JP2005152958A - Joint body of different material of steel material and aluminum material, and its joint method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a joint body of a steel material and an aluminum material, which is spot-welded with high joint strength, and to provide its spot-welding method. <P>SOLUTION: The joint material is a joint body 3 of different materials, which is formed by joining a steel material 1 and an aluminum material 2 by spot-welding. The steel material 1 of the joint body 3 of different materials has a tensile strength of the ≥ 400 MPa and has a plate thickness t<SB>1</SB>in the range of 0.3-2.5 mm. Each amount of Mn and Si that are contained in an oxide film 4 on the surface of the steel material 1 is concentrated respectively twice or more compared with each amount of Mn and Si that are contained in the steel material 1. The plate thickness t<SB>2</SB>of the aluminum material 2 is in the range of 0.5-2.5 mm. The diameter of a nugget 5 formed in spot welding is in the range of 4 × t<SB>2</SB><SP>0.5</SP>-7 × t<SB>2</SB><SP>0.5</SP>with relation to the plate thickness t<SB>2</SB>. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  TECHNICAL FIELD The present invention relates to a dissimilar material joined body of steel material and aluminum material and a joining method necessary for joining the dissimilar material joined body, which is suitable as a structural member such as a transportation field such as an automobile and a railway vehicle, machine parts, and a building structure. .

  In general, spot welding joins metal members of the same type together. For example, an iron-based material (hereinafter simply referred to as a steel material) and an aluminum-based material (generally referred to as pure aluminum and an aluminum alloy). If it can be applied to the joining of dissimilar metal members (dissimilar material joined body), it can significantly contribute to weight reduction and the like.

  However, when a steel material and an aluminum material are joined, it is very difficult to obtain a reliable joint having high strength (joint strength) because a brittle intermetallic compound is easily generated in the joint. Therefore, conventionally, these dissimilar joined bodies (dissimilar metal members) are joined by bolts, rivets or the like, but there are problems such as reliability, air tightness, and cost of the joint joint.

Thus, many studies have been made on spot welding methods for these different types of joined bodies. For example, a method of inserting an aluminum-steel clad material between an aluminum material and a steel material has been proposed (see Patent Documents 1 and 2). In addition, methods of plating or inserting a metal having a low melting point on the steel material side have been proposed (see Patent Documents 3, 4, and 5). Furthermore, a method of sandwiching insulator particles between an aluminum material and a steel material (see Patent Document 6), a method of providing unevenness on a member in advance (see Patent Document 7), and the like have been proposed.
Japanese Patent Laid-Open No. 6-63763 JP 7-178563 A JP-A-4-251676 JP 7-24581 A Japanese Patent Laid-Open No. 4-14383 JP-A-5-228643 JP-A-9-174249

  However, both of these methods are not just spot welding, but require separate processes such as spot welding in multiple layers, plating and processing, and there is a problem that new equipment must be incorporated into the current welding line. Costs also increase. There are also many operational problems such as markedly limited welding conditions.

  In addition, neither of these methods considers the influence of factors on the steel side or aluminum material, which is a material. Even if spot welding conditions are improved, the joining depends on the type of material and the variation in factors on the material side. Since there is a big difference in strength, there is also a big problem that the reproducibility is poor.

  The present invention has been made in order to solve such a problem, and it is a spot having a high bonding strength without newly using another material such as the above-described clad material or requiring a new separate process. The present invention provides a joined body of a steel material and an aluminum material that can be welded and a spot welding method thereof.

In order to achieve the above object, the gist of the dissimilar material joined body of the steel material and the aluminum material in the present invention is a dissimilar material joined body obtained by joining the steel material and the aluminum material by spot welding, and the tensile strength of the steel material is 400 MPa. As described above, the thickness t ₁ is in the range of 0.3 to 2.5 mm, and the respective amounts of Mn and Si contained in the oxide film on the steel surface are the respective amounts of Mn and Si contained in the steel material. compared to, and enriched, respectively 2 times or more, the thickness t ₂ of the aluminum material is in the range of 0.5 2.5 mm, the nugget diameter in the spot welding, the relationship between the thickness t ₂ Therefore, the range is 4 × t ₂ ^{0.5 to} 7 × t ₂ ^0.5 .

In order to achieve the above object, the gist of the spot welding method for a dissimilar material joint between a steel material and an aluminum material in the present invention is a spot welding method for a dissimilar material joint with the aluminum material, which is a plate of aluminum material In relation to the thickness t ₂ , a current of 10 kA or more is allowed to flow for 100 × t ₂ msec or less so that the nugget diameter is in the range of 4 × t ₂ ^{0.5 to} 7 × t ₂ ^0.5 .

