JP2007023330A - Aluminum alloy casting and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an aluminum alloy casting having excellent tensile strength and fatigue strength and suitably used e.g. as a cylinder head for internal-combustion engines and its manufacturing method and further to provide a cylinder head for internal-combustion engines made of the aluminum alloy casting and a cylinder head for internal-combustion engines obtained using the manufacturing method. <P>SOLUTION: In the aluminum alloy casting which has a composition consisting of, by mass, 4.0 to 10.0%, preferably 4.0 to 7.0%, Si, 2.0 to 4.5% Cu, ≤0.5%, preferably 0.25 to 0.45%, Mg, 35 to 200 ppm, preferably >50 to 200 ppm Sr and the balance Al with inevitable impurities, Fe content is reduced to ≤0.25% and the amount of porosity is set to ≤0.3% by area ratio. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an aluminum alloy casting that is excellent in fatigue strength and that is suitably used, for example, a cylinder head used in an internal combustion engine for automobiles, and a method for producing such an aluminum alloy casting.

As an aluminum alloy casting used for a cylinder head for an internal combustion engine, an Al—Cu—Si based aluminum alloy casting defined in JIS H5202 as AC2A, AC2B, and AC4B has been conventionally used (for example, Patent Document 1). Paragraph [0002]).
JP-A-10-205388

  However, in such a cylinder head for an internal combustion engine, the stress tends to increase repeatedly with the recent increase in engine output and the thinning of the cylinder head intended to reduce the weight of the vehicle body. In addition, since it has a structure in which high residual stress generated during T6 or T7 heat treatment is locally concentrated, it cannot be said that the above-described aluminum alloy casting does not have insufficient fatigue strength. The possibility of fatigue cracks from the stress concentrated part of the top deck of the cylinder head and the water jacket cannot be denied.

  The present invention has been made in view of the above-mentioned problems in conventional aluminum alloy cylinder heads for internal combustion engines. The object of the present invention is excellent in tensile strength and fatigue strength, and is suitable, for example, as a cylinder head for internal combustion engines. An aluminum alloy casting that can be used for the above, a manufacturing method of such an aluminum alloy casting, a cylinder head for an internal combustion engine made of the aluminum alloy casting, and a cylinder head for an internal combustion engine manufactured based on the manufacturing method It is to provide.

  As a result of intensive studies on alloy components and casting methods in order to achieve the above object, the present inventors have reduced Fe content as an impurity component and reduced the amount of porosity by reducing the gas component in the molten metal. It has been found that the above-mentioned problems can be solved by reducing the above, and the present invention has been completed.

  That is, the present invention is based on the above knowledge, and the aluminum alloy casting of the present invention has a mass ratio of 4.0 to 10.0%, preferably 4.0 to 7.0% Si and 2%. In addition to 0.0 to 4.5% nCu, 0.5% or less, preferably 0.25 to 0.45% Mg, and 0.25% or less Fe, 35 to 50 ppm Sr, It consists of the balance Al and inevitable impurities, and the porosity is 0.3% or less in terms of area ratio.

  Also, the aluminum alloy casting of the present invention has a mass ratio of 4.0 to 10.0%, preferably 4.0 to 7.0% Si, 2.0 to 4.5% nCu, 0 0.5% or less, preferably 0.25 to 0.45% Mg, and in addition to 0.25% or less Fe, containing 50 to 200 ppm of Sr, the balance being Al and unavoidable impurities, the amount of porosity Similarly, the area ratio is 0.3% or less.

  In the method for producing an aluminum alloy casting of the present invention, when casting the aluminum alloy casting, the mold is preheated to a temperature range of 350 to 400 ° C., and a molten metal having a temperature range of 680 to 720 ° C. is cast. It is desirable to perform T7 treatment (after solution treatment and stabilization treatment) after casting.

  The cylinder head for an internal combustion engine of the present invention is characterized by comprising the above-described aluminum alloy casting of the present invention, and is characterized by being based on the manufacturing method of the present invention.

