Patent Publication Number: US-2013243643-A1

Title: Aluminum alloy having improved oxidation resistance, corrosion resistance, or fatigue resistance, and die-cast material and extruded material prepared by using the aluminum alloy

Description:
TECHNICAL FIELD 
     The present invention relates to an aluminum (Al) alloy, and more particularly, to an Al alloy having improved oxidation resistance, corrosion resistance, or fatigue resistance, and a die-cast material and an extruded material prepared by using the alloy. 
     BACKGROUND ART 
     Currently, magnesium (Mg) is regarded as one of main alloying elements in an aluminum (Al) alloy. Addition of Mg allows an Al alloy to have a high strength, to be favorable to surface treatment, and to have improved corrosion resistance. However, due to Mg having a chemically high oxidizing potential, an oxide or another inclusion may be mixed into molten Al during Mg is alloyed into the molten Al and thus the quality of molten metal may deteriorate. In order to prevent an oxide or another inclusion from being mixed into molten Al, for example, the surface of molten metal may be covered with a protective gas such as SF 6  when Mg is added. 
     However, due to properties of a process of preparing an Al alloy, it may not be easy to completely protect Mg, which is added in large amounts, with a protective gas. Furthermore, since SF 6  used as the protective gas not only is expensive gas but also causes an environmental problem, the use of SF 6  is now being gradually restricted all over the world. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Technical Problem  
     The present invention provides an aluminum (Al) alloy prepared environment friendly and having improved chemical-mechanical properties such as oxidation resistance, corrosion resistance, or fatigue resistance, and a die-cast material and an extruded material prepared by using the Al alloy. The above problem to be solved is provided as an example and the scope of the present invention is not limited thereto. 
     Technical Solution  
     According to an aspect of the present invention, there is provided an aluminum (Al) alloy casted by adding a magnesium (Mg) master alloy, in which a calcium (Ca)-based compound is distributed in an Mg matrix, into molten Al, wherein an Al matrix includes the Ca-based compound, and wherein the Al alloy has superior oxidation resistance, corrosion resistance against salt water, or fatigue resistance to a corresponding Al alloy not including the Ca-based compound. 
     In this case, the Ca-based compound may include at least one of an Mg—Ca compound, an Al—Ca compound, and an Mg—Al—Ca compound, and the Mg—Ca compound may include Mg 2 Ca, the Al—Ca compound may include at least one of Al 2 Ca and Al 4 Ca, and the Mg—Al—Ca compound may include (Mg,Al) 2 Ca. 
     Also, the Mg master alloy may be prepared by adding a Ca-based additive into molten parent material including pure Mg, or an Mg alloy including Al, as a parent material. 
     Furthermore, the Ca-based compound may be formed by dispersively adding a Ca-based additive onto a surface of an upper part of molten Mg, and then exhausting at least a portion of the Ca-based additive in the molten Mg. 
     In this case, the Ca-based compound may be formed by exhausting the Ca-based additive in the molten Mg in such a way that the Ca-based additive does not substantially remain in the Mg master alloy. For this, the upper part of the molten Mg may be stirred and the stirring may be performed at the upper part which is within 20% of a total depth of the molten Mg. 
     Meanwhile, the Ca-based additive may include at least one of calcium oxide (CaO), calcium cyanide (CaCN 2 ), and calcium carbide (CaC 2 ). 
     Also, at least a portion of the Ca-based additive may be exhausted in molten parent material, and the Ca-based compound may be formed due to reaction between Ca supplied from the Ca-based additive and Mg or Al of the parent material. 
     In this case, the Mg master alloy may be added by 0.0001 parts by weight to 30 parts by weight based on 100 parts by weight of Al, and the Ca-based additive may be added by 0.0001 parts by weight to 30 parts by weight based on 100 parts by weight of the parent material. 
     Furthermore, Mg may be dissolved in the Al matrix within a range of 0.1 wt % to 15 wt %. 
     If a content of the Ca-based compound is increased, a weight gain of the Ca-based compound due to oxidation under the same oxidation condition may be reduced. 
     The superior fatigue resistance may refer to a larger cycle number leading fatigue fracture if a cyclic load is applied at a predetermined frequency under stress conditions of 40% to 80% of a tensile strength. 
     According to another aspect of the present invention, there is provided an aluminum (Al) alloy extruded material prepared by extruding the above-described Al alloy, and having a higher strength in comparison to an Al alloy extruded material prepared under the same condition except that the Ca-based compound is not included. 
     According to another aspect of the present invention, there is provided an aluminum (Al) alloy die-cast material prepared by using molten metal of the above-described Al alloy, and having a higher strength in comparison to an Al alloy die-cast material prepared under the same condition except that the Ca-based compound is not included. 
     According to another aspect of the present invention, there is provided a method of preparing an aluminum (Al) alloy extruded material, the method including preparing molten Al including magnesium (Mg); preparing an Al alloy by casting the molten Al; and extruding the Al alloy, wherein the molten Al is prepared by melting Al together with an Mg master alloy in which a calcium (Ca)-based compound combined with at least one of Mg and Al is included in an Mg matrix. 
     In this case, the method may further include performing heat treatment on the Al alloy extruded material after the Al alloy is extruded. 
     According to another aspect of the present invention, there is provided a method of preparing an aluminum (Al) alloy die-cast material, the method including preparing molten Al including magnesium (Mg); and casting the molten Al; and wherein the molten Al is prepared by melting Al together with an Mg master alloy in which a calcium (Ca)-based compound combined with at least one of Mg and Al is included in an Mg matrix. 
     Advantageous Effects  
     If an aluminum (Al) alloy according to the present invention is used, even when a protective gas conventionally used to prevent oxidation of molten Al, for example, SF 6 , is greatly reduced in amount or is not used, a die-cast product may be stably prepared and an extruded material having excellent mechanical properties may be prepared by performing an extruding process. 
     Also, in the Al alloy according to an embodiment of the present invention, since a calcium (Ca)-based compound added when a magnesium (Mg) master alloy is added is distributed in an Al matrix so as to achieve a distribution strengthening effect and a grain refinement effect, mechanical properties of the Al alloy, for example, strength and fatigue resistance, may be remarkably improved. Due to the improvement in casting properties and/or mechanical properties, oxidation resistance and corrosion resistance of the Al alloy may be improved. 
     The effects of the present invention are not limited to the above-described effects and other effects not described above may be understood by one of ordinary skill in the art from the following detailed description of the invention. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flowchart of a method of preparing a magnesium (Mg) master alloy to be added into molten aluminum (Al) so as to prepare an Al alloy, according to an embodiment of the present invention. 
         FIGS. 2A through 2D  are images showing electron probe microanalysis (EPMA) results of microstructures and components of an Mg master alloy, according to an embodiment of the present invention. 
         FIG. 3  is a flowchart of a method of preparing an Al alloy, according to an embodiment of the present invention. 
         FIGS. 4A and 4B  are images showing the surfaces of a molten Al alloy prepared by adding an Mg master alloy including calcium oxide (CaO), according to an embodiment of the present invention, and a molten Al alloy prepared by adding pure Mg. 
         FIGS. 5A and 5B  are images showing the surfaces of a cast material of an Al alloy prepared by adding an Mg master alloy including CaO, according to an embodiment of the present invention, and a cast material of an Al alloy prepared by adding pure Mg. 
         FIGS. 6A and 6B  are images showing analysis results on components of an Al alloy prepared by adding an Mg master alloy including CaO, according to an embodiment of the present invention, and components of an Al alloy prepared by adding pure Mg. 
         FIG. 7A  is an EPMA image of a microstructure of an Al alloy prepared by adding an Mg master alloy including CaO, according to an embodiment of the present invention, and  FIGS. 7B through 7E  are EPMA images showing component mapping results of Al, calcium (Ca), Mg, and oxygen (O), respectively. 
         FIGS. 8 through 10  are images comparatively showing microstructures according to Experimental Examples 2 to 4 and Comparative Examples 2 to 4. 
         FIGS. 11 through 13  are images comparatively showing microstructures according to Experimental Examples 5, 6, and 7 and Comparative Examples 5, 6, and 7. 
         FIG. 14  is a graph showing oxidation resistance of Al alloys based on the content of CaO added to prepare an Mg master alloy. 
         FIG. 15  is a graph showing oxidation resistance of Al alloys according to comparative examples and Al alloys according to embodiments of the present invention, based on the content of Mg. 
         FIGS. 16A through 16G  are images showing oxidation resistance of Al alloys according to comparative examples and Al alloys according to embodiments of the present invention, based on the content of Mg. 
         FIG. 17  is a graph showing corrosion resistance of an Al alloy according to a comparative example and an Al alloy according to an embodiment of the present invention. 
         FIG. 18  is an image showing corrosion properties of an Al alloy according to a comparative example. 
         FIG. 19  is an image showing corrosion properties of an Al alloy according to an embodiment of the present invention. 
         FIG. 20  is a graph showing mechanical properties of an Al alloy used in a fatigue test, according to an embodiment of the present invention. 
         FIG. 21  is a schematic diagram showing decomposition of CaO at an upper part of molten Mg when CaO is added in to the molten Mg. 
     
