Aluminium Alloys

Aluminium alloys versus steels

Aluminium alloys typically have an elastic modulus of around 70 GPa, which is about one third the elastic modulus of steel. For a given load, a part made of an aluminium alloy will therefore show greater elastic deformation than a steel part of identical geometry. Though there are aluminium alloys with higher tensile strengths than commonly used steels, simply replacing steel parts with aluminium alloy equivalents may lead to problems. With new products, design choices are often governed by the special manufacturing technologies that apply to aluminium. Extrusions are particularly important in this regard, owing to the ease of which aluminium alloys, particularly the Al-Mg-Si series, can be extruded from complex profiles.

In general, stiffer and lighter designs can be achieved with aluminium alloys than is feasible with steels. For instance, consider the bending of a thin-walled tube: the second moment of area is inversely related to the stress in the tube wall, i.e. stresses are lower for larger values. The second moment of area is proportional to the cube of the radius times the wall thickness, thus increasing the radius (and weight) by 26% will lead to a halving of the wall stress. For this reason, bicycle frames made of aluminium alloys make use of larger tube diameters than steel or titanium in order to yield the desired stiffness and strength. In automotive engineering, cars made of aluminium alloys employ space frames made of extruded profiles to ensure rigidity. This represents a radical change from the common approach for current steel car design, which depend on the body shells for stiffness, that is a unibody design.

Aluminium alloys are widely used in automotive engines, particularly in cylinder blocks and crankcases due to the weight savings that are possible. Since aluminium alloys are susceptible to warping at elevated temperatures, the cooling system of such engines is critical. Manufacturing techniques and metallurgical advancements have also been instrumental for the successful application in automotive engines. In the 1960s, the aluminium cylinder heads and crankcase of the Corvair earned a reputation for failure and stripping of threads, which is not seen in current aluminium cylinder heads.