JP5997895B2 - MIG welded joint structure - Google Patents

Description

  The present invention relates to a MIG welded joint structure, and more particularly to a joint structure obtained by lap MIG welding of a steel material and an aluminum material which are different metal materials.

  In recent years, from the viewpoint of protecting the global environment and saving energy, in applications such as automobiles, railway vehicles, and other transportation equipment structures, building structures, or machine parts, steel is used where rigidity is required, while weight reduction. However, where it is necessary to make an environmentally friendly structure by using an aluminum material and making it a so-called hybrid structure, in order to manufacture such a structure, Joining of a steel material and an aluminum material will be needed.

  Therefore, various studies have been made on the joining of different kinds of metal materials such as steel and aluminum, and many techniques such as a melting method and a solid phase joining method have been proposed. For example, Japanese Patent Application Laid-Open No. 2004-223548 is proposed for a general MIG welding method as a melting method, and Japanese Patent Application Laid-Open No. 2003-27576 for a general FSW (friction stir welding) as a solid phase bonding method. Publications and the like have been proposed.

  Among these proposed joining methods, the MIG welding method, which is a melting method, is the most common joining method and is well known as a relatively inexpensive joining method. As desired, it has not been easy to obtain a hybrid structure having good joint characteristics by joining steel and aluminum by such a MIG welding technique. That is, when MIG welding a steel material and an aluminum material, if the heat input by arc welding becomes too high, a brittle intermetallic compound is likely to be formed at the joint interface between them. There is a problem that cracks are likely to occur at the existence site, and conversely, if the heat input is too low, the wettability of the melt to the steel surface will deteriorate, and unbonded parts will be scattered at the joint interface. Thus, there is a problem that it is difficult to obtain a good joint. For this reason, a joint interface having a joint strength that can withstand use as a structure cannot be obtained, which hinders application of MIG welding.

JP 2004-223548 A JP 2003-275876 A

  Here, the present invention has been made in the background of such circumstances, the place to be solved is a joint structure obtained by MIG welding an overlapped fillet portion of an aluminum material and a steel material, Provided is a MIG welded joint structure that improves the strength of the joint by effectively improving the soundness of the welded part and that does not cause defects such as cracks and breaks in the welded part during processing such as bending. There is.

  Therefore, in order to solve the above problems, the present inventor has conducted various studies on joint structures obtained by MIG welding the overlapped fillet portion of an aluminum material and a steel material. As a result, the aluminum material and the steel material having a predetermined alloy composition are obtained. In the MIG welded joint structure obtained by using a 4000 series aluminum alloy as a filler material, the maximum thickness of the intermetallic compound layer at the joint interface between the bead and the steel material generated in the welded part, We found the fact that the layer thickness of the intermetallic compound at the end part had a significant effect on the occurrence of defects such as cracks and delamination that occurred in the welded part when bending was performed, and further studies were carried out. As a result, the present invention has been completed.

  That is, the present invention has been completed based on such knowledge, and the gist thereof is a 5000 series or 6000 series aluminum alloy having an Mg content of 3.0% by weight or less. A joint structure obtained by superimposing an aluminum material made of steel on a steel material and MIG welding the overlapped fillet portion using a 4000 series aluminum alloy as a filler material. The maximum thickness of the intermetallic compound layer formed at the bonding interface between the bead and the steel material is 0.5 to 10 μm, and the thickness of the intermetallic compound layer at the bead toe portion is 0.5 to 3 0.0 μm.

According to one of the preferred embodiments of the MIG welded joint structure according to the present invention, the intermetallic compound layer has a plurality of phases of Al—Fe-based intermetallic compounds, and the Fe ₂ Al ₅ phase thereof. However, it is configured so as to be a layer having a thickness not exceeding 6 μm at the bonding interface.

According to another preferred embodiment of the present invention, the intermetallic compound layer has a plurality of Al—Fe-based intermetallic phases, and an Fe ₂ Al ₅ phase of the intermetallic compound layer is 60 at the bonding interface. % Of the thickness of the layer.

