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Manufacturing Processes > Forming Processes > Powder metallurgy

 

Powdar Metllurgy

 

Process description

Die compaction of a blended powdered material into a ‘green’ compact which is then sintered with heat to increase the bond strength. Usually secondary operations are performed to improve dimensional accuracy, surface roughness, strength and/or porosity.

Materials

  • All materials, typically metals and ceramics. Iron, copper alloys and refractory metals most common.
  • Can process materials not formable by other methods.
  • Powder production: atomization, electrolysis and chemical reduction methods.
Powdar Metallurgy

Process variations

  • Cold die compaction: performed at room temperature. Gives high porosity and low strength.
  • Hot forging: deformation of reheated sintered compact to final density and shape.
  • Continuous compaction: for strip or sheet product. Slower than conventional rolling.
  • Isostatic compaction (hot or cold): compaction of powder in a membrane using pressurized fluid (oil, water) or gas. Permits more uniform compaction and near-net shapes. Undercuts and reverse tapers possible, but not transverse holes. Used for ceramics mainly.
  • Extrusion: high pressure ram forces powder through an orifice determining the section profile.
  • Injection molding: fine powder coated with thermoplastic injected into dies. Relatively complex shapes with thin walls achievable.
  • Spark sintering: gives magnetic and electrical properties.
  • Pressureless compaction: for porous components.
  • Secondary operations include: repressing, sizing and machining.

Economic considerations

  • High production rates, small parts up to 1800 pieces/hour.
  • Cycle times dictated by sintering mechanisms.
  • Lead times several weeks. Dies must be carefully designed and made.
  • Production quantities of 20 000+ preferred, but may be economic for 5000 for simple parts.
  • Material utilization very high. Less than 5 per cent lost in scrap. ki87r
  • Powders expensive to produce.
  • Automation of process common.
  • New set of die and punches required for each new product, i.e. flexibility low.
  • Tooling costs very high. Dedicated tooling.
  • Equipment costs high. Sintering equipment not dedicated though.
  • Labor costs low to moderate. Some skilled labor may be required.
  • Finishing costs generally low.
  • Final grinding may be more economical than sizing for very close tolerances.

Typical applications

  • Cutting tools
  • Small arms parts
  • Bearings
  • Filters (porous)
  • Lock components (keys, barrels)
  • Machine parts (ratchets, pawls, cams, gears)

Design aspects

  • Complexity and part size limited by powder flow through die space (powders do not follow hydrodynamic laws) and pressing action.
  • Near-net shapes generated.
  • Concentric, cylindrical shapes with uniform, parallel walls preferred.
  • Multiple-action tooling can be used to create complex parts.
  • Complex profiles on one side only.
  • Parts can be quenched, annealed and surface treated like wrought products to alter mechanical properties.
  • Inert plastics can be impregnated for pressure sealing or a low melting point metal for powder forging.
  • Densities typically between 90 per cent and 95 per cent of original material.
  • Density can be controlled for special functional properties, e.g. porosity for filters.
  • Spheres approximated. Complicated radial contours possible.
  • Marked changes in section thickness should be avoided.
  • Narrow slots, splines, long thin section, knife-edges and sharp corners should be avoided. Use secondary processing operations.
  • Threads not possible.
  • Tapered, blind and non-circular holes, vertical knurls possible in direction of powder compaction.
  • Grooves, cutouts and off-axis holes perpendicular to the pressing direction can not be produced directly.
  • Undercuts perpendicular to compaction direction not possible. Better to secondary process.
  • Radii should be as generous as possible.
  • Chamfers preferred to radii on part edges.
  • Maximum length to diameter ratio =4:1.
  • Maximum length to wall thickness ratio =8:1.
  • Inserts possible at extra cost.
  • Draft angles can be zero.
  • Minimum section can be as low as 0.4 mm, but 1.5mm typically.
  • Sizes ranging 10 g–15 kg in weight or 4mm2–0.016m2 in projected area.

Quality issues

  • Density and strength variations in product can occur with asymmetric shapes. Can be minimized by die design.
  • High densities required for subsequent welding of sintered parts.
  • Porosity in sintered parts means excessive absorption of braze and solder fillers
  • Product strength determinable by powder size, compacting pressure, sintering time and temperature, but generally, lower mechanical properties than wrought materials.
  • Can give a highly porous structure, but can be controlled and used to advantage, e.g. filters and bearing lubricant impregnation (10–30 per cent oil by volume). Also, resin impregnation can greatly improve machinability of sintered products.
  • Generally, lower mechanical properties than wrought materials.
  • Sharp edges on tools should be avoided. Causes excessive tool wear.
  • Remnants of contaminates at grain boundaries may act as crack initiators.
  • Oxide film may impair properties of finished part, for example: chromium and high temperature superalloys.
  • Surface detail good.
  • Surface roughness in the range 0.2–3.2 µm Ra.
  • Process capability charts showing the achievable dimensional tolerances are provided.
  • Repressing, coining and sizing improves surface finish, density and dimensional accuracy. Also for embossing.

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