JP6493564B2 - Friction stir welding method and apparatus - Google Patents

Description

  According to the present invention, the rotary tool is inserted into an unjoined portion between the workpieces and moved while rotating, and the workpiece is softened by frictional heat with the rotary tool, and the softened portion is stirred by the rotary tool. The present invention relates to a friction stir welding method in which joining is performed without adding a filler material by using the generated plastic flow, and an apparatus for realizing the friction stir welding method.

  As a friction welding method, in Patent Document 1, by rotating both or one of a pair of metal materials, the metal material generates frictional heat and softens, while the softened portion is stirred to cause plastic flow. Thus, a technique for joining metal materials is disclosed.

  However, since this technique rotates the metal material to be joined, there is a limit to the shape and size of the metal material to be joined.

  In Patent Document 2, a tool made of a material that is substantially harder than a workpiece is inserted into an unjoined portion of the workpiece, and the tool is moved while being rotated. A method is disclosed in which workpieces are continuously joined in the longitudinal direction by heat and plastic flow.

  The friction welding method described in Patent Document 1 is a method in which workpieces are rotated and welded by frictional heat between workpieces. The friction stir welding method disclosed in Patent Document 2 is a method of joining by moving a tool while rotating a joining member in a fixed state. Thus, in the friction stir welding method, since the tool is moved and joined, even a member that is substantially infinitely long with respect to the welding direction has an advantage that it can be continuously solid-phase joined in the longitudinal direction. Moreover, since it is a solid-phase joining using the plastic flow of the metal by the frictional heat of a tool and a joining member, it can join, without melt | dissolving a junction part. Furthermore, since the heating temperature is low, deformation after joining is small, the joint is not melted, so there are few defects, and in addition, there are many advantages such as not requiring a filler material.

  The friction stir welding method is widely used in the fields of aircraft, ships, railway vehicles, automobiles and the like as a method of joining low melting point metal materials typified by aluminum alloys and magnesium alloys. The reason for this is that these low melting point metal materials are difficult to obtain satisfactory characteristics of the joints by conventional arc welding methods, and the productivity is improved and the quality is high by applying the friction stir welding method. It is because a junction can be obtained.

  On the other hand, the application of friction stir welding to structural steel, which is mainly applied as a structural material such as buildings, ships, heavy machinery, pipelines, and automobiles, is subject to solidification cracking and hydrogen cracking, which are problems in conventional fusion welding. Can be avoided, and the structural change of the steel material can be suppressed, so that it can be expected that the joint performance is excellent. Further, in the friction stir welding method, a clean interface is created by stirring the bonding interface with a rotating tool and the clean surfaces are brought into contact with each other. Therefore, an advantage that a preparatory step such as diffusion bonding is unnecessary can be expected. Thus, the application of the friction stir welding method to structural steel is expected to have many advantages. However, since there is a problem in joining workability such as suppression of defect generation during joining and an increase in joining speed, the friction stir welding method has not been widely used in structural steel compared to low melting point metal materials.

In friction stir welding of structural steel, as described in Patent Document 3 and Patent Document 4, high wear resistance such as polycrystalline boron nitride (PCBN) and silicon nitride (Si ₃ N ₄ ) is used as a rotating tool. Wearable material is used. Since these ceramics are brittle, the thickness of the steel plates to be joined and the construction conditions thereof are significantly limited in order to prevent damage to the rotary tool.

  Patent Document 5 and Patent Document 6 disclose a joining method in which a heating means is added for the purpose of improving joining workability.

  For example, Patent Document 5 includes a heating unit using an induction heating device, and by heating the workpieces before and after joining, friction that increases the joining speed and eliminates cracks in the joined part. A stir welding method is disclosed.

  Patent Document 6 has a heating means using a laser device, and the workpiece is partially heated immediately before joining, thereby suppressing the microstructure change around the heating region due to preheating and increasing the joining speed. A friction stir welding apparatus which is designed to be simplified is disclosed.

  However, the techniques of Patent Document 5 and Patent Document 6 do not take into consideration the surface temperature, depth, and the like of the heated region of the workpieces by heating before bonding, and therefore, sufficient bonding workability cannot be obtained. Furthermore, the microstructure around the heating region may change due to overheating, which may adversely affect the properties of the joint joint, particularly the joint strength.

  In Patent Document 7, the position of the heating region, the surface temperature, the depth, and the like are limited with respect to partially heating the workpieces immediately before bonding, and sufficient strength is obtained and the bonding workability is improved. A friction stir welding method is disclosed. However, the relationship between the position of partial heating of the workpiece and the frictional heat generated by the dynamic friction coefficient between the rotating tool material or the material coated on the surface of the rotating tool and the workpiece is effective for bonding workability. No influence is taken into account.

JP 62-183979 A JP 7-505090 Gazette Special table 2003-532542 gazette Japanese translation of PCT publication No. 2003-532543 JP 2003-94175 A JP 2005-288474 A International Publication No. 2015/045299

  The present invention has been made in view of the above-mentioned present situation, and at the time of friction stir welding, an object of the present invention is to solve the plastic flow failure due to insufficient heating of work materials and to improve the joining workability with sufficient strength. To do. In particular, the relationship between the position of partial heating of the workpiece and the frictional heat generation due to the dynamic friction coefficient between the rotating tool material or the material coated on the surface of the rotating tool and the workpiece affects the workability. In view of this, it is an object of the present invention to provide a friction stir welding method in which the pre-heat treatment process conditions are closely scrutinized and an apparatus for realizing the friction stir welding method.

The inventors obtained the following knowledge as a result of intensive studies to solve the above problems.
a) In ordinary friction stir welding, the only heat source required for welding is frictional heat generated between the rotary tool and the workpiece. Therefore, when the structural steel is joined by the friction stir welding method, the amount of heat necessary to soften the structural steel that is the workpiece cannot be secured. As a result, a sufficient plastic flow cannot be obtained at the joint, and there is a concern about the deterioration of joining workability such as a reduction in joining speed and the occurrence of joining defects.

In order to avoid the deterioration of joining workability, which is very important in industrializing the above technology, it is considered that a pre-heat treatment process before friction stir welding is effective.
b) However, when the preheating heat amount before the friction stir welding is performed, if the amount of preheating heat is excessive, there arises a problem that the microstructure around the heating region changes. In particular, in the case of a high-tensile steel sheet strengthened with a martensite structure, even when the periphery of the heating region is heated below the ferrite-austenite transformation temperature, softening occurs due to tempering of the martensite, and the joint joint strength Is significantly reduced.

