JP6867657B2 - Manufacturing method of magnesium alloy powder - Google Patents

Description

The present disclosure relates to a method for producing a magnesium alloy powder.

Magnesium (Mg) is an abundant resource on the earth, and is excellent in recyclability like aluminum (Al). Therefore, the development of materials using Mg is being promoted in various fields.
For example, recently, in the medical field, the biosafety of WE43 (Mg-4Y-3RE) alloy has been confirmed by Western organizations, and it has been developed into biological structural materials such as bioabsorbable stent materials and biohard tissue implant materials. Is expected.

In addition, in the field of transportation equipment such as automobiles, railroad vehicles, and aircraft, it is essential to reduce the weight of vehicles in order to _{save energy by improving fuel efficiency and environmental measures by reducing CO 2 gas emissions.} Therefore, there is an increasing demand for conversion from conventional steel-based materials to lightweight metal materials such as Mg alloys and Al alloys. In response to such demands, Mg alloys are attracting attention as materials suitable for weight reduction of transportation equipment because they have the lowest specific gravity and high specific strength among typical metals used as structural materials. .. Further, the Mg alloy has high vibration damping property and electromagnetic wave shielding property. Therefore, it is considered that it can be an effective material from the viewpoint of vibration countermeasures peculiar to transportation equipment or noise countermeasures associated with electronic control of drive systems and the like.

Structural materials containing Mg alloys are roughly classified into molten materials and powder metallurgy materials according to the molding method of the structure (molded body). The molten material is used for a method of casting a molten Mg alloy into a mold and casting it into a shape close to the final product, or a method of hot or cold plastic working of a cast material of an Mg alloy having a simple shape such as a billet. Be done. On the other hand, the powder metallurgy material is used in a method in which Mg alloy powder is filled in a mold or the like, pressure-molded, and then sintered.

In general, a molded product obtained by sintering a metal powder (hereinafter referred to as a sintered part) has a finer and more uniform structure than a molded product obtained by casting (hereinafter referred to as a cast product). Therefore, it is easy to obtain strong toughness. Further, in the sintered part, the ceramic particles or the intermetallic compound particles are mixed with the metal powder as the base material, so that the ceramic particles or the intermetallic compound particles are uniformly finely dispersed in the base material. Is possible. In addition, for example, it is possible to design a structure having dispersion shading, such as making the dispersion concentration of each of the above particles near the surface layer higher than that inside. In this way, in the sintered part, in the tissue design, not only the strength but also various functions such as improvement of wear resistance and impartation of affinity with bone cells can be easily added depending on the application. It has the advantage of being able to.

As a typical method for producing a metal powder used as a powder metallurgy material, an atomizing method or a plasma rotating electrode method is known. In these methods, a metal powder is obtained by refining and quenching the molten metal. However, the molten metal of Mg alloy is active and easily burns in the atmosphere. Therefore, the powdering of the Mg alloy by the above method is usually carried out in an atmosphere of an expensive inert gas such as argon or helium.
The atomizing method is a method of obtaining a metal powder by refining and quenching a molten metal using gas or water, and is excellent in mass productivity. However, when the Mg alloy is pulverized by the gas atomization method, it is necessary to use a large amount of expensive inert gas, which increases the operating cost and makes the Mg alloy powder very expensive. Therefore, there is a demand for a method for producing Mg alloy powder, which can carry out powdering of Mg alloy at low cost.

On the other hand, a method for producing Mg alloy powder by a water atomizing method has been proposed in which a molten metal in which calcium (Ca) is added to an Mg alloy is sprayed and powdered using a high-speed rotating water stream (Patent Document 1). ~ 3).

Japanese Unexamined Patent Publication No. 2014-162991 Japanese Unexamined Patent Publication No. 2014-167136 JP-A-2017-61753

In the production of sintered parts, shear stress is applied to the contact interface between powders by pressurization in the molding step of metal powder or in the sintering step after molding, or hot working after sintering. Therefore, even if a thin oxide film is present on the powder surface, the oxide film is broken in a normal treatment step, and a strong metal bond can be obtained. However, when the oxide film has a thickness on the order of several microns, it becomes difficult to sufficiently break the oxide film in a normal treatment step, and the sinterability tends to decrease.
According to the above-mentioned method for producing an Mg alloy powder by the water atomizing method, water is used instead of an expensive inert gas, so that the production cost can be reduced. However, in the above-mentioned production method, since the Mg alloy is powdered using water and the powder is recovered in water, a thick oxide film on the order of several microns is formed on the surface of the powder, and excellent sinterability is obtained. It is considered difficult to obtain.
Therefore, in view of the above situation, the present disclosure provides a method for producing an Mg alloy powder at low cost and capable of improving sinterability.

In order to reduce the production cost of the metal powder by the gas atomization method, it is conceivable to apply the air atomization method using air instead of the expensive inert gas when spraying the molten metal. However, since the molten metal of Mg alloy is active, it is usually difficult to apply the air atomization method.
On the other hand, the present inventors have conducted various studies on the powdering of Mg alloys, and by using a melting raw material of Mg alloys containing a specific element, the flame retardancy of the molten metal can be enhanced, and air atomization can be performed. We found that the law could be applied. Further, they have found that when the air atomizing method is applied to the production of Mg alloy powder, the formation of an oxide film in the powdering step can be controlled, and the present invention has been completed.

That is, the present invention relates to the embodiments described below. However, the present invention is not limited to the embodiments described below, and includes various embodiments.
In one embodiment, a melting raw material of an Mg alloy containing Mg as a main component and at least one selected from the group consisting of Y, Al, and Ca as a first sub-component is melted to melt a molten Mg alloy. The present invention relates to a method for producing an Mg alloy powder, which comprises a step of obtaining the above-mentioned molten metal of the Mg alloy using air and a step of obtaining the Mg alloy powder.

