This document discusses various methods for producing metal powders, including mechanical, atomization, electrochemical, and chemical methods. Mechanical methods include chopping, abrasion, milling and the cold stream process. Atomization methods include gas, water, centrifugal atomization and using a rotating electrode. Factors that influence particle size and shape from atomization are also covered. Electrochemical production involves electrolysis of molten metals. Chemical methods decompose metal compounds with heat or catalysts. The document provides details on the principles, equipment used, advantages and limitations of each production method.

1. Manufacturing Technology II
(ME-202)
Production
&
Characteristics
of Metal
Powder
Dr. Chaitanya Sharma
PhD. IIT Roorkee

2. Powder Metallurgy
Lesson Objectives
In this chapter we shall discuss the following:
1. Powder properties
2. What is powder metallurgy (PM)
3. Need of PM
4. Advantages, Limitations & Application of
PM
5. Basic steps in PM
6. Powder Production and its characteristics
7. Design considerations in PM
Learning Activities
1. Look up
Keywords
2. View Slides;
3. Read Notes,
4. Listen to
lecture
Keywords: Powder metallurgy, powder, Blending, Compacting,
Sintering, Particle size and shape, Atomization, Communication etc.

3. Steps In Making P/M
Fig : Outline of processes and operations involved in making powder-metallurgy parts.

4. Metal Powders
• A powder can be defined as a finely divided particulate solid.
• Engg powders include metals, refractory & ceramics such as
– Metals: Alloys of iron, steel, copper, nickel and aluminum
– Refractory : such as molybdenum and tungsten
– Metallic carbides: such as tungsten carbide
• Metallic powders are classified as either
− Elemental - consisting of a pure metal
− Pre-alloyed - each particle is an alloy
• Geometric features of engineering powders:
• Particle size and distribution
• Particle shape and internal structure
• Surface area
• In general powder surface are covered with oxides, silica, adsorbed
organic materials, and moisture; these films must be removed prior to
shape processing

5. Some Applications of Metal Powders
and many more…

6. Particle Shapes in Metal Powders

7. Powder Particles
Fig (a) SEM photograph of iron-powder particles made by atomization.
(b) Nickel-based superalloy powder particles made by the rotating electrode process
(a) (b)

8. Interparticle Friction &
Flow Characteristics
• Friction between particles affects ability of a powder
to flow readily and pack tightly.
• A common test of interparticle friction is the angle of
repose, which is the angle formed by a pile of powders as
they are poured from a narrow funnel.
• Larger angle indicate greater interparticle friction

9. Knowledge Check
• Which powder (fine or coarse) does experience
greater friction?.
– Smaller particle sizes generally show greater
friction and steeper angles
• Which particle shape does offer lowest interpartical
friction?
– Spherical shapes have the lowest interpartical
friction

10. How to Measure Particle Size?
• Most common method uses screens of different mesh sizes.
• Mesh count - refers to the number of openings per linear inch
of screen.
− A mesh count of 200 means there are 200 openings per
linear inch.
− Since the mesh is square, the count is the same in both
directions, and the total number of openings per square inch
is 2002 = 40,000.
− Higher mesh count means smaller particle size
Particle size that would
pass through mesh
Particle size that would
not pass through mesh

11. Particle Density Measures
• True density - density of the true volume of the
material
− The density of the material if the powders were
melted into a solid mass
• Bulk density - density of the powders in the loose state
after pouring
− Because of pores between particles, bulk density
is less than true density

12. Porosity
• Ratio of the volume of the pores (empty spaces) in the
powder to the bulk volume
• In principle, Porosity + Packing factor = 1.0
• The issue is complicated by the possible existence of
closed pores in some of the particles
• If internal pore volumes are included in above porosity,
then equation is exact.
• Mixing particles of different sizes allows decreased
porosity and a higher packing ratio.

13. Packing Factor
• Packing Factor = Bulk Density/ True Density
• Typical values for loose powders range between 0.5 & 0.7
• Powders of various sizes give higher packing factor.
• Packing can be increased by vibrating the powders, causing
them to settle more tightly
• Pressure applied during compaction
greatly increases packing of
powders through rearrangement
and deformation of particles

14. Production of Metal Powders
• The selection of materials in powder metallurgy
is determined by two factors.
i) The alloy required in the finished part.
ii) Physical characteristics needed in the powder.
• Both of these factors are influenced by the
process used for making powder.

