JP4519508B2 - Dissimilar joints of steel and aluminum - Google Patents

Description

The present invention, automotive, transportation areas like railway vehicles, machine parts, suitable as such a structural member, such as a building structure, about the dissimilar materials bonded body of the steel and the aluminum material.

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 materials 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, a method of sandwiching insulator particles between an aluminum material and a steel material (see Patent Document 3), a method of applying unevenness to a member in advance (see Patent Document 4), and the like have been proposed.
Japanese Patent Laid-Open No. 6-63763 (Claims) JP-A-7-178563 (Claims) JP-A-5-228643 (Claims) JP-A-9-174249 (Claims)

  However, any of these methods is not a mere spot welding and is not a practical method because it significantly hinders the efficiency of the spot welding line. For example, it is necessary to perform spot welding in multiple layers, to install a new facility for sandwiching a resistor such as an insert material in a welding line, or to perform another process such as processing of a member. For this reason, there are many work problems such as high welding costs and extremely limited welding conditions.

  In addition, none of these methods considers the influence of factors on the steel or aluminum material side, and even if spot welding conditions are improved, depending on the type of material and the variation in factors on the material side, There is a big problem that the reproducibility is poor because of the large difference in the bonding strength.

As a method different from these methods, there has been proposed a method of providing an intermediate layer or a plating film of a metal such as Mg or Al having a melting point lower than that of the steel material on the steel material side (Patent Documents 5 and 6). 7, 8, 9). These methods aim to join different materials of steel and aluminum by direct spot welding like the same kind of joining of steel and steel without adding special means such as the insert. Yes. For this reason, by forming a low melting point film or layer between steel and aluminum, it is possible to reduce the brittle interface reaction layer or form irregularities at the interface to increase the bonding strength. It is said.
JP-A-4-143833 (Claims) JP-A-4-251676 (Claims) JP-A-7-24581 (Claims, FIG. 1) JP-A-7-178565 (Claims, pages 1-5, pages 8-15, FIG. 1) JP 2003-145278 (Claims, pages 1 to 6, FIG. 1)

  If different materials of steel and aluminum can be joined directly by spot welding, the spot welding line between existing steel and steel can be used as it is regardless of the welding conditions, which is advantageous.

  However, the method for forming the low melting point film or layer between the steel material and the aluminum material still has advantages and disadvantages, and is not practical. For example, when an intermediate layer of Mg or the like is provided (Patent Document 5), the bonding strength is insufficient at about 80 kgf even when the base material is broken.

  When Al or the like is coated (Patent Document 6), the nugget is not formed although the shear tensile strength is higher than that of the Mg or the like. For this reason, even if only the anchor effect by microscopic bonding can secure the shear tensile strength, the cross tensile strength (peel strength) cannot be ensured, and the application is limited to special applications that only produce shear tension. Is done.

  When a resistance heating element is inserted after aluminum plating is applied to the steel material (Patent Document 7), it is still necessary to incorporate new equipment for sandwiching the resistance heating element in the welding line, and other processes such as processing of the member. is necessary.

  Patent Document 8 that stipulates the melting point of the intermediate layer metal in relation to the melting point of the steel material and the aluminum material eliminates the intermetallic compound formed at the joining interface and directly joins the active surfaces of the steel material and the aluminum material. Yes. For this reason, a fracture in the base material is obtained with a cross tensile strength (peel strength). However, since no nugget is formed and there is no adhesion layer (intermediate layer) due to mutual diffusion, only the anchor effect due to microscopic bonding is obtained as in the case of Patent Document 6. Therefore, although the base material has a certain cross tensile strength due to the fracture of the base material, the joint strength is inevitably low, and a sufficient cross tensile strength commensurate with the required characteristics cannot be ensured.

  Further, the above-mentioned Patent Document 9 describes the area of the area where the intermetallic compound formed at the joint interface between the aluminum-plated steel material and the aluminum material is present, with a constant value (60% Stipulated below. As a result, an aluminum melting portion supplied from the aluminum material side is formed around the Al—Fe intermetallic compound. And this aluminum fusion part has obtained the fracture | rupture in the base material in a shear tensile strength, and the ductile fracture energy 8J or more by carrying out a metal bond directly with steel materials, without interposing an Al-Fe intermetallic compound. .

  However, even this method does not consider the influence of the component composition on the base material side such as an aluminum-plated steel material or aluminum material, or the component composition or thickness of the aluminum plating. As will be described later, these greatly affect the bonding strength when forming the nugget and the required welding heat input. For this reason, many problems still remain, such as reproducibility in which high joint strength can be stably obtained and reduction in joint strength due to an increase in the amount of thinning of the aluminum material due to an increase in the required welding heat input.

The present invention has been made in order to solve such problems, and when joining different materials of steel and aluminum by direct spot welding, the reproducibility is good, and a new amount of reduction in the thickness of the aluminum is increased. The present invention provides a dissimilar material joined body of a steel material and an aluminum material that can perform spot welding with high joint strength without causing problems.

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 steel material is Mn: 0.1 to 3.0% by mass, the thickness t ₁ is in the range of 0.3-2.5 mm, the aluminum material contains Si: 0.4-2% by mass, the thickness t ₂ is in the range of 0.5-2.5 mm, It has a plating film made of Zn and / or Al with a film thickness of 3 to 15 μm and a melting point of 350 to 950 ° C. on the joining side surface of the steel or aluminum material, and the nugget diameter in the spot welding is the plate thickness in relation to t _2, 4 in the range of × t ₂ ^0.5 to 7-× t ₂ ^0.5, and the minimum remaining sheet thickness of the aluminum material in the spot weld, more than 50% of the original thickness t ₂ and it is.