  In order to solve the above-described problems of the prior art, the present inventors have made extensive studies focusing on the influence of material conditions, particularly the strength of steel materials and oxide films, when joining different materials by spot welding. As a result, in spot welding, there is a close relationship between the oxide film on the surface of the steel material and the bonding strength, that is, if the composition or properties of the oxide film on the surface of the steel material in contact with the molten aluminum is controlled, It was found that the joint strength in welding was improved.

  That is, it has been found that the fact that the deformation of the steel material is small and the oxide film is not easily broken even by pressurization in spot welding is effective in improving the joining strength in the joining of different materials. And as a means by which the deformation of the steel material is small and the oxide film is not destroyed by pressurization in this spot welding, in the present invention, the strength of the steel material is increased and Mn and Si are concentrated in the oxide film of the steel material. Selected.

  Furthermore, with regard to the spot welding method for dissimilar joints of steel and aluminum, generally, in order to perform spot welding with high joint strength, the formation of nuggets may be promoted. It is known that both the shear strength and the cross tensile strength increase as the nugget diameter increases. The nugget diameter is related to the amount of heat input. The higher the amount of current, the longer the time. For this reason, generally, a bonded body with high bonding strength is obtained by controlling the nugget diameter by the amount of heat input. Of course, if the nugget diameter is too large, melting reaches the surface of the material to be welded and dust is formed, so it is important to obtain an appropriate nugget diameter.

  However, when joining different materials of steel and aluminum, steel has a higher melting point, higher electrical resistance and lower thermal conductivity than aluminum, so the heat generation on the steel side increases, and the low melting point aluminum melts first. To do. Next, the surface of the steel material melts, and as a result, an Al—Fe-based brittle intermetallic compound layer is formed at the interface, so that high bonding strength cannot be obtained.

  Furthermore, when the melting reaches the aluminum material surface and dust is formed, the amount of thinning of the aluminum material increases, and high bonding strength cannot be obtained. That is, in order to obtain a high bonding strength, it is necessary to add a high heat input that forms a certain nugget diameter, but on the contrary, the melting of the steel material is suppressed to the minimum at the bonding interface, and It is necessary to minimize the occurrence. In addition to this, it is better if the embrittlement of the intermetallic compound layer formed at the interface can be suppressed.

  For this reason, when joining the dissimilar material of steel materials and aluminum materials, it discovered that it was possible to suppress generation | occurrence | production of dust, while obtaining a big nugget diameter about spot-welding conditions by making it welding at high current for a short time. Moreover, in addition to such an effect, it was also found that the melting of the steel material at the bonding interface can be suppressed, and as a result, the interface reaction layer can be thinned and the bonding strength is increased. Conversely, when welding is performed for a long time, a large nugget diameter can be obtained, but the amount of thinning of the aluminum material is large due to generation of dust. Furthermore, the melting of the steel material increases at the bonding interface, and the interface reaction layer also becomes thick, so the bonding strength is low.

  As described above, the present invention improves the bonding strength by controlling the strength and oxide film of the steel material, which are the raw material conditions, when different materials are joined by spot welding. As for spot welding conditions, the bonding strength is improved as welding at a high current for a short time. As a result, spot welding with high joint strength can be performed in a heterogeneous joint of steel and aluminum without using another material as in the prior art or requiring a separate process. The effect that can be achieved.

(Heterogeneous)
FIG. 1 is a sectional view showing a heterogeneous joined body defined by the present invention. In FIG. 1, reference numeral 3 denotes a dissimilar material joined body in which a steel material (steel plate) 1 and an aluminum material (aluminum alloy plate) 2 are joined by spot welding. 4 is an oxide film on the surface of the steel material 1. Reference numeral 5 denotes a nugget having an interface reaction layer 6 in spot welding, and has a nugget diameter indicated by an arrow in the horizontal direction in the drawing. t ₁ is the thickness of the steel material, t ₂ is the thickness of the aluminum material 2, and Δt is the minimum remaining thickness of the aluminum material after spot welding. This FIG. 1 shows the nugget diameter as well as the example of the invention described later, while suppressing the generation of dust and maintaining the minimum remaining thickness of the aluminum material, and further minimizing the melting of the steel material. The joined state is shown.