    
    
     BEST MODE 
     The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to one of ordinary skill in the art. 
     According to an embodiment of the present invention, an aluminum (Al) alloy is prepared by preparing a master alloy into which a predetermined additive is added, and then adding the master alloy into Al. Here, any of pure magnesium (Mg) and an Mg alloy may be used as a parent material of the master alloy, and is denoted as an Mg master alloy. Pure Mg refers to Mg into which no alloying element is intentionally added, and is substantially defined to include an impurity that is inevitably added during Mg is prepared. 
     An Mg alloy is an alloy prepared by intentionally adding another alloying element such as Al into Mg. The Mg alloy including Al as an alloying element may be referred to as an Mg—Al alloy. In this case, the Mg—Al alloy may further include another alloying element other than Al. 
       FIG. 1  is a flowchart of a method of preparing an Mg master alloy, according to an embodiment of the present invention. Referring to  FIG. 1 , the method of preparing an Mg master alloy includes forming molten Mg (S1), adding an additive (S2), stirring or holding (S3), and casting (S4). 
     In operation S1, pure Mg or an Mg alloy is put into a crucible and is heated to form the molten Mg. In this case, a heating temperature may be, for example, in a range of 400° C. to 800° C. Although pure Mg may melt at a temperature higher than or equal to 600° C., since a melting point is lowered due to alloying, the Mg alloy may melt at a temperature lower than or equal to 600° C. and higher than or equal to 400° C. Here, if the temperature is lower than 400° C., the molten Mg may not be easily formed. If the temperature is higher than 800° C., the molten Mg may be sublimated or ignited. 
     The Mg alloy used in operation S1 may include one selected from the group consisting of AZ91 D, AM20, AM30, AM50, AM60, AZ31, AS141, AS131, AS121X, AE42, AE44, AX51, AX52, AJ50X, AJ52X, AJ62X, MR1153, MR1230, AM-HP2, Mg—Al, Mg—Al—Re, Mg—Al—Sn, Mg—Zn—Sn, Mg—Si, Mg—Zn—Y, and equivalents thereof. However, the Mg alloy is not limited thereto. Any Mg alloy that is commonly used in the industrial field may be used. 
     Meanwhile, a small amount of a protective gas may be optionally provided in order to prevent the molten Mg from being ignited. The protective gas may typically use SF 6 , SO 2 , CO 2 , HFC-134a, Novec™612, an inert gas, an equivalent thereof, or a gas mixture thereof, and may suppress the molten Mg from being ignited. 
     Then, in operation S2, a calcium (Ca)-based additive is added to the molten Mg. In this case, the added Ca-based additive may include at least one of calcium oxide (CaO), calcium cyanide (CaCN 2 ), and calcium carbide (CaC 2 ). The Ca-based additive may improve oxidation resistance in the molten Mg and thus a protective gas required to melt Mg may be greatly reduced in amount or may not be used. As such, if an Mg master alloy is prepared according to an embodiment of the present invention as described above, a problem caused by the use of a protective gas such as SF 6  that is restricted due to an environmental reason may be solved. 
     Also, since oxidation resistance of the molten Mg is improved, ignition resistance may be increased and thus an oxide or another inclusion may be suppressed from being mixed into the molten Mg. Accordingly, the cleanliness of molten metal may be significantly improved and thus mechanical properties of an Mg alloy casted by using the molten metal may be improved. 
     At least a portion of the Ca-based additive may be exhausted in the molten Mg. Under an appropriate condition, substantially all of the Ca-based additive may be exhausted in the molten Mg. For example, the Ca-based additive may be reduced in the molten Mg so as to be decomposed to Ca. For example, as a Ca-based additive, CaO may be decomposed into Ca and O. In this case, the decomposed oxygen may be discharged from the molten Mg into the air in the form of a gas or may float on the molten Mg in the form of dross or sludge. 
     Meanwhile, Ca decomposed from CaO may form a compound due to various reactions in molten metal. The compound may be an intermetallic compound formed due to a chemical reaction between metals. The reduced Ca may react with another element(s) in a parent material, e.g., Mg and/or Al, and thus may form a Ca-based compound. 
     Accordingly, the Ca-based additive is a source of Ca used to form a Ca-based compound in the Mg master alloy and is an additive element added into a molten parent material formed to prepare a master alloy. The Ca-based compound is a compound newly formed due to a reaction between Ca supplied from the Ca-based additive and another element(s) in a parent material. Although Ca has solubility with respect to Mg, it is uncovered that Ca, which is reduced from the Ca-based additive in the molten Mg as in the present invention, is only partially dissolved in the molten Mg and mostly forms the Ca-based compound. 
     If a parent material of the Mg master alloy is pure Mg, a formable Ca-based compound may be an Mg—Ca compound, e.g., Mg 2 Ca. Also, if a parent material of the Mg master alloy is an Mg alloy, e.g., an Mg—Al alloy, a formable Ca-based compound may include at least one of an Mg—Ca compound, an Al—Ca compound, and an Mg—Al—Ca compound. For example, the Mg—Ca compound may be Mg 2 Ca, the Al—Ca compound may include at least one of Al 2 Ca and Al 4 Ca, and the Mg—Al—Ca compound may be (Mg,Al) 2 Ca. 
     The decomposition and reaction of the Ca-based additive may be more inactivated due to stirring, and a detailed description thereof will be provided below in relation to operation S3. 
     The Ca-based additive is favorable to have a large surface area in order to improve reactivity and thus is favorable to have a form of powder. However, the Ca-based additive is not limited thereto and may have a form of pellets or masses formed by allowing powder to agglomerate in order to prevent scattering of powder. 
     The Ca-based additive may have a size of 0.1 μm to 500 μm, and more particularly, 0.1 μm to 200 μm. If the Ca-based additive has a size less than 0.1 μm, the size is too small and thus the Ca-based additive may be scattered due to hot air of sublimated Mg and thus may not be easily put into a crucible. Also, the Ca-based additive may agglomerate and thus may not be easily mixed with molten metal having a form of a liquid. Such agglomerates reduce a surface area for reaction and thus are not preferable. If the Ca-based additive has a size greater than 500 μm, a surface area for reaction may be reduced and, furthermore, the Ca-based additive may not react with the molten Mg. 
     The Ca-based additive may be added by 0.001 wt % to 30 wt %, and more particularly, by 0.01 wt % to 15 wt %. If the Ca-based additive is added by less than 0.001 wt %, mechanical properties of the Mg alloy are slightly or hardly improved. Also, if the Ca-based additive is added by more than 30 wt %, intrinsic properties of Mg may not appear. 
     The Ca-based additive may be added into the molten Mg all at once or separately with time intervals. Also, a required amount of the Ca-based additive may be added all at once or the Ca-based additive may be divided into appropriate amounts and may be added separately with time intervals. If the Ca-based additive has a form of powder having fine particles, the Ca-based additive may be added separately with time intervals so as to reduce the possibility that the Ca-based additive agglomerates and to accelerate reaction of the Ca-based additive. 
     In order to accelerate decomposition and reaction of the Ca-based additive, stirring may be performed in operation S3. In this case, the stirring may be started at the same time when the Ca-based additive is added or after the added Ca-based additive is heated to a certain temperature in molten metal. 
     In a typical metal alloying process, molten metal and an alloying element are actively stirred by using, for example, a convection method, in order to cause a reaction in the molten metal. However, in the current embodiment, if an active reaction of the Ca-based additive is induced, the Ca-based additive reacts less efficiently and thus is greatly remains in the ultimate molten metal in an un-decomposed state. If the Ca-based additive remains in the ultimate molten metal as described above, the Ca-based additive may be included in a casted Mg alloy and thus mechanical properties of the Mg alloy may deteriorate. 
       FIG. 21  is a schematic diagram showing decomposition of CaO at an upper part of molten Mg when CaO is added in to the molten Mg. Referring to  FIG. 21 , CaO is decomposed into O and Ca at the upper part of the molten Mg. In this case, the decomposed oxygen may be discharged outside in the form of a gas (O 2 ) or may float on the molten Mg in the form of dross or sludge. Meanwhile, the decomposed calcium may react with another element(s) in molten metal to form various compounds. 
     Accordingly, in the current embodiment, rather than mixing the Ca-based additive into the molten Mg, forming a reaction environment for allowing the Ca-based additive to react on the surface of the molten metal is more critical. For this, in order to allow the added Ca-based additive to stay on the surface of the molten metal as long as possible and to be exposed to the air, the upper part of the molten Mg may be stirred. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Addition 
                 Addition 
                 Addition 
               
