Method for producing aluminum alloy ingot

Abstract

To provide a method for producing an aluminum alloy ingot that allows easier reduction of the Si content in the aluminum alloy ingot.SOLUTION: A molten aluminum alloy comprising Si: 1.0 mass% to 11.0 mass% and Sr: 50 mass ppm to 1500 mass ppm is cooled in a container 2 composed of a non-magnetic material, while being stirred by means of a rotating magnetic field, to a temperature at which the solid phase ratio of the aluminum alloy is 0.2 to 0.7, thereby causing a solid S of the aluminum alloy containing primary crystals to precipitate in the molten metal M. Subsequently, by pressing the contents of the container 2, the molten metal M and the solid S are separated, thereby obtaining an aluminum alloy ingot.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for producing an aluminum alloy ingot. [Background technology]

[0002] Aluminum alloy ingots are obtained by casting a molten aluminum alloy having a chemical composition appropriate for the intended use. The raw materials for the aluminum alloy ingots include virgin aluminum and aluminum scrap.

[0003] In recent years, from the viewpoint of reducing the environmental impact during the production of aluminum alloy ingots and promoting the reuse of aluminum scrap, it has been desired to reduce the proportion of virgin aluminum in the casting raw material and to use a wider variety of aluminum scrap as the casting raw material. However, aluminum scrap that has not been used as a casting raw material until now sometimes contains a large amount of Si (silicon). When such aluminum scrap is used as a casting raw material, it has sometimes been difficult to adjust the Si content of the aluminum alloy ingot to fall within the desired range.

[0004] To address this problem, a method has been proposed for producing an aluminum alloy ingot with a lower Si content than the molten metal, utilizing the phenomenon that in an Al-Si alloy with a hypoeutectic composition, the α-Al phase solidifies at a higher temperature than the eutectic phase of Al and Si. For example, Patent Document 1 describes a method for refining aluminum scrap, which comprises cooling molten aluminum scrap made of an aluminum alloy at a rate of 20°C / min or less to a temperature below the liquidus temperature of the alloy and above the solidus or eutectic temperature of the alloy while stirring, and maintaining the temperature to uniformly generate primary crystal particles throughout the molten metal. Thereafter, the molten metal is pressed with a pressure of 1 to 15 MPa using a pressure stamp to separate a liquid phase above the stamp, solidifying the liquid phase and a compressed primary crystal portion below the stamp, and then recovering the solidified compressed primary crystal portion. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 7-54065 Summary of the Invention [Problem to be solved by the invention]

[0006] However, there is a limit to how much the refining method of Patent Document 1 can reduce the Si content in aluminum alloy ingots, and there is a strong demand for a technology that can further reduce the Si content in aluminum alloy ingots.

[0007] The present invention has been made in view of the above background, and aims to provide a method for producing an aluminum alloy ingot that can more easily reduce the Si content in the aluminum alloy ingot. [Means for solving the problem]

[0008] In one aspect of the present invention, a molten aluminum alloy containing 1.0 mass% or more and 11.0 mass% or less of Si (silicon) and 50 mass ppm or more and 1500 mass ppm or less of Sr (strontium) is cooled to a temperature at which a solid fraction of the aluminum alloy is 0.2 or more and 0.7 or less while being stirred using a rotating magnetic field in a container made of a non-magnetic material, thereby crystallizing a solid aluminum alloy containing primary crystals in the molten aluminum alloy; The method for producing an aluminum alloy ingot comprises squeezing the contents of the vessel to separate the molten metal from the solid, thereby obtaining an aluminum alloy ingot. [Effects of the Invention]

[0009] In the method for producing the aluminum alloy ingot (hereinafter referred to as "ingot"), first, a molten metal containing the specific amounts of Si and Sr is cooled to a temperature within the specific range in a vessel while being stirred using a rotating magnetic field. By cooling the molten aluminum alloy containing Sr while stirring in this manner, primary crystals with a relatively low Si concentration are crystallized in the vessel, and Si can be concentrated in the molten metal. Thereafter, the contents of the vessel are squeezed to separate the Si-enriched molten metal from the solid aluminum alloy containing the primary crystals, thereby easily obtaining an ingot with a low Si content.

