CN106498251B - A kind of biological medical magnesium alloy and preparation method thereof - Google Patents

Abstract

A biomedical magnesium alloy and a preparation method thereof belong to the field of biomedical metal materials. The composition and mass percentage of the alloy are: Mn 0.05-0.2%, Zn 1-4%, Ca 0.05-1%, wherein the molar ratio of Zn/Ca is greater than 1.5, and the rest is Mg. The invention prepares a novel human body degradable medical alloy through certain key technical parameters such as smelting process and alloy composition ratio. The alloy does not contain rare earth elements and mainly has Mg-Zn-Ca phase, which is mainly distributed in the grain boundary and a small amount is distributed in the interior of the matrix. The alloy has excellent biocompatibility, outstanding mechanical properties, excellent plastic processing properties and controllable degradation rate. The application of the alloy is characterized in that: 1) a degradable cardiovascular support material; 2) preparation of orthopedic implants, bone plates and bone nails, and the like.

Description

A kind of biomedical magnesium alloy and preparation method thereof

technical field

The invention belongs to the field of biomedical metal materials, and in particular relates to a Mg-Zn-Mn-Ca series biomedical magnesium alloy and a preparation method thereof.

Background technique

As a new type of biodegradable medical metal material, magnesium alloy has attracted extensive attention in the field of medical materials in recent years. Compared with traditional medical metal materials such as stainless steel, titanium alloy, and cobalt-chromium alloy, magnesium alloy has the following advantages as a medical metal material: 1. It has good biocompatibility, is non-toxic and can be degraded in the human body, and its degradation products will not Harmful to the human body. 2. The elastic modulus of magnesium alloy is about 45GPa, which is close to that of human bone. As an orthopedic implant, it can effectively reduce the "stress shielding effect". 3. It has high specific strength and specific stiffness, and good processing performance, which can meet the requirements of medical implant materials. 4. Abundant resources and low prices. Therefore, magnesium alloys have broad application prospects as biodegradable medical metal materials.

At present, the research hotspots on medical magnesium alloys mainly focus on Mg-Al and Mg-Re alloys. Al is a neurotoxic element, and rare earth elements are easy to accumulate in the brain, endangering human health. Therefore, it is necessary to develop magnesium alloys with better biocompatibility. In addition, the rapid degradation rate of magnesium alloy is also an important factor restricting its application. Alloying is an effective way to improve the corrosion performance of magnesium alloys and solve the problem of rapid degradation of magnesium alloys. As a biomedical magnesium alloy, the microalloying method should be used to improve the corrosion resistance of the magnesium alloy while avoiding the introduction of harmful elements. Therefore, it is of great significance to develop medical magnesium alloys with controllable degradation by selecting alloying elements with good biocompatibility.

Contents of the invention

The purpose of the present invention is to provide a non-toxic and degradable medical magnesium alloy, and at the same time provide a method for preparing the alloy.

The purpose of the present invention is achieved through the following technical solutions:

A biomedical magnesium alloy, characterized in that the components of the magnesium alloy and their mass percentages are: Zn 1-4%, Mn 0.05-0.2%, Ca 0.05-1%, and the molar ratio of Zn/Ca Greater than 1.5, the rest are Mg and unavoidable impurity elements.

Further preferably, the total weight percentage of components Zn, Mn, Ca and unavoidable impurity elements in the magnesium alloy is not more than 5%.

The corrosion rate of the biomedical magnesium alloy of the present invention is 0.2mm/y-1.2mm/y in the simulated body fluid of a human body such as Hank's, which can be regulated as required.

The preparation method of the medical magnesium alloy comprises the following steps:

(1) Raw material preparation: raw materials are pure magnesium (99.9wt.%), pure zinc (99.99wt.%), Mg-Ca master alloy (preferably Mg-9.4wt.% Ca master alloy) and Mg-Mn master alloy (preferably Mg-7.4wt.%Mn master alloy); grinding and cleaning the surface of the raw material with a grinding wheel to remove surface oxides to reduce the generation of smelting impurities.