In the above embodiment, the content of the first sub-component is preferably 0.5 to 20% by mass based on the total mass of the dissolved raw material of the Mg alloy.

The dissolution raw material of the Mg alloy preferably further contains a second subcomponent containing one or more selected from the group consisting of Zn, Mn, Si, Ni, Cu, Zr, and Sn.

The content of the second sub-component is preferably 0.01 to 12% by mass based on the total mass of the dissolution raw material of the Mg alloy.

The melting raw material of the Mg alloy may be an Mg—Zn—Y alloy material, an Mg—Al—Ca alloy material, an Mg—Al—Zn—Ca alloy material, or an Mg—Al—Zn alloy material. preferable.

According to the present invention, it is possible to provide a method for producing an Mg alloy powder at low cost and capable of improving sinterability.

FIG. 1 is a schematic cross-sectional view showing a structural example of an air atomizing device. FIG. 2 is an SEM photograph showing the shape of the Mg alloy powder produced in Example 1. FIG. 3 is an SEM photograph showing the shape of the Mg alloy powder produced in Example 2. FIG. 4 is an SEM photograph showing the shape of the Mg alloy powder produced in Example 3. FIG. 5 is a STEM photograph showing a state (cross section) of the Mg alloy powder produced in Example 1 near the surface. FIG. 6A shows an image of element mapping (O) by EDX analysis (STEM / EDX) of the region corresponding to the STEM photograph shown in FIG. FIG. 6B shows an image of element mapping (Y) by EDX analysis (STEM / EDX) of the region corresponding to the STEM photograph shown in FIG. FIG. 7 is a STEM photograph showing a state (cross section) of the Mg alloy powder produced in Example 2 near the surface. FIG. 8A shows an image of element mapping (O) by EDX analysis (STEM / EDX) of the region corresponding to the STEM photograph shown in FIG. FIG. 8B shows an image of element mapping (Ca) by EDX analysis (STEM / EDX) of the region corresponding to the STEM photograph shown in FIG. FIG. 8C shows an image of element mapping (Al) by EDX analysis (STEM / EDX) of the region corresponding to the STEM photograph shown in FIG. FIG. 9 is a STEM photograph showing a state (cross section) of the Mg alloy powder produced in Example 3 near the surface. FIG. 10A is an image showing element mapping (O) by EDX analysis (STEM / EDX) of the region corresponding to the STEM photograph shown in FIG. FIG. 10B is an image showing element mapping (Al) by EDX analysis (STEM / EDX) of the region corresponding to the STEM photograph shown in FIG. FIG. 11 is a graph showing the result of line analysis of Al distribution in the Mg alloy powder produced in Example 2. FIG. 12 is a graph showing the result of line analysis of Al distribution in the Mg alloy powder produced in Example 3.

The present invention will be specifically described with reference to embodiments. However, the present invention is not limited to the embodiments described below.
For example, when an Mg alloy powder is produced by a gas atomization method using argon gas, it is generally said that the cost of argon gas accounts for 90% of the operating cost of atomization (powdering).
On the other hand, the method for producing the Mg alloy powder according to the embodiment of the present disclosure is selected from the group consisting of (a) Mg as a main component, yttrium (Y), calcium (Ca), and aluminum (Al). A step of dissolving a melting raw material of an Mg alloy containing at least one of these as a first subcomponent to obtain a molten metal of the Mg alloy, and (b) spraying the molten metal of the Mg alloy using air. It has a step of obtaining Mg alloy powder. In the manufacturing method of the above embodiment, since the molten metal is sprayed using air instead of expensive argon gas, the operating cost can be significantly reduced. In one embodiment, the Mg alloy powder preferably contains more than 68% by mass of Mg as a main component. The Mg alloy powder more preferably contains more than 78% by mass of Mg, and even more preferably contains more than 85% by mass of Mg.

In powdering by the air atomization method, in the case of a molten metal of pure Mg, it is considered that an MgO film is formed on the surface of the droplet. However, since MgO is porous, the surface coating of MgO cannot shield the liquid phase Mg inside the droplet from the air, and the Mg droplet burns violently. Therefore, it is difficult to apply the air atomization method as a method for pulverizing pure Mg or Mg alloy.
In order to suppress the combustion of Mg droplets, it is necessary to form a dense coating such as an oxide coating that can sufficiently shield the atmosphere from the liquid phase inside the droplets, and to improve the flame retardancy in powdering. On the other hand, in the production method of the above-described embodiment, in the above-mentioned step (a), Mg is the main component and at least one selected from the group consisting of Y, Ca, and Al is contained as the first sub-component. By using the melting raw material of the Mg alloy, the Mg alloy can be pulverized by the air atomizing method.

In one embodiment, the dissolution raw material of the Mg alloy preferably contains 0.5 to 20% by mass of the first subcomponent based on the total mass, and contains Mg and an unavoidable component as the balance thereof. In one embodiment, the content of the first subcomponent is more preferably 1 to 20% by mass, further preferably 2 to 15% by mass, and even more preferably 3.5 to 12% by mass.

