ALUMINUM ALLOY FOIL AND METHOD FOR PRODUCING SAME

TECHNICAL FIELD

[0001]The present invention relates to an aluminum alloy foil and a method for producing the same.

[0002]The present application claims priority on Japanese Patent Application No. 2022-140851 filed on Sep. 5, 2022, the content of which is incorporated herein by reference.

BACKGROUND ART

[0003]Aluminum alloy foils, which are used as packaging materials for food, lithium ion batteries, and the like, are formed by being significantly transformed by press molding or the like and are thus required to have high elongation. For the aluminum alloy foils having high elongation and good formability, it is considered that fine and uniform crystal grains and randomness of the crystal texture are important.

[0004]For example, Patent Document 1 discloses an aluminum alloy foil containing Fe and Mg and having an amount of Si regulated to 0.10% or less, as an aluminum alloy foil having excellent formability.

[0005]In addition, Patent Document 2 discloses an aluminum alloy foil containing Fe and Si, in which the upper limits of the amounts of Cu and Mn are regulated to 0.2%, and the size of crystal grains is regulated.

CITATION LIST

Patent Documents

SUMMARY OF INVENTION

Technical Problem

[0008]In view of the above-described background, the present inventors are conducting research and development on an aluminum alloy foil suitable as an exterior foil for a lithium ion battery.

[0009]In a case where the present inventors have studied aluminum alloy foils for packaging materials, regarding elongation, the aluminum alloy foils are not transformed in one direction, and so-called stretch forming is often performed. Therefore, it is required that elongation is high not only in a direction parallel to the rolling direction, which is used as an elongation value of a general material, but also in each direction such as the direction of 45° or 90° with respect to the rolling direction.

[0010]In addition, in recent years, thickness reduction of packaging materials has progressed in the battery packaging material field and the like.

[0011]However, the aluminum alloy foil described in Patent Document 1 has a problem in that the uniformity of elongation in each direction is not sufficient, and it is difficult to uniformly transform the aluminum alloy foil by stretch forming.

[0012]In addition, in the aluminum alloy foil described in Patent Document 2, a sufficient value is shown for elongation, but an example in which Cu and Mn are added in an amount of 0.2% or less is shown in order to refine crystal grains. Even in a case where these elements are added in a trace amount, the rollability deteriorates, and the risk of breaking during rolling increases due to the generation of edge cracks; and therefore, there is a concern of the deterioration of productivity.

[0013]In addition, in an aluminum alloy material containing Fe in an amount close to 1.5%, an Al—Fe—Mn-based crystallized substance is coarsened even when a small amount of Mn is added; and therfore, there is a concern that the risk of the coarsened crystallized substance acting as a starting point for breaking during rolling or the generation of holes during forming may increase in a case where the foil thickness is thin.

[0014]An objective of the present invention is to provide an aluminum alloy foil having good workability and high formability.

Solution to Problem

Advantageous Effects of Invention

[0022]According to an aspect of the present invention, it is possible to provide an aluminum alloy foil having good workability and high formability.

BRIEF DESCRIPTION OF DRAWINGS

[0023]FIG. 1 A plan view showing a first embodiment of an aluminum alloy foil according to the present invention.

[0024]FIG. 2 A cross-sectional view showing an example of a continuous casting device for producing a cast plate (slab) serving as a base of the aluminum alloy foil according to the present invention.

DESCRIPTION OF EMBODIMENTS

[0025]Hereinafter, an example of an embodiment of the present invention will be described in detail based on the accompanying drawings. In the drawings to be used in the following description, there are cases where a characteristic portion is shown in an enlarged manner for convenience in order to facilitate the understanding of the characteristics.

[0026]FIG. 1 is a plan view showing an embodiment of an aluminum alloy foil according to the present invention.

[0027]An aluminum alloy foil 1 shown in FIG. 1 is a foil obtained by obtaining a cast plate by a continuous casting method described below and cold-rolling this cast plate and is drawn in FIG. 1 as a strip-like body having a certain width and a length direction oriented left and right.

