WO2025046955A1 - Method for producing al alloy for casting - Google Patents

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

The present invention relates to a method for producing aluminum alloys for casting.

Conventionally, aluminum alloys (hereafter referred to as Al alloys) are widely used as materials for components in automobiles, industrial machinery, aircraft, home appliances, and various other fields, due to their light weight, excellent formability, and mass production. For example, large castings in the automobile field, such as suspension towers, side beams, and battery cases, are generally manufactured by die casting using AlSi10Mg (Al-Si-Mg alloys) and then T7 treatment (solution treatment by heating and quenching, and stabilization treatment (overaging treatment)). 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 can also 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.

Japanese Patent Application Publication No. 11-12673

Incidentally, this type of Al alloy casting requires high mechanical properties as an important safety component, but at the same time, strict cost reduction is also required. One cost reduction measure, for example, is to omit 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 levels, making it difficult to achieve both high mechanical properties and cost reduction.

For example, Patent Document 1 discloses a method for producing an Al alloy casting that has high mechanical properties even in the as-cast state (without heat treatment after casting) using an Al alloy that contains 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 remainder being substantially Al, and satisfies the condition Cu+2.5Mg≧1.25 wt%.

However, the Al alloy casting obtained by the manufacturing method described in Patent Document 1 has a high Cu content of 0.4 wt.% or more, which means it has the disadvantage of being poor in corrosion resistance.

The technical objective of the present invention is to provide a manufacturing method for aluminum alloys for casting that has been improved based on the current situation described above.

The manufacturing method of 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 remainder being Al.  

The sum of the contents of Fe and Mn is limited to 1.0 mass % or less. The Sr is not added in the melting step, but is added in the molten metal treatment step for removing Al oxides and _H2 gas from the molten Al alloy obtained in the melting step.

In the manufacturing method of the casting Al alloy of the present invention, the Mg may not be added in the melting step, but may be added during the molten metal treatment step.

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 a 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 can be 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.

Furthermore, Mg is not added in the melting process, but is added in the melt treatment process to remove Al oxides and _H2 gas from the molten Al alloy obtained in the melting process, so that Mg can be prevented from being burned by the heat of the molten Al alloy, and the effect of adding Mg can be reliably exhibited in the casting Al alloy and Al alloy castings. Therefore, the quality and yield of the casting Al alloy and Al alloy castings can be improved. 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 can be performed without any problems, and the performance will be improved.

FIG. 2 is a flow chart showing the manufacturing process of the present invention. 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. 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. 1 is a graph showing the relationship between the temperature of molten Al alloy during die casting and the tensile strength and yield strength of the casting. 1 is a graph showing the relationship between the temperature of molten Al alloy during die casting and the elongation rate of the casting. 1 is a graph showing the relationship between the die temperature during die casting and the tensile strength and yield strength of the casting. 1 is a graph showing the relationship between the die temperature during die casting and the elongation rate of a casting. 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. 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. FIG. 1 is a diagram showing the component compositions of a heat-treatment-free casting and a T6-treated casting produced by a low-pressure casting method. 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.

Below, an embodiment of the present invention will be described with reference to the drawings.

As shown in Figure 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.

Si 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% to 10.0% by mass. If the Si content is too low (less than 8.0% by mass), the fluidity will be insufficient and the fluidity will not be ensured, and casting cracks will be more likely to occur. Conversely, if the alloy contains excessive Si, exceeding 10.0% by mass, it will reduce the elongation of the Al alloy casting.

Mg exists mainly in the form of a solid solution in the Al matrix in the casting Al alloy or as Mg ₂ Si, and is an alloy 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 Mg is contained within the above range, the mechanical properties such as tensile strength and yield strength of the Al alloy casting 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 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.

Fe is an alloy component that acts to prevent seizure on the mold during casting. The Fe content in the entire Al alloy for casting is preferably in the range of 0.30 mass% to 0.50 mass%. If the Fe content is too high (over 0.50 mass%), Al-Si-Fe needle-like crystals (ternary compound) are generated, significantly reducing the elongation of the Al alloy casting. If the Fe content is 0.50 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.

