Formability refers to the ability of sheet metal to be formed into a desired shape without necking or cracking. Necking is localized thinning of the metal that is greater than the thinning of the surrounding metal. Necking precedes cracking.
From the metallurgical perspective, the formability of a particular metal depends on the metal’s elongation, which is the total amount of strain measured during tensile testing. A metal with a large elongation has good formability because the metal is able to undergo a large amount of strain (work) hardening.
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Strain hardening results in an increase of the load-carrying capacity of a metal as it deforms. It also prevents strains from being localized during forming, so the deformation is uniformly distributed throughout a particular section of the material that is exposed to a specific set of forming stresses. As a result, each localized region of the metal thins uniformly during the forming process.
The load carrying capacity of the metal as it deforms is opposed by the reduction in cross-sectional area of the metal as it thins. There is a maximum load where the increase in stress due to the decrease in the metal cross-sectional area becomes greater than the increase in the load-carrying ability of the metal due to strain hardening. Necking begins at this point as the metal starts to thin more in a localized region. Any additional deformation is concentrated in the necking area, while the loads in the surrounding areas decrease.
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Strength vs. ductility and elongation
Anything done to increase a metal’s yield and tensile strength does so at the expense of ductility, and therefore elongation. As the elongation of a particular alloy decreases, there is a decrease in the amount of deformation before necking occurs. Strengthening treatments include cold rolling working, through hardening and age hardening heat treatments, and solid solution strengthening. Also, strength increases and elongation decreases as grain size decreases.
Once necking begins, the loads on a metal are concentrated in the necked region. The amount of deformation of the necked area before a crack forms depends on the microstructure of the metal and the state of stress on the metal.
Cracks that form during metal forming occur by a fracture process that involves the formation and growth of voids around second-phase particles and inclusions in the metal. This is shown in figure below. The voids form, grow, and coalesce to form a crack. So, the presence of second-phase particles and inclusions reduces a metal’s formability because they are sites where cracks nucleate.
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Second-phase particles are often present as a result of adding certain alloying elements or strengthening heat treatments. This is the case for iron carbide particles in steel or precipitates in age hardened alloys. Inclusions are particles comprised of impurity elements in an alloy. For example, in most steels sulfur is present as an impurity that usually appears as manganese sulfide inclusions. In aluminum alloys iron and silicon present as impurities react with the other elements in the alloys to form hard particles. For an alloy that contains inclusions, reducing the impurity content will help reduce the number of inclusions, and improve formability.
Stress state and formability
Finally, the formability of a metal also depends on the state of stress on a metal during forming. The state of stress depends on the shape of the component being fabricated and the process used to form the component. Forming limit diagrams are used to predict whether the forming strains to which a metal will be exposed will result in necking or cracking.