US20200238434A1 - Double-side friction stir welding method and double-side friction stir welding device for metal sheets or metal plates - Google Patents

Double-side friction stir welding method and double-side friction stir welding device for metal sheets or metal plates Download PDF

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Publication number
US20200238434A1
US20200238434A1 US16/635,016 US201816635016A US2020238434A1 US 20200238434 A1 US20200238434 A1 US 20200238434A1 US 201816635016 A US201816635016 A US 201816635016A US 2020238434 A1 US2020238434 A1 US 2020238434A1
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Prior art keywords
metal
metal plates
rotating tools
metal sheets
pair
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US16/635,016
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English (en)
Inventor
Muneo Matsushita
Rinsei Ikeda
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JFE Steel Corp
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JFE Steel Corp
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Publication of US20200238434A1 publication Critical patent/US20200238434A1/en
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K20/00—Non-electric welding by applying impact or other pressure, with or without the application of heat, e.g. cladding or plating
    • B23K20/12—Non-electric welding by applying impact or other pressure, with or without the application of heat, e.g. cladding or plating the heat being generated by friction; Friction welding
    • B23K20/122—Non-electric welding by applying impact or other pressure, with or without the application of heat, e.g. cladding or plating the heat being generated by friction; Friction welding using a non-consumable tool, e.g. friction stir welding
    • B23K20/1245—Non-electric welding by applying impact or other pressure, with or without the application of heat, e.g. cladding or plating the heat being generated by friction; Friction welding using a non-consumable tool, e.g. friction stir welding characterised by the apparatus
    • B23K20/1255—Tools therefor, e.g. characterised by the shape of the probe
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K20/00—Non-electric welding by applying impact or other pressure, with or without the application of heat, e.g. cladding or plating
    • B23K20/12—Non-electric welding by applying impact or other pressure, with or without the application of heat, e.g. cladding or plating the heat being generated by friction; Friction welding
    • B23K20/122—Non-electric welding by applying impact or other pressure, with or without the application of heat, e.g. cladding or plating the heat being generated by friction; Friction welding using a non-consumable tool, e.g. friction stir welding
    • B23K20/1225—Particular aspects of welding with a non-consumable tool
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K20/00—Non-electric welding by applying impact or other pressure, with or without the application of heat, e.g. cladding or plating
    • B23K20/12—Non-electric welding by applying impact or other pressure, with or without the application of heat, e.g. cladding or plating the heat being generated by friction; Friction welding
    • B23K20/122—Non-electric welding by applying impact or other pressure, with or without the application of heat, e.g. cladding or plating the heat being generated by friction; Friction welding using a non-consumable tool, e.g. friction stir welding
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K20/00—Non-electric welding by applying impact or other pressure, with or without the application of heat, e.g. cladding or plating
    • B23K20/12—Non-electric welding by applying impact or other pressure, with or without the application of heat, e.g. cladding or plating the heat being generated by friction; Friction welding
    • B23K20/122—Non-electric welding by applying impact or other pressure, with or without the application of heat, e.g. cladding or plating the heat being generated by friction; Friction welding using a non-consumable tool, e.g. friction stir welding
    • B23K20/123—Controlling or monitoring the welding process
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K20/00—Non-electric welding by applying impact or other pressure, with or without the application of heat, e.g. cladding or plating
    • B23K20/12—Non-electric welding by applying impact or other pressure, with or without the application of heat, e.g. cladding or plating the heat being generated by friction; Friction welding
    • B23K20/122—Non-electric welding by applying impact or other pressure, with or without the application of heat, e.g. cladding or plating the heat being generated by friction; Friction welding using a non-consumable tool, e.g. friction stir welding
    • B23K20/1245—Non-electric welding by applying impact or other pressure, with or without the application of heat, e.g. cladding or plating the heat being generated by friction; Friction welding using a non-consumable tool, e.g. friction stir welding characterised by the apparatus
    • B23K20/125—Rotary tool drive mechanism
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K20/00—Non-electric welding by applying impact or other pressure, with or without the application of heat, e.g. cladding or plating
    • B23K20/12—Non-electric welding by applying impact or other pressure, with or without the application of heat, e.g. cladding or plating the heat being generated by friction; Friction welding