According to the present invention, since both Fe and porosity (porosity) adversely affecting the mechanical performance such as tensile strength, fatigue strength and elongation of the aluminum alloy casting are reduced, the mechanical performance of the aluminum alloy casting is reduced. Especially, fatigue strength can be improved.
In the manufacturing method of the present invention, the molten alloy at a predetermined temperature is cast into a mold preheated to the above temperature range, so that the molten metal can be secured and a healthy aluminum alloy casting with a small casting hole is obtained. be able to.

  Therefore, by applying such an aluminum alloy casting or aluminum alloy casting manufacturing method to a cylinder head for an internal combustion engine, the fatigue strength of the cylinder head can be improved, such as a top deck or a water jacket of the cylinder head. Even in the stress concentration part, an excellent effect of preventing the occurrence of fatigue cracks is brought about.

  Hereinafter, the reasons for limitation of the aluminum alloy castings of the present invention, such as alloy components and manufacturing conditions, will be described together with their actions. In the present specification, “%” represents mass percentage unless otherwise specified.

(1) Si: 4.0 to 10.0%
Si (silicon) is an element that has the effect of increasing tensile strength, fatigue strength, and hardness, and has the effect of imparting fluidity to the molten metal and improving castability, but when it is less than 4.0% Cannot fully exhibit such an action, and addition exceeding 10% tends to reduce elongation, so it is added in the range of 4.0 to 10.0%.
In order to minimize the adverse effect of the decrease in elongation due to the addition, the upper limit is preferably set to 7.0%.

(2) Cu: 2.0 to 4.5%
Cu (copper) is an element that has the effect of increasing tensile strength, fatigue strength, and hardness by heat treatment, but tends to decrease elongation. In particular, it is added in the range of 2.0 to 4.5% in order to achieve both the tensile strength having a high correlation with the high cycle fatigue strength and the high elongation necessary for preventing low cycle fatigue strength and cracking during heat treatment.

(3) Mg: 0.5% or less Mg (magnesium) is an element effective for increasing tensile strength, fatigue strength, and hardness by heat treatment, similarly to Cu. However, if added excessively, Mg ₂ Si precipitates and tends to reduce elongation, so the upper limit value needs to be 0.5%.
In addition, in order to make the said effect by addition of Mg more reliable and to minimize the adverse effect of the addition, it is desirable to add in the range of 0.25 to 0.45%.

(4) Fe: 0.25% or less
Fe (iron) precipitates as an acicular Fe compound, and has an adverse effect on the overall strength, such as tensile strength, fatigue strength, and elongation. Therefore, the upper limit is set to 0.25%.
In addition, since Fe is a component harmful to the mechanical performance as described above, its content is preferably as small as possible.

(5) Sr: 35 to 200 ppm
Since Sr has the effect of refinement of eutectic Si particles and shape improvement, 35 ppm or more is added as necessary. However, when added in excess, the Sr compound crystallizes and causes a decrease in strength and elongation, so the upper limit value needs to be 200 ppm. In addition, in order to make the said effect by Sr addition more reliable, it is desirable to set it in the range of more than 50 ppm and 200 ppm or less.

(6) Porosity amount: 0.3% or less Porosity correlates with the gas content in the molten metal, and generally has an adverse effect on tensile strength, fatigue strength, elongation, and the like. In particular, since it becomes the starting point of fatigue cracks and reduces fatigue strength, in order to remove the toxicity of the above-mentioned Fe compound and improve fatigue strength, it is necessary to simultaneously reduce the porosity amount to 0.3% or less by area ratio. is there.

Here, in order to reduce the amount of porosity, it is necessary to reduce the gas content in the molten metal as described above. For example, an inert gas such as a sufficiently dehumidified nitrogen (N ₂ ) gas is rotated. The gas content can be reduced by stirring while blowing into the molten metal with a blower of the type, and then sufficiently calming until the blown gas rises, and in this state, pouring the mold into the mold can be started. In this case, the porosity will decrease.
In addition, as a standard for setting the porosity amount to 0.3% or less, it has been confirmed that the analysis value of the gas (hydrogen amount) content after solidification of the molten metal should be 0.25 cc / 100 g or less. .

