Aluminum alloy for casting, aluminum alloy casting, and method for producing same

Abstract

The present invention provides an aluminum alloy for casting that can be suitably used for a casting method having a low casting speed and makes it possible to obtain high mechanical properties without applying a solution treatment or quenching, even in a casting having a thin wall part. An aluminum alloy for casting according to the present invention is characterized by containing 4.6-5.5 wt% of Mg, 0.4-1.5 wt% of Si, 0.2-0.6 of wt% Cu, 0.2-1.5 wt% of Mn, and 0.01-0.5 wt% of Ti, the balance being Al and inevitable impurities. The aluminum alloy for casting according to the present invention preferably furthermore contains more than 0 wt% and up to 0.2 wt% of one or more from among B, Zr, and V.

Description

Aluminum alloy for casting, aluminum alloy casting, and manufacturing method thereof

[0001] The present invention relates to an aluminum alloy for casting, particularly an aluminum alloy for casting that can be suitably used in casting methods with slow casting speeds, an aluminum alloy casting made from said aluminum alloy for casting, and a method for producing said aluminum alloy casting.

[0002] The greatest feature of casting is its high degree of freedom in shape, making it easy to mold even complex-shaped products. However, when producing aluminum alloy castings, the introduction of strain due to the temperature history during casting can sometimes be a problem. For example, if strain accumulates inside the aluminum alloy casting, it becomes difficult to demold, and if the aluminum alloy casting has thin-walled portions, the thin-walled portions will deform during demolding.

[0003] On the other hand, although it is effective to reduce the casting speed in order to suppress distortion during casting, it is difficult to impart excellent mechanical properties to the aluminum alloy casting obtained under such casting conditions. Furthermore, in order to enhance the effect of aging treatment and improve the strength of the aluminum alloy casting, solution treatment and quenching are generally performed, but if there are thin-walled portions, these heat treatments may cause cracks or deformation in the aluminum alloy casting.

[0004] Here, the JIS AC7A Al-Mg alloy is known as a highly tough, non-heat-treatable alloy, but given the increasing demands for strength and yield strength in aluminum alloy castings, it is difficult to say that it exhibits sufficient mechanical properties.Al-Mg alloys also have poorer fluidity than Al-Si alloys, and when producing castings with large thickness variations, there is a risk of cracks occurring due to poor fluidity or insufficient replenishment of molten metal during solidification.

[0005] In contrast, Patent Document 1 (Japanese Patent Laid-Open Publication No. 62-89849) proposes a method for producing an aluminum alloy casting that has excellent castability, medium strength, and high ductility, and whose anodized coating is not discolored, and proposes a method for producing an aluminum alloy casting, comprising: subjecting an aluminum alloy containing 2.5 to 4.5 wt. % Mg, 0.1 to 1.0 wt. % Si, 1.0 wt. % or less Mn, 0.05 to 0.3 wt. % Ti, 0.02 wt. % or less B, 1.0 wt. % or less Fe, 0.001 to 0.1 wt. % Be, 0.1 to 1.5 wt. % Zn, 0.1 to 0.5 wt. % Cu, and 0.05 to 0.4 wt. % Cd, with the balance being Al and inevitable impurities, to a solution treatment at 560 to 600°C, followed by a tempering treatment at 100 to 300°C.

[0006] The manufacturing method of aluminum alloy castings described in Patent Document 1 above is described as follows: "By reducing the Mg content, adding Si, and adding an appropriate amount of one or more of Zn, Cu, and Cd while adjusting the amounts of other additive elements and by increasing the temperature of solution treatment, Si, Cu, and Cd are sufficiently dissolved in the matrix, and by subsequent tempering, Mg-Si compounds and Al-Cu compounds are precipitated in fine form, making it possible to obtain an aluminum alloy casting with sufficient tensile strength and yield strength without reducing elongation."