  In order to solve the above-mentioned problems of the prior art, the inventors of the present invention have the effect of material conditions when joining different materials by spot welding, in particular, the component composition and thickness on the base material side such as steel and aluminum, welding, etc. Intensive research has been conducted focusing on the relationship between factors such as the melting point, components, and film thickness of the plating film provided in advance and the interface reaction layer formed during bonding. As a result, it has been found that these factors have a great influence on the bonding strength when forming the nugget. In other words, it has been found that if these factors are controlled, the joint strength of the dissimilar material joint in spot welding is improved.

  In the present invention, by controlling these factors and controlling the melting of the steel material at the joining interface, the interface reaction layer is suppressed to the optimum thickness, and generation of dust is suppressed to a minimum amount. In addition, by controlling the Mn content of the steel material and the Si content of the aluminum material, the Mn and Si are concentrated in the interfacial reaction layer, so that the interfacial reaction layer is not easily destroyed. Form a layer.

  In general, in order to perform spot welding with a high joint strength, it is only necessary to promote the formation of a nugget, and it is known that the larger the nugget diameter, the higher the shear strength and the cross tensile strength. The nugget diameter is related to the amount of heat input during welding, and the nugget diameter increases as the amount of current during welding increases and the time increases. For this reason, generally, by controlling the nugget diameter by the amount of heat input, an interface diffusion layer is formed at the time of nugget formation to obtain a bonded body with high bonding strength. Of course, if the nugget diameter becomes 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 dissimilar materials of steel and aluminum, steel has a higher melting point and electric resistance than aluminum, and its thermal conductivity is small, so the heat generation on the steel side increases. For this reason, the low melting point aluminum material is first melted, and then the surface of the steel material is melted. As a result, an Al—Fe-based brittle intermetallic compound layer is formed at the interface, so that high bonding strength cannot be obtained. In addition, when the melting reaches the surface of the aluminum material and dust is formed, the amount of thinning of the aluminum material increases, and this also prevents high bonding strength from being obtained.

  On the other hand, by interposing an intermediate layer between the molten aluminum and the steel material in contact with the steel, the time required for the interfacial reaction layer, which is an intermetallic compound of steel and aluminum, to be controlled, the appropriate interfacial reaction layer thickness It can be. As this intermediate layer, a plating film having a melting point close to that of the aluminum material is applied in advance to the surface of the steel material or aluminum material before welding. This is a conventional method of applying a low melting point plating film on the steel material side described above.

  However, a brittle intermetallic compound layer that easily breaks the interfacial reaction layer can be obtained depending on changes in the material conditions and spot welding conditions only by interposing an intermediate layer, such as applying this plating film in advance before welding. It may be formed.

  On the other hand, in the dissimilar material joined body of the present invention, as described above, the Mn content of the steel material and the Si content of the aluminum material are controlled, and Mn and Si are concentrated in the interfacial reaction layer, so The formation of the compound layer is suppressed to form an intermetallic compound layer in which the interface reaction layer is not easily destroyed.

  Further, in the dissimilar material joined body of the present invention, by controlling factors such as the thickness of the base material side such as steel or aluminum, the melting point of the plating film provided in advance before welding, the component, the film thickness, etc. By controlling the melting, the interface reaction layer is suppressed to an optimum thickness, and a large nugget diameter is obtained, and generation of dust is suppressed to a minimum amount.

  As described above, in the present invention, when different materials of a steel material and an aluminum material are directly joined by spot welding, the reproducibility is improved, and a new problem such as an increase in the thickness reduction of the aluminum material does not occur. It has the effect of enabling spot welding with high strength.

  Hereinafter, embodiments of the present invention and reasons for limiting the requirements of the present invention will be specifically described.

(Heterogeneous)
FIG. 1 is a cross-sectional view showing an embodiment of a heterogeneous joined body defined by the present invention. In FIG. 1, 3 is 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. Reference numeral 4 denotes a plating film provided in advance on the joining side surface of the steel material 1. In addition, for the purpose, the plating film 4 is provided at least on the surface of the joining side of either the steel material or the aluminum material. Of course, it may be provided on the surface (on both surfaces of steel or aluminum). 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.

(Steel)
In the present invention, the shape of the steel material to be used is not particularly limited, and an appropriate shape and material such as a steel plate, a steel shape member, and a steel pipe, which are generally used for structural members or selected from structural member uses, are used. Is possible. However, in the present invention, the Mn content of the steel material and the plate thickness t ₁ are defined. That is, in the present invention, in order to improve the bonding strength, the steel is Mn: 0.1 to 3.0 include mass%, the plate thickness t ₁ is required to be in the range of 0.3 to 2.5 mm.

(Mn content of steel)
Mn in the steel material has a barrier effect that delays the contact between the molten aluminum and the steel material by being present at the bonding interface together with Si in the aluminum material described later. Furthermore, it has the effect of concentrating with Si in the interfacial reaction layer, minimizing the formation of brittle intermetallic compounds and forming an intermetallic compound layer in which the interfacial reaction layer is not easily destroyed. If the Mn content in the steel material is less than 0.1% by mass, this effect cannot be obtained, the formation of the interface reaction layer cannot be suppressed, and the bonding strength decreases. On the other hand, when the Mn content in the steel material exceeds 3.0% by mass, the toughness is deteriorated and the steel is easily cracked around the welded portion, so that the joint strength is lowered. Therefore, the Mn content in the steel material is in the range of 0.1 to 3.0 mass%, preferably 0.5 to 3.0 mass%, more preferably 1 to 2.5 mass%.

  In the present invention, the component composition and structure of the steel material other than Mn are not limited, but in order to obtain the strength of the steel material, high-tensile steel (high tensile) is preferable. In this respect, it is preferable that C: 0.05 to 1% and Si: 0.5 to 3% are contained as basic components of the high-tensile steel in addition to the above Mn. Furthermore, one or two or more of Cr: 0 to 1%, Mo: 0 to 0.2%, Nb: 0 to 0.1%, V: 0 to 0.1%, Ti: 0 to 0.1% are optionally selected. Steel contained in can also be applied. Cr, Mo, and Nb improve hardenability and improve strength, and V and Ti improve strength by precipitation hardening. However, excessive addition of these elements reduces the toughness around the weld and tends to cause nugget cracks.

