JP4093221B2 - Aluminum alloy for casting, aluminum alloy casting and method for producing the same - Google Patents

Description

  The present invention relates to an aluminum alloy casting excellent in practical fatigue characteristics such as fatigue strength and heat fatigue resistance, a method for producing the same, and an aluminum alloy for casting suitable for the production.

  Various members are shifting to aluminum alloys due to demands for weight reduction. Even if the member is already made of an aluminum alloy, the thickness of each part can be reduced for further weight reduction and the like. Therefore, the aluminum alloy is required to have higher reliability in terms of strength, fatigue resistance, and the like than before. In particular, when aluminum alloys are used for automobile engine parts, etc., they are often used in high-temperature environments, so not only heat resistance such as high-temperature strength and creep properties, but also high fatigue resistance for cold cycles. Characteristics (heat fatigue resistance) are required. An example of such a member is a cylinder head of a reciprocating engine.

  Since the cylinder head is complicated and large in size, it is usually manufactured by casting. There are AC2A, AC2B, AC4B, AC4C (JIS), and the like as the aluminum alloy for castings, but many other aluminum alloys have been developed. For example, the following patent documents disclose the disclosure. Many of the aluminum alloys of Examples disclosed in the following Patent Documents 1 to 4 contain Cu and Mg. This is because Cu and Mg strengthen the precipitation of the matrix phase and increase the strength of the cylinder head. On the other hand, in Patent Document 5, Cu and Mg are treated as impurities from the viewpoint of obtaining a cast with high heat fatigue resistance, and their upper limit is restricted to 0.2% by mass. The reason for this is that Cu and Mg generate thermally unstable precipitates, and the precipitates become coarse during use of the casting to reduce the ductility and toughness of the casting, resulting in a decrease in thermal fatigue resistance. It is thought that.

Japanese Patent Laid-Open No. 10-251790 Japanese Patent Laid-Open No. 11-199960 JP 2001-303163 A Japanese Patent No. 3415346 Japanese Patent No. 3164587

  However, since the aluminum alloy disclosed in Patent Document 5 does not substantially contain Cu and Mg, its hardness and strength are extremely low, and the practical strength as a base material tends to be insufficient. . Therefore, in Patent Document 5, while using another high-strength casting aluminum alloy as a base material, a portion where thermal strain is concentrated and high heat fatigue resistance is required (for example, a valve bridge portion of a cylinder head or a secondary combustion). The aluminum alloy is build-up bonded between the chamber hole and the valve hole. That is, the aluminum alloy disclosed in Patent Document 5 is only limitedly used in a portion where high heat fatigue resistance is necessary. Thus, changing the type of aluminum alloy to be used depending on the part is not preferable because the manufacturing cost of castings such as cylinder heads is very high.

  The present invention has been made in view of such circumstances, and provides an aluminum alloy for castings that is excellent in heat fatigue resistance while ensuring the strength and fatigue resistance required for a cylinder head and the like. With the goal. Moreover, it aims at providing such an aluminum alloy casting and its manufacturing method.

  As a result of intensive studies to solve this problem and repeated trial and error, the inventor does not necessarily reduce the ductility and toughness of the casting even when Mg is contained and the entire casting is strengthened. In addition, the inventors have newly discovered that strength and fatigue resistance as a base material and high heat fatigue resistance can both be achieved, and the present invention has been completed.

(Aluminum alloy for casting)
That is, the aluminum alloy for castings of the present invention has silicon (Si): 4 to 12% by mass, copper (Cu): 0.2% by mass or less, and magnesium (Mg) when the total is 100% by mass. : More than 0.2% by mass and 0.5 % by mass or less ; nickel (Ni): 0.2-3% by mass; iron (Fe): 0.1-0.7% by mass; titanium (Ti) : 0.15 to 0.3 mass%, and the balance is aluminum (Al) and inevitable impurities, and an aluminum alloy casting excellent in practical fatigue characteristics is obtained.

  The aluminum alloy casting cast using the casting aluminum alloy of the present invention has high strength and high fatigue strength (fatigue resistance) and also exhibits high heat fatigue resistance. If this aluminum alloy for castings is used, for example, a casting such as a cylinder head that requires high strength as a whole and partially requires high heat fatigue resistance, the same alloy can Casting becomes possible, and the manufacturing cost of the casting can be greatly reduced. For example, the aluminum alloy for casting of the present invention is optimal for casting of a high performance cylinder head for gasoline engines and a cylinder head for diesel engines that require high strength and high fatigue resistance.

(Aluminum alloy casting)
The present invention can be grasped not only as an aluminum alloy for castings but also as an aluminum alloy casting excellent in practical fatigue characteristics.
That is, when the present invention is 100% by mass as a whole, Si: 4 to 12% by mass, Cu: 0.2% by mass or less, Mg: more than 0.2% by mass and 0.5 % by mass or less Ni: 0.2-3 mass%, Fe: 0.1-0.7 mass%, Ti: 0.15-0.3 mass%, and the balance consisting of Al and inevitable impurities, It has a metal structure composed of a base phase mainly composed of α-Al and a skeleton phase crystallized so as to surround the base phase in a network, and the base phase is excellent in practical fatigue characteristics by precipitation strengthening with Mg. It is good also as an aluminum alloy casting characterized by this.

