JP2007260777A - Different material joint body of steel material and aluminum material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a joint body of a steel material and an aluminum material and its spot welding method capable of performing a spot welding high in welding strength. <P>SOLUTION: In the different material joint body 3, a galvanized steel material 1 and an aluminum material 2 of the specific plate thickness are welded by the spot welding. An Al<SB>5</SB>Fe<SB>2</SB>-based compound layer is provided on the steel material side, and an Al<SB>3</SB>Fe-based compound layer is provided on the aluminum material side, on a welding interface of this joint body of different materials. An area S<SB>1</SB>occupying in the surface direction of the specific thickness part of these two layers occupies a portion of ≥50% of a nugget area S. Furthermore, an area S<SB>3</SB>occupying in the cross sectional direction of a Zn-Fe based compound layer contained in each of these two layers is set to be ≤10% of the sum of S2+S3, where S2 is an area occupying in the cross sectional direction of the specific thickness part of these two layers. The high welding strength is thus obtained. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a dissimilar material joined body that is a joined body of dissimilar metal members of an iron-based material and an aluminum-based material in the transportation field of automobiles, railway vehicles, and the like, machine parts, and building structures.

  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 a joined body of different kinds of metal members (dissimilar material joined body), it can significantly contribute to weight reduction of an automobile body or the like. In other words, for automobile bodies and other panels where the inner and outer panels are joined together, the joining of steel materials is the mainstream, and if either the inner or the outer can be made of aluminum, the weight of the panel can be reduced. Can do.

  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 material joined bodies (dissimilar metal member joined bodies) are joined by bolts, rivets or the like, but there are problems such as reliability, air tightness, and cost of the joint joint.

  Therefore, many studies have been made on spot welding methods for these dissimilar material joints. 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 for 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 JP-A-4-143833 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, a more serious problem is that, in a galvanized steel sheet that is widely used on the steel material side such as the automobile body panel, spot weldability is worse in a dissimilar joint of steel material and aluminum material than in a bare steel plate. Is a point.

  This is because, in addition to the brittle intermetallic compound produced in the joint of dissimilar materials, in the dissimilar material joining of galvanized steel sheet (galvanized steel material) and aluminum material, brittle Zn-Fe compound derived from galvanization This is because the layer will inevitably generate. Since this Zn-Fe-based compound layer is brittle, it becomes a starting point of fracture and remarkably decreases the bonding strength.

  The present invention has been made to solve such a problem, and provides a joined body of a steel material and an aluminum material that can perform spot welding with a high joint strength even with a galvanized steel sheet (steel material). is there.

In order to achieve the above object, the gist of the dissimilar material joined body of steel material and aluminum material in the present invention is a galvanized steel material having a plate thickness t ₁ of 0.3 to 2.5 mm and an average thickness of a galvanized layer of 3 to 19 μm. And a dissimilar material joined body obtained by spot welding an aluminum material having a thickness t ₂ of 0.5 to 2.5 mm, and at the joining interface of the dissimilar material joined body, an Al ₅ Fe ₂ based compound layer on the steel material side, Total of the Al ₃ Fe-based compound layers on the aluminum material side, and the total thickness of these two layers in the nugget depth direction that occupies the plane direction at the joining interface on the aluminum material side in the range of 0.5 to 10 μm The area occupies a ratio of 50% or more of the area in the plane direction at the joining interface on the aluminum material side of the nugget, and the nugget depth of the Al ₅ Fe ₂ compound layer and the Al ₃ Fe compound layer by SEM Average thickness of the sum of these two layers in the direction In the cross-sectional observation of the bonding interface portion of 0.5 to 10 μm, the total area in the cross-sectional direction of the Zn-Fe compound layer contained in each of these two layers is 0.5 to 10 μm. It is assumed that the ratio is 10% or less of the area in the cross-sectional direction of a certain part.

  Here, in order to further increase the bonding strength, in addition to the above summary, the average diameter at the bonding interface on the aluminum material side of the nugget is 7 mm or more, and occupies the planar direction of the Zn layer at the bonding interface in contact with the nugget The total area is preferably 30% or less of the area occupied in the plane direction at the joining interface on the aluminum material side of the nugget.

  Similarly, the minimum remaining plate thickness on the aluminum material side in the dissimilar material joint is preferably 50% or more of the original aluminum material plate thickness.

Further, similarly, it is preferable that the plate thickness ratio t ₁ / t ₂ between the steel material and the aluminum material is 1 or more.

  In order to spot-weld the same kind of materials such as steel materials and aluminum materials with high joint strength, it is generally only necessary to promote the formation of nuggets. The larger the nugget area, the greater the shear strength and the cross tensile strength. It is known to be higher.

  In addition, the nugget area is related to the heat input amount during spot welding, and the larger the current amount, the larger the time, so in general, by controlling the nugget diameter with the heat input amount during spot welding. A bonded body with high bonding strength is obtained. Of course, if the nugget area 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 area.

  However, when joining different materials of steel and aluminum, the steel has a higher melting point, higher electrical resistance and lower thermal conductivity than the aluminum, so the heat generation on the steel side increases, and the low melting point aluminum first Melts. Next, the surface of the steel material is melted, and as a result, an Al—Fe based brittle intermetallic compound layer is formed at the joining interface (welding interface).

The intermetallic compound formed by spot joining of steel and aluminum is roughly divided into two layers, and the Al ₅ Fe ₂ compound (meaning intermetallic compound Al ₅ Fe ₂ defined in Table 2 below) on the steel side, It is known that an Al ₃ Fe-based compound (meaning intermetallic compound Al ₃ Fe defined in Table 2 and the like described later) is formed on the aluminum material side. Since these intermetallic compounds are very brittle, it has been conventionally impossible to obtain high bonding strength.

In addition to this, in the spot joining of a galvanized steel sheet (galvanized steel material) and an aluminum material, as described above, a Zn-Fe compound derived from galvanization (intermetallic compound Fe ₃ defined in Table 2 and the like described later). (Meaning Zn ₇ ) layer is formed and is inevitably contained in the compound layer. Since this Zn-Fe-based compound layer is brittle, it becomes a starting point of fracture and significantly reduces the bonding strength.

  Therefore, particularly when joining dissimilar materials of galvanized steel and aluminum by spot welding, it is necessary to add a high heat input to form a certain nugget diameter in order to obtain high joint strength, Furthermore, it is very important to control the thickness and structure of the interface reaction layer at the joint interface (welding interface).

In particular, as a result of investigating the thickness of the interfacial reaction layer on the dissimilar material joint strength by spot welding between galvanized steel and aluminum, the findings that the thinner the better the interfacial reaction layer behavior is, I found it different. That is, if the relationship between the thickness and area of the Al ₅ Fe ₂ -based compound layer on the steel material side and the Al ₃ Fe -based compound layer on the aluminum material side constituting the interface reaction layer is controlled within the optimum range, for example, the interface reaction layer However, it was found that the bonding strength is increased to a practical level even if it is composed of these two layers of intermetallic compounds.

