Method for manufacturing metal joined body

Abstract

To provide a novel manufacturing method with which it is possible to obtain a metal joined body in which a high tensile strength steel plate or a hot stamp steel plate, etc. is firmly joined with an aluminum base body.SOLUTION: The present invention is a method for manufacturing a metal joined body, the method comprising an energization step for energizing a joining section of a steel plate sandwiched and held by electrodes and an aluminum base body. The thickness of a to-be-joined section of the steel plate is 1-3.5 mm. The energization step is carried out by energizing 3-9.5 kA for 300-1200 ms. The energization step may be composed of a first energization step for energizing 200-400 ms in 3-8 kA and a second energization step for energizing 300-700 ms in 5-9 kA. According to the manufacturing method of the present invention, a metal joined body is obtained in which a steel sheet and an aluminum base body are joined with an intermetallic compound layer interposed therebetween.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a method for producing a metal joined body comprising a steel plate and an aluminum substrate. [Background technology]

[0002] In order to respond to weight reduction, high functionality, structural changes, etc., components in which dissimilar metals are bonded (called "metal bonded bodies") are used. For example, roofs, outer panels, etc. in which lightweight aluminum alloy plates that form the exterior surface are bonded to steel plates that form the skeleton or frame are used in automobiles.

[0003] Up until now, dissimilar metals have mainly been joined by riveting (self-piercing rivet joining, etc.) or clinching. Such mechanical joining methods require auxiliary materials (rivets, etc.) and dedicated equipment, which increases the weight and cost of the joined body. In addition, protrusions that form at mechanically joined parts can cause interference with surrounding parts and reduce workability.

[0004] Recently, therefore, there have been many proposals to perform resistance welding (eg, spot welding) of dissimilar metals (eg, a steel plate and an aluminum alloy plate), and for example, there are related descriptions in the following patent documents. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent Publication No. 2022-63070 [Patent Document 2] Patent Publication No. 2023-66438 Summary of the Invention [Problem to be solved by the invention]

[0006] In both patent documents, a current of about 13 kA is passed to join a steel plate and an aluminum alloy plate (

[0057] in Patent Document 1 /

[0076] in Patent Document 2). In addition, the steel plate joined to the aluminum alloy plate is a thin low-carbon steel plate (270 MPa class / plate thickness: 0.8 mm) (

[0051] in Patent Document 1 /

[0054] in Patent Document 2).

[0007] The above current value is sufficiently smaller than the large current value (over 15 kA) that directly melts the aluminum alloy sheet itself by resistance heating alone, as in the past. However, it has been newly discovered that when, for example, thick high-tensile steel or hot stamped steel is spot welded to an Al alloy sheet at the current values ​​described in both patent documents, spatter (surface spatter caused by molten beads scattering from between the electrode and the joined material, internal spatter caused by molten beads scattering from the joint between the joined materials, etc.) may occur.

[0008] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a new manufacturing method for obtaining a metal joined body of a steel plate and an aluminum substrate. [Means for solving the problem]

[0009] As a result of intensive research into solving this problem, the inventor came up with the idea of ​​melting the area near the joining surface of the aluminum base by heat transfer from a resistance-heated steel plate, and successfully realizing this idea. By expanding on this result, the inventor has completed the present invention, which will be described below.

[0010] <<Method for manufacturing a metal bonded body>> The present invention is a manufacturing method including a current passing step of passing a current to a joint portion of a steel sheet and an aluminum base held between electrodes, the joint portion of the steel sheet having a thickness of 1 to 3.5 mm, and the current passing step is performed by passing a current of 3 to 9.5 kA for 300 to 1200 ms, to obtain a metal joined body in which the steel sheet and the aluminum base are joined via an intermetallic compound layer.

[0011] According to the manufacturing method of the present invention, when a thick steel plate having a high electrical resistance is resistance-welded to an aluminum substrate, it is possible to obtain a metal joint excellent in strength and reliability while suppressing the generation of spatter. The reason or mechanism for the manifestation of such excellent effects is currently considered to be as follows.

