ALUMINUM ALLOY FOR CASTING, ALUMINUM ALLOY MEMBER, AND METHOD FOR MANUFACTURING ALUMINUM ALLOY MEMBER

Abstract

An aluminum alloy member having a satisfactory mechanical strength, an aluminum alloy for casting from which such an aluminum alloy member having a satisfactory mechanical strength can be manufactured, and a method for manufacturing such an aluminum alloy member are provided. In an aspect according to the present disclosure, an aluminum alloy for casting contains, in mass %, Si: 4.0-7.5%, Cu: 1.5-4.0%, Zn: 1.0% or more, and Mg: 0.2-0.8%, in which a remainder of the aluminum alloy for casting consists of Al and unavoidable impurities. Note that a content of Zn may be limited to, in mass %, 1.0% or more and 5.0% or less. Further, a content of Mg may be limited to, in mass %, 0.5% or more and 0.8% or less.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from Japanese patent application No. 2022-181764, filed on Nov. 14, 2022, the disclosure of which is incorporated herein in its entirety by reference.

BACKGROUND

The present disclosure relates to an aluminum alloy for casting, an aluminum alloy member, and a method for manufacturing an aluminum alloy member.

As an example of the method for manufacturing an aluminum alloy member, there are a method for manufacturing an aluminum alloy member by performing a solution treatment on an aluminum alloy casting and a method for manufacturing an aluminum alloy member disclosed in Japanese Unexamined Patent Application Publication No. 2019-157231. In the method for manufacturing an aluminum alloy member disclosed in Japanese Unexamined Patent Application Publication No. 2019-157231, an aluminum alloy-cast material is heated to and kept in a solid-liquid coexisting temperature range, and then rapidly cooled.

SUMMARY

The inventors of the present application have found the following problem.

There has been a demand for an aluminum alloy member having a satisfactory mechanical strength. Further, there has been a demand for an aluminum alloy for casting from which an aluminum alloy member having a satisfactory mechanical strength can be manufactured, and a method for manufacturing such an aluminum alloy member. Further, regarding the aluminum alloy for casting and the method for manufacturing an aluminum alloy member, it is desirable to manufacture an aluminum alloy member having a satisfactory mechanical strength without performing a solution treatment in order to reduce material costs, save energy, and reduce CO₂ emission.

The present disclosure has been made in view of the above-described problem, and an object thereof is to provide an aluminum alloy for casting from which an aluminum alloy member having a satisfactory mechanical strength can be manufactured, an aluminum alloy member having a satisfactory mechanical strength, and a method for manufacturing an aluminum alloy member having a satisfactory mechanical strength.

In an aspect according to the present disclosure, an aluminum alloy for casting contains, in mass %, Si: 4.0-7.5%, Cu: 1.5-4.0%, Zn: 1.0% or more, and Mg: 0.2-0.8%, in which a remainder of the aluminum alloy for casting consists of Al and unavoidable impurities.

By the above-described composition and the like, it is possible to manufacture an aluminum alloy member having a satisfactory mechanical strength by increasing the contents of Zn and Mg.

Further, in the above-described aluminum alloy for casting, a content of Zn may be limited to, in mass %, 1.0% or more and 5.0% or less. Further, in the above-described aluminum alloy for casting, a content of Mg may be limited to, in mass %, 0.5% or more and 0.8% or less.

By the above-described composition and the like, it is possible to stabilize the mechanical strength by raising the lower limits of the contents of Zn or Mg.

Further, in the above-described aluminum alloy for casting, a content of Si may be limited to, in mass %, 4.0% or more and less than 5.0%, or a content of Cu may be limited to, in mass %, 1.5% or more and 2.0% or less.

By the above-described composition and the like, it is possible to prevent or reduce the increase in material cost by limiting the content of Si or Cu.

In another aspect according to the present disclosure, an aluminum alloy member is made of an aluminum alloy for casting containing, in mass %, Si: 4.0-7.5%, Cu: 1.5-4.0%, Zn: 1.0% or more, and Mg: 0.2-0.8%, in which a remainder of the aluminum alloy for casting consists of Al and unavoidable impurities, in which the aluminum alloy member has a tensile strength of 230 MPa or larger, a 0.2% proof stress of 180 MPa or larger, a high-cycle fatigue strength of 100 MPa or larger, and a low-cycle fatigue strength of 8,000 cycles or more.

By the above-described composition and the like, it is possible to manufacture an aluminum alloy member having a satisfactory mechanical strength by increasing the contents of Zn and Mg.

