METHOD FOR PRODUCEING A WELDED LINK

Abstract

Method for producing a welded member (100) in which a workpiece (1) made of a high-strength material and a weld object (2) are welded together, wherein the workpiece (1) is designed as a plate-shaped component having a first and a second side, wherein the first and the second side are opposite each other and spaced apart in a thickness direction of the workpiece (1), the method comprising: welding the workpiece (1) and the weld object (2) together, such that welded sections (25A) are formed between the first side of the workpiece (1) and the weld object (2), by performing projection welding;and applying a cancellation stress (F1) to the welded member (100) after projection welding, such that a stress (F2, F4) generated in the welded sections (25A) after projection welding and acting in such a direction as to pull the welded sections (25A) closer together is cancelled, wherein the cancellation stress (F1) is generated by applying a load (W1) to a load target location (1, 2) which is a target section of the welded member (100) to which the load is applied, wherein the load target location is a nut (2) which is the weld object, and the cancellation stress (F1) is achieved by using a member arranged along a central axis of a threaded hole (21) of the nut (2) to apply a load (W1) to the nut (2) in a direction in which the nut (2) moves away from the workpiece (1) is moved away, is produced.;

Description

BACKGROUND The present disclosure relates to a method for manufacturing a welded link. For example, JP 2013-78784A describes a method for welding a weld nut around a hole in a workpiece using projection welding. In the welded section between the workpiece and the weld nut formed by projection welding, stress is generated due to post-weld cooling. This residual stress can lead to delayed fracture in the welded section. The unexamined Japanese patent application Publication No. JP 2012 - 179 646 A discloses a structural component for a motor vehicle and a method for its manufacture with excellent properties with regard to delayed fracture of a welded section between a workpiece and a weld nut, which is welded by projection welding. The structural component for a motor vehicle uses a weld nut with projections on a weld surface, which is a surface welded to the workpiece. The projections are melted by an electric current applied by the projection welding process to form the welded section between the workpiece and the weld nut. To reduce residual stress that causes delayed fracture of the welded section during projection welding, a recess is provided around each projection on the weld surface in the weld nut.The total volume of each depression is in the range of 0.7 to 1.3 times the total volume of the respective projections. SUMMARY The method for reducing delayed fracture of a section welded by projection welding, as described above, requires the use of a weld nut with a novel shape, featuring depressions in addition to projections, for projection welding. However, it is desirable to reduce delayed fracture of the welded section by projection welding even when using a weld nut that does not have the special shape described above. In one aspect of the present disclosure, it is desirable to provide a novel method for producing a welded link in which delayed fracture of the welded sections is reduced by projection welding. One aspect of the present disclosure is a method for producing a welded member in which a workpiece made of a high-strength material and a weld object are welded together. The method comprises welding the workpiece and the weld object together, such that welded sections are formed between the workpiece and the weld object, by performing projection welding. The method further comprises applying a lifting stress to the welded member after the projection welding, such that a stress which is generated in the welded sections after the projection welding and which acts in such a direction as to draw the welded sections closer together is lifted. In the configuration described above, a cancellation stress is generated in the welded section after projection welding. This cancellation stress is applied in such a way as to cancel any stress induced in the welded sections formed by the projection welding process. Thus, the cancellation stress reduces the residual stress in the welded sections that could cause delayed failure. This, in turn, reduces delayed failure of the welded sections caused by projection welding. Furthermore, reducing delayed failure of welded sections during projection welding requires consideration of complex factors such as heat input. However, when applying a cancellation stress in a post-projection welding process, these complex