Sheet Metal Forming Processes Introduction Ratio Surface Area

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 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)