  As described above, in the MIG welded joint between the aluminum material and the steel material according to the present invention, the intermetallic compound layer formed at the joint interface between the bead and the steel material generated at the welded portion obtained by MIG welding them. The maximum thickness is within the range of 0.5 to 10 μm, and the thickness of the intermetallic compound layer at the bead toe portion is configured to be 0.5 to 3.0 μm. The soundness of the welded portion can be effectively increased, and the joint strength can be advantageously improved. Even if such a welded joint is subjected to bending or the like, the welded portion can be welded. The occurrence of problems such as cracking and peeling at the site can be advantageously prevented or avoided.

In particular, in such a MIG welded joint structure, among the phases of a plurality of Al—Fe intermetallic compounds constituting the intermetallic compound layer, the phase that affects the properties of the bonding interface is the Fe ₂ Al ₅ phase. Thus, when it is present as a layer of a certain thickness or more, it has become clear that the properties of the resulting joint will be deteriorated. ₂ Al ₅ phase is configured to be a layer having a thickness not exceeding 6 μm and / or a layer having a thickness of 60% or less in the whole area of the intermetallic compound layer, It has become possible to more effectively prevent the occurrence of problems such as cracks and fractures in the weld during bending of the joint.

It is an isometric view explanatory drawing which shows an example of the MIG welding joint of the aluminum material and steel materials according to this invention. It is II-II cross-section expansion explanatory drawing in FIG. It is a cross-sectional enlarged explanatory drawing which shows an example of the welding part in the MIG welded joint of the aluminum material and steel materials according to this invention, (a) is explanatory drawing equivalent to the A section in FIG. 2, (b) is ( It is an expansion explanatory view equivalent to the B section in a). An Fe-Al based compound showing the results obtained by analyzing the layer structure of the intermetallic compound layer formed at the weld interface between the bead and the steel in the weld zone by high resolution EPMA element mapping and TEM electron diffraction identification. It is a distribution map of a phase. It is longitudinal cross-sectional explanatory drawing which shows 1 process of the MIG welding technique for obtaining the MIG welding joint according to this invention. It is a cross-sectional photograph of the sample obtained by performing the stress concentration type bending test with respect to the MIG welded joint according to Test Example 2 obtained in the example, and (a) is a photograph showing the entire cross-section of the sample. (B) is the photograph which expands and shows the welding site | part to which the bending process was given. It is a cross-sectional photograph of the sample obtained by carrying out the stress concentration type bending test with respect to the MIG welded joint according to Test Example 9 obtained in the example, and (a) is a photograph showing the whole sample. (B) is the photograph which expands and shows the welding site | part by which the bending process is given.

  Hereinafter, in order to clarify the MIG welded joint structure according to the present invention more specifically, representative embodiments of the present invention will be described in detail with reference to the drawings.

  First, FIGS. 1 and 2 schematically show an example of an MIG welded joint of an aluminum material and a steel material according to the present invention in the form of a perspective view and a longitudinal sectional view, respectively. In these drawings, the MIG welded joint 10 is configured so that the flat aluminum material 12 and the flat steel material 14 having different thicknesses are positioned above the aluminum material 12 at the respective end portions. Thus, the overlapped fillet portion is subjected to MIG welding (lap fillet weld) in the overlapped state to form the welded portion 16, thereby being integrated. That is, the MIG welding operation is performed on the end surface portion of the laminated aluminum material 12, and the end surface portion of the aluminum material 12 and the surface of the steel material 14 are melted together with the filler material, and there is a molten metal. The bead 18 which consists of is formed, and the welding part 16 is comprised.

  Here, as shown in FIG. 1, the aluminum material 12 and the steel material 14 are MIG welded over the entire length of the overlapped portion, and the welded portion 16 is formed by a bead 18 generated thereby. Along the end face of the aluminum material 12, the aluminum material 12 is formed so as to extend continuously over the entire length. Further, as shown in FIG. 2, the welded portion 16, specifically, the bead 18, has a substantially triangular cross section that connects two orthogonal surfaces, that is, the end surface of the aluminum material 12 and the upper surface of the steel material 14. It has a shape.

