JP2008274403A - Aluminum alloy for casting, aluminum alloy casting, and method for manufacturing aluminum alloy casting - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an aluminum alloy casting excellent in strength and fatigue property. <P>SOLUTION: The aluminum alloy casting includes, when the sum total is taken as 100% by mass, 4-12% Si, 1.0-3.0% Cu, 0.2-0.6% Mg, 0.2-3% Ni, 0.1-0.7% Fe, 0.1-0.3% Ti, and 0.03-0.5% Zr with the balance being Al and unavoidable impurities, and has a metallic structure, comprising a base phase mainly consisting of α-Al, a skeleton phase crystallized so as to surround the base phase in a network configuration, and first and second precipitation phases precipitated in the base phase and having different particle sizes. The aluminum alloy casting has an excellent thermal fatigue strength as well as high strength and high fatigue strength as a base material. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an aluminum alloy casting excellent in practical fatigue characteristics such as fatigue strength and heat fatigue strength as well as strength, 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 an aluminum alloy is used for an automotive engine member, etc., it is often used in a high temperature environment, so that not only mere room temperature strength but also heat resistance such as high temperature strength and creep characteristics is required. In addition, high fatigue strength (heat-resistant fatigue strength) corresponding to the cold cycle is 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.

JP-A-9-263867 Japanese Patent Laid-Open No. 10-317085 JP 2001-303163 A Japanese Patent Laid-Open No. 2004-217953 JP 2004-225134 A JP 2005-139552 A

  The casting aluminum alloys disclosed in Patent Documents 1 and 2 are precipitation strengthened by the contained Cu and Mg. However, as is clear from the examples of each patent document, the content of Cu in any aluminum alloy exceeds 3%. In the composition disclosed there, Cu appears as a thermally unstable precipitate, and the precipitate becomes coarse during use of the aluminum alloy casting. As a result, stress and strain tend to concentrate on the local area, the ductility and toughness of the aluminum alloy casting are lowered, and the heat fatigue resistance is also lowered.

  Patent Documents 3 to 5 disclose an aluminum alloy for casting in which the content of Cu is at most about 1%, the hardening by precipitation strengthening is suppressed, the ductility of the casting is improved, and the strength and fatigue strength are increased. ing. However, when considering the heat fatigue strength, in the aluminum alloys disclosed in those patent documents, when a strain that is amplified by a cooling cycle is applied, local deformation of the aluminum alloy casting is not suppressed, and eventually the point of thermal fatigue life. However, the actual situation is that a large improvement effect has not been obtained.

  Further, Patent Document 6 discloses an aluminum alloy that does not substantially contain Cu (0.2 mass% or less) and optimizes the contents of Ni, Fe, and Ti together with the contents of Mg. Patent Document 6 discloses that, after casting using this aluminum alloy, an aluminum alloy casting excellent not only in strength and fatigue strength but also in heat resistance fatigue strength can be obtained by performing solution heat treatment and aging heat treatment. Yes. Here, in patent document 6, the method of (hot) water cooling after a heating is employ | adopted as solution heat treatment.

  However, according to the inventor's research, when water cooling is performed by solution heat treatment on a complicated and large casting such as a cylinder head, the cooling rate is too high depending on the part of the casting. As a result, a large amount of residual distortion is likely to occur inside the casting. Since this residual strain causes a decrease in practical fatigue strength (described later), even in the aluminum alloy casting of Patent Document 6, there is room for further improvement in terms of coexistence of fatigue strength and heat-resistant fatigue strength in the working environment. It was. In particular, the cylinder heads and engine blocks of diesel engines that are used under severe conditions such as high pressure, high temperature, and high vibration require high durability, not only high strength and fatigue strength, but also high heat fatigue over a considerable period of time. Strength is required. Therefore, there has been a demand for an aluminum alloy for castings that can provide an aluminum alloy casting that is more excellent in strength, fatigue strength, heat-resistant fatigue strength, and the like that can be applied to such members.

By the way, when the present inventor conducted various tests, when the solution heat treatment in which the aluminum alloy of Patent Document 6 is air-cooled after heating is applied, the precipitation of the Mg2Si phase proceeds inside the casting during the cooling, and is sufficiently Casting with a sufficient static strength and fatigue strength could not be obtained.
Moreover, when air-cooling the Al-Si-Cu-Mg alloy of Patent Document 5 and Patent Document 6 by solution heat treatment, although corresponding static strength and fatigue strength can be obtained, the alloy is subjected to a thermal cycle. It was found that the strain amplitude caused local non-uniform deformation and the heat fatigue resistance was insufficient.
It has also been found that even when Ni, Ti, Zr or the like is added to these alloys and subjected to an aging heat treatment, sufficient hardness cannot be obtained, and static strength and fatigue strength tend to decrease rather.

The present invention has been made in view of such circumstances. That is, it aims at providing the aluminum alloy for castings from which the casting excellent in not only intensity | strength and fatigue strength but heat-resistant fatigue strength is obtained. In particular, an object of the present invention is to provide an aluminum alloy for casting that can obtain stable heat fatigue strength and the like by solution heat treatment and aging heat treatment even for a complex and large aluminum alloy casting such as a cylinder head of a diesel engine. And
In addition, another object of the present invention is to provide an aluminum alloy casting and a manufacturing method thereof.

  As a result of intensive studies to solve this problem and repeated trial and error, the present inventors reviewed the content of Cu, which is one of the basic elements, and appropriately set other elements and their contents, New aluminum alloy for castings that can provide sufficient strength and fatigue strength without requiring water cooling in solution heat treatment, and also has a sufficiently high heat-resistant fatigue strength and excellent balance in all aspects. As a result, the present invention has been completed.

(Aluminum alloy for casting)
That is, the aluminum alloy for castings of the present invention is composed of the following elements and unavoidable impurities when the whole is 100% by mass (hereinafter simply referred to as “%”), and is an aluminum alloy casting excellent in practical fatigue characteristics. It is characterized by being obtained.
Silicon (Si): 4 to 12%,
Copper (Cu): 1.0-3.0%
Magnesium (Mg): 0.2-0.6%
Nickel (Ni): 0.2-3%,
Iron (Fe): 0.1-0.7%
Titanium (Ti): 0.1-0.3%
Zirconium (Zr): 0.03-0.5%
Aluminum (Al): balance

(Function and effect)
If the aluminum alloy for castings of the present invention is used, the properties of strength, fatigue strength (fatigue resistance) and heat fatigue strength (heat fatigue resistance), which have been difficult to achieve with conventional aluminum alloys for casting, coexist at a high level. An aluminum alloy casting is obtained. The aluminum alloy casting having such excellent characteristics is obtained for the first time when all of the above-mentioned elements all act synergistically within the above-mentioned specific ranges, and the present invention newly provides such a composition. It has epoch-making value in terms of discovery.

