This document provides an overview of powder metallurgy, including:
1) The topics that will be covered related to powder metallurgy processes and properties including powder manufacturing, sintering, and applications.
2) The basic steps in powder metallurgy including mixing powders, compacting, and sintering to produce parts from metal powders.
3) The advantages of powder metallurgy which include a wide range of possible alloys and properties, close control over dimensions, and high material utilization.
1. UNIT IV
Powder Metallurgy
3ME4A
Er. Mohit Ostwal
Assistant Professor
Department of Mechanical Engineering
Jodhpur Institute of Engineering and Technology, Jodhpur
2. Topics to be covered
• Introduction
• Properties of Powder Processed Materials
• Powder Manufacturing
• Mechanical Pulverization
• Sintering
• Electrolytic Process
• Chemical reduction
• Atomization
• Properties of Metal Powders
• Compacting of Powders sintering
• Advantages and Application
3. Introduction – Powder Process
• Process of producing metal powders and making finished
/semi finished objects from mixed or alloyed powders with or
without the addition of nonmetallic constituents.
• Powder metallurgy (PM) is a metal working process for
forming precision metal components from metal powders. The
metal powder is first pressed into product shape at room
temperature. This is followed by heating (sintering) that
causes the powder particles to fuse together without melting.
4. Why Powder Processing ?
• The parts produced by PM have adequate physical and mechanical
properties while completely meeting the functional performance
characteristics.
• The cost of producing a component of given shape and the required
dimensional tolerances by PM is generally lower than the cost of
casting or making it as a wrought product, because of extremely
low scrap and the fewer processing steps.
• Parts can be produced which are impregnated with oil or plastic, or
infiltrated with lower melting point metal. They can be
electroplated, heat treated, and machined if necessary.
• The rate of production of parts is quite high, a few hundreds to
several thousands per hour.
5. Process in Powder Metallurgy
1. Mixing (Blending)
2. Compacting
3. Sintering
Fig.1. Basic steps in Powder Metallurgy process
6. Process in Powder Metallurgy
Fig.2. Typical Set for Powder Metallurgy Tools
7. ProductionofPowders(PowderManufacturing)
• Widely used Metal Powders
Pure Metals Alloys
Aluminum Aluminum-iron
Antimony Brass
Beryllium Copper-zinc-nickel
Bismuth Nickel-chromium
Cadmium Nickel-chromium-iron
Cobalt Nickel-copper
Copper Nickel-iron
Iron Silicon-iron
Lead, Molybdenum Solder
Manganese, Zinc Stainless steel
Molybdenum, Tungsten Composites
Nickel, Silicon, Tin, Titanium etc. Borides, Carbides, Nitrides etc.
8. Metal Powder Characteristics
• Apparent Density - The apparent density or specific gravity of a powder is expressed
in kg/m3. It should be kept constant. This means that the same amount of powder should
be fed into the die each time.
• Compressibility - Compressibility is the ratio of the volume of initial powder to the
volume of the compressed piece. It varies considerably and is affected by the particle-size
distribution and shape. Compressibility affects the green strength of a compact.
• Fineness - Fineness refers to the particle size and is determined by passing the powder
through a standard sieve or by microscopic measurement.
• Flow ability – It is the characteristic of a powder that permits it to flow readily and
conform to the mold cavity. It can be described as the rate of flow through a fixed orifice.
• Particle Size Distribution – It refers to the amount of each standard particle size in
the powder. It influences the Flowablity and apparent density as well as porosity of the
product.
• Sintering ability - It is the suitability of a powder for bonding of particles by the
application of heat.
9. 1. Mechanical Method
Mechanical forces such as compressive forces, shear or impact to facilitate particle
size reduction of bulk materials.
Milling:
a. Impact – Striking of one powder particle against another.
b. Attrition - production of wear debris due to the rubbing action
between two particles.
c. Shear – Cutting of particles resulting in fracture.
d. Compression forces – Particles broken into fine particles -squeezing.
Mechanism of Milling
a. Microforging - particles are impacted repeatedly so that they flatten with very less
change in mass.
b. Fracture - Individual particles deform and cracks initiate and propagate resulting in
fracture.
c. Agglomeration - Mechanical interlocking due to atomic bonding or Vander Waals
forces.
d. Deagglomeration - Breaking of agglomerates.
PowderProductionMethods
10. Milling Equipment's
1. Crushers - ceramic materials such as oxides of metals.
a. Jaw Crushers
b. Gyratory Crushers
c. Roll Crushers
2. Grinders - reactive metals such as titanium, zirconium, niobium,
tantalum.