(Steel thickness)
In the present invention, it is necessary that the thickness t ₁ of the steel is a conjugate is 0.3 to 2.5 mm. If the thickness t ₁ of the steel product is less than 0.3 mm, it can not be ensured the necessary strength and rigidity as the structure member or structural material is improper. 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 length exceeds 2.5 mm, other joining means are employed as the structural member or structural material described above, and therefore, there is little need to perform joint by spot welding. For this reason, there is no need to increase the thickness t _{1 of the} steel material beyond 2.5 mm.

(Strength of steel)
The strength of the steel material is not limited in the present invention, but the tensile strength is preferably a high strength of 400 MPa or more, and more preferably a high strength of 500 MPa or more. When the tensile strength is less than 400 MPa, the deformation of the steel material is increased by pressurization, and the reaction with aluminum is promoted. As a result, an intermetallic compound is easily formed. Mild steel is generally low-alloy steel, which facilitates diffusion of Fe and Al, and tends to form brittle intermetallic compounds. Furthermore, the tensile strength is 400 MPa or more, preferably 500 MPa or more in order to reduce the weight of the relatively thin plate thickness t ₁ while satisfying the required rigidity and strength of the joined body as a structural member. The strength is preferred. For example, when the tensile strength of a steel material such as mild steel is low, it is difficult to reduce the weight by satisfying the required rigidity and strength of the joined body as a structural member after setting the plate thickness t ₁ to be relatively thin.

(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 present invention defines a Si content of the aluminum material steel and the plate thickness t _2. That is, in the present invention, in order to improve the bonding strength, it is necessary that the aluminum material contains Si: 0.4 to ₂ % by mass and the plate thickness t2 is in the range of 0.5 to 2.5 mm.

(Si content of aluminum material)
Si in the aluminum material, together with Mn in the steel material described above, has a barrier effect that delays the contact between the molten aluminum and the steel material by being present at the bonding interface. Furthermore, it has the effect of concentrating with Mn in the interfacial reaction layer, minimizing the formation of brittle intermetallic compounds and forming an intermetallic compound layer in which the interfacial reaction layer is not easily destroyed. If the Si content in the aluminum material is less than 0.4% by mass, this effect cannot be obtained, the formation of the interface reaction layer cannot be suppressed, and the bonding strength decreases. On the other hand, when the Si content in the aluminum material exceeds 2% by mass, the toughness is deteriorated, and it becomes easy to crack around the nugget, and the bonding strength is lowered. Therefore, the Si content in the aluminum material is in the range of 0.4 to 2% by mass.

(Aluminum thickness)
Thickness t ₂ of the aluminum material used in the present invention is in the range of 0.5 to 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 missing, not 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, if it exceeds the thickness t ₂ is 2.5mm aluminum material, as in the case of the thickness of the above-described steel, because other joining means are employed as a structural member or structural material, subjected to spot welding Less need to be joined. Therefore, there is no need to increase the plate thickness t ₂ of the aluminum material beyond 2.5 mm.

(Type of aluminum material)
The strength of the aluminum material is not limited in the present invention, but it is desirable that the strength (yield strength) is high in order to suppress deformation due to pressure, as in the case of the steel material. Also, in terms of satisfying the requirements stiffness, strength of the bonded body as a structural member, relatively as the thickness t ₂ of the thin-walled, in order to lighten, who strength (yield strength) is high is preferable. 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.

(Plating film)
In the present invention, a plating film made of Zn and / or Al having a film thickness of 3 to 15 μm and a melting point of 350 to 950 ° C. is previously provided on the joining side surface of either the steel material or the aluminum material described above before welding. . Control the time for the interfacial reaction layer, which is an intermetallic compound of steel and aluminum, by applying a plating film with an appropriate film thickness that is close to the melting point of the aluminum material so that it becomes an intermediate layer between the steel and aluminum materials. In addition, an appropriate interfacial reaction layer thickness of 0.1 to 5 μm can be obtained.

In this regard, when the thickness of the plating film is less than 3 μm or the melting point of the plating film is less than 350 ° C., the plating film melts and discharges out of the system at the initial stage of bonding, and the formation of a brittle interface reaction layer cannot be suppressed. On the contrary, when the thickness of the plating film exceeds 15 μm or the melting point of the plating film exceeds 950 ° C., a large amount of welding heat input is required for melting and discharging the plating film to the outside of the system. For this reason, the melting amount of the aluminum material is increased, and the amount of thinning of the aluminum is increased due to generation of dust. Therefore, the thickness of the plating film is in the range of 3 to 15 μm, preferably in the range of 5 to 10 μm, the melting point of the plating film is in the range of 350 to 950 ° C., preferably 400 to 900 ° C., and more preferably in the range of the melting point of aluminum to 900 ° C. or less. And

The film thickness of the plating film is obtained by cutting a sample after plating, embedding in a resin, polishing, and performing SEM observation. In this case, it is preferable to measure the three-point thickness with a field of view of 2000 times and average them.

As for the types of plating that satisfy the above conditions, when applied to steel, the corrosion resistance of the steel can be secured, and since plating can be easily applied to both steel and aluminum, Zn and Al are used. A plating film containing the main component, preferably a plating film containing at least 80% in total of one or two of Zn and Al is desirable.

If a plating film mainly composed of Zn or Al is provided on the steel material side in advance, the corrosion resistance of the steel material as the structural member can be improved. Steel is usually used with a paint applied, but even if this paint is scratched, Zn and Al in the plating film are preferentially corroded, so the steel that is the base material must be protected. Can do. Furthermore, since the potential difference between the steel material and the aluminum material is reduced, contact corrosion of dissimilar metals, which is one of the problems in dissimilar joints, can be suppressed.

