ALUMINUM ALLOY INGOT AND METHOD FOR MANUFACTURING SAME

Abstract

 An aluminum-alloy ingot comprises: an aluminum matrix (1); and TiB₂ aggregates (2), which are dispersed in the aluminum matrix (1) and in which TiB₂ particles (3) are aggregated. The average value of the circle-equivalent diameters of the TiB₂ aggregates (2) in the state in which the TiB₂ aggregates (2) are exposed from the aluminum matrix (1) is 3.0 µm or less and the average value of the circularities is 0.20 or more.

Description

TECHNICAL FIELD

[0001] The present invention relates to an aluminum-alloy ingot and to a manufacturing method thereof.

BACKGROUND ART

[0002] When an ingot is to be manufactured by casting an aluminum alloy, a crystal-grain refining agent is sometimes added to the melt of the aluminum alloy for the purpose of refining the crystal particles in the ingot. An Al-Ti-B refining agent, in which a Ti-B (titanium-boron) compound, such as TiB₂, has been dispersed in a base composed of aluminum, is used as the crystal-grain refining agent (Patent Document 1).

[0003] When a solid solution of the Al-Ti-B refining agent is formed in the melt of an aluminum alloy, a state results in which solid Ti-B compound has been dispersed in the melt. When the melt is caused to solidify in this state, the Ti-B compound functions as heterogenous nuclei, and therefore crystal particles can be caused to grow in the aluminum matrix, with the Ti-B compound serving as the starting points. As a result, the crystal particles in the aluminum matrix can be refined.

PRIOR ART LITERATURE

Patent Document

[0004] Patent Document 1
Japanese Laid-open Patent Publication 2001-191654

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

[0005] However, with regard to existing aluminum-alloy ingots, in which the crystal particles are refined using an Al-Ti-B refining agent, linear defects sometimes occur when performing flattening work, such as rolling or extruding, and there is a risk that it will lead to degradation in the surface quality of the final product.

[0006] The present invention was conceived considering this background, and an object of the present invention is to provide an aluminum-alloy ingot, comprising fine grain sizes and in which the occurrence of linear defects when performing flattening work can be curtailed, and a manufacturing method thereof.

MEANS FOR SOLVING THE PROBLEMS

[0007] One aspect of the present invention is an aluminum-alloy ingot comprising: an aluminum matrix; and

TiB₂ aggregates, which are dispersed in the aluminum matrix and in which TiB₂ particles are aggregated;

wherein the average value of the circle-equivalent diameters of the TiB₂ aggregates in the state in which the TiB₂ aggregates are exposed from the aluminum matrix is 3.0 µm or less and the average value of the circularities is 0.20 or more.

[0008] Another aspect of the present invention is a method of manufacturing the aluminum-alloy ingot of the above-mentioned aspect, comprising:

a melting step, in which a crystal-grain refining agent, in which a base composed of aluminum contains TiB₂ particles and the average value of the center-to-center distances between adjacent TiB₂ particles is 0.60 µm or more, is melted in a melt of aluminum alloy; and

a casting step, in which, after the melting step, the melt is cast.

[0009] Yet another aspect of the present invention is a method of manufacturing the aluminum-alloy ingot according to the above-mentioned aspect, comprising:

a melting step, in which a crystal-grain refining agent, in which a base composed of aluminum contains TiB₂ aggregates in which TiB₂ particles are aggregated, is melted in a melt of an aluminum alloy; and

a casting step, in which, after the melting step, the melt is cast;

wherein, in the situation in which projection-surface areas of 2,000 or more TiB₂ aggregates have been measured in the state in which TiB₂ aggregates were exposed from the base, the TiB₂ aggregates in the crystal-grain refining agent have a particle-size distribution in which the average value of the circle-equivalent diameters of TiB₂ aggregates, which have a projection-surface area in the 95^th percentile or higher, is 3.0 µm or less.

EFFECTS OF THE INVENTION

[0010] The aluminum-alloy ingot has TiB₂ aggregates, in which TiB₂ particles are the primary particles, in an aluminum matrix. In addition, the average value of the circle-equivalent diameters and the average value of the circularities of the TiB₂ aggregates are within the above-mentioned specific ranges, respectively. This means that the aluminum-alloy ingot will be manufactured by casting a melt that contains the above-mentioned specific TiB₂ aggregates.

[0011] The performance of the TiB₂ aggregates, in which the average value of the circle-equivalent diameters and the average value of the circularities are within the respective specific ranges, in refining the crystal particles in the aluminum matrix is high. Consequently, by forming the TiB₂ aggregates in the melt, the crystal particles in the aluminum matrix can be sufficiently refined. In addition, because the particle sizes of the TiB₂ aggregates are comparatively small, the occurrence of linear defects when performing flattening work on the aluminum-alloy ingot can be curtailed.

[0012] For this reason, according to the above-mentioned aspect, an aluminum-alloy ingot, having a fine grain size and in which the occurrence of linear defects during flattening work can be curtailed, can be provided.

