Metals and alloys are processed into different shapes by various manufacturing methods.
Some of the most important industrial processes are casting, rolling, extruding, wire drawing,
forging, and deep drawing.
When a uniaxial stress is applied to a long metal bar, the metal deforms elastically at first and then plastically, causing permanent deformation. For many engineering designs the engineer is interested in the 0.2 percent offset yield strength, ultimate tensile strength, and elongation (ductility) of a metal or alloy. These quantities are obtained from the engineering stress-strain diagram originating from a tensile test. The hardness of a metal may also be of engineering importance. Commonly used hardness scales in industry are Rockwell B and C and Brinell (BHN).
Grain size has a direct impact on the properties of a metal. Metals with fine grain size are stronger and have more uniform properties. The strength of metal is related to its grain size through an empirical relationship called the Hall–Petch equation. Metals with grain size in the nano range (nanocrystalline metals) are expected to have ultra high strength and hardness as predicted by the Hall–Petch equation.
When a metal is plastically deformed by cold working, the metal becomes strainhardened,
resulting in an increase in its strength and a decrease in its ductility. The strain hardening can be removed by giving the metal an annealing heat treatment. When the strain-hardened metal is slowly heated to a high temperature below its melting temperature, the processes of recovery, recrystallization, and grain growth take place, and the metal is softened. By combining strain hardening and annealing, large thickness reductions of metal sections can be accomplished without fracture.
By deforming some metals at high temperature and slow loding rates, it is possible to achieve superplasticity i.e., deformation of the order of 1000-2000%. The grain size must be ultrafine to achieve superplasticity.
Plastic deformation of metals takes place most commonly by the slip process, involving the movement of dislocations. Slip usually takes place on the closest-packed planes and in the closest-packed directions. The combination of a slip plane and a slip direction constitutes a slip system. Metals with a high number of slip systems are more ductile than those with only a few slip systems. Many metals deform by twinning when slip becomes difficult.
Grain boundaries at lower temperatures usually strengthen metals by providing barriers to dislocation movement. However, under some conditions of high-temperature deformation, grain boundaries become regions of weakness due to grain boundary sliding.
