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Powder Metallurgy
Dr. Somkiat TungjitsitcharoenIndustrial Engineering
Chulalongkorn University
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Elemental
or alloy
metal
powders
Additive
(Dielubricants
or
graphite)
Mixing
Hot compactionIsostatic,
Extrusion Die
compacting
Spraying,
Sintering
Pressureless
Cold compaction
Die compacting,
Isostatic, Rolling,Injection molding,
Slip casting
Sintering
Vacuum or
Atmospheric
Optional
manufacturing step
Sizing, Repressing,
Resintering,
Forging, Coining,
Metal infiltration, Oil
Impregnation
Optional finishingsteps
Heat treating,
Tumbling, Plating,
Machining, Stream
treating
Finish
product
Powder
Metallurgy
Processes.
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1. A technique for making parts from high-
melting-point refractory metals which may
be difficult or uneconomical to produce by
other methods.
2. Offer high production rates on relatively
complex parts, by the use of automated
equipment requiring little labor.
Process Capabilities
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3. Offer good dimensional control and
resultant elimination of machining and
finishing operations. Reducing waste,
scrap, and save energy.
4. Offer capability for impregnation and
infiltration for special purposes.
Process Capabilities
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5. Because of wide range of compositions, it
possible to obtain special mechanical and
physical properties such as stiffness,
damping, hardness, density, toughness
etc.
Process Capabilities
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The general sequence of operations
involved in the powder metallurgy process
is shown schematically in figure above.
The component powders are mixed,
together with lubricant, until a
homogeneous mix is obtained.
The mix is then loaded into a die and
compacted under pressure, after which the
compact is sintered.
Production of sintered parts
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An exception is the process for making filter
elements from spherical bronze power here
no pressure is used; the power being
simply placed in a suitably shaped mould in
which it is sintered. This process is know as
loose powder sintering.
Production of sintered parts
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The object of mixing is to provide a
homogeneous mixture and to incorporate the
lubricant.
Popular lubricants are stearic acid, stearin,
metallic stearates, especially zinc stearate,
and increasingly, other organic compounds of
a waxy nature.
Mixing
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The main function of the lubricant is to
reduce the friction between the powder
mass and the surfaces of the tools die
walls, core rods, etc. - along which the
power must slide during compaction, thus
assisting the achievement of the desired
uniformity of density from top bottom of the
compact.
Mixing
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The reduction of friction also makes it easier to
eject the compact and so minimizes the tendency
to from cracks. It has been suggested that an
additional function of the lubricant is to help the
particles to slide over each other, but it seems
doubtful whether this factor is of much
significance:- good compacts can be obtainedwithout any admixed lubricant, e.g. using die wall
lubrication or iso-static pressing.
Mixing
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Care in the selection of lubricant is
necessary, since it may adversely affect
both green and sintered strengthsespecially if any residue is left after the
organic part has decomposed.
Over-mixing should be avoided, since this
increases the apparent density of the mix.
M ixing
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Additionally, over-mixing usually further
reduces the green strength of the subsequent
compacts probably by component coating the
whole surface of the particles, thereby reducing
the area of metal contact on which the green
strength depends. The flow properties also are
impaired good flow is essential for the next
step i.e. loading the powder into the die.
M ixing
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Pressing
The mixed powders are pressed to shape in a
rigid steel or carbide die under pressures of
150-900 MPa. At this stage, the compacts
maintain their shape by virtue of cold-welding
of the powder grains within the mass.
The compacts must be sufficiently strong to
withstand ejection from the die and subsequent
handling before sintering.
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Compacting is a critical operation in theprocess, since the final shape and
mechanical properties are essentially
determined by the level and uniformity ofthe as- pressed density.
Powders under pressure do not behave as
liquids, the pressure is not uniformity
transmitted and very little lateral flow takes
place with the die.
Pressing
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Tool
The basic parts of a tool set are the die in
which the powder is contained, and
punches which are used to apply the
compacting pressure.
Multiple punches acting independently are
used if the component being pressed
different levels.
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The die and core rod (s) from the contour of the
compact parallel to the direction of pressing,
and must, of course, be free from projections
and re-entrants at right angles to the pressing
direction; otherwise it would be impossible to
eject the compact from the die.
