JP7799550B2 - Aluminum alloy foil - Google Patents

Description

This invention relates to aluminum alloy foil for use in forming processes.

Aluminum alloy foils used as packaging materials for food products, lithium-ion batteries, etc. are required to have high formability because they are subjected to large deformations during press forming and other processes.
For example, in Patent Document 1, the range of components is specified, the grain size of crystal grains is specified, and further, the area ratio of Cube orientation is specified, thereby improving formability.
Furthermore, Patent Document 2 states that the ratios of the diffraction intensities representing the (111) plane, the (100) plane, the (110) plane, and the (311) plane are specified to improve moldability.

Japanese Patent Application Laid-Open No. 2018-115376 JP 2012-052158 A

However, conventional aluminum alloy foils do not have sufficient formability.
Incidentally, as the plastic processing accompanying the forming of aluminum foil progresses, unevenness occurs on the surface of the material, and in order to obtain good formability, it is necessary to suppress and reduce this unevenness.
The present invention has been made in light of the above circumstances, and one of its objects is to provide an aluminum alloy foil having high formability.

That is, among the aluminum alloy foils of the present invention, a first embodiment contains Fe: 0.8% by mass or more and 1.8% by mass or less, Si: 0.01% by mass or more and 0.15% by mass or less, Cu: 0.001% by mass or more and 0.05% by mass or less, the inevitable impurity Mn is restricted to 0.01% by mass or less, and the balance is Al and other inevitable impurities, and is characterized in that, when the arithmetic mean roughness before plastic working is R ₀ , the arithmetic mean roughness after plastic working is R _a , and the strain during plastic working is ε, the increase rate α of surface roughness due to plastic working satisfies the following formula:
α=(Ra-R0)/ε≦0.02

Another form of the aluminum alloy foil invention is characterized in that, in the above-mentioned invention, the elongation at angles of 0°, 45°, and 90° relative to the rolling direction is 25% or more.

The following explains the provisions of this invention.

The reasons for limiting each component in the composition of the aluminum alloy foil of the present invention are explained below.

Fe: 0.8% by mass or more and 1.8% by mass or less. Fe crystallizes as an Al-Fe intermetallic compound during casting and serves as a recrystallization site during annealing, thereby refining the recrystallized grains. If the Fe content is low, the distribution density of coarse intermetallic compounds is low, the refining effect is low, and the final crystal grain size distribution becomes non-uniform. On the other hand, if the Fe content is excessive, the effect of refining the crystal grains saturates or decreases, and further, the size of the Al-Fe compounds generated during casting becomes very large, resulting in reduced ductility and rollability of the foil. For this reason, the Fe content is set to a lower limit of 0.8% by mass and an upper limit of 1.8% by mass. For the same reason, it is preferable to set the lower limit to 1.0% by mass and the upper limit to 1.6% by mass.

Si: 0.01% by mass or more and 0.15% by mass or less Si crystallizes coarse intermetallic compounds during casting. It is desirable to limit the amount added to prevent the formation of coarse intermetallic compounds. However, if the content is too low, it is necessary to use high-purity metal, which significantly increases manufacturing costs. On the other hand, if the content is excessive, it may cause coarsening of the compound size and a decrease in distribution density, which may lead to concerns about reduced rollability, elongation, and formability. For this reason, the lower limit of the Si content is set to 0.01% by mass and the upper limit to 0.15% by mass. For the same reason, it is desirable to set the lower limit to 0.01% by mass and the upper limit to 0.08% by mass.

Cu: 0.001% by mass or more and 0.05% by mass or less Cu is an element that increases the strength of aluminum foil and reduces elongation. On the other hand, it has the effect of suppressing excessive work softening during cold rolling, which has been reported for Al-Fe alloys. If the Cu content is too small, the effect of suppressing work softening is low, and if it is too large, elongation is clearly reduced. For this reason, the lower limit of the Cu content is set to 0.001% by mass and the upper limit to 0.05% by mass. For the same reason, it is desirable to set the lower limit to 0.005% and the upper limit to 0.01%.

Mn: 0.01% by mass or less Mn dissolves in the aluminum matrix as an impurity or forms very fine compounds, suppressing aluminum recrystallization. A small amount of Mn can be expected to suppress work softening, similar to Cu, but a high content delays recrystallization during intermediate annealing and final annealing, making it difficult to obtain fine, uniform crystal grains. Therefore, the Mn content is limited to 0.01% or less. For the same reason, a content of 0.005% or less is desirable.

