EP3561097A1

EP3561097A1 - High strength magnesium alloy with excellent flame retardancy, and method for producing same

Info

Publication number: EP3561097A1
Application number: EP17883943.7A
Authority: EP
Inventors: Woo-Jin Park; Hyung-Sub EOM; Dae-Hwan Choi; Sang-Jin Kim
Original assignee: Posco Co Ltd; Research Institute of Industrial Science and Technology RIST
Current assignee: Research Institute of Industrial Science and Technology RIST; Posco Holdings Inc
Priority date: 2016-12-21
Filing date: 2017-12-21
Publication date: 2019-10-30
Also published as: JP2020509196A; US20200087757A1; KR101858856B1; EP3561097A4; WO2018117713A1; CN110114485A

Abstract

An aspect of the present invention relates to a high strength magnesium alloy with excellent flame retardancy, wherein the magnesium alloy comprises 2.0-13.0 wt% of Al, 0.1-0.5 wt% of Mn, 0.0015-0.025 wt% of B, and 0.1-1.0 wt% of Y with the remainder comprising Mg and other unavoidable impurities, and comprises 6.5% or more of an Mg-Al intermetallic compound in terms of volume fraction, the Mg-Al intermetallic compound having an average grain size of 20-500 nm.

Description

[Technical Field]

The present disclosure relates to a high strength magnesium alloy having excellent flame retardancy, and a method of producing the same.

[Background Art]

Magnesium is one of the lightest metals in practical use, and thus, may be applied to portable electronic products such as smart phones, tablet PCs, laptop computers, and structural materials of transportation means for vehicles, trains, aircraft and the like. Magnesium alloys, having various elements added to magnesium, are attracting attention as eco-friendly lightweight metal materials.
Magnesium alloys have excellent castability. Therefore, casting products of magnesium alloy, which are mainly produced by die casting methods such as high pressure casting, low pressure casting or gravity casting, have been mainly applied to practical products. In recent years, development and market expansion of products for use in telegraph materials using magnesium alloys, which may be manufactured through processing such as rolling, extrusion or the like, have been promoted.
In general, the types of alloying elements used in magnesium alloys for casting or magnesium alloys for wrought product are similar. Most commonly used types of magnesium alloys are AZ-based alloys with Al and Zn added, and AM-based alloys with Al and Mn added. The two types of alloys commonly contain Al to improve the castability and tensile strength of magnesium.
The AZ and AM magnesium alloys, which account for most of the commercial magnesium alloys, are suitable for producing various casting products due to improvement in the flowability of molten metal by Al addition, and are also suitable for billet casting and plate casting. However, yield strength or tensile strength thereof is significantly lower than those of competitive aluminum alloys, and there is a problem in that a thickness of the product should be increased or the product shape should be modified.
In addition, the magnesium alloy has a high possibility of ignition due to high oxygen affinity, which limits the use conditions.
Therefore, there is a demand for developing a high strength magnesium alloy having excellent flame retardancy and a method of producing the same.

(Prior Art Document)

(Patent Document 1) Korean Patent Laid-Open Publication No. 10-2015-0077494

[Disclosure]

[Technical Problem]

An aspect of the present disclosure is to provide a high strength magnesium alloy having excellent flame retardancy and a method of producing the same.
On the other hand, the object of the present disclosure is not limited to the above description. It will be understood by those of ordinary skill in the art that there is no difficulty in understanding the additional objects of the present disclosure.

[Technical Solution]

According to an aspect of the present disclosure, a high strength magnesium alloy having excellent flame retardancy includes, by weight%, 2.0 to 13.0% of aluminum (Al), 0.1 to 0.5% of manganese (Mn), 0.0015 to 0.025% of boron (B), 0.1 to 1.0% of yttrium (Y), a remainder of magnesium (Mg), and unavoidable impurities, the high strength magnesium alloy including a Mg-Al intermetallic compound in a volume fraction of 6.5% or more. The Mg-Al intermetallic compound has an average particle diameter of 20 to 500 nm.
According to another aspect of the present disclosure, a method of producing a high strength magnesium alloy having excellent flame retardancy includes:

preparing a molten metal including, by weight%, 2.0 to 13.0% of aluminum (Al), 0.1 to 0.5% of manganese (Mn), 0.0015 to 0.025% of boron (B), 0.1 to 1.0% of yttrium (Y), a remainder of magnesium (Mg), and unavoidable impurities;
casting the molten metal to obtain a magnesium alloy casting material;
subjecting the magnesium alloy casting material to a solution treatment at a temperature ranging from 370 to 490°C for 2 to 20 hours to obtain a magnesium alloy;
cooling the magnesium alloy to 100°C or lower; and
aging the magnesium alloy cooled in the cooling of the magnesium alloy at a temperature of 150 to 250°C for 2 to 48 hours.

