Resistance spot welding method

Abstract

To provide a resistance spot welding method capable of obtaining a metal joined body in which an iron base body and an aluminum base body are firmly joined.SOLUTION: There is provided a resistance spot welding method for an object to be joined in which an iron base body and an aluminum base body are overlapped. The resistance spot welding method includes a first energization step of energizing the object to be joined, and a second energization step of energizing the object to be joined after the first energization step. The object to be bonded has a reaction layer containing Zn between the iron base body and an aluminum base body. In the first energization step, a precursor layer in which Al and Zn are substituted with each other between the aluminum base body and the reaction layer is generated without melting the aluminum base body. In the second energization step, at least the precursor layer is melted to generate a bonding layer composed of an intermetallic compound containing Fe and Al.SELECTED DRAWING: Figure 2D

Description

Technical Field

[0001] The present invention relates to a resistance spot welding method for an iron substrate and an aluminum substrate, etc.

Background Art

[0002] In order to cope with weight reduction, high functionality, structural changes, etc., members obtained by joining dissimilar metals (referred to as "metal joints") are used. For example, a roof, an outer panel, etc. in which a lightweight aluminum alloy plate constituting an exterior surface is joined to a steel plate forming a skeleton or a frame are used in automobiles.

[0003] Dissimilar metals have hitherto been mainly joined by self-piercing rivet joining (such as self-piercing rivet joining) or clinching. Such mechanical joining methods require auxiliary materials (rivets, etc.) and dedicated equipment, increasing the weight and cost of the joined body. In addition, the protrusions formed at the mechanically joined parts may interfere with peripheral members or reduce the assembly workability.

[0004] Therefore, recently, dissimilar metals (for example, a steel plate and an aluminum alloy plate) have also come to be resistance spot welded, and for example, there are proposals related to the following patent documents.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] In Patent Document 1, a 6000 series aluminum alloy plate and a zinc-plated steel plate (plating thickness: about 20 μm) are sandwiched between electrodes with a pressure of 300 kgf, and energization of about 3 kA × 240 ms is performed only once to resistance spot weld the two (

[0048] ,

[0049] ). Due to this energization, the eutectic molten metal generated between the bonding interfaces discharges the oxide film on the bonded surfaces, suppresses the formation of intermetallic compounds, and the exposed fresh surfaces are firmly bonded together (

[0010] ,

[0015] ,

[0024] ,

[0056] , etc.).

[0007] The present invention has been made in view of such circumstances, and an object thereof is to provide a resistance spot welding method or the like that can firmly bond an iron substrate and an aluminum substrate by a mechanism different from the conventional one.

Means for Solving the Problems

[0008] As a result of intensive research, the present inventor has newly found that when a precursor layer containing Al and Zn is previously formed in the vicinity of the bonding interface (bonding portion) between the iron substrate and the aluminum substrate, a bonding layer composed of an intermetallic compound of Fe and Al is sufficiently thickly formed, and the iron substrate and the aluminum substrate can be bonded (spot welded) with high strength. By developing this result, the present invention described below has been completed.

[0009] 《Resistance Spot Welding Method》 The present invention is a resistance spot welding method for a workpiece in which an iron substrate and an aluminum substrate are overlapped, and includes a first energization step of energizing the workpiece and a second energization step of energizing the workpiece after the first energization step. The workpiece has a reaction layer containing Zn between the iron substrate and the aluminum substrate. The first energization step generates a precursor layer in which Al and Zn are mutually substituted between the aluminum substrate and the reaction layer without melting the aluminum substrate. The second energization step is a resistance spot welding method for melting at least the precursor layer to generate a bonding layer composed of an intermetallic compound containing Fe and Al.

[0010] According to the resistance spot welding method of the present invention (also simply referred to as "spot welding"), by passing through the formation of an Al-Zn-based precursor layer, a bonding layer composed of an Fe-Al-based intermetallic compound can be formed thick enough between the iron substrate and the aluminum substrate, enabling the two to be joined with high strength.

[0011] 《Metal bonded body》 The present invention can also be understood as a metal bonded body in which an iron substrate and an aluminum substrate are joined by the above-described resistance spot welding method.