[0010] As described above, according to the above-described embodiment, it is possible to provide a method for producing an aluminum alloy ingot that can more easily reduce the Si content in the aluminum alloy ingot. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a cross-sectional view showing the main parts of an aluminum alloy ingot manufacturing apparatus according to the embodiment. [Figure 2] FIG. 2 illustrates the state in which solid-phase aluminum alloy is precipitated in a vessel in the manufacturing method of the aluminum alloy ingot of the embodiment. [Figure 3] FIG. 3 is an explanatory diagram showing the state in which the contents of the container are compressed in the manufacturing method of the aluminum alloy ingot of the example. DETAILED DESCRIPTION OF THE INVENTION

[0012] In the manufacturing method, a molten aluminum alloy is first prepared by melting a casting raw material. Examples of the casting raw material used in the manufacturing method include discarded aluminum products, aluminum parts separated from discarded products, and aluminum waste such as scraps and chips generated during the manufacturing process of aluminum products and aluminum parts. Furthermore, new aluminum ingots, recycled aluminum ingots, intermediate alloys, etc. can also be used as the casting raw material, as needed. The aluminum waste used as the casting raw material may be a casting material or a wrought material.

[0013] The molten metal is composed of an aluminum alloy containing 1.0 to 11.0 mass% Si and 50 to 1500 mass ppm Sr. By setting the Si content and Sr content in the molten metal within the above-mentioned specific ranges, the Si content in the final ingot can be easily reduced.

[0014] If the Si content in the molten metal is less than 1.0% by mass, the Si content is already sufficiently low, which may make it difficult to concentrate Si in the molten metal and reduce the Si content in the ingot. From the viewpoint of more easily reducing the Si content in the ingot, the Si content in the molten metal is preferably 3.0% by mass or more, more preferably 4.0% by mass or more, even more preferably 5.0% by mass or more, particularly preferably 6.0% by mass or more, and most preferably 6.5% by mass or more.

[0015] On the other hand, if the Si content in the molten metal is excessively high, it may be difficult to crystallize the primary crystals at a temperature higher than the eutectic of Al and Si, and it may be difficult to obtain an aluminum alloy ingot with a lower Si content than the molten metal. By setting the Si content in the molten metal to 11.0 mass% or less, preferably 10.0 mass% or less, more preferably 9.0 mass% or less, even more preferably 8.0 mass% or less, and particularly preferably 7.5 mass% or less, it is possible to crystallize the primary crystals at a temperature higher than the eutectic and easily obtain an aluminum alloy ingot with a lower Si content than the molten metal.

[0016] In determining a preferred range of the Si content in the molten metal, the above-mentioned upper and lower limits of the Si content in the molten metal can be combined in any manner. For example, the preferred range of the Si content in the molten metal may be 3.0% by mass to 11.0% by mass, 4.0% by mass to 10.0% by mass, 5.0% by mass to 9.0% by mass, 6.0% by mass to 8.0% by mass, or 6.5% by mass to 7.5% by mass.

[0017] In the above-described production method, by adding 50 to 1500 ppm by mass of Sr to a molten metal having a Si content within the above-described specific range, an aluminum alloy ingot having a lower Si content than the molten metal can be easily obtained. From the viewpoint of further reducing the Si content in the ingot, the Sr content in the molten metal is preferably 50 to 1300 ppm by mass, more preferably 50 to 1000 ppm by mass, even more preferably 50 to 800 ppm by mass, particularly preferably 50 to 500 ppm by mass, and most preferably 50 to 300 ppm by mass.

[0018] From the viewpoint of further reducing the Si content in the ingot, the molten metal is preferably made of an aluminum alloy containing 3.0 to 11.0% by mass of Si and 50 to 1300 ppm by mass of Sr, more preferably 4.0 to 10.0% by mass of Si and 50 to 1000 ppm by mass of Sr, still more preferably 5.0 to 9.0% by mass of Si and 50 to 800 ppm by mass of Sr, particularly preferably 6.0 to 8.0% by mass of Si and 50 to 500 ppm by mass of Sr, and most preferably 6.5 to 7.5% by mass of Si and 50 to 300 ppm by mass of Sr.

[0019] The molten metal may contain additional elements other than Si, Sr, and Al (aluminum), such as Fe (iron), Cu (copper), Mn (manganese), Mg (magnesium), Cr (chromium), Zn (zinc), and Ti (titanium), if necessary.

[0020] More specifically, the molten metal may have a chemical composition containing 1.0 to 11.0 mass% Si and 50 to 1500 mass% Sr, with the balance being Al. Alternatively, the molten metal may have a chemical composition containing 1.0 to 11.0 mass% Si, 50 to 1500 mass% Sr, and one or more additional elements selected from the group consisting of Fe, Cu, Mn, Mg, Cr, Zn, and Ti, with the total content of the additional elements being 0.5 to 6.0 mass%, with the balance being Al.

