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Super Plastic Forming

 

Process description

Sheet metal is clamped over a male or female form tool and heated to a high enough temperature to give the material high ductility at low strain rates. Pressurized gas (typically argon) on the back face of the sheet forms the material into cavity or over the surface of the tool.

Materials

  • For stainless steels, aluminum and titanium alloys typically.
  • Material must be able to deform at low strain rates and high temperatures and possess a stable microstructure.
Super Plastic

Process variations

  • Either male or female tool: male forming is more complex, but offers greater design freedom and more uniform material distribution.
  • Additional use of tool movement with gas pressure gives deeper parts with more uniform wall thickness.
  • Diaphragm forming: uses additional diaphragm sheet behind material to give better control of thickness distribution.
  • Can be used in conjunction with Diffusion Bonding (Welding) (DFW) to create complex parts.

Economic considerations

  • Production rates low. Long cycle times.
  • Slower than conventional deep drawing.
  • Lead times moderate, typically weeks, depending on complexity of mold.
  • Material utilization good. Some waste may be generated during subsequent trimming operations. Scrap not recyclable directly.
  • Some aspects can be automated.
  • Economically viable for low to moderate production volumes (10–10 000). Can be used for one-offs.
  • Tooling costs high.
  • Equipment costs high.
  • Direct labor costs low to moderate.
  • Finishing costs low to moderate. Trimming typically required.

Typical applications

  • Used to generate deep and intricate forms in sheet metal
  • Aerospace fuselage panels
  • Containers
  • Casings
  • Architectural and decorative work
  • Can also be used to clad other materials

Design aspects

  • Complexity limited to shape of female or male tool and constant thickness parts.
  • Ribs, bosses and recesses possible.
  • Re-entrant features not possible.
  • Radii should be greater than five times the wall thickness.
  • Sharp radii at extra cost can be produced.
  • Draft angles ranging 2 to 3°.
  • Maximum drawing ratio (height to width) =0.6.
  • Maximum dimension =2.5 m.
  • Maximum thickness =4 mm.
  • Minimum thickness =0.8 mm.

Quality issues

  • No spring back exhibited after processing.
  • No residual stresses.
  • Creep performance poor due to small grain sizes produced.
  • Cavitation and porosity can occur in some alloys at high temperatures and low strain rates.
  • Graphite coating on the blank sheet used to reduce friction.
  • Surface detail good.
  • Secondary operations such as heat treatment, paint, powder coating and anodizing commonly used to improve finish.
  • Surface roughness ranging 0.4–6.3 µm Ra.
  • Achievable dimensional tolerances ranging ±0.13mm–±0.25mm up to 25 mm, ±0.45mm– ±0.78mm up to 150 mm. Wall thickness tolerances are typically ±0.25mm.

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