               
                   
                 of 5 wt 
                 of 10 wt 
                 of 15 wt 
               
               
                   
                 % CaO 
                 % CaO 
                 % CaO 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 CaO 
                 No Stirring 
                 4.5 wt 
                 8.7 wt 
                 13.5 wt 
               
               
                 Residues 
                   
                 % CaO 
                 % CaO 
                 % CaO 
               
               
                 in Alloy 
               
               
                   
                 Stirring of 
                 1.2 wt 
                 3.1 wt 
                 5.8 wt 
               
               
                   
                 Inside of 
                 % CaO 
                 % CaO 
                 % CaO 
               
               
                   
                 Molten Metal 
               
               
                   
                 Stirring of Upper 
                 0.001 wt 
                 0.002 wt 
                 0.005 wt 
               
               
                   
                 Part of Molten 
                 % CaO 
                 % CaO 
                 % CaO 
               
               
                   
                 Metal (Present 
               
               
                   
                 Invention) 
               
               
                   
               
            
           
         
       
     
     Table 1 shows results of measuring CaO residues according to a stirring method when CaO is added into molten Mg of AM60B. In this case, the added CaO is 70 μm in size, and CaO is added by 5 wt %, 10 wt %, and 15 wt %. As the stirring method, upper part stirring, internal stirring, and no stirring of the molten Mg are selected. As shown in Table 1, when the upper part of the molten Mg is stirred, unlike the other cases, the most part of the added CaO is reduced to Ca. 
     The stirring may be performed at the upper part which is within 20%, and desirably, within 10%, of a total depth of the molten metal from the surface thereof. If the stirring is performed at a depth greater than or equal to 20%, the decomposition of the Ca-based additive does not easily occur at the surface of the molten Mg. 
     In this case, a stirring time may differ according to the state of an added powder and the temperature of the molten metal, and it is preferable to stir the molten metal sufficiently until the added Ca-based additive is completely exhausted in the molten metal as long as possible. Here, the exhaustion means that decomposition of the Ca-based additive is substantially completed. Decomposition of the Ca-based additive in the molten Mg due to the stirring and Ca formed due to the decomposition may further accelerate a reaction for forming various compounds. 
     After operation S3 is completed, operation S4 for solidifying the molten Mg in a mold is performed to prepare the Mg master alloy. In operation S4, the temperature of the mold may be in a range of room temperature (for example, 25° C.) to 400° C. Also, a master alloy may be separated from the mold after the mold is cooled to room temperature. However, if the master alloy is completely solidified, the master alloy may be separated even before the temperature reaches room temperature. 
     Here, the mold may use any one selected from a metallic mold, a ceramic mold, a graphite mold, and equivalents thereof. Also, the casting method may include sand casting, die casting, gravity casting, continuous casting, low-pressure casting, squeeze casting, lost wax casting, thixo casting, or the like. 
     Gravity casting may denote a method of pouring a molten alloy into a mold by using gravity, and low-pressure casting may denote a method of pouring a molten alloy into a mold by applying a pressure onto the surface of the molten alloy by using a gas. Thixo casting is a casting method performed in a semisolid state and is a combination method adopting advantages of typical casting and forging. However, the present invention is not limited to a mold type and a casting method or process. 
     The above prepared Mg master alloy has a matrix having a plurality of domains divided from each other by boundaries therebetween. In this case, the plurality of domains divided from each other may typically be a plurality of grains divided by grain boundaries therebetween and, as an another example, may be a plurality of phase regions defined by two or more different phase boundaries therebetween. 
     Meanwhile, a Ca-based compound formed during the Mg master alloy is prepared may be dispersed and exist in the matrix of the Mg master alloy. In this case, the Ca-based compound may be one formed due to reaction between the Ca-based additive added in operation S2 and another element(s), e.g., Mg and/or Al, in the Mg parent material. 
     That is, the Ca-based additive is reduced to Ca during the Ca-based additive is added into the molten Mg and is stirred or held. In general, since the Ca-based additive is thermodynamically more stable than Mg, Ca is not expected to be separated from the Ca-based additive due to reduction in the molten Mg. However, according to experiments by the present inventors, it is uncovered that the Ca-based additive is reduced in the molten Mg. The reduced Ca may react with another element(s), e.g., Mg and/or Al, in the parent material, thereby forming a Ca-based compound. 
     Accordingly, the Ca-based additive is a source of Ca used to form a Ca-based compound in the Mg master alloy and is an additive element added into a molten parent material formed to prepare a master alloy. The Ca-based compound is a compound newly formed due to a reaction between Ca supplied from the Ca-based additive and another element(s) in a parent material. 
     Although Ca has solubility with respect to Mg, it is uncovered that Ca, which is reduced from the Ca-based additive in the molten Mg as in the present invention, is only partially dissolved in the molten Mg and mostly forms the Ca-based compound. 
     In the case, if a parent material of the Mg master alloy is pure Mg, a formable Ca-based compound may be an Mg—Ca compound, e.g., Mg 2 Ca. Also, if a parent material of the Mg master alloy is an Mg alloy, e.g., an Mg—Al alloy, a formable Ca-based compound may include at least one of an Mg—Ca compound, an Al—Ca compound, and an Mg—Al—Ca compound. For example, the Mg—Ca compound may be Mg 2 Ca, the Al—Ca compound may include at least one of Al 2 Ca and Al 4 Ca, and the Mg—Al—Ca compound may be (Mg,Al) 2 Ca. 
     In this case, it is highly probable that the Ca-based compound is distributed at grain boundaries, i.e., boundaries between grains, or phase boundaries, i.e., boundaries between phase regions. This is because such boundaries are further open and have a relatively high energy in comparison to the inside of the grains or the phase regions, and thus may provide a favorable site for nucleation and growth of the Ca-based compound. 
       FIGS. 2A through 2D  are images showing electron probe microanalysis (EPMA) results of an Mg master alloy prepared by adding CaO as a Ca-based compound into an Mg—Al alloy, according to an embodiment of the present invention. 
       FIG. 2A  shows a microstructure of the Mg master alloy observed by using back scattered electrons. As shown in  FIG. 2A , the Mg master alloy includes regions surrounded by a compound (bright part), that is, a microstructure having a plurality of grains. In this case, the compound (bright part) is formed along grain boundaries.  FIGS. 2B through 2D  are EPMA images showing component mapping results of Al, Ca, and O, respectively, in a region of the compound (bright part). 
     Al and Ca are detected in the compound (bright part in  FIG. 2A ) as shown in  FIGS. 2B and 2C  but O is not detected as shown in  FIG. 2D . As such, it may be understood that an Al—Ca compound, which is formed due to reaction between Ca separated from CaO and Al included in the parent material, is distributed at grain boundaries of the Mg master alloy. The Al—Ca compound may be Al 2 Ca or Al 4 Ca, which is an intermetallic compound. 
     Meanwhile, the above EPMA results show that an Al—Ca compound is mainly distributed at grain boundaries of the Mg master alloy because the Ca-based compound is probably distributed at grain boundaries rather than the inside of grains due to properties of the grain boundaries having open structures. However, these analysis results do not mean that all Ca-based compounds are distributed only at the grain boundaries. In some cases, the Ca-based compound may exist inside grains. 
     The above prepared Mg master alloy is used to be added into an Al alloy. As described above, an Mg master alloy includes a Ca-based compound, which is formed due to reaction between Ca supplied from the Ca-based additive during an alloying process, and Mg and/or Al. All Ca-based compounds are intermetallic compounds and have a melting point higher than the melting point (658° C.) of Al. For example, the melting points of Al 2 Ca and Al 4 Ca as Al—Ca compounds are 1079° C. and 700° C., respectively, which are higher than the melting point of Al. 
     Accordingly, if a master alloy including the above-described Ca-based compound is added into molten Al, the Ca-based compound may mostly remain without being melted in molten metal. Furthermore, if an Al alloy is prepared by casting the molten metal, the Ca-based compound may be included in the Al alloy. 
     A method of preparing an Al alloy, according to an embodiment of the present invention, will now be described. 
     An Al alloy according to an embodiment of the present invention may be prepared, in order to form molten metal in which an Mg master alloy and Al are melted, melting Al to form molten Al and then adding and melting the Mg master alloy including a Ca-based compound, into the molten Al. As another method, Al and the Mg master alloy may be s put into a melting apparatus such as a crucible, and then may be heated together to be melted. 