(Aluminum alloy casting manufacturing method)
The present invention can also be grasped as a production method suitable for producing the above-described aluminum alloy for castings.
That is, the present invention includes a casting process in which an aluminum alloy molten metal containing Al as a main component is poured into a mold and solidified to obtain an aluminum alloy casting, and a heat treatment process in which the aluminum alloy casting is subjected to solution heat treatment and aging heat treatment. And
The aluminum alloy casting after the heat treatment step is Si: 4 to 12% by mass, Cu: 0.2% by mass or less, Mg: more than 0.2% by mass , and 0.1% when the whole is 100% by mass . 5 mass% or less , Ni: 0.2-3 mass%, Fe: 0.1-0.7 mass%, Ti: 0.15-0.3 mass%, the balance being Al and inevitable impurities And has a metal structure composed of a base phase mainly composed of α-Al and a skeleton phase crystallized so as to surround the base phase in a network shape, and the base phase is precipitated by a precipitate containing Mg. It is good also as a manufacturing method of the aluminum alloy casting characterized by being excellent in the strengthened practical fatigue characteristic.

(Function)
The aluminum alloy of the present invention has achieved both high strength or high fatigue strength of an aluminum alloy casting, which has been difficult in the past, and high heat fatigue resistance. The mechanism that makes this possible is not always clear, but it can be considered as follows. In the following description, both the aluminum alloy for casting as a casting raw material and the aluminum alloy casting that is a cast product are appropriately referred to simply as an aluminum alloy.
Conventionally, when increasing the fatigue strength of an aluminum alloy (casting), first, improvement of the static tensile strength can be considered. In general, a precipitation strengthening element such as Cu or Mg is included as a technique.

  However, by simply applying such a method, even if the strength of the aluminum alloy can be increased, ductility and toughness are reduced. As a result, the fatigue strength affected by the stress concentration, average stress, etc. cannot be increased, and further, the heat fatigue resistance is also lowered due to the decrease in ductility and toughness. Therefore, conventionally, it has been extremely difficult to satisfy the strength, fatigue resistance and heat fatigue resistance of aluminum alloys at a high level. For example, even if the disclosure content of each patent document described above is viewed, there is no one that satisfies them in a high dimension at the same time, and only one of the characteristics is excellent.

On the other hand, the aluminum alloy of the present invention does not substantially contain Cu, and optimizes the content of Ni, Fe and Ti together with the content of Mg, thereby improving the strength, fatigue strength and heat fatigue resistance. Both are compatible. Hereinafter, the action of each element will be described.
First, since the aluminum alloy of the present invention does not substantially contain Cu, the structure of the matrix phase is stabilized, and the brittleness of the matrix phase in a thermal fatigue environment is suppressed, thereby improving the heat fatigue resistance. Contribute. Incidentally, the embrittlement of the matrix phase by Cu is considered to be caused by the coarsening of the Cu compound precipitated in the matrix phase in a thermal fatigue environment.

  However, since the aluminum alloy of the present invention does not substantially contain Cu, precipitation strengthening by Cu cannot be expected. Therefore, the present inventor considered adding Mg into the aluminum alloy to increase its strength. The reason why Mg is selected instead of Cu is that corrosion resistance is taken into consideration.

  At this time, if Mg is contained to the same extent as a conventional aluminum alloy, the strength as a base material can be improved, but as described above, the fatigue strength and thermal fatigue resistance are deteriorated due to the decrease in ductility and toughness of the aluminum alloy. Inevitable. However, as a result of diligent research by the present inventors, by making the Mg content within the scope of the present invention, the hardness, strength, fatigue strength, etc. of the aluminum alloy are increased without substantially inhibiting the heat fatigue resistance of the aluminum alloy. I found something new. Of course, when the Mg content is increased, the ductility and toughness of the aluminum alloy are lowered, and the fatigue strength and heat fatigue resistance may be slightly deteriorated. However, it is considered that the deterioration is sufficiently supplemented by a skeletal phase strengthened by crystallization of a compound such as Ni or Fe. In particular, by appropriately adjusting the Ni content, high heat fatigue resistance equal to or higher than that of conventional aluminum alloys can be obtained. This point will be further described as follows.

  The skeletal phase is networked and surrounds the base phase. By this skeletal phase, the stress and strain acting on the alloy are not easily concentrated locally but easily distributed uniformly. At this time, when the amount of crystallized Ni compound or Fe compound in the skeleton phase increases, stress concentration tends to occur in that portion, and there is a concern that the fatigue strength and the like of the aluminum alloy may decrease. However, in the case of the aluminum alloy of the present invention, the base phase is appropriately softened by substantially not containing Cu, and the content of Mg is not large, so in the crystallization part of the Ni compound or Fe compound. Stress concentration is not a problem.

  The aluminum alloy of the present invention further contains Ti. For this reason, the crystal grain in an aluminum alloy becomes very fine. Along with this, the distribution of the skeletal phase composed of network-like crystallized substances becomes isotropic, and the applied stress and strain are more easily dispersed evenly, improving the fatigue strength and heat fatigue resistance. Contribute. Moreover, since Ti dissolves in the matrix phase and strengthens the matrix phase, it is effective for improving the strength of the aluminum alloy. As described above, the aluminum alloy of the present invention satisfies the strength, fatigue strength, and heat fatigue resistance at a high level, which has been difficult to achieve conventionally, only by optimizing the content and synergistic action of each alloy element. It seems that they are now able to

  It should be noted that the structure of the aluminum alloy casting of the present invention may slightly change depending on the site at the very beginning of use. For example, when the temperature environment that is exposed differs depending on the site, such as a cylinder head, the vicinity of the combustion chamber of the cylinder head becomes relatively high, and, for example, Mg compound precipitated from the base phase becomes coarse in the initial stage of use. It can happen. However, in the case of the present invention, the coarsening of the precipitates ends early, and the ductility and toughness are recovered by further heating. Even if the ductility and toughness are slightly reduced in the initial use, the base phase is reinforced by the crystallized skeletal phase of the Ni compound or the like, so that the thermal fatigue resistance is hardly lowered. On the other hand, the portion of the cylinder head that does not reach a high temperature is in a state where the matrix phase is precipitated and strengthened by the Mg compound, and exhibits sufficient strength and hardness as a base material. As described above, the aluminum alloy of the present invention can satisfy those requirements at the same time even if the required properties differ depending on the parts of the members.