    • B23K20/122—Non-electric welding by applying impact or other pressure, with or without the application of heat, e.g. cladding or plating the heat being generated by friction; Friction welding using a non-consumable tool, e.g. friction stir welding
    • B23K20/1245—Non-electric welding by applying impact or other pressure, with or without the application of heat, e.g. cladding or plating the heat being generated by friction; Friction welding using a non-consumable tool, e.g. friction stir welding characterised by the apparatus
    • B23K20/126—Workpiece support, i.e. backing or clamping
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K20/00—Non-electric welding by applying impact or other pressure, with or without the application of heat, e.g. cladding or plating
    • B23K20/22—Non-electric welding by applying impact or other pressure, with or without the application of heat, e.g. cladding or plating taking account of the properties of the materials to be welded
    • B23K20/227—Non-electric welding by applying impact or other pressure, with or without the application of heat, e.g. cladding or plating taking account of the properties of the materials to be welded with ferrous layer
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K2101/00—Articles made by soldering, welding or cutting
    • B23K2101/18—Sheet panels
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K2103/00—Materials to be soldered, welded or cut
    • B23K2103/02—Iron or ferrous alloys
    • B23K2103/04—Steel or steel alloys

Definitions

  • the present invention relates to a double-side friction stir welding method of welding metal sheets or metal plates to each other, and to a double-side friction stir welding device for performing double-side friction stir welding.
  • a pair of rotating tools facing each other is respectively disposed on a top-surface side and a bottom-surface side of a butted portion or an overlapping portion, which becomes a welded joint of the metal sheets or the metal plates; the pair of rotating tools is moved in a welding direction while being rotated at the butted portion or the overlapping portion; and while the metal sheets or the metal plates are softened by frictional heat generated between the rotating tools and the metal sheets or the metal plates, softened parts are stirred by the rotating tools.
  • a plastic flow is caused to occur at the welded joint to join the metal sheets or the metal plates to each other.
  • aspects of the present invention are directed to overcoming a problem that may occur when the friction stir welding method is applied to joining metal sheets or metal plates to each other or when the friction stir welding device is applied to joining metal sheets or metal plates to each other, that is, the problem that a localized improper plastic flow occurs in the welded joint due to differences in temperature and plastic flow occurring in a thickness direction of the metal sheets or the metal plates at the welded joint. Therefore, aspects of the present invention are advantageous in overcoming welding defects, ensures a sufficient strength (joint strength), and intends to improve welding workability, in particular, to increase welding speed.
  • a butted portion (or an overlapping portion) where the metal sheets or the metal plates (such as steel sheets or steel plates) only butt against each other (or Overlap each other) and are not joined to each other yet is called an “unwelded joint”, whereas a portion where the metal sheets or the metal plates are integrated to each other by being joined to each other due to a plastic flow is called a “welded joint”.
  • Patent Literature 1 discloses a technology in which, while a pair of metal materials is softened by frictional heat generated between the metal materials by rotating one or both of the metal materials, softened parts are stirred to produce a plastic flow to join the metal materials to each other.
  • this technology since the metal materials to be joined to each other are rotated, there is a limit to the shapes and dimensions of the metal materials.
  • Patent Literature 2 discloses a method of continuously joining metal sheets or metal plates to each other in a longitudinal direction by heat generated and a plastic flow between rotating tools, which are made of materials that are substantially harder than the materials of the metal sheets or the metal plates, by inserting the rotating tools into an unwelded joint of the metal sheets or the metal plates and by moving the rotating tools while being rotated.
  • the metal sheets or the metal plates in a fixed state are joined to each other by moving the rotating tools while being rotated. Therefore, this method is advantageous in that even substantially indefinitely elongated members in the welding direction can be continuously subjected to solid-state welding in a longitudinal direction thereof.
  • the solid-state welding uses a plastic flow of metals caused by frictional heat between the rotating tools and the metal sheets or the metal plates, the metal sheets or the metal plates can be joined to each other without melting the welded joint. Further, this method has many advantages such as deformation being small after the welding because the heating temperature is low, and defects being few and a metal filler being unnecessary because the welded joint is not melted.