Increase in surface roughness due to plastic working α
When aluminum foil is plastically deformed, unevenness (surface roughness) occurs on the material surface. Surface roughness is considered to be unevenness in thickness, and by suppressing it, it is possible to prevent a decrease in the forming limit.
When the arithmetic mean roughness before plastic working is R ₀ , the arithmetic mean roughness after plastic working is R _a , and the plastic strain is ε, the increase rate α of surface roughness due to plastic working satisfies the following formula.
α=(R _a −R ₀ )/ε≦0.02
If the increase rate α exceeds 0.02, the forming limit decreases.

The aluminum alloy foil of the present invention preferably has an elongation of 25% or more at angles of 0°, 45°, and 90° relative to the rolling direction. High elongation at these angles provides high formability.

The present invention makes it possible to suppress a decline in the forming limit and achieve excellent formability.

1 is a graph showing the increment of surface roughness (Ra-R ₀ ) relative to plastic strain in an example of the present invention. FIG. 1 is a diagram showing the planar shape of a square punch used in a limit forming height test in an example of the present invention.

Hereinafter, an embodiment of the aluminum alloy foil of the present invention will be described.
In the production of the aluminum alloy foil of this embodiment, first, an ingot adjusted to a predetermined composition is melted. The ingot, or slab, is subjected to a homogenization treatment, followed by hot rolling and then cold rolling. In the cold rolling, one or more intermediate annealings can be performed as desired. In the final cold rolling after the last intermediate annealing, rolling is performed at a predetermined reduction ratio to obtain an aluminum alloy foil of a predetermined thickness. The aluminum alloy foil after cold rolling is subjected to final annealing to obtain the alloy foil of this embodiment. The aluminum alloy foil of this embodiment can be formed. Each step will be described below.

Casting: Slab thickness: 600 mm to 750 mm. Casting to obtain ingots can be performed using conventional methods, but it is desirable to set the slab thickness to a specified thickness. The slab thickness affects the cooling rate during casting, and also the size and distribution of the crystallized products and crystal grains produced during casting. In addition, different slab thicknesses also affect the rolling ratio to the final foil.
It is believed that the refinement and uniformity of the crystal grains after casting contributes to the refinement and uniformity of the foil after final annealing. Furthermore, changes in the rolling ratio due to variations in slab thickness also contribute to the development of texture in the final foil. These crystal grain size and the degree of texture accumulation affect surface roughness, which in turn contributes to formability. Therefore, it is desirable for the slab thickness to be 600 mm or more. However, if the slab thickness exceeds 750 mm, the cooling rate during casting decreases, which is likely to cause crystallized deposits and coarsening of the crystal grain size during casting.

- Predetermined composition The composition of the aluminum alloy foil contains Fe: 0.8% by mass or more and 1.8% by mass or less, Si: 0.01% by mass or more and 0.15% by mass or less, Cu: 0.001% by mass or more and 0.05% by mass or less, the inevitable impurity Mn is restricted to 0.01% by mass or less, and the balance is composed of Al and other inevitable impurities.

Homogenization treatment: 480°C to 540°C x 8 hours or more The homogenization treatment aims to eliminate microsegregation in the ingot and adjust the distribution state of intermetallic compounds, and is an important treatment for obtaining a fine, uniform crystal grain structure in the aluminum alloy foil after final annealing.
If the homogenization temperature is less than 480° C., the crystal grains will not be sufficiently refined, and if it exceeds 540° C., the crystal grains will become coarse. If the treatment time is less than 8 hours, the homogenization will be insufficient.

・Hot rolling: Finishing temperature 230℃ to 280℃
When hot-rolling the ingot after homogenization treatment, the finishing temperature is important. By optimizing the finishing temperature, recrystallization is suppressed (the hot-rolled sheet has a fibrous structure). However, if the finishing temperature exceeds 280°C, recrystallization occurs in part of the sheet after hot rolling, making it difficult to obtain an ideal texture in the final product. Furthermore, a non-uniform structure in which fibrous grains and recrystallized grains coexist contributes to non-uniformity in the crystal grain structure in the final product, which may result in reduced formability. On the other hand, finishing at a rolling temperature below 230°C requires an extremely low temperature during hot rolling, which may cause cracks to form on the sides of the sheet, significantly reducing productivity. Therefore, the finishing temperature of hot rolling is preferably within the above range.

Rolling ratio: 99.2% or more It is desirable to set the rolling ratio from slab to hot rolling finish at 99.2% or more to finely break down the crystallized particles formed during casting. In addition, a high rolling ratio will result in a fibrous structure after hot rolling.

