Mg Alloys Containing Misch Metal Manufacturing Method of Wrought Mg Alloys Containing Misch Metal, and Wrought Mg Alloys Thereby

Abstract

There are provided a magnesium alloy with a misch metal, a method of producing a wrought magnesium alloy with a misch metal, and a wrought magnesium alloy produced thereby, in which a great deal of misch metal is added to magnesium, and thus refractory eutectic phases or multi-phases are formed into a stable network structure or a stable dispersed phase, thereby inhibiting deformation of a magnesium matrix at a high temperature to maintain a high strength. The magnesium alloy with the misch metal has the formula of Mg100-x-y-gAxByCz, where A is zinc (Zn) or aluminum (Al); B is the misch metal; C is at least one element selected from the group consisting of manganese (Mn), nickel (Ni), copper (Cu), tin (Sn), yttrium (Y), phosphor (P), silver (Ag), and strontium (Sr); and x, y and z are the compositions of 0 at %≦x≦6 at %, 0.8 at %≦y≦7 at %, and 0 at %≦z≦2 at %, respectively.

Description

DESCRIPTION

1. Technical Field

The present invention relates to a magnesium alloy with a misch metal, in which a great deal of misch metal is added to magnesium, thereby having a network structure or a dispersed phase which is stable at a high temperature and thus exhibiting excellent mechanical properties. Further, the present invention relates to a method of producing a wrought magnesium alloy which granulates solidification structures, i.e. secondary phases or multi-phases, by means of hot extrusion and hot rolling, and ultra-refines grains of a matrix, and a wrought magnesium alloy produced by the method.

2. Background Art

Nowadays, as environmental and saveenergy problems attract a lot of attention all over the world, it is absolutely required to make parts lighter in weight. There are stronger and stronger requests that an environmental pollution problem resulting from carbon dioxide generated during transportation by road, aviation, and rail should be solved, and that parts or end-products should be made lighter in order to save a transportation fuel. Under this situation, a magnesium alloy suggests a most efficient possibility of making the products lighter, because it has the lowest density among commercial alloys, namely ⅔, and ⅕ times as low density as an aluminum alloy, and a ferrous alloy, respectively. In addition, the magnesium alloy has excellent specific strength, rigidity, vibration absorptivity, machinability, dimension stability, and electromagnetic wave shielding effects, so that it is widely used as sheathings of electronic/telecommunication products such as mobile communication equipment and laptop computers.

In general, the magnesium alloy for a high-temperature structure is classified into two types: a casting magnesium alloy used without heat treatment, and a sand casting magnesium alloy in which high-temperature properties are improved by precipitating a secondary phase in a matrix.

In the casting magnesium alloy, because a molten metal frequently generates an eddy when passing through the gate of a metal mold to enter a cavity, its product contains a number of blowholes. These remaining blowholes results in generating a blister on a surface of the product during heat treatment including solution heat treatment in the future, the product is not typically subjected to the heat treatment. Accordingly, an AZ91 alloy, a Mg—Al alloy, which is widely used as the casting magnesium alloy at the present time, is low in high-temperature properties, especially creep resistance. Hence, the AZ91 alloy has a difficulty in being applied to parts exposed to a high temperature (150° C. or more) such as a transmission case of an automobile. This is because, when aluminum is added to magnesium, room-temperature strength and fluidity of the molten metal are improved, but a phase of Mg₁₇Al₁₂ is formed to deteriorate the creep resistance property at a high temperature. In order to overcome this drawback, either addition of earth-rare elements or addition of calcium (Ca), silicon (Si), strontium (Sr), etc. as disclosed in U.S. Pat. No. 6,264,763 is carried out. However, so far, there is a limit to utility in the aspects of productivity, mechanical properties including the high-temperature creep property and corrosion resistance, and costs.

In the sand casting magnesium alloy, the secondary phase is precipitated in the matrix by the heat treatment, and thereby high-temperature strength and heat-resistant property are improved. Thus, it is possible to obtain a relatively sound casting. For this purpose, addition elements should has a great solubility change in a magnesium matrix according to temperature, and maintain soubility at a temperature of 200° C. or more, which is mainly used. As main addition elements of the sand casting magnesium alloy, silver (Ag), thorium (Th), yttrium (Y), neodymium (Nd), scandium (Sc), etc. are used, each of which is too expensive or contains a radioactive substance. Thus, these elements have been restrictively used for the case of giving a greater weight on performance than a cost.

Meanwhile, conventionally, there is a technical problem on formation of the magnesium alloy. In principle, the magnesium alloy has a hexagonal close packed structure and restrictions of a slip system required for plastic working. For this reason, it is very difficult to form a product at a room temperature. Hence, the product should be formed through hot working.

