o – bulk density) and the rolled strip density 1:
011

Shaping and compaction of powder materials

ρ1 2 R 1- cos α
= +1
ρ0 h1

The value h1 represents the strip thickness, which must not be exceeded at the demanded density ratio
1 /o. It can only be varied by a change of a radius of rolls. The bulk density o is given by powder
properties. The same applies for the angle of repose  , moreover, it can be changed within a narrow
limits by roughing the rolls.
When planning a technical arrangement of powder rolling, there is often a tendency to

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implement subsequent sintering of a green strip, further cold forming, annealing with a possible
further cold rolling into one technological line. However, owing to various speeds of individual
operations, it is sometimes more effective to perform these operations separately.

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For powder materials which can be processed by casting and rolling into strips, rolling is
preferred because thin strips can be made with only low number of passes through the rolling stand.
However, a disadvantage is a higher price of the initial powder material and low rolling speeds,
therefore this process is always considered for individual respective cases.

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The powder rolling technology is often the only possible way of processing for a manufacture
of porous strips or strips of special materials or those difficult to form. For example, this is the
manufacture of strips with special magnetic or electrical properties made of Al-Ni-Co or Cu-Ti alloys.
Further, this method is applied for the manufacture of coated strips for friction and sliding materials
(Fig. 6.13a) and bimetallic (Fig. 6.13b) or trimetallic strips (Fig. 6.13c).

6.2.4 Isostatic pressing

A
At isostatic pressing the powder is encased in a sheath on which pressure transferred by gas,
rubber, plastic material or liquid is applied. In this case, hydrostatic pressing is used.
The main advantage of this method lies in a fact, that friction between powder and die walls,
which resulted in uneven distribution of pressure, tension and compact density, is removed. The
sheath, in which the powder is encased, has to have special properties. At high pressure, it has to
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behave as a liquid medium in order to transfer the pressure to the powder isostatically. At standard
pressure, it has to behave as a solid material to maintain its shape, on which accuracy of the compact
depends, even after filling with heavy powder.
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Isostatic pressing enables to reach higher densities of pressed materials (Fig. 6.15), to
manufacture components of larger dimensions and to process powders difficult to compress - e.g. a
manufacture of molybdenum rods.
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Fig. 6.15 Density increase by isostatic pressing [3].

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Shaping and compaction of powder materials

6.2.4.1 Cold isostatic pressing (CIP)

This isostatic pressing procedure is carried out in a steel pressure vessel. After closing, the
required pressure ranging from 200 MPa (for ceramics and graphite) up to 400 MPa for metal powders
is developed through appropriate equipment. For compaction, a delay on the maximum pressure for
only several seconds is sufficient, but its decrease has to be performed slowly (especially in lower
pressure regions), so that cracks cannot occur.
In term of a structural design, there are two basic concepts of the pressure vessel. The ASEA
system - closing the pressure vessel with a cover hold down by a specially adapted frame (“wet bag”

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method – Fig. 6.16).
When pressing smaller parts, substantially higher pressing outputs can be achieved in a device
called as a “system with a bolt closure with a dry sheath” (“dry bag” method – Fig. 6. 17). Powder is

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filled into a flexible tube an upper and bottom opening of which is connected to a pressing chamber.
Then the openings are closed and pressure is transferred through a liquid which encloses a flexible
sheath. After finishing the pressing and evacuating the chamber, the flexible sheath is moved to the
pressure housing walls and thus the compact is freed to be taken-out.

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Fig. 6.16 Principle diagram of the cold isostatic pressing – the ASEA system (isostatic press made by
KOBE STEEL, LTD) [8].
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D

Fig. 6.17 Principle diagram of the cold isostatic pressing – the dry bag method with a bolt closure
(isostatic press made by KOBE STEEL, LTD) [9].
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Shaping and compaction of powder materials

The isostatic pressing is used mainly for compacts of larger dimensions and rotating shapes
from materials difficult to form. Products of SiN, ceramics and graphite, cemented carbides (rolls,
dies), molybdenum, tungsten, etc. can be obtained this way. Molybdenum and tungsten powders are
processed by this technology to consumable electrodes for arc remelting, which have a weight of 1 – 2
tons. A principle diagram of individual process steps of the dry bag cold isostatic pressing is depicted
in Fig. 6.18.

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Fig. 6.18 Principle diagram of individual process steps of the dry bag cold isostatic pressing [9].

6.2.4.2 Hot isostatic pressing (HIP)

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For hot isostatic pressing, which is nowadays performed at temperatures up to 2000 °C, a
heating device is installed inside a pressure vessel. An important part is a thermal insulation cylinder
mm

separating a working area (high temperature application) from the steel pressure vessel a temperature
of which must not exceed c. 150 °C, so that tensile properties of the steel housing cannot drop (Fig.
6.19).
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D

Fig. 6.19 Principle diagram of the hot isostatic pressing [10].