       FIG. 3  is a flowchart of a method of preparing an Al alloy, according to an embodiment of the present invention, and more particularly, a method of preparing an Al alloy by forming molten Al and then adding and melting an Mg master alloy prepared by using the above-described method, into the molten Al. 
     As illustrated in  FIG. 3 , the method of preparing an Al alloy includes forming molten Al (S11), adding an Mg master alloy (S12), stirring or holding (S13), and casting (S14). 
     Initially, in operation S11, molten Al is formed by putting Al into a crucible and is heating Al at a temperature ranging from 600° C. to 900° C. In operation S11, Al may be any one selected from pure Al, an Al alloy, and equivalents thereof. The Al alloy, for example, may be any one selected from 1000-series, 2000-series, 3000-series, 4000-series, 5000-series, 6000-series, 7000-series, and 8000-series wrought Al, and 100-series, 200-series, 300-series, 400-series, 500-series, and 700-series casting Al. Then, in operation S12, the Mg master alloy prepared by using the above-described method is added into the molten Al. In this case, the Mg master alloy used in operation S12 may be added by 0.0001 parts by weight to 30 parts by weight based on 100 parts by weight of Al. If the added Mg master alloy is added by less than 0.0001 parts by weight, effects (hardness, corrosion resistance, weldability, etc.) achieved by adding the Mg master alloy may be insufficient. Also, if the Mg master alloy is added by more than 30 parts by weight, intrinsic properties of Al alloy may not appear. 
     In this case, the Mg master alloy may be added in the form of an ingot. However, the Mg master alloy is not limited thereto and may be added in various forms such as a form of powder and a form of granules. Also, the size of the Mg master alloy is not limited. 
     When the Mg master alloy is added, a Ca-based compound included in the Mg master alloy is provided together into the molten Al. As described above, the Ca-based compound provided into the molten Al may include at least one of an Mg—Ca compound, an Al—Ca compound, and an Mg—Al—Ca compound. 
     In this case, a small amount of a protective gas may be additionally provided in order to prevent the Mg master alloy from being oxidized. The protective gas may typically use SF 6 , SO 2 , CO 2 , HFC-134a, Novec™612, an inert gas, an equivalent thereof, or a gas mixture thereof, and may suppress the molten Mg from being oxidized. 
     However, in the present invention, the above protective gas is not essentially needed and may not be provided. That is, if the Mg master alloy including the Ca-based compound is added as in the present invention, ignition resistance is increased due to an increase in oxidation resistance of the Mg master alloy and an inclusion of an impurity such as an oxide in molten metal is remarkably reduced in comparison to a typical case when Mg not including a Ca-based compound is added. As such, according to the above-described method, even without using a protective gas, the cleanliness of molten Al may be greatly improved and thus the quality of molten metal may be significantly improved. 
     Then, in operation S13, the molten Al is stirred or held for 1 to 400 minutes. Here, if the stirring or holding time is less than 1 minute, the Mg master alloy is not sufficiently mixed into the molten Al. Otherwise, if the stirring or holding time is more than 400 minutes, the molten Al is stirred or held for a unnecessarily long time. 
     After operation S13 is completed, operation S14 for solidifying the molten Al in a mold is performed to prepare the Al alloy. In operation S14, the temperature of the mold may be in a range of room temperature (for example, 25° C.) to 400° C. Also, a master alloy may be separated from the mold after the mold is cooled to room temperature. However, if the master alloy is completely solidified, the master alloy may be separated even before the temperature reaches room temperature. The casting method is described in detail above in relation to the method of preparing an Mg master alloy and thus is not described here. 
     In this case, the prepared Al alloy may be any one selected from 1000-series, 2000-series, 3000-series, 4000-series, 5000-series, 6000-series, 7000-series, and 8000-series wrought Al, and 100-series, 200-series, 300-series, 400-series, 500-series, and 700-series casting Al. 
     As described above, if an Mg master alloy including a Ca-based compound is added, the cleanliness of molten Al may be improved and thus mechanical properties of a casted Al alloy may be remarkably improved. That is, due to an improved cleanliness of molten metal, an impurity such as an oxide or another inclusion which deteriorates mechanical properties does not exist in an Al alloy casted by using the molten metal, and bubbles inside the casted Al alloy are also reduced remarkably. Since the above-casted Al alloy has a cleaner internal state than a conventional Al alloy, the Al alloy according to the present invention has mechanical properties, for example, yield strength, tensile strength, and elongation, superior to the conventional Al ally. 
     Accordingly, although an Al alloy having the same content of Mg is prepared, according to the present invention, the cleanliness of molten metal may be improved and thus a casted Al alloy may have excellent properties. 
     Also, since the loss of Mg added into Al in molten metal is reduced, even when a smaller amount of Mg is added in comparison to a conventional case, an Al alloy may be economically prepared to have substantially the same content of Mg as the conventional case. 
     Furthermore, if an Mg master alloy according to the present invention is added into molten Al, instability of Mg in the molten Al may be remarkably improved and thus the content of Mg may be more easily increased in comparison to a conventional case. 
     Mg may be dissolved by up to 15 wt % in Al and thus may increase mechanical properties of an Al alloy. For example, if Mg is added into a 300-series or 6000-series Al alloy, strength and elongation of the Al alloy may be improved. 
     However, in a conventional case, due to the above-described high oxidizing potential of Mg, an oxide or another inclusion may be mixed into molten metal and thus the quality of an Al alloy may deteriorate. Since the above problem becomes more serious if the content of Mg is increased, even when a protective gas is used, the content of Mg added into molten Al may not be stably increased. 
     On the other hand, according to the present invention, an Mg master alloy may be stably added into molten Al such that castability may be ensured while increasing the ratio of Mg by easily increasing the content of Mg in an Al alloy in comparison to a conventional case. Accordingly, if an Mg master alloy is added into a 300-series or 6000-series Al alloy according to the present invention, an oxide or another inclusion may be suppressed from being mixed and strength, elongation, and castability may be improved. Furthermore, a 500-series or 5000-series Al alloy which is not substantially used at present may be used. 
     For example, an Al alloy according to the present invention may easily increase a dissolved amount of Mg up to 0.1 wt % or more, up to 5 wt % or more, up to 6 wt % or more, further up to 10 wt % or more, and even further up to 15 wt % corresponding to a limit of solubility. 
     The stability of Mg in an Al alloy may act favorably when a waste of the Al alloy is reused. For example, if the content of Mg is high in the waste used to prepare the Al alloy, a process of reducing the content of Mg to a required ratio (hereinafter referred to as a ‘demagging process’) is performed. If the required ratio of the content of Mg is low, the difficulty and the cost of the demagging process are increased. 
     For example, in a 383 Al alloy, it is technically easy to reduce the content of Mg to 0.3 wt % but it is very difficult to reduce the content of Mg to 0.1 wt %. Also, a chlorine gas (Cl 2 ) is used to reduce the ratio of Mg; however, the chlorine gas is environmentally harmful and requires an additional cost. 
     However, an Al alloy prepared by using an Mg master alloy including a Ca-based compound, according to the present invention, may maintain the ratio of Mg to be more than 0.3 wt % and thus has technical, environmental, and cost advantages. 
     Also, the method of preparing an Al alloy, according to the present invention, may further include adding a small amount of iron (Fe) after operation S11 or S12. In this case, the amount of added Fe may be less than that added in a conventional case. That is, if an Al alloy is conventionally casted, for example, die-casted, due to soldering between a die made of an iron-based metal and an Al cast material, the die may be damaged. In order to solve this problem, 1.0 wt % to 1.5 wt % of Fe has been conventionally added into an Al alloy when the Al alloy is die-casted. However, the addition of Fe may cause another problem of reducing corrosion resistance and elongation of the Al alloy. 
     However, as described above, an Al alloy according to the present invention may include Mg at a high ratio and the conventional soldering problem of a die may be significantly improved even though a considerably small ratio of Fe in comparison to a conventional case is added. Accordingly, the conventional problem of reducing corrosion resistance and elongation may be solved. 
     In this case, the content of Fe added to prepare the Al alloy may be less than or equal to 1.0 wt % (and greater than 0 wt %) with respect to the Al alloy, and more particularly, less than or equal to 0.2 wt % (and greater than 0 wt %). As such, Fe within the above composition range may be included in a matrix of the Al alloy. 
     Properties of an Al alloy prepared by using the method of preparing the Al ally, according to the present invention, will now be described in detail. 