  However, the detailed reason and mechanism why the casting made of the aluminum alloy for casting according to the present invention exhibits such excellent characteristics is not always clear at present. Therefore, the reason and mechanism will be described below in the background to the metal composition of the aluminum alloy for casting of the present invention and the presently conceivable range. Hereinafter, including both an aluminum alloy for casting as a casting raw material and an aluminum alloy casting that is a cast product, it is also simply referred to as “aluminum alloy” as appropriate.

(1) When increasing the strength and fatigue strength of an aluminum alloy, the cast casting is subjected to a solution heat treatment and further subjected to an aging heat treatment to precipitate the contained precipitation strengthening elements such as Cu and Mg. It is common to strengthen. Speaking of solution heat treatment, it is common to form a supersaturated solid solution of precipitation strengthening elements by heating the casting to a temperature higher than the solubility line where precipitation strengthening elements such as Cu and Mg are dissolved, and then rapidly cooling by water cooling or the like. Is.

  However, as described above, when the cooling process after heating in the solution heat treatment (so-called quenching process) is water-cooled, the part that directly contacts water has a high cooling rate, but the inside or the part with poor water circulation is cooled. The speed is small, and the cooling state after heating tends to be uneven depending on the part of the casting. As a result, a portion that rapidly shrinks due to rapid cooling and a portion that does not shrink are formed, and a large amount of residual strain tends to occur inside the casting. This tendency naturally increases as the casting becomes larger and more complicated.

  The present inventor considered that this residual strain may be one of the factors that greatly reduce the practical fatigue strength. It is considered that a metal structure that suppresses generation of the residual strain itself and disperses the residual strain and acting stress almost uniformly throughout the casting is preferable, while precipitating and strengthening with the precipitation strengthening element. It came to develop the aluminum alloy for casting of invention.

(2) Therefore, first, it was considered to reduce or suppress the residual strain or make the distribution uniform by making the cooling of the solution heat treatment gentler than the conventional water cooling or the like. However, since the cooling process of the solution heat treatment is a so-called quenching process, if the cooling rate is simply slowed down, the precipitation strengthening element precipitates as a coarse compound during the cooling process, and sufficient precipitation occurs. It is also expected that strengthening cannot be expected.

  However, the aluminum alloy for castings of the present invention contains an appropriate amount of transition elements such as Ti and Zr. Although the transition elements such as Ti and Zr are in a solid solution state in the matrix phase after casting, they begin to precipitate finely in the heating process of the subsequent solution heat treatment. This is because the precipitation temperature region of transition elements such as Ti and Zr is different from the precipitation temperature region of Cu and Mg and is in a higher temperature region. Then, the precipitate of transition elements such as Ti and Zr is an incentive, and Cu and Mg that have been sufficiently dissolved in the heating step of the solution heat treatment have a slow cooling rate in the subsequent cooling step. Then, it is considered that stable or metastable precipitation is started in advance using a precipitation phase mainly composed of a transition element of Ti or Zr, which has been preliminarily precipitated in the heating step as the previous stage.

That is, the casting aluminum alloy of the present invention contains an appropriate amount of Ti and Zr in addition to an appropriate amount of Cu and Mg. Therefore, in the cooling process where the cooling rate of the solution heat treatment is slower than before, Ti or It is considered that precipitation of strengthening elements such as Cu and Mg was made possible by using the precipitate of Zr as a nucleus.
Then, by performing an aging heat treatment after the solution heat treatment, the remaining Cu or Mg that has been frozen without being precipitated by the solution heat treatment further starts to be originally deposited. By this aging heat treatment, the precipitation amount (volume) of Cu, Mg, etc. is sufficiently ensured.

Here, if the precipitation amount of Cu or Mg is merely increased, it is conceivable to increase the Cu content or the like to the conventional level. However, even if the strength of the aluminum alloy can be increased, the ductility and toughness of the aluminum alloy may decrease, and the fatigue strength affected by stress concentration, average stress, or the like may not be increased. Furthermore, the heat resistance fatigue strength may be reduced due to the decrease in ductility and toughness. Therefore, in the present invention, such a situation is avoided by appropriately setting the contents of Cu and Mg in the aluminum alloy.
Thus, in the present invention, even when the cooling step of the solution heat treatment is performed by air cooling or the like, the precipitation strengthening with an appropriate amount of Cu or Mg is sufficiently advanced, and the decrease in the heat fatigue strength due to residual strain is reduced and suppressed. It is considered that the strength and fatigue strength were ensured, and each specification of not only the strength and fatigue strength of the aluminum alloy but also the heat fatigue strength was satisfied at a high level.

  Incidentally, Cu and Mg precipitates already precipitated in the cooling step of the solution heat treatment include a metastable phase in addition to the stable phase, and the Cu and Mg precipitates can grow by the aging heat treatment. For this reason, when the above-mentioned solution heat treatment and further aging heat treatment are applied to the cast product after casting, the precipitate phase of Cu and Mg has a precipitate phase having a large particle size (first precipitate phase) and a precipitate having a small particle size. Phase (second precipitated phase) may appear. Therefore, the aluminum alloy casting of the present invention subjected to the solution heat treatment and the aging heat treatment as described above is provided with a multiphase precipitate including at least a first precipitation phase and a second precipitation phase having different particle sizes. In addition, the particle size of a 1st precipitation phase is 30-300 nm, for example, and the particle size of a 2nd precipitation phase is 1-20 nm, for example.

(3) In the foregoing, the effects of Cu and Mg (particularly Cu) as precipitation strengthening elements and the effects of Ti and Zr as transition elements have been mainly described. However, the reason why the aluminum alloy of the present invention is excellent in the above-mentioned characteristics is not the only reason. That is, the influence of the metal structure in which the skeleton phase crystallized so as to surround the base phase mainly composed of α-Al in a network shape is finely and substantially isotropically distributed is also very large. Therefore, this skeleton phase will be mainly described below.

  The skeletal phase is formed by crystallization of a compound containing Si, Ni, or Fe, and expands in a network to surround the base phase. In the aluminum alloy of the present invention, the stress and strain acting by providing an appropriate amount of this skeletal phase is easily dispersed without concentrating locally. When the skeletal phase is too small, the stress dispersion is insufficient, and when it is excessive, the ductility is lowered and the fatigue strength is lowered, which is not preferable. Therefore, it is considered that by setting the skeleton phase to an appropriate amount, high strength and high fatigue strength are exhibited together with high heat fatigue strength.

  Moreover, the aluminum alloy of the present invention further contains Ti and Zr. For this reason, the matrix phase in the aluminum alloy and the skeleton phase surrounding it are also composed of extremely fine crystal grains that are isotropically distributed. Therefore, not only the stress and strain acting on the aluminum alloy are dispersed as a whole by the skeleton phase formed in a network shape, but also isotropically and evenly distributed or dispersed.