(2) Melting: Preheat the graphite crucible in a resistance furnace, and after the temperature of the furnace rises to 300-500°C, feed the mixed gas of N ₂ +SF ₆ into the furnace; among them, the volume flow rate of N ₂ and SF ₆ The ratio is preferably 100:1. Put the pure magnesium ingot into the crucible after passing the protective gas for 5 minutes, raise the furnace temperature to 680-760°C, and wait until the magnesium ingot is completely melted, add and melt in the order of pure Zn, Mg-Ca master alloy, and Mg-Mn master alloy; preferably Each time an alloy is added, after the alloy is completely melted, stir at a constant speed and counterclockwise for 5-8 minutes, keep it at 740°C for 15 minutes, and finally lower the temperature of the melt to 720°C, and let it stand for 30 minutes; N ₂ + is continuously introduced throughout the melting process The mixed protective gas of SF ₆ prevents the oxidation or combustion of the magnesium alloy melt.

Further, the addition amount of pure Zn, Mg-Ca master alloy and Mg-Mn master alloy should be higher than the theoretical amount, and the higher amount is used for the burning loss of the process.

(3) Casting: remove slag, then pour the melt into a preheated metal mold at a constant speed, and demould after solidification to obtain an alloy ingot. The preheating temperature of the mold is 200°C. In order to prevent oxidation or burning during casting, a protective gas is introduced into the mold first, and the protective gas is continuously delivered to the liquid flow for protection during the casting process.

The beneficial effects of the present invention are:

1. A biomedical magnesium alloy with good biocompatibility and controllable degradation rate was prepared.

2. By controlling the alloy composition and adjusting the Zn/Ca atomic ratio in the alloy to control the type and volume fraction of the second phase in the alloy, thereby adjusting the degradation rate of the alloy.

3. The second phase of the medical magnesium alloy prepared by the present invention is mainly a Mg-Zn-Ca phase and is mainly distributed at the grain boundary, and a small amount is distributed inside the matrix.

4. The magnesium alloy of the present invention has good mechanical properties. Zn and Mn have dual functions of solid solution strengthening and aging strengthening. In addition, Zn is easy to form compounds with other alloy elements or metal impurities, which can further enhance the effect of solid solution strengthening. The addition of Ca element can improve the formability and strength of magnesium alloy. Therefore, the good mechanical properties of the magnesium alloy are effectively guaranteed.

5. The application of the biomedical magnesium alloy of the present invention is characterized in that it is used to prepare orthopedic implants such as degradable cardiovascular stents, bone nails, and bone plates.

6. The preparation method has the advantages of low raw material cost, simple and easy-to-operate preparation process, and the like.

Description of drawings

Fig. 1 is the optical metallographic photograph of magnesium alloy in embodiment 1;

Fig. 2 is the appearance after the corrosion of magnesium alloy in embodiment 1 10 days;

FIG. 3 is the polarization curve of the magnesium alloy in Example 1.

Detailed ways:

The present invention is further described in conjunction with specific examples of implementation as follows, and it is pointed out that the following examples of implementation are only used to illustrate specific implementation methods of the present invention, and cannot limit the scope of protection of the present invention.

Example 1

Preparation of as-cast Mg-4.0wt% Zn-0.1wt% Mn-0.2wt% Ca magnesium alloy, Zn/Ca (atomic ratio 12.25) in the alloy.

1) Raw material preparation: The test raw materials are 916g of pure magnesium (99.9wt.%), 43.7g of pure zinc (99.99wt.%) (calculated according to 10% burning loss), 27.5g of Mg-9.4wt.%Ca master alloy ( Calculated by 30% burning loss) and 17.4g of Mg-7.4wt.%Mn master alloy (calculated based on 30% burning loss), and the raw materials were polished with grinding wheels to remove surface oxides.

2) Melting:

In order to prevent the oxidation or combustion of the magnesium alloy melt, the mixed protective gas of N ₂ +SF ₆ is continuously introduced into the whole melting process, and the flow ratio of N ₂ and SF ₆ is 100:1;

a) Dry the crucible, slag removal tool, stirring rod and mold in an oven at 200°C for use;

b) Put the treated crucible into a resistance furnace, set the temperature to 300°C, and when the furnace temperature reaches, pass in protective gas;

c) Add the polished high-purity magnesium ingot after feeding the protective gas for 5 minutes, and at the same time, the furnace temperature rises to 760°C;

d) After the magnesium ingot is completely melted, add high-purity zinc. After the zinc is completely melted, stir at a constant speed and counterclockwise for 5 minutes. After the -9.4wt.% Ca master alloy is completely melted, stir it counterclockwise at a constant speed for 5 minutes, keep it at 740°C for 15 minutes, and then add the Mg-7.4wt.% Mn master alloy. After the Mg-7.4wt.% Mn master alloy is completely melted, , Stir counterclockwise for 5 minutes;

e) Furnace temperature is set to 720°C, let stand for 30 minutes, and remove slag.

