Manufacturing method for composite electrode terminal

Abstract

[Problem] To provide a method for manufacturing a composite electrode terminal that has excellent joining strength and electrical characteristics and can also be formed into a terminal shape simultaneously with joining a plurality of metal materials. [Solution] A method for manufacturing a composite electrode terminal including a first metal layer and a second metal layer joined together, the method being characterized by placing a first metal material that will become the first metal layer on top of a second metal material that will become the second metal layer and performing press molding in this state at a predetermined joining temperature and a reduction ratio R (t0 / t1) falling within the range of 1.80-5.0, where t0 mm is the total thickness of the first metal material and the second metal material before joining, and t1 mm is the total thickness of the first metal material and the second metal material after joining.

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a composite electrode terminal made of a plurality of metal layers. [Background technology]

[0002] For example, electrode terminals used in secondary battery cells such as lithium ion batteries (LIBs) generally use Al material for the positive electrode terminal and Cu material for the negative electrode terminal. When using this type of secondary battery as a drive source for electric vehicles, hybrid vehicles, etc., high output is required, so a plurality of secondary battery cells are electrically connected in series by bus bars or the like. In this case, dissimilar materials must be joined, with the Cu material of the negative electrode terminal and the Al material of the positive electrode terminal. However, Cu-Al fusion welding easily produces brittle intermetallic compounds (IMC), and the brittleness of this reaction layer makes practical welding difficult. Therefore, Cu / Al clad bus bars and Cu / Al clad terminals are used to enable fusion welding of the same materials between the cell's electrode terminals and bus bars (the electrode terminals and the bus bars that connect them are joined using the same materials, making fusion welding possible).

[0003] For example, Patent Document 1 discloses a clad terminal made by laminating Cu and Al materials. However, this is made by laminating the Cu and Al materials by rolling, then generating a reaction layer of several μm or more at the bonding interface by diffusion in a heating furnace, metallurgically bonding them, and then forming them by press working, which requires a lot of labor and cannot be said to be productive. Punching (trimming) also results in material waste. Furthermore, metallurgically, IMCs of Cu and Al are formed in this reaction layer, but generally, when the thickness of the IMCs reaches the order of micrometers, their fragility becomes apparent, and even during the formation stage, they are prone to contain defects such as cracks due to thermal stress. In other words, at the bonding interface between dissimilar metals where fragile IMCs on the order of μm are formed by diffusion bonding in such a heating furnace, there are concerns about durability in terms of the occurrence of cracks due to press working after bonding and fatigue failure due to thermal stress during use. At the same time, these crack defects also lead to a decrease in the important functionality (electrical resistance) of electrical components. It has been reported that the electrical resistivity of various Cu-Al IMCs, such as AlCu, Al2Cu, Al3Cu4, and Al4Cu9, is approximately 5 to 6 times higher than that of Al (NMRaj et al.: Transactions of the Indian Institute of Metals, 71, 2018, pp. 107-116.), so it is desirable for IMCs to be as thin as possible, even if they are free of defects such as cracks. Patent Document 2 discloses a clad terminal made up of an Al layer, a Cu layer, and a nickel (Ni) layer, in which the Ni layer prevents cracks during forming by press working and cracks during crimping. Patent Document 3 discloses a terminal made by joining Cu and Al, but the joining method is laser welding. Fusion welding, such as laser welding, generates high temperatures, making it much more difficult to suppress the growth of IMCs in dissimilar metal joining than in solid-state joining. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2021-157897 [Patent Document 2] Japanese Patent Application Publication No. 2021-144792 [Patent Document 3] Japanese Patent Publication No. 2020-095837 Summary of the Invention [Problem to be solved by the invention]

[0005] In view of the above technical problems, the present invention aims to provide a highly productive method for manufacturing a composite electrode terminal that has excellent joining strength and electrical properties and is capable of joining multiple metal materials and simultaneously forming them into a terminal shape. [Means for solving the problem]

[0006] The method for manufacturing a composite electrode terminal according to the present invention is a method for manufacturing a composite electrode terminal consisting of a first metal layer and a second metal layer joined together, and is characterized in that, in a state in which a first metal material that will become the first metal layer is placed on top of a second metal material that will become the second metal layer, the total thickness of the first metal material and the second metal material before joining is t0 mm, and the thickness of the joint between the first metal layer and the second metal layer after joining is t1 mm, the composite electrode terminal is pressure-molded at a predetermined joining temperature so that the reduction ratio R(t0 / t1) is in the range of 1.80 to 5.0.

