Patent ID: 12194562

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

A rotating tool for double-sided friction stir welding according to aspects of the present invention will be described below with reference to the drawings. Note that the present invention is not limited to the following embodiments.

First, a double-sided friction stir welding method to which the rotating tools for double-sided friction stir welding according to aspects of the present invention (hereinafter referred to as “rotating tools”) are applicable will be described.FIG.1illustrates an example of butt welding using a double-sided friction stir welding method.FIG.2illustrates an example of lap welding using a double-sided friction stir welding method.

As illustrated inFIG.1andFIG.2, in a double-sided friction stir welding method, a double-sided friction stir welding apparatus including a pair of rotating tools1and8, a holding device (not illustrated), and a control device (not illustrated) that controls the operations of rotating tools1and8is used. The control device controls, for example, an inclination angle α of each of the rotating tools1and8, the distance between an end portion of the rotating tool1and an end portion of the rotating tool8, the welding speed, the rotational speed of each of the rotating tools1and8, and so forth.

The rotating tools1and8of the double-sided friction stir welding apparatus (the rotating tool that is disposed on the front surface side of metal plates will hereinafter sometimes be referred to as a front-surface-side rotating tool1, and the rotating tool that is disposed on the rear surface side of the metal plates will hereinafter sometimes be referred to as a rear-surface-side rotating tool8) are arranged such that one of them is located on the side on which first surfaces (front surfaces) of metal plates (workpieces, or to-be-welded members)4are present and the other is located on the side on which second surfaces (rear surfaces) of the metal plates4are present. The two metal plates4are arranged so as to be parallel to a joint center line7that is illustrated inFIG.1andFIG.2and are each held by a holding device (not illustrated). On an unwelded portion of the two metal plates4that is located on the joint center line7, the rotating tools1and8move in a welding direction (a direction indicated by an arrow in each of the drawings) while rotating and pressing the metal plates4. As a result, the metal plates4are softened by frictional heat generated between the rotating tools1and8and the metal plates4, and the softened portions of the metal plates4are stirred by the rotating tools1and8in such a manner as to generate plastic flow, so that the metal plates4are welded together. Note that, in the following description, a portion in which the metal plates4have been welded together will be referred to as a welded portion5.

As illustrated inFIG.1andFIG.2, when viewed from the front surface side (or the rear surface side) of the metal plates4, it is preferable that the rotating tool1on the front surface side and the rotating tool8on the rear surface side, which oppose each other, rotate in opposite directions. This enables a rotation torque that is applied by the rotating tool1to the metal plates4and a rotation torque that is applied by the rotating tool8to the metal plates4to cancel each other out. As a result, compared with the case of a friction stir welding method of the related art in which an unwelded portion is pressed and welded by using only one rotating tool disposed on one surface side, the structure of a jig that restrains a to-be-welded member can be further simplified. In the cases illustrated inFIG.1andFIG.2, the direction of rotation of the front-surface-side rotating tool1is indicated by arrow Ts, and the direction of rotation of the rear-surface-side rotating tool8is indicated by arrow Tb.

Note that, if the rotating tool1on the front surface side and the rotating tool8on the rear surface side, which opposes each other, rotate in the same direction, the speed of one of the rotating tools relative to the other of the rotating tools approaches zero. As a result, plastic deformation of the metal plates4becomes smaller as the plastic flow of the metal plates4becomes close to a uniform state, and heat generated by plastic deformation of the material cannot be obtained, so that it is difficult to achieve a favorable welded state. Thus, in order to uniformly obtain a temperature rise and a shearing stress that are sufficient to achieve a favorable welded state in a thickness direction of the metal plates, it is effective to set the directions of rotations of the rotating tool1on the front surface side (the first surface side) and the rotating tool8on the rear surface side (the second surface side), which oppose each other, to be opposite to each other.

Here, types of welding of metal plates will now be described.

Preferred examples of types of welding of metal plates include butt welding and lap welding. In butt welding, as illustrated inFIG.1, in a state where the two metal plates4are placed end to end without overlapping, a butt-joint portion including the end surfaces (abutting surfaces) of the metal plates opposing each other, is pressed by the rotating tools1and8, and the rotating tools1and8are caused to move in the welding direction while rotating, so that the metal plates are welded together. In lap welding, as illustrated inFIG.2, an overlapping portion where end portions of the two metal plates4at least partially overlap each other is pressed by the rotating tools1and8, and the rotating tools1and8are caused to move in the welding direction while rotating, so that the metal plates are welded together. Note that the difference betweenFIG.1andFIG.2is only the type of welding, and the configurations of the other devices and so forth are the same, and thus, a case of butt welding illustrated inFIG.1will be mainly described below.

The rotating tools for double-sided friction stir welding according to aspects of the present invention will now be described.

FIG.4is a diagram illustrating a rotating tool20of the related art that includes a probe.FIG.5toFIG.8are diagrams illustrating the rotating tools1and8in accordance with aspects of the present invention.FIG.5illustrates one of the rotating tools according to the first embodiment of the present invention.FIG.6illustrates one of the rotating tools according to the second embodiment of the present invention.FIG.7illustrates one of the rotating tools according to the third embodiment of the present invention. Note thatFIG.4toFIG.7each has a side view in the upper part thereof and a plan view in the lower part thereof. The front-surface-side rotating tool1and the rear-surface-side rotating tool8have the same shape, and thus, only the front-surface-side rotating tool1is illustrated inFIG.4toFIG.8.

The rotating tool20including a probe (a pin)21, which is an example of the related art, will now be described with reference toFIG.4.FIG.4(a)andFIG.4(b)each illustrate an example of the rotating tool20that includes the probe21formed on a shoulder portion22. For example, in the case of the rotating tool20illustrated inFIG.4(a), the rotating tool20is shaped as follows: the diameter of the shoulder portion22(shoulder diameter) is 12 mm, the diameter of the probe21(pin diameter) is 4 mm, the length of the probe21(pin length) is 0.5 mm, and the depth of a concave surface is 0.3 mm. In the case of the rotating tool20illustrated inFIG.4(b), the rotating tool20is shaped as follows: the shoulder diameter is 20 mm, the pin diameter is 6.7 mm, the pin length is 0.7 mm and the depth of a concave surface is 0.3 mm.

As illustrated inFIG.4(a)andFIG.4(b), an end portion of the rotating tool20of the related art, that is, a portion of the rotating tool20that comes into contact with softened portions of metal plates during welding, includes the shoulder portion22(the area indicated by the shoulder diameter inFIG.4(a)andFIG.4(b)) and the probe21(the area indicated by the pin diameter inFIG.4(a)andFIG.4(b)). The shoulder portion22has a flat shape formed of a substantially planar surface or a gently curved surface. The probe21has a shape that is discontinuous to the shoulder portion22and has a shape protruding substantially vertically toward the metal plates (not illustrated).