  Ti and Zr, as described above, serve as nuclei when Cu and Mg are precipitated as described above, and solidly dissolve in the matrix phase and effectively act to improve the strength of the aluminum alloy.

(4) In this way, in the aluminum alloy of the present invention, strength, fatigue strength, and the like can only be achieved by the synergistic action of each alloy element of Si, Cu, Mg, Ni, FeTi and Zr in addition to Al. It is thought that the heat fatigue strength has reached a high level that exceeds the conventional level at the same time.

  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 to be exposed differs depending on the part, such as a cylinder head, the vicinity of the combustion chamber of the cylinder head becomes relatively high temperature. For example, Cu and Mg compounds precipitated from the base phase are in the initial stage of use. It may be coarse.

  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 stage of use, the base phase is reinforced by the skeletal phase from which the Ni compound or the like has crystallized, so that the thermal fatigue strength is hardly greatly reduced.

Of course, since the matrix phase is also strengthened by precipitation with a multiphase precipitate made of a compound of Cu or Mg, it exhibits sufficient strength and hardness as a base material.
Therefore, for example, the aluminum alloy for casting of the present invention is optimal for a high performance cylinder head for a gasoline engine or a cylinder head for a diesel engine that is required to have high durability under a severe environment. However, it goes without saying that the aluminum alloy of the present invention is suitable for cast members other than the cylinder head, and the use environment (use temperature, etc.) of the cast member is not questioned.

(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, the present invention is Si: 4-12%, Cu: 1.0-3.0%, Mg: 0.2-0.6%, Ni: 0.2-3%, Fe: 0.1 0.7%, Ti: 0.1 to 0.3%, Zr: 0.03 to 0.5%, the balance is composed of Al and inevitable impurities, and the base phase is mainly α-Al; , A skeleton phase crystallized to surround the matrix phase in a network, a first precipitation phase having a particle size of 30 to 300 nm, and a second precipitation phase having a particle size of 1 to 20 nm are precipitated in the matrix phase. It can also be grasped as an aluminum alloy casting characterized by excellent practical fatigue characteristics composed of a metal structure having an object.

(Aluminum alloy casting manufacturing method)
Furthermore, this invention can be grasped | ascertained also as a manufacturing method suitable for the aluminum alloy casting.
That is, the present invention is Si: 4-12%, Cu: 1.0-3.0%, Mg: 0.2-0.6%, Ni: 0.2-3%, Fe: 0.1 A molten aluminum alloy having a composition containing 0.7%, Ti: 0.1-0.3%, Zr: 0.03-0.5% and the balance consisting of Al and inevitable impurities is poured into a mold and solidified. A method for producing an aluminum alloy casting, characterized by comprising a casting step for obtaining an aluminum alloy casting and a heat treatment step for subjecting the aluminum alloy casting to a solution heat treatment and an aging heat treatment. Can also be grasped.

  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 can also 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. Both this heat fatigue resistance and the above-mentioned fatigue resistance are collectively referred to as “practical fatigue characteristics” in this specification. Further, the fatigue strength and the heat-resistant fatigue strength are referred to as “practical fatigue strength” in the present specification by combining both of those described later.
More specifically, thermal fatigue is a fatigue phenomenon that results from constraining thermal expansion and contraction, causing strain in the compression or tension direction during heating and strain in the tension or compression direction during cooling. is there. This fatigue phenomenon includes out-of-phase and in-phase depending on the phase difference between temperature and strain.

  By the way, thermal fatigue is indicated by the thermal fatigue life, and 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. .

  Here, the “heat-resistant fatigue strength” is a breaking strength when an aluminum alloy casting is subjected to a cooling cycle in which heating and cooling are periodically repeated under predetermined restraint conditions (under stress action conditions). However, since thermal fatigue is inherently indicated by the thermal fatigue life under predetermined restraint conditions, it is not easy to measure itself. Therefore, in the present specification, not only the breaking strength itself but also the heat fatigue resistance is almost synonymous, and the expression “heat fatigue strength” may be expressed qualitatively. For example, in the case of “high heat fatigue strength”, it should be noted that the specific breaking strength is not high but it may simply be “high heat fatigue resistance”.

  The present invention will be described in more detail with reference to embodiments of the invention. In addition, the content demonstrated by this specification including the following embodiment is suitably applied also to the aluminum alloy for castings concerning this invention, an aluminum alloy casting, and its manufacturing method. It should also be noted that which embodiment is the best depends on the casting object, the required performance of the casting, and the like.

(1) Composition The chemical composition of the aluminum alloy of the present invention is determined for the following reason.
<Si>
Si in the aluminum alloy of the present invention is preferably 4 to 12% (hereinafter, “%” means mass%).
If Si is too small, 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 is excessive, 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. Further, brittle Si particles increase, the ductility and toughness of the casting decrease, and as a result, the fatigue strength and heat fatigue strength may decrease.

The lower limit of Si is preferably 5%, 6%, 6.5% or even 7%. The upper limit of Si is preferably 11%, 10%, 9%, 8.5% or even 8%. In addition, these lower limits and upper limits can be combined arbitrarily (this also applies to the following other elements).
Si also contributes to the formation of the skeleton phase in the aluminum alloy of the present invention. When Si is within 6 to 9%, for example, the amount of eutectic Si forming the skeleton phase is also appropriate, and an aluminum alloy having excellent strength and ductility is obtained. In particular, when Si is 7 to 8%, an aluminum alloy having an excellent balance of castability, strength, and ductility is obtained, which is optimal.

<Cu>
Cu is preferably 1 to 3%.
If Cu is too small, precipitation strengthening by Cu becomes insufficient, and the desired strength and hardness of the aluminum alloy cannot be obtained.
On the other hand, if the amount of Cu is excessive, the matrix phase can be excessively hardened due to precipitation strengthening. In particular, when the amount of crystallized substances by other elements is large as in the present invention, if Cu is excessive, the fatigue strength due to stress concentration tends to decrease. Further, when Cu is excessive, the porosity in the casting is increased, and the fatigue strength is likely to be lowered similarly.

  The lower limit of Cu is preferably 1.3%, 1.4%, and further 1.5%. The upper limit of Cu is preferably 2.7%, 2.6%, and further 2.5%. For example, Cu is more preferably 1.4 to 2.6%, and further preferably 1.5 to 2.5%.

<Mg>
Mg is preferably 0.2 to 0.6%. Mg is a precipitation strengthening element. In order to secure the strength and fatigue strength of the aluminum alloy as the base material, it is very important to contain an appropriate amount of Mg.
If Mg is too small, the matrix phase of the aluminum alloy is too soft and sufficient strength cannot be obtained. When Mg is excessive, the ductility and toughness of the aluminum alloy are lowered, and sufficient fatigue strength and thermal fatigue strength cannot be obtained.
The upper limit of Mg is preferably 0.5%, 0.4%, and further 0.3%. For example, Mg is preferably 0.2 to 0.5%, more preferably 0.2 to 0.4%.