In another embodiment, the melting raw material of the Mg alloy may further contain a second sub-component in addition to Mg and the first sub-component from the viewpoint of improving the functionality of the Mg alloy. The second subcomponent includes one or more selected from the group consisting of Zn, Mn, Si, Ni, Cu, Zr, and Sn.
From the viewpoint of exhibiting the effect of the second subcomponent, the content thereof is preferably 0.01% by mass or more, more preferably 0.05% by mass or more, still more preferably 0.1% by mass or more. On the other hand, the content of the second sub-component is preferably 12% by mass or less, more preferably 11% by mass or less, still more preferably 10% by mass or less, based on the total mass of the dissolution raw material of the Mg alloy. ..
By adjusting the content of the second subcomponent in the range of 0.01 to 12% by mass, it is easy to obtain the desired functionality in addition to the characteristics of Mg and the flame retardancy in powdering. It becomes. From this point of view, in one embodiment, the dissolved raw material of the Mg alloy contains 0.5 to 20% by mass of the first subcomponent and 0.01% by mass of the second subcomponent based on the total mass. It is preferable to contain 12% by mass, Mg and an unavoidable component as the balance thereof.

The above-mentioned "unavoidable component" means a component originally contained in the Mg raw material or unavoidably mixed in the manufacturing process. Usually, it means an element that does not affect the characteristics of the Mg alloy, and the amount thereof is very small. As used herein, the term "unavoidable component" means an elemental component contained in the dissolved raw material excluding Mg, the first subcomponent, and the second subcomponent. In one embodiment, the content of the unavoidable component is preferably less than 0.01% by mass, based on the total mass of the dissolved raw materials.

Hereinafter, the melting raw material of the Mg alloy will be described more specifically.
In the above production method, the dissolution raw material of the Mg alloy is characterized by containing at least one selected from the group consisting of Y, Ca, and Al as the first subcomponent. When a melting material of an Mg alloy containing at least Y, Ca, or Al is used, excellent flame retardancy can be obtained at the time of atomization, and the Mg alloy can be easily powdered by the air atomization method. In particular, when the dissolution raw material contains Y or Ca, it is considered that a dense oxide film is formed on the surface of the droplets when the molten metal of the Mg alloy is sprayed, and it becomes easy to enhance the flame retardancy.

Without being bound by _theory, standard free energy of oxygen to 1 mole of Y 2 _{O 3} and CaO, respectively, a -605.55kJ and -604.05KJ. On the other hand, the standard free energy of MgO for 1 mol of oxygen is -569.45 kJ. Thus, than _MgO, since towards the standard free energy of Y 2 _{O 3} and CaO is low, in the spray of molten metal, the dense oxide film containing _Y 2 _{O 3} or CaO on the surface of the droplet It is thought to be generated instantly. The presence of molar volume is large Y ₂ O ₃ or CaO as compared to MgO, the oxide film is considered oxide film becomes compressed tendency densified. By forming such a dense oxide film on the surface of the droplet, the reaction between Mg inside the droplet and oxygen in the air is blocked, and combustion of the droplet during atomization (powdering) can be prevented. It becomes possible to safely carry out the production of Mg alloy powder by the air atomizing method.

Further, since the oxide film is dense, the growth rate of the film is slow, and it is considered that the thickness of the oxide film after solidification is thin. As a matter of fact, in the study by the present inventors, in comparison with the Mg alloy powder produced by the water atomization method, according to the above production method, the thickness of the oxide film formed on the surface of the Mg alloy powder can be reduced. It has been confirmed. Therefore, in the production of sintered parts using Mg alloy powder, ordinary processing such as pressure treatment at the time of molding or sintering, or treatment of applying shear stress to the contact interface between powders at the time of hot processing after sintering is common. It is considered that the oxide film on the surface of the Mg alloy powder can be easily broken by the treatment step, a strong metal bond is formed in the production of the sintered part, and it becomes easy to obtain excellent sinterability. ..
Further, in the production of sintered parts, it is considered that the fine dispersion of the oxide in the structure is also effective in improving the strength of the sintered body. Further, since the produced Mg alloy powder has a dense oxide film on the surface, it is considered that oxidation, which contributes to deterioration of characteristics, does not proceed easily even when stored for a long period of time.

In one embodiment, the melting material of the Mg alloy contains at least Y as the first subcomponent. The presence of Y in the dissolution raw material is advantageous not only in the formation of a dense oxide film, but also in promoting the formation of a long-period laminated structure (LPSO) phase in the sintered structure.
Based on the total mass of the dissolved raw material, the content of Y is preferably 0.5 to 10% by mass, more preferably 1 to 9% by mass, still more preferably 2 to 7% by mass. By adjusting the Y content in the dissolved raw material to 0.5% by mass or more, it becomes easy to form a dense oxide film containing Y on the droplet surface during atomization by the air atomization method, and combustion of the molten metal It is possible to avoid the danger caused by. On the other hand, by adjusting the Y content in the dissolved raw material to 10% by mass or less, it is possible to easily suppress a decrease in fluidity due to an increase in the viscosity of the molten metal. As a result, atomization can be performed satisfactorily without causing problems such as nozzle clogging.

The dissolution raw material of the above embodiment preferably contains at least Zn as a second subcomponent. The Mg alloy powder obtained from the dissolved raw material containing Y and Zn can further promote the formation of the LPSO phase, and can further improve the strength of the sintered part.
The Zn content in the dissolved raw material is preferably 0.3 to 5% by mass, more preferably 0.5 to 4% by mass, and 1 to 3% by mass, based on the total mass, from the viewpoint of promoting the formation of the LPSO phase. Is even more preferable.
In a more specific embodiment, the dissolved raw material contains at least 2-10% by mass of Y as the first subcomponent, 1-3% by mass of at least Zn as the second subcomponent, and Mg and unavoidable as the balance thereof. It is preferable to contain a target component.

