Processes that bend, stretch, cut, or fracture relatively thin metal sheets or pieces into a range of diverse shapes are known as sheet metal processes.
CHARACTERISTICS OF SHEET METAL
- Sheet metal has a large surface area to thickness ratio.
- Tensile forces are used to form most materials.
- It’s best to avoid decreasing thickness as much as possible because it can lead to necking and failure.
- Elongation, anisotropy, grain size, residual stresses, spring back, and wrinkling are some of the primary elements that play a role.
Elongation
Elongation refers to a sheet metal’s ability to stretch without necking or failing.
The sheet metal must have a high strain hardening exponent (n) and strain rate sensitivity exponent in order to be stretched (m).
High consistent elongation is desirable for good formability because the material is frequently stretched in sheet forming.
- In the specimen, certain spots extend more than others.
- Strain rate elongation
- Grain size Y.P. elongation
- Leuder’s bands = stretcher strain marks
Necking
- Necking takes place at an angle ‘ ‘
- Localized
- Diffuse
- Depends on the material’s strain rate sensitivity (m).
- With higher values of ‘m,’ post-uniform elongation is greater.
- Elongation totals ‘n’ and ‘m.’
Anisotropy
- Anisotropy (directionality) describes how different planer directions behave differently.
- Sheet metal processing requires two sorts of anisotropic properties.
- Normal anisotropy and planar anisotropy are two types of anisotropy.
- Planar anisotropy occurs in cold rolled sheets as a result of the fibres’ preferred orientation, and it can result in earing during drawing operations. This can be avoided by annealing, although annealing reduces the strength of the material.
- The term “normal anisotropy” refers to the thinning of sheet metal as a result of stretching. This could make deep drawing difficult.
Grain size
- The roughness of stretched sheets is determined by grain size. The coarser the granules, the rougher the surface.
- For ordinary sheet metal forming processes, an ASTM grain size of 7 or finer is preferred.
Residual stresses
- Residual stresses are the result of non-uniform deformation during the forming process.
- Due to the cold working nature of sheet metal manufacturing, residual tensions can cause distortion, corrosion, and cracking.
- Suitable stress relieving methods can minimise the strains, however this adds to the cost and lengthens the production time per item. The part may distort if it is disturbed, such as by removing a bit of it.
Springback
- Springback occurs when a plastically distorted work item recovers its original shape once the (deforming) force is removed.
- It causes distortion, a loss of dimensional precision, and, in certain situations, the product’s entire rejection.
- It’s impossible to estimate how much spring back there will be. To develop suitable components, only trial and error approaches are available. Techniques like overbending and punch bottoming can be used to control it.
- This impact is most noticeable in bending and other forming processes with a high bend radius to thickness ratio, such as automotive body pieces.
Wrinkling
- If the sheet is not correctly gripped or the clearances given are not correct, wrinkles are frequently created during the drawing operation.
- Wrinkles can be avoided by using the right tool and die.
- Wrinkling occurs when compressive forces build in the plane of the sheet (buckling). With unsupported lengths of sheet metal, the likelihood of wrinkling increases. Thickness decreases, thickness is not uniform, Wrinkling can also be caused by trapped lubricants.
Sheared edge quality
Processed sheet metal components may have rough edges with cracks or layers of work hardening.
Post-processing activities such as fine blanking, shaving, and lubrication can increase quality by providing correct clearances between the punch and die, having suitable radii for the punch and die, and performing fine blanking, shaving, and lubrication.
The margins of the sheet may be rough, not square, and contain cracks, residual stresses, and a work hardened layer, all of which affect the sheet’s formability.
Post-processing activities such as fine blanking, shaving, and lubrication can increase quality by providing correct clearances between the punch and die, having suitable radii for the punch and die, and performing fine blanking, shaving, and lubrication.
The margins of the sheet may be rough, not square, and contain cracks, residual stresses, and a work hardened layer, all of which affect the sheet’s formability.
Surface conditioning sheet
Surface tear, nicks, and other defects in sheets generated by rolling can be caused by faulty design and operation of the rolling mill stand, roughness of the rolls, and other factors.
It is dependent on the rolling techniques used. In sheet formation, this is critical since it can lead to ripping and poor surface quality.
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