JP7173081B2 - Friction stir welding method for aluminum alloy plate and steel plate - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for joining an aluminum alloy plate and a steel plate, and more specifically to a friction stir welding method for overlapping and joining an aluminum alloy plate and a steel plate.

In recent years, in the field of transportation equipment such as automobiles, where weight reduction is required, structures and parts (hereinafter referred to as "members") have been developed by combining aluminum alloys and steel and making full use of the characteristics of each material. It is In a member that combines aluminum alloy and steel, it is necessary to join dissimilar metal materials. There is a problem that a brittle intermetallic compound composed of the main elements that do not exist is often formed, and sufficient joint strength cannot be obtained.

Therefore, as a method for strongly joining an aluminum alloy and steel, a solid phase joining method such as a diffusion joining method, which does not form a molten layer, has been developed. However, the diffusion bonding method requires a preparatory step for keeping the surface of the mating surface of the materials clean, which leads to an increase in cost, and thus has the problem of being difficult to implement industrially.

As another diffusion bonding method, there is a friction bonding method. Techniques for joining metal materials by agitating parts to cause plastic flow have been disclosed. However, since this technique requires rotating the metal materials to be joined, there are limitations on the shape and size of the metal materials to which this method is applied.

By the way, in recent years, friction stir welding has been developed as a new solid phase welding method, and has been widely used mainly for butt welding of metal materials of the same kind or similar to each other. For example, in Patent Document 2, a rotating tool made of a material harder than the metal plate is inserted into an unbonded portion of the metal plate, and the rotating tool is moved in the joining direction while rotating to move between the rotating tool and the metal plate. A method has been proposed for continuously joining metal sheets in the longitudinal direction by generating heat and plastic flow in the joint. In this method, the metal plates are joined by rotating and moving the rotating tool while the metal plates are fixed. Therefore, there is an advantage that even an infinitely long member can be continuously solid-phase-bonded along the bonding direction. In addition, since this is a solid-state joining utilizing the plastic flow of metal due to frictional heat between the rotating tool and the metal plate, the joint can be joined without melting, and no filler material is required. Furthermore, since the temperature at which the joint is heated is low and it does not melt, there are many advantages such as less deformation of the joint and fewer defects.

Friction stir welding has been widely used in fields such as aircraft, ships, railway vehicles and automobiles as a method for joining low melting point metals such as aluminum alloys and magnesium alloys. The reason for this is that it is difficult to obtain satisfactory weld characteristics for these low-melting-point metals using the conventional arc welding method. This is because not only can it be obtained, but also productivity can be improved. Furthermore, since the joining interface is agitated by a rotating tool, clean surfaces can be created and the clean surfaces can be brought into contact with each other.

As mentioned earlier, this friction stir welding method is an extremely excellent method for joining metal materials of the same kind or similar to each other. In this case, the two metals are mixed at the joint interface, the melting point is lowered, a molten phase is generated, a coarse solidification structure is generated, and brittle intermetallic compounds are generated during solidification.

As a means of solving this problem, in Patent Document 3, an aluminum alloy plate and a steel plate are superimposed via a Zn-5Al layer or a Zn hot-dip layer, and the surface of the joint is pressed with a rotary tool to rub the aluminum alloy. A diffusion layer consisting of Al, Al-Zn, Zn-Al, Fe-Zn and Fe is formed by stirring and plastically flowing to cause mutual diffusion of the Zn-5Al layer or Zn hot-dip plated layer and the aluminum alloy. Then, a method of joining an aluminum alloy plate and a steel plate is disclosed by forming an Al--Zn--Fe alloy layer through plastic flow, without forming a brittle intermetallic compound.

Further, Patent Document 4 describes a friction stir welding method in which dissimilar metals such as an aluminum alloy and steel are superimposed on each other. Using a melting point plated steel sheet, a joining pin that rotates during friction stir welding is pressed from the surface of the aluminum alloy and inserted to the vicinity of the joining interface where the aluminum alloy and the low melting point plated steel sheet are joined, and is formed on the aluminum alloy side. A friction stir welding method for obtaining a high-strength welded joint by solid-phase joining an aluminum alloy and a low-melting-point plated steel plate by diffusing the coating layer by plastic flow in the plastic flow region and exposing a new surface on the surface of the low-melting-point plated steel plate. and friction stir welding members are disclosed.

On the other hand, as a rotating tool, a double-acting friction stir welding method using a welding tool including a pin member and a substantially cylindrical shoulder member having a hollow into which the pin member is inserted is known. Since the joining tool consisting of the pin member and the shoulder member can independently control the rotation and advance/retreat motions, by adjusting the timing of the advance/retreat motion of the pin member and the advance/retreat motion of the shoulder member, the pin member can be press-fitted. It makes it possible to backfill the formed concave portion.

