Aluminium alloy wheels manufacturing process, materials and design

Aluminium alloy wheels manufacturing process have developed a lot since 1970s. Due to sophisticated wheels design, casting has become the dominant manufacturing process. Alloy wheel material has evolved too: car wheels alloys now contain 7 to 12% silicon content, and varying contents of magnesium in addition to aluminium, in order to meet the demand for metal-mould casting properties, corrosion and fatigue resistance.

History of aluminium car wheels

The first light-alloy sheet aluminium car wheels were used in Daimler-Benz and Auto-Union racing cars in the 1930s. In the 1960s, Porsche began the batch production of sheet wheels, which consisted of a wheel rim and nave. The first high-volume production of sheet wheels in Europe started in 1979 for Daimler-Benz cars destined for the USA.

Through further development of the production process for wheel rims and naves, the manufacturing costs were reduced markedly so that an aluminium sheet wheel has been produced in large numbers for the BMW 5 Series since 1995

Using aluminium wheels on passenger cars began with the upper class or flagships models in order to give them a distinctive personal touch.

Mainly cast at this time, they started in the 1970s to be factory-fitted to mass-produced cars.

Wheels are now representing about 15% of the average aluminium content in passenger cars and light trucks, and if the main motivation has been styling with mainly cast solutions, weight reduction requirements have lead to the development of more technical cast but also forged and fabricated solutions.

These components have, however, critical safety functions and must meet high standards of design, engineering and workmanship.

Design/material selection considerations

Stiffness: Structural stiffness (design dependent) is the basic value to consider when designing an aluminium wheel to achieve at least the same vehicle behaviour as with an equivalent steel wheel.However, material stiffness (Young’s modulus) is very little depending on alloy and temper.

Static behaviour: Yield strength is considered to avoid deformation under maximal axial efforts (accelerations and braking) and radial ones (plus turning). Misuse cases are considered in relation to tensile strength. Yield tests under pressure are also conducted to check this behaviour.

Fatigue behaviour: This is the most important parameter for dimensioning. Finite element software is systematically used during design. Service stresses are considered, including multi-axial stresses as of recently. Rotary bending and rim rolling tests are used to verify these calculations.

Crash worthiness: Mainly, but not only, linked to stress/strain curves in large displacements. Crashworthiness is beginning to be now simulated. However impact tests systematically check the resistance to accidental collisions such as pavements impacts.

Cooling: Whatever the type of wheel (cast, forged, strip, mixed wrought-cast,…), aluminium dissipates heat more quickly than steel. Further, aluminium wheels act as a very efficient heat sink. This results in significant improvements of braking efficiency, and a reduced risk of tyre overheating.

Style – weight saving: Reduction of weight of the unsprung mass of vehicles is a key priority. A compromise has to be accepted if styling requirements dictate different production technologies (s. figure).

Dimensional: A perfect mass balance is a key parameter to avoid significant vibrations. As a result, cast and forged wheels are machined. Lightness also reduces vibrations of aluminium sheet wheels

Corrosion: Cast and forged wheels are painted or lacquered after chemical conversion. Strip wheels are polished and varnished or also painted. Even at the uncoated iron/aluminium disk, or hub interface, no significant corrosion has ever been noticed for any

Manufacturing

Wheel manufacturing process is dominated by aluminum. Aluminium penetration in wheels was in the year 2000 for European vehicles about 30 to 35%, compared to largely more than 50% in USA and Japan. This is representing more than 14% of the average aluminium content of a vehicle and is expected to rapidly increase (foreseen 45% in 2005 and 70% in 2010). In the US, the repartition of aluminium in wheels was in the year 1999:

82% cast, 11% forged (including all vehicles), 4% for sheet and 3% for plate.
In Europe, the share of casting is slightly higher (more than 85%) due to the lesser extent of forged wheels for trucks (including light ones). However many developments are on the way to reduce weight of present aluminium wheels without fully sacrificing style. With this purpose, a really attractive compromise could consist in cast central discs (or forged when competitive), assembled (mainly by welding) to extruded or laminated rims

Casting processes

Among aluminium wheels, cast ones represent more than 80% in Europe, 85% in USA for passenger cars and light trucks, and 93% in Japan.
Their main advantages, when compared to steel or other aluminium wheels are:

1. A high styling versatility
2. Weight (equal or less than steel without styling)
3. Dimensional accuracy (mass distribution)
4. Recycling ability
5. Static and dynamic behaviour

The major casting processes for wheels are:

• Low-pressure die casting (mainly)
• Gravity permanent mould casting (less used)
• Squeeze-casting process (marginally used)

Rarely used are the following processes:

• Counter pressure die casting
• Casting-forging (Cobapress)
• Thixocasting

Processing after casting

After casting, wheels are (a) 100% x-ray inspected and then eventually heat-treated prior to machining. This step is followed by a pressure tightness testing before drilling valves and bold nut holes.
After a cosmetic inspection wheels are then (b) painted or varnished, this operation including a pre-treatment (degreasing, phosphatizing and/or chromating…). 3D dimensional controls (c), dynamic balance checking, (d) bending and rim roll fatigue as well as (e) impact tests are statistically performed.