  Moreover, as a material of the aluminum material 12 positioned on the upper side of the two workpieces to be superposed on each other, the alloy number of JIS name is 5000 series (Al-Mg series) or 6000 series ( Al-Mg-Si-based) is selected from aluminum alloys, thereby ensuring formability and rigidity for obtaining a structure. And in this invention, among such aluminum alloy materials, the aluminum alloy whose Mg content is 3.0% (weight basis) or less is used. However, when the Mg content in the aluminum material 12 increases, the growth rate of the intermetallic compound at the bonding interface between the bead 18 and the steel material 14 increases, and a thick intermetallic compound layer is formed in a short time. Therefore, control of the Mg content in the aluminum alloy that gives the aluminum material 12 becomes important. In addition, it is because the thickness of the intermetallic compound layer produced in the joining interface of the bead 18 and the steel material 14 is likely to exceed 10 μm when the Mg content exceeds 3.0% by weight. .

  Furthermore, in the aluminum alloy in which the Mg content is 3.0% by weight or less, which gives the aluminum material 12 according to the present invention, as the 5000 series aluminum alloy, AA5005 (Mg: 0.50 to 1.1%) , AA5050 (Mg: 1.1 to 1.8%), AA5052 (Mg: 2.2 to 2.8%), etc. are preferably employed. As the 6000 series aluminum alloy, AA6016 (Mg:. 25-0.6%), AA6063 (Mg: 0.45-0.9%), AA6061 (Mg: 0.8-1.2%), etc. will be suitably employed.

  The thickness of the aluminum material 12 that gives the MIG welded joint 10 according to the present invention is not particularly limited as long as it is made of a specific aluminum alloy according to the present invention. The thickness is appropriately selected according to the required characteristics, but when the MIG welded joint 10 is used for a structure, the thickness is in the range of 0.5 mm or more and 3.0 mm or less. Is appropriately selected. This is because if the plate thickness of the aluminum material 12 is less than 0.5 mm, it is difficult to ensure the rigidity of the structure, and if it exceeds 3 mm, the heat for melting the plate material is excessive during MIG welding. This is because it becomes difficult to control the layer thickness of the intermetallic compound formed at the joint interface between the bead 18 and the steel material 14 in the welded portion 16. Moreover, even in the tempering of the aluminum material 12, it is appropriately determined according to the use, but generally, when using a 5000 series aluminum alloy material, the O material is preferably used, In the case of a 6000 series aluminum alloy material, a T4 material, a T6 material, or the like is preferably used.

  Further, the steel material 14 positioned on the lower side is not particularly limited with respect to the material, and can be appropriately selected according to the characteristics required for the intended joint, for example, mild steel. Alternatively, carbon steel, high-tensile steel, stainless steel, or the like can be used. It should be noted that such steel materials include hot galvanized (GI), alloyed hot dip galvanized (GA), aluminum alloy plated, electrogalvanized, etc., and conventionally known zinc or zinc alloy, aluminum or aluminum alloy surfaces. Even if the treatment is applied or not, it is good, but when using a steel material that has been subjected to such a surface treatment, due to the presence of the surface treatment layer formed on the steel material surface, A brittle intermetallic compound layer formed by the metallurgical reaction between aluminum and steel, with no arc or molten metal coming into immediate contact with the steel, thus effectively preventing steel penetration Therefore, the thickness of the intermetallic compound layer according to the present invention can be controlled more advantageously.

  Further, the shape of the aluminum material 12 or the steel material 14 that gives the MIG welded joint 10 as illustrated is not limited to a flat plate shape, and the overlapping portion on which the MIG welding operation is performed is at least a flat plate or thru plate. As long as it is in the form of a face plate, any of various shapes manufactured by a known method such as rolling, extrusion, forging or the like can be adopted. However, in general, a plate material, an extruded material, or an extruded shape material in which a welded portion has a flat plate shape or a face plate shape is advantageously used.