[0013] In addition, in a method of manufacturing the aluminum-alloy ingot, after the crystal-grain refining agent has been melted in the melt of the aluminum alloy, the ingot is manufactured by solidifying the melt. In a method of manufacturing the aluminum-alloy ingot, by melting the crystal-grain refining agent, which has the above-mentioned specific composition, into the melt in the melting step, TiB₂ aggregates for which the average value of the circle-equivalent diameters and the average value of the circularities are within the respective specific ranges, can be formed in the melt. Furthermore, by solidifying such a melt, the crystal particles of the ultimately obtained aluminum-alloy ingot can be easily refined, and the occurrence of linear defects when performing flattening work on the aluminum-alloy ingot can be curtailed.

[0014] In addition, in a method of manufacturing the aluminum alloy, because the quality of the aluminum-alloy ingot can be improved by using methods performed in the past, that is, by melting the crystal-grain refining agent in the melt of the aluminum alloy, there is no need to add special steps or equipment to refine the crystal particles of the aluminum matrix. Therefore, according to a method of manufacturing the aluminum alloy, an aluminum-alloy ingot having excellent quality can be obtained while avoiding an increase in the cost of manufacturing the aluminum-alloy ingot.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] 

FIG. 1 is an enlarged photograph of TiB₂ particles, which are contained in an aluminum-alloy ingot, of Working Example 1.

FIG. 2 is an enlarged photograph of TiB₂ particles, which are contained in an aluminum-alloy ingot, of Comparative Example 1.

MODES FOR CARRYING OUT THE INVENTION

[0016] The chemical composition of the above-mentioned aluminum-alloy ingot is not particularly limited and may be any kind of aluminum alloy. It is noted that the "aluminum alloy" described above is a concept that encompasses pure aluminum. For example, the above-mentioned aluminum-alloy ingot may have a chemical composition classified as A1000-series aluminum or may have a chemical composition classified as A2000-series alloys, A3000-series alloys, A4000-series alloys, A5000-series alloys, A6000-series alloys, A7000-series alloys, or A8000-series alloys.

[0017] From the viewpoint of curtailing the aggregation of TiB₂ particles during casting and making the average value of the circle-equivalent diameters of TiB₂ aggregates small, the above-mentioned aluminum-alloy ingot preferably has a chemical composition that contains one or two or more of elements selected from the group consisting of Si (silicon): 0.01 mass% or more and 14.0 mass% or less, Fe (iron): 0.01 mass% or more and 2.0 mass% or less, Cu (copper): 0.01 mass% or more and 7.0 mass% or less, Mg (magnesium): 0.01 mass% or more and 7.0 mass% or less, Mn (manganese): 0.01 mass% or more and 2.0 mass% or less, and Ti (titanium): 0.003 mass% or more and 0.3 mass% or less, the remainder being Al (aluminum) and unavoidable impurities.

[0018] The aluminum-alloy ingot contains an aluminum matrix and TiB₂ aggregates. Depending on the chemical composition, the aluminum-alloy ingot may contain crystallized products. In addition, the aluminum-alloy ingot may contain unaggregated TiB₂ particles.

[0019] The aluminum matrix contains aluminum atoms and solid-solution elements in accordance with the chemical composition of the above-mentioned aluminum-alloy ingot. In addition, the aluminum matrix is composed of numerous crystal particles. The crystal-particle sizes of the aluminum matrix vary in accordance with the chemical composition of the above-mentioned aluminum-alloy ingot; in an aluminum-alloy ingot that contains the above-mentioned specific TiB₂ aggregates, the mean particle size of the aluminum matrix is usually within the range of 50 µm or more and 5,000 µm or less.

[0020] The TiB₂ aggregates are dispersed in the aluminum matrix, in which aggregates of the TiB₂ particles serve as primary particles. The particle size of each individual TiB₂ particle contained in the TiB₂ aggregates may be, for example, 0.1 µm or more and 5.0 µm or less.

[0021] The average value of the circle-equivalent diameters of the above-mentioned TiB₂ aggregates in the state in which the TiB₂ aggregates are exposed from the above-mentioned aluminum matrix is 3.0 µm or less. By setting the average value of the circle-equivalent diameters of the TiB₂ aggregates to 3.0 µm or less, the occurrence of linear defects when performing flattening work, such as rolling or extruding, on the above-mentioned aluminum-alloy ingot can be curtailed.

[0022] In the situation in which the average value of the circle-equivalent diameters of the TiB₂ aggregates is larger than 3.0 µm, the possibility that coarse TiB₂ aggregates will be present in the aluminum-alloy ingot becomes high. For that reason, in this situation, when flattening work has been performed on aluminum-alloy ingots, there is a risk that linear defects originating at the coarse TiB₂ aggregates will tend to occur.

[0023] From the viewpoint of curtailing the occurrence of linear defects, the lower limit of the average value of the circle-equivalent diameters of the TiB₂ aggregates is not particularly limited, but the average value of the circle-equivalent diameters of the TiB₂ aggregates formed by the manufacturing method of the above-mentioned aspect is usually 1.0 µm or more.

