Aluminum alloy sheet material for lithium-ion battery and method for producing the same

Abstract

An aluminum alloy sheet material for a lithium-ion battery can significantly reduce the number of welding defects (e.g., bead non-uniformity and underfill) that occur during laser welding. The aluminum alloy sheet material includes 0.8 to 1.5 mass % of Mn, 0.6 mass % or less of Si, 0.7 mass % or less of Fe, 0.2 mass % or less of Cu, and 0.2 mass % or less of Zn, with the balance being Al and unavoidable impurities, Al—Mn—Si-based intermetallic compounds having a maximum length of less than 1.0 μm being distributed in a matrix of the aluminum alloy sheet material in a number equal to or larger than 0.25 per μm.sup.2, and the area ratio of the intermetallic compounds being 3.0% or more when a field of view having an area of 5000 μm.sup.2 is subjected to image analysis.

Claims

1. An aluminum alloy sheet material for a lithium-ion battery comprising 0.8 to 1.5 mass % of Mn, 0.6 mass % or less of Si, 0.7 mass % or less of Fe, 0.2 mass % or less of Cu, and 0.2 mass % or less of Zn, with the balance being Al and unavoidable impurities, Al—Mn—Si-based intermetallic compounds having a maximum length of less than 1.0 μm being distributed in a matrix of the aluminum alloy sheet material in a number equal to or larger than 0.25 per μm.sup.2, and an area ratio of the intermetallic compounds being 3.0% or more when a field of view having an area of 5000 μm.sup.2 is subjected to image analysis.

2. The aluminum alloy sheet material according to claim 1, the aluminum alloy sheet material having an electrical conductivity at 25° C. of 45 to 55 IACS %.

3. The aluminum alloy sheet material according to claim 1, the aluminum alloy sheet material having an average surface crystal grain size (circle equivalent diameter) of 50 μm or less.

4. A method for producing an aluminum alloy sheet material for a lithium-ion battery comprising homogenizing an ingot of an aluminum alloy having the composition according to claim 1 at 400 to 550° C. for 3 to 48 hours, hot-rolling the homogenized ingot at a hot-rolling start temperature of 400 to 550° C., cold-rolling the hot-rolled ingot at a reduction ratio of 70% or more to have a specific thickness, and subjecting the cold-rolled ingot to final annealing in a continuous annealing furnace at a temperature increase rate of 100 to 500° C./s and an annealing temperature of 480 to 550° C.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a view illustrating an example of bead non-uniformity that occurs during pulsed laser welding.

DESCRIPTION OF EMBODIMENTS

(2) The effects of each alloy component of the aluminum alloy sheet material for a lithium-ion battery according to the invention, and the reasons for which the content range of each alloy component is limited as described above, are described below.

(3) Mn

(4) Mn is an element that is effective for improving the strength of the aluminum alloy sheet material. The Mn content is preferably 0.8 to 1.5 mass %. If the Mn content is less than 0.8 mass %, the effect of improving the strength of the aluminum alloy sheet material may be insufficient. If the Mn content exceeds 1.5 mass %, coarse intermetallic compounds may be produced during casting, and breakage may occur when forming a casing material, or the formability of the explosion-proof valve may deteriorate when forming a sealing material. Mn is dissolved in the aluminum matrix to lower the electrical conductivity, and improves the thermal conversion efficiency of the applied laser light. Moreover, Mn produces Al—Mn-based intermetallic compounds and Al—Mn—Si-based intermetallic compounds, and improves the laser light absorption rate.

(5) Si

(6) Si produces Al—Mn—Si-based intermetallic compounds when homogenizing the ingot, and improves the dispersibility of the intermetallic compounds. If the Si content exceeds 0.6 mass %, coarse Al—Fe—Si-based intermetallic compounds or Al—Mn—Si-based intermetallic compounds may be easily produced, and welding defects may occur from these intermetallic compounds during laser welding. Therefore, the Si content is preferably limited to 0.6 mass % or less, more preferably 0.35 mass % or less, and still preferably 0.05 mass % or less. However, since it is necessary to use high-purity ground metal when reducing the Si content, the production cost may increase. Moreover, since an increase in electrical conductivity may occur due to a decrease in dissolved Si content, the thermal conversion efficiency of the applied laser light may decrease, and the penetration depth may decrease. Therefore, it is desirable to select an optimum Si content taking account of cost and the desired penetration depth.