The dissolution raw material of the above embodiment may be a mixture in which each metal material is combined so as to have a desired composition, or a commercially available Mg alloy material (billet or ingot) having a desired composition. For example, an Mg—Zn—Y based alloy material can be mentioned as an example of the Mg alloy material that can be used as the dissolution raw material. As specific examples, Mg ₉₇ Zn ₁ Y ₂ (containing 1 atomic% (2.5 mass%) Zn, 2 atomic% (6.8 mass%) Y), Mg ₉₈ Zn ₁ Y ₁ (1 atomic% (2)). .6 mass%) Zn, 1 atomic% (3.5 mass%) Y contained), Mg ₉₅ Zn ₂ Y ₃ (2 atomic% (4.8 mass%) Zn, 3 atomic% (9.9 mass%) ) Containing Y). Among them, Mg ₉₇ Zn ₁ Y ₂ can be preferably used. In the above-mentioned Mg alloy material given as a specific example, the content (mass%) of each element described in parentheses is a typical value, and actually has a range. The same applies to the Mg alloy materials described below.

In another embodiment, the dissolving raw material of the Mg alloy contains at least Ca as a first subcomponent. In addition to being advantageous in forming an oxide film, the presence of Ca in the dissolution raw material can finely precipitate and disperse Mg-Ca-based intermetallic compounds, thereby improving the strength of the sintered body. It is also preferable in terms of points.

Based on the total mass of the dissolved raw material, the Ca content is preferably 0.5 to 10% by mass, more preferably 1 to 8% by mass, and even more preferably 2 to 5% by mass. By adjusting the Ca content in the dissolved raw material to 0.5% by mass or more, it becomes easy to form a dense oxide film containing Ca on the surface of the droplet during atomization, and the danger of burning the molten metal is avoided. can do. On the other hand, by adjusting the Ca content in the dissolution raw material to 10% by mass or less, embrittlement of the sintered body due to an increase in the precipitation amount of Mg-Ca-based intermetallic compounds is suppressed, and excellent processability is obtained. be able to.

The dissolution raw material of the above embodiment preferably further contains Al as a first subcomponent. In the Mg alloy powder obtained from the dissolved raw material containing Ca and Al, _{an intermetallic compound such as (Mg, Al) 2} Ca is precipitated, so that it becomes easy to further improve the strength of the sintered part. Further, when Ca and Al coexist, it is considered that Al is concentrated in the oxide film, so that the oxide film becomes more stable and the flame retardancy is improved. In this case, the total content of Ca and Al is preferably 20% by mass or less. In the case of a dissolution raw material containing Ca and Al, it becomes easier to adjust the precipitation amount of the intermetallic compound by adjusting the ratio of their contents. From the viewpoint of suppressing an increase in the amount of intermetallic compounds precipitated, the Ca content is preferably 0.5 to 10% by mass, more preferably 0.7 to 7% by mass, and even more preferably 1 to 5% by mass. On the other hand, the Al content is preferably 1 to 12% by mass, more preferably 3 to 11% by mass, and even more preferably 5 to 10% by mass.
In a more specific embodiment, the dissolved raw material preferably contains Ca and Al as the first subcomponent in a total amount of 1 to 20% by mass, more preferably 2 to 15% by mass, and 3 to 12% by mass. It is more preferably contained, and Mg and an unavoidable component are contained as the balance thereof.

The dissolution raw material of the above embodiment preferably further contains at least Zn as a second subcomponent. The Zn content in the dissolved raw material is preferably 0.1 to 8% by mass, more preferably 0.2 to 6% by mass, and even more preferably 0.3 to 4% by mass.
In a more specific embodiment, the dissolved raw material contains 1 to 20% by mass of Ca and Al as the first subcomponent in total, 0.1 to 8% by mass of at least Zn as the second subcomponent, and the balance thereof. It is preferable to contain Mg and an unavoidable component.

The dissolution raw material of the above embodiment may be a mixture in which each metal material is combined so as to have a desired composition, or an Mg alloy material (billet or ingot) having a desired composition.
For example, an Mg—Al—Ca based alloy material can be mentioned as an example of the Mg alloy material that can be used as the dissolution raw material. As specific examples, AMX602 (containing 6% by mass Al, 2% by mass Ca), Mg ₈₅ Al ₁₀ Ca ₅ (10 atomic% (10.7% by mass) Al, 5 atomic% (7.9% by mass) Ca. Included).
Another example is an Mg-Al-Zn-Ca based alloy material. Specific examples include AZX912 (containing 9% by mass Al, 1% by mass Zn, and 2% by mass Ca), AZX311 (containing 3% by mass Al, 1% by mass Zn, and 1% by mass Ca), and AZX611 (containing 6% by mass). Al, 1% by mass Zn, 1% by mass Ca is contained). Among them, AZX912 can be preferably used.

In yet another embodiment, the melting material of the Mg alloy contains at least Al as the first subcomponent. The presence of Al in the dissolution raw material is preferable in that it is advantageous in improving the flame retardancy in powdering and also in that it is easy to improve the strength of the sintered body.
Based on the total mass of the dissolved raw material, the Al content is preferably 3.5 to 12% by mass, more preferably 4.5 to 11% by mass, still more preferably 5.5 to 10% by mass. By adjusting the content of Al in the dissolution raw material to 3.5% by mass or more, Al tends to be concentrated in the oxide film and the liquid phase immediately inside the oxide film. Since Al ₂ O ₃ has a larger molar volume than Mg O, densification of the oxide film can be expected. Further, with respect to the liquid phase immediately inside the oxide film, effects such as a decrease in Mg vapor pressure due to the concentration of Al can be expected. From these facts, it is considered that Al can avoid the danger due to the combustion of the molten metal. On the other hand, by adjusting the Al content in the dissolution raw material to 12% by mass or less, embrittlement of the sintered body due to an increase in the precipitation amount of Mg-Al intermetallic compounds is suppressed, and excellent processability is obtained. be able to.

