JP2019177408A - Welded structure and method for manufacturing the same - Google Patents

Abstract

To provide a welded structure in which an aluminium material and a steel material are welded to different materials by a resistance spot welding method, which is excellent in welding strength.SOLUTION: In a welded structure (1A), a steel material (20), a molten aluminium-system plated steel plate (30) and an aluminium material (10) are welded by spot welding, and a ratio of a shortest distance L3 between an outer edge part of an upper nugget (51) and an electrode pressing surface (11) to a board thickness t2 of the aluminium material (10) is 20% or more and 80% or less.SELECTED DRAWING: Figure 5

Description

  The present invention relates to a bonded structure bonded to different materials by resistance spot welding and a method for manufacturing the bonded structure.

  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 excellent in corrosion resistance. For example, in the automobile field, an aluminum material may be used as a structural material of a vehicle body to reduce the weight of the vehicle body.

  The aluminum material may be used only for a part of the vehicle body from the viewpoint of manufacturing cost, productivity, and the like. For example, the aluminum material is used for a part (roof or the like) that does not require much strength. In this case, it is necessary to join the aluminum material constituting a part of the vehicle body and the steel material constituting the other part to each other.

  In general, among various methods used for joining different materials, the resistance spot welding method has advantages such as a reduction in the weight of the joint and high productivity as compared with the mechanical joining method. However, usually, when spot welding is performed on an aluminum material and a steel material, there is a problem in that joint strength may be reduced due to formation of a brittle intermetallic compound at the joint interface between them.

  So far, techniques related to spot welding of aluminum and steel have been proposed. For example, Patent Document 1 describes a method of spot-welding aluminum materials and steel materials with an intermediate material interposed therebetween.

  Patent Document 2 describes a bonded structure obtained by bonding an aluminum material and a hot-dip aluminized steel sheet.

Japanese Patent No. 6094440 (issued on March 15, 2017) Japanese Patent No. 4280671 (issued on June 17, 2009)

  However, the welded portion formed by the method described in Patent Document 1 may not be able to obtain sufficient mechanical strength. Specifically, it is described that the aluminum nitride-based alloy layer as the third metal material included in the intermediate material is a ceramic-based alloy and has a melting point of about 2473K. Further, during spot welding, the aluminum nitride-based alloy layer on the aluminum material side exists without deformation. Accordingly, the nugget formed by melting aluminum and the aluminum nitride-based alloy layer are insufficiently bonded, and the bonding strength of the welded portion may be insufficient.

  Patent Document 2 describes a bonded structure in which an aluminum material and a hot-dip aluminized steel sheet are bonded. This joint structure exhibits good joint strength as a test result of the cross tensile strength. However, it is difficult to directly apply this technique to the dissimilar material joining between the aluminum material and the steel material as described above.

  Various types (steel types) exist as steel materials. For example, when manufacturing a vehicle body, as for the steel material which comprises each part of a vehicle body, the steel type and the thickness of a steel material are selected according to the request | requirement of intensity | strength different for every part of a vehicle body. Further, even when a plurality of steel materials are to be joined, it is required to appropriately spot weld the plurality of steel materials and the aluminum material. The knowledge about joining such steel materials and aluminum materials by spot welding is not disclosed in Patent Documents 1 and 2.

  One aspect of the present invention has been made in view of the above-described conventional problems, and is a bonded structure in which an aluminum material and a steel material are bonded to each other by a resistance spot welding method, and has a high bonding strength. Is to provide.

  The bonded structure in one aspect of the present invention is a bonded structure bonded to a steel material by spot welding by laminating a molten aluminum-based plated steel plate and an aluminum material in this order, and the molten aluminum-based plating The plated layer of the steel plate is an aluminum layer containing 3% by mass to 12% by mass of Si and 0.5% by mass to 5% by mass of Fe, and the thickness of the aluminum material before spot welding is determined. As t1, the nugget diameter of the molten nugget of aluminum formed in the vicinity of the boundary between the aluminum material and the molten aluminum-based plated steel sheet is 5√ (t1) or more. The surface opposite to the surface facing the plated steel plate is the first surface, and (i) in the spot-welded part (Ii) The ratio of the shortest distance between the outer edge of the molten nugget and the first surface with respect to the thickness t2 of the aluminum material when the molten nugget is considered not to exist. Then, the remaining rate is 20% or more and 80% or less.

  In the bonded structure according to an aspect of the present invention, the molten nugget is used as a first nugget, and a molten nugget of steel formed by melting the molten aluminum-based plated steel sheet and the steel material is used as a second nugget. In the cross section when the structure is cut by a plane parallel to the thickness direction, the second nugget and the surface of the molten aluminum-based plated steel sheet on the interface where the molten aluminum-based plated steel sheet and the steel material contact each other When the length from one side of the boundary to the other is referred to as the molten steel nugget diameter, the ratio of the nugget diameter of the first nugget to the molten steel nugget diameter is preferably 1.1 or more and 2.0 or less.

  In the bonded structure in one aspect of the present invention, the molten aluminum-based plated steel sheet is a steel sheet containing N of 25 ppm or more and 200 ppm or less as a base steel, and N: 3 is provided at the interface between the base steel and the molten aluminum plating layer. It is preferable that an N enriched layer of 0.0 atomic% or more is formed.

  In the bonded structure according to an aspect of the present invention, the central region of the bonded interface between the first nugget and the base steel of the molten aluminum-based plated steel sheet is the central region, the outermost peripheral region is the outer peripheral region, When the region between the central region and the outer peripheral region is an intermediate region, the central region is mainly formed with Fe-Al-based intermetallic compounds, and the outer peripheral region is mainly Fe-Al-Si-based. It is preferable that an intermetallic compound is formed, and the intermediate region is a mixture of an Fe—Al—Si intermetallic compound and the N enriched layer.