     An Al alloy prepared by using the method of preparing the Al ally, according to the present invention, includes an Al matrix and a Ca-based compound existing in the Al matrix, wherein Mg may be dissolved in the Al matrix. 
     In this case, Mg may be dissolved in the Al matrix within a range of 0.1 wt % to 15 wt %. Also, Ca may be dissolved in the Al matrix by an amount less than or equal to a limit of solubility, for example, less than or equal to 500 ppm. 
     As described above, Ca reduced from a Ca-based additive added into an Mg master alloy mostly exists in the form of the Ca-based compound and only a part of it is dissolved in an Mg matrix. If the Mg master alloy is added into molten Al, since Ca dissolved in the Mg master alloy is diluted, the amount of Ca substantially dissolved in the matrix of the Al alloy also has a small value less than or equal to the limit of solubility. 
     Accordingly, the Al alloy according to the present invention has a microstructure in which Ca is dissolved in the Al matrix by an amount less than the limit of solubility, for example, less than 500 ppm, and the Ca-based compound is formed separately in the Al matrix. 
     In this case, the Al matrix may have a plurality of domains divided from each other by boundaries therebetween, and the Ca-based compound may exist at the boundaries or the inside of the domains. 
     The Al matrix may be defined as a metal structure body in which Al is a major component and another alloying element is dissolved, or another alloying element other than the Ca-based compound or a compound including the other alloying element is formed as a separate phase. 
     In this case, the plurality of domains divided from each other may typically be a plurality of grains divided by grain boundaries therebetween and, as an another example, may be a plurality of phase regions defined by two or more different phase boundaries therebetween. 
     The Al alloy according to the present invention may improve mechanical properties depending on the Ca-based compound formed in the Mg master alloy. As already described above, if the Mg master alloy is added into the molten Al, the Ca-based compound included in the Mg master alloy is also added into the molten Al. All Ca-based compounds are intermetallic compounds formed due to reaction between Ca and other metal elements and have higher melting points than Al. 
     Accordingly, if a master alloy including the Ca-based compound is added into the molten Al, the Ca-based compound may be remains without being melted in molten metal. Moreover, if an Al alloy is prepared by casting the molten metal, the Ca-based compound may be included in the Al alloy. 
     The Ca-based compound may be dispersed and distributed in the Al alloy in the form of fine particles. The Ca-based compound, as an intermetallic compound, is a high strength material in comparison to Al which is a matrix. Due to the dispersive distribution of such a high strength material, the strength of the Al alloy may be increased. 
     Meanwhile, the Ca-based compound may provide a site for nucleation during phase transition of the Al alloy occurs from a liquid phase to a solid phase. That is, the phase transition from the liquid phase to the solid phase when the Al alloy is solidified will be carried out in the form of nucleation and growth. In this case, since the Ca-based compound itself acts as a heterogeneous nucleation site, nucleation for phase transition to the solid phase is initially occurs on the interface between the Ca-based compound and the liquid phase. The nucleated solid phase grows around the Ca-based compound. 
     If the Ca-based compounds are distributed, solid phases growing at the interfaces of the different Ca-based compounds meet each other to form boundaries, and these boundaries may form grain boundaries or phase boundaries. Accordingly, if the Ca-based compound functions as a nucleation site, the Ca-based compound exists inside grains or phase regions, and the grains or phase regions become finer in comparison to a case when the Ca-based compound does not exist. 
     Also, the Ca-based compound may be distributed at grain boundaries, i.e., boundaries between grains, or phase boundaries, i.e., boundaries between phase regions. This is because such boundaries are further open and have a relatively high energy in comparison to the inside of the grains or the phase regions, and thus may provide a favorable site for nucleation and growth of the Ca-based compound. 
     If the Ca-based compound is distributed at grain boundaries or phase boundaries of an Al alloy, since this Ca-based compound acts as an obstacle to the movement of the grain boundaries or the phase boundaries, the movement of the grain boundaries or the phase boundaries may be suppressed and thus an average size of the grains or the phase regions may be reduced. 
     Accordingly, the Al alloy according to the present invention may have averagely finer and smaller grains or phase regions in comparison to an Al alloy not including the Ca-based compound. These fine and small grains or phase regions due to the Ca-based compound may improve both strength and elongation of the Al alloy. 
     The Al matrix may be any one selected from 1000-series, 2000-series, 3000-series, 4000-series, 5000-series, 6000-series, 7000-series, and 8000-series wrought Al, and 100-series, 200-series, 300-series, 400-series, 500-series, and 700-series casting Al. 
     In the Al alloy according to the present invention, a total amount of Ca may be 0.0001 parts by weight to 10 parts by weight based on 100 parts by weight of Al. The total amount of Ca is a sum of the amount of Ca dissolved in the Al matrix and the amount of Ca existing in the Ca-based compound. 
     In this case, most of Ca existing in the Al alloy exists in the Ca-based compound and the amount of Ca dissolved in the Al matrix is small. That is, as described above, most of Ca reduced from the Ca-based additive added into the Mg master alloy is not dissolved in the Mg matrix and forms the Ca-based compound. Accordingly, if the Mg master alloy is added to form Al, since the amount of Ca dissolved in the Mg master alloy is small, the amount of Ca dissolved in the Al matrix through the Mg master alloy is also small, for example, less than or equal to 500 ppm. 
     Meanwhile, the Al matrix may include dissolved Mg by 0.1 wt % to 15 wt %, by 5 wt % to 15 wt %, further by 6 wt % to 15 wt %, and even further by 10 wt % to 15 wt %. That is, as described above, if the Mg master alloy prepared by adding the Ca-based additive, according to the present invention, is used, the amount of Mg added into the molten Al may be stably increased. Accordingly, the amount of Mg dissolved in the Al matrix may also be increased. 
     The increase in the amount of dissolved Mg may greatly contribute to improvement of strength of the Al alloy according to solid solution strengthening and heat treatment, and may achieve superior castability and excellent mechanical properties in comparison to a conventional commercial alloy. 
     Furthermore, an Al alloy according to an embodiment of the present invention has an improved oxidation resistance in comparison to an Al alloy corresponding to the Al alloy according to an embodiment and not including the above-described Ca-based compound. As will be described below, the oxidation resistance of the Al alloy according to the present invention may be increased if the content of the Ca-based additive added to prepare the Mg master alloy is high. The improvement in oxidation resistance is related to an improvement in quality of an Al alloy and/or a distribution of the Ca-based compound in a matrix. 
     Here, an Al alloy corresponding to the Al alloy according to an embodiment may refer to a typical Al alloy including the same additive elements other than the Ca-based additive as the Al alloy according to an embodiment, for example, the same elements according to the standards of the Aluminum Association of America. 
     For example, if the Al alloy according to an embodiment is prepared by adding an Mg master alloy including a Ca-based compound, instead of Mg, into a typical 6061 alloy, an Al alloy corresponding to the Al alloy according to an embodiment may be the typical 6061 alloy. 
     Meanwhile, in a narrow sense, an Al alloy corresponding to the Al alloy according to an embodiment may refer to an Al alloy including the same-composition additive elements other than the Ca-based additive as the Al alloy according to an embodiment. For example, if the Al alloy is a new Al alloy that does not belong to those according to the standards of the Aluminum Association of America, an Al alloy corresponding to the Al alloy may refer to an Al ally having substantially the same contents of additive elements (except for the Ca-based additive) as the new Al alloy. Here, the same does not mean mathematically the same but means practically the same in consideration of, for example, an experimental error range. 
     Experimental examples will now be provided for better understanding of the present invention. The experimental examples described below are only for better understanding of the present invention and the present invention is not limited by the experimental examples below. 
     Table 2 comparatively shows cast properties of an Al alloy prepared by adding an Mg master alloy including CaO as a Ca-based additive into Al (Experimental Example 1) and an Al alloy prepared by adding pure Mg including no Ca-based additive into Al (Comparative Example 1). 
     Specifically, the Al alloy of Experimental Example 1 is prepared by adding 305 g of the Mg master alloy into 2750 g of Al, and the Al alloy of Comparative Example 1 is prepared by adding 305 g of pure Mg into 2750 g of Al. The Mg master alloy used in Experimental Example 1 employed an Mg—Al alloy as a parent material, and a weight ratio of CaO with respect to the parent material is 0.3. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Experimental 
                 Comparative 
               