The dissolution raw material of the above embodiment preferably contains at least Zn as a second subcomponent. The Mg alloy powder obtained from a dissolution raw material containing Al and Zn as subcomponents can easily obtain a good balance between mechanical properties and processability. The Zn content in the dissolved raw material is preferably 0.1 to 8% by mass, more preferably 0.2 to 6% by mass, and even more preferably 0.3 to 4% by mass from the viewpoint of improving mechanical properties.
In a more specific embodiment, the dissolved raw material contains at least 3.5-12% by mass of Al as the first subcomponent, 0.1 to 8% by mass of at least Zn as the second subcomponent, and the balance thereof. Contains Mg and unavoidable components.

The dissolution raw material of the above embodiment may be a mixture in which each metal material is combined so as to have a desired composition, or a commercially available Mg alloy material (billet or ingot) having a desired composition. For example, an Mg—Al—Zn-based alloy material can be mentioned as an example of the Mg alloy material that can be used as the dissolution raw material. Specific examples include AZ91D (containing 9% by mass Al and 1% by mass Zn), AZ61A (containing 6% by mass Al and 1% by mass Zn), AZ63 (containing 6% by mass Al and 3% by mass Zn), and AZ81A. (Contains 8% by mass Al and 1% by mass Zn). Among them, AZ91D can be preferably used.

The method for producing the Mg alloy powder of the above embodiment can be carried out by applying a typical technique used in the gas atomizing method. The production of metal powder by the gas atomization method is generally carried out using a powdering apparatus having classifications such as dissolution, atomization, classification and accumulation. The production method of the above embodiment can be carried out by using at least the equipment constituting the dissolution classification and the atomization classification, and in the atomization classification, at least air is used to spray the molten metal.
More specifically, in the above production method, the step (a) of melting the melting raw material to obtain a molten Mg alloy can be carried out using a melting furnace equipped with a graphite crucible or the like. In the step (a), the molten Mg alloy can be obtained by putting the melting raw material described above into a melting furnace and heating it.

The melting temperature can be adjusted according to the melting raw material used, the characteristics of the apparatus, and the target powder particle size. From the viewpoint of melting the melting raw material of the Mg alloy and obtaining a preferable liquid phase state when spraying the molten metal, the melting temperature should be set in consideration of the temperature drop during the movement of the molten metal from the melting furnace to the sprayed position. Is preferable. Usually, it is preferable to set the melting temperature so that the temperature equal to or higher than the liquidus temperature can be secured at the position where the molten metal is sprayed. Generally, the dissolution temperature is preferably set to a temperature 50 to 300 ° C. higher than the liquidus temperature of the dissolution raw material.

When a nozzle with a stopper is attached to the bottom of the melting furnace, it is possible to suppress the temperature drop in the movement of the molten metal from the melting furnace to the nozzle, control the molten metal temperature at the atomized position, and suppress the oxidation of the molten metal. Becomes easier. Further, when the atmosphere in the melting furnace is pressurized, it becomes easy to control the outflow rate of the molten metal from the molten metal nozzle, and the quality such as the particle size can be stabilized. In addition, it is easy to improve operability such that the nozzle is less likely to be blocked, and the quality of the obtained powder is stable.

In the above manufacturing method, (b) the step of spraying the molten metal of the Mg alloy with air to obtain the Mg alloy powder can be carried out using a general atomizing apparatus. The air injection type at the time of atomization may be either a free fall type or a confined type. Regardless of which method is used, air can be efficiently injected from the periphery of the molten metal flow, so that a finer Mg alloy powder can be produced. The direction of hot water from the dundish is generally downward, but it can also be sideways or upward. When the hot water discharge direction is set to be sideways or upward, it is possible to prevent combustion due to dropping of the molten metal on the accumulated powder, and atomization can be carried out more safely.
In one embodiment, from the viewpoint of suppressing the production cost, it is preferable to spray the molten metal using only air, but if necessary, an inert gas may be mixed into the air. When an inert gas is mixed, it is considered that it becomes easy to control the film thickness of the oxide film. As the inert gas, argon gas or helium gas can be typically used, but nitrogen gas can also be used.

FIG. 1 is a schematic cross-sectional view showing a structural example of an air atomizing device. In the atomizing apparatus shown in FIG. 1, in order to simplify the structure, the molten raw material is heated in a melting furnace to obtain a molten metal, and then the molten metal is manually transferred to the tundish of the atomizing apparatus. In addition, an air nozzle is installed so that air can be injected sideways with respect to the flow of the molten metal falling from the molten metal nozzle under the tundish.

Hereinafter, the step (b) will be described more specifically by taking the case where the atomizing device 9 shown in FIG. 1 is used as an example.
First, the molten metal 1 obtained in the step (a) is transferred to the tundish 2, and the molten metal 1 is discharged from the nozzle 3 provided under the tundish 2. On the other hand, the molten metal 1 is sprayed by injecting air 5 from the air nozzle 4 with respect to the flow of the molten metal to form droplets (sprayed molten metal) 6. A powder is obtained by solidifying the droplet 6. The molten metal that has fallen without being atomized is collected by the molten metal saucer 7. Further, the powder formed by atomization (not shown) is collected by the powder saucer 8.

It is preferable to set the temperature at the time of transferring the molten metal to the tundish, that is, at the time of pouring, in consideration of the temperature decrease while the molten metal moves from the tundish to the sprayed position. It is preferable to adjust the temperature at the time of pouring so that the temperature equal to or higher than the liquidus temperature can be secured at the position where the molten metal is sprayed.

