Patent Application
20240209487
This application claims priority to Japanese Patent Application No. 2022-206214 filed on Dec. 23, 2022, incorporated herein by reference in its entirety.
The present disclosure relates to a cast, hot-worked product of an Al—Mg—Si aluminum alloy, and a method for producing the cast, hot-worked product.
Patent Document 1 discloses a method for producing an Al—Zn—Mg (7000 series) casting material that is resistant to stress corrosion cracking (SCC).
In some cases, conventional aluminum alloys cannot prevent the development of cracks when a certain tensile load is applied in high humidity. In the prior art, stress corrosion cracking (SCC) testing of aluminum alloys has been discussed; however, humid gas stress corrosion cracking (HG-SCC) testing of aluminum alloys has not been discussed.
The present disclosure was achieved in light of the above circumstances. An object of the present disclosure is to provide a cast, hot-worked product of an Al—Mg—Si aluminum alloy which is configured to suppress the development of cracks when a certain tensile load is applied in high humidity. Another object of the present disclosure is to provide a method for producing the cast, hot-worked product.
The cast, hot-worked product of the present disclosure is a cast, hot-worked product of an Al—Mg—Si aluminum alloy, wherein the aluminum alloy comprises Si and Cu, and wherein an Si/Cu ratio, which is a ratio of a mass % of the Si to a mass % of the Cu, is less than 3.3.
In the method for producing the cast, hot-worked product of the Al—Mg—Si aluminum alloy according to the present disclosure, the method may comprise obtaining an ingot by casting the Al—Mg—Si aluminum alloy, and obtaining the cast, hot-worked product by hot-working the ingot, and a reduction of the cast, hot-worked product in the hot working may be 10% or more.
In the present disclosure, the hot working may be hot forging.
According to the present disclosure, a cast, hot-worked product of an Al—Mg—Si aluminum alloy which is configured to suppress the development of cracks when a certain tensile load is applied in high humidity, and a method for producing the cast, hot-worked product are provided.
In the accompanying drawings,
The cast, hot-worked product of the present disclosure is a cast, hot-worked product of an Al—Mg—Si aluminum alloy, wherein the aluminum alloy comprises Si and Cu, and wherein an Si/Cu ratio, which is a ratio of a mass % of the Si to a mass % of the Cu, is less than 3.3.
For downsizing the caps and valves of high pressure hydrogen tanks for next-generation fuel cell electric vehicles, a high-strength aluminum alloy has been under development. In recent years, in the case of using an aluminum alloy other than A6061 material, the aluminum alloy needs to pass four kinds of hydrogen embrittlement tests relating to the materials that can be used in high-pressure hydrogen environments, which are tests specified in the international regulations and specifications that are expected to be established. It has been confirmed that the aluminum alloy passed the following three tests: an atmospheric tension test, a slow strain rate technique (SSRT) tension test and a fatigue test both in high-pressure hydrogen. However, it has not been confirmed that the aluminum alloy passed the remaining HG-SCC test. Existing research reported that aluminum alloys with compositions different from those of the A6061 material can meet the acceptance criteria of the test by controlling their Si and Cu contents within a specified range. However, it was found that the aluminum alloy produced within the specified range sometimes fails the test.
The HG-SCC test is a test for examining whether stress corrosion cracking (SCC) occurs, which is specific to aluminum alloys and occurs when a certain load is applied in high humidity. When an aluminum alloy fails the test, a crescent-shaped fracture surface appears on the aluminum alloy. The crescent-shaped fracture surface is a fracture surface shape that is likely to be formed on a low-toughness material. In the case of a high-toughness material, fracture surfaces extend in parallel. In the HG-SCC test, accordingly, there is not only a large influence of the stress corrosion cracking phenomenon but also a large influence of the toughness of the aluminum alloy. Also, existing research reported that even if a large amount of Si is added to the aluminum alloy, the aluminum alloy can meet the acceptance criteria of the test by adding at least 0.3% of Cu. There is no research on the range of the Si/Cu ratio where the aluminum alloy fails the HG-SCC test, and it is not clear whether all the confirmed cracks are caused by insufficient toughness of the aluminum alloy. There is a possibility that they are cracks produced by the stress corrosion cracking phenomenon.
In the present disclosure, on the assumption that the results of the HG-SCC test are largely influenced by the toughness of the aluminum alloy, the conditions that make it possible to produce a standard product containing Si and Cu components and having the reduction of hot working, both of which have an influence of the toughness of the aluminum alloy, and to suppress the development of cracks when a certain tensile load is applied in high humidity, were found.
In the present disclosure, the method for producing the aluminum alloy was found, in which the length of the cracks developed by applying a certain tensile load to a test piece (TP) in high humidity for 90 days, is equal to or less than a predetermined length.
For improving the toughness of the aluminum alloy, it is effective to employ hot working in the production method. In the present disclosure, it was tried to improve the toughness of the aluminum alloy by the hot working.
