JP2013078804A - Dissimilar metal bonding method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a dissimilar metal bonding method, by which a dissimilar metal-bonded member can be efficiently produced which has high joint strength while being provided with an intermetallic compound layer on the bonding interface.SOLUTION: The method includes: carrying out bonding that has rapid heating and rapid cooling following the rapid heating when a dissimilar metal material is bonded by forming a compound layer in which a diffusion layer in a bonded portion partially contains at least one intermetallic compound; and thereafter carrying out heat treatment at temperature at which a main component metal in a low-melting-point-side metallic material eliminates dislocation loops and voids formed by aggregating atomic vacancies of a rapidly cooled composition in the bonded portion and at temperature equal to or lower than 1/2 of the melting point expressed by absolute temperature of the lowest-melting-point intermetallic compound among intermetallic compounds generated.

Description

  In the present invention, when both materials are heated and joined, different metal materials in which an intermetallic compound of both materials is generated at the joining interface, for example, a steel material and an aluminum alloy material, a steel material and a titanium alloy material, an aluminum alloy The present invention relates to a joining technique for joining materials and titanium alloy materials easily, inexpensively, and with high strength, and relates to a dissimilar metal joining method for obtaining a joining member having high joining strength.

  Regarding the joining of dissimilar metals as described above, for example, Non-Patent Document 1 discloses that in the dissimilar metal joining made of steel and aluminum material, Si and O are contained at appropriate concentrations in the intermetallic compound reaction layer generated at the joining interface. It has been shown that the presence of the present invention can suppress the overgrowth of the reaction layer and increase the bonding strength. By using a steel plate whose internal oxidation is adjusted to 1.5 μm, aluminum having a thickness of 1.6 mm is used. It is disclosed that a cross tensile strength of 1.4 kN at the maximum can be obtained in electrical resistance spot welding combined with an alloy plate (A6022).

In Non-Patent Document 2, when spot welding is performed by combining a 980 MPa class galvannealed steel plate material having a plate pressure of 1.2 mm and an aluminum alloy plate (A6022) having a plate thickness of 1.0 mm, two-stage energization is performed. To promote softening and melting of the plated layer, thereby generating a wedge-shaped intermetallic compound Al ₃ Fe ₂ in the reaction interface layer, which can provide a cross tensile strength as high as 1.2 kN. Have been described.

“Overview of National Welding Society”, Japan Welding Society, September 2005, 77th, p. 320-321 “Overview of the National Welding Society of Japan”, Japan Welding Society, April 2006, Vol. 78, p. 162-163

However, in the case of the method described in Non-Patent Document 1, it is necessary to adjust the composition of the alloy in order to control the structure of the compound at the bonding interface, so that the bonding condition range is narrow and the conditional restrictions are severe. was there.
Further, the method described in Non-Patent Document 2 has a problem that energization time becomes long because it is necessary to perform energization in two stages in two-layer spot joining.

  In this way, in joining conventional dissimilar metal materials with diffusion, in order to make the compound layer thin in order to increase the joint strength, industrially unsuitable inefficient joining conditions must be adopted. When the efficient conditions are employed, there is a conflicting problem that the strength cannot be ensured by forming a thick compound layer containing an intermetallic compound having a large particle size.

  The present invention has been made in view of the above-mentioned problems in conventional dissimilar metal material joining accompanied by a diffusion phenomenon. The object of the present invention is to provide high joining strength while having an intermetallic compound layer at the joining interface. An object of the present invention is to provide a dissimilar metal joining method capable of efficiently producing a dissimilar metal joining member provided.

  As a result of intensive studies aimed at achieving the above object, the present inventors have controlled the thickness and size of the intermetallic compound layer formed at the joint interface between different metal materials, thereby achieving the above-mentioned problem. The inventors have found that this can be solved, and have completed the present invention.

  The present invention is based on the above knowledge, and in the dissimilar metal joining method of the present invention, a diffusion layer in a joint part of dissimilar metal material by a junction having a rapid heating process and a subsequent quenching process, for example, electrical resistance joining. A dislocation loop formed by forming a compound layer containing at least one intermetallic compound in a part of the metal and bonding the atomic vacancies of the quenching structure of the main component metal of the low melting point metal material More than half the melting point expressed by the absolute temperature of the main component metal of the low melting point metal material, and the melting point of the intermetallic compound to be generated is the highest. The temperature of the melting point expressed by the absolute temperature of the low intermetallic compound is ½ or less, preferably the temperature of ½ of the melting point and the temperature at which the low melting point metal material is softened by the disappearance or recrystallization of the precipitation strengthening phase. Any of them It is characterized by performing the heat treatment at a lower temperature below the temperature range.