The injection of air can be carried out by applying a usual injection method. For example, air pressurized by a compressor is injected from an air nozzle. The flow velocity of air at the time of injection is adjusted by the injection pressure. In contrast to the water atomization method, the method of atomizing using air can suppress oxidation because it uses air as a medium, and there is no need to worry about the generation of hydrogen due to the reaction between powder and water. .. Further, since a large pump for pressurizing water and a water recovery / purification device are not required, and a drying step for the recovered powder is not required, the device and the process can be greatly simplified.

The shape and size of the Mg alloy powder can be adjusted by the shape of the molten metal nozzle, the molten metal temperature, the injection pressure of air, and the like. In one embodiment, the Mg alloy powder obtained by the above production method preferably has an irregular shape (amorphous non-spherical shape). The irregularly shaped powder increases the strength of the green compact after molding due to the anchor effect between the powder particles. Further, there is an advantage that the molded product is not easily deformed even after the binder is decomposed and dissipated after the degreasing step.

In the production of metal powder by the gas atomization method, the droplet immediately after atomization has an irregular shape, but usually, a spherical powder is obtained by forming a spherical shape by surface tension before the droplet solidifies. On the other hand, in the method for producing the Mg alloy powder of the above embodiment, since air is used, an oxide film is formed on the surface of the droplet immediately after atomization, and it is easy to obtain a powder that maintains an irregular shape.
In the atomization method using argon gas, a spherical Mg alloy powder is usually obtained, which is considered to be because the formation of an oxide film on the droplet surface is suppressed by the argon gas. From this point of view, the air atomizing method can be said to be a method suitable for producing an irregularly shaped Mg alloy powder because the surface of the droplet is rapidly oxidized by air.

The Mg alloy powder produced by the production method of the above embodiment has a composition corresponding to the dissolution raw material used at the time of production. The Mg alloy powder can be used for various purposes. Normally, in the Mg alloy powder produced by the water atomization method, the oxide film on the powder surface exceeds 1 μm, whereas according to the production method of the above embodiment, the thickness of the oxide film is controlled to less than 1 μm. It is easy to do. The thickness of the oxide film can be observed by a scanning transmission electron microscope (STEM) in a cross section near the surface of the powder.

In one embodiment, the thickness of the oxide film on the surface of the Mg alloy powder is preferably 800 nm or less, more preferably 500 nm or less, still more preferably 200 nm or less, from the viewpoint of obtaining excellent sinterability. According to the above manufacturing method, it is easy to control the thickness of the oxide film within the above range. Therefore, in one embodiment, the Mg alloy powder of the above embodiment has excellent sinterability and can be suitably used as a powder metallurgy material. By using the Mg alloy powder of the above embodiment, sintered parts having excellent various characteristics can be easily manufactured.

Sintered parts can be manufactured by filling a mold or the like with Mg alloy powder, pressure molding, and then sintering. Since the thickness of the oxide film on the surface of the Mg alloy powder of the above embodiment is thin, strong bonds between the metals are formed at the time of sintering, and it is easy to obtain a sintered part having high strength. The sintering temperature is preferably adjusted as appropriate according to the composition of the Mg alloy powder. For example, in the case of Mg alloy powder having a composition of Mg—Zn—Y alloy, it is preferable to perform sintering at a temperature of 500 to 550 ° C. By appropriately adjusting the temperature at the time of sintering, it becomes easier to obtain a sintered part having high strength. Further, by adding a hot working process after that, the size is further refined, and the strength of the sintered body is further improved by introducing strain. Sintering by hot pressing or pulse energization pressure sintering method is also effective for densification and high strength.

The sintered parts are not particularly limited and can be used for various purposes. Due to the characteristics of Mg alloy, sintered parts include parts for transportation equipment such as automobiles, railroad vehicles, and aircraft, parts for electronic equipment such as personal computers and mobile phones, and biological parts such as artificial bones and implants. It can be suitably used for such purposes.

Hereinafter, the present invention will be specifically described with reference to Examples, but the present invention is not limited to the following Examples.
(Examples 1 to 4, Comparative Example 1)
1. 1. Production of Mg Alloy Powder The Mg alloy material shown in Table 1 was used as a dissolution raw material, and air atomization was carried out according to the procedure shown below.

First, as a melting raw material, a commercially available billet or ingot of the Mg alloy material shown in Table 1 was cut out to an appropriate size. About 80 g of the cut out billet or ingot was weighed, placed in a graphite crucible, placed in a heating furnace, and heated to 650 ° C. in an argon gas atmosphere to dissolve the billet or ingot.
Next, the graphite crucible was promptly taken out from the heating furnace, and the molten metal was poured into the tundish of the in-house air atomizing device (see FIG. 1). For the flow of the molten metal falling from the molten metal nozzle set below the tundish, air was injected from the air nozzle installed sideways, and the molten metal was sprayed to atomize (powder). The diameter of the molten metal nozzle was φ10 mm, the injection pressure of air was 10 kgf / cm ² , and the diameter of the air nozzle was φ1.8 mm.

In Examples 1 to 4, combustion was not confirmed during atomization, and the Mg alloy powder obtained after atomization could be recovered. However, in Example 4, the atomization was partially insufficient and the molten metal scattered near the molten metal saucer turned blackish, but the fine powder sprayed far away showed a silvery white color. The yields of the Mg alloy powders in Examples 1, 2, 3, and 4 were approximately 50 g, 40 g, 40 g, and 20 g, respectively.
On the other hand, in Comparative Example 1, the molten metal burned violently when the graphite crucible was taken out from the heating furnace and poured into the tundish. The experiment was interrupted because it was determined that the injection of air would make the combustion even more intense.
From the above, when the dissolved raw material contains at least one element selected from the group consisting of Y, Ca, and Al, the flame retardancy of the Mg alloy is improved, and the Mg alloy powder is produced by the air atomization method. It turns out that is possible.