<Ni>
Ni is preferably 0.2 to 3%. Ni crystallizes the Ni compound and strengthens the network-like skeleton phase.
When Ni is too small, 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 is excessive, coarse Ni compounds are likely to be produced, and ductility and toughness may be significantly reduced. In any case, it is not preferable because the fatigue strength and heat fatigue strength of the aluminum alloy can be lowered.

The lower limit of Ni is preferably 0.5%, 0.7%, and further 0.8%. The upper limit of Ni is preferably 2%, 1.5%, and further 1.2%. In particular, when Ni exceeds 2%, the Ni compound becomes slightly large and the homogeneity of the structure starts to deteriorate. It is more preferable that Ni is 0.5 to 2%, since a solidified structure having an appropriate amount of Ni crystallized and its size can be obtained. Furthermore, it is more preferable that Ni is 0.7 to 1.5%, and a stable high heat fatigue resistant aluminum alloy is obtained.
The Ni compound is a general term for compounds containing Ni. For example, the Ni compound includes an Al—Ni compound, an Al—Ni—Cu compound, and an Al—Fe—Ni compound.

<Fe>
Fe is suitably 0.1 to 0.7%.
If Fe is too small, the formation of an Fe compound may be small, and the formation of a network-like skeleton phase composed of crystallized substances may be insufficient. If Fe is excessive, a coarse Fe compound is likely to be produced, and ductility and toughness may be significantly reduced. In any case, it is not preferable because the fatigue strength and heat fatigue strength of the aluminum alloy can be lowered.

The lower limit of Fe is preferably 0.2%, more preferably 0.3%. The upper limit of Fe is preferably 0.6 mass and further 0.5%. For example, Fe is more preferably 0.2 to 0.6%, more preferably 0.3 to 0.5%. When Fe is within this range, the crystallization amount and size of the Fe compound are optimized, and higher heat fatigue strength is obtained.
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>
Ti is preferably 0.1 to 0.3%. Ti refines crystal grains and strengthens the matrix phase by solid solution strengthening or precipitation strengthening. Further, Ti sufficiently refines the crystal grains to make the network-like skeletal phase made of crystallized material more isotropic.
Further, when Ti dissolves or precipitates in the matrix phase, the matrix phase is appropriately cured, and strain concentration in the matrix phase is suppressed. As a result, the strain distribution in the aluminum alloy becomes more uniform, and the fatigue strength and heat fatigue strength are improved.
Further, Ti, together with Zr, acts as a nucleus of a precipitation phase containing Cu and Mg. For this reason, since the precipitation phase containing Cu and Mg is precipitated as a thermally stable phase in the high temperature region of the cooling process with the precipitation phase containing the transition element of Ti compound or Zr compound as the nucleus, the heat resistance of the aluminum alloy of the present invention It is thought that the fatigue strength is improved.

When Ti is excessively small, crystal grains are insufficiently refined, a dendrite structure peculiar to a cast structure is developed, and an isotropic network-like skeleton phase is hardly obtained.
On the other hand, when Ti is excessive, Ti dissolved in the matrix phase increases, the matrix phase becomes too hard, and the casting may be sheared. In addition, coarse Ti compounds are generated in the matrix phase, which may significantly reduce the ductility and toughness of the casting. In any case, the fatigue strength and heat fatigue strength are reduced, which is not preferable. Such Ti is more preferably 0.15 to 0.25% and further preferably 0.18 to 0.24%.

  Note that Ti can be contained by adding an Al—Ti 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.

<Zr>
Zr is preferably 0.03 to 0.5%. Zr makes crystal grains fine and prevents dendrite alignment, thereby making the network-like skeletal phase of crystallized material more isotropic.
Further, Ti dissolves or precipitates in the matrix phase to appropriately improve the high-temperature strength of the matrix phase, and suppresses strain concentration in the matrix phase to make the strain distribution more uniform.
Further, Zr, together with Ti, acts as a nucleus of a precipitation phase containing Cu and Mg. For this reason, since the precipitation phase containing Cu and Mg is precipitated as a thermally stable phase at a high temperature in the cooling process with the precipitation phase containing the transition element of the Ti compound and Zr compound as the nucleus, the heat resistance fatigue of the aluminum alloy of the present invention Strength is improved.

If Zr is too small, the above-described effects cannot be obtained sufficiently. When Zr is excessive, a coarse primary crystal compound is generated, the ductility and toughness of the aluminum alloy are remarkably lowered, and the fatigue strength and heat fatigue strength are lowered. Further, if Zr is excessive, uniform melting becomes difficult unless the molten metal temperature is raised, which is not preferable.
The lower limit of Zr is preferably 0.05%, 0.07%, and further 0.08%. The upper limit of Zr is preferably 0.4 mass, 0.3%, 0.2%, and further 0.15%. For example, it is more preferable that Zr is 0.03 to 0.3%, more preferably 0.05 to 0.15%.

  Incidentally, when the total content of both elements exceeds 0.5%, a coarse Ti compound containing Ti is generated. For this reason, not only the ductility and toughness of the aluminum alloy casting are lowered, but the amount of Ti effective for refining the crystal grains described above is reduced, and the crystal grains may be coarsened. As a result, the isotropy and homogeneity of the metal structure of the casting may be hindered, and the strength, fatigue strength, and thermal fatigue strength of the aluminum alloy may be reduced. Thus, for example, Zr is more preferably 0.03 to 0.15% so that the total of Ti and Zr is less than 0.5%.

<Mn>
Mn is preferably 0.1 to 0.7%. Mn strengthens the crystallized skeleton phase as a Mn compound. Further, Mn prevents the Fe compound from becoming coarse needles, and hinders the deterioration of the ductility and toughness of the casting. Further, Mn, together with Zr and i, acts as a nucleus of a precipitated phase containing Cu and Mg. For this reason, since the precipitation phase containing Cu and Mg is precipitated as a thermally stable phase at a high temperature in the cooling process with the precipitation phase containing the transition element of the Ti compound and Zr compound as the nucleus, the heat resistance fatigue of the aluminum alloy of the present invention Strength is improved.

If Mn is too small, the effect cannot be obtained sufficiently. If Mn is excessive, a coarse Mn compound is generated, the ductility and toughness of the casting are remarkably lowered, and the fatigue strength and heat-resistant fatigue strength of the aluminum alloy may be lowered.
The lower limit of Mn is preferably 0.2%, more preferably 0.25%. The upper limit of Mn is preferably 0.6 mass, 0.5%, and further 0.4%. For example, if the Mn is 0.2 to 0.5%, more preferably 0.25 to 0.4%, the above-described effects are most exhibited and it is more preferable.