  By the way, “strength” as used in the present specification is a breaking strength at the initial use of an aluminum alloy. This strength is substantially maintained in the temperature range of room temperature to 150 ° C. This strength may be indicated by tensile strength, but may be indicated by the hardness of the entire alloy. In addition, when the below-mentioned fatigue strength is high, it is generally considered that this tensile strength is also high.

  “Fatigue” is the strength against general high cycle fatigue, and “fatigue resistance” is the resistance to fatigue. “Fatigue strength” is the breaking strength when a repeated stress is applied to an aluminum alloy casting at a predetermined temperature. It is indexed by average stress, stress amplitude, number of repetitions (life to break).

  “Thermal fatigue” is a type of low cycle fatigue, and is fatigue that occurs when temperature and strain change periodically. “Heat fatigue resistance” is resistance to fatigue. More specifically, thermal fatigue is a fatigue phenomenon that results from constraining thermal expansion and contraction to cause strain in the compression or tension direction during heating and strain in the tension or compression direction during cooling. There are out-of-phase and in-phase depending on the phase difference between temperature and strain. This thermal fatigue is indexed by the thermal fatigue life. The test method will be described later. In particular, in the case of an aluminum alloy, since the thermal expansion coefficient is large, out-of-phase thermal fatigue in which compression distortion occurs during heating and tensile strain occurs during cooling due to thermal expansion constraints, and resistance to this is required. . Note that both the fatigue resistance and the heat fatigue resistance are collectively referred to as “practical fatigue characteristics” in this specification.

  The present invention will be described in more detail with reference to embodiments of the invention. It should be noted that the contents described in this specification, including the following embodiments, are applicable as appropriate to the aluminum alloy for castings according to the present invention, the aluminum alloy casting, and the manufacturing method thereof. In addition, it should be noted that which embodiment is the best depends on the casting object, the required performance of the casting, and the like.

(1) Composition As for Si in the aluminum alloy of this invention, 4-12 mass% is suitable. When Si is less than 4% by mass, castability is poor and casting defects are likely to occur in the casting. In addition, the thermal expansion coefficient of the casting is increased. On the other hand, when Si exceeds 12 mass%, directivity increases when the molten alloy solidifies, and the metal structure becomes inhomogeneous. In addition, a large amount of casting defects may occur in the final solidified portion of the casting. Furthermore, brittle Si particles increase and the ductility and toughness of the casting may be reduced.

  More preferably, Si is 5 to 9% by mass. When the amount of Si is within this range, the most stable castability can be obtained. Moreover, the amount of eutectic Si forming the skeleton phase is also appropriate, and an aluminum alloy casting excellent in strength and ductility can be obtained. Furthermore, the optimal amount of Si is 7-8 mass%. When the amount of Si is within this range, the castability becomes more stable, and the balance between strength and ductility becomes optimum.

  Cu is preferably 0.2% by mass or less. When Cu exceeds 0.2% by mass, many thermally unstable precipitates are generated in the alloy in a high temperature range where a cylinder head or the like is used. The precipitates are gradually coarsened during use of the aluminum alloy casting, resulting in a decrease in ductility and toughness, and there is a risk that the heat fatigue resistance of the aluminum alloy casting will be significantly reduced. Moreover, when Cu exceeds 0.2 mass%, a base phase will harden | cure excessively by the precipitation strengthening. In particular, when the amount of crystallized material is large as in the aluminum alloy of the present invention, there is a concern that the fatigue strength is reduced due to stress concentration. The Cu content is preferably as low as possible, and the upper limit is preferably 0.1% by mass, and more preferably 0.05% by mass. In particular, when Cu is present as an unavoidable impurity, it is optimal that Cu is 0% by mass.

  The above-described tendency of lowering thermal fatigue resistance due to lowering of ductility and toughness is not limited to Cu, but also to Mg described later. However, if the amount of Mg is small, the precipitate is slightly coarsened at the initial stage of use, but the structural change due to subsequent heating is small, and the ductility and toughness are recovered early. Cu has a strong tendency to corrode aluminum alloys. Therefore, from the viewpoint of ensuring corrosion resistance, the Cu amount is preferably within the above range. However, considering recyclability, manufacturing cost, etc., Cu having an impurity level may be present in the aluminum alloy. Therefore, in the present invention, the upper limit of the amount of Cu is not substantially 0% by mass, but is 0.2% by mass. Thereby, the manufacturing cost of an aluminum alloy casting is reduced, and the recyclability is also improved.

  It is preferable that the lower limit of Mg is 0.1% by mass, 0.15% by mass, further 0.2% by mass, and the upper limit is 0.5% by mass, further 0.4% by mass. For example, Mg is preferably 0.1 to 0.5 mass%, more preferably 0.2 to 0.4 mass%.

  As described above, the aluminum alloy of the present invention does not substantially contain Cu, which is a precipitation strengthening element. Accordingly, it is very important to contain an appropriate amount of Mg in order to ensure the strength and fatigue strength of an aluminum alloy that is a base material for a cylinder head or the like. If Mg is too small, the base phase is too soft and its effect is weak. If Mg is excessive, the ductility and toughness of the aluminum alloy will be reduced, resulting in a decrease in thermal fatigue resistance.

