Duralumin

In addition to aluminium, the main materials in duralumin are copper, manganese and magnesium. For instance, Duraluminium 2024 consists of 91-95% aluminium, 3.8-4.9% copper, 0.3-0.9% manganese, 1.2-1.8% magnesium, <0.5% iron, <0.5% silicon, <0.25% zinc, <0.15% titanium, <0.10% chromium and no more than 0.15% of other elements together.[1]

Duralumin was developed by the German metallurgist Alfred Wilm at Dürener Metallwerke AG. In 1903, Wilm discovered that after quenching, an aluminium alloy containing 4% copper would slowly harden when left at room temperature for several days. Further improvements led to the introduction of duralumin in 1909.[2] The name is mainly used in popular science to describe the Al-Cu alloy system, or '2000' series, as designated by the International Alloy Designation System (IADS) originally created in 1970 by the Aluminum Association.

Duralumin sample from the USS Akron (ZRS-4)

The first mass-production aircraft to make extensive use of duralumin, the armored Junkers J.I sesquiplane of WW I.

Duralumin, its composition and heat treatment, was openly published in the German scientific literature before World War I. Despite this, it was not adopted beyond Germany until after World War I. Reports of German use during World War I, even in technical journals such as Flight, could still mis-identify its key alloying component as magnesium rather than copper.[3] In the UK there was little interest in its use until after the war.[4]

The earliest known attempt to use duralumin for a heavier-than-air aircraft structure occurred in 1916, when Hugo Junkers first introduced its use in the creation of the Junkers J 3's airframe, a single-engined monoplane "technology demonstrator" that marked the first use of the Junkers trademark duralumin corrugated skinning. Only the covered wings and tubular fuselage framework of the J 3 were ever completed, before the project was abandoned. The slightly later, solely IdFlieg-designated Junkers J.I armoured sesquiplane, known to the factory as the Junkers J 4, had its all-metal wings and horizontal stabilizer made in the same manner as the J 3's wings had been, along with the experimental and airworthy all-duralumin Junkers J 7 single-seat fighter design, which led to the Junkers D.I low-wing monoplane fighter, introducing all-duralumin aircraft structural technology to German military aviation in 1918.

Its first use in aerostatic airframes was in rigid airship frames eventually including all those of the "Great Airship" era of the 1920s and 1930s: the British built R-100, the German passenger Zeppelins LZ 127 Graf Zeppelin, LZ 129 Hindenburg, LZ 130 Graf Zeppelin II, and U.S. Navy airships USS Los Angeles (ZR-3, ex-LZ 126), USS Akron (ZRS-4) and USS Macon (ZRS-5).[5][6]

Duralumin was used to manufacture bicycle components and framesets from the 1930s to 1990s. Several companies in Saint-Étienne, France stood out for their early, innovative adoption of duralumin: in 1932, Verot et Perrin developed the first light alloy crank arms; in 1934, Haubtmann released a complete crankset; from 1935 on, Duralumin freewheels, derailleurs, pedals, brakes and handlebars were manufactured by several companies.

Complete framesets followed quickly, including those manufactured by: Mercier (and Aviac and other licensees) with their popluar Meca Dural family of models, the Pelissier brothers and their race-worthy La Perle models, and Nicolas Barra and his exquisite mid-twentieth century “Barralumin” creations. Other names that come up here also included: Pierre Caminade, with his beautiful Caminargent creations and their exotic octagonal tubing, and also Gnome et Rhône, with its deep heritage as an aircraft engine manufacturer that also diversified into motorcycles, velomotors and bicycles after World War Two.

Mitsubishi Heavy Industries, which was prohibited from producing aircraft during the American occupation of Japan, manufactured the “cross” bicycle out of surplus wartime duralumin in 1946. The “cross” was designed by Kiro Honjo, a former aircraft designer responsible for the Mitsubishi G4M.[7]

Duralumin use in bicycle manufacturing faded in the 1970s and 1980s. Vitus nonetheless released the venerable “979” frameset in 1979, a “Duralinox” model that became an instant classic among cyclists. The Vitus 979 was the first production aluminium frameset whose thin-wall 5083/5086 tubing was slip-fit and then glued together using a dry heat-activated epoxy. The result was an extremely lightweight but very durable frameset. Production of the Vitus 979 continued until 1992.[8]

Although the addition of copper improves strength, it also makes these alloys susceptible to corrosion. For sheet products, corrosion resistance can be greatly enhanced by metallurgical bonding of a high-purity aluminium surface layer. These sheets are referred to as alclad, and are commonly used by the aircraft industry.[9][10]

Aluminium alloyed with copper (Al-Cu alloys), which can be precipitation hardened, are designated by the International Alloy Designation System as the 2000 series. Typical uses for wrought Al-Cu alloys include:[11]

• 2011: Wire, rod, and bar for screw machine products. Applications where good machinability and good strength are required.
• 2014: Heavy-duty forgings, plate, and extrusions for aircraft fittings, wheels, and major structural components, space booster tankage and structure, truck frame and suspension components. Applications requiring high strength and hardness including service at elevated temperatures.
• 2017 or Avional (France): Around 1% Si.[12] Good machinability. Acceptable resistance to corrosion in air and mechanical properties. Also called AU4G in France. Used for aircraft applications between the wars in France and Italy.[13] Also saw some use in motor-racing applications from the 1960s,[14] as it is a tolerant alloy that could be press-formed with relatively unsophisticated equipment.
• 2024: Aircraft structures, rivets, hardware, truck wheels, screw machine products, and other structural applications.
• 2036: Sheet for auto body panels
• 2048: Sheet and plate in structural components for aerospace application and military equipment