<V>
V is preferably 0.01 to 0.5%. V strengthens the solid phase of the aluminum alloy base phase and improves the high temperature strength of the base phase.
If V is too small, this effect cannot be obtained sufficiently. When V is excessive, a coarse primary crystal compound is generated, the ductility and toughness of the casting are remarkably reduced, and the fatigue strength and heat fatigue strength of the aluminum alloy may be reduced.

  The lower limit of V is preferably 0.015%, 0.02%, and more preferably 0.05%. The upper limit of V is preferably 0.3 mass and further 0.15%. For example, it is more preferable that V is 0.015 to 0.3%, more preferably 0.02 to 0.15%.

<Sr, Sb, Na>
Strontium (Sr), antimony (Sb), sodium (Na), etc. refine eutectic Si. When an appropriate amount of such an element is contained, the thermal fatigue life, that is, the thermal fatigue strength of the aluminum alloy casting is further improved. Of course, if the content of any element is too small, the effect of refining the eutectic Si particles cannot be sufficiently obtained.

Sr is preferably 0.003 to 0.05%. If the amount of Sr is excessive, the effect of refining the eutectic Si particles is saturated, and the gas absorption becomes intense. Therefore, Sr is more preferably 0.003 to 0.01%.
Sb is preferably 0.02 to 0.2%. If the amount of Sb is excessive, the Sb compound crystallizes to lower the ductility of the aluminum alloy, and the fatigue strength and heat fatigue strength can be deteriorated. Therefore, Sb is more preferably 0.05 to 0.12%.
Na is preferably 0.001 to 0.03%. When Na is excessive, the toughness of the aluminum alloy is lowered, and the fatigue strength and heat fatigue strength can be deteriorated. Therefore, Na is more preferably 0.001 to 0.01%.

(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 alloy elements (Ti, Zr, Cu, Mn, etc.) dissolved therein, precipitated compound particles (for example, precipitated particles of Cu compound or 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 (Ni, Fe, etc.) 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 strengthened particles depending on the compounding elements and the raw materials used.

  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 The 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 the cast metal having a hypoeutectic structure is obtained by solidifying the molten metal in a bowl shape, local concentration of porosity is suppressed. Further, a situation in which the stress is concentrated at the porosity concentration portion and the fatigue characteristics of the casting are deteriorated is also 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, the amount of Si is less than the eutectic point of the Al—Si binary alloy, so that primary Si is relatively difficult to crystallize. However, depending on the alloy element other than Si and its content, 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.

  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 appropriate amounts of Cu and Mg, not only heat fatigue resistance but also hardness as a base material, Strength, fatigue resistance, etc. are also secured. The initial hardness when the base phase is used is, for example, 80 HV or higher, more preferably 85 HV or higher in terms of Vickers hardness (HV). The upper limit of the hardness is approximately 120 HV, although it varies depending on the content of Cu, Mg, etc., as well as the heat treatment conditions.

  Incidentally, the “initial hardness at the time of use” is a hardness before the aluminum alloy casting receives a thermal 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 95 HV or higher, more preferably 100 HV or higher.

(3) Manufacturing method The aluminum alloy casting of the present invention includes a casting step of injecting and solidifying a molten aluminum alloy having the same composition as the above-described aluminum alloy for casting into a mold to obtain an aluminum alloy casting, and a solution in the aluminum alloy casting. And a heat treatment step for performing an aging heat treatment and an aging heat treatment.

  Here, the solution heat treatment is a treatment in which a casting is held at a high temperature and then rapidly cooled to form a supersaturated solid solution. 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, strength, ductility and toughness are highly balanced, and a casting having not only strength but excellent fatigue strength and heat fatigue strength can be obtained.

  Furthermore, the corners of the crystallized material are rounded, and it is expected that the practical fatigue characteristics will be improved by reducing the stress concentration. In the case of the present invention, by the heat treatment, Cu in the matrix phase is a compound (mainly Al-Cu, Al-Cu-Si-Mg-based compound), and Mg is a compound (mainly Al-Mg-Si-based compound). 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. The temperature during the heating step of the solution heat treatment is more preferably 490 to 530 ° C., and the holding time is 1 to 3 hours, from the viewpoint of each characteristic and cost of the casting. 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. The temperature of the aging heat treatment is 160 to 200 ° C., and the holding time is 1 to 5 hours, which is more preferable in terms of each characteristic and cost of the casting.

  Here, in the case of the aluminum alloy casting of the present invention, the rapid cooling step of the solution heat treatment can be replaced with conventional water cooling or the like, and the aluminum alloy casting after the heating step can be cooled in a gas. The gas may be an inert gas or the like, but since it is usually air, the rapid cooling process can be an air cooling process. It should be noted that the case where the casting made of the aluminum alloy for casting according to the present invention is subjected to solution heat treatment such as water cooling is not excluded from the scope of the present invention.

  Since the solution heat treatment is essentially a solid solution treatment, it is necessary to perform rapid cooling rather than furnace cooling like annealing. This is because the precipitation strengthening elements Cu and Mg, which are sufficiently and uniformly dissolved in the matrix phase in the heating process, are temporarily frozen, and then the precipitation phase of Cu and Mg is made fine and uniform in the subsequent aging heat treatment. This is to make it appear. If a long cooling process, such as furnace cooling, is employed in the solution heat treatment, so-called baking does not occur, Cu, Mg, etc. appear as coarse compounds, and an aluminum alloy with the necessary characteristics cannot be obtained. .

  However, in the case of the aluminum alloy of the present invention, the base phase is composed of Cu, Mg, etc., which are stable at high temperatures in the matrix phase, while the cooling rate of the rapid cooling process in the solution heat treatment is made slower than the conventional water cooling, etc. A precipitated phase can appear. This is due to the fact that transition elements such as Ti, Zr and Mn already dissolved in the matrix phase by casting begin to precipitate finely in advance in the heating step of the solution heat treatment. That is, if the cooling rate in the rapid cooling process of the solution heat treatment is relatively slow, the preceding precipitation phase of those transition elements becomes the nucleus, so that Cu, Mg, etc. are stable phases in the high temperature region during the cooling process. It precipitates and becomes a preceding phase (mainly the first precipitated phase) and is dispersed in the matrix phase.

  However, not all Cu, Mg, and the like contained in the matrix phase are precipitated during the rapid cooling process, but there are also precipitation strengthening elements such as Cu and Mg that have been frozen in the rapid cooling process. Such a precipitation strengthening element starts its original precipitation in the aging heat treatment after the solution heat treatment. Since this aging heat treatment is generally at a lower temperature and a longer time than the previous quenching step, a fine precipitated phase composed of Cu, Mg or the like is dispersed in the matrix phase. As a result, it is considered that a multiphase precipitate formed by dispersing a large-sized stable first precipitation phase and fine second precipitation phase is formed in the matrix phase.

