TWI888188B - 7xxx aluminum alloy, method of forming the same, and structure material of automobile - Google Patents

Abstract

A forming process for 7XXX aluminum alloys is developed. The process combined process condition of an aluminum manufacturer and a forming plant, thereby adjusting alloy composition to satisfy 10 wt%＜Zn+Mg+4Mn+Cu＜12 wt% without changing condition of paint baking. Subsequently, a suitable process is applied to obtain an aluminum coil with specific thickness. Then, a forming and aging process is performed by controlling a temperature of a mold to cool down a high-temperature aluminum sheet to a nucleation temperature range. As such, precipitation of undesired cluster GPI may be avoided. Then, a crystal nucleus stabilization treatment is performed to ensure that suitable cluster GPII, which is advantage for bake-hardening, can be grown. Therefore, room-temperature inventory time can be extended to more than half year, and strength of the aluminum alloy can meet safety requirements after baking.

Description

7XXX series aluminum alloy, its manufacturing method and automobile structural material

The present invention relates to an aluminum alloy and a method for manufacturing the same, and in particular to an aluminum alloy used as an automobile component and a method for manufacturing the same.

Energy conservation and carbon reduction have made the issue of automobile lightweighting continue to heat up, and the application of aluminum alloys has gradually extended from exterior components to internal components. Among them, the specific strength of 7XXX series aluminum-zinc-magnesium-copper (Al-Zn-Mg-Cu) aviation aluminum alloy is no less than that of 15B22 hot stamping steel. It can not only replace steel in the car to protect the driver and passengers' safety, but also further reduce the weight of the car. Therefore, the penetration rate of aluminum in vehicles has increased year by year. However, the domestic demand and economic scale of Taiwan's automobile or electric vehicle industry are relatively small, and most of them are exported. The parts produced need to be transported by sea to designated car factories around the world. According to the material usage habits of car manufacturers, different car models result in different component designs, so they will not be assembled immediately, but will be produced together according to the manufacturer and accumulated in sufficient quantity, resulting in a continuous extension of room temperature storage time.

Unfortunately, the 7XXX series Al-Zn-Mg-Cu aluminum alloy is a precipitation hardening aluminum alloy, which is prone to aging hardening at room temperature, and the hardness will increase with the storage time. When the paint treatment is applied after the vehicle is assembled, the baking strength increase will be insufficient and fail to meet the safety regulations. Therefore, most advanced aluminum factories adopt the strategy of being close to car factories. For example, Japan has set up factories near North America, Mexico, China and Thailand. However, the investment amount of this move is too high and is not suitable for the attributes of Taiwan's industrial ecology (Taiwan has no automobile industry chain, and most of them need to be sent to various parts of the world for assembly).

In addition, the room temperature formability of 7XXX series aluminum alloys is generally not good. For structural sheets with large strokes and complex shapes, they are prone to cracking, so it is necessary to increase the forming temperature to overcome this problem. For example, the known technologies with patent publication numbers WO 2008059242A2, WO 2016067045A1 and CN 102216484A integrate high-temperature solid solution and processing into one by integrating the process, and then transfer the aluminum material in a high temperature state to the mold (cold mold) for simultaneous forming and quenching. In addition, the formed components need to be subjected to lengthy artificial aging heat treatment in accordance with the specifications of AMS-2772, such as at a temperature of about 121°C for 24 hours, in order to improve the strength. However, this is not only time-consuming and energy-consuming, but also increases carbon production. It also occupies a large amount of furnace space, which reduces productivity. In addition, after being assembled into a car, it still needs to be painted, which softens the strength and ultimately fails to meet safety requirements, making it impossible to ship smoothly.

In order to shorten the problem of too long artificial aging time, Chinese patent publication number CN 110191970A adopts the metallurgical concept, and immediately conducts low-temperature and short-time heat treatment on the components after high-temperature forming and rapid cooling to generate sufficient nuclei, so that the strength can be continuously improved when the car factory is baking paint, thereby replacing the lengthy aging treatment. The disadvantage is that the types of nuclei formed are more diverse, and a considerable proportion of undesirable atomic groups GPI are naturally mixed, which is easy to derive the problem of inventory hardening. If the shipping or inventory time is a little longer, the strengthening effect after baking paint will be greatly reduced and not meet the standards, greatly increasing the instability of product properties.

