Alloys or metallic compositions – Magnesium base – Lanthanide containing
Reexamination Certificate
2000-06-16
2001-10-09
Ip, Sikyin (Department: 1742)
Alloys or metallic compositions
Magnesium base
Lanthanide containing
C148S420000
Reexamination Certificate
active
06299834
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a heat-resistant magnesium alloy and, more particularly, to a heat-resistant magnesium alloy with excellent castability.
2. Description of Related Art
Recently, there has been increased need for the weight reduction of industrial materials, and as the lightweight material, magnesium alloys lighter than aluminum alloys have drawn researchers attention. Magnesium alloys are the lightest among commercial metals, and now being used as the material for automobiles as well as air planes. For example, magnesium alloys have been already used as the material for road wheels and engine head covers of automobiles.
In addition, there has been great need for the weight reduction of all automobile parts, and the application range of magnesium alloys tends to be enlarging further. For example, it has been proposed to apply magnesium alloys even to structural parts such as engine blocks or the like, and functional parts such as pistons or the like, of which the temperatures tend to be elevated. If the pistons, for example, which have conventionally been made of aluminum alloys are made of magnesium alloys, the weight thereof could be reduced, and the inertia d force or the like could be also reduced, thereby enabling weight reduction of other parts, too.
Magnesium alloy products are generally produced by casting procedure including die-casting. So, to enlarge the application range of magnesium alloys, the improvement of castability thereof is needed. Furthermore, to reduce the mass production costs, the improvement of yield rate is also needed by restraining and preventing the occurrence of casting cracks (not cracking or not tears) or any casting defects.
Under these circumstances, various kinds of magnesium alloys have been developed. Japanese Unexamined Patent Publication No. Sho 61(1986)-3863 discloses Mg—Al—Si—Mn alloys and Japanese Unexamined Patent Publication Nos. Hei 6(1994)-25791 and Hei 7(1995)-18364 disclose Mg—Zn—Ca alloys, which exhibit improved heat resistance superior to that of conventional Mg—Al—Zn—Mn alloys.
In addition, various kinds of Mg—RE—Zn alloys (RE: rare earth element(s)), which are different from the above-described alloys, have been also developed. Mg—RE—Zn alloys have been reported to exhibit good heat resistance and castability superior to those of the Mg—Zn—Ca alloys, and are disclosed in British patent publications Nos. P637040, P1378281, Japanese Unexamined Patent Publication No. Hei 9(1997)-256099 and Publication of translation of International patent application No. Hei 10-513225 (WO96/24701), etc. British patent publications Nos. P-637040 and P1378281 disclose that the addition of 0.1 to 0.9% by weight of zirconium as a grain-refining element is preferable.
Furthermore, Japanese Unexamined Patent Publication No. Hei 9(1997)-256099 discloses that 0.5 to 3% by weight of calcium can restrain the occurrence of casting cracks in the conventional Mg—Zn—Ca alloys.
Upon further investigations and researches on the magnesium alloys disclosed in Japanese Unexamined Patent Publication No. Hei 9(1997)-256099, the present inventors have clarified that in the case of magnesium alloys, when casting strain is small during casting, no crack occurs, but when casting strain becomes greater during casting, cracks may occur. The reason of the occurrence of casting strain can be considered as follows.
During casting, thermal contraction occurs due to solidification shrinkage and cooling of cast products. This thermal contraction causes the occurrence of casting strain. Provided that the cooling process is identical, the amount of casting strain depends only on the configuration of castings. For example, at corners or the like, under strong constraint, the amount of casting strain increases. However, actually, the cooling process varies from place to place due to the variation of thickness and the casting design in addition to the configuration of casting. Consequently, in low cooling rate parts, a larger casting strain may occur, and consequently cracks may occur, as compared to high cooling rate peripheral parts. Thus, casting cracks may occur in ribs, bosses and corners where the thickness greatly varies.
Publication of translation of international patent application No. Hei 10-513225 (WO96/24701) discloses that calcium operates as a castability modifier, but does not disclose any specific embodiment. This publication merely discloses that the calcium content ranges from 0 to 1% by weight. Furthermore, the present inventors have recognized that calcium makes ignition-resistant molten alloys, and consequently serves to improve the work efficiency in melting and casting operations, but may cause defects such as casting cracks, resulting in cast products being difficult to form.
Upon these investigations by the present inventors, no conventional technique of severely adjusting the calcium content to highly effect both the heat resistance and castability of Mg—RE—Zn alloys has been found yet.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide heat-resistant magnesium alloys capable of exhibiting excellent heat resistance and castability of Mg—RE—Zn alloys.
To attain this object, the present inventors have earnestly researched and carried out various systematic experiments. As a result, we have found that by properly adjusting the calcium content in magnesium alloys, Mg—RE—Zn alloys capable of exhibiting improved heat resistance and castability can be obtained, and have developed heat-resistant magnesium alloys of the present invention.
The heat-resistant magnesium alloy in accordance with the present invention contains 1.0 to 6.0% by weight of zinc, 0.4 to 1.0% by weight of zirconium, 1.5 to 5.0% by weight of rare earth element(s), up to 0.3% by weight of calcium, magnesium being as the balance, and unavoidable impurities.
With the present invention, by mixing proper amounts of zinc, rare earth element(s), zirconium and calcium, both highly good heat resistance and castability of magnesium alloys can be effected, as have conventionally been difficult to be done.
Especially, by mixing a proper amount of calcium, the heat resistance can be improved, as compared to those of the conventional alloys, and by adjusting the calcium content to less than those of the conventional alloys, the casting cracks can be restrained, as compared to the conventional Mg—RE—Zn alloys.
The rare earth element(s) to be included in the heat-resistant magnesium alloy in accordance with the present invention are elements capable of improving the heat resistance due to their solid solution and crystallization and precipitation in grain boundaries. When the rare earth element (s) content is less than 1.5% by weight, the heat resistance cannot be sufficiently improved. On the other hand, when the rare earth element(s) content exceeds 5.0% by weight, the toughness deteriorates to cause the casting cracks.
To improve the heat resistance sufficiently, and ensure high toughness, the rare earth element(s) content is preferably adjusted to 1.5 to 4.0% by weight, and more preferably 2.0 to 4.0% by weight.
The rare earth element(s) include lanthanum, cerium, praseodymium, neodymium or the like. One of these rare earth element(s) or a mixture of at least two of them will do as the rare earth element(s) to be included in the heat-resistant magnesium alloy of the present invention. Furthermore, mischmetal which is a mixture of rare earth element(s) such as lanthanum, cerium, praseodymium, neodymium or the like may be used.
Zinc serves to strengthen the &agr;-Mg phase as a based phase with its solid solution strengthening, and improve the strength of the magnesium alloy at room temperature.
When the zinc content is less than 1.0% by weight, the static strength of the magnesium alloy remarkably decreases so as to be less practical. On the other hand, when the zinc content exceeds 6.0% by weight, the amount of the solid solution of zinc increases to accelerate the diffusion, and a large amount of fusible crysta
Horie Toshio
Iwahori Hiroaki
Kitano Tomoyasu
Sugiyama Yoshio
Suzuki Takamasa
Ip Sikyin
Kabushiki Kaisha Toyota Chuo Kenkyusho
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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