Materials used are hardened tool steels or hard
metals (cemented carbides).
Tool
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The use of the more expensive carbide is
increasing because of the life it affords, and the
increasing cost of tool changes both in lost
production and tool setters wages.
PM high-speed steels are finding, increasing
application in this field.
For short runs, ordinary steel dies may, of
course, be more economical.
Tool
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The importance of precise dimensions and
high quality of the surface finish scarcely
needs emphasis bearing in mind that one of
the major features justifying the use of sintered
parts is the ability to produce such parts
accurately as regards size and with a surfacefinish that obviates the necessity for
subsequent machining operations.
Tool
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Die life is another important aspect, and here it is
impossible to give more than an indication.
The life depends not only on what material is
being pressed, and to what density, what
lubrication is provided and the degree of die wear
that can be tolerated, but also on the skill of the
tool setter, and the complexity of the tool.
Tool
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Sintering
Sintering is a key part of the operation.
The compact acquires the strength needed to
fulfill the intended role as an engineering
component.
In general , sintering requires heat.
Suffice to say that atomic diffusion takes place
and the welded areas formed duringcompaction grow until eventually may be lost
completely.
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Recrystallization and grain growth may follow, and
the pores tend to become rounded and the total
porosity, as a percentage of the whole volume
tends to decrease.
The operation is almost invariably carried out
under a protective atmosphere , because of the
large surface areas involved, and at temperatures
between 60 and 90% of the melting point of the
particular metal or alloys.
Sintering
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For powder mixtures, however, the sinteringtemperature may be above the melting-point of
the lower-melting constituent, e.g. copper/tin
alloys, iron/copper structural parts, tungstencarbide/cobalt cemented carbides, so that
sintering in all these cases takes place, hence the
term liquid phase sintering.
Of course essential to restrict the amount of liquid
phase in order to avoid impairing the shape of the
part.
Sintering
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Control over heating rate, time, temperature and
atmosphere is required for reproducible results.
The type of furnace most generally favored is an
electrically heated one through which the
compacts are passed on woven wire mesh belt.
The belt and the heating elements are of a
modified 80/20 nickel/chromium alloy and give a
useful life at temperatures up to 1150C.
Sintering
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For higher temperatures walking beam
furnaces are preferred, and these are
increasingly being used as the demand for
higher strength in sintered parts increases.
Silicon carbide heating elements are used and
can be operated up to 1350C.
Sintering
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For special purposes at still higher
temperature molybdenum heating elements
can be used, but special problems areinvolved, notably the readiness with which
molybdenum forms a volatile oxide.
Molybdenum furnaces must operate in a pure
hydrogen atmosphere.
Sintering
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Dimensional changes during sintering
Generally, the part tends to increase in density
as sintering proceeds and this still further
improves the mechanical properties.
Of course, an overall shrinkage which leads to
complications.
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It is possible, however, to get an increase in
size i.e. growth can result from a number of
factors:
(a) Entrapped gases with the compact;
(b) Water vapor formed within the object by
reduction of oxides;
(c) Decomposition products of the lubricant.
Dimensional changes during sintering
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Rapid heating and green density intensify all
these effects and may lead not only to overall
growth but to blistering and distortion. Should
be avoided.
Another cause of growth is the result of having
mixed powers of different elements.
The growth is most marked above the melting
point of the lower melting constituent.
Dimensional changes during sintering
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Fast heating rates tend to increase growth.
Necessary to allow for this change in the
design and manufacture of the tools , but it is
possible and increasingly practiced so to
balance the composition and sintering regime
that no dimensional change takes place .
Dimensional changes during sintering
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It should however , be noted that dimensional
change is influenced also by compact density;
the lover this is the greater the tendency to
shrink.
This is one of the reasons why uniformity of
density of the compact is of such importance.
Dimensional changes during sintering
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Secondary and Finishing operations
In order to improve the properties of
sintered P/M products further or to impart
special characteristics.
The methods are Coining and Sizing,
Impact forging, Impregnating, Infiltration, or
other
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They are compaction operations, performed
under high pressure in presses.