  In the bonded structure according to an aspect of the present invention, the length of each region in the radial direction from the center of the central region at the bonding interface is set to the length of each of the central region, the intermediate region, and the outer peripheral region. When the width is defined, the region width of the intermediate region is preferably 0.1 mm or more.

  The bonded structure according to one embodiment of the present invention preferably has a bond strength of 1 kN or more as a measurement result of a cross tension test.

  The manufacturing method of the joining structure in one mode of the present invention is a manufacturing method of a joining structure including the spot welding process of carrying out spot welding by laminating a hot-dip aluminum plating steel plate and an aluminum material in this order to steel materials. The plated layer of the molten aluminum-based plated steel sheet is an aluminum layer containing 3% by mass to 12% by mass of Si and 0.5% by mass to 5% by mass of Fe before spot welding. In the spot welding step, the pressure of the electrode is set to t1, and the surface of the aluminum material opposite to the surface facing the molten aluminum-based plated steel sheet is the first surface. Is 1.4 kN or more and 3.9 kN or less, and the value represented by the product of the welding current and the energization time is in the range of 65 or more and 210 or less, and the aluminum The nugget diameter of the molten nugget of aluminum formed in the vicinity of the boundary between the aluminum material and the molten aluminum-based plated steel sheet is 5√ (t1) or more, and (i) in the spot welded portion, (Ii) The ratio of the shortest distance between the outer edge portion of the molten nugget and the first surface to the thickness t2 of the aluminum material when no molten nugget is considered to be present as the residual ratio of the aluminum material Then, spot welding is performed so that the remaining rate is 20% or more and 80% or less.

  A bonded structure in which an aluminum material and a steel material are bonded to each other by a resistance spot welding method and has excellent bonding strength can be provided.

It is sectional drawing which shows the state which laminated | stacked the steel material, the hot-dip aluminum type plated steel plate, and the aluminum material before spot welding. It is a figure which expands and shows the interface vicinity of the base-material steel plate and aluminum plating layer of the hot-dip aluminum type plated steel plate in sectional drawing shown in FIG. It is sectional drawing which shows typically the junction part of the joining structure body in Embodiment 1 of this invention. It is a top view which omits this aluminum material at the time of seeing the above-mentioned joined structure object from the aluminum material side, and is shown typically. It is a figure for demonstrating the method to measure the aluminum alloy residual rate and the area | region width of an intermediate area | region of the joining structure in Embodiment 1 of this invention. It is sectional drawing which shows typically the junction part of the joining structure body in Embodiment 2 of this invention. It is a schematic diagram for demonstrating the cross tension test in an Example, (a) is a top view of a joining structure body, (b) is sectional drawing of a joining structure body.

[Embodiment 1]
Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. The following description is for better understanding of the gist of the invention and does not limit the present invention unless otherwise specified. Unless otherwise specified, “A to B” representing a numerical range means “A or more (including A and greater than A) and B or less (including B and less than B)”. The shape and dimensions (length, depth, width, etc.) of the configuration described in each drawing in the present application do not necessarily reflect the actual shape and dimensions, and are appropriately changed for clarity and simplification of the drawings. is doing.

  The joint structure in the present embodiment is a vehicle component formed by joining, for example, a roof panel and a roof side portion of a vehicle body in the automobile field. However, the bonded structure of the present invention is not necessarily limited to this. The present invention can be appropriately applied to a joint structure for use where there is a demand for joining an aluminum material and a steel material by spot welding.

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

<Knowledge of the present invention>
Conventionally, in the automobile field, the use of aluminum materials for roofs and the like has been promoted as described above. On the other hand, when spot welding is performed on an aluminum material and a steel material, a fragile Fe—Al binary alloy layer is generated at the joint interface. There is a problem in that joint strength decreases due to the occurrence of peeling at the Fe-Al binary alloy layer.

  Until now, the technique regarding the joining structure which joined the aluminum material and the hot dip galvanized steel sheet has been reported (patent document 2). However, this technique cannot be applied as it is, for example, to the manufacture of vehicle parts in the automobile field. This is because the welding conditions are essentially different between spot welding of an aluminum material and spot welding of a steel material. Aluminum materials need to be welded in a short time with a relatively large welding current because of their low specific resistance and high thermal conductivity. Further, for example, when manufacturing a vehicle component, it may be necessary to weld an aluminum material and a plurality of steel materials.

  Therefore, in addition to aluminum materials and hot-dip aluminum-based plated steel sheets, steel materials were laminated, and joint structures were manufactured by spot welding to join them to each other. . Moreover, the case where there are a plurality of steel materials was completely unknown.

  Under such circumstances, the present inventors tried to manufacture a joined structure having high joint strength by spot welding an aluminum material, a molten aluminum-based plated steel plate, and a steel material. And it was earnestly investigated what kind of structure as a joining structure could raise joint strength. As a result, the following knowledge was obtained and this invention was implement | achieved.

  That is, as the molten aluminum-based plated steel sheet, a molten aluminum-based plated steel sheet containing a predetermined amount of Si and Fe in the plating layer is used. In this case, in the molten aluminum-based plated steel sheet, an Fe—Al—Si ternary alloy layer (Fe—Al—Si intermetallic compound) is formed between the base steel sheet and the plating layer. When such a molten aluminum-based plated steel sheet is used, the area ratio of the Fe—Al binary alloy layer generated in the strong heat input region (described later) at the joining interface between the aluminum material and the molten aluminum-based plated steel sheet is 70% or more. It can be suppressed to 90% or less.

  Moreover, let the nugget produced | generated in the joining interface of an aluminum material and a molten aluminum type plated steel plate by spot welding be a molten Al nugget. Generally, in order to increase the bonding strength of the bonded structure, it is necessary to increase the molten Al nugget. When the welding current at the time of spot welding is increased, the molten Al nugget can be increased by increasing the heat input. On the other hand, when the residual ratio of the aluminum material in the joint portion is small, the fracture strength of the aluminum material is small, and thus the joint strength is lowered. In the bonded structure according to an aspect of the present invention, the nugget diameter of the molten Al nugget and the Al alloy residual ratio in the spot-welded portion satisfy predetermined conditions (specifically described later). Thereby, it can prevent that the fracture strength of an aluminum material becomes low, and can make the joint strength of a joining structure high.

