CN109518027B

CN109518027B - Preparation method and application of fine-grain Mg-Al-Ti-C intermediate alloy

Info

Publication number: CN109518027B
Application number: CN201811464048.2A
Authority: CN
Inventors: 董天顺; 冯阳; 刘利; 李国禄; 王冉
Original assignee: Hebei University of Technology
Current assignee: Hebei University of Technology
Priority date: 2018-12-03
Filing date: 2018-12-03
Publication date: 2020-10-09
Anticipated expiration: 2038-12-03
Also published as: CN109518027A

Abstract

The invention relates to a preparation method and application of a fine-grain Mg-Al-Ti-C intermediate alloy. The method is changed on the basis of casting magnesium alloy, a coarse-grain Mg-Al-Ti-C intermediate alloy is prepared by a two-step method (the main principle is that a TiC-containing Mg-Al-Ti-C intermediate alloy is prepared in an additional mode), the coarse-grain Mg-Al-Ti-C intermediate alloy is treated by a copper mold spray casting method to obtain a fine-grain Mg-Al-Ti-C intermediate alloy, and the magnesium alloy is subjected to modification treatment by the fine-grain Mg-Al-Ti-C intermediate alloy. The invention reduces the tendency of the coarse-grain intermediate alloy to enhance particle agglomeration in the modification treatment of the magnesium alloy, and is beneficial to improving the bonding strength of the magnesium alloy; the second phase reinforcing particles TiC of the intermediate alloy are uniformly distributed, and the hardness of the magnesium alloy is improved.

Description

Preparation method and application of fine-grain Mg-Al-Ti-C intermediate alloy

Technical Field

The invention relates to the technical field of metal material casting magnesium alloy, in particular to a preparation method and application of fine-grain Mg-Al-Ti-C intermediate alloy.

Background

Cast magnesium alloys, represented by AZ91D, are currently the least dense and widely used metallic material. The wide application of the catalyst has important significance for saving energy, reducing emission and slowing down greenhouse effect. In addition, in recent years, the development potential and application advantages of magnesium alloys have attracted great attention of many countries, governments, enterprises and research institutions to magnesium alloy research, and a great deal of manpower, material resources and financial resources are invested to achieve remarkable results. The application range of the magnesium alloy is more and more extensive, and the current range relates to the fields of automobile industry, communication electronics industry, aerospace industry and the like. It is noteworthy that the use of magnesium alloys is currently limited to non-structural components only. One of the important reasons is that the mechanical properties and corrosion resistance of magnesium alloys are poor. Therefore, it is imperative that magnesium alloys be treated to enhance their mechanical properties and corrosion resistance. The magnesium alloy is refined and modified by adding the intermediate alloy, so that the mechanical property of the magnesium alloy is improved. In addition, it is also an effective method to enhance the mechanical properties of the magnesium alloy by adding the master alloy on the basis of the additional reinforcing particles and utilizing the second phase of the master alloy. However, the magnesium alloy is mostly modified by using the coarse-grain master alloy at present, the performance of the magnesium alloy is difficult to improve by the method, meanwhile, the agglomeration of reinforced particles is easily caused by the adoption of the second phase of the coarse-grain master alloy to reinforce the mechanical property of the magnesium alloy, and reports on modification treatment of the magnesium alloy by using the fine-grain master alloy are few.

At present, the main methods for preparing the intermediate alloy at home and abroad comprise: water quenching, melt spinning, copper mold spray casting, and atomization rapid solidification.

1) A water quenching method:

the water quenching method is a rapid solidification method which has a long history and is simple to operate. The main process flow is as follows: putting the master alloy into a quartz tube, vacuumizing the quartz tube, introducing high-purity argon for protection, and sealing the quartz tube; and heating the quartz tube by adopting a high-frequency induction heating device to melt the master alloy, and after the master alloy is completely melted, putting the intermediate alloy into water to carry out water quenching and quenching so as to obtain the alloy with refined structure.

By adopting the process method, a metal tube can be used for replacing a quartz tube in order to further improve the cooling speed. The heating rate is improved, and the quartz tube is put into water and stirred earlier, so that the reaction probability of the master alloy and the quartz tube can be reduced.