In addition, the solution of the above-mentioned problems does not list all features of the present disclosure. The various features of the present disclosure and the advantages and effects thereof may be understood in more detail with reference to the following specific embodiments.

[Advantageous Effects]

According to an embodiment of the present disclosure, there is provided an effect of providing a high strength magnesium alloy having excellent flame retardancy and a method of producing the same.

[Description of Drawings]

FIGS. 1A and 1B are images of microstructures of magnesium alloy casting materials of comparative material 1(a) and inventive material 7(b).
FIG. 2 is an image of a microstructure after a solution treatment of comparative material 1 is completed.
FIG. 3 is an image of a microstructure after a solution treatment of inventive material 7 is completed.
FIGS. 4A and 4B are graphs illustrating the results of measuring hardness values of comparative material 1(a) and inventive material 7(b) according to the aging time at 200°C.
FIGS. 5A to 5C are images illustrating microstructures of a magnesium alloy after an aging treatment of comparative member 1(a), inventive material 7(b), and comparative material 5(c).
FIG. 6 is a graph illustrating a change in hardness value based on the aging time of Inventive material 7 and a change in Mg-Al intermetallic compound size within a crystal grain.
FIG. 7 is a graph illustrating a volume fraction of Mg-Al intermetallic compound based on the aging time of Inventive Material 7.

[Best Mode for Invention]

Hereinafter, embodiments of the present disclosure will be described. However, the embodiments of the present disclosure may be modified into various other forms, and the scope of the present disclosure is not limited to the embodiments described below. Further, the embodiments of the present disclosure are provided to more fully explain the present disclosure to those skilled in the art.
As a result of deep research to solve the problems of ignition characteristics and low strength of magnesium alloys, the present inventors have found that a large amount of intermetallic compounds may be finely distributed by adding B and Y in combination and performing an aging treatment thereon, and thus excellent flame retardancy and high strength thereof may be secured, and the present disclosure has been obtained.

High Strength Magnesium Alloy having Excellent Flame Retardancy

Hereinafter, a method of producing a high strength magnesium alloy having excellent flame retardancy according to an embodiment in the present disclosure will be described in detail.
According to an embodiment in the present disclosure, a high strength magnesium alloy having excellent flame retardancy includes, by weight%, 2.0 to 13.0% of Al, 0.1 to 0.5% of Mn, 0.0015 to 0.025% of B, 0.1 to 1.0% of Y, a remainder of Mg, and unavoidable impurities, and includes a Mg-Al intermetallic compound in a volume fraction of not less than 6.5%. An average grain size of the Mg-Al intermetallic compound is 20 to 500 nm.
First, the alloy composition according to an embodiment will be described in detail. Hereinafter, the unit of each element content refers to weight% unless otherwise specified.

Al: 2.0 to 13.0%

Al increases the tensile strength or yield strength, and improves the castability by improving the fluidity of alloy molten metal.
If the Al content is less than 2.0%, the above-mentioned effect is insufficient. On the other hand, if the Al content exceeds 13.0%, the brittleness may be increased and the workability and ductility may be reduced. Therefore, the Al content may be 2.0 to 13.0%.
Further, in detail, a lower limit of the Al content may be 2.5%, and in further detail, the lower limit thereof may be 6.5% to secure a tensile strength of 160 MPa or more. In detail, an upper limit of the Al content may be 12.0%, and in more detail, the upper limit thereof may be 11.0%.

Mn: 0.1 to 0.5%

Mn is an element contributing to an increase in tensile strength by forming an intermetallic compound with Al to allow for fine grains. In addition, Mn serves to lower a corrosion rate of magnesium by lowering Fe, which is a typical impurity element unnecessary for a magnesium alloy, through intermetallic compound formation.
If the Mn content is less than 0.1%, the above-mentioned effect is insufficient. On the other hand, if the Mn content exceeds 0.5%, brittleness due to excessive formation of an acicular intermetallic compound may be caused. Therefore, the Mn content may be 0.1 to 0.5%.
Further, in more detail, a lower limit of the Mn content may be 0.11%, and in further detail, an upper limit thereof may be 0.45%.

