JP2024138817A - High-strength aluminum alloy material with excellent SCC resistance and manufacturing method - Google Patents

Abstract

To provide an aluminum alloy material having excellent SCC resistance despite high strength of tensile strength 650 MPa or more, and a manufacturing method of the alloy.SOLUTION: A casting material uses aluminum alloy which consists of Zn:8.0% to 10.0%,Mg:1.5 to 3.5%,Cu:0.20 to 2.50%,Zr:0.15 to 0.25% in mass% and balance in Al and inevitable impurities, where an average crystal grain size is controlled to 50 μm or less by homogenizing treatment.SELECTED DRAWING: Figure 1

Description

The present invention relates to an aluminum alloy material that has excellent stress corrosion cracking resistance (SCC resistance) and high strength, and a manufacturing method thereof.

In the fields of vehicles and industrial machinery, there is a demand for weight reduction, and therefore Al-Zn-Mg high strength materials are being investigated.
Furthermore, structural members used in vehicles and industrial machinery are sometimes subjected to large stresses, and therefore are required to have excellent stress corrosion cracking resistance (SCC resistance).
However, in the case of Al-Zn-Mg alloy materials, the higher the strength, the more likely it is that the lower the SCC resistance will be.

For example, Patent Document 1 describes an extruded material using an Al-Zn-Mg alloy, in which the thickness of the surface recrystallization is 7% or less of the wall thickness and the average grain size of the surface recrystallization is 150 μm or less, thereby improving stress corrosion cracking resistance.
However, the strength is at a level of about 450 MPa, which is not sufficiently high.

Patent No. 2928445

The present invention aims to provide an aluminum alloy material and manufacturing method that has high strength (tensile strength of 650 MPa or more) and excellent SCC resistance.

In the present invention, the aluminum alloy casting material for use in extrusion processing is a casting material using an aluminum alloy containing, in mass %, the following: Zn: 8.0% to 10.0%, Mg: 1.5 to 3.5%, Cu: 0.20 to 2.50%, Zr: 0.15 to 0.25%, with the balance being Al and unavoidable impurities, and is characterized in that the average crystal grain size is controlled to 50 μm or less by homogenization treatment.
In this way, when a cast material having an average crystal grain size of 50 μm or less is extruded under the processing conditions described below, a high-strength aluminum alloy material having excellent SCC resistance can be obtained.

Such casting materials are obtained by using molten metal of the above composition at 710-750°C, casting it into a round bar shape (billet), and then homogenizing it at 450-500°C for 10-30 hours.

The manufacturing method of the high-strength aluminum alloy material with excellent SCC resistance according to the present invention is characterized in that the extruded material is extruded using the aluminum alloy casting material described in claim 1, and then subjected to solution treatment and quenching treatment at 450 to 550°C, followed by three-stage aging treatment.

The purpose of the solution treatment is to maintain the average crystal grain size at 50 μm or less while sufficiently resolving the crystallized substances that precipitated during the extrusion process. Then, a quenching treatment is performed by water cooling or the like, and then a three-stage aging treatment (artificial aging treatment) is performed in a temperature range of 100 to 180° C.
For example, it is preferable to form fine nuclei at a relatively low temperature of 100 to 130°C in the first stage, then conduct a second heat treatment at a high temperature of 140 to 180°C, and further conduct a third heat treatment at 100 to 130°C.

In the present invention, the extruded aluminum alloy material is characterized in that, in high-magnification observation of the metal structure, the number density of dispersed particles at the nm level within crystal grains is 13,000 particles/μm2 or ^more , and the number density of dispersed particles at grain boundaries is 0.01 particles/nm or less, and as a result, a tensile strength of 650 MPa or more and a 0.2% yield strength of 600 MPa or more can be obtained.

The composition of the aluminum alloy will be described.
<Zn and Mg Components>
Zn does not reduce extrudability even at relatively high concentrations and contributes to improving strength, and the addition of Mg causes _MgZn2 to precipitate in the structure, increasing strength.
However, if the amount of Mg added is large, the extrudability decreases, and the amount of _MgZn2 precipitated becomes too large, which may decrease the toughness.
Therefore, a combination of Zn: 8.0 to 10.0% and Mg: 1.5 to 3.5% is preferable.
<Cu Component>
The addition of Cu is effective in improving strength due to the solid solution effect, but if the amount added is too large, general corrosion resistance decreases, so the Cu content is preferably in the range of 0.20 to 2.50%.
<Zr Component>
The Zr component is a transition element, which is effective in suppressing the depth of recrystallization formed on the surface of the extruded material during extrusion processing and in making the crystal grains finer.
This improves stress corrosion cracking resistance.
In the present invention, Zr is added in an amount of 0.15 to 0.25%.
＜Other＞
The Ti component is effective in refining crystal grains when a billet for use in extrusion is cast, and a very small amount of B is also generally added.
Ti: A small amount of 0.005 to 0.05% is sufficient.
In the casting process of 7000 series aluminum alloys, Fe and Si are often contained as impurities. If the amounts are large, they affect the extrudability, stress corrosion cracking resistance, etc., so it is preferable to restrict Fe to 0.2% or less and Si to 0.1% or less.
In the present invention, the unavoidable impurities refer to other components that may be mixed in to the extent that they do not affect the SCC resistance and strength, and may be contained in an amount of 0.1% or less.

The present invention uses a cast material made of an aluminum alloy with a specified chemical composition, and performs a three-step aging treatment after solution treatment and quenching, thereby obtaining a high-strength aluminum alloy material while maintaining excellent SCC resistance.