In this manner, in order to develop intermediate products and end-products of the magnesium alloy, it is absolutely necessary to improve formability. It is the most effective method that is to refine a crystal structure of the magnesium alloy to improve an elongation. In addition, the magnesium alloy of the fine grain structure should be substantially produced by an industrial method. The demand for a magnesium alloy plate is gradually increased. However, the magnesium alloy plate having a required fine grain structure is not efficiently produced using a currently commercialized magnesium alloy.

In the case of an existing AZ31 alloy, a reduction in thickness during rolling should be increased in order to refine the grain. In this case, the reduction in thickness is restricted due to serious cracking, and thus the grain refinement is restricted. In other words, this solid solution alloy is restricted in a source capable of generating recrystallization in its interior, and thus has a limit to the grain refinement.

DISCLOSURE

Technical Problem

It is an objective of the present invention to provide a magnesium alloy with a misch metal, in which a great deal of misch metal is added to magnesium, and thus refractory eutectic phases or multi-phases are formed into a stable network structure or a stable dispersed phase, thereby inhibiting deformation of a magnesium matrix at a high temperature, and which other elements are additionally added, and thus precipitation/solid-solution is strengthened in a matrix structure or the network structure is strengthened, thereby having excellent mechanical properties in which high strength is maintained at a high temperature.

It is another objective of the present invention to provide a method of producing a wrought magnesium alloy, in which a secondary phase or multiphase magnesium alloy to which a misch metal is added granulates solidification structures, i.e. secondary phases or multi-phases, by means of hot extrusion and hot rolling, and recrystallizes a matrix, thereby refining grains.

It is another objective of the present invention to provide a wrought magnesium alloy, which has a fine grain structure, exhibits mechanical properties such as high strength and high toughness in a room-temperature area in which the alloy is substantially used, having a good elongation at a temperature at which formation is substantially carried out, and thus is improved in formability.

Technical Solution

To accomplish these objectives, a magnesium alloy with a misch metal according to the present invention has the formula expressed by Mg_100-x-y-zA_xB_yC_z, where A is zinc (Zn) or aluminum (Al); B is the misch metal; C is at least one element selected from the group consisting of manganese (Mn), nickel (Ni), copper (Cu), tin (Sn), yttrium (Y), phosphor (P), silver (Ag), and strontium (Sr); and x, y and z are the compositions of 0 at %≦x≦6 at %, 0.8 at %≦y≦7 at %, and 0 at %≦z≦2 at %, respectively.

Further, the misch metal may be a didymium-based misch metal or a cerium-based misch metal.

Here, the didymium-based misch metal may be a rare earth alloy composition including neodymium (Nd) and praseodymium (Pr).

Also, the cerium-based misch metal may contain 45 wt %≦Ce≦65 wt %, 20 wt %≦La≦30 wt %, 5 wt %≦Nd≦15 wt %, and 0 wt %≦Pr≦10 wt %.

Furthermore, the magnesium alloy may further comprise calcium of 2 at % or less.

A method of producing a wrought magnesium alloy with a misch metal according to the present invention comprises the steps of: fusion-casting a magnesium alloy composition having the formula of Mg_100-x-y-zA_xB_yC_z, where A is zinc (Zn) or aluminum (Al); B is the misch metal; C is at least one element selected from the group consisting of manganese (Mn), nickel (Ni), copper (Cu), tin (Sn), yttrium (Y), phosphor (P), silver (Ag), and strontium (Sr);

and x, y and z are the compositions of 0 at %≦x≦6 at %, 0.8 at %≦y≦7 at %, and 0 at %≦z≦2 at %, respectively; and hot-extruding the cast, and refining grains through granulation and dispersion of other phases than magnesium in the cast, and recrystallization of a matrix.

Here, the method may further comprise a step of hot-rolling the hot-extruded product to form a plate.

Further, the hot-extruding step may be performed under the extrusion conditions of a temperature range from 350° C. to 450° C., and a ratio of reduction in section of 5˜80:1.

Also, the hot-rolling step may be performed under the rolling conditions of a temperature range from 350° C. to 500° C., and a percentage of single reduction in thickness from 25% to 50%.

Further, a wrought magnesium alloy with a misch metal is produced by the steps of: fusion-casting a composition having the formula of Mg_100-x-y-zA_xB_yC_z, where A is zinc (Zn) or aluminum (Al); B is the misch metal; C is at least one element selected from the group consisting of manganese (Mn), nickel (Ni), copper (Cu), tin (Sn), yttrium (Y), phosphor (P), silver (Ag), and strontium (Sr); and x, y and z are the compositions of 0 at %≦x≦6 at %, 0.8 at %≦y≦7 at %, and 0 at %≦z≦2 at %, respectively; hot-extruding the cast, and refining grains through granulation and dispersion of other phases than magnesium in the cast, and recrystallization of a matrix; and hot-rolling the hot-extruded product to from a wrought product.

Here, the other phases than magnesium may have a size of 20 μm or less.

