Method for producing al alloy for casting

Abstract

The present invention addresses the problem of providing an Al alloy for casting, the Al alloy being capable of achieving both high mechanical properties and cost reduction.　An Al alloy for casting according to the present invention contains 8.0-10.0% by mas of Si, 0.25-0.40% by mass of Mg, 0.30-0.50% by mass of Fe, 0.28-0.52% by mass of Mn, 0.08-0.22% by mass of Cu, 0.04-0.15% by mass of Ti, and 0.0075-0.028% by mass of Sr, with the balance being made up of Al. The sum of the content of Fe and the content of Mn is limited to 1.0% by mass or less. Sr is not added in the melting step but is added in the molten metal treatment step for removing Al oxide and an H2 gas from the molten Al alloy obtained in the melting step.

Description

Manufacturing method of aluminum alloy for casting

[0001] The present invention relates to a method for producing an Al alloy for casting.

[0002] Aluminum alloys (hereinafter referred to as "Al alloys") have traditionally been widely used as materials for components in automobiles, industrial machinery, aircraft, household appliances, and various other fields due to their lightweight, excellent formability, and mass-producibility. For example, large castings in the automotive field, such as suspension towers, side beams, and battery cases, are typically produced by die-casting using AlSi10Mg or similar (Al-Si-Mg alloys), followed by T7 treatment (solution treatment by heating and quenching, and stabilization treatment (overaging)). Large castings produced in this manner have an elongation of 8% or more, a tensile strength of 200 to 230 MPa, and a yield strength of 120 to 140 MPa (which can also be referred to as 2% proof stress). Incidentally, for structural parts in the automotive field, giga die-casting (large die-castings) requires an elongation of 8% or more, a tensile strength of 180 MPa or more, and a yield strength of 120 MPa or more.

[0003] Japanese Patent Application Publication No. 11-12673

[0004] In this type of Al alloy casting, as an important safety component, high mechanical properties are required, but at the same time, strict cost reduction is also required. One cost reduction measure is, for example, omitting heat treatment after casting. However, when an Al-Si-Mg alloy, which is classified as a heat-treatable alloy, is not heat-treated (i.e., without heat treatment), the mechanical properties such as elongation, tensile strength, and yield strength do not reach the required levels, making it difficult to achieve both high mechanical properties and cost reduction.

[0005] For example, Patent Document 1 discloses a method for producing an Al alloy casting that has high mechanical properties even in an as-cast state (without heat treatment after casting), using an Al alloy containing 4 to 8 wt % Si, 0.4 to 1.0 wt % Cu, 0.2 to 0.4 wt % Mg, 0.05 to 0.3 wt % Fe, 0.002 to 0.02 wt % Sr, 0.0005 to 0.1 wt % Zr, with the balance being substantially Al, and that satisfies the condition Cu+2.5Mg≧1.25 wt %.

[0006] However, the Al alloy casting obtained by the manufacturing method of Patent Document 1 has a high Cu content of 0.4 wt % or more, and therefore has the disadvantage of being poor in corrosion resistance.

[0007] The present invention has as its technical object the provision of a method for producing an Al alloy for casting which has been improved by examining the current situation as described above.

[0008] The method for producing a casting Al alloy according to the present invention comprises 8.0 to 10.0 mass% Si, 0.25 to 0.40 mass% Mg, 0.30 to 0.50 mass% Fe, 0.28 to 0.52 mass% Mn, 0.08 to 0.22 mass% Cu, 0.04 to 0.15 mass% Ti, and 0.0075 to 0.028 mass% Sr, with the balance being Al.

[0009] The sum of the contents of Fe and Mn is limited to 1.0 mass % or less. Sr is not added in the melting step, and Sr is added to the molten Al alloy obtained in the melting step to remove Al oxides and H. ２ It is added during the molten metal treatment process to remove gases.

[0010] In the method for producing an Al alloy for casting according to the present invention, the Mg may not be added in the melting step, but may be added in the molten metal treatment step.