2) A melt spinning method:

the melt-spun preparation process principle is as follows: adding the original alloy into a quartz tube, and vacuumizing; adopting an induction coil to perform high-frequency induction heating on a quartz tube to melt the original alloy, introducing high-purity argon into the quartz tube, and spraying the molten metal from a small hole formed in the bottom of the quartz tube by using the pressure of the high-purity argon; the sprayed molten metal meets a copper roller rotating at high speed to form a strip-shaped rapid solidification alloy. The method can change the cooling speed by adjusting the rotating speed of the copper roller, and has the defect that the massive rapid solidification alloy cannot be prepared. The nano-crystalline Al-5Ti-B alloy is prepared by adopting a melt-spinning method in the Guojinyang of Zhengzhou university, and the aluminum alloy is subjected to modification treatment, so that a good effect is obtained.

3) Copper mold spray casting:

the technological principle of the copper mold spray casting method for preparing the rapid solidification alloy is as follows: placing the small-block original alloy in a quartz tube with a small hole at the bottom, vacuumizing, and introducing argon for protection; and (3) melting the intermediate alloy by adopting high-frequency induction, and spraying molten metal into a copper mold right below by utilizing the vertical pressure difference in a quartz tube to be rapidly cooled after the intermediate alloy is completely melted, thereby obtaining the rapidly solidified alloy.

The copper die has high heat conductivity and small sample size, so that the obtained alloy structure is relatively fine. This method is lower in cooling rate than the strip casting method, but it can produce bulk rapidly solidified alloys of smaller size.

4) Atomization rapid solidification method:

the atomization method utilizes the combined action of high-speed fluid impact force, centrifugal force and mechanical striking force to disperse alloy melt into fine-sized fog-shaped molten drops, and simultaneously the molten drops are contacted with fluid or a substrate to be rapidly solidified, so that a refined alloy structure is obtained. The method has the advantages of low cost, high production efficiency and suitability for large-scale production, but also has the defect of unstable cooling effect.

The atomization method is researched more at home and abroad. Ruthardt and Klaus Bauckhage abroad think that the alloy can be directly atomized by using static capillary waves, and the atomized metal liquid has small particle size and high evaporation and condensation rate. The domestic Zhang Shuguang adopts an ultrasonic atomization method to prepare the spherical fine-grain Sn-Pb alloy.

Disclosure of Invention

The invention aims to provide a preparation method and application of a fine-grain Mg-Al-Ti-C intermediate alloy aiming at the defects in the current magnesium alloy casting technology. The method is changed on the basis of casting magnesium alloy, a coarse-grain Mg-Al-Ti-C intermediate alloy is prepared by a two-step method (the main principle is that a TiC-containing Mg-Al-Ti-C intermediate alloy is prepared in an additional mode), the coarse-grain Mg-Al-Ti-C intermediate alloy is treated by a copper mold spray casting method to obtain a fine-grain Mg-Al-Ti-C intermediate alloy, and the magnesium alloy is subjected to modification treatment by the fine-grain Mg-Al-Ti-C intermediate alloy. The invention reduces the tendency of the coarse-grain intermediate alloy to enhance particle agglomeration in the modification treatment of the magnesium alloy, and is beneficial to improving the bonding strength of the magnesium alloy; the second phase reinforcing particles TiC of the intermediate alloy are uniformly distributed, and the hardness of the magnesium alloy is improved.

The technical scheme of the invention is as follows:

a preparation method of a fine-grain Mg-Al-Ti-C intermediate alloy comprises the following steps:

preparing Al powder, TiC powder and a pure magnesium ingot according to the components of the alloy, wherein the mass percentage of each component is as follows: 0.68-1.2% of C, 2.72-4.8% of Ti, 3.42-5.6% of Al and the balance of Mg, wherein the molar ratio of Ti: c is 1: 1;

the first step is as follows: preparing prefabricated blocks

Mixing Al powder and TiC powder, and carrying out ball milling treatment on the mixed powder for 3.5-4.5 h by using a ball mill; then, pressing the powder subjected to ball milling into a precast block under the pressure of cold pressing of 12-16 MPa;

the second step is that: preparation of coarse-grain Mg-Al-Ti-C intermediate alloy

Putting the precast block and a pure magnesium ingot together, introducing high-purity argon for protection, heating to 820-860 ℃ by using a vacuum induction smelting furnace, preserving heat for 1-1.5 h, and cooling along with the furnace to obtain a coarse-grain Mg-Al-Ti-C intermediate alloy; the purity of the high-purity argon is 99.999 percent;

the third step: preparation of fine-grained Mg-Al-Ti-C intermediate alloy

Cutting the coarse-grain intermediate alloy into blocks, placing them in quartz tube whose bottom end is equipped with hole, vacuumizing the copper mould spray-casting furnace body to make its vacuum degree be not less than 1.2 × 10^-3pa, and filling high-purity argon for protection; utilizing an induction coil to perform high-frequency induction heating to melt the coarse-grain intermediate alloy, then introducing high-purity argon, and spraying the molten metal into a copper mold to obtain a fine-grain intermediate alloy;

wherein the heating temperature is 750-760 ℃, and the metal liquid is blownAir pressure of 7.5 × 10⁴～8.5×10⁴pa；

The range of the size of the prefabricated block in the first step is phi 18-22 mm multiplied by 13-17 mm;

and the size of the coarse-grain master alloy block in the third step is 1-1.2 mm.