[0012] The amount of heat generated by passing electricity through the materials to be joined (Q) is roughly determined by the electrical resistance (R), the current value (I), and the time the current is passed (t) (Q=I 2 × R × t). Electrical resistance (R) is determined by the material (components, structure, etc.) and thickness of the materials being joined. In particular, steel plate has a much higher electrical resistance than an aluminum substrate of roughly the same thickness, and the higher the strength, the higher the electrical resistance can become.

[0013] When a current of a conventional level is applied to a thick steel plate having a high electrical resistance, the steel plate is rapidly overheated and spatters are likely to occur. In the present invention, a current of a relatively low value is applied to such a steel plate for a long period of time, so that spatters are unlikely to occur.

[0014] Even if the materials to be joined do not melt due to the current flow, the vicinity of the joining surface of the aluminum base in contact with the steel plate can melt due to the heat conducted from the steel plate (especially near the center) which has become hot due to resistance heating. This initiation of melting causes a melting reaction (for example, solid-liquid interdiffusion reaction: SLID) near the joining surface, generating an intermetallic compound layer between the steel plate and the aluminum base, allowing the two to be firmly joined (welded).

[0015] 《Metal joint》 The present invention can also be understood as a metal joined body. For example, the present invention may be a metal joined body in which a steel plate having a thickness of 1 to 3.5 mm is resistance-welded to an aluminum base. An example of the manufacturing method thereof is as described above.

[0016] The bonding layer of the joined materials (steel plate and aluminum substrate) does not necessarily have to be an intermetallic compound (layer) in its entirety. A part of the joined materials (e.g., iron substrate or aluminum substrate) may remain or be mixed in the bonding layer. Furthermore, the bonding layer may contain components of the coating layer that was on the joined part (e.g., Zn, O, etc.).

[0017] "others" (1) Unless otherwise specified, the thickness and electrical resistance of the "joined parts" in this specification are specified in the state before joining (before the current application process). When it is not possible to directly measure the joined parts (for example, after joining), it is advisable to estimate the characteristics of the joined parts by measuring the areas where the state before joining is almost maintained (the outer periphery of the joint or heat-affected zone (HAZ)).

[0018] (2) Unless otherwise specified, "x to y" in this specification includes a lower limit value x and an upper limit value y. Any numerical value included in the various numerical values ​​or numerical ranges described in this specification may be used as a new lower limit or upper limit value to create a new range such as "a to b". Furthermore, unless otherwise specified, "x to ykA" in this specification means xkA to ykA. The same applies to other unit systems (ms, etc.). [Brief description of the drawings]

[0019] [Figure 1] FIG. 1 is a schematic diagram illustrating an overview of spot welding. [Diagram 2] 4 is an example of a time chart relating to the spot welding. [Diagram 3] 1 is a micrograph showing a fracture surface of a joint (sample 1). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] One or more components selected arbitrarily from the present specification may be added to the above-mentioned components of the present invention. The contents described in this specification may be appropriately applied to not only the manufacturing method but also the product (metal bonded body, etc.). Which embodiment is best depends on the target, required performance, etc.

[0021] 《Steel plate》 The steel plate according to the present invention may have the following characteristics.

[0022] (1) Thickness The thickness of the steel plate (plate thickness) is, for example, 1 to 3.5 mm, 1.4 to 3.2 mm, 1.6 to 3 mm, or 1.8 to 2.6 mm. The plate thickness in this specification refers to the thickness of the parts to be joined before being used for joining.

[0023] If the plate thickness is too large, the amount of heat generated when current is applied is also too large, making it easier for spatters to occur. If the plate thickness is too small, the amount of heat generated is reduced and the amount of heat dissipated is increased, making it difficult to sufficiently heat the area near the surface to be joined of the aluminum substrate.