15. How to Select Method
for Powder Production?
• The choice of a specific technique for powder
production depends on following factors:
1. Particle size,
2. Shape,
3. Microstructure and chemistry of powder and
4. Cost of the process.

16. Methods for Producing Metal
Powders
The different methods used for producing metal powder are:
1. Mechanical methods of powder production:
i) Chopping or Cutting ii) Abrasion methods
iii) Machining methods iv) Milling
v) Cold-stream Process.
2. Atomization methods of powder production:
i) Gas atomization ii) Water atomization
iii) Atomization with a rotating consumable electrode; and
iv) Centrifugal atomization
3. Chemical methods of powder production:
i) Reduction of oxides ii) Precipitation from
iii) Thermal decomposition of compounds
iv) Hydride decomposition v) Thermite reaction
vi) Electro- chemical methods
4. Electrolysis Process:

17. Mechanical Methods for
Powder Production
1. Chopping or Cutting:
 In this process, strands of hard steel wire, in
diameter as small as 0.0313 inches are cut up into
small pieces by means of a milling cutter.
 This technique is actually employed in the
manufacturing of cut wire shots which are used for
peening or shot cleaning.
Limitations:
• Difficult and costly to make powders
• No control on purity of powders

18. Mechanical Methods Continued…
2. Rubbing or Abrasion Methods:
a) Rubbing of Two Surfaces: When we rub two surfaces
against each other, hard surface removes the material
from the surface of soft material.
* Contamination
b) Filing: Filing as a production method has been frequently
employed, especially to alloy powders, when supplies from
conventional sources have been unobtainable.
Such methods are also used for manufacture of coarse
powders of dental alloys.
Filing can also be used to produce finer powder if its teeth
are smaller.
* commercially not feasible.

19. Rubbing or Abrasion Methods
continued...
c) Scratching: If a hard pin is rubbed on some soft metal
the powder flakes are produced. Scratching is a technique
actually used on a large scale for the preparation of coarse
magnesium powders. Either by hardened steel pin/
scratching belt against mg block/revolving drum.
Advantages:
•For consuming scrap from another process, machining is a
useful process.
•Presently the machined powder is used with high carbon
steel and some dental amalgam powders.
Disadvantages:
• Lack of control on powder characteristics, including
chemical contamination such as oxidation, oil and other metal
impurities.
• The shape of the powder is irregular and coarse.

20. Mechanical Comminution
to Obtain Fine Particles
Figure 17.6 Methods of mechanical comminution to obtain fine
particles: (a) roll crushing, (b) ball mill, and (c) hammer milling.

21. Milling Process
Milling Process is a high production rate of mechanical
method of powder production.
Principle:
The basic principal of milling process is the application of
impact and shear forces between two materials, a hard and
a soft, causing soft material to be ground into fine
particles.
Milling techniques are suitable for brittle materials.
Two types of milling are;
i) Ball Milling
ii) Attrition Milling.

22. Ball Milling
Ball milling is an old and relatively simple method for grinding
large lumps of materials into smaller pieces and powder form.
Principle of the process:
The principle is simple and is based on the impact and shear
forces.
Hard balls are used for mechanical comminution of brittle
materials and producing powders.
Milling Unit:
The basic apparatus consists of the following;
• A ball mill or jar mill which mainly consists of
a rotating drum lined from inside with a hard material.
• Hard balls, as a grinding medium, which continue to impact the
material inside the drum as it rotates/rolls.

23. Important Parameters of Milling
1. The most important parameter to consider is the speed of rotation of the drum. An
optimum/critical speed is adjusted for maximum impact velocity. * Critical speed is the
speed above which the ball will centrifuge.
• Very slow speed of rotation will not carry the balls to the top, these will roll back down the
drum sides.
• Very fast speed (higher than critical speed) will not let the balls drop down as they will be
carried around due to centrifugal forces. Thus, an optimum speed is required. This speed of
rotation varies with the inverse square root of the drum diameter.
2. The material of grinding media and its size and density.
• The size and density of the milling medium is selected according to the deformation and
fracture resistance for metals.
• For hard and brittle materials large and dense media is used. Whereas, small balls are used
for finer grinding.
• As a general rule, the balls should be small and their surface should be a little rough. The
material of the balls and lining of the drum should be same as that of the material being
ground.
3. The rate of milling of a powder is a function of quantity in the total space between the balls.
4. Lubricants and surface active agents are used to nullify the welding forces which causes
agglomeration.

