WO2011122776A2

WO2011122776A2 - Melting method of magnesium-based metal and magnesium alloy manufactured using the same

Info

Publication number: WO2011122776A2
Application number: PCT/KR2011/001845
Authority: WO
Inventors: Shae K Kim
Original assignee: Korea Institute Of Industrial Technology
Priority date: 2010-03-29
Filing date: 2011-03-16
Publication date: 2011-10-06
Also published as: WO2011122776A3; KR20110108775A; KR101145124B1

Abstract

Provided is a melting method of a magnesium-based metal. In the melting method, a solid magnesium-based metal is covered with at least one substance of an alkali metal oxide, an alkali metal compound, an alkaline earth metal oxide, an alkaline earth metal compound, and mixtures thereof, before applying heat for melting. Then, the magnesium-based metal is melted by applying heat thereto.

Description

MELTING METHOD OF MAGNESIUM-BASED METAL AND MAGNESIUM ALLOY MANUFACTURED USING THE SAME

The present invention relates to a melting method of magnesium or magnesium-based metal and a magnesium alloy manufactured using the same.

Magnesium alloy solution, i.e., molten magnesium alloy, which is prepared through high-temperature heating, is liable to be ignited. To prevent molten magnesium alloys from being ignited, flux or protective gas is used.

Flux inhibits the reaction between molten magnesium alloy and oxygen to prevent ignition during processing. A flux composition may be mixtures of MgCl_2, KCl, and other metal chlorides, and may also include small amounts of CaF₂ and MgO in some cases. A protective gas minimizes the exposed area of molten magnesium alloy through densification and characteristic variation of an oxide film with respect to a surface of the molten magnesium alloy, to thus protect the molten magnesium alloy. Examples of protective gases include SO₂, CO₂, inert gas, SF₆, HFC-134a, Novec^TM612, and mixtures thereof.

However, using flux causes not only a portion of molten magnesium alloy to be lost due to the reaction between the molten magnesium alloy and the flux, but also mechanical and corrosion-resistant properties of the final product to be deteriorated because reaction products are introduced during casting.

Also, protective gas used to prevent the ignition of molten magnesium alloy is harmful to humans and causes corrosion of metal equipment. In particular, protective gases are classified as greenhouse gases, and their use is now being strictly regulated. For example, SF₆ has a global warming potential of 23,900 times that of CO₂. In advanced countries, therefore, regulations restricting the use of SF₆ are being established.

The present invention is directed to securing an environment-friendly melting technique for promoting the non-flammability of magnesium alloy and inhibiting the use of protective gases.

The present invention has been made in an effort to solve the above-mentioned problems of the prior art, and it is an object of the present invention to provide a magnesium alloy and a manufacturing method thereof, in which the used amount of protective gas is reduced or no protective gas is used. That is, the object of the present invention is to increase the resistance of a molten magnesium alloy to oxidation and ignition.

Another object of the present invention is to prevent the contamination of molten magnesium alloy which may be caused by protective gas. The present invention is to provide a method of manufacturing a magnesium alloy having improved cleanness in a furnace, or cleanness of a molten magnesium alloy during transporting or pouring.

Objects of the present invention are not limited to the aforesaid, and other objects not described herein will be clearly understood by those skilled in the art from descriptions below.

In accordance with an exemplary embodiment of the present invention, a method of melting a magnesium-based metal includes: covering a solid magnesium-based metal with at least one substance of an alkali metal oxide, an alkali metal compound, an alkaline earth metal oxide, an alkaline earth metal compound, and mixtures thereof, before applying heat for melting; and melting the magnesium-based metal by applying heat thereto.

The melting of the magnesium-based metal may further include supplying the alkali metal oxide, the alkali metal compound, the alkaline earth metal oxide, the alkaline earth metal compound, or the mixtures thereof to a surface of the magnesium-based metal.

In accordance with another exemplary embodiment of the present invention, a method of melting a magnesium-based metal includes: supplying SF₆ or a gas mixture of SF₆ and CO2 over a solid magnesium-based metal before applying heat for melting or at a temperature before the magnesium-based metal is melted; melting the solid magnesium-based metal by applying heat; covering the molten magnesium-based metal with at least one substance of an alkali metal oxide, an alkali metal compound, an alkaline earth metal oxide, an alkaline earth metal compound, and mixtures thereof; and stirring the molten magnesium-based metal until said at least one substance covering the molten magnesium-based metal is exhausted through a reaction with the molten magnesium-based metal.

An added amount of the alkali metal oxide, the alkali metal compound, the alkaline earth metal oxide, the alkaline earth metal compound, or the mixtures thereof may be 30 wt% based on a total weight of the molten magnesium-based metal.

The alkali metal oxide, the alkali metal compound, the alkaline earth metal oxide, the alkaline earth metal compound, or the mixtures thereof, which is supplied over the molten magnesium-based metal, may cover the molten magnesium alloy in succession at predetermined time intervals.

In accordance with yet another exemplary embodiment of the present invention, a method of melting a magnesium-based metal includes: melting a magnesium-based metal by applying heat thereto; and applying at least one substance of an alkali metal oxide, an alkali metal compound, an alkaline earth metal oxide, an alkaline earth metal compound, and mixtures thereof, over a surface of a molten magnesium-based metal before the magnesium-based metal being melted is ignited.

The alkali metal oxide, the alkali metal compound, the alkaline earth metal oxide, the alkaline earth metal compound, or the mixtures thereof, which is supplied over the molten magnesium-based metal, may include a calcium-based compound.

The alkali metal oxide, the alkali metal compound, the alkaline earth metal oxide, the alkaline earth metal compound, or the mixtures thereof, which is supplied over the molten magnesium-based metal, may be in the form of powders having a particle size of 0.1 ㎛ to 200 ㎛.