[0024] The average value of the circle-equivalent diameters of the TiB₂ aggregates described above is a value that is calculated by the following method. First, a test piece is collected from the interior of the aluminum-alloy ingot. Subsequently, the aluminum matrix on the surface of the test piece is removed by a method such as dip etching. By observing the surface of the test piece using an electron microscope or the like, an enlarged photograph of the TiB₂ aggregates is taken. The circle-equivalent diameter of each individual TiB₂ aggregate is calculated based on the projection-surface area of the TiB₂ aggregate in the enlarged photograph that was taken. By performing the above operation on a plurality of randomly selected TiB₂ aggregates and calculating the arithmetic mean of the circle-equivalent diameters thereof, the average value of the circle-equivalent diameters of the TiB₂ aggregates can be obtained. The number of TiB₂ aggregates used when calculating the average value of the circle-equivalent diameters should be, for example, 3 or more.

[0025] In addition, the average value of the circularities of the above-mentioned TiB₂ aggregates in the state in which the TiB₂ aggregates are exposed from the above-mentioned aluminum matrix is 0.20 or more. The circularity of a TiB₂ aggregate is a value that becomes an indication of whether the shape of the TiB₂ aggregate is close to a sphere and means that, the closer the circularity is to 1, the closer the shape of the TiB₂ aggregate is to a sphere.

[0026] In casting processes of the above-mentioned aluminum-alloy ingot, TiB₂ aggregates having a variety of shapes are usually formed in the melt. Furthermore, the more that the TiB₂ aggregates in the melt have a shape in which the circularity is large and therefore the shape is close to a sphere, the more the TiB₂ aggregates have the property of tending to function as heterogenous nuclei. Accordingly, by setting the average value of the circularities of the TiB₂ aggregates to 0.20 or more, it is possible to increase the proportion of the TiB₂ aggregates, among the TiB₂ aggregates formed during casting, that can function as heterogenous nuclei.

[0027] In the situation in which the average value of the circularities of the TiB₂ aggregates is less than 0.20, the proportion of the TiB₂ aggregates, among the TiB₂ aggregates formed during casting, that can function as heterogenous nuclei tends to become low. Consequently, in this situation, there is a risk that the refinement of the crystal particles in the aluminum matrix will become insufficient.

[0028] From the viewpoint of increasing the effect of refining the crystal particles of the TiB₂ aggregates, the upper limit of the average value of the circularities of the TiB₂ aggregates is not particularly limited, but the average value of the circularities of the TiB₂ aggregates formed by the manufacturing method of the above-mentioned aspect is usually 0.8 or less.

[0029] The average value of the circularities of the TiB₂ aggregates described above specifically is a value that is calculated by the following method. First, the method of calculating the circle-equivalent diameters is the same, and an enlarged photograph of the TiB₂ aggregates is taken. The circularity of each individual TiB₂ aggregate is described by the equation below, using the surface area S [µm²] and the circumferential length L [µm] of each TiB₂ aggregate in the enlarged photograph that was taken.

[0030] By performing the above operation on a plurality of randomly selected TiB₂ aggregates and calculating the arithmetic mean of the circularities thereof, the average value of the circularities of the TiB₂ aggregates can be obtained. The number of TiB₂ aggregates used when calculating the average value of the circularities should be, for example, 3 or more.

[0031] In addition, by setting the average value of the circle-equivalent diameters and the average value of the circularities of the TiB₂ aggregates contained in the above-mentioned aluminum-alloy ingot to within the above-mentioned specific ranges, respectively, the functions and effects thereof described above can be increased, and the amount of the crystal-grain refining agent added in the casting process can be decreased while ensuring the effect of refining the crystal particles.

[0032] That is, in casting processes of the above-mentioned aluminum-alloy ingot as described above, by setting the average value of the circle-equivalent diameters of the TiB₂ aggregates formed in the melt to within the above-mentioned specific range, the number of coarse TiB₂ aggregates can be decreased and the total number of TiB₂ aggregates in the melt can be made large. In addition, by setting the average value of the circularities of the TiB₂ aggregates to within the above-mentioned specific range, the proportion of the TiB₂ aggregates in the melt that can function as heterogenous nuclei can be increased.

[0033] Accordingly, in casting processes of the above-mentioned aluminum-alloy ingot, by setting the average value of the circle-equivalent diameters and the average value of the circularities of the TiB₂ aggregates to within the above-mentioned specific ranges, respectively, the number of TiB₂ aggregates that can function as heterogenous nuclei can be made large and, in turn, the effect of refining the crystal particles in the aluminum matrix can be increased.