factors, such as heat input, are not necessary.Delayed fracture of the welded sections can easily be reduced. In one aspect of the present disclosure, the lifting stress can be generated by applying a load to a load target location, which is a target section of the welded member to which the load is applied. The above configuration allows the generation of a lifting stress after projection welding in a relatively simple manner by directly applying the load to the load target location of the welded member. In one aspect of the present disclosure, the load target location can be a nut, which is the weld object. The lifting stress can be generated by using a link arranged along a central axis of a threaded hole in the nut to apply a load to the nut in a direction in which the nut moves away from the workpiece. The configuration itself causes plastic deformation around the welded sections in the welded link by pushing or pulling the nut in the direction in which the nut moves away from the workpiece. In particular, a stress that causes downward warping and bending acts around the welded sections in the workpiece. This can generate a lifting stress in the welded link.Furthermore, since the link arranged along the central axis is used, a lifting stress is readily generated which can uniformly reduce delayed fracture of the welded sections. In one aspect of the present disclosure, the load target location can be the workpiece. The welded object can be a nut. The lifting stress can be generated by compressing surrounding parts of the nut in the workpiece in a thickness direction of the workpiece. The configuration itself causes plastic deformation around the welded sections in the welded member by compressing the workpiece. In particular, since a stress that causes contraction around the welded sections in the workpiece is present, a stress that causes downward warping and bending also acts around the welded sections in the workpiece. This can generate a lifting stress in the welded member. In one aspect of the present disclosure, the lifting stress can be generated by pressing the nut in a direction in which the nut moves away from the workpiece. With this configuration, the lifting stress can be generated simply by pressing the nut without further ado. In one aspect of the present disclosure, the lifting stress can be generated by pulling the nut in a direction that moves the nut away from the workpiece. With this configuration, the lifting stress can be generated simply by pulling the nut without any further action. BRIEF DESCRIPTION OF THE DRAWINGS An exemplary embodiment of the present disclosure is described below with reference to the accompanying drawings; in the drawings: Fig. 1 shows a schematic diagram of a welded link in which a workpiece and a weld nut are welded together; Fig. 2 shows a bottom view of the weld nut; Fig. 3A shows a schematic diagram of the weld nut being welded to the workpiece by projection welding; Fig. 3B shows a schematic diagram of the welded link immediately after welding; Fig. 3C shows a schematic diagram of the welded link inverted in an upward-downward direction; Fig. 3D shows a schematic diagram of the inverted welded link placed on a support stand; Fig. 3E shows a schematic diagram of the welded link arranged downwards of a pin for applying a load; and Fig.Fig. 3F a schematic diagram showing the pin that applies the load to the weld nut to generate a lifting stress in the welded member; Fig. 4 a schematic cross-sectional view showing the pin that applies the load to the weld nut; Fig. 5 a schematic diagram showing the lifting stress generated in the welded member; Fig. 6 a schematic diagram showing delayed fracture of the welded sections caused by stress generated due to the cooling of the welded sections after projection welding; Fig. 7 a schematic diagram showing the generation of lifting stress by pressing on surrounding parts of the weld groove in the workpiece to compress the workpiece in a thickness direction; Fig.Figure 8 shows a schematic diagram illustrating the generation of the lifting stress by inserting a screw into a threaded hole of the weld nut and pulling the screw in a direction in which the weld nut moves away from the workpiece; and Figure 9 shows a schematic diagram illustrating the generation of the lifting stress by applying shot peening to the surrounding parts of the weld nut in the workpiece to compress the workpiece in the thickness direction. DETAILED DESCRIPTION OF EXAMPLE EXECUTIONS [1. Configuration] < Welded link > A welded link 100, shown in Fig. 1, is a link in which a workpiece 1 and a weld nut 2, which is a weld object, are welded together by projection welding. The workpiece 1 is a plate-shaped element to which the weld nut 2 is welded. The