Sheet Metal Forming Processes

Introduction Ratio – Surface Area:Volume is very high for sheet metal process. Plates – thickness > 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.

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

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.

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.

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.’

Yield Point Elongation Low carbon steel Certain points elongate more in the specimen. Strain rate  elongation  Grain size  Y.P. elongation  Leuder’s 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.

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.

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)