  Therefore, the inventors examined various preheat treatment process conditions before friction stir welding.

as a result,
c) By using a heat source with a high energy density such as a laser, the surface temperature, area, and position of the heating region in the pre-heat treatment process are strictly controlled, and the temperature in the thickness direction of the heating region is also adjusted as necessary. Control appropriately. As a result, it has been found that the joining workability can be improved without causing deterioration of the jointed joint properties such as the jointed joint strength.
d) In particular, with respect to the position of partial heating of the workpiece, the relationship between the frictional heating controlled by the dynamic friction coefficient between the workpiece of the rotary tool or the material coated on the surface of the rotary tool and the workpiece. Thus, the knowledge that the region where the effect of improving the bonding workability is generated is obtained.
e) In normal friction stir welding, since the joint is naturally cooled after the completion of joining, there is a problem that the microstructure control by thermal history management that is performed in the rolling process at the time of steel production cannot be applied. there were. However, immediately after the completion of the joining, it was found that the joint joint characteristics can be further improved by performing a process that combines heat treatment and cooling treatment on the joint.

  The present invention is based on the above knowledge, and in particular, when the friction stir welding method is applied to the joining of structural steel, the plastic flow failure due to insufficient heating of the work material is solved, and sufficient In addition to high strength, it is intended to improve the joining workability.

That is, the gist configuration of the present invention is as follows.
[1] A shoulder portion and a pin portion that is arranged on the shoulder portion and shares the rotation axis with the shoulder portion, and the shoulder portion and the pin portion are made of a material harder than a steel plate that is a workpiece. The rotating tool is inserted into an unjoined portion between the steel plates and rotated to move in the joining direction, and the softened portion is softened by the frictional heat between the rotating tool and the steel plate while the softened part is moved with the rotating tool. A friction stir welding method in which steel plates are joined to each other by causing plastic flow by stirring, and the dynamic friction coefficient between the material of the rotating tool or the material coated on the surface of the rotating tool and the steel plate is 0. .6 or less, and a region where the surface temperature T _S (° C.) of the steel sheet heated by the heating means provided in front of the rotating tool in the joining direction satisfies the following formula (1) is defined as a heating region. The heating area and the front The minimum distance to the rotating tool is not more than the diameter of the shoulder of the rotating tool, the area of the heating region is not more than the area of the maximum diameter portion of the pin portion of the rotating tool, and is 65% of the area of the heating region. % Of the surface of the steel plate is a straight line that passes through the rotation axis of the rotary tool and is parallel to the welding direction, and is parallel to the welding center line and to the retreating side of the pin portion of the rotary tool. A friction stir welding method located between a straight line separated by the same distance as the maximum radius.
T _S ≧ 0.8 × T _A1 (1)
T _A1 is a temperature represented by the following formula (2).
T _A1 (° C.) = 723-10.7 [% Mn] −16.9 [% Ni] +29.1 [% Si] +16.9 [% Cr] +290 [% As] +6.38 [% W] · (2)
Said [% M] is content (mass%) of M element in the steel plate which is a workpiece, and is set to 0 when not containing.
[2] When the maximum depth from the surface of the steel sheet in the region where the temperature T _D (° C.) in the thickness direction of the heating region satisfies the following formula (3) is the depth D of the heating region, the heating The friction stir welding method according to claim 1, wherein the depth D of the region is 30% or more of the thickness of the steel plate.
T _D ≧ 0.8 × T _A1 (3)
[3] The friction stir welding method according to [1] or [2], wherein the heating means is a laser heating device.
[4] Back heating means is provided at the rear of the rotating tool in the joining direction, and the rear heating means heats the joining portion of the steel sheet according to any one of [1] to [3]. Friction stir welding method.
[5] A friction stir welding method according to [4], wherein a cooling means is provided behind the rear heating means in the joining direction, and the cooling means cools the joint heated by the rear heating means. .
[6] A cooling means is provided behind the rotating tool in the joining direction, and the cooling means cools the joining portion of the steel sheet. Friction stirring according to any one of [1] to [3] Joining method.
[7] A friction stir welding method according to [6], wherein a rear heating unit is provided behind the cooling unit in the joining direction, and the rear heating unit heats the joint cooled by the cooling unit. .
[8] A friction stir welding apparatus for joining unjoined portions between steel plates as workpieces, comprising a shoulder portion and a pin portion arranged on the shoulder portion and sharing the rotation axis with the shoulder portion. The shoulder portion and the pin portion are made of a material harder than the steel plate, and move in the joining direction while rotating in a state where the shoulder portion and the pin portion are inserted in the unjoined portion between the steel plates, so that the steel plate is caused by frictional heat. A rotating tool that causes plastic flow by stirring the softened portion while being softened, a heating unit that is provided in front of the rotating tool in the joining direction, and that heats the steel sheet, and realizes the following state 1 A friction stir that has a dynamic friction coefficient of 0.6 or less between the material of the rotary tool or the material coated on the surface of the rotary tool and the steel plate. Joining device.
(State 1)
When a region where the surface temperature T _S (° C.) of the steel sheet heated by the heating unit satisfies the following formula (1) is a heating region, the minimum distance between the heating region and the rotating tool is the rotation It is not more than the diameter of the shoulder portion of the tool, the area of the heating region is not more than the area of the maximum diameter portion of the pin portion of the rotating tool, and 65% or more of the area of the heating region is the surface of the steel plate A joint center line that is a straight line that passes through the rotation axis of the rotary tool and is parallel to the joint direction, and a straight line that is parallel to the joint center line and separated from the retreating side by the same distance as the maximum radius of the pin portion of the rotary tool. , Located between.
T _S ≧ 0.8 × T _A1 (1)
T _A1 is a temperature represented by the following formula (2).
T _A1 (° C.) = 723-10.7 [% Mn] −16.9 [% Ni] +29.1 [% Si] +16.9 [% Cr] +290 [% As] +6.38 [% W] · (2)
Said [% M] is content (mass%) of M element in the steel plate which is a workpiece, and is set to 0 when not containing.
[9] The friction stir welding apparatus according to [8], wherein the control unit controls the rotating tool and the heating unit so as to realize the following state 2.
(State 2)
When the maximum depth from the surface of the steel sheet in the region where the temperature T _D (° C.) in the thickness direction of the heating region satisfies the following formula (3) is the depth D of the heating region, the depth of the heating region The thickness D is 30% or more of the thickness of the steel plate.
T _D ≧ 0.8 × T _A1 (3)
[10] The friction stir welding apparatus according to [8] or [9], wherein the heating means is a laser heating apparatus.
[11] The apparatus according to any one of [8] to [10], further including a rear heating unit that heats the joint portion of the steel plates, the rear heating unit provided at a rear side in the joining direction of the rotary tool. Friction stir welding device.
[12] The friction stir welding apparatus according to [11], further including a cooling unit that cools the joint, and the cooling unit is provided behind the rear heating unit in the joining direction.
[13] The friction stirrer according to any one of [8] to [10], further including a cooling unit that cools a bonded portion of the steel plates, the cooling unit being provided at the rear in the bonding direction of the rotary tool. Joining device.
[14] The friction stir welding apparatus according to [13], further including a rear heating unit that heats the joint, and the rear heating unit is provided behind the cooling unit in the joining direction.

