JP2012097321A - High-strength aluminum alloy forged product excellent in stress corrosion cracking resistance and forging method for the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the corrosion resistance and stress corrosion cracking resistance of a high-strength aluminum alloy.SOLUTION: The high-strength aluminum alloy forged product is produced by forging an aluminum alloy containing 0.8-2.2 mass% Si (hereinafter described as %), 0.7-1.5% Cu, 0.8-1.8% Mg, 0.5-1.1% Mn, and 0.05-0.30% Zr, with the balance composed of aluminum and unavoidable impurities in such a manner that the Zener-Hollomon parameter Z falls within the range: 2.9×10≤Z≤6.6×10, wherein a fibrous structure composed of crystal grains of an aspect ratio of 10 or more occupies 80% or more of the cross-sectional area of the forged product.

Description

  The present invention relates to a high-strength aluminum alloy forged product having excellent stress corrosion cracking resistance and a method for producing the forged product.

  In recent years, various parts used in equipment such as automobiles have been promoted to reduce the recyclability and weight of constituent materials in consideration of environmental problems. Especially for automobiles, the purpose is to reduce the increase in weight caused by incidental facilities such as safety devices, and weight reduction of materials has become an important issue, and aluminum alloys are beginning to be used for body seats and underbody parts.

  In vehicles such as automobiles, aluminum alloy is frequently used for undercarriage parts because the weight reduction under the floor is particularly effective for fuel economy. It is important for the undercarriage parts that strength and workability as well as strength do not decrease due to corrosion. That is, these parts are required to have excellent stress corrosion cracking resistance.

  As aluminum high-strength materials used as structural members, 2000-series (Al-Cu-series) and 7000-series (Al-Mg-Zn-series) aluminum alloys are known. Since it is strong, the number of forgings increases. In contrast, the 6000 series aluminum alloy is excellent in workability, and is therefore an effective alloy for materials for products such as forging. However, the strength is generally lower than that of the alloy series. As a high-strength material of the 6000 series aluminum alloy, 6066 alloy is known, but the crystal grain of the product after forging may be coarsened by this forging process. When precipitates exist at the grain boundaries, intergranular corrosion may occur. In the stress corrosion test, the progress of cracks due to corrosion at the grain boundaries becomes significant by applying stress in a corrosive environment.

  Among the recrystallized products, in particular, Cu-added alloys may be corroded by precipitated Cu, which often adversely affects product characteristics. Further, when stress is applied in a corrosive environment, the progress of cracks due to corrosion at the grain boundaries becomes remarkable. In particular, there is concern over the occurrence of problems due to stress corrosion cracking in parts where stress is applied, such as parts around automobile foot.

Regarding high-strength 6000 series aluminum alloys, Patent Document 1 describes strength and toughness, that is, corrosion resistance used for vehicle structural members to obtain high strength of 350 MPa or more and high toughness of Charpy impact value of 20 J / cm ² or more. An excellent Al—Mg—Si based aluminum alloy forging is described. Here, intergranular corrosion is prevented by prescribing Mn and Zr with specific amounts of Mg, Si, and Cu, and by specifying a method for producing a forged material (forging conditions and heat treatment conditions for the forged product).

Patent Document 2 describes Al-Mg-Si based on strength and toughness, that is, Al-Mg-Si excellent in corrosion resistance used as a vehicle structural member for obtaining a high strength of 400 MPa or more and a high toughness of Charpy impact value of 25 J / cm ² or more. The aluminum alloy forging is described. Here, similarly to Patent Document 1, the amount of Mg, Si, and Cu is specified, and Mn, Cr, and Zr are added, and the forging production method (forging conditions, area reduction rate, heat treatment conditions for forgings) And the grain boundary corrosion is prevented by limiting the ratio of the average crystal grain size and the sub-grain structure in the central part of the cross-sectional thickness of the forged material.
JP-A-19-169699 JP 19-177308 A

  As high-strength aluminum alloys, Al—Cu, Al—Zn—Mg, and Al—Zn—Mg—Cu alloys are known. However, considering the strength, workability, and corrosion resistance, the Al—Mg—Si based alloy has an excellent balance of these properties, and is a useful alloy for processed products such as forging. However, the strength is never high, and an alloy having higher strength, excellent workability and corrosion resistance is desired. In Al-Mg-Si alloys, Al-Mg-Si alloys include alloys with Cu added for the purpose of improving strength, such as JIS6066 alloy, but crystal grains become coarse after forging. It has been confirmed. Moreover, corrosion resistance falls under the influence of deposited Cu. This is a concern that the corrosion resistance and stress corrosion cracking resistance may be lowered when considering use as an undercarriage structural member of a vehicle such as an automobile. Therefore, it is necessary to improve the corrosion resistance or the stress corrosion cracking resistance.

