US20110116966A1

US20110116966A1 - Aluminum alloy, method of casting aluminum alloy, and method of producing aluminum alloy product

Info

Publication number: US20110116966A1
Application number: US13/003,124
Authority: US
Inventors: Hiroshi Kawahara; Hideaki Matsuoka; Hiroaki Iwahori; Taiki Kanou; Chikatoshi Maeda; Tokujiro Konishi
Original assignee: Toyota Motor Corp
Current assignee: Toyota Motor Corp
Priority date: 2008-07-14
Filing date: 2009-07-06
Publication date: 2011-05-19
Also published as: WO2010007484A1; WO2010007484A8; CN102089450A; JP2010018875A

Abstract

An aluminum alloy is composed of 15% or more and 7.5% or less by mass of silicon, 0.45% or more and 0.8% or less by mass of magnesium, 0.05% or more and 0.35% or less by mass of chromium, and aluminum, assuming that the total amount of the alloy is 100% by mass.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to an aluminum alloy that has excellent castability, workability, and mechanical characteristics, a method of casting the aluminum alloy, and a method of producing a product of the aluminum alloy.
2. Description of the Related Art
Forged products of 6061 alloy aluminum, which has excellent strength and toughness and high corrosion resistance, are used for wheels, suspension arms and so on for weight reduction of vehicles. However, since the 6061 alloy has poor castability, near-net-shape blanks with complicated shapes are difficult to obtain and extrusion products are often used as blanks instead. Thus, the production cost tends to increase when parts with complicated shapes are produced. For this reason, casting aluminum alloys such as AC4C alloy and AC4CH alloy are used in some cases. When net-shape castings that are formed by casting such a casting aluminum alloy, or near-net-shape blanks that are produced from such a casting aluminum alloy are formed into final shape by forging by taking advantage of their castability, products with complicated shapes can be produced at low production cost. However, the above casting aluminum alloys have poor workability, compared to the 6061 alloy.
To solve the above problem, Japanese Patent Application Publication No. 9-125181 (JP-A-9-125181) discloses an Al—Si—Mg—Fe based alloy that has improved hot forgeability. Also, Japanese Patent Application Publication No. 7-109537 (JP-A-7-109537) discloses an Al—Si—Mg—Ti—B based alloy that has improved mechanical characteristics.
In Al—Si based alloys that are excellent in castability, Si is crystallized into brittle crystals via eutectic reaction. Thus, the mechanical characteristics, especially ductility, of an Al—Si based alloys always pose a challenge. In this case, to improve the ductility of Al—Si based alloy, it is necessary to finely crystallize Si crystals which are formed as a result of eutectic reaction (which are hereinafter referred to as “eutectic Si”). Thus, Sr, Na, Sb, and Ca are added singly or in combination as property improving elements for grain refinement of eutectic Si. However, while these property improving elements are effective even when present in an extremely small amount, each element has a unique problem such as absorption of gas or reaction with fire-resistant materials. Also, since the improving capacity decreases as the property improving elements are consumed with time after the addition, component management often cause trouble. Such property improving elements are also added for grain refinement of eutectic Si in the above related arts, but it is preferred that grain refinement of eutectic Si is stably achieved by addition of elements other than the above property improving elements.
A solute element such as Mg is often added to Al—Si based alloys to obtain mechanical strength, especially proof strength, comparable to that of the 6061 alloy. However, since improvement in strength of Al—Si based alloys cannot be achieved only by adjusting the amounts of Si and Mg, Cu has been added in combination with Mg for improved strength. However, when Cu is added, Cu compounds may precipitate or crystallize and decrease the corrosion resistance of the Al—Si based alloys. Especially, the addition of Cu tends to segregation in casting products and may impair the corrosion resistance thereof. Cu is added to the Al—Si based alloy for improving strength also in the related art disclosed in JP-A-9-125181 and JP-A-7-109537, but it is desired to improve the strength of Al—Si based alloys without adding Cu in order to maintain the corrosion resistance thereof.

