METHOD FOR PRODUCING CASTING Al ALLOY

Abstract

To provide a casting Al alloy capable of achieving both high mechanical properties and a cost reduction.SOLUTION: A cast aluminum alloy of the present invention contains 8.0 to 10.0 mass% of Si, 0.25 to 0.40 mass% of Mg, 0.30 to 0.50 mass% of Fe, 0.28 to 0.52 mass% of Mn, 0.08 to 0.22 mass% of Cu, 0.04 to 0.15 mass% of Ti, and 0.0075 to 0.028 mass% of Sr, with the balance being Al. The total content of Fe and Mn is restricted to 1.0 mass% or less. Sr is not added during the melting step but during the melting treatment step to remove aluminum oxides and H2 gas from the aluminum alloy melt obtained in the melting step.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to an Al alloy for casting and an Al alloy casting using the same. [Background technology]

[0002] Conventionally, aluminum alloys (hereinafter referred to as Al alloys) are widely used as materials for components in automobiles, industrial machinery, aircraft, home appliances, and various other fields, because they are lightweight and have excellent formability and mass production. For example, large castings such as suspension towers, side beams, and battery cases in the automobile field are generally manufactured by die-casting using AlSi10Mg or the like (Al-Si-Mg alloys), followed by T7 treatment (solution treatment by heating and quenching, and stabilization treatment (overaging treatment)). The large castings manufactured in this way have an elongation rate of 8% or more, a tensile strength of 200 to 230 MPa, and a yield strength of 120 to 140 MPa (which may be called 2% proof stress). Incidentally, for structural parts in the automobile field, giga die casting (large die castings) requires an elongation rate of 8% or more, a tensile strength of 180 MPa or more, and a yield strength of 120 MPa or more. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 11-12673 Summary of the Invention [Problem to be solved by the invention]

[0004] Incidentally, this type of Al alloy casting is required to have high mechanical properties as an important safety part, and at the same time, strict cost reduction is also required. One of the cost reduction measures is, for example, omitting the heat treatment after casting. However, when an Al-Si-Mg alloy, which is classified as a heat-treatable alloy, is not heat-treated (no heat treatment is performed), the mechanical properties such as elongation, tensile strength, and yield strength do not reach the required level, so there is a problem that it is 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 4-8 wt% Si, 0.4-1.0 wt% Cu, 0.2-0.4 wt% Mg, 0.05-0.3 wt% Fe, 0.002-0.02 wt% Sr, 0.0005-0.1 wt% Zr, with the balance being substantially Al, and satisfying 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 a disadvantage of being poor in corrosion resistance. [Means for solving the problem]

[0007] The technical object of the present invention is to provide an improved Al alloy for casting, based on a consideration of the current situation as described above, and an Al alloy casting using the same.

[0008] The casting Al alloy according to the present invention contains 8.0-10.0 mass% Si, 0.25-0.40 mass% Mg, 0.30-0.50 mass% Fe, 0.28-0.52 mass% Mn, 0.08-0.22 mass% Cu, 0.04-0.15 mass% Ti, and 0.0075-0.028 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. Moreover, the Sr is not added in the melting step, but in the molten metal treatment step for removing Al oxides and H2 gas from the molten Al alloy obtained in the melting step.

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

[0011] The present invention also includes an Al alloy casting produced using the casting Al alloy described in claim 1 or 2. Effect of the Invention

[0012] According to the present invention, even if giga die casting (large die casting) or large low pressure casting parts are cast in an as-cast state (without heat treatment after casting), a casting Al alloy and an Al alloy casting exhibiting good castability and mechanical properties (elongation, tensile strength, yield strength, etc.) can be obtained, and costly heat treatment can be omitted, so that a casting Al alloy and an Al alloy casting can be provided at low cost. In particular, in the present invention, Sr is not added in the melting process, but is added in the molten metal treatment process for removing Al oxides and H2 gas from the molten Al alloy obtained in the melting process, so that Sr is prevented from being burned by the heat of the molten Al alloy, and the effect of adding Sr can be reliably exerted in the casting Al alloy and the Al alloy casting. Therefore, the quality and yield of the casting Al alloy and the Al alloy casting can be improved.