Cold Rolling Hot rolling is followed by cold rolling, which may be interrupted by one or more intermediate anneals.
Intermediate annealing: 300-400°C x 3 hours or more This softens the material hardened by cold rolling (recovering rollability) and also promotes the precipitation of Fe, reducing the amount of dissolved Fe.
If the intermediate annealing temperature is less than 300°C, there is a risk that recrystallization will not be completed and the crystal grain structure will become non-uniform. Furthermore, if the intermediate annealing temperature is higher than 400°C, the recrystallized grains will become coarse, and the final crystal grain size will also become large. Furthermore, at higher temperatures, the amount of Fe precipitation decreases and the amount of dissolved Fe increases. If the amount of dissolved Fe is large, recrystallization during final annealing is suppressed, resulting in a significant increase in the density of Cu orientation and R orientation. Even if the treatment time is less than 3 hours, there is a risk that recrystallization will be incomplete and Fe precipitation will be insufficient.

For intermediate annealing, batch annealing is performed in which the coil is placed in a furnace and held there for a certain period of time.
There are two types of methods: continuous annealing (CAL annealing) and continuous annealing line (CAL annealing), which rapidly heats and cools the material. Either method is acceptable when intermediate annealing is applied.
For example, in batch annealing, conditions such as 300 to 400°C for 3 hours or more can be adopted, and in CAL annealing, conditions such as a temperature rise rate of 100 to 250°C/sec, a heating temperature of 420 to 470°C, no holding time or a holding time of 5 seconds or less, and a cooling rate of 20 to 200°C/sec can be adopted. However, in this embodiment, the presence or absence of intermediate annealing, the conditions when intermediate annealing is performed, etc. are not limited to specific ones.

Final cold rolling: rolling ratio of 95% or more. Since crystal grains are refined during the cold rolling process (grain subdivision), the higher the final cold rolling ratio from intermediate annealing to the final thickness, the finer the crystal grains. Furthermore, the higher the cold rolling ratio, the more the Cu orientation and R orientation can be developed. Therefore, a higher final cold rolling ratio is desirable, and specifically, a final cold rolling ratio of 95% or more is desirable. However, if the final cold rolling ratio is less than 95%, the recrystallized grain size after final annealing becomes coarse and non-uniform, resulting in worse surface roughness, making it difficult to achieve high ductility and therefore high formability.

Thickness after final cold rolling: A desired thickness can be achieved by final cold rolling. In this embodiment, the thickness is not particularly limited, but a thickness of 10 to 40 μm can be shown, for example.

Final annealing: 250°C to 350°C x 10 hours or more Final annealing is performed to completely soften the foil after final cold rolling. Final annealing after foil rolling may be performed at, for example, 250°C to 350°C. If the final annealing temperature is low, softening is insufficient. If the temperature exceeds 350°C, problems such as foil deformation and reduced economic efficiency arise. If the final annealing time is less than 10 hours, the effect of the final annealing is insufficient.

In the aluminum alloy foil of the embodiment, the increase rate α {(Ra−R ₀ )/ε} of surface roughness due to plastic working is 0.02 or less.
When the arithmetic mean roughness before plastic working is R ₀ , the arithmetic mean roughness after plastic working is Ra, and the strain during plastic working is ε, the increase rate α of surface roughness due to plastic working is 0.02 or less.
The strain (ε) in the calculation formula represents plastic strain and is calculated according to the deformation mode, such as tension, compression, shear, torsion, etc. For example, ε due to uniaxial tension is calculated as follows:
First, before the tensile test, two lines are drawn vertically in the center of the test piece and the distance between them (l ₀ ) is measured. After deformation, the distance between the two lines (l i ) is measured again and calculated as ε = ln(l i /l ₀ ).
The above-mentioned requirement can be achieved by setting the slab thickness to 600 mm to 750 mm in the manufacturing process, controlling the size and distribution of crystallized particles and crystal grains generated during casting, and the rolling ratio up to the final foil, thereby optimizing the crystal grain size and texture in the final foil.

Elongation at 0°, 45°, and 90° relative to the rolling direction is 25% or more The aluminum alloy foil of the embodiment preferably has elongation at 0°, 45°, and 90° relative to the rolling direction of 25% or more.
The elongation of the aluminum alloy foil of this embodiment does not necessarily have to satisfy this condition, but it is desirable that it does. Furthermore, it is even more desirable that the elongation in the above three directions is 30% or more.
The above elongation characteristics can be obtained by appropriately controlling the slab thickness and rolling ratio in the manufacturing process, similar to the surface roughness described above.