Argon is used as a medium for pressure transfer, while working with pressures up to 200 MPa.
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Shaping and compaction of powder materials

Non-porous sintered materials can be hot isostatic pressed without any further actions. Porous
compacts and powders have to be placed into a sheath of plate (commonly steel or titanium) or glass
and they are evacuated.
Hot isostatic pressing makes use of a combination of elevated temperature and isostatic
pressing by gas. A principle diagram is depicted in Fig. 6. 20. Powder is placed into a flexible sheath
and vacuum degassed. The sheath is closed hermetically and inserted into the working area of a press.
Pressure is transferred through an inert gas or an auxiliary medium. While heating the container,
pressure is increasing. The sheath can be removed by machining (simple shapes) or acid leaching
(complex shapes) – Fig. 6.21.

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The HIP technology is used e.g. for high-speed steels, Ti-alloys and superalloys. Its advantage
is a possibility of processing large sintered parts with very low (residual) porosity. For example, in

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cemented carbides the content of pores after pressing and sintering is 1 – 2 %. The powder compaction
to close-to-finished (near-net) dimensions allows reaching a high material utilization.

.B
A
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Fig. 6.20 Particular steps during the hot isostatic pressing [7].

Another application of the hot isostatic pressing is an additional compaction of castings by

mm

which a homogenous structure without pores and shrinkage holes can be obtained (Fig. 6. 22). An
advantage of this process, i.e. a combination of casting and HIP, is reaching higher static and
particularly dynamic strength, better ductility, better machinability with higher quality of a work-piece
surface, higher corrosion resistance as well as longer operating life of parts.
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a) b)

Fig. 6.21 Encasing of powder material (a) and the finished product after annealing and machining (b)
[11].
015

Shaping and compaction of powder materials

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Fig. 6.22 Additional compaction of castings.

6.2.5 Powder forging

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This technology combines powder metallurgy advantages with die forging processes. This
way quite complicated shapes with high density (up to 99.5 %) and product accuracy can be produced
(Fig. 6. 23). In general, either forging of a non-sintered compact (i.e. powder forging) or of a sintered
compact, mostly in a hot protective atmosphere, can be performed. Technological operations of
forging:
a) preparation of a powder mixture – mixing of powder metal with pressing additives (lubricants,
graphite, etc.);
b) pressing of preforms;
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c) heating in order to remove lubricant under a protective atmosphere;
d) sintering (in the case of a sintered compact forging);
e) forging;
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f) finishing operations (heat treatment, smoothing etc.).
mm
r. A

Fig. 6.23 Connecting rods manufactured by powder forging [12].

6.2.6 Explosive compaction

When powder compacting by explosive detonation, methods using explosion either on a free
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area (Fig. 6. 24) or in closed pressure vessels can be used.

Technical solutions for explosive compaction on a free area can be as follows:
a) Powder materials are directly compacted (the so-callled contact system) – a container with
powder is in a direct contact with a charge, resulting in rod shaped or tubular compacts.
b) Explosive pressing with a flat charge – a manufacture of plate shaped semi-products.
Forming powders by detonation in closed pressure vessels is in principle very similar to
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Shaping and compaction of powder materials

conventional pressing of metal powders. Usual systems cylinder – piston can be used, whereas an
impulse is imposed on a punch either through a pressure wave or a shock wave of a steel cylinder set
to motion by detonation of an appropriate explosive charge.
This dynamic and high-energy process of forming enables enhancement of mechanical
properties of powder products.

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a)

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Fig. 6.24 Principle diagram of the explosive compaction of powder materials (a) and the industrial
equipment for the explosive compaction (b) [13].

6.2.7 Metal injection moulding

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Powder injection moulding (PIM) is based on a technically developed plastic injection
moulding technology which is combined with a classical powder metallurgy. Depending on a type of
the used injected powder, PIM includes two modifications: 1) Ceramic powders (CIM – Ceramic
mm

Injection Moulding) and 2) metal powders (MIM – Metal Injection Moulding)

This technology allows obtaining products with precise dimensional tolerances which cannot be
obtained by conventional pressing and sintering ( 0.08 to 12 mm). This accuracy usually enables to
leave out further processing. In principle, very fine powders are used, so that sintering times are as
short as possible. The procedure of the manufacture of parts using MIM technology is shown in Fig.
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6.25.
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Fig. 6.25 Particular phases of the metal injection moulding technology [14].
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Shaping and compaction of powder materials

It consists of the following phases:

a) Mixing powder with a binder (wax, polymer) – powder with particle size  25 µm and a
spherical shape is mixed with a proper bonding agent, which determines rheological properties of
the mixture. A particle surface has to be clean and smooth to enable a formation of a binder thin
film. Metal powders are prepared by gas or water atomization, or possibly by carbonyl
decomposition.
b) Granulation – forming powder material to a specific shape to be handled more easily.
c) Injection moulding – includes heating a mixture above a binder melting point (150 – 180 °C)

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(binder plastificatioon), injecting the melt into a mould under pressure of 15 – 30 MPa and cooling
the mould down to solidify the powder and binder mixture (Fig. 6. 26). The compact is called a

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“green body”.

