Download - 13577 Powder Metallurgy
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Manufacturing Technology
MEC205
Powder Metallurgy
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Powder Metallurgy
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Powder Metallurgy
Metal
Powder
Metal Product
Powder metallurgy work materials:
Largest tonnage of metals are alloys of iron, steel, and aluminumOther PM metals include copper, nickel, and refractory metals such as molybdenum
and tungsten
Metallic carbides such as tungsten carbide are often included within the scope of
powder metallurgy
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Feasible when
1. the melting point of a metal is too high such as W, Mo
2. the reaction occurs when melting such as Zr and for
superhard tool materials.
3. Certain alloy combinations and cermets made by PM cannot be
produced in other ways
Powder Metallurgy (PM) (around 1800s)
Pressing Powders are compressed into the desired shape
in a press-type machine using punch-and-die tooling
designed specifically for the part.
Sintering- Heating at a temperature well below melting.
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Advantages of P/M:
1. Versatile in numerous industries
2. Eliminates or minimizes machining
3. Minimizes scrap
4. Maintains close dimensional
tolerances
5. Permits a wide variety of alloy
systems
6. Facilitates manufacture of complex
shapes which would be impractical
with other processes
7. Provides excellent part to part
repeatability
8. Cost Effective
9. Energy and environmentally efficient
Limitations of P/M
1. High tooling and equipment costs
2. Metallic powders are expensive
3. Problems in storing and handling
metal powders( as powder degraded
over time)
4. Limitations on part geometry because
metal powders
5. do not readily flow laterally in the die
during pressing
6. Variations in density throughout part
may be a problem, especially for
complex geometries
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Characteristic of engineering powder:
Apowdercan be defined as a finely divided particulate solid
Engineering powders include metals and ceramics
Geometric features of engineering powders:
1. Particle size and distribution
2. Particle shape and internal structure
3. Surface area
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1. Particle size and distribution:
Refers dimension of individual powders.
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 equal in both directions, and the
total number of openings per square inch is 2002= 40,000
Higher mesh count = smaller particle size
The procedure of separating powder by size called as classification
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Particle size (PS)=
MC= mesh count, opening per linear inch
Tw= wire thickness of mesh in inches
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2. Particles shape and internal structure:
Aspect ratio: maximum dimension to minimum dimension.
For spherical particle aspect ration is 1.
Volume of loose powder called as open pores
Volume of close powder called closed pores
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3. Surface area:
Assuming that particle shape is perfect sphere then,
Area and volume of sphere given as ( Area A= 4 r)( Volume= 4/3 r)
Where D = diameter of sphere in mm
Area to volume ration is given as,
And this area to volume ration is expressed as particle shape.
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Other features:
Interparticle Friction and Powder Flow:
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
Large angle of response shows greater
friction
Lubricants are often added to powders to
reduce interparticle friction and facilitate
flow during pressing
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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
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Packing Factor:
Porosity + Packing factor = 1.0
Packing factor is Bulkdensity divided by true density
Typical values for loose powders range between 0.5 and 0.7
If powders of various sizes are present, smaller powders will fit into
spaces between larger ones, thus 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
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Production of Metallic Powders:
In general, producers of metallic powders are not the same companies
as those that make PM parts
Any metal can be made into powder form
Three principal methods by which metallic powders are commercially
produced
1. Atomization
2. Chemical
3. Electrolytic
In addition, mechanical methods are occasionally used to reduce powder
sizes
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Atomization Method
Involves the conversion of molten metal into a spray of droplets that solidify into
powders.Most versitile and popular method of producing metal powder.
Applicable to all metals and alloys.
There are two method of atomization (1.gas atomization and 2. water atomization)
1. Gas atomization:High velocity gas stream flows through expansion nozzle, siphoning molten metal
from below and spraying it into container
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Fig d: centrifugalatomization
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Other methods of producing powder:
-Chemical reduction of particulate compounds
-Electrolytic deposition( like electrolytic cell)
-Pulverization or grinding of brittle materials
-Thermal decomposition
-Precipitation from solutions
-Condensation of metal vapors
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Conventional pressing and sintering:
After the metallic powder has been produced, PM consist of three steps.
1. Blending and mixing of powders2. Compaction - pressing into desired shape
3. Sintering - heating to temperature below melting point to cause
solid-state bonding of particles and strengthening of part
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1. Blending and mixing of powders:
Blending: Combining powders of the same material but possibly different particle
sizes
.Blending accomplished by
a) rotating drum b)Rotation in double cone container c) agitation in screw mixer.
d) stirring in a blade mixer
Best result obtained when container is 20 to 40% full
Some additives are added including lubricants(loosely held Al and Zn) to reduce
friction.Additivesto provide strength and deflocculentto agglomerate the powder
Mixing: Combining powders of different materials
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2. Compaction:
Application of high pressure to the powders to form them into the required
shape
Conventional compaction method ispressing, in which opposing punches
squeeze the powders contained in a die
The workpart after pressing is called a green compact, the word green
meaning not yet fully processed
The green strengthof the part when pressed is adequate for handling but
far less than after sintering
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Conventional Pressing in PM:
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Example of a Powder Press:
Presses are used in PM compaction are mechanical, hydraulic or
combination of both.
Achieved by pressing from one direction or from both direction.
Force required given as, Ap= projected area of part mm2Pc= compaction pressure required MPa
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3. Sintering:
Heat treatment to bond the metallic particles, thereby increasing strength
and hardness
Usually carried out at between 70% and 90% of the metal's melting point
(absolute scale)
Generally agreed among researchers that the primary driving force for
sintering is reduction of surface energy
Part shrinkage occurs during sintering due to pore size reduction
Conventional sintering called as solid state/phase sintering.
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Sintering Sequence
Sintering on a microscopic scale: (1) particle bonding is initiated at
contact points; (2) contact points grow into "necks"; (3) the poresbetween particles are reduced in size; and (4) grain boundaries develop
between particles in place of the necked regions