workpiece 1 is, for example, a thin plate for forming a vehicle body. The workpiece 1 can be made of a high-strength steel, i.e., a high-strength material with a tensile strength of 340 MPa or more. Among such high-strength materials, the workpiece is preferably made of an ultra-high-strength steel, i.e., an ultra-high-strength material with a tensile strength of 980 MPa or more. The workpiece 1 has at least one hole 11 that passes through the workpiece 1 in one thickness direction. The weld nut 2 is a nut made, for example, of a metal such as iron and used for projection welding. As shown in Fig. 1 and Fig. 2, the weld nut 2 has a threaded hole 21, a first end face 22, a second end face 23, a flange 24, three projections 25, and a conical section 26. The threaded hole 21 is designed to pass through the weld nut 2. Although this is not shown in Fig. 1, an internal thread is provided on the inner circumferential surface of the threaded hole 21, which can be screwed in place with a screw. The first end face 22 and the second end face 23 are designed to surround openings positioned at opposite ends of the threaded hole 21. The first end face 22 is flat. The flange 24 is designed to surround an opening 21A of the threaded hole 21 of the first end face 22. The projections 25 are sections to be melted by projection welding. The projections 25 are provided at the first end face 22 and protrude from the first end face 22. In the present embodiment according to the illustration in Fig. 2, three projections 25 are provided such that they surround the opening 21A at approximately equal intervals. The projections 25 each have a triangular shape in plan view. In the following description, welded sections between the workpiece 1 and the weld nut 2, which are formed by melting the projections 25 by projection welding and crushing them by applying pressure, are referred to as welded sections 25A. The conical section 26 is provided on the side of the opening 21A of the threaded hole 21. The conical section 26 is an inclined surface that connects the inner circumferential surface of the threaded hole 21 and the first end face 22 and widens towards the opening 21A of the threaded hole 21. <Method for producing a welded link> With reference to Figs. 3A to 3F, a method for manufacturing a welded link is explained in more detail. First, as shown in Fig. 3A, the workpiece 1 is positioned on a lower electrode 30A, which is contained in a welding machine used for projection welding. Then, the hole 11 of the workpiece 1 and the threaded hole 21 of the weld nut 2 are aligned, and the weld nut 2 is positioned on the workpiece 1 such that the projections 25 contact the workpiece 1. At this point, a gap is formed between the first end face 22 of the weld nut 2 and a surface of the workpiece 1 that does not contact the lower electrode 30A. A positioning pin, not shown, provided in the welding machine and passing through the hole 11 and the threaded hole 21, can be used to position the weld nut 2 on the workpiece 1. Next, with the weld nut 2 positioned on the workpiece 1, the workpiece 1 and the weld nut 2 are clamped by the lower electrode 30A and an upper electrode 30B to apply pressure, as shown in Fig. 3A. Projection welding is then performed, with electric current being applied to the projections 25, which are the joining sections, via the lower electrode 30A and the upper electrode 30B, and pressure being applied to the projections 25 to perform the joining while the projections 25 are heated by resistance heating. As the projection welding is performed, the three projections 25 are melted, and the three welded sections 25A are formed between the workpiece 1 and the weld nut 2 to obtain the welded link 100 immediately after welding, as shown in Fig. 3B, where the workpiece 1 and the weld nut 2 are welded together.In the welded link 100, the weld nut 2 is positioned over the workpiece 1 immediately after welding. Next, the welded link 100 is removed from the welding machine immediately after welding. Then, the welded link 100 is inverted so that the weld nut 2 is positioned below the workpiece 1, as shown in Fig. 3C. The inverted welded link 100 is then placed on the support stand 200, as shown in Fig. 3D. The support stand 200 is configured to contact parts located near the weld nut 2 in the workpiece 1 in order to support the welded link 100. The support stand 200 can have a cylindrical shape so that it surrounds the weld nut 2. Next, as shown in Fig. 3E, a loading device is adjusted to apply a load such that the welded link 100 is positioned under a pin 300 of the loading device. The pin 300 may have a floating bearing mechanism that allows the pin 300 to be suspended in a horizontal direction. Next, the pin 300, as shown in Fig. 