In accordance with still another exemplary embodiment of the present invention, a method of melting a magnesium-based metal includes: supplying an ignition preventing gas over a solid magnesium-based metal before applying heat for melting; melting the solid magnesium-based metal by applying heat thereto; covering a molten magnesium-based metal with at least one substance of an alkali metal oxide, an alkali metal compound, an alkaline earth metal oxide, an alkaline earth metal compound, and mixtures thereof; and stirring the molten magnesium-based metal until said at least one substance covering an upper layer portion of the molten magnesium-based metal is exhausted through a reaction with the molten magnesium-based metal.

The stirring of the molten magnesium-based metal may induce a surface reaction.

In accordance with even another exemplary embodiment of the present invention, a method of melting a magnesium-based metal includes: supplying a protective gas over a solid magnesium-based metal at 300 ℃ or higher after applying heat for melting; melting the solid magnesium-based metal by applying heat thereto; covering a molten magnesium-based metal with at least one substance of an alkali metal oxide, an alkali metal compound, an alkaline earth metal oxide, an alkaline earth metal compound, and mixtures thereof; and stirring the molten magnesium-based metal until said at least one substance covering the molten magnesium-based metal is exhausted through a reaction with the molten magnesium-based metal.

In accordance with further exemplary embodiment, a magnesium alloy is manufactured through the above-described methods.

As described above, according to the present invention, it is possible to prevent ignition by increasing the ignition temperature when melting magnesium alloy for manufacturing magnesium alloy, and also reduce or eliminate the use of protective gas, which is classified as a greenhouse gas, by adding an additive that inhibits oxidation.

Furthermore, according to the present invention, since impurities produced by ignition during the manufacture of a magnesium alloy are fundamentally removed, it is possible to improve cleanness in a furnace, and cleanness of molten magnesium alloy during transporting or pouring.

In addition, an additive (at least one of an alkali metal oxide, an alkali metal compound, and an alkaline earth metal compound) added during melting of a magnesium alloy can be purchased at low cost, and therefore, the total cost can be decreased by cost reduction of the additive itself as well as cost reduction resulting from reduction or removal of protective gases.

Exemplary embodiments can be understood in more detail from the following description taken in conjunction with the accompanying drawings, in which:

Figure 1 is a flowchart illustrating a melting method of a magnesium-based alloy according to an embodiment of the present invention;

Figure 2 is a flowchart illustrating a melting method of a magnesium-based alloy according to another embodiment of the present invention;

Figure 3 is a flowchart illustrating a reaction procedure when an additive, alkaline earth metal oxide (e.g., CaO) is used in the case of Figure 1 (in the case of applying a protective gas first);

Figure 4 is a schematic view exemplarily showing dissociation of an alkaline earth metal oxide (CaO) which is an additive, through stirring of an upper layer portion of a molten magnesium alloy;

Figures 5 and 6 are graphs showing results of protective gas reduction experiments of a molten magnesium alloy prepared by a manufacturing method of a magnesium-based alloy according to the present invention;

Figure 7 is a graph showing thermogravimetric analysis (TGA) experimental results of a magnesium alloy prepared by a manufacturing method of a magnesium-based alloy according to the present invention;

Figures 8 and 9 are graphs showing atomic emission spectroscopy (AES) experimental results of a magnesium alloy prepared by a manufacturing method of a magnesium-based alloy according to the present invention;

Figure 10 is a graph showing ignition temperatures of magnesium alloys in an ambient atmosphere and a nitrogen atmosphere, respectively, which are prepared by a manufacturing method of a magnesium-based alloy according to the present invention;

Figure 11 is a graph showing ignition temperatures of magnesium alloys in an ambient atmosphere and a nitrogen atmosphere, respectively, which are prepared by a manufacturing method of a magnesium-based alloy according to the present invention;

Figure 12 is a photograph showing a billet surface of a magnesium alloy prepared by a manufacturing method of a magnesium-based alloy according to the present invention;

Figure 13 is a graph showing an ignition temperature of a magnesium alloy prepared by a manufacturing method of a magnesium-based alloy according to the present invention;

Figure 14 is a graph showing an ignition temperature of a magnesium alloy prepared by a manufacturing method of a magnesium-based alloy according to the present invention;

Figure 15 is a graph showing an ignition temperature of a magnesium alloy prepared by a manufacturing method of a magnesium-based alloy according to the present invention;

Figure 16 is a graph showing an ignition temperature of a magnesium alloy prepared by a manufacturing method of a magnesium-based alloy according to the present invention;

Figures 17 and 18 are graphs showing ignition experimental results of magnesium alloys prepared by a manufacturing method of a magnesium-based alloy according to the present invention;

Figures 19 and 20 are graphs showing results of protective gas reduction experiments of a molten magnesium alloy prepared by a manufacturing method of a magnesium-based alloy according to the present invention;

Figures 21 and 22 are graphs showing ignition experimental results of magnesium alloys prepared by a manufacturing method of a magnesium-based alloy according to the present invention;

Figures 23 to 25 are photographs showing surfaces of magnesium alloys which are respectively prepared without a protective gas, with a protective gas, and with a method according to the present invention;

Figure 26 is a graph showing results of a DTA test carried out in a dry air atmosphere for investigating how the added amount of CaO has an effect on ignition characteristics of an AZ91D magnesium alloy according to the present invention;

Figure 27 is a graph showing results of a DTA test carried out in a dry air atmosphere for investigating how the added amount of SrO has an effect on ignition characteristics of an AZ31 magnesium alloy according to the present invention; and

Figure 28 illustrates results obtained by measuring a time taken for ignition by heating specimens with a torch under the same conditions in order to compare ignition characteristics of a CaO added magnesium alloy according to the present invention with those of commercially available high-temperature magnesium alloys.