As the double-acting friction stir welding method, for example, in Patent Document 5, a press-fitting pin harder than an aluminum alloy plate is press-fitted into a plurality of superimposed aluminum alloy plates with a friction welding tool with a triple structure and stirred. After that, the aluminum alloy spilled out from the press-fitting hole that was generated was confined between the press-fitting pin and the outer ring. A method for point joining an aluminum alloy is disclosed in which the surface of the alloy plate is pressed and the aluminum alloy overflow portion is flowed and embedded in the press-fitting hole.

Furthermore, as another method for joining dissimilar metal members by a double-acting friction stir welding method, for example, in Patent Document 6, from the side of an aluminum plate superimposed on a steel plate, a probe of a rotary tool reaches directly above the steel plate. When inserting the aluminum plate and the steel plate by friction stir welding, a double-acting rotary tool in which the probe and the shoulder member can be moved separately in the axial direction is used as the rotary tool, and the probe is inserted into the aluminum plate. After performing friction stir welding, the probe is pulled out from the friction stir part formed in the aluminum plate, and the probe hole created by such pulling out is filled by material flow from other parts of the friction stir part. A method for joining metal members is disclosed.

JP-A-62-183979 Japanese Patent Publication No. 07-505090 JP-A-2002-66759 JP 2007-253172 A Japanese Patent Application Laid-Open No. 2001-259863 JP 2010-260109 A

By the way, in Patent Documents 3 and 4, there is a method for solving the generation of brittle intermetallic compounds at the joint interface, which is a problem when dissimilar metals such as an aluminum alloy plate and a steel plate are joined by friction stir welding. Although disclosed, Patent Documents 5 and 6 relating to the double-acting friction stir welding method do not consider the above problems at all. For example, Patent Document 5 discloses a technique related to a member in which aluminum alloy plates are superimposed, and Patent Document 6 discloses a technique related to a joining method of a member in which an aluminum alloy plate and a steel plate are superimposed. It only discloses that the joint characteristics are improved by backfilling the recess formed by press-fitting, and does not disclose any solution to the problem of formation of fragile intermetallic compounds at the joint interface.

The present invention was developed in view of the above-mentioned problems of the conventional double-acting friction stir welding method. The object of the present invention is to propose a double-acting friction stir welding method capable of suppressing the formation of an intermetallic compound at the joint interface where the two come into contact with each other and efficiently forming a joint having sufficient joint strength.

The inventors have made intensive studies to solve the above problems, and as a result, have obtained the following new findings.
a) A rotating tool is inserted into an object to be joined in which an aluminum alloy plate and a steel plate are superimposed from the surface of the aluminum alloy plate while being rotated, and the rotating tool is moved in the joining direction to disperse the joining material by friction stirring. When aluminum alloy plate and steel plate are friction stir welded by plastic flow, inserting the tip of the probe from the mating surface of the aluminum alloy plate and steel plate to the steel plate side causes the new surfaces of both materials to come into contact with each other. It is possible to form a bonding interface that is compatible with each other, and to ensure a metallurgical bonding state.
b) In the process of forming the joint interface and achieving a metallurgical joint state, iron and aluminum, which are the main components of both materials, diffuse and become distributed in both materials across the joint interface. If a brittle intermetallic compound composed of iron and aluminum is generated within this diffusion range, it causes a decrease in bonding strength.
c) In order to suppress the formation of the intermetallic compound, it is necessary to suppress the diffusion of iron and aluminum distributed in both materials across the bonding interface. It is necessary to control the peak temperature (maximum temperature reached) immediately after heating and the subsequent cooling rate within an appropriate range.
d) In friction stir welding, the joint is heated by frictional heat and plastic heat generated when the material to be welded is friction-stirred by a rotating tool. The heat generated by the probe and the shoulder can be independently controlled by using a double-acting rotary tool in which the shoulder and the shoulder are separately constructed and each can independently control the rotation speed. Therefore, by applying the above-described double-acting rotary tool to friction stir welding of an aluminum alloy plate and a steel plate, the peak temperature and cooling rate of the weld interface can be controlled within appropriate ranges.
e) Specifically, by setting the rotation speed of the shoulder of the rotating tool higher than the rotation speed of the probe, frictional stirring of the mating surface between the aluminum alloy plate and the steel plate by the tip of the probe is minimized, and the probe is The peak temperature at the joint interface can be suppressed by supplementing the insufficient heat input due to the decrease in rotation speed with friction stirring by the shoulder rotating at high speed.
f) Furthermore, a probe with an optimized tip shape is inserted to the mating surface of the aluminum alloy plate and steel plate or to the steel plate side of the mating surface, and the two materials that are in contact at the mating surface are friction-stirred to create a new surface of both materials. When forming a joining interface where the two come into contact with each other, a homogeneous new surface can be efficiently formed, and an excessive temperature rise can be avoided to suppress the formation of fragile intermetallic compounds.
The present invention has been developed based on the above-mentioned new findings with further improvements.

