Aluminum alloys

Aluminum alloys (aluminium alloys in British English) are alloys of aluminum with other elements, mainly magnesium, silicon, copper, zinc and manganese, which raise the strength of pure aluminum. Their density is around 2.7 g/cm³, about a third that of steel; they conduct heat and electricity well and resist corrosion naturally thanks to their oxide layer. Most of them machine very well. The modulus of elasticity is about 70 GPa, roughly a third that of steel.

They are divided into wrought alloys (sheet, plate, bar and extrusions) and casting alloys. Wrought alloys carry a four-digit number whose first digit gives the series by main alloying element; Europe adds the prefix EN AW (EN 573), and in the US the same numbers are registered by the Aluminum Association. The 5xxx series (Al-Mg, for example 5754 and 5083) is not heat-treatable, welds well and resists even seawater, so it is used for sheet and welded structures. The 6xxx series (Al-Mg-Si, for example 6060, 6061 and 6082) is the all-round choice for extrusions and machined parts. The 7xxx series (Al-Zn, for example 7075) reaches the highest strengths. The 2xxx series (Al-Cu, for example 2017A and 2024) is historically known as duralumin, a name often used loosely for any strong aluminum alloy.

The alloy number is followed by the temper (EN 515, with the same letters as the US system under ANSI H35.1): O means annealed, H strain-hardened by cold working (for example H111 or H32) and T heat-treated. The most common temper, T6, means solution heat-treated and artificially aged; T651 adds stress relief by stretching, an advantage for plate that will be machined. As a guide, 6082-T6 and 6061-T6 reach a tensile strength of around 300 MPa, 7075-T6 typically 500 to 570 MPa and annealed 5754 around 200 MPa.

Casting alloys are designated EN AC in Europe and usually contain more silicon, for example AlSi9Cu3(Fe) for die casting (close to the US alloy A380) or AlSi10Mg, which is also used in metal 3D printing.

Aluminum alloys are the default choice for machined prototypes and low-volume parts: plates, brackets, flanges, housings, machine frames, heat sinks, jigs and fixtures. 6061-T6 in the US and 6082-T6 in Europe are readily available, affordable, and weld and anodize well. Choose 7075 for highly stressed parts, molds for short runs and wherever strength-to-weight ratio is decisive. For precision plates and fixtures there is cast tooling plate with low internal stress; for sheet metal parts, 5754 or 5083 (in the US typically 5052).

Aluminum saves weight mainly where the part can have a larger cross-section. Stiffness relative to weight is almost the same for aluminum and steel, so simply swapping steel for aluminum while keeping the dimensions leads to roughly three times the deflection.

Do not write just "aluminum" on the drawing, but the alloy and temper, for example 6061-T6 or EN AW-6082-T6. Alloys 7075 and 2024 are unsuitable for fusion welding, and even 6061-T6 or 6082-T6 loses roughly a third to a half of its strength in the heat-affected zone of a weld, which the structural analysis must account for. Unlike steel, aluminum alloys have no distinct fatigue limit, so for cyclically loaded parts use fatigue data for the specific number of cycles.

Anodizing gives the best results on the 6xxx series. On 2xxx and 7xxx alloys the color tends to be less clean, and on high-silicon casting alloys the layer is dark and uneven. Threads in aluminum wear with frequent assembly, so consider thread inserts. In contact with stainless steel, copper or carbon fiber composite in a damp environment, aluminum is prone to galvanic corrosion. On thin-walled parts, expect distortion from internal stresses in the stock.