Powder manufacture

Powder manufacture

1. mixing elemental, alloy or ceramic refractory powders-compact mixture in die

   • shapes then heated or sintered in a controlled atmosphere furnace to bond particles

2. advantages

   • form materials not readily formed by other means
   • make materials with high melting point.
     • ceramic materials
     • tungsten alloys
   • make blends of materials that are not easily alloyed
   • Net shape forming process
     • low waste
     • high tolerances
     • complex shapes
   • No segregation-consistent/predictable properties

3. limitations

   • compaction problems with high length/width ratio
   • rapid section changes causes cracking during sintering
   • small components-limited strenght

4. 95% raw material utilisation, 29 Mj per kg to finish part

5. Powder process route

   • 6 options when powder manufacturing
     • comminution of solid metal
     • precipitation from solution of salt
     • thermal decomposition of a chemical compound
     • reduction of a compound, usually the oxide, in the solid state
     • electro-deposition of Cu
     • atomization of molten metal
       • water/air atomisation
         • irregular shaped particles with surface oxide film.
         • cheap-ideal for general PM
           • oxide must be reduced during subsequent sintering
       • inert-gas atomisation
         • oxide free, spherical powder for non sturctural applications
       • Important powder characteristics
         • particle size, size distribution, particle shape, structure, surface condition

6. PM blending, compaction and sintering

   • mixing/blending
     • homogeneous mixture/incorporate lube
     • lube reduces friction between powder mass and tool surface
       • assisting uniform density within compact
       • easier to eject component
   • compact/press - green state
     • uniform strength sufficient to be handled
   • sintering
     • densify, develop desired properties
       • use protective atmosphere-N2 or Ar
         • or reducing atmosphere -H2, NH3,CH4
       • Temp = 70-90% of Tmelt
         • particles weld together-increase strength and ductility
           • removes porosity(5-10% remain)-10-25% shrinkage
     • through sinter time, density, strength and ductility all increase
       • density goes from 80-100%, strength 10-80%, ductility- 0-45%!!!
   • minimal final finishing!
     • post sinter operations
       • reduce porosity and increase strength, dimensional accuracy
         • re-pressing
         • Forging
         • infiltration by liquid metal
         • impregnate with organic material
         • Heat Treatment

7. other powder processes

   • cold isostatic pressing-CIP
   • Hot isostatic pressing-HIP
   • Direct metal laser sintering
   • powder forging
   • powder rolling
   • metal injection moulding