  • the friction stir welding method As a welding method of welding low-melting-point metal sheets or metal plates, metal sheets or metal plates made of an aluminum alloy or a magnesium alloy being typical examples, the friction stir welding method is becoming widely used in the fields of, for example, airplanes, ships, railroad vehicles, and automobiles. This is because, although such low-melting-point metal sheets or metal plates do not easily allow a welded joint to have satisfactory characteristics in existing arc welding methods, when the friction stir welding method is applied, such low-melting-point metal sheets or low-melting-point metal plates allow productivity to be increased and allow a high-quality welded joint to be provided.
  • the application of the friction stir welding method to structural steel sheets or structural steel plates primarily used as material for structures, such as buildings, ships, heavy machines, pipelines, or automobiles allows solidification cracking or hydrogen cracking, which are problems in existing melt welding, and structural changes in steel sheets or steel plates to be suppressed from occurring. Therefore, it is possible to expect an increase in joint performance.
  • the friction stir welding method since by stirring a joining interface by the rotating tools, clean surfaces can be created and can be brought into contact with each other, the advantage that a previous preparation step, such as diffusion bonding, is not required can be expected. In this way, when the friction stir welding method is applied to structural steel sheets or structural steel plates, many advantages are expected.
  • An example of the main causes of defects in the friction stir welding method described in Patent Literature 2 is differences in temperature and plastic flow occurring in a thickness direction of the metal sheets or the metal plates.
  • the surface side that is pushed by a shoulder part of the rotating tool is subjected to a large deformation at high temperatures due to a sufficient temperature increase and shear-stress load caused by rotating the shoulder part, as a result of which clean surfaces are created at a welding interface.
  • Patent Literature 2 When the technology of the friction stir welding method described in Patent Literature 2 is applied to structural steel sheets or structural steel plates, since the strength of the structural steel sheets or structural steel plates under high temperatures is high, in the case of low-heat input and high welding speed, there is a strong tendency that a state of a sufficient plastic flow described above cannot be realized. Therefore, it is difficult to increase the welding speed while suppressing defects at the time of welding from occurring.
  • Patent Literature 3 discloses a double-side friction stir welding method.
  • shoulder parts of a pair of rotating tools facing each other is respectively pushed against a top-surface side and a bottom-surface side of a joint portion of metal sheets or metal plates, and two surfaces of the joint portion of the metal sheets or the metal plates are subjected to a large deformation at high temperatures due to a sufficient temperature increase and shear stress caused by rotating the shoulder parts. Therefore, it is possible to realize a plastic flow sufficient for realizing a homogeneously welded state with respect to the thickness direction of the metal sheets or the metal plates, and to increase the welding speed while suppressing defects at the time of welding from occurring.
  • Patent Literature 3 when pushing the shoulder parts of the pair of rotating tools facing each other respectively against the top-surface side and the bottom-surface side of the joint portion of the metal sheets or the metal plates, a gap between the shoulder parts of the pair of rotating tools, which is important in realizing a temperature increase and shear stress sufficient for realizing the welded state, is not considered at all.
  • An object according to aspects of the present invention is to provide a friction stir welding method that overcomes the problems of the related art, and a friction stir welding device that is suitable for performing friction stir welding. That is, in accordance with aspects of the present invention, when performing double-side friction stir welding, shoulder parts of a pair of rotating tools facing each other is respectively pushed against a top-surface side and a bottom-surface side of a joint portion of metal sheets or metal plates, and two surfaces of the joint portion of the metal sheets or the metal plates are subjected to a large deformation at high temperatures due to a sufficient temperature increase and shear-stress caused by rotating the shoulder parts, so that it is possible to realize a plastic flow sufficient for realizing a homogeneously welded state with respect to a thickness direction of the metal sheets or the metal plates.
  • an object according to aspects of the present invention is to provide a friction stir welding method in which a gap between shoulder parts of a pair of rotating tools facing each other, which is important in realizing a sufficient temperature increase and shear stress for attaining a welded state, is closely examined; and a friction stir welding device for realizing this.
  • welded state refers to a state in which crystal grains are continuously formed across a welded joint interface of the metal sheets or the metal plates and the interface of the metal sheets or the metal plates has microscopically disappeared. “Attaining a welded state” or “realizing a welded state” means that the aforementioned state is attained or the aforementioned state is realized.
  • the present inventors have assiduously conducted studies to overcome the above-described problems and, as a result, have gained the findings (a) to (d) mentioned below.