  Preferably, the molten aluminum-based plated steel sheet according to the present embodiment contains a predetermined amount of nitrogen (N) in the base steel sheet, and is concentrated at the interface between the base steel sheet and the plating layer by a specific pretreatment. A layer is formed. Using this molten aluminum-based plated steel sheet, spot welding is performed on the aluminum material, the molten aluminum-based plated steel sheet, and the steel material under welding conditions that provide appropriate heat input. Thereby, the joint strength of the joint structure can be further increased. For example, a joint structure having a joint strength of 1 kN or more can be manufactured as a measurement result of a cross tension test.

<Joint structure>
Hereinafter, the bonded structure according to the embodiment of the present invention will be described with reference to FIGS. First, each plate material before spot welding will be described with reference to FIG. FIG. 1 is a cross-sectional view showing a state in which a steel material, a molten aluminum-based plated steel plate, and an aluminum material are laminated before spot welding.

  As shown in FIG. 1, before the spot welding, the steel material 20, the hot-dip aluminum-based plated steel plate 30, and the aluminum material 10 are stacked in this order. A bonded structure 1A (see FIG. 3) is manufactured by spot welding these plate materials using the electrode 40.

  In the present embodiment, a joined structure having one steel material will be described for ease of explanation. However, the bonded structure according to another embodiment of the present invention may include a plurality of steel materials. For example, the number of steel materials may be two or three. Below, the case where there is one steel material may be referred to as a three-piece joined structure, and the case where there are two steel materials is sometimes referred to as a four-piece joined structure.

  Note that the joint structure in one embodiment of the present invention is not limited to one manufactured by spot welding at one place. A joined structure manufactured by spot welding at a plurality of locations is also included in the scope of the present invention.

(Steel)
For example, in the automotive field, the body is assembled by changing the strength of the material at the right place, and the SPC 440 class and SPC 590 class having higher strength than the SPC 270 class having general material strength may be used. The steel type and 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, galvannealed steel sheet (GA steel sheet), and the like. The plate | board thickness of the steel material 20 exists in the range of 0.8 mm-1.4 mm, for example.

(Aluminum material)
In this specification, the term aluminum material is used to include both pure aluminum (however, it is allowed to contain inevitable impurities) and an aluminum alloy.

  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 1000 series, 3000 series, 5000 series, 6000 series, and 7000 series aluminum alloys may be used.

  The aluminum material 10 preferably has a Fe concentration of 1.0% by mass or less in consideration of corrosion resistance, workability, and the like. Fe contained in the aluminum material 10 has a much smaller influence on the generation and growth of the Fe—Al binary alloy layer than Fe in the aluminum plating layer 32 described later.

In addition, it is preferable that the aluminum material 10 has about 1% by mass of Si and 0.01 to 1.5% by mass of Mg 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. Moreover, when 1.5-6.0 mass% Mg is added, high intensity | strength will be obtained by solid solution strengthening.

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

  Moreover, the plate | board thickness of the aluminum material 10 may be suitably set according to the intensity | strength calculated | required as the aluminum material 10, and although it does not specifically limit, For example, it may be 0.6 mm-1.4 mm. In general, when an aluminum material is used for a roof panel of an automobile, the thickness is about this. Moreover, the plate | board thickness of the aluminum material 10 may be 0.8 mm-1.3 mm, and 1.0 mm-1.3 mm may be sufficient.

(Molten aluminum-based plated steel sheet)
The molten aluminum-based plated steel sheet 30 will be described with reference to FIGS. 1 and 2. FIG. 2 is an enlarged view of a part (A1) of the cross-sectional view shown in FIG.

  As shown in FIGS. 1 and 2, a molten aluminum-based plated steel sheet 30 includes a base steel sheet (base steel) 31 and an aluminum plating layer 32 formed on the surface of the base steel sheet 31. The aluminum plating layer 32 includes an alloy layer 33 formed at the 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 an 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, steel types to which Si, Mn, Cr, Ni, etc. are added may be used. Moreover, 25 ppm or more and 200 ppm or less of N are added to the base steel plate 31 in this embodiment. Thus, when heat treatment is performed under specific conditions after plating, an N enriched layer 34 is generated at the interface between the base steel plate 31 and the alloy layer 33. An example of the specific condition is a heat treatment that is held at a temperature of 520 ° C. for 6 hours.

  The N enriched layer 34 is a very thin layer formed on the surface portion of the base steel plate 31, and is a layer having a thickness of about 5 nm, for example. The N enriched layer 34 is more concentrated than the base steel sheet 31 (N is concentrated), and contains, for example, 3.0 atomic% or more of N. The N enriched layer 34 suppresses interdiffusion 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 is increased when the heat treatment conditions after plating are made constant. However, when the base steel plate 31 contains an excessive amount of N exceeding 200 ppm, the manufacturability of the base steel plate 31 itself decreases.

  Further, the molten aluminum-based plated steel sheet 30 is manufactured as follows. That is, the base steel plate 31 is immersed and passed through a molten aluminum plating bath containing aluminum as a main component. Thereby, 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 onto a steel strip immediately after being pulled up from a molten aluminum plating bath and controlling the amount of plating adhesion. By making the thickness of the aluminum plating layer 32 in 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.

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

  The aluminum plating layer 32 in the present embodiment is an aluminum layer containing 3% by mass to 12% by mass of Si and 0.5% by mass to 5% by mass of Fe. 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 Si concentration is regulated to 12% by mass.

  In order to sufficiently generate the Fe—Al—Si ternary alloy layer in the alloy layer 33, the amount of Fe and Si eluted from the base steel plate 31 to the molten Al is insufficient when immersed in the aluminum plating bath. Therefore, Fe and Si are replenished from the aluminum plating layer 32 to the bonding interface by increasing the Fe and Si contents of the aluminum plating layer 32. 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. Thereby, the amount of Si necessary for generating the Fe—Al—Si ternary alloy layer is ensured.