That is, in the present invention, an aluminum alloy plate and a steel plate are superimposed, and a backing jig is abutted on the steel plate side to fix the body to be joined. Using a double-acting rotary tool configured as a body and capable of independently setting the number of revolutions, the probe of the double-acting rotary tool is rotated and inserted into the object to be joined from the aluminum alloy plate side, and the double-acting rotary tool is rotated. Friction stir welding method in which the shoulder of a dynamic rotary tool is brought into contact with the surface of an aluminum alloy plate while being rotated, and in this state, the rotary tool is moved in the welding direction to frictionally stir the object to be welded, causing plastic flow and friction stir welding. In the above, the probe has a circular cross section and the tip is formed into a plane or a convex curved surface with a radius of curvature of 10 mm or more, and the tip of the probe is moved from the surface of the aluminum alloy plate to the aluminum alloy Friction stirring characterized by inserting the mating surface of the plate and the steel plate or from the mating surface to the steel plate side, and setting the rotational speed SS (rotations/minute) of the shoulder higher than the rotational speed PS (rotations/minute) of the probe. Joining method.

The double-acting rotary tool used in the friction stir welding method of the present invention is characterized in that the tip of the probe has a spiral recess formed in a direction opposite to the direction of rotation.

In the double-acting rotary tool used in the friction stir welding method of the present invention, the surface of the shoulder contacting the aluminum alloy plate is ring-shaped, flat or convexly curved, and the rotation direction are characterized by having spiral recesses formed in opposite directions.

The friction stir welding method of the present invention is represented by the number of rotations SS (rotations/minute) of the shoulder, the diameter SD (mm) of the shoulder, the diameter PD (mm) of the probe, and the welding speed JS (mm/minute). A parameter that correlates with the amount of heat generated by the shoulder per unit joint length: SS × (SD ³ - PD ³ ) / JS is the following formula (1) with respect to the thickness t ₁ (mm) of the aluminum alloy plate;
3000×t ₁ ≦SS×(SD ³ −PD ³ )/JS≦34000×t ₁ (1)
and correlated with the amount of heat generated by the probe per unit bonding length represented by the number of rotations PS (rotations/minute) of the probe, the diameter PD (mm) of the probe, and the bonding speed JS (mm/minute) : PS×PD ³ /JS is the following formula (2) with respect to the thickness t ₁ (mm) of the aluminum alloy plate;
100×t ₁ ≦PS×PD ³ /JS≦1100×t ₁ (2)
is characterized by satisfying

Further, in the friction stir welding method of the present invention, the diameter SD (mm) of the shoulder of the rotating tool and the diameter PD (mm) of the probe are the following (3) with respect to the thickness t ₁ (mm) of the aluminum alloy plate. and (4) formula;
4×t ₁ ≦SD≦15×t ₁ (3)
t ₁ ≤ PD ≤ 5 x t ₁ (4)
is characterized by satisfying

Further, in the friction stir welding method of the present invention, the insertion amount P (mm) from the mating surface of the aluminum alloy plate and steel plate to the steel plate side of the probe tip of the rotating tool that is inserted while rotating from the surface of the aluminum alloy plate. , 0 mm or more and 0.5 mm or less.

Further, the friction stir welding method of the present invention is characterized in that the rotation axis of the rotary tool that is rotated and inserted from the surface of the aluminum alloy plate is set perpendicular to the surface of the aluminum alloy plate.

According to the present invention, in the friction stir welding method for superimposing and joining an aluminum alloy plate and a steel plate, a probe coaxially arranged at the tip and a shoulder are configured separately as rotating tools, and the number of rotations is set separately. By using a double-acting rotary tool that allows setting of Furthermore, it is possible to prevent an excessive temperature rise at the joint interface and suppress the formation of an intermetallic compound, so that a high-strength lap joint can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure explaining the double-acting friction stir welding method of this invention used for joining an aluminum alloy plate and a steel plate. FIG. 2 is a cross-sectional view taken along line AA of FIG. 1, and is a diagram for explaining a region where a jointed body in which an aluminum alloy plate and a steel plate are superimposed is friction-stirred by a double-acting rotating tool. FIG. 4 is a diagram illustrating the shape of a double-acting rotary tool that can be used in the present invention; FIG. 4 is a diagram illustrating the shape of a double-acting rotary tool that can be used in the present invention; FIG. 4 is a diagram illustrating the shape of a double-acting rotary tool that can be used in the present invention; FIG. 4 is a diagram illustrating the shape of a double-acting rotary tool that can be used in the present invention; It is a figure explaining the shape of the double acting rotary tool of a comparative example. It is a figure explaining the tensile test piece which measures a shear strength.

