ALUMINUM ALLOY CASTING AND METHOD OF MANUFACTURING SAME

Abstract

An aluminum (Al)-magnesium (Mg)-silicon (Si)-based aluminum alloy casting includes: at least boron (B) and phosphorus (P). The boron (B) and the phosphorus (P) satisfy B/P≥8 in percentage by weight.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority under 35 U.S.C. § 119 to Japanese Patent Application 2020-158839, filed on Sep. 23, 2020, the entire content of which is incorporated herein by reference.

TECHNICAL FIELD

This disclosure relates to an Al—Mg—Si-based aluminum alloy casting and a method of manufacturing the same.

BACKGROUND DISCUSSION

In the related art, in order to increase mechanical strength of aluminum, an Al—Mg—Si-based aluminum alloy casting in which magnesium and silicon are added to aluminum and alloyed is known (for example, see JP 2017-210653A (Reference 1)). It is known that when phosphorus as an impurity is contained in the aluminum alloy casting, an action of refining a eutectic of an Mg—Si-based compound is inhibited.

The aluminum alloy casting described in Reference 1 is subjected to a dephosphorization treatment before casting to reduce a phosphorus content to 0.0002 wt % or less. Further, in the aluminum alloy casting described in Reference 1, by setting a manganese content to 0.2 wt % to 2 wt %, a fine Al—Mn—Si-based crystallized product becomes a crystallization nucleus of an Mg—Si-based crystallized product, and the Mg—Si-based crystallized product is refined. Accordingly, an aluminum alloy casting having excellent toughness is obtained.

Examples of the dephosphorization treatment in the related art include a method described in JP 2002-80920A (Reference 2). In the dephosphorization treatment method described in Reference 2, magnesium is added to molten aluminum, and chlorine gas is blown into the molten aluminum to float MgCl₂ that absorbs Mg₃P₂ in the molten aluminum thereby removing phosphorus from the surface of the molten aluminum.

When the dephosphorization treatment is performed before casting as in the aluminum alloy casting described in Reference 1, it is necessary to use the dephosphorization treatment method described in Reference 2. However, since chlorine gas is extremely toxic, handling thereof requires attention, and it is pointed out as a causative substance of ozone holes, which may lead to an environmental problem. In addition, in order to float and remove MgCl₂ absorbing Mg₃P₂, magnesium is wasted, and a facility for detoxifying chlorine gas is additionally required, which may lead to an increase in manufacturing cost. Further, in a general molten metal facility, P₂O₅ is contained in a refractory binder and a coating agent of a ruddle, and phosphorus generated by P₂O₅ is mixed in the molten metal, which makes it difficult to refine the Mg—Si-based crystallized product.

A need thus exists for an Al—Mg—Si-based aluminum alloy casting and a method of manufacturing the same which are not susceptible to the drawback mentioned above.

SUMMARY

A characteristic configuration of an aluminum alloy casting according to this disclosure is that the aluminum alloy casting is an aluminum (AO-magnesium (Mg)-silicon-(Si)-based aluminum alloy casting including at least boron (B) and phosphorus (P), and the boron (B) and the phosphorus (P) satisfies B/P≥8 in percentage by weight.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and additional features and characteristics of this disclosure will become more apparent from the following detailed description considered with the reference to the accompanying drawings, wherein:

FIG. 1 is a diagram showing preconditions of a casting test;

FIG. 2 is an enlarged photograph of an aluminum alloy casting according to Example 1;

FIG. 3 is an enlarged photograph of an aluminum alloy casting according to Example 2;

FIG. 4 is an enlarged photograph of an aluminum alloy casting according to Example 3;

FIG. 5 is an enlarged photograph of an aluminum alloy casting according to Example 4;

FIG. 6 is an enlarged photograph of an aluminum alloy casting according to Example 5;

FIG. 7 is an enlarged photograph of an aluminum alloy casting according to Comparative Example 1;

FIG. 8 is an enlarged photograph of an aluminum alloy casting according to Comparative Example 2; and

FIG. 9 is an enlarged photograph of an aluminum alloy casting according to Comparative Example 3.