  Adjusting the composition of the aluminum plating layer 32 as described above not only promotes the formation of the Fe—Al—Si ternary alloy layer, but also increases the Fe and Si concentration of molten Al generated during spot welding. This also occurs. The nugget formed by rapidly cooling molten Al having a high Fe and Si concentration is hardened by the effect of solid solution strengthening of Fe and Si and the rapid cooling effect. Therefore, the joint strength of the joined structure 1A can be increased. And generation | occurrence | production of a weak Fe-Al binary alloy layer is suppressed, and 1 A of joining structures with a highly reliable joint strength are obtained.

  The composition of the above-described aluminum plating layer 32 is a composition of a region that does not include 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), such as Ti, Sr, B, Cr, Mn, Zn, etc. that do not significantly affect the Fe-Al interdiffusion reaction. Elements can be appropriately included in the aluminum plating layer 32.

(Joint part)
1 A of joining structures of this embodiment are formed by carrying out spot welding in the state which each plate laminated | stacked as mentioned above. The joint part 2A of the joint structure 1A will be described with reference to FIGS. FIG. 3 is a cross-sectional view schematically showing a joint portion 2A of the joint structure 1A in the present embodiment. FIG. 4 is a plan view schematically showing the bonded structure 1 </ b> A with the aluminum material 10 omitted when viewed from the aluminum material 10 side.

  As shown in FIG. 3, an upper nugget (first nugget) 51 that joins an aluminum material 10 and a molten aluminum-based plated steel plate 30, a molten aluminum-based plated steel plate 30, and a steel material is joined to the joint 2 </ b> A of the bonded structure 1 </ b> A. A lower nugget (second nugget) 52 that joins 20 is formed. In the cross-sectional view shown in FIG. 3, the upper nugget 51 is a molten nugget of aluminum 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 fan shape. The lower nugget 52 is formed at a boundary portion between the molten aluminum-based plated steel plate 30 and the steel material 20, and is formed in an elliptical shape extending to both of them.

  In the bonded structure 1 </ b> A, a strong heat input region (central region) 70 is formed at the interface between the upper nugget 51 and the base steel plate 31. As shown in FIG. 4, the strong heat input region 70 is formed at the center of the upper nugget 51 when the bonded structure 1 </ b> A is seen through from the aluminum material 10 side. A center point 51A shown in FIG. 4 is a central portion of energization during spot welding.

  Further, 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 upper 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.

  If it demonstrates on the basis of the center point 51A, as shown in FIG. 4, the upper nugget 51 will be formed so that it may expand from the center point 51A in circular shape. At the interface between the upper 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 arranged in this order on the outer side. Existing.

  These nuggets and each region will be described below.

(nugget)
The upper nugget 51 is formed as follows. That is, when the electrode 40 (see FIG. 1) is energized, the boundary surface between the aluminum material 10 and the molten aluminum-based plated steel sheet 30 is resistance-heated. Thereby, aluminum melts. As the molten aluminum solidifies, an upper nugget 51 is formed. Therefore, the upper nugget 51 is mainly made of aluminum and is affected by the composition of the aluminum material 10 and the aluminum plating layer 32.

  The influence of heat due to resistance heating becomes weaker as the distance from the center of energization increases. Therefore, the strong heat input region 70 is a region affected by relatively large heat input, and the weak heat input region 80 is a region affected by relatively small heat input.

  Further, the Fe and Si concentrations of the upper nugget 51 can vary 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 serving as the Fe and Si supply source to the upper nugget 51, the higher the Fe and Si concentration of the upper nugget 51 increases.

  For example, consider a case where spot welding is performed using a 1% chromium copper alloy tip having a curved surface with a diameter of 6 mm and a curvature radius of 40 mm under the conditions of welding current: 12.5 kA and energization time: 12 cycles. 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 upper nugget 51 are both higher by 0.1 mass% or more than the Fe and Si contents of the aluminum material 10. As a result, the upper nugget 51 is hardened and the bonding strength of the bonded structure 1A is improved.

  Further, the lower nugget 52 is formed by resistance heating of the boundary surface between the molten aluminum-based plated steel sheet 30 and the steel material 20 by energization, and the molten iron is solidified by raising the temperature to the melting point of Fe.

  Here, 1 A of joining structures in this embodiment are manufactured by laminating | stacking the steel material 20, the hot-dip aluminum type plated steel plate 30, and the aluminum material 10, and carrying out spot welding. When spot welding is performed under the same conditions as in the case of spot welding a two-piece set made of the molten aluminum-based plated steel sheet 30 and the aluminum material 10 with respect to such a three-piece set 1A, there are the following problems. That is, it is difficult to form both the upper nugget 51 and the lower nugget 52 so as to satisfy desired properties. 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 intensive studies, the present inventors have found various requirements for the joint portion 2A in order that the three-piece joint structure 1A has high joint strength.

  In the cross-sectional view shown in FIG. 3 (a cross-section when the bonded structure 1A is cut in a plane parallel to the thickness direction), the plate thickness of the aluminum material 10 (the aluminum material 10 in the undented portion) before spot welding is shown. The length from one to the other at the boundary between the upper nugget 51 and the alloy layer 33 is defined as an upper nugget diameter W1. In the bonded structure 1A of the present embodiment, the upper nugget diameter W1 is 5√ (t1) or more.

  When the upper nugget diameter W1 is less than 5√ (t1), since the upper nugget 51 is too small, the bonding strength of the bonded structure is lowered.