B: 0.0015 to 0.025%

B (boron) has a significantly high melting point, and solubility thereof in a magnesium solid phase or liquid phase is close to zero. Thus, B is known as an element not commonly used in general magnesium alloys.
However, in an embodiment of the present disclosure, B is added to ensure flame retardancy and high strength. In detail, B contributes to forming a large amount of Mg-Al intermetallic compound by adding B and Y in combination to a magnesium alloy and performing an aging treatment thereon, thereby improving tensile strength. In addition thereto, in this case, flame retardancy and strength may be further improved as compared with the case in which B is added alone. In addition, since B may contribute to prevention of oxidation of molten metal to reduce the amount of expensive SF₆ gas used for preventing oxidation of molten metal and SO₂ gas which may cause environmental pollution, B may contribute to reduction in production costs and environmental protection.
If the B content is less than 0.0015%, the above-mentioned effect is insufficient. On the other hand, if the B content exceeds 0.025%, there is a problem in which an Al-B compound is formed on grain boundaries, reducing ductility. Therefore, the B content may be 0.0015 to 0.025%.
Further, in detail, a lower limit of the B content may be 0.002%, and in more detail, an upper limit thereof may be 0.02%.

Y: 0.1 to 1.0%

Y bonds with Al to form a precipitate, to contribute to the improvement of strength, and is an element that has a high oxygen affinity to firmly protect a surface protective film of molten metal to suppress oxidation of the molten metal. In addition, Y is an element to improve flame retardancy even after solidification.
In addition, as described above, Y is added together with B in combination to be subjected to an aging treatment, thereby contributing to formation of a large amount of Mg-Al intermetallic compound to improve tensile strength. Furthermore, in this case, flame retardancy may be further improved as compared with the case in which Y is added alone.
If the Y content is less than 0.1%, the above-mentioned effect is insufficient. On the other hand, if the Y content exceeds 1.0%, ductility may be reduced due to formation of a coarse Al-Y compound. Therefore, the Y content may be 0.1 to 1.0%.
Further, in detail, a lower limit of the Y content may be 0.11%, and in more detail, an upper limit thereof may be 0.95%.
In the embodiment of the present disclosure, the remainder component is magnesium (Mg). Further, in the ordinary producing process, impurities which are not intended may be mixed from a raw material or a surrounding environment, which cannot be excluded. These impurities are known to any person skilled in the art, and thus, are not specifically mentioned in this specification. For example, the impurities may be Fe, Cu, Ni, Ca, Na, Ba, F, S, N or the like.
In this case, in addition to the above alloy composition, 0.3 to 3.0 wt% of Zn may be further included.