  Moreover, in the dissimilar material joining by spot welding of a galvanized steel material and an aluminum material, on the other hand, the characteristic brittle Zn-Fe-based compound layer generated by galvanizing suppresses this and increases the joining strength.

  By these, even if it is a galvanized steel plate (galvanized steel material), the dissimilar-material joining property or spot weldability with an aluminum material improves. For this reason, sufficient joint strength or joint strength of the dissimilar material joined body can be obtained even in spot welding of a large number of continuous spots. In addition, the contact state between the steel material and the aluminum material of the electrode at each spot is stabilized, the electrode life is remarkably improved, and efficient spot welding with a large number of consecutive spots is guaranteed.

  Moreover, without using another material newly like the said prior art, and without requiring a new process, the joining strength of a galvanized steel plate (galvanized steel material) and an aluminum material is high, It also has the effect that different materials can be joined by spot welding.

(Dissimilar material joint)
FIG. 1 is a sectional view of a dissimilar material joined body (joined part) defined by the present invention. In FIG. 1, 3 is a dissimilar material joined body in which a galvanized steel material (galvanized steel plate) 1 and an aluminum material (aluminum alloy plate) 2 are joined by spot welding. Reference numeral 4 denotes a galvanized film or oxide film on the surface of the steel material 1.

  5 at the center of the joint is a nugget having a joint interface (interface reaction layer) 6 in spot welding, and has a nugget diameter indicated by an arrow in the horizontal direction in the figure. Further, the nugget 5 has an area (hereinafter simply referred to as a nugget area S) in the plane direction (left-right direction in the drawing) at the joining interface on the aluminum material side represented by S.

t ₁ is the thickness of the galvanized steel material, t ₂ is the thickness of the aluminum material 2, and Δt is the minimum remaining thickness of the aluminum material after joining by spot welding. 9 is a corona bond portion around the nugget.

  FIG. 1 shows a joining state in which the nugget diameter is secured, the generation of dust is suppressed, the minimum remaining thickness of the aluminum material is maintained, and the melting of the steel material is minimized. The joined body of the invention is also in a joined state as shown in this figure.

  Below, the reason for limitation of each requirement of this invention and its effect | action are demonstrated.

(Thickness of galvanized steel)
In the present invention, the plate thickness t ₁ of the galvanized steel material to be joined needs to be selected from a range of 0.3 to 2.5 mm depending on the plate thickness on the aluminum material side. Without increasing the current value or energizing time in spot welding conditions by increasing the thickness of a single steel material, or by increasing the thickness of the steel material 1 side by overlapping the steel materials directly, etc. Heat input due to resistance heat generation of steel material increases. Further, the heat input distribution in the radial direction of the nugget also changes, and these combined effects tend to cause an increase in temperature on the nugget end side while preventing a decrease in the remaining plate thickness Δt of the aluminum material 2. For this reason, the zinc layer-derived Zn layer 10 is effectively melted and discharged at the bonding interface (corona bond portion) 9 in contact with the nugget 5. As a result, the unique brittle Zn—Fe 2 -based compound layer generated from galvanization is suppressed, and the ratio of the remaining Zn layer 10 is also reduced. For this reason, the direct joining area | region of the steel material 1 and the aluminum material 2 increases, and joining strength increases.

On the other hand, 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 the 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.

(Steel thickness ratio between steel and aluminum)
Here, in order to further increase the bonding strength, the thickness ratio t ₁ / t ₂ between the steel material 1 and the aluminum material 2 in FIG. 1 is preferably 1 or more. By increasing the thickness of the steel material 1 side, the heat input due to the resistance heat generation of the steel material increases without increasing the heat input amount under the spot welding conditions. Further, the heat input distribution in the radial direction of the nugget is also changed, and as described above, the direct joint region between the steel material 1 and the aluminum material 2 is increased by the combined effect, and thereby, the nugget which is a preferable condition of the present invention. It is guaranteed that the total area S ₄ of the Zn layer 10 at the bonding interface portion in contact with 5 is 30% or less of the area S ₂ of the nugget 5. As a result, the bonding strength can be increased.

In contrast, in FIG. 1, when the thickness ratio t ₁ / t ₂ of the steel product 1 and the aluminum material 2 is less than 1, to reduce the proportion of the residual Zn layer 10, directly between the steel 1 and the aluminum material 2 In order to increase the joining region, it is necessary to increase the amount of heat input under spot welding conditions. As a result, even if the ratio of the remaining Zn layer 10 can be reduced, it is not possible to prevent the decrease in the remaining thickness Δt of the aluminum material. As a result, the remaining aluminum material thickness Δt is significantly reduced, and the bonding strength is lowered accordingly.

In this respect, the minimum residual sheet thickness Δt of the aluminum material side in the dissimilar joint, it is preferable that the original 50% or more of aluminum material thickness t _2.

(Galvanized steel)
In the present invention, a galvanized steel material having a double-sided or single-sided galvanized steel having an average thickness of the galvanized layer of 3 to 19 μm is an object of the joined body. In the case of a single-sided galvanized steel material, a surface that is not galvanized may partially appear on the joining side by spot welding. In the present invention, a steel material having a galvanized layer thickness thinner than this or no galvanized layer is not considered.

(Zinc plating layer)
The galvanized layer itself of the steel material may be either galvanized or electroplated, or may be galvanized or alloy plated with iron. However, the average thickness of the galvanized layer is 3 to 19 μm. If the average thickness of the galvanized layer is less than 3 μm, the effect of the galvanized layer itself, such as anticorrosion, cannot be exhibited, and there is no significant difference from the bare steel material, which is meaningless. In addition, when the average thickness of the galvanized layer exceeds 19 μm, the formation of brittle Zn-Fe compound layers and Zn layers derived from galvanizing cannot be suppressed, and these areas are within the scope of the present invention. It becomes difficult to hold on. As a result, the bonding strength is weakened.

(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, when a high-strength steel material is required for the structural member, it is preferable to use a high-strength steel material having a tensile strength of 400 MPa or more.

  In general, low-strength steels are often low-alloy steels, and the oxide film is almost iron oxide. Therefore, Fe and Al are easily diffused, and brittle intermetallic compounds are easily formed. For this reason, it is preferable that the tensile strength is 400 MPa or more, 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, in order to improve the hardenability and precipitation hardening in terms of steel components, steels selectively containing Cr, Mo, Nb, V, Ti, etc. in addition to C can also be applied. 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: 0.1 to 2.5%, Si: 0.001 to 1.5%, Cr: 0 to 1%, Mo : 0 to 0.4%, Nb: 0 to 0.1%, V: 0 to 0.1%, Ti: 0 to 0.1%, or preferably one or two or more of them may be selectively contained as necessary. And it is preferable that the remainder composition of these steel materials consists of Fe and an unavoidable impurity.

(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 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, 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.

