JP2024090353A - Hot-worked cast products of Al-Mg-Si aluminum alloys and their manufacturing method - Google Patents

Abstract

[Problem] To provide a hot-worked cast product of an Al-Mg-Si aluminum alloy capable of suppressing crack growth when a constant tensile load is applied in high humidity, and a manufacturing method thereof. [Solution] A hot-worked cast product of an Al-Mg-Si aluminum alloy, the aluminum alloy containing Si and Cu, and having a Si/Cu ratio, which is the ratio of the mass % of Si to the mass % of Cu, of less than 3.3. [Selected Figure] Figure 1

Description

This disclosure relates to cast hot-worked products of Al-Mg-Si-based aluminum alloys and their manufacturing methods.

Patent Document 1 discloses a method for producing Al-Zn-Mg (7000 series) casting material that is resistant to stress corrosion cracking (SCC).

JP 2002-348631 A

Conventional aluminum alloys may not be able to suppress crack growth when a constant tensile load is applied in high humidity. In conventional technology, SCC tests have been considered for aluminum alloys, but HG-SCC (humid gas stress corrosion cracking) tests have not been considered for aluminum alloys.

This disclosure was made in consideration of the above-mentioned circumstances, and its main objective is to provide a hot-worked cast product of an Al-Mg-Si aluminum alloy that can suppress crack propagation when a constant tensile load is applied in high humidity, and a method for manufacturing the same.

The hot-worked cast product of the present disclosure is a hot-worked cast product of an Al-Mg-Si-based aluminum alloy, the aluminum alloy containing Si and Cu, and the Si/Cu ratio, which is the ratio of the mass % of Si to the mass % of Cu, is less than 3.3.

In the present disclosure, there is provided a method for producing a cast hot-worked product of an Al-Mg-Si-based aluminum alloy, the method comprising: casting the Al-Mg-Si-based aluminum alloy to obtain an ingot; and hot working the ingot to obtain the cast hot-worked product, and the reduction ratio of the cast hot-worked product during the hot working may be 10% or more.

In the present disclosure, the hot working may be hot forging.

This disclosure provides a hot-worked cast product of an Al-Mg-Si aluminum alloy that can suppress crack propagation when a constant tensile load is applied in high humidity, and a method for manufacturing the same.

1 is a graph showing the relationship between the Si/Cu ratio and the initial crack length. 1 is a graph showing the relationship between the rolling reduction and the initial crack length.

1. Cast hot-worked product The cast hot-worked product of the present disclosure is a cast hot-worked product of an Al-Mg-Si-based aluminum alloy, the aluminum alloy containing Si and Cu, and a Si/Cu ratio, which is the ratio of the mass% of Si to the mass% of Cu, is less than 3.3.

High-strength aluminum alloys are being developed to miniaturize the nozzles and valve parts of high-pressure hydrogen tanks for next-generation fuel cell vehicles (FCEVs). In recent years, international regulations regarding materials that can be used in high-pressure hydrogen environments are scheduled to be enacted, and when using aluminum alloys other than A6061 materials, they must pass four hydrogen embrittlement investigation tests. Aluminum alloys have already been confirmed to pass three tests: tensile tests in air, slow strain rate tensile delayed fracture tests (SSRT: Slow Strain Rate Technique), which are tensile tests in hydrogen, and fatigue tests in hydrogen, but the remaining HG-SCC test has not been confirmed to pass. Existing research has reported that even aluminum alloys outside the range of 6061 components can meet the test passing conditions if the Si and Cu components are adjusted to within the specified range, but it has been found that even aluminum alloys manufactured within the specified range may fail.

The HG-SCC test is an investigation test to determine whether or not stress corrosion cracking (SCC), which is specific to aluminum alloys, occurs when a certain load is applied in high humidity. On the other hand, crescent-shaped fractures were confirmed in aluminum alloys that failed the test. Crescent-shaped fractures are a fracture shape that is likely to form in materials with low toughness. In materials with high toughness, the fractures extend parallel to one another. Therefore, it is believed that the HG-SCC test is greatly affected not only by the stress corrosion cracking phenomenon, but also by the toughness of the aluminum alloy. In addition, existing investigations have reported that even if a large amount of Si is added to an aluminum alloy, the test passing conditions are met if 0.3% or more Cu is added, but there is no knowledge that has investigated the range of the actual Si/Cu ratio that results in failure, and it is unclear whether all of the confirmed cracks are due to insufficient toughness of the aluminum alloy, and they may have been caused by the stress corrosion cracking phenomenon.