The probe21has a function of improving a stirring performance in the vicinity of center portions of the metal plates in a plate-thickness direction by entering softened portions of the metal plates further toward the center in the plate-thickness direction during welding. On the other hand, there is a problem in that the probe21that is positioned further forward in the plate-thickness direction (toward the center of the plate-thickness) receives a stress greater than the stress received by the shoulder portion22. Consequently, there is another problem in that repair is required due to the above-mentioned breakage and wear of a rotating tool.

As a result of extensive studies, the inventors of the present invention have developed rotating tools for double-sided friction stir welding that are capable of suppressing occurrence of a defect in a welded portion and increasing a welding speed without having a probe that is especially likely to break or become worn by nature due to a greater stress applied thereto.

As illustrated inFIG.5toFIG.7, an end of each of the rotating tools1and8for double-sided friction stir welding according to aspects of the present invention (the rotating tools1and8opposing each other) is formed of simply an end portion11. Unlike the configuration of the rotating tool of the related art, which is illustrated inFIG.4, the end portion11of each of the rotating tools according to aspects of the present invention does not include the probe21. The end portion11of each of the rotating tools1and8is formed in any one type of a planar shape11a(seeFIG.5), a convex curved shape11b(seeFIG.6), and a concave curved shape11c(seeFIG.7). In addition, each of the end portions11is formed to have a circular cross section.

Here, the end portions11of the rotating tools1and8(an end portion2of the front-surface-side rotating tool and an end portion9of the rear-surface-side rotating tool, which are illustrated inFIG.1and so forth) are portions that come into contact with the metal plates4and with flowing portions (softened portions) of the metal plates4during welding. Thus, the end portions11of the rotating tools1and8are made of a material harder than the metal plates4in a high-temperature environment to which the end portions11are exposed during welding. Accordingly, during welding, the rotating tools1and8can deform the metal plates4while the shapes of the end portions11are maintained. As a result, a high stirring performance can be continuously obtained, and suitable welding can be performed.

Note that test methods for Vickers hardness at elevated temperatures may be used for hardness comparison. The rotating tools1and8may be formed such that only their end portions have the above-mentioned hardness or such that the entire rotating tools1and8have the above-mentioned hardness.

In accordance with aspects of the present invention, in addition to the above-described configuration, it is preferable that the end portion11of each of the rotating tools1and8has vortex-shaped (spiral-shaped) stepped portion12. It is preferable that vortices (spirals) forming the stepped portions12of the rotating tool1be provided so as to extend in a direction opposite to the direction of rotation of the rotating tool1, and it is preferable that vortices (spirals) forming the stepped portions12of the rotating tool8be provided so as to extend in a direction opposite to the direction of rotation of the rotating tool8. It is preferable to provide one or more vortices forming the stepped portions12. Note that if the number of vortices forming the stepped portions12is greater than six, an effect of improving material flow decreases, and in addition, there is a possibility that the end portions11of the rotating tools1and8may easily break as a result of their shapes becoming complex. Therefore, it is preferable that the number of vortices forming the stepped portions12be six or smaller. Note that, in the cases illustrated inFIG.5(b),FIG.6(b), andFIG.7(b)and in the case illustrated inFIG.8(a), four vortices are provided.

From the standpoint of preventing breakage of the end portions11of the rotating tools1and8while improving material flow, the number of vortices forming the stepped portions12can be adjusted in accordance with the diameter of each of the end portions11. More specifically, it is preferable to increase the number of vortices as the diameter of each of the end portions11becomes larger, and it is preferable to decrease the number of vortices as the diameter of each of the end portions11becomes smaller.

Each of the stepped portions12has a shape that is recessed on the other surface (planar surface or curved surface) of a corresponding one of the end portions. By providing such recessed stepped portions12, when the rotating tools1and8press and stir the metal plates4, a metal material softened by frictional heat is caused to flow from the outside of the rotating tools1and8toward the inside, and the metal material can be suppressed from flowing out of the portion pressed by the rotating tools1and8. This can facilitate plastic flow of the pressed portion. In addition, a decrease in the thickness of a welded portion in comparison with a base member can be suppressed, and a beautiful surface of the welded portion can be formed with no burr. Note that the above advantageous effects, which are obtained as a result of providing the stepped portions, are obtained by forming the vortex-shaped stepped portions12such that the stepped portions12extend in the direction opposite to the direction of rotation of the rotating tools1and8. In other words, it is preferable that the end portions of the rotating tools according to aspects of the present invention do not have a vortex-shaped stepped portion, or it is preferable that the end portions of the rotating tools according to aspects of the present invention each have vortex-shaped stepped portions that are formed to extend in a direction opposite to the rotation direction of the corresponding rotating tool.

Note that, advantageous effects similar to those described above can be obtained by providing the vortex-shaped stepped portions12, each of which is formed to extend in the direction opposite to the rotation direction of the corresponding rotating tool, in one or more tiers.

The stepped portions12will now be described more specifically with reference toFIG.8.FIG.8(a)is a plan view of the rotating tool (front-surface-side rotating tool)1that includes the end portion11having the convex curved shape11b, andFIG.8(b)andFIG.8(c)are sectional views taken along line B-B′ ofFIG.8(a).

As illustrated inFIG.8(a), when viewed in plan view, each of the stepped portions12is formed to extend in the direction opposite to the rotation direction. In other words, the direction of the curve of each of the stepped portions12extending from the circumference of a circle toward the center of the circle is set to be opposite to the rotation direction of the rotation tool.

As illustrated inFIG.8(a), when viewed in plan view, each of the vortex-shaped stepped portions12forms a curve extending from the vicinity of the center of the circle toward the circumference of the circle. It is preferable that the length of each vortex be 0.5 turns or more and 2 turns or less when the length of the outer periphery of the end portion11is one turn. The length of each vortex can also be adjusted in accordance with the diameter of the end portion11, and it is preferable to increase the length of each vortex as the diameter of the end portion11becomes larger and to decrease the length of each vortex as the diameter of the end portion11becomes smaller.

Specific examples of the stepped portions12may be step portions12bthat are illustrated inFIG.8(b)and groove portions12cthat are illustrated inFIG.8(c). In the case illustrated inFIG.8(b), similar to the convex curved surface of the end portion11of the rotating tool1, the step portions12bform substantially horizontal steps such that the heights of the steps gradually increase from the circumference of a circle toward the center of the circle. In accordance with aspects of the present invention, the vortex-shaped steps may be formed in one or more tiers from the standpoint of obtaining the above-described advantageous effects. In the case illustrated inFIG.8(b), the formed stepped portions12each have a vortex-like shape when viewed in plan view as illustrated inFIG.8(a).

Note that, although not illustrated, in the case of a rotating tool that includes an end portion having a concave curved shape, when the step portions12bare each formed in the concave curved shape, steps may be formed along the concave curved shape such that the heights of the steps gradually decrease from the circumference of a circle toward the center of the circle.