3F, is used to apply a load to the welded member 100. At this point, the pin 300 is positioned along a central axis A of the threaded hole 21 of the weld nut 2. For example, a servo motor, such as an electric motor or a hydraulic motor, is used to apply the load through the pin 300. As shown in Fig. 4, a conical pin section 301 is provided at a front end of the pin 300. The conical pin section 301 is an inclined surface that widens towards the front end. The pin 300 is inserted into the hole 11 of the workpiece 1. When the conical pin section 301 of the pin 300 contacts the conical section 26 of the weld nut 2, the weld nut 2 is pressed downwards by the pin 300.By pressing the weld nut 2 with the pin 300, a load W1 is applied to the weld nut 2 along the central axis A in a direction in which the weld nut 2 moves away from the workpiece 1. The load W1 is a force that causes plastic deformation of the flange 24 of the weld nut 2 or around the welded sections 25A of the weld nut 2, which are slightly deformed. As the weld nut 2 is pressed downwards and the load W1 is applied to the weld nut 2, a stress acts that causes downward distortion and bending around the welded sections 25A in the workpiece 1. This generates a lifting stress F1 in the welded member 100, as shown in Figures 4 and 5. The lifting stress F1 is a force that acts in such a direction that a stress F3, described later, is lifted, as shown in Figure 6. When the three welded sections 25A, formed on the welded member 100 after projection welding, are cooled, a stress F2 is generated as shown in Fig. 6. The stress F2 acts in such a direction that the three welded sections 25A are pulled close together. In other words, the stress F2 is generated by the cooling of the three welded sections 25A, formed on the welded member 100 after projection welding, and acts in a direction towards the central axis A with respect to the welded sections 25A. The stress F2 acts in such a way that it causes deformation around the welded sections 25A in the welded member 100. In particular, the stress F3, which causes upward distortion, acts around the welded sections 25A in the workpiece 1 due to the stress F2.However, since workpiece 1 is made of a high-strength material and does not follow a direction of stress F3, deformation of workpiece 1 is less likely. Thus, delayed fracture, such as a crack C as shown in Fig. 6 around the welded sections 25A in workpiece 1, can occur. In the present embodiment, however, the pin 300 is used to apply the load W1 to the weld nut 2 in order to generate the lifting stress F1 and to release the stress F3, as described above. This makes it possible to release the stress F2, which acts in such a way as to generate the stress F3. [2. Effect] The present embodiment, which has been described in more detail above, has the following effects. (2a) In the present embodiment, the cancellation stress F1 is generated in the welded member 100 after projection welding. The cancellation stress F1 acts in such a way as to cancel the stress F3, which is generated by the stress F2 produced by the cooling of the three welded sections 25A formed by projection welding. Thus, the stress F2, which remains in the welded sections 25A and causes delayed failure of the welded sections 25A, can be reduced by the cancellation stress F1. This reduces delayed failure of the welded sections 25A caused by projection welding. Furthermore, reducing delayed fracture of the welded sections 25A during projection welding requires consideration of complex conditions such as heat input. In the present embodiment, however, it is not necessary to consider such complex conditions, since the breaking stress F1 is applied in a process after projection welding. Delayed fracture of the welded sections 25A can therefore be reduced without difficulty. (2b) In the present embodiment, the weld nut 2 is pressed downwards by the pin 300 to apply the load W1 to the weld nut 2. This results in plastic deformation around the welded sections 25A of the welded member 100. In particular, a stress acts around the welded sections 25A in the workpiece 1, causing downward distortion and bending. This generates the breaking stress F1 in the welded member 100. The breaking stress F1 can be generated relatively easily by pressing the weld nut 2 downwards in the welded member 100. Furthermore, since the pin 300 is arranged along the central axis A of the threaded hole 21, the breaking stress F1, which can uniformly reduce the delayed fracture of the three welded sections 25A, is easily generated. In the present embodiment, the weld nut 2 corresponds to an example of a welded object and a nut. The weld nut 2 and the workpiece 1 correspond to an example of a load target location. The pin 300 corresponds