  As described above, a forged product used for an undercarriage member of a vehicle such as an automobile is desired to have a strength and a fibrous structure rather than a coarse recrystallized structure. However, in actual manufacturing, a recrystallized structure often occurs. Therefore, it is desired to minimize the propagation to the grain boundary when stress is applied by limiting the recrystallized structure to the minimum on the surface layer and forming a fibrous structure inside the material.

  In view of the above circumstances, as a result of intensive studies, the present inventors have improved the strength by making the inside of a forged product into a fibrous structure by controlling the components and the production process in an Al-Mg-Si alloy. However, it has been found that an aluminum alloy forged product having excellent stress corrosion cracking resistance can be obtained.

That is, according to the first aspect of the present invention, Si: 0.8 to 2.2 mass% (hereinafter referred to as%), Cu: 0.7 to 1.5%, Mg: 0.8 to 1. An aluminum alloy containing 8%, Mn: 0.5 to 1.1%, Zr: 0.05 to 0.30%, with the balance being aluminum and unavoidable impurities, has a Zener-Holomon variable Z of 2.9. × 10 ¹⁰ ≦ Z ≦ 6.6 × 10 ¹¹ is a forged product manufactured such that a fibrous structure composed of crystal grains having an aspect ratio of 10 or more occupies 80% or more of the cross-sectional area of the forged product. It is a forged product made of an aluminum alloy excellent in stress corrosion cracking resistance.

  The invention according to claim 2 of the present invention is the method for manufacturing a forged product according to claim 1, wherein the extruded material manufactured by a conventional method is heated to 490 to 510 ° C, and the extruded material is heated to 150 to 200 ° C. This is a forged product that has been forged with a die. After heating the forged product at 530 to 550 ° C. for 3 to 10 hours, it is cooled with water or warm water, and further subjected to aging treatment at 170 to 190 ° C. for 6 to 10 hours. It is the manufacturing method of the forged product excellent in the stress corrosion cracking resistance characterized by having performed.

  ADVANTAGE OF THE INVENTION According to this invention, the forged product made from aluminum alloy excellent in the high intensity | strength and stress-corrosion cracking resistance can be provided, and weight reduction of an undercarriage part etc. of a motor vehicle is attained.

It is explanatory drawing of the processing rate at the time of upset forging. This is a method for measuring the recrystallized grain thickness of extruded products and forged products. This is a cantilever stress corrosion test.

Hereinafter, embodiments of the present invention will be described.
First, the role of each additive element of the present invention will be described.
Si is an element that forms Mg ₂ Si together with Mg and contributes to strength. If Si is less than 0.8%, the amount of Mg ₂ Si formed is reduced and the effect is reduced. If it exceeds 2.2%, the surface properties during extrusion and the deformation resistance during forging increase, so the workability deteriorates. Therefore, Si is 0.8 to 2.2%. More preferably, it is 0.8 to 2.0%.

  Cu contributes to strength improvement. If it is less than 0.7%, the effect of improving the strength is small, and if it exceeds 1.5%, the extrusion processability is lowered. Further, it is considered that the corrosion resistance is lowered due to precipitation at the grain boundary, and cracks along the grain boundary are likely to proceed when stress is applied. Therefore, Cu is set to 0.7 to 1.5%. More preferably, it is 0.8 to 1.2%.

Mg forms Mg ₂ Si together with Si and contributes to strength improvement. If it is less than 0.8%, the effect is small, and if it exceeds 1.8%, the extrusion processability is lowered. Therefore, Mg is set to 0.8 to 1.8%. More preferably, it is 0.8 to 1.2%.

  Mn and Zr are effective for refining crystal grains and contribute to the improvement of strength. By adding 0.5 to 1.1% of Mn and 0.05 to 0.3% of Zr, coarsening of crystal grains can be prevented even after extrusion, and a fibrous structure can be maintained. However, if the addition amount is too much than the predetermined amount, a coarse compound is formed, resulting in a decrease in strength and a decrease in toughness. Moreover, the said effect will not appear when there is little addition amount. Accordingly, the component ranges of Mn and Zr are 0.5 to 1.1% for Mn and 0.05 to 0.3% for Zr. More preferably, Mn is 0.5 to 0.7% and Zr is 0.15 to 0.2%.

  Further, the aluminum alloy of the present invention may contain one or more of Fe, Zn, Cr, Ti and the like within a range not impairing the effects of the present invention. Specifically, it is possible to add 0.5% or less for Fe, 0.25% or less for Zn, 0.4% or less for Cr, and 0.2% or less for Ti.

  Further, other than the above elements are composed of aluminum and inevitable impurities. Each inevitable impurity is 0.05% or less, and the total amount is preferably 0.15% or less.

