JP2013174022A - Aluminum alloy for casting and aluminum alloy casting - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide aluminum alloy for casting, which is excellent in elongation that is an alternative property of high cycle fatigue strength and thermal fatigue strength, and is excellent in hardness and can be preferably used, for example, for a cylinder head for an internal combustion engine, and to provide an aluminum alloy casting made of the aluminum alloy and a cylinder head for an internal combustion engine that is a typical example of such a casting.SOLUTION: Alloy having a high cycle fatigue strength of ≥100 MPa, an elongation of ≥10% and a hardness of ≥50 HRB is obtained by incorporating, by mass ratio, 4.0-6.0% of Si, 0.5-2.0% of Cu, 0.25-0.5% of Mg, 0.002-0.02% of Sr, 0.005-0.2% of Ti, and ≤0.5% of each of Fe and Mn into aluminum alloy for casting.

Description

  The present invention relates to an aluminum alloy for casting, and more specifically, an aluminum alloy suitably used for a member that requires both high cycle fatigue strength and thermal fatigue strength, an aluminum alloy casting made of the alloy, and an example thereof As a cylinder head for an internal combustion engine.

  As casting alloys having complicated shapes and required mechanical properties, conventionally, Al-Cu-Si systems defined as AC2A, AC2B, and AC4B in JIS H5202 and AC4C and AC4CH are defined. Al-Mg-Si based aluminum alloy castings are used. These cast products include a cylinder head for an internal combustion engine and a cylinder block.

  In these cast products, in order to increase the strength and toughness, the cast body is subjected to T6 treatment (aging treatment at the tempering temperature at which the maximum strength is obtained after solution treatment and quenching treatment) or T7 treatment (overaging after solution treatment and quenching treatment). In many cases, it is used after being subjected to a process for ensuring dimensional stability.

JP 2006-169594 A

  However, in such a conventional cylinder head for an internal combustion engine, the stress tends to increase repeatedly as the output of the engine is increased in recent years and the thickness of the cylinder head is reduced to reduce the weight of the vehicle body. In addition to having a structure in which high residual stress generated during T6 or T7 heat treatment is locally concentrated, the above aluminum alloy castings have an alternative characteristic of high cycle fatigue strength and thermal fatigue strength. There is a problem that fatigue cracks are increasing from the stress concentrated part of the top deck of the cylinder head and the water jacket and the high temperature part between the combustion chamber valves. It was.

  The present invention has been made by paying attention to the above-mentioned problems in conventional aluminum alloy castings, and the object thereof is excellent in elongation, which is an alternative characteristic of high cycle fatigue strength and thermal fatigue strength. Castings that require high performance, for example, an aluminum alloy for casting that can be suitably used for a cylinder head for an internal combustion engine, an aluminum alloy casting made of the aluminum alloy, and a cylinder head for an internal combustion engine that is a representative example of the casting Is to provide.

  As a result of intensive studies on alloy components and heat treatment methods to achieve the above object, the inventors of the present invention specified the respective contents of Si, Cu, and Mg, or performed T7 treatment on the obtained alloy castings. As a result, the present inventors have found that the above problems can be solved, and have completed the present invention.

That is, the present invention is based on the above knowledge, and the aluminum alloy for casting of the present invention has a mass ratio of 4.0 to 6.0% Si, 0.5 to 2.0% Cu, 0 .25 to 0.5% Mg, 0.002 to 0.02% Sr, 0.005 to 0.2% Ti, 0.5% or less of Fe and Mn respectively, with the balance being Al and It is an unavoidable impurity and has a high cycle fatigue strength of 100 MPa or more, an elongation of 10% or more, and a hardness of 50 HRB or more.
Also, by mass ratio, 5.0 to 6.0% Si, 0.8 to 1.3% Cu, 0.3 to 0.4% Mg, 0.002 to 0.02% Sr, 0.005 to 0.2% Ti, each containing 0.2% or less of Fe and Mn, the balance being Al and inevitable impurities, similarly high cycle fatigue strength of 100 MPa or more, elongation of 10% or more The hardness is 50 HRB or more.