Further, in the above-described aluminum alloy member, Mg₅Si₆, which can be measured by using TEM (Transmission Electron Microscopy), may be precipitated.

By the above-described composition and the like, the mechanical strength can be improved by the precipitation of Mg₅Si₆.

In another aspect according to the present disclosure, a method for manufacturing an aluminum alloy member includes:

• • casting an aluminum alloy for casting into an aluminum-cast material, the aluminum alloy for casting containing, in mass %, Si: 4.0-7.5%, Cu: 1.5-4.0%, Zn: 1.0% or more, and Mg: 0.2-0.8%, in which a remainder of the aluminum alloy for casting consists of Al and unavoidable impurities; and
  • performing an artificial aging treatment on the aluminum-cast material by heating the aluminum-cast material to a heating temperature of 185 to 205° C. and keeping it at the heating temperature.

By the above-described composition and the like, it is possible to manufacture an aluminum alloy member having a satisfactory mechanical strength by using an aluminum alloy for casting having increased contents of Zn and Mg.

According to the present disclosure, it is possible to manufacture an aluminum alloy member having a satisfactory mechanical strength.

The above and other objects, features and advantages of the present disclosure will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present disclosure.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flowchart of a method for manufacturing an aluminum alloy member according to a first embodiment;

FIG. 2 shows an example of a heat treatment pattern showing changes in temperature over time;

FIG. 3 is a graph showing an example of results of tensile strength tests performed on T5 materials;

FIG. 4 is a graph showing another example of results of tensile strength tests performed on T5 materials;

FIG. 5 is a graph showing an example of results of tensile strength tests performed on T6 materials;

FIG. 6 is a graph showing an example of results of hardness tests performed on F materials;

FIG. 7 is a graph showing an example of results of high-temperature low-cycle fatigue tests;

FIG. 8 is a graph showing an example of results of a comparison between fatigue lives;

FIG. 9 is a graph showing an example of results of high-temperature low-cycle fatigue tests performed on T6 materials;

FIG. 10 is a graph showing an example of results of high-temperature low-cycle fatigue tests;

FIG. 11 is a graph showing an example of results of high-cycle fatigue tests performed on T5 materials;

FIG. 12 is a graph showing an example of results of high-cycle fatigue tests performed on T6 materials;

FIG. 13 is a graph showing an example of results of a comparison between high-cycle fatigue strengths;

FIG. 14 is a graph showing a relationship between required heat treatment times and contents of Zn;

FIG. 15 is a graph showing results of an EPMA analysis of an aluminum alloy member according to Example 4;

FIG. 16A is a graph showing results of a TEM analysis of the aluminum alloy member according to Example 4;

FIG. 16B is an enlarged view showing results of a TEM analysis of the aluminum alloy member according to Example 4;

FIG. 17 shows results of an EPMA analysis of an aluminum alloy member according to Comparative Example 1;

FIG. 18A shows results of a TEM analysis of the aluminum alloy member according to Comparative Example 1; and

FIG. 18B is an enlarged view showing a result of a TEM analysis of the aluminum alloy member according to Comparative Example 1.

DESCRIPTION OF EMBODIMENTS

Specific embodiments to which the present disclosure is applied will be described hereinafter in detail with reference to the drawings. However, the present disclosure is not limited to the below-shown embodiments. Further, the following description and the drawings are simplified as appropriate for clarifying the explanation.

First Embodiment

An aluminum alloy for casting according to a first embodiment will be described.

An aluminum alloy for casting according to the first embodiment contains, in mass %, Si: 4.0-7.5%, Cu: 1.5-4.0%, Zn: 1.0% or more, and Mg: 0.2-0.8%, and the remainder of the aluminum alloy for casting consists of Al and unavoidable impurities. Examples of such unavoidable impurities include Fe and Mn

When the content of Si in the chemical composition of the aluminum alloy for casting according to the first embodiment is within a suitable range, certain castability can be ensured. Therefore, casting defects such as cracking and shrinkage cavities are unlikely to occur in the aluminum alloy-cast material. On the other hand, when the content of Si is too much, a large number of fragile Si particles crystallize in the aluminum alloy-cast material, so that mechanical properties thereof such as an elongation at break and a strength are likely to deteriorate. Therefore, the content of Si is preferably within a range between 4.0% and 7.5% inclusive in mass %. Further, the content of Si is preferably limited to, in mass %, 4.0% or more and less than 5.0%. In general, Si is more expensive than Zn and Mg. Therefore, it is preferred if the content of Si can be limited as described above because, by doing so, it is possible to reduce the material cost of the aluminum alloy for casting while ensuring a satisfactory mechanical strength thereof.