[0021] In the manufacturing method, after preparing the molten metal, the molten metal is cooled while being stirred using a rotating magnetic field in a container made of a non-magnetic material. When a rotating magnetic field is applied to the container, the molten metal in the container flows so as to rotate around the rotation axis of the rotating magnetic field. By cooling the molten metal in the container while stirring it in this way, temperature deviations in the molten metal can be reduced and primary crystals can be uniformly crystallized in the molten metal. Furthermore, because the primary crystals crystallized in this way have a fine spherical shape, an increase in pressure loss during squeezing can be suppressed and the aluminum alloy solid containing the primary crystals can be efficiently separated from the molten metal.

[0022] In the manufacturing method, the molten metal is cooled, while being stirred, to a temperature at which the solid fraction of the aluminum alloy is 0.2 or more and 0.7 or less. By cooling the molten metal to a temperature within the specific range, primary crystals can be crystallized while suppressing the crystallization of eutectic crystals. As a result, a solid aluminum alloy containing primary crystals can be crystallized in the molten metal. Note that the eutectic may be crystallized in the molten metal cooled to a temperature within the specific range. The "solid fraction" mentioned above refers to the ratio of the mass of solids crystallized from the molten metal to the total mass of the contents of the container.

[0023] More specifically, the temperature of the molten metal can be set by the following method. First, using thermodynamic equilibrium calculation software (such as "JMatPro (registered trademark)" manufactured by Sente Software, Inc.), the solid fraction of the molten metal at a given temperature is calculated based on the chemical components of the molten metal. Next, based on these calculation results, the temperatures at which the solid fraction of the molten metal is 0.2 and 0.7 are determined. Then, the temperature of the molten metal at the completion of cooling can be set so that it is within the range from the temperature at which the solid fraction of the molten metal is 0.2 to the temperature at which the solid fraction of the molten metal is 0.7, as determined in this manner.

[0024] The vessel used for stirring and cooling the molten metal is not particularly limited as long as it is made of a non-magnetic material. From the viewpoint of heat resistance and strength, the vessel is preferably made of non-magnetic stainless steel. Examples of such stainless steel include SUS304 as specified in JIS G 4303:2015.

[0025] The direction of the central axis of the rotating magnetic field is not particularly limited, but from the viewpoint of more efficient stirring of the molten metal, it is preferable that the rotation axis of the rotating magnetic field is parallel to the vertical direction of the container. From the same viewpoint, it is more preferable that the rotation axis of the rotating magnetic field passes through the center of the molten metal and is parallel to the vertical direction of the container. The method for generating the rotating magnetic field is not particularly limited, and any known method can be used. For example, by arranging three stator coils around the container at equal intervals and passing sinusoidal alternating currents through each stator coil so that the phase difference between them is 120°, a rotating magnetic field can be generated with the center of gravity of the three stator coils as the rotation axis.

[0026] The frequency of the rotating magnetic field is preferably 1 Hz or higher, and more preferably 5 Hz or higher. In this case, primary crystals having the desired shape can be more easily crystallized in the molten metal. From the same viewpoint, the frequency of the rotating magnetic field is preferably 100 Hz or lower.

[0027] In the above-described manufacturing method, it is preferable to apply a rotating magnetic field to the molten metal so that the maximum value of the magnetic flux density at the position farthest from the rotation axis of the rotating magnetic field is 3 mT or more. In this case, the power required for stirring the molten metal can be more easily reduced.

[0028] In the manufacturing method, the temperature of the molten metal is cooled to the specified range, and then the contents of the vessel are pressed. This allows an aluminum alloy solid containing primary crystals to be separated from the molten metal, thereby obtaining an aluminum alloy ingot. The Si content in the ingot thus obtained is lower than the Si content in the molten metal before cooling.

[0029] In the above-described manufacturing method, the reason why the Si content in the ingot can be reduced by adding Sr is not entirely clear at present, but the following reasons are considered, for example.

[0030] Sr has the effect of increasing the degree of supercooling required for the crystallization of the eutectic phase, and is thought to have the effect of increasing the time lag between the crystallization of the primary crystal and the crystallization of the eutectic phase when the molten metal is cooled. Therefore, it is thought that adding Sr to a molten metal with a hypoeutectic composition can increase the proportion of primary crystals in the solid aluminum alloy that crystallizes in the molten metal.

[0031] In addition, Sr is thought to have the effect of suppressing the formation and growth of Si particles in the molten metal, and therefore, by adding Sr to a molten metal having a hypoeutectic composition, it is thought that the crystallization of Si in the molten metal as Si particles can be suppressed, and Si can be concentrated in the molten metal.