Forging

Forged aluminium wheels are one-piece wheels formed from a single block of metal by hot forging, hot or cold spinning and machining operations. The forging process permits flexibility in design of the styled disk, similar to cast wheels.

The standard alloys used are the heat- treatable wrought alloys:

• EN AW-AlSi1MgMn (6082) in Europe
• AA-6061(AlSiMgCu) in USA

The manufacturing process permits a maximum brake caliper room in combination with tight dimensional tolerances, low weight and high strength and toughness.

Forging aligns the grain structure along the direction of the material’s flow, thereby permitting exploitation of strength and toughness properties of the alloy to the maximum extent. As a result, forged wheels are more damage tolerant w/r to misuse.

In relation to castings forged materials exhibit decidedly higher fatigue resistance due to absence of pores and because of a fine, homogeneous microstructure. While cast wheels are performing according to the same load and endurance specifications as forged wheels, the latter are more tolerant to overloads as may be experienced in sports cars.

In addition, the dense wrought microstructure permits high gloss diamond machining and polishing of the decorative hub faces.

The traditional wheel forging concept included several forging operations, rough machining, splitting, flow turning, heat treatment, final machining and numerous additional finishing steps, depending on design requirements. As a result, styling dominates weight and costs are considerable, (s. LINK). On the other hand, if low weight and low costs are prime targets, then fabrication technologies must dictate the styling limits. Following this reasoning rigorously, a production concept “Light Forged Wheel” was developed (Otto-Fuchs Metallwerke) and these wheels are used by Audi, BMW, DaimlerChrysler, Jaguar and Volkswagen. Several millions of these wheels have been produced since 1995, with the following steps:

• 1-step forging, coining, piercing
• flow turning (hot spinning)
• solution heat treatment and ageing
• machining, drilling, deburring (optional diamond turning)
• etching and painting.

The 2-piece sheet metal process

• A strip of sheet metal, cut to the required length, is made into a round with the ends butt welded together using a pressure welding machine. After removal of the weld flash, the rims are shaped in a series of rolling operations.
• The wheel nave is formed in several steps on a transfer press using a deep drawing process or stamped on a forging machine.
• Joining the rim to the nave is done by means of a pulsed MIG process. After joining, the wheels are surface treated, i.e. pre-treatment to produce a conversion coating followed by an electro-dip coating.

Materials

The alloys employed have to meet a range of sometimes conflicting requirements:

• Good metal-mould casting properties (castability, susceptibility to hot tearing and shrinkage characteristics)
• Ability to withstand physical impact (elongation and impact strength)
• Corrosion resistance (normal and saline atmospheres)
• Fatigue resistance

These requirements have led to the widespread use of hypoeutectic Al-Si primary alloys with 7 to 12% silicon content, varying contents of magnesium (strength-elongation compromise), low iron and minor impurity concentrations.

In the US and in Japan, the T6 heat treated AlSi7Mg0.3 alloy is used quite exclusively.
In Europe, the proportion of heat treated wheels is increasing but is still a long way from 100%; for these ones, the same AlSi7Mg0.3 primary alloy is preferred.
Non-heat treated wheels are cast either in AlSi7Mg0.3, mainly in France, or in AlSi11Mg, containing less magnesium, mostly in Germany and Italy; this alloy is less favourable in terms of fatigue limit, but it has a better castability and different shrinkage characteristics. It is not, however, suitable for wheels to be heat treated.

Static and fatigue characteristics were measured on representative permanent mould (P.M.) test-pieces, for the AlSi7Mg primary alloy in the T6 temper, and various Mg contents. In every case, the modifier was Na. Results clearly show that the AlSi7Mg0.3 alloy offers the best compromise between fatigue strength and elongation. The same investigations have been carried out with different silicon contents. They clearly demonstrated that an increase in Si content also has an adverse effect on ductility, particularly at low rates of solidification (thick hubs). Nevertheless, alloys with 9-11% Si are still acceptable if better castability is needed.

An increase in magnesium content does not clearly improve fatigue strength but significantly reduces elongation.
(above): Fatigue limits from rotating bending permanent mould (P.M.) test pieces were measured for Na-modified, heat-treated AlSi7Mg0.3. Micrographics and results on test-pieces found max. pore size to be the most closely correlating parameter with fatigue.

Effect of Mg-Content on Strength:
Tensile strength and yield strength vary in parallel with increasing magnesium content over the range of 0 to 0.3%.

Effect of Mg-Content on Ductility:
Elongation varies inversely with tensile strength and yield strength, and clearly proves the superiority of Sb modification.

Effect of Mg-Content on Fatigue:
Magnesium content greatly enhances fatigue strength and the AlSi7MgSb alloys exhibit higher values than the AlSi11MgSr