One-side spot welding workpiece structure evaluation method

Abstract

To provide a method that can evaluate whether a work-piece to be one-sided spot-welded has a required rigidity or not.SOLUTION: By simulation, a size of a gap (separation gap δ) formed between an upper plate 101 and a lower plate 102 is acquired when pressing an overlapped part 103 between the upper plate 101 and the lower plate 102 with a prescribed pressurizing force from only one side in a thickness direction, and on the basis of the size of the gap, a work-piece structure is evaluated.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for evaluating, by simulation, the structure of a workpiece to be subjected to one-side spot welding. [Background technology]

[0002] Direct spot welding, in which a pair of electrodes sandwiches the overlapping portions of multiple metal sheets and current is applied, is a commonly used spot welding method. However, if the electrodes can only access the overlapping portions of the metal sheets from one side in the thickness direction (for example, if the other side in the thickness direction of the overlapping portions is covered by another metal sheet), the overlapping portions cannot be joined by direct spot welding. In such cases, "one-sided spot welding" is sometimes used, in which current is applied while the electrode is pressed against the overlapping portions of the metal sheets from only one side in the thickness direction (see, for example, Patent Document 1).

[0003] In indirect spot welding, which is a type of one-sided spot welding, as shown in FIG. 5 , welding electrode 130 is used to press against planned joining portion P, which is located at overlapping portion 103 of upper sheet 101 and lower sheet 102, from above, while earth electrode 140 is in contact with another portion of the workpiece. Current is then passed between electrodes 130 and 140, welding planned joining portion P. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2020-116630 Summary of the Invention [Problem to be solved by the invention]

[0005] In one-side spot welding, the overlapping portion 103 is pressed only from above by the welding electrode 130 without being supported from below, which makes the overlapping portion 103 prone to deformation. The pressure applied to the to-be-joined portion P depends on the rigidity of the workpieces. However, currently, no established criteria exist for determining the level of rigidity required for workpieces to be joined by one-side spot welding, making it difficult to define the structure of the workpiece that will result in a good weld. This often results in excessively thick lower plate 102 or excessive beads, which increases costs and weight and reduces the advantages of one-side spot welding.

[0006] Therefore, an object of the present invention is to provide a method for evaluating, by simulation, whether a workpiece to be subjected to one-side spot welding has the required rigidity. [Means for solving the problem]

[0007] As shown in Fig. 5, when one-side spot welding is performed on an overlapping portion 103 between an end 101a of an upper sheet 101 and an end 102a of a lower sheet 102, the upper sheet 101 is pressed downward by a welding electrode 130, and the end 101a of the upper sheet 101 further presses down the lower sheet 102, as shown in an exaggerated manner in Fig. 6. Therefore, even if there is no gap between the upper sheet 101 and the lower sheet 102 before pressing, as shown in Fig. 5, pressing with the welding electrode 130 causes the upper sheet 101 and the lower sheet 102 to separate, as shown in Fig. 6. If the rigidity of the overlapping portion 103 between the upper sheet 101 and the lower sheet 102 is insufficient, the gap between the upper sheet 101 and the lower sheet 102 becomes excessive when pressed with the welding electrode 130, resulting in a poor weld. Thus, in one-side spot welding, not only the gap between the sheets before pressing with the welding electrode but also the gap between the sheets after pressing with the welding electrode have a significant effect on the weld quality. The inventors focused on this point and thought that it might be possible to evaluate the structure of the workpiece based on the size of the gap between the upper sheet 101 and the lower sheet 102 formed when pressing with the welding electrode (hereinafter referred to as the "separation gap δ").

[0008] Then, one-side spot welding was performed on various samples, and the separation gap δ and the size of the joint (specifically, the diameter of the fractured part when destructive testing was performed using the chisel test specified in JIS Z 3144:2013) were measured for each sample when the plate assembly was pressed with the electrode (the state immediately before the start of current flow). As a result, as shown in Figure 7, it was revealed that there is a correlation between the separation gap δ and the size of the joint (peel diameter).

[0009] Therefore, the present invention provides a method for evaluating, by simulation, a structure of a workpiece including an upper plate and a lower plate when performing one-side spot welding in which an electrode is pressed with a predetermined pressure from only one side in the thickness direction against an overlapping portion between the upper plate and the lower plate, the method comprising: a step of obtaining, by simulation, the size of a gap formed between the upper plate and the lower plate when the overlapping portion between the upper plate and the lower plate is pressed with a predetermined pressure from only one side in the thickness direction; and evaluating the structure of the workpiece based on the size of the gap.

[0010] As described above, in the present invention, when performing one-side spot welding, the structure of the workpiece is evaluated based on the sheet gap (bead gap) when pressure is applied, obtained by simulation. That is, if the bead gap is within a predetermined range, it is determined that the overlapping portion of the workpiece has sufficient rigidity. If the bead gap is outside the predetermined range, it is determined that the overlapping portion of the workpiece does not have sufficient rigidity. In this way, it is possible to determine whether the workpiece to be subjected to one-side spot welding has sufficient rigidity based on the bead gap, which facilitates the design of the minimum workpiece structure (sheet thickness, bead shape, etc.) necessary to obtain a good weld.

[0011] To obtain the separation gap through simulation as described above, it is possible to create a model similar to the actual workpiece, but in this case, the contact state between the upper and lower plates must be taken into account, which makes the simulation complicated and takes a long time to calculate.