[0024] (2) Tensile strength / composition The steel sheet is, for example, a high tensile strength steel sheet (so-called high tensile strength steel sheet) or a hot stamp steel sheet. The tensile strength of the high tensile strength steel sheet is, for example, 590 MPa or more, 780 MPa or more, 980 MPa or more, or 1180 MPa or more. The tensile strength of the hot stamp steel sheet after quench forming is, for example, 780 MPa or more, 980 MPa or more, 1180 MPa or more, 1380 MPa or more, or 1580 MPa or more.

[0025] Such steel sheets contain a large amount of alloy elements such as Ni, Si, or Mn in addition to C, and the composition is usually adjusted taking into consideration formability as well as strength. Furthermore, the steel sheets have a metal structure according to the strengthening mechanism (solid solution strengthening, precipitation strengthening, quenching, etc.).

[0026] (3) Covering layer The steel sheet may have a coating layer (or a surface treatment layer) at least on the aluminum substrate side of the joined portion. Of course, the coating layer may be provided entirely on one or both sides of the steel sheet. The sheet thickness and electrical resistance of the steel sheet (before joining) referred to in this specification include the coating layer. The thickness of the coating layer is usually 1 to 30 μm.

[0027] The coating layer is, for example, a zinc plating layer, a zinc oxide layer, a Zn-Fe alloy layer, etc. Examples of steel sheets having such a coating layer include galvanized steel sheets, such as hot stamp steel sheets, hot-dip galvanized steel sheets, electrogalvanized steel sheets, and alloyed hot-dip galvanized steel sheets, each having a coating layer containing Zn (such as a Zn-Fe alloy layer). The alloyed hot-dip galvanized steel sheet may be an aluminum-zinc alloy-plated steel sheet (so-called Galvalume Steel Sheet (registered trademark)).

[0028] Incidentally, the more the coating layer is alloyed or oxidized, the greater the electrical resistance and the greater the tendency for it to generate heat. When a relatively low current is applied for a long period of time as in the present invention, the occurrence of spatters can be suppressed even with such a coating layer. On the contrary, such a coating layer can promote a melting reaction or the formation of an intermetallic compound layer near the joining surfaces.

[0029] <Aluminum base> The aluminum substrate may be a wrought material or a cast material. Wrought materials include, for example, rolled materials, (hot) extruded materials, forged materials, etc. Cast materials include, for example, die-cast materials, metal mold cast materials, sand cast materials, etc. The aluminum substrate may have any form (shape, size), thickness of the joined portion, electrical resistivity, strength, composition, coating treatment, etc.

[0030] The aluminum substrate is preferably made of an aluminum alloy containing Mg and / or Si. Such an aluminum alloy is easily melted at the interface to be joined with the steel plate. Examples of wrought materials include A4000, A5000, and A6000 (JIS) types. Examples of cast materials include ADC10, ADC12, and AC4 (JIS) types. The aluminum alloy may contain other alloying elements (Cu, Mn, Zn, Fe, Ti, etc.).

[0031] The aluminum substrate is not limited to a plate material. If at least the portion to be joined of the aluminum substrate is plate-shaped, resistance welding becomes easy. The thickness of the portion to be joined is not important, but is, for example, 1.4 to 4 mm, 1.8 to 3.6 mm, or 2.2 to 3.2 mm.

[0032] 《Metal joint》 The metal joined body is not limited to one in which a single steel plate having the above-mentioned characteristics is joined to an aluminum substrate. The metal joined body may have another substrate (e.g., a steel plate) on the back side of the above-mentioned steel plate (the side opposite to the aluminum substrate). The other substrate may have any shape, thickness, electrical resistivity, strength, composition, coating treatment, etc. The other substrate may be a steel plate (hot-rolled steel plate, cold-rolled steel plate, high-strength steel plate, hot-stamped steel plate, etc.) or a metal body (plate) different from the steel plate.