(Compound in the interface reaction layer)
Based on the premise of the above-described dissimilar material joined body of steel and aluminum, the present invention defines an intermetallic compound (at the joining interface 6 in FIG. 1) in the dissimilar material joined body after spot welding.

  The cross section of the bonding interface 6 at the center of the nugget of the bonded joint of dissimilar materials is shown in FIGS. 2, 3, and 4 for the intermetallic compound defined in the present invention. FIG. 2 is a schematic diagram of a 5000 times SEM photograph of the bonding interface 6 in FIG. FIG. 4 is a 5000 times TEM photograph of the same bonding interface 6. 3 and 4 show Invention Example 8 of Table 4 in Examples described later.

As shown in each of these figures, at the joint interface 6, a layered Al ₅ Fe ₂ compound layer is formed on the steel material side, and a wedge-shaped (or rod-shaped or needle-shaped) Al ₃ Fe compound layer is formed on the aluminum material side. Have.

(Compound layer definition of the present invention)
Referring to FIGS. 1 and 2, the gist of the compound layer definition in the present invention is as follows. First, on the steel material 1 side, the joint interface 6 of the dissimilar material joined body 3 having the preconditions such as the galvanized layer and the plate thickness described above. Each has an Al ₅ Fe ₂ compound layer and an Al ₃ Fe compound layer on the aluminum material 2 side (generated).

(Thickness of compound layer)
In FIG. 2, the total average thickness l in the nugget depth direction (bonding interface cross-sectional direction, vertical direction in the figure) of these two compound layers at the bonding interface is the Al ₅ Fe ₂ compound layer on the steel material 1 side. Is the sum of the average thickness l ₂ in the nugget depth direction at each measurement point and the average thickness l ₁ in the nugget depth direction at each measurement point of the Al ₃ Fe-based compound layer on the aluminum material 2 side.

(Area regulation of compound layer part-plane direction)
Here, first, the nugget area in the planar direction in the present invention and the area definition of the compound layer portion having a constant thickness at the bonding interface will be described with reference to FIG. As shown in FIG. 1, compound layer average of the Al ₃ Fe-based compound layer of Al ₅ Fe ₂ compound layer of the steel material 1 side and the aluminum material 2 side thickness l (l ₁ + l ₂₎ is 0.5 10 .mu.m S ₁ (mm ² ) is defined as the total area of the portion in the plane direction at the joining interface on the aluminum material 2 side. As shown in FIGS. 6 and 7, which will be described later, assuming that the nugget area occupied in the plane direction at the bonding interface on the aluminum 2 material side of the nugget 5 is S (mm ² ), the aluminum of the bonding interface compound layer portion having this constant thickness The area ratio of the total area S ₁ in the planar direction at the joining interface on the material 2 side to the nugget area S ₁ is obtained. In the present invention, the total area S ₁ is defined as occupying a ratio of 50% or more of the nugget area S 2.

(Area regulation of compound layer part-cross-sectional direction)
Further, the area definition in the cross-sectional direction of the Zn—Fe 2 -based compound layer in the present invention will be described with reference to FIGS. Figures 2 and 4 are partial cross-sectional observation results of the joint interface part where the total average thickness l of the Al ₅ Fe ₂ compound layer and Al ₃ Fe compound layer is 0.5 to 10 μm, using a 5000x SEM. It shows.

Here, the total of the Zn—Fe-based compound layers in the cross-sectional direction (vertical direction in the figure) contained (generated) in the two layers of the Al ₅ Fe ₂ -based compound layer and the Al ₃ Fe-based compound layer, respectively. Let the area be S ₃ (μm ² ). Further, the area occupied in the cross-sectional direction of the compound layer portion in which the total average thickness of these two layers is 0.5 to 10 μm is defined as S ₂ (μm ² ). In the present invention, the total area S ₃ of the Zn—Fe-based compound layer is defined as a ratio of 10% or less of the area S ₂ + S _{3 in} the cross-sectional direction of the specific thickness portion of the three compound layers. To do.

(Zn layer)
Next, the Zn layer will be described. The Zn layer is the remaining amount of the galvanized layer on the steel material surface. For this reason, when the Zn layer remains, as shown in FIG. 1, it exists at the junction interface 6 at the nugget end portion (peripheral portion). This Zn layer has a thickness equivalent to or thinner than a relatively thick galvanized layer present at the peripheral edge thereof. If this Zn layer remains at the bonding interface 6 (if it remains), that portion means that the steel material 1 and the aluminum material 2 are not directly bonded. Therefore, in the present invention, the total area S ₄ (mm ² ) in the planar direction of the Zn layer 10 in the bonding interface 6 in contact with the nugget is 30% or less of the area S (mm ² ) in the planar direction of the nugget described above. It is prescribed that it is. The total area S ₄ occupied in the planar direction of the Zn layer 10 is an area occupied in the planar direction at the joining interface on the aluminum material 2 side, similarly to the nugget area S described later.

(Al ₃ Fe compound layer)
In the present invention, in order to increase the bonding strength, the average thickness l ₁ in the nugget depth direction in the nugget center portion of the Al ₃ Fe-based compound layer on the aluminum material 2 side in FIG. 2 is set to a range of 0.5 to 10 μm. It is preferable to do.

The Al ₃ Fe-based compound is an intermetallic compound Al ₃ Fe formed on the aluminum material 2 side, and is formed in a wedge shape as shown in FIGS. In the central part (nugget central part), the size of individual compound grains (or the length of wedge-shaped or acicular compound grains) is large, and as they go toward the end of the nugget (left-right direction in Figs. 2, 3, and 4), Gradually the thickness (grain, needle size and distribution) decreases.

Such an Al ₃ Fe-based compound has a wedge (anchor) effect including the effect of the above-described shape, improves the adhesion between the aluminum material 2 and the Al ₅ Fe ₂ -based compound layer, and increases the bonding strength. This effect is not exhibited when the Al ₃ Fe-based compound layer is too thin. In particular, when l ₁ is less than 0.20 μm, the wedge effect is insufficient, the adhesiveness with the Al ₅ Fe ₂ -based compound layer is poor, the interlaminar fracture tends to occur, and there is a possibility of fracture at a smooth interface. For this reason, it is preferable that the average thickness l ₁ in the nugget depth direction at the nugget central portion of the Al ₃ Fe-based compound layer on the aluminum material 2 side is 0.20 μm or more.

On the other hand, if the Al ₃ Fe-based compound layer grows too much and forms a layer that is too thick, the individual compound grains become the starting point for destruction. This tendency is particularly noticeable when l ₁ exceeds 10 μm. For this reason, it is preferable that the upper limit of the average thickness l ₁ in the nugget depth direction in the nugget center portion of the Al ₃ Fe-based compound layer is 10 μm or less.

