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Tuesday, 12 November 2013

Hot-Working Processes - Extrusion

Points : Extrusion, Definition Definition The extrusion process is now used for shaping a variety of ferrous and non-ferrous metals and alloys. Its most important feature is that we are able to force the metal through a die, and, in a single process from the cast billet, to obtain quite complicated sections of tolerably accurate dimensions.
The metal billet is heated to the required extrusion temperature (350-500°C for aluminum alloys; 700-800°C for brasses; 1100-1250°C for steels) and placed in the container of the extrusion press. The ram is then driven hydraulically with sufficient pressure to force the metal through a hard alloy-steel die. The solid metal section issues from the die in a manner similar to the flow of toothpaste from its tube. Using this process, a wide variety of sections can be produced. Including round rod, hexagonal brass rod (for parting off as nuts), brass curtain rail, small- diameter rod (for drawing still further to wire), tubes in many alloys including stainless steels; and many hollow stress-bearing sections in aluminum alloys (mainly for aircraft construction).

The most serious defect from which extruded sections suffer is called the extrusion defect’ or ‘back-end defect’, because of its occurrence in the last part of the section extruded. It is caused by some of the surface scale of the original billet being drawn into the core of the section by the turbulent flow of metal in the container of the extrusion press. A number of methods are effective in reducing it, including the ‘indirect’ process of extrusion, where, in effect, the die is pushed into the billet, instead of, as in the ordinary direct process, the billet being forced through the die, In the latter process there will be relative movement between the billet and the walls of the container, setting tip a turbulent flow of metal due to frictional forces, and so drawing surface scale into the main stream of extruded metal. Since relative movement between billet and container is reduced to a minimum in the indirect process, frictional forces are much smaller. Consequently, not only is less turbulence produced in the surface layers of the metal being extruded but also the power requirements are less than in the direct process where much more energy is lost in overcoming friction between the skin of the billet and the surface of the container.

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