Further, the other phases than magnesium may be contained from a solid-solution limit to a eutectic point or a hyper-eutectic area.

ADVANTAGEOUS EFFECTS

As described above, in the magnesium alloy with the misch metal according to the present invention, the misch metal is added, and thus refractory eutectic phases or multi-phases are formed into a stable network structure or a stable dispersed phase, thereby inhibiting deformation of a magnesium matrix at a high temperature. Further, other elements are additionally added, and thus precipitation/solid-solution is strengthened in a matrix structure or the network structure is strengthened, thereby having excellent mechanical properties in which a high strength is maintained at a high temperature.

Further, in the method of producing the wrought magnesium alloy according to the present invention, a secondary-phase or multiphase magnesium alloy to which the misch metal is added is recrystallized by the hot extrusion and hot rolling, and the grains are refined.

In addition, the wrought magnesium alloy with the misch metal according to the present invention has a fine grain structure, and thus exhibits mechanical properties such as a high strength and a high toughness in a room-temperature area in which the alloy is substantially used. Further, the wrought magnesium alloy has a good elongation at a temperature at which formation is substantially carried out, and thus is improved in formability.

In this manner, the magnesium alloy with the misch metal having excellent mechanical properties according to the present invention satisfies the requirements of high strength and heat resistance which are required for power transmission parts of a vehicle.

Further, by adding calcium (Ca) to the magnesium alloy with the misch metal according to the present invention, fusion in air and casting are possible, so that it is possible to promote saving of production costs.

Also, the magnesium alloy with the misch metal according to the present invention exhibits a high-temperature strength better than a heat-resistant magnesium alloy produced by existing heat treatment, so that it can be applied to parts for the vehicle and aircraft.

In addition, the magnesium alloy with the misch metal according to the present invention exhibits relatively better corrosion resistance than a previously commercialized heat-resistant magnesium alloy, so that it is used for lightweight parts capable of enduring severe conditions such as high temperature and corrosion.

Meanwhile, in the method of producing the wrought magnesium alloy according to the present invention, a magnesium alloy plate containing a great deal of ultra fine particles can be produced, and the produced plate has fine grains and very excellent formability. Accordingly, the wrought magnesium alloy of the present invention can make it lighter road, aviation, and rail transportations, and be widely used as sheathings of electronic/telecommunication products such as mobile communication equipment and laptop computers.

DESCRIPTION OF DRAWINGS

FIG. 1 is a scanning electron microscope photograph showing a network structure of an alpha magnesium structure and a Mg₁₂Ce phase in Alloy 3 of Table 1;

FIG. 2 is a photograph of a molten metal, in which 2 wt % calcium (Ca) is added to Alloy 9 of Table 1 and fused in air;

FIG. 3 is a photograph showing a cast structure of an magnesium alloy around a eutectic point, according to a fifth embodiment;

FIG. 4 is a microstructure photograph showing a wrought product obtained by performing hot extrusion on a magnesium alloy at a temperature of 450° C., an extrusion speed of 2 mm/sec, a ratio of reduction in section of 6:1, according to a fifth embodiment;

FIG. 5 is a microstructure photograph showing a wrought product formed by rolling a magnesium alloy under a roll temperature of 100° C. with a single reduction in thickness of 40% at a temperature of a test piece of 400° C., according to a fifth embodiment;

FIG. 6 is a photograph showing wrought products that is prepared on the conditions of hot extrusion and rolling performed in a fifth embodiment, and subjected to a high-temperature tension test with strains 1×10⁻³ s⁻¹, 1×10⁻² s⁻¹, 1×10⁻¹ s⁻, and 1×10⁻⁰ s⁻¹ at a temperature of 500° C., together with elongations of the wrought products;

FIG. 7 is a microstructure photograph showing a wrought product obtained by performing hot extrusion on a magnesium alloy at a temperature of 450° C., an extrusion speed of 2 mm/sec, a ratio of reduction in section of 6:1, according to a sixth embodiment;

FIG. 8 is a microstructure photograph showing a wrought product formed by rolling a magnesium alloy under a roll temperature of 100° C. with a single reduction in thickness of 40% at a temperature of a test piece of 400° C., according to a sixth embodiment;

FIG. 9 is a microstructure photograph showing a wrought product obtained by performing hot extrusion on a magnesium alloy at a temperature of 450° C., an extrusion speed of 2 mm/sec, a ratio of reduction in section of 6:1, according to a seventh embodiment; and

FIG. 10 is a microstructure photograph showing a wrought product formed by rolling a magnesium alloy under a roll temperature of 100° C. with a single reduction in thickness of 40% at a temperature of a test piece of 400° C., according to a seventh embodiment.

BEST MODE

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the attached drawings.