[0007] Furthermore, Patent Document 2 (WO 2003 / 023080) aims to "provide an aluminum alloy that exhibits castability suitable for producing thin-walled castings and the like, and that exhibits high strength and excellent ductility even in an as-cast state, and a method for producing castings made from said aluminum alloy," and proposes "an aluminum alloy characterized by containing, when the whole is taken as 100% by mass (mass percentage), 4.0 to 6.0% magnesium (Mg), 0.3 to 0.6% manganese (Mn), and 0.5 to 0.9% iron (Fe), with the balance being aluminum (Al) and unavoidable impurities."

[0008] In the aluminum alloy described in Patent Document 2, it is stated that "by containing Mg, Mn, and Fe in an appropriate composition ratio, castability is improved and high strength and high ductility can be achieved." It also states that "the crystallization temperature range of primary Al crystals is narrowed, and without allowing the dendrites of primary Al crystals to grow significantly, Al-Mn-Fe eutectic is crystallized between the network gaps of primary Al crystals that have completed crystallization. Then, in this state, connections between the solid phases rapidly progress, which is thought to make it less likely for casting cracks to occur."

[0009] Japanese Patent Application Laid-Open No. 62-89849 International Publication No. 2003 / 023080

[0010] However, when solution treatment and quenching are performed as in the manufacturing method of aluminum alloy castings described in Patent Document 1, cracks and deformations may occur in the aluminum alloy castings due to these treatments, particularly in aluminum alloy castings having thin-walled portions.

[0011] Furthermore, the aluminum alloy described in Patent Document 2 is primarily intended for die casting, in which the alloy is cooled and solidified at a cooling rate of 20°C / s or more in the solidification step. When a low casting speed is used to suppress distortion during casting, the effects of the invention cannot be fully obtained.

[0012] In view of the above-mentioned problems in the prior art, an object of the present invention is to provide an aluminum alloy for casting that can be suitably used in a casting method with a slow casting speed and that can provide high mechanical properties even in castings having thin-walled portions without solution treatment or quenching. Another object of the present invention is to provide an aluminum alloy casting made of the aluminum alloy for casting of the present invention, and a simple and efficient method for producing the same.

[0013] In order to achieve the above object, the present inventors have conducted extensive research into the composition and casting conditions of aluminum alloys for casting, and as a result have found a composition range that allows high mechanical properties to be obtained without solution treatment or quenching even when cast at a slow cooling rate of 20°C / s or less, and have arrived at the present invention.

[0014] Specifically, the present invention provides an aluminum alloy for casting, comprising: 4.6 to 5.5 wt% Mg, 0.4 to 1.5 wt% Si, 0.2 to 0.6 wt% Cu, 0.2 to 1.5 wt% Mn, and 0.01 to 0.5 wt% Ti, with the balance consisting of Al and inevitable impurities. By strictly controlling the additive elements and their contents, high strength and yield strength can be imparted by dispersion strengthening of precipitates and crystallized particles and solid solution strengthening of Mg, without the need for solution treatment or quenching, simply by subjecting the cast aluminum alloy casting to aging treatment after casting.

[0015] The aluminum alloy for casting of the present invention preferably contains more than 0 wt % and not more than 0.2 wt % of one or more of B, Zr, and V. Addition of these elements can make the cast structure finer.

[0016] The present invention also provides an aluminum alloy casting made of the aluminum alloy for casting of the present invention, characterized in that it has a 0.2% yield strength of 150 MPa or more and a fracture elongation of 3% or more. Since the aluminum alloy casting of the present invention is made of the aluminum alloy for casting of the present invention, it can be kept free from cracks and deformation even when it has thin-walled portions.

[0017] Furthermore, the present invention also provides a method for producing an aluminum alloy casting, comprising: a casting step of casting a molten aluminum alloy made of the aluminum alloy for casting of the present invention to obtain an aluminum alloy casting; and an aging treatment step of holding the aluminum alloy casting at 150 to 300°C for 1 hour or more, wherein in the casting step, casting is performed at a casting speed such that the cooling rate is 20°C / s or less.

