13577 powder metallurgy

8/12/2019 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

13577 powder metallurgy

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