(7) Mold preheating temperature: 350 to 400 ° C, molten metal temperature: 680 to 720 ° C
The melting point of the aluminum alloy is about 620 to 630 ° C., however, a mold in which the molten metal temperature is maintained in a temperature range of 680 to 720 ° C., which is higher by about 50 to 100 ° C., and preheated to 350 to 400 ° C. By casting into the mold, the formation of the cast hole is prevented and the strength of the product is improved.

That is, when the mold temperature is less than 350 ° C., the fluidity of the molten metal decreases and the surroundings of the molten metal increase, and conversely, when it exceeds 400 ° C., the cooling rate of the molten metal and the solid alloy casting is increased. However, the mold preheating temperature is set in the range of 350 to 400 ° C.
Similarly, the molten metal temperature is set in the temperature range of 680 to 720 ° C. from the viewpoint of fluidity and cooling rate. At this time, it is also possible to increase the cooling rate by using a means such as partially cooling the mold as necessary, thereby reducing the cast holes and making the crystal grains finer.

(8) T7 treatment (after solution treatment, stabilization treatment)
In general, a cylinder head made of an aluminum alloy is often subjected to T6 treatment (after solution treatment and artificial aging treatment) in order to increase the strength. In the present invention, however, the strength is slightly higher than that of the T6 treatment. Although it is inferior, T7 treatment (after solution treatment and stabilization treatment) is performed to improve elongation necessary for the cylinder head, reduce residual stress, and secure dimensional stability.

As specific conditions for such T7 treatment, conditions are employed in which a solution treatment at 500 to 510 ° C. for 4 to 8 hours is followed by a stabilization treatment at 210 to 220 ° C. for 6 to 10 hours. be able to.
And, when the aluminum alloy of the present invention is applied to a cylinder head by such a T7 treatment, the required hardness of 54 HRB or more and 70 HRB or less is obtained from the viewpoint of preventing the head bolt seat surface and the gasket seal surface from sagging. Can do.

Similarly, for aluminum alloy castings having a Mg content of 0.25 to 0.45%, after a solution treatment at 500 to 510 ° C. for 4 to 8 hours, a slightly higher temperature of 230 to 240 ° C. A stabilization process for 6 to 10 hours can be performed.
In this case, since the temperature is higher than the stabilization treatment of the alloy casting, the residual stress is reduced, and the effect of improving the fatigue strength can be obtained.

  Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

Example 1
Aluminum alloys having component compositions as shown in Table 1 were respectively melted by an electric furnace, and after degassing treatment by blowing dry nitrogen gas, component composition / gas analysis samples were collected.
Next, molten metal having a temperature of 700 ° C. was poured into a mold preheated to 380 ° C. to obtain six types of aluminum alloy castings having a size of a JIS boat mold (190 × 23 × 40 mm).

  Each of the obtained aluminum alloy castings was subjected to T7 treatment (solution treatment: 500 ° C. × 6 hours, stabilization treatment: 210 ° C. × 8 hours), and then a tensile test piece and a fatigue test piece were cut out and tested. Carried out.

  In addition, a hardness test piece is cut out from the grip portion of the tensile test piece after completion of the test, and a B-scale Rockwell hardness (HRB) is measured. It cut out and calculated | required the area ratio of the porosity using the optical microscope (20 time).

These results are also shown in Table 1. The “gas amount” in the table represents the result of a hydrogen analyzer by the Ransley method, and indicates the amount of hydrogen (cc) in 100 g of the solidified aluminum alloy. In addition, the difference in the gas amount of each aluminum alloy casting is due to the degree of degassing treatment.
Further, FIG. 1 shows the relationship between the Fe content in the aluminum alloy casting and the porosity amount, which affects the fatigue strength (10 ⁷ times).

