Joining metals structuraly by welding, brazing, or
soldering, or by such mechanical means as riveting or
bolting, is a tremendous help in design and fabrication.
When all other properties are equal, material that can be
welded has the advantage.
Shock and Fatigue Properties
Aircraft metals are subject to both shock and fatigue
(vibrational) stresses. Fatigue occurs in materials that
are exposed to frequent reversals of loading or
repeatedly applied loads, if the fatigue limit is reached
or exceeded. Repeated vibration or bending will
ultimately cause a minute crack to occur at the weakest
point. As vibration or bending continues, the crack
lengthens until complete failure of the part occurs. This
is termed shock and fatigue failure. Resistance to this
condition is known as shock and fatigue resistance. It is
essential that materials used for critical parts be resistant
to these stresses.
The preceding discussion of the properties and
qualities of metals is intended to show why you must
know which traits in metals are desirable and which are
undesirable to do certain jobs. The more you know about
a given material, the better you can handle airframe
METAL WORKING PROCESSES
When metal is not cast in a desired manner, it is
formed into special shapes by mechanical working
processes. Several factors must be considered when
determining whether a desired shape is to be cast or
formed by mechanical working. If the shape is very
complicated, casting will be necessary to avoid
expensive machining of mechanically formed parts. On
the other hand, if strength and quality of material are the
prime factors in a given part, a cast will be
unsatisfactory. For this reason, steel castings are seldom
used in aircraft work.
There are three basic methods of metal working.
They are hot-working, cold-working, and extruding.
The process chosen for a particular application depends
upon the metal involved and the part required, although
in some instances you might employ both hot- and
cold-working methods in making a single part.
Almost all steel is hot-worked from the ingot into
some form from which it is either hot- or cold-worked
to the finished shape. When an ingot is stripped from its
mold, its surface is solid, but the interior is still molten.
The ingot is then placed in a soaking pit, which retards
loss of heat, and the molten interior gradually solidifies.
After soaking, the temperature is equalized throughout
the ingot, which is then reduced to intermediate size by
rolling, making it more readily handled.
The rolled shape is called a bloom when its sectional
dimensions are 6 x 6 inches or larger and approximately
square. The section is called a billet when it is
approximately square and less than 6 x 6 inches.
Rectangular sections that have width greater than twice
the thickness are called slabs. The slab is the
intermediate shape from which sheets are rolled.
HOT-ROLLING.Blooms, billets, or slabs are
heated above the critical range and rolled into a variety
of shapes of uniform cross section. The more common
of these rolled shapes are sheets, bars, channels, angles,
I-beams, and the like. In aircraft work, sheets, bars, and
rods are the most commonly used items that are rolled
from steel. As discussed later in this chapter, hot-rolled
materials are frequently finished by cold-rolling or
drawing to obtain accurate finish dimensions and a
bright, smooth surface.
FORGING.Complicated sections that cannot be
rolled, or sections of which only a small quantity is
required, are usually forged. Forging of steel is a
mechanical working of the metal above the critical range
to shape the metal as desired. Forging is done either by
pressing or hammering the heated steel until the desired
shape is obtained.
Pressing is used when the parts to be forged are large
and heavy, and this process also replaces hammering
where high-grade steel is required. Since a press is slow
acting, its force is uniformly transmitted to the center of
the section, thus affecting the interior grain structure as
well as the exterior to give the best possible structure
Hammering can be used only on relatively small
pieces. Since hammering transmits its force almost
instantly, its effect is limited to a small depth. Thus, 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 forging surface will be concave. If
the center is properly worked, the surface will be convex