Further, when the content of Cu is within a suitable range, an Al—Cu-based compound(s) and/or an Al—Mg—Si—Cu-based compound(s) may be precipitated in the metallographic structure of the aluminum alloy member by an artificial aging treatment. The Al—Cu-based compound is, for example, CuAl₂. The mechanical strengths, such as a tensile strength and a 0.2% proof stress, of the aluminum alloy member can be improved by these compounds or the like. On the other hand, when the content of Cu is too much, the ductility and toughness of the aluminum alloy member may deteriorate. Therefore, the content of Cu is preferably in a range of 1.5 to 4.0% in mass %. Further, the content of Cu is preferably limited to, in mass %, 1.5% or more and less than 2.0%. In general, Cu is more expensive than Zn and Mg. Therefore, it is preferred if the content of Cu can be limited as described above because, by doing so, it is possible to reduce the material cost of the aluminum alloy for casting while ensuring a satisfactory mechanical strength thereof. Further, the technology for removing Cu from aluminum alloys has not been established. Therefore, it is preferred if the content of Cu can be limited as described above in view of recycling.

Further, when the content of Zn is within a suitable range, Zn atoms are solid-dissolved (or solid-soluted) in aluminum crystals in the aluminum alloy member, so that the mechanical strength of the aluminum alloy member can be improved. Further, since Zn atoms, which are solid-dissolved in aluminum crystals, are precipitation nuclei of compounds during the artificial aging treatment, it is possible to impart the mechanical strength required for the aluminum alloy member to the aluminum alloy member in a short artificial aging treatment time. That is, the artificial aging treatment time of the aluminum alloy-cast material can be shortened. Accordingly, the content of Zn is preferably 1.0% or more in mass %. Further, the content of Zn is preferably limited to, in mass %, 1.0% or more and 8.0% or less. This is because when the content of Zn exceeds 8.0%, the effect for shortening the artificial aging treatment time of the aluminum alloy-cast material is no longer substantially obtained. Further, this is because when the content of Zn is 8.0% or less, the material cost can be reduced.

Further, the content of Zn is preferably limited to, in mass %, 1.0% or more and 5.0% or less. This is because it is possible to reduce the material cost can while ensuring satisfactory mechanical properties by reducing the content of Zn.

Further, when the content of Mg is within a suitable range, an Al—Mg—Si—Cu-based compound(s) and/or an Mg—Si-based compound(s) are precipitated in the metallographic structure of the aluminum alloy member by an artificial aging treatment. The Mg—Si-based compound is, for example, Mg₅Si₆. Regarding the mechanical strength of the aluminum alloy member, for example, a tensile strength, hardness, and the like can be improved by the above-described precipitation of the Mg—Si-based compound. On the other hand, when the content of Mg is too much, the elongation may deteriorate. Therefore, the content of Mg is preferably 0.2% or more and 0.8% or less.

Further, the content of Mg may be limited to, in mass %, 0.5% or more to 0.8% or less. When the content of Mg is 0.5% or more in mass %, an Mg—Si-based compound(s) is stably precipitated in the metallographic structure of the aluminum alloy member by an artificial aging treatment. In this way, it is possible to further improve the mechanical strength of the aluminum alloy member.

Note that examples of the unavoidable impurities contained in the remainder of the aluminum alloy for casting according to the first embodiment are Fe and Mn. When the aluminum alloy for casting according to the first embodiment contains Fe or Mn as unavoidable impurities, the content of Fe may be limited to, in mass %, 1.0% or less, and the content of Mn may be limited to, in mass %, 1.0% or less.

Method for Manufacturing Aluminum Alloy Member

Next, an example of a method for manufacturing an aluminum alloy member by using an aluminum alloy for casting according to the first embodiment will be described with reference to FIG. 1.

An aluminum alloy for casting is cast into an aluminum alloy-cast material (Step ST1). Specifically, the aluminum alloy for casting is heated and melted, so that a molten aluminum alloy is obtained. The molten aluminum alloy is poured into a mold, and cooled and solidified in the mold, so that an aluminum alloy-cast material is formed. The aluminum alloy-cast material is released from the mold. The aluminum alloy-cast material has any of various shapes, such as a shape of a cylinder head, a shape of turbo component (i.e., a component for a turbo charger), and the like.