  Ni is preferably 0.2 to 3% by mass. Ni crystallizes the Ni compound and strengthens the network-like skeleton phase. When Ni is less than 0.2% by mass, the amount of Ni compound produced is small, and the formation of a network-like skeleton phase composed of crystallized substances may be insufficient. When Ni exceeds 3 mass%, a coarse Ni compound is likely to be generated, and the ductility and toughness may be significantly reduced. In particular, when Ni exceeds 2% by mass, the Ni compound becomes slightly large and the homogeneity of the structure starts to deteriorate. Therefore, it is preferable to set Ni to 0.5 to 2% by mass, since a solidified structure with an appropriate amount of Ni crystallized and its size can be obtained. Furthermore, the optimum amount of Ni is 0.7 to 1.5% by mass. When the Ni content is within this range, the crystallization amount and size of the Ni compound are optimized, and the heat fatigue resistance is stably increased. The Ni compound is a general term for compounds containing Ni. Examples of the Ni compound include an Al—Ni compound, an Al—Ni—Cu compound, and an Al—Fe—Ni compound.

  Fe is suitably 0.1 to 0.7% by mass. If Fe is less than 0.1% by mass, the formation of an Fe compound is small and the formation of a network-like skeletal phase composed of a crystallized product may be insufficient. If Fe exceeds 0.7% by mass, a coarse Fe compound is likely to be formed, and ductility and toughness may be significantly reduced. More preferably, Fe is 0.2 to 0.6 mass%. Furthermore, the optimum amount of Fe is 0.3 to 0.5% by mass. When the amount of Fe is within this range, the crystallization amount and size of the Fe compound are optimized, and the effect of improving heat fatigue resistance is maximized. The Fe compound is a generic name for compounds containing Fe. Examples thereof include an Al—Si—Fe—Mn compound, an Al—Si—Fe compound, and an Al—Fe—Ni compound.

  Ti is preferably 0.15 to 0.3% by mass. Ti refines crystal grains and strengthens the matrix phase in solid solution. When the crystal grains are sufficiently refined, the network-like skeletal phase composed of the crystallized material becomes isotropic. Further, since Ti dissolves in the matrix phase, the matrix phase is appropriately cured, strain concentration in the matrix phase is suppressed, and the strain distribution becomes more uniform. Thus, the stress and strain distribution acting on the casting is made uniform, and the fatigue strength is improved. If Ti is less than 0.15% by mass, refinement of crystal grains becomes insufficient, a dendrite structure peculiar to a cast structure develops, and an isotropic network-like skeleton phase cannot be obtained. When Ti exceeds 0.3% by mass, Ti dissolved in the matrix phase increases, the matrix phase becomes too hard, and the casting may cause shear fracture. In addition, a coarse Ti compound is generated in the matrix phase, which may significantly reduce the ductility and toughness of the casting. Ti is more preferably 0.2 to 0.3% by mass, and most preferably 0.2 to 0.25% by mass.

Note that Ti can be contained by adding an Al—Ti alloy, an Al—Ti—B alloy, an Al—Ti—C alloy, or the like in the final step of melting the raw material. When Ti is added as a mother alloy (aluminum alloy) in this way, aggregation of Ti compounds and the like can be suppressed, and sufficient refinement of crystal grains, isotropic and homogenization of the metal structure can be easily achieved. When an Al—Ti—B alloy or the like is used as an additive for Ti, boron (B) or the like is mixed in the alloy. When B increases, the heat resistance of the aluminum alloy decreases, so the content is preferably 0.01% by mass or less.
Incidentally, the ratio d / DAS between the crystal grain size d of the aluminum alloy of the present invention and the secondary dendrite arm interval DAS is, for example, about 5 to 20. This crystal grain size d is obtained by measuring, for example, according to JlS-H-0501 “Method for testing grain size of copper-stretched products”.

  The aluminum alloy of the present invention preferably further contains 0.1 to 0.7% by mass of manganese (Mn). Mn crystallizes as a Mn compound and strengthens the skeleton phase. If Mn is less than 0.1% by mass, the effect is small. If Mn exceeds 0.7% by mass, a coarse Mn compound is generated, and the ductility and toughness of the casting may be significantly reduced. Mn also has the effect of preventing the Fe compound from becoming coarse needles and hindering the reduction in ductility and toughness of the casting. It is more preferable that Mn is 0.2 to 0.5% by mass. Furthermore, the optimum amount of Mn is 0.3 to 0.5% by mass. When the amount of Mn is within this range, the above effect can be maximized. Note that. The Mn compound is a general term for compounds containing Mn. Examples thereof include an Al—Si—Fe—Mn compound, an Al—Si—Mn compound, and an Al—Mn compound.

  The aluminum alloy of the present invention preferably further contains one or both of 0.03 to 0.5% by mass of zirconium (Zr) and 0.02 to 0.5% by mass of vanadium (V). Both elements have the effect of making crystal grains fine, preventing dendrite alignment, and making the network-like skeleton phase of crystallized substances more isotropic. Moreover, both elements strengthen the matrix phase in solid solution, and moderately improve the high-temperature strength of the matrix phase. It also has the effect of preventing strain concentration on the base phase. If the contents of both elements are too small, these effects are weak. If the contents of both elements are excessive, a coarse primary crystal compound is generated, and the ductility and toughness of the casting are significantly reduced. Moreover, when content of both elements becomes excessive, uniform melt | dissolution will become difficult unless the temperature of a molten metal is raised. Furthermore, if the content of both elements exceeds 0.5% by mass, a coarse Ti compound containing Ti is generated, which not only lowers the ductility and toughness of the casting, but also reduces the crystal grains described above. There is a possibility that the effective Ti amount decreases and the crystal grains become coarse. As a result, the isotropy and homogeneity of the metal structure of the casting can be hindered. Zr is more preferably 0.03 to 0.15% by mass, and V is preferably 0.02 to 0.15% by mass, and more preferably both elements are contained.

