Magnesium alloy for use in casting and having a narrower solidification temperature range

A magnesium alloy for use in casting is disclosed herein, which contains zinc and a rare earth metal component and has a solidification temperature range of at most 50.degree. C. The magnesium alloy comprises 8.5 to 1.9 % by weight of a rare earth metal mixture consisting essentially of cerium and lanthanum as the rare earth metal component, 6.4 to 4.2% by weight of zinc, and the balance of magnesium, based on the total weight of the magnesium alloy.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a magnesium alloy improved in castability 
by having a narrower solidification temperature range of at most 
50.degree. C. 
2. Description of the Prior Art 
Magnesium alloys are lightweight, and some magnesium alloys have sufficient 
strength. However, the magnesium alloys have a wider solidification 
temperature range, i.e., a wider solid-liquid coexistence temperature 
range. For this reason, they are liable to produce cracks in casting, and 
particularly, it is difficult to produce a large-sized product in a 
casting manner. Therefore, no prior art has succeeded in industrially 
carrying out the manufacture of a relatively large-sized cast product made 
of a magnesium alloy in spite of the many efforts by those skilled in the 
art. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a 
magnesium alloy suitable for use in casting and having a narrower 
solidification temperature range so that the casting thereof can be easily 
carried out and no cracks will be produced in it. 
The present inventors have found that the above object can be achieved by 
providing a magnesium alloy containing a specified amount of zinc and a 
specified amount of a rare earth metal mixture having a specified 
composition. 
Thus, according to the present invention, there is provided a magnesium 
alloy for use in casting, which contains zinc and a rare earth metal 
component and has a solidification temperature range of at most 50.degree. 
C., said magnesium alloy comprising 8.5 to 1.9% by weight of a rare earth 
metal mixture consisting essentially of cerium and lanthanum as the rare 
earth metal component, 6.4 to 4.2% by weight of zinc, and the balance of 
magnesium, based on the total weight of the magnesium alloy. 
The rare earth metal mixture contained in the alloy of the present 
invention may consist essentially of cerium and lanthanum, but it is 
particularly preferable that the mixture consists of at least 55% by 
weight of cerium, at least 18% by weight of lanthanum, and the balance of 
praseodymium and/or neodymium, based on the total weight of the mixture. 
With the magnesium alloy of the present invention, it is possible to 
suppress production of cracks which may often be produced with the prior 
art magnesium alloy and to produce a lightweight magnesium alloy product 
in a casting manner regardless of the size. This significantly contributes 
to the development of the industry. 
DETAILED DESCRIPTION OF THE INVENTION 
The magnesium alloy according to the present invention is suitable for use 
in a metal mold casting including lower pressure casting, die casting and 
the like. 
Even if the contents of cerium and lanthanum are beyond the abovedescribed 
ranges, it is possible to provide a solidification temperature range 
narrowed down to some extent, but within the above ranges, a particularly 
narrower solidification temperature range being able to be achieved (see 
Comparative Example 3). If the mount of the rare earth metal mixture 
contained in the magnesium alloy of the present invention is out of the 
above-defined range, a resulting magnesium alloy has a significantly 
widened solidification temperature range and hence, it is impossible to 
achieve the object of the present invention (see Comparative Example 1). 
The zinc contained in the magnesium alloy of the present invention serves 
to improve the castability of the magnesium alloy. If the content of zinc 
is less than the above range, a resulting magnesium alloy exhibits a 
insufficient castability (see Comparative Example 2). If the content of 
zinc is more than the above-defined range, a resulting magnesium alloy has 
a considerably increased solidification temperature range and a reduced 
mechanical strength. 
The magnesium alloy for use in casting according to the present invention 
can be produced by a process known for an alloy containing a rare earth 
metal.

The present invention will now be described in detail by way of of Examples 
and Comparative Examples. 
EXAMPLES 
As used in the following Examples and Comparative Examples, % is by weight, 
unless it is otherwise defined. 
Example 1 
3 Parts by weight of granular cerium (having a purity of 92.2%) is mixed 
with 2 parts by weight of a granular misch metal free of cerium (having a 
lanthanum content of 46.0%). The mixture has a composition of 55.4% of Ce, 
19.2% of La, 14.6% of Nd and 5.0% of Pr, the balance consisting of 
impurities such as Fe, Si, Cr and the like. 
250 Grams of the rare earth metal mixture and 450 grams of a zinc piece are 
added to 9,300 grams of molten magnesium at about 680.degree. C. and 
melted. 
The resultant molten material is poured into a mold for an oil pump body 
having the following dimensions and a bottle gourd-shaped crosssection 
having two opened holes of the same size (R 50 mm) are provided in two 
raised portions of the bottle gourd shape): 
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Maximum width 
250 mm Minimum width 
80 mm 
Height 100 mm Diameter of hole 
40 mm 
Distance between centers of two holes 
150 mm 
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The solidification of the molten material was started from about 
540.degree. C. and completed at about 500.degree. C. Therefore, the 
solidification temperature range was about 40.degree. C. The material was 
subjected to an artificial aging at a temperature of 200.degree. C. for 5 
hours. 
Ten cast products of the same type were produced in the same manner, and as 
a result, there were no cracks and no surface depressions produced in any 
of the cast products. 
Comparative Example 1 
Using the same rare earth metal mixture as in Example 1, a similar oil pump 
body was produced in the same manner as in Example 1, except that 100 g of 
the rare earth metal, 450 g of zinc and 9,450 g of magnesium were used. 
Ten similar cast products were produced using this magnesium alloy, and 
there were cracks produced in two of the cast products. The solidification 
behavior was as follows: 
Solidification starting temperature: about 610.degree. C. 
Solidification finishing temperature: about 530.degree. C. 
Solidification temperature range: about 80.degree. C. 
Comparative Example 2 
Using the same rare earth metal mixture as in Example 1, a similar oil pump 
body was produced in the same manner as in Example 1, except that 150 g of 
the rare earth metal, 250 g of zinc and 9,600 g of magnesium were used. 
Ten similar cast products were produced using this magnesium alloy, and 
there were cracks and surface depressions produced in two of the cast 
products. With the magnesium alloy in Comparative Example 2, the viscosity 
of the molten metal during casting was too high, and it was difficult to 
pour the molten metal for casting. The solidification behavior was as 
follows: 
Solidification starting temperature: about 620.degree. C. 
Solidification finishing temperature: about 550.degree. C. 
Solidification temperature range: about 70.degree. C. 
Comparative Example 3 
A magnesium alloy was produced in the same manner as in Example 1, and an 
oil pump body was produced in the same manner as in Example 1, except for 
the use of a rare earth metal having a composition consisting of 40.6% of 
Ce, 19.8% of La, 29.0% of Nd and 6.7% of Pr, the balance consisting of 
impurities such as Fe, Si, Cr and the like. 
The amounts of the rare earth metal mixture, zinc and magnesium and the 
process are as defined in Example 1. Ten similar cast products were 
produced using such a magnesium alloy. There were cracks produced in one 
of the cast products, and surface depressions produced in two of the cast 
products. The solidification behavior was as follows: 
Solidification starting temperature: about 560.degree. C. 
Solidification finishing temperature: about 480.degree. C. 
Solidification temperature range: about 80.degree. C.