[0028]The rolling direction of the aluminum alloy foil 1 is the left-right direction (the length direction of the strip-like foil 1) shown in FIG. 1, and for convenience, a 0° direction with respect to the rolling direction means the left-right direction of FIG. 1, a 45° direction with respect to the rolling direction means the arrow direction expressed as 45° shown in FIG. 1, and a 90° direction with respect to the rolling direction means the arrow direction expressed as 90° shown in FIG. 1. In other words, the 90° direction with respect to the rolling direction in the aluminum alloy foil 1 means the width direction of the strip-like aluminum alloy foil 1 (vertical direction in FIG. 1).

[0029]The aluminum alloy foil 1 shown in FIG. 1 is provided to have a thickness of, for example, about 0.1 μm to 0.2 mm. The thickness of the aluminum alloy foil 1 may be a general thickness used as a foil.

[0030]This aluminum alloy foil 1 is, for example, made of an aluminum alloy having a composition including: Fe in an amount of 1.2% by mass or more and 2.5% by mass or less; and other elements including at least Si in a total amount of 0.5% by mass or less, with a remainder being Al and unavoidable impurities.

[0031]In addition, in the aluminum alloy foil 1, for example, all of elongation in the 0° direction, elongation in the 45° direction, and elongation in the 90° direction with respect to the rolling direction are 20% or more, and the average crystal grain size is less than 20 μm.

[0036]Si forms Al—Fe—Si-based intermetallic compounds together with Fe; however, in a case where an excess amount of the other elements including Si are added, the size of the compound is coarsened, and the distribution density is decreased. In a case where the amount of the other elements including Si exceeds the upper limit, there is a concern that the elongation and the formability may deteriorate due to the coarse crystallized substance and the uniformity of the recrystallized grain size distribution after final annealing may deteriorate. For these reasons, the total amount of the other elements including at least Si is determined to be 0.5% by mass or less. For the same reasons, it is desirable that the upper limit of the amount of Si is set to be 0.4% by mass.

[0039]The aluminum alloy foil 1 that is used for packaging materials is three-dimensionally transformed by press forming. Therefore, the aluminum alloy foil is required to have not only elongation in the rolling direction but also good elongation in various directions. In a case where the elongation in any one of the above-described directions is less than 20%, the elongation in the direction limits the rate, and the formability of the aluminum alloy foil 1 deteriorates.

[0040]In order to maintain the formability of the aluminum alloy foil 1, it is required that the elongation is 20% or more in all directions with respect to the rolling direction. The elongation being excellent in all directions in the aluminum alloy foil 1 of the present embodiment means that, for example, with respect to the rolling direction, the elongation in the 0° direction, the elongation in the 45° direction, and the elongation in the 90° direction are excellent.

[0045]In an Al—Fe alloy, continuous recrystallization preferentially occurs due to annealing after rolling, and grain boundaries are present at a high density in the microstructure after annealing. In a case where there are many grain boundaries with a misorientation of 2° or more, stress concentration is likely to occur during forming, and the formability deteriorates. On the other hand, crystal grains surrounded by grain boundaries with a misorientation of 15° or more are considered to significantly contribute to the formability and need to be fine. In consideration of a balance therebetween, it is necessary that the grain boundary length with a misorientation of 2° or more per area of 1 μm² and the average grain size of the crystal grains surrounded by the grain boundary with a misorientation of 15° or more satisfy Expression (1). The grain boundary length and the average grain size preferably have a relationship of L×S≤10.

[0048]The intermetallic compound mentioned in the present embodiment is mainly an Al—Fe-based intermetallic compound or an Al—Fe—Si-based intermetallic compound dispersed in the metallic microstructure.

[0049]Examples of the intermetallic compound other than the above-described intermetallic compound include an Al—Mg—Si-based intermetallic compound, an Al—Fe—Mn-based intermetallic compound, and an Al—Mn-based intermetallic compound. It is preferable that the average grain size of the intermetallic compound is in a range of 0.50 to 0.80 μm, and the dispersed number density is in a range of about 150000 to 320000 particles/mm².

[0050]In a case where the grain size of the intermetallic compound is too small or the dispersed number density is too low, it is difficult to contribute to the refinement of the crystal grains, and good formability cannot be obtained. On the contrary, in a case where the grain size of the intermetallic compound is large, and in particular, coarse intermetallic compounds are present, the particles are likely to serve as starting points of cracks during forming, and the formability is impaired. In addition, in a case where the dispersed number density of the intermetallic compounds is too high, work hardening during forming becomes large, and stress concentration is also likely to occur; and thereby, the formability is also impaired.