[0013] Furthermore, Mg is not added in the melting process, but is added in the molten metal treatment process for removing Al oxides and H2 gas from the molten Al alloy obtained in the melting process. This prevents Mg from being burned by the heat of the molten Al alloy, and ensures the effect of adding Mg in the Al alloy for casting and Al alloy castings. This improves the quality and yield of the Al alloy for casting and Al alloy castings. Note that the present invention does not exclude heat treatment after casting at all. In applications requiring higher mechanical properties, for example, heat treatment of parts after large-scale low-pressure casting is acceptable and improves performance. [Brief description of the drawings]

[0014] [Figure 1] FIG. 2 is a flow diagram showing the manufacturing process of the present invention. [Diagram 2] FIG. 2 is a diagram showing the composition ranges of alloys of other companies and the present invention, and the component compositions of examples and comparative examples. [Diagram 3] FIG. 1 is a diagram showing the results of measuring the tensile strength, yield strength, and elongation of a casting obtained by a die casting method. [Figure 4] 1 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. [Diagram 5] 1 is a graph showing the relationship between the temperature of molten Al alloy during die casting and the elongation rate of the casting. [Figure 6] 1 is a graph showing the relationship between the die temperature during die casting and the tensile strength and yield strength of the casting. [Figure 7] 1 is a graph showing the relationship between the die temperature during die casting and the elongation rate of a casting. [Figure 8] 1 is a graph showing the relationship between the degree of vacuum in a die during die casting and the tensile strength and yield strength of a casting. [Figure 9] 1 is a graph showing the relationship between the degree of vacuum in a die during die casting and the elongation rate of a casting. [Figure 10] FIG. 1 is a diagram showing the component compositions of a heat-treatment-free casting produced by a low-pressure casting method and a casting that has been T6-treated. [Figure 11]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. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, specific embodiments of the present invention will be described with reference to the drawings.

[0016] As shown in FIG. 2, the composition of the casting Al alloy and Al alloy casting of 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 remainder being Al (aluminum) and unavoidable impurities.

[0017] Silicon is an important alloy component (element) that contributes to improving castability, especially fluidity. As mentioned above, the Si content in the entire Al alloy for casting should be 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 and the fluidity cannot be ensured, and casting cracks are likely to occur. Conversely, if the alloy contains excessive Si exceeding 10.0% by mass, the elongation of the Al alloy casting will decrease.

[0018] Mg exists mainly in the form of a solid solution in the Al matrix in the casting Al alloy or as Mg2Si, and is an alloying component effective for improving tensile strength and yield strength. The content of Mg in 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 such as the tensile strength and yield strength of the Al alloy casting can be improved without significantly affecting the castability or the elongation of the Al alloy casting. If the Mg content is too low (less than 0.25% by mass), seizure to the mold is likely to occur, and if the Mg content is too high (more than 0.40% by mass), the elongation of the Al alloy casting tends to decrease.

[0019] Fe is an alloy component that acts to prevent seizure against the mold during casting. The Fe content in the entire Al alloy for casting is preferably in the range of 0.30% by mass to 0.50% by mass. If the Fe content is too high (over 0.50% by mass), Al-Si-Fe needle-like crystals (ternary compounds) are generated, significantly reducing the elongation of the Al alloy casting. If the Fe content is 0.50% by mass or less, the generation of needle-like crystals is suppressed, and the adverse effect on the elongation of the Al alloy casting is suppressed.

[0020] Mn is an alloying component added to prevent seizure against the mold during casting and to suppress the formation of needle-like crystals made of Al-Si-Fe, thereby ensuring the elongation of the Al alloy casting. The Mn content in the entire Al alloy for casting is preferably in the range of 0.28% by mass to 0.52% by mass. If the Mn content is set to 0.39% by mass or less, the mold releasability from the mold is improved. If the Mn content exceeds 0.52% by mass, the 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 the elongation of the Al alloy casting.

