JP7047543B2 - Joined structure and its manufacturing method - Google Patents

Description

The present invention relates to a joint structure in which dissimilar materials are joined by resistance spot welding and a method for manufacturing the same.

Aluminum-based materials such as aluminum and aluminum alloys (hereinafter referred to as aluminum materials) are used in various fields because they are lightweight and have excellent corrosion resistance. For example, in the automobile field, an aluminum material may be used as a structural material of a vehicle body in order to reduce the weight of the vehicle body.

From the viewpoint of manufacturing cost, productivity, etc., the aluminum material may be used only for a part of the vehicle body, for example, for a part (roof, etc.) where strength is not so required. In this case, it is necessary to join the aluminum material that constitutes a part of the vehicle body and the steel material that constitutes the other part to each other as different materials.

In general, among various methods used for joining dissimilar materials, the resistance spot welding method has advantages such as weight reduction of the joint portion and high productivity as compared with the mechanical joining method. However, usually, when the aluminum material and the steel material are simply spot welded, there is a problem that the joint strength may be lowered due to the formation of a fragile intermetallic compound at the bonding interface between the aluminum material and the steel material.

Patent Document 1 describes a spot welding method for increasing the joining strength of a plurality of steel plates having different plate thicknesses. In this method, spot welding is performed using electrodes made of different materials, and a nugget is formed near the interface between the steel plates to increase the joint strength.

Japanese Unexamined Patent Publication No. 2013-173155 (issued on September 5, 2013)

However, in general, since aluminum material is a material that is more difficult to spot weld than steel for various reasons, the method described in Patent Document 1 cannot be directly applied to spot welding between aluminum material and steel material.

By the way, as a spot welding method, a single-phase alternating current method is widely used. On the other hand, when spot welding an aluminum material, a large welding current is required, so that the single-phase alternating current method is rarely used.

If a bonded structure can be manufactured by spot welding an aluminum material and a steel material by a single-phase AC method, the manufacturing cost can be reduced. This is because the equipment is relatively inexpensive and existing equipment can be used.

In order to increase the bonding strength of such a bonded structure, it is necessary to increase the molten nugget formed by melting the aluminum material (hereinafter, may be referred to as a molten nugget of the aluminum material). When the welding current at the time of spot welding is increased, the heat input increases, so that the molten nugget of the aluminum material can be increased. On the other hand, the molten nugget of the aluminum material may reach the surface layer of the aluminum material, and in this case, the corrosion resistance of the bonded structure is lowered.

One aspect of the present invention has been made in view of the above-mentioned conventional problems, and is a joint structure in which an aluminum material and a steel material are joined to each other by a single-phase alternating current resistance spot welding method, and the corrosion resistance is improved. The purpose is to provide an excellent joint structure.

The bonded structure according to one aspect of the present invention is a bonded structure bonded by spot welding an aluminum material and a molten aluminum-based plated steel plate, and is a boundary portion between the aluminum material and the molten aluminum-based plated steel plate. A molten aluminum nugget is formed in the first surface, and the surface of the aluminum material opposite to the surface of the molten aluminum-based plated steel plate facing the surface thereof is used as the first surface, and the outer edge portion of the molten nugget and the aluminum material are used. The shortest distance from the first surface of the aluminum is 0.01 mm or more.

Further, in the joint structure according to one aspect of the present invention, the aluminum material and the molten aluminum-based plated steel plate are laminated on the steel material in this order and spot welded, whereby the steel material and the molten aluminum-based plated steel plate are spot-welded. And the aluminum material are joined, the molten nugget is the first nugget, and the nugget formed at the boundary between the molten aluminum-based plated steel plate and the steel material is the second nugget. It is preferable that the ratio of the nugget diameter of the first nugget to the nugget diameter is 1.1 or more and 2.0 or less.

The method for manufacturing a bonded structure according to one aspect of the present invention is a method for manufacturing a bonded structure including a spot welding step of spot welding an aluminum material and a molten aluminum-based plated steel plate, and the molten aluminum of the aluminum material. The surface opposite to the surface facing the system-plated steel plate is used as the first surface, and in the spot welding step, the pressing force of the electrode is 1.4 kN or more and 4.9 kN or less, and the welding current and the energization time are The value represented by the product is in the range of 54 or more and 240 or less, the outer edge portion of the molten nugget formed at the boundary portion between the aluminum material and the molten aluminum-based plated steel plate, and the first surface of the aluminum material. It is characterized in that the aluminum material and the molten aluminum-based plated steel plate are spot-welded by a single-phase AC method so that the shortest distance between them is 0.01 mm or more.

Further, in the method for manufacturing a joint structure according to one aspect of the invention, it is preferable that the joint strength is 0.8 kN or more as a measurement result of a cross tensile test of the joint structure formed in the spot welding step.

It is possible to provide a joint structure in which an aluminum material and a steel material are joined by a resistance spot welding method of a single-phase alternating current method, and which is excellent in corrosion resistance.

It is a schematic diagram which shows the structure of the single-phase alternating current type spot welder used for manufacturing the junction structure in Embodiment 1 of this invention. It is a schematic diagram which shows the shape of the electrode provided in the said spot welder. FIG. 3 is an enlarged view showing the vicinity of the interface between the base steel plate of the hot-dip aluminum-based plated steel sheet and the aluminum-plated layer in the cross-sectional view shown in FIG. It is sectional drawing which shows typically the joint part of the joint structure in Embodiment 1 of this invention. It is a top view which shows typically the case which the said joint structure is seen from the side of an aluminum material, omitting the aluminum material. It is a figure for demonstrating the method of measuring the residual Al thickness in the said joint structure. It is sectional drawing which shows typically the joint part of the joint structure in Embodiment 2 of this invention. It is a figure for demonstrating the method of measuring the residual Al thickness in the said joint structure. It is sectional drawing which shows typically the joint part of the joint structure in Embodiment 3 of this invention. It is sectional drawing which shows typically the joint part of the joint structure in Embodiment 4 of this invention. It is a schematic diagram for demonstrating the cross tension test in an Example, (a) is a plan view of a joined structure, and (b) is a sectional view of a joined structure.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The following description is intended to better understand the gist of the invention, and does not limit the present invention unless otherwise specified. Further, unless otherwise specified in the present specification, "A to B" representing a numerical range means "A or more (including A and larger than A) and B or less (including B and smaller than B)". The shapes and dimensions (length, depth, width, etc.) of the configurations described in each drawing in this application do not necessarily reflect the actual shapes and dimensions, and are appropriately modified for the sake of clarity and simplification of the drawings. are doing.

The joint structure according to the embodiment of the present invention is, for example, a vehicle component formed by joining a roof panel and a roof side portion of a vehicle body in the field of automobiles. However, the joint structure of the present invention is not necessarily limited to this. The present invention can be appropriately applied to a joint structure for which there is a demand for joining different materials of an aluminum material and a steel material by a single-phase alternating current resistance spot welding method.

In the following, for the sake of simplification of the description, one joint portion formed by spot welding in the joint structure will be described. However, it goes without saying that a joint structure having a plurality of joint portions is also included in the scope of the present invention.

Further, since the molten aluminum-based plated steel sheet has a plated layer formed on the surface of the base steel sheet, it may be treated as a kind of steel material in the present specification.

