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 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
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Clegg, A.J. (1991). Precision Casting Processes. Pergamon Press
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Heine, R.W. et al. (1976) Principles of Metal Casting. Tata McGraw-Hill
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Breithaupt. J. (1999). Physics. Palgrave Foundations
Contents
- Introduction
- Casting Processes
- Gravity Die Processes
- Pressure Die Casting, Low Pressure Casting
- Squeeze Casting, Billeting
- Forging, Conclusion
- References
