Abstract:
One exemplary embodiment includes a cast alloy including Al present in an amount of about 6.5 wt % to about 9.0 wt %; Sn present in an amount of about 1.0 wt % to about 3.0 wt %; Ce present in an amount of about 0 wt % to about 1.0 wt %; and, Mg comprising a balance of the alloy minus an amount of minor and trace elements wherein Mg is present at an amount of greater than about 85 wt %.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part Application of U.S. patent application Ser. No. 11/749,201 filed on May 16, 2007, which claims the benefit and priority of U.S. Provisional Application No. 60/801,632, filed May 18, 2006. 
     
    
     TECHNICAL FIELD 
       [0002]    The disclosure generally relates to metal alloys and more particularly to Mg—Al—Sn Cast alloys. 
       BACKGROUND 
       [0003]    There are currently two major alloy systems, Mg—Al—Zn (AZ) and Mg—Al—Mn (AM) that are generally used for automotive casting applications. AZ91 (Mg-9% Al-1% Zn) is used in many non-structural and low-temperature components where strength is desired, such as brackets, covers, cases and housings; providing essentially the same functionality with significant mass savings compared to steel, cast iron, or aluminum alloys. For structural applications such as instrument panel beams, steering systems, and radiator support, where crashworthiness is important, AM50 (Mg-5% Al-0.3% Mn) or AM60 (Mg-6% Al-0.3% Mn) offer unique advantages due to their higher ductility (10-15% elongation) and higher impact strength compared to die cast magnesium alloy AZ91 or aluminum alloy A380, but at the expense of strength. 
       SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION 
       [0004]    The present invention provides advantages and alternatives over the prior art by providing magnesium-aluminum-tin (Mg—Al—Sn) based cast alloys suitable for structural applications including cerium (Ce)-containing and yttrium (Y)-containing Mg—Al—Sn alloys that provide superior strength and ductility compared to Mg—Al—Sn alloys without Ce or Y, as well as conventional alloys such as AZ91. The Mg—Al—Sn alloys according to the present invention provide a desired combination of strength and ductility and are advantageously useable for automotive structural applications, for example, including engine cradles, control arms, and wheels. 
         [0005]    In one embodiment, the invention includes a cast alloy including Al present in an amount of about 6.5 wt % to about 9.0 wt %; Sn present in an amount of about 1.0 wt % to about 3.0 wt %; Ce present in an amount of about 0 wt % to about 1.0 wt %; and, Mg comprising a balance of the alloy minus an amount of minor and trace elements wherein Mg is present at an amount of greater than about 85 wt %. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    Exemplary embodiments of the invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
           [0007]      FIG. 1  shows the effect of Al concentration on the mechanical properties of an Mg—Al—Sn alloy without Ce according to one exemplary embodiment. 
           [0008]      FIG. 2  shows the effect of Sn concentration on the mechanical properties of an Mg—Al—Sn alloy without Ce according to one exemplary embodiment. 
           [0009]      FIG. 3  shows the effect of Ce addition on the mechanical properties of Mg—Al—Sn alloys according to one exemplary embodiment and contrasted with conventional alloys. 
           [0010]      FIG. 4  shows the effect of yttrium addition on the mechanical properties of Mg—Al—Sn alloys according to one exemplary embodiment and contrasted with conventional alloys. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0011]    The following description of the embodiment(s) is merely exemplary (illustrative) in nature and is in no way intended to limit the invention, its application, or uses. 
         [0012]    One exemplary embodiment may include a cast alloy comprising Ce (Cerium) from about 0 to about 1 wt %, preferably from about 0.05 wt % to about 0.8 wt %, more preferably from about 0.05 wt % to about 0.5 wt %, and most preferably from about 0.2 wt % to about 0.5 wt %, has the unexpected result of significantly increasing the strength of Mg—Al—Sn alloys. 
         [0013]    In one embodiment, the Mg—Al—Sn alloys may include Al at about 6.5 wt % to about 9 wt %, more preferably about 6.8 wt %, to about 9 wt %. 
         [0014]    In another embodiment, the Mg—Al—Sn alloys may include Sn at about 1 wt % to about 3 wt %. 
         [0015]    In another embodiment, the Mg—Al—Sn alloys may include Mn at about 0.2 wt % to about 0.6 wt %. 
         [0016]    In the embodiment listed above, it will be appreciated that Mg makes up a balance amount of the alloy minus other minor or trace amounts of elements. 
         [0017]    For example, it will be appreciated that Mg—Al—Sn alloys according to select embodiments may include Zn, Si, Cu, Ni, Fe, and other unnamed elements in trace or minor amounts. For example, referring to Table I is shown an exemplary embodiment of the Mg—Al—Sn alloys including maximum amounts of other (unnamed) elements 
         [0000]    
       
         
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                 Element 
                 Amount (wt %) 
               
               
                   
                   
               
             
             
               
                   
                 Mg 
                 Balance 
               
               
                   
                 Al 
                 6.8-9.0 
               
               
                   
                 Sn 
                 1.0-3.0 
               
               
                   
                 Mn 
                 0.2-0.6 
               
               
                   
                 Ce 
                 0.05-0.5  
               
               
                   
                 Zn 
                   0.3 maximum 
               
               
                   
                 Si 
                  0.01 maximum 
               
               
                   
                 Cu 
                  0.01 maximum 
               
               
                   
                 Ni 
                 0.002 maximum 
               
               
                   