Below, the reason for limitation of each requirement of this invention and its effect | action are demonstrated.
(Tensile strength of steel)
In the present invention, the shape and material of the steel material to be used are not particularly limited, and an appropriate shape and material, such as a steel plate, a steel shape member, a steel pipe, which are generally used for structural members or selected from structural member applications Can be used. However, it is necessary that the steel material has a tensile strength of 400 MPa or more and a plate thickness t ₁ of 0.3 to 2.5 mm.

  As for the strength of the steel material, when the tensile strength is less than 400 MPa, the deformation of the steel material (in the plate thickness direction) is large due to pressurization by spot welding, and the oxide film is easily destroyed. For this reason, as described above, the reaction with molten aluminum is promoted. As a result, a brittle intermetallic compound is easily formed.

  In addition, mild steel generally has many low alloy steels, and the oxide film is made of iron oxide. Therefore, Fe and Al are easily diffused, and brittle intermetallic compounds are easily formed. For this purpose, the tensile strength needs to be 400 MPa or more, and desirably 500 MPa or more.

In this invention, although the component of steel materials is not limited, in order to acquire the intensity | strength of the said steel materials, it is preferable that it is high-tensile steel (high ten). In addition to steel, steel containing selectively Cr, Mo, Nb, V, Ti, etc. can also be applied as a component of steel in order to enhance hardenability and precipitation hardening. Cr, Mo, and Nb improve hardenability and improve strength, and V and Ti improve strength by precipitation hardening. However, the addition of a large amount of these elements reduces the toughness around the weld and tends to cause nugget cracks. For this reason, as a component of steel, basically, in mass%, C: 0.05 to 0.5%, Mn: 1 to 2.5%, Si: 0.5 to 1.5%, Cr: 0 to 1%, Mo : 0 to 0.2%, Nb: 0 to 0.1%, V: 0 to 0.1%, Ti: 0 to 0.1%, preferably 1 or 2 or more types are preferably selectively contained. And it is preferable that the remainder composition of these steel materials consists of Fe and an unavoidable impurity.
(Steel thickness)
In the present invention, it is necessary that the steel sheet has a thickness t ₁ of 0.3 to 2.5 mm. When the thickness t _{1 of the} steel material is less than 0.3 mm, the strength and rigidity necessary for the structural member and structural material described above cannot be secured, which is inappropriate. In addition, since the steel material is largely deformed by pressurization by spot welding and the oxide film is easily destroyed, the reaction with aluminum is promoted. As a result, an intermetallic compound is easily formed. On the other hand, when the thickness exceeds 2.5 mm, other joining means are employed as the above-described structural member or structural material, so that there is little need to join by spot welding. For this reason, it is not necessary to increase the thickness t _{1 of the} steel material beyond 2.5 mm.
(Oxide film of steel)
Each content of Mn and Si in the oxide film 4 on the steel material surface needs to be concentrated twice or more with respect to each content of Mn and Si in the base steel material.

  Although the action of Mn and Si in these oxide films is not certain, the oxide film on the steel surface originally has a barrier effect that delays the contact between the molten aluminum and the steel material. Here, it is presumed that the barrier effect of this oxide film becomes remarkably large as Mn and Si are concentrated in the oxide film. That is, it is assumed that Mn and Si strengthen the oxide film and suppress breakage due to the pressure of spot welding.

  Furthermore, these Mn and Si in the oxide film melt even after the oxide film itself is destroyed, preventing the diffusion of Fe and Al at the joint interface, and minimizing the formation of brittle intermetallic compounds. It is thought that there is.

  If the Mn and Si contents in the oxide film are concentrated less than twice the respective Mn and Si contents of the steel material, these effects are reduced and the bonding strength cannot be increased. The amount of Mn and Si in the oxide film depends on the amount of Mn and Si in the steel material. In this regard, in order to enhance the effect of Mn and Si concentration in the oxide film, the higher the Mn and Si concentration in the oxide film, the better. For that purpose, the more Mn and Si are contained in the steel material Good. However, on the other hand, the inclusion (addition) of a large amount of Mn and Si in the steel material lowers the toughness of the steel material around the welded portion, and the nugget crack is likely to occur. For this reason, the Mn content of the steel material is preferably in the range of 1 to 2.5%, and the Si content is preferably in the range of 0.5 to 1.5%.

  In order to increase the concentration of Mn and Si in the oxide film, the amount of Mn and Si in the steel material may be kept low, and Mn and Si may be concentrated only on the steel material surface.

