The shape of the forging must promote smooth material flow. Therefore, a parting line, is chosen with proper consideration of the fiber structure of the finished forging. Fibers, caused by alignment of inclusions, second-phase particles, and micro segregation, should flow the contour of the forging as far as possible, because this ensures greatest toughness and ductility. At the parting line fibers are unavoidably cut through when the flash is trimmed. Therefore, the parting line is best placed where minimum stresses arise in the service of the forging. After the parting line is located, the cavity walls are given sufficient draft to allow removal of the forging from the die cavity, The internal draft is greater than the external draft because the forging tends to sink on to bosses of dies prior to its removal from the die. Fillets and corners must be given appropriate radii to assure both material flow and reasonable die life.
Die forgings are usually formed hot. The hot material cannot stay in contact in with the die too long, for the forging then will not be hot enough for the next operation. Also if the hot material is in contact with the die too long, it will overheat the die and so cause excessive wear, softening and breakage. The rapidity with which the part can be formed into the uniform shapes and with uniform properties permits the die-forging operation to compete with other processes in high-quality parts.
The use of closed-impression dies improve both the strength and toughness of the metal in all directions. The fiber structure characteristic of metal can be formed so as to improve the mechanical properties in areas where it is most needed to meet specific service conditions. Tools, gear blanks, crankshafts, connecting rods, and a great variety of machine parts are produced by the forgings.
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