Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

GTBL042-14 GTBL042-Callister-v3 October 2, 2007 13:45

2nd Revised Pages

570 • Chapter 14 / Synthesis, Fabrication, and Processing of Materials

Metal fabrication techniques

Forming operations Casting Miscellaneous

Forging Rolling Extrusion Drawing Sand Die Investment Lost foam Continuous Powder

metallurgy

Welding

Figure 14.1 Classification scheme of metal fabrication techniques discussed in this chapter.

When deformation is achieved at a temperature above that at which recrys-

hot working tallization occurs, the process is termedhot working(Section 8.13); otherwise, it

is cold working. With most of the forming techniques, both hot- and cold-working

procedures are possible. For hot-working operations, large deformations are possi-

ble, which may be successively repeated because the metal remains soft and ductile.

Also, deformation energy requirements are less than for cold working. However,

most metals experience some surface oxidation, which results in material loss and

cold working a poor final surface finish.Cold workingproduces an increase in strength with the

attendant decrease in ductility, since the metal strain hardens; advantages over hot

working include a higher quality surface finish, better mechanical properties and a

greater variety of them, and closer dimensional control of the finished piece. On oc-

casion, the total deformation is accomplished in a series of steps in which the piece is

successively cold worked a small amount and then process annealed (Section 14.5);

however, this is an expensive and inconvenient procedure.

The forming operations to be discussed are illustrated schematically in Figure

14.2.

Forging

forging Forgingis mechanically working or deforming a single piece of a normally hot metal;

this may be accomplished by the application of successive blows or by continuous

squeezing. Forgings are classified as either closed or open die. For closed die, a force

is brought to bear on two or more die halves having the finished shape such that

the metal is deformed in the cavity between them (Figure 14.2a). For open die, two

dies having simple geometric shapes (e.g., parallel flat, semicircular) are employed,

normally on large workpieces. Forged articles have outstanding grain structures and

the best combination of mechanical properties. Wrenches, automotive crankshafts

and piston connecting rods are typical articles formed using this technique.

Rolling

rolling Rolling,the most widely used deformation process, consists of passing a piece of

metal between two rolls; a reduction in thickness results from compressive stresses

exerted by the rolls. Cold rolling may be used in the production of sheet, strip, and

foil with high quality surface finish. Circular shapes as well as I-beams and railroad

rails are fabricated using grooved rolls.