[0012] 《Others》 Unless otherwise specified, "x to y" as used in this specification includes the lower limit value x and the upper limit value y. For any numerical value included in the various numerical values or numerical ranges described in this specification, a new range such as "a to b" can be newly established with the new numerical value as the lower limit value or the upper limit value. Also, unless otherwise specified, "x to y kA" as used in this specification means x kA to y kA. The same applies to other unit systems (such as ms).

Brief description of the drawings

[0013]

Figure 1A

Figure 1B

Figure 1C

Figure 1D

Figure 2A

Figure 2B

Figure 2C

Figure 2D

Modes for carrying out the invention

[0014] One or more components arbitrarily selected from this specification can be added to the components of the present invention described above. The content described in this specification can also be appropriately applied to manufacturing methods and objects (such as metal joints). Whether any embodiment is the best depends on the object, required performance, etc.

[0015] 《Iron substrate and aluminum substrate》 (1) The iron substrate and the aluminum substrate are not limited by the material (component composition) or form. The iron substrate may be pure iron or an iron alloy (especially steel). The aluminum substrate may be pure aluminum or an aluminum alloy. The iron substrate and the aluminum substrate may have the same form or different forms. Hereinafter, for the sake of convenience of explanation, while taking a steel plate and an aluminum alloy plate (referred to as "Al alloy plate") as representative examples of the iron substrate and the aluminum substrate, the explanation will be given.

[0016] The steel plate and the Al alloy plate may each be one sheet, or at least one of them may be a plurality of sheets. The plurality of plate materials may have the same or different thicknesses, component compositions, surface treatment states (presence or absence of plating, etc.).

[0017] (2) The steel plate is, for example, a cold-rolled steel plate, a hot-rolled steel plate, a high-tensile steel plate (so-called High Tensile Strength Steel Sheets), a hot-stamped (pressed) steel plate, etc. The high-tensile steel plate has, for example, a tensile strength of 590 MPa or more, 780 MPa, 980 MPa or more, or 1180 MPa or more. The tensile strength of the hot-stamped steel plate after heat treatment (quenching and forming) is, for example, 780 MPa, 980 MPa or more, 1180 MPa or more, 1470 MPa or more, or 1580 MPa or more.

[0018] The thickness of the steel plate (plate thickness) is, for example, 0.4 to 3.5 mm, 0.8 to 3.2 mm, 1 to 3 mm, 1.4 to 2.8 mm, or 1.6 to 2.6 mm. The iron substrate may be a single steel plate with a desired thickness or a stack of multiple steel plates with a desired thickness. Note that the thickness (plate thickness) referred to in this specification is the thickness before joining, and is the thickness of the region to be spot welded (the portion clamped by the electrodes). That is, this specification does not target regions or parts not related to spot welding, and their forms and characteristics are not questioned.

[0019] (3) The aluminum substrate may be a wrought material or a cast material. The wrought material is, for example, a rolled material, a (hot) extruded material, a forged material, etc. For the wrought material, for example, A4000 series, A5000 series, A6000 series (JIS), etc. are used. The cast material is, for example, a die-cast material, a permanent mold casting material, a sand mold casting material, etc. For the cast material, for example, ADC10 series, ADC12 series, AC4 series (JIS), etc. are used. The aluminum substrate is not questioned in terms of form (shape, size), thickness, electrical resistivity, strength, composition, coating treatment, etc.

[0020] The aluminum substrate only needs to have a form that can be clamped by a pair of electrodes, and it does not have to be entirely a plate material. The thickness of the aluminum substrate is not questioned, but for example, it is 1 to 4 mm, 1.8 to 3.6 mm, or 2.2 to 3.2 mm.

[0021] 《Reaction layer》 The reaction layer may be between the joint surfaces of the iron substrate and the aluminum substrate (between the joint interfaces). The reaction layer only needs to contain Zn that can be replaced with Al in the aluminum substrate, and may be composed of any of pure zinc, zinc alloys (Zn-Fe alloy, Zn-Al alloy, etc.), zinc compounds, etc.

[0022] The reaction layer is, for example, a surface treatment layer (e.g., a plating layer) provided on at least the joint surface of a steel plate. Specific examples include zinc-based plated steel plates such as hot-dip galvanized steel plates, electro-galvanized steel plates, and alloyed hot-dip galvanized steel plates. Since such plated steel plates also serve as the iron substrate and the reaction layer, there is no need to separately provide the reaction layer. The reaction layer may be one in which the initial surface treatment layer has its components, structure, etc. changed by heating (quenching, etc.) before spot welding, forming, or the like.