[0034] As described above, by setting the average value of the circle-equivalent diameters and the average value of the circularities of the TiB₂ aggregates contained in the above-mentioned aluminum-alloy ingot to within the above-mentioned specific ranges, respectively, the effect due to the decrease in the circle-equivalent diameters and the effect due to the increase in circularity can be made to function synergistically. As a result, when manufacturing the above-mentioned aluminum-alloy ingot, the amount of the crystal-grain refining agent added during casting can be decreased more than in the past while maintaining the effect of refining the crystal particles.

[0035] For example, the content of the TiB₂ aggregates in the above-mentioned aluminum-alloy ingot can be set to 0.0001 mass% or more and 0.0010 mass% or less based on the boron atoms.

[0036] With regard to the manufacture of the above-mentioned aluminum-alloy ingot, a method can be used that comprises: a melting step, in which the crystal-grain refining agent is caused to melt in the melt of the aluminum alloy; and, after the melting step, a casting step that casts the melt.

[0037] The crystal-grain refining agent added into the melt in the above-mentioned manufacturing method has a base composed of aluminum. The shape of the base is not particularly limited and may have, for example, a rod shape, a plate shape, or the like.

[0038] In addition, the base contains TiB₂ particles. The TiB₂ particles are dispersed within the base and may exist in an unaggregated state. In addition, TiB₂ aggregates may be formed in the base by aggregation of a plurality of the TiB₂ particles. More specifically, all the TiB₂ particles in the base may exist in an unaggregated state, or all the TiB₂ particles in the base may exist in a TiB₂ aggregate state. Furthermore, both TiB₂ particles and TiB₂ aggregates may exist in the base in an unaggregated state.

[0039] With regard to at least some of the TiB₂ particles in the base, after the crystal-grain refining agent has melted into the melt in the melting step, the TiB₂ particles aggregate in the melt to form TiB₂ aggregates. In addition, with regard to the TiB₂ aggregates in the base, when the crystal-grain refining agent has melted in the melt in the melting step, the TiB₂ aggregates migrate into the melt while maintaining the aggregated state. For that reason, by causing the crystal-grain refining agent, which contains TiB₂ particles, to melt in the aluminum melt, TiB₂ aggregates can be formed in the melt.

[0040] In the above-mentioned melting step, for example, the crystal-grain refining agent according to any of the aspects below can be melted in the melt. That is, in a first aspect of the crystal-grain refining agent, the average value of the center-to-center distances of adjacent TiB₂ particles is 0.60 µm or more in an arbitrary cross section of the above-mentioned crystal-grain refining agent. By setting the center-to-center distances of the TiB₂ particles in the crystal-grain refining agent to within the above-mentioned specific range, the aggregation of TiB₂ particles and the growth of TiB₂ aggregates when the crystal-grain refining agent has been melted in the melt can be curtailed. As a result, the average value of the circle-equivalent diameters and the average value of the circularities of the TiB₂ aggregates formed in the melt can be easily set to within the above-mentioned specific ranges, respectively.

[0041] In the situation in which the average value of the center-to-center distances of adjacent TiB₂ particles is less than 0.60 µm, it becomes easy for aggregates to form between TiB₂ particles when the crystal-grain refining agent has been melted in the melt. As a result, there is a risk that coarse TiB₂ aggregates will tend to be formed in the melt, which will lead to a decrease in the effect of refining the crystal particles and to an increase in the occurrence frequency of linear defects during flattening work.

[0042] It is noted that the average value of the center-to-center distances of the TiB₂ particles described above is a value that is calculated by the following method. First, the crystal-grain refining agent is cut to expose a cut surface. This cut surface is observed using an electron microscope or the like, and an enlarged photograph of the cut surface is taken. With regard to the enlarged photograph that was taken, the center of gravity of each TiB₂ particle present in the enlarged photograph is determined. It is noted that both the TiB₂ particles present in an undispersed state and the TiB₂ particles that constitute a portion of the TiB₂ aggregates are included in the TiB₂ particles present in the enlarged photograph.

[0043] Next, the TiB₂ particles that will become the objects to be measured for the center-to-center distances are determined from among the TiB₂ particles present in the enlarged photograph. Furthermore, the distance between the center of gravity of the TiB₂ particle that is the object to be measured and the center of gravity of the TiB₂ particle closest to the TiB₂ particle that is the object to be measured is measured, and that value is set as the center-to-center distance of the TiB₂ particle that is the object to be measured. By performing the above operation on all TiB₂ particles present in the enlarged photograph, the arithmeticmean value of the obtained center-to-center distances is set as the average value of the center-to-center distances of the TiB₂ particles.

[0044] In a second aspect of the crystal-grain refining agent, in the situation in which the projection-surface areas of 2,000 or more TiB₂ aggregates have been measured in the state in which the TiB₂ aggregates are exposed from the base, the TiB₂ aggregates in the crystal-grain refining agent have a particle-size distribution in which the average value of the circle-equivalent diameters of the TiB₂ aggregates, which have a projection-surface area in the 95^th percentile or higher, is 3.0 µm or less. In crystal-grain refining agents containing TiB₂ aggregates, the TiB₂ aggregates having large circle-equivalent diameters effectively function as heterogenous nuclei in the aluminum melt.