The purpose of these operations are
Imparting dimension accuracy to the
sintered part
Improving its strength and surface finish
by further densification
Coining and Sizing
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Utilization of preformed and sintered alloy
powder compacts, which are subsequently cold
and hot forged to the desired final shape.
Good surface finish, good dimensional
tolerance and a uniform and fine grain size.
Suitable for such application as highly stressed
automotive and jet-engine components.
Impact forging
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The inherent porosity of P/M can be utilized by
impregnating them with fluid.
Bearing and Bushing that are internally
lubricated, with up to 30% oil by volume, are
made by this method.
So that the parts have continuous supply of
lubricant during their service lives.
Impregnation
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This is a process whereby a slag of a lower-
melting-point metal is placed against the
sintered part and then the assembly is heated
to a temperature sufficient to melt the slag.
Infiltration
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The molten metal infiltrates the pores to
produce a relatively pore-free part having
good density and strength.
The most common application is the
filtration of iron-base compacts by copper.
Infiltration
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The advantages of infiltration are
Improve hardness and tensile strength.
Prevent moisture penetration which could
cause corrosion.
Infiltration with Lead, which is low shear
strength, the parts develop lower frictional
characteristics.
Infiltration
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Heat treating : improve hardness and
strength
Machining : produce various geometry
Grinding : improve dimensional accuracy
and surface finish Plating : improve appearance and
resistance to wear and corrosion.
Other methods
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A second pressing
operation, repressing, can
be done prior to sintering to
improve the compactionand the material properties.
The properties of this solid are similar to cast
or wrought materials of similar composition.
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Porosity can be adjusted by the amount of
compaction.
Usually single pressed products have high
tensile strength but low elongation. These
properties can be improved by repressing
as in the following table.
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Material Tensile(MPa)
Tensile
as Percent ofWrought Iron
Tensile
Elongationon 20 mm.
Elongation as
percent ofWrought iron
elongation
Wrought Iron,Hot Rolled
331 100% 30% 100%
Powder Metal,
84% density
214 65% 2% 6%
Powder Metal,
repressed, 95
% density283 83% 25% 83%
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Powder metallurgy is useful in making parts
that have irregular curves, or recesses thatare hard to machine.
It is suitable for high volume production with
very little wastage of material. Secondary
machining is virtually eliminated.
Typical parts include cams, ratchets,
sprockets, pawls, sintered bronze and iron
bearings (impregnated with oil) and carbide
tool tips.
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Design Considerations
Part must be so designed to allow for easy
ejection from the die. Sidewalls should be
perpendicular; hole axes should be parallel
to the direction of opening and closing of thedie.
Holes, even complicated profiles, are
permissible in the direction of compressing.
The minimum hole diameter is 1.5 mm
(0.060 in).
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The wall thickness should be compatible
with the process typically 1.5 mm (0.060 in)
minimum. Length to thickness ratio can be
up to 18 maximum-this is to ensure that
tooling is robust. However, wall thicknesses
do not have to be uniform, unlike otherprocesses, which offers the designer a great
amount of flexibility in designing the parts.
Design Considerations
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Drafts are usually not desirable except for
recesses formed by a punch making a blind
hole. In such a case a 2-degree draft is
recommended. Note that the requirement of
no draft is more relaxed compared to other
forming processes such as casting, molding
etc.
Design Considerations
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Undercuts are not acceptable, so designs
have to be modified to work around this
limitation. Threads for screws cannot be
made and have to be machined later.
Tolerances are 0.3 % on dimensions. If
repressing is done, the tolerances can be as
good as 0.1 %. Repressing, however,
increases the cost of the product.
Design Considerations
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Why you choose PM : Economic
A sintered PM component of comparable
quality may be cheaper than a cast or
wrought component.
PM typically uses more than 97% of the
starting raw material in the finished part.
Suited to high volume components
production requirements.
Long-term performance reliability in critical
applications.
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Because of P/M parts can eliminate many
secondary manufacturing and assembly
operations, it has become increasinglycompetitive with casting, forging and
machining.
But P/M have to invest in punches, dies,
and equipments so much.
Economics of P/M
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Economics of P/M So that production volume must be sufficient high
to warrant the investment.
