Patent Description:
Casting alloys are known (such as alloy A356 specified by the American Society for Testing and Materials (ASTM)), which are aluminum (Al) alloys containing silicon (Si) and having added magnesium (Mg) for improving mechanical properties of an Al-Si aluminum alloy with favorable castability. The Mg added for improved strength may be oxidized and depleted in a molten state, thereby promoting oxide production and gas absorption. The addition of beryllium (Be) to the Al-Si-Mg aluminum alloy is known to inhibit Mg depletion.

The addition of antimony (Sb) to the Al-Si-Mg aluminum alloy AC4C or AC4A specified in Japanese Industrial Standards (JIS) H <NUM>, for example, is known to improve (refine) a Si phase in a eutectic structure and thus improve an elongation property (refer to Patent Literature <NUM>).

However, when the Al-Si-Mg aluminum alloy with added Sb undergoes high-temperature heat treatment such as solution treatment, a cast surface may turn black, thereby damaging its appearance. Proposals for inhibiting the blackening of the cast surface include the addition of a large amount of Be to the Al-Si-Mg aluminum alloy to which Sb has been added, and the combined addition of Be and Ca (refer to Patent Literature <NUM> and Patent Literature <NUM>).

As per Patent Literature <NUM>, the blackening is inhibited when a Be content is <NUM> mass% or larger. Adequate care is required in handling Be because Be is a rare metal and therefore expensive, and because Be dust is highly toxic.

The present invention has been made in consideration of the above matters and is directed to providing a method for manufacturing an Al-Si-Mg aluminum alloy casting material with a low Be content and an excellent appearance after heat treatment.

According to the present invention, a method for manufacturing an Al-Si-Mg aluminum alloy casting material comprises:
performing a heat treatment to sequentially perform:.

According to the present invention, the method for manufacturing an Al-Si-Mg aluminum alloy casting material can be provided.

An embodiment according to the present invention is described below with reference to the drawings but the present invention is not limited thereto. Constituent elements of the embodiment described below can be combined as appropriate. In some cases, part of the constituent elements may not be used. The constituent elements in the embodiment described below include elements that can be easily conceived of by a person skilled in the art, elements substantially equivalent thereto, and elements within a so-called range of equivalents.

AnAl-Si-Mg aluminum alloy for casting of the present embodiment contains <NUM> mass% or larger and <NUM> mass% or smaller of Si, <NUM> mass% or larger and <NUM> mass% or smaller of Mg, <NUM> mass% or larger and <NUM> mass% or smaller of Sb, and <NUM> mass% or larger and <NUM> mass% or smaller of Be with the remainder consisting of Al and unavoidable impurities.

Si contributes to castability and mechanical properties. The castability improves considerably when the Si content is <NUM> mass% or larger. The castability is important in making a large cast such as an automobile part. Because the addition of Si makes a Si crystallized matter more likely to coarsen and makes the elongation property more likely to drop, the Si content needs to be kept to <NUM> mass% or smaller. During aging treatment, Si is precipitated along with Mg as an Mg-Si compound, contributing to improved strength.

Because Mg is precipitated together with Si as the Mg-Si compound in the Al-Si-Mg aluminum alloy for casting of the present embodiment during the aging treatment, Mg provides the effect of improving strength. This effect is significant when the Mg content is <NUM> mass% or larger and even more so when it is <NUM> mass% or larger. Conversely, the Mg content of larger than <NUM> mass% deteriorates the elongation property and promotes oxide production, thereby causing hard spots and other defects. Thus, the Mg content is more preferably <NUM> mass% or larger and <NUM> mass% or smaller, which improves the strength, prevents the deterioration of the elongation property, and inhibits the oxide production.

Sb provides the effects of refining Si in the eutectic structure and improving the elongation property. These effects are significant when the Sb content is <NUM> mass% or larger. When the Sb content is larger than <NUM> mass%, a coarse Mg-Sb compound may be created, which may result in the deterioration of the elongation property.

As described in Patent Literature <NUM>, the blackening of the cast surface has been considered unavoidable unless the Al-Si-Mg aluminum alloy contains a large amount of Be. Through extensive research, the inventors of the present invention have discovered that a relation between the Be content in the Al-Si-Mg aluminum alloy and the blackening of the cast surface is not a simple inverse proportional relation. More specifically, they have found that the blackening of the cast surface is unlikely to occur until the Be content in the Al-Si-Mg aluminum alloy reaches a prescribed threshold value; that the blackening occurs easily when the Be content is higher than the prescribed threshold value; and that the blackening is inhibited when the Be content further increases, for example, to <NUM> mass% or larger.