  The aluminum alloy casting of the present invention is characterized by comprising the above-mentioned aluminum alloy of the present invention, and 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

  According to the present invention, since Si, Cu, Mg, Sr, and Ti contained in the aluminum alloy for casting are limited to specific ranges, respectively, the elongation of the casting by the alloy can be increased, and high cycle fatigue is achieved. An aluminum alloy casting excellent in both strength and thermal fatigue strength, for example, a cylinder head for an internal combustion engine can be obtained.

It is a graph which shows the influence of Si content and Cu content which have on the generation amount of a casting defect as a result of the ductility test of the aluminum alloy for casting.

  Hereinafter, the reasons for limiting the components in the casting aluminum alloy of the present invention, the heat treatment conditions applied to the aluminum alloy casting made of the alloy, and the like will be described in more detail. In the present specification, “%” represents mass percentage unless otherwise specified.

(1) Si content: 4.0-6.0%
Since Si (silicon) has an effect of improving castability, when casting a complicated shape such as a cylinder head or a thin-walled portion, Si is considered from the viewpoint of fluidity of the molten metal, that is, moldability of the cast product. It is necessary to add more than a certain amount. That is, when Si is less than 4.0%, the fluidity of the molten metal becomes insufficient. In addition, the quasi-solid phase region widens, the shrinkage nests are dispersed, porosity is generated, and shrinkage cracks are easily generated. Si also has the effect of improving the mechanical strength, wear resistance, and vibration resistance of the cast material.
However, as Si increases, the thermal conductivity and ductility of the alloy decrease and the thermal fatigue properties deteriorate. When the Si content exceeds 6.0%, the elongation decreases remarkably, and since the shrinkage nests begin to concentrate, the occurrence of nests may be observed.

FIG. 1 shows the result of a shrinkage test, that is, the result of measuring the specific gravity at the center of the bottom surface when casting a conical test piece by the Archimedes method and measuring the casting defect rate by the difference from the standard specific gravity. Thus, it can be seen that when the Si content is 4.0 to 6.0%, the casting defects (total of porosity and nests) are the smallest, and when the Cu content is smaller, the casting defect amount is smaller.
In addition, as Si content, it is more preferable to exist in the range of 5.0 to 6.0%.

(2) Cu content: 0.5-2.0%
Cu (copper) has the effect of improving the mechanical strength of the aluminum alloy. This effect becomes prominent when Cu is contained in an amount of 0.5% or more. However, as Cu is increased, the thermal conductivity and stretchability are lowered, and the thermal fatigue characteristics are deteriorated. Further, as the Cu content increases, the solidification form becomes bowl-like, the shrinkage nests are dispersed, and porosity is generated.
As is clear from FIG. 1, if the amount of Si is the same, the amount of casting defects increases as the amount of Cu increases, and these adverse effects become prominent when Cu exceeds 2.0%. The range is 0.0%, more preferably 0.8 to 1.3%.

(3) Mg: 0.25 to 0.5%
If Mg (magnesium) is added, the tensile strength and hardness are increased by heat treatment, and the thermal fatigue strength and elongation tend to decrease. If added excessively, it precipitates as Mg ₂ Si and lowers the thermal fatigue strength and elongation, so it is added in the range of 0.25 to 0.5%, more preferably 0.3 to 0.4%.
By adding in this range, it contributes to matrix strengthening by aging precipitation of the intermediate phase of Mg ₂ Si. Moreover, when Mg content exceeds 0.5%, the surface oxidation amount of a molten metal will increase notably and the malfunction that an inclusion defect increases will arise.

(4) Fe: 0.5% or less Fe (iron) precipitates as an acicular iron-based compound, and has an adverse effect on the overall such as tensile strength, fatigue strength, thermal fatigue strength, and elongation. The value is 0.5%.
In addition, since Fe is a harmful component as described above, it is desirable that it is as small as possible, and it is preferably 0.2% or less, and ideally includes the case where it is substantially 0%.