               
                   
                 Example 1 
                 Example 1 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Amount of Dross 
                 206 g 
                 510 g 
               
               
                 (Impurity Floating on Surface 
               
               
                 of Molten Metal) 
               
               
                 Content of Mg in Al alloy 
                 4.89% 
                 2.65% 
               
               
                 Fluidity of Molten Metal 
                 Good 
                 Bad 
               
               
                 Hardness 
                 92.6 
                 92 
               
               
                 (HR Load 60 kg, 1/16″ Steel Ball) 
               
               
                   
               
            
           
         
       
     
     Referring to Table 2, the amount of an impurity floating on the surface of molten metal (the amount of dross) is remarkably smaller in the case when the Mg master alloy is added (Experimental Example 1) than the case when pure Mg is added (Comparative Example 1). Also, the content of Mg in the Al alloy is larger in the case when the Mg master alloy is added (Experimental Example 1) than the case when pure Mg is added (Comparative Example 1). As such, it may be seen that, according to the present invention, the loss of Mg is remarkably reduced in comparison to a method of adding pure Mg. 
     Also, the fluidity of the molten metal and the hardness of the Al alloy are superior in the case when the Mg master alloy is added (Experimental Example 1) than the case when pure Mg is added (Comparative Example 1). 
       FIGS. 4A and 4B  are images showing the states of molten metal according to Experimental Example 1 and Comparative Example 1. Referring to  FIGS. 4A and 4B , the state of the molten metal is good in Experimental Example 1 ( FIG. 4A ), but the surface of the molten metal changes to black due to oxidation of Mg in Comparative Example 1 ( FIG. 4B ). 
       FIGS. 5A and 5B  are images comparatively showing the surfaces of cast materials of the Al alloys according to Experimental Example 1 and Comparative Example 1. 
     Referring to  5 A and  5 B, the surface of the cast material of the Al alloy prepared by adding the Mg master alloy according to Experimental Example 1 ( FIG. 5A ) is cleaner than that of the cast material of the Al alloy prepared by adding pure Mg according to Comparative Example 1 ( FIG. 5B ). This is because castability is improved by CaO added into the Mg master alloy. That is, when the Al alloy prepared by adding pure Mg (Comparative Example 1) is casted, ignition marks are shown on the surface due to oxidation of pure Mg during casting. However, when the Al alloy prepared by adding the Mg master alloy including CaO (Experimental Example 1) is casted, ignition is suppressed and thus a clean surface may be obtained. 
     As such, it may be seen that the quality of the molten metal is remarkably improved and thus castability is improved in the case when the Mg master alloy is added in comparison to the case when pure Mg is added. 
       FIGS. 6A and 6B  are images showing results of energy dispersive spectroscopy (EDS) analysis on the Al alloys according to Experimental Example 1 and Comparative Example 1 by using a scanning electron microscope (SEM). Referring to  FIGS. 6A and 6B , in the Al alloy prepared by adding pure Mg according to Comparative Example 1 ( FIG. 6B ), only Mg and Al are detected. However, in the Al alloy prepared by adding the Mg master alloy including CaO according to Experimental Example 1 ( FIG. 6A ), Ca exists. Also, Mg and Al are detected at the same position and oxygen is hardly detected. As such, it may be seen that Ca exists as a Ca-based compound by reacting with Mg and/or Al after being reduced from CaO. 
       FIG. 7A  is an EPMA image of a microstructure of the Al alloy of Experimental Example 1, and  FIGS. 7B through 7E  are EPMA images showing component mapping results of Al, Ca, Mg, and O, respectively. 
     Ca and Mg are detected at the same position in an Al matrix as shown in  FIGS. 7B  through 7D, and O is not detected as shown in  FIG. 7E . 
     The above result corresponds to the result of  FIG. 6A , and thus it may be seen once again that Ca exists as a Ca-based compound by reacting with Mg and/or Al after being reduced from CaO. 
     Table 3 comparatively shows mechanical properties of die-cast alloys according to comparative examples and die-cast alloys according to experimental examples of the present invention. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Tensile Strength 
                 Yield Strength 
                 Elongation 
               
               
                   
                 (MPa) 
                 (MPa) 
                 (%) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Experimental 
                 314 to 366 
                 228 to 306 
                 6 to 8 
               
               
                 Example 2 
               
               
                 Comparative 
                 201 to 258 
                 184 to 204 
                 1.4 to 2.3 
               
               
                 Example 2 
               
               
                 Experimental 
                 357 to 435 
                 270 to 390 
                  5 to 17 
               
               
                 Example 3 
               
               
                 Comparative 
                 300 to 350 
                 210 to 300 
                  5 to 20 
               
               
                 Example 3 
               
               
                   
               
            
           
         
       
     
     Experimental Example 2 shows a die-cast binary Al—Mg alloy and prepared by adding 10 wt % of an Mg master alloy including CaO into Al. Comparative Example 2 shows, as a commercial Al alloy, a Magsimal-59 alloy including 5.0 wt % to 6.0 wt % of Mg, which is a quite high content of Mg in a commercial Al alloy. 
     Referring to Table 3, when Experimental Example 2 and Comparative Example 2 including relatively high contents of Mg are compared, the tensile strength, the yield strength, and the elongation according to Experimental Example 2 are higher than those according to Comparative Example 2. In particular, in Experimental Example 2, although 10 wt % of Mg is included in Al, a high tensile strength greater than 360 Mpa and a high elongation of 8% are obtained. 
     As described above, if the content of Mg included in molten Al is increased, the quality of molten metal is reduced due to oxidation of Mg. In an actual case, if 10 wt % of Mg is included in Al, commercialization through die-casting may not be easily acheived. However, according to Experimental Example 2, although the content of Mg is increased to 10 wt %, since the molten metal is maintained in a good state, it may be seen that a bad influence due to addition of Mg is suppressed and an improvement in mechanical properties due to addition of Mg is acheived. 
       FIG. 8  is an image showing a microstructure according to Experimental Example 2. Referring to  FIG. 8 , in the alloy according to Experimental Example 2, although the content of Mg is 10 wt %, an impurity such as an oxide or another inclusion is not observed and very fine grains are obtained. The size of the grains according to Experimental Example 2 is very small in comparison to a general commercial alloy. These clean and fine-grain properties are regarded as one factor for achieving excellent mechanical properties of Experimental Example 2. 
     Comparative Example 3 shows a die-cast material prepared by using a 7xx alloy including Mg by about 2 wt % to about 3.5 wt %, and Experimental Example 3 shows a die-cast material the same as that of Comparative Example 3 except that an Mg master alloy including CaO is added instead of pure Mg as an alloying element. 
     In comparison to Comparative Example 3, Experimental Example 3 achieves a significantly higher tensile strength and yield strength and achieves an equivalent elongation. As described above, it is regarded that the above result is related to an improvement in cleanliness of molten metal when an Al alloy according to an experimental example of the present invention is prepared. 
       FIGS. 9A and 9B  are images comparatively showing microstructures according to Experimental Example 3 and Comparative Example 3. Referring to  FIGS. 9A and 9B , in comparison to the alloy according to Comparative Example 3, the alloy according to Experimental Example 3 has much finer grains. As described above, it is regarded that these fine grains of the die-cast Al alloys according to the experimental examples are achieved because growth at grain boundaries is suppressed by the Ca-based compound distributed at the grain boundaries or because the Ca-based compound functions as a site for nucleation during solidification. Also, these fine grains are regarded as one factor for achieving excellent mechanical properties of the Al alloys according to the experimental examples. 
     In Table 4, Comparative Example 4 shows a die-cast material of an ALDC12 alloy that is the most commonly used as a commercial die-cast alloy and its mechanical properties are as shown in the ASM handbook. Experimental Examples 4-1 and 4-2 show die-cast materials having the same composition as that of Comparative Example 4 except that an Mg master alloy including CaO is added instead of pure Mg as an alloying element. In Experimental Example 4-1, 0.3 wt % of the Mg master alloy including 0.5 wt % of CaO is added to Al. In Experimental Example 4-2, 0.5 wt % of the Mg master alloy including 1.0 wt % of CaO is added to Al. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 Tensile Strength 
                 Yield Strength 
                 Elongation 
               