[0011] According to the present invention, even when giga die casting (large die casting products) or large low-pressure cast parts are cast in an as-cast state (without post-casting heat treatment), an Al alloy for casting and an Al alloy casting product exhibiting good castability and mechanical properties (elongation, tensile strength, yield strength, etc.) can be obtained, and costly heat treatment can be omitted, making it possible to provide an Al alloy for casting and an Al alloy casting product at low cost. In particular, in the present invention, Sr is not added in the melting step, and Sr is used to reduce Al oxides and H in the molten Al alloy obtained in the melting step. ２ Since Sr is added during the molten metal treatment process for removing gases, it is possible to prevent Sr from being burned by the heat of the molten Al alloy, and the added effect of Sr can be reliably exerted in Al alloys for casting and Al alloy castings, thereby improving the quality and yield of Al alloys for casting and Al alloy castings.

[0012] Furthermore, Mg is not added in the melting process, and the Al oxides and H in the molten Al alloy obtained in the melting process are also added. ２ If Mg is added during the molten metal treatment process to remove gases, it is possible to prevent Mg from being burned by the heat of the molten Al alloy, and the effects of adding Mg can be reliably exerted in Al alloys for casting and Al alloy castings. Therefore, the quality and yield of Al alloys for casting and Al alloy castings can be improved. Note that the present invention does not completely exclude heat treatment after casting. In applications requiring higher mechanical properties, for example, heat treatment of parts after large-scale low-pressure casting can be performed without any problems, resulting in improved performance.

[0013] FIG. 1 is a flow diagram showing the manufacturing process of the present invention. FIG. 2 is a diagram showing the composition ranges of competitor's alloys and the present alloy, and the component compositions of Examples and Comparative Examples. FIG. 3 is a diagram showing the results of measuring the tensile strength, yield strength, and elongation of castings obtained by die casting. FIG. 4 is a graph showing the relationship between the temperature of the molten Al alloy during die casting and the tensile strength and yield strength of the casting. FIG. 5 is a graph showing the relationship between the temperature of the molten Al alloy during die casting and the elongation of the casting. FIG. 6 is a graph showing the relationship between the mold temperature during die casting and the tensile strength and yield strength of the casting. FIG. 7 is a graph showing the relationship between the mold vacuum during die casting and the tensile strength and yield strength of the casting. FIG. 8 is a graph showing the relationship between the mold vacuum during die casting and the elongation of the casting. FIG. 9 is a diagram showing the component compositions of a heat-treatment-less casting and a T6-treated casting produced by low-pressure casting. FIG. 1 is a diagram showing the results of measuring the tensile strength, yield strength, and elongation of a heat-treatment-free casting obtained by a low-pressure casting method and a T6-treated casting.

[0014] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, specific embodiments of the present invention will be described with reference to the accompanying drawings.

[0015] As shown in FIG. 2 , the composition of the casting Al alloy and Al alloy casting according to the present invention includes, by mass, 8.0 to 10.0% Si (silicon), 0.25 to 0.40% Mg (magnesium), 0.30 to 0.50% Fe (iron), 0.28 to 0.52% Mn (manganese), 0.08 to 0.22% Cu (copper), 0.04 to 0.15% Ti (titanium), and 0.0075 to 0.028% Sr (strontium), with the balance being Al (aluminum) and inevitable impurities.

[0016] Silicon is an important alloy component (element) that contributes to improving castability, particularly fluidity. As described above, the Si content of the entire cast aluminum alloy is preferably in the range of 8.0% by mass to 10.0% by mass. If the Si content is too low (less than 8.0% by mass), the fluidity is insufficient, making it difficult to ensure fluidity, and casting cracks are likely to occur. Conversely, if the aluminum alloy contains excessive silicon (more than 10.0% by mass), the elongation of the aluminum alloy casting will decrease.