[0023] 《Preliminary Layer》 The preliminary layer is a mixed layer of Zn and Al (an alloy layer and / or a compound layer). The preliminary layer is formed by the mutual substitution of Zn contained in the reaction layer and Al contained in the aluminum substrate in contact with the reaction layer between the contacting reaction layer and the aluminum substrate. It is considered that the substitution of Zn and Al occurs due to the solid-phase reaction between the reaction layer and the aluminum substrate, causing mutual diffusion of Zn and Al.

[0024] The preliminary layer is generated at least across the contact interface between the reaction layer and the aluminum substrate. The entire reaction layer may become the preliminary layer.

[0025] 《Bonding Layer》 The bonding layer mainly consists of an Fe-Al-based intermetallic compound (also simply referred to as "IMC"). The IMC is generated by the melting reaction between the iron substrate and the aluminum substrate. It is considered that the preliminary layer itself or the Zn contained in the preliminary layer acts like a catalyst to promote the formation of the IMC.

[0026] The IMC is, for example, Fe 2 Al 5 or FeAl 3 etc. On the iron substrate side, for example, a (first) bonding layer mainly composed of Fe 2 Al 5 is formed. On the aluminum substrate side, for example, a (second) bonding layer mainly composed of FeAl 3 is formed.

[0027] The overall thickness of the bonding layer or IMC layer is, for example, 2 to 15 μm, 3 to 10 μm or 4 to 8 μm. Depending on the energization conditions, for example, the thickness of the first bonding layer is 1 to 10 μm, 2 to 7 μm, and further 3 to 5 μm.

[0028] The bonding layer may contain protrusions remaining in an unmelted state from the iron substrate side or the aluminum substrate side. The thickness of the bonding layer and the form of the protrusions (size, shape, distribution, etc.) are evaluated based on an image (SEM image) obtained by observing the cross-section of the joint portion with a microscope.

[0029] 《Spot Welding》 Spot welding is performed by applying the following energization to a pair of electrodes sandwiching an iron substrate and an aluminum substrate.

[0030] (1) First Energization Step The first energization step is performed to form a precursor layer in which Al and Zn are mutually substituted between the reaction layer and the aluminum substrate. In the first energization step, since it is not necessary to melt the iron substrate or the aluminum substrate, less input heat is required than in the second energization step. Such a first energization step may be performed by energizing for 200 to 400 ms or 250 to 350 ms within, for example, 3 to 8 kA or 3.5 to 7.5 kA. Also, the first energization step may be performed by energizing for 30 to 80 ms or 40 to 70 ms within 8 to 10 kA or 8.5 to 9.5 kA.

[0031] (2) Second Energization Step The second energization step is performed to cause a melting reaction between the iron substrate and the aluminum substrate sandwiching the precursor layer to generate an Fe-Al based intermetallic compound (bonding layer). The second energization step has a larger input heat than the first energization step, but does not require as large an input heat (amount of energization) as directly melting the aluminum substrate by resistance heating (Joule heating). Such a second energization step may be performed by energizing for 300 to 700 ms or 400 to 600 ms within, for example, 6 to 9 kA or 6.5 to 8.5 kA. Also, the second energization step may be performed by energizing for 200 to 400 ms or 250 to 350 ms within 11 to 15 kA or 12 to 14 kA.

[0032] Although a pre - energization process for adapting the contact state between the iron substrate and the aluminum substrate may be separately performed, the first energization process may also serve as the pre - energization process. The current value during the energization process may be constant or variable. At least one of the energization processes may be, for example, up - slope energization in which the current is gradually increased.

[0033] The first energization process and the second energization process may be performed continuously, or may be performed with a cooling process for interrupting (non - energizing) or reducing the energization intervening between the processes. The joint after the second energization process is subjected to natural air cooling, forced cooling through an electrode, etc. This cooling may be performed by down - slope energization in which the current value is monotonically decreased.

[0034] 《Electrode》 The electrode for spot welding may be a detachable one (cap - tip type) on the shank or an integrated one (integral type) with the shank.