SUMMARY OF THE INVENTION

The present invention provides a novel aluminum alloy which has excellent castability and workability and has high mechanical characteristics. The present invention also provides a method of producing a casting that is formed from the aluminum alloy of the present invention and has high mechanical characteristics, and a method of producing an aluminum alloy product that has high mechanical characteristics which is obtained by machining the casting.
A first aspect of the present invention relates to an aluminum alloy that is composed of 3.5% or more and 7.5% or less by mass of silicon (Si); 0.45% or more and 0.8% or less by mass of magnesium (Mg); 0.05% or more and 0.35% or less by mass of chromium (Cr); and aluminum (Al).
According to the above configuration, the aluminum alloy having the above composition has excellent castability and can be cast into a complicated net or near-net shape.
Also, since the Mg contained in an effective amount in the aluminum alloy contributes to grain refinement of eutectic Si, grain refinement of eutectic Si can be achieved without relying on the property improving elements such as Sr as described above. As grain refinement of eutectic Si can be achieved, the aluminum alloy exhibits high ductility and has excellent workability.
In addition, the strength of the aluminum alloy is improved when it contains an effective amount of Mg and contains Cr. Therefore, the aluminum alloy has improved strength and excellent corrosion resistance even if it is free from Cu.
The aluminum alloy according to this aspect may further contain unavoidable impurities. The unavoidable impurities may include iron (Fe), and the content of iron in the aluminum alloy may be 0.3% or less by mass.
The aluminum alloy according to this aspect may further contain 0.05% or more and 0.3% or less by mass of titanium (Ti).
The aluminum alloy according to this aspect may be free from copper.
The aluminum alloy according to this aspect may further contain at least one of 0.003% or more and 0.05% or less by mass of strontium (Sr); 0.001% or more and 0.03% or less by mass of sodium (Na); and 0.05% or more and 0.15% or less by mass of antimony (Sb).
In the aluminum alloy according to this aspect, when the silicon is crystallized via eutectic reaction, crystallized silicon may have an average grain size of 5 um or smaller.
A second aspect of the present invention relates to a casting method of an aluminum alloy. The casting method includes: pouring molten alloy comprising 3.5% or more and 7.5% or less by mass of silicon; 0.45% or more and 0.8% or less by mass of magnesium; 0.05% or more and 0.35% or less by mass of chromium; and aluminum; and allowing the molten alloy to cool and solidify.
In the casting method according to this aspect, the alloy may further contain unavoidable impurities. The unavoidable impurities may include iron, and the content of iron in the aluminum alloy may be 0.3% or less by mass.
In the casting method according to this aspect, the alloy may further contain at least one of 0.003% or more and 0.05% or less by mass of strontium; 0.001% or more and 0.03% or less by mass of sodium; and 0.05% or more and 0.15% or less by mass of antimony (Sb).
In the casting method according to this aspect, solidification of the molten alloy may be achieved by cooling the molten alloy at a cooling rate of PC/sec or faster.
A third aspect of the present invention relates to a method of producing an aluminum alloy product including performing cold working and/or hot working on an aluminum alloy casting manufactured by using the casting method according to the second aspect.
According to the above configuration, an aluminum alloy casting and an aluminum alloy product having excellent castability and workability and having high mechanical characteristics can be obtained. In addition, when a solution heat treatment and an aging heat treatment are performed on the aluminum alloy casting or aluminum alloy product, spheroidization of eutectic Si is promoted and greater ductility develops, and the Mg is precipitated as magnesium silicide (Mg₂Si) and mechanical strength such as tensile strength and proof strength improves.
In the method for producing an aluminum alloy product according to this aspect, the cold working and/or hot working are performed on the aluminum alloy casting at a processing rate that provides a cumulative area reduction of 30% or more.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further objects, features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein:

FIG. 1 is a photograph that is substituted for a drawing and shows the metallic structure of test sample 4-3 of this embodiment; and

FIG. 2 is a graph that shows the changes in ductility which were observed when an casting (blank) formed from a high-strength aluminum alloy according to this embodiment was hot-rolled at different rolling reductions.