  The degree of Mn and Si concentration in the oxide film can be analyzed by TEM-EDX analysis from the cross section of steel. In the present invention, the component ratio of Mn and Si excluding oxygen in the oxide film is converted, and the degree of concentration of Mn and Si is calculated by comparison with the ratio of Mn and Si components in the original steel And

  Generally, a steel material is continuously annealed during rolling, and conditions are set so as to suppress the formation of an oxide film as much as possible. Even if an oxide film is formed, it is usually removed by a subsequent pickling process. In this invention, although the manufacturing conditions of steel materials are not limited, the oxide film which concentrated Mn and Si can be formed by performing water cooling after annealing at 800-900 degreeC, for example. Further, the steel film having an oxide film enriched with Mn and Si as defined in the present invention can be obtained by not performing further pickling or not completely removing the oxide film even by pickling. In addition, in order to control the degree of concentration of Mn and Si, as described above, depending on the Mn and Si contents in the original steel material, by controlling the annealing temperature, time, atmosphere, etc. Is also possible.

  The film thickness of the oxide film enriched with Mn or Si may be about several tens of nm to 1 μm, and the oxide film need not be extremely thick. With the oxide film enriched with Mn and Si defined in the present invention, the barrier effect can be obtained with such a film thickness. Moreover, if it is the oxide film of these thickness and a component, the said barrier effect can be acquired, without having a bad influence on welding. In this respect, the comparatively simple operation of controlling the oxide film on the steel material side makes it possible to perform spot welding with high joint strength without greatly changing the conventional conditions and methods on the steel material side and the welding side. It is also an effect.

(Aluminum material)
The aluminum material used in the present invention is not particularly limited in the type and shape of the alloy, and depending on the required characteristics as each structural member, commonly used plate materials, profiles, forging materials, casting materials, etc. It is selected appropriately. However, the strength of the aluminum material is desirably higher in order to suppress deformation due to pressurization during spot welding, as in the case of the steel material. In this respect, the use of A5000 series, A6000 series, etc., which are high in strength among aluminum alloys and are widely used as this kind of structural member, is optimal.

However, the thickness t ₂ of the aluminum material used in the present invention is in the range of 0.5 2.5 mm. If the thickness t ₂ of the aluminum material is less than 0.5 mm, in addition to the strength as a structural material is inappropriate in shortage, no nugget diameter can be obtained easily can dust easily melting reaches an aluminum material surface Therefore, high bonding strength cannot be obtained. On the other hand, when the thickness t ₂ of the aluminum material exceeds 2.5 mm, spot welding is performed because other joining means are adopted as the structural member and structural material, as in the case of the steel thickness described above. Less need to be joined. For this reason, it is not necessary to increase the thickness t ₂ of the aluminum material beyond 2.5 mm.

(Nugget diameter)
Spot welding is performed so that the nugget diameter in spot welding is in the range of 4 × t ₂ ^{0.5 to} 7 × t ₂ ^0.5 in relation to the plate thickness t ₂ of the aluminum material. In other words, it is necessary to select spot welding conditions such that the nugget diameter is in the range of 4 × t ₂ ^{0.5 to} 7 × t ₂ ^0.5 .

Conventionally, when spot-welding the same kind of metal material, it is said that 5 × t ^0.5 is optimal for the metal material t in terms of strength, workability, and economy. ing. However, there is no report about joining of dissimilar metal materials like this invention. In this regard, the present inventors have obtained a sufficient joint strength when the nugget diameter in spot welding is in the range of 4 × t ₂ ^{0.5 to} 7 × t ₂ ^0.5 in relation to the thickness t ₂ of the aluminum material. In addition, it was newly clarified that both workability and economy are excellent.
That is, in the case of joining different metal materials as in the present invention, the optimum nugget diameter depends on the plate thickness of the aluminum material, and the influence of the plate thickness of the steel material is so small that it can be ignored. In addition, there is a wide optimum range larger than the optimum nugget diameter at the time of spot welding of the same kind of metal material.

Here, when the nugget diameter is less than 4 × t ₂ ^0.5 , the nugget diameter is small and the bonding strength is insufficient. Also, if the nugget diameter exceeds 7 × t ₂ ^0.5 , the nugget diameter is sufficient to obtain bonding strength, but dust tends to occur and the amount of thinning of the aluminum material is large. To do. That is, the nugget diameter needs to be in the range of 4 × t ₂ ^{0.5 to} 7 × t ₂ ^0.5 , and ^{preferably in} the range of 5 × t ₂ ^{0.5 to} 7 × t ₂ ^0.5 .

