Magnesium alloy and method of producing the same

Details Magnesium alloy and method of producing the same Magnesium alloy and method of producing the same

1999-07-30

2002-05-28

Ip, Sikyin (Department: 1742)

Alloys or metallic compositions

Magnesium base

Manganese containing

C420S407000, C420S409000, C420S412000, C420S410000, C148S538000, C148S666000, C148S420000

Reexamination Certificate

active

06395224

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a magnesium alloy, and more particularly to a magnesium alloy having fine grain structure and a method of producing the same.
2. Description of the Related Art
Grain refinement of a magnesium alloy is extremely effective in improving mechanical properties of the magnesium alloy. A variety of attempts have been made and the following three methods have been put into practical use.
(1) The Method of Adding Zirconium to a Magnesium Alloy
This method is effective in refining grain, and magnesium alloys containing zirconium are standardized, for example, in the ASTM.
(2) The Method of Adding C
2
Cl
4
(tetrachloroethylene), C
2
Cl
6
(hexachloroethane) or the Like to a Magnesium-aluminum Alloy
It is supposed that Al
4
C
3
is formed by adding these carbon-containing compounds, and that the grain refinement of the magnesium alloy is realized by this aluminum carbide.
(3) The Method of Superheating a Molten Magnesium Alloy Just Before Pouring it Into a Mold.
It is assumed that in this method of superheating, rapid cooling and pouring a molten magnesium alloy in a mold, grain is refined by the effect of the iron and/or carbon which have dissolved from a crucible or the like into a molten magnesium alloy
These methods can attain the grain refinement of a magnesium alloy. However, there remain following problems.
The method (1) is highly effective in grain refining of a magnesium alloy, but is available only for alloys not containing aluminum. When zirconium is added to an alloy containing aluminum, its refining effect disappears. This is because zirconium reacts with aluminum to form a compound. The zirconium-aluminum compound does not have an effect of refining the grain of the magnesium alloy. In addition, zirconium is often added into a molten magnesium alloy, using a mgnesium-zirconium mother alloy. This mother alloy is prepared from magnesium ingots and zirconium chloride based flux. In this case, harmful chloride gas generates during the addition process. Also the chloride remaining in the magnesium alloy causes corrosion of the magnesium alloy. In addition, when the molten magnesium alloy is held quietly in order to remove impurities, zirconium also settles (or precipitates) and is removed from the magnesium alloy. As a result, its refining effect disappears.
The method (2) of adding tetrachloroethylene, hexachloroethane, or the like has a similar chloride gas problem to that of the method (1) of adding zirconium. That is to say, harmful chloride gas generates and the chloride remaining in the magnesium alloy causes corrosion. In addition to this, when the molten magnesium alloy is held quietly in order to remove impurities, carbon volatilizes from the magnesium alloy and the refining effect decreases.
In the method (3) of superheating, rapid cooling and pouring a molten magnesium alloy into a mold, there is a need to superheat a molten magnesium alloy to approximately 900° C. and then take out a crucible from a melting furnace and rapidly cool the molten alloy to around a temperature at which the molten alloy is suitable to be poured into a mold. Thus, the casting procedure becomes very complicated. In addition, the grain is not refined when the cooling rate of the molten alloy is not high enough. Also, unless the molten alloy is poured into a mold immediately after being rapidly cooled, the grain is not refined. Also, the obtained magnesium alloy deteriorates in corrosion resistance because the iron content in the obtained magnesium alloy increases.
Further, Japanese Unexamined Patent Publication (KOKAI) No. H9-157782 and Japanese Patent Registration No.2705844 disclose magnesium alloys containing 0.001 to 0.02 weight % boron.
The magnesium alloy disclosed in the former publication contains 0.5 to 2 weight % zirconium. Accordingly, there still exist the above-described problems such as chloride gas generation, corrosion caused by chlorides, grain refining effect disappearance caused by holding the molten magnesium alloy, and the former publication discloses no solution for these problems. Besides, because zirconium reacts with aluminum to form a compound, zirconium does not contribute to grain refinement of widely used magnesium-aluminum alloys.
On the other hand, the magnesium alloy disclosed in the latter publication contains 0.005 to 0.1 weight % titanium. The present inventors' examinations and researches demonstrate that titanium does not refine the grain of magnesium alloys. When titanium and boron are contained simultaneously in magnesium alloys, an adverse effect of increasing the grain size of the magnesium alloys was observed.
SUMMARY OF THE INVENTION
The present invention has been conceived in view of the above problems. It is an object of the present invention to provide a magnesium alloy which can have fine grain easily under ordinary casting conditions, and which can maintain fine grain even when the molten alloy is held at high temperatures for a long time with the purpose of removing impurities and inclusions or waiting until casting is carried out, or even after the solidified alloy is melted again.
In order to achieve this object, the present inventors have conducted intensive researches and various systematic experiments. As a result, the present inventors have found and confirmed that the grain of a magnesium alloy can be refined by including boron and manganese at the same time.
The present inventors have also established a method of producing a magnesium alloy having fine grain by cooling and solidifying a molten magnesium alloy which has been prepared to include boron and manganese.
(Magnesium Alloy)
The magnesium alloy according to the present invention comprises magnesium as a main component, boron of 0.0005 weight % or more, manganese of 0.03 to 1 weight %, and substantially no zirconium and no titanium.
Boron is highly effective in refining grain of magnesium alloys. The boron of 0.0005 weight % or more is necessary for grain refinement because when the boron content is less than 0.0005%, almost no grain refining effect is obtained.
In order to obtain a sufficient grain refining effect, it is suitable to set the boron content to be 0.001 weight % or more.
On the other hand, an increase in the boron content does not cause any problem in refining grain. However, excessive boron settles (precipitates) as a compound, and boron sediment (or precipitation) does not contribute to grain refinement. Also, an increase in the boron content is disadvantageous in terms of production costs. The present inventors have confirmed that the effect of grain refinement almost reaches the maximum when the boron nominal content is approximately 0.5%. Accordingly, it is preferable to set the boron content to be 0.5% or less.
Therefore, it is more preferable that the boron content is in a range from 0.001 to 0.5 weight %.
Manganese is an element which is necessary to achieve the grain refinement of a magnesium alloy when boron is contained in the magnesium alloy simultaneously. When the manganese content is less than 0.03 weight %, almost no grain refining effect is obtained.
On the other hand, when the manganese content exceeds 1 weight %, excess manganese exists as insoluble manganese or an insoluble manganese compound, and the insoluble manganese or manganese compound absorb boron and make the grain refining effect of boron decrease or disappear. Therefore, it is preferable to set the manganese content in a range from 0.03 to 1 weight %.
In order to refine the grain of a magnesium alloy further and efficiently, it is more suitable to set the manganese content in a range from0.05 to 0.8 weight %.
The details of the mechanism as to how grain is refined have not been clarified yet, but it is supposed that the grain refining effect largely attributes to a multiplier effect of boron and manganese. This will be discussed later.
Zirconium unfavorably reacts with aluminum to form a compound, so zirconium has no grain refining effect in a magnesium alloy including aluminum. Besides, zirconium is

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