More specifically, Be forms a dense passive oxide film on the molten metal surface of the aluminum alloy and inhibits oxidation of the molten aluminum alloy. Be inhibits Mg depletion in the aluminum alloy. For enhanced effects, the Be content needs to be <NUM> mass% or larger. However, if the Be content is larger than <NUM> mass%, the cast surface easily blackens when an ingot is subjected to, after casting, solution treatment, water quenching, and aging treatment, or so-called temper designation T6 heat treatment stipulated in JIS H <NUM> (hereinafter referred to as T6 heat treatment). This is presumably because the aluminum oxide layer on the cast surface becomes thick by the T6 heat treatment, which leads to the blackening of the cast surface. In the present embodiment, the Be content of <NUM> mass% or larger and <NUM> mass% or smaller inhibits the blackening of the cast surface by the T6 heat treatment.

The Al-Si-Mg aluminum alloy for casting of the present embodiment may also contain boron (B) as a refining material of the cast structure, where B ≤ <NUM>% hold.

The Al-Si-Mg aluminum alloy for casting of the present embodiment permits inevitable impurities, but iron (Fe), which gets easily mixed in, is kept to <NUM>% or smaller, and other elements of the inevitable impurities are kept to <NUM>% or smaller.

The Al-Si-Mg aluminum alloy for casting of the present embodiment permits calcium (Ca), which inevitably gets mixed in. However, if the Ca content is <NUM> mass% or larger, gas absorption becomes intensified and fluidity worsens. Therefore, the Ca content in the Al-Si-Mg aluminum alloy for casting of the present embodiment is <NUM> mass% or larger and smaller than <NUM> mass%, and more favorably kept to <NUM> mass% or larger and <NUM> mass% or smaller.

The following describes an example of the method for manufacturing a casting material using the Al-Si-Mg aluminum alloy for casting of the present embodiment described above.

An aluminum alloy with an alloy composition containing <NUM> to <NUM> mass% inclusive of Si, <NUM> mass% or larger and <NUM> mass% or smaller of Mg, <NUM> mass% or larger and <NUM> mass% or smaller of Sb, and <NUM> mass% or larger and <NUM> mass% or smaller of Be, and the remainder consisting of Al and inevitable impurities is produced by melting with a known method.

The resulting aluminum alloy molten metal undergoes molten metal treatment, such as component adjustment, slag removal, degassing and the like. If Ti and B are contained as refining materials, a rod hardener (refining material) formed with an Al-Ti-B alloy, for example, is added to the aluminum alloy molten metal before casting. In the present invention, Ti is not used as refining material.

The aluminum alloy molten metal obtained in the melting step is poured into a mold to obtain an ingot.

The ingot obtained in the casting step undergoes the T6 heat treatment to obtain the Al-Si-Mg aluminum alloy casting material of the present embodiment. The T6 heat treatment is heat treatment in which the ingot is subjected to solution treatment, quenching treatment, and aging treatment in sequence.

As conditions of the solution treatment, a solution treatment temperature is held at <NUM> or higher and <NUM> or lower for <NUM> hours or longer and <NUM> hours or shorter. As an example of the solution treatment conditions, the solution treatment temperature is held at <NUM> for <NUM> hours. If the solution treatment temperature is lower than <NUM> or temperature hold time is shorter than <NUM> hours, the effect of the solution treatment is small. If the solution treatment temperature is higher than <NUM>, local melting (burning) may occur. Even if the temperature hold time exceeds <NUM> hours, no change is seen in amounts of elements of Mg and Si in solid solution, but the costs increase.

The ingot subjected to the solution treatment is water-cooled as the quenching treatment. Water used for the quenching treatment may be warm water.

After the quenching treatment, the ingot forming supersaturated solid solution is subjected to the aging treatment. As conditions for the aging treatment, the aging temperature is held at <NUM> or higher and <NUM> or lower for <NUM> hours or longer and <NUM> hours or shorter. As an example of conditions for the aging treatment, the aging temperature is held at <NUM> for <NUM> hours.

The Al-Si-Mg aluminum alloy for casting and the Al-Si-Mg aluminum alloy casting material of the present embodiment, having undergone the T6 heat treatment, are less blackened after the heat treatment and are excellent in appearance. In the Al-Si-Mg aluminum alloy for casting and the Al-Si-Mg aluminum alloy casting material of the present embodiment, Mg contributes to the mechanical strength as there is little Mg depletion in the molten metal and the temper designation T6 refining stipulated in JIS H <NUM> is performed, thereby making the tensile strength <NUM> MPa or higher and the elongation <NUM>% or greater, for example. The Al-Si-Mg aluminum alloy casting material of the present embodiment, having undergone the T6 heat treatment, is manufactured as an automobile part, for example.