In the case illustrated inFIG.8(c), in the curved surface (convex curved surface) of the end portion11of the rotating tool1, each of the groove portions12chas a groove that is substantially U-shaped when viewed in cross section so as to be recessed on the other surface. In accordance with aspects of the present invention, one or more groove portions12cmay be formed from the standpoint of obtaining the above-described advantageous effects. In the case illustrated inFIG.8(c), the formed groove portions12ceach have a long and narrow shape extending in a vortex-like manner when viewed in plan view as illustrated inFIG.8(a).

Note that, although not illustrated, in the rotating tool1that includes the end portion11having the concave curved shape11cor the planar shape11a, also when the groove portions12care formed in the concave curved surface or the planar surface, grooves each of which is substantially U-shaped when viewed in cross section may be formed.

In accordance with aspects of the present invention, in addition to the above-described configuration, it is preferable that a diameter D (mm) of the end portion11of each of the rotating tools1and8satisfy the following relational formula (3):
4×t≤D≤20×tformula (3)
where t stands for thickness of each metal plate (mm).

By controlling the diameter of each of the end portions11, the rotating tools1and8can provide a temperature rise and a shearing stress uniformly and effectively in the plate-thickness direction of the metal plate4. It is preferable that the diameter D of the end portion11of the rotating tool1be controlled in accordance with the thickness of each of the metal plates4(the total thickness of the metal plates4in the case of lap welding). In other words, it is effective to set the diameter D (mm) of the end portion11of each of the rotating tools1and8in accordance with the above formula (3): 4×t≤D≤20×t.

If the diameter D (mm) is less than 4×t (mm), uniform plastic flow in the plate-thickness direction may sometimes not be effectively obtained. In contrast, in the case where the diameter D (mm) is greater than 20×t (mm), a region in which plastic flow is generated is unnecessarily widened, and an excessive load is applied to the apparatus. Therefore, this case is not preferable.

The rotating tools according to the first to third embodiments of the present invention will be described in detail below. Note that only the front-surface-side rotating tool1is illustrated inFIG.5toFIG.7.

First Embodiment

As illustrated inFIG.5(a)andFIG.5(b), each of the rotating tools1and8according to the first embodiment of the present invention ends with circular peripheries, and each of these ends is formed of simply the end portion11having the planar shape11a. Each of the end portions11formed in the planar shape has an end surface that comes into contact with the metal plates and that is formed of a single planar surface perpendicular to the rotation axis of a corresponding one of the rotating tools1and8. Unlike a rotating tool of the related art, each of the end surfaces does not have a probe protruding toward the metal plates. In addition, as illustrated inFIG.5(b), in each of the rotating tools1and8, the above-mentioned vortex-shaped (spiral-shaped) stepped portions12, which extend in the direction opposite to the rotation direction, can be formed in one or more tiers in the end portion11. Note that the step portions12bor the groove portions12c, which have been described above, are formed as the stepped portions12.

Second Embodiment

As illustrated inFIGS.6(a) and6(b), the rotating tools1and8of the second embodiment have circular ends each of which is formed of only the end portion11having the convex curved shape11b, and these ends of the rotating tools are convexed. Although a rotating tool of the related art includes a probe protruding toward metal plates and has a discontinuous shape formed of a shoulder portion and the probe, each of the end portions11having the convex curved shape has a continuous shape without having a probe and forms an approximately uniform inclined surface. In other words, each of the end portions1having the convex curved shape has an end surface that comes into contact with the metal plates and that is formed of a single curved surface (a parabolic surface, a prolate surface, or a spherical surface) being convexed in the direction toward the center, and the end surface forms a curve having an approximately uniform radius of curvature in a cross-sectional shape including the rotation axis in a direction vertical to the metal plates. In addition, as illustrated inFIG.6(b), in each of the rotating tools1and8, the above-mentioned vortex-shaped (spiral-shaped) stepped portions12, which extend in the direction opposite to the rotation direction, can be formed in one or more tiers in the end portion11. Note that the step portions12bor the groove portions12c, which have been described above, are formed as the stepped portions12.

In addition, in the case where the end of each of the rotating tools1and8is formed of the end portion11having the convex curved shape11b, when the convex curved surface (convex surface) has a height dv (mm), and the end portion of the rotating tool has the diameter D (mm), it is preferable that the rotating tool satisfy the following relational formula (4).
dv/D≤0.06  formula (4)

When the end portions come into contact with the metal plates within a range in which the above formula (4) is satisfied (i.e., the value of dv/D is 0.06 or smaller), pressure can be effectively applied to the flowing portions. As a result, plastic flow that is sufficient for welding can be generated by rotations of the rotating tools. In contrast, in the case of exceeding the range of the above formula (4) (i.e., the value of dv/D exceeds 0.06), the front and rear surfaces of the welded portion become notably recessed, and the thickness of the welded portion becomes notably small with respect to the thickness of each of the metal plates, so that it may sometimes be difficult to ensure the joint strength. Therefore, this case is not desirable. Note that, in order to effectively apply pressure to the flowing portions, it is preferable to set a lower limit of the value of dv/D to 0.01 or larger.

Third Embodiment

As illustrated inFIGS.7(a) and7(b), the rotating tools1and8of the third embodiment have circular ends each of which is formed of simply the end portion11having the concave curved shape11c, and these ends of the rotating tools are concaved. Although a rotating tool of the related art includes a probe protruding toward metal plates and has a discontinuous shape formed of a shoulder portion and the probe, each of the end portions11having the concave curved shape has a continuous shape without having a probe and forms an approximately uniform inclined surface. In other words, each of the end portions1having the concave curved shape has an end surface that comes into contact with the metal plates and that is formed of a single curved surface (a parabolic surface, a prolate surface, or a spherical surface) being concaved in the direction toward the center, and the end surface forms a curve having an approximately uniform radius of curvature in a cross-sectional shape including the rotation axis in the direction vertical to the metal plates. In addition, as illustrated inFIG.7(b), in each of the rotating tools1and8, the above-mentioned vortex-shaped (spiral-shaped) stepped portions12, which extend in the direction opposite to the rotation direction, can be formed in one or more tiers in the end portion11. Note that the step portions12bor the groove portions12c, which have been described above, are formed as the stepped portions12.