DETAILED DESCRIPTION OF EMBODIMENTS

The present inventors have found that the strength of an aluminum alloy containing silicon can be significantly improved without addition of copper (Cu) by adding chromium to the aluminum alloy in addition to adding magnesium in such an amount that the ductility of the aluminum alloy is not adversely affected to improve strength. The present inventors have also found that magnesium contributes not only to improvement of the strength of the aluminum alloy but also to grain refinement of eutectic Si.
Description is hereinafter made of an embodiment to implement the present invention.
An aluminum alloy according to this embodiment is an aluminum alloy that has excellent castability and workability, and contains, assuming that the total amount of the alloy is 100% by mass, 3.5% or more and 7.5% or less by mass of silicon (Si), 0.45% or more and 0.8% or less by mass of magnesium (Mg), 0.05% or more and 0.35% or less by mass of chromium (Cr), and the remainder of aluminum (Al) and unavoidable impurities.
When the Si content is not within the above range, the aluminum alloy has low castability. When the Si content is less than 3.5% by mass, the amount of shrinkage during casting increases. Then, casting defects tend to occur and defects such as casting cracks tends to occur in the casting. The Si content is preferably 4.5% or more by mass, more preferably 5.5% or more by mass. When the Si content is more than 7.5% by mass, shrinkage defects tend to aggregate in the portions of the casting that solidify last. In addition, when the Si content is too high, crystallization of brittle Si grains increases, and the ductility and mechanical strength decreases. The Si content is preferably 7% or less by mass, more preferably 6.5% or less by mass.
The content of Mg in the aluminum alloy is 0.45% or more and 0.8% or less by mass, assuming that the total amount of the aluminum alloy is 100% by mass.
The Mg undergoes eutectic reaction and contributes to grain refinement of Si crystals in the aluminum alloy. The reason for it is not fully understood, but it is considered that the existence of Mg changes the interfacial energy between Al and Si, and concentration of Mg occurs at the growth interface of Si and inhibits the growth of Si crystals. The Mg precipitates as Mg₂Si during a heat treatment step, which is described later, and improves the mechanical strengths such as tensile strength and proof strength. That is, the aluminum alloy needs to contain an appropriate amount of Mg to strengthen the α-Al matrix phase (the structure of which is described later). When the Mg content is less than 0.45% by mass, the grain refinement of eutectic Si cannot be fully achieved and the matrix phase does not have sufficient strength. The Mg content is preferably 0.5% or more by mass, more preferably 0.55% or more by mass. When the Mg content is too high, a portion of the Mg do not form a solid solution and remains as Mg compounds even after a heat treatment, which decreases the ductility and toughness of the aluminum alloy. The Mg content is preferably 0.7% or less by mass, more preferably 0.65% or less by mass.
Cr is contained in an amount of 0.05% or more and 0.35% or less by mass, assuming that the total amount of the aluminum alloy is 100% by mass. The Cr forms solid solution or precipitates as Cr compounds in the α-Al matrix phase and strengthens the matrix phase to improve the mechanical strengths such as tensile strength and proof strength. While it is considered that the improvement of the aluminum alloy is mainly due to precipitation of Mg₂Si phase, it is also considered that precipitation of Cr compounds produces a synergistic effect or the existence of Cr influences the precipitation state of the Mg₂Si phase. Especially, when the aluminum alloy is used for machining blanks, Cr is an effective element to prevent recrystallization during machining. When the Cr content is less than 0.05% by mass, the α-Al matrix phase does not have sufficient strength. The Cr content is preferably 0.1% or more by mass, more preferably 0.12% or more by mass. When the Cr content is more than 0.35% by mass, coarse crystals of Cr compounds are formed and the ductility and toughness tend to decrease. The Cr content is preferably 0.25% or less by mass, more preferably 0.2% or less by mass.
The aluminum alloy of this embodiment may contain various property improving elements as long as the advantages and effects of this embodiment are not impaired. Specific examples of the property improving elements includes titanium (Ti), strontium (Sr), sodium (Na), antimony (Sb), and iron (Fe).
Ti is preferably contained in an amount of 0.05% or more by mass and 0.3% or less by mass, assuming that the total amount of the aluminum alloy is 100% by mass. The Ti aids in the formation of fine crystal grains, and provides solid solution strengthening or precipitation strengthening of the α-Al matrix phase. When the Ti content is 0.05% or more by mass, sufficiently fine crystal grains are formed and crystallized grains tends to be isotropically distributed in the alloy. Since columnar crystals tend to grow when the directionality from the casting mold is strong, Ti may be added in view of the workability in using the resulting casting as a machining blank. When the Ti content is 0.05% or more by mass, the mechanical characteristics are improved since shrinkage and so on are distributed finely in the alloy. More preferred Ti content is 0.1% or more by mass. When the Ti content is too high, coarse crystals of Ti compounds are formed in the metallic structure and the mechanical characteristics decrease. Thus, the Ti content is preferably 0.3% or less by mass, more preferably 0.25% or less by mass, much more preferably 0.2% or less by mass.
Sr, Na, and Sb aid in grain refinement of eutectic Si. While grain refinement of eutectic Si is achieved by adding Mg in the aluminum alloy of this embodiment, grain refinement of eutectic Si is promoted and the mechanical characteristics are further improved when one or more of these elements are added. Especially, when a casting of the aluminum alloy of this embodiment is used as a machining blank, it exhibits excellent workability. The aluminum alloy of this embodiment preferably contains at least one of 0.003% or more and 0.05% or less by mass of Sr, 0.001% or more and 0.03% or less by mass of Na, and 0.05% or more and 0.15% or less by mass of Sb, assuming that the total amount of the aluminum alloy is 100% by mass. When the content of Sr is excessive, fine and coarse eutectic Si are mixed in the resulting alloy and crystallization of Sr compounds tend to occur. Also, gas absorption increases and promotes the formation of cavities which may decrease the ductility. Thus, more preferred Sr content is 0.01% or less by mass. When the content of Na is excessive, fine and coarse eutectic Si are mixed in the resulting alloy and the ductility may decrease. Thus, more preferred Na content is 0.01% or less by mass. When the content of Sb is excessive, coarse eutectic Si are mixed in the resulting alloy and crystallization of Sb compounds, which may reduce the ductility, occurs. Thus, more preferred Sb content is 0.12% or less by mass.
Fe is an unavoidable impurity which may be derived from the raw material. Thus, the Fe content is preferably 0.3% or less by mass, or 0.2% or less by mass, assuming that the total amount of the aluminum alloy is 100% by mass. When the Fe content is more than 0.3% by mass, crystallization of Fe compounds increases and the ductility decreases.
The aluminum alloy of this embodiment has improved mechanical strength and is substantially free from copper (Cu), which decreases corrosion resistance as described before. If it is necessary to determine the Cu content, it should be less than 0.01% by mass. The aluminum alloy is preferably free from Cu in view of its corrosion resistance.
Manganese (Mn) is generally used to prevent recrystallization of aluminum alloys. However, the aluminum alloy of this embodiment may be free from Mn since it contains Cr. This is because Mn decreases the amount of Si in solid solution in the α-Al matrix phase. Boron (B) is generally used in combination with Ti as an additive element which contributes to grain refinement of metallic structure. However, the aluminum alloy of this embodiment may be free from B since it forms TiB, which decreases machinability.