                 Fe 
                 0.002 maximum 
               
               
                   
                 Others  
                  0.02 maximum 
               
               
                   
                   
               
             
          
         
       
     
         [0018]    Referring to  FIG. 1  is shown the effect of Al concentration on Mg—Al—Sn alloys without Ce according to exemplary embodiments. It is seen that ultimate tensile strength (UTS) and tensile yield strength (YS) increase with Al concentration while % elongation (ductility) decreases. 
         [0019]    Referring to  FIG. 2  is shown the effect of Sn concentration on Mg—Al—Sn alloys without Ce according to exemplary embodiments. It is seen that ultimate tensile strength (UTS) and tensile yield strength (YS) increase with Al concentration over the range of about 1 to about 3 wt % while % elongation (ductility) decreases. 
         [0020]    Although it has been found that Mg—Al—Sn cast alloys according to exemplary embodiments have a desirable combination of strength and ductility for automotive structural applications, it has been found that certain Mg—Al—Sn alloys such as AT72 (Mg-8% Al-2% Sn) and AT82 (Mg-8% Al-2% Sn) (in wt %) have a strength that is below that of a conventional structural cast alloy AZ91 (Mg-9% Al-1% Zn), discussed above. 
         [0021]    Referring to Table II is shown an exemplary embodiment of Ce containing Mg—Al—Sn alloys and the results of tensile testing carried out according to ASTM E21-92 procedures at an initial strain rate of 0.001 s −1 . Shown are resulting tensile yield strength (YS) in MPa, ultimate tensile strength (UTS) in MPa, and percent elongation (Ef %) of the AT82 alloy (Mg-8% Al-2% Sn). The AT82 alloy with additions of Ce at about 0.2 wt % and about 0.5 wt % is shown contrasted with AT82 without Ce, as well as the conventional alloys AM60 (Mg-6% Al-0.3% Mn) and AZ91 (Mg-9% Al-1% Zn). 
         [0022]    To prepare experimental sample alloys listed in Table II, a commercially available AM50 alloy, pure aluminum, pure tin, and Mg-26% Ce master alloy were used. The experimental alloys were prepared and melted in a 30 lb steel crucible under SF 6 /CO 2  protection. All alloying additions were made at about 650° C., and the melt was stirred for about 2 minutes and kept for 20 minutes to ensure homogenization. The alloy melts were heated to a desired casting temperature between 680° C. and 720° C. A mild steel tensile bar (10 mm diameter and 25 mm gage length) permanent mold was heated to about 280-300° C., and the molten metal was poured under the SF 6 /CO 2  protective gas. 
         [0023]    Tensile testing was carried out at room temperature according to ASTM E21-92 procedures at an initial strain rate of 0.001 s −1 . For each condition, at least three specimens were tested and the average properties are listed in Table II. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE II 
               
               
                   
               
               
                 Alloy 
                 Ce wt % 
                 YS MPa 
                 UTS MPa 
                 Ef % 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 AT82 
                 0 
                 90.4 
                 122.4 
                 1.6 
               
               
                 AT82 
                 0.2 
                 124 
                 152.8 
                 0.9 
               
               
                 AT82 
                 0.5 
                 115.1 
                 161.3 
                 1.1 
               
               
                 AM60 
                 0 
                 77.3 
                 153 
                 4.3 
               
               
                 AZ91 
                 0 
                 96.7 
                 150.4 
                 1.5 
               
               
                   
               
             
          
         
       
     
         [0024]    It can be seen from Table II that addition of Ce to Mg—Al—Sn alloys according to exemplary embodiments advantageously increases the yield strength by up to about 38% as well as increases the strength relative to the conventional structural cast alloys AZ91 at the selected levels of Ce addition while maintaining a ductility comparable to AZ91. 
         [0025]    Referring to  FIG. 3  is shown graphical results for YS, UTS, and Ef % for the alloys listed in Table II. The results graphically demonstrate the significant increase in both YS and UTS with Ce addition to Mg—Al—Sn alloys according to exemplary embodiments compared to Mg—Al—Sn alloys without Ce as well as compared to conventional structural cast alloys. 
         [0026]    Thus, both the Ce containing Mg—Al—Sn alloys and the non-Ce containing Mg—Al—Sn alloys according to exemplary embodiments are desirably useable in structural applications, such as automotive structural applications, for example, engine cradles, control arms and wheels, where the Ce containing Mg—Al—Sn alloys provide superior strength compared to non-Ce containing Mg—Al—Sn alloys as well as conventional structural cast alloys. 
         [0027]    It will further be appreciated that the Ce and non-Ce containing Mg—Al—Sn alloys according to the exemplary embodiments may be advantageously die cast according to conventional methods to form structural components 
         [0028]      FIG. 4  shows the effect of yttrium additions on the tensile properties of AT82 alloy. The results show that 0.2% Y addition to AT82 alloy can improve the mechanical properties, 10% increase in yield strength, 32% increase in ultimate tensile strength, and 44% increase in ductility. On the other hand, the effect of 0.5% Y addition has less effect on the properties of AT82 alloy. Compared to the AZ91 alloy, the new Mg—Al—Sn—Y alloy, namely ATY8202, has significantly higher strength but lower ductility. Various embodiment may include 0-1% Y, 0.1 to 0.5% Y, or 0.1-0.3% Y. 
         [0029]    The above description of embodiments of the invention is merely exemplary in nature and, thus, variations thereof are not to be regarded as a departure from the spirit and scope of the invention.