2. Analysis of Mg alloy powder (1) Observation of powder shape by scanning electron microscope (SEM) With respect to the Mg alloy powder produced by the air atomization method in Examples 1 to 3, the powder shape of each was observed using SEM. The observation was carried out by irradiating an electron beam under the condition of an acceleration voltage of 15 kV using a Quanta200 3D FEG manufactured by FEI. Each SEM photograph is shown in FIG. 2 (Example 1), FIG. 3 (Example 2), and FIG. 4 (Example 3). As is clear from the SEM photograph, it was found that all of the Mg alloy powders produced in Examples 1 to 3 had an irregular shape.

(2) Element distribution by X-ray photoelectron spectroscopy (XPS) XPS is effective for analyzing the abundance ratio and state of elements in the surface layer within about 5 nm from the surface. The Mg alloy powder produced in Examples 1 to 3 by the air atomization method was adhered to the surface of a 1 cm square carbon tape attached on the sample table without any gaps. Then, after lightly pressing the surface to flatten it, it was used as an analysis sample, and elemental analysis was performed by XPS. Therefore, the analytical values shown in Table 2 show the average value of a plurality of powder particles existing in the area irradiated with X-rays.
In the analysis, PHI 5000 VersaProbe II manufactured by ULVAC PFI Co., Ltd. was used. The measurement conditions were X-ray: monochromatic AlKα ray (1486.6 eV), detection angle: 45 °, and analysis area: φ200 μm. The pass energies during wide scan and narrow scan were 187.85 eV and 29.35 eV, respectively.

As shown in Table 2, it was found that the abundance ratio of oxygen (O) was as high as about 50 atomic% on the surfaces of all the powders, and the surface of the Mg alloy powder was covered with an oxide film. The abundance ratio of carbon (C) on the surface of the powder shown in Table 2 is mainly derived from the contamination by hydrocarbons in the atmosphere adhering to the surface of the Mg alloy powder.
Further, in the Mg alloy powder of Example 1, the yttrium (Y) abundance ratio on the powder surface was 12.3 atomic%. On the other hand, since the Y content of the dissolved raw material is 2.1 atomic% (converted from the Y content of the dissolved material in Table 1 of 6.8% by mass), Y is concentrated on the powder surface. I understood. Further, from the state analysis, the peak of binding energy of Y ₂ O ₃ was confirmed.

For the Mg alloy powder of Example 2, the calcium (Ca) abundance ratio on the powder surface was 4.2 atomic%. On the other hand, since the Ca content of the dissolved raw material is 3.3 atomic% (converted from the Ca content of the dissolved raw material of 2.06% by mass in Table 1), Ca is concentrated on the powder surface. I understood. In addition, the peak of the binding energy of CaO was confirmed by the state analysis. On the other hand, Al was not detected and was found to be less than 0.1 atomic%.

The Mg alloy powder of Example 3, the state analysis, a peak binding energy of MgO and _MgO X (X <1) was confirmed as an oxide. On the other hand, Al was not detected and was found to be less than 0.1 atomic%.

(3) Analysis of cross section of Mg alloy powder In order to analyze the state (cross section) of the Mg alloy powder produced in Examples 1 to 3 near the surface, each Mg alloy powder is subjected to focused ion beam (FIB) processing. , Foil-shaped test pieces (thickness about 100 nm) were collected respectively. The FIB processing was carried out using a composite beam processing observation device "JIB4501" manufactured by JEOL Ltd. under the conditions of ion beam: Ga ion, acceleration voltage: 30 kV, and protective film: carbon.

For each of the test pieces of Examples 1 to 3 collected as described above, a scanning transmission electron microscope (STEM) "JEM-2100F" manufactured by JEOL Ltd. was used under the condition of an acceleration voltage of 200 kV. Each cross section was observed. In addition, element mapping was analyzed using an energy-dispersed X-ray detector "JED-2300" manufactured by JEOL Ltd. in correspondence with the measurement area of the STEM photograph, and the distribution state of the elements was observed.

FIG. 5 shows a STEM photograph showing the state (cross section) of the Mg alloy powder produced in Example 1 near the surface. In addition, images of element mapping by EDX analysis (STEM / EDX) corresponding to the measurement region of the STEM photograph shown in FIG. 5 are shown in FIGS. 6A and 6B. FIG. 6A is an image showing the elemental mapping of O, and FIG. 6B is an image showing the elemental mapping of Y.

FIG. 7 shows a STEM photograph showing the state (cross section) of the Mg alloy powder produced in Example 2 near the surface. In addition, images of element mapping by STEM / EDX corresponding to the STEM photograph shown in FIG. 7 are shown in FIGS. 8A to 8C. 8A is an image showing the elemental mapping of O, FIG. 8B is an image showing the elemental mapping of Ca, and FIG. 8C is an image showing the elemental mapping of Al.

FIG. 9 shows a STEM photograph showing the state (cross section) of the Mg alloy powder produced in Example 3 near the surface. Further, images of element mapping by STEM / EDX corresponding to the STEM photograph shown in FIG. 9 are shown in FIGS. 10A and 10B. FIG. 10A is an image showing the elemental mapping of O, and FIG. 10B is an image showing the elemental mapping of Al.

In the Mg alloy powder test piece produced in Example 1, as can be seen from the scanning transmission electron microscope (STEM) photograph shown in FIG. 5, a black layer was present on the surface side (upper part of the image) of the test piece. In the element mapping (O) shown in FIG. 6A, the concentration of oxygen (O) was confirmed, and it was found that the above layer was an oxide film having a thickness of about 50 to 100 nm. Further, as can be seen from the element mapping (Y) shown in FIG. 6B, the concentration of Y was also confirmed in the oxide film.