  According to the present invention, after performing bonding by a rapid heating process and a subsequent rapid cooling process, for example, by electrical resistance bonding, the main component metal of the low-melting-point-side metal material is the vacancy of the rapid cooling structure of the bonded portion. More than half the melting point expressed by the absolute temperature of the main component metal of the low melting point metal material, and more than the temperature at which the dislocation loops and voids formed by aggregation disappear. The melting point of the intermetallic compound having the lowest melting point of the compound is not more than ½ of the melting point represented by the absolute temperature. Since the heat treatment is performed in a temperature range equal to or lower than the lower one of the temperatures at which the metal material softens, the bonding strength of the dissimilar metal bonding member can be greatly improved.

It is a graph which shows the relationship between the thickness of the metal compound layer formed in the joining interface of steel and an aluminum alloy, and cross tensile strength. 1 is an overall view of a spot welding apparatus used for joining dissimilar metal materials according to the present invention. It is a top view which shows the shape of the cross tension test piece used for evaluation of joining strength in this invention. It is a graph which shows the relationship between the area ratio in the nugget of the intermetallic compound layer of thickness of 0.5-3.2 micrometers, and joining strength. It is a graph which shows the relationship between the maximum value of the crystal grain diameter of an intermetallic compound layer, and joining strength. It is sectional drawing which shows the joining structure of the galvanized steel plate and aluminum alloy which were obtained by Example 1. FIG. (A) It is a transmission electron micrograph which shows the state before heat processing of the joining structure | tissue obtained by Example 1. FIG. (B) It is a transmission electron micrograph which shows the state after heat processing of the joining structure | tissue obtained by Example 1. FIG. (C) It is an external appearance photograph which shows the fracture state of the test piece by the cross tension test of Example 1. FIG. 2 is a transmission electron micrograph showing the joint structure obtained in Comparative Example 1. FIG. (A) It is an external appearance photograph which shows the fracture state of the test piece by the cross tension test of the comparative example 2. FIG. (B) It is an external appearance photograph which shows the fracture state of the test piece by the cross tension test of Example 2. FIG. (C) It is an external appearance photograph which shows the fracture state of the test piece by the cross tension test of Example 3. FIG. (D) It is an external appearance photograph which shows the fracture state of the test piece by the cross tension test of the comparative example 3. FIG. (A) It is a general view which shows the joining procedure of the dissimilar metal material by Example 4 with a spot welding apparatus. (B) It is sectional drawing which shows the joining structure of the galvanized steel plate obtained by Example 4, and aluminum alloy. (C) It is a transmission electron micrograph which shows the joining structure | tissue obtained by Example 4. FIG. 6 is a transmission electron micrograph showing the joint structure obtained in Example 5. FIG.

  Hereinafter, the dissimilar metal joining method of the present invention and the dissimilar metal joining member obtained thereby will be described in more detail and specifically.

FIG. 1 shows various joining conditions using an AC type spot welding apparatus as shown in FIG. 2 in which a steel plate having a thickness of 0.55 mm and a 6000 series aluminum alloy plate having a thickness of 1.0 mm are overlapped. After joining under (current: 20000-30000A, pressure 150-600 kgf, energization time: 250 milliseconds or less), heat treatment (treatment temperature: 140-500 ° C., treatment time: 20 minutes-under various conditions) 7 hours), the relationship between the thickness of the metal compound layer formed at the bonding interface and the cross tensile strength is arranged. Among the above conditions, in order to obtain the relationship of FIG. 1, the condition that 90% or more falls within a difference of 1 μm or less so that the thickness of the intermetallic compound layer formed at the joint interface after welding is as uniform as possible. Selected and plotted.
In addition, the cross tension test was performed according to the method prescribed | regulated to JISZ3137 using the test piece of a dimension shape as shown in FIG.

As is apparent from the results of FIG. 1, in order to obtain a high strength dissimilar metal joining member, it is necessary to control the thickness of the metal compound layer formed by diffusion joining at a certain area ratio, In order to obtain a joining strength based on the strength of an aluminum alloy sheet having a tensile strength of 210 MPa, that is, a cross tensile strength of 0.6 kN or more, the control range of the thickness of the intermetallic compound is 0.5 to 3.2 μm. In order to obtain a cross tensile strength of 0.9 kN or more, a range of 0.6 to 2.8 μm, and in order to obtain a high strength of 1.2 kN or more, the thickness of the intermetallic compound layer is It can be seen that the thickness needs to be in the range of 0.8 to 2.5 μm.
That is, when the thickness of the intermetallic compound layer exceeds 3.2 μm, the contribution to the strength decreases, and even when the thickness is less than 0.5 μm, sufficient strength cannot be obtained. Therefore, the thickness of the intermetallic compound layer must be at least 0.5 μm or more and 3.2 μm or less.

