Patent Publication Number: US-11045898-B2

Title: Friction stir spot welding method and friction stir spot welding apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to a friction stir spot welding method and a friction stir spot welding apparatus. 
     BACKGROUND ART 
     Friction stir spot welding (FSSW) is expected to replace, for example, spot welding and rivet connection. In friction stir spot welding, a rotary tool is plunged into workpieces that are stacked together. At the time, frictional heat is generated, which softens part of the workpieces, and the softened part of the workpieces is stirred. Consequently, the workpieces are jointed together. Examples of the friction stir spot welding include single-acting friction stir spot welding and refill friction stir spot welding. In the single-acting friction stir spot welding, a rotary tool in which a shoulder and a pin are fixed is used. In the refill friction stir spot welding, a rotary tool in which a shoulder and a pin move relative to each other is used. In the refill friction stir spot welding, a backfilling process is performed after the plunging process. In the backfilling process, a hole formed as a result of plunging the rotary tool into the workpieces (i.e., a plunging hole) is backfilled with the stirred materials, and thereby a mark formed due to the joining can be made less noticeable (see Patent Literature 1). 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Laid-Open Patent Application Publication No. 2012-196682 
     SUMMARY OF INVENTION 
     Technical Problem 
     There are cases where the surface of a workpiece is subjected to surface treatment, such as anodizing treatment, or a protective layer made of a material that is different from the base material of the workpiece may be formed on the surface of the workpiece by primer coating. It has been found that in a case where workpieces having such a protective layer formed thereon are joined together by refill friction stir spot welding, the strength of the joint is lower than in a case where workpieces having no protective layer formed thereon are joined together by the same refill friction stir spot welding. The inventors of the present invention have further discovered that the lowering of the strength of the joint is caused by how the protective layer component is distributed in the refill friction stir spot welding. 
     The present invention has been made in view of the above. An object of the present invention is to provide a friction stir spot welding method that makes it possible to suppress the lowering of the strength of the joint when workpieces having a protective layer formed thereon are joined together by refill friction stir spot welding. 
     Solution to Problem 
     A friction stir spot welding method according to one aspect of the present invention is a refill friction stir spot welding method of joining a first workpiece and a second workpiece together by friction stir spot welding by using a rotary tool that includes a cylindrical shoulder and a columnar pin movable within the shoulder, the first workpiece and the second workpiece being such that a protective layer is formed on a joint surface of at least one of the first workpiece and the second workpiece. The friction stir spot welding method includes: performing a plunging process of moving the shoulder toward the second workpiece from the first workpiece side while rotating the rotary tool to plunge the shoulder into the first and second workpieces, such that a material of the first workpiece and a material of the second workpiece are partly stirred to form a stirred portion, and concurrently, a component of the protective layer flows into an inside of the shoulder together with other stirred materials; and performing a backfilling process of pushing the stirred materials that have flowed into the inside of the shoulder in the plunging process out of the shoulder by the pin while rotating the rotary tool, and concurrently, moving the shoulder backward to backfill a plunging hole with the stirred materials, the plunging hole being formed by the plunging of the shoulder. In the plunging process, the shoulder is plunged to a position that is shifted from a boundary between the first workpiece and the second workpiece to the second workpiece side by 1 mm or more, such that the component of the protective layer is concentrated in a central portion of the stirred portion when the backfilling process is completed. 
     In the plunging process, in which the stirred portion is formed, the component of the protective layer does not spread over the entire stirred portion, but a layer in which the density of the component of the protective layer is high, i.e., a layer (remaining layer) whose main component is the component of the protective layer, is formed within the stirred portion. In a conventional friction stir spot welding method, as a result of performing the backfilling process, the remaining layer is moved to a position close to the original position of the protective layer. In this case, when force is applied to the joint, a crack tends to occur in the remaining layer. The inventors of the present invention have found through experiments that this is the cause of the lowering of the joint strength. In this respect, according to the above-described method, in the backfilling process, the remaining layer can be concentrated in the central portion of the stirred portion. This makes it possible to prevent the formation of cracks in the remaining layer and suppress the lowering of the joint strength. 