Examples of the type of plating film mainly composed of Zn or Al include Al, Al—Zn, Al—Si, Zn, and Zn—Fe. In the present invention, Mn and Si are concentrated in the interface reaction layer to suppress the formation of a brittle intermetallic compound layer, thereby forming an intermetallic compound layer in which the interface reaction layer is not easily destroyed. For this purpose, it seems that it is more efficient to contain Si or Mn on the plating side provided in advance. In this regard, if Mn and Si are to be concentrated in the interface reaction layer, it is necessary to contain 5% by mass or more of Si and Mn in the plating containing Zn or Al as a main component. On the other hand, however, if there is too much Si or Mn in the plating, cracks are likely to occur around the nugget. This tendency is particularly noticeable when Mn is contained in the plating. In this sense, plating containing Zn or Al containing Mn or Si as the main component other than the Al—Si plating film is not practical. In other words, this point also shows the significance of concentrating Mn and Si in the interface reaction layer by controlling the Mn content of the steel material and the Si content of the aluminum material.

  In consideration of these matters, among the plating films mainly composed of Zn or Al, a zinc plating film containing 88% by mass or more of Zn is particularly recommended for the plating film of the present invention. Further, it is recommended that a galvanized film containing 8 to 12% by mass of Fe is further applied to the surface of the steel material.

  When a galvanized film containing Zn of 88% by mass or more is applied to the steel surface, the corrosion resistance of the steel material is particularly high, and the galvanized film can easily control the melting point to 350 to 950 ° C. In addition, in the case of zinc plating containing 88% by mass or more of Zn and further containing 8 to 12% by mass of Fe, the Fe component remaining at the time of melting and discharging the plating film reacts efficiently with the aluminum material. Thereby, formation of the interface reaction layer can be controlled in a short time, and an appropriate interface reaction layer thickness of 0.1 to 5 μm can be obtained. Of course, the corrosion resistance is also high, and contact corrosion of different metals can be suppressed.

  In addition, you may contain other components other than an above-described component in a plating film. However, in order to prevent the melting point range from deviating from the range of the present invention and to prevent the corrosion resistance from being inferior, the additive component and the addition amount are set so as not to cause poor welding such as cracking around the nugget.

  The plating method of the steel material or aluminum material is not limited in the present invention, but an existing wet or dry plating method can be used. In particular, in the galvanization of steel materials, methods such as electroplating, hot dipping, and alloying after hot dipping are recommended. Moreover, in galvanization of an aluminum material, electroplating, zinc displacement plating, etc. are illustrated.

Although the essential or preferable conditions on the material side of spot welding have been described above, the essential or preferable conditions of the spot welded part will be described below.

(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 the spot welding conditions so 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 the bonding strength, but dust tends to occur and the amount of thinning of the aluminum material is large, so the bonding strength is reduced. To do. Therefore, 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 .

(Interface reaction layer)
In the present invention, as described above, the interface reaction layer generated in spot welding is controlled. As a control measure, the average thickness of the interface reaction layer is preferably 0.1 to 5 μm. When the average thickness of the brittle interfacial reaction layer exceeds 5 μm, the bonding strength is significantly reduced. On the other hand, when the average thickness of the interfacial reaction layer is less than 0.1 μm, the influence of the oxide film on the steel material increases, and the contact between the steel material and the aluminum material is reduced. Decreases. Therefore, the average thickness of the interface reaction layer is preferably 0.1 to 5 μm, and more preferably 0.1 to 3 μm.
For the analysis of the thickness of the interface reaction layer, it is easy to observe the cross section of the welded part by SEM, measure the thickness at a plurality of locations, and average it.

Further, in the present invention, Mn and Si are concentrated to a desired level in the interface reaction layer to increase the bonding strength. The level of concentration of Mn and Si for increasing the bonding strength is preferably specified by the amount of Mn element and the amount of Si element at the middle point (thickness center) in the thickness direction in the interface reaction layer. . Then, as a rough indication for increasing the bonding strength, at the intermediate point in the thickness direction in the interface reaction layer, the amount of Mn element is more than 1.5 times the ratio of the amount of Mn element of steel, and the amount of Si element is aluminum. It is preferable that the ratio to the Si element amount of the material is 1.1 times or more and the ratio to the Si element amount of the steel material is 1.1 times or more.

  In order to concentrate these Mn and Si to the above desired level in the interface reaction layer, it is necessary to optimize the Mn and Si contents and spot welding conditions of the steel material and the aluminum material. In actual testing, Mn can be increased up to 2.5 times and Si up to 1.8 times by optimizing spot welding conditions. As Si is concentrated, the bonding strength tends to increase.

  The degree of concentration of Mn and Si in the interfacial reaction layer can be analyzed by TEM-EDX analysis from the cross section of the welded joint and each secondary ion intensity analysis by SIMS, but SIMS (secondary ion mass spectrometry) It is recommended to analyze the secondary ion intensity of Mn and Si using a device because there are few errors. Based on the ratio of Mn and Si strength at the midpoint of the interfacial reaction layer and the Mn and Si strengths obtained by SIMS in the same way using SIMS, the amount of Mn element in the interfacial reaction layer / steel material The amount of Mn element, the amount of Si element in the interface reaction layer / the amount of Si element in the steel material, and the amount of Si element in the interface reaction layer / the amount of Si element in the aluminum material are obtained respectively.

(Aluminum material thickness reduction)
As described above, in order to secure the bonding strength, it is desirable to reduce the thickness of the aluminum material after bonding by spot welding as much as possible. As one guideline, it is desirable that the minimum remaining thickness Δt of the aluminum material 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 this 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 the heterogeneous joint of the present invention. 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.

