CN111155012B

CN111155012B - High-fluidity high-thermal conductivity rare earth magnesium alloy suitable for die-casting ultrathin parts and preparation method thereof

Info

Publication number: CN111155012B
Application number: CN202010187474.7A
Authority: CN
Inventors: 李远发; 李法兵; 陈善荣; 宋卓能; 李国强; 樊晓泽; 刘志翔; 赖福宏; 朱杰
Original assignee: Ka Fung Industrial Technology Huizhou Co ltd
Current assignee: Ka Fung Industrial Technology Huizhou Co ltd
Priority date: 2020-03-17
Filing date: 2020-03-17
Publication date: 2022-02-18
Anticipated expiration: 2040-03-17
Also published as: CN111155012A

Abstract

The invention discloses a high-fluidity high-heat-conductivity rare earth magnesium alloy suitable for die-casting ultrathin parts and a preparation method thereof. The high-fluidity high-heat-conductivity rare earth magnesium alloy suitable for the die-casting ultrathin part comprises the following components in percentage by mass: 1.0-5.0% of Al, 1.0-5.0% of RE, 0.1-1.0% of Si, 0.1-1.0% of Ca, 0.01-0.5% of Mn and the balance of Mg. The magnesium alloy has high thermal conductivity and high die-casting flowability on the basis of ensuring good mechanical property through the compatibility design of doping elements such as Al, RE, Si, Ca, Mn and the like, and is suitable for high-yield die-casting production of ultrathin parts such as 3C and the like.

Description

High-fluidity high-thermal conductivity rare earth magnesium alloy suitable for die-casting ultrathin parts and preparation method thereof

Technical Field

The invention relates to the technical field of magnesium alloy, in particular to a high-fluidity high-heat-conductivity rare earth magnesium alloy suitable for die-casting ultrathin parts and a preparation method thereof.

Background

The rapid development of modern industrial technology, especially in recent years, 5G communication, 3C products, automobile electronics and other parts, puts increasing demands on the heat conductivity of materials to ensure and improve the service life and working stability of products. The heat conductivity coefficient of pure magnesium is 158W/m.k, which is second to that of pure copper and pure aluminum metal materials, but the strength of pure magnesium is low, and the die-cast magnesium alloy with high heat conductivity is obtained by alloying, so that the magnesium alloy has special development advantages in the fields of 5G communication, 3C and automobile products and the like which need high heat dissipation performance.

However, poor fluidity of magnesium alloy causes a series of problems of low yield, poor surface quality and the like when forming thin-walled parts. Although the existing magnesium alloy is easy to be die-cast, for a thin-wall and ultrathin shell (the thickness is less than 0.6mm), the kinetic viscosity of a magnesium alloy melt is obviously increased along with the reduction of the temperature in the die-casting process, and the defect of poor fluidity is further shown. At present, AZ91D magnesium alloy with better fluidity is generally used for die-casting and molding 3C products such as notebook computer shells, but the heat conductivity coefficient of the die-cast AZ91D magnesium alloy is only 53W/m.k. With the technical progress, the functions of the notebook computer are more and more powerful, and because the internal space of the notebook computer is narrow, a large number of heat dissipation components (such as a CPU, a hard disk and a mainboard) are arranged in a very limited space. If the heat cannot be discharged in time, the running speed of the machine is reduced, the machine is halted, and some internal electronic components are damaged, so that higher requirements are brought to heat dissipation of electronic devices such as a CPU (central processing unit) and the like, and particularly, the heat dissipation function of the shell is challenged.

Therefore, the development of the novel die-casting magnesium alloy with high fluidity and high heat conductivity has extremely important significance for improving the yield and the heat dissipation capacity of ultrathin magnesium products such as 3C products and the like.

Disclosure of Invention

The invention aims to provide a high-fluidity high-heat-conductivity rare earth magnesium alloy suitable for die-casting ultrathin parts, aiming at the defects or shortcomings in the prior art. The magnesium alloy has high heat conductivity, high fluidity and die-casting performance, and is suitable for die-casting production of ultrathin parts including 3C and the like.

The invention also aims to provide a method for preparing the high-fluidity high-heat-conductivity rare earth magnesium alloy suitable for die-casting ultrathin parts.