The cast, hot-worked product of the present disclosure is a cast, hot-worked product of an Al—Mg—Si aluminum alloy.
The aluminum alloy of the Al—Mg—Si aluminum alloy comprises Si and Cu, and the Si/Cu ratio, which is the ratio of the mass % of the Si to the mass % of the Cu, is less than 3.3. The Si/Cu ratio may be 1.6 or more.
The Al—Mg—Si aluminum alloy used in the present disclosure is a 6000 series aluminum alloy.
The chemical composition (mass %) of the Al—Mg—Si aluminum alloy used in the present disclosure, may be as follows: Mg is from 0.80 mass % to 3.00 mass %; Si is from 0.40 mass % to 1.26 mass %; Cu is from 0.15 mass % to 0.52 mass %; Fe is 0.70 mass % or less; Cr is from 0.04 mass % to 0.35 mass %; Mn is 0.15 mass % or less; Zn is 0.25 mass % or less; Ti is 0.15 mass % or less; and the residual components are aluminum and unavoidable impurities.
Mg is effective in improving mechanical properties. Insufficient Mg leads to poor mechanical properties. Excess Mg leads to a deterioration in castability, forgeability, stress corrosion crack resistance and elongation. Mg may be 1.20 mass % or less.
Si may be 0.79 mass % or more. When the content of Si is too large, a deterioration in mechanical properties occurs.
Cu is effective in improving mechanical properties and stress corrosion crack resistance. Insufficient Cu leads to poor mechanical properties and poor improvement in stress corrosion crack resistance. Excess Cu leads to a deterioration in corrosion resistance and elongation. Cu may be 0.30 mass % or more.
Fe is an impurity that is likely to be included during the refining and casting of aluminum. When the content of Fe is large, a deterioration in mechanical properties occurs. Fe may be 0.35 mass % or less, or it may be 0.25 mass % or less.
In the aluminum alloy of the present disclosure, Cr, Mn, Zn and Ti are optional components.
Cr and Mn are effective in preventing recrystallization in a heating process such as heat treatment, and it is also effective in improving stress corrosion crack resistance and mechanical properties. Excess Cr and Mn lead to a stagnation in these effects. In addition, excess Cr and Mn may lead to an increase in the amount of insoluble compounds and thus to a deterioration in mechanical properties.
Zn is effective in improving mechanical properties. Insufficient Zn leads to poor mechanical properties. Excess Zn leads to increased opportunities for casting cracks and the like and thus to a deterioration in castability, forgeability, stress corrosion crack resistance and elongation.
Ti is effective in reducing the grain size in a solidification microstructure. Insufficient Ti leads to the production of coarse grains and thus to the production of cracks during the casting step and the production of orange peel surfaces during the forging step. Excess Ti leads to a stagnation in the effect. In addition, excess Ti leads to an increase in the amount of an insoluble compound and thus to a deterioration in mechanical properties.
In the present disclosure, the crack length (HG-SCC crack length) of the cast, hot-worked product measured by the humid 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.
In the method for producing the cast, hot-worked product of the Al—Mg—Si aluminum alloy according to the present disclosure, the method may comprise obtaining an ingot by casting the Al—Mg—Si aluminum alloy, and obtaining the cast, hot-worked product by hot working the ingot, and a reduction of the cast, hot-worked product in the hot working may be 10% or more.
In the production method of the present disclosure, the casting step and the hot-working step are conducted in this order.
In the production method of the present disclosure, the casting step, a homogenization treatment step, the hot-working step, and a T6 heat treatment step may be conducted in this order.
The casting step is a step of obtaining an ingot by casting the Al—Mg—Si aluminum alloy.
The Al—Mg—Si aluminum alloy comprises Si and Cu, and the Si/Cu ratio, which is the ratio of the mass % of the Si to the mass % of the Cu, is less than 3.3. The Si/Cu ratio may be 1.6 or more.
The ingot may be a continuous-cast rod.
In the casting step, the casting material may be obtained by melting the aluminum alloy at a temperature of 750° C., and the molten alloy is cast in a metal mold at a temperature of 150° C., and then solidifying the molten alloy in the mold.
The temperature of the molten aluminum alloy is not particularly limited, as long as it is a temperature at which the aluminum alloy can be molten.
The homogenization treatment step is a step of homogenizing the ingot.
In the homogenization treatment, the ingot may be heated in hot state at 430° C. to 470° C. for 5 to 10 hours.
The hot-working step is a step of obtaining the cast, hot-worked product by hot-working the ingot.
The hot working may be extrusion and rolling of the ingot in hot state; it may be hot forging; or both of them may be conducted. In the case of hot forging, a test piece (a cut material) having a given length and a rectangular cross-sectional surface which has a given width and a given thickness, may be cut from the obtained cast material to subject the test piece to forging to a predetermined reduction. A cast, hot-forged product may be obtained by hot forging of the cut material by applying strong pressure thereto in a forging mold having a predetermined shape. The cut material may be forged at a temperature ranging from 300° C. to 470° C., for example. The temperature of the forging mold may be 200° C., for example. In the case of the hot forging, the hot-forging temperature may be equal to or more than the recrystallization temperature, or it may be from 300° C. to 480° C.