In this disclosure, it is predicted that the results of the HG-SCC test are significantly affected by the toughness of the aluminum alloy, and standard products are produced by varying the hot working reduction rate and the Si and Cu components, which affect the toughness of the aluminum alloy, and conditions are found that can suppress crack propagation when a constant tensile load is applied in high humidity.
In the present disclosure, a method for producing an aluminum alloy has been discovered in which a constant tensile load is applied to a test piece (TP) in high humidity for 90 days, and the length of the crack that grows is equal to or less than a predetermined length.
In order to improve the toughness of an aluminum alloy, it is effective to perform hot working during the manufacturing process, and in this disclosure, an attempt is made to improve the toughness of an aluminum alloy by hot working.

The cast, hot-worked product of the present disclosure is a cast, hot-worked product of an Al-Mg-Si based aluminum alloy.
The Al-Mg-Si-based aluminum alloy contains Si and Cu, and has a Si/Cu ratio, which is a ratio of the mass% of Si to the mass% of Cu, of less than 3.3. The Si/Cu ratio may be 1.6 or more.
The Al-Mg-Si based aluminum alloy used in this disclosure is a 6000 series aluminum alloy.
The chemical components (mass %) of the Al-Mg-Si-based aluminum alloy used in the present disclosure may be 0.80-3.00 mass % Mg, 0.40-1.26 mass % Si, 0.15-0.52 mass % Cu, 0.70 mass % or less Fe, 0.04-0.35 mass % Cr, 0.15 mass % or less Mn, 0.25 mass % or less Zn, 0.15 mass % or less Ti, with the balance being aluminum and unavoidable impurities.

Mg: 0.80-3.00% by mass
Mg is effective in improving mechanical properties. If Mg is insufficient, the mechanical properties are insufficient. If Mg is excessive, castability, forgeability, stress corrosion cracking resistance, and elongation are reduced. The Mg content may be 1.20 mass% or less.

Si: 0.40-1.26% by mass
The Si content may be 0.79 mass % or more. If the Si content is too high, the mechanical properties are deteriorated.

Cu: 0.15-0.52% by mass
Cu is effective in improving mechanical properties and stress corrosion cracking resistance. If Cu is insufficient, the mechanical properties are insufficient and the improvement of stress corrosion cracking resistance is insufficient. If Cu is excessive, the corrosion resistance and elongation are reduced. Cu may be 0.30 mass% or more.

Fe: 0.70% by mass or less Fe is an impurity that is easily mixed in during the process of refining and casting aluminum, and if the content is high, it deteriorates the mechanical properties. The Fe content may be 0.35% by mass or less, or may be 0.25% by mass or less.

Cr: 0.04-0.35% by mass, Mn: up to 0.15% by mass, Zn: up to 0.25% by mass, Ti: up to 0.15% by mass Cr, Mn, Zn, and Ti are optional components in the aluminum alloy of the present disclosure.
Cr and Mn are effective in preventing recrystallization during heating such as heat treatment, and are also effective in improving stress corrosion cracking resistance and mechanical properties. If Cr and Mn are excessive, the effect reaches a plateau, and insoluble compounds increase, which can deteriorate mechanical properties.
Zn is effective in improving mechanical properties. If the amount of Zn is insufficient, the mechanical properties will be insufficient. If the amount of Zn is excessive, casting cracks will easily occur, and castability, forgeability, stress corrosion cracking resistance, and elongation will decrease.
Ti is effective in making the crystal grains of the solidification structure finer. If there is insufficient Ti, the crystal grains become coarse, causing cracks during casting and rough surfaces during forging. If there is too much Ti, the effect reaches a plateau and insoluble compounds increase, deteriorating mechanical properties.