  In addition, when the overlapped fillet portion of the aluminum material 12 and the steel material 14 is MIG welded, a 4000 series aluminum alloy is used as the filler material with an alloy number of JIS name, and the intended welded portion 16 is used. Will be formed. Since such a 4000 series aluminum alloy has a relatively low melting point, it becomes easy to control the temperature of the melted portion. For this reason, the intended welded portion 16 and thus the bead 18 can be advantageously formed. It is. Moreover, the diameter of such a filler material is generally in the range of 0.8 mm or more and 1.6 mm or less. This is because if the diameter of the filler material becomes too thin, it becomes difficult to handle, and the MIG welding operation may be hindered, and a filler metal having a large diameter exceeding 1.6 mm is used. In addition, a higher heat is required to melt it, and since the temperature of the droplets is higher, it becomes difficult to cool, and the thickness of the intermetallic compound layer formed at the joint interface between the bead 18 and the steel material 14 is controlled. It is difficult to do. In addition, as a 4000 series aluminum alloy which provides this filler material, the thing of materials, such as AA4043 and AA4047, can be mentioned, for example.

  Moreover, the above-described aluminum material 12 and the steel material 14 are overlapped, and the overlapped fillet portion is MIG welded using the above-described filler material, so that the end portion of the aluminum material 12 and the surface of the steel material 14 are between. When the welded portion 16 is formed and joined to each other, a brittle metal of Al and Fe is formed at the joining interface between the bead 18 made of the weld metal of the welded portion 16 and the steel material 14 as shown in FIG. An intermetallic compound layer 20 is formed. This intermetallic compound layer 20 has a thickness as shown in FIG. 3 according to local changes in the welding state in the direction (the left-right direction in FIG. 3) in which the welded portion 16 extends from the aluminum material 12 onto the surface of the steel material 14. Will change.

And when this inventor variously examined about the thickness of this intermetallic compound layer 20, in order to improve the soundness of the welding part 16 and to ensure the outstanding joint strength, the thickness of this intermetallic compound layer 20 is sufficient. In particular, it has become clear that the maximum thickness (t) shown in FIG. 3 (a) needs to be configured within the range of 0.5 to 10 μm. is there. If the maximum thickness (t) of the intermetallic compound layer 20 is less than 0.5 μm, there is no guarantee that the intermetallic compound layer 20 is continuously formed, and the bead 18 and the steel material 14 are not guaranteed. This is because unbonded portions may be formed in some places at the joint interface with each other, which makes it impossible to ensure the joint strength, and in the case of exceeding 10 μm, among the intermetallic compounds, Fe _This is because the thickness of the ₂ Al ₅ phase is increased, and problems such as breakage and cracking are caused from there.

Further, the intermetallic compound layer 20 formed at the joint interface between the bead 18 and the steel material 14 is allowed to exist at a predetermined thickness until reaching the toe end portion (left end portion in FIG. 3) of the bead 18. The joint strength can be improved and the occurrence of defects such as breakage and cracking can be prevented or avoided. That is, in the present invention, the thickness of the intermetallic compound layer 20 at the toe portion of such a bead 18 is configured to be 0.5 to 3.0 μm. Here, when the thickness of the intermetallic compound layer 20 in such a bead toe part is less than 0.5 μm, in the bead toe part, in the joint interface between the bead 18 and the steel material 14, unjoined parts are sometimes present. If the thickness exceeds 3.0 μm, the layer thickness of the Fe ₂ Al ₅ phase at the bead toe portion becomes thick, and there is a risk that it will easily break. . The thickness of the intermetallic compound layer 20 at the bead toe portion is an average thickness between the toe end (tip) of the bead 18 and a predetermined distance (d) as shown in FIG. In general, 100 μm is adopted as the predetermined distance (d), and it is obtained as an average value of the thicknesses at a plurality of locations, for example, five locations at equal distances.

Furthermore, due to the implementation of MIG welding, the intermetallic compound formed at the joint interface between the bead 18 and the steel material 14 is composed of a plurality of Al—Fe-based materials due to the material of the materials to be joined. From the bead 18 toward the steel material 14, the intermetallic compound layer 20 is formed in a layered form composed of an FeAl ₃ phase, an Fe ₂ Al ₅ phase, an FeAl phase, and an Fe ₃ Al phase sequentially. Such a layer structure of the intermetallic compound layer 20 can be easily recognized from the distribution diagram shown by the high-resolution EPMA element mapping shown in FIG. 4 and the identification result of the TEM electron diffraction. As a result of the study of the layer structure of such an intermetallic compound layer 20, the Fe ₂ Al ₅ phase layer therein is the most brittle and, therefore, the Fe ₂ Al ₅ phase layer breaks down. Therefore, in the present invention, the thickness of the Fe ₂ Al ₅ phase is 6 μm in the entire region of the intermetallic compound layer 20, in other words, at the joint interface between the bead 18 and the steel material 14. A configuration that does not exceed the range is preferably employed. This is because if the thickness of the Fe ₂ Al ₅ phase exceeds 6 μm, the Fe ₂ Al ₅ phase layer may be easily broken.