In the joint nugget formed by the spot welding apparatus, the intermetallic compound is not necessarily formed in the entire area of the nugget (joint surface), and the distribution of the compound thickness is recognized in the nugget.
Therefore, the relationship between the area ratio and the bonding strength at which the thickness of the intermetallic compound layer falls within the above range was investigated.

  That is, the area ratio at which the intermetallic compound layer is formed to the above thickness and is firmly bonded is measured by observing the central cross section of the bonded portion and measuring the length within which the intermetallic compound layer is in the thickness range. From the distance from the center of the nugget, the ratio of the sum of the areas assumed to form a concentric donut-shaped junction region to the nugget area (joint area) was calculated. The result is shown in FIG.

  According to FIG. 4, when the area ratio with respect to the bonding area of the region in which the intermetallic compound layer is formed to a thickness of 0.8 to 2.5 μm is 52% or more, the bonding strength is good even at the lower limit of variation. It was confirmed that when the area ratio was 70% or more, the joint strength was further improved, and a cross tensile strength of 0.9 kN or more was obtained.

  FIG. 5 shows the influence of the maximum value of the crystal grain size (major axis) of the intermetallic compound on the bonding strength. Even when the intermetallic compound layer is slightly thicker, the crystal grains are fine. It can be seen that the bonding strength is slightly improved.

The change in the structure where the crystal grains become finer at the joint interface of the intermetallic compound layer due to the heat treatment following the joining process is influenced by the lattice defects introduced by the joining process involving rapid heating and quenching processes and the recovery of crystals. It is thought that there is.
In order for crystal grains to grow or disappear, it is necessary for atoms to move through the material, that is, to cause diffusion. The structure formed by the rapid heating process and the rapid cooling process is in a state where the energy of the system is high. For example, it is a state in which a gradient of components and many vacancies and dislocations are introduced by a rapid cooling process.
Such a metal structure in a high energy state changes to a lower energy state by heat treatment. Typical examples include uniform component gradients, crystal grain growth and disappearance, dislocation loops and voids formed by a collection of atomic vacancies, and crystal grain boundaries changing to low energy grain boundaries. It is a phenomenon.

Under high temperature conditions, the lattice contains many atomic vacancies, but when quenched from such a high temperature state, vacancy remains in the cooled lattice at a high concentration, and such supersaturation occurs. These vacancies also increase the energy of the system. When heat treating a normal structure, since there are only pores that are thermodynamically acceptable, diffusion caused by the heat treatment is also limited by the amount of holes and the movement speed of the holes. It is considered that diffusion that does not occur in normal heat treatment can occur by heat-treating a structure containing various vacancies.
In other words, in order to cause the growth and disappearance of crystal grains, it is necessary to increase the temperature and take time to secure a sufficient amount of vacancies and atoms, but the amount of vacancies is sufficient. Since it exists in the crystal lattice, it is considered that sufficient diffusion and reaction can occur even at a relatively low temperature.

The refinement of crystal grains, which is characteristic as a change in structure in the present invention, can occur under heat treatment conditions where the number (frequency) of crystal nucleation is high and the grain growth rate is not so fast. The heat treatment at a low temperature of the quenched structure is considered to be a condition in which excessive vacancies increase the number (frequency) of nucleation and subsequent crystal growth does not cause coarsening of crystal grains.
In order to perform heat treatment under such conditions, heat treatment at a high temperature that leads to coarsening of the crystal grains should be avoided, and it is necessary and sufficient to move the vacancies introduced by the rapid heating and quenching processes. It is considered that the temperature should be the lower limit of the heat treatment temperature. That is, as a recovery of the metal structure, the heat treatment should be performed at a temperature at which voids formed by collecting atomic vacancies or dislocation loops disappear, not at a temperature at which electrical resistance recovers.

Detailed description of the recovery of the crystal can be found on pages 229 to 235 of “Dislocation theory” edited by the Japan Institute of Metals published by Maruzen Co., Ltd.
It can be said that the condition for the disappearance of voids formed by the collection of atomic vacancies or the disappearance of dislocation loops for the recovery of the metal structure is to perform a heat treatment at a temperature equal to or higher than the V stage of the recovery of the metal structure.