[0045] However, when the circle-equivalent diameters of the TiB₂ aggregates in the crystal-grain refining agent become excessively large, coarse TiB₂ aggregates tend to mix into the aluminum melt when the crystal-grain refining agent has been melted in the aluminum melt. As a result, when flattening work is performed on the aluminum-alloy ingot after it has been cast, there is a risk that linear defects originating from the coarse TiB₂ aggregates will tend to occur.

[0046] Accordingly, by setting the particle-size distribution of the TiB₂ aggregates in the crystal-grain refining agent to the above-mentioned specific aspect, the possibility that the crystal-grain refining agent will contain coarse TiB₂ aggregates can be made low and, in turn, the mixing of the coarse TiB₂ aggregates into the melt can be curtailed. As a result, the average value of the circle-equivalent diameters and the average value of the circularities of the TiB₂ aggregates formed in the melt can be set easily to within the above-mentioned specific ranges, respectively.

[0047] In the situation in which the average value of the circle-equivalent diameters of the TiB₂ aggregates, which have a projection-surface area in the 95^th percentile or higher, is larger than 3.0 µm, the possibility that the crystal-grain refining agent will contain coarse TiB₂ aggregates becomes high. Consequently, when the crystal-grain refining agent has been melted in the melt, the coarse TiB₂ aggregates tend to mix into the melt, and there is a risk that this will lead to a decrease in the effect of refining the crystal particles and to an increase in the occurrence frequency of linear defects during flattening work.

[0048] It is noted that the average value of the circle-equivalent diameters of the TiB₂ aggregates described above is a value calculated by the following method. First, the crystal-grain refining agent is cut to expose a cut surface. The cutting direction of the crystal-grain refining agent is not particularly limited. For example, in the situation in which the crystal-grain refining agent is rod shaped, the crystal-grain refining agent should be cut through its center in a plane perpendicular to the longitudinal direction.

[0049] Next, after polishing the cut surface of the crystal-grain refining agent, the TiB₂ aggregates are exposed from the base by removing a peripheral portion of the TiB₂ aggregates at the base. For example, a method such as dip etching can be used as the method of removing the base.

[0050] Subsequently, the TiB₂ aggregates that have been exposed from the base are observed using an electron microscope or the like, and an enlarged photograph of the TiB₂ aggregates is taken. The surface area of each TiB₂ aggregate in the enlarged photograph is taken as the projection-surface area of the TiB₂ aggregate. It is noted that an image-analyzing apparatus or the like can be used in the calculation of the projection-surface area and the calculation of the circle-equivalent diameter of each TiB₂ aggregate.

[0051] The above operation is performed for 2,000 or more TiB₂ aggregates randomly selected from the TiB₂ aggregates present at the cut surface of the crystal-grain refining agent. Based on the projection-surface areas of the TiB₂ aggregates obtained in this manner, the 95^th percentile of the projection-surface areas is calculated. It is noted that, in the situation in which a plurality of numerical values is sorted in order from the smallest numerical value to the largest numerical value, the percentile is a numerical value in which the number of numerical values counting from the smallest numerical value is the desired percentage of the total number of numerical values. In the situation in which there is no such numerical value, the largest value of the values, in which the number of numerical values counting from the smallest numerical value is less than the desired percentage of the total number of the numerical values, is taken as the percentile. More specifically, the 95^th percentile of the projection-surface areas is the value in which the number of TiB₂ aggregates having a projection-surface area in the 95^th percentile or higher is 5% of the total number of TiB₂ aggregates for which the projection-surface area was measured.

[0052] After the 95^th percentile of the projection-surface areas has been determined as described above, the circle-equivalent diameters of the TiB₂ aggregates having a projection-surface area in the 95^th percentile or higher are calculated. It is noted that a circle-equivalent diameter is the diameter of a circle having a surface area equal to the projection-surface area of a TiB₂ aggregate. Furthermore, by calculating the mean average of the circle-equivalent diameters of the obtained TiB₂ aggregates, the average value of the circle-equivalent diameters of the TiB₂ aggregates having a projection-surface area in the 95^th percentile or higher can be obtained.

[0053] The content of the TiB₂ particles in the crystal-grain refining agent can be set to, for example, 0.5 mass% or more and 3.2 mass% or less. In this situation, the average value of the center-to-center distances of adjacent TiB₂ particles tends to become large. As a result, the effect of curtailing the aggregation of the TiB₂ particles when the crystal-grain refining agent has been melted in the melt can be exhibited more reliably.

[0054] In the above-mentioned manufacturing method, the above-mentioned crystal-grain refining agent is melted in the melt of an aluminum alloy having a desired chemical composition. At this time, the melt may be stirred as needed to evenly disperse the TiB₂ particles in the crystal-grain refining agent in the melt.