(Aluminum material thickness reduction)
As described above, in the sense of ensuring the bonding strength, thinning of the aluminum material after joining by spot welding as small as possible, it is desirable minimum residual thickness Δt is 50% or more of the original thickness t _2. More preferably, it is a good minimum residual thickness Δt is more than 90% of the original thickness t _2. The minimum remaining thickness Δt of the aluminum material is obtained by observing the cross section with an optical microscope or SEM, measuring the thickness reduction thickness, and taking the difference from the original thickness.

(Spot welding)
FIG. 2 illustrates an embodiment of spot welding for obtaining a heterogeneous joined body. In FIG. 2, 1 is a steel plate, 2 is an aluminum alloy plate, 3 is a dissimilar joint, 5 is a nugget, and 7 and 8 are electrodes.

As spot welding conditions, it is necessary to flow an adjusted current of 10 kA or more so that the nugget diameter falls within the above specified range in relation to the plate thickness t ₂ of the aluminum material of 100 × t ₂ msec or less.

As described above, high-current and short-time spot welding satisfies all of securing the nugget diameter, reducing dust, and reducing the interface reaction layer. When the current is less than 10 kA, there is not enough heat input to form and grow nuggets. In order to keep the nugget diameter within the specified range of the present invention, it is necessary to greatly increase the current amount to 10 kA or more. In addition, in the case of a long time exceeding 100 × t ₂ msec, the nugget diameter can be secured, but since the generation of dust and the growth of the interface reaction layer are brought about, the bonding strength is lowered.

  In addition, it is preferable to increase not only the energization time but also the amount of current within the above ranges as the thickness of the steel material or aluminum material to be joined increases as in the case of welding with the same kind of metal.

  The pressure applied during spot welding is not particularly specified, but it stabilizes the electrical contact between dissimilar materials, electrodes and materials, supports the molten metal in the nugget at the unmelted area around the nugget, and further In order to suppress the occurrence of this, a certain high pressure is required. However, since the nugget diameter tends to decrease as the applied pressure is increased, the amount of current needs to be increased accordingly.

  The shape of the electrode for spot welding is not particularly specified, but an R-shaped electrode having a large R is particularly desirable for the electrode 8 on the aluminum material side in order to increase the current efficiency in the initial energization. The steel-side electrode 7 may be either a dome-shaped R-type or F-type, but similarly, a larger R is desirable. Although the polarity is not specified, when using DC spot welding, it is desirable to use the aluminum material side as an anode and the steel material side as a cathode.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples, and the present invention is not limited to the following examples. Of course, it is also possible to implement them, and they are all included in the technical scope of the present invention.

  Test steels containing the chemical components (mass%) shown in Table 1 were melted and rolled to a plate thickness of 1.2 mm to obtain thin steel plates. In continuous annealing, oil annealing or water washing was performed after annealing at 500 to 1000 ° C., and then adjusted to the target strength of each test steel by tempering. Thereafter, the pickling time in sulfuric acid was changed to control the thickness of the oxide film and the concentration of Mn and Si in the oxide film.

  The annealing temperature and pickling time were preliminarily investigated by TEM-EDX from the cross section, and the Mn and Si concentrations in the oxide film were investigated and adjusted to 1.0, 1.5, 2.0, and 3.0. .

  The concentration of Mn and Si in the oxide film was determined by performing three quantitative analyzes using EDX on the oxide film part, re-converting the component ratio of Mn and Si, excluding only oxygen, and converting the Mn and Si of the original steel material. The ratio with the component ratio was calculated and averaged. In this example, Mn and Si were concentrated to the same extent, and only one of them was not concentrated. As the aluminum material, a commercially available A6022 aluminum alloy plate having a thickness of 1 mm was used.

  These steel plates (steel materials) and aluminum alloy plates (aluminum materials) were processed into the shape of a cross tensile test piece described in JIS A 3137, and then spot welded to prepare dissimilar joined bodies.

  For spot welding, a DC resistance welding tester was used, and the welding pressure was 1.5 kN, and welding was performed at one point with the welding current and time shown in Table 2. A dome-shaped electrode made of a Cu-Cr alloy was used, and the anode was made of aluminum and the cathode was made of steel.

  Evaluation of the minimum remaining thickness of the aluminum material is carried out by cutting the spot welded sample at the center of the weld, embedding it in resin, polishing it, measuring the plate thickness using an optical microscope, and averaging the results. Asked.

  The evaluation of the nugget diameter was measured on the steel material side using a method described in JIS A 3137 using a sample of a heterogeneous joint after the cross tension test. In this example, the nugget diameter and the minimum remaining thickness of the aluminum material hardly changed depending on the difference in the oxide film of the steel material.

