Module DMV20173

8.0 POWDER PROCESSING





8.2.1 METHODS OF POWDER PRODUCTION


1. There are several methods of producing metal powders, and most of them can be

produced by more than one method.

2. The choice depends on the requirements of the end product.

3. The microstructure, bulk and surface properties, chemical purity, porosity, shape and
size distribution of the particles depend on the particular process used.

4. These characteristics are important because they significantly affect the flow and

permeability during compaction and in subsequent sintering operations.

5. Particle sizes produced range from0.1 to1000 μm.

6. Example methods of producing metal powders are:

• atomization;

• reduction;
• electrolytic deposition;
• carbonyls;

• comminution;
• mechanical alloying;

• miscellaneous methods;
• nanopowders;

• microencapsulated powders.























Figure 8.3 Metal powders.




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9.2.2 BLENDING METAL POWDERS



1. Blending refers to when the powders of the same chemical composition but possibly
different particle size are intermingled.
2. Different particle are blended to reduce porosity.

3. Mixing refers to powders of different chemistries being combined.
4. An advantage of PM technology is the opportunity to mix various metals into alloys

that would be difficult or impossible to produce by other process.
5. The process accomplished by mechanical means such as, rotation in drum, rotation
in a double-cone container, agitation in a screw mixer and stirring in a blade mixer

(Figure 8.4).
6. To achieve best result in mixing, the container will be filled between 20% - 40% full.

7. The containers are usually designed with internal baffles or other ways of preventing
free-fall during blending of different size of powder, because variations in settling

rates between sizes result in segregation.

8. Other ingredients are added during blending or mixing process:
a. Lubricant, zinc and aluminum is added in small amounts as to reduce friction

between particles and at the die wall during compaction.
b. Binder, which are required in some cases to achieve adequate strength in the
pressed but unsintered part.

c. Deflocculants, which inhibit agglomeration of powder for better flow
characteristics during subsequent processing.



















Figure 8.4 Mixing and blending processes of metal powders. (a) crushing (b) mixing (c)
hammering.




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9.2.3 SINTERING


1. The last process is sintering, the process after pressing, the purpose of this process
in improve strength and hardness of green compact.

2. This green compact is easily crumbled under low stress.
3. Sintering is a hear treatment operation performed on the compact to bond its metallic
particles, thereby increasing strength and hardness.

4. Since powder metallurgy application is usually involve medium to high production,
most sintering furnace are designed with mechanized flow-through capability for the
work part.

5. The process of heat treatment consists of three steps where the furnace is
segregated into three chambers according to respective process:
a. Preheat, in which the lubricants and binders are burned off.

b. Sinter.
c. Cool down.


Typical sintering temperatures and times are given for the selected metals in Table 8.1.


Table 8.1 Typical sintering temperature and times for selected powder metals.
Metal °C Time (min)

Brass 850 25
Bronze 820 15
Copper 850 25

Iron 1100 30
Stainless steel 1200 45

Tungsten 2300 480

6. In modern sintering process, the atmosphere in the furnace is controlled. The

purposes of requiring controlled atmosphere:
a. Protection from oxidation.

b. Providing a reducing atmosphere to remove existing oxides.
c. Providing a carburizing atmosphere.
d. Assisting in removing lubricants and binders used is pressing.


7. Common sintering atmosphere are inert gas, nitrogen-based, dissociated ammonia,

hydrogen, and natural gas based.



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8. Vacuum atmosphere are used for certain metals, such as stainless steel and
tungsten.

8.2.4 SECONDARY OPERATION


1. In order to further improve the properties of sintered powder metallurgy products, or
to impart special characteristic, several additional operations may be carried out after

sintering.
2. The function of secondary operation are varied, they include densification, sizing,
impregnation, infiltration, heat treatment and finishing.



Example 8.1

Describe the stages to produce compacted product using powder metallurgy

technique in a form of flow chart.



Metal Powder Blending Compaction



Example 8.2

What hazards are involved in powder metallurgy processing? Explain their causes.

Answer.

Because of their high surface area to volume ratio, metal powders can be explosive,

particularly aluminum, magnesium, titanium, zirconium and thorium. Great care must
be exercised both during blending and in storage and handling. Precautions include

(a) grounding equipment, (b) preventing sparks (by using non-sparking tools) and
avoiding friction as a source of heat, and (c) avoiding dust clouds, open flames, and
chemical reactions.



EXERCISE 8.1


1. Describe briefly the production steps involved in making powder
metallurgy parts.

2. Why blending the metal powder is essential in powder metallurgy

processing.




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9.3 PROCESSING OF CERAMIC POWDERS



1. Ceramics are compounds of metallic and nonmetallic elements.
2. Ceramics can be divided into two general categories:

(a) Traditional ceramics—ceramic materials that obtained naturally in minerals
such as clay, bauxite, shales.
(b) Industrial ceramics (also called engineering, high-tech, or fine ceramics) -

cermic materilas that obtained in chemical processing such as nitride,
carbide, alumina, zirconia, silicon.


Example 8.3:
State some applications of the traditional ceramics and engineering

ceramics?