The aluminum alloy of this embodiment has a metallic structure that is composed of α-Al matrix phase, and crystallized phase that contains fine eutectic Si crystallized in a network structure that surrounds the matrix phase. The crystallized phase contains crystallized Fe compounds and so on in addition to the eutectic Si. The matrix phase contains precipitated compound grains (such as precipitated grains of Mg compounds and Cr compounds) in addition to the alloy elements (Si, Mg, Cr, Ti, etc.) in a state of solid solution. The eutectic Si contained in the crystallized phase preferably has an average grain size of 5 μm or smaller, more preferably 4 μm or smaller, much more preferably 3.5 μm or smaller. The average grain size of the eutectic Si is the arithmetic average of the values of the maximum length (maximum diameter) of a plurality of eutectic Si that is measured by image analysis of a microscope image obtained by metallographic observation under an optical microscope.
A method of producing a casting of the aluminum alloy of this embodiment is described below. The method of producing a casting of the aluminum alloy of this embodiment essentially includes a pouring step and a solidifying step.
The pouring step is a step of pouring molten alloy that is composed of, assuming that the total amount of the alloy is 100% by mass, 3.5% or more and 7.5% or less by mass of silicon (Si), 0.45% or more and 0.8% or less by mass of magnesium (Mg), 0.05% or more and 0.35% or less by mass of chromium (Cr), and the remainder of aluminum (Al) and unavoidable impurities into a casting mold. The method of producing a casting of the aluminum alloy of this embodiment is not limited to typical gravity casting and pressurized casting, and may be die-casting. The casting mold for use in the casting may be of any type such as a sand mold or metal mold.
The solidifying step is a step of cooling the molten alloy after the pouring step to solidify it. Grain refinement of eutectic Si can be achieved by properly selecting the material and wall thickness of the casting mold, the dimensions of the casting (or dimensions of the mold cavity of the casting mold), the cooling method and so on to increase the cooling rate (solidification rate). For example, the average grain size of eutectic Si can be reduced by selecting a cooling rate of, for example, 1° C./see or higher, preferably 5° C./sec or higher.
The method preferably further includes a heat treatment step of subjecting the aluminum alloy after the solidifying step to a solution heat treatment and/or an aging heat treatment. The heat treatment step promotes the spheroidization of the eutectic Si and improves the ductility of the aluminum alloy after the solidifying step.
Here, the solution heat treatment is a heat treatment in which the aluminum alloy is maintained at a high temperature and then cooled rapidly to form super-saturated solid solution. The aging heat treatment is a heat treatment in which the aluminum alloy is heated and maintained at a relatively low temperature to cause the elements in the super-saturated solid solution to precipitate in order to impart the aluminum alloy with a suitable degree of hardness. By these heat treatments, fine precipitates are uniformly distributed and the eutectic Si are spheroidized, whereby an aluminum alloy having highly balanced strength, ductility and toughness can be obtained. The conditions of the heat treatments may be selected based on the composition and required properties and so on of the casting. For example, the casting may be heated and maintained at 450° C. to 550° C. for 0.5 to 10 hours and then cooled rapidly in the solution heat treatment process. The heating temperature and retention time are preferably 490° C. to 535° C. and 0.5 to 3 hours, respectively, for a good balance between cost and properties. The casting may be heated and maintained at 140° C. to 250° C. for 1 to 20 hours in the aging heat treatment process. The heating temperature and retention time are preferably 160 to 200° C. and 1 to 5 hours, respectively, for a good balance between cost and properties.
An aluminum alloy product is obtained by subjecting the aluminum casting that is obtained by the above procedure to a processing step. That is, the method of producing an aluminum alloy product of this embodiment essentially include a pouring step and a solidifying step as described above and a processing step.
The pouring step and the solidifying step are the same as described above. The processing step involves cold-working and/or hot-working the aluminum alloy casting after the solidifying step to obtain an aluminum alloy product. The method cold working and/or hot working is not particularly limited. For example, the cold working and/or hot working may be by forging (extend forging, swaging, etc.), rolling, spinning or the like. The cold working and/or hot working may be either performed once or repeated twice or more. Either cold working or hot working may be performed, or cold working may be performed after hot working.
The processing step is preferably a step in which the aluminum alloy casting is processed at a processing rate that provides a cumulative area reduction of 30% or more, preferably 50% or more. When processing is performed twice or more, it is preferred that the cumulative area reduction after all the stages of processing is 30% or more, preferably 50% or more. By increasing the processing rate, the cast structure is broken up and finer eutectic Si are formed and uniformly dispersed in the metallic structure. As a result, an aluminum alloy product that has high ductility can be achieved.
The method of producing an aluminum alloy product of this embodiment preferably further includes a heat treatment step of subjecting the aluminum alloy product after the processing step to a solution heat treatment and an aging heat treatment. The heat treatment step is the same as described before.
A homogenizing treatment may be performed on the aluminum alloy casting before the processing step as needed. The homogenizing treatment is a treatment for incorporation of crystallized phase that is not incorporated in the solid solution and spheroidization of the crystallized phase, and improves the workability in the processing step thereafter. As the homogenizing treatment, the aluminum alloy casting may be heated and maintained at 450° C. to 550° C. for 0.5 to 10 hours. Cooling after the heating is not particularly limited. The heating temperature and retention time are preferably 490° C. to 535° C. and 0.5 to 3 hours, respectively, for good balance between cost and properties.
The aluminum alloy of this embodiment is suitably used for a casting product or forged product which is required to have high strength and corrosion resistance or a material (such an ingot) from which they are formed. Examples of such products include suspension systems of vehicles. Examples of suspension systems include upper arm, lower arm, knuckle, axle carrier, disk wheel, and cross member. When the aluminum alloy of this embodiment is applied to these members, significant weight reduction and performance improvement of the vehicles can be achieved.
Examples of the aluminum alloy, the method of producing an aluminum alloy casting and the method of producing an aluminum alloy product of this embodiment are described in further detail.
As Test Example 1, test samples 1-1 to 1-9 composed of aluminum alloys having different compositions as shown in Table 1 were prepared and their mechanical characteristics were evaluated.
In the pouring step and solidifying step, the ingredients were mixed to obtain different alloy compositions, and each mixture was melted to prepare molten alloy, which was then pored into a copper mold with cavity dimensions of 80 mm×70 mm×15 mm and was allowed to cool and solidify to obtain an aluminum alloy casting.
In the heat treatment step, a heat treatment designated as T6 was performed on the obtained castings. In the T6 heat treatment, the castings were subjected to a solution treatment at 535° C. for 1 hour and then quenched into warm water at 50° C., followed by an aging heat treatment at 170° C. for 3 hours, thereby obtaining castings as test samples 1-1 to 1-9.
The tensile strength, proof strength and ductility of the test samples 1-1 to 1-9 were evaluated. A flat plate tensile test piece with a thickness of 3 mm was obtained from a thick central portion of each of the castings obtained by the above procedure. The tensile test was performed at a cross head speed of 0.3 mm/min using an autograph manufactured by Shimazu Corporation. The 0.2% proof strength was obtained from a stress-strain curve calculated from the displacement and load measured with a video extensometer. The tensile test was done at room temperature. The results are summarized in Table 2.