In addition, the Chinese patent publication number CN 104959437A preheats the mold first, heats the temperature of the die to 160℃ to 250℃, and keeps the temperature of the punch at room temperature, and places the 6XXX series-T4 sheet with a thickness of 0.8mm to 1.5mm on the mold without heating for forming. After the forming is completed, the mold needs to be pressure-maintained, and the pressure-maintaining process does not exceed 5 minutes. Then the formed sheet is taken out and air-cooled to room temperature to obtain the main strengthening phase. The disadvantage is that in order to increase the strength of the sheet during the subsequent baking, it is necessary to maintain pressure for about 5 minutes after the mold is formed, but too long a pressure-maintaining time will reduce the production rate. In addition, the raw material of the aforementioned patent is required to be in a heat-treated T4 state, which is different from the general cold-rolled material of the present invention, so it is very different from the design concept of this case.

Taiwan Patent Announcement No. TW I728287B can effectively solve the above-mentioned problems of artificial aging and storage hardening at the same time, but its design is aimed at aluminum coils produced by aluminum factories, not the requirements of aluminum factories, car factories or OEM molding factories at the same time as the present invention. Furthermore, its strength is relatively low, only half of the standard requirements, so it cannot meet the safety requirements.

In view of this, the present invention integrates the processes of aluminum factories, first-tier suppliers (tier 1) in the automotive industry, and equipment manufacturers. Without increasing the cost burden, the ratio of the main strengthening elements is appropriately adjusted to 10wt% < Zn + Mg + 4Mn + Cu < 12wt%, and after the necessary production process, an aluminum coil with a thickness of 1mm to 4mm is obtained. Then, in the forming plant, high-temperature forming is used with the appropriate mold temperature to adjust the cooling rate, so that the aluminum material can be cooled to the optimal nucleation temperature range of the strengthening phase (for example, 100℃ to 150℃), avoiding direct cooling to room temperature, which will produce undesirable atomic group GPI precipitation that weakens the strength of the paint. In addition, considering the annual production requirements, the mold closing and pressure holding time must not exceed 15 seconds. Therefore, the subsequent crystal nucleus stabilization treatment at a temperature of 120℃ to 160℃ for 60 minutes to 300 minutes is required to ensure that the appropriate atomic group GPII that is conducive to baking strengthening can grow smoothly, so as to achieve the purpose of changing the type and size of the crystal nucleus, thereby improving the stability of the material, and thus extending the room temperature storage time to more than half a year. After the car factory paint, the strength of the aluminum alloy can be further improved to the safety requirements, which is enough to meet the various needs of the global automotive industry.

One aspect of the present invention is to provide a method for manufacturing an aluminum alloy, which prolongs the room temperature storage time of the aluminum alloy by controlling the composition of the aluminum blank and performing a crystal nucleus stabilization treatment.

Another aspect of the present invention is to provide an aluminum alloy, which is prepared using the above aspect.

Another aspect of the present invention is to provide an automobile structural material, which comprises the aluminum alloy of the above aspect.

According to one aspect of the present invention, a method for manufacturing an aluminum alloy is provided, which includes providing an aluminum blank; performing a rolling operation on the aluminum blank to obtain a cold-rolled aluminum coil; performing a forming operation on the cold-rolled aluminum coil to obtain a formed aluminum sheet; and performing a crystal nucleus stabilization treatment on the formed aluminum sheet to obtain an aluminum alloy. Based on 100wt% of the aluminum billet, the aluminum billet comprises 3.5wt% to 7.5wt% of zinc, 1.0wt% to 4.0wt% of magnesium, no more than 3.0wt% of copper, no more than 0.5wt% of silicon, no more than 0.5wt% of iron, no more than 0.3wt% of titanium, no more than 0.5wt% of manganese, no more than 1.5wt% of impurities and a balance of aluminum, and the zinc (Zn), magnesium (Mg), copper (Cu) and manganese (Mn) in the aluminum billet satisfy the following formula: 10wt%<Zn+Mg+4Mn+Cu<12wt%. The forming operation comprises placing the cold rolled aluminum coil in a mold, and the mold clamping temperature is 100°C to 150°C. The stabilization treatment temperature of the aforementioned nucleus stabilization treatment is 120°C to 160°C, and the stabilization treatment time of the nucleus stabilization treatment is 60 minutes to 300 minutes. The aluminum alloy is a 7XXX series aluminum alloy.

