JP4445425B2 - Dissimilar joints of steel and aluminum - Google Patents

Description

  The present invention relates to a dissimilar material joined body of dissimilar metal members of an iron-based material and an aluminum-based material in a transportation field such as an automobile and a railway vehicle, a machine part, a building structure, and the like.

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

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

  Thus, many studies have been made on spot welding methods for these different types of joined bodies. For example, a method of inserting an aluminum-steel clad material between an aluminum material and a steel material has been proposed (see Patent Documents 1 and 2). In addition, methods of plating or inserting a metal having a low melting point on the steel material side have been proposed (see Patent Documents 3, 4, and 5). Furthermore, a method of sandwiching insulator particles between an aluminum material and a steel material (see Patent Document 6), a method of providing unevenness on a member in advance (see Patent Document 7), and the like have been proposed.

Japanese Patent Laid-Open No. 6-63763 (full text) Japanese Patent Laid-Open No. 7-178563 (full text) JP-A-4-251676 (full text) JP 7-24581 A (full text) Japanese Patent Laid-Open No. 4-14383 (full text) Japanese Patent Laid-Open No. 5-228643 (full text) JP 9-174249 A (full text)

  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.

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

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 steel material having a plate thickness t ₁ of 0.3 to 2.5 mm and an aluminum plate having a plate thickness t ₂ of 0.5 to 2.5 mm. This is a dissimilar material joined by spot welding, with an Al ₅ Fe ₂ compound layer on the steel material side and an Al ₃ Fe compound and Al ₁₉ Fe ₄ Si ₂ on the aluminum material side at the weld interface of the joined material. Each having a layer with an Mn-based compound, and the average thickness in the nugget depth direction within the range of the nugget center ± 0.1 mm of the layer of the Al ₃ Fe-based compound and the Al ₁₉ Fe ₄ Si ₂ Mn-based compound. Is 0.5 to 10 μm.

  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.

  Also, the nugget area is related to the amount of heat input. The higher the amount of current and the longer the time, the larger the nugget area. Therefore, in general, bonding with high bonding strength is achieved by controlling the nugget diameter by the amount of heat input during spot welding. Get the body. 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 melts, and as a result, a brittle intermetallic compound layer of Al-Fe system is formed at the interface.

The intermetallic compound formed by spot welding of steel and aluminum is roughly divided into two layers, with Al ₅ Fe ₂ compound on the steel side and Al ₃ Fe or Al ₁₉ Fe ₄ Si ₂ Mn on the aluminum side. It is known that compounds form. Since these intermetallic compounds are very brittle, it has been conventionally considered that high bonding strength cannot be obtained.

  The present inventor, when joining different materials of 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, In addition, it was considered necessary to control the thickness and structure of the interface reaction layer at the bonding interface.

Therefore, as a result of investigating the thickness of the interfacial reaction layer on the bonding strength in detail, it was found that the behavior of the interfacial reaction layer is significantly different from the conventional knowledge that the thinner the better. That is, the thickness and area of the Al ₅ Fe ₂ compound layer on the steel material side and the layer of Al ₃ Fe compound and Al ₁₉ Fe ₄ Si ₂ Mn compound on the aluminum material side constituting the interface reaction layer It has been found that if the relationship is controlled within the optimum range, the bonding strength increases even if the interface reaction layer is composed of this two-layer intermetallic compound.

More specifically, as a structure of the interface reaction layer, in particular, a layer of an Al ₃ Fe compound and an Al ₁₉ Fe ₄ Si ₂ Mn compound on the aluminum material side with respect to the Al ₅ Fe ₂ compound layer on the steel material side. By controlling the size (thickness in the nugget depth direction) to the optimum range and forming a layer of Al ₃ Fe compound and Al ₁₉ Fe ₄ Si ₂ Mn compound controlled to this optimum range in a large area It has been found that the bonding strength can be dramatically improved.

  As described above, the present invention forms an interfacial reaction layer in the optimum thickness range on the aluminum material side in a large area while obtaining a relatively large nugget area when joining different materials by spot welding. Thereby, a large nugget area can also be obtained, and the joining strength of the dissimilar material joined body can be improved. As a result, spot welding with high joint strength can be performed in a heterogeneous joint of steel and aluminum without using another material as in the prior art and without requiring a separate process. The effect that can be achieved.

