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Postby subham » Thu Nov 01, 2012 3:03 pm

Forging processes are among the most important means of producing parts for highper formance applications. Forging is typical of a group of bulk deformation processes in which a solid billet is forced under high pressure by the use of a press to undergo extensive plastic deformation into a final near-to-finished shape. 46 Other examples of deformation processes are extrusion , in which a long object with a high L/D ratio is produced by pushing a metal billet through a die, drawing , in which a billet is pulled through a die, and rolling, in which a slab is passed through rolls to make a thin sheet.Forging usually is carried out on a hot work piece, but other deformation processes such as cold extrusion or impact extrusion may be conducted cold, depending upon the material. Because of the extensive plastic deformation that occurs in forging, the metal undergoes metallurgical changes. Any porosity is closed up, and the grain structure and second phases are deformed and elongated in the principal directions of working, creating a “fiber structure.” The forging billet has an axial fiber structure due to hot working, but this is redistributed depending upon the geometry of the forging.


The mechanical fibering due to the preferred alignment of inclusions, voids, segregation,and second-phase particles in the direction of working introduces a directionality
(anisotropy) to structure-sensitive properties such as ductility, fatigue strength,and fracture toughness. The principal direction of working (such as the long axis of a
bar) is defined as the longitudinal direction. The long-transverse direction is perpendicular to the longitudinal direction. The variation of reduction of area in the tensile
test (the most sensitive measure of ductility) with the angle that the specimen axis makes with the forging axis. This shows that structure-sensitive mechanical properties like reduction of area, fatigue limit, and fracture toughness exhibit anisotropy as a result of closing up of porosity and alignment of second-phase particles produced by the plastic deformation. The designer needs to realize that some properties may not be the same in all directions of the forging. Therefore, in designing a forging, the direction of maximum plastic deformation (longitudinal) should be aligned with the direction of the part that needs to carry the maximum stress.
Forgings are classified into open- or closed-die forgings. Open dies, usually flat dies, are used to impose localized forces for deforming billets progressively into simple
shapes, much as the blacksmith does with his hammer and anvil. Closed-die forging or impression die forging uses mechanical presses or hammers to force the metal to flow into a closed cavity to produce complex shapes to close dimensional tolerances. A wide variety of shapes, sizes, and materials can be made in forging. Table describing the advantages of the common forging processes can be found at http://www.mhhe.com/dieter. With proper forging die design, grain fl ow is controlled to give the best roperties at the critically stressed regions.
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