Magnesium alloy with excellent ignition resistance and mechanical properties, and method of manufacturing the same

Abstract

A magnesium alloy that forms a stable protective film on the surface of molten metal, having excellent ignition resistance restricting natural ignition of a chip thereof as well as having excellent strength and ductility, so that the Mg alloy can be melted and cast in the air or a common inert atmosphere. The magnesium alloy includes, by weight, 7.0% or greater but less than 11% of Al, 0.05% to 2.0% of Ca, 0.05% to 2.0% of Y, greater than 0% but not greater than 6.0% of Zn, and the balance of Mg, and the other unavoidable impurities. The total content of the Ca and the Y is equal to or greater than 0.1% but less than 2.5% of the total weight of the magnesium alloy.

Claims

1. A magnesium alloy manufactured by melt casting, the magnesium alloy consisting of, by weight, 7.0% or greater but less than 9.5% of Al, 0.05% to 0.58% of Ca, 0.05% to 0.4% of Y, greater than 0% but less than 1.2% of Zn, greater than 0% but not greater than 1.0% of Mn, a balance of Mg, and other unavoidable impurities, and an oxide layer including a CaO and Y.sub.2O.sub.3 layer disposed on the magnesium alloy, and a MgO and CaO layer disposed on the CaO and Y.sub.2O.sub.3 layer, wherein a total content of the Ca and the Y is equal to or greater than 0.1% but less than 0.98% of a total weight of the magnesium alloy.

2. The magnesium alloy of claim 1, wherein the total content of the Ca and the Y is equal to or greater than 0.1% but less than 0.7% of the total weight of the magnesium alloy.

Description

DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a view showing variation in the ignition temperature depending on the amount of Ca and Y that is added in comparative example 2 to comparative example 7 and example to example 6, which are cast according to an exemplary embodiment of the invention;

(2) FIG. 2 is a view showing the results of electron probe micro-analysis (EPMA) on an oxide layer on the surface of a molten metal after a magnesium alloy according to example 4, which was cast according to an exemplary embodiment of the invention, was maintained at 670° C. for 10 minutes;

(3) FIG. 3 is a view schematically showing the structure of double composite oxide layers formed on the surface of a solid or liquid phase in an alloy in which Ca and Y are added in combination, the double composite oxide layers serving to block the penetration of external oxygen; and

(4) FIG. 4 is a view showing variation in yield strength, tensile strength and elongation depending on the amount of Ca that is added in comparative example 2 to comparative example 7, which are cast according to an exemplary embodiment of the invention.

BEST MODE

(5) Reference will now be made in detail to exemplary embodiments of a Mg alloy and a method of manufacturing the same according to the present invention. However, it is to be understood that the following embodiments are illustrative but do not limited the invention.

(6) The method of manufacturing a Mg alloy according to an exemplary embodiment of the invention is as follows.

(7) First, raw materials that include Mg (99.9%), Al (99.9%), Zn (99.99%), Ca (99.9%), Y (99.9%) and selectively Mn (99.9%) were prepared, and were then melted. Then, Mg alloy cast materials having the alloy compositions described in comparative example 1 to comparative example 7 and example 1 to example 6 in Table 1 below were produced from the raw materials using a gravity casting method. Specifically, the temperature of a molten metal was increased up to a temperature between 850° C. and 900° C., so that these elements were completely melted, in order to produce an alloy by directly inputting Ca and Y, which have high melting points of 842° C. and 1525° C., respectively, into the molten metal. After that, the molten metal was gradually cooled down to a casting temperature, and then the Mg alloy cast materials were produced by casting the molten metal.

(8) Alternatively, according to an exemplary embodiment of the invention, it is possible to manufacture a Mg alloy by a variety of methods in addition to the method in which casting is performed after a molten metal is formed by simultaneously melting raw materials including Mg (99.9%), Al (99.9%), Zn (99.99%), Ca (99.9%) and Y (99.9%). In an example, it is possible to first form a Mg alloy molten metal using the raw materials of Mg, Al and Zn or alloys thereof, input the raw materials of Ca and Y, or a Ca compound and a Y compound into the Mg alloy molten metal, and then produce a Mg alloy cast material by a suitable casting method. It is also possible to produce a Mg alloy cast material by preparing a Mg, Al, Zn, Ca and Y alloy (master alloy ingot) of which the contents of Ca and Y are higher than final target values, forming a Mg alloy molten metal using raw materials of Mg, Al and Zn or alloys thereof, and then inputting the master alloy ingot into the Mg alloy molten metal. This method is particularly advantageous in that the master alloy ingot can be input at a temperature that is lower than the temperature at which the raw materials of Ca and Y are directly input into the Mg alloy molten metal, since the melting point of the master alloy ingot is lower than those of the raw materials of Ca and Y. In addition, the formation of a Mg alloy according to the invention can be realized by a variety of methods, and all methods of forming a Mg alloy that are well-known in the art to which the invention belongs are included as part of the invention.