[0055] In the manufacturing method, after the crystal-grain refining agent has been melted in the melt, the aluminum-alloy ingot can be manufactured by casting the melt. The casting method is not particularly limited; for example, methods such as semicontinuous casting, continuous casting, and the like can be used. An aluminum-alloy ingot manufactured by these methods can be used in the manufacture of flattened materials such as rolled plates, extruded materials, and the like. In addition, by cooling the aluminum-alloy melt described above after the melt has been poured into a mold or a sand mold, it is also possible to obtain an ingot having a desired article shape or a shape close to the desired article shape.

[0056] In the above-mentioned manufacturing method, after the crystal-grain refining agent has been melted into the melt, it is preferable to cast the melt within 30 min. The specific gravity of the TiB₂ particles is greater than that of the melt. Consequently, when the elapsed time since the point in time when the crystal-grain refining agent was melted into the melt becomes long, the TiB₂ particles precipitate owing to their intrinsic weight, and the TiB₂ particles tend to aggregate at the lower portion of the crucible. As a result, there is a risk that coarse TiB₂ aggregates will tend to be formed. By setting the time from the point in time at which the crystal-grain refining agent was melted into the melt until the casting to within 30 min, such problems can be avoided more easily.

[0057] When viewed from another viewpoint, it is noted that the aluminum-alloy ingot and the manufacturing method thereof described above can also be understood as the invention of a crystal-grain refining agent in which the distribution state of TiB₂ particles is specified.

[0058] That is, a first aspect of the crystal-grain refining agent comprises:

a base composed of aluminum; and

TiB₂ particles, which are present in the base;

wherein the average value of center-to-center distances of the TiB₂ particles that are adjacent in an arbitrary cross section is 0.60 µm or more.

[0059] In addition, a second aspect of the crystal-grain refining agent comprises:

a base composed of aluminum; and

TiB₂ aggregates, in which TiB₂ particles are aggregated and present in the base; and

in the situation in which the projection-surface areas of 2,000 or more of the TiB₂ aggregates have been measured in the state in which the TiB₂ aggregates are exposed from the base, the TiB₂ aggregates have a particle-size distribution in which the average value of the circle-equivalent diameters of the TiB₂ aggregates, which have a projection-surface area in the 95^th percentile or higher, is 3.0 µm or less.

[0060] It is assumed that such a crystal-grain refining agent can be manufactured by, for example, the following manufacturing method.

[0061] That is, in the manufacturing method of the crystal-grain refining agent, a melt of aluminum, which will become the base, is prepared;

the TiB₂ particles are dispesed in the melt by blowing the TiB₂ particles, together with an inert gas, into the melt; and

the melt is subsequently solidified. The crystal-grain refining agent can be obtained by the above.

[0062] It is preferable to use TiB₂ particles having a narrow particle-size distribution range as the TiB₂ particles that are blown into the melt, and it is more preferable to use TiB₂ particles having a particle-size standard deviation of 0.5 µm or less. In addition, for example, nitrogen gas, argon gas, or the like can be used as the inert gas. It is noted that the standard deviation of the particle sizes of the TiB₂ particles is a value that is calculated based on the particle-size distribution on a volume basis. To acquire the particle-size distribution on a TiB₂-particle volume basis, specifically, a laser-diffracting, particle-size-distribution measuring apparatus can be used.

(Working Examples)

[0063] Working examples of the aluminum-alloy ingot and the manufacturing method thereof will be explained below. It is noted that the specific aspects of the aluminum-alloy ingot and the manufacturing method thereof according to the present invention are not limited to the aspects of the working examples, and the configuration can be modified as appropriate within a range that does not impair the gist of the present invention.

(Working Examples 1, 2 and Comparative Examples 1, 2)

[0064] Working Examples 1, 2 and Comparative Examples 1, 2 are examples of aluminum-alloy ingots composed of pure aluminum. In these examples, first, a melt was prepared by melting aluminum metal having a purity of 99.7 mass%. After the temperature of the melt was set to 718°C, a crystal-grain refining agent, in which TiB₂ particles are dispersed in a base composed of aluminum, was added such that the boron atoms reached 10 mass ppm.

[0065] Crystal-grain refining agents used in the present example specifically had a chemical composition containing Ti: 1.0 mass% or more and 5.5 mass% or less and B: 0.1 mass% or more and 1.5 mass% or less, the remainder being Al and unavoidable impurities, and had a base composed of aluminum and TiB₂ particles present in the base. Some of the TiB₂ particles existed in an unaggregated state, the remainder being composed of TiB₂ aggregates.

[0066] The center-to-center distances between adjacent TiB₂ particles in an arbitrary cross section of the crystal-grain refining agent of the present example are the values shown in Table 1. In addition, in the situation in which the projection-surface areas of 2,000 or more TiB₂ aggregates were measured in the state in which the TiB₂ aggregates were exposed from the base, the TiB₂ aggregates in the crystal-grain refining agent had particle-size distributions in which the average values of the circle-equivalent diameters of the TiB₂ aggregates, which have a proj ection-surface area in the 95^th percentile or higher, are the values shown in Table 1.