In this embodiment, it is desirable that the following characteristics be further provided.
For crystal grains surrounded by large-angle grain boundaries with a misorientation of 15° or more, the average grain size is 15 μm or less, and the maximum grain size/average grain size is ≦3.5
By suppressing the surface roughness of the foil that occurs during plastic working, improvements in elongation and formability can be expected. One of the factors that influence this surface roughness is the crystal grain size, and to suppress surface roughness, it is desirable for the average crystal grain size to be 15 μm or less. Furthermore, if the crystal grain size distribution is uneven, it is expected that local deformation will occur more easily, resulting in reduced elongation. Therefore, high formability can be obtained not only by keeping the average crystal grain size 15 μm or less, but also by ensuring that the maximum grain size/average grain size is ≦3.5.
However, when the crystal orientation density is 50 or more, the average crystal grain size may be 25 μm or less.
The above properties can be obtained by appropriately controlling the slab thickness and rolling ratio in the manufacturing process, similar to the surface roughness and elongation mentioned above.

Cube orientation density of 6 or less and Cu orientation density of 30 or more. Texture also affects the surface roughness of foil. Surface roughness occurs frequently near grain boundaries and is therefore related to the deformation and unevenness of individual crystal grains. If the crystal orientation is relatively uniform, the deformation and rotation of crystal grains accompanying deformation are similar. However, if the orientation varies greatly, plastic deformation causes uneven deformation and rotation of each crystal grain, which leads to the development of surface roughness. Therefore, accumulating crystal orientation leads to the suppression of surface roughness. Because foils have a thin material thickness, the rolling ratio during their manufacturing process is relatively high, making it easy for rolling texture to develop. However, if Cube orientation also develops at the same time, the variation in crystal orientation increases, making it unsuitable for suppressing surface roughness. Therefore, a Cube orientation density of 6 or less and a Cu orientation density of 30 or more is desirable.
However, when the average crystal grain size is 6 μm or less, the Cu orientation density may be 15 or more.
The above Cube orientation density and Cu orientation density can be obtained by setting the final cold rolling reduction rate to 95% or more in the manufacturing process.

Examples of the present invention will be described below.
The aluminum alloys shown in Table 1 (the balance being Al and other unavoidable impurities) were melted by a conventional method to obtain slabs having the thicknesses shown in Table 2. The slabs were subjected to a homogenization treatment in which they were held at 500°C for 8 hours or more.
The slabs after the homogenization treatment were hot rolled to a finished thickness of 5 mm by the hot rolling method shown in Table 2, and the hot rolling finishing temperature was 235°C to 284°C.

The hot-rolled material was then cold-rolled. In cold rolling, all specimens except for Test Material No. 10 were subjected to intermediate annealing at a thickness of 2.8 mm (cold rolling reduction of 44.4%). The intermediate annealing was performed in a batch furnace at 360°C for 3 hours. Final cold rolling was then performed to a finished thickness of 40 μm. The reduction ratio of the final cold rolling was 98.6%. Test Material No. 10 was rolled to the finished thickness without intermediate annealing. Therefore, the final cold rolling was 99.2%.
The aluminum alloy foil after the cold rolling was subjected to final annealing under the conditions of 300°C x 20 hours.

The obtained test materials were evaluated for each of the following items, and the evaluation results are shown in Table 3.

The elongation was measured by a tensile test. The tensile test was conducted in accordance with JIS Z2241. JIS No. 5 test pieces were prepared so that the elongation could be measured in the directions of 0°, 45°, and 90° relative to the rolling direction. The test was conducted using a universal tensile tester (Shimadzu Corporation, AGS-X 10 kN) at a tensile speed of 5 mm/min.
The elongation was calculated as follows: First, before the test, two lines were marked at the longitudinal center of the test piece in the direction perpendicular to the test piece, spaced 50 mm apart (the gauge length). After the test, the fracture surfaces of the aluminum alloy foils were butted together to measure the distance between the marks, and the gauge length (50 mm) was subtracted from the distance to obtain the elongation (mm), which was then divided by the gauge length (50 mm) to obtain the elongation (%).