3) Casting: Take out the crucible and start casting; in order to prevent oxidation or burning during casting, a protective gas is introduced into the mold first, and the protective gas is continuously delivered to the liquid flow for protection during the casting process, and the melt is poured into the 200 The metal mold preheated at ℃ is solidified and released from the mold to obtain an alloy ingot.

Alloy corrosion resistance: The ratio of the surface area (cm ² ) of the alloy sample to the volume (ml) of Hank's solution is 1/150, and the corrosion rate of the alloy is 0.31mm/y when immersed in Hank's solution at 37.4°C.

Example 2:

Preparation of as-cast Mg-2.0wt% Zn-0.2wt% Mn-0.5wt% Ca magnesium alloy, Zn/Ca (atomic ratio 2.45) in the alloy.

1) Raw material preparation: The test raw materials are 882g of pure magnesium (99.9wt.%), 21.9g of pure zinc (99.99wt.%) (calculated according to 15% burning loss), 68.8g of Mg-9.4wt.% Ca master alloy ( 30% burning loss calculation) and Mg-7.4wt.%Mn master alloy 34.9g (calculation is based on 30% burning loss), and the raw material is polished with a grinding wheel to remove surface oxides.

2) Melting:

In order to prevent the oxidation or combustion of the magnesium alloy melt, the mixed protective gas of N ₂ +SF ₆ is continuously introduced into the whole melting process, and the flow ratio of N ₂ and SF ₆ is 100:1;

a) Dry the crucible, slag removal tool, stirring rod and mold in an oven at 200°C for use;

b) Put the treated crucible into a resistance furnace, set the temperature to 300°C, and when the furnace temperature reaches, pass in protective gas;

c) Add the polished high-purity magnesium ingot after feeding the protective gas for 5 minutes, and at the same time, the furnace temperature rises to 760°C;

d) After the magnesium ingot is completely melted, add high-purity zinc. After the zinc is completely melted, stir at a constant speed and counterclockwise for 5 minutes. After the -9.4wt.% Ca master alloy is completely melted, stir it counterclockwise at a constant speed for 5 minutes, keep it at 740°C for 15 minutes, and then add the Mg-7.4wt.% Mn master alloy. After the Mg-7.4wt.% Mn master alloy is completely melted, , Stir counterclockwise for 5 minutes;

e) Furnace temperature is set to 720°C, let stand for 30 minutes, and remove slag.

3) Casting: Take out the crucible and start casting; in order to prevent oxidation or burning during casting, a protective gas is introduced into the mold first, and the protective gas is continuously delivered to the liquid flow for protection during the casting process, and the melt is poured into the 200 The metal mold preheated at ℃ is solidified and released from the mold to obtain an alloy ingot.

Alloy corrosion resistance: The ratio of the surface area (cm ² ) of the alloy sample to the volume (ml) of Hank's solution is 1/150, and the corrosion rate of the alloy is 0.63mm/y when immersed in Hank's solution at 37.4°C.

Example 3:

Preparation of as-cast Mg-1.0wt% Zn-0.05wt% Mn-0.2wt% Ca magnesium alloy, Zn/Ca (atomic ratio 3.06) in the alloy.

1) Raw material preparation: The test raw materials are 924g of pure magnesium (99.9wt.%), 10.9g of pure zinc (99.99wt.%) (calculated according to 15% burning loss), 27.61g of Mg-9.4wt.%Ca master alloy ( Calculated by 30% burning loss) and 8.77g of Mg-7.4wt.% Mn master alloy (calculated by 30% burning loss), and the raw materials were polished with grinding wheels to remove surface oxides.

2) Melting:

In order to prevent the oxidation or combustion of the magnesium alloy melt, the mixed protective gas of N ₂ +SF ₆ is continuously introduced into the whole melting process, and the flow ratio of N ₂ and SF ₆ is 100:1;

a) Dry the crucible, slag removal tool, stirring rod and mold in an oven at 200°C for use;

b) Put the treated crucible into a resistance furnace, set the temperature to 300°C, and when the furnace temperature reaches, pass in protective gas;

c) Add the polished high-purity magnesium ingot after feeding the protective gas for 5 minutes, and at the same time, the furnace temperature rises to 760°C;

d) After the magnesium ingot is completely melted, add high-purity zinc. After the zinc is completely melted, stir at a constant speed and counterclockwise for 5 minutes. After the -9.4wt.% Ca master alloy is completely melted, stir it counterclockwise at a constant speed for 5 minutes, keep it at 740°C for 15 minutes, and then add the Mg-7.4wt.% Mn master alloy. After the Mg-7.4wt.% Mn master alloy is completely melted, , Stir counterclockwise for 5 minutes;

e) Furnace temperature is set to 720°C, let stand for 30 minutes, and remove slag.