In the Mg alloy test piece produced in Example 2, as can be seen from the STEM photograph shown in FIG. 7, a black layer and a light gray layer were present on the surface side (upper part of the image) of the test piece. .. In the element mapping (O) shown in FIG. 8A, the concentration of oxygen (O) could be confirmed over both of the above layers, and thus it was found that the oxide film had a thickness of about 130 nm. Further, as can be seen from the element mapping (Ca) shown in FIG. 8B and the element mapping (Al) shown in FIG. 8C, concentration of Ca and Al was also confirmed in the oxide film.

In the Mg alloy test piece produced in Example 3, as can be seen from the STEM image shown in FIG. 9, a black layer was present on the surface side (upper part of the image) of the test piece. In the element mapping (O) shown in FIG. 10A, the concentration of oxygen (O) was confirmed, and it was found that the layer was an oxide film having a thickness of about 70 nm. As can be seen from the element mapping (Al) shown in FIG. 10B, the clear concentration of Al in the oxide film as seen in the test piece of the Mg alloy produced in Example 2 (see FIG. 8C) is Not confirmed.

(4) Line analysis of Al distribution Line analysis was performed on the Mg alloy powders of Example 2 and Example 3 in order to compare the distribution of Al. The line analysis results of each test piece of Example 2 and Example 3 are shown in FIGS. 11 and 12. Since the oxide film on the powder surface is not a plane perpendicular to the observation surface and the analysis line, the boundary between the oxide film is not sharpened by picking up the information on the outermost surface and the interface between the base materials. Here, the region of the oxygen (O) peak is defined as the region of the oxide film. In the line analysis, element mapping data by EDX was used for a range of 200 nm in width centered on the dotted line (see reference numeral a) shown in each STEM photograph of FIG. 7 (Example 2) and FIG. 9 (Example 3). It is used and converted into a line analysis result.

The absolute value of the strength cannot be compared between FIGS. 11 and 12 due to the influence of the sample thickness and alloy components, but in each case, the Al strength (concentration) starts to increase from the surface to the inside. It was found that the position or peak position was shifted to the inside (inside) of the powder as compared with other elements. This is considered to correspond to the fact that Al is not detected on the powder surface in the analysis by XPS. In the Mg alloy powder of Example 2, a peak of Al was observed in the region of the oxide film (see FIG. 11), and it was found that Al was concentrated. On the other hand, in the Mg alloy powder of Example 3, a broad Al peak was observed near the inner boundary of the oxide film (see FIG. 12), and Al was concentrated from the oxide film to the inner base material. all right.

From the above results, in the Mg alloy powders produced in Examples 1 and 2, the molten metal of the dissolving material was refined and rapidly cooled by air injection to become droplets, and at the same time, Y or Ca was concentrated on the droplet surface. It is considered that a thin and dense oxide film was formed, and the film blocked the contact between the liquid phase inside the droplet and oxygen in the air atmosphere, so that the molten metal could be formed without burning during atomization. Further, when Al is contained at the same time as Ca, it is considered that the concentration of Al in the oxide film is promoted, so that the oxide film is further densified and the flame retardancy is improved.

Further, in Examples 3 and 4, the Mg alloy powder could be obtained by air atomization, and thus it was found that the inclusion of Al is effective in improving the flame retardancy of the molten metal. In the line analysis result of the Mg alloy powder of Example 3, the Al concentrated in the oxide film contributes to the densification of the oxide film, and the Al concentrated in the base material inside the oxide film reduces the vapor pressure of Mg, etc. It is possible that the flame retardancy is improved by contributing to. Further, from the situation at the time of atomization in Example 4, it is considered that the flame-retardant effect in the molten metal became smaller as the Al concentration decreased, and combustion occurred in the pure Mg of Comparative Example 1 to make atomization impossible. ..

As represented by Examples 1 to 3, according to the production method of the above embodiment, an irregularly shaped Mg alloy powder can be obtained, and the thickness of the oxide film on the powder surface is about 50 to 130 nm. Since it is thin, it is considered easy to obtain excellent sinterability. Therefore, in one embodiment, the Mg alloy powder produced by the production method of the above embodiment is considered to be effective for producing sintered parts.

1: Molten, 2: Tundish, 3: Molten nozzle, 4: Air nozzle, 5: Air, 6: Droplet, 7: Molten saucer, 8: Powder saucer, 9: Atomizer

Claims (5)

A step of dissolving a melting raw material of an Mg alloy containing Mg as a main component and at least one selected from the group consisting of Y, Al, and Ca as a first sub-component to obtain a molten metal of the Mg alloy, and a step of obtaining a molten metal of the Mg alloy. A method for producing an Mg alloy powder, which comprises a step of spraying the molten metal of the Mg alloy with air to obtain an Mg alloy powder. The production method according to claim 1, wherein the content of the first sub-component is 0.5 to 20% by mass based on the total mass of the dissolved raw material of the Mg alloy. 1. The melting raw material of the Mg alloy further contains a second subcomponent containing one or more selected from the group consisting of Zn, Mn, Si, Ni, Cu, Zr, and Sn. Or the manufacturing method according to 2. The production method according to claim 3, wherein the content of the second sub-component is 0.01 to 12% by mass based on the total mass of the dissolved raw material of the Mg alloy. Claimed that the melting raw material of the Mg alloy is an Mg—Zn—Y alloy material, an Mg—Al—Ca alloy material, an Mg—Al—Zn—Ca alloy material, or an Mg—Al—Zn alloy material. Item 2. The manufacturing method according to Item 1 or 2.

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