Abstract:
A magnesium alloy consisting essentially of 10 to 15% by weight of Al, 0.5 to 10% by weight of Sn, 0.1 to 3% by weight of Y, and 0.1 to 1% by weight of Mn, the balance being Mg and inevitable impurities.

Description:
CLAIM OF PRIORITY  
       [0001]     The present application claims priority from Japanese application serial No. 2003-394869, filed on Nov. 26, 2003, the content of which is hereby incorporated by reference into this application.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a magnesium alloy whose molten metal exhibits good fluidity. The magnesium alloy exhibits good creep properties. The magnesium alloy is particularly suitable for engine related parts.  
         [0004]     2. Related Art  
         [0005]     Magnesium alloys have been utilized as structure materials or housings for automobiles or portable electronic devices because of their light weight and high specific strength. Parts of these alloys have been manufactured by die-casting or injection molding; as magnesium alloys the following are known. 
    (1) Mg—Al—Zn series such as ASTM: AZ91D     (2) Mg—Al—Mn series such as ASTM: AM60B, AM50A     (3) Mg—Al—Si series such as ASTM: AS41B     (4) Mg—Al-rare earth element series such as ASTM: AE42    
 
         [0010]     Among the alloys the (1) alloy is most widely used as casings for potable telephones and notebook type personal computers. Particularly, it is said that AZ91D has well balanced fluidity, mechanical strength and corrosion resistance. The alloy (2) has improved impact resistance, and alloys (3), (4) have improved mechanical strength such as creep characteristics.  
         [0011]     Patent Document: Japanese Patent Laid-open 2001-158930  
         [0012]     From the viewpoints of energy saving with light-weight of car bodies and recycling of the used products, a large expectation to magnesium alloys is concentrated in recent years. Application of magnesium alloys to engine parts is expected, accordingly. Among the alloy series, though AZ91D has relatively good fluidity, its creep strength is poor and hence application of the alloys to engine parts is not proper.  
         [0013]     The AS41B or AE42 alloys with improved creep properties have poorer fluidity than the AZ91D alloys, resulting in a low molding yield.  
         [0014]     The engine related parts include intake manifolds, cylinder head covers, oil pans, transmission cases, for example. The conventional magnesium alloys such as AZ91D, AM60B, which can be shaped by die-casting and injection molding, show poor heat resistance at high temperatures. For example, bolts for fixing the parts may be loosen. Thus, the magnesium alloys are not suitable for engine parts used at a temperature higher than 100° C.  
         [0015]     Accordingly, magnesium alloys for engine parts should have good creep characteristics such as a small deformation, i.e. a small creep strain at high temperatures. Since the magnesium alloys of the present invention are better in creep characteristics than the conventional magnesium alloys, they can be used under high temperatures and high pressures. Further, the molten metal of the alloys has a good fluidity being almost equal to that of AZ91D, it is possible to produce mass production parts without or almost free from defects by die-casting or injection molding.  
       SUMMARY OF THE INVENTION  
       [0016]     It is a subject of the present invention to provide magnesium alloys with good fluidity and creep toughness.  
         [0017]     In one aspect of the present invention, a magnesium alloy according to the present invention consists essentially of 10 to 15% by weight of Al, 0.5 to 10% by weight of Sn, 0.1 to 3% by weight of Y, 0.1 to 1% by weight of Mn, the balance being Mg and inevitable impurities.  
         [0018]     In another aspect of the present invention, a magnesium alloy consists essentially of 10 to 15% by weight of Al, 0.5 to 10% by weight of Sn, 0.1 to 3% by weight of Y, 0.1 to 1% by weight of Mn, 0.1 to 5% by weight of Zn, the balance being Mg and inevitable impurities. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0019]      FIG. 1  shows a shape of a mold having a test piece for testing fluidity evaluation.  
         [0020]      FIG. 2  is a graph showing the test result of fluidity.  
         [0021]      FIG. 3  is a graph showing the creep test results.  
         [0022]      FIG. 4  is a side view of a intake manifold.  
         [0023]      FIG. 5  is a partial cross sectional view of a cylinder cover.  
         [0024]      FIG. 6  is a perspective view of an oil pan. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0025]     Al lowers a melting point of magnesium alloys thereby to improve fluidity of the alloy. Al forms Mg—Al series compounds to improve strength at room temperature. If an amount of Al is less than 10% by weight, the fluidity is insufficient and the injection molding of the alloy becomes difficult. If an amount of Al content exceeds 15% by weight, a large amount of Mg—Al compound is formed to constitute a network so that the elongation of the alloy lowers.  
         [0026]     Sn lowers a melting point of the alloy to improve fluidity of the alloy. If an amount of Sn is less than 0.5% by weight, the fluidity is insufficient so that casting of the alloy becomes difficult; if an amount of Sn is larger than, the effect of addition of Sn becomes saturated. In addition to that, the specific gravity of the alloy becomes large so that an advantage of light-weight of Mg alloys would be lost.  
         [0027]     Y forms Al—Y compounds having relatively high melting points to improve the creep strength of the alloy. If an amount of Y less than 0.1% by weight, a sufficient creep strength would not be expected. On the other hand, if an amount of Y exceeds 3% by weight, a large amount of Al—Y compounds is formed thereby to increase a melting point of the alloy so that casting of the alloy becomes difficult. Further, since Y is an expensive element, a large amount of Y increases a cost of the alloy.  
         [0028]     Mn forms compound with Al and Fe which causes corrosion of the magnesium alloys and improves corrosion resistance by trapping iron atoms in the compounds. If an amount of Mn is less than 0.1% by weight, the effect of corrosion resistance of the alloy is insufficient. If an amount of Mn exceeds 1% by weight, there is a tendency that an yield of melting of the alloy becomes worse. A further improvement of the corrosion resistance would not be expected if an excess amount of Mn is added. Since Mn has a large specific gravity, it may locally precipitate or precipitate in the bottom of the molten metal vessel.  
         [0029]     Zn may be added in some cases. Zn may lower a melting point of the alloy to improve fluidity. If an amount of Zn exceeds 3% by weight, there is a tendency that casting crack may be generated.  
         [0030]     The present invention provides magnesium alloys that have excellent fluidity and creep properties.  
         [0031]     Other examples of the magnesium alloy compositions are shown in Table 1 below. In the Table 1, numerals represent % by weight.  
                                                                         TABLE 1                                   Al   Y   Mn   Sn   Zn   Mg                                    (1)   10   0.5   0.2   8   2   bal.       (2)   15   3   0.8   3   0.1   bal.       (3)   12   1   0.2   5   0.1   bal.       (4)   15   2   0.8   1   0.5   bal.       (5)   10   1   0.2   2   5   bal.       (6)   12   0.8   0.2   5   0.5   bal.                  
 
