Sheet Metal: High ratio of surface area to thickness Thickness < 6mm Sheet Thickness > 6mm plate

Transcription

1 Sheet Metal: High ratio of surface area to thickness Thickness < 6mm Sheet Thickness > 6mm plate

2 Sheet metal forming is a process that materials undergo permanent deformation by cold forming to produce a variety of complex three dimensional shapes The process is carried out in a plane of sheet by tensile forces with high ratio of surface area to thickness. The raw material for sheet metal manufacturing processes is the output of the rolling process. Typically, sheets of metal are sold as flat, rectangular sheets of standard size. the first step in any sheet metal process is to cut the correct shape and sized blank from larger sheet. Blanks are produced from rolled coils

3 Metal Forming Processes Three major categories of sheet metal processes 1. Shearing Shearing to separate large sheets, or cut part perimeters or make holes in sheets 2. Bending Straining sheet around a straight axis 3. Deep Drawing Forming of sheet into convex or concave shapes 4- Miscellaneous Processes Processes Shearing Bending Spinning Deep drawing Roll bending Stretch bending Hydroforming Explosive forming Blanking Punching

4 Typical Products: car bodies, aircraft fuselages, beverage cans, Shearing Bending Deep Drawing 4

5 applied force is tensile, stretching forming. bending moments shearing forces of to rupture the metal in the plane of shear

6 Spinning

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8 Hydroforming

9 Blanking Pressing

10 7.2. Sheet Metal Characteristics

11 1. Elongation: Determines the capability of the sheet metal to stretch without necking and failure; Uniform extension up to necking high strain-hardening exponent (n)and strain-rate sensitivity exponent (m)desirable. C m

12 Necking

13 2- Yield-point elongation Yield-point elongation Observed with mild-steel sheets; also called Lueder s bands and stretcher strains can be eliminated by temper rolling, but sheet must be formed within a certain time after rolling. Lueders Bands

14 3. Anisotropy (normal): Determines thinning behavior of sheet metals during stretching; important in deep-drawing operations The parameter commonly used to characterize this behavior is the anisotropy coefficient, or Lankford coefficient R, defined in uniaxial tensile tests on rectangular sheet specimens by : R 2 3 where ε2 and ε3 are the plastic strains along the width direction and the thickness direction of the specimen, respectively

15 Experiments show that R depends on the in-plane direction. Having determined the values of r for three directions in the plane of the sheet metal (0, 45, 90 ), the normal anisotropy coefficient is obtained from the equation: R n r r90 r 4

16 Orthotropy axes of the rolled sheet metals: LD longitudinal direction; TD transversal direction; ND normal direction

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18 4. Anisotropy (planar): Exhibits different behavior in different planar directions; present in cold-rolled sheets because of preferred orientation or mechanical fibering; causes earing in drawing; can be reduced or eliminated by annealing but at lowered strength measure of the variation of r with direction is known as the planar anisotropy coefficient: R r 0 r 90 2r 45 2 if the value of the anisotropy coefficient is the same along all the directions in the plane of the sheet metal, the earring phenomenon will not be observed.

19 Variation of the anisotropy coefficient in the plane of the sheet metal

20 5). Grain Size Small Grain Size Large Strength Large Grain Size Rough Surface appearance (orange peel) Determines surface roughness on stretched sheet the coarser the grain, the rougher the appearance (orange peel); also affects material strength 20

21 6-Residual stresses Caused by nonuniform deformation during forming; causes part distortion when sectionedand can lead to stress-corrosion cracking; reduced or eliminated by stress relieving. Stress-Corrosion Cracking Brass and 300-series austenitic stainless steels are particularly susceptible to stress corrosion cracking.

22 7. Springback: Caused by elastic recovery of the plastically deformed sheet after unloading; causes distortion of part and loss of dimensional accuracy; can be controlled by techniques such as overbending

23 8. Wrinkling: Caused by compressive stresses in the plane of the sheet; can be objectionable or can be useful in imparting stiffness to parts; can be controlled by proper tool and die design. 9. Quality of sheared edges: Depends on process used; edges can be rough, not square, and contain cracks, residual stresses, and a work-hardened layer, which are all detrimental to the formability of the sheet.

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