  In order to grow crystal grains of an intermetallic compound in a diffusion bonding interface between metal materials having different melting points, it is necessary that any one of the base metal metals is sufficiently diffused. Since the base material on the low melting point side causes diffusion at a lower temperature, it is considered that the grain growth of the intermetallic compound at the interface is also governed by the diffusion of the base material on the low melting point side.

  For example, in the joining of iron and aluminum, the refinement of crystal grains by heat treatment of a rapid heating / quenching structure typified by electric resistance welding is the recovery of the aluminum base alloy, which is a base material having a lower melting point than that of the iron base alloy. It begins to occur when the Vth stage is reached, that is, at a temperature of 127 ° C. (400K) or higher. This coincides with the temperature at which the dislocation loops and voids formed by the aggregation of vacancies in the quenching structure of the aluminum-based alloy, which is the base material, disappears, and the structure generated in the rapid heating / quenching process is stable. It is thought to be caused by changing to.

As described above, the movement of vacancies in the low-melting-point side metal matrix lattice is the heat treatment condition of the present invention, but sufficient diffusion is required in order to obtain the desired structure efficiently in a shorter time. It is preferable to heat-treat at a temperature that generates In order for the diffusion to occur sufficiently, a temperature of 1/2 or more of the melting point expressed in absolute temperature can be used as a guide.
As described above, since the base material on the low melting point side causes diffusion at a low temperature, it is considered that the grain growth of the intermetallic compound at the interface is also governed by the diffusion of the base material on the low melting point side. A sufficient effect can be expected by setting the melting point of the side base material to a temperature equal to or higher than ½ of the value expressed in absolute temperature.

On the other hand, when the heat treatment is performed at a temperature higher than half the value of the melting point of the low-melting-point side base material in terms of absolute temperature, for example, in the case of a precipitation strengthening type alloy, the precipitated phase disappears. It is also possible to reach temperature. Further, if strength is obtained by work hardening, the strength of the base material may be reduced by recrystallization. If the precipitated phase that plays the main function of strengthening the base material disappears, the bonding strength itself cannot be improved even if the strength of the bonded portion increases.
Therefore, in the heat treatment after bonding according to the present invention, the strength of the base material must be maintained at a strength required for the structure. That is, it is necessary to perform the heat treatment at a temperature equal to or lower than the temperature at which the low melting point metal material softens due to the disappearance or recrystallization of the precipitation strengthening phase.

  In addition, it is necessary to cause sufficient diffusion of the low-melting-point base metal, but if excessive diffusion occurs in the intermetallic compound grains, strength cannot be obtained by coarsening the intermetallic compound grains. there is a possibility. In order to avoid such coarsening of the intermetallic compound grains, sufficient diffusion occurs in the intermetallic compound grains at the joint, that is, a guideline for the temperature at which vacancies can easily move in the intermetallic compound. As described above, it is necessary to perform heat treatment at a temperature equal to or lower than ½ of the melting point represented by the absolute temperature of the intermetallic compound having the lowest melting point among the intermetallic compounds generated for the dissimilar metal bonding.

  Therefore, a preferable range is a temperature at which vacancies move in the low melting point side base material lattice, and the temperature range in which the strength of the base material can be sufficiently obtained without intermetallic compound grains being coarsened. It has been found that heat treatment at the highest possible temperature in the temperature range is industrially most effective. That is, the heat treatment of the present invention is a low melting point metal by the temperature of 1/2 of the melting point expressed by the absolute temperature of the intermetallic compound having the lowest melting point among the generated intermetallic compounds and disappearance or recrystallization of the precipitation strengthening phase. It is necessary to carry out at a temperature equal to or lower than the lower one of the temperatures at which the material softens.

On the other hand, in the intermetallic compound formed in the Al-Fe system, a component ratio close to 3: 1 or 5: 2 is observed in the analysis by EDX, and from the component ratio, Al ₃ Fe, Al ₅ Fe ₂ is determined. The melting points of these intermetallic compounds are higher than the melting point of aluminum 660 ° C., and the melting points of Al ₃ Fe and Al ₅ Fe ₂ are observed to be 1160 ° C. and 1169 ° C., respectively. The temperature that becomes a standard for the movement of γ can be ½ of the melting point expressed in absolute temperature, that is, about 440 ° C.
From experiments in the present invention, it was found that the vacancy of the quenching structure of the low-melting-side substrate was changed to a stable structure without coarsening the intermetallic compound. It is desirable to raise the temperature to eliminate dislocation loops and voids formed by aggregation, more preferably to a temperature of 1/2 or more of the melting point represented by the absolute temperature of the main component metal of the low melting point metal material However, it was found that when the vacancies in the intermetallic compound lattice formed at the interface are heated far beyond the temperature at which the vacancies can easily move, the intermetallic compound grains become coarse.