3) Casting: Take out the crucible and start casting; in order to prevent oxidation or burning during casting, a protective gas is introduced into the mold first, and the protective gas is continuously delivered to the liquid flow for protection during the casting process, and the melt is poured into the 200 The metal mold preheated at ℃ is solidified and released from the mold to obtain an alloy ingot.

Alloy corrosion resistance: The ratio of the surface area (cm ² ) of the alloy sample to the volume (ml) of Hank's solution is 1/150, and the corrosion rate of the alloy is 0.86mm/y when immersed in Hank's solution at 37.4°C.

Example 4:

Preparation of as-cast Mg-4.0wt% Zn-0.1wt% Mn-1.0wt% Ca magnesium alloy, Zn/Ca (atomic ratio 2.45) in the alloy.

1) Raw material preparation: The test raw materials are 806g of pure magnesium (99.9wt.%), 43.6g of pure zinc (99.99wt.%) (calculated according to 15% burning loss), 137.3g of Mg-9.4wt.%Ca master alloy ( Calculated by 30% burning loss) and 17.4g of Mg-7.4wt.%Mn master alloy (calculated based on 30% burning loss), and the raw materials were polished with grinding wheels to remove surface oxides.

2) Melting:

In order to prevent the oxidation or combustion of the magnesium alloy melt, the mixed protective gas of N ₂ +SF ₆ is continuously introduced into the whole melting process, and the flow ratio of N ₂ and SF ₆ is 100:1;

a) Dry the crucible, slag removal tool, stirring rod and mold in an oven at 200°C for use;

b) Put the treated crucible into a resistance furnace, set the temperature to 300°C, and when the furnace temperature reaches, pass in protective gas;

c) Add the polished high-purity magnesium ingot after feeding the protective gas for 5 minutes, and at the same time, the furnace temperature rises to 760°C;

d) After the magnesium ingot is completely melted, add high-purity zinc. After the zinc is completely melted, stir at a constant speed and counterclockwise for 5 minutes. After the -9.4wt.% Ca master alloy is completely melted, stir it counterclockwise at a constant speed for 5 minutes, keep it at 740°C for 15 minutes, and then add the Mg-7.4wt.% Mn master alloy. After the Mg-7.4wt.% Mn master alloy is completely melted, , Stir counterclockwise for 5 minutes;

e) Furnace temperature is set to 720°C, let stand for 30 minutes, and remove slag.

3) Casting: Take out the crucible and start casting; in order to prevent oxidation or burning during casting, a protective gas is introduced into the mold first, and the protective gas is continuously delivered to the liquid flow for protection during the casting process, and the melt is poured into the 200 The metal mold preheated at ℃ is solidified and released from the mold to obtain an alloy ingot.

Alloy corrosion resistance: The ratio of the surface area (cm ² ) of the alloy sample to the volume (ml) of Hank's solution is 1/150, and the corrosion rate of the alloy is 1.12mm/y when immersed in Hank's solution at 37.4°C.

Example 5:

Preparation of as-cast Mg-3.0wt% Zn-0.1wt% Mn-0.5wt% Ca magnesium alloy, Zn/Ca (atomic ratio 3.67) in the alloy.

1) Raw material preparation: The test raw materials are 806g of pure magnesium (99.9wt.%), 32.84g of pure zinc (99.99wt.%) (calculated according to 15% burning loss), 68.82g of Mg-9.4wt.%Ca master alloy ( Calculated by 30% burning loss) and 17.5g of Mg-7.4wt.%Mn master alloy (calculated based on 30% burning loss), and the raw materials were polished with grinding wheels to remove surface oxides.