The present invention relates to a technique for joining an object to be joined in which an aluminum alloy plate 1 and a steel plate 2 are superimposed on each other by friction stir welding, and as shown in FIG. As shown in FIG. The workpiece is inserted while being rotated from the rotary tool 3, and the shoulder 5 of the rotary tool 3 is brought into contact with the surface of the aluminum alloy plate 1 while being rotated. Both materials are joined by friction stirring and plastic flow. At that time, the tip of the probe 4 is inserted to the steel plate side from the mating surface 6 of both materials, and the shape of the tip of the probe 4 of the rotary tool 3 is optimized to align the aluminum alloy plate 1 and the steel plate 2. A joint interface 7 is formed on the surface 6 where the new surfaces of both materials are in contact with each other, and the rotational speeds of the probe 4 and the shoulder 5 of the rotating tool 3 are separately set to generate frictional heat generated by the rotation of the probe 4 and the shoulder 5. and plastic deformation heat are separately controlled to control the peak temperature (maximum temperature reached) of the joint interface 7 and the subsequent cooling rate to suppress the diffusion of the iron and aluminum elements across the joint interface 7, thereby reducing the brittle metal This is a technique for suppressing the formation of intercalants and for obtaining a joint 8 between an aluminum alloy plate and a steel plate having excellent joint strength. In addition, 9 shown in the figure is a jig backing the steel plate side of the object to be joined, in which the aluminum alloy plate 1 and the steel plate 2 are superimposed, and 10 is joined by the probe 4 and the shoulder 5 of the rotary tool 3. It shows the region where the material flows plastically.

First, from the viewpoint of maximizing the effect of the present invention, the thickness of the aluminum alloy plate 1 and the steel plate 2 constituting the object (member) to be joined to which the friction stir welding method of the present invention is applied is It is preferably within the range of 1 to 3 mm, and the steel plate is within the range of 1 to 3 mm. However, the effect of the present invention can be obtained even if the plate thickness is outside the above range.

Further, the double-acting rotary tool 3, in which the probe 4 coaxially arranged at the tip and the shoulder 5 used in the friction stir welding method of the present invention are separately constructed, is used for the steel plate from the mating surface of the aluminum alloy plate and the steel plate. Since the probe is inserted up to the side, the probe tip that directly contacts the steel plate is made of a material that is at least harder than the steel plate, and the probe and shoulder other than the tip that contacts the aluminum alloy plate are at least harder than the aluminum alloy plate. It is important that it is made of a hard material.

The rotating directions of the probe 4 and the shoulder 5 constituting the double-acting rotating tool 3 used in the present invention may be the same or opposite.

Furthermore, in the friction stir welding method of the present invention for overlapping and joining an aluminum alloy plate and a steel plate, the joining conditions are limited to the following ranges to increase the effect of suppressing the formation of intermetallic compounds at the joining interface and increase the joining strength. can be improved.

First, the probe 4 of the double-acting rotary tool 3 must have a circular cross section and a flat tip or a convex curved surface with a radius of curvature of 10 mm or more. Insert the tip of the probe into the mating surface of the aluminum alloy plate and the steel plate or from the mating surface to the steel plate side, and frictionally stir the two materials that are in contact at the mating surfaces to form a joint interface where the new surfaces contact each other. By forming a shape that satisfies the above conditions, the tip of the probe can be evenly brought into contact with the mating surface, and a homogeneous new surface can be formed. When the tip of the probe has a convex curved surface, the radius of curvature is preferably 40 mm or more.

Moreover, the tip of the probe 4 of the double-acting rotary tool 3 preferably has a spiral recess formed in the direction opposite to the direction of rotation. It is preferable that the spiral recess has a shape that is more recessed than the flat portion or the convex curved portion of the probe tip. Examples of the recessed shape include those having a groove-like or step-like cross-sectional shape (a stepped shape having substantially horizontal steps). By providing such a spiral concave portion, the material softened by the frictional heat generated by the stirring by the probe tip that is inserted above the mating surface of the aluminum alloy plate and the steel plate is made to flow from the outside to the inside of the probe tip. , the outflow of the material to the outside due to the pressing of the tip of the probe can be suppressed. In addition, this can promote plastic flow on the mating surfaces, promote the formation of new surfaces, and further suppress the generation of burrs and small pieces protruding from the steel plate side to the aluminum alloy plate side on the mating surfaces. . Since these burrs and small pieces are generated at high temperatures, fragile intermetallic compounds are likely to be generated around them, causing a decrease in the strength of the joint. It should be noted that the above effect of providing the spiral recess can be obtained only by providing the direction of the spiral in the direction opposite to the rotating direction of the probe.

