powder metallurgy (ise)

8/8/2019 Powder Metallurgy (ISE)

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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.



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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.



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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.



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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.



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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 .



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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.



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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.



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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.



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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.



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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



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



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



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



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.

powder metallurgy (ise)

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