(Nugget diameter ratio)
Further, in a cross section (for example, the cross section shown in FIG. 3) when the joined structure 1A is cut along a plane parallel to the thickness direction, the lower nugget 52 on the interface where the molten aluminum-based plated steel sheet 30 and the steel material 20 contact each other. Hereinafter, the length from one side of the boundary between the surface of the molten aluminum-based plated steel sheet 30 to the other is referred to as a first steel nugget diameter (molten steel nugget diameter). In the bonded structure 1A of the present embodiment, the ratio of the nugget diameter of the upper nugget 51 to the first steel nugget diameter is preferably 1.1 or more and 2.0 or less.

  When the ratio is less than 1.1, the nugget diameter of the upper nugget 51 becomes too large (heat input becomes too large) during spot welding, and the aluminum material 10 is greatly thinned. Therefore, a bonded structure having a bonding strength of less than 1 kN can be obtained as a measurement result of the cross tension test.

  On the other hand, when the 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 easily broken at the lower nugget 52 in the cross tension test, and the joint strength can be less than 1 kN.

  The nugget diameters of the upper nugget 51 and the lower nugget 52 can be measured as follows. That is, the size of each of the upper nugget 51 and the lower nugget 52 (boundary with surrounding materials) in the bonded structure 1A is visually determined using, for example, an electron micrograph of a cross section as shown in FIG. Can be prescribed. The width from one end to the other end at the boundary between the upper nugget 51 and the alloy layer 33 is defined as an upper nugget diameter W1. The length from one side of the boundary between the lower nugget 52 and the surface of the molten aluminum-based plated steel sheet 30 to the other on the interface where the molten aluminum-based plated steel sheet 30 and the steel material 20 abut is defined as a first steel nugget diameter W2. In the bonded structure 1A of the present embodiment, the ratio W1 / W2 is 1.1 to 2.0.

(Aluminum alloy remaining rate)
Further, as shown in FIG. 3, the aluminum material 10 has depressions on the surface due to the electrode 40 biting into the aluminum material 10 by the applied pressure. A portion where the depression on the surface of the aluminum material 10 is generated is defined as an electrode pressing surface (first surface) 11. In other words, the electrode pressing surface 11 is a surface on the opposite side of the surface of the joining portion 2A that faces the molten aluminum-based plated steel sheet 30 of the aluminum material 10.

  The aluminum alloy remaining rate of the bonded structure 1A in the present embodiment will be described with reference to FIG.

  As shown in FIG. 5, the plate thickness t2 of the aluminum material 10 at the joint 2A is the electrode pressing surface 11 and the other plate surface (on the side of the molten aluminum-based plated steel plate 30 side) when it is considered that the upper nugget 51 does not exist. Distance).

  The shortest distance between the outer edge of the upper nugget 51 and the electrode pressing surface 11 is L3. The ratio of the distance L3 to the plate thickness t2 is defined as the remaining rate of the aluminum material 10 (sometimes referred to as an aluminum alloy remaining rate in this specification).

  In the bonded structure 1A of the present embodiment, the aluminum alloy remaining rate is 20% or more and 80% or less. When the aluminum alloy residual ratio is less than 20%, the breaking strength of the aluminum material 10 becomes small. That is, when the heat input at the time of spot welding is increased, the upper nugget diameter W1 is increased while the aluminum material 10 is thinned, whereby the aluminum material 10 is easily broken at the electrode pressing surface 11 portion. On the other hand, when the aluminum alloy residual ratio exceeds 80%, the upper nugget 51 is too small, so that the bonded structure cannot obtain a sufficient bonding strength.

  As described above, in the bonded structure 1A of the present embodiment, the Fe-Al-Si ternary alloy layer is formed as the alloy layer 33 on the molten aluminum-based plated steel sheet 30, and the upper nugget diameter W1 and the remaining aluminum alloy remain. Manufactured under welding conditions such that the rate is within the above range. Accordingly, the bonded structure 1A can increase the bonding strength and have a stable bonding strength.

  Moreover, as for the joining structure 1A of this embodiment, the strong heat input area | region 70, the intermediate | middle area | region 60, and the weak heat input area | region 80 are formed in the interface of the upper nugget 51 and the base-material steel plate 31 as mentioned above. . These regions will be described below with reference to FIGS.

  As shown in FIGS. 3 to 5, a weak heat input region 80 is formed at the outermost peripheral portion at the joint interface between the upper nugget 51 and the base steel plate 31. Assuming that the extension line of the center axis of the pair of electrodes 40 (see FIG. 1) is the electrode center line, the heat input tends to be higher at a position closer to the electrode center line at the joint interface during spot welding. It is in. The weak heat input region 80 is a position far from the electrode center line, and is a portion where the heat input during spot welding is small. On the other hand, the strong heat input area | region 70 is a location with large heat input at the time of spot welding. The intermediate region 60 is a heat input amount between the weak heat input region 80 and 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 high-temperature heating, and the N-enriched layer 34 of the alloy layer 33 is also melted and various elements are mutually diffused, so that the base steel plate 31 is melted. . As a result, the upper nugget 51 having a higher Fe concentration than the aluminum material 10 is formed.

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

  Therefore, in the strong heat input region 70, an Fe—Al-based intermetallic compound is mainly formed. However, it is allowed to include various elements supplied from the composition of the aluminum material 10 and the molten aluminum-based plated steel sheet 30 and to include an alloy formed by these elements. In this specification, “mainly Fe—Al-based intermetallic compounds are formed” means that, for example, the intense heat input region 70 is formed using nanoprobe electron beam analysis and energy dispersive X-ray analysis (EDX). When analyzed, it means that data showing that the abundance ratio of the Fe—Al intermetallic compound is the most is obtained.

(Weak heat input area)
On the other hand, the weak heat input region 80 is a region that is heated at the time of spot welding although it is relatively cooler than the strong heat input region 70. 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). In the weak heat input region 80, the Fe—Al—Si intermetallic compound is mainly formed continuously at the boundary between the upper nugget 51 and the N enriched layer 34 of the base steel plate 31.