[0017] Mg is mainly dissolved in the Al matrix of the casting Al alloy, or in Mg ２ Mg exists as Si and is an alloying component effective in improving tensile strength and yield strength. The Mg content of the entire casting Al alloy is preferably 0.25% by mass or more and 0.40% by mass or less. If the Mg content is within the above range, the mechanical properties of the Al alloy casting, such as tensile strength and yield strength, can be improved without significantly affecting the castability or elongation of the Al alloy casting. If the Mg content is too low (less than 0.25% by mass), seizure to the mold tends to occur, while if the Mg content is too high (more than 0.40% by mass), the elongation of the Al alloy casting tends to decrease.

[0018] Fe is an alloy component that acts to prevent seizure on the mold during casting. The Fe content of the entire cast Al alloy is preferably in the range of 0.30% by mass to 0.50% by mass. If the Fe content is too high (exceeding 0.50% by mass), Al-Si-Fe needle crystals (ternary compounds) are formed, significantly reducing the elongation of the Al alloy casting. If the Fe content is 0.50% by mass or less, the formation of needle crystals is suppressed, thereby suppressing the adverse effect on the elongation of the Al alloy casting.

[0019] Mn is an alloying component added to prevent seizure on the mold during casting and to suppress the formation of needle-like crystals consisting of Al-Si-Fe, thereby ensuring the elongation of the Al alloy casting. The Mn content of the entire casting Al alloy is preferably in the range of 0.28% by mass to 0.52% by mass. Setting the Mn content to 0.39% by mass or less improves mold releasability. Furthermore, if the Mn content exceeds 0.52% by mass, Al crystal grains become coarse and the elongation decreases. Therefore, if Mn is added in the range of 0.39% by mass or less, the mold releasability can be improved while suppressing the decrease in elongation of the Al alloy casting.

[0020] As described above, Mn, in combination with Fe, acts to suppress the formation of Al-Si-Fe-based needle crystals. According to research conducted by the present inventors, it was found that the effect of suppressing the formation of needle crystals is extremely high when the sum of the contents of Fe and Mn is 1.0 (mass%) or less (Fe+Mn≦1.0).

[0021] Cu is an alloying element that dissolves in the Al matrix and is effective in improving the mechanical properties of Al alloy castings, particularly tensile strength and yield strength. The Cu content of the entire Al alloy for casting is preferably in the range of 0.08% by mass to 0.22% by mass. Cu content within this range effectively exerts its solid solution strengthening effect in the Al alloy without impairing corrosion resistance. If the Cu content is too high (more than 0.22% by mass), the corrosion resistance and elongation of the Al alloy casting will be reduced.

[0022] Ti is an alloying component effective in refining Al crystal grains. If the Ti content is too low (less than 0.04% by mass), the Al crystal grains become coarse, reducing elongation, tensile strength, and yield strength. On the other hand, if the Ti content is too high (more than 0.15% by mass), Si crystal grains (eutectic Si) are concentrated too much at the grain boundaries, resulting in defects, which also reduces elongation, tensile strength, and yield strength. Therefore, the Ti content of the entire cast Al alloy is preferably in the range of 0.04% to 0.15% by mass. This allows the degree of Al crystal grain refinement to be adjusted to a degree that allows dendritic shapes to remain within the Al crystal grains, and the Si crystal grains are finely dispersed throughout the cast structure, resulting in a stable and uniform cast structure.

[0023] Sr is an alloying element that refines Si crystal grains and is effective in improving elongation, tensile strength, and yield strength even in the as-cast state (without post-cast heat treatment). This effect is particularly pronounced when 0.0075% by mass or more of Sr is added. In contrast, excessive Sr addition exceeding 0.028% by mass produces Al-Si-Sr ternary compounds, resulting in over-refining and increased susceptibility to casting defects, leading to reduced mechanical properties such as elongation, tensile strength, and yield strength. The addition of Sr has the effect of spheroidizing and refining Si crystal grains in the as-cast state. Since spheroidizing Si crystal grains suppresses stress concentration, including Sr in the composition of casting Al alloys allows for the omission of heat treatments such as solution treatment, resulting in Al alloy castings that retain sufficient mechanical properties such as elongation, tensile strength, and yield strength, even in the as-cast state without heat treatment. Therefore, the Sr content in the entire Al alloy for casting is preferably in the range of 0.0075 mass % to 0.028 mass %.

