The welding of aluminium and its alloys

Cold working or strain hardening

Cold work, work hardening or strain hardening is an important process used to increase the strength and/or hardness of metals and alloys that cannot be strengthened by heat treatment. It involves a change of shape brought about by the input of mechanical energy. As deformation proceeds the metal becomes stronger but harder and less ductile, as shown in Fig. 2.5, requiring more and more power to continue deforming the metal. Finally, a stage is reached where further deformation is not possible - the metal has become so brittle that any additional deformation leads to frac­ture. In cold working one or two of the dimensions of the item being cold worked are reduced with a corresponding increase in the other dimen - sion(s). This produces an elongation of the grains of the metal in the direc­tion of working to give a preferred grain orientation and a high level of internal stress.

The increase in internal stress not only increases strength and reduces ductility but also results in a very small decrease in density, a decrease in electrical conductivity, an increase in the coefficient of thermal expansion and a decrease in corrosion resistance, particularly stress corrosion resis­tance. The amount of distortion from welding is also likely to be far greater than from a metal which has not been cold worked.

If a cold worked metal is heated a temperature is reached where the internal stresses begin to relax and recovery begins to take place. This restores most of the physical properties of the unworked metal but without any observable change in the grain structure of the metal or any major change in mechanical properties. As the temperature is increased, recrys­tallisation begins to occur where the cold worked and deformed crystals are replaced by a new set of strain-free crystals, resulting in a reduction in strength and an increase in ductility. This process will also result in a fine grain size, perhaps finer than the grain size of the metal before cold working took place. It is possible therefore to grain refine a metal by the correct combination of working and heat treatment. On completion of recrystalli­sation the metal is said to be annealed with the mechanical properties of the non-cold-worked metal restored.

At temperatures above the recrystallisation temperature the new grains begin to grow in size by absorbing each other. This grain growth will result in the formation of a coarse grained micro-structure with the grain size depending upon the temperature and the time of exposure. A coarse grain size is normally regarded as being undesirable from the point of view of both mechanical properties and weldability.