Aluminum alloy, solidified aluminum alloy member, and method for manufacturing the same

Abstract

The present invention provides an alloy exhibiting medium strength, high ductility, and especially toughness, a solidified member, and a method for manufacturing the same. [Solution] The aluminum alloy contains 4.0 to 5.9 mass% Si, 0.46 to 0.80 mass% Mg, 0.05 to 0.50 mass% Cu, one or more of the following as additive elements: 0.20 mass% or less Mn, 0.30 mass% or less Fe, 0.25 mass% or less Ti, 0.030 mass% or less Sr, and 0.0050 mass% or less Be, as well as unavoidable impurities in a total amount of 0.50 mass% or less, with the remainder being Al.

Description

Technical Field

[0001] The present invention relates to an aluminum alloy containing 0.5% or less of trace Cu, a solidified member of an aluminum alloy, and a method for manufacturing the same. The present invention relates to an alloy having high ductility and exhibiting a medium strength with a tensile strength of about 200 to 350 MPa as needed, and various solidified members such as sand-cast members, die-cast members, and high-pressure cast members (low-speed filling squeeze members), and a method for manufacturing the same. In particular, since precipitates responsible for strengthening the alloy are not easily softened even when affected by heat at temperatures exceeding 300°C, the present invention relates to a technology capable of surely achieving desired mechanical properties while suppressing product distortion and generation of residual stress inside the product that occur during heat treatment, i.e., cooling from solution treatment.

Background Art

[0002] Table 1 shows representative compositions of aluminum alloy castings according to JIS standards and aluminum alloy castings according to AA standards. (See Non-Patent Document 1 and Non-Patent Document 2)

[0003]

Table 1

[0004] The alloy types can be roughly classified into a) Al-Si-Mg-based alloys, b) Al-Si-based alloys, c) Al-Si-Cu-based alloys, and d) Al-Si-Mg-Cu. Among these, Al-Si-Mg-based alloys with 6.5% or more of Si and about 0.3% of Mg as the main elements and other elements as impurities have good corrosion resistance. In particular, alloys of the A356 and AC4CH series are widely used for important safety parts of automobiles, such as wheels. Its strengthening mechanism is age hardening by precipitates composed of the main elements Mg and Si. By selecting age treatment among heat treatment conditions and suppressing internal defects, the alloy can exhibit high ductility exceeding 10% and medium strength at the 250 - 300 MPa level.

[0005] However, it is known that when a solidified material is subjected to a solution treatment exceeding 500°C, for example, and then quenched slowly at a slow rate through the 300°C to 400°C temperature range, the Mg-Si precipitates become coarser, reducing tensile strength and elongation, and preventing the acquisition of mechanical properties obtained at normal quenching rates. (See, for example, Non-Patent Literature 3) It can be easily inferred that this phenomenon occurs in Al-Si-Mg alloys, regardless of the Si content, with the same precipitation mechanism resulting in a decrease in tensile strength.

[0006] Al-Si alloys such as A443 and AC3A are known as non-heat-treatable alloys, but because age hardening cannot be expected, their tensile strength is at the 200 MPa level, and since they are not heat-treated, including solution treatment, the eutectic Si is not granular, resulting in low elongation.

[0007] Al-Si-Cu alloys such as AC2B and AC4B are used in manifolds, cylinder heads, and other applications, and can achieve high tensile strengths exceeding 350 MPa through heat treatment. However, these alloys contain trace amounts of Mg, which is considered an impurity, and this high strength is only achievable through aging treatment that utilizes this Mg.

[0008] Furthermore, Al-Si-Cu alloys have poor corrosion resistance due to their high Cu content, and when cast, they have poor molten metal replenishment capabilities to the final solidified area during solidification in the mold. Sand-cast products, in particular, tend to develop numerous minute shrinkage cavities, such as those smaller than 0.5 mm, in the final solidified area. Even with solution treatment, the high Cu content means that, considering the final solidification temperature is approximately 500°C, performing solution treatment above this temperature presents localized melting problems. Consequently, the morphology of the Cu-containing compounds generated during casting, which cause reduced elongation, cannot be made smooth and sufficiently small. Moreover, the granulation of eutectic Si, which is effective in improving elongation, requires a long time. In summary, high elongation cannot be easily obtained due to the effects of shrinkage cavities in the solidified material, the morphology and size of the Cu compounds, and insufficiently granulated eutectic Si.

[0009] Some alloys, such as alloys 324 and 336, are Al-Si-Cu alloys that also contain Mg as an active element. However, like AC2B and AC4B, these alloys do not have good corrosion resistance, and high elongation is not easily achieved due to the influence of shrinkage cavities, Cu compounds, and the morphology of eutectic Si. [Prior art documents] [Non-patent literature]

[0010] [Non-Patent Document 1] Japan Aluminum Association, Aluminum Handbook (6th Edition), JIS H 5202 (Aluminum Alloy Castings) [Non-Patent Document 2] Showa Denko, Aluminum Handbook 2012, AA Standard (based on the International Alloy Symbols) [Non-Patent Document 3] N. Saruwatari: Materials Transaction, 64(2023), 2575-2583 [Overview of the Initiative] [Problems that the invention aims to solve]