11. Milling Equipment's - Grinders
1. Ball Mill
• Cylindrical vessel rotating horizontally along the axis.
• The grinding media may be made of hardened steel, or tungsten
carbide, ceramics like agate, porcelain, alumina, zirconia.
• Process grinds the powder materials by impact/collision & attrition.
• Dry Milling - 25 vol% of powder is added along with about 1 wt% of a
lubricant such as stearic or oleic acid.
• Wet Milling - 30-40 vol% of powder with 1 wt% of dispersing agent
such as water, alcohol or hexane is employed.
• Diameter – 250 mm
12. Milling Equipment's - Grinders
2. Vibratory Ball Mill
• Longer period – Fine Powder.
• High amount of energy is imparted to the particles and milling is
accelerated by vibrating the container.
• Electric motor connected to the shaft of the drum by an elastic
coupling.
• Drum is lined with wear resistant material.
• 80% of container is filled at starting with grinding and starting material.
• Vibratory motion is obtained by an eccentric shaft that is mounted on a
frame inside the mill. The rotation of eccentric shaft causes the drum of
the vibrating mill to oscillate.
• Frequency – 1500 to 3000 oscillation/min.
• Amplitude of oscillation – 2 to 3 mm.
• Grinding Bodies – Steel / Carbide Balls – 10 to 20mm.
• Fined Particle size – 5 to 100 microns.
13. Milling Equipment's - Grinders
3. Attrition Mill
• The charge is ground to fine size by the action of a vertical shaft
with side arms attached to it. The ball to charge ratio may be 5:1,
10:1, 15:1.
• This method is more efficient in achieving fine particle size.
14. Milling Equipment's - Grinders
4. Rod Mill
• Horizontal rods are used instead of balls to grind.
• Granularity of the discharge material is 40-10 mm.
• The mill speed varies from 12 to 30 rpm.
15. Milling Equipment's - Grinders
5. Planetary Mill
• High energy mill widely used for producing metal, alloy, and
composite powders.
16. Milling Equipment's - Grinders
6. Fluid Energy Grinding or Jet Mill
• The basic principle of fluid energy mill is to induce particles to collide
against each other at high velocity, causing them to fracture into fine
particles.
• The pressurized fluid is introduced into the grinding zone through
specially designed nozzles which convert the applied pressure to kinetic
energy. Also materials to be powdered are introduced simultaneously
into the turbulent zone.
• Can create powders of average particle size less than 5 μm.
• The fluid used is either air about 0.7 MPa or stream at 2 MPa.
17. Powder Production Methods
Other Mechanical Processes are:
• Machining – Chips from turning are crushed into powder – Mg, Be, Ag,
Solder and Dental Alloys.
• Shotting - Fine stream of molten metal is poured through a vibratory
screen into air or protective gas medium. When the molten metals falls
through screen, it disintegrates and solidifies as spherical particles. These
particles get oxidized. The particles thus obtained depends on pore size of
screen, temperature, gas used, frequency of vibration. Metal produced by
the method are Cu, Brass, Al, Zn, Sn, Pb, Ni.
• Graining - Same as shotting except that the falling material through sieve
is collected in water; Powders of cadmium, Bismuth, antimony are
produced.
18. Powder Production Method
2. Physical Methods :
a. Electrolytic Deposition:
• Used for producing copper ,iron , zinc, tin, nickel, cadmium, antimony,
silver, lead, beryllium powders.
• Metals of high purity are precipitated from aqueous solution on the
cathode of an electrolytic cell.
• Copper Powder => Solution containing copper sulphate and sulphuric acid
and crude copper as anode.
at anode: Cu -> Cu+ + e-; at cathode: Cu+ + e- ->Cu
• Iron powder => anode is low carbon steel; cathode is stainless steel.
• Mg powder => electrode position from a purified magnesium sulphate
electrolyte using insoluble lead anodes and stainless steel cathodes.
19. Final deposition occurs in three ways:
1. A hard brittle layer of pure metal which is subsequently milled to
obtain powder (eg. iron powder)
2. A soft, spongy substance which is loosely adherent and easily
removed by scrubbing.
3. A direct powder deposit from the electrolyte that collects at the
bottom of the cell.
20. Powder Production Method
b. Atomization:
• This uses high pressure fluid jets to break up a molten metal stream
into very fine droplets, which then solidify into fine particles.
• High quality powders of Al, brass, iron, stainless steel, tool steel,
superalloys.
