Duralumin (also called duraluminum, duralum, duraluminium, duralium or dural) is the trade name of one of the earliest types of age-hardenable aluminium alloys. Its use as a trade name is obsolete, and today the term is mainly used to describe aluminium-copper alloys, designated as the '2000' series by the International Alloy Designation System (IADS).
Duralumin was developed by the German metallurgist Alfred Wilm at Dürener Metallwerke Aktien Gesellschaft.
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. The name is obsolete today, and mainly used in popular science to describe the Al-Cu alloy system, or '2000' series, as designated by the International Alloy Designation System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.
Its first use was in rigid airship frames eventually including all those of the "Great Airship" era of the 1920s and 1930s: 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). Its composition and heat treatment were a wartime secret. With this new rip-resistant mixture, duralumin quickly spread throughout the aircraft industry in the early 1930s, where it was well suited to the new monocoque construction techniques that were being introduced at the same time. Duralumin also is popular for use in precision tools such as levels because of its light weight and strength.
Duralumin, its composition and heat treatment, was openly published in the German scientific literature before WWI. Despite this, it was not adopted beyond Germany until after WWI. Reports of German use during WWI, even in such erudite technical journals as Flight, could still mis-identify its key alloying component as magnesium rather than copper. Only after the war did interest begin in its use in the UK.
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 Junkers J.I armoured sesquiplane 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.
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.
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:
- 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. 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. Also saw some use in motor-racing applications from the 1960s, 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.
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