  In diffusion bonding, if the thickness of the intermetallic compound is too thin, the bonding strength will be insufficient.However, if the process temperature is increased in order to increase productivity, the intermetallic compound grains that are produced tend to become coarse, and the intermetallic compound The brittle nature of the compound may appear. Since diffusion bonding is generally performed in a temperature range where the diffusion rate is sufficiently high, the intermetallic compound crystal grains tend to be large. In order to finely control crystal grains, a long process time is required, and an unrealistic process time is required to apply to industrial production. On the other hand, in melt bonding, the intermetallic compound grows coarsely in order to reach a high temperature reaching the melting point.

  In the present invention, once the electrical resistance heat is generated at the bonding interface, the structure of the non-equilibrium composition is formed with a constant thickness by rapid cooling by metal heat transfer, and then the atomic vacancies of the quenched structure of the low melting point metal gather. The temperature at which the dislocation loops and voids formed are extinguished, more preferably at least half the melting point represented by the absolute temperature of the main component metal of the low melting point metal material, and between the metals generated at the interface By performing heat treatment at a temperature equal to or lower than ½ of the melting point represented by the absolute temperature of the intermetallic compound having the lowest melting point among the compounds, the intermetallic compound layer is formed with a constant thickness at the time of bonding between different metal materials. By securing and refining the compound structure of the nonequilibrium composition thereon, a strong intermetallic compound layer having a fine crystal grain size and a constant thickness can be formed.

In the present invention, a combination of an iron-based alloy and an aluminum-based alloy can be suitably used as the dissimilar metal material as described above.
The iron-based alloy mentioned here means a material mainly composed of iron, and specifically, rolled steel sheets such as carbon steel, alloy steel, mild steel, and high-tensile steel can be suitably used.

Further, the aluminum-based alloy means an alloy containing aluminum as a main component, and for example, a 1000 series to 7000 series aluminum alloy can be suitably used.
In the present invention, the “main component” means a metal most contained in the alloy.

  As a combination of dissimilar metal materials used in the present invention, as in the above-described electrical resistance bonding, a process in which a rapid heating process and a subsequent quenching process by heat removal by the material can be used, and the metal is bonded to the interface by diffusion. There is no particular limitation as long as it is a combination of materials containing a metal that forms a compound as a main component. Examples of combinations of such metal elements include Ti (titanium) and Al (aluminum), and Ti and Fe. The combination of (iron) can be mentioned.

  In the present invention, when the combination of dissimilar metal materials is an iron-based alloy and an aluminum-based alloy, the main component metal of the low-melting-point side metal material is formed by agglomerating atomic vacancies in the joint at the junction. It can be carried out in the temperature range of 130 to 440 ° C. calculated from the temperature at which the dislocation loops and voids disappear, that is, 127 ° C. or more and less than half the absolute temperature of the melting point of the intermetallic compound. Moreover, it is desirable to set it as 190 degreeC or more from a viewpoint of heat-processing in the temperature range below 1/2 of melting | fusing point represented by the absolute temperature of the intermetallic compound with the lowest melting | fusing point among the produced intermetallic compounds. Moreover, the temperature of the melting point represented by the absolute temperature of the intermetallic compound having the lowest melting point among the intermetallic compounds to be generated and the temperature at which the low melting point side metallic material is softened by the disappearance or recrystallization of the precipitation strengthening phase. From the viewpoint of performing the heat treatment in a temperature range lower than the lower one, it is desirable that the temperature be 410 ° C. or lower.

  Further, at this time, it is preferable to use a galvanized steel sheet as the iron-based alloy, which is formed on the surface of the aluminum-based alloy by causing eutectic melting between zinc and aluminum forming the plating layer and applying pressure. The oxide film can be easily removed from the bonding interface together with the eutectic melt having a low melting point, and diffusion bonding between the new surfaces of the two metal materials from which the galvanized layer and the oxide film have been removed becomes possible.

In the dissimilar metal bonding method of the present invention, as a bonding having a rapid heating process and a subsequent rapid cooling process, typically, electrical resistance bonding using a spot welding apparatus as shown in FIG. 2 can be employed. However, it is possible not only to use such resistance heat generation, but also to use, for example, a laser beam as other heating means.
Further, not only spot joining by nugget formation but also seam-like joining can be performed by using a roller-like electrode.