[0007] Here, the composite electrode terminal is expressed as being made up of a first metal layer and a second metal layer, but it also includes composite electrode terminals made up of not only two layers but also three or more layers joined together. The expression "pressure molding" means that it is possible to mold the materials into a predetermined shape and structure at the same time as joining.

[0008] In the present invention, the joining temperature of the metal materials during joining is preferably in the range of 210 to 410° C., taking into consideration the thickness of the IMC, as will be described in detail later. The joining method according to the present invention is characterized in that when the first metal material and the second metal material are stacked and pressure-molded, plastic flow is generated at the joining interface, stretching, dividing, and removing contaminant layers such as oxide films at the joining interface that hinder the interdiffusion of the first and second metal materials, and solid-state joining is carried out at a low temperature, in a short time, and through efficient diffusion on the resulting clean surface. In addition, since the present invention generates strong plastic flow at the joining interface by using a high reduction ratio, as described above, the contaminant layer such as an oxide film becomes very thin or is divided, thereby greatly increasing its surface area, and facilitating the diffusion of contaminant layer elements such as oxygen into the base material. Therefore, almost no obstacle layer that inhibits efficient diffusion remains at the bonding interface (the principle behind ensuring high cleanliness of the bonding interface by using a reduction ratio that achieves efficient diffusion bonding at low temperatures and in a short time).

[0009] In the present invention, when the combination of the first metal layer and the second metal layer is an alloy of copper (Cu) or its alloy and aluminum (Al), the bonding temperature may be in the range of 270 to 410°C, and when the combination of the first metal layer and the second metal layer is copper (Cu) or its alloy and commercially pure aluminum (Al), the bonding temperature may be in the range of 210 to 370°C. Considering the thickness of the IMC, the bonding temperature can be lower in the case of commercially pure aluminum than in the case of aluminum alloys. Here, commercially pure aluminum refers to Japanese JIS 1000 series aluminum, and other aluminum alloys. Typical examples of aluminum alloys include JIS 5000 series and 3000 series. [Effects of the Invention]

[0010] According to the present invention, low-temperature solid-state bonding accompanied by plastic flow at the bonding interface is possible by performing pressure forming at a predetermined temperature so that the reduction ratio R(t0 / t1) is within a predetermined range. The inventors call this method low-temperature forge welding. Conventional forge welding is known as the oldest welding method, as described in the WELDING HANDBOOK Vol. 2 compiled by the American Welding Society (AWS) as "the only welding method that was commonly used until the 19th century." This is primarily used to join low-carbon steels together, and fluxes such as silica sand and borax were used to prevent the formation of oxide scale. With the development of melting processes, this joining method is no longer commonly seen. Examples of practical applications to date include blacksmithing, in which steel is joined to a soft iron blade by repeatedly hitting it with a hammer, and steel pipe forming, in which strip-shaped steel is rolled with rollers and the ends are forged together.However, both of these methods require artisanal skills carried out at extremely high temperatures (from about 1000°C to near the melting point) or are material manufacturing processes that involve large-scale production lines, so their application is limited. Melting point T normalized to absolute temperature m T / T is the ratio of junction temperature T tom is approximately 0.8 or more. The low-temperature forge welding method described in this invention is a new, highly productive multi-material manufacturing method that uses general-purpose industrial presses such as mechanical presses and hydraulic presses to complete forming and compounding at low temperatures and in a short time. The melting point T of high-melting-point materials standardized in absolute temperature m T / T is the ratio of junction temperature T to m is roughly in the range of 0.3 to 0.7. Compared to conventional diffusion bonding or welding using heat treatment, the bonding temperature is significantly lower, but the high reduction ratio eliminates the influence of the diffusion barrier layer, making it possible to achieve solid-state bonding in a short time. By performing this interdiffusion at a low temperature and in a short time, the growth of the brittle reaction layer (IMC), which has traditionally been a problem in dissimilar material joining, can be suppressed to a reaction layer that is significantly thinner than the 1 μm thickness at which brittleness generally becomes a concern, resulting in a composite electrode terminal with excellent joining strength and electrical properties. Although the joining mechanism is a reactive layer, the fragility of the layer is essentially neutralized, and high-quality dissimilar material joining can be completed in an instant along with the forming process. Furthermore, the thickness of the reaction layer can be controlled by the bonding temperature and reduction ratio, which are the basic bonding parameters of this method. Therefore, if there is a record of the bonding temperature, it is possible to guarantee the quality of the bond at all points based on the product dimensions (reduction ratio) even after bonding. [Brief explanation of the drawings]