Preferred embodiments of the present invention will be described below in more detail with reference to the accompanying drawings. In every possible case, like reference numerals are used for referring to the same or similar elements in the description and drawings. Moreover, detailed descriptions related to well-known functions or configurations will be ruled out in order not to unnecessarily obscure subject matters of the present invention.

In the present invention, an additive is input for the purpose of preventing or delaying ignition when a magnesium (Mg)-based metal is being melted. The inputting may be performed in such a way that the additive is applied on a solid magnesium-based metal before applying heat for melting. Alternatively, while the metal is being melted by heating or after a solid metal changes into a liquid metal by fully applying heat, the inputting of the additive may be carried out before ignition occurs. Here, since ignition occurs abruptly when the Mg-based metal is liquefied, an ignition preventing gas may be applied on a solid metal in advance. Also, the additive may be input after applying heat which is insufficient for a solid magnesium-based metal to be melted, then supplying a protective gas over the magnesium-based metal at 300 ℃ or higher, and melting the magnesium-based metal through additional heating.

In the present invention, materials used for the additive may include alkali metal oxide, an alkali metal compound, an alkaline earth metal oxide, an alkaline earth metal compound, and equivalents thereof. Further, mixtures of these additive materials may also be used as the additive. In particular, calcium-based compounds such as calcium oxides (e.g., CaO) and calcium compounds (e.g., Ca(OH)₂) are effective for the additive material. More preferably, calcium oxides may be effective.

Typical protective gases (protecting gas for molten magnesium) minimize an exposed area of a molten magnesium alloy due to the densification and characteristic variation of an oxide film with respect to a surface of the molten magnesium alloy, and thus protect the molten magnesium alloy. Examples of protective gases may be SO₂, CO₂, inert gas, SF₆, HFC-134a, Novec^TM612, and mixtures thereof. Among these gases, SF₆ or a mixture of SF₆ and CO₂ is mainly used.

The present invention is applicable to a process of manufacturing a magnesium alloy using a melting method of a magnesium-based metal or magnesium (hereinafter, collectively referred to as 'magnesium metal' in the Detailed Description and Claims).

As illustrated in Figure 1, a melting method of a magnesium-based metal according to an embodiment of the present invention includes the steps of: applying a protective gas (S1); forming a molten magnesium (S2); adding an additive into the molten magnesium (S3); and stirring the molten magnesium and the additive (S4). Alternatively, as illustrated in Figure 2, a melting method of a magnesium-based metal according to another embodiment of the present invention includes the steps of: applying an additive (S11); forming a molten magnesium (S12); and stirring the molten magnesium (S13). In Figure 1, the additive may be input in parallel with the step of applying the protective gas. As illustrated in Figure 1, using of the additive enables the total used amount of the protective gas to be considerably reduced even though the protective gas is used initially. The reducible amount of the protective gas may vary with retention time of the molten magnesium. However, it is possible to substantially reduce 90% or more of the total amount of protective gas which is typically used during a melting procedure. When the retention time of the molten magnesium is minimized, the used amount of the protective gas can be reduced to even 5% or less of the typically used amount.

In step S1 or S11 of forming the molten magnesium, magnesium is put into a crucible and heated at a temperature ranging from 400 ℃ to 800 ℃ under a protective gas atmosphere. The temperature provided herein for melting magnesium or magnesium alloys means a melting temperature of pure magnesium or magnesium alloys. The melting temperature may vary by alloy type. Separately from the input timing of the additive, it is preferable that magnesium or magnesium alloys may be completely melted in order for the additive and the molten magnesium or magnesium alloy to sufficiently react with each other.

In other words, as illustrated in Figure 2, the additive may be input before heating the solid magnesium-based metal or may be input at a temperature which is not high enough to melt the metal. Alternatively, the additive may also be applied in a state that initial ignition is delayed or prevented using an ignition preventing gas. After the magnesium alloy has been melted, the additive may be further supplied if necessary. In the case where the additive is further supplied, the additive may be used to cover the molten magnesium alloy in succession at predetermined time intervals. In the melting method of Figure 2, applying of the protective gas may be performed in parallel with inputting of the additive if necessary. In the case where the molten magnesium is formed after applying the additive as illustrated in Figure 2, it is basically unnecessary to use the protective gas. Through experiments, it was proven that magnesium can be melted by only initially applying the additive without using the protective gas.

The present invention is applicable to a process of melting any magnesium alloy which is generally available in industries. Examples of the magnesium alloys include AZ91D, AM20, AM30, AM50, AM60, AZ31, AS41, AS31, AS21X, AE42, AE44, AX51, AX52, AJ50X, AJ52X, AJ62X, MRM 53, MRI230, AM-HP2, Mg-Al, Mg-Al-Re, Mg-Al-Sn, Mg-Zn-Sn, Mg-Si, Mg-Zn-Y, and equivalents thereof, however, the present invention is not limited thereto.

The additive used in step S3 or S11 is added to the molten magnesium in the form of powders. It is preferable that the additive be powdered for accelerating the reaction with the magnesium alloy. Desirably, the additive may be added in a powder state so as to increase a surface area for efficient reaction. In the present invention, the addition of the additive may be carried out before heat is applied to solid magnesium, before the solid magnesium is melted even after heat is applied thereto, or before ignition occurs after heat is applied. In the case of using the protective gas, the protective gas may be used prior to heating of magnesium, or initially, however, the protective gas may be supplied at 300 ℃ or higher before melting. This makes it possible to further reduce the used amount of the protective gas.