  Further, in the dissimilar metal joining method of the present invention, when the obtained joining member is coated, the heat treatment subsequent to the joining can be performed together with the baking treatment of the paint in the painting process, whereby the heat treatment process can be performed. It can be omitted, and waste of energy consumption can be eliminated.

  Hereinafter, the present invention will be specifically described based on examples. In addition, this invention is not limited at all by these Examples.

Example 1
Using an AC type spot welding apparatus shown in FIG. 2, a galvanized steel sheet 1 having a thickness of 0.55 mm and a 6000 series aluminum alloy sheet 2 having a tensile strength of 200 MPa and a thickness of 1.0 mm are superposed and added. Resistance spot bonding was performed under the conditions of a pressure of 300 kN, a current of 24,000 A, and a current application time of 0.2 seconds.
At the time of joining, aluminum and zinc in the plating layer are reacted at a temperature of about 400 ° C. to cause eutectic melting, and the eutectic formed as shown in FIG. The oxide film 2a on the surface of the aluminum alloy plate 2 together with the melt is discharged as a discharge D to form a new surface of the aluminum alloy plate 2 and to cause a diffusion reaction without melting the aluminum alloy. A diffusion layer is formed on the aluminum alloy plate 2, Fe and Al are reacted in the diffusion layer to form a thin intermetallic compound layer L, and both dissimilar metal materials 1 and 2 are joined via the intermetallic compound layer L. did.

  As a result, the aluminum oxide film 2a on the surface of the aluminum alloy plate 2 is discharged to the periphery of the nugget together with the reaction phase of zinc and aluminum, that is, the eutectic alloy, and has a thickness of 0.8 to 2.5 μm at the bonding interface. A thin intermetallic compound layer L having a thickness is formed in a region of 48% of the junction area (nugget area), and the intermetallic compound crystal grains of the intermetallic compound layer L have an elliptical cross section, and the major axis is about It was 0.3 μm. A transmission electron micrograph of the joint is shown in FIG.

  Next, when the bonding member thus prepared is subjected to heat treatment at 440 ° C. for 1.5 hours, the total thickness of the intermetallic compound layer becomes 0.8 to 2.5 μm, which is 89% of the bonding area. It was confirmed that it was generated in the range. At the same time, the intermetallic compound grains changed equiaxially, and the crystal grain size could be reduced to 0.1 μm or less. This transmission electron micrograph is shown in FIG.

Then, as a result of measuring the strength of the joining member subjected to the heat treatment at 440 ° C. × 1.5 hours by the cross tension test according to the procedure shown in FIG. 3, a joining strength of 1.60 kN was obtained, and FIG. As shown, it was confirmed that the aluminum alloy fractured.
Note that the intermetallic compound layer L, the result of investigation by EDS and X-ray diffraction, the compound layer is formed by an intermetallic compound according to component ratios of FeAl near FeAl ₃ and _Fe 5 Al _2, and compound It was confirmed that Zn in the plating layer is a compound that is hardly contained so as not to be detected by analysis.

(Comparative Example 1)
When the bonded member obtained under the same conditions as in Example 1 was subjected to a heat treatment of 500 ° C. × 0.5 hours, the intermetallic compound layer had a thickness exceeding 3.2 μm, and the area ratio was 98%, which had an interface having an intermetallic compound layer in the diffusion bonding layer, but there was almost no bonding layer having an intermetallic compound thickness of 3.2 μm or less, and the bonding area was 2%. It was. In addition, the crystal grain changes into an equiaxed portion and a columnar two layer, the crystal grain size is 0.1 to 0.2 μm at the equiaxed grain portion, and the columnar portion extends in the direction perpendicular to the bonding interface, and the maximum major axis Were observed to be close to 1.0 μm. A transmission electron micrograph in this case is shown in FIG.
Moreover, in the cross tension test, it fractured | ruptured in the joining interface and only the intensity | strength of 0.09 kN was obtained.

(Comparative Example 2)
Using the same spot welding apparatus as in Example 1 above, a galvanized steel sheet 1 having a plate thickness of 0.55 mm and a 6000 series aluminum alloy plate 2 having a plate thickness of 1.0 mm were superposed, the pressurizing force being 300 kN, and the current being Example. Resistance spot bonding was performed under the conditions of 21000 A and energization time of 0.2 seconds, which was smaller than the case 1, and both dissimilar metal materials were diffusion bonded.
As a result, the intermetallic compound layer L was formed, and the region in the range of 0.8 to 2.5 μm was formed in 46% of the bonding area. The maximum major axis of the intermetallic compound crystal grains constituting the intermetallic compound layer L was 0.06 μm.