The purpose of the invention is realized by the following technical scheme.

The high-fluidity high-thermal conductivity rare earth magnesium alloy suitable for die-casting ultrathin parts comprises the following components in percentage by mass:

1.0-5.0% of Al, 1.0-5.0% of RE, 0.1-1.0% of Si, 0.1-1.0% of Ca, 0.01-0.5% of Mn and the balance of Mg.

In a preferred embodiment, the RE is one or more of La and Ce.

In a preferred embodiment, RE is La.

In a preferred embodiment, the mass ratio of RE to Al is 1.0-3.0.

In a preferred embodiment, the mass ratio of Si to RE is 0.10-0.20.

In a preferred embodiment, the mass ratio of Ca to RE is 0.06-0.20.

In a preferred embodiment, the mass ratio of Mn to RE is 0.003-0.10.

The method for preparing the high-fluidity high-thermal conductivity rare earth magnesium alloy suitable for the die-casting ultrathin part comprises the following steps:

(1) preparing raw materials: quantitatively preparing Mg ingots, Al ingots, Mg-RE alloys, Mg-Si alloys, Mg-Ca and Mg-Mn alloy raw materials according to the mass percentage;

(2) melting: introducing protective gas into the smelting furnace, heating to melt the Mg ingot, heating to 700-730 ℃, adding the Mg-RE alloy, the Mg-Si alloy and the Mg-Mn alloy in batches, and fully and uniformly stirring after melting; then adding the Al ingot, adding the Mg-Ca alloy after melting, and fully and uniformly stirring after melting to obtain an alloy melt;

(3) refining: standing for 8-12 min, degassing, adding a flux, and refining at 700-710 ℃ for 30-35 min;

(4) pouring: and standing for 1-2 hours after refining, slagging off, and casting to obtain the high-fluidity high-heat-conductivity rare earth magnesium alloy suitable for die-casting ultrathin parts.

In a preferred embodiment, the addition amount of the fusing agent is 0.5-1.0% by mass of the alloy melt.

Compared with the prior art, the invention has the following advantages and beneficial effects:

the magnesium alloy has high heat conductivity (the heat conductivity coefficient is not less than 112W/m.K at room temperature) and simultaneously has die-casting performance and high die-casting fluidity (the pouring fluidity exceeds 900mm), and is suitable for high-yield die-casting production of ultrathin parts including 3C and the like.

Drawings

FIG. 1 is a microstructure electron microscope scan of a magnesium alloy of example 1;

FIG. 2 is a microstructure electron microscope scan of the magnesium alloy of comparative example 1;

FIG. 3 is a microstructure electron microscope scan of the magnesium alloy of comparative example 2;

FIG. 4 is a microstructure electron microscope scan of the magnesium alloy of comparative example 3;

FIG. 5 is a microstructure electron microscope scan of the magnesium alloy of comparative example 4;

FIG. 6 is a microscopic structure electron microscopic scan of the magnesium alloy of comparative example 5.

Detailed Description

The technical solutions of the present invention are further described in detail with reference to specific examples, which are provided for the purpose of making the disclosure of the present invention more thorough and complete, and the scope of protection and the implementation of the present invention are not limited thereto. Also, unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used in the description of the invention herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. In the specific examples, unless otherwise specified, the technical means used are in accordance with the conventional means employed by those skilled in the art of the present invention.

The invention relates to a high-fluidity high-heat-conductivity rare earth magnesium alloy suitable for die-casting ultrathin parts, which comprises the following components in percentage by mass:

In a preferred embodiment, the RE is one or more of La and Ce.

In a preferred embodiment, the mass ratio of RE to Al is 1.0-3.0.

In a preferred embodiment, the mass ratio of Si to RE is 0.10-0.20.

In a preferred embodiment, the mass ratio of Ca to RE is 0.06-0.20.

In a preferred embodiment, the mass ratio of Mn to RE is 0.003-0.10.