(9) TABLE-US-00001 TABLE 1 Alloy Composition Alloy Symbol Al Zn Ca Y Mn Comp. Ex. 1 AZ80 7.76 0.54 0.17 Comp. Ex. 2 AZ91 8.51 0.65 0   0.21 Comp. Ex. 3 AZ91 + 0.2Ca 8.89 0.76 0.20 0.21 Comp. Ex. 4 AZ91 + 0.5Ca 8.35 0.62 0.49 0.22 Comp. Ex. 5 AZ91 + 0.7Ca 8.85 0.67 0.63 0.25 Comp. Ex. 6 AZ91 + 1.0Ca 8.08 0.60 0.91 0.21 Comp. Ex. 7 AZ91 + 2.0Ca 8.42 0.68 2.10 0.21 Example 1 Alloy 1 7.98 0.55 0.61 0.19 0.22 Example 2 Alloy 2 7.94 0.50 0.18 0.12 0.20 Example 3 Alloy 3 8.68 0.65 0.58 0.21 0.21 Example 4 Alloy 4 8.56 0.68 0.97 0.59 0.22 Example 5 Alloy 5 8.56 0.53 0.24 0.10 0.22 Example 6 Alloy 6 8.63 0.72 0.10 0.10 0.20

(10) In this embodiment, a graphite crucible was used for induction melting, and a mixture gas of SF.sub.6 and CO.sub.2 was applied on the upper portion of the molten metal, so that the molten metal did not come into contact with the air, in order to prevent the molten metal from being oxidized before the alloying process was finished. In addition, after the melting was completed, mold casting was performed using a steel mold without a protective gas. A plate-shaped cast material having a width of 100 mm, a length of 150 mm and a thickness of 15 mm was manufactured for a rolling test, a cylindrical billet having a diameter of 80 mm and a length of 150 mm was manufactured for an extrusion test, and a cylindrical billet having a diameter of 55 mm and a length of 100 mm was manufactured for an ignition test of the alloy cast material. Although the Mg alloy was cast by a mold casting method in this embodiment, a variety of casting methods, such as sand casting, gravity casting, squeeze casting, continuous casting, strip casting, die casting, precision casting, spray casting, semi-solid casting, and the like, may also be used. The Mg alloy according to the invention is not necessarily limited to a specific casting method.

(11) Afterwards, the slabs manufactured by selecting some of the alloys that were prepared above were subjected to homogenization heat treatment at 400° C. for 15 hours. In sequence, the materials of comparative example 2 to comparative example 6 and example 4 in Table 1, which were subjected to homogenization heat treatment, were machined into sheet materials having a final thickness of 1 mm via hot working, in which the respective materials were rolled under conditions of a roll temperature of 200° C., a roll diameter of 210 mm, a roll speed of 5.74 mpm, and a reduction ratio of each roll of 30%/pass.

(12) In addition, in comparative example 1 and example 2 in Table 1, rod-shaped extruded materials having a final diameter of 16 mm were manufactured by extruding the billets that were subjected to homogenization heat treatment under conditions including an extrusion speed of 5 m/min, an extrusion ratio of 25:1, and an extrusion temperature of 250° C. The extruded materials had a good surface state.

(13) Although rolling and extrusion were performed after casting and homogenization heat treatment in this embodiment, the materials may be manufactured by a variety of forming methods, such as forging and drawing, without being necessarily limited to a specific forming method.

