For two years he investigated the possible strengthening of Al-Cu alloys by heat treatment, no doubt being frustrated to find that, contrary to carbon steels, these alloys were soft rather than hard after quenching from a high temperature. Then, in 1903, his Centre was commissioned by the German War Munitions factory of Berlin to find an aluminium alloy with the characteristics of brass that could be used for the manufacture of ammunition. By experimenting with an Al-Cu-Mn alloy in 1906, Wilm almost achieved the required strength but the hardness remained too low. He then added 0.5% Mg and prepared some sheet which he heated in a salt bath at 520°C and quenched. This was on a Saturday morning just before closing at noon and Wilm’s assistant, Jablonski, only had time to make a quick hardness measurement before leaving. Wilm himself is said to have spent the remainder of a sunny weekend sailing nearby on the Havel River.
On the following Monday, Jablonski completed his measurements and both men were astounded to find that the hardness was significantly higher than previously recorded. The calibration of the hardness machine was checked and the experiment repeated confirming that the hardness increased for four days, after which it remained constant. A patent was obtained for an
alloy with 3.5-5.5%Cu, plus less than 1% Mg and Mn, and by 1908 experimental work had advanced to a stage that it was considered ready to be released for commercial production. Wilm was allowed to acquire all patent rights and agreed to licence the invention to Durener Metalwerke in Duren, north western Germany. Presumably contractions of the words “Durener” and “aluminium” led to the name “Duralumin” for the alloy, which was copyrighted in 1909, and is still recognized today.