  ADVANTAGE OF THE INVENTION According to this invention, the plastic flow failure by underheating of a workpiece can be eliminated, and the joining workability of friction stir welding can be improved. Furthermore, a change in the microstructure around the heating region is also suppressed, and a high joint strength can be obtained at the joint.

FIG. 1 is a schematic diagram illustrating a friction stir welding method according to the present embodiment. FIG. 2 is a diagram (top view and AA sectional view) showing an example of a heating region in a preheating process, a cooling region and a reheating region in a process performed after bonding. FIG. 3 is a diagram showing the relationship between the temperature and tensile strength of the steel plates to be joined by the friction stir welding method according to this embodiment. FIG. 4 is a diagram showing a cross-sectional dimension of the rotary tool.

Hereinafter, the present invention will be specifically described through embodiments of the present invention. FIG. 1 is a schematic diagram illustrating a friction stir welding method and a friction stir welding apparatus according to the present embodiment. The friction stir welding method according to the present embodiment, as shown in FIG. 1, the rotary tool is moved in the welding direction while rotating and inserted to the unwelded portion between the steel plate, the frictional heat between the rotational tool and the steel sheet While the steel plates are softened by the above, the softened portion is stirred with a rotating tool to cause plastic flow, thereby joining the steel plates together. Here, the rotating tool includes a shoulder portion and a pin portion that is arranged on the shoulder portion and shares the rotation axis with the shoulder portion, and at least the shoulder portion and the pin portion are harder than the steel plate that is the workpiece. It is formed by the material.

  In FIG. 1, reference numeral 1 is a rotary tool, 2 is a rotating shaft, 3 is a steel plate, 4 is a joint, 5 is a heating means, 6 is a cooling means, and 7 is backward heating. Means 8, a shoulder of the rotary tool, 9 a pin part of the rotary tool, and 15 a control means. α indicates the tilt angle of the rotating tool. “AS” indicates an advancing side, and “RS” indicates a retreating side. Here, the advancing side is defined as the side where the tool rotation direction and the joining direction coincide with each other, and the retreating side is defined as the side where the tool rotation direction and the joining direction are opposite to each other.

  In the present embodiment, the butted portion that is not yet joined just by butting the steel plates 3 is described as “unjoined portion”, and the portion joined and integrated by plastic flow is described as “joined portion”. To do.

  In the friction stir welding method of this embodiment, a preheat treatment process in which the steel plate 3 is heated by the heating means 5 provided in front of the rotary tool 1 moving in the joining direction is important. Hereinafter, the conditions of the pre-heat treatment process will be described with reference to FIG.

  FIG. 2 is a diagram (top view and AA sectional view) showing an example of a heating region in a preheating process, a cooling region and a reheating region in a process performed after bonding. In FIG. 2, the joining center line 10 indicates a straight line passing through the rotation axis 2 of the rotary tool 1 on the surface of the steel plate 3 and parallel to the joining direction. The RS line 11 is a straight line parallel to the joining center line 10 and separated to the retreating side by the same distance as the maximum radius of the pin portion 9 of the rotary tool, 12 is a heating area, and 13 is a cooling area. , 14 is a reheating region. a indicates the diameter of the shoulder 8 of the rotary tool, b indicates the maximum diameter of the pin portion 9 of the rotary tool, X indicates the minimum distance between the heating region 12 and the rotary tool 1, and D indicates the depth of the heating region 12. T indicates the thickness of the steel plate 3.

Surface temperature T _S of steel plate in heating region: T _S ≧ 0.8 × T _A1
FIG. 3 is a diagram showing the relationship between the temperature and tensile strength of the steel plates to be joined by the friction stir welding method according to this embodiment. As shown in FIG. 3, the steel plate 3 to be joined by the friction stir welding method of the present embodiment is generally about 30% strength at room temperature at a temperature of about 80% of _TA1 , which is the transformation temperature of steel. It becomes. Moreover, when it becomes higher than this temperature, the intensity | strength of the copper plate 3 will fall further. Therefore, the steel plate 3 is previously softened so that the surface temperature T _S of the steel plate 3 satisfy the above 0.8 × T _A1 ° C., and stirred the steel plate 3, to promote plastic flow. Thereby, the load concerning the rotary tool 1 is reduced and the joining speed can be increased. Therefore, in the friction stir welding method in this embodiment, the surface temperature T _S of the steel plate 3 is the heating region 12 a region which satisfies the following formula (1).

T _S ≧ 0.8 × T _A1 (1)
The transformation temperature T _A1 (° C.) of steel can be obtained by the following formula (2).

T _A1 (° C.) = 723-10.7 [% Mn] −16.9 [% Ni] +29.1 [% Si] +16.9 [% Cr] +290 [% As] +6.38 [% W] · (2)
Said [% M] is content (mass%) of M element in the steel plate 3 which is a workpiece, and is set to 0 when not containing.

If the temperature exceeds 0.8 × T _A1 ° C., the strength of the steel plate 3 tends to decrease as the temperature increases. Therefore, it is preferable to adjust so that the surface temperature T _S of the steel plate 3 in the heating region 12 does not increase excessively. Specifically, in order to secure the heating region 12 in the thickness direction, a temperature gradient (temperature variation on the surface) may exist on the surface of the heating region 12. The high surface temperature is preferably 1.5 × T _M ° C. or less. Furthermore, it is preferable that the surface temperature of the steel plate 3 in the heating region 12 is less than T _M ° C. before contacting the rotary tool 1 that passes through the heating region 12. Thereby, the damage of the rotary tool 1 and the alteration of the microstructure around the heating region 12 due to an excessive increase in the temperature of the joint 4 can be avoided. T _M (° C.) is the melting point of the steel plate 3 as the workpiece.

Minimum distance X between the heating region on the surface of the steel plate and the rotating tool: less than the diameter of the shoulder of the rotating tool If the minimum distance X between the heating region 12 on the surface of the steel plate 3 and the rotating tool 1 becomes too large, heating is performed before joining. The temperature in the region 12 is lowered and the effect of preheating is not sufficiently obtained. For this reason, in the friction stir welding method according to the present embodiment, the minimum distance X between the heating region 12 on the surface of the steel plate 3 and the rotary tool 1 moving in the joining direction is equal to or less than the diameter of the shoulder 8 of the rotary tool.