[0033] The metal joined body may be one in which a plurality of steel plates and an aluminum base are joined by the method of the present invention. There is no restriction on the presence or absence of joining between the plurality of steel plates. In the case where an aluminum base is joined by the method of the present invention to a joint between previously joined steel plates, the joint (thickness) between the steel plates may be considered as the "joined portion (thickness) of the steel plate" in the present invention.

[0034] 《Manufacturing method》 The metal joint is made by passing an electric current between the pressure-welded steel plate and aluminum substrate (electric current passing step).

[0035] (1) Current value The value of the current applied to the portion of the steel sheet to be joined is, for example, 3 to 9.5 kA, 4 to 9 kA, 4.5 to 8 kA, or 5 to 7.5 kA. The current density, calculated by dividing the current value by the tip area of ​​the electrode on the steel sheet side, is, for example, 26.5 to 84.0 A / mm 2 or 44.2~66.3A / mm 2 It is also possible to use the following.

[0036] (2) Power-on time The current application time (total time per joint) is, for example, 300 to 1200 ms, 400 to 1000 mm, or 500 to 900 ms. If the current application time is too short, the intermetallic compound that provides the joint strength is not sufficiently generated on the joint surface of the aluminum base. If the current application time is too long, adhesion occurs between the electrode and the aluminum base, reducing productivity and reducing joint quality.

[0037] (3) Current flow pattern The current flow process may be composed of a plurality of processes. For example, by carrying out a first current flow process in which the current value at the start of current flow is relatively small, followed by a second current flow process in which the current value is relatively large, it becomes possible to form a sufficient joint (nugget) while effectively suppressing spattering.

[0038] The current value in the first current application step is, for example, 3 to 8 kA, 3.5 to 7.5 kA, or 4 to 7 kA. The current application time in the first current application step is, for example, 200 to 400 ms or 250 to 350 ms.

[0039] The current value in the second current application process is, for example, 5 to 9 kA or 6 to 8 kA. The current application time in the second current application process is, for example, 300 to 700 ms or 400 to 600 ms.

[0040] The current value in the first current flow process may be an up-slope current flow that gradually increases the current. The up-slope may be linear or curved. It is preferable that the current values ​​at the start and end of the first current flow process are within the above-mentioned range, and the current value at the end of the first current flow process is approximately equal to the current value at the start of the second current flow process. Such an up-slope current flow shortens the first current flow process and suppresses spattering, enabling efficient joining. The first current flow process may also serve as a pre-current flow process that familiarizes the contact state between the steel sheet and the aluminum substrate.

[0041] (4) Cooling process The parts to be joined after the current application step may be naturally cooled or may be cooled through the electrodes (cooling step). The latter cooling step may be performed by cutting off the current (non-current application), by applying a small current value, or by monotonically decreasing the current value (downslope current application). In the case of joining hot stamped steel sheets, the parts to be joined after the current application step may be rapidly cooled.

[0042] "electrode" Electric current is usually applied through electrodes pressed against the steel sheet and aluminum base. The electrodes are selected according to the characteristics (material, shape, etc.) of the steel sheet and aluminum base to be joined. An example of an electrode for resistance spot welding is described below.

[0043] (1) Form The electrode may be detachable from the shank (cap-tip type) or integral with the shank (integral type). Usually, cap-tip type electrodes (also called "tips") are used to reduce costs.

[0044] The electrode (tip) has, for example, a bottomed, generally cylindrical tip portion and a generally cylindrical body portion continuing from the tip portion. The outer surface (pressure contact surface) of the tip portion may be convex or concave. The size of the electrode is not important. The outer diameter (nominal diameter / D2) of the body portion is, for example, φ10 to 20 mm, or φ12 to 18 mm. The tip diameter (D1) is, for example, φ6 to 14 mm, or φ8 to 12 mm.

[0045] It is advisable to introduce a coolant (cooling liquid / cooling water) into the inner tube at the inside of the tip of the electrode. When the coolant is forcibly circulated, it is possible to suppress the temperature rise of the electrode and to stably cool the plate material through the electrode.