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Fig. 6.26 Schematic depiction of the injection press and its main components: 1 – feed hopper, 2 –
granulate, 3 – reciprocal screw, 4 – heating, 5 – a stationary half of a mould, 6 – a moving half of a
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mould, 7 – melt, 8 – compact [15].

d) Binder removal – this is a key process, critical for product properties, therefore it requires very
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careful control. In this phase, “extraction” of the binder out of the compact occurs, after which a
porous structure is formed, the so-called “brown body”. Time needed for the binder removal
depends on the used method, a maximum thickness of a compact wall and a bonding system and
can last up to several days. Basic methods of the binder removal involve the following: 1) thermal
decomposition – heating a compact, so that the polymer binder can melt, decompose and ultimately
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evaporate; 2) catalytic decomposition of the mixture; 3) dissolving the binder by appropriate

solvents; 4) gelatinization and evaporation of the binder. A change in colour of the compact after
the particular processes is shown in Fig. 6.27.
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Fig. 6.27 Change in colour of the compact after the particular processes: a) the compact after injecting,
b) the compact after the binder removal, c) the sintered finished product [16].
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Shaping and compaction of powder materials

e) Sintering of a compact – a thermal process in which individual separate particles are

agglomerated and form final demanded strength of the product. This phase is related to
considerable shrinking, which may reach as much as 30 % as a result of high porosity of the part
after the binder removal. Relative density typically ranges around 95 % and higher. Mechanical
properties roughly correspond to properties of forged metal of the same composition.

MIM technology products are used mainly in applications for high-volume manufacturing,
where it leads to as much as 70 % production cost reduction. For example, these are applications for

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military industry (triggers, hammers, cartridge ejectors), in electronics for a manufacture of
complicated and precision parts for telecommunication, printers, copying machines, video recorders,
cameras, etc., in health service for scissors, parts of medical and dental instruments, in automotive

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industry for safety elements for airbag systems, fuel injection, turbochargers, etc. (see – Fig. 6. 28).

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Fig. 6.28 Examples of MIM technology applications.

6.3 Forming without external pressure

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All methods of forming without applying external pressure have a common principle, that an
appropriate amount of a proper nonmetallic material is added to metal powder, so that an originated
suspension is either liquid to be cast or formable. This matter has to be utterly removable from a
product, namely under such conditions not to deteriorate a product’s shape before achieving a
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satisfactory strength through sintering. Main advantages are as follows:

a) no need for costly presses and pressing tools, that means low purchasing cost for equipment
b) even smaller batches can be produced at low cost
c) size of manufactured parts can be enlarged.
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6.3.1 Ceramic casting method

Into a plaster mould either a full casting is cast or a hollow one onto a metal core. A hollow
casting can be also obtained in a way that after settling a layer of solid particles in a specific thickness
on a mould walls, the remaining slurry is poured out. This method is wide-spread in ceramics. It is
called a slurry casting method; slurry is a common name for a suspension of fine clay in a carrier
liquid, most frequently in water.
For preparation of the slurry for metal powders the same principles as for ceramics are
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applied. Slurry is the more stable, the finer the powder is and the higher the viscosity of the liquid
carrier is (metal powder to liquid ratio 3:2). As metal powders are often coarser than ceramic ones and
viscosity cannot be increased with regard to casting, stabilizers play a decisive role for the slurry
preparation (e.g. sodium and ammonium salts of alginic acid). The stabilizers allow the slurry to be
processed prior to separation of the solid state suspension from the liquid and settling onto the bottom
of the mould. On the contrary, liquid separation from the suspended phase is necessary in a mould a
front side of which has to be permeated by a continuous capillary system which takes the liquid carrier
away.
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Shaping and compaction of powder materials

6.3.2 Freeze casting of suspensions

Draining the liquid phase away from the suspension through a porous wall is not always
advantageous. For thicker suspensions with a smaller amount of the liquid, a technology of suspension
injection into a metal mould with a following freezing is used. Water with addition of starch or latex is
used as the liquid phase. Water is then removed from frozen castings by vacuum sublimation or slow
drying. Porosity is quite high (30 to 33 %) and in some cases can be reduced by infiltrating metal. For
example, turbine blades of TiC are fed with Cr-Ni-Co based alloy after sintering.

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