Thus, the aluminum alloy casting of the present invention exhibits excellent thermal fatigue strength because the stress in the matrix phase is dispersed with respect to the strain amplitude due to the thermal cycle. In addition, since the cooling rate during the rapid cooling process is relatively moderate, residual strain is hardly generated in the aluminum alloy casting during solution heat treatment (particularly during the rapid cooling process) or there is little residual strain generated. For this reason, even if a repeated stress is applied to the aluminum alloy casting, it is considered that sufficient fatigue strength is exhibited.
However, as described above, it should be noted that the reason and mechanism that the aluminum alloy of the present invention exhibits the above-described excellent practical fatigue characteristics is not necessarily clear at present.

  The rapid cooling step of the solution heat treatment is not limited to the cooling means, but in order to obtain the metal structure as described above, the cooling rate is preferably 20 to 200 ° C./min. If the cooling rate is too low, a fine precipitate phase such as Cu or Mg cannot be obtained. On the other hand, if the cooling rate is excessive, the residual strain in the aluminum alloy casting increases, resulting in a decrease in practical fatigue characteristics. The lower limit of the cooling rate is more preferably 25 ° C./min, more preferably 30 ° C./min. The upper limit of the cooling rate is more preferably 150 ° C./min, further 100 ° C./min. These upper and lower limits can be arbitrarily combined.

  Such a rapid cooling step with a cooling rate can be achieved, for example, by placing the aluminum alloy casting after the heating step of the solution heat treatment in a gas such as air or inert gas forcibly fluidized. That is, in the method for producing an aluminum alloy casting of the present invention, it is preferable that the rapid cooling step is a step of cooling the aluminum alloy casting after the heating step during the solution heat treatment in a gas.

The cooling rate can be appropriately set by adjusting the temperature, flow rate or density (heat capacity) of the gas that is the working fluid. However, in terms of equipment, costs, etc., the rapid cooling process is an air cooling process in which the aluminum alloy casting after the heating process is placed in low-temperature air, or air is forcibly blown to the aluminum alloy casting. Is preferred.
In addition, cooling by solution heat treatment by air cooling or the like increases the degree of freedom of solution heat treatment, which is extremely advantageous in terms of cost reduction when mass-producing aluminum alloy castings.

(4) Aluminum alloy casting As described above, the aluminum alloy casting of the present invention is subjected to precipitation by precipitation strengthening elements such as Cu and Mg during solution heat treatment (specifically during its rapid cooling process) and subsequent aging heat treatment. Occurs in. Although the precipitation phase generated in any process is finely and uniformly distributed, grains precipitated in advance during the solution heat treatment can grow to some extent in the subsequent aging heat treatment. Therefore, in the aluminum alloy casting of the present invention, the first precipitation phase having a particle size of 30 to 300 nm and the second precipitation phase having a particle size of 1 to 20 nm have a metal structure in which a multiphase precipitate is precipitated in the matrix phase. . That is, the aluminum alloy casting of the present invention has the same composition as the aluminum alloy for casting described above, and a base phase mainly composed of α-Al, and a skeleton phase crystallized so as to surround the base phase in a network shape. The first precipitation phase having a particle size of 30 to 300 nm and the second precipitation phase having a particle size of 1 to 20 nm are composed of a metal structure having a multiphase precipitate precipitated in the matrix phase, and have practical fatigue characteristics. It will be excellent.

  Here, the particle sizes of the first precipitation phase and the second precipitation phase may vary depending on the conditions of the solution heat treatment and the aging heat treatment. Therefore, if the particle size of the first precipitated phase is about 30 to 250 nm, more preferably about 30 to 200 nm, and the second precipitated phase is about 1 to 15 nm, further about 1 to 10 nm, not only the strength but also the practical fatigue characteristics are obtained. It becomes more excellent and preferable.

However, each mechanical property of the aluminum alloy casting is not influenced only by the particle size of the precipitated phase, but is also greatly influenced by the amount of precipitation of each of the first and second precipitated phases.
If the amount of precipitation of the first precipitation phase is too small, the state of stress distribution with respect to thermal strain is not uniform, which is not preferable. If the amount of precipitation of the first precipitation phase is excessive, the ductility of the matrix is lowered, leading to a decrease in strength and fatigue strength.

  If the amount of precipitation of the second precipitation phase is too small, the strength and fatigue strength are lowered, which is not preferable. If the amount of precipitation of the second precipitation phase is excessive, the ductility of the matrix is lowered, leading to a decrease in strength and fatigue strength.

  Of course, the theoretical upper limit of the precipitation amount of each precipitation phase is determined by the metal composition. However, the actual amount of precipitation is greatly influenced by each processing condition of solution heat treatment and aging heat treatment after casting. Therefore, in order to secure the respective precipitation amounts as described above, it is preferable to determine the respective treatment conditions for solution heat treatment and aging heat treatment.

  Here, in the case of the aluminum alloy of the present invention, it is considered that the precipitate of Cu or Mg that precipitates with the precipitation phase of transition elements such as Ti and Zr as a nucleus in the rapid cooling step of the solution heat treatment is a stable phase. . This stable precipitation phase (first precipitation phase) has a small contribution to the strength of the aluminum alloy casting compared to the conventional metastable fine precipitates that precipitate in a water-cooled material that is water-cooled by solution heat treatment. Conceivable.

  Therefore, in order to sufficiently secure the strength of the aluminum alloy casting of the present invention, it is necessary to sufficiently increase the volume fraction of fine precipitates (second precipitation phase) by aging heat treatment after solution heat treatment. However, from the viewpoint of thermal fatigue, the stress acting on the aluminum alloy is relieved by the fact that the precipitated phase of the size of the stable phase level such as the first precipitated phase is dispersed in the matrix phase, and heat fatigue It is thought that the strength is improved.

  In connection with the casting process of the aluminum alloy casting of the present invention, castings such as micro main linkage caused by solidification shrinkage, gas porosity caused by melting gas, and pinholes may occur in the casting. If there is a hole such as a cast hole in a stress concentration portion or the like, cracks and fractures are likely to occur from that point. Conversely, an aluminum alloy casting having a lower porosity is superior not only in fatigue strength but also in thermal fatigue strength. Therefore, it is preferable that the number of pores such as a cast hole is small.

However, it is difficult to avoid the occurrence completely, and it is not always necessary. Therefore, it is sufficient that the porosity in the aluminum alloy casting is 0.3% by volume or less. Of course, it is more preferable that the porosity is 0.1 volume% or less, further 0.05 volume% or less.
In many cases, there is no practical problem even if there is a cast hole or the like in a portion where the acting stress is low (other than the stress concentration portion). It is often sufficient that the above-described porosity is achieved at a particularly required site.

  Further, in relation to the above casting process, in order for the aluminum alloy casting of the present invention to have a metal structure in which the matrix phase and the skeletal phase surrounding it are isotropically and uniformly dispersed as described above, The size (DASII) is preferably 40 μm or less, more preferably 35 μm or less, and even more preferably 30 μm or less. If this secondary dendrite arm is excessive, the skeleton size of the strengthening phase constituted by the crystallization phase becomes large, which is not preferable as a structure for uniformly dispersing the load stress.