[0024] In the present invention, alloying elements such as Zn, Ni, Sn, Pb, Ca, Cr, and Cd are treated as inevitable impurities. The Zn content is limited to 0.15 mass% or less because, like Cu, excessive Zn content impairs corrosion resistance. Ca is a harmful alloying element that destabilizes the cast structure and deteriorates the flow of molten metal, so its content is limited to 0.005 mass% or less. It is desirable to keep the contents of other inevitable impurities to 0.1 mass% or less.

[0025] There is no particular limitation on the purity (quality) of the Al raw material used. Sufficient refining makes it easier to obtain Al alloy castings with good mechanical properties. However, since refining requires significant costs, it is acceptable for other unavoidable impurities to be present. For example, high-purity Al ingots and recycled materials may be blended in appropriate proportions.

[0026] In the production of the Al alloy for casting and Al alloy castings of the present invention, raw materials containing the major alloying elements Al, Si, Mg, Fe, Mn, Cu, Ti, and Sr, except for Mg and Sr, are first charged into a melting and holding furnace and melted (melting step, see FIG. 1). Here, a slag removal process is performed by adding a flux. The temperature of the molten Al alloy during the melting step is controlled within the range of 730°C ± 10°C. Raw materials containing Mg and Sr are not charged into the melting and holding furnace.

[0027] Next, the molten Al alloy melted in the melting and holding furnace was transferred to a ladle preheated to a temperature within the range of 750°C ± 50°C, and N ２ By rotating the bubbling gas, Al oxide and H in the molten Al alloy are separated. ２ Gas is removed (molten metal treatment process). The molten metal treatment process is carried out for approximately 10 to 15 minutes, depending on the amount of molten Al alloy. During the molten metal treatment process, raw materials containing Mg and Sr (secondary alloy ingots) are added to the ladle so that the composition of the Al alloy for casting is in the predetermined ratio described above (secondary alloy addition process). After 5 minutes from the start of the molten metal treatment process, flux is added, and N ２By rotating the gas through the bubbling, the Al oxide and H in the molten Al alloy are removed. ２ The gas is removed.

[0028] If the processing time of the molten metal treatment step is, for example, 12 minutes, it is necessary to ensure a time sufficient to uniformly stir Mg and Sr into the molten Al alloy. Furthermore, controlling the molten metal temperature is extremely important, and the temperature of the molten Al alloy after the molten metal treatment step is controlled within the range of 680°C ± 10°C. Since alloy components such as Mg and Sr are easily burned by the heat of the molten Al alloy, in the present invention, they are added in the molten metal treatment step rather than the melting step to prevent the burnout of Mg and Sr.

[0029] Examples of secondary alloy ingots include AlMn20 (containing 20% ​​by mass of Mn) and AlSr10 (containing 10% by mass of Sr). It goes without saying that AlSi50 (containing 50% by mass of Si), AlCu50 (containing 50% by mass of Cu), Mg99.9%, and AlTi5B1 (containing 5% by mass of Ti and 1% by mass of B) may be added to the ladle to adjust the alloy composition. A slag removal process may also be performed during the molten metal treatment process. While it is acceptable to add only the Sr-containing raw material during the molten metal treatment process and the Mg-containing raw material during the melting process, it is preferable to add both raw materials during the molten metal treatment process.

[0030] Next, the molten Al alloy refined through the melt treatment process is transferred from the ladle to a holding furnace (holding furnace process). The temperature of the molten Al alloy in the holding furnace is controlled within a range of 665°C ± 5°C. The refined molten Al alloy is then poured from the holding furnace into a predetermined mold (die) under pressure using a die-casting machine or low-pressure casting machine and solidified to form an Al alloy for casting or an Al alloy casting (casting process). Here, the mold temperature is set within a range of 180°C to 220°C so that the temperature of the molten Al alloy poured from the holding furnace into the mold is maintained at approximately 665°C ± 5°C. When die-casting is employed, the degree of vacuum in the mold is set within a range of 50 mbar to 100 mbar.

