Friction stir welding method, friction stir welding device, and method for manufacturing joined member

Abstract

Provided is a friction stir welding method that makes it possible for good joint characteristics of a joined portion to be obtained in a stable manner even if a steel plate having a high melting point is used as a material to be joined. A predetermined relationship between a load of a rotating tool and a separation distance of the rotating tool is satisfied.

Description

Friction stir welding method, friction stir welding device, and method of manufacturing welded members

[0001] The present invention relates to a friction stir welding method, a friction stir welding apparatus, and a method for manufacturing a welded member.

[0002] In friction stir welding, for example, double-sided friction stir welding, workpieces are joined as follows. That is, a pair of rotary tools is placed on the front and back sides of workpieces having at least two or more metal plates. The rotary tools are then pressed against the front and back sides of the workpieces, respectively, and moved in the joining direction while rotating. This softens the metal plates with frictional heat between the rotary tool and the workpieces, and the softened portions are stirred with the rotary tool. Plastic flow is then generated in the region of the workpieces that will become the joint, and the workpieces are joined. Hereinafter, the region where the workpieces are butted or overlapped and not yet joined is referred to as the "unjoined portion," and the region that has been joined and integrated is referred to as the "joined portion."

[0003] As an example of technology related to such friction stir welding, Patent Document 1 describes a friction stir welding method in which "the coefficient of dynamic friction between the steel plate and the material of the rotary tool or the material coated on the surface of the rotary tool is 0.6 or less, and when the heating region is defined as a region where the temperature TS (°C) of the surface of the steel plate heated by heating means provided ahead of the rotary tool in the welding direction satisfies the following formula (1), the minimum distance between the heating region and the rotary tool is equal to or less than the diameter of the shoulder of the rotary tool, the area of ​​the heating region is equal to or less than the area of ​​the maximum diameter portion of the pin portion of the rotary tool, and 65% or more of the area of ​​the heating region is located between a weld center line, which is a straight line on the surface of the steel plate that passes through the rotation axis of the rotary tool and is parallel to the welding direction, and a straight line that is parallel to the weld center line and separated toward the retreating side by a distance equal to the maximum radius of the pin portion of the rotary tool. TS≧0.8×T Ａ１ ... (1) T Ａ１ is the temperature expressed by the following formula (2): Ａ１(°C) = 723 - 10.7 [% Mn] - 16.9 [% Ni] + 29.1 [% Si] + 16.9 [% Cr] + 290 [% As] + 6.38 [% W] ... (2) The above [% M] is the content (mass%) of the M element in the steel plate, which is the processed material, and is set to 0 if no M element is contained.

[0004] Patent Document 2 states, "A double-sided friction stir welding method characterized in that the shoulder and pin portions of the rotary tool share a rotation axis and are made of a material harder than the steel plate, and the heating area is an area on the surface side of the steel plate heated by a heating device provided forward in the welding direction of one of the rotary tools, and during heating, the steel plate surface temperature TP (°C) in the heating area satisfies formulas (1) to (3), and the gap G (mm) between the opposing shoulder portions and the thickness t (mm) of the steel plate satisfy formula (5), thereby performing friction stir welding. In the range of -0.5 x D ≦ W < -0.1 x D, 40 ≦ TP ≦ 1200 ... (1) In the range of -0.1 x D ≦ W ≦ 0.1 x D, 100 ≦ TP ≦ 1200 ... (2) In the range of 0.1 x D < W ≦ 0.5 x D, 40 ≦ TP ≦ 1200 ... (3) 0.4 x t ≦ G ≦ t ... (5) Here, W (mm) indicates the distance from the center line of the weld in the width direction of the weld, D (mm) indicates the diameter of the shoulder of the rotary tool, G (mm) indicates the gap between the shoulders, and t (mm) indicates the thickness of the steel plate.

[0005] Patent Document 3 states that "a rotary tool is pressed against an unjoined portion, which is an abutting portion or an overlapping portion, of an end of a first electromagnetic steel strip and an end of a second electromagnetic steel strip from both sides of the unjoined portion while rotating in opposite directions, and then the rotary tool is moved in the joining direction to join the first electromagnetic steel strip and the second electromagnetic steel strip, and the diameter D (mm) of the shoulder of the rotary tool satisfies the relationship of the following formula (1), and RS x D, which is expressed by the rotation speed RS (revolutions / min) of the rotary tool, the diameter D (mm) of the shoulder of the rotary tool, and the joining speed JS (mm / min), ３ / JS satisfies the relationship of the following formula (2): 4×TJ≦D≦10×TJ (1) 200×TJ≦RS×D ３ / JS≦2000×TJ (2) where TJ is the average value (mm) of the sheet thickness of the first electromagnetic steel strip and the sheet thickness of the second electromagnetic steel strip when the unjoined portion is a butt joint, and is the thickness (mm) of the overlapping portion when the unjoined portion is an overlapping portion."