     In the above friction stir spot welding method, a surface pressure of the shoulder against the stirred portion in the backfilling process may be set to be lower than the surface pressure of the shoulder against the stirred portion in the plunging process. 
     In a case where the shoulder is plunged to a certain plunging depth or more in the plunging process as described above, the flow of not only the materials in the stirred portion but also the materials around the stirred portion is facilitated. Accordingly, when the shoulder is pressed against the stirred portion in the backfilling process, a phenomenon occurs in which the portion around the stirred portion rises. In this respect, by setting the surface pressure of the shoulder against the stirred portion in the backfilling process to be low as described above, the occurrence of the phenomenon can be prevented. 
     In the backfilling process of the above friction stir spot welding method, the shoulder may be pressed against the stirred portion at a surface pressure of 90 to 175 MPa. 
     By thus setting the surface pressure of the shoulder against the stirred portion in the backfilling process to a relatively low value of 90 to 175 MPa, the rising of the portion around the stirred portion, which may occur when the shoulder is pressed against the stirred portion, can be suppressed. 
     A friction stir spot welding apparatus according to another aspect of the present invention is a friction stir spot welding apparatus for joining a first workpiece and a second workpiece by friction stir spot welding, the first workpiece and the second workpiece being such that a protective layer is formed on a joint surface of at least one of the first workpiece and the second workpiece. The friction stir spot welding apparatus includes: a rotary tool that includes a cylindrical shoulder and a columnar pin movable within the shoulder; and a controller configured to control the rotary tool. The controller: moves the shoulder toward the second workpiece from the first workpiece side while rotating the rotary tool to plunge the shoulder into the first and second workpieces, such that a material of the first workpiece and a material of the second workpiece are partly stirred to form a stirred portion, and concurrently, a component of the protective layer flows into an inside of the shoulder together with other stirred materials; and then causes the pin to push the stirred materials that have flowed into the inside of the shoulder in the plunging process out of the shoulder while rotating the rotary tool, and concurrently, moves the shoulder backward to backfill a plunging hole with the stirred materials, the plunging hole being formed by the plunging of the shoulder. When forming the stirred portion, the controller plunges the shoulder to a position that is shifted from a boundary between the first workpiece and the second workpiece to the second workpiece side by 1 mm or more. 
     Advantageous Effects of Invention 
     The above-described friction stir spot welding method makes it possible to suppress the lowering of the strength of the joint when workpieces having a protective layer formed thereon are joined together. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a general view of a friction stir spot welding apparatus. 
         FIG. 2  is a block diagram of a control system of the friction stir spot welding apparatus. 
         FIG. 3  is a conceptual diagram showing a plunging process in a conventional friction stir spot welding method. 
         FIG. 4  is a conceptual diagram showing a backfilling process in the conventional friction stir spot welding method. 
         FIG. 5  is a conceptual diagram showing a plunging process in a friction stir spot welding method according to one embodiment. 
         FIG. 6  is a conceptual diagram showing a backfilling process in the friction stir spot welding method according to the embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     &lt;Joining Apparatus&gt; 
     First, a friction stir spot welding apparatus (which may hereinafter be simply referred to as “joining apparatus”)  100  used in a friction stir spot welding method (which may hereinafter be simply referred to as “joining method”) according to one embodiment of the present invention is described.  FIG. 1  is a schematic diagram showing the joining apparatus  100 .  FIG. 2  is a block diagram of a control system of the joining apparatus  100 . The joining apparatus  100  is an apparatus for joining a first workpiece  101  and a second workpiece  102  together (hereinafter, these workpieces  101  and  102  are collectively referred to as “workpiece  103 ”) by friction stir spot welding. The joining apparatus  100  includes a body  10 , a tool unit  20 , and a controller  40 . 
     The body  10  includes: a frame  11  formed in a C shape; backing  12  provided on one end portion of the frame  11  (in the lower part of  FIG. 1 ) and supporting the workpiece  103 ; and a tool unit driver  13  provided on the other end portion of the frame  11  (in the upper part of  FIG. 1 ) in a manner to face the backing  12 . 