[0031] The aluminum alloy casting of the present invention can be produced by conventional die casting, low-pressure casting, or gravity casting. The present invention will be specifically described in the following examples. It should be noted that the present invention is not limited to the following examples, and various modifications are possible without departing from the spirit and scope of the present invention.

[0032] Figure 2 shows the composition ranges of a casting Al alloy from another company (hereinafter referred to as the "Company's Alloy") and the casting Al alloy of the present invention (hereinafter referred to as the "Alloy"), as well as the component compositions of Examples and Comparative Examples. Figure 3 shows the results of measurements of tensile strength, yield strength, and elongation as characteristic values ​​for test pieces cut from large castings (giga die castings) obtained by die casting using the same mold for the Company's Alloy, Examples, and Comparative Examples. The values ​​in Figure 3 are average values ​​measured using five test pieces for each.

[0033] Figure 3 also shows the results of evaluations of melt flow during die casting, seizure on the mold, and cracking of the casting. The competitor's alloy shown in Figures 2 and 3 is a casting Al alloy called C370 manufactured by Alcoa, which is considered to correspond to AlSi10MnMg. Comparative Example 1 is a case where the Sr content is near the lower limit of the alloy of the present invention, and Comparative Example 2 is a case where the Sr content is near the upper limit of the alloy of the present invention. The competitor's alloy, Examples, and Comparative Examples are all in an as-cast state without undergoing heat treatment.

[0034] As is clear from Figure 3, Examples 1 and 2 of the present invention both exhibited a tensile strength of approximately 280 MPa, a yield strength of approximately 140 MPa, and an elongation of approximately 14% in the as-cast state, significantly exceeding the mechanical properties of the competitor's alloys and Comparative Examples 1 and 2. The mechanical properties of Examples 1 and 2 far exceed the required standards for structural parts in the automotive industry, for example, and were found to be fully durable despite being in the as-cast state without heat treatment. Furthermore, Examples 1 and 2 exhibited good melt flow during casting and no seizures. Furthermore, no cracks were observed in the large-scale castings, demonstrating extremely good castability overall. Note that in Comparative Examples 1 and 2, occasional cracks were observed in the large-scale castings.

[0035] Figure 4 shows the relationship between the temperature of the molten Al alloy during die casting in the present invention and the tensile strength and yield strength of the Al alloy casting. As can be seen from Figure 4, if the temperature of the molten Al alloy is within the range of 665°C to ±5°C, the Al alloy casting can achieve extremely high tensile strength and yield strength. Figure 5 shows the relationship between the temperature of the molten Al alloy during die casting in the present invention and the elongation of the Al alloy casting. In this case as well, if the temperature of the molten Al alloy is within the range of 660°C to 670°C, the Al alloy casting can achieve extremely high elongation.

[0036] Figure 6 shows the relationship between the mold temperature during die casting and the tensile strength and yield strength of the aluminum alloy casting according to the present invention. As can be seen from Figure 6, when the mold temperature is within the range of 180°C to 220°C, the aluminum alloy casting can achieve extremely high tensile strength and yield strength. Figure 7 shows the relationship between the mold temperature during die casting and the elongation of the aluminum alloy casting according to the present invention. In this case, too, when the mold temperature is within the range of 180°C to 220°C, the aluminum alloy casting can achieve extremely high elongation. Although the details are unclear, it is believed that a mold temperature within the range of 180°C to 220°C makes it easier to maintain the aluminum alloy molten temperature within the range of 660°C to 670°C during die casting. The mold temperature prevents the aluminum alloy molten temperature from decreasing, and it is believed that the aluminum alloy casting has excellent mechanical properties because the aluminum alloy molten temperature is within the range of 660°C to 670°C.

[0037] Figure 8 shows the relationship between the degree of mold vacuum during die casting in the present invention and the tensile strength and yield strength of the Al alloy casting. As can be seen from Figure 8, if the degree of mold vacuum is within the range of 50 mbar to 100 mbar, the Al alloy casting can achieve extremely high tensile strength and yield strength. Figure 9 shows the relationship between the degree of mold vacuum during die casting in the present invention and the elongation of the Al alloy casting. In this case as well, if the degree of mold vacuum is within the range of 50 mbar to 100 mbar, the Al alloy casting can achieve extremely high elongation.