In the present disclosure, the crack length (HG-SCC crack length) of a cast hot-worked product measured by a wet gas stress corrosion cracking (HG-SCC) test may be less than 1.57 mm, may be 1.27 mm or less, may be 0.92 mm or less, may be 0.50 mm or less, or may be 0.16 mm or less.

2. Manufacturing Method of Cast Hot Worked Product The present disclosure relates to a manufacturing method of a cast hot worked product of an Al-Mg-Si based aluminum alloy, the method comprising: casting the Al-Mg-Si based aluminum alloy to obtain an ingot; and hot working the ingot to obtain the cast hot worked product, wherein a rolling reduction rate of the cast hot worked product in the hot working may be 10% or more.

The manufacturing method of the present disclosure is carried out in the order of a casting step and a hot working step.
The manufacturing method of the present disclosure may be carried out in the following order: casting, homogenization, hot working, and T6 heat treatment.

The casting step is a step of casting an Al-Mg-Si based aluminum alloy to obtain an ingot.
The Al-Mg-Si-based aluminum alloy contains Si and Cu, and a Si/Cu ratio, which is a ratio of the mass% of Si to the mass% of Cu, may be less than 3.3 and may be 1.6 or more.
The ingot may be a continuously cast rod.
In casting, the aluminum alloy may be melted at a melting temperature of 750° C., the molten metal may be cast into a metal mold having a mold temperature of 150° C., and solidified to obtain a cast material.
The melting temperature of the aluminum alloy is not particularly limited as long as it is a temperature at which the aluminum alloy can be melted.

The homogenization treatment step is a step of homogenizing the ingot.
The homogenization treatment may involve heating the ingot to a hot state at 430 to 470° C. for 5 to 10 hours, for example.

The hot working step is a step of hot working the ingot to obtain the hot-worked cast product.
The hot working may be hot extrusion or rolling of the ingot, hot forging, or both. In the case of hot forging, a test piece of a predetermined length with a rectangular cross section of a predetermined width and thickness may be cut out from the obtained cast material to apply forging with a predetermined rolling reduction rate, and a cut-out material may be obtained. The cut-out material may be hot forged by strong pressure in a forging die of a predetermined shape to obtain a cast hot forged product. The forging temperature of the cut-out material may be, for example, 300 to 470°C, and the die temperature of the forging die may be, for example, 200°C. The hot forging temperature in the case of hot forging may be equal to or higher than the recrystallization temperature, and may be 300 to 480°C.
In the hot working, the reduction ratio (also referred to as the rolling ratio) of the cast hot worked product may be 10% or more, 35% or more, or 60% or less.
The rolling reduction rate (%) refers to the ratio of the amount of reduction in the thickness of a metal material when the metal material is plastically processed to the original thickness of the metal material.
Specifically, when hot working is performed on an ingot having a thickness A, and the thickness of the cast hot-worked product after hot working is B, the rolling reduction is calculated as follows.
Reduction rate (%) = {(A-B)/A} * 100

The T6 heat treatment step is a step of subjecting the cast hot-worked product to T6 heat treatment.
The T6 heat treatment is a process in which a cast hot-worked product is subjected to solution treatment, quenching, and artificial aging in that order. The T6 heat treatment can increase the strength of the cast hot-worked product.
Solution treatment is a process for uniformly dissolving the additive elements in the cast hot-worked product. In solution treatment, the cast hot-worked product may be heated and held in an air atmosphere at 440°C or higher and at a temperature at which the product does not partially melt for about 3 to 6 hours. If the temperature in solution treatment is too low, a solid solution state cannot be created, and if the temperature is too high, the product will partially melt.
In the quenching, the cast hot-worked product may be quenched in water at 25 to 80° C. after the solution treatment.
Artificial aging is a process in which a cast hot-worked product is heated to a predetermined temperature to precipitate a secondary phase. In the artificial aging process, the cast hot-worked product may be heated and held at 140 to 240°C for 0.3 to 48 hours after quenching.