A magnesium alloy with a misch metal according to the present invention has the formula expressed by Mg_100-x-y-zA_xB_yC_z, where A is zinc (Zn) or aluminum (Al); B is the misch metal; C is at least one element selected from the group consisting of manganese (Mn), nickel (Ni), copper (Cu), tin (Sn), yttrium (Y), phosphor (P), silver (Ag), and strontium (Sr); and x, y and z are the compositions of 0 at %≦x≦6 at %, 0.8 at %≦y≦7 at %, and 0 at %≦z≦2 at %, respectively.

When the magnesium alloy with the misch metal according to the present invention is solidified by casting, a magnsium-rich solid solution (alpha magnesium) constitutes a matrix structure. Secondary phases are crystallized by the misch metal (element B), and form a network structure or a dispersed phase that is compositely constructed with a magnesium matrix. This network structure is stable at a high temperature, and thus provides excellent machanical properties. Further, tertiary phases can be created by element A and C groups, and mainly strengthen solid-solution/precipitation of the magnesium matrix or the network structure, thereby improving the mechanical properties.

In the case in which aluminum (Al) of the element A group is added along with element B in excess of 5 at %, a Mg₁₇A1₁₂ phase is formed in the alpha magnesium matrix, thereby deteriorating the mechanical properties.

Thus, the added aluminum is preferably restricted to 5 at % or less.

Further, zinc (Zn) has a peak solid-solution limit of 2.4% at 340° C. with respect to magnesium (Mg). However, considering an amount dissolved in the secondary or tertiary phase, an addition range of the element A group is preferably restricted to 5 at % or less.

In the magnesium alloy with the misch metal according to the present invention, aluminum (Al) and zinc (Zn) of the element A group having solubility with respect to magnesium are contained in Mg-misch metals, so that multiphases can be obtained. Here, the added misch metal is composed of elements having atomic numbers 57 through 71, and includes a didymium-based misch metal or a cerium-based misch metal. The didymium-based misch metal is a rare earth alloy composition including neodymium (Nd) and praseodymium (Pr). Especially, the cerium-based misch metal refers to a commercialized misch metal alloy which has a main composition of 45 wt %≦Ce≦65 wt %, 20 wt %≦La≦30 wt %, 5 wt %≦Nd≦15 wt %, and 0 wt %≦Pr≦10 wt %, and in which other 15 or more trace elements are present in view of a characteristic in which the misch metal is crystallized. This misch metal (element B) is caused to form a network structure or a dispersed phase which is stable at a high temperature, and improve corrosion and fluidity of the molten metal.

When an addition range of the misch metal (element B) exceeds 7 at %, this is not favorable because a fraction of the secondary phase causing brittleness is increased, so that the elongation of the material is removed at a room temperature. Thus, in the present invention, the addition range of the misch metal (element B) is restricted to 7 at % or less.

In the magnesium alloy with the misch metal according to the present invention, in order to promote solid-solution strengthening or precipitation strengthening or strengthen the network structure in the magnesium matrix structure, an element C group (Si, P, B, Mn, Sr, Y, Ni, Cu, Sn, and Ag) is added. At this time, the added element C group has a strong affinity with magnesium (Mg) or the misch metal. When the element C group is added at a small amount, it can improve the mechanical properties while maintaining the network structure. Exemplary examples of the element C group are phosphor (P), boron (B), manganese (Mn), strontium (Sr), yttrium (Y), nickel (Ni), copper (Cu), tin (Sn), and silver (Ag). Accordingly, an addition range of the element C group is restricted to 2 at % or less so as to be able to expect effects caused by the precipitation/solid-solution strengthening of the matrix structure while maintaining or strengthening the stable network structure at a high temperature.

Further, in the magnesium alloy with the misch metal according to the present invention, a small amount of calcium (Ca) is added, so that the magnesium alloy composition can be fused and cast in air without using a shielding gas or flux. An addition range of calcium is restricted to 2 at % or less, so as to be able to provide favorable effects of calcium (Ca).

MODE FOR INVENTION

Hereinafter, a magnesium alloy with a misch metal according to exemplary embodiments of the present invention will be described in detail.

Embodiment 1

A molten metal of a magnesium alloy composition as given in the following Table 1 was prepared, and a cast was obtained by casting. More specifically, a carbon crucible was heated in an electric induction furnace at a temperature of 700° C. Magnesium was fused in the carbon crucible, and then other addictives were added. Thereby, a molten alloy was formed and poured into a mold, which was pre-heated up to 1200° C. Thereby, the cast was formed.