  The aluminum alloy of the present invention preferably further contains 0.0005 to 0.003% by mass of calcium (Ca). When a small amount of Ca is added together with Ti, Zr or V within the above range, the refinement of crystal grains becomes more stable. If Ca is less than 0.0005 mass%, the effect is not obtained. When Ca exceeds 0.003 mass%, a dendrite structure is likely to develop, and the isotropic property of the network-like skeleton phase made of crystallized material is lost, and the cast structure may be inhomogeneous. Moreover, since the porosity which is a casting defect will also arise easily when Ca content increases, it is more preferable when the upper limit shall be 0.002 mass%. Moreover, it is preferable that there are few inevitable impurities, for example, it is good to set it as less than 0.5 mass%. More preferably, it is optimal to be less than 0.25% by mass, and further less than 0.1% by mass.

(2) Structure The casting of the aluminum alloy casting of the present invention or the aluminum alloy for casting of the present invention (for convenience, the two are collectively referred to as “aluminum alloy casting” or simply “casting”) has a matrix phase and a skeleton. It consists of phases. The base phase is mainly composed of α-Al, and the skeletal phase is composed of a crystallized material surrounding the base phase in a network form (see FIG. 1). Such a metal structure is obtained, for example, by crystallizing a skeleton phase around the matrix phase by a eutectic reaction after the matrix phase is solidified as primary crystals. In this case, the metal structure is mainly a hypoeutectic structure obtained by melting the molten aluminum alloy in a mold.

  The matrix phase includes not only α-Al but also each alloy element dissolved therein, precipitated compound particles (for example, precipitated particles of Mg compound), and the like. Similarly, the skeletal phase includes not only the Al—Si eutectic but also a compound crystallized with the eutectic and each alloy element dissolved therein. The compound particles that recrystallize or precipitate in the skeletal phase to reinforce the skeletal phase are hereinafter referred to as skeletal phase reinforcing particles (see FIG. 1). Examples of such reinforcing particles include Al-Ni compounds, Al-Si-Ni compounds, Al-Fe compounds, Al-Si-Fe compounds, Al-Si-Fe-Mn compounds, and eutectic Si. Etc. Among these, crystallized particles made of Ni compounds or Fe compounds have a great effect as reinforcing particles. In addition, SiC particles, Al2O3 particles, TiB2 particles, and the like can be reinforcing particles.

  Here, the skeletal phase is composed of a crystallized substance having high elasticity and high yield stress or hard reinforcing particles. Since these are connected in a network and surround the base phase, and the structure is fine and uniform, the stress acting on the casting is uniformly distributed in the skeletal phase, and the base phase that is the source of fatigue cracks Stress sharing tends to decrease. As a result, the fatigue properties such as fatigue resistance and heat fatigue resistance of the aluminum alloy casting of the present invention are considered to have improved.

  The aluminum alloy casting of the present invention preferably has a hypoeutectic structure free of primary crystal Si. When casting a large casting having a complicated shape having a hollow inside such as a cylinder head, it is difficult to completely control the directivity of solidification and let the porosity escape to the feeder part outside the casting. Therefore, if a casting made of a hypoeutectic structure is obtained by solidifying the molten metal in the form of a hypoeutectic structure, local concentration of porosity is suppressed, and porosity is concentrated in stress concentration portions and the like, and the fatigue characteristics of the casting are reduced. The situation can be avoided. Further, by using a hypoeutectic structure, the crystallized substances are dispersed and formed in a network, and a skeleton phase is effectively formed even with a small amount of crystallized substances.

  Moreover, primary Si in the casting can be a starting point for fatigue fracture. In particular, in the case of large castings such as cylinder heads, the overall solidification is slow, so the primary Si produced during solidification may float and segregate above the molten metal due to the difference in specific gravity. It tends to be the starting point of destruction. Therefore, it is preferable that primary Si is not substantially present. In the case of the present invention, since the Si amount is less than the eutectic point of the Al—Si binary alloy, primary Si is relatively difficult to crystallize. However, depending on the alloy element other than Si and the content thereof, the eutectic point may move to the low Si side and primary crystal Si may crystallize. In such a case, it is good to adjust Si amount in the range which does not impair castability.

  An aluminum alloy casting may be cast by adding an appropriate amount of an element for refining eutectic Si, such as strontium (Sr), sodium (Na), and antimony (Sb), in the molten aluminum alloy of the present invention. . Thereby, the ductility and toughness of the casting can be further improved. Sr is good to contain 0.003-0.03 mass%. If the content of Sr exceeds 0.03% by mass, the effect of refining the eutectic Si particles is saturated and gas absorption becomes intense. Further, if the Sr content is less than 0.003 mass%, the effect of refining the eutectic Si particles is not sufficiently observed.

  Sb is preferably contained in an amount of 0.02 to 0.3% by mass. If the Sb content exceeds 0.3% by mass, the fluidity of the molten metal is lowered, and there is a risk of poor hot water surroundings. Moreover, if content of Sb is less than 0.02 mass%, the refinement | miniaturization effect of eutectic Si particle | grains will not fully be recognized.

  Na is preferably contained in an amount of 0.003 to 0.03% by mass. If the Na content exceeds 0.03% by mass, the toughness decreases. Moreover, if content of Na is less than 0.003 mass%, the refinement | miniaturization effect of eutectic Si particle | grains will not fully be recognized.