to an example of a member arranged along the central axis A of the threaded hole 21. [3. Other embodiments] Although the embodiment of the present disclosure is described above, the present disclosure is not limited to the embodiment described above and can be implemented in practice in various ways. (3a) In the embodiment described above, a pin 300 is used to apply a load along the central axis A of the threaded hole 21 of the weld nut 2 in order to generate the lifting stress F1. However, the method of generating the lifting stress F1 in the welded member 100 is not limited to this. For example, as shown in Fig. 7, the receiving stand 200 and a pressure stand 400 can be used to press against surrounding parts of the weld nut 2 in the workpiece 1 to compress the workpiece 1 in the thickness direction, thereby generating the lifting stress F1 in the welded member 100. The pressure stand 400 is arranged on the welded member 100 and is configured to contact a section of the workpiece 1 opposite a section of the receiving stand 200 from a side opposite the receiving stand 200. The pressure stand 400 can have a cylindrical shape with an upper surface for surrounding the weld nut 2 from above.In particular, an area outside the area where the weld nut 2 is positioned in the workpiece 1, and near the weld nut 2 in the workpiece 1, is clamped by the holding stand 200 and the pressure stand 400, thereby applying the load W1 to compress the workpiece 1 in the thickness direction. A stress F4, which causes the workpiece 1 to contract due to the load W1, acts around the welded sections 25A in the workpiece 1. The stress F4 acts in such a way as to cause plastic deformation around the welded sections 25A in the welded link 100. In particular, a stress that causes downward warping and bending acts due to the stress F4 around the welded sections 25A in the workpiece 1. When the workpiece 1 is warped and bent, the lifting stress F1 is generated in the welded link 100.The shape of the mounting stand is not limited to a cylindrical form. For example, the mounting stand can be shaped so that the entire base surface of the workpiece 1 supports the welded link 100. The shape of the pressure stand is also not limited to a cylindrical form with a top surface. Furthermore, although not shown, in the welded link 100, for example, the workpiece 1 can be compressed along a direction orthogonal to the thickness direction of the workpiece 1 to generate the lifting stress F1. In other words, in the welded link 100, the workpiece 1 can be compressed in a direction extending towards the center of the weld nut 2, thereby generating the lifting stress F1. In this case, a stress also acts around the welded sections 25A in the workpiece 1, causing downward distortion and bending, as described above. This generates the lifting stress F1 in the welded link 100. Furthermore, for example, in the welded link 100 as shown in Fig. 8, a screw 500 fitted into the weld nut 2 can be used to pull the weld nut 2 in a direction away from the workpiece 1, thereby generating the lifting stress F1. Specifically, the screw 500 is inserted into the threaded hole 21 of the weld nut 2, and after fitting the screw 500 into the threaded hole 21, the screw 500 is pulled upwards. This applies the load W1 along the central axis A to the weld nut 2 in a direction in which the weld nut 2 moves away from the workpiece 1. As the weld nut 2 is pulled upwards and the load W1 is applied to the weld nut 2, a stress acts around the welded sections 25A in the workpiece 1, causing downward distortion and bending. This generates the lifting stress F1 in the welded link 100. Furthermore, for example, in the case of the welded link 100, shot peening can be applied to the surrounding parts of the weld nut 2 in the workpiece 1 to compress the workpiece 1 in the thickness direction, thereby generating the breaking stress F1, as shown in Fig. 9. Shot peening is a process in which countless small spheres S made of steel or the like are caused to collide at high speed to induce compressive residual stress due to plastic deformation. (3b) In the embodiment described above, the weld nut 2 has three projections 25, but the number of projections 25 is not limited to this. For example, there may be two, four, or more projections. (3c) In the embodiment described above, the projections 25 have a triangular shape in plan view, but the shape of the projections is not limited to this. For example, the projections may have a spherical shape that establishes point contact with the workpiece 1. The projections may have a shape that establishes linear contact with the workpiece 1. (3d) In the embodiment described above, the weld nut 2 has the flange 24, but the weld nut need not have a flange. That