Here, the additive may include alkali metal oxide, an alkali metal compound, an alkaline earth metal oxide, an alkaline earth metal compound, and equivalents thereof. Further, mixtures of these additive materials may also be used as the additive.

The alkali metal oxide may be at least one selected from sodium oxide, potassium oxide, and equivalents thereof. The alkaline earth metal oxide may be at least one selected from beryllium oxide, magnesium oxide, calcium oxide, strontium oxide, and equivalents thereof. The alkaline earth metal compound may be at least one selected from calcium carbide (CaC₂), calcium cyanamide (CaCN₂), calcium carbonate (CaCO₃), calcium sulfate hemihydrate (CaSO₄), and equivalents thereof.

In particular, calcium-based compounds (e.g., CaO or Ca(OH)₂) are effective. However, the present invention is not limited to the additive types above. That is, any material may be used as the additive so long as it can increase the ignition temperature of the magnesium alloy, reduce the oxidation of the magnesium alloy or reduce the required amount of the protective gas.

0.0001 to 30% by weight of the additive may be added based on the molten magnesium alloy. If the amount of the additive is less than 0.0001% by weight, the effects expected by the addition of the additive are not achieved. On the contrary, if the amount of the additive is more than 30% by weight, the inherent characteristics of magnesium or magnesium alloy are not exhibited. In the present invention, it was unnecessary to use the additive in an amount exceeding 30 wt% based on the total weight of the molten magnesium alloy. When the additive was used in an amount of 30 wt% or less, magnesium or magnesium alloy could be melted by using the protective gas at an initial stage as illustrated in Figure 1, or even without using the protective gas as illustrated in Figure 2.

The additive may have a particle size of 0.1 ㎛ to 500 ㎛. If the additive is too fine, that is, less than 0.1 ㎛ in size, the additive is liable to be scattered by vaporized magnesium or hot wind, thereby making it difficult to input the additive into a furnace. Further, the additives are agglomerated each other, and thus clustered while not being easily mixed with liquid molten metal. On the contrary, if the additive is too coarse, that is, greater than 500 ㎛ in size, it is undesirable because a total surface area is not increased. It is preferable that an ideal particle size should not exceed 500 ㎛. More preferably, the particle size may be 200 ㎛ or less.

In step S3, the molten magnesium is stirred for 1 second to 60 minutes per 0.1 wt% of the additive.

Here, if the stirring time is less than 1 second/0.1wt%, the additive is not mixed with the molten magnesium sufficiently; and, if the stirring time is more than 60 minutes/0.1wt%, the stirring time of the molten magnesium may be unnecessarily lengthened. In general, the stirring time depends on the volume of the molten magnesium and the input amount of additive.

The additive powders of a required amount may be input at once. However, to accelerate the reaction and reduce agglomeration possibility, it is preferable that the additive powders be re-input after a predetermined time elapses from a first input time, or the additive powders are grouped into several batches of appropriate amounts and the batches are input in sequence.

Stirring Method and Conditions

It is important that the additive used herein covers the molten magnesium uniformly. This induces the reaction between the additive and the molten magnesium, and prevents the molten magnesium from directly contacting the air. It is preferable to stir the molten magnesium for efficient reaction between the molten magnesium and the additive. Of course, the stirring may not be performed if the amount of the additive is too small. A main stirring method is to artificially stir (mechanically stir) the molten magnesium from the outside. In the case of mechanical stirring, the stirring may be performed in such a manner that the additive powders are not agglomerated. The ultimate purpose of the stirring is to induce the molten magnesium and added powders to properly react with each other.

The stirring time may vary with the temperature of a molten metal and the state (pre-heating state or the like) of powders added. Preferably, the stirring may continue to be performed in principle until the powders are not observed on the surface of the molten magnesium. Since the powders are lower in specific gravity than the molten magnesium so that they float on the molten magnesium in a normal state, it can be indirectly determined that the powders and the molten magnesium sufficiently react when the powders are not observed on the molten magnesium any longer. Herein, the term 'sufficiently react' means that all of the additive powders substantially react with the molten magnesium and are exhausted. Through the reaction between the molten magnesium and the additive, the ignition tendency of the molten magnesium can be significantly decreased even though ignition preventing gases or other well-known ignition preventing methods are not used. In the case where the additive is added before the magnesium-based metal is melted, the stirring may be performed after the additive is added and the solid magnesium-based metal is then melted by heating.

Stirring Time

The stirring is effective when it is performed after the molten magnesium is formed by heating the magnesium metal. In addition, the stirring may start after the powders receive heat from the molten magnesium and reach a predetermined temperature or higher, which enables acceleration of the reaction. The stirring continues to be performed until the powders are not observed on the surface of the molten magnesium. After the additive is completely exhausted through the reaction, the stirring is finished.

Surface Reaction

When the additive is input into the molten magnesium, the additive does not sink into the molten magnesium but floats on the surface of the molten magnesium due to a difference in specific gravity.

In the present invention, it is important to create a reaction environment where an oxide reacts on the surface rather than inside the molten magnesium. To this end, it is important not to forcibly stir the additive floating on the surface of the molten magnesium into the molten magnesium. It is important to spread the additives floating on a local region of the molten magnesium surface over other exposed regions of the molten magnesium surface.

Reaction occurred better in the case of stirring the molten magnesium, and also reaction occurred better at an outer surface (surface of an upper layer portion) rather than inside the molten magnesium. That is, the molten magnesium reacted better with the powders exposed to air at the outer surface (surface of an upper layer portion) thereof. It was more effective that one side of the additive was in contact with air. However, results were not satisfactory under a state of vacuum or ambient gas. For sufficient reaction, it is necessary to induce the surface reaction through stirring of the upper layer portion.