[0011] Traditionally, most automotive parts, with the exception of die-cast parts, were thick-walled. However, with the recent shift towards electrification of automobiles, there has been a surge in the study and development of thinner die-cast parts for large vehicle systems, including battery cases, rear and front components. In parallel, the development of large, thin sand-cast parts for preliminary verification is also progressing as needed to confirm the performance of die-cast parts with various mechanical properties. Furthermore, weight reduction is not limited to the automotive sector; it is also being demanded for industrial equipment parts, such as robot arms. [Means for solving the problem]

[0012] To solve the above problems, a first aspect of the present invention provides an aluminum alloy containing 4.0 to 5.9 mass% of Si, 0.46 to 0.80 mass% of Mg, 0.05 to 0.50 mass% of Cu, one or more of the following as additive elements: 0.20 mass% or less of Mn, 0.30 mass% or less of Fe, 0.25 mass% or less of Ti, 0.030 mass% or less of Sr, and 0.0050 mass% or less of Be, and an amount of unavoidable impurities of 0.50 mass% or less in total, with the remainder being Al.

[0013] Alternatively, the aluminum alloy according to the first aspect of the present invention contains 0.50 to 0.70% by mass of Mg and 0.05% by mass or more and less than 0.25% by mass of Cu.

[0014] The aluminum alloy solidification member according to the second aspect of the present invention is made of the aluminum alloy according to the first aspect of the present invention, and is characterized in that it contains independently dispersed eutectic Si in its metallic structure, a portion of which Mg and Si exist as Mg2Si compounds having a smooth shape, and the other portion of which Mg and Si are dissolved or precipitated in the primary aluminum and eutectic aluminum matrix phases.

[0015] Alternatively, the aluminum alloy solidification member according to a second aspect of the present invention is characterized by having a Vickers hardness of 65 Hv or more and 120 Hv or less.

[0016] A third aspect of the present invention is a method for manufacturing an aluminum alloy solidified member made of an aluminum alloy according to the first aspect of the present invention, comprising the steps of: pouring molten aluminum alloy into a sand mold; solidifying the molten aluminum alloy in the sand mold; removing the solidified member from the sand mold; and performing a T6 treatment on the solidified member, which includes solution treatment, water quenching, and artificial aging treatment. When the solidified member has tensile strength a (MPa) and elongation c (%), the quality index expressed as a + 150 × log(c) is 410 or higher, and the Vickers hardness is 70 Hv or higher.

[0017] A fourth aspect of the present invention is a method for manufacturing an aluminum alloy solidified member made of an aluminum alloy according to the first aspect of the present invention, comprising the steps of: pouring molten aluminum alloy into a sand mold; solidifying the molten aluminum alloy in the sand mold; removing the solidified member from the sand mold; and performing a T4 treatment on the solidified member, which includes solution treatment and water quenching, or a T6 treatment, which includes solution treatment, forced air cooling, and artificial aging. When the solidified member has tensile strength a (MPa) and elongation c (%), the quality index expressed as a + 150 × log(c) is 360 or higher, and the Vickers hardness is 70 Hv or higher.

[0018] A fifth aspect of the present invention is a method for manufacturing an aluminum alloy solidified member made of an aluminum alloy according to the first aspect of the present invention, comprising the steps of pouring molten aluminum alloy into a sand mold, solidifying the molten aluminum alloy in the sand mold, removing the solidified member from the sand mold, and performing a T4 treatment on the solidified member, which includes solution treatment and forced air cooling. When the solidified member has tensile strength a (MPa) and elongation c (%), the quality index expressed as a + 150 × log(c) is 360 or higher, and the Vickers hardness is 65 Hv or higher.

[0019] A sixth aspect of the present invention is a method for manufacturing an aluminum alloy solidified member made of an aluminum alloy according to the first aspect of the present invention, comprising the steps of: pouring molten aluminum alloy into a mold; solidifying the molten aluminum alloy in the mold; removing the solidified member from the mold; and performing a T6 treatment on the solidified member, which includes solution treatment, water quenching or forced air cooling, and artificial aging treatment, or a T4 treatment, which includes solution treatment, water quenching or forced air cooling. When the solidified member has tensile strength a (MPa) and elongation c (%), the quality index expressed as a + 150 × log(c) is 410 or higher, and the Vickers hardness is 70 Hv or higher.

[0020] The solution treatment according to the third to sixth aspects of the present invention is carried out at a temperature higher than 500°C and 540°C or lower.

[0021] The forced air cooling according to the third to sixth aspects of the present invention is performed using a fan or a mist fan.

Advantages of the Invention

[0022] The aluminum alloy and the aluminum alloy solidified member according to the present invention contain a slightly higher amount of Mg and a very small amount of Cu than conventional standard alloys. As a result, precipitates containing Cu and Mg that are difficult to soften even during high-temperature holding are developed in the aluminum alloy solidified member, and a predetermined strength is exhibited. Further, in the aluminum alloy solidified member according to the present invention, the amounts of Fe compounds and eutectic Si that serve as the origin and propagation path of cracks are suppressed to a low level, and Sr that refines eutectic Si is added as needed. Further, in the manufacturing method of the aluminum alloy solidified member according to the present invention, Fe compounds and eutectic Si are granulated by solution treatment, and then quenched or forced air-cooled in the atmosphere, and then artificial aging treatment is performed as needed. Thereby, an aluminum alloy solidified member having high strength while having higher ductility than AC4CH and A356 alloys and a manufacturing method thereof are provided.