Next, an artificial aging treatment is performed on the aluminum alloy-cast material (Step ST2). Specifically, after the aluminum alloy-cast material is completely released from the mold, the aluminum alloy-cast material is cooled by blowing air onto it. At the point when the blowing of air on the aluminum alloy-cast material is started, the temperature of the aluminum alloy-cast material is preferably, for example, 300° C. or higher. The cooling rate in this cooling is preferably, for example, 30° C./min or higher. The blowing of air onto the aluminum alloy-cast material is preferably continued until its temperature is lowered to, for example, 110° C. After that, an artificial aging treatment is performed on the aluminum alloy-cast material by heating the aluminum alloy-cast material to an aging treatment temperature and keeping it at the aging treatment temperature for a predetermined time, so that an aluminum alloy member is manufactured. The aging treatment temperature is, for example, 185 to 205° C. The predetermined time is, for example, 180 minutes. Further, the aluminum alloy member is preferably left undisturbed so that it is cooled.

Through the above-described method, the aluminum alloy member can be manufactured. As will be described later, this aluminum alloy member has a satisfactory mechanical strength. This aluminum alloy member is a T5 material because it has been subjected only to the artificial aging treatment without being subjected to a solution treatment. Therefore, it is possible to manufacture an aluminum alloy member having a satisfactory mechanical strength without performing a solution treatment.

Note that an aluminum alloy member may be manufactured by using an aluminum alloy for casting according to the first embodiment and performing only the above-described step ST1 without performing the above-described step ST2. That is, the aluminum alloy-cast material may be used as the aluminum alloy member. In this case, the aluminum alloy member is a casting (i.e., a cast article) that has not been heat-treated, i.e., is an F material.

Alternatively, an aluminum alloy member may be manufactured by using an aluminum alloy for casting according to the first embodiment, and performing the above-described step ST1, performing a solution treatment on the aluminum alloy-cast material, and then performing the above-described step ST2. In this case, the aluminum alloy-cast material is the aluminum alloy member. In this case, the aluminum alloy member is a T6 material.

EXAMPLES

Next, various tests were conducted for examples of the aluminum alloy for casting according to the first embodiment in order to measure the mechanical strengths of them. Results of these tests will be described hereinafter.

Tensile Test

Firstly, results of tensile tests will be described.

In each of Examples 1-5 and 8, an aluminum alloy member was manufactured from an aluminum alloy for casting having a respective constituent composition shown in Table 1 by using an example of the method for manufacturing an aluminum alloy member shown in FIG. 1. Conditions for heat treatments in Examples 1-5 and 8 are shown in Table 2.

TABLE 1
	Constituent Composition [mass %]
	Cu	Si	Mg	Zn	Fe	Mn	Al
Example 1	2.4	5.9	0.3	5.0	0.36	0.28	Bal
Example 2	2.4	5.8	0.6	5.0	0.36	0.28	Bal
Example 3	2.4	5.7	0.5	1.5	0.26	1.00	Bal
Examples 4, 6, 7	2.4	6.0	0.5	3.0	0.36	0.50	Bal
Example 5	1.6	4.2	0.5	3.0	0.84	0.50	Bal
Example 8	2.4	5.9	0.5	3.0	0.85	0.51	Bal
Target	1.5-4.0	4.0-7.0	0.2-0.8	≥1.0	≤1.0	≤1.0	Bal
Composition
Comparative	2.7	6.2	0.3	0.2	0.42	0.29	Bal
Examples 1-3
(JIS AC2B)
(Reference) JIS	2.0-4.0	5.0-7.0	≤0.5	≤1.0	≤1.0	≤1.0	Bal
AC2B

TABLE 2
						Fatigue
	Heat Treatment				Characteristic
	Condition (T5)	Static Strength	High-	Low-
	Aging			0.2%		Cycle	Cycle
	Temper-	Aging	Tensile	Proof	Butt	Fatigue	Fatigue
	ature	Time	Strength	Strength	Elongation	Strength	Life
	[° C.]	[h]	[MPa]	[MPa]	[%]	[MPa]	[cycle]
Example 1	200	2.0-3.0	232	190	1.0	110	9000
Example 2	200	2.0-3.0	272	230	1.1	110	9000
Example 3	200	4.0-5.0	247	220	1.1	110	8500
Example 4	200	3.0-4.0	266	221	1.4	110	9000
Example 5	200	3.0-4.0	266	220	1.0	110	9000
Example 8	200	3.0-4.0	263	220	1.0	110	9000
Comparative	200	6.0	197	165	1.4	80	5500
Example 1
(JIS AC2B)