[0011] [Figure 1] Schematic diagrams showing the structure of a composite electrode terminal in which a first metal material m1 and a second metal material m2 are stacked and pressure-molded to bond the first metal layer M1 and the second metal layer M2. (a) shows an example of a reduction ratio R1.80, (b) R2.10, (c) R2.60, and (d) R3.60. [Figure 2] This is an example of using a mechanical press for pressure molding of a combination of copper (C1020) and aluminum alloy (A5052). [Figure 3]This is an optical microscope image of a cross section near the bonding interface of a sample made by pressure molding a combination of copper (C1020) and aluminum alloy (A5052) at a bonding temperature of 360°C and a reduction ratio of R4.2. [Figure 4] The results of area analysis of each element by an electron probe microanalyzer (EPMA) of the bonded interface of the sample shown in Figure 3 are shown below. Note that CP indicates a backscattered electron image. [Figure 5] (a) shows a field emission scanning electron microscope (FE-SEM) secondary electron image of the bonded interface of the sample shown in Figure 3. (b) shows the results of EPMA line analysis of the bonded interface. [Figure 6] (a) shows the relationship between the reduction ratio R and the maximum oxygen signal intensity in EPMA line analysis of the bonded interface cross section for a combination of copper (C1020) and aluminum alloy (A5052) (bonding temperature 265°C). (b) shows how the contaminated layer at the bonded interface is broken up as the reduction ratio increases, reducing its effect as a diffusion barrier. [Figure 7] This shows the relationship between the joining temperature and the maximum tensile load (breaking load) of the joint and the amount of mating material attached (mass concentration) on each fracture surface for a combination of copper (C1020) and aluminum alloy (A5052) (reduction ratio 4.2). [Figure 8] This shows the relationship between the joining temperature and the maximum tensile load of the joint and the thickness of the IMC for a combination of copper (C1020) and aluminum alloy (A5052) (reduction ratio 4.2). [Figure 9] This shows the results of EPMA surface analysis of the Cu-side fracture surface after a tensile test of a sample made by pressure molding at a joining temperature of 360°C, using a combination of copper (C1020) and aluminum alloy (A5052). [Figure 10] This figure shows the relationship between the joining temperature and maximum tensile load for press-formed samples of a combination of copper (C1020) and commercially pure aluminum (A1200) (reduction ratio 4.4). The open plots in the figure indicate fracture at the joining interface, while the shaded plots indicate failure at the joining interface. [Figure 11]This shows secondary electron images observed by FE-SEM of the thickness of the interfacial reaction layer (IMC) at the bonding temperature for a combination of copper (C1020) and commercially pure aluminum (A1200) (reduction ratio 4.4). [Figure 12] A bright field (BF) image of the cross section of the bonded interface at a bonding temperature of 310° C. in FIG. 11 is observed with a transmission electron microscope (TEM). [Figure 13] The results of line analysis by EPMA of the cross section of the bonded interface at a bonding temperature of 310°C in FIG. 11 are shown. [Figure 14] This graph shows a comparison of the relationship between joining temperature and IMC thickness in pressure molding with copper (C1020) between commercially pure aluminum (A1200) and aluminum alloy (A5052). [Figure 15] This shows the fatigue properties of a sample made by combining copper (C1020) and commercially pure aluminum (A1200) under repeated tensile loads, which was press-formed at a joining temperature of 310°C and a reduction ratio of 4.4. DETAILED DESCRIPTION OF THE INVENTION

[0012] Test samples of composite terminals were manufactured and comparative studies were carried out, and the results are described below, but the present invention is not limited to these.