[0023] Further, unlike the mold solidifying material, the metal additive manufacturing method has an extremely fine metal structure without solution treatment, and a large amount of various elements are dissolved in the aluminum matrix phase. The metal additive manufacturing method exhibits a solidification rate that is 10 to 100 times faster than that of the die casting method, which has a fast solidification rate among mold solidifying materials. However, since the metal additive manufacturing method continues to be heated, for example, at 200°C during the manufacturing process, the lower part of the aluminum alloy formed body containing, for example, 10% by mass of Si and 0.4% by mass of Mg tends to soften due to overaging.

[0024] However, when the alloy according to the present invention is formed by the metal additive manufacturing method, a large amount of Mg and Si are dissolved in the formed body, and even if precipitates are generated, precipitates containing Cu that do not undergo overaging in the 200°C temperature range are generated, so the formed body does not soften. Therefore, when the alloy according to the present invention is formed by the metal additive manufacturing method, variations in mechanical properties and softening in the formed body can be avoided.

Brief Description of the Drawings

[0025] [Figure 1] This is a photographic diagram showing the microstructure of a mold casting product made of Al-4.5%Si-0.52%Mg-0.18%Cu-0.15%Ti-0.15%Fe-0.01%Sr-0.001%Mn alloy before and after solution treatment (before heat treatment, 530°C x 6 hours). [Figure 2] This photograph shows the effect of solution treatment temperature (490°C × 6 hours, 550°C × 6 hours) on the microstructure of mold castings made of Al-4.5%Si-0.52%Mg-0.18%Cu-0.15%Ti-0.15%Fe-0.01%Sr-0.001%Mn alloy. [Figure 3] This is a schematic diagram showing the quality index and heat treatment conditions affecting the hardness of the mold solidification material and sand mold solidification material using the alloy according to this embodiment. [Modes for carrying out the invention]

[0026] [First Embodiment] The aluminum alloy according to the first embodiment of the present invention contains 4.0 to 5.9 mass% of Si, 0.46 to 0.80 mass% of Mg, 0.05 to 0.50 mass% of Cu, one or more of the following as additive elements: 0.20 mass% or less of Mn, 0.30 mass% or less of Fe, 0.25 mass% or less of Ti, 0.030 mass% or less of Sr, and 0.0050 mass% or less of Be, and unavoidable impurities in total amount of 0.50 mass% or less, with the remainder being Al.

[0027] Furthermore, the aluminum alloy according to this embodiment contains 0.50 to 0.70% by mass of Mg and 0.05% by mass or more and less than 0.25% by mass of Cu.

[0028] In this specification, "unavoidable impurities" refer to elements other than Al, Si, Mg, Cu, Mn, Fe, Ti, Sr, and Be, such as V, Cr, P, Pb, Sn, and Ni. Oxygen is not included in the definition of "unavoidable impurities."

[0029] [Second Embodiment] The aluminum alloy solidification member according to the second embodiment of the present invention is made of the aluminum alloy according to the first embodiment, and contains independently dispersed eutectic Si in its metallic structure, with some of the Mg and Si existing as Mg2Si compounds having a smooth shape, and other parts of the Mg and Si being dissolved or precipitated in the primary aluminum and eutectic aluminum matrix phases.

[0030] Furthermore, the aluminum alloy solidified member according to this embodiment has a Vickers hardness of 65 Hv or more and 120 Hv or less.

[0031] [Chemical composition] The cooling rate of solidified members obtained by gravity casting, low-pressure casting, and high-pressure casting with slow filling is 1 / 10 to 1 / 100 of the cooling rate during solidification obtained by die casting. Therefore, solidified members obtained by these high-pressure casting methods do not have as fine a metallic structure as solidified members obtained by die casting, and it is not easy to obtain high ductility without solution treatment.

[0032] First, let's discuss this from the perspective of improving ductility. In the high-pressure casting method described above, where there is no seizing between the mold and the molten metal, unlike the die-casting method, the best way to reduce the amount of Fe is to minimize the amount of iron compounds that cause a decrease in ductility. Then, if necessary, a small amount of Mn is added to change the morphology of the needle-shaped iron compounds to granular ones.

[0033] In this embodiment, where the Mg content is high, some Fe compounds also contain Mg, and when solution-treated, their form becomes granular or smooth. These have the effect of suppressing crack initiation and propagation.

[0034] Furthermore, reducing the Si content is extremely effective in suppressing the causes of crack initiation and propagation. Additionally, granulating the eutectic Si through solution treatment is even more effective in suppressing crack initiation and propagation. However, considering the reduction of product distortion during quenching, the cooling rate from the solution treatment temperature must be slowed. In that case, both strength and elongation will decrease significantly.

[0035] Considering these factors, it is necessary to select an appropriate amount of Si that does not adversely affect the flowability of the molten metal. For this purpose, the Si content of the aluminum alloy solidified member according to this embodiment is set to 4.0 to 5.9%. The Si content may also be 4.0 to 5.5%. Using such an Si content is an effective method for achieving extremely high ductility while exhibiting the required strength through the addition of Cu.

