Technical Report The Construction of Alloy Wheels

Technical Report The Construction of Alloy Wheels

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Technical Report The Construction of Alloy Wheels

Introduction

Aluminium alloy wheels are now used in all branches of Motor Racing,
ranging from Touring cars and Rally cars to Formula One cars. The
Alloy wheels offer far superior mechanical properties than the
conventional steel wheels formerly used. These benefits include
reduced un-sprung weight, i.e. not held by the suspension, providing
more precise steering input and improved responsiveness.

Alloy wheels also improve acceleration and braking due to the
reduction of weight. The added strength of a quality alloy wheel can
significantly reduce wheel/tire deflection in cornering. This is
particularly critical with a vehicle equipped with high performance
tires where lateral forces may approach 1.0g. The metals in alloy
wheels are excellent conductors of heat - improving heat dissipation
from the brakes - reducing risk of brake fade under demanding
conditions. Additionally, alloy wheels can be designed to allow more
air to flow over the brakes, this can help cooling.

The alloy used in the finest road wheels today is a blend of aluminium
and other elements. The term "mag wheel" is sometimes incorrectly used
to describe alloy wheels. Magnesium is generally considered to be an
unsuitable alloy for road usage due to its brittle nature and
susceptibility to corrosion.

Nowadays, there are basically three ways in which alloy automotive
wheels are constructed. The three types can be referred to simply by
cast, billet and forged.

Casting is a relatively inexpensive way to produce a high-quality,
fairly strong alloy wheel; many aftermarket alloy wheels designed for
street use are made this way. Billet wheels are machined from a solid
chunk of material and forging uses intense heat and pressure to
transform a slug of alloy material into the final shape of a wheel.

This report will focus mainly on the casting processes used.

Casting Processes

The most common process of constructing alloy wheels is One-Piece
Casting, there are a number of methods to do this, and the most basic
is Die Casting, this process is used world wide throughout the casting
industry and is not exclusive to alloy wheels.

The Die Casting process uses a permanent mould usually made of metal,
which generally means that there is high tooling costs compared to
other methods of casting, but this high tooling cost is combined with
low production costs. This means that die casting is suitable for
products with a large production number.

There are four main processes in the die casting family, these are:

Gravity Die Casting

Pressure Die Casting

Low Pressure Die Casting

Squeeze Casting

Gravity Die Casting

This process is the most simple of the four, the mould or die, which
is generally made in two halves, is filled with molten metal, in the

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case of alloy wheels an aluminium alloyed with a variety of other
elements, the metal remains in the die until fully cooled and
solidified. The casting is then removed by the separation of the die
and the process can be repeated.

The molten alloy is produced normally by placing the solid metals in a
crucible furnace or pot furnace, and heating the metal to over 1200F
which is the melting point of aluminium; the actual temperature will
be higher to ensure correct viscosity for pouring. Each of these
furnaces has advantages, fuel fired furnaces are of two types: the
indirect flame, where the combustion does not come into contact with
the metal, and direct flame, where there is direct contact between the
combustion and the metal.

Crucible furnaces are made of a clay graphite mixture or of silicon
carbide, lift out crucibles are especially useful for flexibility in
small operations, after the melt has been prepared, the crucible is
lifted out of the furnace using tongs, its temperature is measured and
is poured directly into the die.

Pot furnaces are fairly similar to a crucible except you cannot lift
the pot out and pour, a ladle is needed, or a tilting pot may be used.

The gravity die process has its similarities with sand casting, which
does not use metal as its moulds, but compacted sand, and therefore is
entirely reusable. However, gravity die casting permits the production
of more uniform casting with closer dimensional tolerances.

Castings can be produced to a higher level of manufacturing
consistency than sand casting; normally the surface finish is far
superior to that for sand and the process will generally need less
machining and finishing than sand castings. This type of casting
offers a very reasonable production cost and is a good method for
casting designs that are more visually oriented or when reducing
weight is not a primary concern.

However, die casting does involve higher tooling costs which means
that there is a need for large order quantities and the process is not
as flexible as sand casting in terms of design complexity. Since the
process relies on gravity to fill the mould, the aluminium is not as
densely packed in the mould as some other casting processes. These
cast wheels will have a higher weight to achieve the required
strength.

Pressure Die Casting

Pressure casting is a similar process, but instead of pouring the
molten material into the mould, the molten alloy is drawn up into the
mould using a high-pressure vacuum. This eliminates much of the
trapped gas found in the gravity casting process, producing a stronger
wheel that is much less porous than a gravity-cast one.

The main advantages to pressure casting are:

· The ability to produce castings with close dimensional control.

· Production of a good surface finish.

· High rate of production.

Opposing these advantages are:

· High setup costs.

· High tooling costs.

· Restrictions on the range of alloys that can be cast.

Low Pressure Die Casting

This process is a compromise between gravity and pressure casting; it
tries to eliminate the unwanted properties from both methods.

A mould or die, is mounted on a holding furnace and is connected to
the molten metal by a feed tube or stalk. The furnace is pressurised
by the introduction of air above the surface of the molten metal
causing it to rise steadily in the stalk and gently fill the mould.
The air in the mould cavity is expelled through vents in the die and
when the cavity is filled, solidification starts.