Solution:

Traditional ceramics : Pottery, brick, tiles.
Engineering ceramics : cutting tool, automotive parts, electronic parts.


9.3.1 RAW MATERIALS


1. Raw materials for ceramics mostly are found in nature such as flint (a rock
composed of very fine-grained silica) and feldspar (a group of crystalline minerals

consisting of aluminum silicates plus potassium, calcium, or sodium).
2. Traditional raw materials used for making ceramics is clay, which has a fine-
grained sheet-like structure.

3. Categories of ceramic raw materials include:
a) From natural deposits (clay, bauxite, shales)

b) Refined industrial mineral (feldspar, kyanite, zircon)
c) Industrial chemical processing (alumina, magnesia, nitride)


Think

Some of the ceramic resources mineral in Malaysia.


9.3.2 MANUFACTURING PROCESSES OF CERAMIC MATERIALS

1. Several techniques are available for processing ceramics into useful products
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depending on the type of ceramics involved and their shapes.
2. The procedure to produce ceramic product involves the following steps:

3. Crushing or grinding the raw materials into very fine particles.
4. Mixing them with additives to impart certain desirable characteristics.
5. Shaping, drying, and firing the material.

6. Figure 8.3 shows the processing steps involved in making ceramic parts.






















Figure 8.3 Processing steps in making ceramic parts.

8.3.3 CRUSHING AND MILLING


1. Crushing and milling are the processes used to reduce the ceramic particle size
from coarse to fine particle distribution.
2. It is important because as fine particle distribution obtained, the properties of

ceramic compaction is improve in term of the density, hardness and fracture
toughness etc.

3. Common equipments to perform crushing and milling activities include jaw
crusher, rotary crusher, hammermill, crushing roller etc.(Figure 8.4).



















Figure 8.4 (a)Jaw crusher. ( b) Hammermill.





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8.3.4 ADDITIVIES


The ground particles are then mixed with additives—the functions of which are one or
more of the following:

a) Binder : for holding ceramic particles together.
b) Lubricant : to reduce internal friction between particles during moulding

and to help remove the part from the mould.
c) Wetting agent : to improve mixing.
d) Plasticizer : to make the mix more plastic and formable.

e) Agents : to control foaming and sintering.

8.3.5 FORMING AND SHAPING


1. Forming and shape processes transform the ceramic powders into a green

product.
2. Both processes involve with controlled size, surface quality, density and
microstructure.

3. Some of the major manufacturing methods to produce ceramic parts are slip
casting, extrusion, jiggering, pressing and injection moulding.
4. Table 8.2 shows he advantages and limitations of each process.


I) SLIP CASTING


1. The basic process of slip casting involves the suspension of a powder

material in a solution.
2. The powder and solution mixture (slip) are then cast into a mould .
3. The liquid then removes from the powder by capillary action or drain, leaving

the powder in the form of the mould.
4. This powder shape (green part) can then be removed from the mould and
sintered in an oven to give it strength.

5. Figure 8.5 shows the sequence of operations in slip-casting a ceramic part.

EXERCISE 8.2


State the similarities and differences between slip casting and metal casting.







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(a) (b)
















(d) (c)
Figure 8.5 Sequence of operations in slip-casting (a)(b)(c)(d).


II) EXTRUSION


1. The processing of ceramic also same as the processing of powders where
it can be compacted using extrusion technique.


III) JIGGERING

1. A combination of processes is used to make ceramic plates.

2. In this process, clay slugs first are extruded and formed into a bat over a
plaster mould.

3. They then are jiggered on a rotating mould.
4. Jiggering is a motion in which the clay bat is formed by means of templates
or rollers. The part then is dried and fired.

5. The jiggering process is confined to axisymmetric parts and has limited
dimensional accuracy.

6. The operation is automated for improved productivity.Extrusion
7. Fig 8.6 (a) shows Extruding and (b) jiggering operations.









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Figure 8.6 (a) Extruding and (b) jiggering operations.


Example 8.4


What is jiggering? What shape does it produce?


Solution:
Fine size particles Jiggering is a proses of shaping with rotating mould rolls
on the surface of ceramic compacts.


IV) PRESSING


Pressing is a process where ceramic compacts may be subjected to a

pressure in order to achieve uniform compaction and density. Some of the
pressing processes in ceramic manufacturing are dry pressing, hot pressing,
wet pressing and isostatic pressing.


a) Dry Pressing

i. Dry pressing is used for relatively simple shapes, such as whiteware,
refractories for furnaces, and abrasive products.
ii. Density can vary significantly in dry-pressed ceramics (as in P/M

compaction) because of friction among the particles and at the mould walls.


b) Hot pressing
i. In this process (also called pressure sintering), the pressure and the heat
are applied simultaneously.

ii. This method reduces porosity and, thus, makes the part denser and
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stronger.
iii. Graphite commonly is used as a punch and die material, and protective

atmospheres usually are employed during pressing.

c) Wet Pressing

i. In wet pressing, the part is formed in a mould while under high pressure in a
hydraulic or mechanical press.

ii. This process generally is used to make intricate shapes. Moisture content
usually ranges from 10 to 15%.
iii. Production rates are high; however, (a) part size is limited, (b) dimensional

control is difficult to achieve because of shrinkage during drying, and (c)
tooling costs can be high.


d) Isotatic Pressing
i. Isostatic pressing also is used for ceramics in order to obtain uniform

density distribution throughout the part during compaction.
ii. Used extensively in powder metallurgy.