	TABLE 1

	Composition [% by mass]

Test sample No.	Si	Cu	Mg	Fe	Ti	Cr	Al

1-1	3.5	<0.01	0.60	0.17	0.10	0.25	bal.
1-2	5.6	<0.01	0.69	0.18	0.08	0.05	bal.
1-3	6.9	<0.01	0.69	0.14	0.15	0.16	bal.
1-4	7.5	<0.01	0.59	0.14	0.02	0.19	bal.
1-5	6.9	<0.01	0.45	0.14	0.08	0.10	bal.
1-6	9.0	<0.01	0.45	0.15	0.13	0.12	bal.
1-7	6.9	<0.01	0.89	0.17	0.12	0.11	bal.
1-8	7.3	<0.01	0.59	0.17	0.12	0.01	bal.
1-9	6.9	<0.01	0.35	0.17	0.12	<0.01	bal.

TABLE 2

	Tensile strength	0.2% Proof strength
Test sample No.	[MPa]	[MPa]	Elongation [%]

1-1	320	282	10
1-2	332	281	9.2
1-3	333	292	8.2
1-4	336	290	8.3
1-5	325	280	8.6
1-6	327	280	5.1
1-7	337	295	5.6
1-8	317	256	8.9
1-9	285	240	5.3
(AC4CH)