  • the pair of rotating tools need to be such that the rotation direction on the top-surface side and the rotation direction on the bottom-surface side are opposite each other.
  • aspects of the present invention provide a double-side friction stir welding method in which a pair of rotating tools facing each other is respectively disposed on a top-surface side and a bottom-surface side of a butted portion or an overlapping portion, which comprises a joint portion of two metal sheets or two metal plates; the pair of rotating tools are moved in a welding direction while being rotated at the butted portion or the overlapping portion; and while the metal sheets or the metal plates are softened by frictional heat generated between the rotating tools and the metal sheets or the metal plates, softened parts are stirred by the rotating tools to produce a plastic flow to join the metal sheets or the metal plates to each other, wherein
  • each of the pair of rotating tools includes a shoulder part and a pin part that is disposed at the shoulder part and that along with the shoulder part includes a common rotation axis, and at least the shoulder parts and the pin parts are made of materials that are harder than materials of the metal sheets or the metal plates,
  • the pair of rotating tools is respectively pushed against a top surface and a bottom surface of each metal sheet or metal plate, and the rotating tools are moved in the welding direction while being rotated, and
  • a gap G (mm) between the shoulder parts that is formed by providing a gap g (mm) between ends of the pin parts of the pair of respective rotating tools with respect to a thickness t (mm) of each metal sheet or metal plate for a case of butting or with respect to a total thickness t (mm) of the overlapped metal sheets or the overlapped metal plates for a case of overlapping satisfies
  • the pair of rotating tools is further rotated in opposite directions to perform the friction stir welding.
  • aspects of the present invention also provide a double-side friction stir welding method in which a pair of rotating tools facing each other is respectively disposed on a top-surface side and a bottom-surface side of a butted portion or an overlapping portion, which comprises a joint portion of two metal sheets or two metal plates; the pair of rotating tools is moved in a welding direction while being rotated at the butted portion or the overlapping portion; and while the metal sheets or the metal plates are softened by frictional heat generated between the rotating tools and the metal sheets or the metal plates, softened parts are stirred by the rotating tools to produce a plastic flow to join the metal sheets or the metal plates to each other, wherein
  • each of the pair of rotating tools includes a shoulder part and a pin part that is disposed at the shoulder part and that along with the shoulder part includes a common rotation axis, and at least the shoulder parts and the pin parts are made of materials that are harder than materials of the metal sheets or the metal plates,
  • the pair of rotating tools is pushed against a top surface and a bottom surface of each metal sheet or metal plate, the rotating tools are moved in the welding direction while being rotated, and
  • the rotation axes of the pair of respective rotating tools is inclined at an inclination angle ⁇ (°) on a preceding side with respect to the welding direction from a vertical direction with respect to the metal sheets or the metal plates, the inclination angle ⁇ satisfies
  • a gap G (mm) between the shoulder parts that is formed by providing a gap g (mm) between ends of the pin parts of the pair of respective rotating tools with respect to a thickness t (mm) of each metal sheet or metal plate and a diameter D (mm) of the shoulder part of each rotating tool satisfies
  • the pair of rotating tools is further rotated in opposite directions to perform the friction stir welding.
  • aspects of the present invention provide a double-side friction stir welding device in which a pair of rotating tools facing each other is respectively disposed on a top-surface side and a bottom-surface side of a butted portion or an overlapping portion, which comprises a joint portion of two metal sheets or two metal plates; the pair of rotating tools is moved in a welding direction while being rotated at the butted portion or the overlapping portion; and while the metal sheets or the metal plates are softened by frictional heat generated between the rotating tools and the metal sheets or the metal plates, softened parts are stirred by the rotating tools to produce a plastic flow to join the metal sheets or the metal plates to each other, wherein
  • each rotating tool includes a shoulder part and a pin part that is disposed at the shoulder part and that along with the shoulder part includes a common rotation axis, and at least the shoulder parts and the pin parts are made of materials that are harder than materials of the metal sheets or the metal plates,
  • a gripping device that fixes the metal sheets or the metal plates while the pair of rotating tools are moved in the welding direction while being rotated
  • a gap G (mm) between the shoulder parts that is formed by providing a gap g (mm) between ends of the pin parts of the pair of respective rotating tools with respect to a thickness t (mm) of each metal sheet or metal plate for a case of butting or with respect to a total thickness t (mm) of the overlapped metal sheets or the overlapped metal plates for a case of overlapping satisfies
  • a rotation driving device that further rotates the pair of rotating tools in opposite directions is provided.