  By the way, the aluminum alloy casting of the present invention contains not only the above skeleton phase but also the matrix phase by precipitation strengthening by containing an appropriate amount of Mg, not only heat fatigue resistance but also hardness, strength as a base material, Fatigue resistance is also ensured. The initial hardness when the base phase is used is, for example, 64 VHV or more, more preferably 67 HV or more in terms of Vickers hardness (HV). The upper limit of the hardness varies depending on the amount of Mg, heat treatment conditions, and the like, but is about 100 HV. Incidentally, the “initial hardness at the time of use” is the hardness before the aluminum alloy casting receives a heat history (hardness in a virgin state). Considering an engine cylinder head as an example of an aluminum alloy casting, the “initial hardness in use” is the hardness before the first operation of the engine (that is, before firing).

  When the use environment of the aluminum alloy casting is relatively low (for example, 150 ° C. or less) or when the temperature of a specific part of the casting is low, the hardness of the base phase is maintained almost as above. it is conceivable that. This tendency is the same for the hardness of the entire alloy, and the hardness is 97 HV or higher, more preferably 105 HV or higher.

  When the matrix phase is precipitation strengthened with Mg or the like, it is effective to perform heat treatment. As heat treatment of the aluminum alloy casting, there are solution heat treatment and aging (hardening) heat treatment. The solution heat treatment is a treatment for forming a supersaturated solid solution by holding a casting at a high temperature and then rapidly cooling it by water cooling or the like. The aging heat treatment is a treatment for imparting an appropriate hardness by heating and holding the casting at a relatively low temperature and precipitating an element that has been dissolved in supersaturation. By these heat treatments, fine precipitates are uniformly dispersed, and a casting in which strength, ductility and toughness are highly balanced can be obtained. Further, the corners of the crystallized material are rounded, and the stress concentration is reduced, so that the practical fatigue characteristics are expected to be improved. In the case of the present invention, Mg in the matrix phase is precipitated as a compound (mainly Al—Mg—Si compound) by these heat treatments, and the hardness of the matrix phase is moderately increased.

These heat treatment conditions are appropriately selected according to the composition and desired characteristics of the casting. There are generally T6 processing, T4 processing, T5 processing, T7 processing, etc., depending on the desired processing temperature and processing time. For example, the solution heat treatment may be rapidly cooled after heating and holding at 450 ° C. to 550 ° C. for 1 to 10 hours, for example. In addition, the aging heat treatment may be performed by heating and holding at 140 ° C. to 300 ° C. for 1 to 20 hours, for example.
Moreover, it is preferable that the porosity of the aluminum alloy casting of this invention is less than 0.3 vol%. When the porosity is 0.3 vol% or more, the heat fatigue resistance of the casting cannot be sufficiently exhibited. The lower the porosity, the better the heat fatigue resistance inherent in the alloy. For this reason, the porosity is preferably less than 0.1 vol%, and optimally less than 0.05 vol%. Such a porosity value only needs to be achieved in a portion where the heat fatigue resistance of the casting is particularly required, such as an inter-valve portion of the combustion chamber of the cylinder head.

(3) Use The aluminum alloy for castings of the present invention is naturally used as a raw material for aluminum alloy castings. Although the form of the aluminum alloy for casting is not ask | required, it is an ingot state normally.

  The aluminum alloy casting of the present invention is suitable for a member that is required to have strength, fatigue resistance, heat fatigue resistance, and the like at the same time, regardless of its size, shape, usage environment, and the like. For example, there are engine members, motor members, heat radiating members, and the like. For example, the engine member includes a cylinder head, a turbo rotor, and the like. Since the aluminum alloy casting of the present invention has high corrosion resistance, it is also suitable for exhaust system members (exhaust pipes, exhaust management valves, etc.). Furthermore, since the aluminum alloy casting of the present invention is excellent in fatigue strength and corrosion resistance, it is suitable for members that require both of these performances, such as automobile underbody members, chassis members, and the like. Weight reduction and performance improvement of each member can be achieved. More specifically, examples of the suspension member include a disc wheel, an upper arm, a lower arm, a suspension arm, an axle carrier, and an axle beam. Examples of the chassis member include a side member and a cross member. Moreover, you may use for the brackets, transmission case, etc. which attach the member for engines, and its peripheral member. Furthermore, not only in the automobile field, but also in other fields, if a member requiring corrosion resistance and fatigue strength is manufactured with the aluminum alloy casting of the present invention, the weight reduction and performance improvement can be achieved.

  The aluminum alloy casting of the present invention is particularly suitable for a cylinder head for a high-performance reciprocating engine that requires heat fatigue resistance as well as hardness and strength as a base material. The cylinder head is exposed to a severe cold environment and is repeatedly subjected to thermal strain. In particular, extremely high heat fatigue resistance is required for the valve portion (valve bridge portion) of the combustion chamber. On the other hand, the other base material parts are required to have high strength and high fatigue resistance rather than heat fatigue resistance. Further, in the water jacket portion, high corrosion resistance is also required from the viewpoint of suppressing long-term deterioration in heat transfer performance due to formation of the corrosion-generated film, that is, reduction in cooling efficiency. The cylinder head made of the aluminum alloy casting according to the present invention satisfies all of these requirements at a high level. In addition, the cylinder head and the like are complex and large in size, but the aluminum alloy for castings of the present invention is optimal as a raw material alloy because of its good castability. In addition, the cylinder head is subjected to machining such as cutting and polishing on the cast product after casting to form an assembly surface, a camshaft bearing, etc., but the aluminum alloy casting of the present invention impedes such workability. I don't have to.

  In addition, the casting method of the aluminum alloy casting of the present invention is not particularly limited. Sand casting or die casting may be used, and gravity casting, low pressure casting or high pressure casting may be used. In consideration of mass production of castings, die casting and low pressure casting are suitable.