[0013] Figure 1 shows a schematic diagram of the case where the reduction ratio R is changed when the first metal material m1 and the second metal material m2 are stacked together, with (a) showing an example of R1.80, (b) showing an example of R2.10, (c) showing an example of R2.60, and (d) showing an example of R3.60. The reduction ratio R is defined as the ratio t0 / t1 of the total thickness t0 of the first metal material m1 and the second metal material m2 before joining to the total thickness t1 of the first metal layer M1 and the second metal layer M2 after joining. In the shape diagrammatically shown in FIG. 1, the left side shows the state before bonding, and the right side shows the state after bonding and molding into the terminal shape by pressure molding. By forming the second metal layer M2 so as to encase the first metal layer M1, plastic flow is promoted. Here, for example, the first metal layer M1 may be made of pure copper (Cu) or an alloy thereof, and the second metal layer M2 may be made of pure aluminum (Al) or an alloy thereof.

[0014] Figure 2 shows the time variation of each index (press load, slide stroke, slide speed) when a composite electrode terminal is press-formed using a mechanical press at a reduction ratio of R4.2 and a joining temperature of 360°C. Low-temperature forge welding was performed using an AC servo press (Komatsu Industries H1F200-2) with a shot per minute (SPM) of 50, with an override of 50% (=SPM 25). The pressure application time is approximately 150 ms, making this a high-throughput process. The peak load was approximately 88 kN. The pressure molding shown here is an example of high-speed processing, and in view of the joining principle according to the present invention, the joining time may be extended to the order of several seconds, for example, up to the order of 10 seconds.

[0015] An optical microscope image of the cross section of the joint of the composite electrode terminal fabricated as described above is shown in Figure 3 (only the vicinity of the joint interface is shown). In this example, the first metal phase M1 is Cu (C1020) and the second metal phase M2 is Al (A5052). Figure 4 shows the results of an EPMA surface analysis of the bonding interface. FIG. 4 shows the backscattered electron image (CP) and the results of area analysis of Cu, Al, and oxygen (O). From Figure 4, it can be seen that the cleanliness of the bonded interface is very high, and that no reaction layer thick enough to be recognized with the spatial resolution of EPMA has formed. The bonded interface of this sample was observed in more detail using FE-SEM. Figure 5 shows a secondary electron image acquired at high magnification. FIG. 5(b) shows the results of EPMA line analysis of the bonding interface. From this, a reaction layer presumed to be an IMC between Cu and Al on a submicron scale well below 1 μm can be clearly observed at the bonded interface (confirmation of metallurgical bonding). As with the results in Figure 4, the O chart shows that the bonded interface is extremely clean. The high cleanliness of this bonding interface, in other words, the absence of any contaminant layer that would hinder diffusion, allows diffusion to occur at low temperatures and in a short time. Temperature, which acts as the driving force in diffusion, has a dominant effect on the reaction rate. However, by using low-temperature diffusion in this method, the growth of IMCs is suppressed to a thickness well below 1 μm, making the material essentially IMC-free.

[0016] Next, the relationship between the reduction ratio R and the cleanliness of the bonded interface was investigated. The maximum oxygen signal intensity measured by EPMA line analysis of the cross section of the bonded interface was used as an index of the cleanliness of the bonded interface. The results are shown in Figure 6(a). These are the experimental results for a combination of C1020 and A5052 at a bonding temperature of 265°C. It can be seen that increasing the reduction ratio R improves the cleanliness of the bonded interface. That is, the larger the reduction ratio, the smaller the influence of contaminant layers (layers that hinder the diffusion reaction) such as oxide films, and the more efficiently and healthily the diffusion reaction can be carried out. The cleanliness is high even at the reduction ratio of R1.80 shown in Figure 6(a), but as the reduction ratio increases to R2.60 and R3.60, the cleanliness increases further, becoming almost equal to the base signal intensity (background) of the main line analysis. This can also be confirmed by the fact that the O signal intensity in Figure 4 and Figure 5(b) is the same at the bond interface and in the Al base material. Regarding the improvement in cleanliness of the bonded interface with an increase in the reduction ratio, Fig. 6(b) shows a schematic image of the separation of contaminated layers such as oxide films at the cross section of the bonded interface. As the reduction ratio increases, the diffusibility at low temperatures improves.