(Al ₅ Fe ₂ compound layer)
In the present invention, in order to further increase the bonding strength, the average thickness l ₂ in the nugget depth direction of the Al ₅ Fe ₂ based compound layer which is the intermetallic compound Al ₅ Fe ₂ on the steel material 1 side is also 0.20 to 5 μm. It is preferable that it is the range of these. This Al ₅ Fe ₂ -based compound layer also gradually decreases in thickness (grain size, needle size and distribution) toward the end of the nugget (left and right directions in FIGS. 2, 3, and 4). If the average thickness l _{2 of} this Al ₅ Fe ₂ based compound layer is too thin or too thick, the bonding strength may be reduced. The same as in the case of the Al ₃ Fe based compound layer.

(Area of both compound layers)
2, above described, the average thickness l ₁ of the nugget depth direction of the Al ₃ Fe-based compound layer, the total average thickness and the average thickness of l ₂ of the nugget depth direction of the Al ₅ Fe ₂ compound layer L is the average thickness of these two layers in the nugget depth direction.

In the present invention, in order to increase the bonding strength, the area S ₁ (area S ₁ occupying in the plane direction at the bonding interface on the aluminum material 2 side) of the portion where the total average thickness l is 0.5 to 10 μm is shown in FIG. Increase as shown in. That is, it is defined as 50% or more of the nugget area S (area S occupied in the plane direction at the bonding interface on the aluminum material 2 side, as shown in FIG. 1).

That is, the larger the area S ₁ in the plane direction at the joint interface of the specific thickness portion between the Al ₃ Fe-based compound layer and the Al ₅ Fe ₂ -based compound layer, the higher the bonding strength. When the area S ₁ is small as shown in FIG. 7 and less than 50% of the nugget area S and the same strength, the larger the nugget area S, the more likely the fracture load (joint strength) of the joint will decrease. . On the other hand, when the nugget area S is small, the joint is easily broken at a lower load.

When the area S ₁ between the Al ₃ Fe-based compound layer and the Al ₅ Fe _2- based compound layer is large as shown in Fig. 6, the area of the joint (bonding interface) with high bonding strength is sufficiently large, so a larger breaking load It becomes. As a result, since the joining interface has a sufficiently higher breaking load than the aluminum base material, the aluminum material side breaks without breaking the interface.

  The above-mentioned area regulation of the interface reaction layer having the optimum thickness is intended to control the compound layer on the aluminum material side and the compound layer on the steel material side within the optimum range, from the viewpoint of bonding strength. For this reason, the direction in which the present invention is directed is different from the conventional common sense that the thinner the better, the more actively present it is. In order to improve the bonding strength, the interface reaction layer having the optimum thickness range is formed in a large area, in other words, based on the technical idea of existing in a wide range.

(Zn-Fe compounds)
In the present invention, on the other hand, the compound or compound layer is an impurity, and the Zn—Fe 2 compound, which is an intermetallic compound, is restricted in order to inhibit the bonding strength. Specifically, the Zn—Fe compound contained (generated) in each of the two layers of the Al ₃ Fe compound layer and the Al ₅ Fe ₂ compound layer is regulated.

As shown in Fig. 2, the cross-sectional direction of the Zn-Fe compound layer contained in (generated) each of the two layers of Al ₅ Fe ₂ compound layer and Al ₃ Fe compound layer (vertical direction in the figure) Is the total area S ₃ . Further, the area occupied in the cross-sectional direction of the compound layer portion in which the total average thickness of these two layers is 0.5 to 10 μm is defined as S ₂ . In the present invention, the total area S ₃ of the Zn—Fe based compound layer is defined as a ratio of 10% or less of the area S _{2 in} the cross-sectional direction of the specific thickness portion of the two compound layers.

The area S ₃ of the sum of the brittle Zn-Fe-based compound layer, the sum of S ₃ of the above-described area S ₂ Toko, when more than 10% of S ₂ + S ₃ is significantly reduced bonding strength of the joint To do. The Zn-Fe compound is also called an Fe-Zn compound.

(Zn layer area)
Furthermore, in the present invention, as a preferable condition, as described above, the total area S ₄ in the planar direction of the Zn layer 10 in the bonding interface 6 in contact with the nugget is 30% or less of the area S 2 in the planar direction of the nugget. It prescribes that it exists.

As described above, the Zn layer is the remaining portion of the galvanized layer on the surface of the steel material.If the Zn layer remains at the junction interface 6 at the end of the nugget (periphery) as shown in FIG. In this case, it means that the steel material 1 and the aluminum material 2 are not directly joined. When the total area S ₄ of the fragile Zn layer exceeds 30% of the observed nugget area S in the planar direction, there is a high possibility that the bonding strength of the bonded portion is significantly reduced.

Measurement of the total area S ₄ in the planar direction of the Zn layer 10 at the bonding interface 6 in contact with the nugget is based on the observation results with an optical microscope 200 times the cross-sectional direction at each site where the Zn layer 10 exists. As shown in FIG. 6, the measurement can be performed on the assumption that the Zn layer 10 exists symmetrically in the circumferential direction of the nugget at the bonding interface 6 in contact with the nugget.

(Joint strength and fracture mode)
In the case of the present invention, when the bonding strength is high, the bonding interface does not break, and the bonded portion breaks into a plug shape (the aluminum material is thicker inside than the range where the Al ₃ Fe-based compound layer exists). Break in the direction). In other words, such a fracture form of the joint portion represents the high joint strength of the present invention.

On the other hand, when the bonding strength is low as in the prior art, it breaks at the bonding interface and breaks between the Al ₅ Fe ₂ compound layer and the Al ₃ Fe compound layer or inside one of the compound layers. In other words, such a fracture form of the joint portion represents a low joint strength.

(Identification method of intermetallic compounds)
In the present invention, Al ₃ Fe-based compound layer and Al ₅ Fe _2- based compound layer are identified by using EDX (Energy Dispersive X-ray spectroscopy) with the HAADF-STEM image (5000 times to 10,000 times) of the cross section of the joint. Identified by performing semi-quantitative analysis by point analysis. In other words, unless the joint interface is measured using the HAADF-STEM method described below, it is difficult to identify the intermetallic compound specified in the present invention and to accurately measure the thickness and area of the intermetallic compound layer. I can say that.

To distinguish (identify) these intermetallic compounds, the composition of multiple (as many as possible) measurement points at the joint interface was measured in the above semi-quantitative analysis, and Fe, Al, Zn (at%) was used as a percentage. Depending on the particular composition. That is, as shown in Table 2, the composition of the “Al ₅ Fe ₂ -based compound” is such that the Fe amount ranges from 24.0 to 29.0 at% and the Al amount ranges from 70.0 to 74.0 at%. The composition of the “Al ₃ Fe-based compound” is such that the Fe amount is 18.0 to 24.0 at% and the Al amount is 74.5 to 81.0 at%. Further, the composition of the Zn-Fe-based compound is a typical Fe ₃ Zn ₇ composition, the Fe amount is 31.0 to 40.0 at%, and the Zn amount is 60.0 to 69.0 at%.