The composition of the aluminum alloy used in the test evaluation is shown below, with the balance of the components shown in the table of FIG. 1 being aluminum and unavoidable impurities. The manufacturing conditions for the round bar (billet) are shown below. The extrusion and heat treatment conditions are shown. The evaluation results are shown.

Molten aluminum alloys having the compositions shown in the table of FIG. 1 were prepared, and round bar blanks shown in the table of FIG. 2 were cast using metal molds and comparatively evaluated.
The components shown in the table of FIG. 1 are expressed as the content in the aluminum alloy in mass %, with the remainder being Al and unavoidable impurities.

As shown in the table of FIG. 2, the molten metal temperature was adjusted to 720° C., poured into a metal mold, and gravity casting was performed.
As long as a material suitable for extrusion processing can be obtained, the method is not limited to gravity casting.
The round bar used in the evaluation this time had a diameter of 30 mm and a length of 40 mm, and was subjected to homogenization treatment at a HOMO (homogenization) holding temperature of 470° C. and a HOMO holding time of 24 hours.
When the alloy composition according to the present invention was used, the average grain size of the round bar material was 50 μm or less.

Next, under the conditions shown in the table of FIG. 3, the extruded material was extruded to a thickness of 2 mm and a width of 20 mm, and then subjected to solution treatment, quenching treatment, and artificial aging treatment.
The extrusion conditions were as follows: the temperature of the round bar material was preheated to 370° C., and the material was extruded.
The temperatures of the extruded shapes (extruded materials) were as shown in the table of FIG.
The solution treatment conditions were as follows: the extruded material was held at 475° C. for 60 minutes, and then quenched by water cooling (WQ).

In Examples 1 and 2, a three-stage artificial aging treatment was performed, which included a first stage (1st stp) of heat treatment at 120°C for 100 min, a second stage (2nd stp) of heat treatment at 170°C for 30 min, and a third stage (3rd stp) of heat treatment at 120°C for 100 min.
In Comparative Examples 1 to 12, one-step or two-step artificial aging treatment was performed under the conditions shown in the table of FIG.

The evaluation results are shown in the table of FIG.
The evaluation method is as follows.
<Mechanical properties>
Based on JIS-Z2241, JIS-5 test pieces were prepared, and the tensile strength, σ _0.2 yield strength, and elongation were measured using a tensile tester conforming to the JIS standard.
<Microstructure>
The sample surface was mirror-polished and etched with Keller's reagent.
The metal structure was observed by optical microscopy, and the image at 100 times magnification was processed to determine the average crystal grain size.
<Stress corrosion cracking resistance (SCC resistance)>
The test piece was subjected to a stress of 80% of its yield strength, and the following conditions were counted as one cycle. If no cracks occurred after 720 cycles, the target was achieved.
If cracks occurred during testing, the number of cycles at which they occurred was recorded.
[1 cycle]
The sample is immersed in a 3.5% NaCl aqueous solution at 25° C. for 10 minutes, then left to stand at 25° C. and a humidity of 40% for 50 minutes, and then naturally dried.
<Nanostructure>
Using a TEM or the like, the structure was observed at a magnification of 10,000 times or more, and the number density (number/ ^μm2 ) of nanometer-level particles dispersed within the crystal grains and the number density (number/nm) of nanometer-level particles present at the crystal grain boundaries were measured.

The table in FIG. 4 shows the target values of the present invention for each evaluation item.
Examples 1 and 2 had high strength, such as a tensile strength of 650 MPa or more and a 0.2% yield strength of 600 MPa or more, while also having SCC resistance of 720 cycles or more.
This nanostructure had an intragranular number density of 130,000 particles/μm2 or ^more and a grain boundary number density of 0.01 particles/nm or less.
In contrast, Comparative Example 1 contained a small amount of Zn and did not contain Zr, and Comparative Example 2 contained a small amount of Zn, so that the targets for both strength and SCC resistance were not achieved.
Comparative Example 3 had a low Zn content, and after two-stage artificial aging, the strength and SCC resistance did not reach the targets.
In Comparative Example 4, Zn was not added, and the strength did not reach the target.
In Comparative Example 5, neither Cu nor Zr was added, and the targets for yield strength and SCC resistance were not achieved.
Although the alloy compositions of Comparative Examples 6 to 10 were within the range of the present invention, the artificial aging treatment was one or two steps, and therefore the target SCC resistance was not achieved.
Comparative Examples 11 and 12 had a high Zn content and had sufficient strength, but did not achieve the target SCC resistance.

Claims (4)

A casting material using an aluminum alloy containing, in mass%, Zn: 8.0% to 10.0%, Mg: 1.5 to 3.5%, Cu: 0.20 to 2.50%, Zr: 0.15 to 0.25%, and the balance being Al and unavoidable impurities,
An aluminum alloy casting material, characterized in that the average crystal grain size is controlled to 50 μm or less by homogenization treatment. A method for producing a high-strength aluminum alloy material with excellent SCC resistance, comprising extruding the aluminum alloy casting material according to claim 1, subjecting the extruded material to solution treatment and quenching treatment at 450 to 550°C, and then performing a three-stage aging treatment. 3. The method for producing a high-strength aluminum alloy material with excellent SCC resistance according to claim ² , characterized in that, in high-magnification observation of the metal structure, the number density of dispersed particles at the nm level within crystal grains is 13,000 particles/μm2 or more, and the number density of dispersed particles at grain boundaries is 0.01 particles/nm or less. The method for manufacturing high-strength aluminum alloy material with excellent SCC resistance described in claim 3, characterized in that the tensile strength is 650 MPa or more and the 0.2% yield strength is 600 MPa or more.

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