The application of the fine-grain Mg-Al-Ti-C intermediate alloy is used for modifying the magnesium alloy:

the method comprises the following steps: putting a magnesium alloy ingot into a resistance furnace under a protective atmosphere, heating to 740-780 ℃, removing slag after the ingot is melted, adding fine-grain Mg-Al-Ti-C intermediate alloy, cooling to 700-730 ℃, keeping the temperature for 8-12 min under stirring, removing slag, and casting molten metal to obtain modified magnesium alloy;

wherein the mass of the added fine-grain Mg-Al-Ti-C intermediate alloy accounts for 2-15 wt% of the mass of the modified magnesium alloy.

The magnesium alloy is AZ91D or AZ91 HP.

The purities of the high-purity Al powder, the high-purity TiC powder and the high-purity argon are 99.999 percent, and the purity of the pure Mg ingot is 99.95 percent.

The vacuum induction smelting furnace is a ZG-0-01 type vacuum induction smelting furnace, and the well type crucible resistance furnace is a SG2-5-12 well type crucible resistance furnace.

The protective atmosphere is SF with the volume percentage of 3 percent₆+97％CO₂The mixed gas of (1).

The invention has the substantive characteristics that:

the invention prepares fine-grain Mg-Al-Ti-C intermediate alloy (the magnesium alloy is modified by adopting the coarse-grain intermediate alloy at present) by a two-step method and a copper die spray casting method, and the modification treatment is carried out on the magnesium alloy, which is very favorable for enhancing the mechanical property of the magnesium alloy. The coarse-grain Mg-Al-Ti-C intermediate alloy is treated by a copper die spray casting method, so that the problem of serious agglomeration of reinforced particles is effectively solved, TiC is uniformly dispersed and distributed, and the TiC is vital to uniform distribution of the TiC in the magnesium alloy; and the magnesium alloy is subjected to modification treatment by the fine-grain Mg-Al-Ti-C intermediate alloy, so that TiC particles are uniformly distributed, the problems that the magnesium alloy is treated by the coarse-grain Mg-Al-Ti-C intermediate alloy and a large amount of TiC particles are eccentrically gathered at the grain boundary of the magnesium alloy are solved, and the tensile strength of the magnesium alloy is favorably improved. TiC is reinforced particles, so that the hardness of the magnesium alloy can be greatly improved by adding the TiC.

The invention has the beneficial effects that:

the invention designs a two-step method and a copper mold spray casting method to prepare fine-grain Mg-Al-Ti-C intermediate alloy, and the magnesium alloy is subjected to modification treatment, so that the magnesium alloy has excellent mechanical properties; compared with the traditional cast magnesium alloy, the invention firstly uses a two-step method and a copper mold spray casting method to prepare the fine-grained Mg-Al-Ti-C intermediate alloy, and then the magnesium alloy is subjected to modification treatment, so that the bonding strength is enhanced, and the hardness is improved. Compared with the traditional cast magnesium alloy, the magnesium alloy has the advantages that: the agglomeration phenomenon of the reinforced particles of the magnesium alloy is obviously improved; the bonding strength is improved and is 1.1-1.3 times of that of the traditional cast magnesium alloy; the hardness is 1.2-1.6 times of that of the traditional cast magnesium alloy.

Drawings

The invention is further illustrated with reference to the following figures and examples.

FIG. 1 is an SEM photograph of a macrocrystalline Mg-Al-Ti-C master alloy of example 1.

FIG. 2 is an SEM photograph of the fine grained Mg-Al-Ti-C master alloy of example 1.

FIG. 3 is a metallographic picture of modified AZ91D of example 1 with 5.2 wt% addition of a fine crystalline Mg-Al-Ti-C master alloy.

FIG. 4 is an SEM photograph of modified AZ91D with 5.2 wt% addition of a fine crystalline Mg-Al-Ti-C master alloy of example 1.