<Findings of the present invention>
Conventionally, in the field of automobiles, the use of aluminum materials for roofs and the like has been promoted as described above. On the other hand, when the aluminum material and the steel material are simply spot welded, a fragile Fe—Al binary alloy layer is formed at the bonding interface. There is a problem that the bonding strength of the bonded structure is lowered due to the peeling at the portion of the Fe—Al binary alloy layer.

So far, techniques relating to a joint structure in which an aluminum material and a hot-dip aluminum-plated steel sheet are joined have been reported (see Patent No. 4280671 and the like). The main purpose of this type of conventional technique is to suppress the formation of an Fe—Al binary alloy layer at the bonding interface and increase the bonding strength of the bonding structure. Therefore, during spot welding, the heat input to the welded portion becomes excessive, and the molten nugget of the aluminum material reaches the surface layer of the aluminum material, which may reduce the corrosion resistance of the bonded structure.

The present inventors have conducted diligent studies and attempted to manufacture a bonded structure having excellent bonding strength while maintaining corrosion resistance when joining dissimilar materials by a single-phase AC resistance spot welding method. rice field. Then, they have found a condition as to what kind of structure the joint structure should have to obtain a joint structure having excellent corrosion resistance and joint strength, and have realized the present invention.

[Embodiment 1]
An embodiment of the present invention will be described below with reference to FIGS. 1 to 5.

In the present embodiment, as an example of the joint structure of the present invention, a joint structure in which an aluminum material and a hot-dip aluminum plated steel plate are joined (hereinafter, may be referred to as a two-disc joint structure) will be described.

First, a spot welder used for manufacturing the joint structure of the present embodiment will be described with reference to FIGS. 1 and 2. FIG. 1 is a schematic view showing the configuration of a single-phase AC type spot welder 100 used for manufacturing the joint structure 1A (see FIG. 3) in the present embodiment. FIG. 2 is a schematic view showing an electrode 120 included in the spot welder 100.

As each member included in the spot welder 100, known equipment can be used except as specifically described below. Therefore, in FIGS. 1 and 2, in order to avoid the redundancy of the description, the illustration of each part of the spot welder 100 and the electrode 120 is appropriately omitted. It may be understood that the members for which illustration and description are omitted are the same as those known.

(Spot welder)
As shown in FIG. 1, the spot welder 100 includes a welding control device 110 and a pair of electrodes 120 electrically connected to the welding control device 110. The welding control device 110 includes a single-phase AC power supply 111 and a control unit 112.

The hot-dip aluminum-based plated steel sheet 30 and the aluminum material 10 are placed on top of each other between the pair of electrodes 120. The joint structure 1A (see FIG. 3) is formed by energizing these plate materials using the electrodes 120 and performing resistance spot welding.

The spot welder 100 has, for example, an electrode 120 supported by an arm (not shown), and includes an electrode pressurizing mechanism (not shown) for pressing the electrode 120 against the hot-dip aluminum-based plated steel sheet 30 or the aluminum material 10. ing.

The single-phase AC power supply 111 is formed by connecting one phase of, for example, a commercial three-phase power supply to a welding transformer via a thyristor, and supplies electric power to the electrode 120.

The control unit 112 comprehensively controls each unit of the spot welder 100 based on the welding conditions set by the user. The control unit 112 controls spot welding between the molten aluminum-based plated steel sheet 30 and the aluminum material 10 by controlling, for example, the pressing force, pressurizing time, current, energizing time, and the like of the electrodes.

(electrode)
The shape of the electrode 120 included in the spot welder 100 of the present embodiment will be schematically described with reference to FIG. FIG. 2 is a schematic view showing the shape of the electrode 120.

As shown in FIG. 2, the electrode 120 is a DR (dome radius) shaped electrode chip. A DR-type electrode tip is a radius-shaped (a radius of curvature larger than the above-mentioned curved surface) at the tip of a D-shaped electrode tip (the tip of the electrode is a curved surface and the radius of curvature of the curved surface is 1/2 of the outer diameter of the electrode). A curved surface) is formed. The DR type electrode tip has good shapeability by a tip dresser (tool for polishing electrodes) and is widely used in the automobile industry.

The electrode 120 has an outer diameter D1 of the body portion of 16 mm. The tip of the electrode 120 is a curved surface having a tip diameter D2 of 6 mm and a radius of curvature of 40 mm. Further, the surface of the shoulder portion between the body portion and the tip portion of the electrode 120 is a curved surface having a radius of curvature of 8 mm.

The material of the electrode 120 is a chromium copper alloy (Cr—Cu alloy) containing 0.4% by mass to 1.2% by mass of Cr. The chromium-copper alloy may contain about 1% by mass of Cr, or may contain, for example, 0.8% by mass to 1.1% by mass of Cr.

The electrode 120 is not limited to the above dimensions and materials as long as it is a DR type electrode tip. The outer diameter D1 of the electrode 120 is not limited to 16 mm, and may have an outer diameter appropriately designed according to the shape of the tip portion. The tip diameter D2 of the tip of the electrode 120 is preferably 5 mm to 7 mm. Further, the radius of curvature of the tip of the electrode 120 and the radius of curvature of the surface of the shoulder are not particularly limited, and are appropriately designed according to the values of the outer diameter D1 and the tip diameter D2 of the tip. It's okay.

(Aluminum material)
As used herein, the term aluminum material is used to include both pure aluminum (although it is permissible to contain unavoidable impurities) and aluminum alloys.

The aluminum material 10 may be a wrought material, and the material is not particularly limited. Various aluminum alloys can be used as the aluminum material 10. For example, various aluminum alloys of 1000 series, 3000 series, 5000 series, 6000 series, and 7000 series may be used.

The aluminum material 10 preferably has an Fe concentration of 1.0% by mass or less in consideration of corrosion resistance, processability, and the like. Further, it is preferable that the aluminum material 10 is added with Si of about 1.0% by mass and Mg of 0.01 to 1.5% by mass, and fine Mg ₂ Si is precipitated by heat treatment such as aging treatment. .. In this case, the strength of the aluminum material 10 is improved. From this viewpoint, it is preferable to set the lower limit of the Si content to 0.1% by mass. Further, when 1.5 to 6.0% by mass of Mg is added, high strength can be obtained by strengthening the solid solution.

On the other hand, if the aluminum material 10 contains an excess amount of Mg exceeding 6.0% by mass, defects are likely to occur during spot welding. Further, if an excess amount of Si exceeding 3.0% by mass is contained, coarse precipitates or crystallized substances may be generated inside the aluminum material 10 to reduce the bonding strength. Therefore, it is preferable that the aluminum material 10 has a Mg concentration of 0.01 to 6.0% by mass and a Si concentration of 3.0% by mass or less.

The aluminum material 10 may be, for example, an aluminum alloy 6022. In terms of mass%, the aluminum alloy 6022 contains Si: 0.90% or more and 1.20% or less, Fe: 0.20% or less, Cu: 0.05% or less, Mn: 0.04% or more and 0.12% or less. , Cr: 0.05% or less, Zn: 0.05% or less, Mg: 0.45% or more and 0.70% or less, Ti: 0.05% or less, and the balance consists of Al and unavoidable impurities. The composition.

Further, the aluminum material 10 may be, for example, an aluminum alloy 5052. The aluminum alloy 5052 has Si: 0.25% or less, Fe: 0.4% or less, Cu: 0.1% or less, Mn: 0.1% or less, Cr: 0.15% or more and 0. It contains 35% or less, Zn: 0.1% or less, Mg: 2.2% or more and 2.8% or less, and the balance is composed of Al and unavoidable impurities.