[0035] The basic shape of the tip of the electrode (especially the convex electrode) is defined in JIS C9304 (1999). For example, there are a flat shape (F - shape), a radius shape (R - shape), a dome shape (D - shape), a dome - radius shape (DR - shape), a truncated - cone shape (CF - shape), a truncated - cone radius shape (CR - shape), etc. The DR - shaped and R - shaped electrodes have versatility.

[0036] The size of the electrode is not limited. The outer diameter (nominal diameter / D2) of the body part is, for example, φ10 - 20 mm, and further φ12 - 18 mm. The tip diameter (D1) is, for example, φ6 - 14 mm, and further φ8 - 12 mm. Each electrode in contact with the iron substrate and the aluminum substrate may have the same or different forms (shape, size (diameter)) and materials.

[0037] Refrigerant (coolant / cooling water) may be introduced into the inner cylinder part inside the tip of the electrode. When the refrigerant is forced to circulate, the temperature rise of the electrode can be suppressed and the cooling of the plate material through the electrode can be stably performed.

[0038] The electrode (at least the tip) is preferably made of a material excellent in thermal conductivity, electrical conductivity, strength, etc. The electrode is preferably made of, for example, a copper alloy having a conductivity of 75 to 95% IACS, more preferably 80 to 90% IACS. Such copper alloys include, for example, chromium copper, zirconium copper, chromium-zirconium copper, alumina-dispersed copper, beryllium copper, and the like.

[0039] The pressing force of the electrode sandwiching the iron substrate and the aluminum substrate is, for example, 2 to 7 kN, more preferably 3 to 6 kN. The pressing force may be constant or variable.

[0040] 《Metal Bonded Body》 The metal bonded body of the iron substrate and the aluminum substrate constitutes, for example, a vehicle skeleton (such as a platform), a panel (such as a roof panel, a side outer panel), a housing, a storage chamber, and the like.

Example

[0041] A sample (metal bonded body) in which a steel plate (iron substrate) and an Al alloy plate (aluminum substrate) were spot-welded was fabricated, and the cross-section of the joint was observed. While exemplifying such a specific example, the present invention will be described in more detail.

[0042] [First Embodiment] The outline of the spot welding according to this embodiment is shown in FIG. 1A. As the workpiece, a plate stack in which a first steel plate, a second steel plate, and an Al alloy plate were laminated in this order was used. The spot welding is performed by energizing a pair of electrodes pressed against each surface of the plate stack (the lower surface of the first steel plate and the upper surface of the Al alloy plate). In this embodiment, for the sake of convenience of explanation, unless otherwise specified, the direction of the arrow shown in FIG. 1A is taken as the vertical direction or the horizontal direction (the same applies to other figures).

[0043] 《Fabrication of Sample》 (1) Workpiece For the first steel plate, a cold-rolled steel plate (440 MPa grade / thickness: 1.4 mm) which is a non-plated steel plate was used. For the second steel plate, an alloyed hot-dip galvanized steel plate (270 MPa grade / thickness: 0.8 mm) was used. For the Al alloy plate, a material equivalent to JIS A6022 (elongation material / thickness: 1.2 mm) was used respectively. The plating layer (reaction layer) of the second steel plate was about 8 μm thick. The melting point of the plating layer itself was about 420 °C, and the melting point of the Al alloy plate was about 650 °C.

[0044] Note that the alloyed hot-dip galvanized steel plate (GA steel plate: Galva-Annealed Steel) is obtained by heat-treating a hot-dip galvanized steel plate to alloy zinc in the plating layer with the base iron. This plating layer usually consists of a Zn-Fe alloy containing about 7 - 16% Fe (mass ratio to the whole plating layer).

[0045] Each plate material was directly used for spot welding without performing surface polishing or the like. Each plate material was cut into strip shape (30 mm × 100 mm) for use.

[0046] (2) Electrodes For the pair of electrodes, commercially available chips of the same DR type (JIS C9304) (manufactured by OBARA Corporation) were used. Cooling water (flow rate: 2.7 L / min) was supplied to the inside (inner cylindrical part) of the chip to forcibly cool the chip. The electrodes were made of chromium copper (Cr: 1 mass%, Cu: the balance), and its electrical conductivity was 80% IACS.