2) Melting:

In order to prevent the oxidation or combustion of the magnesium alloy melt, the mixed protective gas of N ₂ +SF ₆ is continuously introduced into the whole melting process, and the flow ratio of N ₂ and SF ₆ is 100:1;

a) Dry the crucible, slag removal tool, stirring rod and mold in an oven at 200°C for use;

b) Put the treated crucible into a resistance furnace, set the temperature to 300°C, and when the furnace temperature reaches, pass in protective gas;

c) Add the polished high-purity magnesium ingot after feeding the protective gas for 5 minutes, and at the same time, the furnace temperature rises to 760°C;

d) After the magnesium ingot is completely melted, add high-purity zinc. After the zinc is completely melted, stir at a constant speed and counterclockwise for 5 minutes. After the -9.4wt.% Ca master alloy is completely melted, stir it counterclockwise at a constant speed for 5 minutes, keep it at 740°C for 15 minutes, and then add the Mg-7.4wt.% Mn master alloy. After the Mg-7.4wt.% Mn master alloy is completely melted, , Stir counterclockwise for 5 minutes;

e) Furnace temperature is set to 720°C, let stand for 30 minutes, and remove slag.

3) Casting: Take out the crucible and start casting; in order to prevent oxidation or burning during casting, a protective gas is introduced into the mold first, and the protective gas is continuously delivered to the liquid flow for protection during the casting process, and the melt is poured into the 200 The metal mold preheated at ℃ is solidified and released from the mold to obtain an alloy ingot.

Alloy corrosion resistance: The ratio of the surface area (cm ² ) of the alloy sample to the volume (ml) of Hank's solution is 1/150, and the corrosion rate of the alloy is 0.43mm/y when immersed in Hank's solution at 37.4°C.

Embodiment 6:

Preparation of as-cast Mg-3.0wt% Zn-0.1wt% Mn-0.2wt% Ca magnesium alloy, Zn/Ca (atomic ratio 9.19) in the alloy.

1) Raw material preparation: The test raw materials are 806g of pure magnesium (99.9wt.%), 32.87g of pure zinc (99.99wt.%) (calculated according to 15% burning loss), 27.55g of Mg-9.4wt.%Ca master alloy ( Calculated by 30% burning loss) and 17.5g of Mg-7.4wt.%Mn master alloy (calculated based on 30% burning loss), and the raw materials were polished with grinding wheels to remove surface oxides.

2) Smelting: In order to prevent the oxidation or combustion of the magnesium alloy melt, the mixed protective gas of N ₂ +SF ₆ is continuously introduced into the whole smelting process, wherein the flow ratio of N ₂ and SF ₆ is 100:1;

a) Dry the crucible, slag removal tool, stirring rod and mold in an oven at 200°C for use;

b) Put the treated crucible into a resistance furnace, set the temperature to 300°C, and when the furnace temperature reaches, pass in protective gas;

c) Add the polished high-purity magnesium ingot after feeding the protective gas for 5 minutes, and at the same time, the furnace temperature rises to 760°C;

d) After the magnesium ingot is completely melted, add high-purity zinc. After the zinc is completely melted, stir at a constant speed and counterclockwise for 5 minutes. After the -9.4wt.% Ca master alloy is completely melted, stir it counterclockwise at a constant speed for 5 minutes, keep it at 740°C for 15 minutes, and then add the Mg-7.4wt.% Mn master alloy. After the Mg-7.4wt.% Mn master alloy is completely melted, , Stir counterclockwise for 5 minutes;

e) Furnace temperature is set to 720°C, let stand for 30 minutes, and remove slag. Casting: Take out the crucible and start casting; in order to prevent oxidation or burning during casting, a protective gas is introduced into the mold first, and the protective gas is continuously sent to the liquid flow for protection during the casting process, and the melt is poured into the 200°C preheated The heated metal mold is demolded after solidification to obtain an alloy ingot.

Alloy corrosion resistance: The ratio of the surface area (cm ² ) of the alloy sample to the volume (ml) of Hank's solution is 1/150, and the corrosion rate of the alloy is 0.26mm/y when immersed in Hank's solution at 37.4°C.

Embodiment 7:

Preparation of as-cast Mg-2.0wt% Zn-0.15wt% Mn-0.05wt% Ca magnesium alloy, Zn/Ca (atomic ratio 24.5) in the alloy.

1) Raw material preparation: The test raw materials are 806g of pure magnesium (99.9wt.%), 21.94g of pure zinc (99.99wt.%) (calculated according to 15% burning loss), 6.9g of Mg-9.4wt.%Ca master alloy ( Calculated by 30% burning loss) and 26.28g of Mg-7.4wt.%Mn master alloy (calculated by 30% burning loss), and the raw materials were polished with grinding wheels to remove surface oxides.