  • aspects of the present invention also provide a double-side friction stir welding device in which a pair of rotating tools facing each other is respectively disposed on a top-surface side and a bottom-surface side of a butted portion or an overlapping portion, which comprises a joint portion of two metal sheets or two metal plates; the pair of rotating tools is moved in a welding direction while being rotated at the butted portion or the overlapping portion; and while the metal sheets or the metal plates are softened by frictional heat generated between the rotating tools and the metal sheets or the metal plates, softened parts are stirred by the rotating tools to produce a plastic flow to join the metal sheets or the metal plates to each other, wherein
  • each of the pair of rotating tools includes a shoulder part and a pin part that is disposed at the shoulder part and that along with the shoulder part includes a common rotation axis, and at least the shoulder parts and the pin parts are made of materials that are harder than materials of the metal sheets or the metal plates,
  • a gripping device that fixes the metal sheets or the metal plates while the pair of rotating tools is moved in the welding direction while being rotated is provided
  • the rotation axes of the pair of respective rotating tools is inclined at an inclination angle ⁇ (°) on a preceding side with respect to the welding direction from a vertical direction with respect to the metal sheets or the metal plates, the inclination angle ⁇ satisfies
  • a gap G (mm) between the shoulder parts that is formed by providing a gap g (mm) between ends of the pin parts of the pair of respective rotating tools with respect to a thickness t (mm) of each metal sheet or metal plate for a case of butting or with respect to a total thickness t (mm) of the overlapped metal sheets or the overlapped metal plates for a case of overlapping and a diameter D (mm) of the shoulder part of each rotating tool satisfies
  • a rotation driving device that further rotates the pair of rotating tools in opposite directions is provided.
  • the diameter D (mm) of each shoulder part with respect to the thickness t (mm) of each metal sheet or metal plate for the case of butting or with respect to the total thickness t (mm) of the overlapped metal sheets or the overlapped metal plates for the case of overlapping satisfy 4 ⁇ t ⁇ D ⁇ 20 ⁇ t, and that the gap g (mm) with respect to the thickness t (mm) of each metal sheet or metal plate for the case of the butting or the total thickness t (mm) of the overlapped metal sheets or the overlapped metal plates for the case of overlapping and the diameter D (mm) of the shoulder part of each rotating tool satisfy
  • the shoulder parts of the pair of rotating tools facing each other is respectively pushed against the top surface and the bottom surface of a joint portion of the metal sheets or the metal plates, and the two surfaces of the joint portion of the metal sheets or the metal plates are subjected to a large deformation at high temperatures due to a sufficient temperature increase and shear stress caused by rotating the shoulder parts. Therefore, it is possible to accelerate a homogeneous plastic flow with respect to the thickness direction of the metal sheets or the metal plates, and to attain a good welded state. As a result, it is possible to increase the welding speed while suppressing defects at the time of welding from occurring, to ensure a sufficient strength, and to improve welding workability, so that the effects are industrially considerably exhibited.
  • FIG. 1 is a schematic perspective view of an exemplary arrangement of rotating tools and metal sheets or metal plates in accordance with aspects of the present invention.
  • FIG. 2 ( 1 ) is a plan view showing a part of the rotating tools and the metal sheets or the metal plates in FIG. 1
  • FIG. 2 ( 2 ) is a sectional view along arrow A-A shown in FIG. 2 ( 1 ).
  • FIGS. 3 ( 1 ) and 3 ( 2 ) are each a sectional view showing the dimensions of cross sections of the rotating tools used in an embodiment.
  • double-side friction stir welding is performed by butting two metal sheets or metal plates against each other or by making the two metal sheets or metal plates overlap each other and by disposing a pair of rotating tools respectively on a top-surface side and a bottom-surface side of a butted portion or an overlapping portion.
  • a pair of rotating tools 1 and 8 is respectively disposed so as to face the top-surface side and the bottom-surface side of each of two metal sheets 3 or metal plates 3 that are butted against each other, the rotating tools 1 and 8 are inserted into an unwelded joint 12 from the top-surface side and the bottom-surface side, respectively, of each metal sheet 3 or metal plate 3 , and the pair of rotating tools 1 and 8 is moved in a welding direction while being rotated.