     The tool unit driver  13  is capable of causing the tool unit  20  to move in a direction toward the workpiece  103  (hereinafter, “move forward”) and to move in a direction away from the workpiece  103  (hereinafter, “move backward”). The tool unit driver  13  is provided with a tool unit pressure sensor  41  (see  FIG. 2 ). The tool unit driver  13  of the present embodiment is configured as a rack-and-pinion mechanism. However, as an alternative, the tool unit driver  13  may be configured as, for example, a ball screw mechanism or a hydraulic actuator mechanism. 
     The tool unit  20  includes: a tool holder  21  connected to the tool unit driver  13  of the body  10 ; a clamp  22  held by the tool holder  21 ; and a rotary tool  23  held by the tool holder  21  and positioned inward of the clamp  22 . 
     The clamp  22  includes a cylindrical pressing member  24  and an urging portion  25  configured to urge the pressing member  24  toward the workpiece  103 . The urging portion  25  of the present embodiment is configured as a spring coil. However, as an alternative, the urging portion  25  may be configured as, for example, a rack-and-pinion mechanism, a ball screw mechanism, or a hydraulic actuator mechanism. 
     The rotary tool  23  is mounted to the tool holder  21  via a rotation motor  26 . Accordingly, the rotary tool  23  rotates relative to the tool holder  21  and the workpiece  103 . The rotary tool  23  includes: a shoulder holder  27  connected to the rotation motor  26 ; a shoulder  28  held by the shoulder holder  27 ; and a pin  29  positioned within the shoulder  28 . 
     The shoulder  28  is positioned inward of the pressing member  24 , and has a cylindrical shape. The central axis of the shoulder  28  coincides with the rotational axis of the rotary tool  23 . When performing friction stir spot welding, the shoulder  28  is plunged into the workpiece  103  while rotating. 
     The pin  29  has a columnar shape. The central axis of the pin  29  coincides with the rotational axis of the rotary tool  23  and the central axis of the shoulder  28 . The pin  29  is connected to the shoulder  28  via a pin driver  30 . The pin driver  30  is capable of causing the pin  29  to move within the shoulder  28  in the axial direction. It should be noted that the position of the pin  29  relative to the workpiece  103  in the axial direction is determined by the moving amount of the tool unit  20  and the moving amount of the pin  29  relative to the shoulder  28 . The pin  29  rotates together with the shoulder  28 . 
     The controller  40  includes a CPU, ROM, RAM, etc. The controller  40  controls the entire joining apparatus  100  including the rotary tool  23 . 
     As shown in  FIG. 2 , the controller  40  is electrically connected to the tool unit pressure sensor  41 , and is capable of obtaining the pressing pressure of the tool unit  20  when the tool unit  20  is pressed against the workpiece  103  (hereinafter, “tool unit pressing pressure”) based on a measurement signal transmitted from the tool unit pressure sensor  41 . It should be noted that, based on the obtained tool unit pressing pressure and the urging force of the clamp  22  (the urging force is calculated from the moving amount of the tool unit  20 ), the pressing pressure of the shoulder  28  when the shoulder  28  is pressed against the workpiece  103  (hereinafter, “shoulder pressing pressure”) can be calculated. The shoulder pressing pressure may be calculated by an external device (e.g., external PC) provided separately from the joining apparatus  100 , or may be calculated by the controller  40 . 
     The controller  40  is electrically connected to the tool unit driver  13 , the rotation motor  26 , and the pin driver  30 , and transmits control signals to these devices. By transmitting the control signals to these devices, the controller  40  can move the tool unit  20  by an intended moving amount and at an intended moving speed, rotate the rotary tool  23  at an intended rotational speed (hereinafter, “tool rotational speed”), and move the pin  29  relative to the shoulder  28  by an intended moving amount and at an intended moving speed. It should be noted that the moving amount and the moving speed of the shoulder  28  are equal to the moving amount and the moving speed of the tool unit  20 . 
     The joining apparatus  100  is as described above. In the above-described joining apparatus  100 , the clamp  22 , the shoulder  28 , and the pin  29  are configured to move forward and backward integrally, and the shoulder  28  and the pin  29  are configured to rotate integrally. However, the clamp  22 , the shoulder  28 , and the pin  29  may be configured to move or rotate independently of each other. Although  FIG. 1  shows the first workpiece  101  and the second workpiece  102  as flat plate-shaped members, the first workpiece  101  and the second workpiece  102  are not limited to flat plate-shaped members. 