  However, if the minimum distance X between the heating region 12 and the rotating tool 1 becomes too small, the rotating tool 1 may be damaged by the heat of the heating means 5, so that it moves in the joining direction with the heating region 12 on the surface of the steel plate 3. The minimum distance X to the rotating tool 1 is preferably 0.1 times or more the diameter of the shoulder 8 of the rotating tool. The diameter of the shoulder 8 of the rotary tool in this embodiment is, for example, about 8 to 60 mm. In order to sufficiently obtain the effect of preheating, the moving speed of the rotary tool 1 is preferably 200 mm / min or more and 3000 mm / min or less.

The area of the heating region on the surface of the steel sheet: not more than the area of the maximum diameter portion of the pin portion of the rotating tool When the heating region 12 becomes too large, the microstructure of the heating region 12 and its peripheral region changes. In particular, in the case of a high-tensile steel sheet strengthened by a martensite structure, even when heating at a temperature below the ferrite-austenite transformation temperature, the martensite is tempered to cause softening and greatly reduce the joint strength. . For this reason, in the friction stir welding method according to the present embodiment, the area of the heating region 12 on the surface of the steel plate 3 is equal to or less than the area of the maximum diameter portion of the pin portion 9 of the rotary tool.

  On the other hand, if the area of the heating region 12 becomes too small, the effect of preheating cannot be sufficiently obtained. Therefore, the area of the heating region 12 on the surface of the steel plate 3 is preferably 0.1 times or more the area of the maximum diameter portion in the pin portion 9 of the rotary tool.

  The maximum diameter of the pin portion 9 of the rotary tool in the present embodiment is, for example, about 2 to 50 mm. The maximum diameter of the pin portion 9 of the rotary tool is the maximum diameter among the diameters obtained at the cut surface when one pin portion is cut in a cross section perpendicular to the axial direction.

  FIG. 4 is a diagram showing a cross-sectional dimension of the rotary tool. As shown in FIG. 4, when the diameter of the pin portion 9 of the rotary tool does not change along the axial direction, the diameter (4 mm in the figure) of the pin portion 9 of the rotary tool is set to the diameter of the pin portion 9 of the rotary tool. The maximum diameter may be used. When the pin portion 9 of the rotating tool has a taper shape or the like and the pin diameter varies depending on the position in the axial direction, the largest diameter may be the maximum diameter of the pin portion 9 of the rotating tool. 4 indicates the probe length, and the probe length is calculated by the difference in height between the tip portion of the pin portion 9 of the rotary tool and the highest position of the shoulder portion 8 of the rotary tool. Length.

  The shape of the heating region 12 may be any shape such as a circle, an ellipse, or a rectangle. The shape of the maximum diameter portion of the pin portion 9 of the rotary tool is usually circular or elliptical.

On the surface of the steel plate, the area of the heating region located between the joining center line and the RS wire: 65% or more of the area of the heating region on the surface of the steel plate In the friction stir welding of the steel plate 3, the plastic flow starts from the advanced side As described above, along the rotational direction of the rotary tool 1, it passes through the joining direction front, the retreating side, and the joining direction rear, and the advanced side is the end point. Since the advanced side is the starting point of plastic flow, insufficient heating of the steel plate 3 as the workpiece is likely to occur. For this reason, when plastic flow is insufficient and defects occur, most of them occur on the advanced side. Therefore, on the surface of the steel plate 3, the advancing side is preferentially heated and the steel plate is softened to promote plastic flow, suppress the occurrence of defects, and increase the joining speed.

  However, when the dynamic friction coefficient between the material of the rotary tool 1 or the material coated on the surface of the rotary tool 1 and the steel plate 3 to be joined is 0.6 or less, it is between the rotary tool 1 and the steel plate 3. The generated frictional heat and plastic flow are reduced. The advanced side is a region that is a starting point of plastic flow in front of the rotary tool 1 and is a region where frictional heat between the rotary tool 1 and the steel plate 3 is greatly generated. However, since the dynamic friction coefficient tends to decrease in a high temperature state, if this part is heated to a high temperature by preheating, if the dynamic friction coefficient between the rotary tool 1 and the steel plate 3 is small, sufficient frictional heat generation cannot be obtained. On the other hand, since the retreating side is located in the middle of the plastic flow, if the plastic flow at this position becomes insufficient, the occurrence of defects on the advanced side that is the end point of the plastic flow is greatly affected. In particular, when the dynamic friction coefficient between the rotary tool 1 and the steel plate 3 is small, sufficient plastic flow cannot be obtained.

  Accordingly, when the dynamic friction coefficient between the material of the rotary tool 1 or the material coated on the surface of the rotary tool 1 and the steel plate 3 is 0.6 or less, 65% of the area of the heating region 12 on the surface of the steel plate 3. The above is positioned between the junction center line 10 and the RS wire 11 parallel to the junction center line 10 to preferentially heat the retreating side. This promotes plastic flow on the retreating side, which is the middle of plastic flow, while ensuring frictional heat generation on the advanced side, which is the starting point of plastic flow, suppresses the occurrence of defects, and increases the joining speed. You can plan. The area range of the heating region 12 located between the bonding center line 10 and the RS line 11 is preferably 70% or more, more preferably 80% or more, and may be 100%.

  Further, from the viewpoint of preferentially heating the retreating side, the center of the heating region 12 is positioned between the RS line 11 and a straight line passing through an intermediate point between the junction center line 10 and the RS line 11. In other words, the center of the heating region 12 is positioned on the retreating side with respect to the bonding center line 10, and the distance from the center of the heating region 12 to the bonding center line 10 is set to 0 of the maximum radius in the pin portion 9 of the rotary tool. It is preferable to be 5 times or more and 1 time or less.

Temperature T _D in the thickness direction region of the heating region: T _D ≧ 0.8 × T _A1
As described above, the steel plate 3 joined by the friction stir welding method of the present embodiment has a strength of about 30% of the strength at normal temperature at a temperature of about 80% of _TA1 , which is the transformation temperature of the steel. Moreover, when it becomes higher than this temperature, the intensity | strength of the steel plate 3 will fall further. Therefore, it is preferable to soften the steel plate 3 in advance in the thickness direction region of the heating region 12 at a temperature of 0.8 × T _A1 ° C. or higher. Thereby, the load concerning the rotary tool 1 is further reduced, and the joining speed can be further increased. Accordingly, the temperature T _D in the thickness direction of the region of the heating region 12 has the depth D of the following formula (3) heating region 12 a depth from the surface of the steel plate 3 in satisfying region.

T _D ≧ 0.8 × T _A1 (3)
T _A1 (° C.) can be obtained by the following formula (2).

T _A1 (° C.) = 723-10.7 [% Mn] −16.9 [% Ni] +29.1 [% Si] +16.9 [% Cr] +290 [% As] +6.38 [% W] · (2)
Said [% M] is content (mass%) of M element in the steel plate 3 which is a workpiece, and is set to 0 when not containing.