In addition, in the case where the end of each of the rotating tools is formed of the end portion11having the concave curved shape, when the concave curved surface (concave surface) has a depth dc (mm), and the end portion of the rotating tool has the diameter D (mm), it is preferable that the rotating tool satisfy the following relational formula (5).
dc/D≤0.03  formula (5)

When the end portions come into contact with the metal plates within a range in which the above formula (5) is satisfied (i.e., the value of dc/D is 0.03 or smaller), the concave curved surfaces of the end portions are filled with the softened metal, so that pressure can be uniformly applied to the flowing portions. As a result, plastic flow that is sufficient for welding can be generated by rotation of the rotating tools. In contrast, in the case of exceeding the range of the above formula (5) (i.e., the value of dc/D exceeds 0.03), it becomes difficult to apply a uniform pressure to the above-mentioned flowing portions, and it may sometimes become difficult to ensure plastic flow sufficient for welding. Therefore, this case is not desirable. Note that, in order to apply a uniform pressure to the flowing portions, it is preferable to set a lower limit of the value of dc/D to 0.01 or larger.

Note that the shapes of base portions of the rotating tools1and8, the base portions being opposite to the end portions of the rotating tools1and8, are not particularly limited as long as the base portions can be attached to a double-sided friction stir welding apparatus that is known in the related art.

A preferred example of a double-sided friction stir welding method using the rotating tools1and8for double-sided friction stir welding according to aspects of the present invention will now be described.

In the double-sided friction stir welding method, by optimizing conditions of the following various parameters, more favorable advantageous effects related to improvement in the durability of the rotating tools, suppression of occurrence of a joint defect, and an increase in welding speed can be obtained.
(1) inclination angle of rotating tool α(°): 0≤α≤3  formula (1)

FIG.3is a diagram illustrating a region in which friction stirring is performed by using the rotating tools according to aspects of the present invention.FIG.3(a)is a diagram illustrating a state where the rotating tools1and8, which are disposed on the front and rear surfaces of the metal plates4such as those illustrated inFIG.1, are moved in the welding direction when viewed in plan view from the front surfaces of the metal plates4.FIG.3(b)is a sectional view taken along line A-A′ ofFIG.3(a).

As illustrated inFIG.3(b), it is preferable to perform welding in a state where the rotation axes of the rotating tools1and8(a rotation axis3of the front-surface-side rotating tool and a rotation axis10of the rear-surface-side rotating tool) are each inclined backward in the welding direction at an angle α° with respect to a vertical line6, which extends in the direction vertical to the metal plates4. In other words, it is preferable that the rotating tools1and8be inclined in such a manner that the leading ends of the rotating tools1and8are positioned further forward than the trailing ends of the rotating tools1and8are in the welding direction. As a result, a load that is otherwise applied to the rotating tools1and8in the horizontal direction (a bending direction) during welding can be dispersed to components of force that compress the rotating tools1and8in the axial directions of the rotating tools1and8.

The rotating tools1and8need to be made of a material that is harder than the metal plates4, and for example, there is a case where a material having poor toughness such as a ceramic is used. In this case, when a force in the bending direction is applied to the rotating tools1and8, there is a possibility that a stress will be locally concentrated, which in turn results in breakage of the rotating tools1and8. In order to avoid such a situation, by arranging the rotating tools1and8in such a manner that their rotation axes3and10are each inclined at the predetermined angle (α°) as described above, components of the load applied to the rotating tools1and8can be received as forces that compress the rotating tools1and8in the corresponding axial directions, and the forces in the bending direction can be reduced. As a result, the durability of each of the rotating tools1and8can be further improved.

The above-mentioned advantageous effects are obtained when the inclination angle α is 0 degrees or greater, and when the inclination angle α exceeds 3°, the front and rear surfaces of the welded portion may become recessed, and this may sometimes adversely affect the joint strength. Thus, it is preferable to set the inclination angle of the rotation axis of each of the rotating tools1and8to be 0≤α≤3.
(2) distanceG(mm) between end portions of pair of rotating tools 1 and 8:0.25×t−0.2×D×sin α≤G≤0.8×t−0.2×D×sin α  formula (2)
where t stands for thickness (mm) of unwelded portion of metal plates4, D stands for diameter (mm) of end portions of rotating tools1and8, and α stands for inclination angle (°) of rotating tools1and8.

In double-sided friction stir welding, when achieving uniform provision of a sufficient temperature rise and a sufficient shearing stress in the plate-thickness direction during welding, it is important to manage the distance G between the end portions of the rotating tools1and8opposing each other. More specifically, it is preferable to manage (adjust) the above-mentioned distance G between the end portions of the rotating tools1and8to be within the range of the above formula (2) by using the thickness t of the unwelded portion of the metal plates4, the diameter D of the end portion of each of the rotating tools1and8, and the inclination angle α of each of the rotating tools1and8.

Note that, in the case where butt welding, which is illustrated inFIG.1, is performed, the thickness of one of the metal plates4may be set as the thickness t of the unwelded portion of the metal plates4. In the case where lap welding, which is illustrated inFIG.2, is performed, the total thickness of the metal plates4overlapping each other may be set as the thickness t. The inclination angle α of each of the pair of rotating tools1and8may be the same angle. In addition, the diameter D of the end portion of each of the rotating tools1and8refers to a diameter (pin diameter) of the end portion11having the planar shape or one of the curved shapes (the concave curved shape and the convex curved surface), which are illustrated inFIG.5toFIG.7, in a cross section including the corresponding rotation axis in the direction vertical to the metal plates.

In the case where the rotating tools1and8are not inclined (i.e., the inclination angle α of each of the rotating tools1and8is 0 degrees), the lower limit and the upper limit of the distance G between the end portions2and9of the rotating tools1and8may be respectively set to 0.25×t and 0.8×t.

In contrast, in the case where the rotating tools1and8are inclined (i.e., the inclination angle α of each of the rotating tools1and8is 0≤α≤3), or in the case where the diameter D of the end portion of each of the rotating tools1and8is increased in order to increase the contact area between the end portion of the rotating tool1and the front surfaces of the metal plates4and the contact area between the end portion of the rotating tool8and the rear surfaces of the metal plates4, the distance G between the rotating tools1and8needs to be set smaller. In this case, as expressed by the above formula (2), the lower limit of G may be obtained by subtracting 0.2×D×sin α from 0.25×t, and the upper limit of G may be obtained by subtracting 0.2×D×sin α from 0.8×t.

As described above, by controlling the distance G between the end portions of the rotating tools1and8to be within the range of the above formula (2), the end portions of the rotating tools1and8opposing each other are pressed against the front and rear surfaces of the metal plates4with a sufficient load, and heat generation and plastic flow in the welded portion are sufficiently facilitated. As a result, plastic flow is facilitated uniformly in the plate-thickness direction, and a joint in a favorable state can be obtained. Note that, when the above-mentioned value of the distance G exceeds the upper limit in formula (2), the end portions of the rotating tools1and8cannot be pressed against the front and rear surfaces of the metal plates4(workpieces) with a sufficient load, and the above-mentioned advantageous effects may sometimes not be obtained. In contrast, when the above-mentioned value of the distance G falls below the lower limit in formula (2), the front and rear surfaces of the welded portion may become recessed, and this may sometimes adversely affect the joint strength.

