Aluminum alloy, aluminum alloy solidified part, and method of manufacturing the same

Abstract

The present invention provides an alloy exhibiting medium strength and high ductility, particularly high toughness, a solidified part, 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, and 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, 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 a trace amount of Cu of 0.5% or less, an aluminum alloy solidified part, and a method for producing the same. The present invention relates to a medium-strength alloy that exhibits high ductility and, as necessary, a tensile strength of approximately 200 to 350 MPa, various solidified parts such as sand-cast parts, permanent mold-cast parts, and high-pressure cast parts (low-speed fill squeeze parts), and a method for producing the same. In particular, the precipitates that strengthen the alloy are resistant to softening even when subjected to heat at temperatures above 300°C, and therefore the present invention relates to a technology that can reliably achieve desired mechanical properties while suppressing distortion and residual stress within the product that occur during cooling from heat treatment, i.e., solution treatment. [Background technology]

[0002] Table 1 shows typical compositions of JIS-standard aluminum alloy castings and AA-standard aluminum alloy castings (see Non-Patent Documents 1 and 2).

[0003] [Table 1]

[0004] Alloys can be roughly classified into a) Al-Si-Mg alloys, b) Al-Si alloys, c) Al-Si-Cu alloys, and d) Al-Si-Mg-Cu alloys. Among these, Al-Si-Mg alloys, which contain 6.5% or more Si and 0.3% Mg as the primary elements with other elements as impurities, have good corrosion resistance. A356 and AC4CH alloys in particular are widely used in critical automotive safety components, such as wheels. Their strengthening mechanism is age hardening due to precipitates composed of the primary elements Mg and Si. By selecting aging treatment among the heat treatment conditions and minimizing internal defects, these alloys can exhibit high ductility exceeding 10% and moderate strength of 250–300 MPa.

[0005] However, it is known that if a solidified material is subjected to solution treatment above 500°C, for example, and then quenched slowly through a temperature range of 300°C to 400°C, the Mg-Si precipitates become coarse, the tensile strength and elongation decrease, and the mechanical properties obtained with a normal quenching rate cannot be obtained (see, for example, Non-Patent Document 3). It is easy to predict that this phenomenon will occur in Al-Si-Mg alloys due to the same precipitation mechanism, regardless of the Si content, 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. Furthermore, because they are not subjected to heat treatment including solution treatment, the eutectic Si is not granular, and so their elongation is low.

[0007] Al-Si-Cu alloys such as AC2B and AC4B are used in manifolds, cylinder heads, etc., and can achieve high tensile strength of over 350 MPa through heat treatment. However, these alloys contain trace amounts of Mg, which is treated as an impurity, and this is limited to the case where aging treatment is carried out utilizing this.

[0008] Furthermore, Al-Si-Cu alloys contain a large amount of Cu, resulting in poor corrosion resistance. Furthermore, during casting, the molten metal is poorly replenished in the final solidification zone during solidification in the mold. Sand-cast products are prone to the frequent occurrence of minute shrinkage cavities, typically 0.5 mm or smaller, especially in the final solidification zone. Furthermore, even if solution treatment is performed, the large amount of Cu content means that, given the final solidification temperature of approximately 500°C, solution treatment above that temperature poses the risk of localized melting. Therefore, the morphology of Cu-containing compounds formed during casting, which cause elongation loss, cannot be smoothed and sufficiently reduced. Furthermore, the granulation of eutectic silicon, which is effective in improving elongation, requires a long time. In summary, high elongation cannot be easily achieved due to the effects of shrinkage cavities in the solidified material, the morphology and size of Cu compounds, and insufficient granulation of eutectic silicon.

[0009] There are alloys such as alloys 324 and 336 that are Al-Si-Cu alloys but contain Mg as an effective element. However, like AC2B and AC4B, these alloys also have poor corrosion resistance and are affected by shrinkage cavities, Cu compounds, and the morphology of eutectic Si, making it difficult to achieve high elongation. [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 international alloy symbols) [Non-patent document 3] N. Saruwatari: Materials Transaction, 64(2023), 2575-2583 Summary of the Invention [Problem to be solved by the invention]