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

  In the present specification, “mainly Fe—Al—Si intermetallic compounds are formed” means the presence of Fe—Al—Si intermetallic compounds as described above for the strong heat input region 70. This means that data indicating that the ratio is the highest can be obtained.

(Intermediate area)
In the intermediate region 60, the amount of heat input is such that the alloy layer 33 (Fe—Al—Si ternary alloy layer) is somewhat melted while spot welding and the N-enriched layer remains. Therefore, in the intermediate region 60, the alloy layer 33 and the N enriched layer 34 are mixed. In other words, the alloy layer 33 is formed discontinuously so as to be scattered at the boundary between the upper nugget 51 and the N enriched layer 34 of the base steel plate 31. In the intermediate region 60, there is no fragile Fe—Al intermetallic compound, or the abundance is small.

  In this specification, “Fe—Al—Si intermetallic compound and N enriched layer 34 coexist” means that when the intermediate region 60 is analyzed, the Fe—Al—Si intermetallic compound and N This means that data indicating the presence of the concentrated layer 34 (for example, data detecting the presence of Fe, Al, Si, and N) is obtained, and the following data is obtained. That is, in the intermediate region 60, it is meant that almost no Fe is detected on the molten Al nugget 51 side, and data indicating that no Fe—Al intermetallic compound is generated is 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.

  For example, when a force that peels off the aluminum material 10 is applied to the bonded structure 1A, the intermediate region 60 stops cracks that develop from the weak heat input region 80 that is the outer peripheral portion of the upper nugget 51. Work to let you. Thereby, when a further force is applied to the aluminum material 10, the bonded structure 1A is likely to be a button break rather than a shear break. That is, the upper nugget 51 has a high bonding strength by the intermediate region 60 acting so as to increase the bonding strength.

  As shown in FIG. 5, the length (region width) of the intermediate region 60 is measured as follows. That is, the length of the gap between the weak heat input region 80 and the strong heat input region 70 is measured in a sectional view as shown in FIG. The length of the right intermediate region 60 is L1, and the length of the left intermediate region 60 is L2. At this time, the length L of the intermediate region 60 is obtained by L1 + L2. In the present embodiment, the length L of the intermediate region 60 is 0.1 mm or more.

  What has been described above can be summarized as follows. The length of each region in the radial direction from the center of the strong heat input region 70 at the joint interface between the upper nugget 51 and the base steel plate 31 is defined as the strong heat input region 70, the intermediate region 60, and the weak heat input region 80. The area width of the intermediate area 60 is 0.1 mm or more.

<Method for producing bonded structure>
The manufacturing method of 1 A of joining structures includes the spot welding process of laminating | stacking the molten aluminum type plated steel plate 30 and the aluminum material 10 on the steel material 20 in this order, and spot-welding using a spot welding machine.

  The applied pressure during spot welding is in the range of 1.4 kN or more and 3.9 kN or less. The amount of 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 from the power supply frequency of the commercial power source connected to the spot welder. The commercial power supply may have a power frequency of 50 Hz or a power frequency of 60 Hz. Other power supply frequencies may be used. The amount of heat input can be controlled by adjusting the welding current according to the difference in power supply frequency. The value of welding current x energization time (cycle) at the time of spot welding is in the range of 65 to 210.

  Moreover, as the electrode 40 used for spot welding, for example, a DR (dome radius) type electrode tip may be mentioned. The DR-shaped electrode tip is a D-shaped electrode tip (the tip of the electrode is a curved surface, and the radius of curvature of the curved surface is ½ of the outer diameter of the electrode). Curved surface). The DR-type electrode tip has good shapeability by a tip dresser (electrode polishing tool) and is widely used in the automobile industry.

  For example, the electrode 40 has a body portion with an outer diameter of 16 mm, a tip diameter of 6 mm, and a tip portion having a curved surface with a curvature radius of 40 mm. Moreover, the surface of the shoulder part between the trunk | drum part of the electrode 40 and the said front-end | tip part is a curved surface with a curvature radius of 8 mm. The material of the electrode 40 is, for example, a chromium copper alloy (Cr—Cu alloy) containing 0.4 mass% to 1.2 mass% of Cr. The chromium copper alloy may contain about 1% by mass of Cr, for example, 0.8% by mass to 1.1% by mass of Cr.

  When using a commercial power supply with a power supply frequency of 60 Hz, 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 term “hold” refers to forging pressurization in which a material to be welded is pressed using the electrode 40 while the electrode 40 is cooled after the electrode 40 is energized. As a spot welder, for example, a single-phase AC welder can be used.

  The electrode 40 is not limited to the above dimensions and materials as long as it is a DR-type electrode chip. The outer diameter of the electrode 40 is not limited to 16 mm, and may be an appropriately designed outer diameter according to the shape of the tip. The tip diameter of the tip portion of the electrode 40 is preferably 5 mm to 7 mm. Further, the radius of curvature of the tip of the electrode 40 and the radius of curvature of the surface of the shoulder are not particularly limited, and may be values appropriately designed according to the values of the outer diameter and the tip diameter of the tip. .

(Advantageous effect)
The bonded structure 1 </ b> A of the present embodiment is manufactured by spot welding in a state where a molten aluminum-based plated steel plate 30 is sandwiched between the aluminum material 10 and the steel material 20. In the molten aluminum-based plated steel sheet 30, an alloy layer 33 made of an Fe—Al—Si-based intermetallic compound is formed on an aluminum plating layer 32. Then, spot welding is performed under welding conditions adjusted so that the residual ratio of the aluminum alloy in the joint is in the range of 20% to 80%.

  Preferably, the nugget diameter ratio between the upper nugget 51 and the lower nugget 52 is adjusted, and the intermediate region 60 exists at the bonding interface between the upper nugget 51 and the base steel plate 31. Thereby, the joint strength in the lower nugget 52 can be increased, and the progress of cracks at the joint interface between the upper nugget 51 and the base steel sheet 31 can be prevented by the intermediate region 60. Accordingly, the bonded structure 1A can increase the bonding strength and have a stable bonding strength.