Here, criteria for determining (identifying) the composition of each interface reaction layer are as follows. That is, “Al ₅ Fe ₂ -based compound” and “Al ₃ Fe-based compound” are layers in which Fe and Al are both detected by 10 mass% or more by EDX point analysis. In other words, the layer in which both Fe and Al are less than 10% by mass was not an interface reaction layer specified in the present invention.

  In addition, the Zn-Fe compound was a layer in which Fe was detected by 27.7% by mass or more and Zn was detected by 72.3% by mass or less by EDX point analysis. Even if Zn was detected, the layer in which Fe was not detected in 27.7 mass% or more was distinguished from the originally existing Zn plating layer or Zn layer 10 and was not an interface reaction layer.

The composition of the Zn-Fe compound is a typical composition of Fe ₃ Zn ₇ [Fe 27.7 to 36.3 mass% (Fe 31.0 to 40.0 at%)], and the other phases contained in the Zn plating layer Distinguished. The other phase, zeta-phase [FeZn _13: Fe5.8~6.2 wt% (Fe6.7 ~7.2at%)], δ1 phase [FeZn _7: Fe7.3 ~11.3 wt% (Fe8.5 ~13.0at %)), Γ1 phase [Fe ₅ Zn ₂₁ : Fe 16.2 to 20.8 mass% (Fe 18.5 to 23.5 at%)], Γ phase [Fe ₃ Zn ₁₀ : Fe 21.2 to 27.7 mass% (Fe 24.0 to 31.0at%)). 31.0at%)) (Exhibiting composition of Zn plating layer: edited by the Japan Iron and Steel Institute, 138th and 139th Nishiyama Memorial Lecture “Surface Treatment Technology Advances and Future Trends” p.15. (Issued May 1)).

In addition, Al ₅ Fe ₂ , Al ₃ Fe, and Fe ₃ Zn ₇ are recognized as the phase of the interfacial reaction layer, but the composition is not necessarily composed of stoichiometric composition and has a certain composition width. . In this regard, the phases were identified from the crystal structure obtained by electron diffraction using TEM, and Fe, Al, Si, Mn, and Zn elements were measured by EDX for each phase. As a result, in practice, the Al ₅ Fe ₂ and Al ₃ Fe phases have a lower Fe content than the stoichiometric composition, and the Fe ₃ Zn ₇ phase has a higher Fe content than the stoichiometric composition. It was found that the composition of was shifted. Based on these results, those in which the ratios of Fe, Al, and Zn satisfy the ranges shown in Table 2 were determined as the phases of the respective reaction layers.

  The HAADF-STEM method (High Angle Annular Dark Field-Scanning Transmission Electron Microscope) is a method for obtaining an image signal by collecting elastically scattered electrons scattered on the high angle side with an annular detector. The HAADF-STEM image is hardly affected by the diffraction contrast, and the contrast is characterized by being approximately proportional to the square of the atomic number (Z), and the obtained image becomes a two-dimensional map having the composition information as it is. Since trace elements can be detected with high sensitivity, it is effective for fine structure analysis of the bonding interface.

More specifically, the average thickness of each compound layer in the interfacial reaction layer was roughly measured by SEM after cutting at the center of the nugget of the bonded body, embedding in a resin so that the cross section could be observed, and mirror polishing. taking measurement. Then, the nugget center and the portion inside the boundary of the layer that is regarded as the Al ₅ Fe ₂ compound, the portion inside and outside the boundary of the layer that is regarded as the Al ₃ Fe compound, the layer that is regarded as each compound By subjecting the inner and outer parts of the part that is expected to exceed the upper limit in the depth direction to FIB to a thickness that allows TEM observation using a focused ion beam processing device (FB-2000A) manufactured by Hitachi, Ltd. Is used as a sample for observation and analysis.

Then, using a JEOL field emission transmission electron microscope (JEM-2010F) equipped with a HAADF detector and observing at an accelerating voltage of 200 kV and a field of view of 100 μm (5000 times to 10,000 times), EDX point analysis is performed for all the different phases, and the Al ₃ Fe compound layer and the Al ₅ Fe ₂ compound layer are identified.

The thickness (length) l ₁ of the Al ₃ Fe compound in Fig. 2 is based on the HAADF-STEM image with a field of view of 100 µm, and all the grains and needles identified as Al ₃ Fe compounds were identified. The length in the depth direction was measured and averaged.

The thickness (length) l ₂ in the depth direction of the Al ₅ Fe ₂ -based compound layer in FIG. 2 was measured and averaged at five points from the same image. The above measurement was performed on all the samples for observation and analysis.

(Measurement method of intermetallic compound planar area)
Based on these measurements, at the joint interface on the aluminum material side of the portion where the total average thickness l in the nugget depth direction of the two layers, Al ₃ Fe compound layer and Al ₅ Fe ₂ compound layer, is 0.5 to 10 μm. The total area S ₁ occupying in the plane direction was determined. Further, the nugget area S occupying in the plane direction at the joining interface on the aluminum material side was also obtained in the same manner. That is, as shown in FIGS. 6 and 7, assuming that these layers are symmetrical in the circumferential direction of the nugget, the area in the plane direction was calculated from the radial position in the cross section, assuming a concentric circle. . Further, the total area S _{4 in} the planar direction of the Zn layer 10 was also calculated in the same manner. That is, as shown in FIG. 1, when the Zn layer 10 exists, it exists along the circumference (periphery) of the nugget. For this reason, when the existence of the Zn layer 10 is confirmed, it is assumed that the Zn layer 10 is symmetric in the circumferential direction of the nugget, and from the radial position in the cross section, it is assumed that it is a concentric circle. The area was calculated.

(Method for measuring cross-sectional area of intermetallic compound)
The portion where the total thickness l in the nugget depth direction of the two layers of the Al ₃ Fe compound layer and the Al ₅ Fe ₂ compound layer identified and measured by EDX point analysis in this way is 0.5 to 10 μm The total area S ₂ in the cross-sectional direction and the total area S _{3 in} the cross-sectional direction of each of the Zn-Fe-based compound layers contained in the two layers are measured by the SEM observation of the identified bonding interface site, 2000 SEM observation was performed at a magnification of 2 to 10000 times (2000 times when the average thickness l is 1 μm or more, 2000 times when the average thickness 1 is less than 1 μm). Specifically, the area in the cross-sectional direction of each reaction layer is measured with a field width of 100 μm at each radial position of 500 μm from the center of the nugget radius, and the measurement is performed up to the position where the reaction layer exists. The measurement was performed in the radial direction, and the total was obtained.

  These results may also be verified by TEM observation at a magnification of 2000 to 10000 times. For example, in the SEM photograph of 5000 times the joint interface shown in Fig. 3 and the 5000 times TEM photograph of the same joint interface shown in Fig. 4, the part indicated by the dotted line is the joint interface. Easy to distinguish visually.