[0011] Traditionally, most automotive parts, with the exception of die-cast parts, have been thick-walled. However, with the recent shift toward electric vehicles, there has been active research and development into thinner die-cast parts for large vehicle bodies, including battery cases, rear and front parts. In parallel with this, development of large, thin sand castings has also been progressing as necessary for pre-testing the performance of die-cast parts with various mechanical properties. Furthermore, weight reduction is required not only in the automotive industry, but also in industrial equipment parts, such as robot arms. [Means for solving the problem]

[0012] In order to solve the above problems, a first aspect of the present invention provides an aluminum alloy containing 4.0 to 5.9 mass% Si, 0.46 to 0.80 mass% Mg, 0.05 to 0.50 mass% Cu, and one or more of 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 additional elements, and unavoidable impurities in a total amount of 0.50 mass% or less, with the balance being Al.

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

[0014] An aluminum alloy solidified portion according to a 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 eutectic Si independently dispersed in the metallographic structure, with part of Mg and Si existing as MgSi compounds having a smooth outer shape, and other parts of Mg and Si being solid-dissolved or precipitated in the primary aluminum and eutectic aluminum parent phase.

[0015] Alternatively, the solidified aluminum alloy portion according to a second aspect of the present invention is characterized in that it has 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 producing an aluminum alloy solidified portion made of the aluminum alloy according to the first aspect of the present invention, comprising the steps of pouring a molten aluminum alloy into a sand mold, solidifying the molten aluminum alloy in the sand mold, removing the solidified portion from the sand mold, and subjecting the solidified portion to a T6 treatment including solution treatment, water quenching, and artificial aging treatment. When the solidified portion has properties of tensile strength a (MPa) and elongation c (%), it has a quality index expressed as a + 150 × log(c) of 410 or more and a Vickers hardness of 70 Hv or more.

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

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

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

[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. [Effects of the Invention]

[0022] The aluminum alloy and aluminum alloy solidified part according to the present invention contain a slightly higher amount of Mg and a very small amount of Cu than conventional standard alloys. This allows the aluminum alloy solidified part to develop precipitates containing Cu and Mg that are resistant to softening even at high temperatures, thereby achieving a desired strength. Furthermore, the aluminum alloy solidified part according to the present invention has low amounts of Fe compounds and eutectic Si, which serve as crack initiation and propagation paths, and optionally contains Sr, which refines the eutectic Si. Furthermore, the method for producing the aluminum alloy solidified part according to the present invention involves granulating the Fe compounds and eutectic Si by solution treatment, followed by water quenching or forced air cooling in the atmosphere, and then optionally artificial aging treatment. This provides an aluminum alloy solidified part and a method for producing the same that exhibits higher ductility and higher strength than AC4CH and A356 alloys.

[0023] Furthermore, unlike mold-solidified materials, metal additive manufacturing (MAM) has an extremely fine metal structure without solution treatment, and various elements are dissolved in large amounts in the aluminum matrix. MAM exhibits a solidification rate 10 to 100 times faster than, for example, die casting, which has a fast solidification rate among mold-solidified materials. However, because MAM continues to be heated to, for example, 200°C during manufacturing, the lower part of an aluminum alloy molded 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 manufactured by metal additive manufacturing, the manufactured body does not soften because large amounts of Mg and Si are dissolved in the body, and even if precipitates are generated, they contain Cu, which does not over-age in the temperature range of 200°C. Therefore, when the alloy according to the present invention is manufactured by metal additive manufacturing, variations in the mechanical properties of the manufactured body and softening can be avoided. [Brief explanation of the drawings]

[0025] [Figure 1] FIG. 1 is a photograph showing the metal structure of a metal mold casting product of an 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] FIG. 1 is a photograph showing the effect of solution treatment temperature (490°C x 6 hours, 550°C x 6 hours) on the metal structure of a metal mold casting product of an 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] FIG. 1 is a schematic diagram showing the influence of heat treatment conditions on the quality index and hardness of a mold-solidified material and a sand-solidified material using the alloy according to this embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0026] [First embodiment] The aluminum alloy according to the first embodiment of the present invention contains 4.0 to 5.9 mass% Si, 0.46 to 0.80 mass% Mg, 0.05 to 0.50 mass% Cu, and as additional elements, one or more of 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, and the balance being Al.

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

[0028] In this specification, "unavoidable impurities" refers to elements other than Al, Si, Mg, Cu, Mn, Fe, Ti, Sr, and Be, such as V, Cr, P, Pb, Sn, Ni, etc. Note that oxygen is not included in "unavoidable impurities."