In addition, the Fe ₂ Al ₅ phase constituting the intermetallic compound layer 20 formed at the joint interface between the bead 18 and the steel material 14 is configured to have a layer thickness not exceeding 6 μm as described above. In addition, or alternatively, at any part of the intermetallic compound layer 20, that is, at the bonding interface between the bead 18 and the steel material 14, the thickness is 60% or less of the total thickness of the intermetallic compound layer 20. It will be configured. If the thickness exceeds 60%, the Fe ₂ Al ₅ phase layer may be easily broken.

Thus, by defining the layer thickness of the Fe ₂ Al ₅ phase in the intermetallic compound layer 20 formed at the joint interface between the bead 18 and the steel material 14 by the MIG welding operation, the soundness of the welded portion 16 is It is possible to further improve the strength of the joint, to realize excellent joint strength, and to more advantageously prevent the occurrence of problems based on defects such as breakage and cracking from the Fe ₂ Al ₅ phase layer. Can be solved.

  By the way, the MIG welded joint 10 according to the present invention as described above employs a known MIG welding technique to form the intermetallic compound layer 20 at a predetermined thickness at the joint interface between the bead 18 and the steel material 14 as described above. Therefore, it can obtain by welding the overlap fillet part of the aluminum material 12 and the steel material 14 so that appropriate heat input is implement | achieved.

  That is, in order to obtain the target MIG welded joint 10, first, as shown in FIG. 5, the plate-like aluminum material 12 and the plate-like steel material 14 are made of the aluminum material 12 at each end portion. It is preferably fixed by an appropriate restraining jig (not shown) so that it is superposed so as to be positioned above, and the superposed state is maintained and it does not move relatively. In such a fixed state, the MIG welding operation is performed on the end surface portion of the aluminum material 12.

  Specifically, when performing such an MIG welding operation, a welding wire 24 as a filler material, which is a consumable electrode, is protruded from the tip opening 30 of the nozzle 28 at a predetermined length. A similar MIG welder is used. In this MIG welding machine, the welding wire 24 is moved independently in the axial direction with respect to the nozzle 28 by a wire supply device (not shown). Is gradually supplied to the welding site side (downward). Further, in order to cut off the molten metal from the atmosphere, an inert gas 32 made of a mixed gas obtained by combining one or more inert gases such as argon gas, helium gas, neon gas and the like from the inside of the nozzle 28 during arc welding. (Indicated by a two-dot chain line in FIG. 5) is sprayed on the welded portion. Further, the welding wire 24 is connected to a positive pole side of a welding power source device (not shown) through a contact tip 34 to be a positive pole (anode), while a grounded workpiece (12, 14) is − It is connected to the pole side and serves as a negative pole (cathode).

  Then, by operating a welding power source device (not shown) and applying a predetermined current and voltage such as a direct current pulse current between the welding wire 24 and the workpieces (12, 14), such a welding wire is obtained. An arc 36 (indicated by a one-dot chain line in FIG. 5) is generated between the distal end portion 26 of the 24 and the material to be welded, while a nozzle 28 (welding wire 24) is moved along a predetermined surface along the end surface of the aluminum material 12. By relatively moving at a speed, MIG welding of the aluminum material 12 and the steel material 14 is advanced.

  At that time, the end surface portion of the aluminum material 12 is melted by the arc 36 generated between the workpieces (12, 14) and the welding wire 24, and the welding wire 24 is also melted. 38 moves on to-be-welded material, and the bead 18 which welds the aluminum material 12 and the steel material 14 with these molten aluminum (molten metal) will be formed.