Zn: 0.3 to 3.0%

Zn is a solid solution strengthening element and is an element which promotes formation of Mg₁₇Al₁₂ phase or improves tensile strength by forming a separate intermetallic compound containing Zn such as in Mg₂Zn or the like.
If the Zn content is less than 0.3%, the above-mentioned effect is insufficient. On the other hand, if the Zn content exceeds 3.0%, a large amount of a separate intermetallic compound including Zn, such as Mg₂Zn or the like, is formed to increase brittleness, which may lead to a decrease in ductility and toughness.
Therefore, the Zn content may be 0.3 to 3.0%. In more detail, the Zn content may be within a range of from 0.5 to 1.5 wt%, considering the improvement of the strength and the reduction in brittleness.
A high-strength magnesium alloy having excellent flame retardancy according to an embodiment in the present disclosure not only satisfies the alloy composition described above, but also contains a Mg-Al intermetallic compound in a volume fraction of 6.5% or more. An average grain size of the Mg-Al intermetallic compound is 20 to 500 nm.
When the main alloy element added to magnesium is Al, an Mg-Al intermetallic compound may be formed, and a typical Mg-Al intermetallic compound is Mg₁₇Al₁₂ phase. The Mg-Al intermetallic compound serves to secure high strength.
Since a maximum addition amount of Al or other alloying elements to be added to a magnesium alloy is smaller than a maximum high capacity of each alloying element to Mg, most Al is solidified in the Mg matrix, rather than inducing intermetallic compound formation in the grain, and thus, formation of an Mg-Al intermetallic compound is not a general phenomenon. Thus, it is difficult to form a large amount of an Mg-Al intermetallic compound. In the case of an embodiment in the present disclosure, a large amount of Mg-Al intermetallic compound may be secured by adding B and Y in combination and performing an aging treatment thereon.
If the volume fraction of the Mg-Al intermetallic compound is less than 6.5%, it is difficult to ensure high strength. Accordingly, the volume fraction of the Mg-Al intermetallic compound may be 6.5% or more, in detail, 7.0% or more, in further detail, 7.5% or more.
An upper limit of the volume fraction of the Mg-Al intermetallic compound is not particularly limited, but if the content thereof exceeds 30%, the grain size of the Mg-Al intermetallic compound may be coarsened, and brittleness may be increased. Thus, the volume fraction of the Mg-Al intermetallic compound may be 30% or less, and in detail, may be 25% or less.
If the average grain size of the Mg-Al intermetallic compound is less than 20 nm, the fraction of the Mg-Al intermetallic compound is low and it is difficult to secure high strength. If the average grain size thereof is more than 500 nm, brittleness increases.
In this case, one or more of an Al-Mn intermetallic compound and an Al-Y intermetallic compound is further included, and the total amount thereof may be 5% or less in a volume fraction. If the total amount thereof exceeds 5%, the Mn and Y contents are excessive and thus, brittleness may increase.
In this case, the magnesium alloy according to an embodiment in the present disclosure may have an ignition temperature of 700°C or higher.
In addition, the magnesium alloy according to an embodiment in the present disclosure may have a hardness of 70 Hv or more.
The magnesia alloy according to an embodiment in the present disclosure may have a tensile strength of 130 MPa or more and an elongation of 3% or more. Further, a tensile strength of 160 MPa or more may be secured by controlling the Al content and the like.

Method of Producing High-strength Magnesium Alloy having Excellent Flame Retardancy

Hereinafter, a method for producing a high strength magnesium alloy having excellent flame retardancy according to another embodiment in the present disclosure will be described in detail.
According to another aspect of the present disclosure, there is provided a method of producing a high strength magnesium alloy having excellent flame retardancy, including: preparing a molten metal satisfying the above-described alloy composition; casting the molten metal to obtain a magnesium alloy casting material; subjecting the magnesium alloy casting material to a solution treatment at a temperature ranging from 370 to 490°C for 2 to 20 hours to obtain a magnesium alloy; cooling the magnesium alloy to 100°C or lower; and aging the cooled magnesium alloy at a temperature of 150 to 250°C for 2 to 48 hours.

Molten Metal Preparation

A molten metal satisfying the above alloy composition is prepared. The molten metal is prepared through general molten metal preparation for magnesium alloy, without particular limitation.
For example, the above-described alloying elements are prepared in accordance with the proposed composition range, and then charged into a crucible for melting, to then be subjected to a melting operation. Since the melting point of the magnesium alloy is relatively low, any method using a gas furnace, an electric furnace, an induction melting furnace, or the like may be used.
In preparing the alloying elements, each alloy element may be prepared in a pure form, but the alloying elements may also be charged into the crucible in the form of a master alloy in which Mn, B and Y are mixed with Mg or Al. B, Y and Mn have high melting points and may thus be charged into the crucible in the form of master alloy mixed with Mg or Al, which is advantageous in terms of dissolving.
In addition, when the prepared dissolving material is charged into the crucible, the material may be charged into the crucible in order from the element having a low melting point, which may be advantageous in terms of dissolving work.

Casting

The molten metal is cast to obtain a magnesium alloy casting material. The casting is not particularly limited as in the case of the molten metal preparation.
For example, a method using a movable mold and a method using a fixed mold may be used. Representative examples of the method using the movable mold include twin roll casting and belt casting using a movable mold such as a twin roll or a twin belt. Also, representative examples of the method using the fixed mold include semi-continuous casting or continuous casting such as billet casting, and may also include mold casting such as high-pressure casting, low-pressure casting and gravity casting.
As the casting process, although various methods as described above may be used, since boron or yttrium having a low solubility with respect to magnesium is added together with aluminum, it may be advantageous to apply a casting method capable of increasing a cooling rate. To this end, the mold should be cooled, with cooling water. When cooling water is used, the mold surface should be maintained at room temperature or more before casting so that the condensed water on the surface of the mold may be removed, and then should be maintained at room temperature or lower after the condensed water is removed.