(Heterogeneous)
FIG. 1 is a cross-sectional view of a heterogeneous bonded 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. 4 is an oxide film on the surface of the steel material 1. Reference numeral 5 denotes a nugget having a weld interface (interface reaction layer) 6 in spot welding, and has a nugget diameter indicated by an arrow in the horizontal direction in the figure. t ₁ represents the thickness of the steel material, t ₂ represents the thickness of the aluminum material 2, and Δt represents the minimum remaining thickness of the aluminum material after joining by spot welding. FIG. 1 shows a joining state in which the generation of dust is suppressed while maintaining the minimum remaining plate thickness of the aluminum material while the nugget diameter is secured, and further the melting of the steel material is minimized. The joined body is also joined as shown in this figure.

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

(Steel thickness)
In the present invention, it is necessary that the steel sheet has a thickness t ₁ of 0.3 to 2.5 mm. When the thickness t _{1 of the} steel material is less than 0.3 mm, the strength and rigidity necessary for the structural member and structural material described above cannot be secured, which is inappropriate. In addition, since the steel material is largely deformed by pressurization by spot welding and the oxide film is easily destroyed, the reaction with aluminum is promoted. As a result, an intermetallic compound is easily formed. On the other hand, when the thickness exceeds 2.5 mm, other joining means are employed as the above-described structural member or structural material, so that there is little need to join by spot welding. For this reason, it is not necessary to increase the thickness t _{1 of the} steel material beyond 2.5 mm.

(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, the tensile strength of the steel material is preferably 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: 1 to 2.5%, Si: 0.5 to 1.5%, Cr: 0 to 1%, Mo : 0 to 0.2%, Nb: 0 to 0.1%, V: 0 to 0.1%, Ti: 0 to 0.1%, preferably 1 or 2 or more types are preferably selectively contained. And it is preferable that the remainder composition of these steel materials consists of Fe and an unavoidable impurity.

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

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

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

  The intermetallic compound defined in the present invention is shown in FIGS. 3 and 4, which are cross-sectional views of the weld interface 6 in the cross section of the joint part of the dissimilar material joint. FIG. 3 is a diagram schematically showing a cross-sectional micrograph of the weld interface 6 of the joint cross section of FIG.

As shown in FIG. 3, at the welding interface 6, a layered Al ₅ Fe ₂ compound layer is formed on the steel material side, and granular or needle-shaped Al ₃ Fe compound and Al ₁₉ Fe ₄ Si ₂ Mn compound are formed on the aluminum material side. Each having a mixed layer.

In the layer where the Al ₃ Fe-based compound aluminum material side and Al ₁₉ Fe ₄ Si ₂ Mn-based compound are mixed in FIG 3, the nugget center ± 0.1mm to l ₁ indicating the length of the nugget depth direction shown by the dashed line The average thickness in the nugget depth direction within the range is shown. Further, S ₁ shown in the plane direction at the joint exhibits an area of _{(Al 3 Fe + Al 19 Fe} 4 Si 2 Mn) compound (area of the plane direction of dissimilar materials bonded body junction) of this thickness range.

In Al ₅ Fe ₂ compound layer formed in layers on the steel material side of FIG. 3, the l ₂ indicated by the length of the nugget depth direction in the range of nugget center ± 0.1mm indicated by the dashed line, a nugget depth direction of the Indicates the average thickness. Also, S ₂ shown in the planar direction at the junction indicates the area of the Al ₅ Fe ₂ compound layer of this thickness range (area of a plane direction of dissimilar materials bonded body junction).

(Compound layer on the aluminum material side)
First, in the present invention, in order to increase the bonding strength, the average thickness in the nugget depth direction within the range of the nugget center ± 0.1 mm of the layer of the Al ₃ Fe compound and the Al ₁₉ Fe ₄ Si ₂ Mn compound. Let l _{1 be} in the range of 0.5-10 μm.