Parts
Weight (Kg) Cost
saving
(%)Forged billet P/M Final part
Fuse large brace 2.8 1.1 0.8 50%
Engine mountsupport
7.7 2.5 0.5 20%
Arrestor hook
support fitting79.4 25.0 12.9 25%
Nacelle frame 143.0 82.0 24.2 50%
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1) Material available by PM.
2) Mechanical Properties - PM components
are designed to meet structural criteria in
many applications
3) Examples of shapes possible using PM
Why you choose PM : Performance
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Material available by PM.
Porous material
Hard metals
Metal with very high melting point
Composite material
Special high duty alloy
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Porous materials: The chief products in the
group are filters and oil-retaining bearingsoften referred to as self-lubricating
bearings. These products cannot readily or
satisfactorily be produced by alternativeprocesses.
Hard metals: (Tungsten carbide bonded
with cobalt), producing a whole range of
cutting tools and wear parts. PM is the only
route to produce them.
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Metals with very high melting points: i.e. the
refractory metals (tungsten, molybdenum,
and tantalum) are very difficult to produce
by melting and casting. It is difficult if not
impossible to make these composite
products except by PM.
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Composite materials: (two or more metals which are
insoluble even in the liquid state, or mixtures ofmetals with non-metallic substances such as oxides
and other refractory materials)
Electrical contact material (copper/tungsten, silver
/cadmium oxide)
Hard metals (cemented carbides)
Tungsten carbide bonded with cobalt
Friction materials
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Composite materials: These dispersion-
strengthened materials have strengths
especially at elevated temperatures
superior to that of case and wrought metals
of similar basic composition.
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Special high-duty alloys: The advantages of
the PM route are a higher yield of usable
material, and a finer uniform microstructure
that confers improved mechanical properties.
The PM process has also allowed the
development of new types of materials having
microcrystalline or even amorphous (glass
like) structures.
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Special high-duty alloys: The final
consolidated product is characterized by
very high strength, ductility, and thermal
stability. Microcrystalline and amorphous
structures can be achieved their use in
aircraft structures would significantly reduce
the weight and increase the payload.
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Mechanical Properties.
Impact strength
Tensile strength
Modulus of
elasticity
Fatigue Ferrous
Compressive
strength
Temperature Effects
Creep
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Impact strength: The impact strength of
ferrous PM alloys can be improved by
increasing density and by infiltrating with
copper. High densities could be achieved by
repressing, or re-sintering.
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Tensile Strength: Where strength of a
feature is critical, it is essential to work with
the PM manufacturer to optimize the design
for manufacturing, determine the strength
that can be reasonably specified, and
establish the test and inspection
procedures for maintaining performance.
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Tensile strength: Improvements in the
strength could be achieved by filling the
surface connected pores with a liquid metal
that has a lower melting point (infiltration)
Elimination of flaws- using hot isostatic
pressing.
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Modulus of Elasticity: MOE indicate lower
values for PM alloys than for equivalent
wrought and cast alloys, higher values than
for die cast alloys, and much higher values
than plastics and most composites.
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Modulus of Elasticity: Higher MOE may
allow opportunities to reduce wall thickness
and eliminate reinforcing features, such as
gussets and ribs, when redesigning
plastics, composites and die-castings for
PM.
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Fatigue Ferrous: PM alloys exhibit an
endurance limit, as do cast and wrought
alloys of similar composition. The
endurance limit increases with increasing
component density. Fatigue performance of
a P/M component relative to tensile
strength is reasonably consistent.
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Fatigue ferrous: An increase in mechanical
properties is achieved when pores are filled
with an organic rather than metallic
material. The operation prevents the entry
of potentially corrosive electrolyte during
subsequent plating operations.
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Compressive strength: This specific
property can be increase leading to the
reduction in porosity of the surface layer,
increasing the surface hardness and the
wear resistance.
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Temperature Effects: The temperature
ranges in which most PM components
operate have little effect on copper, iron,
steel and stainless steel alloys. In those
applications where anticipated tempera-
tures are high or low enough, alloy
formulation can be modified to improve the
properties.
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Temperature Effects: The stability of
mechanical properties at elevated
temperatures often makes PM an
economical alternative to injection moldedplastics, composites and die castings,
which can experience loss of strength and
reduced MOE at approximately the same
temperatures that induce creep.