However, since the strength of the steel plate 3 tends to decrease as the temperature rises if it exceeds 0.8 × T _A1 ° C, it is preferable to adjust so that the temperature of the steel plate 3 in the heating region 12 does not rise too much. Specifically, in order to secure the heating region 12 in the thickness direction, there may be a temperature gradient (temperature variation along the thickness direction) in the thickness direction of the heating region 12, but in that case, the heating region 12 12, the highest temperature in the thickness direction of the steel plate 3 is preferably 1.5 × T _M ° C. or less. Furthermore, in order to avoid damage to the rotary tool 1 due to excessive increase in the temperature of the joint 4 and alteration of the microstructure around the heating region 12, the temperature in the thickness direction of the steel plate 3 in the heating region 12 is It is preferable that the temperature is lower than T _M ° C. before contacting the rotary tool 1 passing through the heating region 12. T _M (° C.) is the melting point of the steel sheet 3 as the workpiece.

The depth D of the heating zone: depth D of 30% of the thickness t or more heating region 12 of the steel sheet, the steel sheet in the region where the temperature T _D in the thickness direction of the heating area 12 is 0.8 × T _A1 ° C. or higher 3 is defined by the maximum depth from the surface. The depth D of the heating region 12 is preferably 30% or more of the thickness t of the steel plate 3. By setting the depth D of the heating region 12 to 30% or more of the thickness t of the steel plate 3, plastic flow is further promoted, which is advantageous for reducing the load applied to the rotary tool 1 and increasing the joining speed. The depth D of the heating region 12 is more preferably 50% or more of the thickness of the steel plate 3.

  However, if the depth D of the heating region 12 exceeds 90% of the thickness t of the steel plate 3, the heating becomes excessive, and there is a concern about changes in the microstructure around the heating region 12. The thickness D is preferably 90% or less of the thickness t of the steel plate 3.

  In order to realize the above-described conditions, the friction stir welding apparatus according to this embodiment includes a control unit 15. The control means 15 controls the operations of the rotary tool 1 and the heating means 5. The control means 15 may control operations of the rear heating means 7 and the cooling means 6.

  Further, the heating means 5 used in the preheat treatment process is not particularly limited, but is preferably a laser heating apparatus. By using a laser having a high energy density as a heat source, it is possible to more accurately control the preheat treatment process conditions, and it is possible to improve the joining workability without impairing the joint characteristics.

  The joining conditions other than those described above are not particularly limited. For example, the moving speed of the heating means 5 used in the preheat treatment process may be approximately the same as the joining speed. Moreover, when using a laser heating apparatus for this heating means 5, the laser output and beam diameter may be suitably set according to joining conditions.

  The preheat treatment process in the friction stir welding method and apparatus according to the present embodiment has been described above. However, in the friction stir welding method and apparatus according to the present embodiment, the cooling means 6 is disposed behind the rotating tool 1 moving in the joining direction. The joint joint strength may be improved by providing the cooling means 6.

  Usually, after joining is completed, the joint 4 is naturally cooled, so that the strength of the joint joint cannot be sufficiently obtained when the hardenability of the steel plate 3 as the workpiece is low. On the other hand, a cooling means 6 is provided behind the rotating tool 1 moving in the joining direction, and the joining portion 4 of the steel plate 3 is cooled by the cooling means 6 and the cooling rate is appropriately controlled. Strength can be improved. As the cooling means 6, for example, a cooling device that ejects an inert gas is preferably used. The cooling rate in this case is preferably 30 to 300 ° C./s in the range of 800 ° C. to 500 ° C., for example. For example, argon gas or helium gas can be used as the inert gas.

  In the case where the hardenability of the steel sheet 3 which is a workpiece is high, there is a possibility that the steel sheet 3 is excessively hardened, which lowers the toughness of the joint joint. In contrast, excessive heating is suppressed by providing a rear heating means 7 for heating a rear portion close to the rotary tool 1 at the rear in the joining direction of the rotary tool 1 and gradually cooling it while appropriately controlling the cooling rate. it can. As the rear heating means 7, it is preferable to use, for example, a high-frequency induction heating or a heating device using a laser as a heat source. The slow cooling rate in this case is preferably 10 to 30 ° C./s in the range of 800 ° C. to 500 ° C., for example.

  The rear heating means 7 may be provided behind the rotating tool moving in the joining direction and behind the cooling means 6, and the joining portion 4 of the steel plate 3 may be reheated by the rear heating means 7. Thereby, when the joining part 4 is quenched by cooling by the cooling means 6 and hardened excessively, the joint characteristics having both strength and toughness can be obtained by suppressing the hardness by tempering by the rear heating means 7. The cooling rate in this case is preferably 30 to 300 ° C./s in the range of 800 ° C. to 500 ° C., for example, and the reheating temperature is preferably 550 to 650 ° C., for example.

  Further, a cooling means 6 may be provided behind the rotating tool 1 moving in the joining direction and behind the rear heating means 7, and the joint 4 of the steel plate 3 may be cooled by the cooling means 6.

  In this case, immediately after joining, slow cooling is performed by the rear heating means 7 and then rapid cooling is performed by the cooling means 6 so that the structure can be combined, and joint characteristics having both strength and ductility can be obtained. The cooling rate in this case is, for example, about 10 to 30 ° C./s in the range of 800 ° C. to 600 ° C. (gradual cooling range), and then 30 to 30 in the range of 600 ° C. to 400 ° C. (rapid cooling range). It is preferably about 300 ° C./s.

  About joining conditions other than the above, a conventional method may be followed. However, the greater the torque of the rotary tool 1, the lower the plastic fluidity of the steel sheet 3, so that defects and the like are likely to occur.

  Therefore, in the friction stir welding method and apparatus of this embodiment, the rotational speed of the rotary tool 1 is set in the range of 100 to 1000 rpm, the torque of the rotary tool 1 is suppressed, and the target is to increase the welding speed to 1000 mm / min or higher. And When the joining speed is increased from 500 mm / min to 1000 mm / min or less, the torque of the rotary tool 1 is preferably suppressed to 90 N · m or less. As a result, it is possible to avoid a state in which the rotary tool 1 is broken during joining or an unjoined portion remains. Further, when the joining speed is set to 500 mm / min or less, the torque of the rotary tool 1 is preferably suppressed to less than 75 N · m. Thereby, the load of the rotary tool 1 can be eased while ensuring plastic fluidity.

  Moreover, as a target steel type of the friction stir welding method of the present embodiment, general structural steel and carbon steel, for example, rolled steel for welded structure of JIS (Japanese Industrial Standards) G 3106, and for mechanical structure of JIS G 4051 Carbon steel or the like can be used. It can also be applied to high-strength structural steel having a tensile strength of 800 MPa or more, and a strength of 85% or more of the tensile strength of the steel plate (base material), and further a strength of 90% or more can be obtained at the joint 4.

