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Aluminum alloy for casting

1. An aluminum alloy for casting purposes which consists essentially of over 6% and up to 13% by weight of silicon, over 2% and up to 5% by weight of copper, over 0.25% and up to 1% by weight of magnesium, over 0.1% and up to 0.5% by weight of nickel and over 0.03% and up to 1% by weight of antimony, the balance being aluminum and impurities, said copper and said magnesium having a ratio by weight of about 3:1 to 8:1.
2. An alloy as set forth in claim 1, further including at least one member selected from the group consisting of; more than 0.1% but not more than 0.5% by weight of zirconium; more than 0.1% but not more than 1% by weight of manganese.
3. An alloy as set forth in claim 1, containing more than 0.03% but not more than 2.0% by weight of titanium.
4. An alloy as set forth in claim 2, containing more than 0.03% but not more than 2.0% by weight of titanium.

Aluminum has recently come to use for a wide range of applications including the manufacture of vehicles and machines. As a result, there has been a strong need for an aluminum alloy for casting purposes which is superior in strength and heat resistance.

The inventors of this invention have succeeded in developing a previously proposed (hereinafter indicated as previous alloy) casting alloy which is superior in castability, strength and heat resistance, as disclosed in Japanese Laid-Open Patent Specification No. 69234/1980. The previous alloy contains over 6% and up to 13% of silicon, over 3% and up to 5% of copper, over 0.2% and up to 1% of magnesium and over 0.03% and up to 1% of antimony, the balance being aluminum and impurities. It has a maximum strength which is as high as 40 kg/cm 2 , and an elongation of 3 to 4%, and is by far superior in thermal shock resistance to any conventionally known alloy of this kind**. It is, therefore, suitable for use in the manufacture of a part of a machine which repeatedly exposed to intense heat, for example, a piston in an engine. FNT **(for example, Japanese Industrial Standard for aluminum casting alloys "AC8A" and "AC8B")

Further research of the inventors has, however, indicated that the previous alloy has a number of defects. They have found that if a piston made of this alloy is used for a long time, that portion of the piston which has been exposed to heat repeatedly undergoes a permanent volumetric shrinkage which is different from that resulting from ordinary thermal expansion and contraction, and that the shrinkage enlarges the clearance between the piston and the cylinder, resulting in a blowby or piston slap. Moreover, the alloy is liable to lamellar abrasion, for example, in a groove in which a piston ring is fitted, resulting in failure of the ring to function properly.

SUMMARY OF THE INVENTION

It is an object of this invention to improve the drawbacks of the alloy as hereinabove described, while maintaining its excellent properties, and thereby provided an aluminum alloy which is suitable for use in the casting of machine parts which are trouble-free even after exposure to heat for a long time.

The inventors have found that the addition to the previous alloy of nickel in a quantity of over 0.1% and up to 0.5% by weight and of copper and magnesium in a ratio by weight of about 3:1 to 8:1 is effective for preventing the aforesaid volumetric shrinkage and improving the wear resistance of the alloy without causing any appreciable reduction in its excellent properties, including strength and thermal shock resistance.

According to this invention, there is, thus, provided an aluminum alloy for casting purposes which contains over 6% and up to 13% by weight of silicon, over 2% and up to 5% by weight of copper, over 0.25% and up to 1% by weight of magnesium, over 0.1% and up to 0.5% by weight of nickel and over 0.03% and up to 1% by weight of antimony, the balance being aluminum and impurities, the copper and magnesium having a ratio by weight of about 3:1 to 8:1.

The alloy of this invention is excellent in thermal properties including thermal shock resistance, and resistance to lamellar abrasion, and substantially free from any permanent volumetric shrinkage even after long exposure to high temperatures. Therefore, it is suitable for use in the manufacture of machine parts which have to be exposed to high temperatures, for example, a piston in an engine.

Copper contributes to improving the strength of the alloy by artificial aging. If only 2% by weight or less of copper is employed, however, it is impossible to expect any effective improvement of the alloy strength, while the use of 5% by weight or more of copper should also be avoided, since too large a quantity of an intermetallic compound formed between aluminum and copper, which is not in the form of a solid solution, is likely to remain in the matrix, and cause a reduction in the toughness and fatigue resistance of the alloy, and a higher susceptibility of a casting to cracking.

Magnesium contributes to improving the strength of the alloy, since an intermetallic compound composed of magnesium and silicon, or aluminum, copper and magnesium is precipitated by artificial aging. If only 0.25% by weight or less of magnesium is employed, however, such precipitation may not take place in a sufficient quantity, while the use of magnesium in a quantity above 1% by weight should also be avoided, since it brings about a drastic reduction in the toughness and thermal shock resistance of the alloy, and seriously impairs the effect of antimony on the alloy structure.