[0018] In the method for producing an aluminum alloy casting of the present invention, the aluminum alloy for casting of the present invention is used as a raw material. Therefore, even if solution treatment or quenching is not performed, an aluminum alloy casting having high mechanical properties can be obtained by simply performing aging treatment. Specifically, an aluminum alloy casting having a 0.2% yield strength of 150 MPa or more and a fracture elongation of 3% or more can be obtained.

[0019] Furthermore, in the method for producing an aluminum alloy casting of the present invention, the cooling rate during casting is slow, at 20°C / s or less, so that accumulation of strain inside the aluminum alloy casting can be suppressed. As a result, even if the aluminum alloy casting has thin-walled portions, deformation during demolding after casting can be suppressed extremely effectively.

[0020] Furthermore, in the method for producing an aluminum alloy casting of the present invention, it is preferable that the treatment temperature in the aging treatment step is 180 to 210°C and the holding time is 2 to 4 hours. By adopting such aging treatment conditions, it is possible to more reliably achieve high levels of strength, proof stress, and fracture elongation of the aluminum alloy casting.

[0021] According to the present invention, it is possible to provide an aluminum alloy for casting that can be suitably used in a casting method with a slow casting speed and that can provide high mechanical properties even in castings having thin-walled portions without solution treatment or quenching. Furthermore, according to the present invention, it is also possible to provide an aluminum alloy casting made of the aluminum alloy for casting of the present invention, and a simple and efficient method for producing the same.

[0022] 1 is an aging curve of an aluminum alloy casting (Example 1); FIG. 2 is an aging curve of an aluminum alloy casting (Example 3); FIG. 3 is a schematic diagram showing the positions where thermocouples were installed and the positions where tensile test specimens were taken.

[0023] The aluminum alloy for casting, the aluminum alloy casting, and the method for producing the same according to the present invention will be described in detail below, but the present invention is not limited to these alone.

[0024] 1. Cast Aluminum Alloy The cast aluminum alloy of the present invention can be suitably used in a casting method with a slow casting speed (slow cooling rate), and the additive elements and their contents are optimized so that high mechanical properties are exhibited without solution treatment or quenching (without T6 treatment). Each component will be described in detail below.

[0025] (1) Essential additive elements Mg: 4.6 to 5.5 wt% Mg dissolves in Al to improve mechanical properties, and when aging treatment is performed, it precipitates with Si as an Mg-Si compound, further improving mechanical properties. This effect is significant at 4.6 wt% or more, but conversely, if it exceeds 5.5 wt%, cracks are more likely to occur during casting. The amount of Mg added is preferably 4.8 to 5.3 wt%, and more preferably 4.9 to 5.1 wt%.

[0026] Si: 0.4 to 1.5 wt% Si precipitates as an Mg-Si compound with Mg during aging treatment, improving the mechanical properties of aluminum alloy castings. This effect becomes significant at 0.4 wt% or more, while exceeding 1.5 wt% reduces the toughness of aluminum alloy castings. The amount of Si added is preferably 0.4 to 1.2 wt%, and more preferably 0.4 to 0.8 wt%. Furthermore, adding 0.4 to 1.5 wt% of Si can improve the fluidity of aluminum alloys.

[0027] Cu: 0.2 to 0.6 wt% Cu precipitates as an Al-Cu compound during aging treatment, improving the mechanical properties of aluminum alloy castings. Cu also has the effect of suppressing stress corrosion cracking. These effects become significant at 0.2 wt% or more, but if added in excess of 0.6 wt%, the corrosion resistance of aluminum alloy castings decreases. The amount of Cu added is preferably 0.3 to 0.5 wt%.

