Engineering & Technology · Manufacturing Processes & Process Selection

Forming, deformation processes and material flow

Pressing a metal sheet into a bend changes its shape without cutting away the bend. Release the load and the angle changes slightly. Forming depends on permanent deformation, while the material’s remaining elastic response helps explain why the tool shape and free part shape can differ.

Material Structure & Properties and Engineering Design Methods & Trade-offs supply deformation, properties, requirements and trade-offs. This chapter recalls volume, stress and percentage arithmetic before using them. The last lesson introduces mean and standard deviation in context. All numerical prices, dimensions, shrinkage factors, times, yields, distributions and resource quantities are original teaching assumptions. They are not supplier data, machine settings, qualified procedures or allowable service values.

Back to the chapter

After reading, you can

Relate permanent deformation, approximate volume conservation and elastic recovery to forming choices.

Move material rather than remove it

In metal forming, sufficiently large stresses produce plastic deformation: some shape change remains after unloading. Rolling squeezes stock between rolls; forging changes shape through compressive tools; extrusion pushes material through a shaped opening; sheet forming bends or draws a thin starting sheet. Different arrangements control where material can flow and where it may stretch or thicken.

The material must tolerate the required deformation without unacceptable cracking, folding, thinning or wrinkling. Force, friction, tool geometry, temperature and material condition affect the result. Formability describes behaviour during manufacture; a high service strength does not automatically make a material easy to form. A condition selected for forming may need further treatment before service.

Track volume with stated assumptions

For a dense solid with negligible porosity, losses and density change, plastic forming can often be approximated as conserving volume. If a uniform bar changes area from A0 to A1 and length from L0 to L1, then A0L0 ≈ A1L1. The equation follows from the same amount of material occupying a new shape, not from a rule that every dimension stays fixed.

Our figure uses flat rectangular prisms with constant 10 mm width. Their longitudinal sections have equal drawn area because both represent the same 8000 mm³ volume at that width. The original bar becomes thinner and longer. An actual extrusion also has entry and end effects, a possible discarded remainder and frictional flow; this simple comparison does not predict required force.

Before: length 100 mm, thickness 8 mm, width 10 mm. After: length 160 mm, thickness 5 mm, width 10 mm. Both volumes 8000 mm³. Longitudinal drawing scale 3 SVG units/mm in both directions; widths into the page are equal and not projected. Area reduction 37.5%; length increase 60%. Original ideal geometry, not a forming-force or production-yield prediction.

Account for recovery and material history

Even after plastic flow, elastic strains remain under load and partly recover on unloading. In bending this recovery is called springback. A dimension measured while clamped need not equal the free dimension. Predicting recovery requires material and geometry information; “bend it a little farther” is not a complete process specification.

Cold working can increase strength through work hardening while reducing capacity for further deformation. At suitable elevated temperatures, recovery and recrystallisation can change that balance. “Hot” and “cold” refer to the material’s metallurgical response, not a universal room-temperature boundary. Heating is not a guarantee of better properties: oxidation, grain change, distortion and the intended final condition must be considered.

Thermoplastic extrusion also produces continuous profiles, but it moves a heated polymer melt through a die and then stabilises the profile by cooling. Its flow and later shrinkage are different from the nearly incompressible solid-plastic model used here. A process name shared across material families does not make their physical models interchangeable.

Check an idealised area reduction

Take a uniform bar 100 mm long with cross-section 10 mm × 8 mm = 80 mm². Its volume is 8000 mm³. If forming gives a uniform 10 mm × 5 mm section, its area is 50 mm².

Conserving volume gives L1 = 8000/50 = 160 mm. Area reduction is (80 − 50)/80 = 37.5%, while length increase is (160 − 100)/100 = 60%. The percentages differ because they have different denominators and one quantity is the inverse of the other under this model.

If 10% of the original volume were instead lost as a discarded end, usable volume would be 7200 mm³ and length 144 mm at the same final area. Record what material is included before treating volume conservation as a production-yield claim.

Try the reasoning

Why do a 37.5% area reduction and a 60% length increase agree?
Show the answer and reasoning
Their products preserve volume: 0.625 × 1.60 = 1. They are percentage changes relative to different initial quantities.
Why can a bent sheet move after it leaves the tool?
Show the answer and reasoning
The part elastically recovers on unloading even though some deformation is permanent; this changes the free geometry.
Does successful forming establish the final service properties?
Show the answer and reasoning
No. Deformation and thermal history change the material state. The intended final properties need appropriate treatment and verification.

What to carry forward

Forming redirects material through plastic flow. Check material condition, losses and recovery before predicting the final geometry.

References

Teaching resources and research supporting this lesson’s explanations and examples.