DETAILED DESCRIPTION

Hereinafter, an embodiment of an Al—Mg—Si-based aluminum alloy casting and a method of manufacturing the same according to this disclosure will be described with reference to the drawings. However, this disclosure is not limited to the following embodiment, and various modifications can be made without departing from the scope of this disclosure.

The present inventors have found a composition of an aluminum alloy casting that can refine an Mg—Si-based crystallized product by detoxifying phosphorus without performing dephosphorization treatment before casting. That is, in the present embodiment, by adding boron to make B/P≥8, phosphorus is detoxified, the Mg—Si-based crystallized product is refined, and the toughness of the aluminum alloy casting can be increased.

The aluminum alloy casting according to the present embodiment contains 2.0 wt % or more and 8.0 wt % or less of magnesium (Mg), 1.0 wt % or more and 5.0 wt % or less of silicon (Si), 0.005 wt % or more and 0.05 wt % or less of boron (B), 0.0003 wt % or more and 0.005 wt % or less of phosphorus (P), and a balance being aluminum (Al) and inevitable impurities. The aluminum alloy casting contains boron and phosphorus so as to satisfy B/P≥8. The aluminum alloy casting may contain 0.3 wt % or less of iron (Fe), 0.3 wt % or more and 0.8 wt % or less of manganese (Mn), 0.2 wt % or less of titanium (Ti), and 0.001 wt % or more and 0.01 wt % or less of beryllium (Be). At this time, titanium is preferably added so as to satisfy (B—8P)/Ti≥0.13.

[Composition]

(Mg: 2.0 wt % or More and 8.0 wt % or Less)

Mg contributes to improvement in tensile strength of the aluminum alloy casting. When the content of Mg is less than 2.0 wt %, an amount of the Mg—Si-based crystallized product decreases, and the tensile strength decreases. On the other hand, when the content of Mg exceeds 8.0 wt %, the amount of the Mg—Si-based crystallized product becomes excessively large, and ductility decreases. When the content of Mg exceeds 8.0 wt %, remarkable oxidation of the molten metal occurs, which is thus not preferred.

(Si: 1.0 wt % or More and 5.0 wt % or Less)

Si contributes to improvement in tensile strength of the aluminum alloy casting. When the content of Si is less than 1.0 wt %, an amount of the Mg—Si-based crystallized product decreases, and the tensile strength decreases. When the content of Si is less than 1.0 wt %, the castability (melt flowability, seizure resistance, and casting cracking resistance) is deteriorated, which is not preferred. On the other hand, when the content of Si exceeds 5.0 wt %, the amount of the Mg—Si-based crystallized product becomes excessively large, and the ductility decreases.

(Fe: 0.3 wt % or Less)

Fe contributes to improvement in tensile strength of the aluminum alloy casting. When the content of Fe exceeds 0.3 wt %, an Al—Si—Fe-based crystallized product is generated, and the ductility decreases.

(Mn: 0.3 wt % or More and 0.8 wt % or Less)

Mn improves the baking property of a mold. When a content of Mn is less than 0.3 wt %, it is difficult to obtain an effect of preventing baking of the aluminum alloy casting with respect to the mold. On the other hand, when the content of Mn exceeds 0.8 wt %, an Al—Si—Mn-based crystallized product is generated, and the ductility decreases.

(B: 0.005 wt % or More and 0.05 wt % or Less)

B detoxifies phosphorus and refines the Mg—Si-based crystallized product. When a content of B is less than 0.005 wt %, a function of detoxifying phosphorus cannot be exhibited. On the other hand, when the content of B exceeds 0.05 wt %, AlB-based coarse compounds crystallize, and the ductility decreases.

(P: 0.0003 wt % or More and 0.005 wt % or Less)

When a content of P is less than 0.0003 wt %, a function of refining a eutectic of the Mg—Si-based compound is inhibited. In order to make the content of P be 0.0003 wt % or more, a dephosphorization treatment is required in advance. On the other hand, when P exceeds 0.005 wt %, an AlP compound serving as a crystallization nucleus of the Mg—Si-based crystallized product crystallizes, and the ductility decreases.