“Method for Producing Aluminum Alloy Foil”

[0051]In order to produce the aluminum alloy foil 1 shown in FIG. 1, an aluminum alloy molten metal satisfying the above-described composition is made, and this aluminum alloy molten metal is used to obtain an aluminum alloy cast plate by a continuous casting method. Next, this aluminum alloy cast plate is processed by cold-rolling into a targeted thickness; and thereby, the aluminum alloy foil 1 can be obtained.

[0052]FIG. 2 shows an example of a continuous casting device suitable for a step of producing the aluminum alloy foil 1, and a continuous casting device A shown in FIG. 2 includes a tundish 3 in which an alloy molten metal M having a targeted composition is stored, a refractory nozzle 5 horizontally provided on a side wall 3A of the tundish 3, an upper roll (transport cooling device) 6 and a lower roll (transport cooling device) 7 disposed on a tip side of the nozzle 5. In addition, a gutter 8 for supplying the alloy molten metal M is provided above the tundish 3. A supply pipe 9 is provided at a lower part of the gutter 8, and this makes it possible to supply the alloy molten metal M to the tundish 3 via the supply pipe 9.

[0053]In FIG. 2, the upper roll 6 and the lower roll 7 are shown in a simplified manner, but these rolls have a double structure having a roll core and a roll shell, and a channel of a cooling medium (not shown) is formed between the roll core and the roll shell so that each roll can be cooled from the inside. In FIG. 2, only a part of the roll shell is shown by a broken line, and the details of each roll are omitted.

[0054]Since the alloy molten metal M can be supplied (poured) between the upper roll 6 and the lower roll 7 from the tip of the nozzle 5, the aluminum alloy cast plate indicated by reference numeral 10 in FIG. 2 can be cast by driving the upper roll 6 and the lower roll 7 to rotate while supplying the alloy molten metal M.

[0055]The aluminum alloy cast plate 10 can be continuously cast by supplying the alloy molten metal from the gutter 8 to the tundish 3 to adjust the amount of the alloy molten metal in the tundish 3 and continuously supplying the aluminum alloy molten metal from the tundish 3 to the space between the rolls 6 and 7 via the nozzle 5.

[0056]The aluminum alloy cast plate 10 can be produced by setting a cooling rate to be in a range of about 50° C./sec to 500° C./sec with the continuous casting device A shown in FIG. 2. The plate thickness of this cast plate 10 can be set to to be in a range of about 4 mm to 10 mm, for example, about 7 mm.

[0057]By adopting a cooling rate in this range, it becomes easy to generate a microstructure satisfying L×S≤15 . . . Expression (1) in the aluminum alloy foil 1 to be described later.

[0058]It is preferable to subject the obtained aluminum alloy cast plate 10 to a homogenization treatment. The homogenization treatment can be performed at a temperature of 550° C. to 620° C. for several hours, for example, at 595° C. for 8 hours. The time of the homogenization treatment is preferably 1 hour to 12 hours.

[0059]Once the aluminum alloy cast plate 10 is produced, cold rolling is performed a required number of times at a required working ratio; and thereby, the aluminum alloy foil 1 having a thickness of about 10 μm to 0.2 mm, for example, a thickness of 40 μm can be obtained. The working ratio per pass of cold rolling (rolling reduction) is preferably 40% or more, and the total working ratio is preferably 60% or more.

[0060]It is preferable that the cold rolling is performed a plurality of times and intermediate annealing is performed between the cold rollings. In the intermediate annealing, the aluminum alloy cast plate 10 is preferably heated at a temperature of 200° C. to 400° C. for about several tens of minutes to several hours. For example, it is preferable that the aluminum alloy cast plate 10 is heated at 360° C. for about 3 hours and then slowly cooled.

[0061]In addition, it is preferable to perform final annealing after the cold rolling. In the final annealing, it is desirable that the aluminum alloy cast plate 10 is heated at a temperature of 220° C. to 350° C. for about 30 minutes to 10 hours and then slowly cooled.