  As an evaluation of the bonding strength, a cross tension test was performed on different types of bonded bodies. The cross tension test is based on the joint strength between A6022 materials = 1.0 kN. If it is 0.9 kN or more, ◎, 0.7 to 0.9 kN, ○, 0.5 to 0.7 kN, Δ, less than 0.5 kN. X.

  In this example, the cross tension test was used for the evaluation of the bonding strength because the cross tension test had a larger difference between the test conditions. The spot welded portion is weaker in the bonding strength in the direction perpendicular to the bonding surface than in the horizontal direction with respect to the bonding surface. For this reason, even if the shear tensile test result of pulling in the horizontal direction with respect to the joint surface is good, the result of the cross tension test of pulling in the vertical direction with respect to the joint surface is not necessarily good. On the other hand, if the result of the cross tensile test is good, it can be said that the result of the shear tensile test is better. Also in this example, the result of the shear tensile test is consistent with the above-mentioned cross tensile test result, and those that obtained the evaluation of ○ and ◎ in the cross tensile test are both higher than 2.5 kN. Shear strength.

  Tables 3 to 6 show the results of cross tension tests of different types of joints after spot welding of each steel type in Table 1 and A6022.

From Tables 3 to 6, it can be seen that when the tensile strength of the steel material is less than 400 MPa, the bonding strength is inferior, and when the tensile strength of the steel material is 400 MPa or more, preferably 500 MPa or more, the bonding strength increases. It can also be seen that when the amount of Mn and Si in the oxide film on the steel surface is concentrated twice or more, the bonding strength is increased, and when the concentration is less than twice, the bonding strength is inferior. Furthermore, the nugget diameter is ^{_{4 × t 2 0.5 ~7 × t}} 2 0.5, it can be seen that the bonding strength is high. It can also be seen that the bonding strength is further increased when the minimum remaining thickness of the aluminum material is 50% or more of the original thickness.

And the spot welding condition that the joint strength is high is that the current value is 10 kA or more and the joining time is 100 × t ₂ msec or less (in the example, the thickness of the aluminum material is 1 mm, the joining time is 100 msec. It can be seen that the following is desirable.

  Therefore, from the results of these examples, the critical significance of each requirement defined in the present invention can be understood.

  According to the present invention, a spot having a high bonding strength without any other material such as a clad material, without a separate process, and without significantly changing the conditions on the steel material side, the aluminum material side, or the spot welding side. Dissimilar joints of steel and aluminum that can be welded can be provided. Such a joined body can be very usefully applied as various structural members in transportation fields such as automobiles and railway vehicles, machine parts, building structures, and the like. Therefore, the present invention greatly expands the use of the heterogeneous joined body of steel and aluminum.

It is sectional drawing which shows the dissimilar joined body of this invention. It is explanatory drawing which shows the aspect of the spot welding for obtaining a dissimilar joined body.

Explanation of symbols

1: steel plate, 2: aluminum alloy plate, 3: dissimilar joined body, 4: oxide film,
5: Nugget, 6: Interfacial reaction layer, 7, 8: Electrode

Claims (3)

A dissimilar material joint obtained by joining a steel material and an aluminum material by spot welding, wherein the steel material has a tensile strength of 400 MPa or more and a thickness t _{1 in} the range of 0.3 to 2.5 mm, and an oxide film on the surface of the steel material Each amount of Mn and Si contained therein is more than twice as concentrated as each amount of Mn and Si contained in this steel material, and the thickness t _{2 of the} aluminum material is 0.5. A steel material and an aluminum, wherein the nugget diameter in the spot welding is in the range of 4 × t ₂ ^{0.5 to} 7 × t ₂ ^0.5 in relation to the plate thickness t ₂ Dissimilar material joint with material. _2. The dissimilar material joined body of steel material and aluminum material according to claim 1, wherein a minimum remaining plate thickness of the aluminum material in the spot welded portion is 50% or more of the original plate thickness t ₂ . A spot welding method for a dissimilar joint of steel material and aluminum material according to claim 1 or 2, wherein the nugget diameter is 4 × t ₂ ^{0.5 to} 7 × t ₂ ^0.5 in relation to the thickness t ₂ of the aluminum material. A spot welding method for a dissimilar material joint between a steel material and an aluminum material, wherein a current of 10 kA or more is allowed to flow for 100 × t ₂ msec or less so as to be within a range.