[0006] International Publication No. 2018 / 070316 JP 2020-124739 A JP 2023-041556 A

[0007] However, in conventional friction stir welding methods such as those described in Patent Document 1, when high-melting-point steel sheets, for example, steel sheets with a Si content of 2.0 to 5.0 mass %, are used as the welded materials, the joint characteristics of the welded portion may deteriorate. Therefore, there is currently a demand for improvement in this respect.

[0008] The present invention has been developed in light of the above-mentioned current situation, and aims to provide a friction stir welding method (hereinafter also referred to as a friction stir welding method with excellent stability of joint characteristics) that makes it possible to obtain stable and excellent joint characteristics at the welded portion even when high-melting-point steel plates are used as the welded materials. Another aim of the present invention is to provide a friction stir welding apparatus that can be suitably used for the above-mentioned friction stir welding method. Furthermore, another aim of the present invention is to provide a method for manufacturing welded members. Note that in this specification, any numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits, respectively.

[0009] The inventors have conducted extensive research to achieve the above-mentioned object. First, the inventors investigated the cause of deterioration in joint characteristics of a weld when high-melting-point steel plates are used as the workpieces in a conventional friction stir welding method. As a result, the inventors have obtained the following findings. That is, when high-melting-point steel plates are used as the workpieces, the rotary tool is prone to wear and damage (hereinafter also referred to as wear). This wear of the rotary tool causes insufficient heat input and poor plastic flow, which leads to deterioration in joint characteristics of the weld.

[0010] Therefore, the inventors conducted further studies based on the above findings. As a result, the inventors discovered the following: - There is a correlation between the progress of wear on the rotary tool and the load on the rotary tool during joining. - By controlling the rotary tool separation distance so as to satisfy a predetermined relationship depending on the load on the rotary tool, it is possible to obtain stable and excellent joint characteristics at the weld, even when high-melting-point steel plates are used as the welded materials. The present invention was completed through further studies based on the above findings.

[0011] That is, the gist of the present invention is as follows: 1. A friction stir welding method in which a pair of opposing rotary tools are pressed against the front and back surfaces of workpieces and the workpieces are welded by moving the rotary tools in the welding direction while rotating, wherein the load on the rotary tools and the separation distance between the rotary tools satisfy the following relationship: - When 0.01≦dL<0.10, dL×0.1≦dD≦dL×1.0 - When 0.10≦dL<0.30, dL×0.2 - 0.01≦dD≦dL×1.2 - 0.02 - When 0.30≦dL<0.70, dL×0.4 - 0.07≦dD≦dL×1.4 - 0.08, where dL = ΔL / Lint, Lint is the initial value of the load on the rotary tool, and ΔL is the amount of change in the load on the rotary tool from Lint. Furthermore, dD = ΔD / Dint, where Dint is the initial value of the separation distance of the rotary tool, and ΔD is the amount of change in the separation distance of the rotary tool from Dint.

[0012] 2. The friction stir welding method according to 1, wherein the load on the rotary tool is a load value in the axial direction of the rotary tool, a load value in the welding direction, or a torque value in the rotational direction.

[0013] 3. The friction stir welding method according to 1 or 2 above, which is butt welding or lap welding.

[0014] 4. A friction stir welding apparatus used in the friction stir welding method according to any one of 1 to 3 above, comprising: a pair of opposing rotating tools; a measuring device that measures the load on the rotating tools; a calculating device that calculates a control range for the separation distance of the rotating tools based on the load on the rotating tools measured by the measuring device and on the formula according to 1 above; and a position adjusting mechanism that changes the position of the rotating tools within the control range for the separation distance of the rotating tools calculated by the calculating device.

[0015] 5. A method for manufacturing a welded member, comprising joining workpieces by the friction stir welding method according to any one of 1 to 3 above to obtain a welded member.

[0016] According to the present invention, even when high-melting-point steel plates are used as the joined materials, it is possible to avoid deterioration of the joint characteristics of the weld and to obtain stable, excellent joint characteristics of the weld. This makes it possible to stably produce high-quality joined members regardless of the type of joined materials, and the industrial value of this invention is extremely great.