According to one embodiment of the present invention, before the rolling operation, the aluminum blank is subjected to a two-stage homogenization treatment, wherein the two-stage homogenization treatment comprises a first stage homogenization treatment, wherein the treatment temperature of the first stage homogenization treatment is 465°C to 485°C, and the treatment time of the first stage homogenization treatment is 12 hours to 36 hours; and a second stage homogenization treatment, wherein the treatment temperature of the second stage homogenization treatment is 470°C to 500°C, and the treatment time of the second stage homogenization treatment is 12 hours to 48 hours.

According to one embodiment of the present invention, the above-mentioned rolling operation includes hot rolling the aluminum billet to obtain a hot-rolled aluminum coil; and cold rolling the hot-rolled aluminum coil to obtain a cold-rolled aluminum coil.

According to one embodiment of the present invention, the thickness of the cold-rolled aluminum coil is 1 mm to 4 mm.

According to one embodiment of the present invention, before the cold-rolled aluminum coil is placed in the mold, the forming operation further includes heating the cold-rolled aluminum coil to 400°C to 500°C.

According to one embodiment of the present invention, after the cold-rolled aluminum coil is placed in the mold, the forming operation further includes a pressure holding operation on the cold-rolled aluminum coil, wherein the pressure holding time of the pressure holding operation is not less than 10 seconds.

According to one embodiment of the present invention, the above method further includes performing a paint baking treatment on the aluminum alloy, wherein the paint baking temperature of the paint baking treatment is 170°C to 205°C, and the paint baking time of the paint baking treatment is 20 minutes to 30 minutes.

According to one embodiment of the present invention, the above-mentioned aluminum alloy has a storage time of no less than six months before being subjected to the paint treatment.

According to another aspect of the present invention, an aluminum alloy is provided, which is produced by the aluminum alloy manufacturing method described in the above aspect.

According to another aspect of the present invention, an automobile structural material is provided, which comprises the aluminum alloy of the above aspect.

The aluminum alloy and its manufacturing method of the present invention are applied to control the composition of the aluminum blank and the conditions of the crystal nucleus stabilization treatment to precipitate specific atomic groups, thereby improving the material stability and extending the room temperature storage time of the aluminum alloy, and having a strength that meets safety standards.

100:Methods

110,120,130,140: Operation

The following detailed description and accompanying drawings will provide a better understanding of the present disclosure. It should be noted that, as is standard practice in the industry, many features are not drawn to scale. In fact, for the sake of clarity of discussion, the dimensions of many features may be arbitrarily scaled.

[Figure 1] is a flow chart showing a method for manufacturing an aluminum alloy according to some embodiments of the present invention.

As used in the present invention, "around", "about", "approximately" or "substantially" generally means within 20%, within 10%, or within 5% of the stated value or range.

Aluminum alloys (such as 7XXX series aluminum alloys) have become one of the main materials for lightweight electric vehicles and fuel vehicles due to their high strength and performance comparable to 1500MPa grade hot stamping steel. However, aluminum alloy materials have the disadvantage of natural aging hardening when stored at room temperature, but car factories usually require a long time of transportation and concentration before assembly. The problem of natural aging hardening will cause the strength of the aluminum alloy to decrease after subsequent baking paint treatment, which does not meet safety regulations. Therefore, the present invention provides an aluminum alloy and a method for manufacturing the same, which controls the composition ratio of the aluminum embryo and the conditions of the crystal nucleus stabilization treatment to precipitate specific atomic groups, thereby improving the material stability and extending the room temperature storage time of the aluminum alloy, and can have a strength that meets safety regulations.