24. ATTRITION MILLING
Attrition is the term which means to wear or rub
away. It is a process of grinding down by friction.
Milling Unit:
•In attrition milling a very high efficiency ball mill
is agitated by a vertical rotating shaft with
horizontal arms.
•In these mills the rotational speeds are nearly 6 –
80 rpm while the size of medium (balls) used is 3 –
6 mm.
•Power is used to rotate the agitator and not the
vessel as in case of ball mills. The central rotating
shaft of attrition mill is equipped with several
horizontal arms. When rotated, it exerts the
stirring action to tumble the grinding medium
randomly throughout the entire chamber.

25. COLD STREAM PROCESS
• This process is based on impact phenomenon caused by impingement of high
velocity particles against a cemented carbide plate.
• The unit consists of:
 A feed container;
 A compressor capable of producing a high velocity stream of air (56 m3/min.)
operating at 7 MPa (1000 psi);
 A target plate, made of cemented tungsten carbide, for producing impact;
 A classifying chamber lined with WC while the supersonic nozzle and target
generally are made of cemented tungsten carbide.
Mechanism of the Process:
The material to be powdered is fed in the chamber and from there falls in front of high
velocity stream of air.
This air causes the impingement of material against target plate, where material due to
impaction is shattered into powder form. This powder is sucked and is classified in the
classifying chamber. Oversize is recycled and fine powder is removed from discharge
area.
* Rapidly expanding gases leaving the nozzle create a strong cooling effect through
adiabatic expansion. This effect is greater than the heat produced by pulverization.

26. Atomization
• In this process the molten metal is forced through an
orifice into a stream of high velocity air, steam or inert
gas .This causes rapid cooling and disintegration into very
fine powder particles and the use of this process is limited
to metals with relatively low melting point.
Major atomization techniques are:
i) Gas atomization
ii) Water atomization
iii) Centrifugal atomization
iii) Atomization with a rotating
consumable electrode; and

27. Gas/ Liquid Atomization
• Atomization breaks molten metal into small droplets by rapidly
freezing them before the droplets come into contact with each other or
with solid surface.
• The atomization is achieved by bringing the thin molten metal stream in
contact with the impact of high energy jets of gas or liquid.
• Air, nitrogen and argon are commonly used gases and water is the most
widely used liquid.
If gases are used for
atomization then it
is known as gas
atomization while
atomization with
water is known as
water atomization

28. Factors Affecting Particle Shape &
size in Gas/ Liquid Atomization
Following factor influences shape and size of powder:
• Rate of solidification determines the particle shape.
– Spherical if low heat capacity gas is employed.
– Highly irregular if water is employed
• Particle size distribution can be controlled by
o Design and configuration of jets
o Pressure and volume of atomizing fluid
o Thickness of metal stream

29. Centrifugal Atomization
• In this technique, molten metal falls on to a disc rotating at
high speed. The liquid metal that is impinging on the disc will
be thrown out rapidly into small droplets by the disc owing to
centrifugal forces. These particles are solidified and
collected in the collection chamber.
Advantages:
i) low operating costs, ii) very good yields in narrow size ranges.
iii)Capability to produce only one size of droplet
iv)Modest energy consumption
Application:
Magnesium powders in the ~0.5mm range for
military pyrotechnic applications. Superalloys
etc.
Disadvantage:
Requires large diameter atomizing chamber owing to
spray with an included angle of 1800

30. Atomization with Rotating
Consumable Electrode• In this method, arc between the rotating consumable
electrode and the stationary electrode is utilized for
melting the metal. Since the electrode is rotating,
the molten metal is atomized by the centrifugal force
and collected in the chamber filled with inert gas.

31. Electrochemical Process
These methods are based on the electrolysis of molten solutions
of metals or fused salts.
The metals are electrically deposited on the cathode of an
electrolytic cell as a sponge or powder or at least in a physical
state form which it can be easily disintegrated into a powder.
Components
The equipment used is an
electrolytic bath made of steel,
and lined from inside with rubber.
Two electrodes are inserted in the
bath.
Cathode is made of lead while
anode is made of the same metal
whose powder is being produced.
Figure: Electrolytic Cell Operation for deposition of Powder

32. Electrochemical Process
Principle & Operation:
•The basic principle is the electrolysis process in which decomposition of a
molten salt/aqueous solution into its ions is obtained by the passage of
electric current.
•The metallic ions are deposited at the cathode which can be removed with a
brush and collected at the bottom.
The electrolytic tanks have conical
bottoms with a valve. Suction pipes
are connected to these bottoms and
powder is removed from the tank.
The efficiency of the tank/process
depends on the deposition rate.