[0017] FIG. 1 is a schematic diagram showing an example of friction stir welding.

[0018] The present invention will be described based on the following embodiments: [1] Friction stir welding method First, a friction stir welding method according to one embodiment of the present invention will be described.

[0019] As described above, in the friction stir welding according to one embodiment of the present invention, as shown in FIG. 1 , a pair of rotary tools are placed on the front and back sides of workpieces having at least two or more metal plates. Hereinafter, the rotary tool placed on the front side of the workpieces will be referred to as the front-side rotary tool. Furthermore, the rotary tool placed on the back side of the workpieces will be referred to as the back-side rotary tool. In FIG. 1 , reference numeral 1 denotes the rotary tool (front-side rotary tool), 2 denotes the rotary tool (back-side rotary tool), and 3 denotes the workpieces. Then, the pair of opposing rotary tools are pressed against the front and back sides of the workpieces, respectively, and the rotary tools are rotated and moved in the joining direction. That is, the front-side rotary tool is pressed against the front side of the workpieces, and the back-side rotary tool is pressed against the back side of the workpieces, respectively, and these rotary tools are rotated and moved in the joining direction. This softens the metal plates with frictional heat between the rotary tool and the workpieces, and the softened portion is stirred with the rotary tool. Then, plastic flow is generated in the region of the workpieces that will become the weld, and the workpieces are joined.

[0020] In friction stir welding according to one embodiment of the present invention, it is extremely important to monitor the load on the rotary tool during welding and control the separation distance of the rotary tool so as to satisfy the relationship of the following equation (1) (consisting of three equations divided into cases) according to the (monitored) load on the rotary tool. When 0.01≦dL<0.10, dL×0.1≦dD≦dL×1.0 When 0.10≦dL<0.30, dL×0.2−0.01≦dD≦dL×1.2−0.02 (1) When 0.30≦dL<0.70, dL×0.4−0.07≦dD≦dL×1.4−0.08 Here, dL=ΔL / Lint. Lint is the initial value of the load on the rotary tool. ΔL is the amount of change in the load on the rotary tool from Lint. Also, dD=ΔD / Dint. Dint is the initial value of the separation distance of the rotary tool, and ΔD is the amount of change in the separation distance of the rotary tool from Dint.

[0021] That is, when friction stir welding is performed using high-melting-point steel plates as the workpieces, wear of the rotary tool is likely to occur. Insufficient heat input and poor plastic flow due to wear of the rotary tool cause deterioration of the joint characteristics of the weld. In this regard, the separation distance of the rotary tool is controlled so as to satisfy the relationship of the above-mentioned formula (1) (consisting of three formulas divided into cases), preferably the relationship of the following formula (2), depending on the load on the rotary tool during welding, which is correlated with the progress of wear of the rotary tool. This makes it possible to stably produce high-quality welded members regardless of the progress of wear of the rotary tool.・When 0.01≦dL<0.10, dL×0.12≦dD≦dL×0.9 ・When 0.10≦dL<0.30, dL×0.24−0.012≦dD≦dL×1.1−0.02 ... (2) ・When 0.30≦dL<0.70, dL×0.48−0.084≦dD≦dL×1.3−0.08

[0022] If dL is less than 0.01, the joining can be performed with the separation distance of the rotary tools kept at Dint, i.e., dD = 0. In addition, at the initial stage of joining, plastically flowed workpieces may adhere to the rotary tools, increasing the tool load. In this case, too, the joining can be performed with the separation distance of the rotary tools kept at Dint, i.e., dD = 0. If dL is 0.70 or more, it is recommended to replace the rotary tools. Here, the separation distance D of the rotary tools is the separation distance (shortest distance) between the front-side rotary tool and the back-side rotary tool in the thickness direction of the workpieces (hereinafter also referred to as the thickness direction), as shown in FIG. 1 .

[0023] Furthermore, Lint may be the load on the rotary tool when, for example, the separation distance of the rotary tool is set to an initial value (Dint) and welding is performed under the same welding conditions as those in actual welding (expected for actual welding) in a state where the rotary tool is not worn, etc. Lint can be obtained, for example, by performing a preliminary welding test in advance.

[0024] ΔL can be expressed as ΔL=Lint−LT, where LT is the load on the rotary tool at time T. In other words, ΔL can be said to be the amount of reduction in the load on the rotary tool from Lint.

