For a car the most important benefit aluminum and aluminum alloy steels above is their low density, or, often said, specific gravity.
Why use aluminum in the car
Density aluminum alloys averages 2,7 in grams per cubic centimeter, compared with 7,87 for steel. Thus, the density of aluminum alloys is only about 35 % the density of steel.
However, the modulus of elasticity of aluminum steels equals only 70 GPa compared to 207 GPa for steels. It means, that for equal bending stiffness aluminum beam must be 43,5 % thicker, than a steel beam. The thing is, that the stiffness of the structural element - beams, profile or sheet - from any material is directly proportional to the product of the elastic modulus of this material by the moment of inertia of the cross section (E ·I) of this element. As a result,, weight loss, which can be obtained by applying aluminum over steel will not be proportional to the difference in density of the two materials. In general, the replacement of the steel beams on an aluminum beam gives a weight reduction of about 50 % (cm. more details here).
and wrought, and casting
As casting, and wrought aluminum alloys are very widely used in cars. Casting aluminum alloys are used primarily for engine, transmission and suspension elements, while wrought alloys in form of sheets, and extruded profiles are widely used in the bodywork. Some car models, for example Audi A8 and Audi A2, They have an all-aluminum body.
Casting aluminum alloys
Cast aluminum alloys, which are used in car, They are basically a series of alloys 300 (Al-Si-Cu and Al-Si-Mg), such as the:
- alloy 319 for intake manifold, cylinder heads and transmission housings;
- alloy 383 for the cylinder block;
- alloy 356 for cylinder head and
- A356 alloy for wheels and suspension arms.
The main alloying element in these alloys is silicon, which provides them with good casting properties, including, high fluidity. These alloys are cast using a number of conventional methods of molding sand mold and casting into steel separable forms to more complex methods of casting, such as, die casting and investment casting. If an aluminum casting high demands on tightness and the number of casting defects, then apply these methods of casting, as vacuum high pressure die casting or casting the semiliquid state.
Alloys of copper
In addition to casting aluminum alloys of the 3xx series, some alloys of the 2xx series (Al-Cu) are also used in cars.. These include alloys 201, 204 and 206, details of which are cast chassis, suspension and some of the engine components. Casting aluminum alloys both series - and 2xx, and 3xx - are thermally hardenable alloys.
Wrought Aluminum Alloys
Aluminum alloys for heat exchangers
Such aluminum alloys, as 1200 and 3005 used in heat exchangers, which include the radiator, evaporator tube and fin. Benefits applying aluminum in these products is not only, that aluminum is very high thermal conductivity, but also in the, he had a significantly higher ratio of strength / density, than copper-based alloys, which are conventional materials for the heat exchanger.
Sheet aluminum alloys
Sheet aluminum alloys, which are used for the body panels, are work-hardened alloys of the 5xxx series (Al-Mg), such, both alloys 5182, 5454 and 5754, as well as heat-hardening alloys of the 6xxx series (Al-Mg-Si), such as , 6009, 6061 and 6111.
5xxx series alloys are thermally reinforcing, that is, they are almost impossible to harden by heat treatment. The sheets from these alloys are supplied in the annealed state "O", and they work hardening is obtained when the punching operation of these sheet metal parts.
Sheets made of alloys 6xxx series are delivered T4 state, that is, in the state after quenching and natural aging. Then they get hardened state T6 due to artificial aging, which occurs during heating in a paint curing oven during the dyeing operation.
5xxx series alloys respond well to forming by plastic deformation. However, during the formation of sheet metal parts from these alloys, traces of plastic deformation by stretching (Luder stripes) may appear on their surface. Therefore these alloys are not used for the exterior panels, but is used for interior panels and frame parts of the body. Sheet 6xxx series alloys are not susceptible to Luder bands and so they are used for interior and exterior panels, and for the body shell elements.
Aluminum alloys for profiles
Alloys for aluminum profiles – extrusion aluminum alloys, which are used in car design, are:
- 6xxx series alloys (Al-Mg-Si) 6005, 6061, 6063 and 6082;
- 7xxx series alloys (Al-Zn-Mg): 7004, 7116, 7029 and 7129.
Profiles of these aluminum alloys are used for manufacturing various elements of the body frame, the gain of the front wings, the motor mounting frame, seat frame, bumper beams, steering parts.
Aluminum alloys of both series - 6xxx and 7xxx - are thermally hardened by heating for hardening (solution treatment) followed by natural or artificial aging. 7xxx series alloys are more difficult to press, than the 6xxx series alloys, especially in case of complex hollow profiles. They - 7xxx series alloys – Moreover, less corrosion resistant and less welded.
Body: aluminum and steel
Details of the car body structure, carrying elements such as roof, require multiple stamping and welding, when they are made of steel. If you use aluminum, it is possible to use only one single-piece extruded aluminum profile, which is subjected to special treatment, for example, hydroforming. The use of only a single extruded section instead of stamped and welded makes it possible to reduce the number of necessary equipment and the cost of assembly work.
Stamping aluminum compared to steel
In general, the ability of aluminum alloys to plastic deformation - plastic molding - is about two thirds of this ability of steel. Due to the lower capacity for forming complex aluminum body panels may require multiple stamping operations or assembly of several stamped parts.
Moreover, due to the low modulus of elasticity of aluminum aluminum parts exhibit higher elastic recoil after the molding operation, for example, bending. Therefore, aluminum stamped parts are more difficult to stamp: they do not closely follow the shape of the stamp, as the steel parts. In addition, the aluminum alloys have a high tendency to form scratches and tool, than steel and therefore require a larger amount of lubricant and higher purity stamping tool surface.
aluminum welding Properties
Although aluminum alloys can be welded by spot welding resistance, as steel, There are some differences between its use for aluminum. When spot welding of aluminum is necessary to apply a high amperage because of its low electrical resistivity and high thermal conductivity. The welding current strength for aluminum alloys is 15-30 kA compared with 8-10 kyloamper to become.
It means, that the increased size needed welding machines for resistance welding of aluminum, as well as increased consumption of electric energy.
Fusion arc welding (TIG and MIG) can also be applied to aluminum alloys. However, due to their high thermal conductivity, they require welding for increased energy consumption.
Among other methods of material compounds, which are used for parts made of aluminum alloys are:
- samoprobivnye rivets,
- adhesive compounds and
- a combination of resistance welding to adhesive bonding.
Источник: Advanced Materials in Automotive Engineering, ed. Jason Rowe, Woodhead Publishing, 2012