tailieunhanh - The Welding of Aluminum & Its Alloys Part 2

Tham khảo tài liệu 'the welding of aluminum & its alloys part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Welding metallurgy 15 Illustration of the effect of cold work on strength hardness and ductility. 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. 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 dimension . 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. 16 The welding of aluminium and its alloys 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 .