It is desirable that the stirring is performed at an upper layer portion of which a depth is about 20% of a total depth of the molten magnesium from the surface. If the depth is beyond 20%, the surface reaction according to a preferred example of the present invention is rarely generated. More preferably, the stirring may be performed in an upper layer portion of which a depth is about 10% of the total depth of the molten magnesium from the surface. The substantially floating additives are induced to be positioned in an upper layer portion of which a depth is 10% of an actual depth of the molten magnesium, thereby minimizing the turbulence of the molten magnesium.

Through the reaction between the molten magnesium and the additive, the additive is completely exhausted so as not to remain in the magnesium alloy at least partially or substantially.

For example, the exhausting of an alkaline earth metal oxide, which is one of the additives, involves removing an oxygen component from the alkaline earth metal oxide. The oxygen component is removed in the form of oxygen gas (O₂) or in the form of dross or sludge through combination with magnesium or alloying components in the molten magnesium. The oxygen component is substantially removed out from the top surface of the molten magnesium by stirring the upper layer portion of the molten magnesium. Figure 3 illustrates the case of melting a magnesium or magnesium alloy after applying a protective gas on the molten magnesium surface. Figure 4 is a schematic view exemplarily showing dissociation of an alkaline earth metal oxide (CaO) which is an additive, through stirring of the upper layer portion of the molten magnesium alloy.

An alkaline metal or alkaline earth metal produced by the exhaustion (dissociation) of the additive reacts with the molten magnesium alloy so as not to at least partially or substantially remain in the magnesium alloy. This means that the alkaline earth metal produced by the dissociation is compounded with at least one of magnesium, aluminum, and other alloying elements (components) in the magnesium alloy, and is thus not left remaining substantially. Here, a compound collectively refers to an intermetallic compound obtained through bonding between metals. In Figure 3, steps S4 and S5 of exhausting the additive are simultaneously performed with step S6 of forming a compound, however, these steps are separately set forth herein for the sake of understanding. That is, when supplying of the alkaline earth metal starts starts in step 4, step S5 is initiated.

In step 5 of exhausting the alkaline earth metal oxide, there is a flash during the reduction reaction of the alkaline earth metal oxide on the surface of the molten magnesium. The flash may be used as an index for confirming whether the reduction reaction is completed or not. In the case of terminating the reaction by tapping the molten magnesium while the flash is being generated, the alkaline earth metal oxide added may not be fully exhausted. That is, the tapping of the molten magnesium is performed after the flash, which can be used as an index for indirectly measuring the reduction reaction, disappears.

The following are reaction formulae respectively expressing that pure magnesium and magnesium alloy are dissociated by calcium oxide (one of the additives).

In pure molten magnesium, magnesium in the molten magnesium reacts with alkaline earth metal to thereby form a magnesium (alkaline earth metal) compound. For example, if the alkaline earth metal oxide is CaO, Mg₂Ca is formed. Oxygen constituting CaO is discharged out of the molten magnesium in the form of oxygen gas (O₂), or combines with Mg to be MgO and is then discharged in the form of dross (see Reaction Formula 1 below).

Reaction Formula 1

Pure Mg + CaO -> Mg (Matrix) + Mg₂Ca ... [O₂ produced + MgO dross produced]

In a molten magnesium alloy, magnesium in the molten magnesium alloy reacts with alkaline earth metal to thereby form a magnesium (alkaline earth metal) compound or an aluminum (alkaline earth metal) compound. Also, an alloying element reacts with alkaline earth metal to form a compound together with magnesium or aluminum. For example, if the alkaline earth metal oxide is CaO, Mg₂Ca, Al₂Ca, or (Mg, Al, other alloying element)₂Ca is formed. Oxygen constituting CaO is discharged out of the molten magnesium in the form of oxygen gas (O₂) as in the pure Mg case, or combines with Mg to be MgO, which is discharged in the form of dross (see Reaction Formula 2 below).

Herein, Ca supplied from the additive is prone to be compounded with elements other than Mg in the magnesium alloy.

Reaction Formula 2

Mg Alloy + CaO -> Mg Alloy (Matrix) + (Mg₂Ca + Al₂Ca + (Mg, Al, other alloying element)₂Ca} ... [O₂ produced + MgO dross produced]

As described above, the method of manufacturing a magnesium-based metal according to the present invention makes it possible to manufacture a magnesium alloy economically when compared to convetinoal methods of manufacturing a magnesium alloy. In the case of melting a magnesium alloy according to the method of the present invention, an ignition temperature ranges from 500 ℃ to 1,500 ℃, and thus flame resistance is improved. Additionally, as described below, the present invention considerably reduces the used amount of a protective gas.

Figures 5 and 6 are graphs showing results of protective gas reduction experiments of a molten magnesium alloy prepared by a method of the present invention.

The results in Figure 5 are obtained by carrying out an experiment at 680 ℃ which is a die casting temperature of a molten magnesium, and the results in Figure 6 is obtained by carrying out an experiment at 720 ℃ (in an over-heating state).

In Figures 5 and 6, “sealed” means a state that external air is not introduced, and “unsealed” means a state that external air is introduced.

As shown in Figure 5, in the unsealed state of 680 ℃ (the atmosphere where external air was introduced), 1000 ppm of SF₆ was required for each of AZ91D, AZ91D-0.04wt% CaO, and AZ91D-0.13wt% CaO in order to prevent ignition. However, in the sealed state where external air was not introduced, about 500 ppm of SF₆ was required for both AZ91D and AZ91D-0.04wt%, and 300 ppm of SF₆ was required for AZ91D-0.13wt% CaO in order to prevent ignition. Therefore, under the conditions of a typical die casting temperature, if air is not introduced, it can be understood that the more the input amount of the additive is, the smaller the required amount of the protective gas for ignition protection is.