Moreover, it is preferable that one or more spiral recesses be formed in the tip of the probe. However, if the number of spiral recesses exceeds 6, not only will the effect of promoting the plastic flow of the material be saturated, but also the probe tip will be likely to break due to the complicated shape. The number of spiral recesses is preferably changed according to the diameter of the probe tip from the viewpoint of preventing breakage of the probe tip while improving the plastic flow of the material. It is desirable that the larger the diameter of the probe tip, the smaller the number. A preferable number is 2 to 4.

Further, the length of one spiral recess provided at the tip of the probe is preferably 0.25 or more and 1 or less when the length of the outer periphery of the tip of the probe is taken as one turn. The length of the spiral recess is also preferably varied according to the diameter of the probe tip, preferably longer for larger probe tip diameters and shorter for smaller probe tip diameters.

FIG. 3 shows a double-acting rotary tool compatible with the present invention, and FIGS. , (b), (d), (f) and (h) are examples in which the tip of the probe is convex. Also, (c) to (h) show an example in which four spiral recesses are provided at the tip of the probe.

On the other hand, the shoulder 5 of the double-acting rotary tool 3 has a ring-shaped cross section, and the surface in contact with the aluminum alloy plate may be a flat surface, a convex curved surface, or a concave curved surface. This is preferable because it enables high-speed bonding at 1000 mm/min or higher. Further, the surface that comes into contact with the aluminum alloy plate preferably has a spiral recess formed in a direction opposite to the direction of rotation, like the tip of the probe. When the surface in contact with the aluminum alloy plate is curved, the radius of curvature of the curved surface is preferably 30 mm or more.

Further, the spiral recess provided on the surface of the shoulder that contacts the aluminum alloy plate must be recessed from the other surface (flat or curved surface) of the tip. Examples of the recessed shape include those having a groove-like or step-like cross-sectional shape (a stepped shape having substantially horizontal steps). By providing such a groove-shaped concave portion, the surface of the shoulder contacting the aluminum alloy plate is pressed against the surface of the aluminum alloy plate, and the material softened by the frictional heat generated when stirring is applied to the aluminum alloy plate of the shoulder. It is possible to prevent the material from flowing outward from the surface of the shoulder contacting the aluminum alloy plate by causing the material to flow from the outside to the inside of the contact surface. In addition, this can promote the plastic flow of the pressed portion, prevent the thickness of the joint from decreasing with respect to the original thickness, and form a beautiful joint surface without burrs. can do. The effect of providing this spiral recess can only be obtained by providing the spiral recess in a direction opposite to the direction of rotation of the shoulder.

Moreover, it is preferable that the number of spiral recesses formed on the surface of the shoulder contacting the aluminum alloy plate is one or more. However, if the number of spiral recesses exceeds 6, not only will the effect of promoting the plastic flow of the material saturate, but there is a risk that the shoulder surface will be easily damaged due to the complicated shape. is preferred. The number of spiral recesses provided on the shoulder surface is preferably changed according to the diameter of the shoulder from the viewpoint of preventing damage to the shoulder surface while promoting plastic flow of the material. It is desirable that the larger the diameter, the larger the number, and the smaller the diameter of the shoulder, the smaller the number.

Further, the length of one spiral recess formed on the surface of the shoulder that contacts the aluminum alloy plate is preferably 0.5 or more and 2 or less when the length of the outer circumference of the shoulder is taken as one turn. . The length of the spiral recess formed on the surface of the shoulder that contacts the aluminum alloy plate is also preferably changed according to the diameter of the shoulder. It is desirable to

As mentioned above, FIG. 3 shows a double-acting rotary tool suitable for the present invention, in which FIGS. is a concave surface, (e) and (f) are flat surfaces, and (g) and (h) are convex surfaces. Also, (e) to (h) show an example in which four spiral recesses are provided on the surface of the shoulder contacting the aluminum alloy plate.

Furthermore, in order to further enhance the effects of the present invention, the friction stir welding method of the present invention includes a shoulder rotation speed SS (rotations/minute), a shoulder diameter SD (mm), a probe diameter PD (mm) and a multiplicity of A parameter that correlates with the amount of heat generated by the shoulder per unit joint length represented by the moving speed of the dynamic rotary tool (that is, the welding speed) JS (mm/min): SS × (SD ³ - PD ³ )/JS is aluminum The following formula (1) for the thickness t ₁ (mm) of the alloy plate;
3000×t ₁ ≦SS×(SD ³ −PD ³ )/JS≦34000×t ₁ (1)
is preferably satisfied.