  • the arrow Q indicates the rotation direction of the rotating tool 1 that is disposed on the top-surface side
  • the arrow R indicates the rotation direction of the rotating tool 8 that is disposed on the bottom-surface side.
  • a welded joint 4 that is obtained in this way is linearly formed in the advancing direction of the rotating tools 1 and 8 .
  • a straight line 7 (hereunder called “welding center line”) extending from the unwelded joint 12 to the center of the width of the welded joint 4 in FIG. 1 matches a path of the rotating tools 1 and 8 that advance in the direction of arrow P (see FIG. 2 ( 1 )).
  • the two metal sheets 3 or metal plates 3 are gripped by a gripping device (not shown) and are fixed at predetermined positions.
  • a gripping device a device that is capable of preventing variations in the positions of the metal sheets 3 or the metal plates 3 as the rotating tools 1 and 8 advance may be used, so that the structure of the gripping device is not particularly limited to certain structures.
  • a gap G (mm) is formed between stepped portions 5 and 9 (hereunder called “shoulder parts”), the stepped portion 5 being formed due to a difference between a diameter D (mm) of the rotating tool 1 and a diameter a (mm) of the pin part 6 and the stepped portion 9 being formed due to a difference between a diameter D (mm) of the rotating tool 8 and a diameter a (mm) of the pin part 10 .
  • the rotating tool 8 on the bottom-surface side is rotated in a direction (that is, the direction of arrow R) opposite to the rotation direction (that is, the direction of arrow Q) of the rotating tool 1 on the top-surface side.
  • a direction that is, the direction of arrow R
  • the rotating tool 8 is rotated counterclockwise.
  • the rotating tool 8 is rotated clockwise.
  • the gap g is provided between the end of the pin part 6 of the rotating tool 1 and the end of the pin part 10 of the rotating tool 8 , the gap G is provided between the shoulder part 5 of the rotating tool 1 and the shoulder part 9 of the rotating tool 8 , and the rotating tool 1 and the rotating tool 8 are rotated in opposite directions. Therefore, it is possible to subject both surfaces of each metal sheet 3 or metal plate 3 to a sufficient temperature increase and shear stress, to reduce differences between the temperatures and between the plastic flows occurring in a thickness direction of each metal sheet 3 or metal plate 3 at the welded joint 4 , and to attain a homogeneous welded state. In addition, it is possible to advantageously overcome welding defects by overcoming plastic flow failures occurring locally inside the welded joint 4 , to ensure a sufficient strength, and to improve welding workability, in particular, to increase welding speed.
  • the rotating tool 1 on the top-surface side includes the shoulder part 5 and the pin part 6 that is disposed at the shoulder part 5 and that includes along with the shoulder part 5 a common rotation axis 2 .
  • the rotating tool 8 on the bottom-surface side includes the shoulder part 9 and the pin part 10 that is disposed at the shoulder part 9 and that along with the shoulder part 9 includes a common rotation axis 11 .
  • At least the shoulder parts 5 and 9 and the pin parts 6 and 10 are made of materials that are harder than the materials of the metal sheets 3 or the metal plates 3 .
  • the rotation directions Q and R of the respective rotating tools 1 and 8 facing each other are such that the rotation direction Q on the top-surface side is opposite to the rotation direction R on the bottom-surface side.
  • adjusting the arrangement of the rotating tools as follows is effective in terms of prolonging the life of each rotating tool, suppressing welding defects from occurring, and increasing the welding speed.
  • the rotating tools 1 and 8 can be subjected to loads with respect to the rotating tools 1 and 8 as component forces that are compressed in the directions of the respective rotation axes 2 and 11 .
  • the pair of rotating tools 1 and 8 need to be made of materials that are harder than the materials of the metal sheets 3 or the metal plates 3 .
  • the rotating tools 1 and 8 are made of materials that lack toughness, such as ceramic, subjecting the pin parts 6 and 10 to bending-direction forces causes localized stress to be concentrated and the pin parts 6 and 10 to break. Therefore, by causing the rotation axes 2 and 11 of the respective rotating tools 1 and 8 to be inclined at the angle ⁇ (hereunder referred as the “inclination angle”), the loads exerted upon the rotating tools 1 and 8 are applied as component forces that are compressed in the directions of the respective rotation axes 2 and 11 , such that it is possible to reduce the bending-direction forces and to prevent the rotating tools 1 and 8 from becoming damaged.