(14) Measurement of Ignition Temperature of Mg Alloy

(15) Afterwards, in order to measure the ignition temperature of the Mg alloys, chips having a predetermined size were produced by machining the outer portion of the cylindrical billets, which were manufactured above, in conditions including a depth of 0.5 mm, a pitch of 0.1 mm, and a constant speed of 350 rpm. 0.1 g chips that were produced by the foregoing method were heated by loading them at a constant speed into a heating furnace, which was maintained at 1000° C. The temperatures at which a sudden rise in temperature begins during this process were determined as ignition temperatures, and the results are presented in Table 2. Each value of the ignition temperatures presented in Table 2 indicates the mean of values measured by test that was performed at least 5 times on the same composition.

(16) TABLE-US-00002 TABLE 2 Ignition Temperature (° C.) Comp. Ex. 1 583 Comp. Ex. 2 565 Comp. Ex. 3 692 Comp. Ex. 4 729 Comp. Ex. 5 744 Comp. Ex. 6 767 Comp. Ex. 7 786 Example 1 742 Example 2 714 Example 3 783 Example 4 810 Example 5 743 Example 6 747

(17) FIG. 1 is a view showing variation in the ignition temperature depending on the content of Ca according to comparative example 2 to comparative example 7 and example 3 to example 6, which were manufactured using the above-described method.

(18) As presented in Table 2 and shown in FIG. 1, the ignition temperature of Mg alloys of comparative example 2 to comparative example 7 suddenly increases as the amount of Ca that is added increases to 1 wt %, and after that, tends to increase at a uniform rate. This is because thin and dense composite oxide films of CaO and MgO formed on the surface of the surface of the solid or liquid alloy acted as a protective film, thereby increasing the ignition temperature.

(19) In Table 2, comparing each ignition temperature of example 3 and example 4 with the respective ignition temperature of comparative example 5 and comparative example 6, it can be appreciated that the ignition temperature is much higher when Y was also added to the Mg alloys than when Ca was added alone to the Mg alloys. This is because a mixed layer of CaO and Y.sub.2O.sub.3 was formed in the portion that was in contact with molten metal due to the addition of Y, as can be seen from the result of electron probe micro-analysis (EPMA) of FIG. 2, and that this layer was able to effectively reduce the oxygen in the air from penetrating into and reacting with the molten metal. In addition, a mixed layer of CaO and MgO was present in the outer portion of the mixed layer of CaO and Y.sub.2O.sub.3. As shown in FIG. 3, these double mixed layers help the molten metal remain more stable by effectively reducing the penetration of oxygen into the molten metal even at high temperatures. In this way, it can be appreciated that the composite oxide layers of CaO and Y.sub.2O.sub.3 were formed between the existing oxide layer and the surface of the alloy due to the addition of a small amount of Y to the alloy in which Ca was added, thereby further improving the ignition resistance of the alloy.

(20) In addition, comparing comparative example 4 with example 5, comparative example 6 with example 3, and comparative example 7 with example 4, it can be appreciated that the ignition temperature was higher when Ca and Y were added in combination than when Ca was added alone, even though the total content of Ca and Y was less than the content of Ca. This shows that a more excellent effect can be realized in terms of increasing ignition resistance when Ca and Y are added in combination than when Ca is used alone in order to increase the ignition temperature of the Mg alloy.

(21) Evaluation of Tensile Properties of Mg Alloy

(22) Samples of a rod-shaped extruded material according to the ASTM-E-8M standard, in which the length of a gauge was 25 mm, were manufactured using the Mg alloys of comparative example 1 to comparative example 7 and example 1 to example 6, which were manufactured by the above-described method, and a tensile test was carried out at room temperature under a strain of 1×10.sup.−3 s.sup.−1 using a common tensile tester. Alternatively, in the case of rolled materials, rolled sheet materials having a thickness of 1 mm were heat-treated at 250° C. for 30 minutes, and then sub-size sheet-shaped samples in which the length of a gauge was 25 mm, were produced. Tensile test was carried out under the same conditions as for the rod-shaped samples. The results are presented in Table 3.