               
                   
                 (MPa) 
                 (MPa) 
                 (%) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Comparative 
                 160 
                 325 
                 4 
               
               
                 Example 4 
               
               
                 Experimental 
                 163 
                 344 
                 5.83 
               
               
                 Example 4-1 
               
               
                 Experimental 
                 180 
                 370 
                 6.19 
               
               
                 Example 4-2 
               
               
                   
               
            
           
         
       
     
     As shown in Table 4, in comparison to the commercial ALDC12 alloy according to Comparative Example 4, Experimental Examples 4-1 and 4-2 according to embodiments of the present invention achieve higher tensile strengths and yield strengths and superior elongations. In particular, in Experimental Example 4-2 including a larger content of CaO, the elongation as well as the strength is superior. As described above, the above result may be related to an improvement in cleanliness of molten metal when an Al alloy according to an experimental example of the present invention is prepared. 
       FIGS. 10A through 10C  are images showing microstructures of the die-cast Al alloy according to Comparative Example 4 and the die-cast Al alloys according to Experimental Examples 4-1 and 4-2. 
     Referring to  FIGS. 10A through 10C , in comparison to Comparative Example 4, Experimental Examples 4-1 and 4-2 achieve finer grains. As described above, it is regarded that these fine grains of the die-cast Al alloys according to the experimental examples are achieved because growth at grain boundaries is suppressed by the Ca-based compound distributed at the grain boundaries or because the Ca-based compound functions as a site for nucleation during solidification. Also, these fine grains are regarded as one factor for achieving excellent mechanical properties of the Al alloys according to the experimental examples. 
     Table 5 shows mechanical properties of Al alloy extruded materials according to experimental examples and Al alloy extruded materials according to comparative examples. Experimental Examples 5, 6, and 7 respectively show Al alloy extruded materials prepared by adding an Mg master alloy including CaO into a 5056 alloy, a 6061 alloy, and a 7075 alloy which are commercial Al alloys, and Comparative Examples 5, 6, and 7 respectively show the 5056 alloy, the 6061 alloy, and the 7075 alloy. 
     Specimens according to Experimental Examples 5, 6, and 7 are obtained by performing casting, extruding, and then T6 heat treatment, and Comparative Examples 5, 6, and 7 use data according to the ASM standards (T6 heat treatment data). 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                 Tensile Strength 
                 Yield Strength 
                 Elongation 
               
               
                   
                 (MPa) 
                 (MPa) 
                 (%) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Experimental 
                 424 
                 231 
                 34.2 
               
               
                 Example 5 (5056) 
               
               
                 Comparative 
                 290 
                 152 
                 35 
               
               
                 Example 5 (5056) 
               
               
                 Experimental 
                 349 
                 329 
                 17.8 
               
               
                 Example 6 (6061) 
               
               
                 Comparative 
                 310 
                 276 
                 12 
               
               
                 Example 6 (6061) 
               
               
                 Experimental 
                 662 
                 610 
                 13.6 
               
               
                 Example 7 (7075) 
               
               
                 Comparative 
                 572 
                 503 
                 11 
               
               
                 Example 7 (7075) 
               
               
                   
               
            
           
         
       
     
     As shown in Table 5, in comparison to the commercial Al alloy extruded materials not including the Ca-based compound, the Al alloy extruded materials according to experimental examples of the present invention achieve higher tensile strengths and yield strengths and superior or equivalent elongations. In particular, with respect to the 5000-series alloy, in comparison to the commercial Al alloy extruded material (Comparative Example 5), in the Al alloy extruded material according to the present invention (Experimental Example 5), the tensile strength may be greatly increased by about 1.46 times and the elongation may be maintained at an equivalent level. Furthermore, with respect to the 6000-series alloy and the 7000-series alloy, in comparison to the commercial Al alloy extruded materials (Comparative Examples 6 and 7), in the Al alloy extruded materials according to the present invention (Experimental Examples 6 and 7), both of the tensile strength and the elongation may be increased. 
     In more detail, with respect to the tensile strength, Experimental Example 5 is about 1.46 times of Comparative Example 5, Experimental Example 6 is about 1.13 times of Comparative Example 6, and Experimental Example 7 is about 1.16 times of Comparative Example 7. That is, the tensile strengths according to the experimental examples are about 1.13 times to about 1.46 times higher than the tensile strengths according to the comparative examples. Meanwhile, with respect to the elongation, Experimental Example 5 is about 0.98 times of Comparative Example 5, Experimental Example 6 is about 1.48 times of Comparative Example 6, and Experimental Example 7 is about 1.24 times of Comparative Example 7. 
     In general, if the strength of an alloy is increased, the elongation of the alloy is relatively reduced. However, the Al alloys according to the experimental examples of the present invention have ideal properties for increasing the strength and the elongation. As described above, the above result may be related to an improvement in cleanliness of molten metal of an Al alloy. 
       FIGS. 11 through 13  are images comparatively showing microstructures according to Experimental Examples 5, 6, and 7 and Comparative Examples 5, 6, and 7 shown in Table 5. 
     Referring to  FIGS. 11A and 11B , grains according to Experimental Example 5 ( FIG. 11A ) have an average size of about 25 μm, and grains according to Comparative Example 5 ( FIG. 11  B) have an average size of about 60 μm. That is, the grain size according to Experimental Example 5 is merely about 0.42 times of the grain size according to Comparative Example 5. Referring to  FIGS. 12A and 12B , grains according to Experimental Example 6 ( FIG. 12A ) have an average size of about 30 μm, and grains according to Comparative Example 6 ( FIG. 12B ) have an average size of about 50 μm. That is, the grain size according to Experimental Example 6 is merely about 0.6 times of the grain size according to Comparative Example 6. Referring to  FIGS. 13A and 13B , grains according to Experimental Example 7 ( FIG. 13A ) have an average size of about 25 μm, and grains according to Comparative Example 7 ( FIG. 13B ) have an average size of about 50 μm. That is, the grain size according to Experimental Example 7 is merely about 0.5 times (half) of the grain size according to Comparative Example 7. 
     Accordingly, it is seen that the Al alloy extruded materials according to the experimental examples of the present invention have much finer grains in comparison to the commercial Al alloy extruded materials. For example, the grain size of the Al alloy extruded materials according to the experimental examples of the present invention is in a range of about 0.42 times to about 0.6 times of the grain size of the commercial Al alloy is extruded materials. 
     It is regarded that these fine grains of the Al alloys according to the experimental examples are achieved because growth at grain boundaries is suppressed by the Ca-based compound distributed at the grain boundaries or because the Ca-based compound functions as a site for nucleation during solidification. Also, these fine grains are regarded as one factor for achieving excellent mechanical properties of the Al alloys according to the experimental examples. 
     Table 6 comparatively shows mechanical properties of Al alloy extruded materials according to experimental examples and Al alloy extruded materials according to comparative examples, based on heat treatment conditions. Comparative Examples 6-1, 6-2, and 6-3 respectively show specimens (extruded materials) prepared by casting, extruding, and heat-treating a commercial 6061 alloy under conditions T1, T5, and T6, and Experimental Examples 6-1, 6-2, and 6-3 respectively show specimens prepared by casting, extruding, and heat-treating an Al alloy, which is prepared by adding an Mg master alloy including CaO into the commercial 6061 alloy, under conditions T1, T5, and T6. In Table 6, an Al alloy substantially refers to an Al alloy extruded material that is completely heat-treated after being extruded. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 6 
               