[0036] Next, we will discuss the improvement of strength. In the solidified aluminum alloy member according to this embodiment, heat treatment causes Mg to form fine precipitates of 1 to several nanometers in size with Si, and Cu to form fine precipitates of 1 to several nanometers in size with Al and Mg, thereby contributing to the improvement of strength.

[0037] Conventionally, when cooling to room temperature after solution treatment, under conditions where the product is air-cooled from the solution temperature, the Mg-Si precipitates sometimes coarseened during the cooling process, failing to contribute to strength improvement, from the perspective of preventing product distortion.

[0038] However, as with the aluminum alloy solidification member according to this embodiment, by containing 0.05% by mass or more of Cu and 0.46% by mass or more of Mg, age hardening by Al-Cu-Mg precipitates progresses through artificial aging after quenching (T6 treatment), natural aging after quenching (T4 treatment: left at room temperature), or aging during temperature decrease during quenching. However, if the amount of Mg added is too high, the ductility will decrease. For this reason, in this embodiment, the amount of Mg is set to 0.46 to 0.80% by mass. Preferably, the amount of Mg is set to 0.50 to 0.70% by mass.

[0039] In the aluminum alloy solidification member according to this embodiment, Cu contributes to strength improvement by forming fine precipitates of 1 to several nanometers in size with Al and Mg. However, if the amount of Cu is too high, corrosion resistance and ductility will decrease. In this embodiment, the amount of Cu is 0.05 to 0.50 mass%. Preferably, the amount of Cu is 0.05 mass% or more and less than 0.25 mass%. More preferably, the amount of Cu is 0.05 to 0.20 mass%.

[0040] In the aluminum alloy solidification member according to this embodiment, Mn is added as needed from the viewpoint of forming a compound with Fe and improving ductility. On the other hand, in this embodiment, the amount of Fe itself is small, so the addition of Mn is hardly necessary, and if it is added, it should be 0.20% by mass or less.

[0041] In the aluminum alloy solidification member according to this embodiment, the Fe content is 0.30% by mass or less from the viewpoint of improving ductility. Preferably, the Fe content is 0.20% by mass or less.

[0042] In the aluminum alloy solidification member according to this embodiment, Ti is added as a crystal refiner to suppress the occurrence of shrinkage cavities caused by insufficient molten metal supply during the solidification process. Adding more than 0.25% by mass does not improve the effect, so in this embodiment, the amount of Ti is 0.25% by mass or less. In sand casting, there is a concern that the elongation may decrease due to the generation of Ti compounds, so it is set to 0.20% by mass or less.

[0043] In the aluminum alloy solidification member according to this embodiment, the amount of Sr required for eutectic Si refinement varies depending on the amount of P impurities contained in the molten alloy and the solidification rate of the molten alloy in the mold. To ensure ductility and guarantee the refinement effect, the amount of Sr should be 0.005 to 0.030 mass% for sand castings and 0.030 mass% or less for die castings. However, if the amount of Sr is too high, it will form Sr oxides that will be mixed into the product, reducing quality and flowability. It will also form Al-Si-Sr compounds, reducing elongation. For this reason, in this embodiment, the amount of Sr is preferably 0.020 mass% or less. In the case of high-pressure castings, the cooling rate during solidification is faster than for die castings, so Sr is added as needed, but it may not be necessary in some cases. The maximum amount of Sr is 0.015 mass% or less.

[0044] In the aluminum alloy solidification member according to this embodiment, Sr and Mg form oxides, which have the effect of reducing the flowability of the molten metal. For this reason, in this embodiment, depending on the size and thickness of the product, Be is added at a rate of 0.0050% by mass or less. The amount of Be is preferably between 0.0005% by mass and 0.0020% by mass.

[0045] [Metal structure] The solidified aluminum alloy member according to this embodiment contains independently dispersed eutectic Si in its metallic structure. Some of the Mg and Si exist as Mg2Si compounds with a smooth shape, while the remaining Mg and Si are dissolved or precipitated in the primary aluminum and eutectic aluminum matrix phases.

[0046] Furthermore, the higher the solution treatment temperature and the longer the treatment time, the more Mg2Si compounds will be dissolved or precipitated in the aluminum matrix. For example, when a solidified material is solution treated at 530°C, only a very small amount of Mg2Si compounds are visible in the metallographic image (see Figure 1(b) below).

[0047] Figures 1(a) and 1(b) are photographic images showing the metallographic structure of a T4 alloy mold casting product with Al-4.5 mass%Si-0.55 mass%Mg-0.18 mass%Cu-0.012 mass%Sr before solution treatment (Figure 1(a)) and after solution treatment at 530°C for 6 hours (Figure 1(b)). Figures 1(a) and 1(b) are the results of observations using an optical microscope. The mold casting was sampled after casting into a boat shape according to JIS H5202.

[0048] Furthermore, the components of the Mg2Si compounds and Fe-containing compounds shown in Figures 1 and 2 were identified by observing backscattered electron (BSE) images using a field emission scanning electron microscope (FE-SEM) and by elemental mapping using energy-dispersive X-ray spectroscopy (EDS) with a scanning transmission electron microscope (STEM). It was confirmed that the Fe-containing compounds were mainly Al-Fe-Si-Mg systems, whose external shape became smooth after solution treatment.