In the composition specified in Table 1, B refers to at % of a cerium-based misch metal. A secondary phase generated by addition of element B is a Mg₁₂Ce phase. FIG. 1 is a scanning electron microscope photograph of Alloy 3, and shows that an alpha magnesium structure and the Mg₁₂Ce phase form a network structure. Because the structures forming the network structure were stable at a high temperature and thus inhibited deformation of the alpha magnesium structure, they exhibited a high strength at a high temperature. Thus, as an amount of element B increased, the Mg₁₂Ce phase increased as well, and both a yield strength and a tensile strength increased at room and high temperatures. Further, in the case of Alloy 10, the Mg₁₂Ce phase, as the secondary phase, as well as a tertiary phase were crystallized in the form of an aluminum compound, and thereby mechanical properties were improved.

TABLE 1
Composition	Strength	Room					Ecorr
(at %)	(MPa)	Temperature	150° C.	200° C.	250° C.	300° C.	(V)
Comparative Example
AZ91D-T6	Yield Strength	135	100	80	65	50	−1.610
	Tensile	280	195	125	98
	Strength
AE42	Yield Strength	113	100	85
	Tensile	194	141	100
	Strength
Embodiment
Alloy 1	Yield Strength	140	120	107	103	92
	Tensile	297	257	197	196
	Strength
Alloy 2	Yield Strength	236	177	171	153	133	−1.600
	Tensile	355	302	275	250
	Strength
Alloy 3	Yield Strength	192	169	140	127	123
	Tensile	352	266	265	228
	Strength
Alloy 4	Yield Strength	280	249	240	204	168
	Tensile	426	370	362	309
	Strength
Alloy 5	Yield Strength	265	192	185	149	125	−1.590
	Tensile	384	297	291	228
	Strength
Alloy 6	Yield Strength	284	199	194	191	132
	Tensile	428	355	304	248
	Strength
Alloy 7	Yield Strength	310	237	215	210	174	−1.552
	Tensile	443	372	368	283
	Strength
Alloy 8	Yield Strength	387	242	236	223	209
	Tensile	518	422	392	320
	Strength
Alloy 9	Yield Strength	400	350	320	290	219	−1.537
	Tensile	537	456	425	358
	Strength
Alloy 10	Yield Strength	259	192	174	152	130	−1.556
	Tensile	404	298	270	217
	Strength
Alloy 11	Yield Strength	447	397	381	360	315
	Tensile	541	526	467	414
	Strength
Alloy 12	Yield Strength	623	574	462	453	445
	Tensile	667	639	503	483
	Strength
Alloy 1: Mg97.5Zn1B1.5,
Alloy 2: Mg97Zn1B2,
Alloy 3: Mg96.5Zn1B2.5,
Alloy 4: Mg95.5Zn1.5B3,
Alloy 5: Mg96Zn2B2,
Alloy 6: Mg95.5Zn2B2.5,
Alloy 7: Mg95Zn2B3,
Alloy 8: Mg94.5Zn2B3.5,
Alloy 9: Mg94Zn2B4,
Alloy 10: Mg94Zn2B4,
Alloy 11: Mg92.5Zn2.5B5,
Alloy 12: Mg89.5Zn3.5B7.

Accordingly, compared to existing magnesium heat-resistant alloys, the magnesium alloys with the misch metal according to the present invention were capable of replacing heat-treatment type sand casting heat-resistant magnesium alloys that maintained a high strength at a temperature of 300° C. or more, and were mainly used at a temperature of 200° C. or more, and magnesium alloys formed by die casting, because the secondary or tertiary phase network structure in which the change in strength depending on the change in temperature was very small was formed.

The values of E_corr given in Table 1 were obtained through potential measurement of an open circuit in a sodium chloride (NaCl) solution of 3.5 wt % for 3 hours. Relative corrosion resistance was checked with respect to the existing heat-resistant alloy, AZ91, which was a comparison target. As a result, it was found that, as the amount of element B increased, the corrosion resistance increased.

Embodiment 2

FIG. 2 is a photograph of a molten metal, in which 2 wt % calcium (Ca) is added to Alloy 9 of Table 1 and fused in air, and shows that a magnesium alloy composition can be fused and cast in air by adding calcium (Ca) to the magnesium alloy composition (e.g. Alloy 9). As can be seen from FIG. 2, it could be found that a thick oxide was not formed on a surface of the molten metal when the magnesium alloy composition was fused in air.

Embodiment 3

TABLE 2
Composition (at %)	Hardness Value (Hv)	Generation of Crack
Alloy 12	165	◯
Alloy 12 + Ni0.3	169	Δ
Alloy 12 + Cu0.3	190	X
Alloy 12 + Sn0.3	176	X
Alloy 12 + Al0.3	185	Δ
Alloy 12 + Mn0.3	195	X
Alloy 12 + Si0.3	191	X
◯: Generation of the crack
Δ: Slight generation of the crack
X: No generation of the crack

In the magnesium alloy with the misch metal according to the present invention, as described, the Mg₁₂Ce phase generated by adding the cerium (Ce)-based misch metal to magnesium (Mg) was an intermetallic compound and had brittleness. Hence, when the Mgl₂Ce phase had a fraction higher than a magnesium matrix, the magnesium alloy has a property that an elongation was lowered. Therefore, in the current embodiment, an attempt was made to improve the property by adding a specific element. In Embodiment 3, Alloy 12 having the highest fraction of the Mg₁₂Ce phase was selected from the compositions given in Table 1, and then it was examined how much the brittleness of the secondary phase was dependent on the addition element.

