METAL POWDER PROCESSING TECHNIQUES

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METAL POWDER PROCESSING TECHNIQUES

• Compaction of Metal Powders
• Different ways of consolidation of metal powders
• With application of pressure which includes
• i) Unidirectional pressing
• (single action or double action pressing)
• ii ) Isostatic pressing
• iii) Powder rolling
• iv) Powder extrusion
• v) Powder swaging
• vi) Powder forging
• vii) Powder Injection Molding

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• (B) Without applying pressure such as
• i) Slip mixing or slip casting
• ii) Vibrational compaction

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• Consolidation generally occurs in three stages
• (a) rearrangement of particles.
• (b) particles contacting by plastic deformation.
• (c) mechanical locking and cold welding of
  particles due to surface shear strains.
• It is, therefore, easier to cold compact
  irregular particles than spherical powder
  particles.

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• Main variables in pressure compaction are
• Method of compaction
• Compaction pressure, time and temperature
• Rate of compaction
• Compacting atmosphere
• Lubricants and other additives of the mix, and
• Die design

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• The main objectives obtained during pressing are
• To achieve the required part shape.
• To obtain the required green density.
• To secure sufficient green strength to permit
  safe handling of the part.
• To provide particle-to-particle contact which is
  necessary for sintering.

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• The basic types of compacting presses are
• Mechanical (single punch or rotary type) presses.
• Hydraulic presses.
• Hybrid-type presses (mechanical presses may make
  use of auxiliary pneumatic or hydraulic devices).

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• For pressing
• The powder metal must first fill the die orifice.
• Filling may be done by hand or automatically from
  the press-hopper.
• A constant volume or constant weight may be used.
• Vibration filling is introduced to create denser
  packing to avoid bridging and high porosity
  defects.
• Pressing may be done automatically.
• The pressure may be applied along more than one
  axis using various punch and die sets designed to
  minimize defects.
• Ejection after pressing may be carried out
  automatically or by hand.

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• Minimum requirements for any powder metal press
• Adequate total pressure capability in the
  direction of pressing and sufficient part
  ejection capability.
• Controlled length and speed of compression and
  ejection strokes.
• Adjustable die fill arrangements.
• Synchronized timing of press strokes.
• Material feed and part removal systems.

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• General classification of powder metallurgy
  parts
• Class I parts with a diameter (or thickness) up
  to 65 mm and one level parts of any contour that
  can be pressed with a force from one direction.
• Class II parts are single level components of any
  thickness and any contour that must be pressed
  from two directions.
• Class III parts are two level components of any
  thickness and contour that must be pressed from
  two directions.
• Class IV parts are multilevel components of any
  thickness and contour that must be pressed from
  two direction.

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• Compacting Presses and Attachments
• The presses systems used are
• Single action press system consisting of
• i) a die to form the outer contour of the part
• ii) an upper punch to form the top surface of
  the part
• iii) a lower punch to form the bottom surface of
  the part
• iv) if required, core rods to form any through
  holes
• (for class I parts).

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• (b) Double action opposed ram system consists of
• a die, upper punch, lower punch and core rods
  (for class I and class II parts).
• (c) Double action floating die system consists of
• moving upper punch, stationary lower punch,
  moving die table and core rods (for class I IV
  parts).
• Further during compaction tooling materials,
  clearances and tolerances require expertise and
  special attention is paid to (i) die design (ii)
  die materials (iii) punch (iv) carbide inserts
  (v) tolerances, clearances and finishes.

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• Forming and sintering in one step
• The simplest forming route for a metal powder is
  loose-powder sintering which is used for forming
  spherical powder.
• In this process the metal powder is filled
  loosely or shaken into the mould by vibration and
  subsequently sintered in the mould.
• The mould can be made of steel or graphite and
  can be used repeatedly.
• Powders which are difficult to compact are often
  formed by simultaneous application of pressure
  and temperature (pressure sintering).

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• Forming and Sintering in Separate Process Steps
• Axial pressing
• The powder is charged by volume filling into a
  closed tool consisting (in principle) of three
  parts a die and an upper and a lower punch, and
  subsequently subjected to a pressure using
  mechanical or a hydraulic press.
• The cycle begins with the tool in the filling
  position (1) in which the upper and the lower
  punches are retracted so that a defined filling
  space arises. The tool is filled with powder from
  the filling shoe, the quantity of powder being
  determined by the volume of the die cavity at
  this stage (2).

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• The height of the filling space is determined by
  the apparent density (filling volume) of the
  powder and by the required final density of the
  component.
• It usually is equal to 2.2 to 3 times the height
  of the component.
• After finishing the filling operation the powder
  is compacted by the counter movement of the lower
  and the upper punches until the tool has reached
  the pressing position (3 4).
• After completion of the pressing step the upper
  punch is withdrawn and the compact is ejected by
  moving the lower punch upwards (5). The ejected
  component is pushed away from the die by the
  filling shoe (6), and the tool is again ready for
  a new pressing cycle.

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Compaction
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Uniaxial Pressing
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• The ejection system
• During compaction according to the ejection
  system with two punches moving in opposite
  direction (figure B) the die remains fixed and
  the two punches carry out the compaction. The
  compact exhibits a symmetrical density
  distribution with a neutral zone in the middle.
  The counter movements of the punches are effected
  by cams and toggle joints in the case of a
  mechanical press and by separately controlled
  upper and lower cylinders in the case of a
  hydraulic press.

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• Floating die system
• A modification of the ejection system is the case
  where single action pressing is combined with a
  motion of the die by friction and spring action
  (figure C).
• The lower punch remains fixed whilst the die
  bolster is supported by springs. The upper punch
  carries out the compaction by downward movement.
• The friction between die and powder is increased
  with increasing compacting pressure. When the
  spring force is exceeded, the die moves in
  relation to the fixed lower punch, the movement
  being controlled by the frictional forces
  generated between the die and the powder and the
  counteracting spring force.
• The position of the neutral zone is determined by
  the friction between the powder and the die in
  relation to the spring force in the other
  direction. The position of the neutral zone can
  hence be adjusted to suit the geometry of the
  component by changing the stiffness of the
  springs.

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• The withdrawal system
• A fixed lower punch, a mobile die and a mobile
  upper punch are used in the withdrawal system
  (figure D).
• In contrast to the ejection system which uses two
  punches moving towards each other (figure B), the
  motion of the die is not only controlled by the
  wall friction but can be controlled from the
  press.
• In this way the neutral zone can be reduced to a
  minimum and moved from the centre of the compact
  to the upper or lower part.

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Powder Compaction and Sintering
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A (825 ton)Mechanical press forcompacting
metalpowder.