Chapter 6 Powder Metallurgy (PM)

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Chapter 6Powder Metallurgy (PM)

• 1302 310
• Engineering Metallurgy
• Dr.Sukangkana Lee

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Introduction

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Material Tensile strength,MPa Tensileas Percent of Wrought Iron Tensile Elongationin 50 mm  (2 in) Elongationas Percent of Wrought Iron Elongation
Wrought Iron, Hot Rolled 331 100 30 100
Powder Metal, 84 density 214 65 2 6
Powder Metal, repressed, 95 density 283 85 25 83
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Benefits

1. PM parts can be fabricated to final or near-net
   shape, thereby eliminating or reducing scrap
   metal, machining and assembly operation
2. Lower overall cost (less scrap lost)
3. High melting point metals and composite materials
   can be produced
4. PM is useful in making parts that have complex
   shapes or difficult to machine
5. Permits a wide variety of alloy systems
6. Produces good surface finishes

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Benefits (Cont.)

• Provides materials which may be heat-treated for
  increased strength or increased wear resistance
• Provides controlled porosity for self-lubrication
  or filtration
• Facilitates manufacture of complex or unique
  shapes which would be impractical or impossible
  with other metalworking processes
• Is suited to moderate- to high-volume component
  production requirements
• Offers long-term performance reliability in
  critical applications
• Is cost-effective

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Disadvantages

1. Porosity originates as the spaces between powder
   particles ? low elongation
2. Expensive powder

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Products and applications
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Raw Materials
Mixing
Forming
Sintering
Optional Operations
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Factors affected the QC of PM

1. Powder characteristic Apparent density (AD), the
   irregularity and porous texture decreases AD
2. Powder preparation most metal powder grains are
   coated by a thin oxide film but will be broken up
   during the pressing, stable oxide films e.g. SiO2
   and Al2O3 cannot reduced during sintering leads
   to abrasive and tool wear
3. Type of compacting press, tool and die
4. Type of sintering furnace, atmosphere, time and
   temperature
5. Heat treatment

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5. Powder Manufacture

• PM Standards Fine powder particles lt 20 µm
• The finer the better preferably 2-10 µm
• Shape of Powders
• Sponge-like for iron powder gives good green
  strength
• Spheroidal particle gives high density and
  uniform distribution (see Fig. 2)

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Powder production

• There are four main processes
• Solid-state reduction
• Atomization
• Electrolysis
• Chemical

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1. Solid state reduction

• Solid state reduction is the most widely used for
  production of iron powder.
• Process
• Selected ore is crushed and mixed with carbon
• Continuous furnace ? Sponge iron
• Further crushing
• Separation of non-metallic material
• Sieving? Irregular fine sponge-like powder

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Milling
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2. Atomization

• Atomization air, nitrogen, argon (for
  oxidisable metal) and water are commonly used.
  The particle shape is controlled by rate of
  solidification of metal droplets.
• Gas atomization gives spheroidal
• Water atomized gives a more irregular shape.
• Each powder produced by this method has the same
  chemical composition.

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GAS ATOMIZATION
Fine powder
Collection chamber
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3. Electrolysis

• By choosing suitable conditions, such as
  electrolyte composition and concentration,
  temperature, and current density, many metals can
  be deposited in a spongy or powdery state.
• Further processingwashing, drying, reducing,
  annealing, and crushingis often required,
  ultimately yielding high-purity and high-density
  powders.

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• Copper is the primary metal produced by
  electrolysis but iron, chromium, and magnesium
  powders are also produced this way.
• Due to its associated high energy costs,
  electrolysis is generally limited to high-value
  powders such as high-conductivity copper powders.

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Electrolytic
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4. Chemical process

• Use for production of a high purity, lt 5 ?m.
• The powders produced can have a great variation
  in properties and yet have closely controlled
  particle size and shape.
• Oxide-reduced powders are often characterized as
  spongy, due to pores present within individual
  particles.

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• Solution-precipitated powders can provide narrow
  particle size distributions and high purity.
• Thermal decomposition is most often used to
  process carbonyls. These powders, once milled and
  annealed, exceed 99.5 percent purity.

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Sponge iron-reduced ore
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Isostatic compaction
Vacuum melting and Cold forging
Hot isostatic pressing Plus hot forging
Direct hot isostatic pressing
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Sinter-HIP

• Sintered metals 92 density is sufficient to
  ensure that open porosity at surface has been
  eliminated ? HIPed to full density.
• This process start from sintering then high
  pressure argon is introduced or vacuum sinter
  followed by HIPing in a separate apparatus for
  hard metal cutting tools.

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Metal injection Moulding (MIM)
Powder Binder
Blending
Granulation
Injection Moulding
Green body
Debinding
Brown body
Sintering
99 Density
Finished part