The molten metals of the magnesium alloy compositions were prepared as in Table 2, and casts were obtained by casting, and subjected to Vickers hardness testing. At this time, the examination was performed while the applied load of indentation was varied from 100 g to 1000 g. In Table 2, it was shown that a hardness value increased by adding nickel (Ni), copper (Cu), tin (Sn), aluminum (Al), manganese (Mn), or silicon (Si) to Alloy 12.

Further, it was found that, when the hardness testing was performed, a degree to which a micro crack was generated around the indentation of a surface was also reduced or disappeared. In Table 2, the increase of the hardness value, or the variation of the generated degree of the crack was caused by the addition element strengthening a network structure. In this manner, according to the current embodiment of the present invention, it can be seen that, when elements, such as a C group of addition elements (Si, P, B, Mn, Sr, Y, Ni, Cu, Sn, and Ag), having a strong affinity with magnesium (Mg) or cerium-based misch metal are added to the magnesium alloy with the misch metal of the present invention, the mechanical properties are improved.

Embodiment 4

	TABLE 3
	Elongation (%)
	Composition	Room
	(at %)	Temperature	150° C.	200° C.
	(Alloy	0.2	2	4
	2)99.5Al0.5
	(Alloy 2)99Al1	0.5	4	6
	(Alloy	1.5	7	10
	2)98.5Al1.5
	(Alloy 6)99Al1	0.2	2	5
	(Alloy 6)98Al2	1	6	11
	(Alloy 6)97Al3	1.8	8	16

Table 3 represents elongations obtained by performing a tensile test on the alloy compositions (e.g. Alloy 2 and Alloy 6) presented in Embodiment 1, to which Al is added. As shown in Table 3, it can be seen that, as a trace of Al is changed, the elongation increases. However, when Al is added in excess of 4 at %, this is not favorable, because the network structure capable of maintaining the high strength at a high temperature is not maintained, and a Mg₁₇A₁₂ phase is created in the magnesium matrix.

As described above, it can be found that the magnesium alloys with the misch metal according to the present invention are high-temperature structural magnesium alloys in which the mechanical properties and the corrosion resistance are greatly improved, compared to the existing heat-resistant magnesium alloys.

Next, a wrought magnesium alloy produced by the magnesium alloy with the misch metal, according to the present invention, will be described.

In the present invention, a magnesium alloy cast to which the misch metal of the foregoing composition is added is extruded and rolled, and thereby a wrought product is formed. In general, the magnesium alloy cannot ensure formability at a room temperature. Hence, in order to obtain a sound wrought product, the magnesium alloy cast is subjected to hot working, and a worked temperature of the magnesium alloy cast is set to a range capable of ensuring soundness of the magnesium alloy cast through a test. In the case of the extrusion, the magnesium alloy cast was pre-heated and extruded within a range from 350 to 500° C. The following extrusion conditions are used: an extrusion ratio:6.5:1, an extrusion die angle:180°, a ram speed:2 cm/min.

This extrusion gives rise to dispersion of a secondary phase and recrystallization of a matrix structure phase, so that an application product in which strength and elongation are improved can be obtained. Further, when the magnesium alloy cast is repetitively rolled with a percentage of reduction in thickness of 40% at 400° C., and thereby a rolled product having a thickness of 1 mm is obtained, the same effect as in the extrusion can be obtained.

A grain refinement mechanism of the magnesium alloy with the misch metal according to the present invention makes use of a phenomenon of dynamic recrystallization in which a nucleus of a new grain is created in a structure during hot working of the magnesium alloy. In the case of the magnesium alloy in which a great deal of particles are distributed, a recrystallization source is increased, and thus grain refinement is conducted in a considerably efficient way. A magnesium alloy in which a volume fraction of other phases than magnesium of a matrix amounts to a range from 5% to 50% in order to maximize such a characteristic is first formed by casting, and the internal phases that are present in the magnesium alloy are effectively dispersed in the magnesium matrix through either hot extrusion or hot extrusion and hot rolling. Thereby, due to the dispersed phases, the dynamic recrystallization is effectively generated, and thus the grain refinement is maximized.