Example 1
[Evaluation of initial crack length]
The range of components of the aluminum alloy was different from that of the A6061 alloy.
An Al-Mg-Si aluminum alloy having the Si/Cu ratio shown in Table 1 was prepared. The aluminum alloy was cast under conditions where no casting cracks would occur, homogenized at 470°C for 7 hours, hot forged at 300 to 470°C, and subjected to T6 heat treatment, which included solution treatment at 555°C for 3 hours, quenching in water, and artificial aging at 180°C for 6 hours, to obtain a T6 material of a cast hot-worked product. The reduction ratio during hot forging is shown in Table 1.
The cast hot-worked product was cut to a predetermined size to prepare a test specimen. A simple HG-SCC test was performed on the prepared test specimen for one day in an atmospheric environment without humidity control. After the test, the test specimen was removed, the cracked surface was observed, and the initial crack length (the crack length that grew in one day) was evaluated. The evaluation results are shown in Table 2.
[HG-SCC test]
The test pieces thus prepared were subjected to an HG-SCC test to evaluate the crack length. The evaluation results are shown in Table 2 as HG-SCC crack length.
The standard for the HG-SCC (wet gas stress corrosion cracking) test method is HPIS E 103:2018 "Standard test method for wet gas stress corrosion cracking of aluminum alloys for compressed hydrogen containers," a standard created by the High Pressure Engineering Society of Japan.
[Other tests]
The tensile strength, yield strength, applied stress, and fracture toughness of the prepared test pieces in an atmospheric environment were measured. The results are shown in Table 2.

(Examples 2 to 6, Comparative Examples 2 to 4)
An Al-Mg-Si aluminum alloy having a Si/Cu ratio shown in Table 1 was prepared, and a cast hot-worked product was obtained under the same conditions as in Example 1, except that hot forging was performed so that the rolling reduction of the cast hot-worked product was the value shown in Table 1. Test pieces were prepared from the obtained cast hot-worked product in the same manner as in Example 1, and the initial crack length, HG-SCC crack length, tensile strength, proof stress, load stress, and fracture toughness value were measured. These results are shown in Table 2.

(Comparative Example 1)
An Al-Mg-Si aluminum alloy having the Si/Cu ratio shown in Table 1 was prepared, and a casting was obtained under the same conditions as in Example 1, except that hot forging was not performed. Test pieces were prepared from the obtained casting in the same manner as in Example 1, and the initial crack length, HG-SCC crack length, tensile strength, proof stress, load stress, and fracture toughness value were measured. These results are shown in Table 2.

[Evaluation results]
FIG. 1 is a graph showing the relationship between the Si/Cu ratio and the initial crack length.
FIG. 2 is a graph showing the relationship between the rolling reduction and the initial crack length.
1 and 2 and Tables 1 and 2, in Comparative Example 1 with a rolling reduction of 0%, the initial crack length is large regardless of the influence of the Si/Cu ratio, and the HG-SCC crack length also exceeds 0.16 mm. In Comparative Example 2 with a rolling reduction of 10%, the initial crack length is large because the Si/Cu ratio is high at 4.0. In Comparative Example 3 with a rolling reduction of 60%, the initial crack length is large because the Si/Cu ratio is high at 4.0.
Moreover, in Examples 2, 3, and 5, the initial crack length and the HG-SCC crack length were 0 mm, and it was found that these satisfied the pass criteria for the HG-SCC test.
Therefore, in order to suppress crack propagation in an aluminum alloy when a constant tensile load is applied in high humidity, it is necessary to form a cast hot-worked product under conditions in which the Si/Cu ratio is at least less than 3.3 and the hot working reduction is 10% or more.

Claims (3)

A hot-worked cast product of an Al-Mg-Si-based aluminum alloy, the aluminum alloy containing Si and Cu, and having a Si/Cu ratio, which is the ratio of the mass % of Si to the mass % of Cu, of less than 3.3. A method for producing a hot-worked cast product of the Al-Mg-Si aluminum alloy according to claim 1, comprising the steps of: casting the Al-Mg-Si aluminum alloy to obtain an ingot; and hot working the ingot to obtain the hot-worked cast product, wherein the reduction ratio of the hot-worked cast product is 10% or more. The method for producing a cast hot-worked product according to claim 2, wherein the hot working is hot forging.

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