The following describes examples of the present invention. In Example <NUM>, Example <NUM> and Comparative Example <NUM>, an aluminum alloy having elements of an alloy composition of Table <NUM> and the remainder of Al was melted to manufacture a molten metal for evaluation. The temperature of each manufactured molten metal for evaluation was held at <NUM>, and the Mg content was measured after <NUM> hours and <NUM> hours. Each measured Mg content was subtracted from the Mg content immediately after the melting to calculate the Mg depletion amounts in the molten metal after <NUM> hours (h) and <NUM> hours (h), and the results are listed in Table <NUM>.

It was confirmed that the Mg depletion amount in the molten metal was obviously smaller in Example <NUM> and Example <NUM> than that in Comparative Example <NUM> with a Be content smaller than <NUM> mass%. Therefore, in Example <NUM> and Example <NUM>, Mg added for strength improvement becomes less oxidized and depleted in the molten metal than Mg in Comparative Example <NUM>, thereby lowering the possibility of promoting oxide production and gas absorption. As a result, in Example <NUM> and Example <NUM>, the molten state is less affected than that in Comparative Example <NUM> and a casting material with improved strength can be stably manufactured.

In Comparative Example <NUM>, Examples <NUM> to <NUM> and Comparative Example <NUM>, casting materials were manufactured with the manufacturing method described above so as to make aluminum alloys having the elements of the alloy composition of Table <NUM> and the remainder of Al. Each casting material was cast into a boat shape using gravity die casting in the same die. Each casting material underwent the T6 heat treatment after water-cooling to sequentially perform the solution treatment, in which the casting material was held at a holding temperature of <NUM> for <NUM> hours, the quenching treatment, and the aging treatment, in which the casting material was held at a holding temperature of <NUM> for <NUM> hours.

Subsequently, a color-difference meter (CR-<NUM> manufactured by Konica Minolta Japan, Inc. ) was used to obtain a body color of the surface of each casting material on the basis of JIS Z <NUM>. A color difference ΔE was calculated for the resulting body colors using the body color of the second comparative example with Be of smaller than <NUM> mass% as a standard on the basis of JIS Z <NUM>.

The resulting color differences ΔE for Examples <NUM> to <NUM> and Comparative Example <NUM> compared to Comparative Example <NUM> are listed in Table <NUM>. <FIG> is a diagram for explaining a relation between a color difference with respect to a Be content in the Al-Si-Mg aluminum alloy for casting and a Mg depletion amount. <FIG> is a diagram illustrating examples of appearances of the side surfaces of casts after the heat treatment.

As can be understood from <FIG>, the Al-Si-Mg aluminum alloy for casting and the Al-Si-Mg aluminum alloy casting material have a Be content of <NUM> mass% or larger and <NUM> mass% or smaller, thereby inhibiting the blackening of the cast surface that has been refined with the temper designation T6 specified in JIS H <NUM>, while inhibiting the depletion amount of Mg in the molten metal.

As illustrated in <FIG>, Comparative Example <NUM> and Example <NUM> are visually recognized as silver and the third comparative example is visually recognized as black. The third comparative example has a Be content larger than <NUM> mass% and it is understood from <FIG> that the surface is blackened. As illustrated in <FIG>, the larger the color difference ΔE from the color of Comparative Example <NUM> is, the more significant the blackening becomes. As can be understood from <FIG> and Table <NUM>, if the color difference ΔE from the color of the second comparative example is <NUM> or larger, the blackness of the cast surface can be easily visually recognized.

Claim 1:
A method for manufacturing an Al-Si-Mg aluminum alloy casting material, the method comprising:
performing a heat treatment to sequentially perform:
a solution treatment on an Al-Si-Mg aluminum alloy casting material containing <NUM> mass% or larger and <NUM> mass% or smaller of Si, <NUM> mass% or larger and <NUM> mass% or smaller of Mg, <NUM> mass% or larger and <NUM> mass% or smaller of Sb, and <NUM> mass% or larger and <NUM> mass% or smaller of Be, <NUM> mass% or smaller of B, and a remainder consisting of Al and unavoidable impurities, wherein the unavoidable impurities include Fe and Ca, the content of Fe in the Al-Si-Mg aluminum alloy casting material is <NUM> mass% or smaller, the content of Ca in the Al-Si-Mg aluminum alloy casting material is <NUM> mass% or larger and smaller than <NUM> mass%, and the total content of the unavoidable impurities except for Fe in the Al-Si-Mg aluminum alloy casting material is <NUM> mass% or smaller;
a quenching treatment; and
an aging treatment, wherein
in the solution treatment, a temperature is held at <NUM> or higher and <NUM> or lower for <NUM> hours or longer and <NUM> hours or shorter, and
in the aging treatment, a temperature is held at <NUM> or higher and <NUM> or lower for <NUM> hours or longer and <NUM> hours or shorter.