By setting the rotation speed SS of the shoulder of the rotary tool higher than the rotation speed PD of the probe, friction stirring of the mating surface between the aluminum alloy plate and the steel plate due to the tip of the probe is minimized, and due to the decrease in the rotation speed of the probe The peak temperature at the joint interface can be suppressed by supplementing the insufficient heat input with friction stirring by the shoulder rotating at high speed. However, when SS×(SD ³ −PD ³ )/JS is less than 3000×t ₁ , heat generation due to friction stirring on the surface of the aluminum alloy plate due to the shoulder becomes insufficient, and the mating surfaces of the aluminum alloy plate and the steel plate are metallurgically joined. It becomes impossible to form a bonded interface in a state of being bonded. On the other hand, when it exceeds 34000 × t ₁ , heat generation due to friction stirring on the surface of the aluminum alloy plate due to the shoulder becomes excessive, and an excessive amount of heat is input around the joint interface between the aluminum alloy plate and the steel plate, resulting in the peak of the joint interface. The temperature (maximum attainable temperature) increases and the cooling rate decreases, thereby promoting the formation of brittle intermetallic compounds. A more preferable range of (SD ³ -PD ³ )/JS is 10000×t ₁ or more and 25000×t ₁ or less.

In addition, the friction stir welding method of the present invention comprises a double-acting rotary tool probe rotation speed PS (times/min), a probe diameter PD (mm), and a double-acting rotary tool movement speed (i.e., welding speed) JS A parameter that correlates with the amount of heat generated by the probe _per unit joint length represented by (mm/min): PS×PD ³ /JS is the following (2 )formula;
100×t ₁ ≦PS×PD ³ /JS≦1100×t ₁ (2)
is preferably satisfied.

When PS×PD ³ /JS is less than 100×t ₁ , frictional stirring of the aluminum alloy plate by the probe becomes insufficient and defects tend to occur, which is not preferable. On the _other hand, if it exceeds 1100 x t1, the tip of the probe frictionally agitates the steel plate side of the mating surface excessively, causing a temperature rise at the joint interface and promoting the formation of brittle intermetallic compounds. A more preferable range of PS×PD ³ /JS is 120×t ₁ or more and 900×t ₁ or less.

Further, in the friction stir welding method of the present invention, the diameter SD (mm) of the shoulder of the double-acting rotary tool is expressed by the following formula (3) with respect to the thickness t ₁ (mm) of the aluminum alloy plate;
4×t ₁ ≦SD≦15×t ₁ (3)
is preferably satisfied.

The shoulder of the double-acting rotary tool is brought into contact with the surface of the aluminum alloy plate while being rotated, and the aluminum alloy plate is friction-stirred to generate frictional heat and plastic heat to heat the joint. However, when the shoulder diameter SD is less than 4×t ₁ mm, uniform plastic flow cannot be obtained in the thickness direction. On the other hand, if it exceeds 15×t ₁ mm, it only unnecessarily widens the region where plastic flow occurs, and an excessive load is applied to the rotary tool. Therefore, by limiting the diameter SD of the shoulder to 4 x t ₁ mm or more and 15 x t ₁ mm or less with respect to the thickness t ₁ (mm) of the aluminum alloy plate, the thickness from the surface of the aluminum alloy plate A necessary and sufficient amount of heat to ensure a state of metallurgical bonding at the bonding interface formed between the mating surfaces of the aluminum alloy plate and the steel plate, while making the temperature distribution in the direction uniform and suppressing the occurrence of defects in the joint. can be supplied. A preferred shoulder diameter SD (mm) is in the range of 6×t ₁ mm or more and 12×t ₁ mm or less.

Furthermore, in the friction stir welding method of the present invention, the diameter PD (mm) of the probe of the double-acting rotary tool is expressed by the following formula (4) with respect to the thickness t ₁ (mm) of the aluminum alloy plate;
t ₁ ≤ PD ≤ 5 x t ₁ (4)
is preferably satisfied.

When lap joining an aluminum alloy plate and a steel plate, by inserting the tip of the probe to the mating surface of the aluminum alloy plate and steel plate or to the steel plate side of the mating surface, the joint interface where the new surfaces of both materials come into contact with each other. can be formed to ensure metallurgical bonding. However, if the diameter PD of the probe is less than t ₁ mm, it is not possible to secure a sufficient bonding interface area and obtain the necessary bonding strength. On the other hand, if it exceeds 5×t ₁ mm, the distance over which the bonding material flows around the probe during bonding becomes long, making it difficult to prevent the occurrence of defects at the bonding interface. Therefore, by limiting the diameter PD of the probe to t ₁ mm or more and 5 × t ₁ mm or less, a necessary and sufficient area of the bonding interface where the metallurgical bonding state is secured between the mating surfaces of both materials is secured, Bonding strength can be increased. A preferable probe diameter PD (mm) is in the range of 2×t ₁ mm or more and 4×t ₁ mm or less.