[0028] Mn: 0.2 to 0.6 wt% Mn crystallizes as an Al-Mn compound during casting, contributing to improving the mechanical properties of aluminum alloy castings. This effect is significant at 0.2 wt% or more, but if added in excess of 0.6 wt%, coarse compounds that can become the starting point for fracture when stress is applied are more likely to form. The amount of Mn added is preferably 0.3 to 0.5 wt%.

[0029] Ti: 0.01 to 0.20 wt% Ti has the effect of refining the cast structure (α phase). This effect is significant at 0.01 wt% or more, but no improvement in effect is observed when added in excess of 0.20 wt%.

[0030] (2) Optional Added Elements One or more of B, Zr, and V: more than 0 wt% and 0.2 wt% or less B, Zr, and V have the effect of refining the cast structure of an aluminum alloy casting, and by including one or more of these elements in an amount of more than 0 wt% and 0.2 wt% or less, this effect can be reliably exhibited.

[0031] (3) Inevitable impurities Fe: 0.2 wt% or less Fe improves the mechanical properties of aluminum alloy castings and also has the effect of preventing seizure on the mold when die-casting. However, Fe also easily forms needle-like compounds that can easily become the starting point for fracture, so the content is restricted to 0.2 wt% or less.

[0032] In addition, unavoidable impurities such as Cr, Ni, Zn, and Be are restricted to 0.1 wt % or less.

[0033] 2. Aluminum Alloy Casting The aluminum alloy casting of the present invention is characterized by being made of the aluminum alloy for casting of the present invention. The shape, size, microstructure, and mechanical properties of the aluminum alloy casting will be described in detail below.

[0034] (1) Shape and Size The shape and size of the aluminum alloy casting of the present invention are not particularly limited as long as the effects of the present invention are not impaired, and various shapes and sizes conventionally known for aluminum alloy castings can be used, but it is preferable that the aluminum alloy casting has a thin-walled portion.

[0035] The thickness of the thin-walled portion is preferably 2 to 5 mm. The aluminum alloy casting of the present invention is made from the aluminum alloy for casting of the present invention, and is cast at a casting speed with a cooling rate of 20°C / s or less, thereby reducing accumulation of strain. In addition, excellent mechanical properties are imparted to the aluminum alloy casting without the need for solution treatment or quenching, so that cracking, deformation, etc. in the thin-walled portion are extremely effectively suppressed. This effect can be reliably manifested by setting the thickness of the thin-walled portion to 2 to 5 mm.

[0036] (2) Metallographic Structure The aluminum alloy casting of the present invention is not subjected to solution treatment or quenching, but is subjected to aging treatment after casting. As a result, the metallographic structure is unique to aluminum alloy castings that have been subjected to T5 treatment, unlike the metallographic structure of aluminum alloy castings that have been given high strength and yield strength by T6 treatment.

[0037] The metal structure of the T6 treatment and that of the T5 treatment can be distinguished by the difference in the element distribution of the matrix and the shape of the crystallized particles. Simply put, compared with the crystallized particles of an aluminum alloy casting having the same composition that has been subjected to the T6 treatment, the crystallized particles of the T5 treatment have rounded corners.

[0038] The method for confirming the shape of the crystallized particles is not particularly limited, and various conventionally known microstructural observation techniques may be used. For example, a mirror-polished cross section of an aluminum alloy casting can be observed with an optical microscope or a scanning electron microscope (SEM).

[0039] (3) Mechanical Properties The aluminum alloy casting of the present invention has high yield strength and excellent elongation at break when subjected to T5 treatment without solution treatment or quenching.

[0040] More specifically, the aluminum alloy casting of the present invention has a 0.2% yield strength of 150 MPa or more and a fracture elongation of 3% or more. The 0.2% yield strength is preferably 160 MPa or more, and more preferably 170 MPa or more. The fracture elongation is preferably 5% or more, and more preferably 6% or more. Furthermore, the tensile strength is preferably 260 MPa or more, more preferably 265 MPa or more, and most preferably 270 MPa or more.