The plate thickness of the aluminum material 10 may be appropriately set according to the strength required for the aluminum material 10, and is not particularly limited, but may be, for example, 0.6 mm to 1.4 mm. Generally, when an aluminum material is used for the roof panel of an automobile, the plate thickness is about this level. Further, the plate thickness of the aluminum material 10 may be 0.8 mm to 1.3 mm or 1.0 mm to 1.3 mm.

(Fused aluminum-based plated steel sheet)
The hot-dip aluminum-based plated steel sheet 30 will be described with reference to FIGS. 1 and 3. FIG. 3 is an enlarged view of a part (A1) of the cross-sectional view shown in FIG.

As shown in FIGS. 1 and 3, the molten aluminum-based plated steel sheet 30 includes a base steel plate (base steel) 31 and an aluminum plating layer 32 formed on the surface of the base steel plate 31. Further, the aluminum plating layer 32 includes an alloy layer 33 formed at an interface between the base steel plate 31 and the aluminum plating layer 32. The alloy layer 33 includes a nitrogen-enriched layer (hereinafter referred to as N-enriched layer) 34 formed at the interface between the alloy layer 33 and the base steel plate 31.

As the base steel plate 31, low carbon steel, medium carbon steel, low alloy steel, stainless steel, or the like can be used. Depending on the application, a steel grade to which Si, Mn, Cr, Ni, etc. are added may be used. Further, N of 25 ppm or more and 200 ppm or less is added to the base steel sheet 31 in the present embodiment. As a result, when heat treatment is performed under specific conditions after plating, an N-concentrated layer 34 is formed at the interface between the base steel plate 31 and the alloy layer 33. Examples of this specific condition include a heat treatment of holding at a temperature of 520 ° C. for 6 hours.

The N-concentrated layer 34 is a very thin layer formed on the surface portion of the base steel plate 31, for example, a layer having a thickness of about 5 nm. The N-concentrated layer 34 is richer in N than the base steel sheet 31 (N is concentrated), and contains, for example, 3.0 atomic% or more of N. The N-concentrated layer 34 suppresses the mutual diffusion of Al and Fe. The effect of suppressing the mutual diffusion of Fe—Al by the N-concentrated layer 34 is improved as the N content of the base steel sheet 31 increases when the heat treatment conditions after plating are kept constant. However, when the base steel plate 31 contains an excess amount of N exceeding 200 ppm, the manufacturability of the base steel plate 31 itself is lowered.

Further, the hot-dip aluminum-based plated steel sheet 30 is manufactured as follows. That is, the base steel sheet 31 is immersed and passed through a hot-dip aluminum-based plating bath containing aluminum as a main component. As a result, the aluminum plating layer 32 is formed on the surface of the base steel plate 31. The thickness of the aluminum plating layer 32 is adjusted, for example, by spraying a wiping gas from the molten aluminum-based plating bath onto the steel strip immediately after being pulled up to control the amount of plating adhesion. By setting the thickness of the aluminum-plated layer 32 within the range of 5 μm or more and 70 μm or less, the workability of the molten aluminum-based plated steel sheet 30 can be improved.

Further, the composition of the aluminum plating layer 32 is the same as the composition of the molten aluminum-based plating bath. Therefore, the composition of the aluminum plating layer 32 can be adjusted by adjusting the composition of the molten aluminum-based plating bath.

The aluminum plating layer 32 in the present embodiment contains an aluminum layer containing 3% by mass or more and 12% by mass or less of Si and 0.5% by mass or more and 5% by mass or less of Fe (note that it contains unavoidable impurities). Tolerate). When the aluminum plating layer 32 contains an excessive amount of Si, the workability of the molten aluminum-based plated steel sheet 30 may be impaired. Therefore, the upper limit of the Si concentration is limited to 12% by mass.

By forming the Fe—Al—Si ternary alloy layer on the alloy layer 33, the bonding strength of the bonding structure 1A can be improved. In order to sufficiently generate the Fe—Al—Si ternary alloy layer, the amount of Fe and Si eluted from the base steel sheet 31 into the molten Al during immersion in the aluminum plating bath is insufficient. Therefore, by increasing the Fe and Si contents of the aluminum plating layer 32, Fe and Si are replenished from the aluminum plating layer 32 to the joining interface. In particular, for Si having a large diffusion coefficient, the Si content of the aluminum plating layer 32 is set as high as 3% by mass or more. This secures the amount of Si required to form the Fe—Al—Si ternary alloy layer.

By adjusting the composition of the aluminum plating layer 32 as described above, not only the formation of the Fe—Al—Si ternary alloy layer is promoted, but also the Fe and Si concentrations of the molten Al generated during spot welding are increased. The action also occurs. The nugget formed by quenching the molten Al having a high concentration of Fe and Si is hardened by the effect of strengthening the solid solution of Fe and Si and the effect of quenching. Therefore, the joint strength of the joint structure 1A can be increased. Then, by suppressing the formation of a fragile Fe—Al binary alloy layer, a joint structure 1A having a highly reliable joint strength can be obtained.

The composition of the aluminum plating layer 32 described above is a composition of a region not including the alloy layer 33 formed at the interface between the base steel plate 31 and the aluminum plating layer 32.

When it is necessary to further improve the characteristics of the bonded structure 1A (characteristics different from those related to spot welding), Ti, Sr, B, Cr, Mn, Zn, etc., which do not significantly affect the mutual diffusion reaction of Fe—Al, etc. The element can be appropriately contained in the aluminum plating layer 32.

(Joint structure)
The joint structure 1A of the present embodiment is formed by spot welding the aluminum material 10 and the hot-dip aluminum-based plated steel sheet 30 using a spot welder 100 in a state where the plate materials are laminated as described above. The joint portion 2A of the joint structure 1A will be described with reference to FIGS. 4 and 5. FIG. 4 is a cross-sectional view schematically showing the joint portion 2A of the joint structure 1A in the present embodiment. FIG. 5 is a plan view schematically showing the joint structure 1A when the aluminum material 10 is viewed from the side of the aluminum material 10 by omitting the aluminum material 10. In FIG. 4, one plate surface of the base steel plate 31 is omitted.

As shown in FIG. 4, a molten Al nugget 51 is formed in the joint portion 2A of the joint structure 1A. The molten Al nugget 51 joins the aluminum material 10 and the molten aluminum-based plated steel sheet 30. In the cross-sectional view shown in FIG. 4, the molten Al nugget 51 is a molten aluminum nugget formed in the vicinity of the boundary between the aluminum material 10 and the molten aluminum-based plated steel sheet 30, and rises toward the inside of the aluminum material 10. It is formed in a substantially fan shape (convex upward).

Further, the surface of the aluminum material 10 is dented because the electrode 120 bites into the aluminum material 10 due to the pressing force. The portion of the surface of the aluminum material 10 where the dent is formed is referred to as the electrode pressing surface (first surface) 11. In other words, the electrode pressing surface 11 is a surface of the joint portion 2A opposite to the surface of the aluminum material 10 facing the hot-dip aluminum-based plated steel sheet 30.