As shown in Table 1 and Table 2, the test samples 1-1 to 1-5, which had a composition within the composition range of the aluminum alloy of this embodiment (Si: 3.5 to 7.5%, Mg: 0.45 to 0.8%, Cr: 0.05 to 0.35%), had a tensile strength of 320 MPa or more, a 0.2% proof strength of 280 MPa or more, and an elongation of 8.0% or more, which indicates that the aluminum alloy casting had both high mechanical strength and high ductility.
In comparison, the test samples 1-6 to 1-9 were not satisfactory in terms of mechanical strength and/or ductility. The test sample 1-6, which contained an excessive amount of Si, had a much lower elongation than the test samples 1-1 to 1-5. The test sample 1-7, which contained an excessive amount of Mg, also had a lower elongation than the test samples 1-1 to 1-5. The test sample 1-8, which was substantially free from Cr, was satisfactory in terms of elongation but deficient in mechanical strength (tensile strength and proof strength). The test sample 1-9 was an Al—Si—Mg based casting alloy (AC4CH) provided in JIS. The test sample 1-9 was not satisfactory in terms of both mechanical strength and ductility as compared to the test samples 1-1 to 1-5.
The test samples 1-1 to 1-5 had almost no defect in the castings and high castability, but the test sample 1-1, which had a Si content of 3.5% by mass, was inferior in castability to the test samples 1-2 to 1-5, which had a Si content of 5.6% or more by mass.
As Test Example 2, test samples 2-1 to 2-4 that were composed of aluminum alloys having different compositions as shown in Table 3 were prepared, and their mechanical characteristics were evaluated.
The ingredients were mixed to obtain different alloy compositions, and each mixture was melted to prepare molten alloy, which was then pored into a copper, mold with cavity dimensions of 80 mm×70 mm×15 mm and was allowed to cool and solidify to obtain an aluminum alloy casting.
A plate-shaped blank with dimensions of 70 mm×15 mm×15 mm was cut from each of the obtained castings, and its surfaces were wet-polished up to the grit #600. Then, the plate-shaped blanks were hot-rolled. The plate-shaped blanks were heated by maintaining them in an electric furnace at 380° C. for 30 minutes and passed between rolls at room temperature. Each of the plate-shaped blanks was passed between rolls seven times in total to obtain aluminum alloy products. Adjustment was made so that the final rolling reduction rate after the seven passes of rolling was approximately 65%.
The obtained products were subjected to the same T6 heat treatment as in the test example 1, to obtain test samples 2-1 to 2-4.
The tensile strength, proof strength, ductility, and hardness of the test samples 2-1 to 2-4 were evaluated. The tensile strength, proof strength, and elongation were measured in the same manner as in Test Example 1. The hardness was measured using a Vickers hardness tester after a load of 5 kg was applied to a thick central portion of each test sample for 25 seconds. The results are summarized in Table 4. In Table 4, the tensile strength, proof strength, ductility, and hardness of a forging alloy 6061 (JIS) quoted from “Aluminum Handbook” are also shown for comparison.

	TABLE 3

	Composition [% by mass]

Test sample No.	Si	Cu	Mg	Fe	Ti	Cr	Sr	Al

2-1	6.9	<0.01	0.59	0.15	0.15	0.16	0.016	bal.
2-2	7.4	<0.01	0.58	0.14	0.05	0.14	—	bal.
2-3	6.9	<0.01	0.89	0.17	0.12	0.11	—	bal.
2-4	6.9	<0.01	0.35	0.12	0.12	<0.01	—	bal.

TABLE 4

Test	Tensile strength	0.2% Proof	Elongation	Hardness
sample No.	[MPa]	strength [MPa]	[%]	(HV)

2-1	367	317	14.3	128
2-2	362	319	14.0	129
2-3	360	314	11.5	129
2-4	310	248	17.0	107
(AC4CH)
6061	315	275	12.0	120
(Published
value)

As shown in Table 3 and Table 4, the test samples 2-1 and 2-2, which had a composition within the composition range of the aluminum alloy of this embodiment, had a tensile strength of 360 MPa or more, a 0.2% proof strength of 310 MPa or more, and an elongation of 14% or more, which indicates that the aluminum alloy product had highly balanced strength and ductility. Also, since the aluminum alloy products were processed at a cumulative area reduction of approximately 65% or more, the tensile strength, proof strength, and ductility were all improved compared to the castings of Test Example 1 (which were not subjected to any processing).
The test sample 2-3, which contained an excessive amount of Mg, exhibited high tensile strength, proof strength, and hardness values. However, the test sample 2-3 exhibited a lower elongation value than the 6061 alloy and did not show significant improvement in ductility in spite of the fact that it was processed at a high processing rate. The test sample 2-4 was the same in composition as AC4CH as in the case with the test sample 1-9 and had excellent ductility, but exhibited lower tensile strength and proof strength values than the 6061 alloy.
As Test Example 3, test samples 3-1 to 3-5 composed of aluminum alloys having different compositions as shown in Table 5 were prepared in the same manner as in Test Example 1, and evaluation was conducted to show how the hardness of castings depends on the Cr content.
The hardness of the test samples 3-1 to 3-5 was measured in the same manner as in Test Example 2. The hardness was measured at a thick central portion of each of the obtained castings. The results are summarized in Table 5.