Please refer to FIG. 1, which is a flow chart of a method 100 for manufacturing an aluminum alloy according to some embodiments of the present invention. First, an operation 110 is performed to provide an aluminum blank. In some embodiments, the composition of the aluminum blank is consistent with the composition of a 7XXX series aluminum alloy. In some embodiments, based on 100wt% of the aluminum blank, the aluminum blank contains 3.5wt% to 7.5wt% of zinc, 1.0wt% to 4.0wt% of magnesium, not more than 3.0wt% of copper, not more than 0.5wt% of silicon, not more than 0.5wt% of iron, not more than 0.3wt% of titanium, not more than 0.5wt% of manganese, not more than 1.5wt% of impurities, and a balance of aluminum. In the aforementioned embodiment, zinc (Zn), magnesium (Mg), copper (Cu) and manganese (Mn) in the aluminum blank satisfy the following formula: 10wt% < Zn + Mg + 4Mn + Cu < 12wt%. If the aluminum blank does not meet the aforementioned conditions, for example, the aforementioned sum is less than or equal to 10wt%, too many coarse herringbone-shaped particles are easily generated, making the aluminum alloy produced subsequently not impact-resistant and easy to break; on the contrary, if the aforementioned sum is greater than or equal to 12wt%, too many AlCuMg phases will be formed, consuming a large amount of strengthening atoms, thereby causing the strength and safety of the aluminum alloy produced subsequently to be insufficient.

Next, operation 120 is performed to perform a rolling operation on the aluminum blank to obtain a cold-rolled aluminum coil. In some embodiments, the rolling operation includes first performing a hot rolling operation on the aluminum blank to obtain a hot-rolled aluminum coil; and performing a cold rolling operation on the hot-rolled aluminum coil to obtain a cold-rolled aluminum coil. In some embodiments, the finished rolled thickness of the cold-rolled aluminum coil is about 1 mm to about 4 mm, but the finished rolled thickness still depends on the product requirements, so the present invention is not limited thereto.

In some embodiments, before operation 120, the aluminum blank may be selectively subjected to a two-stage homogenization treatment to eliminate the low melting point phase and further increase the content of the solid solution atoms. Furthermore, the two-stage homogenization treatment can spheroidize the coarse crystal phase, thereby avoiding cracks or fractures during the rolling operation, and also avoiding the occurrence of cracks due to excessive coarse particles during subsequent forming.

In some embodiments, the two-stage homogenization process includes a first stage homogenization process and a second stage homogenization process, wherein the processing temperature of the first stage homogenization process is about 465°C to about 485°C, and the processing time is about 12 hours to about 36 hours; and the processing temperature of the second stage homogenization process is about 470°C to about 500°C, and the processing time is about 12 hours to about 48 hours.

Next, operation 130 is performed to form the cold-rolled aluminum coil to obtain a formed aluminum sheet. In some embodiments, the forming operation includes heating the cold-rolled aluminum coil to about 400°C to about 500°C. The heating temperature in the aforementioned range is a preferred forming temperature, which can make the resulting formed aluminum sheet have better ductility and not easy to become brittle, so it is not easy to break and can be formed smoothly.

In some embodiments, the forming operation further includes placing the heated cold rolled aluminum coil in a mold for die-closing stamping, wherein the mold is preheated to a die-closing temperature of about 100°C to about 150°C, so as to cool the aluminum sheet to an appropriate nucleation temperature to avoid precipitation of undesirable atomic groups GPI that will weaken the strength in subsequent processes. If the die-closing temperature is too low (e.g., less than 100°C), the cooling speed is too fast, causing the precipitated crystal nuclei to compete with each other, so that the precipitation of undesirable atomic groups GPI cannot be effectively suppressed; on the contrary, if the die-closing temperature is too high (e.g., greater than 120°C), it will directly react into a coarse phase, resulting in the finished product having too low strength and poor impact resistance.

In some embodiments, the forming operation further includes holding the cold-rolled aluminum coil in the mold for a holding time of not less than 10 seconds, preferably about 10 seconds to about 15 seconds. If the holding time is too short (e.g., less than 10 seconds), the nucleation amount of the strengthening atomic group GPII is insufficient, and the undesirable atomic group GPI will precipitate, causing the aluminum alloy to undergo stock hardening in the subsequent process, thereby weakening the strength of the finished product. Therefore, an appropriate holding time can increase the amount of precipitation of the strengthening atomic group GPII and reduce the process cost.

Then, operation 140 is performed to perform a crystal nucleus stabilization treatment on the formed aluminum sheet to obtain an aluminum alloy. In some embodiments, the crystal nucleus stabilization treatment temperature is about 120°C to about 160°C, and the crystal nucleus stabilization treatment time is about 60 minutes to about 300 minutes. If the stabilization treatment temperature is too low (e.g., less than 120°C) or the stabilization treatment time is too short (e.g., less than 60 minutes), the stabilization effect is not good; on the contrary, if the stabilization treatment temperature is too high (e.g., greater than 160°C) or the stabilization treatment time is too long (e.g., greater than 300 minutes), the crystal nucleus will be forced to directly evolve into a coarse phase, and the strength of the aluminum alloy will continue to decrease in the subsequent process, which does not meet the safety regulations.