EXERCISE 8.3

Explain the characteristics of particle powders which produce a high density

product.



V) INJECTION MOULDING


1. Injection moulding is used extensively for the precision forming of ceramics
in high-technology applications, such as for rocket-engine components.

2. The raw material is mixed with a binder, such as a thermoplastic polymer
(polypropylene, polyethylene or ethylene vinyl acetate) or wax.
3. The binder usually is removed by pyrolysis (inducing chemical changes by

heat).
4. The part then is sintered by firing.








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Table 8.2 Advantages and limitations of each process

Process Advantages Limitations
Slip casting Large parts Low production rate
Complex shapes Limited dimensional accuracy

Low equipment cost
Extrusion Hollow shapes and small diameters Parts have constant cross-section

High production rate Limited thickness
Dry pressing Close tolerances Density variation in parts with high
High production rates [with length-to-diameter ratios

automation) Dies require abrasive-wear
resistance

Equipment can be costly
Wet Complex shapes Limited part size and dimensional
pressing High production rate accuracy

Tooling costs can be high
Hot Strong Protective atmospheres

pressing High-density parts required
Short die life
Isostatic Uniform density distribution Equipment can be costly

pressing
Jiggering High production rate with Limited to antisymmetric parts

automation Llimited dimensional accuracy
Low tooling cost
Injection Complex shapes Tooling can be costly

moulding High production rate


8.3.6 DRYING


I) GREEN MACHINING


1. The final shape of ceramic product cannot always be achieved by the
original shaping processes.

2. It is often obtained by secondary operations before the sintering process
that is Green machining process.

3. Green machining refers to shaping operations of the dried unfired ceramic
product. Due to the lower strength in green ceramic product, refinement of



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ceramic shapes are applicable where higher rates of material removal can
be achieved.

4. Some of the operations involve in Green machining are machining, milling,
sanding, grinding and jiggering.


II) FIRING AND SINTERING OF CERAMICS


1. Drying is a critical stage because of the tendency for the part to warp or
crack from variations in the moisture content and in the thickness of the

part.
2. Control of atmospheric humidity and of ambient temperature is important in
order to reduce warping and cracking.

3. Firing (also called sintering) involves heating the part to an elevated
temperature in a controlled environment.
4. Some shrinkage occurs during firing.

5. Firing gives the ceramic part its strength and hardness. This improvement
in properties results from:

a) the development of a strong bond between particles in the ceramic;
b) reduced porosity.
6. Figure 8.7 shows the mechanism of shrinkage of wet clay caused by the

removal of water during drying.
7. Figure 8.8 shows shrinkage of ceramic product from green body to

sintered body.















Figure 8.7 Shrinkage of wet clay caused by the removal of water during

drying.











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(a) (b)
Figure 8.8 Shrinkage of ceramic product from (a) green body to (b) sintered body


Example 8.5:

What is the disadvantage of shaping operations after sintering process.
Solution:
Ceramic product becomes refractory after sintering process. Thus, it is very hard to

reshape the product.


8.3.7 FINISHING OPERATIONS


1. In order to produce near net shape product and enhance material

properties, post processing operations is employed to eliminate
unnecessary voids.
2. Process selection is an important consideration because of the brittle

nature of most ceramics and the additional costs involved in some of these
processes.

3. The finishing processes employed can be one or more of the following
operations:
a) Grinding (using a diamond wheel)

b) Lapping and honing
c) Ultrasonic machining

d) Drilling (by using a diamond-coated drill)
e) Electrical-discharge machining
f) Laser-beam machining

g) Abrasive water-jet cutting
h) Tumbling (to remove sharp edges and grinding marks)








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SUMMARY



In this chapter we have studied that :


1. powder processes is a net-shape forming process consisting of producing metal
powders, blending them, compacting them in dies, and sintering them to impart

strength, hardness, and toughness;
2. the powder processes is capable of producing relatively complex parts
economically, in net-shape form, to close dimensional tolerances, and from a wide

variety of metal and alloy powders;
3. secondary and finishing operations may be performed on powder processes parts
to improve their dimensional accuracy, surface finish, mechanical and physical

properties, and appearance;
4. ceramics are particularly attractive for applications such as heat-engine

components, cutting tools, and components requiring resistance against wear and
corrosion;
5. ceramic processing generally started with crushing and milling of raw materials

and mixed with additive materials to enhance powder characteristics;
6. the next important processes are shaping and forming which involve with slip
casting, extrusion, pressing and injection moulding;

7. after drying, green machining applied to form desired ceramic shape before
sintered at elevated temperature;
8. ceramic processing subsequently may be subjected to further processing to

produce the final desired shapes.


REFERENCES



1. S. Kalpakjian, S.R. Schmid, Manufacturing Engineering and Technology 5 edition,
th
Prentice Hall, 2005.
2. Reed James, Principles of Ceramic Processing












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