(Nugget size)
The average diameter t at the joining interface on the aluminum material side of the nugget 5 of the spot welded portion in FIG. 1 is preferably 7 mm or more in order to ensure the joining strength. In other words, it is preferable to select the spot welding conditions so that the average diameter of the nugget 5 is 7 mm or more.

  If the average diameter of the nugget 5 is less than 7 mm, the nugget area is too small and the bonding strength is likely to be insufficient. On the other hand, the average diameter of the nugget 5 is preferably 12 mm or less. If the average diameter of the nugget 5 exceeds 12 mm, it is sufficient to obtain the joining strength, but dust is easily generated and the amount of thinning of the aluminum material is large, so that the joining strength is reduced.

Conventionally, when spot-welding the same kind of metal material, the area of the nugget 5 in the spot-welded part should be approximately 20 x ^0.5 mm ² with respect to the thickness t of the metal material. It is said that it is optimal both from the viewpoint of economy and economy.

  However, according to the present invention, the nugget area is larger than that of the same kind of metal material for joining different kinds of metal materials. By performing spot bonding so that the average diameter of the nugget 5 in the spot welded portion is 7 mm or more, sufficient bonding strength is obtained, and further, both workability and economy are excellent.

(Measurement of nugget area)
In the present invention, the area S occupying the plane direction in the joining interface of the nugget 5 on the aluminum material 2 side, the area S ₅ occupying the cross section direction of the nugget ₅ , the nugget average diameter ) Can be measured, for example, by observation with a 200 × optical microscope. That is, the nugget on the aluminum material side divided by peeling or cutting at the bonding interface is measured by analyzing a plurality of sample images, and the average is obtained. At this time, the observation surface is a cross section centered on the nugget center. If the nugget shape is approximately circular, cut the joint and observe with an optical microscope from the cross section, measure the diameter of the nugget formed on the aluminum material side joint interface with multiple samples, and average the You may ask for. In that case, the nugget diameter in at least two orthogonal directions is measured.

(Aluminum material thickness reduction)
In order to ensure the joining strength, it is desirable to make the thickness reduction of the aluminum material after joining by spot welding as small as possible. As a guideline, 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 plate thickness Δt of this aluminum material can be obtained by observing it with an optical microscope 200 times larger than the bonded cross section, measuring the plate thickness reduction length, and taking the difference from the original plate thickness.

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

Below, each condition of the spot welding for obtaining this invention dissimilar material joined body is demonstrated.
(Pressure)
Regarding the applied pressure at the time of spot welding, in order to obtain the relatively large required nugget area and the required area of the optimum interface reaction layer, and to be within the optimum range defined in the present invention, It is necessary to apply a high pressure.

Specifically, it is selected from a relatively high pressure range of 1 × t ^0.5 kN to 2.5 × t ^0.5 kN in relation to the thickness t (t ₁ + t _{2 in} FIG. 1) of the entire joint. However, even within this relatively high pressure range, the way of producing the compound differs depending on the material and other welding conditions, and it is not always within the optimum range defined in the present invention. For this reason, it is necessary to select an optimum pressure within the optimum range defined in the present invention from the range of the relatively high pressure according to the material and other welding conditions.

  On the other hand, by applying a relatively large pressing force in the above range, regardless of the shape of the electrode tip, etc., the electrical contact between different materials and between the electrodes is stabilized, and the molten metal in the nugget is moved around the nugget. It can be supported by the unmelted portion, and the relatively large nugget required area and the required area of the optimum interface reaction layer can be obtained. Moreover, generation | occurrence | production of dust can be suppressed.

When the applied pressure is less than 1 × t ^0.5 kN, the applied pressure is too low to obtain such an effect. In particular, in the tip having R 2 at the tip, the contact area decreases, leading to a decrease in nugget area and an increase in current density (= increase in the interface reaction layer), so that the bonding strength decreases. Also, Al ₃ Fe-based compound average thickness of the layer l _1, Al ₅ average thickness of Fe ₂ compound layer l ₂ The likely not be obtained such as the average thickness l of the sum of these two layers .

On the other hand, the nugget area tends to decrease when the applied pressure is increased. When the applied pressure exceeds 2.5 × t ^0.5 kN, a current exceeding the following optimum current is required to obtain the desired nugget area. And the growth of the interface reaction layer, the bonding strength is lowered. Moreover, since the deformation of the aluminum material is large and the joint mark becomes a concave portion, it is not desirable in appearance.

(Current)
In order to obtain the relatively large required nugget area and the required area of the optimum interface reaction layer, it is necessary to control the current during spot welding and to allow a relatively high current to flow for a short time.

Specifically, in relation to the thickness t _{1 of the} entire steel material of the joint (t _{1 in} FIG. ₁ , but when two or more steel materials are laminated, the thickness of the entire steel material) is 12 × It is necessary to flow a relatively high current of t ₁ ^{0.5 to} 35 × t ₁ ^0.5 kA for a short time of 320 × t ₁ ^0.5 msec or less. However, even within this relatively high current and time range, the method of producing the above compound differs depending on the material and other welding conditions, and it is not necessarily within the optimum range defined in the present invention. For this reason, it is necessary to select the optimum current and time within the optimum range defined in the present invention from the relatively high current and time ranges according to the material and other welding conditions.

  In addition, by flowing such a relatively high current for a short time, the electrical contact between different materials and between the electrode and the material is stabilized, and the molten metal in the nugget is supported by the unmelted portion around the nugget. A large nugget required area and a required area of the optimum interface reaction layer can be obtained. Moreover, generation | occurrence | production of dust can be suppressed.

In the case of a low current of less than 12 × t ₁ ^0.5 kA, strictly less than 15 × t ₁ ^0.5 kA, a heat input sufficient to form and grow nuggets cannot be obtained. For this reason, the comparatively large nugget required area and the required area of the optimum interface reaction layer cannot be obtained. In addition, the average thickness l ₁ of the layer of Al ₃ Fe compound and Al ₁₉ Fe ₄ Si ₂ Mn compound, the average of nugget depth direction within the range of nugget center ± 0.1 mm of Al ₅ Fe ₂ compound layer There is a high possibility that the thickness l ₂ , etc. cannot be obtained.

On the other hand, in the case of a high current exceeding 35 × t ₁ ^0.5 kA, extra equipment is required, which is disadvantageous in terms of work and cost. Therefore, from these points, the current is 35 × t ₁ ^0.5 kA or less. Therefore, the working current is in the range of 12 × t ₁ ^{0.5 to} 35 × t ₁ ^0.5 kA, preferably 15 × t ₁ ^{0.5 to} 35 × t ₁ ^0.5 kA.

(Energization time)
The energization time is a relatively short time of 320 × t ₁ ^0.5 msec or less in relation to the thickness t _{1 of the} entire steel material. When the energization time is longer than 320 × t ₁ ^0.5 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. As described above, in order to control the interface reaction layer, the energization time is 320 × t ₁ ^0.5 msec or less, preferably 100 × t ₁ ^0.5 msec to 280 × t ₁ ^0.5 msec. However, as described above, it is necessary to select an optimum time during which the compound control defined in the present invention is within the optimum range in relation to the current, depending on the material and other welding conditions.