In Example 6, an aluminum alloy for casting having the same constituent composition as that of Example 4 was cast into a turbo component, which is an example of the aluminum alloy member, and this turbo component was subjected to a solution treatment and an artificial aging treatment, i.e., was subjected to a T6 treatment. This turbo component was a T6 material. Conditions for the solution treatment were a solution treatment temperature of 490° C. and a treatment time of 2.0 hours. The conditions for the artificial aging treatment were an aging temperature 210° C. and a treatment time of 2.0 hours.

In Example 7, an aluminum alloy for casting having the same constituent composition as that of Example 4 was cast into a turbo component, which is an example of the aluminum alloy member. This turbo component was a casting that has not been heat-treated, i.e., was an F material.

Note that in Comparative Example 1, an aluminum alloy member was manufactured from an aluminum alloy for casting having a constituent composition for Comparative Example 1 shown in Table 1 by using an example of the method for manufacturing an aluminum alloy member shown in FIG. 1. The aluminum alloy for casting according to Comparative Example 1 is an aluminum alloy having a constituent composition that is within the range specified by JIS AC2B. Conditions for heat treatment in Comparative Example 1 are shown in Table 2.

Further, in Comparative Example 2, an aluminum alloy for casting having the same constituent composition as that of Comparative Example 1 was cast into a turbo component. Further, this turbo component was subjected to a solution treatment and an artificial aging treatment, i.e., was subjected to the T6 treatment. This turbo component was a T6 material. Conditions for the solution treatment were a solution treatment temperature of 490° C. and a treatment time of 2.0 hours. The conditions for the artificial aging treatment were an aging temperature 210° C. and a treatment time of 2.0 hours.

In Comparative Example 3, an aluminum alloy for casting having the same constituent composition as that of Comparative Example 1 was cast into a turbo component. This turbo component was a casting that was not heat-treated, i.e., was an F material.

Static strengths and fatigue properties of each of the above-described manufactured aluminum alloy members according to Examples 1 to 8 and Comparative Examples 1 to 3 were measured. The static strengths include a tensile strength, a 0.2% proof stress, and a butt elongation. The fatigue properties include a high-cycle fatigue strength and a high-temperature low-cycle fatigue strength. The high-cycle fatigue strength is a 10⁷ times non-breaking load, i.e., a maximum strength by which a test piece is not broken even when it is applied to the test piece 10⁷ times. The low-cycle fatigue life is the number of cycles of applications of a load of 0.22 (MPa·Strain) until a test piece is broken in a thermal fatigue test. Tensile strengths, butt elongations, 0.2% proof stresses, high-cycle fatigue strengths, and low-cycle fatigue strengths of Examples 1 to 5 and Comparative Example 1 are shown in Table 2. Results of measurements of static strengths and fatigue properties are shown in FIGS. 3 to 13.

In the mechanical strength measurement tests, when the tensile strength is 230 MPa or larger, the 0.2% proof stress is 180 MPa or larger, the butt elongation is 1.0% or more, the high-cycle fatigue strength is 100 MPa or larger, and the low-cycle fatigue strength is 8,000 cycles or more, the mechanical strength was determined to be satisfactory.

As shown in Table 2, each of Examples 1-5 and 8 was determined to have a satisfactory mechanical strength because their tensile strengths, butt elongations, 0.2% proof stresses, high-cycle fatigue strengths, and low-cycle fatigue strengths were equal to or higher than the aforementioned values. In contrast, although the butt elongation of Comparative Example 1 was equal to or higher than the aforementioned value, its tensile strength, 0.2% proof stress, high-cycle fatigue strength, and low-cycle fatigue strength were lower than the aforementioned values. Therefore, the satisfactory mechanical strength of Comparative Example 1 was determined to be unsatisfactory. Note that as shown in Table 2, the tensile strength, 0.2% proof stress, butt elongation, high-cycle fatigue strength, and low-cycle fatigue strength of Example 8 were substantially equal to those of Example 5. As shown in Tables 1 and 2, although the contents of Cu and Si of Example 5 were smaller than those of Example 8, their static strengths and fatigue properties were substantially equal to each other.

Result of Tensile strength Test

Results of tensile strength tests will be described with reference to FIGS. 3 to 5. FIGS. 3 and 4 show examples of results of tensile strength tests performed on the T5 material. FIG. 5 shows an example of results of tensile strength tests performed on the T6 material.