(7) Fe

(8) Fe produces Al—Fe-based intermetallic compounds and Al—Fe—Si-based intermetallic compounds when homogenizing the ingot, and improves the dispersibility of the intermetallic compounds. If the Fe content exceeds 0.7 mass %, coarse Al—Fe-based intermetallic compounds may be produced to a large extent, and breakage may occur when forming a casing material, or the formability of the explosion-proof valve may deteriorate when forming a sealing material. Therefore, it is preferable to limit the Fe content to 0.7 mass % or less.

(9) Cu

(10) Cu is added to adjust the potential of the surface of the material. If the Cu content exceeds 0.2 mass %, local corrosion may easily occur from Al—Cu-based intermetallic compounds (precipitates). Therefore, it is preferable to limit the Cu content to 0.2 mass % or less.

(11) Zn

(12) The Zn content is preferably 0.2 mass % or less. If the Zn content exceeds 0.2 mass %, the potential of the surface of the material may decrease, and uniform corrosion may easily occur.

(13) The advantageous effects of the invention are not impaired even if the aluminum alloy sheet material further includes 0.05 mass % or less of Cr, 0.05 mass % or less of Mg, and/or 0.05 mass % or less of Ti.

(14) The method for producing an aluminum alloy sheet material for a lithium-ion battery according to the invention is described below.

(15) An aluminum alloy having the above composition is melted, and cast. The aluminum alloy may be cast using a normal semi-continuous casting method. In order to reduce production of coarse crystallized products (intermetallic compounds) (that absorb the applied laser light to produce welding defects) during casting, it is desirable to increase the casting speed, or reduce the thickness of the ingot (slab), for example.

(16) The resulting ingot is homogenized. The ingot is homogenized at 400 to 550° C. for 3 to 48 hours so that Al—(Mn, Fe)—Si-based intermetallic compounds (hereinafter referred to as “Al—Mn—Si-based intermetallic compounds”) finely precipitate. If the homogenization temperature is less than 400° C., Al—Mn—Si-based intermetallic compounds may not sufficiently finely precipitate. If the homogenization temperature exceeds 550° C., Al—Mn—Si-based intermetallic compounds may aggregate to form coarse particles, and Mn may be dissolved again. In either case, the advantageous effects of the invention may not be obtained. If the homogenization time is less than 3 hours, Al—Mn—Si-based intermetallic compounds may not sufficiently finely precipitate. If the homogenization time exceeds 48 hours, the homogenization cost may be too high with respect to the precipitation effect.

(17) The homogenized ingot is hot-rolled. The hot-rolling start temperature is set to 400 to 550° C. in order to promote precipitation of Al—Mn—Si-based intermetallic compounds during hot rolling. The hot-rolled ingot is cold-rolled to have a specific thickness. The reduction ratio during cold rolling is preferably set to 70% or more in order to utilize strain introduced during cold rolling as an Al—Mn—Si-based intermetallic compound precipitation site.

(18) Final annealing is performed using a continuous annealing furnace. Since recrystallization does not easily proceed due to the effects of fine precipitation of Al—Mn—Si-based intermetallic compounds, final annealing is performed at a temperature increase rate of 100 to 500° C./s and an annealing temperature of 480 to 560° C. If the annealing temperature is less than 480° C., recrystallization may not sufficiently occur, and cracks or orange peel surfaces may easily occur during drawing/ironing forming or press forming. If the annealing temperature exceeds 560° C., Mn may be dissolved again, and coarse recrystallized grains may be produced.