Afterwards, as shown in Figure 6, in the sealed state of 720 ℃ (the atmosphere where external air is introduced), 3200, 2000 and 1000 ppm of SF₆ gases are required for AZ91D, AZ91D-0.04wt% CaO, and AZ91D-0.13wt% CaO, respectively, in order to prevent ignition. However, in the sealed state where air is not introduced, about 500 ppm of SF₆ gas was required for AZ91D and AZ91D-0.04wt% CaO and about 300 ppm of SF₆ gas was required for AZ91D-0.13wt% CaO in order to prevent ignition. Accordingly, in the case where air is not introduced at an overheating temperature, it can be understood that the more the input amount of the additive is the smaller the required amount of the protective gas for ignition protection is.

Figure 7 is a graph showing thermogravimetric analysis (TGA) experimental results of a magnesium alloy prepared by a manufacturing method of a magnesium alloy according to the present invention.

In Figure 7, the X-axis represents time, and the Y-axis represents weight increment (%). AZ91D shows that weight is sharply increased with the lapse of time. That is, oxidation is rapidly performed. In contrast, AZ91D-0.14wt% CaO shows that weight is slowly increased even with the lapse of time. That is, oxidation progresses slowly. For example, after 400 minutes pass, the weight of AZ91D was increased to 113 and the weight of AZ91D-0.14wt% CaO was increased to 101.

From the experimental results above, therefore, it can be appreciated that the oxidation of the magnesium alloy with the additive added can be prevented.

Figures 8 and 9 are graphs showing atomic emission spectroscopy (AES) experimental results of a magnesium alloy prepared by a manufacturing method of a magnesium alloy according to the present invention.

In Figure 8, the X-axis represents a sputtering time and the Y-axis represents a detected amount. AZ91D shows that oxygen is detected in an amount of about 40 until the sputtering time reaches 9 minutes, and the detected amount of oxygen becomes small thereafter. This means that an oxygen film is relatively thickly formed in AZ91D.

As illustrated in Figure 9, AZ91D-0.7wt% CaO shows that oxygen is detected in an amount of about 26 until the sputtering time reaches 1 minute, and the detected amount of oxygen becomes small thereafter. This means that an oxygen film is relatively thinly formed in AZ91D-0.7wt% CaO.

Accordingly, it can be understood from these experimental results that the oxidation of the magnesium alloy with the additive added is prevented.

Figure 10 is a graph showing an ignition temperature of a magnesium alloy prepared by a manufacturing method of a magnesium alloy according to the present invention.

As shown in Figure 10, it can be understood that the ignition temperature of the magnesium alloy increases as the additive amount of calcium oxide increases. That is, while the magnesium alloy of AZ91D has the ignition temperature of about 490 ℃ in an ambient atmosphere, and about 510 ℃ in a nitrogen atmosphere, the ignition temperature of the magnesium alloy with calcium oxide added is increased by about 100 ℃ according to the additive amount.

(Example 1)

Figure 11 is a graph showing ignition temperatures of magnesium alloys in an ambient atmosphere and a nitrogen atmosphere, respectively, which are prepared by a manufacturing method of a magnesium alloy according to the present invention.

As shown in Figure 11, an AZ91D magnesium alloy into which strontium oxide was not added had the ignition temperature of about 490 ℃ in an ambient atmosphere, and about 510 ℃ in a nitrogen atmosphere. However, the ignition temperature of an AZ91D magnesium alloy with strontium oxide added was increased by about 100 ℃ depending on the amount of strontium oxide added thereto.

(Example 2)

Figure 12 is a photograph showing a billet surface of a magnesium alloy prepared by a manufacturing method of a magnesium alloy according to the present invention.

Specifically, Figure 12 shows images of billet surfaces of AZ91D magnesium alloys for die casting into which alkaline earth metals are added, for example, 0.0001 wt% of beryllium oxide and 0.5 wt% of magnesium alloy are added, respectively. As shown in Figure 12, the billet surface of the AZ91D magnesium alloy was clean without being oxidized or ignited although 0.0001 wt% of beryllium oxide was added. Further, the billet surface of the AZ91D magnesium alloy was also clean without being oxidized or ignited although 0.5 wt% of magnesium oxide was added.

Figure 13 is a graph showing ignition temperatures of magnesium alloys prepared by a manufacturing method of a magnesium alloy according to the present invention.

As definitely observed from the results of Figure 13 on AZ91D magnesium alloys for die casting into which 0.0001 wt% of beryllium oxide and 0.5 wt% of magnesium oxide are added, respectively, the ignition temperature is increased depending on the additive amount of alkaline earth metals such as beryllium oxide and magnesium oxide, thereby improving mechanical and oxidation-resistant properties.

(Example 3)

Figure 14 is a graph showing ignition temperatures of magnesium alloys prepared by a manufacturing method of a magnesium alloy according to the present invention.

As shown in Figure 14, an AZ91D magnesium alloy into which calcium cyanamide, calcium-based compound, is added has the ignition temperature of about 540 ℃ in an ambient atmosphere. That is, the ignition temperature of the AZ91D magnesium alloy without additives is about 510 ℃ in an ambient atmosphere, and resultingly the ignition temperature of the AZ91D magnesium alloy is increased by about 30 ℃ when adding calcium cyanamide.

(Example 4)

Figure 15 is a graph showing ignition temperatures of magnesium alloys prepared by a manufacturing method of a magnesium alloy according to the present invention.

As shown in Figure 15, an AZ91D magnesium alloy into which calcium carbide, calcium-based compound, is added has the ignition temperature of about 540 ℃ in a nitrogen atmosphere. That is, since the AZ91D magnesium alloy without the additive has the ignition temperature of about 510 ℃ in a nitrogen ambient, the ignition temperature of the additive was increased by about 30 ℃.