The ratio d / DASII between the crystal grain size d in the aluminum alloy casting and the secondary dendrite arm interval DASII is preferably about 5 to 20, for example. If this is excessively large, the uniformity of the tissue is impaired, and stress concentration is locally generated, resulting in uneven deformation.
Incidentally, the crystal grain size d is obtained by measurement according to, for example, JlS-H-0501 “Copper grain size test method”.

In the aluminum alloy casting of the present invention, the residual strain inside the casting is preferably 1000 με or less, more preferably 800 με or less, and even more preferably 500 με or less. If the residual strain is excessive, it is considered that in addition to the stress amplitude during actual operation, the equivalent stress corresponding to the residual strain is applied, so that a high average stress acts and the practical fatigue strength decreases.
Incidentally, the residual strain inside the casting referred to in the present invention is after heat treatment. The residual strain is measured by a method in which a strain gauge is attached to the casting surface and a gauge signal generated by releasing the strain in the process of cutting the casting is taken out.

(5) 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 high-performance reciprocating engine member that requires heat fatigue resistance as well as hardness and strength as a base material. In particular, the aluminum alloy casting of the present invention has a heat fatigue strength superior to that of the prior art, and is therefore suitable for diesel engines that require long-term durability under harsh conditions.

  Among these, the aluminum alloy casting of the present invention is suitable for a cylinder head, an engine block, or the like that is exposed to a severe cold environment and repeatedly receives thermal strain. In particular, in the case of a cylinder head, an extremely high heat fatigue resistance is required for the valve portion (valve bridge portion) of the combustion chamber, which is advantageous. However, the other base metal 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 aluminum alloy casting of the present invention satisfies all of the characteristics required for the cylinder head at a high level.

  The cylinder head and the like are complicated in shape and large in size, but the aluminum alloy for castings of the present invention has good castability and is optimal as a raw material alloy. Furthermore, 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 whose mold temperature was adjusted. Hot water was allowed to cool and solidify (casting process).

  The obtained casting was heated at 500 ° C. for 3 hours and then subjected to solution heat treatment by quenching by air cooling at 30 ° C./min. Thereafter, an aging heat treatment was further performed by heating at 170 ° C. for 3 hours (heat treatment step).

A thermal fatigue test piece and a fatigue test piece each having a parallel portion of φ4 mm × length 6 mm were sampled from the cast product after the heat treatment. 1-1 to 1-4 were obtained. The test piece No. The secondary dendrite size (DASII) measured for the 1-4 thermal fatigue test pieces and fatigue test pieces was 25 μm and 35 μm, respectively.
Incidentally, the secondary dendrite arm was measured in accordance with the dendrite arm spacing measurement procedure in the literature “Casting and Solidification Society of Light Metals: 38 (1988) 1.54”.

(2) Evaluation of heat fatigue strength (heat fatigue resistance) The heat fatigue strength 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.

  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%. This total strain range indicates the width of the strain amplitude acting in the heating / cooling cycle, and is the sum of elastic strain and plastic strain (elastic strain + plastic strain).

(3) Evaluation of fatigue strength (fatigue resistance) The fatigue test for determining fatigue strength is a hydraulic servo axial stress load type fatigue test under the conditions of test temperature: room temperature, repetition rate: 100 Hz, minimum strain = 0.1%. The test piece was used in an environment in which the test piece was kept at 150 ° C.
In addition, the test piece used for this test used what grind | polished uniformly the axial direction, adding water with a water resistant abrasive paper of # 1000 about the parallel part of (phi) 4x length 6mm.

(4) Evaluation of Strength (Hardness) Hardness was measured using an Akashi Vickers hardness meter. And hardness was calculated | required from the indentation size after applying 5 kg of loads to a test piece for 30 second. All were performed in a room temperature atmosphere.

(5) Investigation Table 1 shows the results obtained from the above tests for each test piece.
Comparing the test results of the test pieces shown in Table 1, the following can be understood.
First, test piece No. in the composition range of the present invention. It is understood that 1-4 has a sufficiently long thermal fatigue life of 2600 cycles, a fatigue strength (107 times strength) of 50 MPa or more, and a hardness of 90 HV. Therefore, test piece No. It can be seen that 1-4 is an aluminum alloy (casting) in which each characteristic is balanced in a high dimension.

  On the other hand, test piece No. which is out of the composition range of the present invention. None of 1-1 to 1-3 satisfy practically all of the above three characteristics. That is, test piece No. 1-1 and 1-2 do not contain Ni, Zr or the like, and even if the hardness and fatigue strength are sufficient, the thermal fatigue life is 2000 cycles or less, and the heat fatigue strength is low. In addition, test piece No. In 1-3, Cu is not substantially contained and the thermal fatigue life is high, but the hardness is low and the strength is insufficient. The test piece No. About 1-3, since the hardness is low in the first place, the fatigue test is not performed.

(Second embodiment)
(1) Manufacture and Evaluation of Test Specimen As shown in Table 2, test pieces No. 1 and No. 2 were prepared in the same manner as in the first example, using aluminum alloys for castings having different compositions. 2-1 to 2-5 were produced. About each of these test pieces, each characteristic was evaluated similarly to 1st Example, and the result was collectively shown in Table 2. In addition, the composition of each test piece shown in Table 2 mainly differs in the amount of Cu.

(2) Examination When the test results of each test piece shown in Table 2 are compared, the following can be understood.
First, test piece No. in which the amount of Cu is within the composition range of the present invention. It is understood that 2-3 and 2-4 have a sufficiently long thermal fatigue life of 2400 cycles or more, a fatigue strength (107 times strength) of 50 MPa or more, and a hardness of 90 HV or more. . Therefore, test piece No. It can be seen that 2-3 and 2-4 are aluminum alloys (castings) in which each characteristic is balanced in a high dimension.

  By the way, when looking at the relationship between the amount of Cu, hardness, and fatigue strength, first, the hardness increases as the amount of Cu increases. Specimen No. 1 with less than 1% Cu In 2-1 and 2-2, the hardness is 75 HV or less and the strength is insufficient. On the other hand, the test piece no. In 2-5, the hardness is sufficient exceeding 100 HV, but the fatigue strength is reduced to 45 MPa or less.

The test piece No. When 2-1 to 2-3 are observed, it seems that the thermal fatigue life tends to become shorter as the amount of Cu increases. However, specimen no. As seen from 2-3 to 2-5, when the Cu content is 1.5% or more, the thermal fatigue life is relatively stable around 2500 cycles, and the dependence on the Cu content is considered to be small. .
Table 4 shows the porosity in the casting measured for each specimen. It can be seen that if the Cu content exceeds 3%, the porosity in the casting increases.