  Accordingly, it is possible to provide a bonded structure 1A that is a bonded structure in which the aluminum material 10 and the steel material 20 are bonded to each other by the resistance spot welding method and has excellent bonding strength.

(Other configurations)
In this specification, the value obtained by summing the thickness of the base steel plate 31 in the molten aluminum-based plated steel plate 30 and the thickness of each of the plurality of steel materials is referred to as the total steel thickness of the bonded structure. . In the bonded structure according to one aspect of the present invention, the upper limit of the total thickness of the steel material is 3.1 mm. The number of steel materials is not particularly limited as long as the joining structure has a total steel thickness of 3.1 mm or less. In addition, if the total steel plate thickness exceeds 3.1 mm, it is difficult to generate a nugget between the molten aluminum-based plated steel plate 30 and the steel material therebelow. In this case, when the cross tension test is performed, the cross tensile strength of 1 kN or more tends not to be obtained easily at the interface portion between the molten aluminum-based plated steel sheet 30 and the steel material.

[Embodiment 2]
Another embodiment 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 embodiment. FIG. 6 is a cross-sectional view schematically showing the joint portion 2B of the joint structure 1B in the present embodiment.

  In the first embodiment, the three-piece joined structure 1A has been described. The joint structure 1 </ b> B of the present embodiment is a four-piece joint structure including a steel material 21 in addition to the steel material 20.

  As shown in FIG. 6, the joined structure 1 </ b> B includes a steel material 21, a steel material 20, a hot-dip aluminum-based plated steel plate 30, and an aluminum material 10 that are stacked in this order and spot-welded in a state where the respective plate materials are stacked. It is formed by doing.

  The steel material 21 and the steel material 20 may be the same steel type as each other or may be different from each other. Also, the same plate thickness may be used, or different plate thicknesses may be used. The plate | board thickness of the steel material 21 exists in the range of 0.6 mm-2.0 mm, for example.

  In the joint portion 2B of the joint structure 1B, a lower nugget 55 extending to the steel material 21, the steel material 20, and the molten aluminum-based plated steel sheet 30 is formed.

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

  The bonded structure 1B has an aluminum alloy remaining rate of 20% or more and 80% or less, and a W1 / W2 ratio of 1.1 to 2.0. The length of the second steel nugget diameter W3 has little influence on the bonding strength of the bonded structure 1B. Moreover, as for joining structure 1B, the area | region width of the intermediate | middle area | region 60 in the joining interface of the upper nugget 51 and the base-material steel plate 31 is 0.1 mm or more.

  Thereby, as for the joining structure 1B, joining strength can be 1 kN or more as a measurement result of a cross tension test. Therefore, it is a joint structure in which the aluminum material 10, the steel material 20, and the steel material 21 are joined to each other by the resistance spot welding method, and the joint structure 1B having excellent joint strength can be provided.

  The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope shown in the claims, and embodiments obtained by appropriately combining technical means disclosed in 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.

(Cross tension test)
7A and 7B are schematic diagrams for explaining the cross tension test, in which FIG. 7A is a plan view of the joint structure, and FIG. 7B is a cross-sectional view of the joint structure. Here, a four-piece joined structure is shown.

  As shown in FIGS. 7A and 7B, 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 thereon 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. Similarly, the molten aluminum-based plated steel sheet 30, the steel material 20, and the steel material 21 are 50 mm × 150 mm.

  The molten 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. Thereby, a joined structure is formed.

  A cross tension test is performed by applying an upward force to the aluminum material 10 of the bonded structure and applying a downward force to the molten aluminum-based plated steel sheet 30. This evaluated the cross tensile strength (kN). The cross tension test was conducted in accordance with JIS Z 3137.

[Cross-section observation]
Each of the manufactured bonded structures was embedded in an epoxy resin so that the center of the welded portion could be observed, and was polished. After the polishing treatment, etching was performed with 3% NaOH aqueous solution, and 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 a cross section using an optical microscope, the nugget diameters of the aluminum molten nugget and the steel molten nugget and the width L of the intermediate region were measured.

[Spot welder]
In the example shown below, spot welding was performed using a DC inverter type spot welding machine. The initial pressurization was 35 cycles and the hold was 24 cycles. A commercial power source having a power frequency of 60 Hz was used. An electrode having a diameter of 16 mm DR, a tip R of 40 mm, and a tip diameter of 6 mm was used.

(Example 1: 3 sheets)
Lamination of Al alloy (aluminum material) with a plate thickness of 1.2 mm, hot-dip aluminum-plated steel plate with a plate thickness of 0.4 to 0.6 mm, and steel materials of various steel types with a plate thickness of 0.8 mm to 1.4 mm. Then, spot welding was performed under the conditions shown in Table 1 below. The N content in the base steel sheet of the hot-dip aluminum-plated steel sheet was 16 ppm, 28 ppm, 100 ppm, or 193 ppm.

  And about each sample, the cross-sectional observation and the cross tension test were done. The results are shown in Table 1. Comparative Example No. Reference numeral 25 denotes a sample produced in a set of two sheets. Further, the Al melt nugget diameter described in Table 1 below corresponds to the nugget diameter W1 of the upper nugget 51 described above with reference to FIG. 2, and the steel melt nugget diameter corresponds to the first steel nugget diameter W2 of the lower nugget 52. is doing.

  No. in Table 1 As shown in 1-21, in the bonded structure of the example in which the nugget diameter W1, the aluminum alloy residual ratio, and the width of the intermediate region of the upper nugget 51 are within the scope of the present invention, the cross tensile strength is 1 kN or more, Excellent bonding strength.

  On the other hand, Comparative Example No. 1 in which either the nugget diameter W1 of the upper nugget 51 or the aluminum alloy remaining rate is outside the scope of the present invention. 22 and 23, the cross tensile strength was less than 1 kN. Further, Comparative Example No. 1 in which the N content in the base steel sheet of the molten aluminum-based plated steel sheet is 16 ppm. In No. 24, since the intermediate region was not generated, the cross tensile strength was less than 1 kN.