  It should be noted that the maximum thickness of the intermetallic compound layer 20 and the thickness at the bead toe portion at the joint interface between the bead 18 and the steel material 14 in the welded portion 16 formed in such MIG welding operation are within a desired range. As a result, the welding conditions are appropriately adjusted and controlled so that proper heat input can be realized. Therefore, in general, the average current is 20 to 40 A, and the average voltage is 15 to 18 V. In addition to adopting the welding conditions of 40 cm / min to 100 cm / min, the following cooling means and the like will be employed.

That is, as one of the cooling means, for example, as shown in FIG. 5, a copper block is used for the backing 22 that supports the welded portion of the aluminum 12 and the steel material 14, and the inside of the copper block (22) is cooled. By flowing water or supplying liquid nitrogen to the back side of the steel material 14 and cooling the back side of the steel material 14 positioned on the lower side, the steel material 14 and the droplets 38 being welded are kept at 400 ° C. or less. In particular, the excessive growth of the Fe ₂ Al ₅ phase in the intermetallic compound layer 20 is suppressed by cooling to a low temperature.

  It is also effective to increase the flow rate of the inert gas 32, which is a shielding gas, and by increasing the flow rate from about 15 L / min to about 20 L / min to enhance the cleaning effect, the bead is particularly effective. It is also an effective means to improve the wettability of the 18 toe portions to the steel material 14.

  Thus, in MIG welding, the welding conditions are controlled, and the aluminum material 12 and the steel material 14 are welded under appropriate heat input conditions, whereby the intermetallic compound layer 20 according to the present invention is bonded to the bead 18 and the steel material. MIG welded joint 10 formed at the joint interface with No. 14 is advantageously obtained, thereby improving the soundness of the welded portion, realizing excellent weld quality, joint strength, bending work, etc. Even with the processing, it was possible to advantageously prevent or avoid the occurrence of defects such as cracks and breaks in the welded portion 16.

  The exemplary embodiments of the present invention have been described in detail above. However, the embodiments are merely examples, and the present invention is limited in any way by specific descriptions according to such embodiments. It should be understood that it is not interpreted.

  For example, in the above embodiment, as the aluminum material 12, the end surface constituting the overlapped fillet portion is configured as a surface perpendicular to the steel material 14, but the end portion of the aluminum material 12 is subjected to groove processing. Thus, it is possible to apply the present invention to an end surface having a predetermined angle with respect to the steel material 14.

  Further, even in the arrangement form of the nozzle 28 in the MIG welder, the center line of the nozzle 28 is inclined at a predetermined angle forward, backward, left and right with respect to the aluminum material 12 and the steel material 14 as in the prior art. It is also possible for the MIG welding operation to be carried out.

  In addition, although not enumerated one by one, the present invention is carried out in a mode to which various changes, modifications, improvements, etc. are added based on the knowledge of those skilled in the art. It should be understood that all belong to the scope of the present invention without departing from the spirit of the present invention.

  Some examples of the present invention will be shown below to clarify the present invention more specifically. However, the present invention is not limited by the description of such examples. Needless to say.

  First, various aluminum materials, steel materials, and welding wires (filler materials) shown in Table 1 below were prepared and used in combinations shown in Table 1 below. Note that the plate thickness of the aluminum material and the plate thickness of the steel material were 1 mm and 0.7 mm, respectively, and the wire diameter of the welding wire was 1.2 mm.

  In the combination of the aluminum material and the steel material shown in Table 1, after fixing the aluminum material on the steel material so that the overlap margin is 3 mm, as shown in FIG. A welding operation was performed. That is, as the MIG welding machine, a precision control type MIG welding machine equipped with the welding wire shown in Table 1 above is used, and the welding power source apparatus has a positive pole for the welding wire and a negative pole for the material to be welded. After the connection, a DC welding pulse current was passed to generate an arc between the workpieces and MIG welding was performed to obtain various MIG welded joints according to Test Examples 1-14. As shown in Table 2 below, the MIG welding conditions at that time varied the welding current, arc voltage, welding speed, and shielding gas flow rate, and when a copper block was used as the backing, The case where the cooling effect was improved by allowing water to flow or the case where the copper block was not used was selected and adopted, and the MIG welding operation was performed.