Mn is an alloying component that is added to prevent seizure on the mold during casting, as well as to suppress the formation of needle-like crystals made of Al-Si-Fe, ensuring the elongation of the Al alloy casting. The Mn content in the entire Al alloy for casting should be in the range of 0.28% to 0.52% by mass. Setting the Mn content to 0.39% by mass or less improves the mold releasability. Furthermore, 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.

As mentioned above, Mn, in combination with Fe, acts to suppress the formation of Al-Si-Fe acicular crystals. According to the inventors' research, it was found that if the sum of the Fe and Mn contents is 1.0 (mass%) or less (Fe+Mn≦1.0), the effect of suppressing the formation of acicular crystals is extremely high.

Cu is an alloying component that dissolves in the Al base material 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 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.

Ti is an alloying component that is effective in refining Al crystal grains. If the Ti content is too low (less than 0.04 mass%), the Al crystal grains become coarse, reducing 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 reduces 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 the extent that a 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.

Sr is an alloying component that refines the Si crystal grains and is effective in improving the 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. In contrast, when excess Sr is added in an amount exceeding 0.028 weight%, an Al-Si-Sr ternary compound is generated, resulting in over-improvement, and casting defects are more 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 the Si crystal grains in the as-cast state. Since spheroidizing the Si crystal grains suppresses stress concentration, if 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 can be obtained that has sufficient mechanical properties such as elongation, tensile strength, and yield strength, despite being in the as-cast state without heat treatment. Therefore, it is preferable that the Sr content of the entire casting Al alloy be in the range of 0.0075 mass% or more and 0.028 mass% or less.

In the present invention, alloying elements such as Zn, Ni, Sn, Pb, Ca, Cr, and Cd are treated as unavoidable 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 makes the casting structure unstable and deteriorates the flow of molten metal, so its content is limited to 0.005 mass% or less. It is desirable to keep the content of other unavoidable impurities to 0.1 mass% or less.

Incidentally, there are no particular restrictions on the purity (quality) of the Al raw material used. If the Al raw material is sufficiently refined, it becomes easier to obtain Al alloy castings with good mechanical properties. However, because refining is very costly, it is acceptable for other unavoidable impurities to be included. For example, high-purity Al ingots and recycled materials can be mixed in an appropriate ratio.

In the manufacture of the Al alloy for casting and Al alloy castings of the present invention, first, raw materials containing the main alloy 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 Figure 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.

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 casting Al alloy is 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.

If the processing time of the molten metal treatment process is, for example, 12 minutes, it is necessary to ensure that there is enough time to uniformly mix the Mg and Sr into the molten Al alloy. In addition, management of the molten metal temperature is extremely important, and the temperature of the molten Al alloy after the molten metal treatment process 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 during the molten metal treatment process rather than the melting process to prevent the Mg and Sr from burning.

The secondary alloy ingot can be, for example, AlMn20 (containing 20% Mn by mass) or AlSr10 (containing 10% Sr by mass). Needless to say, AlSi50 (containing 50% Si by mass), AlCu50 (containing 50% Cu by mass), Mg99.9%, AlTi5B1 (containing 5% Ti by mass and 1% B by mass), etc. can be added to the ladle to adjust the alloy composition. In addition, a slag removal process can be performed during the molten metal treatment process. It is possible to add only the Sr-containing raw material during the molten metal treatment process and the Mg-containing raw material during the melting process, but it is preferable to add both during the molten metal treatment process.

Then, 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 the range of 665°C ± 5°C. The refined molten Al alloy is then poured from the local holding furnace into a specified metal mold (casting 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 die temperature is set within the 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 approximately 665°C ± 5°C. When die-casting is used, the degree of vacuum in the die is set within the range of 50 mbar to 100 mbar.

The Al 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 examples below, and various modifications are possible without departing from the spirit of the present invention.