As illustrated inFIG.3(b), the above-mentioned distance G corresponds to the shortest length between the end surface of the rotating tool (front-surface-side rotating tool)1and the end surface of the rotating tool (rear-surface-side rotating tool)8, which oppose each other, in the vertical direction.

Note that welding conditions other than those described above may be set in a conventional manner. For example, in the double-sided friction stir welding apparatus and the double-sided friction stir welding method according to aspects of the present invention, the rotational speed of each of the rotating tools1and8is preferably set to 100 to 5,000 r/min and more preferably set to 500 to 3,000 r/min. By setting the rotational speed to be within these ranges, deterioration in mechanical properties due to an excessive amount of heat input can be suppressed while a favorable surface profile is maintained. The welding speed is preferably set to 1,000 ram/min or higher and is more preferably increased to 2,000 mm/min or higher.

Regarding the to-be-welded members, although it is preferable that a welding target be a high-melting-point alloy such as a steel plate, the to-be-welded members are not limited to this case. In addition, a steel plate, which is a type of a metal plate, can be a preferred example of the to-be-welded members.

In the case where the to-be-welded members are steel plates, the types of steels that can preferably be used include common structural steel and carbon steel, examples of which are rolled steels for welded structure of Japanese industrial standards (JIS) G 3106 and carbon steels for machine structural use of JIS G 4051. In addition, it can be favorably applied to high-strength structural steel that has a tensile strength of 800 MPa or greater. Even in this case, in the welded portion, a strength that is 85% or more, preferably 90% or more, and more preferably 95% or more of the tensile strength of a steel plate (a base member) can be obtained.

Double-sided friction stir welding is performed by using a double-sided friction stir welding apparatus that includes the pair of rotating tools1and8according to aspects of the present invention, such as those illustrated inFIG.1and so forth, a holding device (not illustrated), and a control device (not illustrated) that controls the rotating tools. For example, the control device controls the inclination angle of each of the rotating tools1and8, the distance between the end portions of the rotating tools, the welding speed, the rotational speed of each of the rotating tools, and so forth in such a manner as to satisfy the welding conditions of the above formulas (1) and (2).

As described above, according to the rotating tools1and8according to aspects of the present invention, the durability of each of the rotating tools1and8can be improved. In addition, by forming the end portions of the rotating tools into the above-mentioned shapes and causing the rotating tools1and8opposing each other to rotate in the opposite directions, a sufficient temperature rise and a sufficient shearing stress can be given to the metal plates during welding. As a result, the effect that the occurrence of a defect in a welded portion can be suppressed, and the welding speed can be increased. Therefore, by performing the double-sided friction stir welding method using the double-sided friction stir welding apparatus that includes the rotating tools according to aspects of the present invention, the double-sided friction stir welding can be practically applied to welding of structural steel.

EXAMPLE

Operations and effects according to aspects of the present invention will be described below by using an example. Note that the present invention is not limited to the following example.

Double-sided friction stir welding was performed by using steel plates whose plate thickness, chemical composition, tensile strength, and Vickers hardness are shown in Table 1. In the example, lap welding was performed on some of the steel plates, and butt welding was performed on the rest of the steel plates.

In the case of butt welding, two steel plates of the same type were arranged side by side, and butt-joint surfaces forming a so-called square groove, which is not angled, and having a surface condition equivalent to that of a surface machined by a milling machine were formed. After that, welding was performed by pressing the rotating tools against both the first and second surfaces (the front surface and the rear surface) of a butt-joint portion and moving the rotating tools in the welding direction.

In the case of lap welding, two steel plates of the same type were arranged so as to overlap each other, and welding was performed by pressing the rotating tools against both the first and second surfaces (the front surface and the rear surface) of a steel-plate overlapping portion while a single welding length was set to 0.5 m.

Note that, both in the case of butt welding and in the case of lap welding, the pair of rotating tools were caused to rotate in the opposite directions during welding. In other words, the tools rotate in the same direction when the end portion of each of the tools is viewed from the front. Table 2-1 and Table 2-2 show welding conditions for friction stir welding. In addition, here, eight types of rotating tools having the cross-sectional dimensions and the shapes illustrated inFIG.4toFIG.7were used. Each of the rotating tools, which were used, is made of tungsten carbide (WC) having a Vickers hardness of 1,090. The rotating tools that are illustrated inFIG.5(a)toFIG.7(a), each of which does not have a probe or a vortex-shaped stepped portion, were used in examples of the invention. In addition, in the case of using the rotating tools that are illustrated inFIG.5(b),FIG.6(b), andFIG.7(b), each of which does not have a probe but has vortex-shaped stepped portions, since the direction of the vortices shown is the clockwise direction, the rotation direction of each of the rotating tools was set to the counterclockwise direction in examples of the invention, and the rotation direction of each of the rotating tools was set to the clockwise direction in comparative examples. The rotating tools that are illustrated inFIGS.4(a) and4(b), each of which has a probe, were used in other comparative examples.

TABLE 1Platethick-Chemical compositionTensileVickersness(% by mass)strengthhard-Number(mm)CSiMnPS(MPa)ness11.60.30.210.690.0120.003101033722.40.160.070.690.0160.00942514231.20.30.210.690.0120.0031012339