The Al ₃ Fe-based compound and the Al ₁₉ Fe ₄ Si ₂ Mn-based compound are intermetallic compounds formed on the aluminum material side, and are formed in a granular or needle shape as shown in FIGS. In the center (nugget center), the size of individual compound grains (or the length of needle-shaped compound grains) is large, and gradually toward the end of the nugget (left and right in Figs. 3 and 4), the grains and needles gradually Decrease in size and distribution. At this end, the density of the compound grains is small and the compound grains are scattered, but they exist in a larger area than the Al ₅ Fe ₂ compound on the steel material side.

Such Al ₃ Fe-based compounds and Al ₁₉ Fe ₄ Si ₂ Mn-based compounds have a wedge (anchor) effect, including the effects of the above shapes, and adhesion between aluminum materials and Al ₅ Fe ₂ -based compound layers Improve the bonding strength.

Such an effect cannot be exhibited if the layer of the Al ₃ Fe-based compound and the Al ₁₉ Fe ₄ Si ₂ Mn-based compound is too thin. In particular, when l ₁ is less than 0.5 μm, the wedge effect is insufficient, the adhesiveness with the Al ₅ Fe ₂ compound layer is poor, the interlaminar fracture tends to occur, and the fracture occurs at a smooth interface. Therefore, in the present invention, the average thickness l ₁ in the nugget depth direction within the range of the nugget center ± 0.1 mm of the Al ₃ Fe based compound and the Al ₁₉ Fe ₄ Si ₂ Mn based compound layer is 0.5 μm or more. To do.

On the other hand, if the Al ₃ Fe-based compound and the Al ₁₉ Fe ₄ Si ₂ Mn-based compound layer grow too much and the layer is formed too thick, the individual compound grains become the starting point of destruction. In particular, this tendency becomes remarkable when l ₁ exceeds 10 μm. Therefore, in the present invention, the average thickness l ₁ in the nugget depth direction within the range of the nugget center ± 0.1 mm of the Al ₃ Fe-based compound and the Al ₁₉ Fe ₄ Si ₂ Mn-based compound layer is set to 10 μm or less. .

(Compound layer on the steel material side)
In the present invention, in order to further increase the bonding strength after satisfying the intermetallic compound layer condition on the aluminum material side, the average thickness l ₂ in the nugget depth direction of the Al ₅ Fe ₂ compound layer is 0.5. The area S ₂ of the portion in the range of ˜5 μm is preferably 10 × t ₂ ^0.5 mm ² or more, more preferably 20 × t ₂ ^0.5 mm ² or more.

Similarly, in order to further increase the bonding strength, the average thickness l ₂ in the nugget depth direction within the range of the nugget center ± 0.1 mm of the Al ₅ Fe ₂ based compound layer is in the range of 0.5 to 5 μm. It is preferable.

(Correlation between the compound layer on the aluminum material side and the compound layer on the steel material side)
In addition to the individual provisions of the compound layer on the aluminum material side and the compound layer on the steel material side as described above, in order to further increase the bonding strength, the correlation between the compound layer on the aluminum material side and the compound layer on the steel material side Is preferably defined.

That is, at the weld interface of the joined body, the Al ₅ Fe ₂ -based compound layer having an average thickness l ₂ in the range of 0.5 to 5 μm and an Al ₃ Fe system in which the average thickness l ₁ is in the range of 0.5 to 10 μm. compound and Al ₁₉ Fe ₄ Si ₂ layers of Mn-based compound is present and the area S ₁ of the layer of the Al ₃ Fe-based compound and Al ₁₉ Fe ₄ Si ₂ Mn-based compound of the average thickness range is 15 Xt ₂ ^0.5 mm ² or more, more preferably 25 × t ₂ ^0.5 mm ² or more.

The larger the area S ₂ of the layers of the Al ₃ Fe compound and the Al ₁₉ Fe ₄ Si ₂ Mn compound in the average thickness range, the higher the bonding strength. When the area S ₁ is less than 15 × t ₂ ^0.5 mm ^{2 and the} strength is the same, the larger the nugget joining area, the higher the possibility that the fracture load (joining strength) of the joint will decrease. On the other hand, when the joint area of the nugget is small, the joint part is also likely to break at a lower load.

When the area S ₁ of the layer of the Al ₃ Fe compound and the Al ₁₉ Fe ₄ Si ₂ Mn compound in the average thickness range is 25 × t ₂ ^0.5 mm ² or more, a joint having a high bonding force ( Bonding interface) Since the area is sufficiently large, the breaking load is larger. 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.

