sheet metal

What kind of workpiece does sheet metal forming apply to?

Workpieces with a high surface-area-to-thickness ratio

What type of force forms sheet metal shapes, and what can the other force cause?

Tensile forces form the shapes; compressive forces can cause buckling, wrinkling or folds

How does thickness change happen in sheet forming, and why avoid thinning?

Stretching under tensile stress thins the sheet; thinning leads to localized necking and failure

Two material properties to know about the incoming sheet

Previous cold work (it determines ductility) and anisotropy

Why is rolled sheet anisotropic?

Rolling aligns insoluble impurities and dislocations (in hot and cold rolling), so the sheet bends differently depending on direction

If sheet is rolled externally, what must be true of the stock when it arrives?

It must all arrive in the same direction

What deformation do we want in sheet forming, and what shows good formability?

Uniform, permanent deformation (between yield and UTS); high uniform elongation means good formability

What does a high strain hardening coefficient (n) tell you?

n equals the strain at necking, so high n means the material stretches further before necking

Drawback of a high strain hardening coefficient (n)?

Harder material with lower ductility, and stress builds up faster as n increases

What is yield-point elongation (Lueders bands)?

Low-carbon steels have upper and lower yield points, and during yielding local deformation occurs in bands

Why are Lueders bands a problem?

They cause surface inconsistencies (bands, depressions) that affect painting and coating

Two ways to decrease yield-point elongation

Lower the strain rate; increase the grain size (fewer grain boundaries to delay dislocation movement)

How do you eliminate Lueders bands?

Temper rolling: cold rolling by 0.5-1.5% of thickness

Which grain size is preferred in sheet forming, and why?

Fine grains, which increase strength and give a better appearance

What is 'orange peel'?

A rough, bumpy surface from localized stretching of individual large grains. Fine grains make it less noticeable

How is grain size controlled?

Through the correct heat treatment and work process

What happens if residual stress is removed (or more is added)?

Distortions

What can tensile residual stress at the surface lead to?

Warping

How are residual stresses usually relieved?

Stress-relief heat treatment such as annealing

Why does springback occur?

Because of Young's modulus (E) and strength: deformation energy is stored in the atomic bonds, and the elastic recovery shows up as springback