When the metal has solidified as far back into the die as is required,
the pressure is released in the furnace and the molten metal left in
the stalk drops back into the molten pool

A further short cooling period is allowed to ensure that all sections
of the casting are solid, the mould is opened and the casting removed.
The molten metal is contained in a crucible. The crucible can be
topped up with molten metal using a filler port. The whole furnace is
contained in a pressure vessel sealed with a gasket. The riser tube is
connected to the top plate with a riser cap or nozzle. The riser tube
is sunk into the molten metal nearly to the bottom of the pool. The
riser tube is normally made of cast iron coated with a refractory wash
to prevent damage to it from the high temperature of the molten alloy.
These tubes if used regularly will normally last for about six months.

When the holding furnace is at temperature, a little above the melting
point of the alloy being used, it is filled using the filler port,
which is then sealed. When the metal has reached the required
temperature and the die has been pre-heated to its operating
temperature and closed, the inlet valve is opened and dry compressed
air is allowed to fill the sealed furnace causing the molten aluminium
alloy to rise up the tube and fill the die. With the furnace remaining
under pressure, the casting solidifies quickly.

When the metal in the nozzle has solidified, the pressure is released
allowing the still-molten metal in the riser tube to fall back into
the pool. After a further period of cooling to ensure complete
solidification of the casting, the die is then opened and the casting
released into the upper half of the die, from which it is removed,
usually mechanically. Once the sequence has been established, it can
be controlled automatically using temperature and pressure controllers
to supervise more than one die-casting machine.

Although the concept of low pressure casting was developed in the
early part of the century, it was not fully developed for the
production of aluminium castings until the second world war and it was
not until the mid 1950’s that the process was used in the automotive
sector.

This process is by far the most common in not only the production of
automotive alloy wheels, but in any number of components found in a
car engine, from cylinder heads to gearbox covers.

Squeeze Casting

Squeeze casting is a single step process for converting molten metal
into a component with precise dimensional allowances and excellent
surface finish.

This process is a hybrid of casting and forging, molten metal is
poured into the bottom half of a pre-heated die, as the metal starts
solidifying; the upper half closes the die and applies pressure during
the solidification process. The amount of pressure applied is
significantly less than used in forging, therefore parts of great
detail can be manufactured.

Billeting

Billet wheels are machined from a solid chunk, or "billet," of
material. First, a large pole of aluminium alloy is produced is
generally extruded which means that the grain runs through the stock,
much like the fibres within a single strand of wire. The stock
aluminium is then sliced up into sections which are machined down into
either complete wheels or just wheel centres.

Since they retain the grain structure of the extruded stock material,
billet wheels are extremely strong. This grain structure, which is not
present in a cast wheel, gives the final product an increased tensile
strength which means the wheel is even stronger without adding weight.

Billet wheels are also extremely expensive to produce because much of
the original material is wasted. A lot of time is also spent machining
the original stock down to a finished wheel, which adds to the cost of
the final product.

Most billet wheels are actually billet centres bolted into stamped or
spun rim halves. Entire wheels forged from a single billet are so rare
they are almost nonexistent, and are usually seen only on show cars.
Billet centres on multi-piece wheels, however, are common.

Forging

Unlike casting or billeting, forging uses intense heat and pressure to
transform a slug of alloy material into the final shape of a wheel.
Under this heat and pressure, the original grain structure of the
stock material is forced from the centre of the wheel towards the
outer edge. This grain structure is even stronger than the one found
in a billet wheel because it runs along the spokes and further
strengthens the forged wheel's spokes, while the grain in a billet
wheel simply runs through the spokes. Thanks to this process, a forged
wheel can be up to 300 percent stronger than a cast wheel.
Additionally, since forged aluminium is stronger than cast aluminium,
less material is needed to produce the wheel, resulting in a lighter
product.

Because of the basic limitations inherent in forging, most forged
wheels are two or three-piece units. In two-piece construction, a
centre is forged and then welded or bolted into a spun or stamped
outer rim. In a three-piece wheel, the centre is bolted to an inner
and an outer rim half. Three-piece wheels have the advantage of being
easily customizable for a variety of widths and offsets.

Conclusion

A good quality, pressure cast wheel, if made with the right material
(T-6 aluminium), is plenty strong enough for a road racing car, and
certainly for any rally car.

The payoff in forged wheels comes in weight and durability. These
racing wheels cost more, but are generally stronger and lighter than
an equally-sized cast wheel.

Billet and Forged wheels are much lighter and stronger than cast
alloys and because they are not made from one piece they can be
repaired relatively easily. However cast wheels are far less expensive
and are more suited to large order quantities whereas billet and
forged are more suited to one off designs.

References

1. Clegg, A.J. (1991). Precision Casting Processes. Pergamon Press

2. Heine, R.W. et al. (1976) Principles of Metal Casting. Tata
McGraw-Hill

3. Breithaupt. J. (1999). Physics. Palgrave Foundations

4. http://www.efunda.com/processes/metal_processing/squeeze_casting.cfm

5. http://www.fisherpaykel.com/pml/products-&-services/low-pressure-die-casting.cfm

6. http://www.azom.com/details.asp?ArticleID=1392

7. http://www.worldaluminium.org/production/processing/casting.html

8. http://www.factbook.net/Alzo/production.html

9. http://www.scoobyworld.co.uk/trolleyed/6/

10. http://www.alloynet.com/html/faq.html
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