[0062]When the alloy molten metal is subjected to the continuous casting for producing the cast plate at the above-described cooling rate, and the obtained cast plate is subjected to the cold rolling and the final annealing performed under the above-described conditions, it is possible to obtain the aluminum alloy foil 1 capable of satisfying Expression (1).

[0063]In the obtained aluminum alloy foil 1, although a predetermined amount of Fe is contained, Fe affects the crystal texture of the aluminum alloy and contributes to the refinement of the crystal grain size. The aluminum alloy contains 1.2% to 2.5% by mass of Fe, but has good elongation by being made into a cast plate by a continuous casting method even in a case where the amount of Fe is in the above-described range.

[0064]The aluminum alloy used in this production method may contain, in addition to Fe, other elements including at least Si in an amount of about 0.5% by mass or less. An aluminum alloy foil capable of achieving the objective can be obtained even in a case where the other elements such as Si are contained in an amount within the above-described range in the aluminum alloy foil 1 of the present embodiment.

[0065]The production method described above makes it possible to obtain the aluminum alloy foil 1 in which all of elongation in the 0° direction, elongation in the 45° direction, and elongation in the 90° direction with respect to the rolling direction are 20% or more, the average crystal grain size is less than 20 μm, and the Expression (1) is satisfied.

[0066]The aluminum alloy foil 1 described above is suitable for food packaging or molded packaging materials for lithium ion batteries, and is capable of providing an aluminum alloy foil suitable for an application in which large transformation is performed by press forming and an application in which high elongation and formability are required.

[0067]The continuous casting device used in the case of producing the aluminum alloy foil 1 according to the present embodiment is not limited to the twin-roll type shown in FIG. 2. As other continuous casting devices for the continuous casting of an aluminum alloy, methods or devices such as a Hunter method, Lauener Caster I (Alusuisse Caster I), Davey McKee Twin-roll sheet caster, and twin belt casters are also known, and any of these may be used.

EXAMPLES

[0068]An aluminum alloy molten metal was adjusted to have each of alloy compositions Nos. 1 to 46 shown in Table 1 and Table 2, and an aluminum alloy cast plate having a thickness of 7 mm was produced under each of cooling rate conditions of 4 to 483° C./see shown in Table 1 and Table 2 using a twin-roll type continuous casting device shown in FIG. 2. The obtained aluminum alloy cast plate was roll-wound and accommodated in a heating furnace and subjected to a homogenization treatment in which the aluminum alloy cast plate was heated at a temperature of 540° C. to 620° C. for 8 hours as shown in Table 3.

[0069]Next, a cold rolling treatment was performed, and then intermediate annealing was performed by heating a cold-rolled plate at a temperature of 200° C. to 400° C. for 3 hours as shown in Table 3. The cold-rolled plate having a plate thickness of 0.7 mm was subjected to the intermediate annealing. As a result, an aluminum alloy foil having a targeted thickness of 40 μm was obtained. Final annealing was performed by heating the obtained aluminum alloy foil at a temperature of 220° C. to 350° C. for 8 hours as shown in Table 3 and then slowly cooling the aluminum alloy foil; and thereby, a final product was obtained.

[0072]For the measurement of elongation, a tensile test conforming to JIS Z 2241 was performed, a JIS No. 5 test piece was collected from each specimen (foil thickness: 40 ∪m) so that elongation in each of the 0°, 45°, and 90° directions with respect to the rolling direction could be measured, and a test was performed with a universal tensile tester (AGS-X 10 kN, manufactured by Shimadzu Corporation) at a tensile rate of 2 mm/min.

[0073]An elongation rate was calculated as described below. First, before the test, two lines were marked in the vertical direction of the test piece at an interval of 50 mm, which was a gauge length, in the longitudinal center of the test piece. After the test, the broken surfaces of the aluminum alloy foil were matched, the distance between the marks was measured, and the elongation amount (mm) obtained by subtracting the gauge length (50 mm) from the measured distance was divided by the gauge length (50 mm) to obtain the elongation rate (%).