  • the angle ⁇ hereunder referred as the “inclination angle”
  • the inclination angle ⁇ is greater than 0°, the aforementioned effects are obtained.
  • the inclination angle ⁇ is greater than 3°, the front and bottom surfaces of the welded joint become concave-shaped and adversely affect the welding joint strength. Therefore, 30 is the upper limit. That is, it is desirable that the inclination angle be 0° ⁇ 3°. It is more desirable that the inclination angle be 0.5° ⁇ 2.00.
  • the gap G is limited to a range of 0.5 ⁇ t or greater and t or less with respect to thickness t (mm) of the metal sheets 3 or the metal plates 3 in the case of butt-welding or with respect to total thickness t (mm) of the overlapped metal sheets 3 or metal plates 3 in the case of overlap-welding.
  • the shoulder parts 5 and 9 of the respective rotating tools 1 and 8 facing each other respectively push the top-surface side and the bottom-surface side of each metal sheet 3 or metal plate 3 by a sufficient load, and heat generation and plastic flow are accelerated due to friction produced and plastic deformation in a shearing direction by the shoulder parts 5 and 9 of the respective rotating tools 1 and 8 . Therefore, it is possible to accelerate a homogeneous plastic flow with respect to the thickness direction of the metal sheets 3 or the metal plates 3 , and to attain a good welded state.
  • the shoulder parts 5 and 9 of the respective rotating tools 1 and 8 can no longer respectively push the top-surface side and the bottom-surface side of each metal sheet 3 or metal plate 3 by a sufficient load, as a result of the aforementioned effects cannot be obtained.
  • the gap G between the shoulder part 5 of the rotating tool 1 on the top-surface side and the shoulder part 9 of the rotating tool 8 on the bottom-surface side needs to be set small.
  • the gap G needs to be (0.5 ⁇ t) ⁇ (0.2 ⁇ D ⁇ sin ⁇ ) or greater and t ⁇ (0.2 ⁇ D ⁇ sin ⁇ ) or less with respect to, in addition to the thickness t of each metal sheet 3 or metal plate 3 (in the case of butt-welding) or the total thickness t of the overlapped metal sheets or metal plates (in the case of overlap-welding), diameter D (mm) of the shoulder parts 5 and 9 of the respective rotating tools 1 and 8 and the inclination angle ⁇ (°).
  • the gap G When the gap G is less than (0.5 ⁇ t) ⁇ (0.2 ⁇ D ⁇ sin ⁇ ), the front and bottom surfaces of the welded joint become concave-shaped and adversely affect the welding joint strength.
  • the gap G When the gap G is greater than t ⁇ (0.2 ⁇ D ⁇ sin ⁇ ), the shoulder parts 5 and 9 of the respective rotating tools 1 and 8 cannot respectively push the top-surface side and the bottom-surface side of each metal sheet 3 or metal plate 3 by a sufficient load, and the aforementioned effects cannot be obtained. It is desirable that the gap G be (0.6 ⁇ t) ⁇ (0.2 ⁇ D ⁇ sin ⁇ ) ⁇ G ⁇ (0.9 ⁇ t) ⁇ (0.2 ⁇ D ⁇ sin ⁇ ).
  • the gap G is as follows:
  • the gaps g between the end of the pin part 6 and the end of the pin part 10 are less than [0.1 ⁇ 0.09 ⁇ exp ⁇ 0.011 ⁇ (D/t) 2 ⁇ ] ⁇ t, the ends of the pin parts 6 and 10 of the respective rotating tools 1 and 8 facing each other may be brought into contact with each other and may become damaged, which is not desirable.
  • the gap g between the end of the pin part 6 and the end of the pin part 10 is greater than [1 ⁇ 0.9 ⁇ exp ⁇ 0.011 ⁇ (D/t) 2 ⁇ ] ⁇ t, a homogenous plastic flow with respect to the thickness direction cannot be effectively realized.