[0017] Next, the effects of bonding temperature on the bond strength and IMC growth were investigated. Figure 7 shows the results of a product tensile test (room temperature) conducted on a Cu (C1020) / Al (A5052) composite electrode terminal pressed at a reduction ratio of R4.2, examining the relationship between the joining temperature and the maximum tensile load at which the joining surface fractures, as well as the amount of mating material attached (mass concentration) on each fracture surface. In the graph, Al on Cu side indicates the concentration of Al attached to the fracture surface on the Cu material side, and Cu on Al side indicates the concentration of Cu attached to the fracture surface on the Al material side. At a bonding temperature of 35°C (room temperature in summer, without preheating), the bonding interface peeled off after pressing, resulting in a bonding strength of 0N. Furthermore, even those that did not peel after pressing at a joining temperature of 35°C had low strength (approximately 600 N), which is thought to be due to a mechanical joining effect (anchor effect) caused by plastic flow at the edge of the joining interface. It can be seen that at bonding temperatures of 250°C to 450°C, the bonding strength increases significantly compared to the data at room temperature (35°C), with a peak occurring between 330 and 400°C. Furthermore, when the joining temperature becomes too high, as can be seen from the increasing behavior of the mating material adhering to the fracture surface, the diffusion reaction at the joining interface proceeds excessively, and the joining strength decreases. FIG. 8 shows the relationship with the IMC thickness instead of the amount of adhesion of the mating material shown in FIG. At a high bonding temperature of 450°C, the IMC thickness exceeds 1,000 nm (1 μm), and a decrease in bonding strength is observed.

[0018] Figure 9 shows the results of EPMA surface analysis of Cu and Al elements on the fracture surface (near the axis) of the Cu material side obtained in the tensile test with a reduction ratio of R4.2 and a joining temperature of 360°C. It can be seen that Al is adhered to the entire surface of the fracture surface on the Cu material side. It can be seen that the particles are particularly abundant in the form of a fine network. This supports the image of the contaminated layer being split by compression (Fig. 6(b)).

[0019] From the above evaluation results, it can be seen that the reduction ratio R and joining temperature are important factors in the low-temperature forge welding according to the present invention, and from the data in FIG. 6, it can be seen that the reduction ratio R is preferably 1.8 or more. Furthermore, there is an optimum bonding temperature depending on the combination of different materials, and in the case of the combination of C1020 and A5052, it can be said that the bonding temperature range of T℃±50℃, which maximizes the bonding strength, is more preferable (the corresponding IMC thickness is approximately 50nm to 1,000nm (1μm)).

[0020] Next, a test evaluation was performed using copper (C1020) as the first metal layer M1 and commercially pure aluminum (A1200) as the second metal layer M2 (C1020 / A1200). Figure 10 shows the results of investigation into the joining temperature and maximum tensile load at a reduction ratio of R4.4. In the tensile test (at room temperature), the copper side was screwed to a tensile jig, and the pure aluminum side was attached to the tensile jig with an adhesive, and a tensile load was applied in the direction in which the tensile jigs moved apart. Here, the open plots indicate fracture at the bonded interface, and the shaded plots indicate failure at the bonded interface. At bonding temperatures above 150°C, the adhesive broke, and the appropriate bonding temperature range (peak strength) could not be confirmed from this bonding test. At room temperature (25°C), a load of approximately 1,200 N is exerted, but this is due to the mechanical joining effect.