Example 1
Friction stir welding was performed using a steel plate having a plate thickness of 1.6 mm and having a chemical composition and tensile strength shown in Table 1 below. The joint butt surfaces were joined in one pass on one side in a so-called I-shaped groove with no angle, and with a surface condition of the degree of milling. Table 2 shows the welding conditions of the friction stir welding. In Example 1, the rotary tool having the cross-sectional dimensions shown in FIG. 4 (shoulder diameter a: 12 mm, pin portion maximum diameter b: 4 mm, probe length c: 1.4 mm) was used. The rotary tool used in Example 1 is a rotary tool whose surface is coated with titanium nitride (TiN) by physical vapor deposition (PVD) using tungsten carbide (WC) as a raw material. At the time of bonding, the bonded portion was shielded with argon gas to prevent surface oxidation. The coefficient of dynamic friction between the surface of the rotating tool of WC having a TiN coating treatment on the surface and the steel sheet was 0.6 or less.

  The dynamic friction coefficient between the tool material surface and the steel sheet was measured by the following measuring method. Using a ball-on-disk friction and wear tester, a disk made of the target material was pressed against a steel ball having a diameter of 6 mm while rotating with a load of 5 N, and the test was performed at a rotational speed of 100 mm / s and a sliding distance of 300 m. The test was performed at room temperature and without lubrication. The steel ball used for the test is a steel ball made of a material having a chemical component of SUJ2 defined by JIS G 4805 and processed as a steel ball for bearings.

  Prior to the bonding, in order to confirm the heating region by preheating using a laser as a heat source, the steel plate I in Table 1 was subjected to laser irradiation under the irradiation conditions (laser moving speed, laser output, and beam diameter) shown in Table 3. Light was irradiated and the surface temperature was measured by thermography. Furthermore, the cross section of the laser irradiation part was observed, and the microstructure was observed with a nital etchant.

Here, the region having the transformation point (T _A1 ° C) or higher is darkest, and is less than the transformation point (T _A1 ° C) existing outside, but a high hardness structure such as martensite in the base material is tempered. Since the region is etched relatively thin, the region where the transformation point (T _A1 ° C) or higher, the tempering region below the transformation point (T _A1 ° C), and the base material region can be distinguished. is there. Furthermore, from the knowledge of heat treatment of steel, it is known that the tempering region below the transformation point (T _A1 ° C) coincides with the region of 0.8 x T _A1 ° C or more and less than T _A1 ° C. Based on the microstructure observation with such a nital corrosion solution, the depth D _{0 of the} region where the transformation point (T _A1 ° C) or higher and the depth of the region where the temperature becomes 0.8 × T _A1 ° C or higher (of the heating region) Depth D) was measured.

  These measurement results are shown in Table 4.

As shown in Table 4, from the surface temperature measurement result by thermography, in the irradiation condition A, the region of 0.8 × T _A1 ° C or more was a circular shape having a diameter of 3.5 mm. Since the maximum diameter of the pin portion of the rotating tool used here is 4.0 mm, the area of the heating region in the irradiation condition A is equal to or less than the area of the maximum diameter portion of the pin portion of the rotating tool.

In the irradiation condition B, the region of 0.8 × T _A1 ° C. or higher was a circular shape having a diameter of 2.0 mm. Therefore, similarly to the above, the area of the heating region in the irradiation condition B is equal to or smaller than the area of the maximum diameter portion of the pin portion of the rotary tool.

In the irradiation condition C, the region of 0.8 × T _A1 ° C. or higher was a circular shape with a diameter of 4.5 mm. Since the maximum diameter of the pin part of the rotary tool used here is 4.0 mm, the area of the heating region in the irradiation condition C exceeds the area of the maximum diameter part of the pin part of the rotary tool.

In the irradiation condition D, the region of 0.8 × T _A1 ° C. or higher is an ellipse having a major axis in the laser moving direction and a minor axis in the direction perpendicular to the laser moving direction, the major axis is 3.8 mm, and the minor axis is 3. It was 2 mm. Since the maximum diameter of the pin portion of the rotating tool used here is 4.0 mm, the area of the heating region in the irradiation condition D is equal to or less than the area of the maximum diameter portion of the pin portion of the rotating tool.

In the irradiation condition E, the region where the temperature is 0.8 × T _A1 ° C or more is an ellipse having a major axis in the laser moving direction and a minor axis in the direction perpendicular to the laser moving direction. The major axis is 2.2 mm and the minor axis is 1. It was 8 mm. Therefore, similarly to the above, the area of the heating region under the irradiation condition E is equal to or smaller than the area of the maximum diameter portion of the pin portion of the rotary tool.

In the irradiation condition F, the region where the temperature is 0.8 × T _A1 ° C or more is an ellipse having a major axis in the laser movement direction and a minor axis in the direction perpendicular to the laser movement direction. The major axis is 4.9 mm and the minor axis is 4.1 mm. Met. Since the maximum diameter of the pin part of the rotary tool used here is 4.0 mm, the area of the heating region under the irradiation condition F exceeds the area of the maximum diameter part of the pin part of the rotary tool.

Further, as shown in Table 4, the cross-section observation of the laser irradiation unit, the irradiation condition A, the depth of the region becomes T _A1 ° C. or higher and the depth of the turned region D ₀ and 0.8 × T _A1 ° C. or higher The depth (depth D of the heating region) was 0.28 mm and 0.30 mm, respectively. Since the thickness t of the steel plate as the workpiece is 1.6 mm, the depth D of the heating region, which is the depth of the region that is 0.8 × T _A1 ° C or higher, is about the thickness t of the steel plate. 18.8%.

Under irradiation condition B, the depth D _{0 of the} region where T _A1 ° C or higher and the depth of the region where 0.8 × T _A1 ° C or higher (depth D of the heating region) are 0.47 mm and 0, respectively. .50 mm. Since the thickness t of the steel plate as the workpiece is 1.6 mm, the depth D of the heating region is about 31.3% of the thickness t of the steel plate.

Under irradiation condition C, the depth D _{0 of the} region where T _A1 ° C or higher and the depth of the region where 0.8 × T _A1 ° C or higher (depth D of the heating region) are 0.09 mm and 0, respectively. .10 mm. Since the thickness t of the steel plate that is the workpiece is 1.6 mm, the depth D of the heating region is about 6.3% of the thickness t of the steel plate.