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

[0030] The solidified aluminum alloy portion 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 components obtained by gravity casting, low-pressure casting, and low-speed filling high-pressure casting is 1 / 10 to 1 / 100 of the cooling rate during solidification obtained by die casting. Therefore, the solidified components obtained by these high-pressure casting methods do not have as fine a metal structure as the solidified components obtained by die casting, and it is not easy to achieve high ductility without solution treatment.

[0032] First, let's look at improving ductility. Unlike die casting, the high-pressure casting method mentioned above, which does not involve seizure between the mold and the molten metal, is best achieved by minimizing the amount of iron in order to reduce the iron compounds that cause reduced ductility. Then, if necessary, a small amount of manganese is added to change the shape of the needle-like iron compounds to granular.

[0033] In this embodiment, where the Mg content is high, some Fe compounds contain Mg, and when solution-treated, the morphology becomes granular or smooth, which has the effect of suppressing the initiation and propagation of cracks.

[0034] At the same time, reducing the amount of silicon is extremely effective in suppressing the causes of crack initiation and propagation. Furthermore, granulating eutectic silicon through solution treatment is even more effective in suppressing crack initiation and propagation. However, in order to reduce distortion in the product during quenching, it is necessary to slow the cooling rate from the solution treatment temperature. In this case, not only does the strength decrease, but elongation also decreases significantly.

[0035] In consideration of these, it is necessary to select an appropriate Si content that does not adversely affect the fluidity of the molten metal. To this end, the Si content of the aluminum alloy solidified portion according to this embodiment is set to 4.0 to 5.9%. The Si content may be 4.0 to 5.5%. Setting the Si content in this range is an effective method for achieving extremely high ductility while achieving a predetermined strength by adding Cu.

[0036] Next, from the viewpoint of strength improvement, in the aluminum alloy solidified portion according to this embodiment, Mg forms fine precipitates of 1 to several nm in size together with Si, and Cu forms fine precipitates of 1 to several nm in size together with Al and Mg, by heat treatment, thereby contributing to strength improvement.

[0037] Conventionally, when cooling to room temperature after solution treatment, air cooling from the solution temperature was performed to prevent distortion in the product, and the Mg-Si precipitates would coarsen during the cooling process, which sometimes did not contribute to improving strength.

[0038] However, when the aluminum alloy solidified portion according to this embodiment contains 0.05% by mass or more of Cu and 0.46% by mass or more of Mg, age hardening due to 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 reduction during quenching. However, if the amount of Mg added is too large, ductility decreases. Therefore, in this embodiment, the Mg content is set to 0.46 to 0.80% by mass. Preferably, the Mg content is set to 0.50 to 0.70% by mass.

[0039] In the aluminum alloy solidified portion according to this embodiment, Cu forms fine precipitates with Al and Mg, each having a size of one to several nanometers, thereby contributing to improving strength. However, if the Cu content is too high, corrosion resistance and ductility will decrease. In this embodiment, the Cu content is set to 0.05 to 0.50 mass%. Preferably, the Cu content is set to 0.05 mass% or more and less than 0.25 mass%. More preferably, the Cu content is set to 0.05 to 0.20 mass%.

[0040] In the aluminum alloy solidified portion according to this embodiment, Mn is added as needed to improve ductility by forming a compound with Fe. However, since the amount of Fe itself is small in this embodiment, the addition of Mn is hardly necessary, and even if Mn is added, it is limited to 0.20 mass% or less.

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

[0042] In the aluminum alloy solidified part according to this embodiment, Ti is added as a grain refiner to suppress the occurrence of shrinkage cavities caused by insufficient replenishment of molten metal during the solidification process. Since there is no improvement in the effect even if Ti is added in excess of 0.25% by mass, the Ti content is set to 0.25% by mass or less in this embodiment. In sand castings, there is also concern about a decrease in elongation due to the generation of Ti compounds, so the Ti content is set to 0.20% by mass or less.