[0021] Figure 11 shows the results of FE-SEM observation of the bonded interface cross section at different bonding temperatures at a reduction ratio of R4.4. The interfacial reaction layer (IMC) at the bonding interface tended to thicken as the bonding temperature increased, reaching a thickness of approximately 100 nm at a bonding temperature of 210°C, 300 nm at a bonding temperature of 255°C, 300-500 nm at a bonding temperature of 310°C, and 1-2 μm at a bonding temperature of 385°C. Furthermore, at the bonding interface at this bonding temperature of 385°C, some cracks were observed in the IMC. Figure 12 shows a cross-sectional TEM image (BF) of the bonding interface of an electrode terminal press-formed at a reduction ratio of R4.4 and a bonding temperature of 310°C. The reaction diffusion layer consisted of a defect-free two-phase IMC (Al2Cu on the A1200 side and Al4Cu9 on the C1020 side), and the total thickness was approximately 400 nm. Figure 13 shows the results of EPMA line analysis of the bonded interface. As with the combination of C1020 and A5052 (Fig. 5(b)), the O chart confirms that the bonded interface is highly clean. Figure 14 shows a graph comparing the IMC growth behavior versus bonding temperature between commercially pure aluminum A1200 and aluminum alloy A5052 (C1020 / A1200 Vs C1020 / A5052). The reduction ratios were similar at 4.4 and 4.2, respectively, and the cleanliness of the joint interface was sufficiently high (Fig. 5(b), Fig. 13). As a result, it was found that there is a difference in the growth behavior of IMCs between aluminum alloys and commercially pure aluminum. Considering the behavior of bonding strength relative to IMC thickness (Fig. 8), the bonding temperature at which the IMC thickness becomes 50 nm to 1 μm (1,000 nm) is in the range of approximately 270 to 410°C for the combination of C1020 and A5052. Taking A5052 as a representative example of an aluminum alloy, it is found that the bonding temperature should be in the range of 270 to 410°C, and in consideration of quality stability in mass production, a range of 350 to 380°C is best to achieve an IMC thickness of 200 to 500 nm. In contrast, in the case of commercially pure aluminum, which is a combination of C1020 and A1200, if the IMC thickness is 50 nm to 1 μm, the bonding temperature is in the range of approximately 210 to 370°C. Furthermore, taking into account quality stability in mass production, the bonding temperature should be in the range of 260 to 330°C to achieve an IMC thickness of 200 to 500 nm.

[0022] Figure 15 shows the results of a fatigue test conducted on a pressure-molded sample (IMC thickness approximately 400 nm) made of a combination of C1020 and A1200, with a reduction ratio of R4.4 and a joining temperature of 310°C. The test was carried out using a hydraulic servo fatigue testing machine at room temperature under pulsating tension (stress ratio 0) at a repetition frequency of 20 Hz. No fracture was observed at the joint under any of the conditions. [Industrial Applicability]

[0023] The joining method according to the present invention enables low-temperature solid-state joining with excellent joining strength and electrical properties, and furthermore, is extremely superior in productivity compared to conventional methods for manufacturing clad terminals. It is useful for manufacturing composite electrode terminals.

Claims

1. A method for manufacturing a composite electrode terminal consisting of a first metal layer and a second metal layer joined together, comprising: In a state where the first metal material that will become the first metal layer is superimposed on the second metal material that will become the second metal layer, the total thickness of the first metal material and the second metal material before joining is t ０ mm, and the thickness of the joint between the first metal layer and the second metal layer after joining is t １ In mm, At a predetermined joining temperature, the reduction ratio R(t ０ / t １ ) is pressure-molded in a range of 1.80 to 5.0, so that the thickness of the IMC at the bonding interface is 1 μm or less, A method for manufacturing a composite electrode terminal, characterized in that the first metal layer is a combination of pure copper (Cu) and the second metal layer is a combination of commercially pure aluminum (Al), and the bonding temperature is in the range of 210 to 370°C.

2. A composite electrode terminal consisting of a first metal layer and a second metal layer joined together, The total thickness of the first metal layer and the second metal layer before bonding is t ０ mm, and the thickness of the joint between the first metal layer and the second metal layer after joining is t １ In mm, Rolling ratio R(t ０ / t １ ) is in the range of 1.80 to 5.0, and the thickness of the IMC at the bonding interface is 1 μm or less, A composite electrode terminal, characterized in that the first metal layer is made of pure copper (Cu) and the second metal layer is made of commercially pure aluminum (Al).

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