(2-stage energization)
In order to make the reaction layer at the bonding interface like the compound layer defined in the present invention, it is preferable to carry out by two-stage energization or two-stage spot welding instead of the usual one-stage energization. In this way, spot welding is performed in two stages, and the galvanized layer on the steel surface is skipped (removed) by making the second stage energization value lower than the first stage energization value. It becomes easy to join the aluminum material directly.

Moreover, it becomes easier to obtain the compound layer defined by the present compound layer. That is, an Al ₅ Fe ₂ compound layer is easily formed on the steel material side and an Al ₃ Fe compound layer is easily formed on the aluminum material side at the bonding interface of the dissimilar material joined body. Further, the area S ₁ of the specific thickness portion of these two layers tends to be 50% or more of the nugget area S. Furthermore, the area S _{3 of the} Zn—Fe 2 -based compound layer contained in each of these two layers is easily suppressed to 10% or less of the compound area S ₂ + S _{3 in} the cross-sectional direction. In addition, the total area S ₄ of the Zn layer at the bonding interface in contact with the nugget is also easily suppressed to 30% or less of the nugget area.

  Even when spot welding is conducted in two stages, the applied pressure is the same in both the first and second stages, except that the energization value in the second stage is lower than the energization value in the first stage. In addition, it is preferable that the applied pressure, the current value, and the total energization time are within the above-described preferable ranges. Thereby, dissimilar materials can be joined without impairing the efficiency of spot welding.

(Electrode shape)
The shape of the electrode tip for spot welding may be any shape as long as the nugget area and the interface reaction layer can be obtained, and the electrode tips on the steel material side and the aluminum material side may have different shapes or different sizes. However, it is desirable to use a “dome-shaped” electrode tip with R at the tip as shown in Fig. 2 on both the steel and aluminum sides. In the case of such a dome shape, the tip diameter and tip R 1 of the electrode tip need to be 7 mmφ or more and 100 mmR or more in order to achieve both the above-mentioned current density reduction and nugget area increase. 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.

In particular, by using both electrode tips having a tip diameter of 7 mmφ or more and a tip R 1 of 120 mmR or more, the current density reduction and the nugget area increase can both be achieved optimally. When using this chip, the in relation to the steel plate thickness ^{_{t 1, 1.5 × t 1 0.5}} kN~2.5 × t 1 0.5 kN pressing force is applied to, and ^{_{15 × t 1 0.5 ~35 × t}} 1 0.5 It is preferable to apply a current of kA of 320 × t ₁ ^0.5 msec or less.

  The optimum joining condition is the balance of each of the above-described conditions. For example, when the chip diameter, the chip R, or the pressurizing force is increased and the current density is decreased, the current amount is increased accordingly, and the interface reaction is increased. It is necessary to control the layer to an optimum thickness.

  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.

  The dissimilar material joined body was manufactured under the conditions shown in Tables 3, 5, and 7. About each of these manufactured joined bodies, the area ratio of each compound was measured by the measurement method described above, and the bonding strength and the thickness reduction (minimum remaining plate thickness) of the aluminum material were evaluated. As a summary, Table 4 shows the results of spot welding under the conditions of Table 3, Table 6 shows the results of spot welding under the conditions of Table 5, and Table 7 shows the results of spot welding under the conditions of Table 7. Each of them is shown in FIG.

(Steel conditions)
Four types of test steels containing the chemical components (mass%) shown in Table 1 were melted and rolled to a plate thickness of 0.8 to 1.2 mm to obtain thin steel plates. This thin steel plate was annealed at 500 to 1000 ° C. by continuous annealing, then washed with oil or water, and then tempered to obtain steel plates having four types of strength (MPa) shown in Table 1.

(Aluminum material conditions)
As for the aluminum material, a commercially available A6022 aluminum alloy plate (Al-0.6 mass% -1.0 mass% Si-0.08 mass% Mn-0.17 mass% Fe) having a thickness of 1.0 mm was used in common.

(Spot welding conditions)
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 spot-welded under the conditions shown in Table 2 to prepare a dissimilar material joined body.

In spot welding, a direct current resistance welding tester was used, and the correlation between conditions such as pressure, welding current, and time and the control of the average thickness and area of the compound defined in the present invention was investigated in advance. On top of that, in accordance with the thickness t ₂ of an aluminum material, pressure, welding current, set each time, under the conditions shown in each Table we were welded one point.

  The applied pressure, welding current, and energization time were changed within the preferable ranges described in paragraph 0105 in relation to the following electrode tips used.

  And, when the energization of spot welding is made in two stages, the applied pressure is the same in both the first and second stages, except that the energization value in the second stage is lower than the energization value in the first stage. The applied pressure, the current value, and the total energization time were within the preferable ranges described above.

  The electrode tip is a 12mmφ dome shape made of Cu-Cr alloy, the electrode tip has a curvature of 150mmR, the anode is made of aluminum, and the cathode is made of steel.

  The nugget diameter, minimum aluminum sheet thickness, and Zn plating layer remaining ratio were measured by cutting the spot welded sample at the center of the weld, embedding it in resin, polishing and chemical etching, and using a 200x optical microscope. Was observed.

  The thickness of the interface reaction layer was measured by the measurement method described above using the same cross-sectional sample as described above.

(Evaluation of bonding strength)
As evaluation of the joint strength of each joined body, the cross tension test of the dissimilar material joined body was implemented. The cross tension test is based on the joint strength of A6022 materials = 1.0 kN, ◎ if the joint strength is 1.5 kN or more or the fracture mode is aluminum base material fracture, ○ if the joint strength is 1.0 to 1.5 kN, When the bonding strength was 0.5 to 1.0 kN, Δ was marked, and when the bonding strength was less than 0.5 kN, x was marked.

  The reason why the cross tension test was used for the strength evaluation in this example is that the cross tension test had a greater difference between the test conditions than the shear tension test. However, the results of several shear shear tests selected from the inventive examples are consistent with the results of the cross tension test, and the results of evaluation of ○ and ◎ in the cross tension test are both 2.5. High shear strength of kN or higher.

In the results of joining by spot welding in Tables 4, 6, and 8, in each invention example, the area ratio of the total area S ₁ in the planar direction of the joining interface compound layer portion having a constant thickness to the nugget area S is 50% or more. . Further, with respect to the area S ₂ occupied in the cross direction of the joint interface compound layer portion having a constant thickness, the ratio of the total area S ₂ + S ₃ of the Zn-Fe-based compound layer and other two layers is 10% or less. Furthermore, as a preferable condition, the total area S ₄ in the planar direction of the Zn layer at the bonding interface in contact with the nugget is 30% or less of the nugget area S 1 and the nugget average diameter is 7.0 mm or more.