  Although the above-mentioned regulation of the area of the interface reaction layer with the optimum thickness is from the viewpoint of bonding strength, the interrelation between the compound layer on the aluminum material side and the compound layer on the steel material side is determined as the optimum thickness and the optimum area of the interface reaction layer. From this, control is performed within the optimum range. 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 positive it is. Then, as described above, 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.

(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 rounded in the thickness direction outside the range where the Al ₃ Fe-based compound layer exists). Break and break). In other words, such a fractured form of the joint represents the high joint strength of the present invention.

On the other hand, when the bonding strength is low as in the prior art, the fracture occurs at the bonding interface, the layer of wedge-shaped Al ₃ Fe compound and Al ₁₉ Fe ₄ Si ₂ Mn compound is broken, and the Al ₅ Fe ₂ compound layer Fracture occurs between layers of an Al ₃ Fe compound and an Al ₁₉ Fe ₄ Si ₂ Mn compound. In other words, such a fracture form of the joint portion represents a low joint strength.

(Factors affecting bonding strength)
The contribution of each factor that works on the bonding strength described above will be rearranged again.
In order to improve the bonding strength, the effect of the average thickness l ₁ of the Al ₃ Fe-based compound and the Al ₁₉ Fe ₄ Si ₂ Mn-based compound is particularly high, and the average thickness l ₂ is in the optimum range (0.5 to The effect of the area S ₂ of the Al ₅ Fe ₂ -based compound layer in the ₅ μm portion is also high.

As described above, the average thickness l ₁ of the Al ₃ Fe-based compound and Al ₁₉ Fe ₄ Si ₂ Mn-based compound provides adhesion between the aluminum material and the Al ₅ Fe ₂ based compound layer due to the wedge effect. Therefore, it contributes to the improvement of the bonding strength at the center of the nugget. However, the average thickness l ₁ only control of the layers of Al ₃ Fe-based compound with Al ₁₉ Fe ₄ Si ₂ Mn based compound, the Al ₃ Fe-based compound with Al ₁₉ Fe ₄ Si ₂ Mn compounds from ambient There is a possibility that the interface between the layer and the Al ₅ Fe ₂ -based compound layer peels off. Therefore, the breaking load may not be so high.

On the other hand, the area S ₂ of the Al ₅ Fe ₂ -based compound layer where the average thickness l ₂ is in the optimum range (0.5 to ₅ μm) contributes to a wide range of stable adhesion, and the entire spot joint breaks. Has the effect of increasing the load. However, stable strength cannot be obtained by controlling only the area S _{2 of the} Al ₅ Fe ₂ -based compound layer, and the variation in strength is large.

Therefore, in addition to controlling only the average thickness l _{1 of} the Al ₃ Fe-based compound and the Al ₁₉ Fe ₄ Si ₂ Mn-based compound, the area S ₂ of the Al ₅ Fe ₂ based compound layer is controlled. Ensures a high bond strength.

Furthermore, the average thickness l ₂ in the nugget depth direction within the range of the nugget center ± 0.1 mm of the Al ₅ Fe ₂ compound layer, and the Al ₃ Fe compound and Al ₁₉ Fe ₄ Si ₂ Mn system in the optimum thickness range The area S ₁ of the layer with the compound also contributes to the improvement of the bonding strength, but the effect of improving the bonding strength is smaller than the above two factors.
However, even if these factors are each employed alone, the effect of improving the bonding strength is small, but the average thicknesses l ₁ and Al _{5 of the} Al ₃ Fe compound and the Al ₁₉ Fe ₄ Si ₂ Mn compound are small. In addition to controlling the area S ₂ of the Fe ₂ compound layer, the highest bonding strength can be obtained when these controls are performed.

(Method for measuring intermetallic compounds)
In the present invention, an Al ₃ Fe compound and an Al ₁₉ Fe ₄ Si ₂ Mn compound layer or an Al ₅ Fe ₂ compound layer is a HAADF-STEM image (10,000 to 20,000 times) of the cross section of the joint. The semi-quantitative analysis by EDX point analysis is carried out in Fig. 4, and the structure photograph is detected as shown in Fig. 4. 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.