[0021] As described above, Mn, in combination with Fe, acts to suppress the formation of Al-Si-Fe-based needle-like crystals. According to the research and studies conducted by the present inventors, it was found that when the sum of the contents of Fe and Mn is 1.0 (mass%) or less (Fe+Mn≦1.0), the effect of suppressing the formation of needle-like crystals is extremely high.

[0022] Cu is an alloying component that dissolves in the Al base metal and is effective in improving the mechanical properties of Al alloy castings, particularly tensile strength and yield strength. The Cu content in the entire Al alloy for casting is preferably in the range of 0.08% by mass to 0.22% by mass. If Cu is contained within the above range, it can effectively exert its solid solution strengthening effect in the Al alloy without impairing the corrosion resistance. If the Cu content is too high (exceeding 0.22% by mass), it will result in a decrease in the corrosion resistance and elongation of the Al alloy casting.

[0023] Ti is an alloying component effective in refining Al crystal grains. If the Ti content is too low (less than 0.04 mass%), the Al crystal grains become coarse, lowering the elongation, tensile strength, and yield strength, whereas if the Ti content is too high (more than 0.15 mass%), the Si crystal grains (eutectic Si) are concentrated too much at the grain boundaries, forming defects, which also lowers the elongation, tensile strength, and yield strength. Therefore, it is desirable to set the Ti content in the entire casting Al alloy to a range of 0.04 mass% to 0.15 mass%. In this way, the degree of refining of the Al crystal grains is adjusted to a degree that the dendrite shape remains 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.

[0024] Sr is an alloying component that refines Si crystal grains and is effective in improving elongation, tensile strength, and yield strength even in the as-cast state (without heat treatment after casting). This effect is remarkable when 0.0075 mass% or more of Sr is added. On the other hand, when excessive Sr exceeding 0.028 wt% is added, an Al-Si-Sr ternary compound is generated, resulting in over-improvement, and casting defects are likely to occur, resulting in a decrease in 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 the stress concentration is suppressed by spheroidizing Si crystal grains, when Sr is included in the composition of an Al alloy for casting, heat treatment such as solution treatment can be omitted due to its effect, and an Al alloy casting that sufficiently secures mechanical properties such as elongation, tensile strength, and yield strength can be obtained, even though it is in the as-cast state and without heat treatment. Therefore, the Sr content in the entire Al alloy for casting is preferably in the range of 0.0075 mass % or more and 0.028 mass % or less.

[0025] In the present invention, alloying components 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 component that makes the cast structure unstable and deteriorates the flow of molten metal, so its content is limited to 0.005 mass% or less. The contents of other inevitable impurities are preferably kept to 0.1 mass% or less.

[0026] There is no particular restriction on the purity (quality) of the Al raw material used. If the Al raw material is sufficiently refined, it becomes easier to obtain an Al alloy casting with good mechanical properties. However, since refining requires a large cost, it is acceptable for other unavoidable impurities to be included. For example, high purity Al ingots and recycled materials may be mixed in an appropriate ratio.

[0027] In manufacturing the Al alloy for casting and Al alloy castings of the present invention, first, raw materials containing the main alloying components Al, Si, Mg, Fe, Mn, Cu, Ti, and Sr, except for Mg and Sr, are charged into a melting and holding furnace and melted (melting process, see Fig. 1). Here, a flux for removing slag is added to perform a slag removal process. The temperature of the molten Al alloy in the melting process 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.

[0028] Next, the molten Al alloy melted in the melting and holding furnace is transferred to a ladle preheated to within a range of 750°C ± 50°C, and Al oxides and H2 gas in the molten Al alloy are removed by rotary bubbling with N2 gas (molten metal treatment process). The molten metal treatment process is performed for approximately 10 to 15 minutes, depending on the amount of molten Al alloy. During the molten metal treatment process, raw materials (secondary alloy ingots) containing Mg and Sr are added to the ladle so that the composition of the Al alloy for casting has the above-mentioned predetermined ratio (secondary alloy addition process). After 5 minutes from the start of the molten metal treatment process, flux is added and rotary bubbling with N2 gas is performed to remove Al oxides and H2 gas in the molten Al alloy.