33. Advantages of the process:
The technique has a number of advantages, e.g.
• The product is usually of a high commercial purity.
• A considerable range of powder qualities can be obtained by varying
bath compositions.
• Frequently the product has excellent pressing and sintering properties.
• The cost of the operation may in some cases be low.
Limitations:
• Alloy powders cannot be produced.
• The product of process is frequently in active condition (presence of
chemicals on powder particles) which may cause difficulties in
washing and drying it (contamination/oxidation with atmospheric
oxygen may occur).
• The cost of operation may be high in some cases.

34. Powder production at cathode is favored by:
 high current density;
 weak metal concentration;
 addition of acids;
 low temperature;
 avoidance of agitation, and;
 suppression of convection.
* Very fine powder can be obtained when the current flowing is so strong in relation to
the strength of the solution that hydrogen is strongly evolved from the cathode.
• Hydrogen evolution is encouraged by:
(i) increasing cell voltage;
(ii) diminishing the size of the cathode;
(iii) bringing the anode and cathode closer together;
(iv) increasing the temperature;
(v) weakening the strength of the metallic solution
(vi) adding acid

35. • * When metal is deposited without evolution of
hydrogen, the deposit may be ductile and
compact if the current is just not great enough
to cause hydrogen formation, or very hard with
large crystals using strong solutions and large
quantities of electricity, or sandy and brittle
with little cohesion using very small current.

36. Chemical Methods
• Almost all metallic elements can be produced in the form of
powders by suitable chemical reactions or decomposition. For
example all chemical compounds can be decomposed into
their elements if heated to sufficient high temperature. If the
non-metallic redical could be removed, for example by
continuous evacuation or by entrainment in an inert gas, then
practical methods of making metal powders might be feasible.

37. Theory of the process
Mostly chemical methods are based on the decomposition of a
compound into the elemental with heating or with the help of
some catalyst.
In most cases such processes involve at least two reactants.
(i) a compound of the metal
(ii) a reducing agent
Either of the two may be in the state of a solid, liquid (melt), solution
or gas and it would seem therefore that from this point of view at
least sixteen types of such reactions could be possible.
Solid
Liquid
Solid
Solution
Gas

38. REDUCTION OF METAL OXIDES
Manufacturing of metal powder by reduction of oxides is extensively employed,
particularly for Fe, Cu, W and Mo. As a manufacturing technique, oxide reduction
may exhibit certain advantages and disadvantages. These are listed below;
Advantages:
A variety of reducing agents can be used and process can be economical when
carbon is used.
Close control over particle size --- because oxides are generally friable, easily
pulverized and easily graded by sieving.
Porous powders can be produced which have good compressive properties.
Adoptability either to very small or large manufacturing units and either batch or
continuous processes.
Limitations:
 Process may be costly if reducing agents are gases.
 Large volumes of reducing gas may be required, and circumstances where this
is economically available may be limited; in some cases, however, costs may be
reduced by recirculation of the gas.
 The purity of the finished product usually depends entirely upon the purity of
the raw material, and economic or technical considerations may set a limitation

39. Mechanism of Reaction:
Most metal powders manufactured by reduction of oxides are produced using solid
carbon or hydrogen, cracked ammonia, carbon monoxide, or mixture of such
gases. As a reducing agent for metal oxides, carbon holds an important and
peculiar position – because of its general cheapness and availability, and peculiar
for the following reasons.
According to circumstances and temperature, three carbon/oxygen reactions can
occur: (i) C + O2 = CO2
In this reaction, the number of gaseous molecules remain constant and the entropy
change is very small. The free energy change of the reaction is almost constant
from room temperature to 2000 oC.
(ii) 2CO + O2 = 2CO2
The reaction is accompanied by a decrease in the number of gas molecules and in
entropy with a considerable free energy change.
(iii) 2C + O2 = 2CO
This reaction involves an increase in the number of gaseous molecules and a
considerable increase in entropy and a considerable free energy change. This
implies that within temperatures normally used metallurgically, carbon
monoxide becomes increasingly stable the higher the temperature.
Consequently, the free energy change temperature curves for these reactions
intersect ------- at about 700 oC

40. Further reading
• Fundamentals of powder metallurgy W. D. Jones
• Powder Metallurgy: Principles & Applications F. V. Lenel
• Fundamentals of P/M I. H. Khan