Solution Treatment

The magnesium alloy casting material is subjected to a solution treatment at a temperature ranging from 370 to 490°C for 2 to 20 hours to obtain a magnesium alloy. Since a Mg-Al intermetallic compound is formed in the magnesium alloy casting material, but is formed in the form of coarse Mg-Al or mixed with a Mg matrix (Lamellar Mg-Al), the solution treatment is performed to enable such an adverse Mg-Al intermetallic compound to a solid solution treatment.
If the solution treatment temperature is less than 370°C or the holding time is less than 2 hours, the entire Mg-Al intermetallic compound is difficult to be solidified. If the solution treatment temperature exceeds 490°C or the retention time is more than 20 hours, production costs may be increased and productivity may be lowered, and an ignition phenomenon by oxidation may occur before B and Y are added. Therefore, in more detail, the solution treatment may be carried out within a temperature range of 400 to 460 °C for 2 to 20 hours.

Cooling

The magnesium alloy is cooled to 100°C or lower, to significantly reduce a natural aging phenomenon that may appear before aging treatment.
In this case, the cooling rate may be 1 to 100°C/second, and the natural aging phenomenon that may occur during cooling is significantly reduced and the solidified Al element may be prevented from being precipitated at random. For example, rapid cooling may be preferably performed by forced blowing, water cooling, oil cooling, or the like.

Aging Treatment

The cooled magnesium alloy is aged at 150 to 250°C for 2 to 48 hours. In the microstructure of the cooled magnesium alloy after the solution treatment, most of the Al element added as the alloying element does not form a separate intermetallic compound in a state of solid solution solidified in the Mg matrix, so that the strength of the material may not be efficiently increased. Therefore, in the case of an embodiment of the present disclosure, a large amount of Mg-Al intermetallic compound is precipitated through aging to increase the strength and to secure excellent flame retardancy. For example, when B and Y are combined in the range suggested according to an embodiment in the present disclosure, a large amount of Mg-Al intermetallic compound may be precipitated through the aging treatment described above.
Since the precipitation by the aging treatment is a solid-phase reaction proceeding in the solid phase, an Mg-Al intermetallic compound having a particle form, an average particle diameter, a volume fraction, and the like favorable for the improvement of strength and flame retardancy may be formed.
If the aging treatment temperature is less than 150°C or the holding time thereof is less than 2 hours, it is difficult to sufficiently secure the Mg-Al intermetallic compound. On the other hand, if the aging treatment temperature is more than 250°C or the retention time is more than 48 hours, the Mg-Al intermetallic compound may be solidified, resulting in increased production costs and decreased productivity. Therefore, the aging treatment may be performed at 150 to 250°C for 2 to 48 hours. In more detail, the temperature and the holding time may be increased within the above temperature and holding time depending on the amount of Al added.

[Mode for Invention]

Hereinafter, an embodiment in the present disclosure will be described in more detail by way of example. It should be noted, however, that the following examples are intended to illustrate the present disclosure in more detail and not to limit the scope of the present disclosure. The scope of the present disclosure is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

(Embodiment 1)

A magnesium alloy casting material having a thickness of 10 mm was cast by casting a molten metal having the composition shown in Table 1 below. The magnesium alloy casting material was solution-treated at 420°C for 4 hours, cooled to 20°C, and aged at 200°C for 12 hours to produce a magnesium alloy.
Mechanical properties of a Mg-Al intermetallic compound of the magnesium alloy were measured and are shown in Table 1 below. The size of the Mg-Al intermetallic compound was measured by an average size obtained by measuring a circle-equivalent diameter.
Except for the alloying element shown in Table 1, it was magnesium, and Mg-Al means a Mg-Al intermetallic compound.

The ignition temperature was measured at a temperature at which the ignition occurred while raising the temperature inside the furnace body while leaving a sample of 10 g in the form of a chip in the furnace under the atmosphere.

