Aluminium alloy

Welded aluminium alloy bicycle frame, made in the 1990s.

Aluminium alloys are alloys in which aluminium is the main metal used. It is usually alloyed with elements like copper, magnesium, manganese, silicon, tin and zinc. Aluminium alloys are classified into casting alloys and wrought alloys. About 85% of aluminium is used for wrought products. Cast aluminium alloys give cheap products because of their low melting point. Aluminium alloys are widely used in engineering structures.

Engineering use & properties

Aluminium alloy bicycle wheel. 1960s Bootie Folding Cycle.

Aluminium alloys, classified by ANSI or names of main constituents (DIN and ISO), have diverse properties for engineering structures. Choosing the right alloy involves evaluating tensile strength, density, ductility, formability, workability, weldability, and corrosion resistance. Historically, aluminium alloys are crucial in aircraft for their high strength-to-weight ratio, as pure aluminium is too soft for such applications.

Aluminium alloys vs steel

Aluminium alloys have an elastic modulus of about 70 GPa, one-third that of steel, meaning they deform more under the same load. Design choices for new metal products are often influenced by manufacturing technology, with extrusions being crucial for forming complex profiles, especially in Al-Mg-Si alloys. Aluminium alloys can achieve stiffer, lighter designs compared to steels. For example, in thin-walled tubes, increasing the radius by 26% halves the wall stress. Consequently, aluminium bicycle frames use larger tube diameters for desired stiffness. In automotive engineering, aluminium cars use extruded space frames for rigidity, shifting away from the steel unibody design.

Aluminium alloys are commonly used in automotive engines for weight savings, especially in engine blocks and crankcases. Their susceptibility to warping at high temperatures makes effective cooling systems essential. Advances in manufacturing and metallurgy have enabled successful applications, despite past failures like the Chevrolet Corvair's aluminium cylinder heads. However, aluminium alloys have lower fatigue strength than steel, lacking a fatigue limit, which limits their use in high-cycle (over 10^7 stress cycles) components.

Heat sensitivity

Aluminium's heat sensitivity complicates routine procedures, as it melts without glowing red, unlike steel. Heat treatment effects can be negated by blow torches, which may internally damage the material, rendering heat-treated chains unsafe. Aluminium frames, if improperly welded, can twist from internal stresses, prompting the aerospace industry to favor rivets, fasteners, or adhesives instead. Overheated aluminium can be annealed but may still distort, with misalignment potentially correctable if designed for rigidity. Despite its heat intolerance, aluminium is used in rocketry, such as in the RM-81 Agena engine, where its high thermal conductivity prevents melting under extreme heat.