More specifically, the wrought magnesium alloy with the misch metal according to the present invention is expressed by an ordinary chemical formula Mg_100-x-y-zA_xB_yC_z, where A is zinc (Zn) or aluminum (Al), B is the misch metal, C is at least one element selected from the group consisting of manganese (Mn), nickel (Ni), copper (Cu), tin (Sn), yttrium (Y), phosphor (P), silver (Ag), and strontium (Sr), and x, y and z are the compositions of 0 at %≦x≦6 at %, 0.8 at %≦y≦7 at %, and 0 at %≦z≦2 at %, respectively. Here, the added misch metal is composed of elements having atomic numbers 57 through 71, and includes a didymium-based misch metal or a cerium-based misch metal. The didymium-based misch metal is a rare earth alloy composition including neodymium (Nd) and praseodymium (Pr), and particularly the cerium-based misch metal refers to a commercialized misch metal alloy which has a main composition of 45 wt %≦Ce≦65 wt %, 20 wt %≦La≦30 wt %, 5 wt %≦Nd≦15 wt %, and 0 wt %≦Pr≦10 wt %, and in which other 15 or more trace elements are present in view of a characteristic in which the misch metal is crystallized.

When at least one of these addition elements, i.e. elements capable of making a eutectic, and particularly Al, Si, Ag, Ca, Ni, Cu, Zn, Y, Sn, La, Ce, Pr, Nd, Ce rich-misch metal, and didymium rich-misch metal, is added to Mg, the magnesium alloy containing the great deal of phases as described above can be formed by casting. When the magnesium alloy is subjected to hot extrusion, its cast structure is subjected to fracture, and other phases than magnesium are granulated and dispersed. For this reason, the dynamic recrystallization phenomenon is effectively generated to refine the grain.

This hot extrusion makes it possible to perform additional, effective fracture and dispersion with respect to particles generated due to impure elements that are inevitably added during the casting of the magnesium alloy.

Hence, an extruded product of the magnesium alloy can be made more stable.

When the extruded product is subjected to hot rolling, a plate of the magnesium alloy in which a great deal of different phases are present in the magnesium matrix can be formed. In general, when the cast in which a great deal of different phases are present in the magnesium alloy is subjected to hot rolling, the phases are fractured to act as crack sources, and thus the cast cannot be rolled. However, in the case of the extruded product in which the phases are granulated and dispersed, its grains have already been refined, and furthermore their size is restricted to a particle size even when the crack is generated. Thus, the size does not exert a great influence on the hot rolling, so that the extruded product is easily subjected to the hot rolling. Further, even when the temperature for the hot rolling is increased in order to effectively perform the hot rolling, the grain growth is inhibited due to the great deal of distributed particles, and thus the hot rolling is easily performed.

Subsequently, exemplary embodiments of a method of producing the wrought magnesium alloy with the misch metal according to the present invention will be described.

In the following embodiments, production of extruded products and plates of magnesium alloys around a eutectic point, or a solid-solution limit, and in a hyper-eutectic or hypo-eutectic area will be described.

Embodiment 5

In the present embodiment, an extruded product and a plate are formed using a Mg—Ce based misch metal-Zn alloy around a eutectic point.

Mg 93.75%, Ce based mish metal 4.25%, and Zn 2.0% by atomic weight were subjected to mixing and fusion casting, and thereby formed into a slab. FIG. 3 is a photograph of a cast structure of the Mg—Ce based misch metal-Zn alloy. In order to effectively distribute these eutectic phases to obtain a magnesium matrix of fine grains, hot extrusion was performed at a temperature of 450° C., an extrusion speed of 2 mm/sec, a ratio of reduction in section of 6:1.

FIG. 4 is a microstructure photograph of a hot-extruded product of the present embodiment. As observed, no crack exists in an internal structure, and the grains are very fine, an average size of which is less than 14 μm.

In general, in a particle-free magnesium alloy, this grain size cannot be obtained through single hot extrusion.

Further, the plate of the magnesium alloy was formed by rolling the extruded product under a roll temperature of 100° C. with a single reduction in thickness of 40% at a temperature of 400° C. FIG. 5 is a microstructure photograph of a rolled plate. As observed, there is no crack, and the grains are very fine, an average size of which is less than 8 μm.

The formed plate was subjected to a high-temperature tension test. A photograph of test pieces of the tested plate is shown in FIG. 6. The high-temperature tension test was performed with strains 1×10⁻³ S⁻¹, 1×10⁻² S⁻¹, 1×10⁻¹ s⁻¹, and 1×10⁻⁰ s⁻¹ at a temperature of 500° C., and the test pieces exhibited high elongations of 580%, 370%, 340%, and 250%, respectively. Accordingly, it could be found that the magnesium alloy having the great deal of phases around the eutectic point was stably subjected to the hot extrusion and the hot rolling, and had excellent formability due to the grain refinement.

Embodiment 6

The present embodiment relates to production of an extruded product and a plate of a Mg−Ce misch metal of a hypo-eutectic area.

