Forged Wheels versus Cast Process
All GM accessory wheels are forged: Made from a one-piece
ingot of high-quality aluminum alloy. Forging aligns the
metal grain so it flows along the contours of the wheel
for maxiumum strength. After forging, each wheel is heat-treated
for strength and hardness
Forging
Forged wheels take advantage of what happens when metal
is cold worked. Cold working doesn't necessarily mean
you'd want to touch the materials while they're in process,
it means the procedures are done at a temperature below
the point where the metal starts to melt and regrow a
new crystal structure. Just as the spaces between a metal's
crystals may hold flaws, the crystals themselves are full
of imperfections called discontinuities. They may take
a variety of forms, but discontinuities all share one
important quality. By traveling through the crystal lattice
of the individual grains, they allow the metal to change
shape without fracturing like a diamond. When a load is
applied to a metal object, it deforms slightly. When the
load is removed, it regains its original shape. This happens
because discontinuities move a little, and move back.
If the load is high enough, the discontinuities will move
until they reach the edge of their crystal, or until they
run into another discontinuity.
Generally, discontinuities move one atom at a time, and
their movement is guided by the regular structure of the
crystal. If a discontinuity in the structure runs into
another, the regularity is interrupted, and they may become
tangled, and can't return to their starting position.
This has two effects. 1) When the load is removed from
the metal, it will not return to its original shape. 2)
The metal is more resistant to deformation in the future,
because there are fewer discontinuities available to move
around. This description of the process is a single case
of what is actually happening by the billions.
What we can measure is the average of them all. The idea
behind forging is to get, on average, the right number
of discontinuities tangled around each other, with crystals
oriented in the right direction, so that the metal is
very strong and resistant to further deformation. This
is a delicate balance, because too much cold working makes
the metal brittle, so that it fractures instead of absorbing
loads. You can see how this works for yourself - Bend
a paper clip back and forth many times until it breaks.
It begins soft, then gets stiffer, before finally fracturing.
Forging also changes the shape and alignment of the crystal
structure. When molten metal solidifies, its grain structure
is non-directional, amorphous, grains in the sense of
"grains" of sand. As metal is forged, these
grains are stretched in the direction of deformation,
making them more like the "grain" of wood. The
metal is formed so the grain goes in the directions where
strength is needed most. Think of particle board versus
real wood. One is cheap, heavy, and easily formed into
a variety of shapes. The other is strong and light. The
forging process, because of the vast pressures involved,
also compacts the metal, eliminating porosity and the
voids that can be a source of cracks or corrosion. The
result is that less metal is required to achieve a given
strength, meaning lighter, stronger wheels can be made.
Casting
The quality of cast wheels varies dramatically, depending
on process, and sometimes on variables beyond the control
of the manufacturer, such as ambient temperature or even
humidity. In general, pressure casting, in which the metal
is pumped into the mold, is better than just pouring it
in. Castings tend to be porous -- some carburetors actually
leaked fuel through the metal, with no crack or visible
flaw present. Porosity is bad, because it means there
are places where the metal isn't in direct contact with
more metal on all sides. Voids, which tend to form in
the spaces between crystals (a chicken and egg situation),
are where cracks begin. Larger, chunky grains may beget
larger voids, and cracks along crystal boundaries will
have farther to travel. All these points mean that cast
wheels must contain more metal to achieve an acceptable
strength, and are thus heavier. Still, cast wheels can
be made to a high standard with attention to quality processes.
The vast majority of alloy wheels are cast, and provide
many years of good service.