[0032] Therefore, the contents of the container cooled to the temperature within the specific range are composed of a molten metal in which Si is concentrated and a solid aluminum alloy containing primary crystals. Therefore, it is considered that by squeezing the contents of the container to separate the solid aluminum alloy from the molten metal, an ingot with a lower Si content than the molten metal can be obtained.

[0033] The method for squeezing the contents of the container can take various forms. For example, the contents can be squeezed by pressing a filter with through-holes for passing the molten metal against the contents of the container and compressing the contents. In this case, the pressure applied to the contents may be, for example, in the range of 0.5 MPa to 30.0 MPa.

[0034] According to the above-described method, an aluminum alloy ingot with a low Si content can be obtained. The aluminum alloy ingot thus obtained is used to produce wrought materials such as extruded materials and rolled materials. [Example]

[0035] An example of the method for manufacturing the aluminum alloy ingot is described below. In this example, a molten aluminum alloy containing 1.0 to 11.0 mass% Si and 50 to 1500 mass ppm Sr is first cooled to a temperature at which the solid fraction of the aluminum alloy is 0.2 to 0.7 while being stirred in a non-magnetic vessel using a rotating magnetic field. This causes a solid aluminum alloy containing primary crystals to crystallize in the molten aluminum alloy. The contents of the vessel are then squeezed to separate the molten aluminum alloy from the solid. This produces an aluminum alloy ingot.

[0036] 1, the manufacturing apparatus 1 used in the manufacturing method of this example includes a container 2 for holding molten metal, a magnetic field generating unit 3 arranged around the container 2, and a compressing unit 4 for compressing the contents of the container 2. The container 2 is made of non-magnetic stainless steel and has a cylindrical shape with a bottom.

[0037] The magnetic field generating unit 3 has three stator coils 31 arranged at equal intervals around the circumference of the container 2. The magnetic field generating unit 3 is configured to generate a rotating magnetic field within the container 2 by passing a sinusoidal alternating current, which has a phase difference of 120°, through these stator coils 31. The rotating magnetic field generated by the magnetic field generating unit 3 has a rotation axis that is parallel to the vertical direction. In addition, the rotation axis of the rotating magnetic field passes through the center of the container.

[0038] The compression section 4 has a filter 41 with a through hole 411 and a moving device 42 that moves the filter 41 in an up and down direction, and is configured so that the contents of the container 2 can be compressed by moving the filter 41 downward using the moving device 42 while the filter 41 is placed inside the container 2.

[0039] A more specific embodiment of the manufacturing method of this example will be described below. To produce an aluminum alloy ingot, first, casting raw materials are melted in a container 2 of a manufacturing apparatus 1 to produce a molten aluminum alloy (alloys A1 to A4) having the chemical compositions shown in Table 1. In Table 1, "Bal." is a symbol indicating the balance.

[0040] Next, a rotating magnetic field is generated by the magnetic field generating unit 3, and the molten metal in the container 2 is cooled to a temperature where the solid fraction of the molten metal becomes approximately 0.5 while being stirred. The temperature at which the solid fraction of the molten metal of alloys A1 to A4 becomes approximately 0.5 is specifically 585°C. The frequency of the rotating magnetic field is 10 Hz. The maximum value of the magnetic flux density at the position farthest from the rotation axis of the rotating magnetic field is 23 mT. When the molten metal is cooled to the specific temperature while being stirred in this manner, an aluminum alloy solid S containing primary crystals crystallizes in the molten metal M, as shown in Figure 2.

[0041] Thereafter, the filter 41 disposed within the container 2 is moved downward using the moving device 42 of the compressing unit 4, thereby compressing the contents of the container 2 at a pressure of 1.0 MPa. After the filter 41 comes into contact with the contents of the container 2, the filter 41 is further moved downward, and as shown in FIG. 3, the molten metal M passes through the through-holes 411 of the filter 41 and flows out to the top of the filter 41. Meanwhile, the aluminum solid S remaining below the filter 41 is compacted by the filter 41. As a result of the above, aluminum alloy ingots (ingots B1 to B4) can be obtained.

[0042] Table 2 shows the chemical compositions of the thus obtained ingots B1 to B4, the Si reduction rates, and the yields of the aluminum alloy ingots. The Si reduction rates (unit: %) shown in Table 2 are obtained by expressing, as a percentage, the ratio of the difference between the Si content in the molten metal and the Si content in the ingot relative to the Si content in the molten metal. The ingot yields (unit: %) shown in Table 2 are obtained by expressing, as a percentage, the ratio of the mass of the aluminum alloy ingot relative to the mass of the molten metal.