Further, as shown in FIG. 4, in the bonded structure 1A, a strong heat input region (central region) 70 is formed at the interface between the molten Al nugget 51 and the base steel plate 31. As shown in FIG. 5, the strong heat input region 70 is formed in the central portion of the molten Al nugget 51 when the bonded structure 1A is seen through from the side of the aluminum material 10. The center point 51A shown in FIG. 5 is the center of energization of the pair of electrodes 120 during spot welding.

In the bonded structure 1A, a weak heat input region (outer peripheral region) 80 is formed at the interface between the outer edge portion of the molten Al nugget 51 and the base steel plate 31, and the weak heat input region 80 and the strong heat input region 70 are formed. An intermediate region 60 is formed between the two.

Explaining with reference to the center point 51A, as shown in FIG. 5, the molten Al nugget 51 is formed so as to extend in a circle from the center point 51A. At the interface between the molten Al nugget 51 and the base steel plate 31, a region having a certain range including the center point 51A is a strong heat input region 70, and an intermediate region 60 and a weak heat input region 80 are located outside the region. It exists in order.

(Melted Al Nugget)
The molten Al nugget 51 is formed as follows. That is, by energizing using the electrode 120 (see FIG. 1), the boundary surface between the aluminum material 10 and the hot-dip aluminum-based plated steel sheet 30 is resistance-heated. This melts the aluminum. The molten aluminum solidifies to form the molten Al nugget 51. Therefore, the molten Al nugget 51 is mainly made of aluminum and is affected by the composition of the aluminum material 10 and the aluminum plating layer 32.

Further, the Fe and Si concentrations of the molten Al nugget 51 can change depending on the combination of the thickness of the aluminum plating layer 32, the welding current, the energization time, and the electrode shape. The thicker the aluminum plating layer 32 that is the source of Fe and Si to the molten Al nugget 51, the higher the Fe and Si concentrations of the molten Al nugget 51.

For example, consider a case where spot welding is performed under the conditions of a welding current of 12.5 kA and an energization time of 12 cycles using a 1% chrome copper alloy chip having a curved tip having a diameter of 6 mm and a radius of curvature of 40 mm. In this case, if the film thickness of the aluminum plating layer 32 is 5 μm or more, the average Fe and Si concentrations of the molten Al nugget 51 are 0.1% by mass or more higher than the Fe and Si contents of the aluminum material 10. As a result, the molten Al nugget 51 is hardened, and the joining strength of the joining structure 1A is improved.

The nugget diameter of the molten Al nugget 51 can be measured as follows. That is, the size of the molten Al nugget 51 (boundary with surrounding substances) in the bonded structure 1A can be visually defined by using, for example, an electron micrograph of a cross section as shown in FIG. .. The width from one end to the other end at the boundary between the molten Al nugget 51 and the alloy layer 33 is defined as the nugget diameter W1.

Assuming that the extension lines of the mutual center axes of the pair of electrodes 120 (see FIG. 1) are the electrode center lines, the heat input tends to be higher at the position of the bonding interface where the distance from the electrode center lines is shorter during spot welding. It is in. The weak heat input region 80 is a position far from the electrode center line and is a place where heat input during spot welding is small. On the other hand, the strong heat input region 70 is a place where heat input during spot welding is large. The intermediate region 60 is a portion that has received an amount of heat input between the heat input in the weak heat input region 80 and the heat input in the strong heat input region 70.

(Strong heat input area)
In the strong heat input region 70, the aluminum material 10 and the aluminum plating layer 32 are melted by being heated at a relatively high temperature, and the N concentrated layer 34 of the alloy layer 33 is also melted and various elements are mutually diffused. As a result, the base steel plate 31 melts. As a result, the molten Al nugget 51 having a higher Fe concentration than that of the aluminum material 10 is formed.

Although the molten Al nugget 51 is hardened by the concentration of Fe, a fragile Fe—Al binary alloy layer that lowers the joint strength is likely to occur at the joint interface. That is, when the molten Al is rapidly cooled to become the molten Al nugget 51 during spot welding, Fe is reprecipitated from the molten Al to the bonding interface in the strong heat input region 70, and a fragile Fe—Al binary alloy layer is formed.

Therefore, the Fe-Al-based intermetallic compound is mainly formed in the strong heat input region 70. However, it is permissible to include various elements supplied from the composition of the aluminum material 10 and the hot-dip aluminum-based plated steel sheet 30, and to include an alloy formed by these elements. In the present specification, "mainly Fe-Al intermetallic compounds are formed" means that the intense heat input region 70 is defined by using, for example, nanoprobe electron beam analysis and energy dispersive X-ray analysis (EDX). This means that when analyzed, data showing that the abundance ratio of Fe-Al intermetallic compounds is the highest can be obtained.

(Weak heat input area)
On the other hand, the weak heat input region 80 is a region that is heated at a relatively lower temperature than the strong heat input region 70 during spot welding. As a result, in the weak heat input region 80, Fe and Si are slightly dissolved from the alloy layer 33 (Fe—Al—Si ternary alloy layer), and an Fe—Al intermetallic compound is generated. In the weak heat input region 80, Fe-Al-Si-based intermetallic compounds are mainly formed. In the present specification, "mainly the Fe-Al-Si-based intermetallic compound is formed" means the presence of the Fe-Al-Si-based intermetallic compound as described above for the strong heat input region 70. It means that the data showing that the ratio is the highest is obtained.

The weak heat input region 80 is formed in a region having a width of about 0.2 mm from the end portion of the molten Al nugget 51. The width of this region does not change much even if the nugget diameter W1 of the molten Al nugget 51 changes.

(Intermediate region)
In the intermediate region 60, during spot welding, the alloy layer 33 (Fe—Al—Si ternary alloy layer) is melted, but the heat input is such that the N concentrated layer 34 remains. Therefore, in the intermediate region 60, the Fe—Al—Si-based intermetallic compound and the N-enriched layer 34 are mixed. In the present specification, "the Fe-Al-Si-based intermetallic compound and the N-concentrated layer 34 are mixed" means that the Fe-Al-Si-based intermetallic compound and N are analyzed when the intermediate region 60 is analyzed. It means that data indicating the presence of the concentrated layer 34 (for example, data in which the presence of Fe, Al, Si, and N is detected) can be obtained, and the following data can be obtained. That is, it means that Fe is hardly detected on the molten Al nugget 51 side in the intermediate region 60, and data indicating that the Fe-Al intermetallic compound is not generated can be obtained. The analysis means is not particularly limited as long as it can analyze a very small region, and for example, nanoprobe electron beam analysis and EDX can be used.

This intermediate region 60 has, for example, cracks extending from the weak heat input region 80, which is the outer peripheral portion of the molten Al nugget 51, when a force for peeling off the aluminum material 10 is applied to the joint structure 1A. Work to stop. As a result, when a further force is applied to the aluminum material 10, the joint structure 1A tends to break the button instead of breaking the shear. That is, the molten Al nugget 51 has a high bonding strength because the intermediate region 60 works to increase the bonding strength.

(Remaining Al thickness)
FIG. 6 is a diagram for explaining a method of measuring the residual Al thickness in the bonded structure 1A.

As shown in FIG. 6, in the joint structure 1A of the present embodiment, the aluminum material 10 remains on the upper portion of the molten Al nugget 51. The shortest distance between the outer edge of the molten Al nugget 51 and the electrode pressing surface 11 of the aluminum material 10 is defined as the remaining Al thickness L1. The bonded structure 1A of the present embodiment has a residual Al thickness L1 of 0.01 mm or more.