[0043] In the aluminum alloy solidified part according to this embodiment, the amount of Sr required for refining the eutectic Si grains varies depending on the amount of P impurity contained in the molten alloy and the solidification rate of the molten alloy in the mold. To ensure the refining effect and ensure ductility, the Sr content is set to 0.005 to 0.030 mass% for sand castings and 0.030 mass% or less for permanent mold castings. However, too much Sr can form Sr oxides that are mixed into the product, reducing quality and fluidity. It can also form Al-Si-Sr compounds, reducing elongation. For this reason, in this embodiment, the Sr content is preferably set to 0.020 mass% or less. In the case of high-pressure cast products, the cooling rate during solidification is faster than that of permanent mold castings, so Sr is added as needed, but may not be necessary in some cases. The Sr content is a maximum of 0.015 mass% or less.

[0044] In the aluminum alloy solidified portion according to this embodiment, Sr and Mg form oxides that have the effect of reducing the fluidity of the molten metal. Therefore, in this embodiment, Be is added in an amount of 0.0050 mass% or less depending on the size and thickness of the product. The Be content is preferably 0.0005 mass% to 0.0020 mass%.

[0045] [Metal structure] The aluminum alloy solidified portion according to the present embodiment contains eutectic Si dispersed independently in the metallographic structure, with some of the Mg and Si existing as MgSi compounds having a smooth outer shape, and the remaining Mg and Si being dissolved or precipitated in the primary aluminum and eutectic aluminum parent phases.

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

[0047] Figures 1(a) and 1(b) are photographs showing the metallographic structure of a T4 alloy mold casting made of an Al-4.5% by mass Si-0.55% by mass Mg-0.18% by mass Cu-0.012% by mass Sr alloy before (Figure 1(a)) and after (Figure 1(b)) solution treatment at 530°C for 6 hours. Figures 1(a) and 1(b) show the results of observations using an optical microscope. The mold casting was cast into a boat-shaped mold according to JISH5202.

[0048] The components of the Mg2Si compounds and Fe-containing compounds seen in Figures 1 and 2 were identified by observing backscattered electron images (BSE) using a field emission scanning electron microscope (FE-SEM) and elemental mapping using energy dispersive X-ray spectroscopy (EDS) with a scanning transmission electron microscope (STEM). It has been confirmed that the Fe-containing compounds are mainly Al-Fe-Si-Mg compounds, the outer shape of which becomes smooth after solution treatment.

[0049] As shown in Figure 1(a) and Figure 1(b), the metal structure changes before and after solution treatment. The metal structure contains an aluminum matrix, Mg2Si compounds, and compounds containing Fe. In the structure shown in Figure 1, the lightest colored part is the aluminum matrix.

[0050] In the structures shown in Figures 1(a) and 1(b), the darkest areas, indicated by the black arrows, are Mg2Si compounds. For example, as shown in Figure 1(a), before solution treatment, Mg2Si compounds are mainly surrounded by linear shapes, e.g., polygonal shapes. On the other hand, Figure 1(b) shows the microstructure after solution treatment at a temperature of 530°C. The Mg2Si compounds almost completely disappear after solution treatment, and the remaining compounds are no longer polygonal but have smooth shapes. Furthermore, there is almost no burning, indicating local melting, observed in the structures shown in Figures 1(a) and 1(b).

[0051] In the structure shown in Figures 1(a) and 1(b), the area of ​​intermediate brightness between the Mg2Si compound and the aluminum matrix, indicated by the white arrow, is a compound containing Fe. For example, as shown in Figures 1(a) and 1(b), the morphology of a compound containing Fe changes little before and after solution treatment, but extending the solution treatment time results in a morphology with a smoother outer shape.

[0052] [Manufacturing method] The method for producing a solidified aluminum alloy portion according to the present embodiment is, for example, as follows.

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

[0054] The solidified portion is subjected to solution treatment within an appropriate temperature range. Here, the effect of the solution treatment temperature on the aluminum alloy solidified portion cast using a mold will be described. The solidified portion was cast into a boat shape as specified in JISH5202 and collected.

[0055] Figure 2(a) shows the microstructure of a solidified part that underwent solution treatment at 490°C for 6 hours. Because the solution treatment temperature was low, the fibrous eutectic Si shown in Figure 1(a) has become granular, but the Mg2Si compounds still exist in large numbers, although they have a smooth outer shape. No burning (local melting) of the Mg2Si compounds in the metal structure was observed.