[0046] The basic shape of the tip of an electrode (especially a convex electrode) is specified in JIS C9304 (1999). For example, there are flat (F type), radius (R type), dome (D type), dome radius (DR type), truncated cone (CF type), truncated cone radius (CR type), etc. For spot welding of steel plate and aluminum substrate, it is recommended to use, for example, DR-type or R-type electrodes from the viewpoint of versatility.

[0047] (2)Material The electrode (at least the tip) is preferably made of a material having excellent thermal conductivity, electrical conductivity, strength, etc. For example, an electrode made of a copper alloy having an electrical conductivity of 75 to 95% IACS, or even 80 to 90% IACS, is used. Examples of copper alloys include chromium copper, zirconium copper, chromium-zirconium copper, alumina-dispersed copper, and beryllium copper.

[0048] The electrode in contact with the steel plate and the electrode in contact with the aluminum substrate may be the same or different in form (shape, size (diameter)) and material.

[0049] (3) Pressure The electrodes clamp the steel sheet and the aluminum substrate with, for example, 2 to 7 kN, or further 3 to 6 kN. If this pressure is too low, spatter (dust) is likely to occur. If the pressure is too high, deep dents (depressions) are likely to occur due to the electrodes, and deformation and wear of the electrodes are likely to occur. The pressure is usually kept approximately constant, but may vary during the process.

[0050] "others" (1) The metal joint (particularly the joint) may be heat-treated (annealed, tempered, etc.) after welding. The heat treatment can adjust the structure of the joint (nugget) and its vicinity (heat-affected zone), remove residual stress, etc. The heating temperature and heating time may be adjusted depending on the composition and thickness of the steel sheets.

[0051] (2) The metal bonded body can be used for various components and structures. For example, at least a part (metal bonded body) of a vehicle frame, panel, housing, storage compartment, etc. can be obtained by the production method of the present invention. EXAMPLES

[0052] A metal joint was produced by resistance spot welding a steel plate and an aluminum substrate, and the joint strength, fracture surface, etc. were evaluated. The present invention will be described in more detail with reference to such specific examples.

[0053] An overview of spot welding is shown in Figure 1. A sheet assembly in which a steel sheet and an Al alloy sheet are laminated is used as the material to be welded. Spot welding is performed by passing current through a pair of identical electrodes that are pressed against each surface of the sheet assembly (the upper surface of the Al alloy sheet and the lower surface of the steel sheet). In this embodiment, for convenience of explanation, unless otherwise specified, the direction of the arrows shown in Figure 1 is the up-down direction or the left-right direction.

[0054] <Sample Preparation> (1) Materials to be welded The steel plate used was a hot stamped material (thickness: 2.2 mm) made by hot forming boron steel with a zinc plating layer. Both sides of the plate were covered with an Fe-Zn alloy layer (average thickness: 15 μm).

[0055] The aluminum alloy plates used were die-cast material (ADC10 / JIS) and hot-extruded material (A6005C / JIS). The plate thickness for both was 3.0 mm. The melting point (solidus temperature) of the die-cast material was 565°C. The melting point (solidus temperature) of the hot-extruded material was 590°C.

[0056] Each plate material was cut into a rectangular shape (30 mm x 100 mm) and was directly subjected to spot welding without surface polishing or the like.

[0057] (2) Electrode A commercially available DR-type (JIS C9304) tip (manufactured by OBARA Corporation) was used for the electrode. Forced circulating cooling water (flow rate: 2.7 L / min) was supplied to the inside (inner cylindrical part) of the tip to forcibly cool the tip. The electrode was made of chromium copper (Cr: 1 mass%, Cu: balance), and its electrical conductivity was 80% IACS.

[0058] As shown in the enlarged view of Figure 1, the electrode size was as follows: tip diameter (nominal diameter D2): φ16 mm, thickness of the bottom tip: 12 mm, radius of curvature of the tip shoulder: 8 mm, radius of curvature of the tip surface: R1: 40 mm, and tip diameter (D1) was 12 mm.