These intermetallic compounds are identified by measuring the composition of each measurement point 1-1 to 1-24 on the interface of the joint shown in FIG. 4 in the semi-quantitative analysis described above to obtain Al, Fe, Si, Mn, Mg (at %) Is identified by the composition. That is, when the Al amount is 73 to 95 at%, the Fe amount is 5 to 25 at%, and the Si amount is less than 2 at%, the “Al ₃ Fe compound” is obtained. Further, in the same analysis, when the Al amount is 70 to 78 at%, the Fe amount is 10 to 30 at%, and the Si amount is 2 to 15 at%, “Al ₁₉ Fe ₄ Si ₂ Mn compound” is obtained. Furthermore, when the Al amount is 60 to 73 at%, the Fe amount is 25 to 35 at%, and the Si amount is less than 2 at%, the “Al ₅ Fe ₂ compound” is obtained.

  The HAADF-STEM method (High Angle Annular Dark Field Scanning Transmission Electron Microscope) is a technique 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 of the interfacial reaction layer is roughly calculated by SIM after cutting at the nugget central part of the bonded body, embedding in a resin so that the cross section can be observed, and mirror polishing. taking measurement. Then, inside and outside of the nugget center and the portion inside the layer where the Al ₅ Fe ₂ compound is seen, the inside and outside of the layer where the Al ₃ Fe compound and the Al ₁₉ Fe ₄ Si ₂ Mn compound are seen The TEM observation can be performed using the focused ion beam processing device (FB-2000A) manufactured by Hitachi, Ltd., when the length in the depth direction of each compound and the layer seen as the compound exceeds the upper limit. The sample is thinned by applying FIB processing to a suitable thickness and used as a sample for observation and analysis.

Using a JEOL field-emission transmission electron microscope (JEM-2010F) equipped with a HAADF detector, observation was performed at an acceleration voltage of 200 kV in a field of view of 100 μm (10,000 to 20,000 times). EDX point analysis is performed for all of the above, and the layer of the Al ₃ Fe compound and the Al ₁₉ Fe ₄ Si ₂ Mn compound and the Al ₅ Fe ₂ compound layer are identified.

Al ₃ Fe-based compound and Al ₁₉ Fe ₄ Si ₂ Mn-based compound in the depth direction of the thickness (length) from the HAADF-STEM image of the resulting field 100 [mu] m, and all Al ₃ Fe-based compound Al ₁₉ The lengths in the depth direction of grains and needles identified as Fe ₄ Si ₂ Mn compounds were measured and averaged.

The thickness (length) of the Al ₅ Fe ₂ -based compound layer in the depth direction was measured by averaging five points from the same image. The above measurement was performed on all the samples for observation and analysis. In spot welding joining using a dome-shaped tip, the thickness of both compounds from the center decreased as they reached the end. Therefore, the diameter of the point (part) exceeding the upper limit of the length in the depth direction of each compound and the diameter of the point lower than the lower limit were determined and converted into an area that would be the optimum thickness range of each compound.

(Nugget area)
The area of the spot welded nugget 5 in FIG. 1 is preferably spot-bonded so as to be in the range of 20 × t ₂ ^0.5 to 70 × t ₂ ^0.5 mm ² in relation to the thickness t ₂ of the aluminum material. . In other words, it is preferable to select the spot welding conditions so that the nugget area is in the range of 20 × t ₂ ^0.5 to 70 × t ₂ ^0.5 mm ² .

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. Adequate bonding strength is achieved by spot bonding so that the area of the nugget 5 in the spot weld zone is in the range of 20 × t ₂ ^0.5 to 70 × t ₂ ^0.5 mm ² in relation to the thickness t ₂ of the aluminum material. In addition, it is excellent in workability and economy.

  In the case of joining different kinds of metal materials as in the present invention, the optimum nugget diameter depends on the plate thickness on the aluminum material side, and is characterized in that the influence of the plate thickness of the steel material is so small that it can be ignored.