(B/P≥8)

A ratio of boron and phosphorus in the above-described range is B/P≥8. By adding boron in the range, phosphorus is detoxified, the Mg—Si-based crystallized product is refined, and the toughness of the aluminum alloy casting can be increased. It is presumed that this is because a B—P based compound (detoxification of phosphorus), which is more energetically stable than an AlP compound serving as a crystallization nucleus of the Mg—Si-based crystallized product, is generated, and crystallization and growth of the Mg—Si-based crystallized product using the AlP compound as a nucleus are prevented.

In the present embodiment, boron having toxicity similar to that of salt is used. Since boron detoxifies phosphorus rather than removing phosphorus, magnesium is not wasted. Moreover, since it does not require equipment for detoxifying chlorine gas as in the related art, it is environmentally friendly and the toughness of the aluminum alloy casting can be increased at low cost. In particular, in the present embodiment, a required amount of boron for detoxifying phosphorus is defined as B/P≥8, and the phosphorus detoxifying function of boron is enabled even if P₂O₅ is contained in a molten metal facility.

(Ti: 0.2 wt % or Less)

Ti is known as a substance that refines a primary crystal a (Al) phase. When the content of Ti exceeds 0.2 wt %, the phosphorus detoxifying function of boron is inhibited. When the content of Ti exceeds 0.2 wt %, AlTi-based coarse compounds crystallize and the ductility decreases.

((B−8P)/Ti≥0.13)

In general, it is known that, when titanium and boron are simultaneously added, the primary crystal a (Al) phase is refined. However, the present inventors have found that titanium exceeding a predetermined amount inhibits the phosphorus detoxifying function of boron. Therefore, when B/P≥8 is assumed and (B−8P)/Ti≥0.13 is satisfied, phosphorus detoxification of boron functions, the Mg—Si-based crystallized product is refined, and the toughness of the aluminum alloy casting can be further increased.

(Be: 0.001 wt % or More and 0.01 wt % or Less)

Be exerts an effect of preventing oxidation of the molten metal. When Be is less than 0.001 wt %, it is difficult to obtain the effect of preventing the oxidation of the molten metal. On the other hand, even if Be is contained in an amount exceeding 0.01 wt %, there is no change in the effect of preventing the oxidation of the molten metal.

[Manufacturing Method]

A method of manufacturing the aluminum alloy casting according to the present embodiment includes a melting step of melting a starting material of the aluminum alloy casting to generate a molten metal, and a casting step of casting the molten metal generated in the melting step with a mold to manufacture the aluminum alloy casting. The casting step includes a cooling step of cooling the molten metal at 50° C./s or more. The aluminum alloy casting is used for vehicle body parts, engine parts, and the like. When the molten metal generated in the melting step is cooled at 50° C./s or more as in the present method, the Mg—Si-based crystallized product is subjected to homogeneous nucleation due to supercooling, and a refining effect can be obtained. The method of manufacturing the aluminum alloy casting according to the present embodiment may be gravity casting or die casting.

EXAMPLES

FIG. 1 shows a composition of the aluminum alloy casting, a ratio of B/P, a ratio of (B−8P)/Ti, and a cooling rate of Examples and Comparative Examples according to the present embodiment. The starting material of the aluminum alloy casting having the composition shown in FIG. 1 was placed into a crucible and melted at a melting temperature of 760° C. to 780° C. using an electric melting furnace (melting step). At this time, the inside of the crucible was stirred 50 times, and after bubbling argon gas at 2 L/min for 15 minutes, the crucible was allowed to stand for 15 minutes. The molten metal generated in the melting step was placed into a copper mold and cast at a casting temperature of 720° C. to obtain the aluminum alloy casting (casting step). In the casting step, the molten metal was cooled at the cooling rate shown in FIG. 1 and held for a predetermined time (cooling step).

FIGS. 2 to 9 show enlarged photographs of the metal structures of respective aluminum alloy castings. A dark gray needle-like structure in the photograph is the Mg—Si-based crystallized product, and the light gray part is the aluminum base material.