Next, the manufacturing method of this invention is demonstrated.
The extruded material is produced by a conventional method. In the ordinary method, an ingot having the alloy composition manufactured by continuous casting is subjected to a homogenization treatment at 400 to 490 ° C. for 3 to 14 hours, and then cooled and cut into billets. Then, the billet is heated to 450 to 500 ° C., extruded, cooled, cut, and used as the extruded material. Then, after setting the extruded material heated to 490 to 510 ° C. in a mold heated to 150 to 200 ° C. in advance, the forged product subjected to forging is further subjected to a solution treatment for 3 to 10 hr at 530 to 550 ° C. It is necessary to carry out aging treatment at 170 to 190 ° C. for 6 to 8 hours after applying the above.

  The homogenization treatment is necessary to eliminate segregation at the time of casting. Except for this condition, the surface pressure is increased during the subsequent extrusion, and the surface quality of the forged product, which is the final product, is reduced. . Heating during extrusion and heating during forging are necessary to reduce deformation resistance. If the temperature is lower than the lower limit temperature, the workability deteriorates, and if the temperature is higher than the upper limit temperature, characteristics such as workability do not change. Therefore, it is not necessary to raise the temperature more than necessary from the viewpoint of productivity.

  If the mold temperature during forging is less than the lower limit temperature, the set material temperature is lowered, and the workability is lowered. On the other hand, if the upper limit temperature exceeds the material temperature, the balance between processing heat generated by the material during processing and air cooling will be lost, and the mold temperature will be higher than necessary. Therefore, by setting to a predetermined temperature, a balance between material temperature, processing heat generation, and air cooling can be achieved, and stable forging can be performed.

  The solution treatment of the forged product is necessary for dissolving the compound precipitated in the extrusion and forging processes. If the temperature is less than the lower limit temperature or less than the lower limit time, the solid solution does not proceed sufficiently, and the temperature or upper limit time is higher than the upper limit temperature. A longer time is not preferable in terms of production cost because the progress of solid solution is saturated. The aging treatment step is necessary to obtain strength, and sufficient strength cannot be obtained at a temperature or time other than the above conditions.

The performance of the alloy of the present invention varies depending on forging conditions (forging ratio, forging speed, temperature, etc.). In order to fully exhibit the characteristics of this alloy, the Zener-Holomon variable Z (Z factor), which is a temperature compensated strain rate factor, is 2.9 × 10 ¹⁰ ≦ Z ≦ 6.6 × 10 ¹¹ , It is necessary to control the strain rate and temperature. Here, the Z factor is ε as strain rate, Q as activation energy for plastic change, R as gas constant, and T as absolute temperature.
Z = εexp (Q / RT)
Indicated by The subcrystal grain size ds is
ds ⁻¹ = a + blogZ (a, b: experimental constant, Z: Z factor)
Therefore, by controlling the strain rate and temperature, it is possible to control the subcrystal grain size to obtain a fibrous structure. Here, the fibrous structure is assumed to have an aspect ratio of 10 or more given by the following formula.
Aspect ratio = (longest crystal length [longitudinal]) / (shortest crystal length [perpendicular to longitudinal direction])
Crystal grains having an aspect ratio of less than 10 cause intergranular corrosion.

  Therefore, in order to obtain a forged product having a fibrous structure, it is necessary to control the temperature and strain rate during forging (that is, the forging rate) and set the Z factor within the above range. When the Z factor is outside the above range, the ratio of the fibrous structure decreases in the cross-section after forging, that is, the ratio of recrystallization increases, and as a result, the stress corrosion cracking resistance is adversely affected.

  Next, based on an Example, this invention is demonstrated in detail.

  An alloy having a composition shown in Table 1 was melted to obtain an ingot having a diameter of 220 mm. The ingot was subjected to homogenization treatment at 490 ° C. for 4 hours, and then cooled and cut to produce a billet. The billet was extruded at 500 ° C. to produce an extruded round bar having a diameter of 60 mm. The extruded round bar was cut to a length of 150 mm, heated to 485 to 510 ° C., and the extruded round bar was laid sideways with a mold heated to 200 ° C., at a processing rate of 30 mm / s and 270 mm / s. % Upset forging. The forged product was further subjected to solution treatment at 530 to 550 ° C. for 2.5 to 10 hours, immediately quenched with hot water at 50 ° C., and further subjected to artificial aging treatment at 160 to 180 ° C. for 6 to 10 hours. Was T6.

Here, the processing rate of 50% is a value calculated by (r ₁ −r ₂ ) / r ₁ × 100 in FIG. r ₁ is the diameter of the aluminum alloy rod before upset forging, and r ₂ is the diameter of the aluminum alloy rod after upset forging.

  The test alloy forged product thus obtained was subjected to a tensile test, structure observation, and stress corrosion cracking resistance evaluation. The results are shown in Table 2.