[0059] (3) Welding Spot welding was performed using a servo pressure spot welding machine (ARO PA235KVAMF). An example of the current pattern is shown in Figure 2. The electrode pressure (F) for the plate assembly was kept constant at 4kN. Current was applied by controlling the DC current as follows:

[0060] A first current flow process was performed with a first current value of 4 to 7 kA and a first current flow time of 300 ms. The first current value was monotonically increased at a constant rate of change from 4 kA at the start of current flow to 7 kA at the end of current flow (linear upslope current flow).

[0061] Following the first current application process, a second current application process was carried out with a second current value of 7 kA (constant) and a second current application time of 500 ms.

[0062] After the second current application process, the current was cut off, and the electrode was removed from the workpiece after 200 ms had elapsed. In this way, a joint of the hot stamped material and the die cast material (Sample 1) and a joint of the hot stamped material and the hot extruded Al alloy material (A6005C) (Sample 2) were obtained.

[0063] "measurement" Each sample was subjected to a tensile shear test (JIS Z 3136) to measure the load at break. This measurement was repeated three times to obtain the tensile shear strength (average value). For example, sample 1: 3894N (-3σ: 277N). When the tensile shear strength was divided by the area of ​​the broken part, sufficient bond strength exceeding 80 to 100 MPa was ensured for all samples.

[0064] "observation" Figure 3 shows a photograph of the bonded surface (fracture surface on the steel plate side) of sample 1 after the tensile test, observed with an inverted metallurgical microscope (Olympus GX53). An intermetallic compound layer was present on the bonded surface (outer circular area). Remnants of the broken die-cast material were also observed near the center of the bonded surface (inner circular area). As can be seen from Figure 3, the steel plate and the Al alloy plate were bonded with a sufficiently large intermetallic compound layer (nugget), and it was also confirmed that the bond strength was equal to or greater than the strength of the bonded materials (Al alloy plates).

[0065] From the above, it was confirmed that according to the manufacturing method of the present invention, even if a thick steel plate having a high electrical resistivity is used, a dissimilar metal joint body in which the steel plate is firmly joined to the aluminum substrate can be obtained.

Claims

1. The joining step includes a current application step of applying current to the steel plate and the aluminum substrate that are sandwiched between the electrodes, The joined portion of the steel plate has a thickness of 1 to 3.5 mm, The current application step is carried out by applying a current of 3 to 9.5 kA for 300 to 1200 ms, The method for producing a metal joined body is provided in which the steel plate and the aluminum substrate are joined via an intermetallic compound layer.

2. 2. The method for producing a metal joined body according to claim 1, wherein the steel plate is a high-tensile steel plate having a tensile strength of 590 MPa or more.

3. 3. The method for producing a metal joined body according to claim 1, wherein the steel sheet is a hot stamp steel sheet, a hot dip galvanized steel sheet or a galvannealed steel sheet.

4. The method for producing a metal joined body according to claim 3, wherein the joined portion of the hot stamped steel sheet has a coating layer containing Zn at least on the aluminum base side.

5. 2. The method for producing a metal joined body according to claim 1, wherein the joined portion of the aluminum base has a thickness of 1.4 to 4 mm.

6. 6. The method for producing a metal joined body according to claim 1, wherein the aluminum substrate is made of a wrought material or a cast material.

7. The method for producing a metal joined body according to claim 1 , wherein the aluminum substrate contains Mg and / or Si.

8. The current application step includes a first current application step of applying current at 3 to 8 kA for 200 to 400 ms; A second current application process of applying current at 5 to 9 kA for 300 to 700 ms; The method for producing a metal joined body according to claim 1 , comprising:

9. The method for producing a metal joint body according to claim 8 , wherein the first current application step is performed while gradually increasing the current.

10. 2. The method for producing a metal joined body according to claim 1, wherein the electrode presses the joined parts with a pressure of 2 to 7 kN.

11. The method for producing a metal joined body according to claim 1 , wherein the joined parts are resistance spot welded.

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