In the case where Ti is not contained, in Comparative Example 1 in which P is 0.0011 mass % and B is not added (B/P=0, (B−8P)/Ti=∞, cooling rate: 50° C./s), as shown in FIG. 7, a Mg—Si-based eutectic is coarse (dark gray needle-like structure is thick and long), whereas in Example 1 in which 0.0010 mass % of P and 0.0080 mass % of B are added (B/P=8, (B−8P)/Ti=∞, cooling rate: 50° C./s), and in Example 2 in which 0.0011 mass % of P and 0.0200 mass % of B are added (B/P=18, (B−8P)/Ti=∞, cooling rate: 50° C./s), as shown in FIGS. 2 and 3, the Mg—Si-based eutectics are refined (dark gray needle-like structure is thin and short). In Comparative Example 2 in which 0.0010 mass % of P and 0.0060 mass % of B are added (B/P=6, (B−8P)/Ti=−∞, cooling rate: 50° C./s), as shown in FIG. 8, the Mg—Si-based eutectic is coarse. From above, it can be understood that when B/P≥8 in the case where Ti is not contained, the Mg—Si-based eutectic is refined, and when (B−8P)/Ti≥0.13 in the case where Ti is contained, the Mg—Si-based eutectic is refined.

In the case where Ti is contained, in Example 3 in which 0.04 mass % of Ti, 0.0017 mass % of P, and 0.0377 mass % of B are added (B/P=22, (B−8P)/Ti=0.6, cooling rate: 50° C./s), and in Example 4 in which 0.13 mass % of Ti, 0.0021 mass % of P, and 0.0334 mass % of B are added (B/P=16, (B−8P)/Ti=0.13, cooling rate: 50° C./s), as shown in FIGS. 4 and 5, the Mg—Si-based eutectic is refined. In Example 5 in which 0.13 mass % of Ti, 0.0018 mass % of P, and 0.0147 mass % of B are added (B/P=8, (B−8P)/Ti=0.002, cooling rate: 50° C./s), coarse Mg—Si-based eutectic is present in a very small part of the aluminum alloy casting (not shown), whereas the Mg—Si-based eutectic is roughly refined as shown in FIG. 6. From above, it is found that, in the case where Ti is contained, when (B−8P)/Ti≥0.13, the Mg—Si-based eutectic can be reliably refined.

In Comparative Example 3 in which the cooling rate is as low as 10° C./s, chemical components are the same as those in Example 1 in which the cooling rate is 50° C./s, whereas the Mg—Si-based eutectic is coarse as shown in FIG. 9. That is, it can be understood that when the cooling rate is 50° C./s or more in the cooling step, the Mg—Si-based eutectic is refined.

INDUSTRIAL APPLICABILITY

This disclosure is applicable to an Al—Mg—Si-based aluminum alloy casting and a method of manufacturing the same.

A characteristic configuration of an aluminum alloy casting according to this disclosure is that the aluminum alloy casting is an aluminum (Al)-magnesium (Mg)-silicon-(Si)-based aluminum alloy casting including at least boron (B) and phosphorus (P), and the boron (B) and the phosphorus (P) satisfies B/P≥8 in percentage by weight.

The present inventors have found a composition of an aluminum alloy casting that can refine an Mg—Si-based crystallized product by detoxifying phosphorus without performing a dephosphorization treatment before casting. That is, in this configuration, by adding boron to make B/P≥8, phosphorus is detoxified, the Mg—Si-based crystallized product is refined, and the toughness of the aluminum alloy casting can be increased. It is presumed that this is because a B—P-based compound (detoxification of phosphorus), which is more energetically stable than an AlP compound serving as a crystallization nucleus of the Mg—Si-based crystallized product, is generated, and crystallization and growth of the Mg—Si-based crystallized product using the AlP compound as a nucleus are prevented.

In this configuration, boron having toxicity similar to that of salt is used. Since boron detoxifies phosphorus rather than removing phosphorus, magnesium is not wasted. Moreover, since it does not require equipment for detoxifying chlorine gas as in the related art, it is environmentally friendly and the toughness of the aluminum alloy casting can be increased at low cost.