(19) In order to improve the laser light absorption rate to reduce the number of welding defects, it is important to cause a specific amount of Mn-containing intermetallic compounds (Al—Mn—Si-based intermetallic compounds) to precipitate in the matrix. In an aluminum alloy sheet material produced by the above production method, Al—Mn—Si-based intermetallic compounds having a maximum length of less than 1.0 μm are distributed in the matrix in a number equal to or larger than 0.25 per μm.sup.2, and the area ratio of the intermetallic compounds is 3.0% or more when a field of view having an area of 5000 μm.sup.2 is subjected to image analysis. The aluminum alloy sheet material has an electrical conductivity at 25° C. of 45 to 55 IACS %, and has an average surface crystal grain size (circle equivalent diameter) of 50 μm or less. The advantageous effects of the invention can be achieved by satisfying these properties.

(20) When Al—Mn—Si-based intermetallic compounds having a maximum length of less than 1.0 μm are distributed in the matrix in a number equal to or larger than 0.25 per μm.sup.2, laser light is uniformly absorbed during laser welding, and welding defects are suppressed. When the area ratio of the intermetallic compounds is 3.0% or more when a field of view having an area of 5000 μm.sup.2 is subjected to image analysis, laser light is more uniformly absorbed, and laser welding can be performed without producing welding defects.

EXAMPLES

(21) The invention is further described below by way of examples and comparative examples to demonstrate the advantageous effects of the invention. Note that the following examples are intended for illustration purposes only, and the invention is not limited to the following examples.

Example 1

(22) Each of aluminum alloys A to G having the composition shown in Table 1 was melted, and cast using a normal semi-continuous casting method. The resulting ingot (thickness: 500 mm) was homogenized at 500° C. for 12 hours, and each rolling target surface was machined to a depth of 8 mm. The ingot was then hot-rolled (hot rolling start temperature: 500° C., hot rolling finish temperature: 270° C.) to obtain a hot-rolled sheet having a thickness of 5.0 mm. The hot-rolled sheet was cold-rolled to a thickness of 1.0 mm, heated to 500° C. at a temperature increase rate of 200° C./s, and held at 500° C. for 120 seconds to obtain a specimen (Specimens 1 to 7).

Comparative Example 1

(23) Each of aluminum alloys H to M having the composition shown in Table 1 was melted, and cast using a normal semi-continuous casting method. The resulting ingot was treated in the same manner as in Example 1 to obtain a specimen (Specimens 8 to 13).

Example 2

(24) The ingot (thickness: 500 mm) of the aluminum alloy F that was cast in Example 1 was homogenized under the conditions shown in Table 2, and each rolling target surface was machined to a depth of 8 mm. The ingot was subjected to hot rolling, cold rolling, and final annealing under the conditions shown in Table 2 to obtain a specimen (Specimens 14 to 28).

Comparative Example 2

(25) The ingot (thickness: 500 mm) of the aluminum alloy F that was cast in Example 1 was homogenized under the conditions shown in Table 2, and each rolling target surface was machined to a depth of 8 mm. The ingot was subjected to hot rolling, cold rolling, and final annealing under the conditions shown in Table 2 to obtain a specimen (Specimens 29 to 36).