[0049] As shown in Figures 1(a) and 1(b), the metal structure changes before and after solution treatment. The metal structure consists of an aluminum matrix, a Mg2Si compound, and an Fe-containing compound. In the structure shown in Figure 1, the lightest colored area is the aluminum matrix.

[0050] In the microstructures shown in Figures 1(a) and 1(b), the darkest areas, indicated by black arrows, are Mg2Si compounds. For example, as shown in Figure 1(a), before solution treatment, Mg2Si compounds mainly have a linear outline, or polygonal shape. On the other hand, Figure 1(b) shows the microstructure after solution treatment at 530°C. After solution treatment, the Mg2Si compounds almost completely disappear, and the remaining compounds have a smooth outline rather than a polygonal shape. Furthermore, in the microstructures shown in Figures 1(a) and 1(b), burning indicating localized melting is hardly observed.

[0051] In the microstructure shown in Figures 1(a) and 1(b), the areas with brightness intermediate between the Mg2Si compound and the aluminum matrix, indicated by white arrows, are Fe-containing compounds. For example, as shown in Figures 1(a) and 1(b), the Fe-containing compounds show little change in morphology before and after solution treatment, but by increasing the solution treatment time, they develop a morphology with a smoother outer shape.

[0052] [Manufacturing method] The method for manufacturing the aluminum alloy solidification member according to this embodiment is as follows, for example.

[0053] Molten aluminum alloy is poured into a mold such as a sand mold or metal mold, and the molten aluminum alloy is allowed to solidify within the mold. Within the solidification temperature range in which the aluminum alloy changes from liquid to solid, eutectic Si, Mg2Si compounds, Fe-containing compounds, Al-Cu-Mg compounds, etc., are allowed to crystallize in the solidified material. Note that Al-Cu-Mg compounds also include Al-Cu-Mg-Si compounds.

[0054] The solidified material is subjected to solution treatment within an appropriate temperature range. Here, we will explain the effect of the solution treatment temperature on the solidified aluminum alloy material cast using a mold. The solidified material was sampled by casting it into a boat shape as described in JIS H5202.

[0055] Figure 2(a) shows the microstructure of the solidified material after solution treatment at 490°C for 6 hours. Due to the low solution treatment temperature, the fibrous eutectic Si shown in Figure 1(a) has become granular, but the Mg2Si compounds are still present in large numbers, although they have a smooth shape. No burning (localized melting) of the Mg2Si compounds in the metal structure is observed.

[0056] Figure 2(b) shows the microstructure of the solidified material after solution treatment at 550°C for 6 hours. When solution treatment is performed at 550°C, the burning phenomenon becomes pronounced, and countless holes larger than 10 μm are observed due to localized melting. The solidified material treated at 550°C has high tensile strength, yield strength, and Vickers hardness, but its elongation is lower than that of the solidified material at 530°C because of the occurrence of minute holes larger than 10 μm.

[0057] This effect of solution temperature on mechanical properties is similar to that observed in solidified components using sand molds.

[0058] In the method for manufacturing an aluminum alloy solidified member according to this embodiment, solution treatment is performed in a temperature range of over 500°C and up to 540°C. Within this temperature range, the required heat treatment time varies depending on the treatment temperature. For example, the approximate heat treatment time for solution treatment is 6 to 12 hours at 510°C, 3 to 10 hours at 520°C, 1 to 8 hours at 530°C, and 0.5 to 3 hours at 540°C.

[0059] Solidified members treated with a solution treatment at 530°C have a large amount of Mg2Si compound dissolved in the aluminum matrix as Mg and Si, and have almost no minute pores. Therefore, solidified members treated with a solution treatment at 530°C have high tensile strength, yield strength, Vickers hardness, and elongation.

[0060] After this solution treatment, the material is quenched in water or forced-cooled in the atmosphere (natural aging), and artificial aging is performed as needed.

[0061] Artificial aging treatment is carried out, for example, at 130°C to 200°C. This type of aging treatment causes precipitates to form that are 1 to several nanometers in size.

[0062] Furthermore, in the case of water quenching, it is possible that Mg2Si precipitates, in addition to Al-Cu-Mg precipitates, also contribute to age hardening.

[0063] Furthermore, in the case of forced air cooling, the Mg2Si precipitates coarseen during the cooling process from the solution treatment temperature, reducing their contribution to age hardening, and the Al-Cu-Mg precipitates become the main contributor to age hardening.

[0064] When water quenching or forced air cooling is performed after solution treatment, the guideline for cooling time from the solution treatment temperature to 50°C is 20 seconds or less, and 2 minutes to 30 minutes, respectively.

[0065] Figure 3 shows the quality index and hardness of the aluminum alloy solidified member according to this embodiment when cast using a sand mold or metal mold and treated under various heat treatment conditions. Hereinafter, the quality index in this specification is defined as the value indexed as Quality Index = a + 150 × log(c) when the member exhibits tensile strength a (MPa) and elongation c (%).

[0066] For example, as shown in Figure 3(a), when a solidified member cast using a sand mold is subjected to T6 treatment, which includes solution treatment, water quenching, and artificial aging treatment, the solidified member exhibits a quality index of 410 or higher and a Vickers hardness of 70 Hv or higher.