As a result, in each of the inventive examples, as shown in Tables 4, 6, and 8, a high joint strength of the dissimilar material joined body was obtained. Among the inventive examples, the bonding strength of the dissimilar material bonded body is higher as the bonding interface compound layer portion having a certain thickness is larger or the Zn-Fe-based compound layer portion is smaller. Also preferred is a condition, as the nugget average diameter is large, the smaller the total area S ₄ occupied in the plane direction of the Zn layer, a high bonding strength of the dissimilar materials bonded body.

On the other hand, in the results of joining by spot welding in Tables 4, 6 and 8, each comparative example is a joining interface compound layer in which the area ratio of the total area S _{1 in} the planar direction to the nugget area S is less than 50% or has a constant thickness. The ratio of the total area S ₂ + S ₃ of the Zn—Fe-based compound layer and the other two layers to the area S _{2 in} the cross-sectional direction of the portion exceeds 10%. That is, there are too few bonding interface compound layer portions having a constant thickness or too many Zn-Fe-based compound layer portions. For this reason, compared with each corresponding invention example, the joining strength of a dissimilar material joined body is remarkably low.

Here, a specific comparison of the bonding interface structures of Invention Example 8 and Comparative Example 7 in Table 4 is performed. 3 and 4 show the bonding interface structure of Invention Example 8, and FIG. 3 is an SEM photograph of 5000 times the bonding interface. Figure 4 is a 5000 times TEM photograph of the same joint interface. As can be seen from FIGS. 3 and 4, the bonded interface structure of Invention Example 8 has an Al ₅ Fe ₂ compound layer on the steel material side and an Al ₃ Fe compound layer on the aluminum material side.

On the other hand, FIG. 9 shows a bonded interface structure of Comparative Example 7, and shows a 5000 times TEM photograph of the bonded interface. As can be seen from FIG. 9, the bonded interface structure of Comparative Example 7 has an Al ₃ Fe compound layer on the aluminum material side, but there is no Al ₅ Fe ₂ compound layer on the steel material side. I understand.

5, and Comparative Example 7 to these inventive examples 8, a total average thickness of the two layers of the Al ₃ Fe-based compound layer and Al ₅ Fe ₂ based compound layer, the distribution by distances from nugget center Show. In FIG. 5, the portion where the total average thickness l of these two layers is 0.5 to 10 μm is shown within the range of two dotted lines.

  In Invention Example 8 indicated by the black triangle, except for the nugget center portion where the average thickness is too thick, the portion where the average thickness l is 0.5 to 10 μm extends to the portion where the distance from the nugget center portion is 4000 μm. ing. This is shown in a plan view at the joining interface on the aluminum material side as shown in FIG. That is, in Invention Example 8, the area ratio to the area S in the plane direction at the joining interface of the nugget on the aluminum material side is 80%, and almost only the nugget (area S) is obtained except for the nugget center and the nugget periphery. Approximate (overlap).

  On the other hand, in Comparative Example 7 indicated by a black circle below Invention Example 8, the portion where the average thickness l is 0.5 to 10 μm is up to the portion where the distance from the center portion of the nugget is 2000 μm. This is shown in a plan view at the joining interface on the aluminum material side as shown in FIG. That is, in Comparative Example 7, the area ratio to the area S in the planar direction at the joining interface on the aluminum material side of the nugget is 36%, and only the nugget central portion and its peripheral portion overlap with the nugget (area S). Only.

  Therefore, the above examples support the critical significance of the thickness and structure of the interface reaction layer at the bonding interface in the present invention in order to obtain a high bonding strength of the dissimilar material bonded body.

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. It is possible to provide a dissimilar joint of steel and aluminum that can be welded. 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 dissimilar material joined body of steel and aluminum.

It is sectional drawing which shows the junction part interface of this invention dissimilar material joined body. It is explanatory drawing which expanded and modeled the cross-sectional structure | tissue (FIG. 3) of the junction part interface of this invention dissimilar material assembly. It is a drawing substitute SEM photograph which shows the cross-sectional structure | tissue of the junction part interface of this invention dissimilar material joined body. It is a drawing substitute TEM photograph which shows the cross-sectional structure | tissue of the junction part interface of this invention dissimilar material joined body. It is explanatory drawing which shows the thickness distribution of the junction interface reaction layer of this invention dissimilar material joined body. It is explanatory drawing which shows distribution of the fixed thickness of the planar direction of the junction part interface reaction layer of this invention dissimilar material assembly. It is explanatory drawing which shows distribution of the fixed thickness of the planar direction of the junction part interface reaction layer of a comparative example dissimilar material joined body. It is explanatory drawing which shows the aspect of the spot welding for obtaining a dissimilar-material joined body. It is a drawing substitute TEM photograph which shows the cross-sectional structure | tissue of the junction part interface of a comparative example dissimilar material joined body.

Explanation of symbols

1: Steel plate, 2: Aluminum alloy plate, 3: Dissimilar material joined body, 4: Oxide film,
5: Nugget, 6: Interfacial reaction layer, 7, 8: Electrode

Claims (4)

In the plate thickness t ₁ is 0.3 2.5 mm, dissimilar conjugated with galvanized steel average thickness of the galvanization layer is 3 ~19μm, plate thickness t ₂ is the aluminum material is 0.5 2.5 mm by spot welding A joined body of this dissimilar material joined body having an Al ₅ Fe ₂ compound layer on the steel material side and an Al ₃ Fe compound layer on the aluminum material side, and these two layers in the nugget depth direction The total area occupied in the plane direction at the joint interface on the aluminum material side of the portion having a total average thickness of 0.5 to 10 μm is 50% or more of the area occupied in the plane direction at the joint interface on the aluminum material side of the nugget. In the cross-sectional observation of the joint interface portion where the total average thickness of these two layers in the nugget depth direction of the Al ₅ Fe ₂ compound layer and the Al ₃ Fe compound layer is 0.5 to 10 μm by SEM Zn-Fe compound contained in each of these two layers The total area occupied in the cross-sectional direction of the layer is the sum of the area occupied in the cross-sectional direction of the portion where the total average thickness of these two layers is 0.5 to 10 μm and the total area occupied in the cross-sectional direction of the Zn-Fe compound layer. A dissimilar joint of steel and aluminum, characterized by a ratio of 10% or less.   The average diameter at the joining interface on the aluminum material side of the nugget is 7 mm or more, and the total area occupied in the planar direction of the Zn layer at the joining interface in contact with the nugget is the area occupied in the planar direction at the joining interface on the aluminum material side of the nugget The dissimilar material joined body of the steel material and the aluminum material according to claim 1, which is 30% or less.   3. The dissimilar material joined body of steel material and aluminum material according to claim 1, wherein a minimum remaining plate thickness on the aluminum material side in the dissimilar material joint portion is 50% or more of the original aluminum material plate thickness. 4. The dissimilar material joined body of steel material and aluminum material according to claim 1, wherein a thickness ratio t ₁ / t _{2 of the} steel material and aluminum material is 1 or more. 5.

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