[0025] Dint may be determined based on the materials to be joined and the joining conditions planned for actual joining. For example, Dint is preferably 0.01 x t to 0.5 x t. Dint is more preferably 0.05 x t or greater. Dint is more preferably 0.3 x t or less. Here, t is the thickness (mm) of the materials to be joined. In the case of butt joining, t is the average thickness of the materials to be joined, and in the case of lap joining, t is the thickness of the overlapping portion of the materials to be joined.

[0026] ΔD can be expressed as ΔD=Dint−DT, where DT is the separation distance of the rotary tool at time T. In other words, ΔD can be said to be the amount of decrease in the separation distance of the rotary tool from Dint.

[0027] The load of the rotary tool is preferably, for example, a load value in the axial direction of the rotary tool, a load value in the welding direction, or a torque value in the rotational direction. Note that the load value in the axial direction of the rotary tool, the load value in the welding direction, or the torque value in the rotational direction also include values ​​derived from these values ​​(for example, values ​​for which the control range of dD according to the above formula (1) is the same as when the load value in the axial direction of the rotary tool, the load value in the welding direction, or the torque value in the rotational direction is used). The load value in the axial direction of the rotary tool, the load value in the welding direction, or the torque value in the rotational direction can be measured using a general load measuring device or torque measuring device.

[0028] In addition, when the load on the rotary tool differs between the front side rotary tool and the back side rotary tool, the relationship of the above formula (1) (preferably the above formula (2)) is satisfied for both the front side rotary tools.

[0029] Note that monitoring the load of the rotary tool means measuring the load of the rotary tool at predetermined intervals. The measurement interval is not particularly limited, and is preferably set to, for example, 0.1 to 1.0 seconds. Note that the measurement interval may or may not be constant.

[0030] The joining conditions other than those described above are not particularly limited and may be performed in accordance with conventional methods. For example, the rotation speed of the rotary tool is preferably 300 to 2000 rpm. Furthermore, the rotation directions of the rotary tools are preferably opposite between the front-side rotary tool and the back-side rotary tool. From the viewpoint of improving the efficiency of joining, the joining speed is preferably 0.5 m / min or more, more preferably 0.8 m / min or more. The positions of the tips of the front-side rotary tool and the back-side rotary tool may be appropriately set in accordance with conventional methods so as to satisfy the relationship of the above-mentioned formula (1). For example, the positions of the tips of the front-side rotary tool and the back-side rotary tool are preferably symmetrical in the thickness direction with respect to the center position of the thickness of the workpieces.

[0031] Examples of joining methods include butt joining and lap joining. In butt joining, the end faces of the materials to be joined are placed opposite each other, and a rotary tool is pressed against the butt joint, including the end faces (butt joints), of the materials to be joined, while rotating. Then, in this state, the rotary tool is moved in the joining direction to join the materials to be joined. In lap joining, at least a portion of the ends of the materials to be joined are overlapped, and the rotary tool is pressed against the overlap joint while rotating. Then, in this state, the rotary tool is moved in the joining direction to join the materials to be joined.

[0032] The rotary tool to be used is not particularly limited, and a general rotary tool used for double-sided friction stir welding can be used. It is preferable to use a material for the tip of the rotary tool that is harder than the materials to be welded.

[0033] The metal plate used for the workpiece is not particularly limited. For example, a high-melting-point steel plate with a melting point of 1350°C or higher (particularly 1400°C or higher), which is particularly susceptible to wear of the rotary tool, is suitable, such as a steel plate with a Si content of 2.0 to 5.0 mass%. An example of the chemical composition of such a steel plate is, in mass%, C: 0.1% or less, Si: 2.0 to 5.0%, Al: 2.0% or less, and Mn: 1.0% or less, with the balance being Fe and unavoidable impurities. Examples of unavoidable impurities include P: 0.2% or less, S: 0.01% or less, and N: 0.01% or less. The above-mentioned composition may optionally contain, in mass %, at least one selected from the group consisting of Cr: 1% or less, Ni: 1% or less, Cu: 1% or less, Sn: 0.2% or less, Sb: 0.2% or less, Ca: 0.01% or less, REM: 0.05% or less, and Mg: 0.01% or less. Elements other than Si and Fe may all be 0% by mass. The C content is more preferably 0.001% by mass or more. The Mn content and Al content are each more preferably 0.01% by mass or more. The metal plates used as the welded materials may be made of the same steel type or different steel types. Furthermore, the thickness of each metal plate used as the welded material is preferably 0.2 to 3.2 mm.