As is clear from the results in Table 1, when the gas content is large and the porosity amount is large, even if the Fe content which is an impurity is reduced, it has been found that there is no improvement effect in tensile strength, ductility and fatigue strength. (Comparative Examples 1 and 3).
In addition, when the amount of Fe is the same as the conventional one, it was confirmed that by reducing only the gas content and the amount of porosity, a certain degree of improvement effect can be obtained, but the effect is not sufficient (Comparative Examples 1 and 2). ).

And when the amount of porosity and the amount of Fe which is an impurity were both small, the most remarkable improvement effect was recognized (Invention example 1 and Comparative example 1).
This is because when the fatigue strength is improved by reducing the amount of Fe compound that can be used as a starting point for fatigue or reducing the size, the amount and size of porosity that can also be used as a starting point can be reduced as well. It is thought that it is necessary to make it small.

(Example 2)
Based on the composition of Invention Example 1 shown in Table 1, aluminum alloys having the composition shown in Table 2 were similarly dissolved in an electric furnace, and after degassing treatment, cast in the same manner, and after heat treatment Each analysis sample and test piece were collected in the same manner as in the above example, and the same test was performed.
The results are also shown in Table 2.

The aluminum alloy casting of Invention Example 2 is based on Invention Example 1 of the above-described example, and is obtained by adding Sr in order to obtain further eutectic Si refinement and shape improvement effect. It was confirmed that the material had superior tensile strength, elongation, and fatigue strength than those of Example 1 above.
When the Mg content is 0.25 to 0.45% (Invention Examples 3 and 4), a T7 treatment comprising a solution treatment at 500 ° C. × 8 hours and a stabilization treatment at 230 ° C. × 8 hours. Carried out.

  The aluminum alloy casting of Invention Example 3 is obtained by increasing the amount of Mg based on Invention Example 1 of the above example, but has substantially the same tensile strength, elongation, fatigue strength, and hardness as in the case of Invention Example 1. It turned out to have.

The aluminum alloy casting of Invention Example 4 is based on the alloy composition of Invention Example 3 above, and further added with Sr in order to obtain the eutectic Si refinement and shape improvement effect. It was confirmed that the material had even better tensile strength, elongation, fatigue strength, and hardness than the case.
In the case of Invention Example 4 as well, as in the case of Invention Example 3, T7 treatment conditions comprising a solution treatment at 500 ° C. × 8 hours and a stabilization treatment at 230 ° C. × 8 hours are employed. .

It is a graph showing the relationship between aluminum alloy fatigue strength of the casting Fe content on the ⁽¹⁰⁷ times) and porosity volume.

Claims (8)

  In mass ratio, Si: 4.0-10.0%, Cu: 2.0-4.5%, Mg: 0.5% or less, Fe: 0.25% or less, Sr: 35-50ppm An aluminum alloy casting characterized in that the balance is substantially made of Al and the porosity is 0.3% or less in terms of area ratio.   2. The aluminum alloy casting according to claim 1, wherein the Si content is 4.0 to 7.0%, and the Mg content is 0.25 to 0.45%.   In mass ratio, Si: 4.0-10.0%, Cu: 2.0-4.5%, Mg: 0.5% or less, Fe: 0.25% or less, Sr: more than 50 to 200 ppm An aluminum alloy casting characterized in that the balance is substantially made of Al and the porosity is 0.3% or less in terms of area ratio.   4. The aluminum alloy casting according to claim 3, wherein the Si content is 4.0 to 7.0% and the Mg content is 0.25 to 0.45%.   When producing the aluminum alloy casting according to any one of claims 1 to 4, a molten metal having a temperature range of 680 to 720 ° C is cast into a mold preheated to a temperature range of 350 to 400 ° C. The manufacturing method of the aluminum alloy casting characterized by these.   6. The method for producing an aluminum alloy casting according to claim 5, wherein the obtained casting is subjected to a T7 treatment.   A cylinder head for an internal combustion engine, comprising the aluminum alloy casting according to any one of claims 1 to 4.   A cylinder head for an internal combustion engine manufactured by the method according to claim 6.

Images