[0041] The aluminum alloy casting of the present invention preferably has a Vickers hardness of 80 HV or more, more preferably 82 HV or more, and most preferably 84 HV or more.

[0042] 3. Manufacturing Method of Aluminum Alloy Casting The manufacturing method of an aluminum alloy casting of the present invention comprises a casting step of casting a molten aluminum alloy made of the aluminum alloy for casting of the present invention to obtain an aluminum alloy casting, and an aging treatment step of holding the aluminum alloy casting at 150 to 300°C for one hour or more. Each step will be described in detail below.

[0043] (1) Casting Step The casting method in the casting step is not particularly limited as long as it does not impair the effects of the present invention, and various conventionally known casting methods (sand casting, metal mold casting, gravity casting, low-pressure casting, etc.) can be used. That is, the aluminum alloy casting of the present invention is not limited to one cast by a specific casting method, but it is preferable to use a casting method in which the cooling rate during casting is slow.

[0044] The greatest feature of the casting process of the present invention is that the casting is performed at a casting speed that results in a cooling rate of 20°C / s or less. By slowing the cooling rate during casting, it is possible to suppress the accumulation of strain inside the aluminum alloy casting. As a result, even if the aluminum alloy casting has thin-walled portions, it is possible to extremely effectively suppress deformation during demolding after casting.

[0045] Furthermore, other casting conditions are not particularly limited as long as they do not impair the effects of the present invention, and various conventionally known casting conditions can be used.

[0046] (2) Aging Treatment Step The aging treatment step is a step for imparting a 0.2% yield strength of 150 MPa or more and a fracture elongation of 3% or more to the aluminum alloy casting obtained in the casting step by holding the aluminum alloy casting obtained in the casting step at 150 to 300°C for 1 hour or more. More preferred aging treatment conditions are a treatment temperature of 80 to 210°C and a holding time of 2 to 4 hours.

[0047] The method for producing an aluminum alloy casting according to the present invention is characterized in that solution treatment and quenching are not performed as pretreatments before the aging treatment step. As a result, the occurrence of cracks and deformation in the aluminum alloy casting during solution treatment and quenching can be suppressed. Furthermore, there is no need to pay attention to burning during solution treatment.

[0048] Representative embodiments of the present invention have been described above, but the present invention is not limited to these, and various design modifications are possible, all of which are included in the technical scope of the present invention.

[0049] Example: Raw materials formulated to obtain the composition of the cast aluminum alloy of the present invention were melted (melt weight: 10 kg), and 0.2 wt% of molten metal cleaning flux was added relative to the melt weight. Furthermore, Al-5Ti-1B master alloy was added so that the Ti content of the cast aluminum alloy was 0.10 wt%. After slag removal treatment using the molten metal cleaning flux and degassing treatment by injecting Ar gas, the alloy was cast into a JIS No. 4 boat at a casting temperature of 740°C. The mold temperature was 150°C, and two castings were performed using the same raw materials. The cooling rate during casting was measured and found to be 20°C / s.

[0050] The compositions of the aluminum alloy castings obtained from the raw materials blended in Examples 1 to 4 were measured by emission spectrometry. The results are shown in Table 1.

[0051]

[0052] Next, in order to examine appropriate heat treatment conditions, aging curves were obtained for the aluminum alloy castings having the compositions of Example 1 and Example 3. Specifically, the Vickers hardness of the aluminum alloy castings was measured when the aging temperatures were 160°C, 180°C, and 200°C and the aging times were 2 to 16 hours.

[0053] The test pieces used to create the aging curves were 10 mm thick cut from JIS No. 4 boat-shaped ingots. Cross-sectional samples were prepared by mirror polishing the aging-treated test pieces, and Vickers hardness was measured on the cross sections at a load of 5 kgf and a holding time of 15 seconds. The aging curves of the aluminum alloy castings having the composition of Example 1 and Example 3 are shown in Figure 1 and Figure 2, respectively.