FIG. 5 is an EDS analysis map of point A in FIG. 4.

FIG. 6 is SEM morphology and EMPA line scan analysis of TiC particles of modified AZ91D with 5.2 wt% fine crystalline Mg-Al-Ti-C master alloy added to example 1; FIG. 6a is an SEM photograph of modified AZ91D particles with 5.2 wt% of fine crystalline Mg-Al-Ti-C master alloy added; FIG. 6b shows the analysis result of Mg element in EMPA line scan; FIG. 6c shows the analysis result of Al element in EMPA line scan; FIG. 6d shows the analysis result of Ti element in EMPA line scan; FIG. 6e is the analysis result of element C in the EMPA line scan; FIG. 6f shows the analysis result of O element in EMPA line scan.

FIG. 7 is a graph comparing the tensile strength of modified AZ91D with AZ91D for example 1 with the addition of 5.2 wt% of a fine crystalline Mg-Al-Ti-C master alloy.

FIG. 8 is a graph comparing the Vickers hardness of modified AZ91D with AZ91D for example 1 with 5.2 wt% addition of a fine-grained Mg-Al-Ti-C master alloy.

FIG. 9 is a metallographic picture of modified AZ91D of example 2 with 2.7 wt% addition of a fine crystalline Mg-Al-Ti-C master alloy.

FIG. 10 is a graph comparing the tensile strength of modified AZ91D with AZ91D for example 2 with the addition of 2.7 wt% of a fine crystalline Mg-Al-Ti-C master alloy.

FIG. 11 is a graph comparing the Vickers hardness of modified AZ91D with AZ91D for example 2 with the addition of 2.7 wt% of a fine-grained Mg-Al-Ti-C master alloy.

FIG. 12 is a metallographic picture of modified AZ91D with 12.5 wt% addition of a fine crystalline Mg-Al-Ti-C master alloy of example 3.

FIG. 13 is a graph comparing the tensile strength of modified AZ91D with AZ91D for example 3 with 12.5 wt% addition of a fine crystalline Mg-Al-Ti-C master alloy.

FIG. 14 is a graph comparing the Vickers hardness of modified AZ91D with AZ91D for example 3 with 12.5 wt% addition of a fine-grained Mg-Al-Ti-C master alloy.

Detailed Description

Example 1

Example 1 a fine-grained Mg-Al-Ti-C master alloy was prepared and 5.2 wt% of the fine-grained Mg-Al-Ti-C master alloy was used to modify AZ91D, the main constituents of which are shown in table 1.

TABLE 1 AZ91D magnesium alloy chemical composition

The first step is as follows: preparing prefabricated blocks

Mixing Al powder and TiC powder according to the mass ratio of 1:1, wherein the purity of the Al powder and the purity of the TiC powder are 99.999%, and carrying out ball milling treatment on the mixed powder by using a ball mill for 4 hours. And then pressing the powder subjected to ball milling into a precast block by using a cold pressing die under the pressure of 15MPa, wherein the size of the precast block is phi 20mm multiplied by 15 mm. The mol ratio of TiC powder is Ti: c is 1: 1;

And (3) mixing the precast block with a pure magnesium ingot according to the mass ratio of 1: 9, putting together, wherein the purity of the pure magnesium ingot is 99.95 percent, introducing high-purity argon for protection, wherein the purity of the high-purity argon is 99.999 percent, heating to 850 ℃ by adopting a ZG-0-01 type vacuum induction smelting furnace (the main technical parameters are shown in table 2), preserving the heat for 1 hour, and cooling along with the furnace to prepare the coarse-crystal Mg-Al-Ti-C intermediate alloy.

TABLE 2 main technical parameters of ZG-0-01 type vacuum induction melting furnace

The third step: preparation of fine-grained Mg-Al-Ti-C intermediate alloy

Cutting the coarse-grain master alloy into blocks with the size of 1.2mm by using a wire cutting machine, placing the blocks into a quartz tube with a small hole with the diameter of 1.5mm at the bottom end, and vacuumizing a copper mold spray casting furnace body to ensure that the vacuum degree is not lower than 1.2 × 10^-3Pa, introducing high-purity argon for protection, melting the coarse-grained master alloy by high-frequency induction heating with an induction coil, and introducing 8 × 10⁴pa of high purity argon gas, the molten metal being injected under pressure into the copper mold to obtain a fine-grained master alloy (chemical composition shown in table 3).