  • the gap g be [0.1 ⁇ 0.09 ⁇ exp ⁇ 0.011 ⁇ (D/t) 2 ⁇ ] ⁇ t ⁇ g ⁇ [1 ⁇ 0.9 ⁇ exp ⁇ 0.011 ⁇ (D/t) 2 ⁇ ] ⁇ t. It is more desirable that the gap g be [0.12 ⁇ 0.09 ⁇ exp ⁇ 0.011 ⁇ (D/t) 2 ⁇ ] ⁇ t ⁇ g ⁇ [0.9 ⁇ 0.9 ⁇ exp ⁇ 0.011 ⁇ (D/t) 2 ⁇ ] ⁇ t.
  • the diameter D (mm) When the diameter D (mm) is less than 4 ⁇ t, a homogenous plastic flow with respect to the thickness direction of the metal sheets 3 or the metal plates 3 cannot be effectively obtained.
  • the diameter D (mm) when the diameter D (mm) is greater than 20 ⁇ t, only a region where unwanted plastic flow occurs is widened, as a result of which an excessive load is applied to the device, which is not desirable. That is, the diameter D is 4 ⁇ t ⁇ D ⁇ 20 ⁇ t.
  • the thickness t refers to the thickness t (mm) of each metal sheet 3 or metal plate 3 in the case of butt welding or the total thickness t (mm) of the overlapped metal sheets 3 or the overlapped metal plates 3 in the case of overlap-welding. It is more desirable that the diameter D be 5 ⁇ t ⁇ D ⁇ 18 ⁇ t.
  • Length b of the pin part 6 of the rotating tool 1 on the top-surface side and the pin part 10 of the rotating tool 8 on the bottom-surface side is determined as appropriate in accordance with the inclination angle ⁇ , the gap G between the shoulder parts, the gap g between the ends of the pin parts, the diameter D of the shoulder parts, and the thickness t.
  • the welding conditions other than those described above are in accordance with ordinary methods. This makes it possible to set the number of rotations of the rotating tools 1 and 8 facing each other in the range of 100 to 5000 rotations/minute and to increase the welding speed to 1000 mm/min or greater.
  • the metal sheets 3 or metal plates 3 to which aspects of the present invention are applied are suitably applicable to general structural steels or carbon steel sheets or carbon steel plates, such as steel sheets or steel plates in correspondence with JIS G 3106 or JIS G 4051.
  • the metal sheets 3 or metal plates 3 are also advantageously applicable to high-strength structural steel sheets or high-strength structural steel plates having a tensile strength of 800 MPa or greater. Even in this case, the welded joint can have a strength that is greater than or equal to 85% of the tensile strength of the steel sheets or steel plates, or greater than or equal to 90% of the tensile strength of the steel sheets or steel plates.
  • the welding conditions of the friction stir welding are shown in Table 2.
  • Table 2 The welding conditions of the friction stir welding are shown in Table 2.
  • the welding was performed for the case where the rotation direction of the rotating tool on the top-surface side was clockwise and the rotation direction of the rotating tool on the bottom-surface side was counterclockwise, and for the case where the rotation direction of the rotating tool on the top-surface side and the rotation direction of the rotating tool on the bottom-surface side were both clockwise.
  • the rotating tools using as materials two types of tungsten carbide (WC) having cross-sectional shapes shown in FIGS. 3 ( 1 ) and 3 ( 2 ) were used.
  • Table 3 shows whether or not surface defects existed in the observations of appearances of joints at the time of the welding, whether or not internal defects existed in the observations of cross sections of the joints, and tensile strengths when tension tests were performed by extracting tensile specimens having the dimensions of specimen No. 1 prescribed in JIS Z 3121 from the acquired welding joints.
  • specimens were provided as follows. That is, parts in which the welding speeds of the acquired welding joints having the values shown in Table 2 were cut so as to provide a cross section in a location that was 20 mm from an end portion of a welding start side, a cross section in a location that was 20 mm from an end portion of a welding completion side, and a cross section in a location that was intermediate between both the end portions. Whether or not internal defects existed was evaluated by using an optical microscope (magnification: 10 ⁇ ) based on whether or not unwelded states produced in the interior of the welded joints due to a lack of plastic flow was observed.
  • the joint strengths were 70% or less of the tensile strengths of the steel sheets or the steel plates, which were the base materials.

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