• Mechanism for atomization:
In conventional (gas or water) atomization, a liquid metal is produced by
pouring molten metal through a tundish with a nozzle at its base. The
stream of liquid is then broken into droplets by the impingement of high
pressure gas or water. This disintegration of liquid stream is shown in fig.
This has five stages-
i) Formation of wavy surface of the liquid due to small disturbances
ii) Wave fragmentation and ligament formation
iii) Disintegration of ligament into fine droplets
iv) Further breakdown of fragments into fine particles
v) Collision and coalescence of particles
21. Atomization
• Lower surface tension of molten metal, high cooling rate =>
formation of irregular surface => like in water atomization
• High surface tension, low cooling rates => spherical shape formation
=> like in inert gas atomization
22. Atomization
1. Water Atomization
• High pressure water jets.
• Water is used because of high
viscosity and quenching ability.
• Small scale purpose.
2. Gas Atomization
• Use of Argon, Nitrogen, Helium
24. Atomization Unit
• Melting and Superheating Facility - Air melting, inert gas or vacuum
induction melting. Metal is transferred to Tundish, which serves as reservoir
for molten metal.
• Atomization Chamber - Atomization nozzle system is necessary, to avoid
oxidation of powders, the tank is purged with inert gas like nitrogen.
-> Twin Jet Nozzle
-> Annular Jet Nozzle
• Powder Collection Chamber – Dry or wet collection, needs to be cooled for
long run operations.
25. Powder Production
• Other Methods
a. Reduction – Hydrogen/Carbon monoxide (spongy and
porous having uniform sized spherical or angular shapes).
b. Carbonyls – Metal carbonyls such as Fe(CO)5 and Ni(CO)4
(uniform spherical particles with high purity).
c. Precipitation from a chemical solution.
d. Vapour condensation.
26. Compacting
• Blended Powders are pressed into shapes in dies.
• In general- without application of heat.
• Pressed powder is known as GREEN COMPACT.
• Hydraulic or Mechanical presses.
• High Density – High Strength – High Resistance to External
Forces.
• Compacting Pressure.
• Particle Size – Introduction of Small particles.
• Tungsten carbide dies are mostly used – punches are of
same material.
30. Compacting Methods
• Isostatic Pressing - Hot and Cold
• Rolling
• Extrusion – Encased in metal container
P=400MPa
Neoprene Rubber
Urethane
Polyvinyl chloride
Other elastomers
31. Sintering
• Green compacts are heated – controlled atmosphere – below the
melting point – high enough for fusion/bonding of individual
particles.
• Green compact – green strength is low.
• Sintering time – 10 min.(Fe & Cu alloys) to 8 Hrs.'(Tungsten &
Tantalum)
• Mechanical bonds to stronger metal bonds.
32. Sintering
• Carried out in three stages:
a. Melting of minor constituents in powder particles.
b. Diffusion between the powder particles.
c. Mechanical Bonding.
33. Sintering Methods
1. Solid State Sintering
• Neck formation at contact points b/w particles - diffusion.
• Neck Growth – Skeleton of solid particle is formed.
• Pore size is reduced – shrinkage – fully sintered part.
34. Sintering Methods
2. Liquid Phase sintering
• One particle is having low M.P. than other – surface tension.
• Neck formation by liquid phase material transport – particle
rearrangement at liquid phase.
• Solution and precipitation state – small particles dissolve from
contact
• Final sintered product is obtained.
35. Sintering Environment
• Prevent undesirable reactions
• Reduction of surface oxides
• Composition control
• Managing the impurity levels.
• Ex – Pure hydrogen, Ammonia, inert gases, vacuum, nitrogen
based mixtures with carburizing agents.
37. Advantages of PM
• Virtually unlimited choice of alloys,
composites, and associated properties
• Refractory materials are popular by this
process
• Controlled porosity for self lubrication
or filtration uses
• Can be very economical at large run
sizes (100,000 parts)
• Long term reliability through close
control of dimensions and physical
properties
• Wide latitude of shape and design
• Very good material utilization
• 500 parts per hour not uncommon for
average size part
• 60,000 parts per hour achievable for
small, low complexity parts in a rolling
press
38. Disadvantages of PM
• Limited in size capability due to large forces
• Specialty machines
• Need to control the environment – corrosion concern
• Will not typically produce part as strong as wrought
product. (Can repress items to overcome that)
• Cost of die – typical to that of forging, except that design
can be more – specialty
• Less well known process
39. Application of PM
• Cutting tools and dies – Tungsten Carbide
• Machinery Parts – Gears for garden tractors
• Bearing and bushes
• Magnets
• Electrical Parts