(26) Specimens 1 to 7 and 14 to 28 produced in Examples 1 and 2 and Specimens 8 to 13 and 29 to 36 produced in Comparative Examples 1 and 2 were subjected to intermetallic compound (Al—Mn—Si-based intermetallic compound) image analysis, electrical conductivity measurement, crystal grain size measurement, and laser welding stability evaluation using the following methods. The results are shown in Tables 3 and 4. In Tables 1 to 4, the values that fall outside the scope of the invention are underlined. Image analysis: The number and the average maximum length of intermetallic compounds captured within an optical micrograph (magnification: 1000, three fields of view, 5000 μm.sup.2) were measured using an image analyzer “LUZEX-AP” (manufactured by NIRECO Corporation) (after intermetallic compounds having a maximum length of 1.0 μm or more were excluded). Electrical conductivity: The electrical conductivity (at 25° C.) of the specimen was measured at five points using a tester “Sigmatest 2.069” (manufactured by Foerster Japan Limited), and the average value of the values measured at three points excluding the maximum value and the minimum value was taken as the measured value. Crystal grain size: The average crystal grain size of the surface of the specimen was calculated from an optical micrograph (magnification: 100, three fields of view) using the ASTM comparative method. Laser welding stability (number of bead non-uniformities): Laser welding (length: 500 mm) was performed using a laser “YLR-2000” (ytterbium fiber laser) (manufactured by iPG) (fiber diameter: 0.1 mm, frequency: 120 Hz, peak output: 1.6 kW, welding speed: 15 mm/s, shielding gas: Ar (0.25 L/s)). The laser welding stability was evaluated by measuring the number of bead non-uniformities in which the bead size locally increased (see FIG. 1). A case where the number of bead non-uniformities was less than 0.10 per mm was evaluated as “Acceptable”.

(27) TABLE-US-00001 TABLE 1 Chemical component (mass %) Alloy Mn Si Fe Cu Zn A  0.86 0.22 0.41 0.15 0.03 B 1.4 0.23 0.52 0.16 0.02 C 1.1 0.03 0.44 0.15 0.01 D 1.1 0.57 0.43 0.13 0.01 E 1.2 0.21 0.66 0.15 0.02 F 1.2 0.23 0.58 0.18 0.02 G 1.1 0.25 0.40 0.12 0.17 H  0.71 0.30 0.42 0.13 0.02 I 1.6 0.23 0.51 0.11 0.03 J 1.2 0.70 0.43 0.15 0.02 K 1.1 0.20 0.74 0.12 0.02 L 1.2 0.26 0.57 0.25 0.03 M 1.2 0.23 0.52 0.16 0.27

(28) TABLE-US-00002 TABLE 2 Hot rolling Cold rolling Final annealing Homogenization treatment Start Final Final Reduction Temperature Holding Temperature Time temperature thickness thickness ratio increase rate Temperature time Specimen Alloy (° C.) (h) (° C.) (mm) (mm) (%) (° C./s) (° C.) (s) 14 F 420 12 500 5.0 1.0 90 200 500 120 15 F 540 12 500 5.0 1.0 90 200 500 120 16 F 500  3 500 5.0 1.0 90 200 500 120 17 F 500 48 500 5.0 1.0 90 200 500 120 18 F 500 12 410 5.0 1.0 90 200 500 120 19 F 500 12 540 5.0 1.0 90 200 500 120 20 F 500 12 500 5.0 1.5 70 200 500 120 21 F 500 12 500 5.0 1.0 90 100 500 120 22 F 500 12 500 5.0 1.0 90 200 480 120 23 F 500 12 500 5.0 1.0 90 200 540 120 24 F 500 12 500 5.0 1.0 90 200 500 60 25 F 500 12 500 5.0 1.0 90 200 500 180 26 F 500 12 500 5.0 1.0 90 200 500 0 27 F 500 12 500 5.0 1.0 90 200 500 5 28 F 500 12 500 5.0 1.0 90 200 500 15 29 F 390 12 500 5.0 1.0 90 200 500 120 30 F 500  1 500 5.0 1.0 90 200 500 120 31 F 600  6 580 5.0 1.0 90 200 500 120 32 F 500 12 380 5.0 1.0 90 200 500 120 33 F 500 12 500 5.0 2.0 60 200 500 120 34 F 500 12 500 5.0 1.0 90  75 500 120 35 F 500 12 500 5.0 1.0 90 200 450 120 36 F 500 12 500 5.0 1.0 90 200 560 120

