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Innovative aluminium alloys

Recent R & D has shown that low Cu additions in the order of 0.30 to 0.50 to the basic AlSi7Mg0.6 combined to specific aging treatments lead to a potentially attractive compromise, while the resistance to stress corrosion cracking is still good (the alloy passed the ASTM test of 60 days under a stress equal to 75% of the YS).

A further reduction in silicon to 5% appears to improve the elongation without reducing the strength. This small addition of Cu does not materially alter the Fs (T) curves around 50% solid, but leads to lower final eutectics. Tendency to micro shrinkage will have to be checked.

The reduction of Si from 7 to 5 leads to a solid fraction of » 65% if one chooses to operate the NRC process just above the eutectic at 575°C. Another possibility would be to work at 590°C to retain 50% liquid, which would ultimately solidify as a mixture of fine µAl dendrites + µAl-Si eutectic.

AlSi7CuMg, code named ¿87S1¿, and AlSi5CuMg, ¿85S1¿, should be tested on the automobile components of this program.

The alloy 25Z used for the experiments has been obtained from the alloy Al Cu5MgTi received from Pechiney (500 kg about), by adding pure Zn, Mn 75% mother alloy and Cu 33% mother alloy. The initial composition of the ingots received, the final molten bath composition and the theorical composition indicated by Pechiney are indicated in table 4: 2 elements are out of tolerance, the Si due to the small quantity of 87S alloy remained into the holding furnace and the iron, already present into the ingots.

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