According to the functions of various elements in the die-casting magnesium alloy, particularly the synergistic effect of different elements, the addition types and the addition amounts of the various elements are designed based on the physical and chemical principles of the material, and the high-fluidity high-heat-conductivity rare earth magnesium alloy suitable for die-casting ultrathin parts is obtained. Based on the knowledge of the heat conductivity of the alloy and the action mechanism of solid solution and precipitated phases in the alloy, La and Ce rare earth elements with low solid solubility in the magnesium alloy are selected to ensure the high heat conductivity coefficient characteristic of the magnesium alloy, and meanwhile, a large amount of stable second-phase particles (intermetallic compounds) are formed/precipitated in the solidification process of the alloy by utilizing the special strengthening, purifying and activating effects of the rare earth elements to achieve the effect of dispersion strengthening. Meanwhile, Al element is added, so that the melting point and solidus temperature of the alloy are reduced, latent heat is increased, and fluidity is improved. In addition, addition of Si element to magnesium alloy produces intermetallic compound Mg with high stability₂Si can obviously improve the microstructure of the magnesium alloy and improve the fluidity while strengthening; on the other hand, the fluidity of the magnesium alloy melt is close to the oxidation resistance thereofThe stronger the oxidation resistance, the better the fluidity of the melt, and the addition of Ca can form a compact oxidation film consisting of a CaO layer and a CaO/MgO mixed layer on the surface of the alloy, thereby hindering the further oxidation of the alloy, improving the oxidation resistance of the magnesium alloy and further improving the fluidity of the alloy.

The magnesium alloy has the advantages that through the compatibility design of doping elements such as Al, RE, Si, Ca and Mn, the magnesium alloy has high heat conductivity (the heat conductivity coefficient is high, the heat conductivity coefficient at room temperature is more than or equal to 112W/m.K) and high die casting fluidity (the pouring fluidity exceeds 900mm) on the basis of ensuring good mechanical properties (the tensile strength is more than 240MPa, the yield strength is more than 155MPa and the elongation is less than 10.5 percent), and is suitable for high-yield die casting production of ultrathin parts such as 3C. Wherein, the yield of the ultrathin notebook computer shell with the thickness of less than 0.6mm formed by die casting exceeds 80 percent.

In the specific embodiment, the raw materials used are as follows:

mg ingot: the purity is 99.5%. Al ingot: the purity is 99.5%.

The alloy types specifically adopted are as follows:

Mg-La alloy: mg-20La alloy. Mg-Si alloy: mg-20Si alloy. Mg-Ca alloy: mg-20Ca alloy. Mg-Mn alloy: mg-10Mn alloy. Mg-Ce alloy: mg-20Ce alloy.

The following is a detailed description with reference to specific examples.

Example 1

The high-fluidity high-thermal conductivity rare earth magnesium alloy suitable for the die-casting ultrathin part comprises the following components in percentage by mass:

5.0% of Al, 5.0% of La, 1.0% of Si, 1.0% of Ca, 0.5% of Mn and the balance of Mg.

The preparation method of the high-fluidity high-thermal conductivity rare earth magnesium alloy suitable for die-casting ultrathin parts comprises the following steps:

(1) preparing raw materials: quantitatively preparing Mg ingots, Al ingots, Mg-La alloys, Mg-Si alloys, Mg-Ca alloys and Mg-Mn alloy raw materials according to the mass percentage;

(2) melting: after the crucible is preheatedCharging Mg ingot, and introducing SF₆+CO₂(volume ratio is 1: 400) protective gas;

heating to melt the Mg ingot, heating to 710 ℃, adding the Mg-La alloy, the Mg-Si alloy and the Mg-Mn alloy in batches, and fully stirring for 5min after melting; then adding the Al ingot, adding the Mg-Ca alloy after melting, fully and uniformly stirring after melting, and slagging off to obtain an alloy melt;

(3) refining: standing for 8min, stirring and degassing by a graphite rotor degassing machine, synchronously scattering a flux (accounting for 0.2% of the mass of the melt) for covering and refining, and refining at 710 ℃ for 35 min; wherein, the refining gas adopts Ar gas, the rotating speed is 30r/min, and the gas flow is 10L/min;

(4) pouring: and standing for 1h after refining, slagging off, and casting into ingots to obtain the die-casting high-thermal-conductivity flame-retardant magnesium alloy.

And (2) putting the prepared magnesium alloy ingot into a machine side furnace for melting, heating to 700 ℃, fishing out surface scum, and carrying out die-casting molding under the injection specific pressure of 35MPa and the filling speed of 3m/s, wherein the temperature of a die-casting mold is 300 ℃.