As shown in FIG. 3, the tensile strengths and 0.2% proof stresses of Examples 1, 2 and 5 were all higher than those of Comparative Example 1. One of the reasons for these differences is that the contents of Zn and Mg of each of Examples 1, 2 and 5 were larger than those of Comparative Example 1.

As shown in FIG. 4, the tensile strength and 0.2% proof stress of Example 4 were higher than those of Comparative Example 1. One of the reasons for this difference is that the contents of Zn and Mg of Example 4 were larger than those of Comparative Example 1. The butt elongation according to Example 4 was 1.4%, which was substantially equal to that of Comparative Example 1 and considered to be satisfactory.

As shown in FIG. 5, the tensile strength and 0.2% proof stress of Example 6 were substantially equal to those of Comparative Example 2 and considered to be satisfactory. The butt elongation of Example 6 was higher than that of

Comparative Example 2. One of the reasons for this difference is that the content of Zn and Mg of Example 6 were larger than those of Comparative Example 2.

Results of Hardness Test

Results of hardness tests performed on the F material will be described with reference to FIG. 6. FIG. 6 shows an example of results of hardness tests performed on F materials.

As shown in FIG. 6, the Brinell hardness of Example 7 is higher than that of Comparative Example 3, and the Brinell hardness of Example 7 is about 130% of that of Comparative Example 3. That is, the Brinell hardness of Example 7 is equal to a value 30% higher than that of Comparative Example 3. In general, the Brinell hardness is in proportion to the mechanical strength. Therefore, it can be inferred that Example 7 has a higher mechanical strength than that of Comparative Example 3. One of the reasons for this difference is that the contents of Zn and Mg of Example 7 were larger than those of Comparative Example 3.

Results of High-Temperature Low-Cycle Fatigue Test

Results of high-temperature low-cycle fatigue tests will be described with reference to FIGS. 7 to 9. FIG. 7 is a graph showing an example of results of high-temperature low-cycle fatigue tests. FIG. 8 is a graph showing an example of results of a comparison between fatigue lives. FIG. 9 is a graph showing an example of results of high-temperature low-cycle fatigue tests performed on T6 materials.

FIG. 7 shows S—N curves of Examples 4 and 5 and Comparative Example 1 as examples of results of high-temperature low-cycle fatigue tests. The S—N curves of Examples 4 and 5 were calculated based on hysteresis energy HE for the number N_f of repetitions (hereinafter also referred to as the repetition number N_f) in Examples 4 and 5. The hysteresis energy HE for the repetition number N_f in Example 4 and that in Example 5 have roughly the same tendency, and the S—N curves in Example 4 and that in Example 5 roughly coincide with each other. Although the contents of Cu and Si of Example 4 are different from those of Example 5, their results of the high-temperature low-cycle fatigue tests are roughly equal to each other. As shown in FIG. 7, regarding the S—N curves of Examples 4 and 5, the repetition number N_f when the hysteresis energy HE is equal to or lower than 0.22 MPa·Strain is larger than that in Comparative Example 1. Further, regarding the S—N curves of Examples 4 and 5, the repetition number N_f when the hysteresis energy HE is equal to or lower than 0.22 MPa·Strain is roughly equal to that in Comparative Example 2.

As shown in FIG. 8, the repetition number N_f, i.e., the fatigue life, of each of Examples 4 and 5 when the hysteresis energy HE is 0.22 MPa·Strain is larger than that of Comparative Example 1. Note that the fatigue life of each of Examples 4 and 5 is about 30% longer than that of Comparative Example 1. One of the reasons for this difference is that the contents of Zn and Mg of Examples 4 and 5 were larger than those of Comparative Example 1.

Further, the repetition number N_f and the fatigue life of each of Examples 4 and 5 at the hysteresis energy HE of 0.22 MPa·Strain are not much different from those of Comparative Example 2. While each of Examples 4 and 5 is a T5 material, Comparative Example 2 is a T6 material. Therefore, each of Examples 4 and 5 was not subjected to a solution treatment and had a high fatigue life as in the case of the T6 material.

As shown in FIG. 9, the repetition number N_f, i.e., the fatigue life, of Example 6 is roughly equal to that of Comparative Example 2. Although the aluminum alloy for casting of Example 6 had the same constituent composition of as that of Example 4, unlike Example 4, it was subjected to a solution treatment. Even though Example 6 was subjected to a solution treatment, the fatigue life of Example 6 is not much different from that of Comparative Example 2 and hence is considered to be satisfactory.

