Several types of Al-Cu-Mg alloys including other metals and elements, such as silicon, have already been proposed for the manufacture of drawn materials.
For example, the alloy AU.sub.2 G additionally contains very small quantities of other constituents, namely 2 to 2.80% Cu, 0.2 to 0.6% Mg and less than 0.4% Si. This alloy, which is used to a certain extent in the motor industry, sometimes has insufficient drawing capacity when the shape of the desired part is complicated. Furthermore, its mechanical characteristics under traction, which generally are good for the matured, hardened state, are easily altered when the shaped part is subjected to heating, for example, during the baking of paint in the case of car body panels. A loss of yield strength and of tensile strength of the order of 4 Kg/mm.sup.2 is observed after treatment for 15 minutes to 3 hours at 150.degree. to 180.degree. C. Finally, this alloy does not allow good welding of the shaped parts, since it is very susceptible to shrinkage cracking during welding with an oxyacetylene blow-pipe or under neutral gas.
Applicants have disclosed, in French Patent Specification No. 7,239,081, filed Oct. 29, 1971, Al-Cu-Mg alloys which have a greater proportion of Mg, this is to say 1.80 to 2.60%. These alloys have a range of interesting properties but also like the AU.sub.2 G alloy mentioned above, have disadvantages in relation to weldability and shaping in difficult cases.
Altenpohl, in his work "Aluminum und Aluminum-legierungen" has referred (pages 710 to 713), as being a product suitable for drawn pieces, to an Al-Cu-Mg alloy, which conforms with the German Standard DIN 1725 Bl 1 and contains 2 to 3% Cu and up to 0.5% Mg and possibly also includes silicon (less than 0.8%). As will be shown in the present description, alloys of this type or those in which the quantity of Mg can be increased to 1.5% (according to Altenpohl), cannot be regarded as suitable for drawn sheet-metal bodywork panels, since the resultant parts cannot be both hemmed and welded.
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