[Table 1]

Classification	Alloy Composition (wt%)				Mg-Al		Mechanical Properties			Ignition Temperature (°C)
Classification	Al	Mn	B	Y	Size (nm)	Fraction (vol %)	TS (MPa)	El (%)	Hardness (Hv)	Ignition Temperature (°C)
Comparative Material 1	9	0.34	-	-	193	2.6	113	5.2	59	530
Comparative Material 2	3	0.05	0.013	1.23	178	1.7	113	5.2	48	624
Comparative Material 3	6	0.21	0.001	0.41	205	2.1	121	4.2	53	590
Comparative Material 4	9	0.45	0.412	0.05	232	3.8	124	3.1	63	647
Comparative Material 5	3	0.74	-	0.72	184	1.4	114	4.3	50	634
Comparative Material 6	6	0.12	0.003	1.21	212	2.4	123	3.4	56	660
Comparative Material 7	9	0.33	-	0.21	228	3.1	125	3.2	61	612
Inventive Material 1	2.8	0.12	0.018	0.92	134	7.5	135	5.8	71	721
Inventive Material 2	2.7	0.32	0.012	0.63	128	8.1	138	6.2	72	712
Inventive Material 3	3.1	0.44	0.006	0.31	133	8.7	142	6.1	74	718
Inventive Material 4	3.4	0.42	0.003	0.13	129	9.8	144	5.7	78	715
Inventive Material 5	5.6	0.12	0.017	0.12	153	13.3	151	4.6	84	747
Inventive Material 6	6.2	0.3	0.012	0.54	161	14.4	155	5.8	87	733
Inventive Material 7	6.2	0.4	0.006	0.81	154	14.7	158	4.7	88	742
Inventive Material 8	5.8	0.42	0.002	0.88	139	13.8	152	4.4	85	739
Inventive Material 9	8.5	0.12	0.013	0.14	176	19.1	164	3.5	103	783
Inventive Material 10	10.6	0.3	0.003	0.51	184	21.6	167	4.6	107	776
Inventive Material 11	9.8	0.4	0.009	0.94	179	20.3	172	4.2	106	782
Inventive Material 12	8.7	0.42	0.019	0.45	172	19.4	168	3.7	104	785

It can be confirmed that the inventive materials satisfying the alloy composition and the producing conditions proposed according to an embodiment in the present disclosure include a Mg-Al intermetallic compound in a volume fraction of 6.5% or more and the average particle diameter of the Mg-Al intermetallic compound satisfies 20 to 500 nm. Also, it can be confirmed that the flame retardancy is excellent as an ignition temperature is 700°C or more, and the mechanical properties are also superior to the comparative materials.
Meanwhile, it can be confirmed that the comparative materials satisfied the producing conditions proposed according to an embodiment in the present disclosure, but did not satisfy the alloy composition, so that Mg-Al intermetallic compounds were not sufficiently secured. In addition, it can be confirmed that the flame retardancy is poor and the mechanical properties are also inferior to those of the inventive materials.

(Embodiment 2)

The changes in the producing process of the comparative material 1 and the inventive material 7 shown in Table 1 were observed more closely.
FIGS. 1A and 1B are images of the microstructures of the magnesium alloy casting materials of comparative material 1 (a) and inventive material 7(b). The casting structure of the comparative member 1 is composed of a Mg matrix and coarse Mg-Al intermetallic compound, a Mg matrix + Mg-Al intermetallic compound mixed structure (Lamellar Mg-Al), and an Al-Mn intermetallic compound. In the case of the casting structure of the inventive material 7 to which yttrium and boron were added, an Al-Y intermetallic compound (Al-Y) was observed in addition to the above-mentioned structure, and no boron-containing intermetallic compound was observed separately.
When the comparative material and the inventive material having the above casting structure are subjected to the solution treatment, most of the Mg-Al intermetallic compounds except for Al-Mn or Al-Y intermetallic compounds are solidified as the base structure as illustrated in FIGS. 2 and 3, and thus, is not observed. The optical structures of the casting structure and the solution treatment material are almost similar except for the presence or absence of the Al-Y intermetallic compound, but show a large difference in the aging treatment material.
These differences may be confirmed first in the hardness measurement results according to aging time. As illustrated in FIGS. 4A and 4B, the hardness values of the comparative material 1(a) and the inventive material 7(b) were measured at 200°C according to the aging time. As a result, it can be confirmed that the hardness of the inventive material is significantly high. Also, the hardness value of the comparative material 1 barely changes according to the aging time, but it can be confirmed that the hardness value of the inventive material greatly increases when the aging time exceeds 1 hour. In detail, after the lapse of 3 hours, a maximum hardness value corresponding to peak aging is shown, and the hardness value at this time is a value of 97 to 107 Hv which is a value increased 60% or more, as compared with the average hardness value of the casting material aged for 1 hour or less. In addition, a maximum hardness value of the inventive material 7 is about two times the maximum hardness value of the comparative material.
FIGS. 5A to 5C are images showing the microstructures of the magnesium alloys after the aging treatment of the comparative material 1(a), the inventive material 7 (b) and the comparative material 5(c). It can be confirmed that a large amount of Mg-Al intermetallic compound having a size of several tens of nanometers is precipitated in the inventive material 7, which shows that the hardness value of the inventive material is significantly increased.
FIG. 6 illustrates changes in the hardness value (rhombus) and the size (square) of the Mg-Al intermetallic compound in the crystal grains with respect to the aging time of the inventive material 7, and FIG. 7 illustrates a volume fraction of Mg-Al intermetallic compounds with aging time. As illustrated in FIGS. 6 and 7, when the inventive material 7 is aged for 3 hours or more, it can be seen that the Mg-Al intermetallic compounds are grown to 20 nm or more and 10 vol% or more, respectively, in the average size and the volume fraction.
While embodiments have been illustrated and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the scope of the present disclosure as defined by the appended claims.