(23) TABLE-US-00003 TABLE 3 Yield Tensile Strength Strength Elongation (MPa) (MPa) (%) Remarks Comp. Ex. 1 101.7 137.3 2.3 Cast material 167.1 295.6 25.1 Extruded material Comp. Ex. 2 102.2 156.2 3.6 Cast material 283 383 11.7 Rolled material Comp. Ex. 3 104.5 154.7 3.3 Cast material Comp. Ex. 4 100.2 160.6 3.9 Cast material Comp. Ex. 5 104.3 135.3 1.9 Cast material Comp. Ex. 6 103.2 138.9 2.1 Cast material 277 349 8.4 Rolled material Comp. Ex. 7 101.3 139.3 2.3 Cast material Example 1 97.1 138.0 2.8 Cast material Example 2 194.5 317.9 20.1 Extruded material Example 3 102.0 153.4 3.1 Cast material Example 4 277 352 8.2 Rolled material Example 6 99.2 155.0 3.1 Cast material

(24) As shown in FIG. 4, comparing the tensile properties of the cast materials of comparative example 2 to comparative example 7, it can be appreciated that all of the yield strength, the tensile strength and the elongation were increased due to minute effects caused by the addition of Ca as the amount of Ca that was added was increased to 0.5 wt % but were decreased when the amount of Ca that was added was 0.7 wt % or greater. In particular, the elongation of the alloy in which Ca was added in an amount of 0.7 wt % or greater decreased to be smaller than the elongation of comparative example 2 in which Ca was not added. In order to ensure safety in the case of melting in the condition of being exposed to the air and chip machining, an increase in the ignition temperature is essential. For this purpose, at least 1 wt % or greater of Ca must be added. However, in this case, a sudden decrease in the elongation is problematic.

(25) However, as presented in Table 2, comparing comparative example 5 and comparative example 3, it can be appreciated that the tensile strength and elongation of the cast materials were increased when 0.2 wt % of Y was added, if Ca was used in similar contents of 0.63 wt % and 0.58 wt %. This means that the addition of Y can greatly increase the ignition temperature without inducing deterioration in the tensile properties. In fact, the ignition temperature of example 3 in which 0.2 wt % of Y was added was 783° C., increased about 40° C. from the ignition temperature of example 5. This is similar to the ignition temperature of comparative example 7 in which 2.1 wt % of Ca was added. Therefore, the alloy in which 0.58 wt % of Ca and 0.21 wt % of Y are added in combination can have ignition resistance that is the same as that of an alloy in which 2.1 wt % of Ca is added alone as well as tensile properties that are similar to the tensile properties of an ally in which Ca is not added, which are about in the middle of the tensile properties of an alloy in which 0.49 wt % of Ca is added alone and the tensile properties of an alloy in which 0.63 wt % of Ca is added alone.

(26) In addition, comparing comparative example 6 and example 4, it can be appreciated that the tensile properties of the rolled material in the alloy in which the content of Ca was about 1 wt % as in the above were not substantially influenced by the addition of 0.59 wt % of Y. However, due to the addition of Y, the ignition temperature of example 4 was 810° C., which was about 43° C. higher than that of comparative 6. This is also higher than the ignition temperature of comparative example 7 in which 2.1 wt % of Ca was added. Therefore, also for the rolled materials, it can be appreciated that the ignition temperature of the rolled material can also be greatly increased without the decrease in the tensile properties, due to the addition of Y.

(27) As presented in Table 2 and Table 3, comparing comparative example 1 and example 1, it can be appreciated that, even in the alloys in which the respective contents of Al and Zn were decreased to 8 wt % and 0.55 wt %, when both 0.61 wt % of Ca and 0.19 wt % of Y were added, the tensile strength and elongation of the cast material were increased to be slightly greater than those of the alloy in which Ca was not added and the ignition temperature thereof was 742° C., which was increased about 160° C. from that of the alloy in which Ca was not added. In addition, as presented in Table 3, comparing the tensile properties of the extruded materials of comparative example 1 and example 2, it can be appreciated that the yield strength and tensile strength of the alloy in which 0.18 wt % of Ca and 0.12 wt % of Y were added were increased but the elongation thereof were decreased from those of the alloy in which Ca was not added. Nevertheless, the extruded material of example 2 still shows a high value of elongation of about 20%.

(28) As such, it can be appreciated that the ignition resistance of the alloy in which both Ca and Y are added is greatly improved and the tensile properties thereof are also improved from those of an alloy in which Ca is added alone.

(29) The Mg alloy and the method of manufacturing the same according to exemplary embodiments of the present invention have been described above in detail with reference to the accompanying drawings. However, it will be apparent to a person having ordinary skilled in the art to which the present invention belongs that the foregoing embodiments are merely examples of the invention and various modifications and variations are possible. Therefore, it should be understood that the scope of the invention shall be defined only by the appended claims.