Sheet Metal Forming Processes

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Sheet Metal Forming Processes
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Introduction

• Ratio Surface AreaVolume is very high for
  sheet metal process.
• Plates thickness gt 0.25 inches
• Boilers, bridges, ships, etc. use relatively
  thick plates
• Usually sheet forming is done on relatively thin
  material.
• Shapes can be of various size
• Beverage can, car body
• Sheet metal forming includes many widely used
  processes.

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• Sheet Metal Characteristics
• Formability
• Methods
• Sheet metal is formed as coils of sheets (by
  rolling) or plates.
• Blanks are created for required shapes.
• Shearing sheets
• Flame cutting - plates

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Sheet Metal Characteristics

• Main forces acting in sheet metal forming
  tensile
• No (or very low) compressive load. Can lead to
  buckling or folding and wrinkling.
• What is the big difference between bulk
  deformation and sheet metal?
• What factors affect stretching and bending?
• Elongation, Yield point elongation, anisotropy,
  grain size, residual stresses, springback,
  wrinkling.

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Elongation

• Uniform elongation region
• Postuniform elongation region
• True stress, true strain
• Necking begins at
• Uniform strain ? strain hardening index n
• Large n, uniform elongation is better.

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• Necking occurs at an angle ?
• Localized
• Diffuse
• Depends on strain rate sensitivity m of the
  material.
• Postuniform elongation is higher with higher
  values of m.
• Total elongation ? n and m.

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Yield Point Elongation

• Low carbon steel
• Certain points elongate more in the specimen.
• Strain rate ? elongation ?
• Grain size ? Y.P. elongation ?
• Leuders bands stretcher strain marks
• May not be acceptable in many cases
• To avoid skin rolling (cold rolling)
• Reduces thickness of sheet 0.5 to 1.5
• Strain aging can cuase the Y.P elongation to
  appear after a few days.

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• Residual Stresses
• Because of non uniform deformation. Cause
  distortion if some parts are removed.
• Stress corrosion cracking
• Springback
• Thin considerable springback
• Wrinkles
• Due to compressive load in certain areas.

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• Anisotropy
• Previous history of the sheet metal can cause
  anisotropy.
• Crystallographic grain orientation
• Mechanical Fibering - alignment of impurities,
  inclusions
• Grain Size
• Mechanical properties
• Appearance coarse grain causes rough surface
  appearance (orange peel)