TABLE 2-1Front and rear surface side welding toolsDiameter DHeight dv ofDepth dc ofof endconvexconcaveportions ofsurfaces ofsurfaces ofThicknessfront andfront and rearfront and rearSampleof sampleTypeVickersrear surfacesurface sidesurface sidesteelsteel plateofhardness ofside rotatingrotating toolsrotating toolsplate(mm)jointShapetool materialtools (mm)(mm)(mm)dv/DInvention11.6ButtTool without pin, End10908—0.2—example 1having concave surfaceVortex-shaped steps(clockwise direction)[FIG. 7(b)]Invention11.6ButtTool without pin, End109013—0.3—example 2having concave surfaceVortex-shaped steps(clockwise direction)[FIG. 7(b)]Invention11.6ButtTool without pin, End109030—0.3—example 3having concave surfaceVortex-shaped steps(clockwise direction)[FIG. 7(b)]Invention11.6ButtTool without pin, End10908—0.2—example 4having concave surfaceNo vortex-shaped step[FIG. 7(a)]Invention11.6ButtTool without pin, End109013—0.3—example 5having concave surfaceNo vortex-shaped step[FIG. 7(a)]Invention11.6ButtTool without pin, End109030—0.3—example 6having concave surfaceNo vortex-shaped step[FIG. 7(a)]Invention22.4ButtTool without pin, End109013———example 7having flat surfaceVortex-shaped steps(clockwise direction)[FIG. 5(b)]Invention22.4ButtTool without pin, End109020———example 8having flat surfaceVortex-shaped steps(clockwise direction)[FIG. 5(b)]Invention22.4ButtTool without pin, End109040———example 9having flat surfaceVortex-shaped steps(clockwise direction)[FIG. 5(b)]Invention11.6ButtTool without pin, End10908———example 10having flat surfaceNo vortex-shaped step[FIG. 5(a)]Invention11.6ButtTool without pin, End109013———example 11having flat surfaceNo vortex-shaped step[FIG. 5(a)]Invention11.6ButtTool without pin, End109030———example 12having flat surface Novortex-shaped step[FIG. 5(a)]Invention11.6ButtTool without pin, End109080.4—0.050example 13having convex surfaceVortex-shaped steps(clockwise direction)[FIG. 6(b)]Invention11.6ButtTool without pin, End1090130.5—0.038example 14having convex surfaceVortex-shaped steps(clockwise direction)[FIG. 6(b)]Invention11.6ButtTool without pin, End1090300.5—0.017example 15having convex surfaceVortex-shaped steps(clockwise direction)[FIG. 6(b)]Invention22.4ButtTool without pin, End1090130.5—0.038example 16having convex surfaceNo vortex-shaped step[FIG. 6(a)]Invention22.4ButtTool without pin, End1090200.5—0.025example 17having convex surfaceNo vortex-shaped step[FIG. 6(a)]Invention22.4ButtTool without pin, End1090400.5—0.013example 18having convex surfaceNo vortex-shaped step[FIG. 6(a)]Invention11.6ButtTool without pin, End109012—0.4—example 19having concave surfaceVortex-shaped steps(clockwise direction)[FIG. 7(b)]Invention11.6ButtTool without pin, End10906———example 20having flat surfaceVortex-shaped steps(clockwise direction)[FIG. 5(b)]Arrangement of rotatingtoolInclinationGap Gangle α ofbetweenRotation directionfront andshoulders ofof each rotatingRotational speed of rotatingFront and rear surfacerear surfacefront and reartool when endtool RSside welding toolssidesurface sideportion of rotatingFront surfaceRear surfaceWeldingCondition ofrotatingrotating toolstool is viewed fromsidesidespeed TSdc/Dstepped portionstools (°)(mm)front(time/min)(time/min)(m/min)Invention0.025Step-shaped00.80Counterclockwise300030001.2example 1[FIG. 8-(b)]Invention0.023Groove-shaped1.51.00Counterclockwise130013002.0example 2[FIG. 8-(c)]Invention0.010Groove-shaped1.51.00Counterclockwise8008001.2example 3[FIG. 8-(c)]Invention0.025—00.80Counterclockwise300030001.2example 4Invention0.023—1.51.00Counterclockwise130013002.0example 5Invention0.010—1.51.00Counterclockwise8008001.2example 6Invention—Step-shaped01.40Counterclockwise300030001.0example 7[FIG. 8-(b)]Invention—Groove-shaped1.51.40Counterclockwise250025001.5example 8[FIG. 8-(c)]Invention—Groove-shaped1.51.40Counterclockwise150015001.0example 9[FIG. 8-(c)]Invention——00.80Counterclockwise300030001.0example 10Invention——1.51.00Counterclockwise130013001.5example 11Invention——1.51.00Counterclockwise8008001.0example 12Invention—Groove-shaped00.60Counterclockwise300030001.2example 13[FIG. 8-(c)]Invention—Step-shaped00.60Counterclockwise130013002.0example 14[FIG. 8-(b)]Invention—Step-shaped00.60Counterclockwise8008001.2example 15[FIG. 8-(b)]Invention——01.40Counterclockwise300030001.0example 16Invention——01.40Counterclockwise250025001.2example 17Invention——01.40Counterclockwise150015001.0example 18Invention0.033Step-shaped1.50.80Counterclockwise130013002.0example 19[FIG. 8-(b)]Invention—Groove-shaped1.50.80Counterclockwise300030001.0example 20[FIG. 8-(c)]

TABLE 2-2Front and rear surface side welding toolsDiameter DHeight dv ofDepth dc ofof endconvexconcaveportions ofsurfaces ofsurfaces ofThicknessVickersfront andfront andfront andSampleof sampleTypehardnessrear surfacerear surfacerear surfacesteelsteel plateofof toolside rotatingside rotatingside rotatingplate(mm)jointShapematerialtools (mm)tools (mm)tools (mm)dv/Ddc/DInvention11.6ButtTool without pin,109090.6—0.067—example 21End having convex surfaceVortex-shaped steps(clockwise direction)[FIG. 6(b)]Invention11.6ButtTool without pin, End109012—0.4—0.033example 22having concave surfaceNo vortex-shaped step[FIG. 7(a)]Invention11.6ButtTool without pin, End10906————example 23having flat surfaceNo vortex-shaped step[FIG. 5(a)]Invention11.6ButtTool without pin, End109090.6—0.067—example 24having convex surfaceNo vortex-shaped step[FIG. 6(a)]Invention31.2LapTool without pin, End109020————example 25having flat surfaceVortex-shaped steps(clockwise direction)[FIG. 5(b)]Invention31.2LapTool without pin, End109040————example 26having flat surfaceVortex-shaped steps(clockwise direction)[FIG. 5(b)]Invention31.2LapTool without pin, End1090130.5—0.038—example 27having convex surfaceNo vortex-shaped step[FIG. 6(a)]Comparative11.6ButtTool without pin, End10908—0.2—0.025example 1having concave surfaceVortex-shaped steps(clockwise direction)[FIG. 7(b)]Comparative22.4ButtTool without pin, End109013————example 2having flat surfaceVortex-shaped steps(clockwise direction)[FIG. 5(b)]Comparative11.6ButtTool without pin, End1090300.5—0.017—example 3having convex surfaceVortex-shaped steps(clockwise direction)[FIG. 6(b)]Comparative31.2LapTool without pin, End109020————example 4having flat surfaceVortex-shaped steps(clockwise direction)[FIG. 5(b)]Comparative11.6ButtTool with pin, Shoulder109013—0.3—0.023example 5diameter of 12 mm, Pinlength of 0.5 mm[FIG. 4(a)]Comparative11.6ButtTool with pin, Shoulder109013—0.3—0.023example 6diameter of 12 mm, Pinlength of 0.5 mm[FIG. 4(a)]Comparative11.6ButtTool with pin, Shoulder109013—0.3—0.023example 7diameter of 12 mm, Pinlength of 0.5 mm[FIG. 4(a)]Comparative22.4ButtTool with pin, Shoulder109020—0.3—0.015example 8diameter of 20 mm, Pinlength of 0.7 mm[FIG. 4(b)]Comparative22.4ButtTool with pin, Shoulder109020—0.3—0.015example 9diameter of 20 mm, Pinlength of 0.7 mm[FIG. 4(b)]Comparative31.2LapTool with pin, Shoulder109020—0.3—0.015example 10diameter of 20 mm, Pinlength of 0.7 mm[FIG. 4(b)]Arrangement of rotatingtoolGap GFront and rearInclinationbetweenRotational speed ofsurface sideangle α ofshoulders ofRotation direction ofrotating tool RSwelding toolsfront andfront andeach rotating toolFrontRearWeldingCondition ofrear surfacerear surfacewhen end portion ofsurfacesurfacespeedsteppedside rotatingside rotatingrotating tool issidesideTSportionstools (°)tools (mm)viewed from front(time/min)(time/min)(m/min)InventionStep-shaped00.80Counterclockwise300030001.0example 21[FIG. 8-(b)]Invention—1.50.80Counterclockwise130013002.0example 22Invention—1.50.80Counterclockwise300030001.0example 23Invention—00.80Counterclockwise300030001.0example 24InventionGroove-shaped1.51.40Counterclockwise250025001.5example 25[FIG. 8-(c)]InventionGroove-shaped1.51.40Counterclockwise150015001.0example 26[FIG. 8-(c)]Invention—01.40Counterclockwise300030001.0example 27ComparativeStep-shaped00.80Clockwise300030001.2example 1[FIG. 8-(b)]ComparativeStep-shaped01.40Clockwise300030001.0example 2[FIG. 8-(b)]ComparativeStep-shaped00.60Clockwise8008001.2example 3[FIG. 8-(b)]ComparativeGroove-shaped1.51.40Clockwise250025001.5example 4[FIG. 8-(c)]Comparative—01.30Counterclockwise8008001.0example 5Comparative—1.51.20Counterclockwise130013002.0example 6Comparative—31.30Counterclockwise8008001.0example 7Comparative—1.51.80Counterclockwise300030001.0example 8Comparative—0.02.00Counterclockwise160016001.0example 9Comparative—0.02.00Counterclockwise160016001.0example 10