Figure 16 is a graph showing ignition temperatures of magnesium alloys prepared by a manufacturing method of a magnesium alloy according to the present invention.

As shown in Figure 16, in an ambient atmosphere, both of magnesium alloys with calcium-based compounds added, i.e., AZ91D-0.007wt%CaCN₂ and AZ91D-0.03wt%CaC₂ are very superior in ignition temperature and heat resistance to the AZ91D magnesium alloy. From this result, it can be understood that the ignition temperature is significantly increased according to the added amount of calcium-based compounds.

(Example 5)

Figures 17 and 18 are graphs showing ignition experimental results of magnesium alloys prepared by a manufacturing method of a magnesium alloy according to the present invention.

As shown in Figure 17, the ignition temperature of an AM50 magnesium alloy without calcium oxide was about 570 ℃ in an ambient atmosphere, whereas the ignition temperature of the AM50 magnesium alloy was increased to about 590 ℃ when about 0.05 wt% of calcium oxide was added. Further, when about 0.15 wt% of calcium oxide was added, the ignition temperature was increased to about 610 ℃.

As shown in Figure 18, the ignition temperature of an AM50 magnesium alloy without calcium oxide was about 550 ℃ in a nitrogen atmosphere, whereas the ignition temperature of the AM50 magnesium alloy was increased to about 570 ℃ when about 0.05 wt% of calcium oxide was added. Further, when about 0.15 wt% of calcium oxide was added, the ignition temperature was increased to about 640 ℃.

(Example 6)

Figures 19 and 20 are graphs showing results of protective gas reduction experiments of a molten magnesium alloy prepared by a manufacturing method of a magnesium alloy according to the present invention.

Results in Figure 19 are obtained by carrying out an experiment at 680 ℃ which is a die casting temperature of a molten magnesium alloy suitable, and results in Figure 20 are obtained by carrying out an experiment at 720 ℃ (in an overheating state).

As shown in Figure 19, in the unsealed state of 680 ℃ (the atmosphere where air was introduced), 500 ppm of SF₆ was required for each of AZ31D, AZ31D-0.05wt% CaO, and AZ31D-0.32wt% CaO in order to prevent ignition. However, in the sealed state where air was not introduced, about 100 ppm of SF₆ was required for both AZ31D and AZ31D-0.05wt% CaO, and about 40 ppm of SF₆ was required for AZ31D-0.32wt% CaO in order to prevent ignition. Therefore, under the condition of a typical die casting temperature, if air is not introduced, it can be understood that the more the input amount of the additive is the smaller the required amount of the protective gas for ignition protection is.

Afterwards, as shown in Figure 20, in the sealed state of 720 ℃(the atmosphere where air is introduced), 1000 ppm of SF₆ gas was required for each of AZ31D, AZ31D-0.05wt% CaO, and AZ31D-0.32wt% CaO in order to prevent ignition. However, in the sealed state where air is not introduced, about 200 ppm of SF₆ gas was required for AZ31D, about 100 ppm of SF₆ gas was required for AZ31D-0.05wt% CaO, and 40 ppm of SF₆ gas was required for AZ31D-0.32wt% CaO in order to prevent ignition. Accordingly, in the case where air is not introduced at an overheating temperature, it can be understood that the more the input amount of the additive is the smaller the required amount of the protective gas for ignition protection is.

Figures 21 and 22 are graphs showing ignition experimental results of magnesium alloys prepared by a manufacturing method of a magnesium alloy according to the present invention.

As illustrated in Figure 21, the ignition temperature of an AZ31 magnesium alloy without calcium oxide was about 570 ℃ in an ambient atmosphere, whereas the ignition temperature was increased to about 610 ℃ when about 0.3 wt% of calcium oxide was added into the AZ31 magnesium alloy.

As illustrated in Figure 22, the ignition temperature of an AZ31 magnesium alloy without calcium oxide was about 640 ℃ in a nitrogen atmosphere, whereas the ignition temperature was increased to about 690 ℃ when about 0.3 wt% of calcium oxide was added into the AZ31 magnesium alloy.

Figures 23 to 25 are photographs showing surfaces of thin-plate castings of magnesium alloys which are respectively prepared without a protective gas, with a protective gas, and with a method according to the present invention.

As illustrated in Figure 23, when a magnesium alloy is prepared without the protective gas, it can be observed that the surface is blackened by ignition.

Also, as illustrated in Figure 24, even in the case where a magnesium alloy is prepared by applying a protective gas, it can be observed that cracking caused by oxide inclusion was generated.

In the case of using the inventive method of manufacturing a magnesium alloy with calcium oxide added, however, ignition and cracking were not observed, as illustrated in Figure 25.

Figure 26 is a graph showing results of a DTA test carried out in a dry air atmosphere for investigating how the added amount (wt%) of CaO has an effect on ignition characteristics of an AZ91D magnesium alloy. As seen from the experimental results of Figure 26, the ignition temperature increases as the additive amount of CaO increases. When the added amount of calcium oxide is 15.7 wt%, the AZ91D magnesium alloy has a high ignition temperature of about 1300 ℃. However, when the added amount of CaO exceeds 15.7 wt%, the ignition temperature is increased slowly and converged to a predetermined value.

Figure 27 is a graph showing results of a DTA test carried out in a dry air atmosphere for investigating how the added amount of SrO has an effect on ignition characteristics of an AZ31 magnesium alloy. As seen from the experimental results of Figure 27, the ignition temperature increases as the additive amount of strontium oxide increases. When the added amount of strontium oxide is 15.9 wt%, the ignition temperature is increased to about 1400 ℃. Up to 25.8 wt% from 15.9 wt% of SrO, the ignition temperature is maintained to a certain value.