[0056] Figure 2(b) shows the microstructure of a solidified component that underwent solution treatment at 550°C for 6 hours. When solution treatment was performed at 550°C, the burning phenomenon became more pronounced, and numerous holes larger than 10 μm in size due to local melting were observed. The solidified component solidified at a solution treatment temperature of 550°C had high tensile strength, proof stress, and Vickers hardness, but due to the occurrence of microscopic holes larger than 10 μm, its elongation was lower than that of the component solidified at 530°C.

[0057] The effect of the solution temperature on the mechanical properties also shows a similar tendency in solidified parts made using a sand mold.

[0058] In the method for producing an aluminum alloy solidified portion according to this embodiment, solution treatment is performed at a temperature in the range of more than 500°C and not more than 540°C. Note that 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] In the solidified specimen solidified at a solution treatment temperature of 530°C, the MgSi compound is dissolved in the aluminum matrix as Mg and Si in large amounts, and there are almost no micropores. Therefore, the solidified specimen solidified at a solution treatment temperature of 530°C has high tensile strength, proof stress, Vickers hardness, and elongation.

[0060] After such solution treatment, the material is water quenched or forced air cooled in the atmosphere (natural aging), and if necessary, artificial aging treatment is carried out.

[0061] The artificial aging treatment is carried out, for example, at 130° C. to 200° C. Such an aging treatment causes precipitates having a size of 1 to several nm to be precipitated.

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

[0063] In the case of forced air cooling, the Mg2Si precipitates coarsen during the cooling process from the solution treatment temperature, reducing their contribution to age hardening, and the Al-Cu-Mg precipitates mainly contribute to age hardening.

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

[0065] 3 shows the quality index and hardness when the aluminum alloy solidified part according to this embodiment is cast using a sand mold or a metal mold and subjected to various heat treatment conditions. Hereinafter, the quality index in this specification refers to a value indexed by the formula "quality index = a + 150 × log(c)" when the part has the characteristics of tensile strength a (MPa) and elongation c (%).

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

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

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

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

[0070] As shown in Figure 3, the solution treatment temperature is higher than 500°C and not higher than 540°C to ensure strength and ductility. Before heat treatment, the solidified alloy according to this embodiment contains extremely small Al-Cu-Mg compounds (e.g., approximately 3 μm) that can be barely seen with an electron microscope. These compounds are dissolved even after treatment at a relatively low solution treatment temperature of 490°C for two hours. This means that Cu atoms are contained in the aluminum matrix. However, to dissolve the complex polygonal Mg2Si compounds (e.g., approximately 5 μm) observed in the solidified alloy into the aluminum matrix, it is necessary to maintain the temperature above this temperature for a long period of time. After solution treatment, the alloy undergoes precipitation hardening by leaving it at room temperature or by artificial aging. The precipitates that cause hardening are thought to be Al-Cu-Mg precipitates and Mg-Si precipitates, but if the cooling rate to near room temperature after solution treatment is slow (air cooling rather than water cooling), it is the Al-Cu-Mg precipitates that contribute to hardening.

[0071] [Example] (Comparative Examples and Examples of Various Solidified Materials) The solidified portion had a plate-like shape with a wall thickness of 3 mm, a width of 100 mm, and a length of 100 mm, and the cast weight was 400 g. This 3 mm portion was used for testing. The sand-molded solidified portion and the mold-solidified portion both had the same product shape and size. However, the cast weights differed. Furthermore, the forced air cooling described below as a quenching condition is fan air cooling using, for example, a fan or mist fan. The forced air cooling described in Tables 2 to 4 and 6 is fan air cooling. The forced air cooling described in Table 5 is mist fan air 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 typical casting alloy known for its medium strength and high ductility, containing 7.0 mass% Si. Comparative Example 1 was subjected to air cooling after solution treatment at 530°C to prevent distortion. Comparative Example 1 exhibits high elongation almost equivalent to that of Example 6, which was also a molded product and underwent the same heat treatment. However, the tensile strength and yield strength were both low, and the hardness was less than 70 Hv.

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

[0076] Example 2 exhibits a high quality index of over 410, but as shown in FIG. 2(b), numerous local meltings (burning) are observed, raising concerns about the variation in fatigue strength.

[0077] Examples 3, 4, and 5 are sand mold products, and the solution temperature is 500° C. or less or exceeds 540° C. Although Examples 3, 4, and 5 were subjected to T6 treatment, the quality index was less than 410 and the elongation was low.