  For optimal control of the interfacial reaction layer, the plating film is not left in the interfacial reaction layer, the thickness of the interfacial reaction layer is controlled to the above-mentioned thickness, and further, Mn and Si are concentrated in the interfacial reaction layer. It is important to control spot welding conditions.

(Welding heat input)
At this time, the most important is the heat input of welding. For optimal control of the interface reaction layer, the total amount of heat input in spot welding is in the range of 1000 to 10,000 Asec, preferably in the range of 2500 to 7500 Asec, as the total heat input in the spot welding process.

  When this total heat input is less than 1000 Asec, strictly less than 2500 Asec, the plating film layer is not completely melted and discharged from the interface, the interface reaction layer is not formed, the plating film layer remains, or enters the interface reaction layer The remaining plating film component increases, and nuggets are not sufficiently formed. For this reason, joint strength becomes low.

   On the other hand, when this total heat input exceeds 10,000 Asec, strictly exceeding 7500 Asec, the plating film component is sufficiently melted and discharged, but the interface reaction layer grows thick. Moreover, since the generation of dust increases, the thickness of the aluminum material decreases. For this reason, joining strength becomes low.

The total heat input in the spot welding process is as follows: welding current is I ₁ , I ₂ ,... (Unit A), welding time is T ₁ , T ₂ ,. Step) is expressed as I ₁ × T ₁ + I ₂ × T ₂ +.

As described above, in order for the nugget diameter to be in the range of 4 × t ₂ ^{0.5 to} 7 × t ₂ ^0.5 , it is preferable that the process selected for spot welding is a high current process. By providing such a high current process, cracking of the nugget is also suppressed.

More specifically, spot welding is composed of a plurality of processes, and has two or more processes having different welding current values and / or welding times, and at least one process other than the first process is a high current process. In this high current process, an adjusted current of 10 kA or more is allowed to flow for 100 × t ₂ msec or less in relation to the thickness t ₂ of the aluminum material.

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. In the case of a low current of 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.

(High current process and total heat input)
Furthermore, when providing such a high current process, it is preferable that the total amount of heat input by spot welding up to the previous process of the high current process is in the range of 1000 to 10,000 Asec for optimal control of the interface reaction layer.

  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.

(Pressure)
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.

(Electrode shape)
The electrode shape of spot welding is not particularly specified, but particularly the electrode 8 on the aluminum material side is preferably an R-shaped electrode having a large R in order to increase 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 after rolling, oil washing 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.

  As for the aluminum material, a commercially available A1050-O with a thickness of 1 mm (Si: 0.05% by mass, Mn: not contained, Al: 99.50 or more), A6022-T4 (Si: 1.01% by mass, Mn: 0.07% by mass, Mg : 0.6 mass%) and A6061-T4 (Si: 0.7 mass%, Mn: 0.07 mass%, Mg: 1.0 mass%).

In the case of galvanizing the steel material, after various pickling and activation with 10% sulfuric acid for 5 minutes, various plating was performed under the following conditions. Electrogalvanizing is performed at a current of 20 A / dm ² in a zinc plating bath with a pH of 3 by adding sulfuric acid to zinc sulfate 400 g / l, aluminum sulfate 30 g / l, sodium chloride 15 g / l, boric acid 30 g / l. The galvanization was performed to 10 μm.
In the Zn-10% Ni plating, 10 μm of Zn-10% Ni plating was applied by flowing a current of 10 A / dm ² in a bath in which nickel sulfate and nickel chloride were added to the above zinc plating bath. Also, by passing a current of 10A / dm ² using a Watts bath as the comparative example was subjected 10μm Ni plating.

  Hot dip plating was performed only on steel materials, and using various molten metals, Al plating, Al-9% Si plating, and Zn-Fe plating (each of Fe amounts 5, 8, 10, and 15) were each performed by 10 μm. In hot-dip Zn-10% Fe plating, the film thickness was adjusted to 1, 3, 10, 15, and 20 μm by changing the Fe component, temperature, and pulling temperature in the molten metal.

  In addition, when galvanizing aluminum material, after pickling with 10% nitric acid for 30 seconds, sodium hydroxide 500g / l, zinc oxide 100g / l, ferric chloride 1g / l, Roselle salt 10g / l The zinc (including zinc alloy) electroplating was performed after the zinc replacement treatment for 30 seconds in the above treatment solution.

  Each plating film thickness was obtained by cutting a sample after plating, embedding in a resin, polishing, and performing SEM observation. At this time, the three-point thickness was measured in a 2000-fold field of view and averaged.

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 welding was performed to prepare dissimilar joined bodies.

  In spot welding, as shown in Table 2, three welding processes of welding processes 1 to 3 were selectively performed at one welding current (kA) and welding time (msec). . At this time, the welding process 2 was set as a high current process. In each spot welding process, a DC resistance welding tester was used, and in common, the pressure was 1.5 kN, a dome-shaped electrode made of a Cu-Cr alloy was used, the anode was an aluminum material, and the cathode was a steel material. .

  The nugget diameter, the minimum remaining thickness of the aluminum material, the thickness of the interfacial reaction layer, the concentration of Mn and Si in the interfacial reaction layer, the bonding strength, the corrosion resistance, and the like were evaluated. These results are shown in Tables 3 to 7, respectively.

  The nugget diameter was measured on the steel material side by a cross-sectional analysis using an optical microscope by a method described in JIS A 3137 using a sample of a heterogeneous joint after the following cross tensile test.

 The minimum aluminum remaining plate thickness was also measured by a cross-sectional analysis using an optical microscope by the method described in JIS A 3137, using a sample of a dissimilar joint after the following cross tensile test, but on the aluminum material side. 3 points were measured and averaged. And it calculated | required by ratio (%) with the plate | board thickness of the original aluminum material. 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.