Evaluations were conducted in the following manner by using obtained welding joints.

(I) Existence or Nonexistence of Surface Defect when Appearance of Joint is Observed

Observation was performed on portions of the obtained welding joints in each of which the welding speed was one of the values shown Table 2-1 and Table 2-2. Regarding the existence or nonexistence of a surface defect, it is visually determined whether there is a groove-shaped portion that is in an unwelded state due to insufficient plastic flow or whether a welded portion is formed in a recessed manner because the gap G between the shoulder portions of the welding tools is too narrow. When a groove-shaped portion in the unwelded state or a welded portion formed in a recessed manner was seen as a surface defect, a depth Dd(mm) of the portion was measured by using a laser displacement meter and evaluated by using the following criteria.

<Criteria>

No: None of the above-mentioned surface defects are seen.

Acceptable: Although one of the above-mentioned surface defects is seen, the ratio (Dd/t) between the above-mentioned depth Dd(mm) and the thickness t (mm) of each steel plate was 0.1 or smaller.

Yes: One of the above-mentioned surface defects was seen, and the ratio (Dd/t) between the above-mentioned depth Dd(mm) and the thickness t (mm) of each steel plate exceeded 0.1. Alternatively, a groove-shaped portion in the unwelded state extended from the front surface to the rear surface. Note that, in the case where the groove-shaped portion in the unwelded state extended from the front surface to the rear surface, it is considered that the welding has failed, and evaluations of internal defect and joint strength are not conducted.

(II) Existence or Nonexistence of Internal Defect when Cross Section of Joint is Observed

Observation was performed on portions of the obtained welding joints in each of which the welding speed was one of the values shown Table 2-1 and Table 2-2, and the portions were cross-sectionally cut at a position 20 mm from a welding starting end, a position 20 mm from a welding terminating end, and an intermediate position between these ends so as to be used as test specimens. Regarding the existence or nonexistence of an internal defect, whether an unwelded state formed in the welded portion due to insufficient plastic flow is seen was evaluated by using an optical microscope (magnification: 10 times) on the basis of the following criteria.

<Criteria>

No: An unwelded state having a tunnel-like shape is not seen at any of the three positions mentioned above.

Acceptable: An unwelded state formed in the welded portion was seen at one of the three positions mentioned above.

Yes: A portion in the unwelded state formed in the welded portion was seen at two or more of the three positions mentioned above.

Table 3 shows the results of (I) determination of the existence or nonexistence of a surface defect by observing the appearance of a joint when a welding operation was performed once with a welding length of 0.5 m and the results of (II) determination of the existence or nonexistence of an internal defect by observing the cross section of a joint. In addition, tensile test pieces each of which had the dimensions of a No. 1 test specimen defined by JIS Z 3121 were taken from the obtained welding joints, and Table 3 shows tensile strengths obtained when a tensile test (JIS Z 3121) was performed by using the test pieces.

It was confirmed from Table 3 that, in the butt joints of Invention Examples 1 to 24 and the lap joints of Invention Examples 25 to 27, even when the welding speed was increased to 1.0 m or higher, a strongly-welded state was obtained with no surface defect found by observation of the appearance of each joint and no internal defect found by observation of the cross section of each joint. In addition, the obtained joint strengths were each 95% or more of the tensile strength of the steel plates serving as base materials.

In contrast, in the butt joints of Comparative Examples 1 to 3, welding was performed by using rotating tools each of which does not have a probe and each of which has vortex-shaped stepped portions extending in the clockwise direction while the direction of rotation of each of the rotating tools was set to the clockwise direction. A surface defect and an internal defect were observed in the obtained joints, and a strongly-welded state was not obtained. In addition, the obtained joint strengths were each 70% or less of the tensile strength of the steel plates serving as base materials.

In the lap joint of Comparative Example 4, welding was performed by using rotating tools each of which does not have a probe and each of which has vortex-shaped stepped portions extending in the clockwise direction while the direction of rotation of each of the rotating tools was set to the clockwise direction. A surface defect and an internal defect were observed in the obtained joint, and a strongly-welded state was not obtained. In addition, the obtained joint strength was 70% or less of the tensile strength of the steel plates serving as base materials.

In the butt joints of Comparative Examples 5 to 9, rotating tools each having a pin were used on the condition that all of D (the diameter (mm) of the end portion of each rotating tool), a (the inclination angle)(° of each rotating tool), and G (the distance (mm) between the end portions of the pair of rotating tools) satisfied the above formulas (1), (2) and (3).

In the butt joints of Comparative Examples 5 to 9, although it was confirmed that, even when the welding speed was increased to 1.0 m or higher, a strongly-welded state was obtained with no surface defect found by observation of the appearance of each joint and no internal defect found by observation of the cross section of each joint, it was confirmed that the durability of each of the rotating tools was unfavorable.

In the lap joint of Comparative Example 10, although it was confirmed that, even when the welding speed was increased to 1.0 m or higher, a strongly-welded state was obtained with no surface defect found by observation of the appearance of the joint and no internal defect found by observation of the cross section of the joint, it was confirmed that the durability of each of the rotating tools was unfavorable.

