Page 362 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Chapter 14 Metal-Forging Processes and Equipment

Upper die UPPe"
punch

Lower die Die Lower Die

1. Start of stroke 2. End of stroke 1. Start of stroke 2. End of stroke
(a) (b)

FIGURE l4.9 Comparison of (a) closed-die forging with flash and (b) precision or flashless
forging of a round billet. Source: After H. Takemasu, V Vazquez, B. Painter, and T. Altan.

Regardless, the term closed-die forging is often applied to impression die forg-
ing with flash generation, whereas open-die forging generally applies to operations
with simple dies and tooling and with large deformations.
Precision Forging. In order to reduce the number of additional finishing opera-
tions required-hence the cost-the trend has been toward greater precision in
forged products (net-shape forming). Typical precision-forged products are gears,
connecting rods, and turbine blades. Precision forging requires (a) special and more
complex dies, (b) precise control of the blank’s volume and shape, and (c) accurate
positioning of the blank in the die cavity. Also, because of the higher forces required
to obtain fine details on the part, this process requires higher capacity equipment.
Aluminum and magnesium alloys are particularly suitable for precision forging be-
cause ofthe relatively low forging loads and temperatures that they require; however,
steels and titanium also can be precision forged.

Forging Practice and Product Quality. A forging operation typically involves the
following sequence of steps:
Prepare a slug, billet, or preform by processes such as shearing (cropping),
sawing, or cutting off. If necessary, clean surfaces by such means as shot
blasting.
2. For hot forging, heat the workpiece in a suitable furnace and then, if necessary,
descale it with a wire brush, water jet, steam, or by scraping. Some descaling
also may occur during the initial stages of forging, when the scale (which is
brittle) falls off during deformation.
3. For hot forging, preheat and lubricate the dies; for cold forging, lubricate the
blank.
4. Forge the billet in appropriate dies and in the proper sequence. If necessary, re-
move any excess material (such as flash) by trimming, machining, or grinding.

5. Clean the forging, check its dimensions, and (if necessary) machine it to final
dimensions and specified tolerances.
6. Perform additional operations, such as straightening and heat treating (for
improved mechanical properties). Also, perform any finishing operations that
may be required, such as machining and grinding.
7. Inspect the forging for any external and internal defects.