     &lt;Conventional Joining Method&gt; 
     Next, before describing a joining method according to the present embodiment, a conventional joining method is described.  FIG. 3  and  FIG. 4  are conceptual diagrams showing a conventional joining method.  FIG. 3  and  FIG. 4  show only the shoulder  28 , the pin  29 , the first workpiece  101 , and the second workpiece  102 , which are essential components (similarly,  FIG. 5  and  FIG. 6  also show only these essential components). 
     The first workpiece  101  and the second workpiece  102 , which are to be joined together, are made of an aluminum alloy. A protective layer  104  is formed on a surface (joint surface) of the first workpiece  101 , the surface contacting the second workpiece  102 . The protective layer  104  is formed also on a surface (joint surface) of the second workpiece  102 , the surface contacting the first workpiece  101 . It should be noted that examples of the protective layer  104  include an anodized layer, a coated layer, a clad layer, and a sealing layer. 
     The conventional joining method includes a plunging process (see  FIG. 3 ) and a backfilling process (see  FIG. 4 ). Each process is performed as a result of the controller  40  controlling the operation of the rotary tool  23  by transmitting control signals to the tool unit driver  13 , the rotation motor  26 , and the pin driver  30 . 
     In the plunging process, the shoulder  28  is plunged into the workpiece  103 . Specifically, as shown in  FIG. 3 , while rotating the rotary tool  23 , the shoulder  28  is moved forward toward the second workpiece  102  from the first workpiece  101  side. At the time, the pin  29  is kept moved backward. It should be noted that the shoulder  28  is plunged to a position that is shifted from the boundary between the first workpiece  101  and the second workpiece  102  to the second workpiece  102  side by 0.1 to 0.3 mm. 
     As a result of performing the plunging process, the material of the first workpiece  101  and the material of the second workpiece  102  are partly softened by frictional heat, and the softened materials are stirred by the rotation of the shoulder  28 . Consequently, a stirred portion  105  (a shaded portion in the drawing) is formed, which is made of the materials stirred by the shoulder  28  (stirred materials). At the time, the base material component of the first workpiece  101 , the base material component of the second workpiece  102 , and the components of the protective layers  104  flow into the inside of the shoulder  28 . 
     In the plunging process, the components of the protective layers  104  do not spread over the entire stirred portion  105 , but flow in a relatively non-spreading manner. For this reason, as shown in  FIG. 3 , a layer in which the density of the components of the protective layers  104  is high, i.e., a layer whose main components are the components of the protective layers  104  (hereinafter, “remaining layer  106 ”), is formed. As a result of performing the plunging process, the remaining layer  106  flows into the inside of the shoulder  28 . 
     It should be noted that, in the above plunging process, since the shoulder  28  is plunged into the workpiece  103 , if the shoulder  28  is simply moved backward and the stirred portion  105  is cooled down and solidified, a plunging hole will be formed in the portion previously plunged by the shoulder  28 . For this reason, in order to prevent the formation of the plunging hole in the stirred portion  105 , a backfilling process described below is performed following the plunging process. 
     In the backfilling process, the pin  29  is moved forward relative to the shoulder  28  while rotating the rotary tool  23 . As a result, the stirred materials that have flowed into the inside of the shoulder  28  are pushed out of the shoulder  28  by the pin  29 . Then, the stirred materials that have been pushed out from the inside of the shoulder  28  apply force to the end surface of the shoulder  28  at the forward moving direction side (i.e., to the lower surface of the shoulder  28  in  FIG. 3 ), causing the shoulder  28  to move backward, and thereby the stirred materials flow into the portion previously plunged by the shoulder  28  (i.e., into the plunging hole). The operation thus described is continued until the position of the surface of the pin  29 , the surface contacting the workpiece  103 , and the position of the surface of the shoulder  28 , the surface contacting the workpiece  103 , coincide with each other in the axial direction. In this manner, the plunging hole is backfilled with the stirred materials, and thereby the surface of the first workpiece  101  is flattened. 