  Comparative Example No. 2 produced in a set of 2 sheets. 24, Example No. Spot welding was performed under the same welding conditions as in No. 8, but since the total thickness of the steel material was thin, the amount of heat generation was small, and no nugget was generated.

(Example 2: 4-pack)
An aluminum alloy (aluminum material) having a thickness of 1.2 mm, a hot-dip aluminum-based plated steel plate having a thickness of 0.5 mm, a first steel material of various steel types having a thickness of 0.8 mm or 1.4 mm, and a thickness of 1 A second steel material of various steel types of 0.0 mm or 1.2 mm was laminated and spot-welded under the conditions shown in Table 2 below. The total steel plate thickness was 2.3 mm or 3.1 mm, and the N content in the base steel plate of the hot-dip aluminum-plated steel plate was 100 ppm.

  And about each sample, the cross-sectional observation and the cross tension test were done. The results are shown in Table 2.

  No. in Table 2 As shown in 31 to 39, in the bonded structure of the example in which the nugget diameter W1 of the upper nugget 51, the aluminum alloy residual ratio, and the width of the intermediate region are within the scope of the present invention, the cross tensile strength is 1 kN or more, Excellent bonding strength.

  On the other hand, Comparative Example No. 1 in which either the nugget diameter W1 of the upper nugget 51 or the aluminum alloy remaining rate is outside the scope of the present invention. In 40 and 41, the cross tensile strength was less than 1 kN.

1A and 1B joint structure 2A and 2B joint portion 10 aluminum material 20 and 21 steel material 30 molten aluminum-based plated steel plate 31 base material steel plate (underlying steel)
32 Aluminum plating layer (molten aluminum plating layer)
33 Alloy layer 34 N enriched layer 40 Electrode 51 Upper nugget (first nugget)
52 Lower nugget (second nugget)
60 Middle area 70 Strong heat input area (central area)
80 Weak heat input area (outer peripheral area)

Claims (7)

A bonded structure bonded to a steel material by spot welding by laminating a molten aluminum-based plated steel plate and an aluminum material in this order,
The plated layer of the molten aluminum-based plated steel sheet is an aluminum layer containing 3% by mass to 12% by mass of Si and 0.5% by mass to 5% by mass of Fe,
When the thickness of the aluminum material before spot welding is t1, the nugget diameter of the aluminum molten nugget formed in the vicinity of the boundary between the aluminum material and the molten aluminum-based plated steel sheet is 5√ (t1) or more. And
The surface of the aluminum material opposite to the surface facing the molten aluminum-based plated steel sheet is the first surface,
(I) The shortest distance between the outer edge of the molten nugget and the first surface with respect to the thickness t2 of the aluminum material when the molten nugget is regarded as not present in the spot-welded portion. When the rate of distance is the residual rate of the aluminum material,
The bonded structure according to claim 1, wherein the remaining ratio is 20% or more and 80% or less. The molten nugget is the first nugget,
The molten nugget of steel formed by melting the molten aluminum-based plated steel sheet and the steel material as a second nugget,
The surface of the second nugget and the molten aluminum-plated steel sheet on the interface where the molten aluminum-based plated steel sheet and the steel material abut in a cross section when the joined structure is cut by a plane parallel to the thickness direction. When the length from one side of the boundary to the other is referred to as the molten steel nugget diameter,
2. The bonded structure according to claim 1, wherein a ratio of a nugget diameter of the first nugget to the molten steel nugget diameter is 1.1 or more and 2.0 or less.   The molten aluminum-based plated steel sheet uses a steel sheet containing N of 25 ppm or more and 200 ppm or less as a base steel, and an N enriched layer of N: 3.0 atomic% or more is formed at the interface between the base steel and the molten aluminum plated layer. The bonded structure according to claim 2, wherein the bonded structure is formed. A central region at the joint interface between the first nugget and the base steel of the hot-dip aluminum-based plated steel sheet is a central region, an outermost peripheral region is an outer peripheral region, and a region between the central region and the outer peripheral region. Is the middle region
In the central region, mainly Fe-Al intermetallic compounds are formed,
The outer peripheral region is mainly formed of Fe-Al-Si intermetallic compounds,
The junction structure according to claim 3, wherein the intermediate region is a mixture of an Fe—Al—Si intermetallic compound and the N enriched layer. When the length of each region in the radial direction from the center of the central region at the bonding interface is defined as the region width of the central region, the intermediate region, and the outer peripheral region,
The joining structure according to claim 4, wherein a region width of the intermediate region is 0.1 mm or more.   The joining structure according to any one of claims 1 to 5, wherein the joining strength is 1 kN or more as a measurement result of the cross tension test. A method for manufacturing a bonded structure, comprising a spot welding step of spot welding, laminating a molten aluminum-based plated steel sheet and an aluminum material in this order to a steel material,
The plated layer of the molten aluminum-based plated steel sheet is an aluminum layer containing 3% by mass to 12% by mass of Si and 0.5% by mass to 5% by mass of Fe,
The plate thickness of the aluminum material before spot welding is t1, and the surface of the aluminum material opposite to the surface facing the molten aluminum-based plated steel plate is the first surface.
In the spot welding process,
The pressure of the electrode is 1.4 kN or more and 3.9 kN or less, and the value represented by the product of the welding current and the energization time is in the range of 65 or more and 210 or less,
The nugget diameter of the aluminum molten nugget formed in the vicinity of the boundary between the aluminum material and the molten aluminum-based plated steel sheet is 5√ (t1) or more, and
(I) The shortest distance between the outer edge of the molten nugget and the first surface with respect to the thickness t2 of the aluminum material when the molten nugget is regarded as not present in the spot-welded portion. When the distance ratio is the residual ratio of the aluminum material, spot welding is performed so that the residual ratio is 20% or more and 80% or less.

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