金属的塑性变形课件.ppt

Nature of Plastic deformation in Metals ;Mechanical working processes are used to achieve optimum mechanical properties in the metal. Metal working reduces any internal voids or cavities present and thus makes the metal dense. The impurities present in the metal also get elongated with the grains and in the process they get broken and dispersed throughout the metal. This decreases the harmful effects of the impurities and the improves mechanical strength. ;Plastic deformation is the deformation which is permanent and beyond the elastic range of the material. ;The necessary deformation in a metal can be achieved by application of large amount of mechanical force only or by heating the metal and then applying a small force.;The deformation of metals, which is caused by the displacement of the atoms is achieved by one or both of the processes called slip and twinning. ;On the macroscopic scale, when plastic deformation occurs, the metal appears to flow in the solid state along specific directions, which are dependent on the type of processing and the direction of applied force. The crystals or grains of the metal are elongated in the direction of metal flow. This flow of metal can be seen under microscope after polishing and suitable etching of the metal surface. These visible lines are called fibre flow lines. ;Since the grains are elongated in the direction of flow, they would be able to offer more resistance to stresses acting across them. As a result, the mechanically worked metals called wrought products would be able to achieve better mechanical strength in specific orientation, that of the flow direction. ;Since it is possible to control these flow lines in any specific direction by careful manipulation of the applied forces, it is possible to achieve optimum mechanical properties. The metal, of course, would be weak along the flow lines.;Hot Working and Cold Working The metal-working processes are traditionally divided into hot-working and cold-working