[0034] [2] Friction stir welding apparatus Next, a friction stir welding apparatus according to one embodiment of the present invention will be described. The friction stir welding apparatus according to one embodiment of the present invention is a friction stir welding apparatus that can be suitably used in the friction stir welding method of [1] above, and includes a pair of opposing rotary tools, a measuring device that measures the load on the rotary tools, a calculating device that calculates a control range for the separation distance of the rotary tools based on the load on the rotary tools measured by the measuring device, based on the above formula (1), preferably the above formula (2), and a position adjusting mechanism that changes the position of the rotary tools within the control range for the separation distance of the rotary tools calculated by the calculating device.

[0035] The aspects of the rotary tool are as described above.

[0036] The type of the measuring device is not particularly limited. Examples of the measuring device include a general load measuring device or torque measuring device that can measure the load value in the axial direction of the rotary tool, the load value in the welding direction, or the torque value in the rotational direction. In particular, a load measuring device or torque measuring device that can monitor (measure at predetermined intervals) the load value in the axial direction of the rotary tool, the load value in the welding direction, or the torque value in the rotational direction is suitable.

[0037] Examples of the computing device include an input unit that inputs data such as various settings and the load of the rotary tool measured at predetermined intervals by a measuring device, a computing unit that processes the input data, a storage device that stores the data, etc., and an output unit that outputs an operation signal to the position adjustment mechanism based on the results of the processing by the computing unit.

[0038] The position adjustment mechanism is not particularly limited in aspect, and may be, for example, an electric drive device that moves the rotary tool in the axial direction.

[0039] The device configuration other than that described above is not particularly limited, and may be the same as that of a conventionally known friction stir welding device.

[0040] [3] Method for Manufacturing a Joined Member Next, a method for manufacturing a joined member according to one embodiment of the present invention will be described. The method for manufacturing a joined member according to one embodiment of the present invention involves joining workpieces using a friction stir welding method to obtain a joined member. Here, the joined member has two or more base materials (workpieces) and a joint between the base materials (a joint that joins the base materials). Each base material is composed of a metal plate that serves as the workpiece. The joint is a region that has undergone hot working due to frictional heat and plastic flow between the rotary tool and the workpieces, resulting in a recrystallized structure. Conditions other than those described above are not particularly limited, and may be conventional. The number of base materials joined at one joint may be two or more. The number of base materials joined at one joint may be, for example, two, or between three and five.

[0041] Two steel plates with a melting point of 1450°C (Si content: 4.0% by mass) and a thickness of 2 mm were butted together and subjected to double-sided friction stir welding under the conditions shown in Table 1. The welding was performed under each condition until the dL of the rotary tool reached 0.70. A WC flat-tip rotary tool was used for both the front-side rotary tool and the back-side rotary tool. Conditions not specified were as described above or in accordance with standard practice. In both cases, the welding conditions were essentially the same on both sides, and therefore, only one side is listed in Table 1. Additionally, the appearance observations described below were performed on both sides of the joint.

[0042] Here, in the invention example No. 1, the friction stir welding apparatus described above in [2] was used. Then, while monitoring the load on the rotary tool, the rotary tool separation distance was controlled to satisfy the above formula (1) according to the load on the rotary tool, and welding was performed. Furthermore, Lint was the rotary tool load measured in a preliminary joining test. That is, in the preliminary joining test, welding was performed under the joining conditions planned for actual joining, with the rotary tool separation distance set to 0.400 mm, without any wear on the rotary tool, and the rotary tool load at that time was measured. Meanwhile, in the comparative example No. 2, welding was performed with the rotary tool separation distance kept at Dint. Furthermore, in the comparative example No. 3, welding was performed by appropriately changing the rotary tool separation distance outside the range of the above formula (1). Conditions other than those described above were standard. The load on the rotary tool was the axial load value of the rotary tool.

[0043] The appearance of the welded portions of the thus obtained welded members was observed and the joint efficiency was evaluated at each position where dL was 0.05, 0.09, 0.10, 0.20, 0.30, 0.50, and 0.69. The evaluation results are also shown in Table 1. When the appearance observation and the joint efficiency evaluation were "good" at all positions, it was determined that excellent stability of the joint characteristics was obtained.

[0044] In the appearance observation, the joints of the joining members were evaluated as "good" when no burrs or defects were visually observed at each position, and as "poor" when either burrs or defects were observed.