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Creep: Creep is very slow plastic
deformation occurring at stress levels
below the yield point. PM components may
offer economical alternatives to injection-
molded plastics, composites and die-
castings designed for temperatures to 400
F (205 C).
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Creep: For example, a redesign for PM may
require fewer or smaller fasteners, require
less thread depth in tapped holes, allow
threaded fasteners in tapped holes rather
than through bolts and nuts, and eliminate
the need for steel inserts to distribute
concentrated loads.
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Examples Application.
Aerospace
Automotive
Filtration system
Tooling
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Aerospace
Needing excellence fatigue and stress
rupture properties, which require fully dense
alloy and careful testing.
Needing withstand elevated temperatures
in aggressive environments. This is require
Ni Co or Ti alloy.
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Automotive
Sensor ring for the
antiskid brake sensor
for an automotive
control system
A Cu-steel automotive
flange pulley that
includes a sensor on
the face
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Automotive
Multilevel Ni-steel
components are weldedto form an assembly as a
case on the axel of a
tandem truck
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Automotive
A sequence of automobile transmission
sprockets formed by repressing the teeth tonear full density, followed by vacuum
carburization to increase the fatigue strength
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Filtration system
Various filtration designs formed from stainless steel
using pressing and sintering technology. For high
surface area, multiple filter elements are combined into
a single structure to give more filtration.
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Tooling
An injection-molded
iron-nickel computer
connection.
Tool steel spindle formed byinjection molding and sintered
to full density using
supersolidus liquid phase
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Shaping Ceramics
The procedure involves the following steps:
1. Crushing or grinding the raw material into
very fine particles
2. Mixing them with additives to impart certain
desirable characteristics
3. Shaping, Drying, and Firing the material
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Shaping Ceramics
Raw materials
Forming
Shaping Drying
Firing
SinteringFinishing
CrushingMilling
Additives
Binder
Lubricant
Wetting agent
Slip casting, Extrusion,
Pressing, Injection molding
Green
machining
Machining
Grinding
Lapping
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It is generally done in ball mill, either dry or
wet.
Wet crush is more effective because of
keeping particles together and preventing
the suspension of fine particles in the air.
The particles may be sized (sieved), filtered
and washed.
Crushing or Milling
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Crushing or Milling
Ball mill
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The ground particles are then mixed withadditives one or more of the following:
a) Binderfor ceramic particles
b) Lubricant for aiding mold release andreducing internal friction between particles
during molding
c) Wetting agent for improving mixing
d) Plasticizer for making the mix more plasticand formable
e) Deflocculent for making ceramic-water
suspension more uniform
Crushing or Milling
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Shaping Ceramics
Slip casting, Extrusion,
Pressing, Injection molding
Raw materials
Forming or
Shaping Drying
Firing or
SinteringFinishing
CrushingMilling
Additives
Binder
Lubricant
Wetting agent
Green
machining
Machining
Grinding
Lapping
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The forming processes for ceramic are
1. Casting: Slip casting or drain casting
2. Plastic forming: Extrusion, injection
molding, and jiggering
3.
Pressing: Dry pressing, Wet pressing,Isostatic pressing, Jiggering, Injection
molding, Hot pressing
Forming or Shaping process
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Technique for forming processing
Process Advantages Disadvantages
Slip casting
Large parts,
complex shapes,
low equipment cost
Low production rate, limited
dimension accuracy
Extrusion
Hollow shapes and
small diameters
high production rate
Parts have constant cross
section, limited thickness
Dry pressing
Close tolerance,
high production rate
with automation
Density variation in parts with high
length-to-diameter ratio, dies
require high abrasive-wear
resistance, equipment can be
costly
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Technique for forming processing(2)
Process Advantages Disadvantages
Wet pressingComplex shapes, high
production rate
Part size limited, limited
accuracy, tooling costs can be
high.
Hot pressing Strong, high-density partsProtective atmospheres
required, die life can be short
Isostatic pressingUniform density
distributionEquipment can be costly
Jiggering
High production rate with
automation, low cost
tooling
Limited to axisymetry parts,
limited dim. accuracy
Injection moldingComplex shapes, high
production rateCostly Tool
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Slip casting
A slip is a suspension of colloidal (small
particles that do not settle) ceramic
particles in an immiscible (insoluble in each
other) liquid, which is generally water.