Here, the position of the electrode pressing surface 11 (position in the thickness direction of the joint structure 1A) may change depending on the depth recessed from the surface of the aluminum material 10. This depth is affected by the applied pressure and heat input (magnitude of welding current, energization time, etc.) during spot welding. Further, since the pressing force at the time of spot welding is related to the adhesion (micro contact area) between the aluminum material 10 and the molten aluminum-based plated steel sheet 30, it is also related to the current density of energization. Therefore, the height of the molten Al nugget 51 is also affected by the pressing force and heat input during spot welding. The relationship between the welding conditions, the position of the electrode pressing surface 11, and the height of the molten Al nugget 51 changes depending on the plate thickness and type of the aluminum material 10 and the plate thickness and type of the molten aluminum-based plated steel sheet 30. ..

Therefore, (i) the joint strength of the joint portion is increased (for example, the joint strength in a cross tensile test is 0.8 kN or more), and (ii) the residual Al thickness L1 is 0.01 mm or more. It is not easy to meet both conditions.

The joint structure 1A of the present embodiment is realized by the following manufacturing method.

(Manufacturing method of joint structure)
The method for manufacturing the bonded structure 1A in the present embodiment includes a spot welding step of spot welding the aluminum material 10 and the hot-dip aluminum-based plated steel sheet 30 using a single-phase AC spot welder 100.

(Spot welding process)
The heat input in spot welding can be adjusted by the product of the welding current and the energization time. Here, one cycle of the energization time in spot welding is a time determined by the power frequency of the commercial power source connected to the spot welder 100. The commercial power supply may have a power frequency of 50 Hz or may have a power frequency of 60 Hz. Further, it may be another power frequency. Heat input can be controlled by adjusting the welding current according to the difference in power supply frequency.

In the spot welding step, the pressing force of the electrode 120 is controlled to be 1.4 kN or more and 4.9 kN or less, and the value represented by the product of the welding current and the energization time is controlled to be within the range of 54 or more and 240 or less. To. Then, the aluminum material 10 and the hot-dip aluminum-based plated steel sheet 30 are spot-welded by a single-phase AC method so that the residual Al thickness L1 is within the range of the present invention.

The value expressed by the product of the pressing force of the electrode 120 and the welding current and the energization time (that is, heat input) has a complicated effect on the value of the residual Al thickness L1. For example, the value of heat input suitable for setting the residual Al thickness L1 to 0.01 mm or more varies depending on various conditions.

When the pressing force of the electrode 120 is in the range of 1.4 kN or more and 4.9 kN or less and the value represented by the product of the welding current and the energization time is smaller than 54, the residual Al thickness L1 becomes large while the remaining Al thickness L1 becomes large. The nugget diameter W1 of the molten Al nugget 51 becomes too small, and sufficient bonding strength cannot be obtained. Further, when the value represented by the product of the welding current and the energization time is larger than 240, the nugget diameter W1 of the molten Al nugget 51 can be increased, but the heat input becomes excessive and the molten Al nugget 51 becomes. It reaches the electrode pressing surface 11.

In the spot welding process, when a commercial power source having a power frequency of 60 Hz is used, the initial pressurization is, for example, 35 cycles, and the hold is 24 cycles. The initial pressurization may be 10 cycles or more, and the hold may be 5 cycles or more. The hold means forging pressurization in which the electrode 120 is energized and then the material to be welded is pressed by using the electrode 120 while cooling the electrode 120.

Further, in the manufacturing method of the present embodiment, the DR type electrode 120 (see FIG. 2) as described above is used as the electrode. While using such an electrode, spot welding is performed under the above-mentioned conditions. According to the manufacturing method of the present embodiment, the residual Al thickness L1 is 0.01 mm or more even if the plate thickness and material of the aluminum material, the plate thickness of the hot-dip aluminum-based plated steel sheet, etc. change. It is possible to manufacture a bonded structure 1A having excellent corrosion resistance. Then, the joint strength of the joint structure 1A can be increased, and for example, a joint structure having a joint strength of 0.8 kN or more can be manufactured as a measurement result of a cross tensile test. Such high joint strength is an effect obtained by increasing the nugget diameter W1 of the joint structure 1A and forming a specific region (intermediate region 60) in the joint portion 2A.

[Embodiment 2]
Other embodiments of the present invention will be described below. For convenience of explanation, the same reference numerals are given to the members having the same functions as the members described in the above-described embodiment, and the description thereof will not be repeated.

In the first embodiment, a two-disc joint structure has been described. On the other hand, in the present embodiment, a three-sheet joint structure including a steel material in addition to the above-mentioned aluminum material 10 and the hot-dip aluminum-based plated steel sheet 30 will be described. It should be noted that the joint structure is not limited to a 3-disc set, and a 4-disc or more joined structure is also included in the scope of the present invention. In the present specification, the value obtained by totaling the plate thickness of the base steel plate in the molten aluminum-based plated steel plate 30 and the plate thickness of each of the plurality of steel materials is referred to as the total plate thickness of the steel material in the bonded structure. In the joint structure according to one aspect of the present invention, the upper limit of the total plate thickness of the steel material is 4.3 mm. The number of steel materials in the joint structure is not particularly limited as long as the total thickness of the steel materials is within the range of 4.3 mm or less. If the total thickness of the steel material exceeds 4.3 mm, it becomes difficult to generate a nugget between the hot-dip aluminum-based plated steel sheet 30 and the steel material under the molten aluminum-based plated steel sheet 30. In this case, when the cross tensile test is performed, it is easy to break at the interface portion between the molten aluminum-based plated steel sheet 30 and the steel material, and there is a tendency that a cross tensile strength of 0.8 kN or more cannot be obtained.

For example, when manufacturing a vehicle part, it may be necessary to weld an aluminum material and a plurality of steel materials. Since the aluminum material has low intrinsic resistance and high thermal conductivity, it is necessary to weld it in a short time with a relatively large welding current.

The joint structure of the present embodiment is a joint structure in which different materials are joined by laminating a steel material in addition to an aluminum material and a hot-dip aluminum-based plated steel sheet and spot welding them by a single-phase AC method. .. There was no past record and no knowledge about the production of such a bonded structure using a single-phase AC spot welder. Furthermore, it was completely unknown when there were multiple steel materials.

(Joint structure)
The joint structure 1B and the joint portion 2B of the present embodiment will be described with reference to FIGS. 7 and 8. FIG. 7 is a cross-sectional view schematically showing a joint portion 2B of the joint structure 1B according to the second embodiment of the present invention. FIG. 8 is a diagram for explaining a method of measuring the residual Al thickness L1 in the bonded structure 1B.

The bonded structure 1B in the present embodiment is manufactured by laminating a hot-dip aluminum-based plated steel sheet 30 and an aluminum material 10 on a steel material 20 in this order and spot welding them.

(Steel material)
For example, in the field of automobiles, the vehicle body is assembled by changing the strength of the material in the right place, and SPC440 class or SPC590 class having higher strength than SPC270 class having general material strength may be used. The steel type and plate thickness of the steel material 20 may be appropriately set according to the strength required for the steel material 20, and are not particularly limited.