(5) Mn: 0.5% or less By adding Mn (manganese), the shape of the crystallized substance containing Fe can be changed from a needle shape that tends to cause a decrease in strength to a lump shape that does not easily cause stress concentration. it can.
Even if the amount of Mn is more than necessary, the amount of iron-based compound (Al—Fe, Mn—Si) increases, so it is 0.5% or less, preferably 0.2% or less. The Fe: Mn ratio is preferably 1: 1 to 2: 1.

(6) Sr: 0.002 to 0.02%
In particular, for the cylinder head material, in order to improve its heat fatigue resistance, it is desirable to add a predetermined amount of Sr to refine the Si grains of the cast structure.
This Si grain improvement treatment improves mechanical properties such as tensile strength and elongation, and also improves thermal fatigue strength. However, when a large amount of Sr is added, a crystallization region of a band-like coarse Si phase called overmodification is generated and the strength may be lowered. Therefore, the amount added is 0.002 to 0.02%. The range. In addition, it is desirable to cool and solidify the combustion chamber surface where thermal fatigue strength becomes a problem, and to reduce the dendrite arm spacing to 30 μm or less.

(7) Ti: 0.005 to 0.2%
Since Ti is an effective component for refining the crystal grain of the cast structure, it is added in the range of 0.005 to 0.2%. Moreover, in the component range with a large amount of casting defects, the addition of Ti disperses the nests and improves the shrinkage nests.
If the addition amount of Ti is less than 0.005%, there is no effect, and if it exceeds 0.2%, the risk of agglomeration of Al—Fe, TiB, etc., which are the cores of crystal grains, increases.

(8) Heat treatment In the casting made of the aluminum alloy of the present invention, although the strength is slightly inferior to the T6 treatment (after solution treatment, artificial age hardening treatment), the thermal fatigue strength improvement and residual stress required for the cylinder head In order to reduce the thickness and to obtain dimensional stability, it is desirable to perform T7 treatment (after solution treatment, stabilization treatment).
Specifically, for the casting aluminum alloy of the present invention having the above component composition, solution treatment is performed at 500 to 550 ° C. for 2.0 to 8.0 hours, and aging treatment is performed at 190 to 250 ° C. at 2.0. By carrying out the conditions for ˜6.0 hours, target values of high cycle fatigue strength (100 MPa or more), elongation (10.0% or more), and hardness (50 HRB or more) are achieved. The high cycle fatigue strength referred to here means a stress amplitude value when the repetition index until fracture by the rotational bending fatigue test at a rotational speed of 3600 rpm is 10 ⁷ times.

By such a T7 treatment, the head bolt seat surface, gasket seal surface is prevented from sagging, a fastening surface with the cylinder block, the wear resistance in the camshaft sliding portion, etc., a required hardness of 50 HRB or more is obtained. be able to.
If a sufficient solution treatment time is taken, the eutectic Si has a rounded shape due to diffusion, which reduces stress concentration and improves mechanical properties such as toughness.

  EXAMPLES Hereinafter, although this invention is demonstrated further in detail based on an Example, this invention is not limited to these Examples.

(1) Boat type casting test An aluminum alloy having the composition shown in Table 1 was melted in an electric furnace, and after a refinement process and a Si improvement process, a boat type of 190 × 40 × 25 mm was cast, and T7 treatment (530 ° C. × 8 After a solution treatment for a period of time, an aging treatment for 6 hours at a predetermined temperature between 180-260 ° C.), a fatigue test piece and a tensile test piece were cut out, and high cycle fatigue strength and elongation at break were measured respectively. B scale Rockwell hardness (HRB) was measured.
These results are also shown in Table 1. These target values are 100 MPa or more for high cycle fatigue strength, 10.0% or more for elongation, which is an alternative characteristic of thermal fatigue strength, and 50 HRB or more for hardness.