[0067] For example, as shown in Figures 3(b) and 3(c), when a solidified member cast using a sand mold is subjected to a T4 treatment including solution treatment and water quenching, or a T6 treatment including solution treatment, forced air cooling, and artificial aging, the solidified member exhibits a quality index of 360 or higher and a Vickers hardness of 70 Hv or higher.

[0068] For example, as shown in Figure 3(d), when a solidified member cast using a sand mold is subjected to a T4 treatment including solution treatment and forced air cooling, the solidified member exhibits a quality index of 360 or higher and a Vickers hardness of 65 Hv or higher.

[0069] For example, as shown in Figures 3(e), 3(f), 3(g), and 3(h), when a solidified member cast using a mold is subjected to a T6 treatment including solution treatment, water quenching or forced air cooling, and artificial aging treatment, or a T4 treatment including solution treatment, water quenching or forced air cooling, the solidified member exhibits a quality index of 410 or higher and a Vickers hardness of 70 Hv or higher.

[0070] As shown in Figure 3, the solution treatment temperature should be higher than 500°C but lower than 540°C to ensure strength and ductility. In the solidified member of the alloy according to this embodiment before heat treatment, there are extremely small (e.g., about 3 μm) Al-Cu-Mg compounds that can only be confirmed with an electron microscope, but these solidify even at a relatively low solution treatment temperature of 490°C for 2 hours. In other words, Cu atoms are included in the aluminum matrix. However, in order to solidify the Mg2Si compounds (e.g., about 5 μm) with complex polygonal shapes observed in the solidified member into the aluminum matrix, it is necessary to maintain a temperature higher than that and for a long period of time. After the solution treatment, precipitation hardening occurs by leaving it at room temperature or by artificial aging. While Al-Cu-Mg and Mg-Si precipitates are considered to be the precipitates that undergo hardening, if the cooling rate to near room temperature after solution treatment is slow (air cooling level rather than water cooling), Al-Cu-Mg precipitates contribute to hardening.

[0071] [Examples] (Comparative examples and examples of various solidification materials) The solidified material is a flat plate with a thickness of 3 mm, a width of 100 mm, and a length of 100 mm, and its casting weight is 400 g. The 3 mm portion was used for testing. Both the sand mold solidified material and the die-molded material have the same shape and size as the final product. However, their casting weights differ. Furthermore, the forced air cooling described below as a quenching condition refers to fan cooling using, for example, a fan or mist fan. Note that the forced air cooling described in Tables 2-4 and 6 is fan cooling. The forced air cooling described in Table 5 is mist fan cooling.

[0072] Table 2 shows the effect of the solution temperature on the mechanical properties of the alloy according to this embodiment.

[0073] [Table 2]

[0074] Comparative Example 1 is a molded product made of AC4CH, a casting alloy known for its typical medium strength and high ductility, and contains 7.0 mass% Si. Comparative Example 1 underwent solution treatment at 530°C followed by air cooling to prevent distortion. Comparative Example 1 exhibits nearly the same high elongation as Example 6, which is also a molded product and underwent similar heat treatment, but its tensile strength and yield strength are lower, and its hardness is less than 70 Hv.

[0075] Examples 1 and 2 are molded products with a solution treatment temperature of 500°C or below, or above 540°C. The quality index of Example 1 is less than 410.

[0076] Although Example 2 shows a high quality index of over 410, as shown in Figure 2(b), there are concerns about variability in fatigue strength due to the numerous localized melting (burning) areas.

[0077] Examples 3, 4, and 5 are sand-cast products with a solution treatment temperature of 500°C or below, or above 540°C. Despite undergoing T6 treatment, Examples 3, 4, and 5 have a quality index of less than 410 and low elongation.

[0078] Examples 6 and 7 are molded products with a solution treatment temperature exceeding 500°C but not exceeding 540°C. Examples 6 and 7 have a quality index of 410 or higher and high elongation.

[0079] Examples 8, 9, and 10 are sand-cast products with a solution temperature exceeding 500°C and being 540°C or lower. Examples 8, 9, and 10 have a quality index of 410 or higher and high elongation.

[0080] To summarize, the aluminum alloy according to this embodiment has the following characteristics. The alloy according to this embodiment exhibits a higher quality index for both sand-cast and molded products when solution-treated at a temperature between 500°C and 540°C. In particular, the alloy according to this embodiment exhibits the highest elongation and a high quality index when solution-treated at a temperature of 530°C.

[0081] The alloy according to this embodiment, which has been solution-treated at 530°C, maintains high elongation even when the quenching rate after solution treatment is slow, and exhibits higher strength than the standard alloy AC4CH alloy.

[0082] Table 3 shows the effect of Si content on the mechanical properties of the alloy according to this embodiment.

[0083] [Table 3]

[0084] Comparative Examples 2, 3, and 4 are sand-cast products with Si content of 6.3, 7.0, and 8.0 mass%, respectively. Comparative Examples 2, 3, and 4 underwent T6 treatment (water quenching), but their quality index did not meet 410. In Comparative Examples 2, 3, and 4, as the Si content increased, the strength did not change, but only the elongation decreased.

