POWDER METALLURGY

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POWDER METALLURGY

• Lecture 8

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ATOMISATION TECHNIQUES

• In this process, molten metal is separated into
  small droplets and frozen rapidly before the
  drops come into contact with each other or with a
  solid surface.
• Typically, a thin stream of molten metal is
  disintegrated by subjecting it to the impact of
  high-energy jets of gas or liquid.

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Atomisation Technique

• In principle, the technique is applicable to all
  metals that can be melted and is used
  commercially for the production of iron copper
  alloy steels brass bronze low-melting-point
  metals such as aluminum, tin, lead, zinc, and
  cadmium and, in selected instances, tungsten,
  titanium, rhenium, and other high-melting-point
  materials.
• Production rate of commercial atomisation units
  are 400 kg/min.

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Atomisation Technique

• Gas atomisation
• Water Atomisation
• Centrifugal Atomisation

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GAS ATOMISATION

• The liquid metal stream is disintegrated by rapid
  gas expansion out of a nozzle.
• Gas used are air, N2, He or Ar.
• Application nickel-base superalloys and many
  other highly alloyed materials.
• There are horisontal and vertical gas atomiser.
• The particle shape is spherical with a fairly
  wide distribution.
• The product is homogeneous and has good packing
  properties.

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Horisontal Gas Atomisation

• Low temperature atomisers. High velocity gas
  emerging from the nozzle creates a siphon,
  pulling molten metal into the gas expansion zone.
  A high gas velocity aids breakup of the metal.
  During flight through collection chamber, the
  droplets lose heat and solidify.

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VERTICAL GAS ATOMISATION

• The melt is prepared by induction melting and is
  poured into the nozzle. The melt must be
  superheated over Tm. The gas jets can be formed
  by multiple nozzles arranged in a circle around
  the melt stream.

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Operating Variables in the Gas Atomisation
Process

• The Melt
• Melt temperature
• Melt viscosity as it enters the nozzle.
• Alloy type,
• Metal feed rate
• Nozzle geometry
• The nozzle-to-melt distance

• The Gas
• Gas type
• Residual atmosphere
• Gas pressure
• Gas feed rate and velocity
• Gas temperature

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The Production Parameters for Ni-based
Superalloy Powder
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The Formation of Metal Powder by Gas Atomisation
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Changes of the Droplet Shape

• The droplet shape sequence, with distance from
  the nozzle, occurs as cylinder cone sheet
  ligament sphere. Depending on the amount of
  superheat and other variables, solidification can
  produce one of these shapes.

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• The satellites present (Fig. 3.19 left) are due
  to turbulence and smooth particles (Fig. 3.19
  right) formed by control of the flow near the
  nozzle. The elimination of satellites is
  importand for good packing and flow attributes.

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WATER ATOMISATION

• The most common technique for producing elemental
  and alloy powders from metals which melt lt
  1600oC.
• High pressure water jets (single jet, multiple
  jets or an annular ring) are directed against the
  melt stream, forcing disintegration and rapid
  solidification.

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WATER ATOMISATION

• The process is similar to gas atomisation, except
  for the rapid quenching and differing fluid
  properties.
• Because of rapid cooling, the powder shape is
  irregular and rough, with some oxidation.
  Chemical segregation within an alloy particle
  tends to be quite limited.
• Milling may be required after atomisation to
  eliminate particle bonding.

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Four Possible Particle Generation Mechanisms

• Particles are generated by cratering, splashing,
  stripping, and bursting mechanism.
• Bursting mechanism results in the smallest
  powders.
• Typical mass flow rate 5 kg water/kg metal
  powder.

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Process Control Variable

• Water pressure
• The nozzle-to-melt distance

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• A 3.5 carbon steel is water atomised, oxidised,
  milled, decarburised, and annealed to form a
  sponge which is ground to form an iron powder.
• Other materials produced by water atomisation
  include stainless steel, copper, brass, bronze,
  tool steel, cobalt, nickel alloys, precious
  metals, and low melting temperature metals (Pb,
  Sn, and Zn alloys).

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A Comparison Between Gas and Water Atomisation
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Water vs Gas Atomisation

• The powder shape
• The surface contamination
• Oxide coating can be removed by heating in
  hydrogen after atomisation.

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CENTRIFUGAL ATOMISATION

• Reason for development of centrifugal atomisation
  is the desire to control particle size and the
  difficulties in fabricating powders from reactive
  metals.
• Combination of a fusion process and a centrifugal
  force. The centrifugal force throws off the
  molten metal as a fine spray which solidifies
  into a powder.

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• A consumable electrode (anode) made from the
  desired material and rotates at velocities up to
  50,000 rpm. The electrode is melted at its end by
  either plasma arc or stationary tungsten
  electrode.

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Spherical Steel Powder Formed by Centrifugal
Atomisation
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The Droplet Formation Events
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Some Examples of Centrifugal Atomisers
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Atomisation Limitations
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Forming Specific Metal Powders
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PRESS-AND-SINTER PROCESS

• In this process, custom-blended metal powders are
  fed into a die, compacted into the desired shape,
  ejected from the die, and then sintered
  (solid-state diffused) at a temperature below the
  melting point of the base material in a
  controlled atmosphere furnace.
• The compaction step requires the part to be
  removable from the die in the vertical direction
  with no cross movements of the tool members. The
  sintering step creates metallurgical bonds
  between the powder particles, imparting the
  necessary mechanical and physical properties to
  the part.
• Conventional PM offers many advantages over the
  other consolidation methods. It offers the lowest
  manufacturing cost, including modest tooling
  costs. It also produces the closest tolerances in
  the finished parts. Since it is a net-shape
  processing technology, it yields parts requiring
  little or no secondary machining operations.
  Lastly, it makes available to designers and
  fabricators a wide variety of material systems
  from which to choose.
• Parts produced through the press-and-sinter
  process are subject to certain limitations as
  well. Tooling and the maximum press tonnage
  capabilities impose size and shape constraints on
  parts that can be fabricated. Annual production
  quantities dictate how quickly the costs of tool
  set-ups and maintenance can be amortized.
  Finally, the presence of residual porosity in the
  parts will cause certain physical and mechanical
  properties to be lower than those of the wrought
  material.
• Typical Press-and-Sinter Productsgears,
  sprockets, cams, ratchets, levers, clutch plates,
  pressure plates, housings, pole pieces, bearings,
  bushingsTypical Markets Using Press-and-Sinter
  Partsautomotive, appliances, power tools,
  hydraulics, lawn and garden, agriculture,
  off-road equipment, motors, firearms,
  recreational equipment, hardware, business
  equipment

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