The reduction of the cast, hot-worked product in the hot working may be 10% or more, may be 35% or more, or may be 60% or less.
The reduction (%) means the ratio between the thickness reduction quantity of a metallic material in plastic forming and the original thickness of the metallic material.
More specifically, when the ingot having a thickness of A is subjected to hot working and the thickness of the cast, hot-worked product subjected to the hot working is defined as B, the reduction is calculated as follows.
The T6 heat treatment step is a step of T6 heat treatment of the cast, hot-worked product.
In the T6 heat treatment, the cast, hot-worked product is subjected to a solution treatment, quenching, and an artificial aging treatment in this order. By the T6 heat treatment, the strength of the cast, hot-worked product is increased.
The solution treatment is a treatment to uniformly dissolve the added elements of the cast, hot-worked product. In the solution treatment, the cast, hot-worked product may be heated in an air atmosphere for 3 to 6 hours at a temperature which is 440° C. or more and at which the cast, hot-worked product does not partially melt. In the solution treatment, when the temperature is too low, a solid-solution state cannot be made. On the other hand, when the temperature is too high, the cast, hot-worked product partially melts.
In the quenching, the cast, hot-worked product may be quickly cooled down in a water at a temperature ranging from 25° C. to 80° C. after the solution treatment.
The artificial aging treatment is a treatment to precipitate a secondary phase by heating the cast, hot-worked product at a predetermined temperature. In the artificial aging treatment, the cast, hot-worked product may be heated at a temperature ranging from 140° C. to 240° C. for 0.3 to 48 hours after the quenching.
The composition range of the aluminum alloys was different from the typical composition range of A6061 alloy.
Al—Mg—Si aluminum alloys having different Si/Cu ratios shown in Table 1 were prepared. The aluminum alloys were cast in the conditions where no casting cracks occurred; the cast aluminum alloy ingots were homogenized at 470° C. for 7 hours; the homogenized aluminum alloy samples were hot-forged at a temperature ranging from 300° C. to 470° C.; and the hot-forged aluminum alloy samples were subjected to the T6 heat treatment (a solution treatment at 555° C. for 3 hours, quenching in water, and an artificial aging treatment at 180° C. for 6 hours), thereby obtaining T6 samples (cast, hot-worked products). The reductions in the hot forging are shown in Table 1.
Test pieces were produced by cutting the cast, hot-worked products into a predetermined size. In the air environment state, a simple one-day HG-SCC test was conducted on the produced test pieces, without humidity control. After the test, the test pieces were retrieved; the fracture surface of cracks was observed; and the initial crack length (the crack length developed in one day) was evaluated. The evaluation results are shown in Table 2.
HG-SCC tests were conducted on the produced test pieces, and the crack length was evaluated. The evaluation results are shown in Table 2 as “HG-SCC crack length”.
The standard of the HG-SCC test method is HPIS E 103: 2018 “Standard Test Method for Humid Gas Stress Corrosion Cracking of Aluminium Alloys for Compressed Hydrogen Containers”.
Also, the tensile strength, proof stress, applied stress intensity factor and fracture toughness of the produced test pieces in the air environment were measured. The results are shown in Table 2.
Cast, hot-worked products were obtained in the same conditions as Example 1, except that Al—Mg—Si aluminum alloys having Si/Cu ratios shown in Table 1 were prepared and then hot-forged so that the reductions of the cast, hot-worked products were values shown in Table 1. In the same manner as Example 1, test pieces were produced from the obtained cast, hot-worked products, and the initial crack length, HG-SCC crack length, tensile strength, proof stress, applied stress intensity factor and fracture toughness of the test pieces were measured. The results are shown in Table 2.
A cast product was obtained in the same conditions as Example 1, except that an Al—Mg—Si aluminum alloy having a Si/Cu ratio shown in Table 1 was prepared without hot forging. In the same manner as Example 1, a test piece was produced from the obtained cast product, and the initial crack length, HG-SCC crack length, tensile strength, proof stress, applied stress intensity factor and fracture toughness of the test piece were measured. The results are shown in Table 2.
As shown in
In Examples 2, 3 and 5, it is clear that the initial crack length and the HG-SCC crack length are 0 mm, which fulfill the acceptance criterion of the HG-SCC test.
To suppress the development of cracks when a certain tensile load is applied in high humidity to the aluminum alloy, accordingly, it is needed to produce the cast, hot-worked product in at least the following conditions: the Si/Cu ratio is less than 3.3, and the reduction of the hot working is 10% or more.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2022-206214 | Dec 2022 | JP | national |
