duralumin

Duralumin is a wrought aluminum alloy containing roughly 93-97% aluminum, with copper as the primary strengthening element at around 3.5-4.5%, plus smaller amounts of manganese (0.4-0.8%), magnesium (1.2-1.8%), and traces of iron and silicon. It was developed in Germany in 1906 and named for the Dürener Metallwerke factory where it originated. The alloy combines the low density of aluminum with strength approaching that of mild steel, making it roughly twice as strong as pure aluminum while remaining 30% lighter than steel.

The critical difference between duralumin and other aluminum alloys lies in its heat treatment. The alloy undergoes solution heat treatment at around 500°C, then quenching in water, followed by artificial aging at 130-180°C. This aging process, called precipitation hardening, allows copper-rich phases to form within the grain structure, substantially increasing hardness and tensile strength. Without this aging step, the alloy remains relatively soft. Strength typically reaches 420-480 megapascals tensile strength after proper heat treatment, with good fatigue resistance.

Variants and limitations

Modern specifications distinguish several duralumin variants. British and American standards (2017-T4 and similar designations) describe the alloy in different temper conditions. The T4 temper represents solution-treated and naturally aged material; T6 represents solution-treated and artificially aged for maximum strength. Variations in copper content and trace elements produce different properties suited to specific aerospace applications, from skin panels requiring good formability to structural beams needing maximum strength.

The alloy has real drawbacks. Its corrosion resistance is poor compared to pure aluminum; seawater and chloride exposure cause pitting and stress corrosion cracking, particularly along grain boundaries. Aircraft fabricated from duralumin require protective measures including clad layers of pure aluminum on the surface or chemical conversion coatings. The high copper content also makes duralumin expensive and difficult to recycle with other aluminum scrap.

Though modern aerospace increasingly uses 7075 aluminum alloys and composite materials for higher strength-to-weight ratios, duralumin remains in service on older aircraft and in applications where its combination of moderate strength, reasonable corrosion resistance, and workability suits the design. Its historical importance rests on enabling all-metal aircraft construction in the early twentieth century, replacing fabric and wood frames.