As shown in FIG. 10, a reference line for S-N curves that passes through a reference repetition number N_f1 at reference hysteresis energy HE1 is defined. The repetition number N_f in the S-N curve of Comparative Example 1 is smaller than the reference line. In contrast, the repetition number N_f in the S-N curve in Example 4 is larger than the reference line when the hysteresis energy is equal to or lower than the reference hysteresis energy HE1. The repetition number N_f of Example 4 is larger than that of Comparative Example 1 when the hysteresis energy is equal to or lower than the reference hysteresis energy HE1.

Results of High-Cycle Fatigue Test

Next, results of rotational bending fatigue tests will be described with reference to FIGS. 11 to 13. FIG. 11 is a graph showing an example of results of high-cycle fatigue tests performed on T5 materials. FIG. 12 is a graph showing an example of results of high-cycle fatigue tests performed on T6 materials. FIG. 13 is a graph showing an example of results of a comparison between high-cycle fatigue strengths.

As shown in FIG. 11, the high-cycle fatigue strength of Comparative Example 2 was about 80 MPa. Meanwhile, the high-cycle fatigue strength of Example 4 was about 110 MPa, i.e., was higher than that of Comparative Example 2.

As shown in FIG. 12, the high-cycle fatigue strength of Example 6 was about 80 to 110 MPa, i.e., was higher than that of Comparative Example 2. As shown in FIG. 13, the high-cycle fatigue strengths of Examples 4 and 6 were both about 110 MPa, i.e., were about 130% of that of Comparative Example 2, and about 30% higher than that of Comparative Example 2. One of the reasons for these differences is that the contents of Zn and Mg of Examples 4 and 6 were larger than those of Comparative Example 2. While Example 4 was a F material, Example 6 was a T6 material. The high-cycle fatigue strengths of Examples 4 and 6 were both satisfactory irrespective of whether the solution treatment was performed or not.

Required Heat Treatment Time for Zn Content

Next, for each of aluminum alloys for casting having different contents of Zn, a heat treatment time that was required before the hardness of the aluminum alloy for casting reached a predetermined value will be described with reference to FIG. 14.

Aluminum alloys for casting having, for elements other than Zn, constituent compositions that are within a target composition range shown in Table 1 were prepared. The contents of Zn in these prepared aluminum alloys for casting were about 0.2%, about 1.0%, about 1.5%, about 3.0%, about 5.0%, about 6.0%, about 8.0%, respectively.

Aluminum alloy members were manufactured from these prepared aluminum alloys for casting by using an example of the method for manufacturing an aluminum alloy member shown in FIG. 1. The hardness of each of the manufactured aluminum alloy members was periodically measured over the heat treatment time of the step ST2 in the example of the method for manufacturing an aluminum alloy member shown in FIG. 1, i.e., was measured every time the artificial aging treatment time had elapsed. This measurement was repeated until the hardness of the manufactured aluminum alloy member reached a predetermined value. FIG. 14 shows, for each of the manufactured aluminum alloy members, a heat treatment time that was required before the hardness of the aluminum alloy for casting reached the predetermined value.

As shown in FIG. 14, the required heat treatment time decreases as the content of Zn in the aluminum alloy for casting increases. For example, when the content of Zn in the aluminum alloy for casting was about 0.2%, it was longer than five hours. In contrast, when the content of Zn in the aluminum alloy for casting was about 1.0%, it was four hours. When the content of Zn in the aluminum alloy for casting was about 5.0%, it was longer than three hours, and when the content of Zn in the aluminum alloy for casting was about 8.0%, it was shorter than three hours. When the content Zn of the aluminum alloy for casting exceeded the predetermined value, e.g., about 5.0% or about 8.0%, the effect for reducing the required heat treatment time became smaller.

Results of Metallographic Observations

Next, results of metallographic observations will be described with reference to FIGS. 15 to 18B. Each of FIGS. 15, 16A and 16B shows results of EPMA and TEM analyses of the aluminum alloy member according to Example 4. Each of FIGS. 17, 18A and 18B shows results of EPMA and TEM analyses of the aluminum alloy member according to Comparative Example 1.

As shown in FIG. 17, no Zn atom was detected at all over the entire cross section of the aluminum alloy member according to Comparative Example 1. One of reasons for this fact is that the content of Zn of the aluminum alloy member according to Comparative Example 1 was limited.