Figure 28 illustrates results obtained by measuring a time taken for ignition by heating specimens with a torch under the same conditions in order to compare ignition characteristics of a CaO added magnesium alloy according to the present invention with those of commercially available high-temperature magnesium alloys. It can be observed from images of Figure 28 that CaO added magnesium alloys are not ignited for a long time compared to other commercially available magnesium alloys, which proves that the addition of CaO can prevent magnesium alloys from being ignited. It can also be observed that a Mg-3Al-1.13CaO alloy containing 1.13 wt% of CaO exhibits the best ignition resistance. That is, the Mg-3Al-1.13CaO alloy was ignited after the lapse of 200 seconds. In addition, it can be observed that when 1 wt% or more of CaO is added into magnesium alloys, all of the CaO added magnesium alloys are superior in ignition characteristic to commercially available high-temperature magnesium alloys (AS21, AE44, MRI153 and MRI230 in Figure 28) regardless of Al content. In Figure 28, Mg-3Al-1.13CaO alloy means a magnesium alloy which is obtained by adding 1.13 wt% of CaO as an additive into a Mg-3Al magnesium alloy according to the above-described method of the present invention (this indication is identically applied to Mg-6Al-1CaO and Mg-9Al-1.02CaO).

While the present invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. Therefore, the scope of the invention is defined not by the detailed description of the invention but by the appended claims, and all differences within the scope will be construed as being included in the present invention.

Claims

A method of melting a magnesium-based metal, the method comprising:

covering a solid magnesium-based metal with at least one substance of an alkali metal oxide, an alkali metal compound, an alkaline earth metal oxide, an alkaline earth metal compound, and mixtures thereof, before applying heat for melting; and

melting the magnesium-based metal by applying heat thereto.
The method of claim 1, wherein the melting of the magnesium-based metal further comprises supplying the alkali metal oxide, the alkali metal compound, the alkaline earth metal oxide, the alkaline earth metal compound, or the mixtures thereof to a surface of the magnesium-based metal.
A method of melting a magnesium-based metal, the method comprising:

supplying SF₆ or a gas mixture of SF₆ and CO2 over a solid magnesium-based metal before applying heat for melting or at a temperature before the magnesium-based metal is melted;

melting the solid magnesium-based metal by applying heat;

covering the molten magnesium-based metal with at least one substance of an alkali metal oxide, an alkali metal compound, an alkaline earth metal oxide, an alkaline earth metal compound, and mixtures thereof; and

stirring the molten magnesium-based metal until said at least one substance covering the molten magnesium-based metal is exhausted through a reaction with the molten magnesium-based metal.
The method of claim 3, wherein an added amount of the alkali metal oxide, the alkali metal compound, the alkaline earth metal oxide, the alkaline earth metal compound, or the mixtures thereof is 30 wt% based on a total weight of the molten magnesium-based metal.
The method of claim 4, wherein the alkali metal oxide, the alkali metal compound, the alkaline earth metal oxide, the alkaline earth metal compound, or the mixtures thereof, which is supplied over the molten magnesium-based metal, cover the molten magnesium alloy in succession at predetermined time intervals.
A method of melting a magnesium-based metal, the method comprising:

melting a magnesium-based metal by applying heat thereto; and

applying at least one substance of an alkali metal oxide, an alkali metal compound, an alkaline earth metal oxide, an alkaline earth metal compound, and mixtures thereof, over a surface of a molten magnesium-based metal before the magnesium-based metal being melted is ignited.
The method of claim 6, wherein the alkali metal oxide, the alkali metal compound, the alkaline earth metal oxide, the alkaline earth metal compound, or the mixtures thereof, which is supplied over the molten magnesium-based metal, comprises a calcium-based compound.
The method of claim 7, further comprising, before the magnesium-based metal being melted is ignited, stirring the molten magnesium-based metal until the alkali metal oxide, the alkali metal compound, the alkaline earth metal oxide, the alkaline earth metal compound, or the mixtures thereof, is exhausted through a reaction.
The method of claim 8, wherein the alkali metal oxide, the alkali metal compound, the alkaline earth metal oxide, the alkaline earth metal compound, or the mixtures thereof, which is supplied over the molten magnesium-based metal, is in the form of powders having a particle size of 0.1 ㎛ to 200 ㎛.
A method of melting a magnesium-based metal, the method comprising:

supplying an ignition preventing gas over a solid magnesium-based metal before applying heat for melting;

melting the solid magnesium-based metal by applying heat thereto;

covering a molten magnesium-based metal with at least one substance of an alkali metal oxide, an alkali metal compound, an alkaline earth metal oxide, an alkaline earth metal compound, and mixtures thereof; and

stirring the molten magnesium-based metal until said at least one substance covering an upper layer portion of the molten magnesium-based metal is exhausted through a reaction with the molten magnesium-based metal.
The method of claim 10, wherein the stirring of the molten magnesium-based metal induces a surface reaction.
A method of melting a magnesium-based metal, the method comprising:

supplying a protective gas over a solid magnesium-based metal at 300 ℃ or higher after applying heat for melting;

melting the solid magnesium-based metal by applying heat thereto;

covering a molten magnesium-based metal with at least one substance of an alkali metal oxide, an alkali metal compound, an alkaline earth metal oxide, an alkaline earth metal compound, and mixtures thereof; and

stirring the molten magnesium-based metal until said at least one substance covering the molten magnesium-based metal is exhausted through a reaction with the molten magnesium-based metal.
A magnesium alloy manufactured through the methods of any one of claims 1 to 12.