[0078] Examples 6 and 7 are molded products, and the solution treatment temperature is more than 500° C. and not more than 540° C. Examples 6 and 7 have a quality index of 410 or more and high elongation.

[0079] Examples 8, 9, and 10 are sand mold products, and the solution temperature is greater than 500° C. and equal to or less than 540° C. Examples 8, 9, and 10 have a quality index of 410 or more and high elongation.

[0080] To summarize the above, 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 molded products and die molded products when solution treated at a temperature above 500°C and not higher than 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 the present 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 AC4CH alloy.

[0082] Table 3 shows the effect of the 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 mold products with Si contents of 6.3, 7.0, and 8.0 mass%. Comparative Examples 2, 3, and 4 were subjected to T6 treatment (water-quenching), but did not satisfy the quality index of 410. In Comparative Examples 2, 3, and 4, as the Si content increases, the strength does not change, but only the elongation decreases.

[0085] Examples 11, 12, 13, and 14 were sand molded products with Si contents of 4.1, 4.5, 5.5, and 5.8 mass%. Examples 11, 12, 13, and 14 underwent T6 treatment (water-quenching quenching), and all had a quality index of 410 or higher and a hardness of 70 Hv or higher. In Examples 11, 12, 13, and 14, as the Si content decreased, the tensile strength increased, and the high elongation also increased without any decrease.

[0086] The phenomenon that the lower the Si content, the higher the strength and the remarkably high elongation are exhibited, is also evident in molded products.

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

[0088] Example 21 is a molded product, has a Si content of 4.5 mass%, and has been subjected to T4 treatment. Example 21 has a quality index of 410 or more and a hardness of 70 Hv or more. 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 molded products and have undergone the same T4 treatment (forced air cooling). The sand molded products shown in Comparative Example 7 and Example 20 also show differences in tensile strength and hardness, similar to the T4 treatment and molded products shown in Comparative Example 8 and Example 21.

[0090] Examples 15 and 16 are sand mold products that have undergone T6 treatment (water-quenching). Examples 15 and 16 have a long aging time of 5 hours, so they exhibit high tensile strength and yield strength, but still exhibit high elongation, and all satisfy a quality index of 410 or higher and a hardness of 70Hv or higher.

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

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

[0093] [Table 4]

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

[0095] Examples 22, 23, 24, and 25 are sand mold products with Mg contents of 0.46, 0.55, 0.65, and 0.75% by mass, and have been subjected to T6 treatment (water-quenched quenching). All of Examples 22, 23, 24, and 25 have a quality index of 410 or higher and a hardness of 70 Hv or higher. Example 23, which contains 0.55% Mg, exhibits a particularly high quality index.

[0096] Example 26 is a sand mold product, and has the same chemical composition as Example 23, but has been subjected to T4 treatment (forced air cooling). Example 26 exhibits a quality index of 360 or more and a hardness of 65 HV or more.

[0097] Comparative Example 12 is a sand mold product with a low Mg content of 0.41 mass %, and has been subjected to the same T4 treatment (forced air cooling) as in 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 Cu content on the mechanical properties of the alloy according to this embodiment.

[0099] [Table 5]

[0100] Comparative Examples 13 and 14 are sand mold products, and the Cu content is 0.001% by mass. Comparative Example 13 was subjected to T6 treatment (water-quenching), but the Cu content was small, so the quality index was less than 410. Comparative Example 14 was subjected to T4 treatment (forced air cooling), and the quality index was less than 360.

[0101] Examples 27 to 32 are sand mold products, and the Cu amounts are 0.05, 0.09, 0.16, 0.5, 0.16, and 0.16 mass %.

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

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

[0104] Example 32 was artificially aged immediately after solution treatment. Since it was not left at room temperature after quenching, it had a higher quality index than Example 29 and exhibited high tensile strength.

[0105] Examples 29 and 32 contain a small amount of Be, which is effective in improving the flow of molten metal and preventing oxidation of Mg.

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

[0107] [Table 6]

[0108] Examples 34 to 38 are examples in which larger amounts of Fe, Mn, Ti, and Sr were added compared to Example 33. Compared with Example 33, Examples 34 to 38 did not show significant changes in quality index, tensile strength, elongation, etc.

[0109] The reason why the decrease in elongation is small even in examples with a large Fe content, such as Example 36, is thought to be because the alloy according to this embodiment contains a large amount of Mg, so Mg is contained in the Fe compound, and the external shape of the compound becomes granular due to the solution treatment.