The steel material 20 may be a general cold-rolled steel plate, a special cold-rolled steel plate, a high-strength steel plate, a hot-rolled steel plate, or the like, or may be a plated steel plate. The steel material 20 may be, for example, SPC270C, SPC270D, SPC440, SPC590, an alloyed galvanized steel sheet (GA steel sheet), or the like. The plate thickness of the steel material 20 is, for example, in the range of 0.8 mm to 1.4 mm.

(Joint part)
As shown in FIG. 7, the joint portion 2B of the joint structure 1B has a lower nugget (second nugget) 52 that joins the molten Al nugget (first nugget) 51, the molten aluminum-based plated steel sheet 30, and the steel material 20. And are formed. In the cross-sectional view shown in FIG. 7, the lower nugget 52 is formed at the boundary between the hot-dip aluminum-based plated steel sheet 30 and the steel material 20, and is formed in an elliptical shape extending over both of them.

The lower nugget 52 is formed by resistance-heating the interface between the molten aluminum-based plated steel sheet 30 and the steel material 20 by energization and solidifying the molten iron by raising the temperature to the melting point of Fe.

It is not easy to form both the molten Al nugget 51 and the lower nugget 52 so as to satisfy the desired bonding strength for the three-disc bonded structure 1B. This is because the behavior of the two nuggets during spot welding is different due to the difference in melting point between iron and aluminum.

As a result of diligent studies, the present inventors have found the following requirements required for the joint portion 2B in order for the three-disc joint structure 1B to have high joint strength.

That is, in the bonded structure 1B of the present embodiment, the ratio of the nugget diameter W1 of the molten Al nugget 51 (ratio of W1 / W2) to the nugget diameter W2 of the lower nugget 52 is 1.1 or more and 2.0 or less. There is. Here, the length of the long axis in the elliptical shape of the lower nugget 52 is defined as the lower nugget diameter W2.

The nugget diameter W2 of the lower nugget 52 can be measured in the same manner as the nugget diameter W1 of the molten Al nugget 51 described above. That is, the size of the lower nugget 52 (boundary with surrounding substances) in the bonded structure 1B can be visually defined by using, for example, an electron micrograph of a cross section as shown in FIG. 7.

The lower nugget 52 does not always have an elliptical shape as shown in FIG. 7. Therefore, the lower nugget diameter W2 of the lower nugget 52 is defined in more detail as follows. That is, the lower nugget diameter W2 is the length from one to the other of the boundary between the lower nugget 52 and the surface of the molten aluminum-based plated steel sheet 30 on the interface where the molten aluminum-based plated steel sheet 30 and the steel material 20 abut. ..

When the ratio of W1 / W2 is less than 1.1, the nugget diameter W1 of the molten Al nugget 51 becomes too large due to the large heat input during spot welding, and the aluminum material 10 is greatly reduced in wall thickness. Therefore, as a result of the measurement of the cross tensile test, a joint structure having a joint strength of less than 0.8 kN can be obtained. That is, when the heat input during spot welding is increased, the nugget diameter W1 is increased while the aluminum material 10 is thinned, so that the aluminum material 10 is likely to break at the portion of the electrode pressing surface 11.

On the other hand, when the above ratio exceeds 2.0, the heat input during spot welding is small, and the nugget diameter of the lower nugget 52 is not sufficiently large. Therefore, the joint structure is liable to break at the portion of the lower nugget 52 in the cross tensile test, and the joint strength may be less than 0.8 kN.

Further, as shown in FIG. 8, in the joint structure 1B of the present embodiment, the aluminum material 10 remains on the upper portion of the molten Al nugget 51. The shortest distance between the outer edge of the molten Al nugget 51 and the electrode pressing surface 11 of the aluminum material 10 is defined as the remaining Al thickness L1. The bonded structure 1A of the present embodiment has a residual Al thickness L1 of 0.01 mm or more.

(Manufacturing method of joint structure)
In the method for manufacturing the joint structure 1B in the present embodiment, the molten aluminum-based plated steel plate 30 and the aluminum material 10 are laminated in this order on the steel material 20, and spot welded using a single-phase AC type spot welder 100. Includes a spot welding process.

In the spot welding step, the pressing force of the electrode 120 is controlled to be 1.4 kN or more and 4.9 kN or less, and the value represented by the product of the welding current and the energization time is controlled to be within the range of 54 or more and 240 or less. To. Then, the aluminum material 10 and the hot-dip aluminum-based plated steel sheet 30 are spot-welded by a single-phase AC method so that the residual Al thickness L1 is within the range of the present invention.

As a result, even in the three-disc bonded structure 1B, the residual Al thickness L1 can be 0.01 mm or more, and a bonded structure having excellent corrosion resistance can be manufactured. Then, the joint strength of the joint structure 1B can be increased, and for example, a joint structure having a joint strength of 0.8 kN or more can be manufactured as a measurement result of a cross tensile test.

[Embodiment 3]
Other embodiments of the present invention will be described below with reference to FIG. The configurations other than those described in the present embodiment are the same as those in the first and second embodiments. FIG. 9 is a cross-sectional view schematically showing the joint portion 2C of the joint structure 1C in the present embodiment.

In the second embodiment, the three-disc joint structure 1B has been described. On the other hand, the joint structure 1C of the present embodiment is a four-disc joint structure including the steel material 21 in addition to the steel material 20.

As shown in FIG. 9, in the joint structure 1C of the present embodiment, the steel material 21, the steel material 20, the hot-dip aluminum-based plated steel plate 30, and the aluminum material 10 are laminated in this order, and the plate materials are laminated. It is formed by spot welding using a single-phase AC type spot welder 100.

The steel material 21 and the steel material 20 may be of the same steel type or different from each other. Further, the plate thickness may be the same as each other, or the plate thickness may be different from each other. The plate thickness of the steel material 21 is, for example, in the range of 0.8 mm to 1.4 mm.

The joint portion 2C of the joint structure 1C is formed with a lower nugget 55 extending over a steel material 21, a steel material 20, and a hot-dip aluminum-based plated steel sheet 30.

In this case, the length from one to the other of the boundary between the lower nugget 55 and the surface of the hot-dip aluminum-based plated steel sheet 30 on the interface where the hot-dip aluminum-based plated steel sheet 30 and the steel material 20 abut is the nugget diameter (first steel). Nugget diameter) W2. Further, the length from one to the other of the boundary between the lower nugget 55 and the surface of the steel material 20 (or the steel material 21) on the interface where the steel material 20 and the steel material 21 abut is defined as the second steel nugget diameter W3.

In the bonded structure 1C of the present embodiment, the ratio of W1 / W2 is 1.1 to 2.0. The length of the second steel nugget diameter W3 has a small influence on the joining strength of the joining structure 1C. Further, in the joint structure 1C, an intermediate region 60 is formed at the joint interface between the molten Al nugget 51 and the base steel plate 31.

The steel material 21, the steel material 20, the molten aluminum-based plated steel sheet 30, and the aluminum material 10 are spot-welded by a single-phase AC method so that the residual Al thickness L1 is within the range of the present invention.

As a result, even in the 4-disc bonded structure 1C, the residual Al thickness L1 can be 0.01 mm or more, and a bonded structure having excellent corrosion resistance can be manufactured. Then, for example, the joint structure 1C having a joint strength of 0.8 kN or more can be manufactured as a measurement result of the cross tensile test.

