Magnesium based alloy

Abstract

A magnesium based alloy consisting of, by weight: 2-5% rare earth elements, wherein the alloy contains lanthanum and cerium as rare earth elements and the lanthanum content is greater than the cerium content; 0.2-0.8% zinc; 0-0.15% aluminium; 0-0.5% yttrium or gadolinium; 0-0.2% zirconium, 0-0.3% manganese; 0-0.1% calcium; 0-25 ppm beryllium; and the remainder being magnesium except for incidental impurities.

Description

FIELD OF THE INVENTION[0001]The present invention relates to magnesium based alloys and, more particularly, to magnesium based alloys which can be cast by high pressure die casting (HPDC).BACKGROUND TO THE INVENTION[0002]With the increasing need to limit fuel consumption and reduce harmful emissions into the atmosphere, automobile manufacturers are seeking to develop more fuel efficient vehicles. Reducing the overall weight of vehicles is a key to achieving this goal. Major contributors to the weight of any vehicle are the engine and other components of the powertrain. The most significant component of the engine is the cylinder block, which makes up 20-25% of the total engine weight. In the past significant weight savings were made by introducing aluminium alloy cylinder blocks to replace traditional grey iron blocks, and further weight reductions of the order of 40% could be achieved if a magnesium alloy that could withstand the temperatures and stresses generated during engine op...

AI Technical Summary

Benefits of technology

[0026]The total lanthanum and cerium content of the alloy is preferably 1.5-3.5% by weight, more preferably 1.8-3.0%, and most preferably 2.0-2.8%. Without wishing to be bound by theory, the lanthanum and cerium improve the castability and also the creep strength of the alloy. Again, without wishing to be bound by theory, a greater lanthanum content than cerium content further improves the castability of the alloy, particularly the hot tearing resistance of the alloy. Higher ratios of lanthanum to cerium typically give the alloy greater ductility and even greater resistance to hot tearing. Typically, a higher total lanthanum and cerium content is beneficial to the creep resistance of the alloy with a concomitant reduction in the ductility of the alloy.

[0027]The rare earth element content of the alloy may optionally contain neodymium, in which embodiment the rare earth element content is predominantly lanthanum, cerium and neodymium. Without wishing to be bound by theory, the inclusion of neodymium improves the creep resistance of the alloy. However, the neodymium content of the alloy may be reduced to improve the castability of the alloy, in particular its resistance to hot tearing. When present, the neodymium content is preferably 0.5-2.0% by weight of the alloy, more preferably 0.5-1.5% by weight, more preferably about 1% by weight.

[0030]Yttrium is an optional component which may be included. Without wishing to be bound by theory, the inclusion of yttrium is believed to be beneficial for both melt protection and creep resistance. However, the yttrium content of the alloy may be reduced to improve the castability of the alloy, in particular its resistance to hot tearing. When present, the yttrium content is preferably 0.005%-0.5% by weight, more preferably 0.01-0.4% by weight, more preferably 0.05-0.3% by weight, and most preferably 0.1-0.2% by weight.

[0035]The aluminium content is preferably 0.05-0.15% by weight, more preferably 0.08-0.12% by weight, more preferably about 0.1% by weight. Without wishing to be bound by theory, the inclusion of these small amounts of aluminium in the alloys of the present invention is believed to improve the creep properties of the alloys.

[0036]The beryllium content is 0-25 ppm. When present, the beryllium content is preferably 4-20 ppm, more preferably 4-15 ppm, more preferably 6-13 ppm, such as 8-12 ppm although beryllium is preferably absent when yttrium is present as yttrium has a similar effect to beryllium at low yttrium levels. When present, beryllium would typically be introduced by way of an aluminium-beryllium master alloy, such as an Al-5% Be alloy. Without wishing to be bound by theory, the inclusion of beryllium is believed to improve the die castability of the alloy. Again, without wishing to be bound by theory, the inclusion of beryllium is also believed to improve the oxidation resistance of the molten alloy and in particular improves the retention of the rare earth element(s) in the alloys against oxidation losses.

Problems solved by technology

Alloys according to WO2006 / 105594 have demonstrated excellent high temperature creep properties but have proven somewhat difficult to die cast.

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