If the nugget area is less than 20 × t ₂ ^0.5 mm ² , more strictly less than 30 × t ₂ ^0.5 mm ² , the nugget area is too small and the bonding strength is insufficient. Also, if the nugget area exceeds 70 × t ₂ ^0.5 mm ² , it is sufficient to obtain bonding strength, but dust is easily generated and the amount of thinning of the aluminum material is large. To do. Therefore, the nugget area is in the range of 20 × t ₂ ^0.5 to 70 × t ₂ ^0.5 mm ² , preferably in the range of 30 × t ₂ ^0.5 to 70 × t ₂ ^0.5 mm ² .

(Measurement of nugget area)
The nugget area in the present invention is obtained by measuring the area of the interface where the steel material-aluminum material is joined. The method for measuring the area of the bonding interface can be obtained by image-analyzing the aluminum material side separated by peeling or cutting at the bonding interface and measuring the area of the nugget. When the nugget shape is substantially circular, the joint may be cut and observed with an optical microscope from a cross section, and the diameter at the interface of the nugget formed may be measured to obtain the area. 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 thickness Δt of this aluminum material can be obtained by observing the cross section with an optical microscope or SEM, measuring the thickness reduction thickness, and taking the difference from the original thickness.

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

Below, each condition of spot welding for obtaining the dissimilar joined body of the present invention will be described.
(Pressure)
Regarding the pressure applied during spot welding, in order to obtain the relatively large required nugget area and the required area for the optimum interfacial reaction layer, and between the Al ₃ Fe compound and the Al ₁₉ Fe ₄ Si ₂ Mn compound, average thickness l _1, Al ₅ Fe ₂ compound layer of nugget center ± 0.1mm nugget depth direction of the average thickness of l ₂ within the scope of the layers, Al ₃ Fe-based compound of the optimum thickness range and Al ₁₉ Fe ₄ Si ₂ area S ₁ of the layer of the Mn compound, Al ₅ Fe ₂ compound layer area S _2, performs control such as, the to within the optimum range defined in the present invention, relatively high It is necessary to apply a pressing force.

Specifically, in relation to the thickness t ₂ of the aluminum material is selected from a relatively high pressure range of ^{_{2 × t 2 0.5 kN~4 × t}} 2 0.5 kN. 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 2 × t ₂ ^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, the average thickness l _{1 of} the Al ₃ Fe compound and Al ₁₉ Fe ₄ Si ₂ Mn compound layer, and the average nugget depth direction within the range of the nugget center ± 0.1 mm of the Al ₅ Fe ₂ compound layer There is a high possibility that the thickness l ₂ , etc. cannot be obtained.

On the other hand, when the applied pressure is increased, the nugget area tends to decrease.When the applied pressure exceeds 4 × t ₂ ^0.5 kN, a current exceeding the following optimum current is required to obtain the desired nugget area. Bonding strength is lowered because of generation of dust and growth of an interface reaction layer. 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)
Regarding the current at the time of spot welding, in order to obtain the relatively large required nugget area and the required area for the optimum interface reaction layer, the Al ₃ Fe compound and the Al ₁₉ Fe ₄ Si ₂ Mn compound average thickness l _1, Al ₅ Fe ₂ compound layer of nugget center ± 0.1mm nugget depth direction of the average thickness of l ₂ within the scope of the layers, Al ₃ Fe-based compound of the optimum thickness range and Al ₁₉ Fe ₄ Si ₂ area S ₁ of the layer of the Mn compound, Al ₅ Fe ₂ compound layer area S _2, performs control such as, the to within the optimum range defined in the present invention, relatively high It is necessary to pass the current for a short time.

Specifically, a relatively high current of 15 × t ₂ ^0.5 to 30 × t ₂ ^0.5 kA is allowed to flow for a short time of 100 × t ₂ ^0.5 msec or less in relation to the thickness t ₂ of the aluminum material. is necessary. 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 15 × t ₂ ^0.5 kA, strictly less than 18 × t ₂ ^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 30 × t ₂ ^0.5 kA, extra equipment is required, which is disadvantageous in terms of work and cost. Therefore, the current is 30 × t ₂ ^0.5 kA or less from these points. Therefore, the working current is in the range of 15 × t ₂ ^0.5 to 30 × t ₂ ^0.5 kA, preferably 18 × t ₂ ^0.5 to 30 × t ₂ ^0.5 kA.

