Page 369 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Section 14.6 Die Design, Die Materials and Lubrication
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Die Die
Rib Web
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1. Blocked forging 2. Begin finishing 3. Web buckles 4. Laps in finished
forging
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1. Forging begins 2. Die cavities are 3. Cracks develop 4. Cracks propagate
being filled in ribs through ribs
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FIGURE I4.l6 Examples of defects in forged parts. (a) Laps formed by web buckling during
forging; web thickness should be increased to avoid this problem. (b) Internal defects caused
by an oversized billet. Die cavities are filled prematurely, and the material at the center flows
past the filled regions as the dies close.

The various radii in the forging-die cavity can significantly influence the for-
mation of such defects. Internal defects also may develop from (a) nonuniform de-
formation of the material in the die cavity, (b) temperature gradients throughout
the workpiece during forging, and (c) microstructural changes caused by phase
transformations. The grain-flow pattern of the material in forging also is impor-
tant. The flow lines may reach a surface perpendicularly, as shown in Fig. 14.12.
In this condition, known as end grains, the grain boundaries become exposed directly
to the environment and can be attacked by it, developing a rough surface and also
acting as stress raisers.
Forging defects can cause fatigue failures, and they also may lead to such prob-
lems as corrosion and wear during the service life of the forged component. The im-
portance of inspecting forgings prior to their placement in service, particularly in
critical applications, such as aircraft, is obvious. Inspection techniques for manufac-
tured parts are described in Chapter 36 _

l4.6 Die Design, Die Materials, and Lubrication

The design of forging dies requires considerable knowledge and experience regard-
ing the shape and complexity of the workpiece, its ductility, its strength and sensi-
tivity to deformation rate and temperature, and its frictional characteristics. Die
distortion under high forging loads is also an important design consideration, par-
ticularly if close dimensional tolerances are required. The most important rule
in die design is the fact that the part will flow in the direction of least resistance.
Thus, the workpiece intermediate shapes should be planned so that they properly
fill the die cavities. An example of the intermediate shapes for a connecting rod is
given in Fig. 14.7a.