Powder Metallurgy

Powder Metallurgy

1. Four basic steps

   • 1. Powder manufacture
     • Some important properties and characteristics:
       • Chemistry and purity
       • Particle size
       • Size distribution
       • Particle shape
       • Surface texture
     • Melt Automization
       • Atomization is a process where liquid metal is fragmented into small droplets that cool and solidify into particles
       • Majority of commercial powder produced this way
     • Useful for producing prealloyed powders
       • Each powder particle can have the desired alloy composition
     • Methods
       • Chemical reduction of particulate compounds
       • Eletrolytic deposition
       • Pulverization or grinding
       • Thermal decomposition of particulate hydrides
       • Precipitation from solution
       • Condensation of metal vapors
     • Almost any metal or alloy can be converted into powder
     • Rapidly Solidification Powder (Microcrystalline and Amorphous)
       • -If the cooling rate of an atomized liquid is increased, ultra-fine or microcrstalline sized grains can form -Some metals can solidify without becoming crystalline (called amorphous materials) -Amorphous materials can have high strength, improved corrosion resistance, and reduced energy to induce reverse magnetization
     • Testing & Evaluation
       • Should be evaluated for suitability for further processing
       • Flow rate
         • Measures the sease with which poiowder can be fed and distributed into a die
       • Apparent Density
         • The measure of a powder's ability to fill available space without external pressure
       • Compressibility
         • the effectiveness of applied pressure
       • Green Strength
         • Used to describe the strength of the pressed powder after compacting
   • 2. Mixing or blending
     • -Majority of powders are mixed with other powders, binders, lubricants to achieve the desired characteristics in the finished product -Sufficient diffusion must occur during sintering to ensure a uniform chemistry and structure -Unique composites can be produced Blending o mixing operations can be done either wet or dry
   • 3. Compacting
     • Loose powder is compacted and densified into a shape, known as green compact
     • Powders do not flow like liquid. They compress until an equal and opposing force is created
       • Opposing force is created from a combination of (1) resistance by the bottom punch and (2) friction between the particles and die sufrace
     • Most compacting is done with mechanical presses and rigid tools
       • Hydraulic and pneumatic presses also used
     • Additional Considerations
       • When pressure is applied by only one punch, the maxiumum density occurs right below the punch surface and decreases....
   • 4. Sintering
     • Pressed-powder compacxts are heated in a controlled atmosphere to right below the melting point
     • Three Steps
       • 1. Burn-off (purge)
         • Combusts and air and removes lubricants or binders that would interfere with good bonding
       • 2. High-temperature
         • Desired solid state diffusion and bonding occurs
       • 3. Cooling period
         • Lowers temperature of the product in a controlled atmpsphere
       • Must be conducted in oxygen-free conditions

2. Hot-Isostatic Prressing

   • Combined powder compaction and sintering into a single operation
     • Gas-pressure squeezing...