[Embodiment 4]
Other embodiments of the present invention will be described below with reference to FIG. The configurations other than those described in the present embodiment are the same as those in the first to third embodiments. FIG. 10 is a cross-sectional view schematically showing the joint portion 2D of the joint structure 1D in the present embodiment.

In the third embodiment, the four-disc joint structure 1C has been described. On the other hand, the joint structure 1D of the present embodiment is a 5-disc joint structure including the steel material 22 in addition to the steel material 20 and the steel material 21.

As shown in FIG. 10, in the joint structure 1D, the steel material 22, the steel material 21, the steel material 20, the hot-dip aluminum-based plated steel plate 30, and the aluminum material 10 are laminated in this order, and each plate material is laminated. It is formed by spot welding using a single-phase AC type spot welder 100.

The steel material 22, the steel material 21, and the steel material 20 may each have the same steel grade or may have different steel grades. Further, each may have the same plate thickness, or may have different plate thicknesses. The plate thickness of the steel material 22 is, for example, in the range of 0.8 mm to 1.4 mm.

The joint portion 2D of the joint structure 1D is formed with a lower nugget 58 extending over a steel material 22, a steel material 21, a steel material 20, and a hot-dip aluminum-based plated steel sheet 30. The length from one to the other of the boundary between the lower nugget 58 and the surface of the steel material 21 (or the steel material 22) on the interface where the steel material 21 and the steel material 22 abut is defined as the third steel nugget diameter W4.

The joint structure 1D has a ratio of W1 / W2 of 1.1 to 2.0. The lengths of the second steel nugget diameter W3 and the third steel nugget diameter W4 have a small influence on the joint strength of the joint structure 1D. Further, in the joint structure 1D, an intermediate region 60 is formed at the joint interface between the molten Al nugget 51 and the base steel plate 31.

The steel material 22, the steel material 21, the steel material 20, the molten aluminum-based plated steel sheet 30, and the aluminum material 10 are spot-welded by a single-phase AC method so that the residual Al thickness L1 is within the range of the present invention.

As a result, even in the 5-disc bonded structure 1D, the residual Al thickness L1 can be 0.01 mm or more, and a bonded structure having excellent corrosion resistance can be manufactured. Then, for example, the joint structure 1D having a joint strength of 0.8 kN or more can be manufactured as a measurement result of the cross tensile test.

The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the claims, and the embodiments obtained by appropriately combining the technical means disclosed in the different embodiments. Is also included in the technical scope of the present invention.

Hereinafter, the bonded structure of the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited thereto.

[Spot welder]
In the example shown below, spot welding was performed using the above-mentioned spot welder 100. The initial pressurization was 35 cycles and the hold was 24 cycles. As a commercial power source, a power source with a power frequency of 60 Hz was used. As the electrode 120, a DR type electrode tip (see FIG. 2) having a φ16 mm DR, a tip R40 mm, and a tip diameter of 6 mm was used.

[Cross Tensile Test]
11A and 11B are schematic views for explaining a cross tensile test, where FIG. 11A is a plan view of the joint structure and FIG. 11B is a cross-sectional view of the joint structure. Here, a 4-disc joint structure is shown.

As shown in FIGS. 11A and 11B, the steel material 21 and the steel material 20 are laminated, and the molten aluminum-based plated steel sheet 30 and the aluminum material 10 are laminated in this order. As the aluminum material 10, a material having a width W10 of 50 mm and a length L10 of 150 mm is used. Further, the molten aluminum-based plated steel sheet 30, the steel material 20, and the steel material 21 are similarly 50 mm × 150 mm.

The hot-dip aluminum-based plated steel sheet 30 and the aluminum material 10 are laminated so as to form a cross. Then, an electrode is applied to the welding point 3 and spot welding is performed. As a result, a joint structure is formed.

A cross tensile test is performed by applying an upward force to the aluminum material 10 of the bonded structure and a downward force to the molten aluminum-based plated steel sheet 30. Thereby, the cross tensile strength (kN) was evaluated. A cross tensile strength of 0.8 kN or more was judged to be acceptable, and was evaluated as ◯. The cross tensile test was performed in accordance with JIS Z 3137.

[Cross section observation]
Each of the manufactured joint structures was embedded in an epoxy resin so that the center of the welded portion could be observed, and polished. After the polishing treatment, etching was performed with a 3% NaOH aqueous solution, and then further etching was performed with ethanol in which 3% nitric acid was dissolved. This made it possible to measure the nugget diameter at the weld. By observing the cross section using an optical microscope, the nugget diameters of the molten aluminum nugget and the molten steel nugget, and the residual Al thickness L1 were measured.

[Corrosion resistance]
For each of the manufactured bonded structures, a salt spray test (SST) specified in JIS Z2371: 2000 was performed, and the white rust generation time was measured. The corrosion resistance of the bonded structure was evaluated according to the following criteria, and the ○ evaluation was judged to be acceptable.

◯: No white rust occurred in 10 hours ×: White rust occurred in 10 hours.

(Example 1: 2-disc set)
Aluminum alloy 6022 with a plate thickness of 1.0 mm or aluminum alloy 5052 (aluminum material) with a plate thickness of 1.0 mm or 3.0 mm, and hot-dip aluminum-based plated steel plates with a plate thickness of 0.5 mm, 1.0 mm, or 1.6 mm. , And spot welded under the conditions shown in Table 1 or Table 2 below. Then, a cross-section observation, a cross tensile test, and a salt spray test were performed on each sample.

As the hot-dip aluminum-based plated steel sheet, the above-mentioned hot-dip aluminum-based plated steel sheet 30 was used. Therefore, the base steel sheet contains a predetermined amount of N, and the hot-dip aluminum-based plated steel sheet contains an N-concentrated layer. This also applies to the description of the other examples below.

Table 1 shows the results when the aluminum alloy 6022 was used as the aluminum material. The Al molten nugget diameter shown in Table 1 below corresponds to the nugget diameter W1 of the molten Al nugget 51 described above using FIG. 4, and the distance from the top of the molten Al nugget to the surface layer of the aluminum alloy is the remaining Al. It corresponds to the thickness L1. This also applies to the description of the other examples below.

No. in Table 1 As shown in 1 to 6, the bonded structure of the example in which the residual Al thickness L1 was within the range of the present invention showed excellent corrosion resistance and excellent cross tensile strength.

On the other hand, Comparative Example No. in which the molten nugget of the aluminum material reached the surface layer of the aluminum material. In 7 to 9, the cross tensile strength is excellent, but the corrosion resistance is inferior.

Table 2 shows the results when the aluminum alloy 5052 was used as the aluminum material.

No. in Table 2 As shown in 11 to 14, the bonded structure of the example in which the residual Al thickness L1 was within the range of the present invention showed excellent corrosion resistance and excellent cross tensile strength.

On the other hand, Comparative Example No. in which the molten nugget of the aluminum material reached the surface layer of the aluminum material. In 15 and 16, the cross tensile strength is excellent, but the corrosion resistance is inferior.

In addition, Example No. 14 and Comparative Example No. From 16, the following can be seen. That is, even when the plate thickness of the aluminum material is as thick as 3 mm, when the heat input during spot welding becomes higher than a specific value, the molten nugget of the aluminum material reaches the surface layer of the aluminum material, and the corrosion resistance is lowered. obtain.