               
                   
                   
               
               
                   
                   
                 Tensile 
                 Yield 
                   
               
               
                   
                 Hardness 
                 Strength 
                 Strength 
                 Elongation 
               
               
                   
                 (HRF) 
                 (MPa) 
                 (MPa) 
                 (%) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Experimental 
                 47.5 
                 117 
                 215 
                 20.9 
               
               
                 Example 6-1 (T1) 
               
               
                 Comparative 
                 43.7 
                 108 
                 200 
                 20.7 
               
               
                 Example 6-1 (T1) 
               
               
                 Experimental 
                 83.4 
                 203 
                 269 
                 16.1 
               
               
                 Example 6-2 (T5) 
               
               
                 Comparative 
                 63.5 
                 160 
                 234 
                 17.8 
               
               
                 Example 6-2 (T5) 
               
               
                 Experimental 
                 92.5 
                 385 
                 405 
                 15.7 
               
               
                 Example 6-3 (T6) 
               
               
                 Comparative 
                 94.1 
                 372 
                 396 
                 14.5 
               
               
                 Example 6-3 (T6) 
               
               
                   
               
            
           
         
       
     
     As shown in Table 6, regardless of the heat treatment conditions, in comparison to the commercial Al alloy extruded materials not including the Ca-based compound (Comparative Examples 6-1, 6-2, and 6-3), the Al alloy extruded materials according to embodiments of the present invention (Experimental Examples 6-1, 6-2, and 6-3) achieve higher tensile strengths and yield strengths and superior or equivalent elongations. 
     In general, if the strength of an alloy is increased, the elongation of the alloy is relatively reduced. However, the Al alloys according to the experimental examples of the present invention have ideal properties for increasing the strength and the elongation. As described above, the above result may be related to an improvement in cleanliness of molten metal of an Al alloy. 
     Meanwhile, after heat treatment is performed under conditions T1 and T5, the Al alloy extruded materials according to embodiments of the present invention (Experimental Examples 6-1 and 6-2) have higher hardness levels than the commercial Al alloy extruded materials (Comparative Examples 6-1 and 6-2). However, after heat treatment is performed under condition T6, the commercial Al alloy extruded material (Comparative Example 6-3) has a higher hardness level than the Al alloy extruded material according to an embodiment of the present invention (Experimental Example 6-3). 
       FIG. 14  is a graph showing oxidation resistance of Al alloys based on the content of CaO added to prepare an Mg master alloy. In this test, oxidation is performed in an O 2  atmosphere at about 550° C. for about 40 hours. The content of CaO added to prepare the Mg master alloy varies from 0 wt %, to 0.35 wt %, to 0.7 wt %, and to 1.0 wt %, and Al alloys prepared by using the Mg master alloy are represented as Al-5Mg, Al-5(Mg-5Al-0.35CaO), Al-5(Mg-5Al-0.7CaO), and Al-5(Mg-5Al-1.0CaO). In these alloys, the contents of additive elements other than CaO are substantially the same. 
     Referring to  FIG. 14 , under the same condition, in comparison to the comparative examples not including CaO, in the experimental examples including CaO according to the present invention, weight gains (%) of the specimens with respect to the increase in isothermal oxidation time are small. Furthermore, if the content of CaO is increased, that is, if the content of the Ca-based compound in the Al alloy is increase, the weight gain of the specimen is small. Considering that the weight of the specimen is increased as oxidation is processed, if the content of CaO is increased or if the content of the Ca-based compound in the Al alloy is increased, oxidation resistance of the Al alloy is increased. 
       FIG. 15  is a graph comparatively showing oxidation resistance of Al alloys according to comparative examples and Al alloys according to embodiments of the present invention, based on the content of Mg.  FIGS. 16A through 16G  are images comparatively showing oxidation resistance of Al alloys according to comparative examples and Al alloys according to embodiments of the present invention, based on the content of Mg. The Al alloys according to embodiments of the present invention have the same contents of additive elements as the Al alloys according to the comparative examples except that a Ca-based additive is added to prepare an Mg master alloy. The Al alloys according to embodiments of the present invention are marked as “Eco” in  FIGS. 15 and 16 . 
     Referring to  FIGS. 15 and 16 , in overall, if the content of Mg is increased, oxidation resistance is reduced. However, with respect to the same content of Mg, the Al alloys according to embodiments of the present invention have higher oxidation resistances than the Al alloys according to the comparative examples. In particular, the Al alloy including 2.5 wt % of Mg according to an experimental example of the present invention (Eco Al-2.5Mg) has a higher oxidation resistance than pure Al. In this sense, the Al alloys according to embodiments of the present invention may be referred to as oxidation-resistant Al alloys in comparison to typical Al alloys. 
     Meanwhile, the Al alloys according to embodiments of the present invention have excellent corrosion resistance.  FIG. 17  is a graph showing corrosion resistance of an Al alloy according to a comparative example and an Al alloy according to an embodiment of the present invention.  FIG. 18  is an image showing corrosion properties of an Al alloy according to a comparative example.  FIG. 19  is an image showing corrosion properties of an Al alloy according to an embodiment of the present invention. 
     As a reaction rate representing corrosion of metal, a corrosion rate may be represented as a corrosion loss for a unit period of time. In  FIG. 17 , the corrosion rate s is calculated in units of milimeters/year (mmy). Here, a K-factor is calculated as 8.75×10 4 . In this test, a commercial 7075 alloy (AA7075) is used in the comparative example, and an Al alloy prepared by adding an Mg master alloy including CaO into the commercial 7075 alloy (Eco 7075) is used in the experimental example. In the corrosion test, a salt spray test is performed by using a 3% NaCl solution, at 25° C. and pH 7.0, for 240 hours. 
     Referring to  FIGS. 17 through 19 , although differences exists based on specimens, a corrosion rate of the Al alloy according to an embodiment of the present invention (Eco 7075) is lower than or equivalent to the Al alloy according to the comparative example (AA7075). In this sense, the Al alloy according to an embodiment of the present invention may be referred to as an oxidation-resistant Al alloy in comparison to a typical Al alloy. 
     Table 7 shows results of a fatigue test of an Al alloy according to an experimental example of the present invention. The present experimental example uses an Al alloy having the same composition as a commercial 7075 alloy except that an Mg master alloy including CaO is added (hereinafter referred to as ECO-7075). The Al alloy according to the present experimental example has a yield strength of 590.89 Mpa (29.92 kN). In the fatigue test, stresses are 40%, 60%, and 80% of the yield strength (590.89 MPa), a stress amplitude is 5 kN, and frequencies are 10 Hz and 2 Hz. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 7 
               
             
            
               
                   
                   
               
               
                   
                 Cycle (N) 
                   
               
            
           
           
               
               
               
            
               
                 Stress 
                 10 Hz 
                 20 Hz 
               
               
                   
               
               
                 80% 
                 1,021,196 
                 2,012,008 
               
               
                 60% 
                 1,784,082 
                 — 
               
               
                 40% 
                 — 
                 — 
               
               
                   
               
            
           
         
       
     
     As shown in Table 7, according to the present experimental example, if a cyclic load is applied under the stress condition of 40% of a tensile strength, fatigue fracture does not occur. Under the stress condition of 80%, if frequency is 10 Hz, fatigue fracture occurs when the test is performed more than about a million times. If the frequency is 2 Hz, fatigue fracture occurs when the test is performed more than about two million times. The above result may not be easily achieved by a commercial Al alloy. 
     Accordingly, the Al alloy according to the experimental example of the present invention has superior fatigue properties to a corresponding commercial Al alloy (i.e., a 7075 alloy). 
       FIG. 20  is a graph showing mechanical properties of an Al alloy used in a fatigue test, according to an experimental example of the present invention. Referring to  FIG. 20 , the Al alloy according to the experimental example has a yield strength of 590.89 Mpa, a tensile strength of 651.9 Mpa, and an elongation of 13.6%. The above strength and elongation are much higher than a typical 7075 alloy. As such, it may be seen that the Al alloy according to the present experimental example has a high strength and excellent fatigue properties in comparison to a conventional Al alloy. 
     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.