2) Melting:

In order to prevent the oxidation or combustion of the magnesium alloy melt, the mixed protective gas of N ₂ +SF ₆ is continuously introduced into the whole melting process, and the flow ratio of N ₂ and SF ₆ is 100:1;

a) Dry the crucible, slag removal tool, stirring rod and mold in an oven at 200°C for use;

b) Put the treated crucible into a resistance furnace, set the temperature to 300°C, and when the furnace temperature reaches, pass in protective gas;

c) Add the polished high-purity magnesium ingot after feeding the protective gas for 5 minutes, and at the same time, the furnace temperature rises to 760°C;

d) After the magnesium ingot is completely melted, add high-purity zinc. After the zinc is completely melted, stir at a constant speed and counterclockwise for 5 minutes. After the -9.4wt.% Ca master alloy is completely melted, stir it counterclockwise at a constant speed for 5 minutes, keep it at 740°C for 15 minutes, and then add the Mg-7.4wt.% Mn master alloy. After the Mg-7.4wt.% Mn master alloy is completely melted, , Stir counterclockwise for 5 minutes;

e) Furnace temperature is set to 720°C, let stand for 30 minutes, and remove slag.

3) Casting: Take out the crucible and start casting; in order to prevent oxidation or burning during casting, a protective gas is introduced into the mold first, and the protective gas is continuously delivered to the liquid flow for protection during the casting process, and the melt is poured into the 200 The metal mold preheated at ℃ is solidified and released from the mold to obtain an alloy ingot.

Alloy corrosion resistance: The ratio of the surface area (cm ² ) of the alloy sample to the volume (ml) of Hank's solution is 1/150, and the corrosion rate of the alloy is 0.83mm/y when immersed in Hank's solution at 37.4°C.

Although the preferred implementation cases have been listed and described in detail here, those skilled in the art know that various improvements, additions, substitutions, etc. can be made without departing from the essence of the present invention, and these contents are all deemed to belong to the claims within the scope of the present invention.

Claims (7)

1. A preparation method of a biomedical magnesium alloy, each component of the magnesium alloy and its mass percentage are: Zn 1-4%, Mn 0.05-0.2%, Ca 0.05-1%, and the molar ratio of Zn/Ca The ratio is greater than 1.5, and the rest are Mg and unavoidable impurity elements; It is characterized in that, comprising the following steps: (1) Raw material preparation: The raw materials are respectively 99.9wt.% pure magnesium, 99.99wt.% pure zinc, Mg-Ca master alloy and Mg-Mn master alloy; the surface of the raw material is polished and cleaned with a grinding wheel, and the surface oxide is removed to reduce Production of smelting impurities; (2) Melting: Preheat the graphite crucible in a resistance furnace. After the furnace temperature rises to 300-500°C, put N ₂ +SF ₆ mixed gas into the furnace; Put it into the crucible, raise the furnace temperature to 680-760°C, wait until the magnesium ingot is completely melted, add and smelt in the order of pure Zn, Mg-Ca master alloy, and Mg-Mn master alloy; continuously feed N ₂ +SF ₆ during the entire smelting process Mixed protective gas to prevent oxidation or combustion of magnesium alloy melt; (3) Casting: remove slag, then pour the melt into the preheated metal mold at a constant speed, and release the alloy ingot after solidification; The protective gas is fed into the mold, and the protective gas is continuously delivered to the liquid flow for protection during the casting process. 2. The method according to claim 1, wherein the Mg-Ca master alloy is a Mg-9.4wt.%Ca master alloy, and the Mg-Mn master alloy is a Mg-7.4wt.%Mn master alloy. 3. The method according to claim 1, characterized in that the volume flow ratio of N ₂ and SF ₆ in the N ₂ +SF ₆ mixed gas is 100:1. 4. according to the described method of claim 1, it is characterized in that, when adding smelting according to the order of pure Zn, Mg-Ca master alloy, Mg-Mn master alloy in the step (2), every time adding a kind of alloy, treat that alloy is completely After melting, stir at a constant speed and counterclockwise for 5-8 minutes, keep it at 740°C for 15 minutes, and finally lower the temperature of the melt to 720°C and let it stand for 30 minutes. 5. according to the described method of claim 1, it is characterized in that, the add-on of pure Zn, Mg-Ca master alloy, Mg-Mn master alloy will be higher than theoretical consumption, and high excess amount is used for the burning loss of process. 6. The method according to claim 1, characterized in that the total weight percentage of components Zn, Mn, Ca and unavoidable impurity elements in the magnesium alloy is not more than 5%. 7. The method according to claim 1, wherein the second phase of the magnesium alloy is a Mg-Zn-Ca phase and is mainly distributed at grain boundaries.