Mg 95.7%, and Ce based mish metal 4.3% by atomic weight were mixed, and then subjected to casting, hot extrusion, and hot rolling on the same condition as in Embodiment 5. FIG. 7 is a microstructure photograph of this test piece hot-extruded on the condition, wherein there is no crack, and the grains are very fine, an average size of which is less than 15 μm. Further, FIG. 8 is a microstructure photograph of a wrought magnesium alloy that is subjected to hot extrusion, wherein there are created very fine grains, an average size of which is less than 8 μm, without a crack.

In the present embodiment, it could be found that the magnesium alloy having the great deal of phases up to the hypo-eutectic area was stably subjected to the hot extrusion, the hot rolling, and thus the grain refinement.

Embodiment 7

The present embodiment relates to production of an extruded product and a plate of a Mg—Ce misch metal-Zn alloy of a hypo-eutectic area.

Mg 97.0%, Ce based mish metal 1.5%, and Zn 1.5% by atomic weight were mixed, and the subjected to casting, hot extrusion, and hot rolling on the same condition as in Embodiment 5. FIG. 9 is a microstructure photograph of this test piece hot-extruded on the condition, wherein there is no crack inside, and an average grain size is less than 20 μm. Further, FIG. 10 is a microstructure photograph of a wrought magnesium alloy that is subjected to hot extrusion, wherein the grains are very fine, an average size of which is less than 9 μm. In the present embodiment, it could be found that the magnesium alloy having the great deal of phases up to the hypo-eutectic area was stably subjected to the hot extrusion, and thus the grain refinement.

Claims

1. A magnesium alloy with a misch metal, having the formula: Mg100-x-y-zAxByCz,

wherein

A is zinc (Zn) or aluminum (Al);

B is the misch metal;

C is at least one element selected from the group consisting of manganese (Mn), nickel (Ni), copper (Cu), tin (Sn), yttrium (Y), phosphor (P), silver (Ag), and strontium (Sr); and

x, y and z are the compositions of 0 at %≦x≦6 at %, 0.8 at %≦y≦7 at %, and 0 at %≦z≦2 at %, respectively.

2. The magnesium alloy according to claim 1, further comprising calcium of 2 at % or less.

3. The magnesium alloy according to claim 1, wherein the misch metal is a didymium-based misch metal or a cerium-based misch metal.

4. The magnesium alloy according to claim 3, wherein the didymium-based misch metal is a rare earth alloy composition including neodymium (Nd) and praseodymium (Pr).

5. The magnesium alloy according to claim 3, wherein the cerium-based misch metal contains 45 wt %≦Ce≦65 wt %, 20 wt %≦La≦30 wt %, 5 wt %≦Nd≦15 wt %, and 0 wt %≦Pr≦10 wt %.

6. A method of producing a wrought magnesium alloy with a misch metal, the method comprising the steps of:

fusion-casting a magnesium alloy composition having the formula of Mg100-x-y-zAxByCz, where A is zinc (Zn) or aluminum (Al); B is the misch metal; C is at least one element selected from the group consisting of manganese (Mn), nickel (Ni), copper (Cu), tin (Sn), yttrium (Y), phosphor (P), silver (Ag), and strontium (Sr); and x, y and z are the compositions of 0 at %≦x≦6 at %, 0.8 at %≦y≦7 at %, and 0 at %≦z≦2 at %, respectively; and

hot-extruding the cast, and refining grains through granulation and dispersion of other phases than magnesium in the cast, and recrystallization of a matrix.

7. The method according to claim 6, further comprising a step of hot-rolling the hot-extruded product to form a plate.

8. The method according to claim 6, wherein the hot-extruding step is performed under the extrusion conditions of a temperature range from 350° C. to 450° C., and a ratio of reduction in section of 5˜80:1.

9. The method according to claim 7, the hot-rolling step is performed under the rolling conditions of a temperature range from 350° C. to 500° C., and a percentage of single reduction in thickness from 25% to 50%.

10. A wrought magnesium alloy with a misch metal, produced by the steps of:

fusion-casting a composition having the formula of Mg100-x-y-zAxByCz, where A is zinc (Zn) or aluminum (Al); B is the misch metal; C is at least one element selected from the group consisting of manganese (Mn), nickel (Ni), copper (Cu), tin (Sn), yttrium (Y), phosphor (P), silver (Ag), and strontium (Sr); and x, y and z are the compositions of 0 at %≦x≦6 at %, 0.8 at %≦y≦7 at %, and 0 at %≦z≦2 at %, respectively;

hot-extruding the cast, and refining grains through granulation and dispersion of other phases than magnesium in the cast, and recrystallization of a matrix; and

hot-rolling the hot-extruded product to from a wrought product.

11. The wrought magnesium alloy according to claim 10, wherein the other phases than magnesium have a size of 20 μm or less.

12. The wrought magnesium alloy according to claim 10, wherein the other phases than magnesium are contained from a solid-solution limit to a eutectic point or a hyper-eutectic area.