  The thickness of the interface reaction layer (6 in Fig. 1) was measured using a sample of a heterogeneous joint after spot welding, cut at the center of the weld, embedded in resin, polished on the surface, and subjected to SEM observation. . At this time, when the thickness of the interface reaction layer is 1 μm or more, measure the thickness at three points with a field of view of 2000 times, and when the thickness of the interface reaction layer is less than 1 μm, with a field of view of 10,000 times, Obtained by averaging.

  Here, the interfacial reaction layer refers to a compound layer of Fe and Al, which is a layer in which both Fe and Al are detected by EDX by 1 wt% or more, and the detected amount of Zn and Ni is lower than that of Fe and Al. That is, a layer in which both Fe and Al were not detected by 1 wt% or more, or a layer in which the detected amount of Zn or Ni was larger than either Fe or Al was not used as an interface reaction layer as a plating layer.

  The degree of concentration of Mn and Si in the interface reaction layer was measured by measuring the secondary ion intensity by SIMS (ims5f manufactured by CAMECA) from the cross section, similarly to the above thickness measurement. 8keV oxygen ions are used as the primary ions. By irradiating oxygen ions in a 50 × 50μm region including the junction interface and detecting the positive secondary ions, the secondary ions of Mn and Si are perpendicular to the interface. The secondary ionic strength was linearly analyzed. The measurement was performed three times, and the ratio of the secondary ion strength of Mn and Si at the midpoint of the interfacial reaction layer to the secondary ion strength of Mn and Si in the aluminum and steel materials was the concentration of Mn and Si, respectively. And averaged.

  As an evaluation of the bonding strength, a cross tension test was performed on different types of bonded bodies. The cross tension test was performed based on a joint strength of 1.0 kN obtained by spot welding of aluminum materials of A6022 obtained in advance. And, when the bonding strength is 0.9 kN or more, ◎, when 0.7 to 0.9 kN, ◯, when 0.5 to 0.7 kN, Δ, and when less than 0.5 kN, ×.

  In the evaluation of the bonding strength, “◯” or more indicates that the present invention can be applied not only to a use in which a shear tension occurs but also to a use in which a cross tension occurs. On the other hand, the level of Δ indicates that the cross tensile strength (peel strength) cannot be ensured even if the shear tensile strength can be secured, and the use is limited to a special use where only shear tension occurs. Show.

  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-described cross tensile test result, and those that obtained an evaluation of ○ and ◎ in the cross tensile test are both higher than 2.5 kN. Shear strength.

  The corrosion resistance test in Table 3 was performed on each of the above-mentioned spot welded dissimilar joints that was subjected to a coating treatment after the zinc phosphate treatment, simulating the use in automobile materials and the like. That is, each heterozygote was subjected to alkaline degreasing, washed with water, and then subjected to a surface conditioning treatment for 30 seconds using a 0.1% aqueous solution of Surffine 5N-10 manufactured by Nippon Paint. Then, zinc ion 1.0g / l, nickel ion 1.0g / l, manganese ion 0.8g / l, phosphate ion 15.0g / l, nitrate ion 6.0g / l, nitrite ion 0.12g / l, toner value 2.5pt Then, zinc phosphate treatment was performed for 2 minutes in a bath having a total acidity of 22 pt, a free acidity of 0.3 to 0.5 pt, and a temperature of 50 ° C. Thereafter, it was coated with a cationic electrodeposition paint (Power Top V50 Gray, manufactured by Nippon Paint Co., Ltd.) and baked at 170 ° C. for 25 minutes to form a 30 μm paint film.

  The corrosion resistance was evaluated by a composite corrosion test of these coating test pieces. The combined corrosion test was repeated 100 times with a cycle of 2 hours of salt spray, 2 hours of drying and 2 hours of wetting. And the joint part after a composite corrosion test was peeled and observed, and the maximum corrosion depth of Al was measured. The corrosion resistance was evaluated as “◯” when the maximum corrosion depth of Al was less than 0.01 mm, “Δ” when 0.01 to 0.1 mm, and “x” when 0.1 mm or more.

  Table 3 shows the influence of the plating conditions on the bonding strength when various plating is performed on the steel material side or the aluminum material side. Specifically, the situation of the welded part when various plating was applied to the 980 MPa class high-tensile steel plate in Table 1 and spot-welded with A6022 aluminum alloy plate under the optimum conditions of the invention example E in Table 2 Indicates the bonding strength.

  From Comparative Example 1 in Table 3, not only the welding strength (cross tension test result) is inferior, but also when spot welding is performed under the optimum conditions, and plating is not provided on the steel material side or the aluminum material side, It can be seen that the corrosion resistance is inferior.

  On the other hand, Invention Examples 3, 4, 7 to 10, and 12 to 14 are Zn and / or on the surface of a steel material or aluminum material, having a film thickness of 3 to 15 μm and a melting point in the range of 350 to 950 ° C. A plating film made of Al is applied in advance. For this reason, the interface reaction layer can be suppressed to the optimum thickness while obtaining a relatively large nugget diameter. As a result, the bonding strength is high. Moreover, since the minimum remaining plate | board thickness of aluminum material is also comparatively thick and the amount of thickness reduction is also comparatively small, it turns out that generation | occurrence | production of dust is suppressed to the minimum amount.

  On the other hand, Comparative Examples 2, 5, 6, 11, 15, and 16 are pre-coated with a plating film made of Zn and / or Al on the surface of the steel or aluminum material, but the film thickness and melting point of the plating Etc. deviate from the scope of the present invention.

  For this reason, for example, Comparative Examples 2, 5, 6, and 16, where the melting point of the plating is too high, even though spot welding joining under optimum conditions, a relatively large nugget diameter is obtained, but the interface reaction layer is almost Not formed. As a result, the bonding strength is extremely inferior. Further, the minimum remaining plate thickness of the aluminum material is relatively thin, and the amount of thickness reduction is relatively large.

  In Comparative Example 11 in which the film thickness of the plating is too thin, similar to Comparative Example 1 in which there is no plating, even if spot-welded bonding is performed under optimum conditions, not only the bonding strength is inferior, but also the corrosion resistance is inferior.

