powder metallurgy
DESCRIPTION
Crushing and milling methods to form powderTRANSCRIPT
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Crushing and Milling Method to form Powder
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Manufacturing of Metal Powders
Some of the important methods are:1. Atomization 2. Machining3. Crushing and Milling 4. Reduction5. Electrolytic Deposition 6. Shotting7. Condensation
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Used for brittle materials. Jaw crushers, stamping mills, ball mills - used to breakdown the metals. In earlier stages of powder preparation gyratory crushers are used. For fine powder, the metal particles are fractured by impact.
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Main objective of milling• Particle size reduction (main purpose),• shape change , • solid state alloying,• mechanical or solid state mixing,• modification of material properties.
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1. Micro forging : Individual particles or group of
particles are impacted repeatedly so that they flatten with very less change in mass
2. Fracture : Individual particles deform and cracks initiate and propagate resulting in fracture
3. Agglomeration : Mechanical interlocking due to atomic bonding or vander Waals forces.
4. Deagglomeration : Breaking of agglomerates.
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Milling equipment: are generally classified as crushers & mills.
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Ball mills•Contains cylindrical vessel rotating horizontally along the axis. •The vessel is charged with the grinding media(hardened steel, or tungsten carbide, porcelain, alumina).
Important Parameters:1. An optimum/critical speed is adjusted for maximum impact velocity.2. The size and density of the milling medium is selected according to the deformation and fracture resistance for metals.
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Vibratory ball mill High amount of energy is imparted to the particlesMilling being accelerated by vibrating the container. Vibration frequency - 1500 to 3000 oscillations/min. Amplitude of oscillations - 2 to 3 mm.The grinding bodies is made of steel or carbide balls, that are 10-20 mm in diameter. The mass of the balls is 8-10 times the charged particles.
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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. More efficient in achieving fine particle size. In these mills the rotational speeds are nearly 6 – 80 rpm while the size of
medium (balls) used is 3 – 6 mm.
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Fluid energy grinding or Jet milling:The fluid used is either air about 0.7 Mpa
or stream at 2 Mpa The pressurized fluid is introduced into the
grinding zone through specially
designed nozzles which convert the
applied pressure to kinetic energy.
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Rod mills Horizontal rods are used instead of balls to grind. Granularity of the discharge material is 40-100 mm. The mill speed varies from 12 to 30 rpm.
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1. Rubbing action causes contamination of powder since balls may also get rubbed.
2. There is a possibility of excessive oxidation of final powder.
3. Quality of powder is poor.4. Agglomeration may take place.