[0038] As can be seen from the above explanation, according to the present invention, even in the as-cast state (without heat treatment after casting), it is possible to obtain an Al alloy for casting, and thus an Al alloy casting, which exhibits castability equivalent to that of an Al-Si-Mg alloy and good mechanical properties (elongation, tensile strength, yield strength, etc.), and it is possible to omit costly heat treatment, thereby providing an Al alloy for casting and an Al alloy casting at low cost.

[0039] In particular, in the present invention, raw materials (secondary alloy ingots) containing Mg and Sr are added during the molten metal treatment process so that the composition of the cast Al alloy is in the above-mentioned predetermined ratio, which prevents alloy components such as Mg and Sr from being burned by the heat of the molten Al alloy and ensures that the effects of adding Mg and Sr are fully exhibited in the cast Al alloy and Al alloy castings, thereby improving the quality and yield of the cast Al alloy and Al alloy castings.

[0040] The present invention does not intend to completely exclude heat treatment after casting. For example, in low-pressure casting applications that require higher mechanical properties (particularly tensile strength and yield strength), T4 treatment (natural aging after solution treatment), T5 treatment (artificial aging hardening after cooling from high-temperature processing), T6 treatment (artificial aging hardening after solution treatment), or T7 treatment may be performed.

[0041] Figure 10 shows the chemical compositions of Example 3 (heat-treated) and Example 4 (T6-treated) of the present alloy. The chemical compositions of Examples 3 and 4 are identical. Figure 11 shows the results of measurements of tensile strength, yield strength, and elongation as quality values ​​for test pieces cut from a heat-treated casting (Example 3) and a T6-treated casting (Example 4) obtained by low-pressure casting using the same mold. The values ​​in Figure 11 are average values ​​measured using five test pieces for each. Figure 11 also shows the results of evaluations of melt flow during die casting, mold seizure, and cracking of the casting.

[0042] As is clear from Figure 11, Example 3 of the present invention achieved a tensile strength exceeding 180 MPa and a yield strength exceeding 120 MPa even in the as-cast state during low-pressure casting. However, the elongation rate was slightly low at 5%. Example 3 exhibited good fluidity during casting, no seizure occurred, and no cracks were observed in the casting. Overall, Example 3 exhibited extremely good castability. The results of Example 3 were found to be nearly equivalent to the performance (mechanical properties) of AC4C after heat treatment as specified in Japanese Industrial Standard JIS H5302.

[0043] In contrast, Example 4, in which a large, low-pressure cast part was subjected to T6 treatment, had a tensile strength of 300 MPa, a yield strength of 242 MPa, and an elongation of 8.0%, which significantly exceeded the mechanical properties of Example 3. The mechanical properties of Example 4 far exceed the performance (mechanical properties) of AC4C after heat treatment, and far exceed the required level for structural parts in the automotive field, for example. It was found that, depending on the application, heat treatment can make the part fully durable for use. Furthermore, Example 4 also had good fluidity during casting, no seizure occurred, and no cracks were observed in the casting.

Claims

1. An alloy containing 8.0 to 10.0 mass% Si, 0.25 to 0.40 mass% Mg, 0.30 to 0.50 mass% Fe, 0.28 to 0.52 mass% Mn, 0.08 to 0.22 mass% Cu, 0.04 to 0.15 mass% Ti, and 0.0075 to 0.028 mass% Sr, with the balance being Al, the sum of the Fe and Mn contents being limited to 1.0 mass% or less, the Sr not being added in the melting step, and the Al oxides and H in the molten Al alloy obtained in the melting step being not added in the melting step. ２ A manufacturing method for casting aluminum alloys, in which gas is added during the molten metal treatment process to remove gas.

2. The method for producing an Al alloy for casting according to claim 1, wherein the Mg is not added in the melting step, but is added in the molten metal treatment step.

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