TABLE 3Existence ofExistence of sur-internal defectface defect whenwhen cross sec-Tensileappearance oftion of jointstrengthjoint was observedwas observed(MPa)Invention example 1NoNo1009Invention example 2NoNo1012Invention example 3NoNo1007Invention example 4NoNo1005Invention example 5NoNo1006Invention example 6NoNo1001Invention example 7NoNo430Invention example 8NoNo433Invention example 9NoNo432Invention example 10NoNo1005Invention example 11NoNo1002Invention example 12NoNo1000Invention example 13NoNo1012Invention example 14NoNo1015Invention example 15NoNo1007Invention example 16NoNo430Invention example 17NoNo433Invention example 18NoNo432Invention example 19NoAcceptable990Invention example 20AcceptableAcceptable999Invention example 21AcceptableNo995Invention example 22NoAcceptable980Invention example 23AcceptableAcceptable987Invention example 24AcceptableNo983Invention example 25NoNo1011Invention example 26NoNo1006Invention example 27AcceptableNo1002ComparativeYes (unweldedYes587example 1portion)ComparativeYes (unweldedYes274example 2portion)ComparativeYes (unweldedYes487example 3portion)ComparativeYes (unweldedYes657example 4portion)ComparativeNoNo1001example 5ComparativeNoNo1003example 6ComparativeNoNo997example 7ComparativeNoNo424example 8ComparativeNoNo422example 9ComparativeNoNo995example 10

A welding operation with a welding length of 0.5 m was repeatedly performed, and Table 4 shows the number of times, out of the cumulative number of the welding operations, that a strong joint was obtained with a probability of 90% or more while no internal defect was found by observation of the cross section of the joint. As shown in Table 4, in the butt joints of Invention Examples 1 to 24 and the lap joints of Invention Examples 25 to 27, the number of the welding operations in which a strong joint was obtained with a probability of 90% or more was 13 or more.

In contrast, in the butt joints of Comparative Examples 1 to 3, the welding operation was performed by using rotating tools each of which does not have a probe and each of which has vortex-shaped stepped portions extending in the clockwise direction while the direction of rotation of each of the rotating tools was set to the clockwise direction. In Comparative Examples 1 to 3, the number of the welding operations in which a strong joint was obtained with a probability of 90% or more was zero.

In the lap joint of Comparative Example 4, the welding operation was performed by using rotating tools each of which does not have a probe and each of which has vortex-shaped stepped portions extending in the clockwise direction while the direction of rotation of each of the rotating tools was set to the clockwise direction. In Comparative Example 4, the number of the welding operations in which a strong joint was obtained with a probability of 90% or more was zero.

In the butt joints of Comparative Examples 5 to 9, the welding operation was performed by using rotating tools each having a pin, and the number of the welding operations in which a strong joint was obtained with a probability of 90% or more was 10 or less.

In the lap joint of Comparative Example 10, the welding operation was performed by using rotating tools each having a pin, and the number of the welding operations in which a strong joint was obtained with a probability of 90% or more was 10 or less.

As described above, it was revealed that, when welding was performed by using the rotating tools, each of which does not have a probe and each of which has vortex-shaped stepped portions extending in a direction the same as the rotation direction of the rotating tool, a defect occurred in the joint, or a problem occurred in the joint strength, and it was revealed that, when the rotating tools each having a pin were used, the durability of each of the rotating tools was unfavorable.

Experiments were conducted under the same conditions except for the existence or nonexistence of the vortex-shaped stepped portions in each pair of the above Invention Examples, that is, Invention Examples 1 and 4, Invention Examples 2 and 5, Invention Examples 3 and 6, Invention Examples 19 and 22, Invention Examples 20 and 23, and Invention Examples 21 and 24, and these experimental results shown in Table 3 have revealed that the use of rotating tools each of which has vortex-shaped stepped portions increased the weld strength of each joint.

In addition, the results obtained from Invention Examples 19 and 22 in which experiments were conducted under conditions that do not satisfy the following formula (5) have revealed that, when exceeding the range of the formula (5), although it is evaluated that there is no surface defect, it affects ensuring of plastic flow that is sufficient for welding, which may lead to occurrence of an internal defect. In other words, it is understood that, when rotating tools each having a concave surface further satisfy the condition of formula (5), occurrence of a surface defect and an internal defect can be more effectively suppressed, so that a joint having a sufficient strength can be obtained.
Dc/D≤0.03  Formula (5)

In addition, the results obtained from Invention Examples 20 and 23 in which experiments were conducted under conditions that fall below the lower limit of the range of the following formula (3) have revealed that, when falling below the lower limit of the range of formula (3), although the evaluations of surface defect and internal defect are acceptable, such conditions affect ensuring of uniform plastic flow in the plate-thickness direction, which may lead to occurrence of a surface defect and an internal defect. In other words, it is understood that, when rotating tools each having a planar surface further satisfy the condition of formula (3), occurrence of a surface defect and an internal defect can be more effectively suppressed, so that a joint having a sufficient strength can be obtained.
4×t≤D≤20×tformula (3)

The results obtained from Invention Examples 21 and 24 in which experiments were conducted under conditions that do not satisfy formula (1) have revealed that, when exceeding the range of formula (4), although the evaluations of surface defect and internal defect are acceptable, such conditions affect the shape of the surface of a welded portion, which may lead to occurrence of a surface defect. In other words, it is understood that, when rotating tools each having a convex surface further satisfy the condition of formula (4), occurrence of a surface defect and an internal defect can be more effectively suppressed, so that a joint having a sufficient strength can be obtained.
dv/D≤0.06  formula (4)

TABLE 4Number of times a strongjoint was obtained witha probability of 90% or moreInvention example 116Invention example 218Invention example 321Invention example 415Invention example 515Invention example 619Invention example 713Invention example 814Invention example 917Invention example 1016Invention example 1115Invention example 1220Invention example 1318Invention example 1421Invention example 1524Invention example 1615Invention example 1716Invention example 1817Invention example 1916Invention example 2015Invention example 2123Invention example 2214Invention example 2314Invention example 2418Invention example 2515Invention example 2616Invention example 2715Comparative example 10Comparative example 20Comparative example 30Comparative example 40Comparative example 510Comparative example 68Comparative example 710Comparative example 87Comparative example 99Comparative example 108

REFERENCE SIGNS LIST

1front-surface-side rotating tool2end portion of front-surface-side rotating tool3rotation axis of front-surface-side rotating tool4metal plate5welded portion6vertical line extending in direction vertical tometal plate7joint center line8rear-surface-side rotating tool9end portion of rear-surface-side rotating tool10rotation axis of rear-surface-side rotating tool11end portion12stepped portion12bstep portion12cgroove portionG distance between end portions of rotating toolsα inclination angle of rotating toolD diameter of end portion of rotating toolt thickness of metal plate