[0085] Examples 11, 12, 13, and 14 are sand-cast products with Si content of 4.1, 4.5, 5.5, and 5.8 mass%, respectively. Examples 11, 12, 13, and 14 underwent T6 treatment (water quenching), and all met the quality index of 410 or higher, and the hardness of 70 Hv or higher. In Examples 11, 12, 13, and 14, as the Si content decreases, the tensile strength increases, and the elongation also increases further without decreasing.

[0086] The phenomenon described above, where lower Si content results in higher strength while exhibiting remarkably high elongation, is also clearly evident in molded products.

[0087] Comparative Example 8 is a molded product with a Si content of 7.0 mass%. Comparative Example 8 shows a high elongation of 22% and a quality index of 410 or higher, but its hardness is less than 70 Hv and its tensile strength remains at 250 MPa.

[0088] Example 21 is a molded product with a Si content of 4.5% by mass and has undergone T4 treatment. Example 21 has a quality index of 410 or higher and a hardness of 70 Hv or higher. The tensile strength and yield strength of Example 21 (molded product, T4) are approximately 30 MPa higher than those of Comparative Example 8 (molded product, T4).

[0089] Comparative Example 7 and Example 20 are sand-cast products that have undergone the same T4 treatment (forced air cooling). In the sand-cast products shown in Comparative Example 7 and Example 20, differences in tensile strength and hardness were confirmed, similar to the T4 treatment and molded products shown in Comparative Example 8 and Example 21.

[0090] Examples 15 and 16 are sand-cast products that have undergone T6 treatment (water quenching). Although Examples 15 and 16 exhibit high tensile strength and yield strength due to their long aging period of 5 hours, they still show high elongation, and both meet the quality index of 410 or higher, and the hardness of 70 Hv or higher.

[0091] Example 17 is a sand-cast product with a Si content of 4.5 mass% and has undergone T4 treatment (water quenching). Example 17 exhibits higher tensile strength and elongation than Example 20, which is also a sand-cast product but has undergone T4 (forced air quenching) treatment. Example 17 meets a quality index of 360 and has a hardness of 70 Hv or higher.

[0092] Table 4 shows the effect of the amount of Mg on the mechanical properties of the alloy according to this embodiment.

[0093] [Table 4]

[0094] Comparative Examples 9, 10, and 11 are sand-cast products with Mg content of 0.32, 0.41, and 0.87 mass%, respectively, and have undergone T6 treatment (water quenching). The quality index of Comparative Examples 9, 10, and 11 is less than 410, or even if the quality index is 410 or higher, the hardness is less than 70 Hv.

[0095] Examples 22, 23, 24, and 25 are sand-cast products with Mg content of 0.46, 0.55, 0.65, and 0.75 mass%, respectively, and have undergone T6 treatment (water quenching). All of Examples 22, 23, 24, and 25 meet the quality index requirement of 410 or higher, and the hardness requirement of 70 Hv or higher. In particular, Example 23, with an Mg content of 0.55%, shows a high quality index.

[0096] Example 26 is a sand-cast product with the same chemical composition as Example 23, but it has undergone T4 treatment (forced air cooling). Example 26 exhibits a quality index of 360 or higher and a hardness of 65HV or higher.

[0097] Comparative Example 12 is a sand-cast product with a low Mg content of 0.41% by mass, and has undergone the same T4 treatment (forced air cooling) as Example 26. Comparative Example 12 has a quality index of less than 360 and a hardness of less than 65 Hv.

[0098] Table 5 shows the effect of the amount of Cu on the mechanical properties of the alloy according to this embodiment.

[0099] [Table 5]

[0100] Comparative Examples 13 and 14 are sand-cast products, and the Cu content is 0.001% by mass. Comparative Example 13 has undergone T6 treatment (water quenching), but because the Cu content is trace, the quality index is less than 410. Comparative Example 14 has undergone T4 treatment (forced air cooling), and the quality index is less than 360.

[0101] Examples 27-32 are sand-cast products, with Cu content of 0.05, 0.09, 0.16, 0.5, 0.16, and 0.16% by mass, respectively.

[0102] Examples 27-30 and 32 underwent T6 treatment (water quenching), and all had a quality index of 410 or higher and a hardness of 70 Hv or higher. In particular, Example 29, with a Cu content of 0.16 mass%, showed a high quality index.

[0103] Example 31 underwent T4 treatment (forced air cooling) and exhibited a quality index of 360 or higher and a hardness of 65 Hv or higher. Example 31 also exhibited higher tensile strength than Comparative Example 14, which underwent similar T4 treatment (forced air cooling).

[0104] In Example 32, artificial aging was performed immediately after solution treatment. Because it was not left at room temperature after quenching, it exhibits a higher quality index and higher tensile strength than Example 29.

[0105] Examples 29 and 32 contain trace amounts of Be. The inclusion of Be is effective in improving the flow of hot water and preventing the oxidation of Mg.

[0106] Table 6 shows the effects of various elemental amounts and applied pressure on the mechanical properties of the alloy according to this embodiment.

[0107] [Table 6]

[0108] Examples 34-38 are examples in which Fe, Mn, Ti, and Sr were added in larger amounts compared to Example 33. Compared to Example 33, the quality index, tensile strength, elongation, etc., did not change significantly in Examples 34-38.