Figure 2 shows the composition ranges of other companies' casting Al alloys (hereinafter referred to as the other companies' alloys) and the casting Al alloy of the present invention (hereinafter referred to as the present alloy), 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 percentage as characteristic values for the other companies' alloys, the examples, and the comparative examples, which were cut from large castings (giga die castings) obtained by die casting using the same mold. The values in Figure 3 are the average values measured using five test pieces for each.

Figure 3 also shows the results of evaluation of the fluidity of the molten metal during die casting, seizure on the mold, 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 be equivalent 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.

As is clear from Figure 3, Examples 1 and 2 of the present invention, both in the as-cast state, exhibited a tensile strength of approximately 280 MPa, a yield strength of approximately 140 MPa, and an elongation rate of approximately 14%, which greatly exceeded the mechanical properties of the competitor's alloys and Comparative Examples 1 and 2. The mechanical properties of Examples 1 and 2 far surpass the required standards 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. Furthermore, Examples 1 and 2 had good fluidity during casting and no seizure occurred. Furthermore, no cracks were observed in the large castings, and overall they showed extremely good castability. However, in Comparative Examples 1 and 2, cracks were occasionally observed in the large castings.

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 rate 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 an extremely high elongation rate.

Figure 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 Figure 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. Figure 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 thought that it is easier to maintain the molten Al alloy temperature within the range of 660°C to 670°C during die casting, and the molten Al alloy temperature is less likely to drop due to the die temperature, and it is understood that the Al alloy casting has good mechanical properties because the molten Al alloy temperature is within the range of 660°C to 670°C.

Figure 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 Figure 8, if the degree of die 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 die vacuum during die casting in the present invention and the elongation rate of the Al alloy casting. In this case too, if the degree of die vacuum is within the range of 50 mbar to 100 mbar, the Al alloy casting can achieve extremely high elongation.

As can be seen from the above explanation, the present invention can provide an Al alloy for casting, and an Al alloy casting, that exhibits castability equivalent to that of an Al-Si-Mg alloy, even in the as-cast state (without heat treatment after casting), and that exhibits good mechanical properties (elongation, tensile strength, yield strength, etc.), and can provide an Al alloy for casting and an Al alloy casting at low cost by omitting costly heat treatment.

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 specified ratio described above, 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 exerted in the cast Al alloy and Al alloy castings. Therefore, the quality and yield of the cast Al alloy and Al alloy castings can be improved.

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

Figure 10 shows the composition of the alloy of Example 3 (heat treatment-free) and Example 4 (T6 treated). The composition of Examples 3 and 4 is exactly the same. Figure 11 shows the results of measuring the tensile strength, yield strength, and elongation as characteristic values for Examples 3 and 4, which were obtained by cutting test pieces from a heat treatment-free 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 the average values measured using five test pieces for each. Figure 11 also shows the results of evaluating the fluidity of the alloy during die casting, seizure on the mold, and cracking of the casting.

As is clear from Figure 11, Example 3 of the present invention, even in the as-cast state during low-pressure casting, has a tensile strength of over 180 MPa and a yield strength of over 120 MPa. 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. Overall, Example 3 showed extremely good castability. It was found that the results of Example 3 were almost equivalent to the performance (mechanical properties) after heat treatment of AC4C as specified in Japanese Industrial Standard JIS H5302.

In contrast, in Example 4, where 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 standards for structural parts in the automotive field, for example, and it was found that, depending on the application, heat treatment can make it fully durable for use. Furthermore, in Example 4, the molten metal flow during casting was also good, there was no seizure, and no cracks were observed in the casting.

Claims (2)

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, and the sum of the Fe and Mn contents being limited to 1.0 mass% or less,
The Sr is not added in the melting step, but is added in the molten metal treatment step for removing Al oxides and _H2 gas from the molten Al alloy obtained in the melting step.
A manufacturing method for casting aluminum alloys. The Mg is not added in the melting step, but is added in the molten metal treatment step.
A method for producing the aluminum alloy for casting according to claim 1.

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