  On the contrary, in Comparative Example 15 in which the film thickness of the plating is too thick, a relatively large nugget diameter is obtained, but the interface reaction layer is hardly formed. As a result, the bonding strength is extremely inferior. Further, the minimum remaining plate thickness of the aluminum material is relatively thin, and the amount of thickness reduction is relatively large.

  From the above results, the present invention has a good reproducibility when joining dissimilar steel and aluminum materials by direct spot welding, without causing new problems such as an increase in the thickness of aluminum materials. It can be seen that it has the effect of enabling spot welding with high strength. Moreover, the critical significance of factors, such as melting | fusing point of a plating film previously provided before welding, a component, and a film thickness, in base material side, such as steel materials and aluminum materials, in this invention dissimilar material joined body is understood.

  Next, Tables 4 to 7 show the influence on the bonding strength by the spot welding conditions when the steel sheets having various strengths in Table 1 are used and the spot welding conditions are changed from A to G in Table 2. More specifically, steel sheets having various strengths from 270 to 980 MPa in Table 1 were preliminarily subjected to hot-dip Zn-10% Fe plating 10 μm, and the same plating conditions were used. The situation of a welding part and joining strength at the time of spot welding joining from A to G are shown.

  Table 4 shows a joined body of dissimilar materials of a combination of a 270 MPa class steel plate and an aluminum alloy plate of A1050-O. Table 5 shows a joined material of different materials composed of a combination of a 440 MPa class steel plate and an aluminum alloy plate of A6022-T4. Table 6 shows the dissimilar joints of a combination of a 590 MPa grade steel plate and an aluminum alloy plate of A6061-T4. Table 7 shows a joined body of dissimilar materials of a combination of a 980 MPa class steel plate and an aluminum alloy plate of A6022-T4. In Tables 4 to 7, all corrosion resistance test results were ○, so they are not shown in each table.

  In Table 4, when a pure aluminum-based aluminum alloy plate of A1050 is used (the Mn content in the steel is within the range of the present invention), the Si content in the material aluminum material is too lower than the range of the present invention. As a result, the Si to aluminum material Si ratio in the interface reaction layer is relatively low, and Si is not concentrated in the interface reaction layer. For this reason, as can be seen from Table 4, high joint strength cannot be obtained regardless of the spot welding conditions from A to G in Table 2.

  In Table 5, the above two comparative examples are joined under the spot welding conditions of A (total heat input is too small) and B (heat input in the high current process 2 is too small) in Table 2. For this reason, an aluminum alloy plate of A6022 is used, and the amount of Si in the material aluminum material is within the scope of the present invention, and even if the amount of Mn in steel is within the scope of the present invention, the interface reaction layer itself is not generated. . As a result, the bonding strength is remarkably low compared to the four invention examples below.

  Further, in Table 5, the lowermost comparative example is joined under spot welding conditions of G (total heat input is too large) in Table 2. For this reason, the average thickness of the interface reaction layer exceeds 5 μm, and the bonding strength is reduced. This is due to the fact that the minimum remaining thickness of the aluminum material is extremely thin, less than 50%, and the amount of thinning is remarkably large.

In contrast, in the four invention examples in Table 5, the nugget diameter is in the range of 4 × t ₂ ^{0.5 to} 7 × t ₂ ^0.5 , and the thickness of the interface reaction layer is also within a preferable range. The concentration of Mn and Si is also relatively high. For this reason, the bonding strength is significantly higher than that of the comparative example.

However, among the four invention examples in Table 5, the invention examples that are joined under the optimum spot welding conditions D and E in Table 2 have processes 1 to 3, or 1, 2 with different welding current values and times. In step 2, a high current process is performed in which a current of 10 kA or more flows for 100 × t ₂ msec or less. In addition, the total amount of spot welding heat input in step 1 which is the previous step of the high current step 2 is in the range of 1000 to 10,000 Asec. As a result, C in Table 2 (Spot welding total heat input small amount of process 1), F (total heat input large amount), etc., compared to the invention example joined under spot welding conditions deviating from these optimum conditions The bonding strength is relatively high.

  Also in Table 6, the above two comparative examples are joined under the spot welding conditions of A (total heat input amount is too small) and B (heat input amount of high current process 2 is too small) in Table 2. For this reason, using the aluminum alloy plate of A6061, even if the Si content in the material aluminum material is within the scope of the present invention, and the Mn content in the steel is also within the scope of the present invention, the interface reaction layer itself under the A welding conditions In the B welding conditions, although the interface reaction layer itself is generated, the concentration of Mn and Si in the interface reaction layer is low. As a result, the bonding strength is remarkably low compared to the four invention examples below.

  Further, in Table 6, the lowermost comparative example is joined under spot welding conditions of G (total heat input is too large) in Table 2. For this reason, the average thickness of the interface reaction layer exceeds 5 μm, and the bonding strength is reduced. This is because the minimum remaining plate thickness of the aluminum material is 0% and the amount of thinning is remarkably large.

On the other hand, in the four invention examples in Table 6, the nugget diameter is in the range of 4 × t ₂ ^{0.5 to} 7 × t ₂ ^0.5 , and the thickness of the interface reaction layer is within a preferable range. The concentration of Mn and Si is also relatively high. For this reason, the bonding strength is significantly higher than that of the comparative example.

However, among the four invention examples in Table 6, the invention examples that are joined under the optimum spot welding conditions of D and E in Table 2 have processes 1 to 3, or 1, 2 with different welding current values and times. In step 2, a high current process is performed in which a current of 10 kA or more flows for 100 × t ₂ msec or less. In addition, the total amount of spot welding heat input in step 1 which is the previous step of the high current step 2 is in the range of 1000 to 10,000 Asec. As a result, the bonding strength is relatively high as compared with the invention examples in which the welding is performed under spot welding conditions deviating from these optimum conditions such as F (large amount of heat input) in Table 2.