[0067] After the crystal-grain refining agent was added into the melt, the melt was stirred for 30 seconds using a graphite rod in the state in which the temperature of the melt was maintained at 718°C, and the crystal-grain refining agent was sufficiently melted. In addition, at the point in time at which 9 minutes 15 seconds had elapsed since the point in time when the crystal-grain refining agent was added, the melt was once again stirred for 15 seconds using a graphite rod.

[0068] After the second stirring was completed, a ladle, which was made of iron and prepared in accordance with the AA-TP1 standard, was immersed into the melt, and the cup portion of the ladle was filled with the melt. Furthermore, at the point in time when 10 min had elapsed since the point in time when the crystal-grain refining agent was added, the ladle was lifted up from the melt, thereby scooping up the melt in the cup portion. Subsequently, the cup portion of the ladle was cooled using a water-cooling apparatus that is compliant with the AA-TP1 standard, and the melt was caused to solidify. The aluminum-alloy ingots for Working Examples 1, 2 and Comparative Examples 1, 2 were obtained by the above. These aluminum-alloy ingots all exhibited a circular, truncated-cone shape.

[0069] The methods of calculating the average values of the circle-equivalent diameters and the average values of the circularities of the TiB₂ aggregates in the aluminum-alloy ingots of Working Examples 1, 2 and Comparative Examples 1, 2 were as follows. These values for the aluminum-alloy ingots of Working Examples 1, 2 and Comparative Examples 1, 2 are shown in Table 1.

• Average Value of Circle-Equivalent Diameters and Average Value of Circularities of TiB₂ Aggregates

[0070] The aluminum-alloy ingot was cut at a cross section, from among the circular end surfaces of the aluminum-alloy ingot, at which the height from the plane having a small diameter (that is, the plane that is adjacent to the bottom surface of the ladle) was 38 mm, thereby exposing the cut surface. After this cut surface was polished, aluminum matrix was removed by performing dip etching, thereby exposing the entirety of the TiB₂ aggregates.

[0071] Subsequently, enlarged photographs of the TiB₂ aggregates were taken using an electron microscope. As one example, FIG. 1 shows an enlarged photograph of TiB₂ aggregates contained in an aluminum-alloy ingot of Working Example 1, and FIG. 2 shows an enlarged photograph of TiB₂ aggregates contained in an aluminum-alloy ingot of Comparative Example 2. As shown in FIG. 1 and FIG. 2, a TiB₂ aggregate 2 in the aluminum matrix 1 exhibits a lump shape, in which numerous TiB₂ particles 3 are aggregated.

[0072] The surface area of each of the TiB₂ aggregates 2 in the enlarged photographs was calculated using image-analyzing software, and the circle-equivalent diameter of each TiB₂ aggregate was calculated based on the surface area of the TiB₂ aggregate 2. The above operation was performed on eight randomly selected TiB₂ aggregates, and the value of the arithmetic mean of these circle-equivalent diameters was taken as the average value of the circle-equivalent diameters of the TiB₂ aggregates. Table 1 shows the average value of the circle-equivalent diameters of the TiB₂ aggregates for each of the aluminum-alloy ingots.

[0073] In addition, the circumferential length, i.e., the length of the contour, of each of the TiB₂ aggregates 2 in the enlarged photograph described above was calculated using image-analyzing software. Furthermore, the circularity of each of the TiB₂ aggregates was calculated based on the surface area and the circumferential length of each of the TiB₂ aggregates 2. The above operation was performed on three or more randomly selected TiB₂ aggregates, and the value of the arithmetic mean of these circularities was taken as the average value of the circularities of the TiB₂ aggregates. Table 1 shows the average value of the circularities of the TiB₂ aggregates for each of the aluminum-alloy ingots.

(Working Example 3)

[0074] Working Example 3 is an example of an aluminum-alloy ingot composed of an A3000-series alloy. In Working Example 3, first, a melt composed of an A3000-series alloy was prepared using a heating furnace, after which a crystal-grain refining agent was added to the melt such that the boron atoms reached 10 mass ppm. The crystal-grain refining agent used in the present example is the same as that in Working Example 1 except for: the point that it had a chemical composition containing Ti: 5.0 mass% and B: 1.0 mass%, the remainder being Al and unavoidable impurities; the point that the center-to-center distances between adjacent TiB₂ particles at an arbitrary cross section of the crystal-grain refining agent are the values shown in Table 1; and the point that the average value of the circle-equivalent diameters of the TiB₂ aggregates, which have a projection-surface area in the 95^th percentile or higher, is the value shown in Table 1.

[0075] After the crystal-grain refining agent was added to the melt, the melt was stirred for 30 seconds using a graphite rod. Subsequently, the melt was cast using a DCcasting method. Thus, the aluminum-alloy ingot of Working Example 3 was obtained. The aluminum-alloy ingot of Working Example 3 exhibits a rectangular-parallelepiped shape.