Incidentally, the porosity was determined from the difference between the two castings by measuring the density of the test castings and the density of the castings containing no separately quenched copper mold by the Archimedes method.
From the above, it can be said that an aluminum alloy casting within the composition range of the present invention is excellent in all of strength, fatigue strength and heat-resistant fatigue strength, and is balanced in a high dimension. In particular, it can be seen that an aluminum alloy casting in which Cu is 1.5 to 2.5% is excellent in all properties and is optimal as a base material for high-strength parts used in engines and the like.

(Third embodiment)
(1) Manufacture and evaluation of test piece As shown in Table 3, test piece No. different in composition from the test piece described above. 3-1 and 3-2 were manufactured in the same manner as in the first example. Specimen No. 3-1, test piece No. described above. In addition to 2-3, it further contains Na. This Na improvement treatment was performed according to the addition method using a commercially available flux. Specimen No. 3-2 is the test piece No. described above. In addition to 2-3, Sr is further contained. This Sr improvement treatment was performed by a method of adding a master alloy of Al-10 mass% Sr. Specimen No. 3-2 is the test piece No. described above. Further, it contains Sb with respect to 2-3. This Sb improvement treatment was performed by a method of adding a master alloy of Al-10 mass% Sb.
The characteristics of this test piece were also evaluated in the same manner as in the first example, and the results are also shown in Table 3. In Table 3, for reference, the test piece No. used as a base is shown. The data of 2-3 was also shown.

(2) Study As can be seen from Table 3, when at least one of Na, Sr, or Sb is added and improved, the thermal fatigue life of the aluminum alloy casting can be significantly improved while maintaining the hardness and fatigue strength high. I understand. Therefore, it can be seen that the aluminum alloy casting improved by Na, Sr or Sb is optimal as a base material for high strength parts such as diesel engines, which require particularly high heat fatigue strength.

(Fourth embodiment)
(1) Manufacture and evaluation of test piece As shown in Table 4, test piece No. different in composition from the test piece described above. 4-1, 4-2 and 4-3 were produced in the same manner as in the first example. The material DASII in this case is 35 μm. The hardness of each test piece was evaluated, and the results are also shown in Table 4. In addition, each test piece shown in Table 4 mainly differs in the composition regarding the amount of Mg.

(2) Study As can be seen from Table 4, the test piece No. in which the amount of Mg is in the composition range of the present invention. It is understood that 4-1 and 4-2 have sufficient hardness, and the hardness increases as the amount of Mg increases. However, test piece No. in which the Mg amount exceeds 0.6% by mass. In 4-3, a sufficiently large hardness is obtained, but the hardness is the test piece No. 4-3. It is not different from 4-2. Therefore, it can be seen that even if Mg is added in an amount of 0.6 mass% or more, the contribution to the hardness is hardly recognized.

(Metal structure)
Specimen No. 1 relating to the aluminum alloy casting of the present invention. The metal structure observed for 2-4 is shown in FIGS. 1A and 1B. This metal structure is the one after the solution heat treatment and the aging heat treatment are performed on the cast product described in the first embodiment.
Incidentally, FIG. 1A is observed with an optical microscope under an observation magnification of 400 times. FIG. 1B is a TEM structure observed with an electron microscope.

From FIG. 1A, the aluminum alloy casting of the present invention is composed of an entirely homogeneous metal structure in which the matrix phase mainly composed of α-Al is surrounded by an isotropic network-like skeleton phase formed of crystallized substances. I understand.
Moreover, it can be seen from FIG. 1B that a precipitated phase having a large particle size, that is, a first precipitated phase and a second precipitated phase appear in the matrix phase.

It is a metallographic photograph of the aluminum alloy casting of the present invention. It is the metallographic photograph which expanded the base phase in FIG. 1A.

Claims (15)

An aluminum alloy for castings characterized in that, when the total is 100% by mass (hereinafter simply referred to as “%”), an aluminum alloy casting comprising the following elements and inevitable impurities and excellent in practical fatigue characteristics can be obtained. .
Silicon (Si): 4 to 12%,
Copper (Cu): 1.0-3.0%
Magnesium (Mg): 0.2-0.6%
Nickel (Ni): 0.2-3%,
Iron (Fe): 0.1-0.7%
Titanium (Ti): 0.1-0.3%
Zirconium (Zr): 0.03-0.5%
Aluminum (Al): balance   Furthermore, the aluminum alloy for castings of Claim 1 containing manganese (Mn): 0.1-0.7%.   Furthermore, the aluminum alloy for castings of Claim 1 containing vanadium (V): 0.01-0.5%.   Further, one or more of strontium (Sr): 0.003-0.05%, antimony (Sb): 0.02-0.2% and sodium (Na): 0.001-0.03% are added. The aluminum alloy for castings according to claim 1, which is contained. It has the same composition as the aluminum alloy for castings according to claim 1,
a base phase mainly composed of α-Al;
A skeleton phase crystallized to surround the base phase like a network;
A first precipitation phase having a particle size of 30 to 300 nm and a second precipitation phase having a particle size of 1 to 20 nm are composed of a metal structure having a multiphase precipitate precipitated in the matrix phase;
An aluminum alloy casting characterized by excellent practical fatigue characteristics.   The aluminum alloy casting according to claim 5, wherein the skeleton phase is a crystallized product of a compound containing Si, Ni, or Fe. The first precipitation phase comprises a Ti compound or Zr compound and a Cu compound or Mg compound,
The aluminum alloy casting according to claim 5, wherein the second precipitation phase comprises a Cu compound or an Mg compound.   The aluminum alloy casting according to claim 5, wherein the porosity is 0.3% or less.   The aluminum alloy casting according to claim 5, wherein the size of the secondary dendrite (DAS II) is 40 µm or less.   6. The aluminum alloy casting according to claim 5, wherein an initial hardness when the base phase is used is 80 HV or more in terms of Vickers hardness (HV).   The aluminum alloy casting according to claim 5, wherein the aluminum alloy casting is a member for an engine.   The aluminum alloy casting according to claim 11, wherein the engine member is a cylinder head of a reciprocating engine. A casting step of injecting and solidifying a molten aluminum alloy having the same composition as the aluminum alloy for castings according to claim 1 to solidify the casting,
A heat treatment step of subjecting the aluminum alloy casting to solution heat treatment and aging heat treatment,
A method for producing an aluminum alloy casting, wherein the aluminum alloy casting according to claim 5 is obtained. The solution heat treatment includes a heating step and a rapid cooling step after the heating step,
The method for producing an aluminum alloy casting according to claim 13, wherein the rapid cooling step has a cooling rate of 20 to 200 ° C./min.   The method of manufacturing an aluminum alloy casting according to claim 13, wherein the rapid cooling step is a step of cooling the aluminum alloy casting after the heating step in a gas.