[0054] It can be seen that in both the compositions of Example 1 and Example 2, a high Vickers hardness of 80 HV or more can be obtained by applying an appropriate aging treatment. Furthermore, even if the aging time is 10 hours or longer, no significant increase in hardness is observed, and a high Vickers hardness can be obtained with an aging time of about 2 to 4 hours. Furthermore, the aging temperature at which hardness was most efficiently increased under these conditions was 180°C.

[0055] Next, the aluminum alloy castings having the compositions of Examples 1 to 4 were subjected to aging treatment under the treatment conditions shown in Table 1, and the tensile properties of the resulting Example Aluminum Alloys 1 to 4 were evaluated. Here, the aging treatment temperature was evaluated by measuring the actual temperature. The installation position of the thermocouple for measuring the actual temperature is shown in Figure 3. A hole was drilled using a φ3 mm drill, and a K-type thermocouple was inserted. After that, aluminum foil was filled and fixed into the gap between the drill hole and the thermocouple. The sampling period for measuring the actual temperature was set to 500 ms.

[0056] The locations where test specimens for tensile testing were taken are shown in Figure 3. The test specimens were round bars with a parallel portion diameter of 10 mm and a parallel portion length of 35 mm. Three test specimens were prepared for each aluminum alloy casting, and the average tensile properties of these test specimens were calculated. The results are shown in Table 1.

[0057] It can be seen that all of the aluminum alloy castings (Example Aluminum Alloy Castings 1 to 4) of the present invention have a 0.2% yield strength of 150 MPa or more and a fracture elongation of 5% or more.

[0058] Comparative Examples Comparative aluminum alloy castings 1 to 8 were obtained in the same manner as in the Examples, except that raw materials were melted so as to obtain the compositions of the aluminum alloys for casting that serve as comparative examples of the present invention. The compositions of comparative aluminum alloy castings 1 to 8 were measured by optical emission spectroscopy, and the results are shown in Table 1.

[0059] Next, comparative aluminum alloy castings 1 to 8 were subjected to aging treatment under the treatment conditions shown in Table 1, and the tensile properties of the obtained comparative aluminum alloy castings 1 to 8 were evaluated in the same manner as in the examples. The obtained results are shown in Table 1.

[0060] For the comparative aluminum alloy castings, the 0.2% yield strength was less than 150 MPa for all compositions. Comparative aluminum alloy castings 3 to 8 had fracture elongation of 5% or more, but a tendency for the 0.2% yield strength to decrease as the fracture elongation increased was observed.

Claims

1. An aluminum alloy for casting, characterized by containing Mg: 4.6-5.5 wt%, Si: 0.4-1.5 wt%, Cu: 0.2-0.6 wt%, Mn: 0.2-1.5 wt%, Ti: 0.01-0.5 wt%, with the balance consisting of Al and unavoidable impurities.

2. The aluminum alloy for casting according to claim 1, characterized in that it contains more than 0 wt% and not more than 0.2 wt% of one or more of B, Zr and V.

3. An aluminum alloy casting made from the aluminum alloy for casting according to claim 1 or 2, characterized in that the 0.2% yield strength is 150 MPa or more and the fracture elongation is 3% or more.

4. A method for producing an aluminum alloy casting, comprising: a casting step of casting a molten aluminum alloy made from the aluminum alloy for casting according to claim 1 or 2 to obtain an aluminum alloy casting; and an aging treatment step of holding the aluminum alloy casting at 150 to 300°C for one hour or more, wherein in the casting step, casting is performed at a casting speed such that the cooling rate is 20°C / s or less.

5. The method for producing an aluminum alloy casting according to claim 4, wherein the treatment temperature in the aging treatment step is 180 to 210°C and the holding time is 2 to 4 hours.

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