[0045] In the evaluation of joint efficiency, joint efficiency (= breaking strength of joint / breaking strength of base material × 100) of more than 90% was evaluated as "excellent," joint efficiency of 70% to 90% was evaluated as "good," and joint efficiency of less than 70% was evaluated as "poor."

[0046] Here, the breaking strength of the joint was measured as follows. That is, test pieces having the same shape as the No. 1 test piece specified in JIS Z 3121 (2013) were taken from each position of the joint of the obtained joint member so that the joining direction and thickness direction were perpendicular to the longitudinal direction of the test piece and the joint part was located at the center of the parallel part. Next, a tensile test was performed in accordance with JIS Z 3121 (2013) using the taken test pieces to determine the maximum test force (N). The determined maximum test force (N) was then multiplied by the cross-sectional area (mm ２ The value obtained by dividing by the value of the fracture strength of the joint at each position was taken as the fracture strength of the joint at each position.

[0047] The breaking strength of the base material can also be referred to as the tensile strength (TS) of the base material (joined material). That is, the breaking strength of the base material can be measured by conducting a tensile test in accordance with JIS Z 2241 (2022). For example, a JIS No. 5 test piece is taken from the base material or the same material as the base material. Next, a tensile test is conducted using the taken test piece at a crosshead speed of 10 mm / min to measure the tensile strength (TS). Note that when the breaking strength differs depending on the base material, the maximum breaking strength of each base material is used in evaluating the joint efficiency.

[0048]

[0049] As shown in Table 1, in the examples of the invention, the welds of the joined components obtained after joining were evaluated as good in appearance observation and joint efficiency at all positions where dL = 0.05, 0.09, 0.10, 0.20, 0.30, 0.50, and 0.69, and excellent stability of joint characteristics of the welds was obtained. On the other hand, in the comparative examples, at least one of the appearance observation and joint efficiency was evaluated as poor at the positions where dL = 0.05, 0.09, 0.10, 0.20, 0.30, 0.50, and 0.69, and the desired stability of joint characteristics of the welds was not obtained.

[0050] Furthermore, when butt joining or lap joining was performed using various high melting point steel plates (various steel plates having the above-mentioned preferred component composition, melting points of 1400 to 1530°C, and thicknesses of 0.2 to 3.2 mm) as the joined materials, excellent stability of joint characteristics was obtained at the welded portion when the load on the rotary tool was controlled so as to satisfy the above formula (1) using the axial load value of the rotary tool, the load value in the joining direction, or the torque value in the rotational direction.

[0051] 1 Rotating tool (surface side rotating tool) 2 Rotating tool (rear side rotating tool) 3 Joining material

Claims

1. A friction stir welding method in which a pair of opposing rotating tools is pressed against the front and back surfaces of workpieces, respectively, and the workpieces are joined by moving the rotating tools in the joining direction while rotating, wherein the load of the rotating tools and the separation distance of the rotating tools satisfy the following relationship: ・When 0.01≦dL<0.10, dL×0.1≦dD≦dL×1.0 ・When 0.10≦dL<0.30, dL×0.2-0.01≦dD≦dL×1.2-0.02 ・When 0.30≦dL<0.70, dL×0.4-0.07≦dD≦dL×1.4-0.08 Here, dL=ΔL / Lint. Lint is the initial value of the load of the rotating tool. ΔL is the amount of change in the load of the rotating tool from Lint. In addition, dD=ΔD / Dint, where Dint is the initial value of the separation distance of the rotary tool, and ΔD is the amount of change in the separation distance of the rotary tool from Dint.

2. The friction stir welding method according to claim 1, wherein the load of the rotating tool is a load value in the axial direction of the rotating tool, a load value in the welding direction, or a torque value in the rotational direction.

3. The friction stir welding method according to claim 1, which is butt welding or lap welding.

4. The friction stir welding method according to claim 2, which is butt welding or lap welding.

5. A friction stir welding apparatus for use in the friction stir welding method according to any one of claims 1 to 4, comprising: a pair of opposing rotating tools; a measuring device for measuring the load on the rotating tools; a calculating device for calculating a control range for the separation distance of the rotating tools based on the load on the rotating tools measured by the measuring device and based on the formula set forth in claim 1; and a position adjustment mechanism for changing the position of the rotating tools within the control range for the separation distance of the rotating tools calculated by the calculating device.

6. A method for producing a welded member, comprising joining workpieces by the friction stir welding method according to any one of claims 1 to 4 to obtain a welded member.

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