Under irradiation condition D, the depth D _{0 of the} region where T _A1 ° C or higher and the depth of 0.8 × T _A1 ° C or higher (depth D of the heating region) are 0.30 mm and 0, respectively. .32 mm. Since the thickness t of the steel plate as the workpiece is 1.6 mm, the depth D of the heating region, which is the depth of the region that is 0.8 × T _A1 ° C or higher, is about the thickness t of the steel plate. 20.0%.

Under irradiation condition E, the depth D _{0 of the} region that is T _A1 ° C or higher and the depth of the region that is 0.8 × T _A1 ° C or higher (depth D of the heating region) are 0.51 mm and 0, respectively. .54 mm. Since the thickness t of the steel plate as the workpiece is 1.6 mm, the depth D of the heating region is about 33.8% of the thickness t of the steel plate.

Under irradiation condition F, the depth D _{0 of the} region that is T _A1 ° C or higher and the depth of the region that is 0.8 × T _A1 ° C or higher (depth D of the heating region) are 0.10 mm and 0, respectively. .11 mm. Since the thickness t of the steel plate as the workpiece is 1.6 mm, the depth D of the heating region is about 6.9% of the thickness t of the steel plate.

  Table 5 shows preheating process conditions by laser irradiation performed before joining the workpieces, and Table 6 shows process conditions performed after joining. Here, in the process performed after joining, cooling by gas ejection was performed, and in the heating (and reheating), induction heating was performed.

  In Tables 5 and 6, “-” in the preheating process condition and the process condition performed after bonding indicates a case where the preheating process and the process after bonding such as cooling and heating are not performed, respectively. In addition, in the description of “(AS)” and “(RS)” in the distance from the junction center line to the center of the heating area, the center of the heating area is located on the advancing side and the retreating side from the junction center line, respectively. Indicates.

  Table 7 shows the measured value of the torque of the rotating tool when the joining is performed and the measured value of the tensile strength of the obtained joint. The tensile strength of the joint joint is the result of taking a tensile test piece having the size of No. 1 test piece defined in JIS Z 3121 and conducting a tensile test. The greater the torque of the rotating tool, the lower the plastic fluidity and the more likely to cause defects.

  From Table 7, in Invention Examples 1 to 10, even when the joining speed was 400 mm / min, a joint strength of 90% or more of the tensile strength of the steel sheet as the base material was obtained. The torque of the rotary tools of Invention Examples 1 to 10 was 72 N · m or less, and the plastic fluidity was also good. In particular, in Invention Examples 6, 7, and 8 in which only cooling / reheating or cooling was performed after joining, joint joint strength equivalent to the tensile strength of the base material was obtained. In Invention Examples 9 and 10 in which only heating / cooling or heating was performed after joining, a joint strength of 93% or more of the tensile strength of the base material was obtained.

  On the other hand, in Comparative Examples 1-6, the torque of the rotary tool was 75 N · m or more, and the plastic fluidity was poor.

  In Invention Examples 11 to 20, even when the joining speed is increased to 1000 mm / min, a joint strength of 85% or more of the tensile strength of the base material is obtained, and the torque of the rotary tool is 90 N · m. It was the following. In particular, in Invention Examples 16, 17, and 18 in which only cooling / reheating or cooling was performed after joining, a joint joint strength of 99% or more of the tensile strength of the base material was obtained. In Invention Examples 19 and 20, in which only reheating / cooling or reheating was performed after joining, a joint joint strength of 95% or more of the tensile strength of the base material was obtained.

On the other hand, in Comparative Example 7, the rotary tool was damaged during the joining and could not be joined. In Comparative Examples 8 to 12, the unjoined portion remained and could not be joined, and a healthy joint was not obtained. For this reason, in Comparative Examples 7-12, measurement of rotating tool torque etc. is not performed.
(Example 2)
Friction stir welding was performed using a steel plate having a plate thickness of 1.6 mm and a chemical composition and tensile strength shown in Table 1 above. The joint butt surfaces were joined in one pass on one side in a so-called I-shaped groove with no angle, and with a surface condition of the degree of milling. The welding conditions for friction stir welding are shown in Table 2 above. In Example 2, the rotary tool having the cross-sectional dimensions (shoulder diameter a: 12 mm, pin portion maximum diameter b: 4 mm, probe length c: 1.4 mm) shown in FIG. 4 was used. The rotary tool used in Example 2 is made of tungsten carbide (WC) as a raw material and is not subjected to coating treatment, and tungsten carbide (WC) is used as a raw material and is coated with titanium nitride (TiN) by physical vapor deposition (PVD). The surface is made of tungsten carbide (WC), the surface is coated with aluminum nitride nitride (AlCrN), or the material is cubic boron nitride (CBN). .

  At the time of bonding, the bonded portion was shielded with argon gas to prevent surface oxidation. The coefficient of dynamic friction between the surface of the rotating tool and the steel sheet is 0.7 when tungsten carbide (WC) is not used as a raw material, and titanium nitride (PVD) is used as titanium nitride (WC) as a raw material. 0.5 for TiN) coating, 0.4 for tungsten nitride (WC) aluminum nitride (AlCrN) coating, and cubic boron nitride (CBN). In the case of the material, it was 0.3.

  The dynamic friction coefficient between the tool material surface and the steel plate was measured by the same measurement method as in Example 1.

  Table 8 shows preheating process conditions by laser irradiation performed before joining the workpieces.

  In Table 8, “WC” is a rotating tool that is not coated with tungsten carbide (WC) as a material, and titanium nitride (TiN) is coated by physical vapor deposition (PVD) with tungsten carbide (WC) as a material. “WC + TiN” as the rotary tool, “WC + AlCrN” as the rotary tool coated with aluminum chromium nitride (AlCrN) using tungsten carbide (WC) as the raw material, and “CBN” as the rotary tool using cubic boron nitride (CBN) as the raw material. ". The laser irradiation conditions in the preheating process conditions are as shown in Table 3, and the surface shape and depth of the heating region formed by each laser irradiation condition are as shown in Table 4.

  In Example 2, the process after joining was not performed. “(AS)” and “(RS)” in the distance from the junction center line to the center of the heating region indicate that the center of the heating region is located on the advansing side and the retreating side from the junction center line, respectively.

  Table 9 shows the measured values of the torque of the rotating tool and the measured values of the tensile strength of the obtained joints when the joining is performed. The tensile strength of the joint joint is the result of taking a tensile test piece having the size of No. 1 test piece defined in JIS Z 3121 and conducting a tensile test. The greater the torque of the rotating tool, the lower the plastic fluidity and the more likely to cause defects.

  From Table 9, in the invention examples 21-26, even if it was a case where a joining speed was 400 mm / min, the joint joint intensity | strength 90% or more of the tensile strength of the steel plate used as a base material was obtained. The torque of the rotary tools of Invention Examples 21 to 26 was 65 N · m or less, and the plastic fluidity was also good.