The slip is poured into a porous mold made
of plaster of paris.
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Slip casting
After mold has absorbed some of the water
from the outer layers of the suspension and
the remaining suspension is poured out
(make hollow object).
The top of the part is then trimmed, the
mold is opened, and the part is removed.
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Slip casting
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Plastic forming
It tends to orient the layered structure of
clay along the direction of material flow and
so tends to cause anisotropic behavior of
material both in subsequent processing
and the final properties of the ceramic
roduct
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Plastic forming
In extrusion, the clay mixture, containing20%-30% water, is forced through a die
opening by screw type equipment
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Pressing: Dry pressing
Similar to P/M compaction
Used for relatively simple shapes
Organic and inorganic binders such as
stearic acid, wax, starch, and polyvinyl
alcohol are usually added to the mixture
and they also act as lubricants.
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Pressing: Wet pressing
The part is formed in a mold while under
high pressure in a hydraulic or mechanical
press
Generally used to make intricate shapes
Moisture content usually range from 10% to
15%
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Pressing: Isostatic pressing
Extensively use in P/M
Used for ceramics to obtain uniform density
distribution throughout the part
Typical part: Automotive spark-plug
insulators.
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Pressing: Jiggering
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Pressing: Jiggering
Clay slug are first extruded, then formed
into a bat over a plaster mold and finally
jiggered on a rotating mold.
Jiggering is a motion in which the clay bat
is formed by means of templates or rollers.
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Pressing: Injection molding
Use extensively for the precision forming ofceramic of high technology application
such as rocket-engine components.
Raw material is mixed with a binder such
as thermoplastic polymer or wax
Thin section of engineering ceramics suchas alumina, zirconia, silicon nitride etc. are
possible.
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Pressing: Hot pressing
Pressure and temperature are appliedsimultaneously
Make part denser and stronger by reducing
porosity
Protective atmospheres are usually
employed, and graphite is a commonlyused punch and die material
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Shaping Ceramics
Slip casting, Extrusion,
Pressing, Injection molding
Raw materials
Forming or
Shaping Drying
Firing or
SinteringFinishing
CrushingMilling
Additives
Binder
Lubricant
Wetting agent
Green
machining
Machining
Grinding
Lapping
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Drying
It is a critical stage because of the
tendency for the part to warp, or crack,
from variations in the moisture content andthe thickness within the part.
Control of atmospheric humidity and of
temperature is important to reduce warping
and cracking.
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Drying
Loss of moisture results in shrinkage of the
part.
In humid environment, the evaporation rate
is low, and so that the moisture gradient
across the thickness of the part is lower
than that in a dry environment.
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Drying
This low moisture gradient, in turn,
prevents a large, uneven gradient in
shrinkage from the surface to the interior
during drying.
This stage the part can be machined
relatively easy.
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Shaping Ceramics
Slip casting, Extrusion,
Pressing, Injection molding
Raw materials
Forming or
Shaping Drying
Firing or
SinteringFinishing
CrushingMilling
Additives
Binder
Lubricant
Wetting agent
Green
machining
Machining
Grinding
Lapping
Fi i
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Firing
Heating the part to an elevatedtemperature in a controlled environment,
similar sintering in P/M.
Give the part harder and higher strength.
This improvement result from
Development of strong bond between thecomplex oxide particles in ceramic.
Reduced porosity.
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Shaping Ceramics
Slip casting, Extrusion,
Pressing, Injection molding
Raw materials
Forming or
Shaping Drying
Firing or
SinteringFinishing
CrushingMilling
Additives
Binder
Lubricant
Wetting agent
Green
machining
Machining
Grinding
Lapping
Fi i hi
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Finishing
Because of firing causes dimensional
changes, additional operations may be
performed to give the ceramic part its final
shape, improve its surface finish and
tolerance, and remove any surface flaws.
Fi i hi
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Finishing
The finishing process used can be one or more ofthe following;
Grinding with diamond wheel
Lapping and honing
Ultrasonic machining
Drilling by use of a diamond-coated drill
Electrical-discharge machining
Laser-beam machining etc.