Patent Application
20210053146
The present invention relates to an apparatus for friction stir spot welding and a method of friction stir spot welding providing a keyhole free welding.
Friction stir welding FSW is a solid-state joining process employing a non-consumable tool to combine two workpieces having adjacent contacting surfaces by heating below melting point of the workpiece materials. The heat is generated by friction between a tapered pin of a rotating tool of welding apparatus and the workpieces. The generated heat leads to plasticization of the work piece materials in a region near the pin.
Friction stir spot welding FSSW is a variant of the FSW where welding is practiced on a point of the workpiece surfaces. In FSSW, two workpieces one on top of the other are disposed on an anvil. A spot weld is created by pressing a rotating tool with high force onto the top surface of overlapping workpieces abutting on the anvil. The frictional heat due to rotation and the high pressure plasticize the workpiece material in the spot, so that a tapered pin of the rotating tool penetrates into a spot (region to be welded) between the two workpieces. The tapered pin of the rotating tool is plunged into the workpieces until a shoulder surrounding the rotating tool is in contact with the surface of the top workpiece. Rotation of the rotating tool also mixes the plasticized materials of workpieces. The shoulder restrains the plasticized materials and the rotating tool applies a high forging pressure, which bonds the workpieces without melting the workpiece material. Finally, the rotating tool is pulled out of the workpieces to emerge spot weld having a weld nugget in form of a valley.
After pulling out the rotary tool, a keyhole (i.e., an exit hole arising from the penetrated volume of the pin when the rotating tool is retracted) is left in the welded workpieces. The keyhole may be filled with an appropriate material with a post processing or using a consumable pin. The post processing and/or using a using consumable pins increase the final cost of the production. Moreover, the filler may break undesirably from the keyhole due to the stress difference between the filler and the workpieces. Another problem of FSSW with keyhole in the state of the art is that since the FSSW provides a relatively less flexible weld, the keyholes constitute a weak point for the weld. Yet another problem of FSSW in the state of the art is that since the pin of the rotating tool must penetrate into workpieces in a certain minimum depth, workpieces having a total wall thickness lower than the certain minimum depth cannot be welded by FSSW. Yet another problem of FSSW in the state of the art is that if pin of the rotating tool is not long enough, then the rotating tool may not penetrate enough into the workpiece close to the anvil. Thus, the said workpiece may not be plasticized and forged by the rotating tool. This lack of penetration can cause defects in the weld which is a potential source of fatigue cracks.
Consequently, there is a need in the state of the art for an apparatus for and a method of FSSW providing a keyhole free welding.
An exemplary embodiment of the present invention is illustrated by way of example in the accompanying drawings to be more easily understood and uses thereof will be clearer when considered in view of the detailed description, in which like reference numbers indicate the same or similar elements, and the following figures in which:
The elements illustrated in the figures are numbered as follows:
Embodiments of the present invention relates to an apparatus (1) for and a method of friction stir spot welding.
The apparatus (1) for friction stir spot welding comprises;
The press head (2) and the rotary tool (6) are configured for coupling with at least one pressing means.
The press head (2) is for pressing a sandwich structure (S) comprising the intermediate piece (5) positioned between the at least one first workpiece (3) and at least one second workpiece (4) against an anvil (A) such that the intermediate piece (5) is trapped between the second workpiece (4) and an embossing (E) formed on the first workpiece (3) when the press head (2) applies pressure.
The rotary tool (6) is for pressing the embossing (E) via the substantially flat end (6.1) while rotating so as to plasticize the intermediate piece (5), at least one part of each of the first workpiece (3) and the second workpiece (4).
Working principle of one embodiment of the apparatus (1) is described in detail below.
A sandwich structure (S) comprising the intermediate piece (5) disposed/positioned between the at least one first workpiece (3) and at least one second workpiece (4) is placed on an anvil (A). The press head (2) presses the sandwich structure (S) against the anvil (A). The end of the press head (2) having the cavity (2.1) pushes surface of the first workpiece (3). Since the intermediate piece (5) and a part/region of the first workpiece (3) adjacent to the intermediate piece (5) (in other words the first workpiece (3) partly covering the intermediate piece (5)) are received into the cavity (2.1), pressure of the press head (2) is not applied to them. Other regions of the first workpiece (3) around vicinity of the intermediate piece (5) are pushed through the second workpiece (4) by the press head (2) through edge of the cavity (2.1). Thus, the intermediate piece (5) and region of the first workpiece (3) adjacent to the intermediate piece (5) (partly covering the intermediate piece (5)) form an embossing (E) on the first workpiece (3).
Then, while spinning, the rotary tool (6) presses the embossing (E) by its substantially flat end (6.1). The substantially flat end (6.1) of the rotary tool (6) pushes the embossing (E) against the second workpiece (4) and the anvil (A) such that the intermediate piece (5), at least one part of the first workpiece (3) and at least one part of the second workpiece (4) are plasticized. Furthermore, due to the rotational motion of the substantially flat end (6.1), the plasticized materials are mixed. Since the pressing end of the rotary tool (6) facing the first workpiece (3) is substantially flat, the pressing flat end (substantially flat end (6.1)) does not penetrate into the first workpiece (3)/the embossing (E). Instead, the substantially flat end (6.1) also applies a forging pressure to plasticize materials and forms a flat weld nugget (W) without a keyhole.
In one embodiment of the apparatus (1) (see
In this embodiment of the apparatus (1), press head (2) in the form of shoulder (7) guarantees to apply the forging pressure to plasticize materials (weld nugget (W)) by restraining the plasticized materials to be flat. The restraining is to keep the plasticized material between the second workpiece (4) and substantially flat end (6.1) of the rotary tool (6). Moreover, the pressure of the press head (2) provides a clamping effect to prevent the relative movement of the first workpiece (3) and second workpiece (4) during the pressing the embossing (E)/plasticization of the material. In this embodiment, the rotary tool (6) must be movable independently from the shoulder (7).
In another embodiment of the apparatus (1) (see
One important difference between the press head (2) in the form of shoulder (7) and the forging shoulder (8) is shape and/or size difference between the cavity (2.1) and recess (8.1). In some applications, appropriate shape/size of the cavity (2.1) for pressing a sandwich structure (S) may be substantially different than appropriate shape/size of the recess (8.1) for forging the plasticized materials to adjust the forging pressure for enhancing the weld. Thus, a press head (2) is used for pressing the sandwich structure (S) and forming the embossing (E). Then, a forging shoulder (8) may be used for forging the plasticized materials. In some applications, press head (2) in the form of shoulder (7) may be used both for pressing the sandwich structure (S) and forming the embossing (E), and for forging the plasticized materials.
The intermediate piece (5) may comprise a same material as or different than the material of the first workpiece (3) or second workpiece (4). The intermediate piece (5) and/or the first workpiece (3) and/or second workpiece (4) may be, for example, but not limited to, metals, polymers, metal matrix composites, polymer matrix composites.
A method of friction stir spot welding comprises following steps;
In one embodiment of the method, in above step b) the sandwich structure (S) is pressed by a press head (2) having a body wherein the body comprising a cavity (2.1) for receiving at least one part of the intermediate piece (5) and the first workpiece (3) such that the intermediate piece (5) is trapped between the second workpiece (4) and the embossing (E) formed on the first workpiece (3).
In another embodiment of the method, the method comprises following step;
In another embodiment of the method, the press head (2) is a shoulder (7) wherein a space (2.2) for receiving the rotary tool (6) is provided (inside the body) through at least one part of the body wherein the cavity (2.1) forges the plasticized first workpiece (3), intermediate piece (5) and second workpiece (4) by guiding during the step c).