  FIG. 9 (a) shows the fracture state of the test piece when the cross tension test is carried out as it is without applying heat treatment to the joining member obtained as described above, and a part of the nugget is on the steel plate side. Although it remained, most of them were peeled and fractured from the bonding interface, and a strength of 0.67 kN was obtained.

(Example 2)
As a result of performing a heat treatment at 440 ° C. for 1.5 hours on the joining member produced under the same conditions as in Comparative Example 2, the area of 0.8 to 2.5 μm thickness of the intermetallic compound layer L is It was confirmed that the area was enlarged to 90% of the area, the crystal grains of the intermetallic compound were changed to be equiaxed, and the grain size was refined to 0.1 μm.
FIG. 9 (b) shows the fracture state of the test piece when the cross tension test was performed after the above heat treatment, and the plug fracture occurred, and the cross tensile strength reached 1.69 kN. .

  As described above, even when the mode is changed to the plug rupture mode, a high rupture strength is obtained as compared with the base metal strength, which is considered to be affected by an increase in the strength of the base material due to aging of the aluminum alloy. Although it was a peel-type fracture before heat treatment, even if a 1.69 kN cross tensile strength was generated, the fracture occurred at the starting point of the base material without breaking from the joint interface. It shows that the strength of the interface is increasing.

(Example 3)
A bonded member produced under the same conditions as in Comparative Example 2 was subjected to heat treatment at 300 ° C. for 7 hours. As a result, the region in the range of 0.8 to 2.5 μm of the intermetallic compound layer L was expanded to be 82% of the junction area. About the crystal grain of the intermetallic compound, it changed to the equiaxed form, and it was confirmed that the particle size is refined | miniaturized to 0.05-0.1 micrometer.
FIG. 9C shows the fracture state of the test piece when the cross tension test is performed after the above heat treatment, the cross tensile strength is 1.50 kN, and the nugget fracture I was able to.

(Comparative Example 3)
As a result of performing a heat treatment at 500 ° C. for 1 hour on the joining member produced under the same conditions as in Comparative Example 2, the intermetallic compound layer L grows to a thickness exceeding 3.2 μm, and the area is the joining area. In contrast, the crystal grain size of the intermetallic compound is changed into an equiaxed portion and a columnar two-layer, and the crystal grain size is 0.1 to 0 in the equiaxed grain portion. It was observed that the maximum major axis was 1.8 μm in the columnar part.
FIG. 9 (d) shows the fracture state of the test piece when the cross tension test was carried out after heat treatment at 500 ° C. for 1 hour. It was only 08 kN.

In this way, by controlling the thickness and area of the intermetallic compound layer within a predetermined range, sufficient strength can be expressed, and a sufficiently thin intermetallic compound layer is formed in the bonding step before heat treatment. In some cases, the strength can be increased by increasing the thickness of the compound layer within a range in which the particle size of the intermetallic compound can be reduced by the subsequent heat treatment, and conversely, the intermetallic compound layer formed in the bonding step is When the thickness is relatively large, the quality of the dissimilar metal joint can be stabilized by selecting the heat treatment conditions that make the crystal grains fine while not thickening the compound layer.
In this way, high quality of dissimilar metal joints can be stably obtained through the process of forming a rapid heating / quenching structure typified by electrical resistance bonding and the subsequent heat treatment process, and the aging conditions of the base material By combining the above, it is possible to increase the strength of the dissimilar metal joint.

Example 4
As shown in FIG. 10 (a), a galvanized steel sheet 1 having a thickness of 0.55 mm and a 6000 series aluminum alloy sheet 2 having a tensile strength of 210 MPa and a thickness of 1.0 mm are thermosetting having electrical insulation. Overlaid via adhesive S, resistance spot bonding is performed under the conditions of 300 kN applied pressure, 24000 A current, and 0.2 second energization time, and both dissimilar metal materials are diffused without melting the aluminum plate 2. Joined.
At this time, aluminum and the zinc of the plating layer are once reacted at a temperature of about 400 ° C. to cause eutectic melting, and the aluminum alloy plate 2 together with the eutectic melt produced by pressurization by the electrode of the welding apparatus. The oxide film 2a and the adhesive are discharged as discharged matter D, a new surface of the aluminum alloy plate 2 is generated, a diffusion layer is formed on the steel plate 1 and the aluminum alloy plate 2, and an intermetallic compound layer L is formed on the bonding interface. As shown in FIG. 10B, the dissimilar metal materials 1 and 2 were joined through the intermetallic compound layer L without melting the aluminum alloy.