[0076]As a pretreatment, the surface of the aluminum alloy foil was mirror-finished by electropolishing. Specifically, electropolishing was performed at a voltage of 20 V for 5 seconds using a solution containing perchloric acid and ethanol at a volume ratio of perchloric acid:ethanol=1:4.

[0077]Next, the crystal orientation was measured and analyzed under the following conditions using a crystal orientation measuring device (scanning electron microscope (SEM)-EBSD).

(Conditions of Electron Microscope)

(Conditions of EBSD Detector)

(Conditions of Measurement and Calculation of Average Crystal Grain Size)

[0094]Crystal grain boundaries with a misorientation of 15° or more between crystal grains were defined as high-angle grain boundaries (HAGBs), and the sizes of crystal grains surrounded by the HAGBs were measured. Images measured at a magnification of 1000 times were connected to analyze a total area of 50000 μm² or more, and the average crystal grain size was calculated. An area method (average by area fraction method) of EBSD was used for the calculation of the average crystal grain size. OIM Analysis (Ver. 8.0) of TSL Solutions, Inc. was used for the analysis.

[0097]In a case where a misorientation between crystal grains, which became a crystal grain boundary, was set in an observation visual field (Grain-Map) in which crystal orientations were measured by the EBSD method, the set crystal grain boundary could be analyzed. The grain boundary length L of a crystal grain boundary with a misorientation of 2° or more in the visual field was analyzed with a set value of the misorientation between the crystal grains, which became a crystal grain boundary, set to min: 2° (max: 180°). Measurement conditions for the grain boundary length L (the conditions of the electron microscope, the conditions of the EBSD detector, and the measurement area) were the same as the measurement conditions for the average crystal grain size except that a boundary with a misorientation between crystal grains of 2° or more was defined as the crystal grain boundary.

[0098]The obtained metallic microstructure was observed by the EBSD method, and the value of L×S was determined; wherein the grain boundary length with a misorientation of 2° or more per area of 1 μm² was represented by L (/μm), and the average grain size of the crystal grains surrounded by grain boundaries with a misorientation of 15° or more was represented by S (μm).

[0101]The average grain size and dispersion density of the intermetallic compounds were measured by observing a cross section (RD-ND surface) obtained by cutting the aluminum alloy foil in a direction perpendicular to the rolling direction (RD) under the following conditions.

(Conditions for Measuring Average Grain Size and Dispersion Density of Intermetallic Compounds)

[0110]The aluminum alloy foil was cut with a cross section polisher (CP). Backscattered electron images (composition images) of the cross section were captured at a magnification of 2000 times with the SEM, and the captured images were connected to obtain an image having a total area of 40000 μm² or more. The average grain size and dispersion density of the intermetallic compounds were analyzed using the obtained image. For the analysis of the average grain size and dispersion density of the intermetallic compounds, for example, image analysis software ImageJ was used, and the density (particles/mm²) of the number of particles in the visual field and the average grain size (μm) in terms of equivalent circle diameter were calculated. The particle number density and the average grain size were calculated under the above-described conditions for all particles having a projected area of 0.5 μm² or more, regardless of the shape of the particles, in the observation visual field.

[0113]The molding height was evaluated by a square tubular molding test. The test was performed with a universal thin plate molding tester (model 142/20 manufactured by ERICHSEN), and an aluminum foil having a thickness of 40 μm and a shape shown in FIG. 1 was molded using a square punch (side length L=37 mm, chamfer diameter R of corner part=4.5 mm). As test conditions, the wrinkle suppressing force was 10 kN, the scale of the punch rising rate (molding rate) was set to 1, and mineral oil was applied as a lubricant to one surface (a surface where the punch was about to hit) of the aluminum foil. The aluminum foil was molded with the punch that rose from the lower portion of the device and hit the aluminum foil, and the maximum height that the punch rose when the aluminum foil could be molded without breaking or pinholes after being continuously molded three times was defined as the limit molding height (mm) of the material. The height of the punch was changed at intervals of 0.5 mm. A case where the stretch height (limit molding height) was 7.0 mm or more was determined as good moldability and rated as “B”. A case where the stretch height (limit molding height) was 9.5 mm or more was determined as particularly good (excellent) moldability and rated as “A”. A case where the stretch height (limit molding height) was less than 7.0 mm was determined as poor moldability and rated as “C”.