(Energization time)
The energization time is set to a relatively short time of 100 × t ₂ msec in relation to the thickness t ₂ of the aluminum material. When the energization time is longer than 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. As described above, in order to control the interface reaction layer, the energization time is set to 100 × t ₂ msec or less, preferably 20 × t ₂ ^0.5 msec to 80 × t ₂ ^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.

(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, in relation to the plate thickness ^{_{t 2, 2.5 × t 2 0.5}} kN~4 × t 2 0.5 to pressure applied in kN, and ^{_{18 × t 2 0.5 ~30 × t}} 2 0.5 kA It is preferable to flow a current of 100 × t ₂ ^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. Further, as long as the nugget area and the area of the interface reaction layer having the optimum thickness are not inhibited, a condition with a lower current may be added before and after the spot welding condition to form a multi-stage current pattern.

  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. 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 a 980 MPa class high strength steel plate.

  As for the aluminum material, two types of commercially available A6022 (6000 series) aluminum alloy plates having a thickness of 1.0 mm (Tables 3, 4, 6) and 1.6 mm (Table 5) were used.

  These steel plates (steel materials) and aluminum alloy plates (aluminum materials) were processed into the shape of a cross tensile test piece described in JIS A 3137 and spot-welded under the conditions shown in Table 2 to prepare dissimilar joints.

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. Then, the welding pressure, welding current, and time were set according to the thickness t ₂ of the aluminum material, and one point was welded under the conditions shown in Table 2.

  The electrode tips are all dome-shaped made of Cu-Cr alloy, and each has three shapes of 50mmR-12mmφ (Table 3), 120mmR-12mm φ (Tables 4 and 5), and 150mmR-12mm φ (Table 6). It was. The anode was an aluminum material and the cathode was a steel material, and the electrode tips on both sides were the same in each welding example.

Further, as for the thickness reduction of the aluminum material, the minimum remaining plate thickness Δt was 50% or more of the original thickness t ₂ in common with each example.

  Attached to each of the manufactured joined bodies, the thickness in the depth direction and the area of the optimum thickness range of each compound were measured by the measurement method described above. These results are shown in Tables 3-6.

  As an evaluation of the bonding strength of each bonded body, a cross tensile test of different bonded bodies was performed. 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.

  Note that the cross tension test was used for strength evaluation in this example because the shear tension test had a greater difference between the test conditions. The tendency of the shear tensile test was consistent with the result of the cross tensile test, and all of the samples that were evaluated as ◯ or ◎ in the cross tensile test had a high shear strength of 2.5 kN or more.

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

In each table of Tables 3-8, within the range of the nugget center ± 0.1 mm of the layer of the Al ₃ Fe compound and the Al ₁₉ Fe ₄ Si ₂ Mn compound, which is the intermetallic compound layer condition of the present invention, It can be seen that each invention example in which the average thickness l ₁ in the nugget depth direction is in the range of 0.5 to 10 μm increases the bonding strength of the dissimilar bonded body.

Furthermore, in addition to controlling the average thickness l _{1 of} the Al ₃ Fe-based compound and Al ₁₉ Fe ₄ Si ₂ Mn-based compound and the area S ₂ of the Al ₅ Fe ₂ -based compound layer, the Al ₅ Fe ₂ based The average thickness l ₂ in the nugget depth direction within the range of the nugget center ± 0.1 mm of the compound layer, and the layer area S of the Al ₃ Fe compound and the Al ₁₉ Fe ₄ Si ₂ Mn compound in the optimum thickness range _The example of the invention controlled with ₁ has the highest bonding strength compared to the other examples. The invention examples E, F, and G in Tables 4 to 6 correspond to this.

On the other hand, in each table of Tables 3-8, each comparative example that does not satisfy the intermetallic compound layer condition of the present invention has low bonding strength of the dissimilar bonded body. In each of Comparative Examples in Tables 3 to 8, the average thickness l ₁ of the layer of the Al ₃ Fe compound and the Al ₁₉ Fe ₄ Si ₂ Mn compound is outside the upper limit or lower limit of the range.