In particular, in this configuration, a required amount of boron for detoxifying phosphorus is defined as B/P≥8, and a phosphorus detoxifying function of boron is enabled even if P₂O₅ is contained in a molten metal facility. As described above, it is possible to provide an Al—Mg—Si-based aluminum alloy casting which is environmentally friendly and can increase the toughness at low cost.

Another characteristic configuration is that the aluminum alloy casting further includes titanium (Ti), and the boron (B), the phosphorus (P), and the titanium (Ti) satisfy (B−8P)/Ti≥0.13 in percentage by weight.

In general, it is known that, when titanium and boron are simultaneously added, a primary crystal a (Al) phase is refined. However, the present inventors have found that titanium exceeding a predetermined amount inhibits the phosphorus detoxifying function of boron. Therefore, when (B−8P)/Ti≥0.13 is satisfied as in this configuration, phosphorus detoxification of boron functions, the Mg—Si-based crystallized product is refined, and the toughness of the aluminum alloy casting can be further increased.

Another characteristic configuration is that the boron (B) is 0.005 wt % or more and 0.05 wt % or less, and the phosphorus (P) is 0.0003 wt % or more and 0.005 wt % or less.

Within the range of boron as in this configuration, phosphorus is reliably detoxified, and a coarse boron compound is not formed.

Another characteristic configuration is that the magnesium (Mg) is 2.0 wt % or more and 8.0 wt % or less, the silicon (Si) is 1.0 wt % or more and 5.0 wt % or less, the titanium (Ti) is 0.2 wt % or less, and a balance includes the aluminum (Al) and inevitable impurities.

Due to the composition according to this configuration, castability (melt flowability, seizure resistance, and casting cracking resistance) and mechanical properties are excellent. Therefore, it is possible to provide an Al—Mg—Si-based aluminum alloy casting which is environmentally friendly and can increase the toughness at low cost.

A method of manufacturing the aluminum alloy casting according to this disclosure is a method of manufacturing the aluminum alloy casting according to any one of the above aspects, the method including: a melting step of melting a starting material of the aluminum alloy casting to generate a molten metal; and a cooling step of cooling the molten metal generated in the melting step at 50° C./s or more.

When the molten metal generated in the melting step is cooled at 50° C./s or more as in the present method, the Mg—Si-based crystallized product is subjected to homogeneous nucleation due to supercooling, and a refining effect can be obtained.

The principles, preferred embodiment and mode of operation of the present invention have been described in the foregoing specification. However, the invention which is intended to be protected is not to be construed as limited to the particular embodiments disclosed. Further, the embodiments described herein are to be regarded as illustrative rather than restrictive. Variations and changes may be made by others, and equivalents employed, without departing from the spirit of the present invention. Accordingly, it is expressly intended that all such variations, changes and equivalents which fall within the spirit and scope of the present invention as defined in the claims, be embraced thereby.

Claims

1. An aluminum (Al)-magnesium (Mg)-silicon (Si)-based aluminum alloy casting comprising:

at least boron (B) and phosphorus (P), wherein

the boron (B) and the phosphorus (P) satisfy B/P≥8 in percentage by weight.

2. The aluminum alloy casting according to claim 1, further comprising:

titanium (Ti), wherein

the boron (B), the phosphorus (P), and the titanium (Ti) satisfy (B−8P)/Ti≥0.13 in percentage by weight.

3. The aluminum alloy casting according to claim 1, wherein

the boron (B) is 0.005 wt % or more and 0.05 wt % or less, and the phosphorus (P) is 0.0003 wt % or more and 0.005 wt % or less.

4. The aluminum alloy casting according to claim 3, wherein

the magnesium (Mg) is 2.0 wt % or more and 8.0 wt % or less, the silicon (Si) is 1.0 wt % or more and 5.0 wt % or less, the titanium (Ti) is 0.2 wt % or less, and a balance includes the aluminum (Al) and inevitable impurities.

5. A method of manufacturing the aluminum alloy casting according to claim 1, comprising:

a melting step of melting a starting material of the aluminum alloy casting to generate a molten metal; and

a cooling step of cooling the molten metal generated in the melting step at 50° C./s or more.