[0110] Furthermore, the Mn added to the alloy according to this embodiment changes the morphology of the acicular Fe compounds to clumps when coexisting with Fe. Therefore, the addition of Mn does not cause a decrease in the quality index or tensile properties, and is expected to actually have an improving effect when the solidification rate is slower than in this example.

[0111] Although Examples 37 and 38 contained slightly larger amounts of Ti and Sr, they exhibited almost the same tensile properties as Example 33, and there was no change in the quality index.

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

Claims

1. The alloy contains 4.0 to 5.9 mass% Si, 0.46 to 0.80 mass% Mg, 0.05 to 0.50 mass% Cu, and 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, as well as unavoidable impurities in a total amount of 0.50 mass% or less, with the balance being Al. A method for producing an aluminum alloy solidified portion made of an aluminum alloy, comprising: Pouring the molten aluminum alloy into a sand mold; solidifying the molten aluminum alloy in the sand mold; removing the solidified portion from the sand mold; performing a T6 treatment on the solidified portion, which includes a solution treatment, a water quenching, and an artificial aging treatment; Including, The solidified portion is When the material has a tensile strength of a (MPa) and an elongation of c (%), the quality index expressed as a + 150 × log (c) is 410 or more; Vickers hardness is 70Hv or more A method for manufacturing a solidified aluminum alloy component.

2. The alloy contains 4.0 to 5.9 mass% Si, 0.46 to 0.80 mass% Mg, 0.05 to 0.50 mass% Cu, and 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, as well as unavoidable impurities in a total amount of 0.50 mass% or less, with the balance being Al. A method for producing an aluminum alloy solidified portion made of an aluminum alloy, comprising: Pouring the molten aluminum alloy into a sand mold; solidifying the molten aluminum alloy in the sand mold; removing the solidified portion from the sand mold; The solidified portion is T4 treatment, which includes solution treatment and water quenching; or T6 treatment, which includes the solution treatment, forced air cooling, and artificial aging treatment A process of performing Including, The solidified portion is When the material has a tensile strength of a (MPa) and an elongation of c (%), the quality index expressed as a + 150 × log (c) is 360 or more; Vickers hardness is 70Hv or more A method for manufacturing a solidified aluminum alloy component.

3. The alloy contains 4.0 to 5.9 mass% Si, 0.46 to 0.80 mass% Mg, 0.05 to 0.50 mass% Cu, and 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, as well as unavoidable impurities in a total amount of 0.50 mass% or less, with the balance being Al. A method for producing an aluminum alloy solidified portion made of an aluminum alloy, comprising: Pouring the molten aluminum alloy into a sand mold; solidifying the molten aluminum alloy in the sand mold; removing the solidified portion from the sand mold; performing a T4 treatment on the solidified portion, the T4 treatment including a solution treatment and forced air cooling; Including, The solidified portion is When the material has a tensile strength of a (MPa) and an elongation of c (%), the quality index expressed as a + 150 × log (c) is 360 or more; Vickers hardness is 65Hv or more A method for manufacturing a solidified aluminum alloy component.

4. The alloy contains 4.0 to 5.9 mass% Si, 0.46 to 0.80 mass% Mg, 0.05 to 0.50 mass% Cu, and 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, as well as unavoidable impurities in a total amount of 0.50 mass% or less, with the balance being Al. A method for producing an aluminum alloy solidified portion made of an aluminum alloy, comprising: Pouring the molten aluminum alloy into a mold; solidifying the molten aluminum alloy in the mold; removing the solidified portion from the mold; The solidified portion is T6 treatment, which includes solution treatment, water quenching or forced air cooling, and artificial aging; or T4 treatment including the solution treatment and the water quenching or forced air cooling A process of performing Including, The solidified portion is When the material has a tensile strength of a (MPa) and an elongation of c (%), the quality index expressed as a + 150 × log (c) is 410 or more; Vickers hardness is 70Hv or more A method for manufacturing a solidified aluminum alloy component.

5. The solution treatment is carried out at a temperature higher than 500°C and not higher than 540°C. The method for producing a solidified aluminum alloy part according to any one of claims 1 to 4.

6. The forced air cooling is performed using a fan or a mist fan. The method for producing a solidified aluminum alloy part according to any one of claims 2 to 4.

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