TABLE 3 Mg-Al-Ti-C Master alloy chemistry

The fourth step: magnesium alloy with modified quality

The fine-grained Mg-Al-Ti-C intermediate alloy modified AZ91D with the mass fraction of 5.2 wt% is smelted by adopting a SG2-5-12 type well-type crucible resistance furnace (the main technical parameters are shown in Table 4), and the method comprises the following specific steps:

(1) drying the crucible, the strainer (for removing slag), the air pipe tee joint, the metal air pipe, the cast ingot to be smelted and the fine-grain Mg-Al-Ti-C intermediate alloy in a drying box at 50 ℃ for 24 hours; preheating a pit-type resistance furnace in advance at the temperature of 150 ℃ for 24 hours to ensure that a hearth is dried when smelting is carried out on the next day; and (3) preserving the heat of the metal model by using a box-type resistance furnace before smelting at the temperature of 300 ℃, and taking out the metal model before smelting and casting.

(2) The protective atmosphere for smelting is 3% SF₆+97％CO₂The mixed gas of (1). Checking gas introduction device before smelting, placing the crucible containing ingot in the resistance furnace hearth after no error, covering the furnace cover, introducing protective gas, and stopping introducing gas after 5 min; electrifying, heating to 760 deg.C (introducing protective gas every 5min for 1min), removing slag, adding intermediate alloy, cooling to 720 deg.C, maintaining for 10min, removing slag, and discharging.

(3) Directing the protective gas flow to the molten metal, taking out the crucible by using crucible tongs, casting in a metal mold, cooling the casting, opening the mold, and taking out the casting rod with the size of phi 22 multiplied by 130 mm.

TABLE 4 Main technical parameters of SG2-5-12 type well-type crucible resistance furnace

It can be seen from FIG. 1 that the microstructure of the Mg-Al-Ti-C master alloy obtained by the two-step method has an obvious TiC structure, which indicates that the method can prepare large-block Mg-Al-Ti-C master alloy. However, the TiC structure has the problems of uneven distribution and agglomeration because TiC particles are relatively small and the surface free energy is relatively large, so that the TiC structure is easy to agglomerate. Comparing fig. 2, it can be found that the copper die injection casting method effectively solves the problem of serious agglomeration of the reinforcing particles, so that TiC is uniformly dispersed and distributed, which is crucial for the uniform distribution of final TiC in AZ 91D.

As can be seen from fig. 3, the TiC particles are uniformly distributed.

It can be seen from fig. 4 and 5 that the ratio of atomic numbers of Ti element and C element in the particles scattered in the AZ91D matrix is close to 1:1, and thus it is determined to be TiC, which is distributed more uniformly in the matrix. During the solidification of the molten metalTiC and Al elements are repelled to grain boundary, resulting in TiC and β -Mg₁₇Al₁₂Are distributed at the grain boundary and are close to each other. TiC distributed on grain boundaries inhibits the growth of grains to some extent, which is advantageous for increasing the strength of AZ 91D.

As can be seen from FIG. 6, the Mg element is restored to the original level after the Mg element is suddenly decreased from top to bottom along the scanning line, the Al element is suddenly decreased from top to bottom, the Ti element and the C element are suddenly increased from top to bottom, and the three elements O, Ti and C are suddenly increased from top to bottom and are basically overlapped in position, which proves that the scanning line is β -Mg in sequence from top to bottom₁₇Al₁₂Phase, TiC particles and magnesium matrix.

From FIGS. 7 and 8 it can be seen that the tensile strength of the AZ91D alloy was 157.1MPa, the tensile strength of the modified AZ91D with 5.2 wt% of the fine-grained Mg-Al-Ti-C master alloy added was 188.2MPa, the Vickers hardness of AZ91D was 65.9HV, and the Vickers hardness of the modified AZ91D with 5.2 wt% of the fine-grained Mg-Al-Ti-C master alloy added was 108.7 HV. For this reason, the increase in tensile strength and hardness of AZ91D depends on the decrease in the crystal grain size of the matrix and the dispersion strengthening effect of the second phase particles (TiC) added to the AZ91D matrix.

Example 2

Example 2 a fine-grained Mg-Al-Ti-C master alloy was prepared and AZ91D was metamorphosed with 2.7 wt% of the fine-grained Mg-Al-Ti-C master alloy.

The other steps were the same as in example 1. AZ91D was obtained close to example 1.

As can be seen in fig. 9, the TiC particles are uniformly distributed, similar to that observed in fig. 3 of example 1.