is, the lifting stress described above can be generated even if a differently shaped weld nut is used to carry out projection welding. (3e) In the embodiment described above, the welded link 100 is a link in which the workpiece 1 and the weld nut 2 are welded together, but the welding object to be welded to the workpiece 1 is not limited to the weld nut 2. (3f) In the configuration of the embodiment described above and the embodiment described above in (3a), a load is applied directly to the weld nut 2 or the workpiece 1 to generate the lifting stress F1. However, the method of generating the lifting stress is not limited to this. For example, the lifting stress can be generated as a result of another machining operation performed on the welded member 100 after welding, whereby a load is applied indirectly. (3g) Functions of a component in the embodiments described above can be achieved by two or more components, and a function of a component can be achieved by two or more components. Furthermore, functions of two or more components can be achieved by one component, and a function achieved by two or more components can be achieved by one component. Additionally, some of the configurations of the embodiments described above may be omitted. At least some of the configurations of the embodiments described above may be added to or replaced by other configurations of the embodiments described above. Any embodiment that is within the technical principles identified by the formulation of the claims is an embodiment of the present disclosure.

Claims

Method for producing a welded member (100) in which a workpiece (1) made of a high-strength material and a weld object (2) are welded together, wherein the workpiece (1) is designed as a plate-shaped component having a first and a second side, wherein the first and the second side are opposite each other and spaced apart in a thickness direction of the workpiece (1), the method comprising: welding the workpiece (1) and the weld object (2) together, such that welded sections (25A) are formed between the first side of the workpiece (1) and the weld object (2), by performing projection welding;and applying a cancellation stress (F1) to the welded member (100) after projection welding, such that a stress (F2, F4) generated in the welded sections (25A) after projection welding and acting in such a direction as to pull the welded sections (25A) closer together is cancelled, wherein the cancellation stress (F1) is generated by applying a load (W1) to a load target location (1, 2) which is a target section of the welded member (100) to which the load is applied, wherein the load target location is a nut (2) which is the workpiece, and the cancellation stress (F1) is achieved by using a member arranged along a central axis of a threaded hole (21) of the nut (2) to apply a load (W1) to the nut (2) in a direction in which the nut (2) moves away from the workpiece (1) is moved away, is produced.; Method for producing a welded member (100) in which a workpiece (1) made of a high-strength material and a weld object (2) are welded together, wherein the workpiece (1) is designed as a plate-shaped component having a first and a second side, wherein the first and the second side are opposite each other and spaced apart in a thickness direction of the workpiece (1), the method comprising: welding the workpiece (1) and the weld object (2) together, such that welded sections (25A) are formed between the first side of the workpiece (1) and the weld object (2), by performing projection welding;and applying a cancellation stress (F1) to the welded member (100) after projection welding, such that a stress (F2, F4) generated in the welded sections (25A) after projection welding and acting in such a direction as to pull the welded sections (25A) closer together is cancelled, wherein the cancellation stress (F1) is generated by applying a load (W1) to a load target location (1, 2) which is a target section of the welded member (100) to which the load is applied, wherein the load target location is the workpiece (1) and the weld object is a nut (2), and the cancellation stress (F1) is generated by compressing parts of the workpiece (1) surrounding the nut (2) in a thickness direction of the workpiece (1) between the first and second sides. Method for producing a welded link (100) according to claim 1, wherein the lifting stress (F1) is generated by pressing the nut (2) in a direction in which the nut (2) moves away from the workpiece (1). Method for producing a welded link (100) according to claim 1, wherein the lifting stress (F1) is generated by pulling the nut (2) in a direction in which the nut (2) moves away from the workpiece (1).

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