Then, the said intermetallic compound layer L is formed by heat-processing at 170 degreeC which is the hardening temperature of the said thermosetting adhesive S, The range of the thickness of 0.8-2.5 micrometers is the junction area. The region was 56%. It was observed that the maximum major axis of the crystal grains of the intermetallic compound was refined to 0.05 to 0.1 μm (see FIG. 10C).
As a result of carrying out a cross tension test in the same manner, a bonding strength of 0.94 kN was obtained.

In this embodiment, the adhesive S can be cured simultaneously with the miniaturization of the intermetallic compound layer and the insulating layer can be formed between the dissimilar metal materials by one heat treatment, thereby increasing the number of processes and the amount of energy input. It is possible to improve the bonding strength and the electric corrosion resistance without any problems.
It goes without saying that such a method can be applied in the same manner even when coating is applied to the joined members.

(Example 5)
When the dissimilar metal joining of the steel plate 1 and the aluminum alloy plate 2 is performed using the AC type spot welding apparatus shown in FIG. 2, the above except that the surface of the aluminum alloy plate 2 is provided with the galvanized layer 2b. By repeating the same operation as in Example 1, a dissimilar metal joining member as shown in FIG. 11 was obtained.
As a result, the intermetallic compound layer L is formed with a thickness of 0.5 to 3.5 μm at the bonding interface, and the range of the thickness of 0.8 to 2.5 μm is 56% of the bonding area. It was. The maximum major axis of the intermetallic compound particles was found to be 0.05 μm. Similarly, as a result of carrying out a cross tension test, a bonding strength of 1.2 kN was obtained.

  Usually, a strong high melting point oxide film is present on the surface of the aluminum alloy plate 2 and removal thereof becomes a problem during diffusion bonding. In this embodiment, the aluminum alloy plate 2 is galvanized. The oxide film on the surface is removed in the process, and the galvanized layer 2b is melted and removed (melting point: 420 ° C.), so that the new surface of the aluminum alloy plate 2 is exposed. Joining becomes possible.

1 Steel plate (dissimilar metal material)
2 Aluminum alloy plate (dissimilar metal material)
1a Zinc plating layer L Intermetallic compound layer

Claims (9)

  A method of joining dissimilar metal materials in which a compound layer containing at least one intermetallic compound is formed on a part of a diffusion layer in a joint, and performing a joining process including a rapid heating process and a subsequent quenching process. After that, the melting point is the lowest among the intermetallic compounds to be produced, at a temperature higher than the temperature at which dislocation loops and voids formed by the aggregation of the vacancies in the quenching structure of the low melting point metal material. A dissimilar metal joining method, wherein heat treatment is performed in a temperature range of 1/2 or less of a melting point expressed by an absolute temperature of an intermetallic compound.   2. The dissimilar metal bonding method according to claim 1, wherein the heat treatment is performed at a temperature equal to or higher than ½ of a melting point represented by an absolute temperature of a main component metal of the low melting point side metal material.   The low-melting-side metal material is softened by the above heat treatment by halving the melting point expressed by the absolute temperature of the intermetallic compound having the lowest melting point among the generated intermetallic compounds and disappearance or recrystallization of the precipitation strengthening phase. 3. The dissimilar metal joining method according to claim 1, wherein the dissimilar metal joining method is performed at a temperature equal to or lower than the lower one of the temperatures.   A method for joining an iron-based alloy and an aluminum-based alloy in which a compound layer containing at least one type of Fe-Al intermetallic compound is formed on a part of a diffusion layer in a joint portion, which includes a rapid heating process and A dissimilar metal bonding method comprising performing a heat treatment in a temperature range of 130 to 440 ° C. after performing bonding having a subsequent rapid cooling process.   The dissimilar metal joining method according to claim 4, wherein the heat treatment temperature is 190 ° C or higher.   The dissimilar metal joining method according to claim 4 or 5, wherein the heat treatment temperature is 410 ° C or lower.   The iron-based alloy is a galvanized steel sheet, which causes eutectic melting between zinc and aluminum to form a low-melting melt, and pressurizes the oxide film on the surface of the aluminum-based alloy together with the melt. The dissimilar metal joining method according to any one of claims 4 to 6, wherein the new surfaces of the two metal materials from which the galvanized layer and the oxide film have been removed are joined together.   The dissimilar metal joining method according to any one of claims 1 to 7, wherein the joining having the rapid heating process and the subsequent rapid cooling process is electrical resistance joining by energization heating.   The dissimilar metal joining method according to any one of claims 1 to 8, wherein a heat treatment subsequent to the joining having a rapid heating process and a subsequent rapid cooling process is performed in combination with a baking treatment in a painting process.

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