         [0032]     Because of good creep strength and good fluidity, the magnesium alloys can preferably be applied to engine related parts such as intake manifolds shown in  FIG. 4 . The intake manifold  1  comprises a collector  4 , blankets  3  and storage chamber  2 . A cylinder head covers shown in  FIG. 5 . The cylinder head cover  6  having hollows  8 ,  9  and an oil storage  15  confined by a rib  16  is fixed to a baffle plate  7 . An oil pan is shown in  FIG. 6 . The oil pan P has a fixing flange  10  having fixing holes  3  and is fixed to a cylinder block. Since the above applications are well known in the art, detailed explanation is omitted to avoid redundancy. These parts are castings, which require good fluidity.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]     Examples of the magnesium alloys according to the present invention will be explained. Magnesium alloy ingots whose compositions were adjusted to be ones shown in Table 1 were cut into alloy chips of 2 to 5 mm diameter (length; about 5 mm or less, diameter; about 3 mm or less) as a raw material of injection molding. The alloy No. 1 is a conventional material AZ91D. An injection molding machine whose a die clamping force is 75 tons was used.  FIG. 1  shows a plane view of a test piece for fluidity evaluation. An injection speed was 1.0 m/sec, and A mold temperature was kept constant at 200° C. The injection temperature was properly controlled. The length of the test pieces injection-molded at different temperatures, the fluidity was evaluated as the length being from the gate to a position where a defect occurs. The results are shown in Table 2.  
                                                                     TABLE 2                                   Ai   Y   Mn   Sn   Zn                                    No. 1   Comparison   9   —   0.2   —   0.8       No. 2   Comparison   12   —   0.2   5   0.7       No. 3   Example   11   1   0.2   5   —       No. 4   Example   12   2   0.2   5   1       No. 5   Comparison   10   2   0.2   —   —       No. 6   Comparison   13   4   0.2   4   —                  
 
         [0034]      FIG. 2  is a graph showing the measurement results of fluidity of the alloys. The abscissa of  FIG. 2  represents a cylinder temperature of injection mold and the ordinate represents a flow length. The alloy No. 3 exhibited a larger flow length and good fluidity than the alloy No. 1 did. The alloy No. 3 is capable of being injection-molded at a temperature lower than 20 to 30° C. than that of the alloy No. 1.  
         [0035]     The same tests were carried out with respect to the alloy Nos. 2, 4, 5 and 6. The fluidity test results of the alloys No. 2, 4, 5 and 6 are shown n Table 2. The alloy Nos. 2, 4, 5 and 6 exhibited better fluidity than the alloy No. 1 (AZ91D). The alloy No. 6 whose content of Y is larger than the alloy of the present invention was hard to be injection-molded because of frequent metallurgical sticking to the injection-mold.  
                                                                         TABLE 3                                   No. 1   No. 2   No. 3   No. 4   No. 5   No. 6                                    Fluidity   Δ   ⊚   ⊚   ◯   Δ   X                  
        ⊚: A flow length of 330 mm or more and a temperature at which the maximum fluidity length is obtained is lower than 600° C.     ∘: A flow length of 330 mm or more and a temperature at which the maximum fluidity length is obtained is 600 to 630° C.     Δ: A flow length of 270 to 330 mm and a temperature at which the maximum fluidity length is obtained is 600 to 630° C.     X: A flow length of less than 270 mm and a temperature at which the maximum fluidity length is obtained is 600 to 630° C.        
 
         [0040]     The creep properties of the alloys No. 1 and alloy Nos. 2 to 4 that exhibited good fluidity were tested and evaluated.  FIG. 3  shows the creep test results conducted at 150° C., 50 MPa.  
         [0041]     The alloy No. 1 exhibited an strain as large as 5% around 50 hours. The alloy No. 2 exhibited a better property than the alloy No. 1, but it showed an strain larger than 4% around 250 hours; thus the alloy No. 2 cannot be applied as engine parts.  
         [0042]     On the other hand, the alloy Nos. 3 and 4 of the present invention exhibited an strain of about 2% around 250 hours; the formers are remarkably better than the alloy Nos. 1 and 2.  
         [0043]     As having been explained, the alloy Nos. 3 and 4 satisfy the fluidity and creep properties. The alloys to be applied to engine parts should have a sufficiently better fluidity than AZ91D and better creep characteristics than the NO. 2 alloy.