Measurement of Surface Roughness The plastic working in this example was carried out by a tensile test. The tensile test was carried out in the same manner as in the elongation measurement in the previous section, using a JIS No. 5 test piece and applying tensile strain using the universal tensile tester.
The surface roughness of the test material was measured in accordance with JIS B0601:2001. The actual measurements were performed using a confocal laser microscope (Keyence Corporation, VK-X100) and an analysis application (Keyence Corporation, VK-H1XA). The observation magnification was 500x, the field of view size was 1000 x 500 μm, and the measurement location was the center of the width and length of the JIS No. 5 test piece. The data scanned by the laser microscope was subjected to noise removal and tilt correction processing, and the surface roughness was measured. For noise removal, the noise detection level was set to "Normal," and for tilt correction, the correction method was selected to "Surface tilt correction (profile)." The surface roughness parameter was calculated based on JIS B0601:2001 using the arithmetic mean roughness of the surface roughness.

The procedure for continuous observation is described below.
First, the surface quality of the test piece before the test is observed using a confocal laser microscope. Then, a tensile test is performed. The test is stopped at any point during plastic deformation, and the surface quality after plastic deformation is observed again (surface roughness measurement). After the observation, the same test piece is further processed, and the test is stopped during plastic deformation to observe the surface quality (surface roughness measurement). This procedure is repeated multiple times (at least three times or more), and surface shape measurements are performed at different arbitrary strain levels. The plastic strain and change in arithmetic mean roughness (R _a -R ₀ ) obtained in each measurement are plotted, and an approximate straight line is drawn using the least squares method to create a graph of the change in surface roughness versus plastic strain, as shown in Figure 1. The increase rate α of surface roughness is calculated from the slope of the approximate straight line.

Crystal grain size After electrolytic polishing of the foil surface, crystal orientation analysis was performed using a scanning electron microscope (SEM)-EBSD. Crystal grain boundaries with an orientation difference between crystal grains of 15° or more were defined as high angle grain boundaries (HAGBs), and the size of the crystal grains surrounded by HAGBs was measured. Three fields of view with a field size of 90 × 180 μm were measured at a magnification of 900x, and the average crystal grain size and the large grain size/average grain size ratio were calculated. The individual crystal grain sizes were calculated as circle equivalent diameters, and the average crystal grain size was calculated using the EBSD Area method (Average by Area Fraction Method). Note that the analysis was performed using TSL Solutions' OIM Analysis.
The results are shown in Table 3 as average crystal grain size and grain size ratio.

Crystal orientation density The Cube orientation was represented by {001}<100>, and the Cu orientation was represented by {112}<111>. The orientation densities were evaluated by measuring the incomplete pole figures of {111}, {200}, and {220} using X-ray diffraction. The results were used to calculate the three-dimensional orientation distribution function (ODF).
The results are shown in Table 3 as Cube orientation density and Cu orientation density.

Square tube overhang height: The square tube overhang height was evaluated in a square tube forming test. The test was performed using a universal sheet metal forming tester (Model 142/20 manufactured by ERICHSEN) and a square punch (side length L = 37 mm, corner chamfer diameter R = 4.5 mm) with a thickness of 40 μm, as shown in Figure 2. The test conditions were a wrinkle suppression force of 10 kN, a scale of 1 for the punch rising speed (forming speed), and mineral oil applied as a lubricant to one side of the foil (the surface where the punch contacts). The foil was formed by a punch rising from the bottom of the device. The maximum punch rise height (mm) that could be formed without cracks or pinholes after three consecutive forming runs was defined as the square tube overhang height of the material. The punch height was varied in 0.5 mm increments.
The measurement results are shown in Table 3.

As shown in the table, the examples of the present invention have a larger rectangular tube overhang height and superior formability compared to the comparative examples. In contrast, comparative examples 11 to 17, in which either the composition of the present invention or the surface roughness increase rate α due to plastic processing, or both, are outside the range, have a smaller rectangular tube overhang height and inferior formability.

Claims (2)

An aluminum alloy foil containing Fe: 0.8% by mass or more and 1.8% by mass or less, Si: 0.01% by mass or more and 0.15% by mass or less, Cu: 0.001% by mass or more and 0.05% by mass or less, the inevitable impurity Mn being restricted to 0.01% by mass or less, with the remainder consisting of Al and other inevitable impurities, characterized in that, when the arithmetic mean roughness before plastic working is R ₀ , the arithmetic mean roughness after plastic working is R _a , and the strain during plastic working is ε, an increase rate α of surface roughness due to plastic working satisfies the following formula:
α=(R _a −R ₀ )/ε≦0.02 The aluminum alloy foil according to claim 1, characterized in that the elongation at angles of 0°, 45°, and 90° relative to the rolling direction is 25% or more.