[0029] For example, if the processing time of the molten metal treatment step is 12 minutes, it is necessary to secure a time sufficient for uniformly stirring Mg and Sr in the molten Al alloy. In addition, control of 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 in the melting step to prevent the burnt damage of Mg and Sr.

[0030] As the secondary alloy ingot, for example, AlMn20 (containing 20% ​​by mass of Mn) or AlSr10 (containing 10% by mass of Sr) may be used. Of course, AlSi50 (containing 50% by mass of Si), AlCu50 (containing 50% by mass of Cu), Mg99.9%, AlTi5B1 (containing 5% by mass of Ti and 1% by mass of B) or the like may be added to the ladle to adjust the alloy composition. In addition, a slag removal process may be performed during the molten metal treatment process. Only the Sr-containing raw material may be added during the molten metal treatment process, and the Mg-containing raw material may be added during the melting process, but it is preferable to add both during the molten metal treatment process.

[0031] Next, the molten Al alloy refined through the molten metal treatment process is transferred from the ladle to a local holding furnace (local holding process). The temperature of the molten Al alloy in the local holding furnace is controlled within a range of 665°C ± 5°C. The refined molten Al alloy is then poured from the local holding furnace into a predetermined metal mold (casting die) under pressure using a die casting machine or a low pressure casting machine, and solidified to form an Al alloy for casting or an Al alloy casting (casting process). Here, the die temperature is set within a range of 180°C to 220°C so that the temperature of the molten Al alloy poured from the local holding furnace into the die is maintained at about 665°C ± 5°C. When the die casting method is adopted, the degree of vacuum in the die is set within a range of 50mbar to 100mbar.

[0032] The aluminum alloy casting of the present invention can be manufactured by the conventionally known die casting method, low pressure casting method, or gravity casting method. The present invention will be specifically explained in each example. Note that the present invention is not limited to the contents of the following examples, and various modifications are possible within the scope of the gist of the present invention.

[0033] Figure 2 shows the composition ranges of the casting Al alloys of other companies (hereinafter referred to as the "other alloys") and the casting Al alloys of the present invention (hereinafter referred to as the "alloys"), as well as the component compositions of the examples and comparative examples. Figure 3 shows the results of measuring the tensile strength, yield strength, and elongation of test pieces cut from large castings (giga die castings) obtained by die casting using the same die for the alloys of other companies, the examples, and the comparative examples as characteristic values. The values ​​in Figure 3 are the average values ​​measured using five test pieces for each.

[0034] Figure 3 also shows the results of evaluation of the fluidity during die casting, seizure to the die, and cracking of the casting. The other alloys shown in Figures 2 and 3 are Alcoa's casting Al alloy called C370, 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 application, and Comparative Example 2 is a case where the Sr content is near the upper limit of the alloy of the present application. The other alloys, examples, and comparative examples are all in an as-cast state without undergoing heat treatment.

[0035] As is clear from FIG. 3, Examples 1 and 2 of the present invention, in the as-cast state, exhibited a tensile strength of about 280 MPa, a yield strength of about 140 MPa, and an elongation rate of about 14%, which greatly exceeded the mechanical properties of the other alloys and Comparative Examples 1 and 2. The mechanical properties of Examples 1 and 2 far surpass the required level for structural parts in the automotive field, for example, and it was found that they were fully durable in use despite being in the as-cast state without heat treatment. In addition, Examples 1 and 2 had good melt flow during casting and no seizure occurred. Furthermore, no cracks were observed in the large castings, and the castings showed extremely good castability overall. In Comparative Examples 1 and 2, cracks were occasionally observed in the large castings.