[0109] As shown in Example 36, the decrease in elongation is small even in examples with a high Fe content, which is thought to be because the alloy according to this embodiment contains a large amount of Mg, and Mg is included in the Fe compound, causing the compound's external shape to become granular due to the solution treatment.

[0110] Furthermore, the Mn added to the alloy according to this embodiment, when coexisting with Fe, changes the morphology of the needle-shaped Fe compound to a massive form. Therefore, the addition of Mn does not cause a decrease in quality index or tensile properties, and is expected to even improve them when the solidification rate is slower than in this embodiment.

[0111] Examples 37 and 38 contained slightly higher amounts of Ti and Sr, but showed almost the same tensile properties as Example 33, and the quality index remained unchanged.

[0112] Examples 39 and 40 are high-pressure cast products (squeezed products). The high-pressure cast products were cast with a pressure of 100 MPa and a gate speed of 1 m / s, and the product wall thickness and size are the same as those of sand-cast products. The solidification rate of Example 39 is about 10 times faster than that of die-cast products, resulting in a finer metal structure and a higher quality index than that of the forced-air-cooled product shown in Example 21. Example 40 underwent aging treatment immediately after solution treatment, resulting in high quality index, strength, and elongation.

Claims

1. It contains 4.0 to 5.9 mass% Si, 0.46 to 0.80 mass% Mg, 0.05 to 0.50 mass% Cu, one or more of the following additive elements: 0.20 mass% or less Mn, 0.30 mass% or less Fe, 0.25 mass% or less Ti, 0.030 mass% or less Sr, and 0.0050 mass% or less Be, and unavoidable impurities in total amount of 0.50 mass% or less, with the remainder being Al. Aluminum alloy.

2. It contains 0.50 to 0.70% by mass of Mg and 0.05% by mass or more and less than 0.25% by mass of Cu. The aluminum alloy according to claim 1.

3. The metal structure contains independently dispersed eutectic Si, Some of the Mg and Si have a smooth outer shape. ２ It exists as a Si compound, The remaining Mg and Si are characterized by being dissolved or precipitated in the primary aluminum and eutectic aluminum matrix phases. An aluminum alloy solidification member made of the aluminum alloy according to claim 1 or claim 2.

4. The Vickers hardness is between 65 Hv and 120 Hv. The aluminum alloy solidification member according to claim 3.

5. A method for manufacturing an aluminum alloy solidification member made of the aluminum alloy described in claim 1, The process involves pouring molten aluminum alloy into a sand mold, The process of solidifying the molten aluminum alloy in the sand mold, The process of removing the solidified material from the sand mold, The solidified member is subjected to a T6 treatment which includes solution treatment, water quenching, and artificial aging treatment. Includes, The solidification member is If a material has tensile strength a (MPa) and elongation c (%), and its quality index is expressed as a + 150 × log(c), then the quality index is 410 or higher. The Vickers hardness is 70 Hv or higher. A method for manufacturing solidified aluminum alloy components.

6. A method for manufacturing an aluminum alloy solidification member made of the aluminum alloy described in claim 1, The process involves pouring molten aluminum alloy into a sand mold, The process of solidifying the molten aluminum alloy in the sand mold, The process of removing the solidified material from the sand mold, The solidification member, T4 treatment including solution treatment and water quenching, or The T6 treatment includes the aforementioned solution treatment, forced air cooling, and artificial aging treatment. The process of performing and Includes, The solidification member is If a material has tensile strength a (MPa) and elongation c (%), and is expressed as a + 150 × log(c), then the quality index is 360 or higher. The Vickers hardness is 70 Hv or higher. A method for manufacturing solidified aluminum alloy components.

7. A method for manufacturing an aluminum alloy solidification member made of the aluminum alloy described in claim 1, The process involves pouring molten aluminum alloy into a sand mold, The process of solidifying the molten aluminum alloy in the sand mold, The process of removing the solidified material from the sand mold, The solidified member undergoes a T4 treatment which includes solution treatment and forced air cooling. Includes, The solidification member is If a material has tensile strength a (MPa) and elongation c (%), and is expressed as a + 150 × log(c), then the quality index is 360 or higher. The Vickers hardness is 65 Hv or higher. A method for manufacturing solidified aluminum alloy components.

8. A method for manufacturing an aluminum alloy solidification member made of the aluminum alloy described in claim 1, The process of pouring molten aluminum alloy into a mold, The process of solidifying the molten aluminum alloy in the mold, A step of removing the solidified material from the mold, The solidification member, T6 treatment including solution treatment, water quenching or forced air cooling, and artificial aging treatment, The T4 treatment includes the solution treatment and the water quenching or forced air cooling. The process of performing and Includes, The solidification member is If a material has tensile strength a (MPa) and elongation c (%), and its quality index is expressed as a + 150 × log(c), then the quality index is 410 or higher. The Vickers hardness is 70 Hv or higher. A method for manufacturing solidified aluminum alloy components.

9. The solution treatment is carried out at a temperature higher than 500°C and lower than 540°C. A method for manufacturing an aluminum alloy solidification member according to any one of claims 5 to 8.

10. The forced air cooling described above is performed using a fan or a mist fan. A method for manufacturing an aluminum alloy solidified member according to any one of claims 6 to 8.

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