[0114]Alloys having any composition shown in Table 1 and Table 2 were used, the production steps shown in any of A to I in Table 3 were adopted, and Examples Nos. 1 to 33 and Comparative Examples Nos. 34 to 46 shown in Table 1 and Table 2 were produced. All of the examples were examples in which the above-described desired composition or production conditions were satisfied. All of the comparative examples were examples in which any of the above-described desired composition or production conditions was not satisfied.

[0115]For Examples Nos. 1 to 33 and Comparative Examples Nos. 34 to 46, the elongation in the 0° direction, the elongation in the 45° direction, the elongation in the 90° direction, the average crystal grain sizes, the grain boundary densities, the densities of the intermetallic compounds, the average grain sizes of the intermetallic compounds, and the forming limit heights were measured, and the measurement results and evaluation results are shown in Table 4 and Table 5 below.

[0116]The unit of the amount of each element shown in Tables 1 and 2 was % by mass, and the remainder was Al and unavoidable impurities.

[0117]As shown by the results shown in Table 4 and Table 5, the aluminum alloy foils made of an aluminum alloy having a composition including Fe in an amount of 1.2% by mass or more and 2.5% by mass or less; and other elements including at least Si in an amount of 0.5% by mass or less, with a remainder being Al and unavoidable impurity, in which, when the grain boundary length with a misorientation of 2° or more per area of 1 μm² measured by the EBSD method was represented by L (/μm), and the average grain size of crystal grains surrounded by grain boundaries with a misorientation of 15° or more was represented by S (μm), L×S≤15 . . . Expression (1) was satisfied, had excellent characteristics. For example, all of the elongation in the 0° direction, the elongation in the 45° direction, and the elongation in the 90° direction were 20% or more, the value of L×S was 15 or less, the value of the forming limit height was 7 mm or more, and the average crystal grain size was less than 20 μm.

[0118]Specifically, in Examples Nos. 1 to 33 shown in Table 4 and Table 5, the average crystal grain sizes were in a range of 8.5 to 19.3 μm, the elongation in the 0° direction, the elongation in the 45° direction, and the elongation in the 90° direction exhibited excellent values of 20% to 35%, and all the forming limit heights were also 7.0 mm or more. In addition, the values of L×S of all of the specimens of the examples were in a range of 5.36 to 14.77.

[0119]In comparison with these examples, in the specimen No. 34 for which the cooling rate during the casting was slow, the elongation was good, but the value of L×S was more than 15, and the forming limit height was inferior to those of the examples. In the specimen No. 36 for which the cooling rate during the casting was slow, the elongation in the 0° direction and the elongation in the 90° direction were less than 20%, the value of L×S was more than 15, and the forming limit height was inferior to those of the examples. In the specimens Nos. 35 and 37 for which the homogenization treatment temperatures were low, all of the elongation in the 0° direction, the elongation in the 45° direction, and the elongation in the 90° direction were less than 20%, and the forming limit heights was inferior to those of the examples.

[0120]In the specimens Nos. 38 and 39 for which the amounts of Fe were small, the values of L×S were more than 15, any of the elongation in the 0° direction, the elongation in the 45° direction, or the elongation in the 90° direction was less than 20%, and the forming limit heights were inferior to those of the examples. In the specimens Nos. 40 and 41 for which the amounts of Fe were large, any of the elongation in the 0° direction, the elongation in the 45° direction, or the elongation in the 90° direction was less than 20%, and the forming limit heights were inferior to those of the examples.

[0121]In the specimens Nos. 42 to 46, any one of Si, Mg, Cu, Mn, and Zn was added in an amount of 0.5% by mass or more, and the total amounts of the additive elements other than Fe including other additive elements were more than 0.5% by mass. In these specimens, any of the elongation in the 0° direction, the elongation in the 45° direction, or the elongation in the 90° direction was less than 20%, and the values of L×S were more than 15 even in a case where the elongation was 20% or more, and the forming limit heights was thus inferior to those of the examples.

INDUSTRIAL APPLICABILITY

[0122]The aluminum alloy foil of the present embodiment is suitably applied as a packaging material for food or lithium ion batteries.

REFERENCE SIGNS LIST