TABLE 5

Test
sample	Composition [% by mass]

No.	Si	Cu	Mg	Fe	Ti	Cr	Al	Hardness (HV)

3-1	7.0	<0.01	0.59	0.14	0.15	0.01	bal.	115
3-2	7.0	<0.01	0.60	0.12	0.14	0.05	bal.	123
3-3	7.0	<0.01	0.61	0.17	0.15	0.10	bal.	125
3-4	7.1	<0.01	0.59	0.17	0.15	0.30	bal.	126
3-5	7.0	<0.01	0.58	0.15	0.15	0.40	bal.	124

As shown in Table 5, the test samples 3-2 to 3-4, which had a composition within the composition range of the aluminum alloy of this embodiment, had a HV of 120 or greater. The test sample 3-1, which was substantially free from Cr, was deficient in hardness. The test sample 3-5, which contained an excessive amount of Cr, exhibited a high hardness value but coarse Cr compounds (not shown) were observed when its metallic structure was examined under an optical microscope. Thus, it is considered that the test sample 3-5 had low ductility and toughness.
As Test Example 4, test samples 4-1 to 4-4 that were composed of aluminum alloys having different compositions as shown in Table 6 were prepared in the same manner as in Test Example 1, and evaluation was conducted to show how the average grain size of eutectic Si depends on the Mg content.
The average grain size of eutectic Si was obtained by observing the metallic structure in a section taken through a thick central portion of each casting under an optical microscope. Several fields of view of the metallic structure were photographed at a 200 fold magnification (600 μm×480 μm) and a 400 fold magnification (300 μm×240 μm) by an optical microscope. One sample photograph is shown in FIG. 1. The grain size of eutectic Si was measured on photographs substituted for drawings as FIG. 1 using image analysis software “Image-Pro.” The maximum lengths (maximum diameters) of eutectic Si grains in the fields of view were measured and an average grain size was calculated by obtaining the arithmetic average thereof. The result is summarized in Table 6.

TABLE 6

Test	Composition [% by mass]	Size of

sample No.	Si	Mg	Fe	Ti	Cr	Al	eutectic Si [μm]

4-1	7.0	0.30	0.14	0.15	0.15	bal.	6.0
4-2	7.0	0.46	0.14	0.15	0.15	bal.	3.1
4-3	6.8	0.70	0.17	0.15	0.14	bal.	2.6
4-4	7.1	0.89	0.17	0.15	0.15	bal.	2.5

As shown in Table 6, in the test samples 4-2 and 4-3, which had a composition within the composition range of the aluminum alloy of this embodiment, the average grain size of eutectic Si was as small as approximately 3 μm. In the test sample 4-1, which had a low Mg content, the average grain size of eutectic Si was as large as 6 μm. In the test sample 4-4, which contained an excessive amount of Mg, the average grain size of eutectic Si was as small as 2.5 μm, but metallographic observation reveals that Mg compounds, which did not incorporated into the solid solution by the heat treatment, were present in the metallic structure. Thus, it is considered that the ductility of the test sample 4-4 was low as in the case with the test sample 1-7 (Table 2).
As Test Example 5, test samples 5-1 to 5-8 that were composed of aluminum alloys having different compositions as shown in Table 7 were prepared, and evaluation was conducted to show how the average grain size of eutectic Si depends on the cooling rate.
The ingredients mixed to obtain different alloy compositions, and each mixture was melted to prepare molten alloy, which was pored into a casting mold with cavity dimensions of 80 mm×70 mm×T mm in wall thickness and was allowed to cool and solidify to obtain an aluminum alloy casting. The cooling rate was changed by using copper and silica sand shell molds with different wall thicknesses T (15 mm, 22 mm, and 44 mm). The cooling rate for each test sample (actual value measured in a central portion of the casting) is shown in Table 7.
The casting were subjected to the T6 heat treatment in the same manner as in Test Example 1, thereby obtaining castings as test samples 5-1 to 5-8.
The average grain size of eutectic Si was obtained by observing the metallic structure in a section taken through a thick central portion of each casting under an optical microscope. The average grain size of the eutectic Si was obtained in the same manner as in Test Example 4. The results are summarized in Table 7.

TABLE 7

Test		Cooling	Size of
sample	Composition [% by mass]	rate	eutectic

No.	Si	Mg	Fe	Ti	Cr	Al	[° C./Sec]	Si [um]

5-1	7.0	0.46	0.15	0.15	0.15	bal.	0.27	8.9
5-2	7.0	0.46	0.15	0.15	0.15	bal.	1.0	3.3
5-3	7.0	0.46	0.15	0.15	0.15	bal.	5.3	2.6
5-4	7.0	0.46	0.15	0.15	0.15	bal.	8.5	2.7
5-5	7.0	0.30	0.14	0.15	0.15	bal.	0.27	11
5-6	7.0	0.30	0.14	0.15	0.15	bal.	1.0	6.8
5-7	7.0	0.30	0.14	0.15	0.15	bal.	5.3	5.9
5-8	7.0	0.30	0.14	0.15	0.15	bal.	8.5	6.0