(Example 2: 3-disc set)
The following table is made by laminating an aluminum alloy (aluminum material) with a plate thickness of 1.2 mm, a hot-dip aluminum-based plated steel plate with a plate thickness of 0.5 mm, and steel materials of various steel types with a plate thickness of 0.8 mm to 1.4 mm. Spot welding was performed under the conditions shown in 3. The total thickness of the steel plate was 1.3 mm to 1.9 mm. The steel molten nugget diameter shown in Table 3 below corresponds to the nugget diameter W2 of the lower nugget 52. An aluminum alloy 6022 was used as the aluminum material.

No. in Table 3 As shown in 21 to 34, the bonded structure of the example in which the residual Al thickness L1 was within the range of the present invention showed excellent corrosion resistance and excellent cross tensile strength.

On the other hand, Comparative Example No. in which the molten nugget of the aluminum material reached the surface layer of the aluminum material. In 35 and 36, the cross tensile strength is excellent, but the corrosion resistance is inferior.

(Example 3: 4-disc set)
Aluminum alloy (aluminum material) with a plate thickness of 1.2 mm, hot-dip aluminum-based plated steel plate with a plate thickness of 0.5 mm, GA steel plate with a plate thickness of 0.8 mm (first steel material), and various types with a plate thickness of 1.0 mm. The second steel material of the steel type of No. 1 was laminated and spot welded under the conditions shown in Table 4 below. The total thickness of the steel plate was 2.3 mm. An aluminum alloy 6022 was used as the aluminum material.

In the 4-disc bonded structure, (i) the diameter of the molten Al nugget and (ii) the diameter of the molten steel nugget between the molten aluminum-based plated steel sheet and the steel material adjacent to the steel sheet (to the above-mentioned nugget diameter W2). The ratio of the nugget diameter was calculated using (equivalent) and.

No. in Table 4 As shown in 41 to 44, the bonded structures of Examples in which the ratio of the residual Al thickness L1 to the nugget diameter was within the range of the present invention showed excellent corrosion resistance and excellent cross tensile strength.

(Example 4: 5-disc set)
An aluminum alloy (aluminum material) having a plate thickness of 1.2 mm, a hot-dip aluminum-based plated steel plate having a plate thickness of 0.5 mm, and three steel materials were laminated and spot-welded under the conditions shown in Table 5 below. The three steel materials include a first steel material of various steel grades having a plate thickness of 0.8 mm or 1.4 mm, a second steel material of various steel grades having a plate thickness of 1.0 mm, and a plate thickness of 1.0 mm or 1. It is a third steel material of various steel types of 4 mm. The total thickness of the steel plate was 3.3 mm or 4.3 mm. An aluminum alloy 6022 was used as the aluminum material.

In the case of a 5-disc bonded structure, as in the case of a 4-disc set, it is formed between (i) the diameter of the molten Al nugget and (ii) the molten aluminum-based plated steel sheet and the steel material adjacent to the steel sheet. The ratio of the nugget diameter was calculated using the diameter of the molten steel nugget (corresponding to the above-mentioned nugget diameter W2).

No. in Table 5 As shown in 51 to 56, the bonded structures of Examples in which the ratio of the residual Al thickness L1 to the nugget diameter was within the range of the present invention showed excellent corrosion resistance and excellent cross tensile strength.

1A to 1D Joint structure 2A to 2D Joint part 10 Aluminum material 11 Electrode pressing surface (first surface)
20 Steel material 30 Hot-dip aluminum-based plated steel sheet 31 Base steel sheet (base steel)
34 N Concentrated Layer 51 Fused Al Nugget (1st Nugget)
52 Lower nugget (second nugget)
120 electrodes

Claims (4)

It is a joint structure that is joined by spot welding an aluminum material and a hot-dip aluminum-based plated steel sheet.
A molten aluminum nugget is formed at the boundary between the aluminum material and the molten aluminum-based plated steel sheet.
The surface of the aluminum material opposite to the surface of the hot-dip aluminum-based plated steel sheet facing the hot-dip aluminum-based plated steel sheet is designated as the first surface.
The shortest distance between the outer edge of the molten nugget and the first surface of the aluminum material is 0.01 mm or more .
The hot-dip aluminum-based plated steel sheet is made of a steel sheet containing N of 25 ppm or more and 200 ppm or less as a base steel, and an N-concentrated layer of N: 3.0 atomic% or more is formed at the interface between the base steel and the hot-dip aluminum plated layer. Has been
At the joint interface between the molten nugget and the base steel of the molten aluminum-based plated steel sheet, the central region is defined as the central region, the outermost peripheral region is defined as the outer peripheral region, and the region between the central region and the outer peripheral region is defined as the outer peripheral region. As an intermediate region,
Fe-Al-based intermetallic compounds are mainly formed in the central region.
Fe-Al-Si-based intermetallic compounds are mainly formed in the outer peripheral region.
The intermediate region is a bonded structure characterized in that a Fe—Al—Si-based intermetallic compound and the N-concentrated layer are mixed . The hot-dip aluminum-based plated steel sheet and the aluminum material are laminated on the steel material in this order and spot-welded to join the steel material, the hot-dip aluminum-based plated steel sheet, and the aluminum material.
It is assumed that the molten nugget is the first nugget and the nugget formed at the boundary between the molten aluminum-based plated steel sheet and the steel material is the second nugget.
The bonded structure according to claim 1, wherein the ratio of the nugget diameter of the first nugget to the nugget diameter of the second nugget is 1.1 or more and 2.0 or less. A method for manufacturing a bonded structure including a spot welding step of spot welding an aluminum material and a hot-dip aluminum-based galvanized steel sheet.
The surface of the aluminum material opposite to the surface facing the hot-dip aluminum-based plated steel sheet is designated as the first surface.
The hot-dip aluminum-based plated steel sheet is made of a steel sheet containing N of 25 ppm or more and 200 ppm or less as a base steel, and an N-concentrated layer of N: 3.0 atomic% or more is formed at the interface between the base steel and the hot-dip aluminum plated layer. Has been
In the spot welding process,
The pressing force of the electrode is 1.4 kN or more and 4.9 kN or less, and the value represented by the product of the welding current (kA) and the energization time (cycle) is within the range of 54 or more and 240 or less . One cycle is a time determined from the power frequency of the commercial power supply connected to the spot welder used in the spot welding process.
(I) The shortest distance between the outer edge of the molten nugget formed at the boundary between the aluminum material and the molten aluminum-based plated steel sheet and the first surface of the aluminum material is 0.01 mm or more, and the minimum distance is 0.01 mm or more .
(Ii) At the joint interface between the molten nugget and the base steel of the molten aluminum-based plated steel sheet, the central region is the central region, the outermost peripheral region is the outer peripheral region, and between the central region and the outer peripheral region. If the area of is the intermediate area,
Fe-Al intermetallic compounds are mainly formed in the central region.
Fe-Al-Si-based intermetallic compounds are mainly formed in the outer peripheral region.
In the intermediate region, the Fe-Al-Si intermetallic compound and the N-concentrated layer are mixed so as to be mixed.
A method for manufacturing a bonded structure, which comprises spot welding the aluminum material and the hot-dip aluminum-based plated steel sheet by a single-phase alternating current method. The method for manufacturing a joint structure according to claim 3, wherein the joint strength is 0.8 kN or more as a measurement result of a cross tensile test of the joint structure formed in the spot welding step.

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