Furthermore, in the friction stir welding method of the present invention, the insertion amount P of the probe tip inserted while rotating the double-acting rotary tool from the surface of the aluminum alloy plate is based on the mating surface of the aluminum alloy plate and steel plate (0 mm). In this case (see FIG. 2), it is preferable to set the distance from 0 mm to 0.5 mm toward the steel plate.

By inserting the tip of the probe to the mating surface of the aluminum alloy plate and steel plate or to the steel plate side of the mating surface, it is possible to form a joint interface where the new surfaces of both materials come into contact with each other on the mating surface. can be secured. However, if the distance is less than 0 mm, the tip of the probe does not reach the mating surface, and a new surface cannot be formed by stirring the material at the tip of the probe. On the other hand, if it exceeds 0.5 mm, the tip of the probe frictionally stirs the steel plate side of the mating surface excessively, which causes an excessive temperature rise at the formed joint interface and promotes the formation of a brittle intermetallic compound, which is preferable. do not have. A preferable insertion amount P is in the range of 0.2 to 0.4 mm.

In addition, in the friction stir welding method of the present invention, it is preferable that the rotation axis of the rotating tool 3 which is rotated and inserted from the surface of the aluminum alloy plate 1 is perpendicular to the surface of the aluminum alloy plate 1 . For example, if the rotation axis of the rotary tool is inclined in the direction opposite to the welding direction with respect to the perpendicular to the surface of the aluminum alloy plate, the load received by the rotary tool is divided into the compressive stress in the direction of the rotation axis and the load in the direction perpendicular to the rotation axis. Since the force in the bending direction that the rotating tool receives can be reduced, damage to the rotating tool can be prevented. However, in that case, when the probe tip is inserted to the mating surface of the aluminum alloy plate and steel plate or to the steel plate side of the mating surface, and the two materials in contact with the mating surfaces are frictionally stirred to form the joint interface, the probe tip to the mating surface The contact is biased, and it becomes difficult to form a homogeneous new surface. Therefore, even when the rotation axis of the rotary tool is inclined in the direction opposite to the joining direction with respect to the normal to the surface of the aluminum alloy plate, the inclination angle is preferably suppressed to 5° or less.

Double-acting friction stir welding was applied to the objects to be joined, which are shown in Table 1, in which an A5052 aluminum alloy plate specified by JIS H 4000 and a high-strength cold-rolled steel plate for automobiles of 590 MPa class are superimposed. , a joining experiment was conducted with a joining length of 0.3 m.
For the double-acting friction stir welding, a double-acting rotary tool having the shape and dimensions shown in FIG. 3 was used. As a comparative example, a double-acting rotary tool having the shape and dimensions shown in FIG. 4 was used. The specifications of these rotary tools are summarized in Table 2, and the probe diameter PD and shoulder diameter SD of these rotary tools satisfy formulas (3) and (4), respectively.
In addition, for the probe and shoulder of the rotary tool, tool steel (SKD61) with a Vickers hardness Hv of 530, which is harder than the aluminum alloy plate and steel plate listed in Table 1, which is the joining material, was used as the raw material. . During joining, both the shoulder and the probe of the double-acting rotary tool were rotated clockwise.
Table 3 shows other joining conditions. In addition, the inclination angle α shown in Table 3 is an angle inclined in the direction opposite to the joining direction with respect to a line perpendicular to the surface of the aluminum alloy plate.

With respect to the bonded joint thus obtained, confirmation of the success or failure of the bonding state, measurement of the thickness of the intermetallic compound at the bonding interface, and tensile testing of the bonded joint were performed in the following manner.
<Success or failure of bonding state>
The success or failure of the bonded state was confirmed by confirming whether or not the manufactured bonded joint was in a state of spontaneous separation after bonding. When there was no peeling, the bonding state was established, and "O" was indicated.
<Thickness of intermetallic compound>
The thickness of the intermetallic compound is determined by cutting the prepared joint in the direction across the joint (perpendicular to the joint direction), and examining the central portion of the joint interface exposed in the cross section with a scanning electron microscope at a magnification of 5000. Three or more points were observed, the thickness of the intermetallic compound was measured, and the average value was obtained.
<Tensile test of bonded joint>
In the tensile test of the bonded joint, a tensile test piece with a width of 20 mm including the joint perpendicular to the tensile direction is taken from the manufactured bonded joint as shown in FIG. 5, and the tensile test is performed. , the shear strength was measured.