  On the other hand, in Comparative Examples 13 and 14, the torque of the rotary tool was 75 N · m or more, and the plastic fluidity was poor.

  From Table 9, in Invention Examples 27 to 32, even when the joining speed is increased to 1000 mm / min, a joint strength of 85% or more of the tensile strength of the base material is obtained, and the torque of the rotary tool is obtained. Was 81 N · m or less.

  On the other hand, in Comparative Examples 15 and 16, the unbonded portion remained and bonding could not be performed. For this reason, in Comparative Examples 15 and 16, measurement of torque and the like of the rotary tool is not performed.

DESCRIPTION OF SYMBOLS 1 Rotating tool 2 Rotating shaft 3 Steel plate 4 Joining part 5 Heating means 6 Cooling means 7 Backward heating means 8 Rotating tool shoulder 9 Rotating tool pin part 10 Joining center line 11 RS line 12 Heating area 13 Cooling area 14 Reheating area 15 Control means a Diameter of the shoulder of the rotating tool b Maximum diameter of the pin of the rotating tool c Probe length of the rotating tool X Minimum distance between the heating area and the rotating tool D Depth of the heating area t Thickness of the steel sheet α Rotating tool Tilt angle

Claims (14)

A rotating tool comprising a shoulder portion and a pin portion arranged on the shoulder portion and sharing the rotation axis with the shoulder portion , wherein the shoulder portion and the pin portion are made of a material harder than a steel plate as a workpiece. , Inserting in the unjoined part between the steel plates and moving in the joining direction while rotating, and softening the steel plate by frictional heat between the rotating tool and the steel plate, stirring the softened part with the rotary tool A friction stir welding method for joining steel plates by causing plastic flow,
The material of the rotating tool, or the dynamic friction coefficient between the steel sheet and the material coated on the surface of the rotating tool is 0.6 or less,
When the region where the surface temperature T _S (° C.) of the steel sheet heated by the heating means provided in front of the rotating tool in the joining direction satisfies the following formula (1) is defined as the heating region, the heating region and the The minimum distance to the rotating tool is not more than the diameter of the shoulder of the rotating 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,
65% or more of the area of the heating region is a joining center line that is a straight line passing through the rotation axis of the rotating tool on the surface of the steel plate and parallel to the joining direction, parallel to the joining center line, and to the retreating side. A friction stir welding method positioned between a straight line separated by the same distance as the maximum radius of the pin portion of the rotating tool.
T _S ≧ 0.8 × T _A1 (1)
T _A1 is a temperature represented by the following formula (2).
T _A1 (° C.) = 723-10.7 [% Mn] −16.9 [% Ni] +29.1 [% Si] +16.9 [% Cr] +290 [% As] +6.38 [% W] · (2)
Said [% M] is content (mass%) of M element in the steel plate which is a workpiece, and is set to 0 when not containing. When the maximum depth from the surface of the steel sheet in the region where the temperature T _D (° C.) in the thickness direction of the heating region satisfies the following formula (3) is the depth D of the heating region, the depth of the heating region The friction stir welding method according to claim 1, wherein the thickness D is 30% or more of the thickness of the steel plate.
T _D ≧ 0.8 × T _A1 (3)   The friction stir welding method according to claim 1 or 2, wherein the heating means is a laser heating device.   The friction stir welding according to any one of claims 1 to 3, wherein a rear heating unit is provided at a rear side of the rotating tool in a joining direction, and the rear heating unit heats a joint portion of the steel plate. Method.   The friction stir welding method according to claim 4, wherein a cooling means is provided behind the rear heating means in the joining direction, and the cooling means cools the joint heated by the rear heating means.   The friction stir welding method according to any one of claims 1 to 3, wherein a cooling means is provided behind the rotating tool in the joining direction, and the cooling means cools the joining portion of the steel plate.   The friction stir welding method according to claim 6, wherein a rear heating unit is provided at a rear side of the cooling unit in the joining direction, and the rear heating unit heats the joint portion cooled by the cooling unit. A friction stir welding apparatus that joins unjoined portions between steel plates that are workpieces,
A shoulder portion and a pin portion arranged on the shoulder portion and sharing the rotation axis with the shoulder portion, and the shoulder portion and the pin portion are made of a material harder than the steel plate, A rotating tool that causes plastic flow by stirring the softened part while softening the steel plate by frictional heat by moving in the joining direction while rotating in a state inserted in the joint part,
A heating means provided in front of the rotating tool in the joining direction, for heating the steel plate;
Control means for controlling the rotating tool and the heating means so as to realize the following state 1;
The friction stir welding apparatus, wherein the dynamic friction coefficient 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.
(State 1)
When a region where the surface temperature T _S (° C.) of the steel sheet heated by the heating unit satisfies the following formula (1) is a heating region, the minimum distance between the heating region and the rotating tool is the rotation Less than the diameter of the shoulder of the 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,
65% or more of the area of the heating region is a joining center line that is a straight line passing through the rotation axis of the rotating tool on the surface of the steel plate and parallel to the joining direction, parallel to the joining center line, and to the retreating side. And a straight line separated by the same distance as the maximum radius of the pin portion of the rotating tool.
T _S ≧ 0.8 × T _A1 (1)
T _A1 is a temperature represented by the following formula (2).
T _A1 (° C.) = 723-10.7 [% Mn] −16.9 [% Ni] +29.1 [% Si] +16.9 [% Cr] +290 [% As] +6.38 [% W] · (2)
Said [% M] is content (mass%) of M element in the steel plate which is a workpiece, and is set to 0 when not containing. The friction stir welding apparatus according to claim 8, wherein the control means controls the rotary tool and the heating means so as to realize the following state 2.
(State 2)
When the maximum depth from the surface of the steel sheet in the region where the temperature T _D (° C.) in the thickness direction of the heating region satisfies the following formula (3) is the depth D of the heating region, the depth of the heating region The thickness D is 30% or more of the thickness of the steel plate.
T _D ≧ 0.8 × T _A1 (3)   The friction stir welding apparatus according to claim 8 or 9, wherein the heating means is a laser heating apparatus. It further has a rear heating means for heating the joined portion of the steel sheet,
The friction stir welding apparatus according to any one of claims 8 to 10, wherein the rear heating means is provided at a rear side in the joining direction of the rotary tool. A cooling means for cooling the joint;
The friction stir welding apparatus according to claim 11, wherein the cooling means is provided at a rear side in the joining direction of the rear heating means. A cooling means for cooling the joint of the steel plates;
The friction stir welding apparatus according to any one of claims 8 to 10, wherein the cooling means is provided behind the rotating tool in the joining direction. A rear heating means for heating the joint;
14. The friction stir welding apparatus according to claim 13, wherein the rear heating means is provided behind the cooling means in the joining direction.

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