[0043] In addition, ingot D1 shown in Table 2 is an ingot for comparison with ingots B1 to B4. The manufacturing method for ingot D1 is the same as the manufacturing method for ingots B1 to B4, except that a molten metal having the chemical composition shown in alloy C1 in Table 1 is used.

[0044] [Table 1]

[0045] [Table 2]

[0046] As shown in Table 1, the Si content and Sr content of alloys A1 to A4 are each within the specified ranges. Therefore, by producing an aluminum alloy ingot using a molten metal made of these alloys, it is possible to easily obtain an ingot with a low Si content, such as ingots B1 to B4 in Table 2. Among these ingots B1 to B4, ingots B1 to B2, in which the Sr content in the molten metal is 50 ppm by mass or more and 300 ppm by mass or less, can have an even lower Si content than ingots B3 to B4.

[0047] On the other hand, ingot D1 was produced using a molten metal that did not contain Sr (Table 1, alloy C1), and therefore the Si content in ingot D1 is higher than ingots B1 to B4, which were produced using molten metals that contained Sr.

[0048] Although the above has described embodiments of the method for producing an aluminum alloy ingot based on the examples, the specific embodiments of the method for producing an aluminum alloy ingot according to the present invention are not limited to the embodiments, and the configuration can be appropriately changed within the scope that does not depart from the spirit of the present invention.

[0049] For example, the method for producing the aluminum alloy ingot can take the following aspects [1] to [5].

[0050] [1] A molten aluminum alloy containing 1.0 mass% or more and 11.0 mass% or less of Si and 50 mass ppm or more and 1500 mass ppm or less of Sr is cooled to a temperature at which a solid fraction of the aluminum alloy is 0.2 or more and 0.7 or less while being stirred using a rotating magnetic field in a container made of a non-magnetic material, thereby crystallizing a solid aluminum alloy containing primary crystals in the molten aluminum alloy; The method for producing an aluminum alloy ingot comprises squeezing the contents of the container to separate the molten metal from the solid, thereby obtaining an aluminum alloy ingot.

[0051] [2] The method for producing an aluminum alloy ingot according to [1], wherein the Si content in the molten metal is 3.0 mass % or more and 11.0 mass % or less. [3] The method for producing an aluminum alloy ingot according to [1] or [2], wherein the frequency of the rotating magnetic field is 1 Hz or more.

[0052] [4] The method for producing an aluminum alloy ingot according to any one of [1] to [3], wherein the rotating magnetic field is applied to the molten metal so that the maximum value of the magnetic flux density at the position farthest from the axis of rotation of the rotating magnetic field is 3 mT or more. [5] The method for producing an aluminum alloy ingot according to any one of [1] to [4], wherein the molten metal further contains one or more additive elements selected from the group consisting of Fe, Cu, Mn, Mg, Cr, Zn, and Ti, and the total content of the additive elements is 0.5% by mass or more and 6.0% by mass or less. [Explanation of symbols]

[0053] 1 Manufacturing equipment 2 containers 3 Magnetic field generation unit 4. Compression section M molten metal S Aluminum alloy solid

Claims

1. a molten aluminum alloy containing 1.0 mass % or more and 11.0 mass % or less of Si and 50 mass ppm or more and 1500 mass ppm or less of Sr is cooled to a temperature at which a solid fraction of the aluminum alloy is 0.2 to 0.7 while being stirred using a rotating magnetic field in a container made of a non-magnetic material, thereby crystallizing a solid aluminum alloy containing primary crystals in the molten aluminum alloy; The method for producing an aluminum alloy ingot comprises squeezing the contents of the container to separate the molten metal from the solid, thereby obtaining an aluminum alloy ingot.

2. The method for producing an aluminum alloy ingot according to claim 1, wherein the Si content in the molten metal is 3.0 mass % or more and 11.0 mass % or less.

3. 3. The method for producing an aluminum alloy ingot according to claim 1, wherein the frequency of the rotating magnetic field is 1 Hz or more.

4. 3. The method for producing an aluminum alloy ingot according to claim 1, wherein the rotating magnetic field is applied to the molten metal so that a maximum value of magnetic flux density at a position farthest from a rotation axis of the rotating magnetic field is 3 mT or more.

5. 3. The method for producing an aluminum alloy ingot according to claim 1, wherein the molten metal further contains one or more additive elements selected from the group consisting of Fe, Cu, Mn, Mg, Cr, Zn, and Ti, and the total content of the additive elements is 0.5% by mass or more and 6.0% by mass or less.

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