The results of the above evaluation test are also shown in Table 4. The results show the following.
First, No. 1, which satisfies the conditions of the present invention. Joints Nos. 1 to 8 (Invention Examples 1 to 8) had a joint state, and the thickness of the intermetallic compound at the joint interface was 0.9 μm or less, so a tensile strength of 6.5 kN or more ( shear tensile strength) could be obtained.
On the other hand, No. In the joints of Nos. 13 and 14 (Comparative Examples 5 and 6), the insertion amount P from the mating surface of the aluminum alloy plate and the steel plate at the tip of the probe to the steel plate side was smaller than the range of the present invention. After that, the bonding state was not established, and the bonding state was detached by itself.
Also, No. In the joints Nos. 11 and 12 (Comparative Examples 3 and 4), the rotation speed SS of the shoulder and the rotation speed PS of the probe were the same, and the conditions deviated from the present invention. Since the amount was excessive and the thickness of the intermetallic compound at the joint interface became 1.1 μm or more, the tensile strength of the joint became 4.9 kN or less.
Also, No. Joints 9 and 10 (comparative examples 1 and 2) are examples using the rotating tool shown in FIG. Therefore, it was not possible to form a bonded interface having a homogeneous new surface. As a result, although a joint state was established and the intermetallic compound at the joint interface was reduced to 0.7 μm or less, the joint tensile strength was 5.3 kN or less because the area of the homogeneous new surface was narrow. rice field.

The technology of the present invention can be applied not only to automobile members, but also to railway vehicles, aircraft, ships, building structures, electrical equipment, and the like.

1: aluminum alloy plate 2: steel plate 3: double-acting rotary tool 4: probe 5: shoulder 6: mating surface of aluminum alloy plate and steel plate 7: joint interface 8: joint 9: backing jig 10: plastic flow region

Claims (7)

An aluminum alloy plate and a steel plate are superimposed on each other, and a backing jig is abutted on the steel plate side to fix the object to be joined. Using a double-acting rotary tool whose number can be set separately, the probe of the double-acting rotary tool is inserted into the object to be joined from the aluminum alloy plate side while rotating, and the shoulder of the double-acting rotary tool is rotated and brought into contact with the surface of the aluminum alloy plate, and in this state, the rotating tool is moved in the welding direction to frictionally stir the object to be welded, and plastic flow is performed to perform friction stir welding,
The probe has a circular cross section and a flat tip or a convex curved surface with a radius of curvature of 10 mm or more,
Inserting the tip of the probe from the surface of the aluminum alloy plate to the mating surface between the aluminum alloy plate and the steel plate or from the mating surface to the steel plate side,
A friction stir welding method, wherein the number of rotations SS (rotations/minute) of the shoulder is higher than the number of rotations PS (rotations/minute) of the probe. 2. The friction stir welding method according to claim 1, wherein the double-acting rotary tool has a spiral recess formed in the tip of the probe in a direction opposite to the direction of rotation. The double-acting rotary tool has a ring-shaped surface that contacts the aluminum alloy plate of the shoulder and is formed into a plane or a convex curved surface, and has a spiral recess formed in a direction opposite to the direction of rotation. The friction stir welding method according to claim 1 or 2, characterized by: The amount of heat generated by the shoulder per unit bonding length represented by the number of shoulder rotations SS (times/min), the shoulder diameter SD (mm), the probe diameter PD (mm), and the bonding speed JS (mm/min) Correlating parameter: SS × (SD ³ - PD ³ ) / JS satisfies the following formula (1) with respect to the thickness t ₁ (mm) of the aluminum alloy plate, and the number of rotations of the probe PS (times/minute ), the probe diameter PD (mm) and the welding speed JS (mm/min), a parameter that correlates with the amount of heat generated by the probe per unit welding length: PS×PD ³ /JS is the thickness of the aluminum alloy plate. The friction stir welding method according to any one of claims 1 to 3, wherein the following formula (2) is satisfied for the thickness t ₁ (mm).
3000×t ₁ ≦SS×(SD ³ −PD ³ )/JS≦34000×t ₁ (1)
100×t ₁ ≦PS×PD ³ /JS≦1100×t ₁ (2) The diameter SD (mm) of the shoulder of the rotary tool and the diameter PD (mm) of the probe satisfy the following formulas (3) and (4) with respect to the thickness t ₁ (mm) of the aluminum alloy plate. The friction stir welding method according to any one of claims 1 to 4.
Note 4 x t ₁ ≤ SD ≤ 15 x t ₁ (3)
t ₁ ≤ PD ≤ 5 x t ₁ (4) The insertion amount P (mm) of the probe tip of the rotating tool that is inserted from the surface of the aluminum alloy plate while rotating it from the mating surface of the aluminum alloy plate and steel plate to the steel plate side is within the range of 0 mm or more and 0.5 mm or less. The friction stir welding method according to any one of claims 1 to 5, characterized in that: Friction stir welding according to any one of claims 1 to 6, wherein the rotation axis of the rotating tool that is rotated and inserted from the surface of the aluminum alloy plate is perpendicular to the surface of the aluminum alloy plate. Method.

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