For the production of its tube-sets in aluminum Columbus uses light alloys like Al-Zn-Mg (series 7000) and Al-Si-Mg (series 6000). The classification derives from the American Aluminum Association. The alloy elements: Mg = magnesium, Zn = zinc and Si = silicium that, properly combined between each other, form intercrystalline compounds which give the mechanical properties to the alloy. Both alloys acquire their maximum mechanical properties through a heat treatment cycle.



The alloy 7000, used by Columbus for the tube sets Metal Altec2 Plus, Zonal and Airplane, is defined as self-tempering: it is air-hardened. This means that in the areas overheated by the welding the supersaturated structure is restored, which, owing to the natural ageing (which takes place at ambient temperature), allows for a 75% recovery of the initial properties after a lapse of time of about 3 weeks. Nevertheless, Columbus advises to carry out a precipitation hardening treatment in the oven in order to give the structure a bigger homogeneity of the precipitates, with consequent improvement of the fatigue behaviour of the frame. The special alloy of the series 6000, used for the Starship series, is not self-tempering and thus necessitates the heat-treatment of the welded frame. In this way, the distribution of the precipitates is homogeneous even in the areas which have been heated by the welding and, by means of the hardening in the oven, load values are reached which make it possible to realize extremely light and reliable frames. The alloy used for the XLR8R represents the most recent product at the top of the Columbus range together with Starship. This is a sophisticated development of the 7000 Al-Zn-Mg, which through the addition of Zirconium (element used in nuclear reactors, to give an example of just one of its uses), achieving highly superior mechanical and resistance characteristics. The zirconium modifies the kinetics of the intermetallic composites in the matrix during the precipitation stage, conferring a very fine grain to the structure. In practical terms this means:

a)	Superior mechanical characteristics of the frame: Rm=550 Mpa, Rs=510 Ap5>12%
b)	Increase of the recrystallization temperature
c)	Greater resistance to the Hot Cracking phenomenon – the weakening of the material due to thermal alterations caused by welding
d)	Near complete recovery of the mechanical characteristics after welding through the use of a simple artificial aging thermal treatment.

For an idea of this alloy’s degree of sophistication, consider that it is produced in a controlled environment (i.e. the equipment is meticulously cleaned before and after processing to avoid contamination by other components), and that heat treatment and filler materials were developed specifically for the welding.