     In the backfilling process, the shoulder  28  moves backward as a result of the shoulder  28  being pushed back and lifted by the stirred materials. Thus, the shoulder  28  moves backward while the shoulder  28  is pressing the workpiece  103 . That is, the shoulder  28  moves in a direction indicated by outlined arrows in  FIG. 4 , and also, the shoulder  28  presses the workpiece  103  (stirred portion  105 ) in a direction reverse to the moving direction. At the time, the pressing pressure of the shoulder  28  (shoulder pressing pressure) is about 200 MPa. 
     When the backfilling process is completed, the material of the first workpiece  101  and the material of the second workpiece  102 , which have been partly stirred in the stirred portion  105 , are cooled down and solidified, and thereby a columnar joint is formed. In this manner, the first workpiece  101  and the second workpiece  102  are joined together. 
     Here, as shown in  FIG. 4 , in the conventional joining method, when the plunging hole is backfilled with the stirred materials in the backfilling process, the remaining layer  106  is pushed back to a position close to the original position of the protective layers  104 . Accordingly, when the joining of the first workpiece  101  and the second workpiece  102  is completed, the outer peripheral portion of the remaining layer  106  reaches the outer peripheral surface of the joint (i.e., the outer peripheral surface of the stirred portion  105 ) or the vicinity thereof, and the position of the outer peripheral portion of the remaining layer  106  in the axial direction is close to the position of the boundary between the first workpiece  101  and the second workpiece  102  in the axial direction. In this case, when force is applied to the joint, a crack tends to occur in the outer peripheral portion of the remaining layer  106 . For this reason, in a case where workpieces having a protective layer formed thereon are joined together, the strength of the joint is lower than in a case where workpieces having no protective layer formed thereon are joined together. 
     Joining Method According to Embodiment 
     Next, a joining method according to the present embodiment is described.  FIG. 5  and  FIG. 6  show the joining method according to the present embodiment. It is assumed herein that the protective layer  104  is formed on the joint surface of each of the first workpiece  101  and the second workpiece  102 . 
     Similar to the conventional joining method, the joining method according to the present embodiment includes a plunging process (see  FIG. 5 ) and a backfilling process (see  FIG. 6 ). Each process is performed as a result of the controller  40  controlling the rotary tool  23  by transmitting control signals to the tool unit driver  13 , the rotation motor  26 , and the pin driver  30 . 
     In the plunging process of the present embodiment, similar to the conventional plunging process, while rotating the rotary tool  23 , the shoulder  28  is moved forward toward the second workpiece  102  from the first workpiece  101  side to plunge the shoulder  28  into the workpiece  103 . However, as shown in  FIG. 5 , the plunging depth of the shoulder  28  into the workpiece  103  in the present embodiment is greater than the plunging depth in the conventional joining method (see  FIG. 3 ). Specifically, in the present embodiment, the shoulder  28  is plunged to a position that is shifted from the boundary between the first workpiece  101  and the second workpiece  102  to the second workpiece  102  side by 1 mm or more. 
     In a case where the plunging depth of the shoulder  28  is thus great, the stirred portion  105  expands to the second workpiece  102  side compared to a case where the plunging depth is small. For this reason, among the stirred materials flowing into the inside of the shoulder  28 , the amount of the base material component of the first workpiece  101  and the amount of the components of the protective layers  104  are substantially the same as those in the conventional plunging process, but the amount of the base material component of the second workpiece  102  is significantly greater than that in the conventional plunging process. 
     It should be noted that if the plunging of the shoulder  28  in the plunging process is stopped near the boundary between the first workpiece  101  and the second workpiece  102 , the stirred material of the first workpiece  101  is merely pushed into the second workpiece  102  side, and the first workpiece  101  and the second workpiece  102  are not mixed together sufficiently. This tendency is noticeable particularly in a case where impurities such as the protective layers  104  are present at the boundary between the first workpiece  101  and the second workpiece  102 . 
     Next, in the backfilling process of the present embodiment, similar to the conventional backfilling process, while rotating the rotary tool  23 , the stirred materials that have flowed into the inside of the shoulder  28  are pushed out of the shoulder  28  by the pin  29 , and thereby the shoulder  28  is moved backward. 