Example 2

4.0% of Al, 2.5% of La, 1.7% of Ce, 0.8% of Si, 0.7% of Ca, 0.2% of Mn and the balance of Mg.

(1) preparing raw materials: quantitatively preparing Mg ingots, Al ingots, Mg-La alloys, Mg-Ce alloys, Mg-Si alloys, Mg-Ca alloys and Mg-Mn alloy raw materials according to the mass percentage;

(2) melting: preheating a crucible, filling Mg ingot, and introducing SF₆+CO₂(volume ratio is 1: 400) protective gas;

heating to melt the Mg ingot, heating to 700 ℃, adding the Mg-La alloy, the Mg-Ce alloy, the Mg-Si alloy and the Mg-Mn alloy in batches, and fully stirring for 5min after melting; then adding the Al ingot, adding the Mg-Ca alloy after melting, fully and uniformly stirring after melting, and slagging off to obtain an alloy melt;

(3) refining: standing for 12min, stirring and degassing by a graphite rotor degassing machine, synchronously scattering a flux (accounting for 0.2% of the mass of the melt) for covering and refining, and refining at 700 ℃ for 30 min; wherein, the refining gas adopts Ar gas, the rotating speed is 30r/min, and the gas flow is 10L/min;

And putting the prepared magnesium alloy ingot into a machine side furnace for melting, heating to 690 ℃, fishing out surface scum, and carrying out die-casting molding under the filling pressure of 35MPa and the filling speed of 3m/s, wherein the temperature of a die-casting mold is 300 ℃.

Example 3

3.0% of Al, 3.8% of La, 0.6% of Si, 0.5% of Ca, 0.05% of Mn and the balance of Mg.

heating to melt the Mg ingot, heating to 730 ℃, adding the Mg-La alloy, the Mg-Si alloy and the Mg-Mn alloy in batches, and fully stirring for 5min after melting; then adding the Al ingot, adding the Mg-Ca alloy after melting, fully and uniformly stirring after melting, and slagging off to obtain an alloy melt;

(3) refining: standing for 10min, stirring and degassing by a graphite rotor degassing machine, synchronously scattering a flux (accounting for 0.2% of the mass of the melt) for covering and refining, and refining at 700 ℃ for 35 min; wherein, the refining gas adopts Ar gas, the rotating speed is 30r/min, and the gas flow is 10L/min;

Example 4

2.0% of Al, 3.0% of Ce, 0.3% of Si, 0.2% of Ca, 0.01% of Mn and the balance of Mg.

(1) preparing raw materials: quantitatively preparing Mg ingots, Al ingots, Mg-Ce alloys, Mg-Si alloys, Mg-Ca alloys and Mg-Mn alloy raw materials according to the mass percentage;

heating to melt the Mg ingot, heating to 700 ℃, adding the Mg-Ce alloy, the Mg-Si alloy and the Mg-Mn alloy in batches, and fully stirring for 5min after melting; then adding the Al ingot, adding the Mg-Ca alloy after melting, fully and uniformly stirring after melting, and slagging off to obtain an alloy melt;

(3) refining: standing for 10min, stirring and degassing by a graphite rotor degassing machine, synchronously scattering a flux (accounting for 0.2% of the mass of the melt) for covering and refining, and refining at 710 ℃ for 30 min; wherein, the refining gas adopts Ar gas, the rotating speed is 30r/min, and the gas flow is 10L/min;

(4) pouring: and standing for 2 hours after refining is finished, slagging off, and casting into ingots to obtain the die-casting high-heat-conductivity flame-retardant magnesium alloy.

Comparative example 1

5.2% of Al, 0.5% of La, 1.2% of Si, 1.1% of Ca, 0.6% of Mn and the balance of Mg.

The magnesium alloy of this comparative example was prepared according to the preparation method of example 1 and die-cast.

Comparative example 2

5.2% of Al, 5.2% of La, 0.01% of Si, 0.08% of Ca0, 0.6% of Mn and the balance of Mg.

Comparative example 3

5.2% of Al, 5.8% of La, 0.08% of Si, 1.3% of Ca, 0.6% of Mn and the balance of Mg.