The present invention will be described more specifically with reference to examples.
(First embodiment)
(1) Manufacture of test pieces As shown in Table 1, after melting molten aluminum alloys for castings and preparing molten metal, they were poured into a mold for preparing JIS No. 4 test pieces and allowed to cool. And solidified (casting process). The obtained casting was heated at 530 ° C. for 5.5 hours, and then subjected to solution heat treatment by quenching in 50 ° C. warm water. Thereafter, an aging heat treatment was performed by heating at 160 ° C. for 5 hours. A thermal fatigue test piece having a parallel portion of φ4 mm × length 6 mm was taken from the cast product after the heat treatment, and the test piece Nos. 1-1 to 1-8 were obtained.

(2) Evaluation of heat fatigue resistance The heat fatigue resistance of each test piece was evaluated as follows.

  Each test piece was attached to a restraint holder made of a low thermal expansion alloy, and heating and cooling were repeated. This test temperature range was 50 ° C. to 250 ° C., and the repetition rate was 5 min / cycle of heating 2 min and cooling 3 min. In addition, the details of the thermal fatigue test method are described in, for example, JP-A-7-20031, “Materials”, Vol. 45 (1996), pp. 125-130, “Light Metal”, Vol. 45 (1995), p. 671-676.

  Table 1 also shows the thermal fatigue life of each test piece obtained in the thermal fatigue test. Incidentally, the total strain range at the initial stage of the test measured by attaching a high-temperature strain gauge to a JIS-AC2B aluminum alloy test piece was about 0.6%.

  As can be seen by comparing the test pieces in Table 1, the thermal fatigue life is remarkably extended when Cu is 0.2 mass% or less and contains appropriate amounts of Ni, Fe, Mn, and Ti. In particular, specimen no. 1-1 to 1-6 and test piece No. As can be seen from a comparison with 1-8, the thermal fatigue life is greatly extended by containing 0.2 to 3 mass% of Ni even if Cu is 0.2 mass% or less.

  In addition, test piece No. 1-1, 1-5, and test piece No. As can be seen by comparing 1-2 and 1-6, it was also found that the test piece containing appropriate amounts of Mn, Zr and V has a significantly longer thermal fatigue life than the other test pieces.

(Second embodiment)
As shown in Table 2, test pieces No. 2 were used in the same manner as in the first example, using aluminum alloys for castings having different compositions. 2-1 to 2-6 were produced. These test pieces have different amounts of Mg.

  The hardness of these test pieces was measured. The hardness was measured using a Vickers hardness meter or a micro Vickers hardness meter in a room temperature atmosphere. In Table 2, “overall average hardness” is measured by placing a large indentation at a load of 10 kgf and a load time of 30 sec, and indicates the average hardness of the entire test piece. “Initial matrix phase hardness” is measured by placing a small indentation in the center of the matrix phase on a test piece before heating at a load of 100 g and a loading time of 30 sec. “Hardness of base phase after heating” is obtained by measuring the hardness of the base phase of a test piece heated at 250 ° C. × 100 hr by the same method as the above-mentioned “hardness of initial base phase”.

  As can be seen from Table 2, the overall hardness and the hardness of the initial matrix phase are large in a sample having a Mg content of more than 0.1% by mass. In particular, the test piece No. in which Mg exceeds 0.2% by mass. In 2-1 to 2-3, the “total average hardness” does not depend much on the amount of Mg, and both are as large as 100 HV or more.

On the contrary, test piece No. in which the Mg amount is 0.1% by mass or less. 2-4 and 2-5 do not depend on the amount of Mg, and “total average hardness” is extremely low. These tendencies are the same for “the hardness of the initial base phase”.
For this reason, castings whose Mg amount exceeds 0.2% by mass maintain high hardness and high strength other than those heated to high temperatures. For example, high strength for engines such as cylinder heads and exhaust system parts It is also considered preferable as a base material for parts.

The “hardness of the matrix phase after heating” is lower in any test piece than the “hardness of the initial matrix phase” before heating. In particular, when the amount of Mg exceeds 0.2% by mass, the amount of decrease is large. However, “the hardness of the base phase after heating” itself is settled to the same extent regardless of the amount of Mg in any test piece. Therefore, even in a casting containing an appropriate amount of Mg, it is considered that the base phase is sufficiently softened and the ductility as an alloy is improved as in the casting containing substantially no Mg. In other words, even if 0.5% by mass or less of Mg is included in the alloy in order to increase the hardness, strength, fatigue strength, etc. as a base material, the heat fatigue resistance of the part exposed to a high temperature of 250 ° C. It is thought that it cannot be a factor that greatly impedes. For example, in the case of a cylinder head containing 0.2 mass% to 0.5 mass% of Mg, it exhibits excellent heat fatigue resistance in a portion exposed to a high temperature environment, and a portion having a relatively low temperature around it. Then, it is thought that high initial strength etc. are maintained.
The fact that the aluminum alloy of the present invention exhibits such excellent characteristics is considered to be a synergistic effect of containing appropriate amounts of Mg and Ni, as can be seen from Tables 1 and 2.

(Third embodiment)
As shown in Table 3, using the casting aluminum alloys having different compositions, the test piece No. 3-1 to 3-3 were produced. These test pieces have different amounts of Cu.
These test pieces were subjected to a salt spray test to evaluate the corrosion resistance of each test piece. The salt spray test was conducted according to JIS Z2371-1994, with a salt water concentration of 5%, a sprayed salt water temperature of 35 ° C., and a test time of 100 hours. The surface of each test piece was polished uniformly with a # 600 water-resistant abrasive paper before the test.