[0047] As shown in the enlarged view of Fig. 1A, the size of the electrode was as follows: chip diameter (nominal diameter D2): φ16 mm, thickness at the tip bottom was 12 mm, radius of curvature (R) of the tip shoulder was 8 mm, radius of curvature (R1) of the tip face was 40 mm, and the tip diameter (D1) was 12 mm.

[0048] (3) Welding conditions Spot welding was performed using a servo-press type spot welding machine (PA235KVAMF manufactured by ARO). The pressing force (F) of the plate assembly by the electrodes was set to 5 kN (constant). The energization was performed by controlling the direct current along the pattern shown in Fig. 1B. Specifically, it was as follows. It was performed as follows.

[0049] After performing the first energization step (pre-energization step) with a current value of 9 kA (constant) and an energization time of 50 ms, the electrodes were left non-energized for 100 ms to cool the plate assembly through both electrodes (cooling step).

[0050] Thereafter, a second energization step was performed with a current value of 13 kA (constant) and an energization time of 300 ms. During each energization step, no generation of large splashes or the like was observed. Thus, a metal bonded body (specimen 1) was obtained.

[0051] 《Observation》 (1) The cross-section of the (bonded) joint portion (near the contact interface) of specimen 1 was observed with a scanning electron microscope (SEM: S-3600N manufactured by Hitachi High-Technologies Corporation) and an energy dispersive X-ray spectrometer (EDX) attached thereto. The SEM image and the elemental mapping image by EDX (referred to as "EDX image") of the (bonded) joint portion after the first energization step are shown in Fig. 1C. The SEM image and the EDX image of the joint portion after the second energization step are shown in Fig. 1D.

[0052] (2) As is clear from Fig. 1C, in the (bonded) joint portion after the first energization step, the interfaces between the steel plate, the plating layer, and the Al alloy plate remained in a substantially uniform state. Also, a large amount of Al was observed in the region where the plating layer was present, and a large amount of Zn was observed near the bonded surface of the Al alloy plate. Furthermore, almost no Zn or Al was observed on the steel plate side.

[0053] From these, it was confirmed that Al and Zn mutually solid-state diffused between the plating layer and the Al alloy plate (near the bonded surface), and a layer (precursor layer) containing Al and Zn was formed. The thickness of this precursor layer was about 2 to 30 μm.

[0054] As is clear from Fig. 1D, after the second energization step, it was confirmed that melting occurred between the steel plate containing the plating layer and the Al alloy plate, and a bonding layer composed of an Fe-Al based intermetallic compound (IMC) was formed. The thickness of this bonding layer was about 1 to 10 μm.

[0055] Also, it was revealed from Fig. 1D that Zn, which was the main component of the plating layer, hardly existed in the bonding layer (IMC). The IMC constituting the bonding layer is mainly Al 5 Fe 2 phase (the first intermetallic compound / the first layer) and Al 3 Fe phase (the second intermetallic compound / the second layer) on the Al alloy plate side, which has been separately confirmed.

[0056] Incidentally, the metal bonded body of Sample 1 had a nugget (molten and solidified part) of sufficient size between the first steel plate and the second steel plate. That is, it was confirmed that the first steel plate and the second steel plate were also firmly bonded by the above-described first energization step and second energization step.

[0057] [Second Embodiment] The outline of the spot welding according to this embodiment is shown in Fig. 2A. A laminate in which an iron substrate and an aluminum substrate were stacked on each other was used as the workpiece. Specifically, it is as follows.

[0058] 《Fabrication of Samples》 (1) Workpiece For the iron substrate, a hot stamp material (boron steel / plate thickness: 2.2 mm) with both sides covered with a plating layer (reaction layer / average thickness: 15 μm) made of Zn-Fe alloy was used. The plating layer is formed when the zinc plating layer is hot formed.

[0059] For the aluminum substrate, a die-cast material (ADC10 / JIS) or a hot-extruded material (A6005C / JIS) was used. In both cases, the plate thickness was 3.0 mm. The die-cast material had a melting point (solidus temperature) of 565°C. The hot-extruded material had a melting point (solidus temperature) of 590°C.

