sectional dimensions are less than 6 x 6 inches.
sections that have widths greater than twice their thicknesses are
The slab is the intermediate shape from which sheets
Pouring a molten metal, or mixture of molten metals, into a mold
where it is allowed to solidify is called casting.
Two types of
molds are used: single-purpose and permanent molds.
molds have to be specially prepared, sometimes by machines, from
Using metal or permanent molds offers many advantages
where the process is applicable.
For example, the constantly
repeated cost of sand molding is eliminated; but the initial cost of
the metal mold or die is high. The cost is justified only when the
same casting is required in great numbers. However, for many metals,
the metal mold has a distinct advantage: the structure and strength
that result from the relatively rapid solidification in a metal or
For the nonferrous metals, chill casting is quite
practical, especially with some aluminum alloys.
Blooms, billets, or slabs are heated above the critical range and
rolled into a variety of shapes of uniform cross section.
common of these rolled shapes are sheet bars, channels, angles, and
Sheet bar and rods rolled from steel are used extensively
in aircraft work. Hot-rolled material is frequently finished by cold
rolling or drawing to obtain accurate finish dimensions and a bright,
Complicated sections that cannot be rolled or sections of which only
a small quantity is required are usually forged.
Forging is an
important hot-forming process. It is used in producing components of
all shapes and sizes from quite small items to large units weighing
The metal, preheated to the appropriate forging
temperature, is shaped mainly by upsetting (compressive deformation)
between impact surfaces or pressure surfaces, that is, by hammering
or pressing the heated metal until the desired shape is obtained.
Hammering can be used only on relatively small pieces.
hammering transmits its force almost instantly, its effect is
limited to a small depth. Therefore, it is necessary to
use a very heavy hammer or to subject the part to repeated blows
to ensure complete working of the section. If the force applied
is too weak to reach the center, the finished forged surface will
be convex or bulged. The advantage of hammering is that the