In each of Tables 3 to 8, the nugget area often deviated from the above recommended range in the comparative example. For example, 50mmR-12mmφ (Table 3) is all in the range of 12 × t ₂ ^{0.5 to} 19 × t ₂ ^0.5 mm ² , and 120mmR-12mm φ (Tables 4 and 5), 150mmR-12mm φ (Table 6) In condition B, all were in the range of 17 × t ₂ ^{0.5 to} 20 × t ₂ ^0.5 mm ² and the nugget area was small. Also, in condition C of 120mmR-12mmφ (Tables 4 and 5) and 150mmR-12mmφ (Table 6), both are in the range of 72 × t ₂ ^{0.5 to} 89 × t ₂ ^0.5 mm ² , and the nugget area is reversed It was big.

In contrast, in all of the inventive examples, the nugget area was within the above-mentioned recommended range. As described above, in the present invention, it is desirable that the nugget area be in the range of 20 × t ₂ ^0.5 to 70 × t ₂ ^0.5 mm ² .

However, the condition A of 120 mm R-12 mm φ (Tables 4 and 5) and 150 mm R-12 mm φ (Table 6), which are comparative examples, is in the range of 28 × t ₂ ^{0.5 to} 38 × t ₂ ^0.5 mm ² . Despite the nugget diameter almost equivalent to the conditions D, E, and G, which are the invention examples in the table, the bonding strength was low.

  From the above, in order to obtain a high bonding strength, it is necessary to form a certain nugget area, but in addition to that, it is possible to control the thickness and structure of the interface reaction layer at the bonding interface. It can also be seen that it is necessary.

Therefore, the critical significance of each requirement prescribed | regulated by this invention and a preferable requirement is understood from the result of these Examples.

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. FIG. 5 is an explanatory diagram schematically showing FIG. 4. It is a drawing substitute photograph which shows the cross-sectional structure | tissue of the welding interface of the dissimilar-junction junction part cross section of this invention.

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 (6)

And a steel plate thickness t ₁ is 0.3 2.5 mm, an aluminum material thickness t ₂ is 0.5 2.5 mm and a dissimilar materials bonded body formed by bonding by spot welding, the weld interface of the joined body, steel Al ₅ Fe ₂ compound layer on the side, and Al ₃ Fe compound and Al ₁₉ Fe ₄ Si ₂ Mn compound layer on the aluminum material side, respectively, and this Al ₃ Fe compound and Al ₁₉ Fe ₄ Si A dissimilar material joint of steel and aluminum, characterized in that the average thickness in the nugget depth direction within the range of the nugget center ± 0.1 mm of the layer of ₂ Mn compound is 0.5 to 10 μm. The area of the Al ₅ Fe ₂ -based compound layer in which the average thickness in the nugget depth direction is in the range of ^{0.5 to 5} μm in the range of 100 μm when the cross section including the nugget center is observed is 10 × t ₂ ^0.5 dissimilar materials bonded body of the steel and the aluminum material according to claim 1 mm ² or more. Steel and an aluminum material according to claim 2 the average thickness of the nugget depth direction of the Al ₅ Fe ₂ based compound layer is the area of the portion in the range of 0.5~5μm is 20 × t ₂ ^0.5 mm ² or more And dissimilar materials. The steel material and aluminum according to any one of claims 1 to 3, wherein an average thickness in the nugget depth direction in the range of the nugget center ± 0.1 mm of the Al ₅ Fe ₂ based compound layer is 0.5 to 5 µm. Dissimilar material joined body. In the weld interface of the joined body, the Al ₅ Fe ₂ based compound layer, the average thickness in the range of a viewing 100μm when observing the cross section including the nugget center in the region in the range of 0.5 to 5 [mu] m, including nugget center There is a layer of Al ₃ Fe compound and Al ₁₉ Fe ₄ Si ₂ Mn compound having an average thickness in the range of 0.5 to 10 μm in the range of 100 μm when the cross section is observed , and this average thickness range 5. The steel material and the aluminum material according to claim 1, wherein an area of a layer of the Al ₃ Fe compound and the Al ₁₉ Fe ₄ Si ₂ Mn compound is 15 × t ₂ ^0.5 mm ² or more. Dissimilar material joined body. Dissimilar between steel and an aluminum material according to claim 5 area of the layer is 25 × t ^{₂ 0.5} mm ₂ or more and the Al ₃ Fe-based compound having an average thickness range and Al ₁₉ Fe ₄ Si ₂ Mn-based compound Joined body.

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