Comparative example 4

5.2% of Al, 6.5% of La, 1.5% of Si, 1.5% of Ca, 0.6% of Mn and the balance of Mg.

Comparative example 5

The magnesium alloy of the present embodiment is AZ91D magnesium alloy.

Performance testing

1. Microstructure observation

The microstructure of the magnesium alloys of examples 1 to 4 and comparative examples 1 to 5 was observed by an electron microscope, and the observation results are shown in fig. 1 to 6.

Wherein, fig. 1 is a microstructure electron microscope scanning image of the magnesium alloy of the embodiment 1, and the microstructure observation results of the embodiments 2 to 4 are shown in fig. 1. As shown in fig. 1, the magnesium alloys of examples 1 to 4 have a large amount of network-like precipitated phases in the as-cast structure, and a large amount of granular particle phases are precipitated in the intergranular region and uniformly distributed, and the microstructure not only facilitates the improvement of the mechanical properties of the alloys, the reduction of electromigration resistance, but also the improvement of the heat conductivity of the alloys, and also greatly improves the molding fluidity of the alloys.

FIG. 2 is a microstructure electron microscope scanning image of the magnesium alloy of comparative example 1, and it is shown in FIG. 2 that the alloy short rod-like second phase of the magnesium alloy of this comparative example grows large and a local aggregation state occurs. The microstructure easily causes local stress concentration, so that the mechanical property of the alloy is reduced; meanwhile, the microstructure characteristic also increases the electron migration resistance, so that the heat-conducting property of the alloy is reduced.

Fig. 3 is a microstructure electron microscope scanning image of the magnesium alloy of comparative example 2, and it is shown from fig. 3 that the magnesium alloy of comparative example 2 has a microstructure characteristic that a reinforcing phase (short rod-like second phase) with a large size is concentrated at grain boundaries, similar to that of comparative example 1, and further the mechanical properties and the thermal conductivity of the alloy are reduced.

FIG. 4 is a microstructure electron microscope scan of the magnesium alloy of comparative example 3, and it can be seen from FIG. 4 that the distribution morphology of the strengthening phase (i.e., the short rod-like second phase) is similar but the ratio is increased in the magnesium alloy of comparative example 3 compared to comparative examples 1 and 2, and thus the strength of the alloy is higher than that of comparative examples 1 and 2. However, the increase in the proportion of the short rod-like second phase increases the resistance to free electron migration, thereby further degrading the thermal conductivity of the alloy.

FIG. 5 is a microstructure electron microscope scan of the magnesium alloy of comparative example 4, and it is shown from FIG. 5 that comparative example 4 has the highest alloying degree and the highest proportion of short rod-like second phases, contributing more to the strength of the alloy, while the same alloy has a greater decrease in the thermal conductivity.

FIG. 6 is an electron microscope scanning image of the microstructure of AZ91D magnesium alloy of comparative example 5, and it is shown from FIG. 6 that the microstructure of the magnesium alloy has an α -Mg matrix phase of a large amount of solid-soluted Al and coarse Mg precipitated along grain boundaries as dissociable eutectic crystals₁₇Al₁₂The appearance of the phase obviously reduces the heat conductivity coefficient and the mechanical property of the magnesium alloy.

2. Mechanical Property test

Mechanical property tests (tensile property test is shown in GB/T228.1-2010) are carried out on the magnesium alloys of examples 1-4 and comparative examples 1-5, and the test results are shown in Table 1.

TABLE 1 results of mechanical Properties test of magnesium alloys of examples 1 to 4 and comparative examples 1 to 5

The test results in table 1 show that the magnesium alloys of examples 1 to 4 have overall comprehensive mechanical properties superior to those of the magnesium alloys of comparative examples 1 to 5, are significantly superior to the commercial AZ91D alloy listed in comparative example 5, and have excellent plasticity, which indicates that the magnesium alloy of the present invention has excellent comprehensive mechanical properties and good practicability.

3. Test of Heat conductivity

The magnesium alloys of examples 1 to 4 and comparative examples 1 to 5 were subjected to a thermal conductivity test (reference to GB/T22588-.