(29) TABLE-US-00003 TABLE 3 Number of Al—Mn—Si-based Area ratio of Al—Mn—Si-based Electrical intermetallic compounds intermetallic compounds conductivity at Number of bead having maximum length of less having maximum length of less Crystal grain size 25° C. non-uniformities Specimen Alloy than 1.0 μm (per μm.sup.2) than 1.0 μm (%) (μm) (LACS %) (per mm) 1 A 0.28 3.2 50 53 0.05 2 B 0.62 4.2 40 46 0.08 3 C 0.36 3.0 50 45 0.04 4 D 0.65 4.8 40 49 0.08 5 E 0.56 4.6 40 46 0.07 6 F 0.47 3.8 45 47 0.04 7 G 0.45 3.7 45 47 0.05 8 H 0.23 2.8 55 56 0.13 9 I 0.66 4.5 40 43 0.38 10 J 0.69 5.0 40 50 0.09.sup.(1) 11 K 0.60 4.9 40 46 0.17 12 L 0.45 3.6 45 48 0.05.sup.(2) 13 M 0.42 3.4 45 47 0.06.sup.(3) .sup.(1)Breakage occurred. .sup.(2)Local corrosion occurred. .sup.(3)Uniform corrosion occurred.

(30) TABLE-US-00004 TABLE 4 Number of Al—Mn—Si-based Area ratio of Al—Mn—Si-based Electrical intermetallic compounds intermetallic compounds conductivity at Number of bead having maximum length of less having maximum length of less Crystal grain size 25° C. non-uniformities Specimen Alloy than 1.0 μm (per μm.sup.2) than 1.0 μm (%) (μm) (LACS %) (per mm) 14 F 0.68 5.2 50 51 0.03 15 F 0.26 3.1 40 45 0.09 16 F 0.38 3.3 50 46 0.07 17 F 0.55 4.7 45 49 0.04 18 F 0.53 4.6 50 50 0.04 19 F 0.40 3.4 40 46 0.08 20 F 0.45 3.6 50 47 0.07 21 F 0.45 3.8 50 46 0.05 22 F 0.48 4.0 45 49 0.04 23 F 0.44 3.5 50 46 0.07 24 F 0.46 3.7 45 47 0.05 25 F 0.45 3.7 50 46 0.06 26 F 0.48 3.8 40 48 0.04 27 F 0.48 3.8 45 47 0.04 28 F 0.46 3.7 45 47 0.05 29 F 0.21 2.2 50 44 0.23 30 F 0.23 2.5 55 45 0.18 31 F 0.22 2.4 40 43 0.73 32 F 0.23 2.3 60 48 0.17 33 F 0.24 2.5 55 46 0.12 34 F 0.48 4.0 60 49 0.06 35 F 0.49 4.1 Recrystallization 49 0.04 did not occur 36 F 0.43 3.6 60 44 0.38

(31) As shown in Tables 3 and 4, Specimens 1 to 7 and 14 to 28 that fall under the scope of the invention exhibited excellent laser welding stability (i.e., the number of bead non-uniformities was 0.09 or less per mm).

(32) As shown in Table 3, Specimens 8 to 13 having an alloy composition that falls outside the scope of the invention were not suitable as an aluminum alloy sheet material for a lithium-ion battery from the viewpoint of formability, laser welding stability, corrosion resistance, and the like.

(33) As shown in Table 4, Specimen 29 showed insufficient precipitation of Al—Mn—Si-based intermetallic compounds due to a low homogenization temperature, and exhibited poor laser welding stability. Specimen 30 showed insufficient precipitation of Al—Mn—Si-based intermetallic compounds due to a short homogenization time. Specimen 31 showed insufficient precipitation of Al—Mn—Si-based intermetallic compounds due to a high homogenization temperature and a high hot rolling start temperature. Specimen 33 had a small number of Al—Mn—Si-based intermetallic compound precipitation sites due to a small reduction ratio during cold rolling, and exhibited poor laser welding stability. Specimen 32 showed insufficient precipitation of Al—Mn—Si-based intermetallic compounds due to a high hot rolling start temperature, and exhibited poor laser welding stability.

(34) Specimens 34 to 36 showed insufficient recrystallization, and exhibited poor formability due to a low temperature increase rate during final annealing, a low final annealing temperature, and a high final annealing temperature, respectively.