Claims

A high strength magnesium alloy having excellent flame retardancy, comprising:
by weight%, 2.0 to 13.0% of aluminum (Al), 0.1 to 0.5% of manganese (Mn), 0.0015 to 0.025% of boron (B), 0.1 to 1.0% of yttrium (Y), a remainder of magnesium (Mg), and unavoidable impurities,

the magnesium alloy comprising a Mg-Al intermetallic compound in a volume fraction of 6.5% or more,

wherein the Mg-Al intermetallic compound has an average particle diameter of 20 to 500 nm.
The high strength magnesium alloy having excellent flame retardancy of claim 1, wherein the magnesium alloy further comprises 0.5 to 1.5 weight% of zinc (Zn).
The high strength magnesium alloy having excellent flame retardancy of claim 1, wherein the magnesium alloy further comprises one or more of an Al-Mn intermetallic compound and an Al-Y intermetallic compound, and a total content of the Al-Mn intermetallic compound and the Al-Y intermetallic compound is 5% or less in a volume fraction.
The high strength magnesium alloy having excellent flame retardancy of claim 1, wherein the magnesium alloy has an ignition temperature of 700°C or higher.
The high strength magnesium alloy having excellent flame retardancy of claim 1, wherein the magnesium alloy is a hardness of 70 Hv or more.
The high strength magnesium alloy having excellent flame retardancy of claim 1, wherein the magnesium alloy has a tensile strength of 130 MPa or more and an elongation of 3% or more.
A method of producing a high strength magnesium alloy having excellent flame retardancy, the method comprising:
preparing a molten metal including, by weight%, 2.0 to 13.0% of aluminum (Al), 0.1 to 0.5% of manganese (Mn), 0.0015 to 0.025% of boron (B), 0.1 to 1.0% of yttrium (Y), a remainder of magnesium (Mg), and unavoidable impurities;

casting the molten metal to obtain a magnesium alloy casting material;

subjecting the magnesium alloy casting material to a solution treatment at a temperature ranging from 370 to 490°C for 2 to 20 hours to obtain a magnesium alloy;

cooling the magnesium alloy to 100°C or lower; and

aging the magnesium alloy cooled in the cooling of the magnesium alloy at a temperature of 150 to 250°C for 2 to 48 hours.
The method of producing a high strength magnesium alloy having excellent flame retardancy of claim 7, wherein the molten metal further comprises, by weight%, 0.5 to 1.5% of Zn.
The method of producing a high strength magnesium alloy having excellent flame retardancy of claim 7, wherein the preparing of the molten metal is performed by charging a crucible with Mn, B and Y mixed with Mg or Al in the form of a master alloy.
The method of producing a high strength magnesium alloy having excellent flame retardancy of claim 7, wherein the preparing of the molten metal is performed by charging the crucible sequentially from an element having a low melting point.
The method of producing a high strength magnesium alloy having excellent flame retardancy of claim 7, wherein the cooling is performed at a cooling rate of 1 to 100°C/second.