1) Tensile test Tensile test pieces were sampled from the center of the forged product so that the longitudinal direction of the extruded bar was in the length direction of the test piece, and a JIS No. 4 test piece was prepared and tested.
Here, the tensile strength (TS = 395 MPa) of 6066 alloy T6, which is a general alloy, was used as a reference, a tensile strength equal to or higher than this was indicated by ◯, and the strength not satisfying the reference was indicated by ×.

2) Microstructure observation The ratio of the area occupied by the fibrous tissue to the cross-sectional area was determined by cross-sectional observation. In the forging process, processing strain concentrates on the surface layer portion, and when the cross-section of the forged product is observed, recrystallized grains are observed on the surface layer portion. FIG. 2 shows a model diagram in the case where the difference in structure occurs uniformly. The ratio of the fibrous structure was calculated from the following equation by measuring the total cross-sectional area S _{0 of} the forged product and the cross-sectional area S _r of the recrystallized portion of the forged product by image analysis.
Ratio of fibrous structure: (S ₀ -S _r ) / S ₀ × 100
The case where the ratio of the fibrous structure was 80% or more and the aspect ratio was 10 or more was rated as ◯, and the other cases were marked as x. Here, the aspect ratio is represented by the ratio of the long side to the short side of the crystal grain. The aspect ratio was measured by observing the cross-sectional macro and then observing the fibrous tissue portion with an optical microscope. The fibrous tissue part was cut out from the part near the outer periphery (close to the recrystallized structure part) and the center, observed with an optical microscope at a magnification of 200 to 100, the long side and the short side length were measured, and the ratio was determined.

3) Stress corrosion cracking resistance A test piece was prepared in accordance with JIS 8711 “Testing method for stress corrosion cracking of aluminum alloy”, stress of 90% of the proof stress value was applied, and an alternate immersion test was performed. In addition, a chromic acid immersion test at about 95 ° C. and a cantilever stress corrosion cracking test were also conducted. In the cantilever stress corrosion cracking test, as shown in FIG. 3, a test piece is set on a piece-like jig and a current is applied to the test piece to forcibly break it. In this test, the test was performed by applying a current of 5 mA / cm ^{2 in} a 25 ° C. saline solution. A sample in which no cracks were observed after 100 hours had elapsed from the start of the stress corrosion cracking test, and a sample in which fine cracks were observed before or after being covered with corrosive organisms were marked with ×. .

  In Examples 1 to 15, the mechanical properties satisfy A6066-T6 tensile strength of 395 MPa or more, the fibrous structure occupancy of the product cross-section after forging is 80% or more, and stress corrosion cracking does not occur. . On the other hand, in Comparative Examples 16 to 25, the components are out of the scope of the present invention, or the heat treatment conditions for solution treatment or artificial aging treatment after forging are out of the specified range, so the mechanical properties of the A6066 alloy (T6) In Comparative Examples 16-19 and 26-30, the standard tensile strength of 395 MPa or more was not satisfied, and the Zr addition amount contributing to the refinement of the structure was below the specified range, or the Z value was outside the control range Therefore, the fiber structure occupation ratio of the product cross-section after forging was 80% or less, and as a result, micro cracks were observed in the stress corrosion cracking test.

  The present invention relates to the addition of Mn and Zr to an Al—Mg—Si based alloy by adding not only Mg and Si, but also Cu, and adding Mn and Zr by adding Cu and the Z factor. It becomes a forged product fibrous structure by controlling. As a result, the alloy of the present invention has characteristics of high strength and excellent stress corrosion cracking resistance, and can be used for parts such as automobile undercarriage to reduce the weight of the automobile. It has a remarkable effect.

1: Aluminum alloy rod 2: Die 3: Fibrous structure part 4: Recrystallized structure part

Claims (2)

Si: 0.8-2.2 mass% (hereinafter referred to as%), Cu: 0.7-1.5%, Mg: 0.8-1.8%, Mn: 0.5-1.1 %, Zr: 0.05 to 0.30%, and the balance Zer-Holomon variable Z is 2.9 × 10 ¹⁰ ≦ Z ≦ 6.6 × 10 ¹¹ , with the balance being aluminum and inevitable impurities. It is a forged product manufactured so that it has excellent stress corrosion cracking resistance, characterized in that a fibrous structure comprising crystal grains having an aspect ratio of 10 or more occupies 80% or more of the cross-sectional area of the forged product Forged product made of high-strength aluminum alloy.   The method for producing a forged product according to claim 1, wherein the extruded material produced by a conventional method is heated to 490 to 510 ° C, and the extruded material is forged with a mold heated to 150 to 200 ° C. The forged product is heated at 530 to 550 ° C. for 3 to 10 hours, then cooled with water or warm water, and further subjected to aging treatment at 170 to 190 ° C. for 6 to 10 hours, which is characterized by resistance to stress corrosion cracking For producing high-strength aluminum alloy forgings with excellent resistance.

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