Each test piece No. 1 washed with water after the salt spray test. Surface photographs 3-1 to 3-3 are shown in FIGS. 2 (a) to 2 (b), respectively. It can be seen that the test piece having a larger amount of Cu is more corroded and the test piece having a smaller amount of Cu is less corroded. In particular, the test piece No. with Cu of 0.2% by mass or less was used. 3-1 was hardly corroded and showed extremely high corrosion resistance.
Therefore, for example, a cylinder head made of the aluminum alloy of the present invention has high corrosion resistance in addition to the above-described high strength, high heat fatigue resistance, etc., and can be said to have very high reliability.

(Fourth embodiment)
As shown in Table 4, test pieces Nos. Were obtained in the same manner as in the first example, using aluminum alloys for castings having different compositions. 4-1 to 4-3 were produced. These test pieces have different B amounts. These test pieces were heated at 150 ° C. for 100 hours, and then their Vickers hardnesses were measured. The results are also shown in Table 4. This hardness measurement was performed in a room temperature atmosphere.

  From the results in Table 4, it can be seen that the smaller the amount of B, the greater the hardness after long-time heating. Therefore, it can be said that it is preferable to limit the upper limit of B to 0.01% by mass or less as an impurity.

(5th Example)
As shown in Table 5, test pieces Nos. Were obtained in the same manner as in the first example, using aluminum alloys for castings having different compositions. 5-1 to 5-4 were produced. These test pieces have different Ca contents.

  The solidified structure of each test piece was examined with an optical microscope. In Table 5, the presence or absence of the homogeneity of the structure is indicated by ○, Δ, and X. In the case where an isotropic network structure composed of crystallized substances is formed, the mark is given as ◯. When the dendrite structure is developed, the mark is marked as ◯.

  Test piece No. Ca with 0.0005 to 0.003 mass% Ca was obtained. 5-1 and 5-2 were homogeneous structures in which an isotropic network-like skeletal phase formed by crystallized substances was formed as a whole. On the other hand, test piece No. with a Ca content of less than 0.0005% by mass 5-3 was a slightly heterogeneous structure in which dendritic alignment was observed in part. Specimen with a Ca content exceeding 0.003 mass%. 5-4 was a heterogeneous structure in which dendrite alignment was observed throughout the structure. Therefore, it can be said that the Ca amount is preferably 0.0005 to 0.003 mass%.

It is explanatory drawing which showed typically the metal structure of the aluminum alloy casting of this invention. It is a photograph which shows the corrosion condition when performing the salt spray test to the aluminum alloy casting which changed Cu amount, The figure (a) is Cu: 0 mass%, The figure (b) is Cu: 0.5 mass. %, (C) in the figure is Cu: 5% by mass.

Claims (12)

When the whole is defined as 100% by mass, silicon (Si): 4 to 12% by mass, copper (Cu): 0.2% by mass or less, magnesium (Mg): more than 0.2% by mass and 0.5 % Mass% or less , nickel (Ni): 0.2-3 mass%, iron (Fe): 0.1-0.7 mass%, titanium (Ti): 0.15-0.3 mass% The balance consists of aluminum (Al) and inevitable impurities,
An aluminum alloy for castings characterized in that an aluminum alloy casting excellent in practical fatigue characteristics can be obtained.   Furthermore, the aluminum alloy for castings of Claim 1 containing manganese (Mn): 0.1-0.7 mass%.   Furthermore, the aluminum alloy for castings of Claim 1 containing 1 or more types of zirconium (Zr): 0.03-0.5 mass% and vanadium (V): 0.02-0.5 mass%.   Furthermore, boron (B): The aluminum alloy for castings of Claim 1 which is 0.01 mass% or less.   Furthermore, the aluminum alloy for castings of Claim 1 or 3 containing calcium (Ca): 0.0005-0.003 mass%. When the whole is defined as 100% by mass, Si: 4 to 12% by mass, Cu: 0.2% by mass or less, Mg: more than 0.2% by mass and 0.5 % by mass or less , Ni: 0.00%. 2-3% by mass, Fe: 0.1-0.7% by mass, Ti: 0.15-0.3% by mass, the balance consisting of Al and inevitable impurities,
It has a metal structure composed of a base phase mainly composed of α-Al and a skeleton phase crystallized so as to surround the base phase in a network shape,
An aluminum alloy casting characterized in that the matrix phase is excellent in practical fatigue properties obtained by precipitation strengthening with a precipitate containing Mg.   The aluminum alloy casting according to claim 6, wherein the skeleton phase is strengthened by reinforcing particles made of at least a Ni compound and a Fe compound.   The aluminum alloy casting according to claim 6, wherein the initial hardness when the base phase is used is 64 HV or more in terms of Vickers hardness (HV).   The aluminum alloy casting according to claim 6, wherein the metal structure does not include primary crystal Si.   The aluminum alloy casting according to claim 6, wherein the aluminum alloy casting is an engine member.   The aluminum alloy casting according to claim 10, wherein the engine member is a cylinder head of a reciprocating engine. A casting process in which an aluminum alloy molten metal mainly composed of Al is poured into a mold and solidified to obtain an aluminum alloy casting,
A heat treatment step for subjecting the aluminum alloy casting to solution heat treatment and aging heat treatment,
The aluminum alloy casting after the heat treatment step is Si: 4 to 12% by mass, Cu: 0.2% by mass or less, Mg: more than 0.2% by mass , and 0.1% when the whole is 100% by mass . 5 mass% or less , Ni: 0.2-3 mass%, Fe: 0.1-0.7 mass%, Ti: 0.15-0.3 mass%, the balance being Al and inevitable impurities And consist of
It has a metal structure composed of a base phase mainly composed of α-Al and a skeleton phase crystallized so as to surround the base phase in a network shape,
A method for producing an aluminum alloy casting, characterized in that the matrix phase is excellent in practical fatigue properties that are precipitation strengthened by precipitates containing Mg.

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