[0060] (2) Spot Welding The pressing force (F) of the laminate by the electrode was set to 5 kN (constant), and spot welding was performed by controlling the DC current. The energization was performed along the pattern shown in Fig. 2B. Specifically, it is as follows. Other welding conditions were basically the same as those in the case of the first embodiment.

[0061] A first energization process was performed with a first current value of 4 to 7 kA and a first energization time of 300 ms. The first current value was monotonically increased with a constant rate of change over time from 4 kA at the start of energization to 7 kA at the end of energization (linear up-slope energization).

[0062] Subsequent to the first energization process, a second energization process was performed with a second current value of 7 kA (constant) and a second energization time of 500 ms. During each energization step, no occurrence of large spatter or the like was observed.

[0063] After the second energization process, the energization was interrupted, and the electrode was separated from the work piece after 200 ms had elapsed. In this way, a metal joint (specimen 21) between the hot stamp material and the die-cast material, and a metal joint (specimen 22) between the hot stamp material and the hot-extruded Al alloy material (A6005C) were obtained.

[0064] 《Observation》 (1) SEM images of cross-sections of the nugget of specimen 21 observed at three locations (left part, central part, right part) using the aforementioned microscope are shown in Fig. 2C. Also, the SEM image and EDX image related to the central part are shown in Fig. 2D.

[0065] (2) As is clear from Fig. 2C and Fig. 2D, it was confirmed that the bonding layer mainly consisted of an Fe-Al-based intermetallic compound (IMC). The thickness of this bonding layer was about 20 to 30 μm. Also, almost no Zn was contained in the bonding layer. Furthermore, most of the Zn contained in the Al-Zn layer (precursor layer) that existed before the formation of the bonding layer had moved to the Al matrix side of the bonding layer to form a thin zinc oxide film.

[0066] The cross-section of the joint of sample 22 also showed the same tendency as that of sample 21. In addition, when tensile shear tests (JIS Z 3136) were performed on each sample (metal joint), the tensile shear strength (average value of three measurement values) of each sample was 4255 N for sample 1 and 3894 N for sample 22. From these results, it was also confirmed that a thick joint layer was formed by the formation of the precursor layer, ensuring sufficient joint strength. Incidentally, when a non-plated steel plate and an Al alloy plate were spot welded without forming a precursor layer, the joint layer had a thickness of about 0.5 to 2 μm and a tensile shear strength of only about 3100 N.

[0067] From the above, according to the resistance spot welding method of the present invention, it was confirmed that the iron substrate and the aluminum substrate were firmly joined by a joint layer composed of a sufficiently thick Fe-Al intermetallic compound.

Claims

1. A resistance spot welding method for a workpiece in which an iron base and an aluminum base are overlapped, comprising: a first energization step of energizing the workpiece; a second energization step of energizing the workpiece after the first energization step, wherein the workpiece has a reaction layer containing Zn between the iron base and the aluminum base, the first energization step generates a precursor layer in which Al and Zn are mutually substituted between the aluminum base and the reaction layer without melting the aluminum base, and the second energization step is a resistance spot welding method for melting at least the precursor layer to generate a bonding layer composed of an intermetallic compound containing Fe and Al.

2. The resistance spot welding method according to claim 1, wherein the first energization step is performed by energizing at 8 to 10 kA for 30 to 80 ms, and the second energization step is performed by energizing at 11 to 15 kA for 200 to 400 ms.

3. The resistance spot welding method according to claim 1, wherein the first energization step is performed by energizing at 3 to 8 kA for 200 to 400 ms, and the second energization step is performed by energizing at 6 to 9 kA for 300 to 700 ms.

4. The resistance spot welding method according to claim 1, wherein the reaction layer is a zinc layer or an alloy layer containing Zn and Fe.

5. The resistance spot welding method according to any one of claims 1 to 4, wherein the iron base is a high-tensile steel sheet having a tensile strength of 590 MPa or more.

6. The resistance spot welding method according to any one of claims 1 to 4, wherein the iron base is made of a hot stamping material.

7. The resistance spot welding method according to any one of claims 1 to 4, wherein the iron base has a thickness of 0.4 to 3.5 mm.

8. The resistance spot welding method according to any one of claims 1 to 4, wherein the iron base is composed of a stack of a plurality of steel sheets.

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