As can be seen from FIGS. 10 and 11, the modified AZ91D with the addition of 2.7 wt% of the fine-grained Mg-Al-Ti-C master alloy had a tensile strength of 169.5MPa and the modified AZ91D with the addition of 2.7 wt% of the fine-grained Mg-Al-Ti-C master alloy had a Vickers hardness of 81.2 HV. The reason for this tendency is the same as in fig. 7 and 8 of embodiment 1, and will not be described again here.

Example 3

Example 3 a fine-grained Mg-Al-Ti-C master alloy was prepared and AZ91D was metamorphosed with 12.5 wt% of the fine-grained Mg-Al-Ti-C master alloy.

As can be seen from FIG. 12, when the mass fraction of the fine-grained Mg-Al-Ti-C master alloy is increased to 12.5 wt%, a relatively significant agglomeration of TiC occurs₁₇Al₁₂Cannot be inhibited, so β -Mg₁₇Al₁₂Eventually growing into a connected network.

As can be seen from FIGS. 13 and 14, the modified AZ91D with the addition of 12.5 wt% of the fine-grained Mg-Al-Ti-C master alloy had a tensile strength of 183.9MPa, and the modified AZ91D with the addition of 12.5 wt% of the fine-grained Mg-Al-Ti-C master alloy had a Vickers hardness of 96.5HV, both of which were reduced in tensile strength and hardness as compared with FIGS. 7 and 8 in example 1. The reason is that as the volume fraction of TiC increases, the viscosity of the melt increases and the casting fluidity deteriorates, leading to increased casting defects in the cast bar and a decrease in the bond strength and hardness.

Example 4

Example 4 a fine-grained Mg-Al-Ti-C master alloy was prepared and the AZ91HP was metamorphosed with 5.2 wt% of a fine-grained Mg-Al-Ti-C master alloy, the main constituents of which are shown in table 5.

The other steps were the same as in example 1. AZ91HP was obtained close to example 1.

TABLE 5 AZ91HP magnesium alloy chemistry

The above examples illustrate the invention of preparing a fine-grained Mg-Al-Ti-C master alloy and modifying a magnesium alloy with a fine-grained Mg-Al-Ti-C master alloy. Wherein, the mixture ratio in the embodiment 1 is C1 wt%, Ti4 wt%, Al5 wt% and Mg90 wt%, when 5.7 wt% of modified AZ91D of fine-grain Mg-Al-Ti-C intermediate alloy is added, the tensile strength of the modified AZ91D is 188.2MPa, the Vickers hardness of the modified AZ91D is 108.7HV, and the effect is best.

The invention is not the best known technology.

Claims

1. A method of making a fine-grained Mg-Al-Ti-C master alloy, characterized in that the method comprises the steps of:

the first step is as follows: preparing prefabricated blocks

Putting the precast block and a pure magnesium ingot together, introducing high-purity argon for protection, heating to 820-860 ℃ by using a vacuum induction smelting furnace, preserving heat for 1-1.5 h, and cooling along with the furnace to obtain a coarse-grain Mg-Al-Ti-C intermediate alloy;

the third step: preparation of fine-grained Mg-Al-Ti-C intermediate alloy

wherein the heating temperature is 750-760 ℃, and the air pressure for blowing the molten metal is 7.5 × 10⁴～8.5×10⁴Pa ；

the size of the coarse-grain master alloy block in the third step is 1-1.2 mm;

the purity of the high-purity argon in the second step is 99.999%.

2. Use of the fine-grained Mg-Al-Ti-C master alloy produced by the production method according to claim 1 for producing a modified magnesium alloy.

3. Use of a fine-grained Mg-Al-Ti-C master alloy prepared by the method of claim 2, characterized by comprising the steps of: putting a magnesium alloy ingot into a resistance furnace under a protective atmosphere, heating to 740-780 ℃, removing slag after the ingot is melted, adding fine-grain Mg-Al-Ti-C intermediate alloy, cooling to 700-730 ℃, keeping the temperature for 8-12 min under stirring, removing slag, and casting molten metal to obtain modified magnesium alloy;

4. Use of a fine-grained Mg-Al-Ti-C master alloy prepared by the preparation method according to claim 3, characterized in that the magnesium alloy is AZ91D or AZ91 HP.

5. Use of a fine-grained Mg-Al-Ti-C master alloy prepared according to the preparation method of claim 3, characterized in that the protective atmosphere is 3% SF by volume₆And 97% CO₂The mixed gas of (1).