Pressure die-casting process of magnesium alloys

Details Pressure die-casting process of magnesium alloys Pressure die-casting process of magnesium alloys

2000-06-05

2002-10-22

Elve, M. Alexandra (Department: 1725)

Metal founding

Process

Shaping liquid metal against a forming surface

C164S133000

Reexamination Certificate

active

06467527

ABSTRACT:

BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to a process for pressure die-casting a magnesium alloy excellent in the low-temperature strength and high-temperature strength and more specifically to a process for pressure die-casting a magnesium alloy having sufficient strength even at a high temperature up to about 523K, which is required for reducing the weight of, for instance, parts for motorcar engines.
(b) Description of the Prior Art
There has recently been required for the improvement in the fuel consumption of motorcars because of the increased demand for the environmental protection of the earth and accordingly, there has strongly been required for the development of lightweight materials for motorcars.
The magnesium alloy has the lowest density among the metal materials, which have recently been put into practical use, and has intensively been expected as a future lightweight material for motorcars. The magnesium alloys, which have most commonly been used, are Mg—Al—Al—Mn alloys such as AZ91 alloys (Mg—9Al—0.7Zn—0.2Mn) or AM50 alloys (Mg—5Al—0.2Mn). The peripheral techniques such as the techniques for casting these alloys have almost been completely established and therefore, this alloy has, first of all, been investigated in order to reduce the weight of motorcars.
However, the strength of these alloys are reduced at a temperature of not less than 393K and these magnesium alloys are not suitable for the applications such as parts of engines, which must have desired heat resistance and creep resistance at a temperature of up to about 473K. Under such circumstances, there have been developed magnesium alloys, to which silicon is added, such as AS41 (Mg—4Al—1Si—0.2Mn); those, to which rare earth metals are added, such as AE42 alloys (Mg—4Al—2RE—0.2Mn (wherein RE represents a rare earth metal)). Moreover, there has more recently been developed and proposed magnesium alloys, to which calcium is added (Japanese Un-Examined Patent Publication (hereunder referred to as “J.P. KOKAI”) No. Hei 6-25790); and those containing calcium and rare earth metals (J.P. KOKAI Nos. Hei 6-200348 and Hei 7-11347).
All of these magnesium alloys possess the predetermined heat resistance and creep resistance required for the parts of motorcar engines whose temperature is raised up to about 473K or even to a level on the order of about 523K, but they have not been widely used practically except for a part thereof. This is because these alloys are susceptible to hot tearing, shrinkage tearing and/or are liable to form shrinkage cavities and further suffer from problems such as the entrainment of oxides and the formation of film play (wrinkles) on the surface of castings. More specifically, elements such as silicon, rare earth metals and calcium are quite active and they are liable to form oxides and compounds are formed at grain boundaries. For this reason, it would be assumed that these alloys are highly susceptible to hot tearing and shrinkage tearing. However, the most fundamental cause of these drawbacks would be such that there has not yet been elucidated any casting conditions, while taking measures to eliminate these problems.
SUMMARY OF THE INVENTION
The present invention has been developed to solve the problems associated with the conventional techniques for developing lightweight materials for motorcars and accordingly, it is an object of the present invention to provide a process for casting, according to the pressure die-casting (hereunder simply referred to as “die-casting”) process, a die cast product, which is free of any hot tearing, shrinkage tearing and/or shrinkage cavity, using a cold chamber type die-casting machine, starting from a high temperature magnesium alloy, which is suitable as a material for preparing parts for motorcar engines, which should satisfy both requirements for high temperature strength and low temperature strength.
The inventors of this invention have conducted various studies to accomplish the foregoing object. As a result, the inventors have found that the problems concerning hot tearing, shrinkage tearing and shrinkage cavity conventionally observed when casting a high temperature magnesium alloy can be solved and the problems concerning the entrainment of oxides and the formation of film play on the surface of castings can also be solved, by casting a die cast product from a magnesium alloy having a specific alloy composition under specific casting conditions using a cold chamber type die-casting machine. Consequently, the inventors have completed the present invention on the basis of the foregoing finding.
According to a first aspect of the present invention, there is provided a die-casting process for casting a magnesium alloy, which is a method for casting a die cast product free of any hot tearing, shrinkage tearing and formation of shrinkage cavities, starting from a magnesium alloy, which comprises i) 1 to 10% by weight of aluminum; ii) at least one member selected from the group consisting of 0.2 to 5% by weight of a rare earth metal, 0.02 to 5% by weight of calcium, and 0.2 to 10% by weight of silicon and iii) not more than 1.5% by weight of manganese and the balance of magnesium and inevitable impurities, using a cold chamber type die-casting machine, wherein a) the temperature of the molten magnesium alloy is maintained at 650 to 750° C.; b) the charging velocity of the molten metal is set at {fraction (1/100)} to {fraction (10/100)} sec; and c) the intensified pressure after the charging is set at a level of not less than 200 kgf/cm
2
.
According to a second aspect of the present invention, there is provided a die-casting process for casting a magnesium alloy, which is a method for casting a die cast product free of any hot tearing, shrinkage tearing and formation of shrinkage cavities, starting from a magnesium alloy, which comprises i) 1 to 10% by weight of aluminum; ii) at least one member selected from the group consisting of 0.2 to 5% by weight of a rare earth metal, 0.02 to 5% by weight of calcium, and 0.2 to 10% by weight of silicon and iii) not more than 1.5% by weight of manganese and the balance of magnesium and inevitable impurities, using a cold chamber type die-casting machine, wherein d) the temperature of the mold is maintained at 150 to 350° C. e) the surface temperature of the mold at its cavity portions in which the die cast product is susceptible to shrinkage tearing is reduced by not less than 10K compared with the temperature of the peripheral portions of the mold; f) the air pressure in the mold during the die-casting step is controlled to a level of not more than 100 mmHg; and g) an additive for a releasing agent to be applied onto the internal wall of the mold is at least one member selected from the group consisting of graphite, BN, water glass, mica, silica gel, magnesium hydroxide and magnesium oxide.
According to a third aspect of the present invention, there is provided a die-casting process for casting a magnesium alloy, which is a method for casting a die cast product free of any hot tearing, shrinkage tearing and formation of shrinkage cavities, starting from a magnesium alloy, which comprises i) 1 to 10% by weight of aluminum; ii) at least one member selected from the group consisting of 0.2 to 5% by weight of a rare earth metal, 0.02 to 5% by weight of calcium, and 0.2 to 10% by weight of silicon and iii) not more than 1.5% by weight of manganese and the balance of magnesium and inevitable impurities, using a cold chamber type die-casting machine, wherein any oxidation of the molten magnesium alloy is inhibited, the flowability of the molten metal is improved and the entrainment of oxides and the formation of film play are prevented by h) using a melting furnace in which a closed protective atmosphere for inhibiting combustion and oxidation is established over the surface of the molten magnesium alloy and i) pumping out the molten magnesium alloy at a portion not less than 100 mm apart from the surface of the molten alloy.
The die-casting method for casting a magnes

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