Next, each of the various MIG welded joints according to Test Examples 1 to 14 thus obtained is evaluated for bending characteristics, while high-resolution EPMA element mapping and identification of TEM electron diffraction are performed from the cut surface of the welded part. To determine the maximum thickness of the Fe ₂ Al ₅ phase in the intermetallic layer formation region, along with the maximum thickness (total) and the thickness at the bead toe (total). The results are shown in Table 3 below. In addition, the thickness of the intermetallic compound layer at the bead toe portion is obtained by calculating the thickness at five locations (at a length of d = 100 μm) at a position of every 20 μm from the bead toe portion, and is indicated by an average value thereof. Yes. In addition, for the characteristic evaluation of each welded joint, as a stress concentration type bending test, a 90 ° surface bending is performed by a push bending test using a former having a radius of curvature of 10 mm, and the occurrence of openings and cracks in the welded part. The test obtained by examining the presence or absence was adopted, and the obtained results are also shown in Table 3 below. Furthermore, the cross-sectional photograph of the sample of the welded joint which concerns on the test example 2 and the test example 9 obtained in this bending test was shown in FIG.6 and FIG.7, respectively.

  As is apparent from the results of Table 3, in any of the MIG welded joints according to Test Examples 1 to 8 according to the present invention, the maximum of the intermetallic compound layer formed at the joint interface between the bead and the steel material. The thickness is in the range of 0.5 to 10 μm, and the thickness of the intermetallic compound layer in the bead toe portion is in the range of 0.5 to 3.0 μm. Since the wettability of the steel surface can be advantageously increased while setting the thickness of the intermetallic compound layer as an appropriate value, even if a stress concentration type bending test is carried out, the welded part is Defects such as cracks are not caused, and problems such as opening of the welded part are not caused. The fact is shown in FIG. 6 that cross-sectional photographs of the MIG welded joint according to Test Example 2 subjected to the stress concentration type bending test are shown, and the fact is easily and clearly shown from these photographs. Can be recognized.

On the other hand, in the MIG welded joint according to Test Example 9, the maximum thickness of the intermetallic compound layer is 10 μm or more, and the maximum layer thickness of the Fe ₂ Al ₅ phase is 8 μm. According to the stress concentration type bending test, as shown in the photograph shown in FIG. 7, it is recognized that peeling / breaking is caused in the welded portion, and the bead toe portion is greatly opened. Further, in the welded joint according to Test Example 10, heat input is not sufficient in MIG welding, so that the wettability of the steel surface is deteriorated and the presence of an intermetallic compound layer at the bead toe portion is not recognized. For this reason, a problem arises in that peeling occurs at the bead toe portion, resulting in opening. Further, as in Test Examples 11 and 12, an aluminum material made of an aluminum alloy having a Mg content of 3% by weight or more is used, or as in Test Examples 12 to 14, an alloy alloy of the welding wire is 5000 series aluminum alloy. In such a case, the maximum thickness of the intermetallic compound layer becomes too thick, or the thickness of the intermetallic compound layer at the bead toe portion becomes inappropriate. As a result, problems such as opening at the toe end of the bead are caused.

DESCRIPTION OF SYMBOLS 10 MIG welded joint 12 Aluminum material 14 Steel material 16 Welded part 18 Bead 20 Intermetallic compound layer 22 Backing 24 Welding wire 26 Tip part 28 Nozzle 30 Tip opening part 32 Inert gas 34 Contact tip 36 Arc 38 Droplet

Claims (1)

An aluminum material made of a 5000-series or 6000-series aluminum alloy and having an Mg content of 3.0% by weight or less is superimposed on a steel material, and the overlapped fillet portion is used as a filler material to 4000 A joint obtained by joining the end surface portion of the aluminum material and the upper surface portion of the steel material by MIG welding using an aluminum alloy and forming a bead extending continuously along the end surface of the aluminum material Structure,
The maximum thickness of the intermetallic compound layer formed at the joint interface between the bead and the steel material generated in the welded portion is 0.5 to 10 μm, and the thickness of the intermetallic compound layer in the bead toe portion is The intermetallic compound layer has a plurality of Al—Fe-based intermetallic compound phases, and the Fe ₂ Al ₅ phase does not exceed 6 μm at the bonding interface. An MIG welded joint structure characterized by being configured to be a layer having a thickness and a layer having a thickness of 60% or less at the joint interface.