In some embodiments, the aluminum alloy can be selectively subjected to a paint treatment. In one embodiment, the paint temperature of the paint treatment is about 170°C to about 205°C, and the paint time is about 20 minutes to about 30 minutes. In some embodiments, the aluminum alloy can be stored at room temperature for not less than six months before the paint treatment. In other words, the aluminum alloy produced has good aging stability.

The aluminum alloy produced by method 100 can be used as automotive aluminum parts. After the body assembly and paint treatment, its strength can meet the safety requirements and has good stability over time. Therefore, it can take into account the needs of safety and lightweight at the same time and can also meet the material usage habits of car manufacturers.

Several embodiments are used below to illustrate the application of the present invention, but they are not intended to limit the present invention. People with ordinary knowledge in the technical field of the present invention can make various changes and modifications without departing from the spirit and scope of the present invention.

Implementation Example 1

Embodiment 1 utilizes an aluminum billet that complies with the international standard AA7075, and based on the aluminum billet being 100wt%, the Zn+Mg+4Mn+Cu in the aluminum billet is 11.1wt%. The aluminum billet is subjected to a two-stage homogenization treatment, wherein the treatment temperature of the first stage of homogenization treatment is 465℃ to about 485℃, and the treatment time is about 12 hours to about 36 hours; and the treatment temperature of the second stage of homogenization treatment is about 470℃ to about 500℃, and the treatment time is about 12 hours to about 48 hours. Then, the aluminum billet after the two-stage homogenization treatment is sequentially hot-rolled and cold-rolled to obtain a cold-rolled aluminum coil with a thickness of 1mm to 4mm.

Then, the cold-rolled aluminum coil is heated to about 400℃ to about 500℃, and then placed in a mold preheated to 103℃ and maintained under pressure for 12 seconds to obtain a formed aluminum sheet. Then, the formed aluminum sheet is subjected to a stabilization treatment at a stabilization temperature of 121℃ for 120 minutes to obtain an aluminum alloy. The aluminum alloy is placed at room temperature and after a six-month storage time, it is painted at a paint temperature of 170℃ to 205℃ and a paint time of 20 minutes to 30 minutes. Finally, the finished product after the paint treatment is tested for strength and three-point bending crash resistance. The test results are shown in Table 1 below. The crash resistance that meets the specifications is indicated by V, and the non-compliance is indicated by X.

Examples 2 to 3 and Comparative Examples 1 to 7

Examples 2 to 3 and Comparative Examples 1 to 7 use a similar process to Example 1 to manufacture aluminum alloys, with the only difference being the total content of Zn+Mg+4Mn+Cu in the aluminum blank, mold clamping temperature, pressure holding time, and crystal nucleus stabilization treatment conditions. The process conditions and test results of Examples 2 to 3 and Comparative Examples 1 to 7 are shown in Table 1 below.

As shown in Table 1, the aluminum alloys obtained in Examples 1 to 3 all have strength and impact resistance that meet the standards. In contrast, the element combination of Example 1 is lower than the requirement, and it is easy to generate too many coarse fishbone-shaped particles, making the aluminum alloy obtained later not impact-resistant and easy to break; while the element combination of Example 2 is higher than the requirement, and too many AlCuMg phases will be formed, consuming a large amount of strengthening atoms, thereby causing insufficient strength.

In comparison example 3, the mold closing temperature is too low, so the cooling speed is too fast, which causes the types of precipitated crystal nuclei to compete with each other, so the precipitation of the bad atomic group GPI cannot be effectively suppressed; while in comparison example 4, the mold closing temperature is too high, so the crystal nuclei directly react into a coarse phase, which cannot meet the requirements of strength and crash resistance. In comparison example 5, the pressure holding time is too short, the nucleation amount of the strengthening atomic group GPII is insufficient, and the bad atomic group GPI will be precipitated, which will cause the aluminum alloy to undergo inventory hardening in the subsequent process, thereby weakening the strength of the finished product.

