This application claims benefit of International Application No. PCT/EP99/00939, filed Feb. 12, 1999.
(1) Field of the Invention
The invention relates to a process of treating an aluminum alloy consisting of
0,5-2,5% by weight of an alloying mixture of magnesium and silicon, the molar ration of Mg/Si lying between 0,70 and 1,25,
an additional amount of Si equal to ⅓ of the amount of Fe, Mn and Cr in the alloy, as expressed by % by weight,
other alloying elements and unavoidable impurities, and
the rest being made up of aluminium,
which alloy after cooling has been submitted to homogenising, preheating before extrusion and ageing, which ageing takes place after extrusion as a dual step ageing operation to a final hold temperature between 160xc2x0 C. and 220xc2x0 C.
(2) Description of the Related Art
A process for ageing aluminum alloys containing magnesium and silicon (Alxe2x80x94Mgxe2x80x94Si) is described in WO 95.06769. According to this publication the ageing is performed at a temperature between 150 and 200xc2x0 C., and the rate of heating is between 10-100xc2x0 C./hour preferably 10-70xc2x0 C./hour. As an alternative to this, a two-step heating schedule is proposed, wherein a hold temperature in the range of 80-140xc2x0 C. is suggested in order to obtain an overall heating rate within the above specified range.
It is generally known that higher total amounts of Mg and Si will have a positive effect on the mechanical properties of the final product, whereas this has a negative effect on the extrudability of the aluminium alloy. It has previously been anticipated that the hardening phase in the Alxe2x80x94Mgxe2x80x94Si alloys had a composition close to Mg2Si. However, it was also known that an excess of Si produced higher mechanical properties.
Later experiments have shown that the precipitation sequence is quite complex and that except for the equilibrium phase, the phases involved do not have the stoichiometric ratio Mg2Si. In a publication of S. J. Andersen, et. al, Acta mater, Vol. 46 No. 9 p. 3283-3298 of 1998 it has been suggested that one of the hardening phases in Alxe2x80x94Mgxe2x80x94Si alloys has a composition close to Mg5Si6.
The present invention provides an aluminum alloy and a process for treating the aluminum alloy which results in the alloy having better mechanical properties and better extrudability as compared to traditional aluminium alloys. In particular, the alloy contains 0.5 to 2.5% by weight of an alloying mixture of magnesium and silicon, in which the molar ratio of Mg/Si is 0.70 to 1.25, the alloy optionally containing an additional amount of silicon up to about ⅓ of any iron, manganese and chromium in the alloy, as expressed by weight percent, the balance of the alloy being aluminum, optional alloying elements and unavoidable impurities. The process for treating this alloy entails an ageing technique that includes a first stage in which an extrusion of the aluminum alloy is heated with a heating rate above 100xc2x0 C./hour to a temperature between 100-170xc2x0 C., a second stage in which the extrusion is heated with a heating rate between 5 and 50xc2x0 C./hour to a final hold temperature, and in that the total ageing cycle is performed in a time of 3 to 24 hours.