[0036] Fig. 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 Fig. 4, if the temperature of the molten Al alloy is within the range of 665°C to ±5°C, the Al alloy casting can have extremely high tensile strength and yield strength. Fig. 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 have extremely high elongation.

[0037] FIG. 6 shows the relationship between the die temperature during die casting in the present invention and the tensile strength and yield strength of the Al alloy casting. As can be seen from FIG. 6, if the die temperature is within the range of 180°C to 220°C, the Al alloy casting can have extremely high tensile strength and yield strength. FIG. 7 shows the relationship between the die temperature during die casting in the present invention and the elongation rate of the Al alloy casting. In this case, too, if the die temperature is within the range of 180°C to 220°C, the Al alloy casting can have an extremely high elongation rate. Although the details are unclear, if the die temperature is within the range of 180°C to 220°C, it is considered that the Al alloy molten metal temperature during die casting can be easily maintained within the range of 660°C to 670°C. Thanks to the die temperature, the Al alloy molten metal temperature is unlikely to decrease, and since the Al alloy molten metal temperature is within the range of 660°C to 670°C, the Al alloy casting can have good mechanical properties.

[0038] Fig. 8 shows the relationship between the degree of die 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 Fig. 8, if the degree of die vacuum is within the range of 50 mbar to 100 mbar, the Al alloy casting can have extremely high tensile strength and yield strength. Fig. 9 shows the relationship between the degree of die 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 die vacuum is within the range of 50 mbar to 100 mbar, the Al alloy casting can have extremely high elongation.

[0039] 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 provide an Al alloy for casting and an Al alloy casting at low cost by omitting costly heat treatment.

[0040] 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, so that alloy components such as Mg and Sr can be prevented from being burned by the heat of the molten Al alloy, and the added effects of Mg and Sr can be reliably 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.

[0041] 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 (especially 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.

[0042] FIG. 10 shows the composition of the alloy of the present application in Example 3 (heat treatment-free) and Example 4 (T6-treated). The composition of Examples 3 and 4 is exactly the same. FIG. 11 shows the results of measuring the tensile strength, yield strength, and elongation percentage as characteristic values ​​for Examples 3 and 4, which were obtained by cutting out test pieces from a heat treatment-free casting (Example 3) and a T6-treated casting (Example 4) obtained by a low pressure casting method using the same die. The values ​​in FIG. 11 are average values ​​measured using five test pieces each. FIG. 11 also shows the results of evaluating the flowability of the molten metal during die casting, the seizure on the die, and the cracking of the casting.

[0043] As is clear from FIG. 11, Example 3 of the present invention has 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 only showed a slightly low value of 5%. Example 3 had good fluidity during casting, no seizure occurred, and no cracks were observed in the casting. Example 3 showed extremely good castability overall. It was found that the results of Example 3 were almost equivalent to the performance (mechanical properties) after heat treatment of AC4C specified in the Japanese Industrial Standard JIS H5302.

[0044] In contrast, in Example 4, in which a large low-pressure cast part was subjected to T6 treatment, the mechanical properties were significantly superior to those of Example 3, with a tensile strength of 300 MPa, a yield strength of 242 MPa, and an elongation of 8.0%. The mechanical properties of Example 4 far exceed the performance (mechanical properties) of AC4C after heat treatment, and far surpass the required level for structural parts in the automotive field, for example, and it was found that, depending on the application, heat treatment can make it sufficiently durable for use. Note that, in Example 4 as well, the molten metal flow during casting was good, no seizure occurred, and no cracks were observed in the casting.

Claims

1. 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 is limited to 1.0 mass% or less, The Sr is not added in the melting step, and is used to reduce Al oxides and H in the molten Al alloy obtained in the melting step. ２ It is added during the molten metal treatment process to remove gases. Aluminum alloy for casting.

2. The Mg is not added in the melting step, but is added in the molten metal treatment step.

2. The casting aluminum alloy according to claim 1.

3. Casting is performed using the casting Al alloy according to claim 1 or 2. Aluminum alloy castings.

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