As shown in Table 7, in the test samples 5-1 to 5-4, which had a composition within the composition range of the aluminum alloy of this embodiment, the average grain size of eutectic Si was smaller than 9 μm, Above all, the average grain size of eutectic Si was smaller than 5 μm for the test samples, which were cooled at a cooling rate of 1° C./sec or higher, and 3 μm or smaller for the test samples, which were cooled at a cooling rate of 5° C./sec or higher. The average grain size of eutectic Si in the test samples 5-5 to 5-8, which had a low Mg content, exceeded 5 μm even if the cooling rate was 1° C./sec or higher, or even 5° C./sec or higher.
As Test Example 6, a test sample 6-1 that was composed of aluminum alloys having a compositions shown in Table 8 was prepared, and evaluation was conducted to show how the ductility depends on the rolling reduction in the rolling process.
The ingredients mixed to obtain the alloy composition shown in Table 8 were melted to prepare molten alloy, which was then pored into a copper mold with cavity dimensions of 80 mm×70 mm×15 mm and was allowed to cool and solidify to obtain an aluminum alloy casting.
Five plate-shaped blanks with dimensions of 70 mm×15 mm×15 mm were cut from the obtained casting, and their surfaces were wet-polished up to grit #600. Then, the plate-shaped blanks were hot-rolled. The plate-shaped blanks were heated by maintaining them in an electric furnace at 380° C. for 30 minutes and passed between rolls at room temperature. The rolling was performed on the five plate-shaped blanks in such a manner that the final rolling reduction rates of the five plate-shaped blanks were 0% (not processed), 20%, 30%, 40%, and 65%, respectively.
The T6 heat treatment was performed in the same manner as in Test Example 1.
A tensile test was conducted on the test samples after the heat treatment in the same manner as in Test Example 1 to measure the elongation. The result is shown in FIG. 2.

TABLE 8

Test
sample	Composition [% by mass]

No.	Si	Mg	Fe	Ti	Cr	Al

6-1	6.9	0.59	0.15	0.15	0.16	bal.

When castings (blanks) having a composition within the composition, range of the aluminum alloy of this embodiment were reduced in thickness by 30% or more by rolling, the ductility was improved. The test samples processed at a rolling reduction of 40% or more exhibited an elongation of approximately 14%. That is, it was found that the ductility of the aluminum alloy of this embodiment can be significantly improved when it is processed at a cumulative area reduction of 30% or more, and that a more desirable result can be obtained when the cumulative area reduction is 40% or higher and 65% or lower.
The aluminum alloy of this embodiment is suitable for castings or machining blanks that have complicated shapes. In this specification, aluminum alloy castings include near-net-shape machining blanks as well as net-shape castings.
While some embodiments of the invention have been illustrated above, it is to be understood that the invention is not limited to details of the illustrated embodiments, but may be embodied with various changes, modifications or improvements, which may occur to those skilled in the art, without departing from the spirit and scope of the invention.

Claims

1. A casting aluminum alloy comprising:

5.6% or more and 7.5% or less by mass of silicon;

0.45% or more and 0.8% or less by mass of magnesium;

0.05% or more and 0.35% or less by mass of chromium; and aluminum.

2. The aluminum alloy according to claim 1, further comprising unavoidable impurities.

3. The aluminum alloy according to claim 2,

wherein the unavoidable impurities comprises iron, and the content of iron in the aluminum alloy is 0.3% or less by mass.

4. The aluminum alloy according to claim 1, further comprising 0.05% or more and 0.3% or less by mass of titanium.

5. The aluminum alloy according to claim 1, the aluminum alloy being free from copper.

6. The aluminum alloy according to claim 1,

further comprising: at least one of

0.003% or more and 0.05% or less by mass of strontium;

0.001% or more and 0.03% or less by mass of sodium; and

0.05% or more and 0.15% or less by mass of antimony.

7. The aluminum alloy according to claim 1,

wherein when the silicon is crystallized via eutectic reaction, crystallized silicon has an average grain size of 5 μm or smaller.

8. A casting method of an aluminum alloy, comprising:

pouring molten alloy comprising 5.6% or more and 7.5% or less by mass of silicon;

0.45% or more and 0.8% or less by mass of magnesium; 0.05% or more and 0.35% or less by mass of chromium; and aluminum; and

allowing the molten alloy to cool and solidify.

9. The casting method according to claim 8, wherein the alloy contains unavoidable impurities.

10. The casting method according to claim 9, wherein the unavoidable impurities comprises iron, and the content of iron in the aluminum alloy is 0.3% or less by mass.

11. The casting method according to claim 8,

wherein the alloy further comprises at least one of 0.003% or more and 0.05% or less by mass of strontium; 0.001% or more and 0.03% or less by mass of sodium; and 0.05% or more and 0.15% or less by mass of antimony.

12. The casting method according to claim 8, wherein solidification of the molten alloy is achieved by cooling the molten alloy at a cooling rate of 1° C./sec or faster.

13. The casting method according to claim 8, further comprising performing a solution heat treatment and an aging heat treatment on a solidified aluminum alloy casting.

14. A method of producing an aluminum alloy product, comprising

performing cold working and/or hot working on an aluminum alloy casting manufactured by using the casting method according to claim 8.

15. The method of producing an aluminum alloy according to claim 14,

wherein the cold working and/or hot working are performed on the aluminum alloy casting at a processing rate that provides a cumulative area reduction of 30% or more.