  In addition, the invention example joined under the spot welding conditions deviating from the optimum condition of C (total spot welding heat input in step 1) is the same as the invention example joined under the optimum spot welding conditions of D and E. As shown in Table 5, the joining strength is high, and the results are different from those of the invention example in which joining is performed under the spot welding condition of C. This is because even if the total heat input is small, the strength of the steel sheet in Table 6 is relatively high, and if the total heat input in step 1 is small, using a higher strength steel sheet, Advantages including relaxation of welding constraints are shown.

  Also in Table 7, the above two comparative examples are joined under the spot welding conditions A (total heat input is too small) and B (heat input in the high current process 2 is too small) in Table 2. Therefore, using the aluminum alloy plate of A6022, even if the amount of Si in the material aluminum material is within the scope of the present invention, and the amount of Mn in the steel is also within the scope of the present invention, under the A welding conditions, the interface reaction layer itself In the B welding conditions, although the interface reaction layer itself is generated, the concentration of Mn and Si in the interface reaction layer is low. As a result, the bonding strength is remarkably low compared to the three invention examples below.

  In Table 7, the lowermost comparative example is joined under spot welding conditions of G (total heat input is too large) in Table 2. For this reason, the average thickness of the interface reaction layer exceeds 5 μm, and the bonding strength is reduced. This is because the minimum remaining plate thickness of the aluminum material is 0% and the amount of thinning is remarkably large.

On the other hand, in the three invention examples in Table 7, the nugget diameter is in the range of 4 × t ₂ ^{0.5 to} 7 × t ₂ ^0.5 , and the thickness of the interface reaction layer is also within a preferable range. The concentration of Mn and Si is also relatively high. For this reason, the bonding strength is significantly higher than that of the comparative example. Of the three invention examples, two of the invention examples joined in the optimum spot welding conditions of D and E in Table 2 have steps 1 to 3, or 1, 2 with different welding current values and times. Process 2 is a high current process in which a current of 10 kA or more flows for 100 × t ₂ msec or less. In addition, the total amount of spot welding heat input in step 1 which is the previous step of the high current step 2 is in the range of 1000 to 10,000 Asec.

  In addition, the invention example joined under the spot welding conditions deviating from the optimum condition of C (total spot welding heat input in step 1) is the same as the invention example joined under the optimum spot welding conditions of D and E. High bonding strength. This is the same strength effect of the steel sheet as in Table 6, and shows the advantage of using a higher-strength steel sheet when the total heat input in step 1 is small, including relaxation of welding constraints.

  Note that the comparative example in which welding is performed under spot welding conditions deviating from the optimum condition of F (total heat input) in Table 2 in Table 7 has a relatively low bonding strength as compared to other invention examples in Table 7. . This result is different from the results of the invention examples joined in the spot welding conditions of the same F in Tables 5 and 6 above. Further, in the comparative example, the minimum remaining plate thickness of this aluminum material is remarkably thin at 30%, and the thickness reduction is remarkably large. This is presumed that when the strength is high, such as the 980 MPa grade steel sheet in Table 7, the adverse effect on the thinning amount of the aluminum material becomes larger when the total heat input is large.

From the results in Tables 4 to 7 above, it is understood that the total heat input is indispensable as the spot welding conditions that can obtain high joint strength after satisfying the material conditions. In addition, it is desirable that a high current process in which a current value is 10 kA or more and a joining time is 100 × t ₂ msec or less (in the example, the aluminum material is 1 mm in thickness because it is 100 msec or less) is effective. I understand.

  Even if the amount of Mn in steel and the amount of Si in the aluminum material are within the scope of the claims, depending on the spot welding conditions, the thickness of the interface reaction layer exceeds 5 μm, or the minimum remaining plate thickness of the aluminum material is less than 50% In this case, it can be seen that when the Mn, Si concentration and the nugget diameter of the interface reaction layer do not satisfy the claims of the present invention, the bonding strength decreases. In other words, by satisfying the gist stipulated in the present invention, the interfacial reaction layer can be controlled within the optimum range, the Mn and Si concentration of the interfacial reaction layer can be increased, and the amount of thinning of the aluminum material is suppressed, which is high A bonded body having a bonding strength can be obtained.

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 (4)

A steel material and an aluminum material a dissimilar materials bonded body formed by bonding by spot welding, the steel product Mn: 0.1 to 3.0 include mass%, the plate thickness t ₁ is in the range of 0.3 to 2.5 mm, the aluminum material There Si: 0.4 to 2 comprises a mass%, in the range plate thickness t ₂ of 0.5 to 2.5 mm, the steel material or bonding surface of the aluminum material, with a thickness of 3 to 15 [mu] m, a melting point of from 350 to 950 ° C. 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, and a plating film made of Zn and / or Al of the minimum residual sheet thickness of the aluminum material in the spot welds, dissimilar materials bonded body of the steel and the aluminum material, characterized in that at least 50% of the original thickness t _2.   2. The dissimilar material joined body of steel material and aluminum material according to claim 1, wherein the plating film is a zinc plating film containing Zn of 88% by mass or more, which is applied to a steel material bonding side surface.   The dissimilar material joined body of steel materials and aluminum materials according to claim 2 in which said galvanization film contains 8-12 mass% Fe.   It has an interface reaction layer with a thickness of 0.1 to 5 μm at the interface between the nugget and the steel material by spot welding, and at the midpoint of the thickness direction in this interface reaction layer, the amount of Mn element and the amount of Mn element of the steel material The ratio of Si is 1.5 times or more, the Si element amount is 1.1 times or more with respect to the Si element amount of the aluminum material, and the ratio with respect to the Si element amount of the steel material is 1.1 times or more. The dissimilar-materials joined body of the steel materials and aluminum materials as described in a term.

Images