In comparison example 6, the stabilization temperature used in the crystal nucleus stabilization treatment is too low, so the stabilization effect is not good; while in comparison example 7, the stabilization temperature used in the crystal nucleus stabilization treatment is too high, forcing the crystal nucleus to directly evolve into a coarse phase, and the strength of the aluminum alloy continues to decrease after the paint treatment, which does not meet the safety regulations.

According to the above-mentioned embodiments, the aluminum alloy and its manufacturing method provided by the present invention are to adjust the specific element composition, and perform forming operation and crystal nucleus stabilization treatment under specific conditions, so that the aluminum alloy precipitates more strengthening atomic groups, thereby increasing the room temperature storage time of the aluminum alloy, and having sufficient strength, so it can be applied to automobile parts.

Although the present invention has been disclosed as above with several embodiments, they are not intended to limit the present invention. Anyone with ordinary knowledge in the technical field to which the present invention belongs can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention shall be subject to the scope of the patent application attached hereto.

100:Methods

110,120,130,140: Operation

Claims (9)

A method for manufacturing an aluminum alloy, comprising: Providing an aluminum billet, wherein based on the aluminum billet being 100 wt%, the aluminum billet comprises: 3.5 wt% to 7.5 wt% zinc; 1.0 wt% to 4.0 wt% magnesium; Not more than 3.0 wt% copper; Not more than 0.5 wt% silicon; Not more than 0.5 wt% iron; Not more than 0.3 wt% titanium; Not more than 0.5 wt% manganese; Not more than 1.5 wt% impurities; and A balanced amount of aluminum, and the zinc (Zn), magnesium (Mg), copper (Cu) and manganese (Mn) in the aluminum billet satisfy the following formula: 10 wt%＜Zn+Mg+4Mn+Cu＜12 wt%； Perform a rolling operation on the aluminum billet to obtain a cold-rolled aluminum coil; Perform a forming operation on the cold-rolled aluminum coil to obtain a formed aluminum sheet, wherein the forming operation includes: Heating the cold-rolled aluminum coil to 400℃ to 500℃; and Placing the heated cold-rolled aluminum coil in a mold, wherein a mold closing temperature of the mold is 100℃ to 150℃; and The formed aluminum sheet is subjected to a crystal nucleus stabilization treatment to obtain the aluminum alloy, wherein a stabilization treatment temperature of the crystal nucleus stabilization treatment is 120°C to 160°C, and a stabilization treatment time of the crystal nucleus stabilization treatment is 60 minutes to 300 minutes, and the aluminum alloy is a 7XXX series aluminum alloy. The manufacturing method of the aluminum alloy as described in claim 1 further comprises: Before the rolling operation, the aluminum billet is subjected to a two-stage homogenization treatment, wherein the two-stage homogenization treatment comprises: A first stage homogenization treatment, wherein a treatment temperature of the first stage homogenization treatment is 465°C to 485°C, and a treatment time of the first stage homogenization treatment is 12 hours to 36 hours; and A second stage homogenization treatment, wherein a treatment temperature of the second stage homogenization treatment is 470°C to 500°C, and a treatment time of the second stage homogenization treatment is 12 hours to 48 hours. The method for manufacturing an aluminum alloy as described in claim 1, wherein the rolling operation comprises: performing a hot rolling operation on the aluminum billet to obtain a hot-rolled aluminum coil; and performing a cold rolling operation on the hot-rolled aluminum coil to obtain the cold-rolled aluminum coil. A method for manufacturing an aluminum alloy as described in claim 1, wherein a thickness of the cold-rolled aluminum coil is 1 mm to 4 mm. The manufacturing method of aluminum alloy as described in claim 1, wherein after the cold-rolled aluminum roll is placed in the mold, the forming operation further includes: Performing a pressure holding operation on the cold-rolled aluminum roll, wherein a pressure holding time of the pressure holding operation is not less than 10 seconds. The manufacturing method of the aluminum alloy as described in claim 1 further comprises: Performing a paint baking treatment on the aluminum alloy, wherein a paint baking temperature of the paint baking treatment is 170°C to 205°C, and a paint baking time of the paint baking treatment is 20 minutes to 30 minutes. A method for manufacturing an aluminum alloy as described in claim 6, wherein the aluminum alloy has a storage time of not less than six months before the paint treatment. An aluminum alloy is produced by the method for producing an aluminum alloy as described in any one of claims 1 to 7. An automobile structural material comprises the aluminum alloy described in claim 8.

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