Casting vs Forging
There are a variety of methods to produce
a metal part. Each has its own advantages and disadvantages. The
casting and forging are the most common fabrication methods for iron
and steel parts. Herein, we make a simple comparison to their
The casting process consists of pouring or injecting molten metal
into a mold containing a cavity with the desired shape of the
casting. Metal casting processes can be classified either by the
type of mold or by the pressure used to fill the mold with liquid
metal. Sand mold is the most common one.
Characters of Casting
Casting is a solidification process. Therefore, the microstructure
can be finely tuned, such as grain structure, phase transformations
and precipitation. However, defects such as shrinkage porosity,
cracks and segregation are also intimately linked to solidification.
These defects can lead to lower mechanical properties. A subsequent
heat treatment is often required to reduce residual stresses and
optimize mechanical properties.
Forging is a manufacturing process where metal is shaped by plastic
deformation under great pressure into high strength parts.
Characters of forging
Forging or cold forming are metal forming processes. There is no
melting and consequent solidification involved. Plastic deformation
produces an increase in the number of dislocations resulting in a
higher state of internal stress. Indeed, strain hardening is
attributed to the interaction of dislocations with other
dislocations and other barriers (such as grain boundaries).
Simultaneously, the shape of primary crystals (dendrites) changes
after plastic working of the metal. Dendrites are stretched in the
direction of metal flow and thus form fibers of increased strength
along the direction of flow.
We may distinguish hot working from cold working. Hot working is
performed above the recrystallization temperature; cold-working is
performed below it. In hot working strain hardening and distorted
grain structure are very rapidly eliminated by the formation of new
strain-free grains as the result of recrystallization. Rapid
diffusion at hot working temperatures aids in homogenizing the
perform. Initial porosity can also be significantly reduced,
eventually completely healed.
Metallurgical phenomena such as strain hardening and
recrystallization are important because these changes in structure
result in an increase in ductility and toughness over the cast
Most modern performance wheels are made from aluminum by casting or
forging. Forged wheels are manufactured in multiple steps compared
to the one step in the casting process.
Casting has the advantage of allowing the designer more styling
freedom because the process is a more flexible method. Until
recently, most wheels have been gravity cast (heavier and thicker).
Today, low pressure die casting techniques are used to substantially
reduce porosity. Indeed, castings tend to contain porosity which
strongly influences the mechanical integrity of the component. Thus,
cast wheels are generally designed larger and heavier in order to
achieve an acceptable structural strength for a given application.
The forged wheel, because of the enormous pressures involved,
compacts the metal, eliminating porosity and the voids that can be a
source for cracks or corrosion. The result is that less metal is
required to achieve a given strength, meaning lighter wheels can be
made. Furthermore, due to the density of the grain structure, the
polished forged wheel will maintain its luster for much longer than
a polished cast wheel which is very porous.
To summarize, forging yield wheels with higher strength to weight
ratio but the tooling due to the multiple steps process and the
based alloy are comparatively more expensive than in casting
Furthermore, with lighter weight wheels, you will benefit from
increased fuel savings, and better acceleration due to less amount
of inertial weight at the rotational axis. For those reasons,
usually forged wheels are only used for high performance
Some of the important factors affecting the selection of a process
include the following:
- Quantity of the material required
- Design of the part
- Tolerances required
- Metal specification
- Surface finish required
- Tooling costs
- Economics of machining versus process costs
- Delivery requirements