Note that the high cycle fatigue test, using a fatigue tester Ono-type rotating bending, the rotational speed and 3600 rpm, number of repetitions to failure and rated the fatigue strength by the stress amplitude value when the 10 ⁷ times.

As is clear from Table 1, in Examples 1 to 9 having alloy components in a predetermined range and subjected to T7 treatment at an aging temperature of 200 to 240 ° C., both high cycle fatigue strength, breaking elongation and hardness are good. It was confirmed that it showed a good performance.
In contrast, Comparative Examples 1 to 8 in which the alloy components are outside the specified range of the present invention, Comparative Examples 9 and 10 subjected to aging treatment at 180 ° C. or 260 ° C., and further used as conventional cylinder head materials. In the conventional materials 1 and 2 using the AC4CH alloy and the AC2A alloy, at least one of the characteristics of fatigue strength, elongation at break and hardness is low, and satisfactory strength is obtained as a cylinder head material for a high performance engine. It turned out not to be.

(2) Cylinder head casting test Of the above-mentioned examples and comparative examples, picking up the alloy components whose results of the boat-type casting test were relatively good, and casting the cylinder head body, and corresponding T7 treatment After the test, fatigue test pieces and tensile test pieces were cut out from the vicinity of the combustion chamber surface, and similarly high cycle fatigue strength and elongation at break were measured, and B scale Rockwell hardness (HRB) was measured.
These results are shown in Table 2. As target values in this case, the high cycle fatigue strength is 85 MPa or more, and the hardness is 50 HRB or more.

  Furthermore, as a target value of the thermal fatigue strength, a simple thermal fatigue test was performed in which 40 ° C.-270 ° C.-40 ° C. was set as one cycle under a completely restrained condition using a flat notched test piece, and as the target value, It was set to 100 cycles or more, which is the thermal fatigue life of a TIG remelted product using a conventional AC2A alloy.

  As is apparent from the results shown in Table 2, also in the cast product of the cylinder head entity, Examples 2-2 and 6-2 corresponding to Examples 2 and 6 of the boat-type casting test have high cycle fatigue strength, Both fatigue life and hardness were good, and it was confirmed that the required characteristics as a cylinder head were met at a high level.

On the other hand, in Comparative Examples 4-2 and 8-2 corresponding to Comparative Examples 4 and 8 of the boat type casting test, comparatively good evaluation results were obtained in the boat type, but Comparative Example 4 Regarding -2, since the cylinder head body was thick, the fatigue strength and thermal fatigue life were lowered due to the influence of casting defects that did not appear in the boat type.
On the other hand, Comparative Example 8-2, which was not able to achieve the target value slightly in the boat-type casting test, also has low strength in the substantial test, and this is because Si has not been improved by Sr. it is conceivable that.

Claims (4)

  By mass ratio, Si: 4.0-6.0%, Cu: 0.5-2.0%, Mg: 0.25-0.5%, Sr: 0.002-0.02%, Ti: 0.005 to 0.2%, Fe: 0.5% or less, Mn: 0.5% or less, the balance being Al and inevitable impurities, high cycle fatigue strength: 100 MPa or more, elongation: 10% As described above, an aluminum alloy for casting characterized by having a hardness of 50 HRB or more.   By mass ratio, Si: 5.0 to 6.0%, Cu: 0.8 to 1.3%, Mg: 0.3 to 0.4%, Sr: 0.002 to 0.02%, Ti: 0.005 to 0.2%, Fe: 0.2% or less, Mn: 0.2% or less, the balance being Al and inevitable impurities, high cycle fatigue strength: 100 MPa or more, elongation: 10% As described above, an aluminum alloy for casting characterized by having a hardness of 50 HRB or more.   An aluminum alloy casting comprising the aluminum alloy for casting according to claim 1 or 2.   A cylinder head for an internal combustion engine, comprising the aluminum alloy casting according to claim 3.

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