     As previously described, in the plunging process of the joining method according to the present embodiment, the amount of the base material component of the second workpiece flowing into the inside of the shoulder  28  is greater than in the conventional plunging process. For this reason, in the backfilling process, in accordance with the backward movement of the shoulder  28 , a large amount of the base material component of the second workpiece flows into the portion previously plunged by the shoulder  28  (i.e., into the plunging hole). Consequently, as shown in  FIG. 6 , when the backfilling process is completed, the base material component of the first workpiece  101  or the second workpiece  102  spreads near the outer peripheral surface of the stirred portion  105  (i.e., the outer peripheral surface of the joint), and the remaining layer  106  is enclosed within the stirred portion  105 . That is, the components of the protective layers  104  are concentrated in the central portion of the stirred portion  105 . In other words, near the central axis of the stirred portion  105 , the proportion of the components of the protective layers  104  to the stirred materials is higher than in the outer peripheral portion of the stirred portion  105 . 
     Therefore, the components of the protective layers  104  hardly reach the outer peripheral portion of the stirred portion  105 . The position of the outer peripheral portion of the remaining layer  106  in the axial direction is also away from the position of the boundary between the first workpiece  101  and the second workpiece  102  in the axial direction. Consequently, the joint formed by the joining method according to the present embodiment has higher strength than the joint formed by the conventional joining method. 
     It should be noted that large part of the base material of the second workpiece  102  directly contacts the shoulder  28 , and the base material of the second workpiece  102  has relatively high fluidity. For this reason, by quickly moving the shoulder  28  backward in the backfilling process, the base material component of the second workpiece  102  can be caused to preferentially flow into the space that is formed when the shoulder  28  moves backward (i.e., into the plunging hole), and thereby the remaining layer  106  can be covered with the base material of the second workpiece  102 . 
     In the present embodiment, the plunging depth of the shoulder  28  is great, and a large amount of heat is generated by the friction. This facilitates the flow of the materials, and allows the materials to flow in a more spreading manner. Therefore, in the backfilling process, if the shoulder pressing pressure is set to be substantially the same as the shoulder pressing pressure in the conventional joining method, the portion outward of the portion directly pressed by the shoulder  28  rises against the urging force of the clamp  22 . As a result, a large step is formed between the joint and the portion around the joint. 
     In view of the above, the shoulder pressing pressure in the backfilling process in the present embodiment is set to be lower than in the conventional joining method. For example, in the present embodiment, the surface pressure of the shoulder  28  (shoulder pressing pressure) against the stirred portion  105  in the backfilling process is set to be lower than the surface pressure of the shoulder  28  (shoulder pressing pressure) against the stirred portion  105  in the plunging process. More specifically, the shoulder  28  is pressed against the stirred portion  105  with a surface pressure of 90 to 175 MPa. This makes it possible to reduce or eliminate the step that may be formed between the joint and the portion around the joint in the backfilling process. 
     It should be noted that the portion that may rise in the backfilling process is the portion positioned radially outward of the stirred portion  105 , i.e., the portion corresponding to the clamp  22 . Therefore, it is conceivable to suppress the rising by setting the pressing pressure of the clamp  22  high. In this case, however, a mechanism for driving the clamp  22  needs to be installed separately, which may cause the joining apparatus  100  to increase in size and become complex. Therefore, the above-described method, in which the shoulder pressing pressure in the backfilling process is set to be low, is effective also in terms of being able to avoid the problem of causing the joining apparatus  100  to increase in size and become complex. 
     The joining method according to the present embodiment is as described above. Although the above description of the joining method describes only the plunging process and the backfilling process, the joining method may include other processes in addition to the plunging process and the backfilling process. For example, the joining method may include a process that is performed before the plunging process. In the process, the shoulder  28  and the pin  29  that are rotating may be brought into contact with the surface of the first workpiece  101  to heat the surface. 
     REFERENCE SIGNS LIST 
       23  rotary tool 
       28  shoulder 
       29  pin 
       40  controller 
       100  friction stir spot welding apparatus 
       101  first workpiece 
       102  second workpiece 
       104  protective layer 
       105  stirred portion