TABLE 2 Heat conductivity coefficient test results of the magnesium alloys of examples 1 to 4 and comparative examples 1 to 5

The test results in table 2 show that the magnesium alloys of examples 1 to 4 have higher thermal conductivity than those of comparative examples 1 to 5, and have great advantages over the conventional commercial AZ91D listed in comparative example 5, indicating that the magnesium alloy of the present invention has good thermal conductivity and good practicability as a heat dissipation material.

4. Die casting performance test

The flow length of the die casting was tested and the test results are shown in table 3.

TABLE 3 die casting fluidity test results of the magnesium alloys of examples 1 to 4 and comparative examples 1 to 5

From the test results in table 3, it can be seen that the magnesium alloys of examples 1 to 4 have good die casting fluidity, and the die casting flow length is over 935mm, which is significantly better than the magnesium alloys of comparative examples 1 to 4 and the commercial AZ91D alloy of comparative example 5.

5. Die-casting die-bonding observation

The magnesium alloys of examples 1 to 4 and comparative examples 1 to 5 were observed for sticking when the samples were taken out after completion of die casting and opening the dies, and the results of the observation are shown in table 4.

TABLE 4 die-casting die-bonding observation results of magnesium alloys of examples 1 to 4 and comparative examples 1 to 5

From the observation results in table 4, it is clear that the magnesium alloys of examples 1 to 4 are excellent in die-casting releasability and free from die sticking and galling. The magnesium alloy of the invention has good die-casting fluidity under the condition of controlling the compatibility of lower rare earth element content, and has good demoulding performance, thereby being beneficial to the implementation of industrial production and having good practicability.

In conclusion, the magnesium alloy has high thermal conductivity and die-casting performance on the basis of ensuring good mechanical property through the compatibility design of doping elements such as Al, RE, Si, Ca, Mn and the like, has high die-casting fluidity and good die-casting demolding performance, is suitable for high-yield die-casting production of ultrathin parts such as 3C and the like, has very large competitiveness in communication electronics and heat dissipation equipment, and has great market value as a high-thermal-conductivity magnesium alloy material with excellent die-casting performance.

It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, as in the examples La, Ce, Si, Ca, Mn, etc., may be incorporated in the form of Mg-La alloys, Mg-Ce alloys, Mg-Si alloys, Mg-Ca alloys, and Mg-Mn alloys, and in other alternative embodiments, may be incorporated in other possible forms of alloys or materials, and that the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Furthermore, it should be understood that although the present description refers to embodiments, not every embodiment may include only a single embodiment, and such description is for clarity only, and those skilled in the art should make the description as a whole, and the embodiments may be appropriately combined, changed, replaced, or modified to form other embodiments understood by those skilled in the art.

Claims

1. The high-fluidity high-heat-conductivity rare earth magnesium alloy suitable for die-casting ultrathin parts is characterized by comprising the following components in percentage by mass: 5.0% of Al, 5.0% of La5.0%, 1.0% of Si, 1.0% of Ca, 0.5% of Mn and the balance of Mg.

2. The high-fluidity high-heat-conductivity rare earth magnesium alloy suitable for die-casting ultrathin parts is characterized by comprising the following components in percentage by mass: 4.0% of Al, 2.5% of La, 1.7% of Ce, 0.8% of Si, 0.7% of Ca, 0.2% of Mn and the balance of Mg.

3. The high-fluidity high-heat-conductivity rare earth magnesium alloy suitable for die-casting ultrathin parts is characterized by comprising the following components in percentage by mass: 3.0% of Al, 3.8% of La, 0.6% of Si, 0.5% of Ca, 0.05% of Mn and the balance of Mg.

4. The high-fluidity high-heat-conductivity rare earth magnesium alloy suitable for die-casting ultrathin parts is characterized by comprising the following components in percentage by mass: 2.0% of Al, 3.0% of Ce, 0.3% of Si, 0.2% of Ca, 0.01% of Mn and the balance of Mg.

5. The method for preparing the high-fluidity high-thermal conductivity rare earth magnesium alloy suitable for die-casting ultrathin parts as claimed in any one of claims 1 to 4 is characterized by comprising the following steps:

6. The production method according to claim 5, wherein the amount of the flux added is 0.5 to 1.0% by mass of the alloy melt.