Meanwhile, as shown in FIG. 15, Zn atoms were, compared to other elements, uniformly distributed over the entire cross section of the aluminum alloy member according to Example 4. Specifically, most of Zn atoms are distributed in Al crystal grains. Based on these facts, it can be determined that most of Zn atoms were solid-dissolved in Al crystal grains. As described above, the static strengths and fatigue properties of Example 4 are superior to those of Comparative Example 1. One of the reasons for these differences is that the solid solution was enhanced by Zn atoms.

As shown in FIGS. 18A and 18B, it was confirmed that an Al—Cu-based compound(s), an Al—Mg—Si—Cu-based compound(s), and an Al—Si-based compound(s) were precipitated on the cross section of the aluminum alloy member according to Comparative Example 1. The Al-Cu-based compound is, for example, Al₂Cu.

Meanwhile, as shown in FIGS. 16A and 16B, it was confirmed that an Al—Cu-based compound(s), an Al—Mg—Si—Cu-based compound(s), an Al—Si-based compound(s), and an Mg—Si-based compound(s) were precipitated on the cross section of the aluminum alloy member according to Example 4. The Al—Cu-based compound is, for example, Al₂Cu. The Mg—Si-based compound is, for example, Mg₅Si₆. It was confirmed that unlike Comparative Example 1, the Mg—Si-based compound(s) was precipitated on the cross section of the aluminum alloy member according to Example 4. As described above, the static strengths and fatigue properties of Example 4 are superior to those of Comparative Example 1. One of the reasons for these differences is the precipitation hardening of the Mg—Si-based compound, i.e., the precipitation of Mg₅Si₆.

Note that it was confirmed that when the content of Mg of an aluminum alloy for casting according to any of the other examples exceeded 0.5%, the Mg—Si-based compound was stably precipitated on the cross section of the aluminum alloy member in this example. Therefore, when the content of Mg of the aluminum alloy for casting according to any of the other examples exceeded 0.5%, the static strengths and fatigue properties of the aluminum alloy member could be improved in this example.

Note that the present disclosure is not limited to the above-described examples, and they can be modified as appropriate without departing from the scope and spirit of the disclosure. Further, the present disclosure may be carried out by combining any two or more of the above-described embodiments and examples thereof as desired.

From the disclosure thus described, it will be obvious that the embodiments of the disclosure may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the disclosure, and all such modifications as would be obvious to one skilled in the art are intended for inclusion within the scope of the following claims.

Claims

1. An aluminum alloy for casting containing, in mass %, Si: 4.0-7.5%, Cu: 1.5-4.0%, Zn: 1.0% or more, and Mg: 0.2-0.8%, wherein a remainder of the aluminum alloy for casting consists of Al and unavoidable impurities.

2. The aluminum alloy for casting according to claim 1, wherein a content of Zn is limited to, in mass %, 1.0% or more and 5.0% or less.

3. The aluminum alloy for casting according to claim 1, wherein a content of Mg is limited to, in mass %, 0.5% or more and 0.8% or less.

4. The aluminum alloy for casting according to claim 1, wherein a content of Si is limited to, in mass %, 4.0% or more and less than 5.0%, or a content of Cu is limited to, in mass %, 1.5% or more and 2.0% or less.

5. An aluminum alloy member made of an aluminum alloy for casting containing, in mass %, Si: 4.0-7.5%, Cu: 1.5-4.0%, Zn: 1.0% or more, and Mg: 0.2-0.8%, wherein a remainder of the aluminum alloy for casting consists of Al and unavoidable impurities, and wherein the aluminum alloy member has a tensile strength of 230 MPa or larger, a 0.2% proof stress of 180 MPa or larger, a high-cycle fatigue strength of 100 MPa or larger, and a low-cycle fatigue strength of 8,000 cycles or more.

6. The aluminum alloy member according to claim 5, wherein Mg5Si6, which can be measured by using TEM, is precipitated.

7. A method for manufacturing an aluminum alloy member comprising: casting an aluminum alloy for casting into an aluminum-cast material, the aluminum alloy for casting containing, in mass %, Si: 4.0-7.5%, Cu: 1.5-4.0%, Zn: 1.0% or more, and Mg: 0.2-0.8%, and a remainder of the aluminum alloy for casting consisting of Al and unavoidable impurities; andperforming an artificial aging treatment on the aluminum-cast material by heating the aluminum-cast material to a heating temperature of 185 to 205° C. and keeping it at the heating temperature.