New aluminium alloy for wheels

Many drivers opt for light alloy wheels for their car, mainly because of the original design and the tough, sporty look that attracts attention and emphasises the character of the car. But are light alloy wheels not just an attractive accessory? Rio Tinto, one of the world's leading aluminium producers, has recently developed a new revolutionary alloy with interesting properties for the automotive industry.

You may have guessed already: although the popular aluminium wheels primarily provide a nice appearance, they also have a considerable impact on the operation of cars. They are in fact an important factor for the stiffness of the wheels, the normal operation of the suspension and even the braking system (cooling). Well adapted wheels can guarantee our safety and driving comfort.

Aluminium as a construction material

Compared to iron alloys, aluminium alloys are about three times lighter, but generally less strong. At least, this is the opinion of most people. By applying a special heat treatment to hardenable aluminium types, they can be made strong enough for applications in constructions. Some examples are the aluminium profiles used for windows and doors, scaffolding and, last but not least, the applications for aircraft construction. Since the rise of aviation some 100 years ago, the many advantages of aluminium have been successfully applied: due to its low specific gravity, relatively high strength and good corrosion resistance, this metal was ideal for use in aviation and aerospace. 

However, the share of aluminium in cars has been limited to cast cylinder blocks, in some cases it was used for body panels and for wheels. Lately, this share has been increasing because car manufacturers want to drastically reduce the total weight of cars. The arrival of the electric car with its relatively heavy battery also caused a revival of aluminium in the automotive sector. Light alloys, which are used for example in aluminium wheels, are also excellent conductors, which has a significant positive effect on the effective heat transfer of the brakes. This makes an important contribution to maintaining optimum braking power under difficult conditions. In addition, the specific design of alloy wheels can improve the functionality and durability of brake discs and brake saddles.

Aluminium alloys

There are two groups of alloys: the wrought alloys and the cast alloys. The first group is mainly used for the production of sheets and profiles, the second group is mainly used for all kinds of castings, including wheels.

The aluminium alloys and their specificities are complex because of the many compositions and different designations used in various countries and continents. In Europe, the traditional four-digit designation for wrought alloys, e.g. 1XXX, 2XXX to 8XXX, is used according to the Aluminium Association. It is usually referred to as the 1-series up to and including the 8-series (see figure).  DIN standards and British Standards use different designations. In addition to this composition, there is also a coding for the condition F (‘as fabricated’), H (cold formed), O (annealed) and T (heat-treated). These codes therefore define the treatment and the resulting strength properties.

Cast alloys have a different designation when their composition is equivalent to that of wrought alloys.

Hardenable types

In order to harden aluminium, certain alloying elements are required. These elements usually have a limited solubility in the Al-grid. This means that they dissolve well at high temperatures (but below the melting point) and deposit when the metal is cooled quickly. This deposit or precipitation forms the basis of the hardenable Al species. Thus, a structural change does not occur as with steel (the so-called martensite transformation), but a reinforcement through the deposit of hard particles in the aluminium matrix. The alloys of the 2-, 6- and 7-series are the most commonly used hardenable types. There are also alloys that are not thermally treated and still are very strong, for example, due to cold deformation. These are the Al species from the 1, 3 and 5 series.

For cast aluminium wheels, AlSi and AlSiMg alloys are mostly used, the 6-series for forged wheels. In that case, casting is usually done in low-pressure injection moulds. The advantage is that these moulds allow complex shapes (spokes).

New alloy

Rio Tinto recently developed a new aluminium casting alloy specifically for the automotive sector. Because of its special properties, this alloy was called 'Revolution-Al'. The exact composition and treatment of Revolution-Al are currently still secret. The casting alloy is said to belong to the AlSiMg group, with properties up to 15 percent higher than the classic AlSi7Mg alloy A356.2. Certain alloying elements would prevent the formation of brittle phases and thus increase the ratio between strength and ductility. By adjusting the composition of the alloy, for example, increasing the Mg and Mn content in comparison with the classic alloy A357.1, the iron-rich phase would be promoted in comparison with the existing alloy A357.1 and therefore the ratio between strength and utility would be greatly improved.

Due to the higher tensile strength, a weight saving of 7 percent can be achieved for the same strength compared to other similar alloys. Linked to this, the alloy also has high toughness and fatigue properties. The latter characteristic is particularly important, as aluminium generally has a low fatigue limit compared to steel. All these advantages ensure that the new alloy will soon be used for wheels and possibly other applications.

In its test lab in Zwijnaarde, Sirris is able to perform various analyses and tests on various metals, including aluminium alloys. In addition, you can also receive advice on material selection and related heat treatments.

Sources

(Source picture: https://www.mining.com)