(Working Example 4)

[0076] Working Example 4 is an example of an aluminum-alloy ingot composed of an A1000-series aluminum. In Working Example 4, the melt of the aluminum alloy was cast using the same method as that in Working Example 3, except for the point that the melt composed of the A1000-series aluminum was prepared using a heating furnace. Thereby, the aluminum-alloy ingot of Working Example 4 was obtained. The aluminum-alloy ingot of Working Example 4 exhibited a rectangular-parallelepiped shape, the same as in Working Example 3.

[0077] With regard to the aluminum-alloy ingots of Working Example 3 and Working Example 4, the methods of calculating the average value of the circle-equivalent diameters and the average value of the circularities of the TiB₂ aggregates were as follows.

• Average Value of Circle-Equivalent Diameters and Average Value of Circularities of the TiB₂ Aggregates

[0078] The aluminum-alloy ingot is cut and a test piece is collected from a center portion in the width direction and the thickness direction. A surface of the test piece is polished, after which the test piece is subject to dip etching to remove aluminum matrix, thereby exposing the entirety of the TiB₂ aggregates. Thereafter, the average value of the circle-equivalent diameters and the average value of the circularities of the TiB₂ aggregates should be calculated using the same method as in Working Example 1, etc. Table 1 shows these values for the aluminum-alloy ingots of Working Examples 3, 4.

Table 1

	Alloy Type	Crystal-Grain Refining Agent		TiB2 Aggregates in Ingot
		Center-to-Center Distances between Adjacent TiB2 Particles (µm)	Average Value of Circle-Equivalent Diameters of TiB2 Aggregates Having a Projection Surface in the 95th Percentile or Higher (µm)	Average Value of Circle-Equivalent Diameters (µm)	Average Value of Circularities
Working Example 1	Al: 99.7 mass%	0.68	2.95	2.4	0.3
Working Example 2	Al: 99.7 mass%	0.62	2.64	2.2	0.2
Working Example 3	A3000-series alloy	0.68	2.95	2.6	0.3
Working Example 4	A1000-series aluminum	0.63	2.73	2.3	0.2
Comparative Example 1	Al: 99.7 mass%	0.58	3.01	3.8	0.2
Comparative Example 2	Al: 99.7 mass%	0.48	3.97	3.5	0.2

[0079] As shown in Table 1, the average value of the circle-equivalent diameters and the average value of the circularities of the TiB₂ aggregates contained in the aluminum-alloy ingots of Working Examples 1-4 were within the above-mentioned specific ranges, respectively. For this reason, with regard to these aluminum-alloy ingots, the crystal particles in the aluminum matrix were sufficiently refined, there were few coarse TiB₂ aggregates, and the occurrence of linear defects when performing processing such as rolling could be curtailed.

[0080] The average value of the circle-equivalent diameters of the TiB₂ aggregates contained in the aluminum-alloy ingots of Comparative Examples 1, 2 were above the above-mentioned specific ranges, respectively. Consequently, these aluminum-alloy ingots tended to contain coarse TiB₂ aggregates, and linear defects tended to occur during flattening work.

Claims

1. An aluminum-alloy ingot comprising:

an aluminum matrix; and

TiB₂ aggregates, which are dispersed in the aluminum matrix and in which TiB₂ particles are aggregated;

wherein the average value of the circle-equivalent diameters of the TiB₂ aggregates in the state in which the TiB₂ aggregates are exposed from the aluminum matrix is 3.0 µm or less and the average value of the circularities is 0.20 or more.

2. The aluminum-alloy ingot according to claim 1, wherein the content of the TiB₂ aggregates is 0.0001 mass% or more and 0.0010 mass% or less based on the boron atoms.

3. A method of manufacturing the aluminum-alloy ingot according to claim 1 or 2, comprising:

a melting step, in which a crystal-grain refining agent, in which a base composed of aluminum contains TiB₂ particles and the average value of the center-to-center distances between adjacent TiB₂ particles in an arbitrary cross section is 0.60 µm or more, is melted in a melt of aluminum alloy; and

a casting step, in which, after the melting step, the melt is cast.

4. A method of manufacturing the aluminum-alloy ingot according to claim 1 or 2, comprising:

a melting step, in which a crystal-grain refining agent, in which a base composed of aluminum contains TiB₂ aggregates in which TiB₂ particles are aggregated, is melted in a melt of an aluminum alloy; and

a casting step, in which, after the melting step, the melt is cast;

wherein, in the situation in which projection-surface areas of 2,000 or more TiB₂ aggregates were measured in the state in which TiB₂ aggregates were exposed from the base, the TiB₂ aggregates in the crystal-grain refining agent have a particle-size distribution in which the average value of the circle-equivalent diameters of TiB₂ aggregates, which have a projection-surface area in the 95^th percentile or higher, is 3.0 µm or less.

5. The method of manufacturing the aluminum-alloy ingot according to claim 3 or 4, wherein the melt is cast within 30 min after the crystal-grain refining agent has been melted in the melt.

Drawing