mm 357 - lecture 2 - powder manufacturing - for class 3
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MM 357 – Ceramics and Powder Metallurgy
Powder Metallurgy: Lecture 2
Powder Manufacturing
Sudhanshu Mallick
1
German, P 85
Mechanical Powder Fabrication Techniques
Machining: Coarse powders are produced during
machining of metals. The powders can be further refined by grinding.
• The rapid heating and cool down cycle
that the machining produced particles can cause the particle surface to get oxidized.
• It also results in the formation of various phases (different from the original metal these particles were machined from) which may give them different properties
2
Mechanical Powder Fabrication Techniques
Milling: Milling is process of reducing particle size by mechanical
impaction using hard balls.
• Useful for brittle materials. Ductile particles cold weld and “stick together”
• Optimal conditions: – Rotation speed fast enough to carry the balls to the top
of the jar. Too slow – insufficient impact force, Too fast – Balls stick to the jar surface by centrifugal force
– Ball diameter approx 30 times the powder size – Volume of balls approx half the jar volume – Volume of powder approx 25% of jar volume – fills the
interstitials between the grinding balls.
• Contamination from jar and milling balls is a concern
3
German, P 86
German, P 89
Mechanical Powder Fabrication Techniques
Mechanical alloying: Though not really a “Powder
formation technique”, Its being discussed here because of continuity.
• Powders of different materials
are taken in appropriate proportions and mixed in a stirring mill with the aid of milling balls.
• The repeated impact, cold welding and fracture produce the desired composite powder
4
Gas Atomization - Horizontal
• Typically used for low melting point metals
• Gas emerging from the nozzle creates a siphon effect .
• High velocity gas breaks up the metal into droplets
• During flight the droplets loose heat, solidify and settle down
5
German, P 100
German, P 101
Gas Atomization - Vertical • Typically used for high melting
point metals
• Closed inert gas filled chamber is used to prevent oxidation.
• High velocity gas breaks up the metal into droplets
• During flight the droplets loose heat, solidify before touching the chamber walls and settle down
• Depending on the superheating, gas flow rate, nozzle geometry, particles of various shapes can be formed
6
Water Atomization High pressure jets are directed against stream of molten metals
causing disintegration and rapid solidification.
• Synthetic oils or other non-reactive liquids can also be used instead of water to achieve desired particle shape and prevent oxidation
7
German, P 107 German, P 108
Centrifugal Atomization Use centrifugal force to spray molten metal which solidifies into
a powder.
• Better control over particle size
• Typically done in inert atmosphere to prevent oxidation
• Gives coarse particle sizes
8
German, P 111
German, P 113
Powder formation by Electrolysis Used the conventional electrolysis principle (dissolution of Anode,
deposition at Cathode) to precipitate out powders • Metallic ions are generated by the dissolution of the Anode. • Transport of the ions through the electrolyte • Ions redeposit on the cathode • Cathode is washed and ground to produce powders • Depending on the conditions of the electrolytic cell (temp, voltage etc),
powders of different morphologies can be prepared
9
German, P 90
Powder manufacturing - Chemical methods
• Reduction / decomposition of a solid
• Thermal decomposition (carbonyl)
• Hydride – De hydride
• Reaction from a liquid
• Precipitation from a liquid
• Pechini process
• Sol-gel method
Precipitation from a liquid
• Precipitate from a super saturated solution
• Large quantity of a soluble component is dissolved at a higher temperature
• At supersaturation, a large number of nuclei form suddenly
• All nuclei grow at same rate – uniform size distribution
• Precipitation of mixed oxides is possible:
– Fabrication of Nickel ferrite (magnetic) – aqeous solution of iron and nickel sulfate in solution at 80C, when pH is increased to 11 with Ammonium hydroxide, a mixed hydroxide precipitates – washed and dried.
Precipitation from a liquid
Pechini method
Sol-Gel synthesis
Sol-Gel method • Metal alkoxide is formed by reaction of meals with alcohol
• nROH + Me = (RO)nMe + (n/2) H2
• Eg Al reacts with isopropanol at 80C in presence of HgCl2 catalyst
• Alkoxides are commercially available.
• Shape formation
• Gelation
• Calcination
• High chemical homogeneity
Key variables:
• Rate of Hydrolysis and condensation influence the microstructure.
• Type of alkoxide, Reaction temperature, pH, Amount of water added (affects degree of polymerization)
15
Sol-Gel method
17
Rotary Calciner
18
Moulson, P 102
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