[0012] Therefore, the present invention provides a method for evaluating, by simulation, a structure of a workpiece including an upper plate and a lower plate when performing one-side spot welding in which an electrode is pressed with a predetermined pressure from only one side in the thickness direction against an overlapping portion between the upper plate and the lower plate, the method comprising: a step of creating a lower plate model having higher rigidity than the lower plate by simulation, and acquiring a deformation amount of the lower plate model when the lower plate model is pressed from one side in a thickness direction with the predetermined pressure force; and evaluating the structure of the workpiece based on the deformation amount of the lower plate model.

[0013] As described above, the inventors omitted the upper plate model in the simulation of one-sided spot welding and evaluated the workpiece structure based on the deformation amount when the lower plate model was directly pressed. This simplifies the simulation because it is not necessary to consider the contact state between the upper and lower plates. However, when performing one-sided spot welding on an actual workpiece, the rigidity of the upper plate also affects the deformation state of the workpiece. Therefore, simply omitting the upper plate model in the simulation results in a deviation from the actual deformation state of the workpiece. Therefore, by creating a lower plate model with higher rigidity than the lower plate as described above, the deformation amount of the lower plate model is calculated taking into account the rigidity of the upper plate, thereby reducing the deviation from the actual deformation state (separation gap amount). Using the deformation amount of the lower plate model calculated in this way makes it possible to evaluate the workpiece structure through a simple simulation.

[0014] For example, by setting the thickness of the lower plate model to the sum of the thickness of the lower plate and the thickness of the upper plate, it is possible to create a lower plate model that takes into account the rigidity of the upper plate. [Effects of the Invention]

[0015] As described above, according to the present invention, it is possible to evaluate, by simulation, whether a workpiece to be subjected to one-side spot welding has the required rigidity, thereby facilitating the design of the workpiece. [Brief explanation of the drawings]

[0016] [Figure 1] 2A and 2B are cross-sectional views of an upper plate model and a lower plate model created by simulation in the workpiece structure evaluation method according to the first embodiment of the present invention. [Figure 2] FIG. 2 is a cross-sectional view showing the model of FIG. 1 in a pressurized state. [Figure 3] FIG. 10 is a cross-sectional view of a lower plate model created by simulation in a workpiece structure evaluation method according to a second embodiment of the present invention. [Figure 4] FIG. 4 is a cross-sectional view showing a state in which the lower plate model of FIG. 3 is pressed. [Figure 5] FIG. 1 is a cross-sectional view of a workpiece to be spot welded on one side. [Figure 6] FIG. 6 is a cross-sectional view showing a state in which the workpiece of FIG. 5 is pressed by an electrode. [Figure 7] 1 is a graph showing the correlation between the separation gap volume and the peeled diameter. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0018] In the method for evaluating the structure of a workpiece according to the first embodiment of the present invention, first, a model having a similar configuration to the workpiece to be welded (see FIG. 5 ) is created by simulation. Specifically, as shown in FIG. 1 , an upper plate model 1 having a shape, tensile strength, and thickness similar to that of an upper plate 101, a lower plate model 2 having a shape, tensile strength, and thickness similar to that of a lower plate 102, and a hat-shaped plate model (not shown) having a shape, tensile strength, and thickness similar to that of a metal plate 104 having a hat-shaped cross section are created. A joint portion P is provided in an overlapping portion 3 between the upper plate model 1 and the lower plate model 2. In the illustrated example, the plate thickness t1 of the upper plate model 1 is greater than the plate thickness t2 of the lower plate model 2. Furthermore, the tensile strengths of the upper plate model 1 and the lower plate model 2 are equal (they are made of the same material). In the illustrated example, the joint portions P are provided in the overlapping portions 3 near the end 1 a of the upper plate model 1 and the end 2 a of the lower plate model 2. In this case, the width W of the overlapping portion 3 (the distance between the end 1a of the upper plate model 1 and the end 2a of the lower plate model 2) is small, for example, three times or less the diameter D of the welding electrode (W≦3D).

[0019] Of the metal plates forming the overlapping portion, the metal plate on one side in the thickness direction is referred to as the "upper plate" and the metal plate on the other side in the thickness direction is referred to as the "lower plate," but these terms are not intended to specify the position during welding (vertically upper or lower).

[0020] Then, a portion to be joined P provided in the overlapping portion 3 of the upper plate model 1 and the lower plate model 2 is pressed from one side in the thickness direction (upward in the figure) with a predetermined pressure F. Then, as shown in FIG. 2 , the upper plate model 1 is pressed down by this pressure F, and the end 1 a of the upper plate model 1 presses down the lower plate model 2. As a result, a gap is formed between the upper plate model 1 and the lower plate model 2, and this gap is acquired. In this embodiment, the maximum gap within a cylindrical region R of a diameter D of the welding electrode 130 centered on a straight line L in the pressure direction that passes through the portion to be joined P is acquired as the separation gap δ. Specifically, the maximum gap within the cylindrical region R is the gap in the pressure direction between the models 1 and 2 at the point in the cylindrical region R farthest from the end 1 a of the upper plate model 1 (the right end in FIG. 2 ).

[0021] Next, it is determined whether the separation gap δ obtained above is within a predetermined range. For example, various samples having a configuration similar to that shown in FIG. 5 and differing in the rigidity (sheet thickness, tensile strength, etc.) of the upper and lower sheets 101 and 102 are joined by one-sided spot welding at the intended joining portion P, and the relationship between the separation gap before joining and the quality of the weld (the size of the peel diameter) after joining is recorded for each sample. Then, a range of separation gaps that will result in a good weld (i.e., a peel diameter equal to or greater than a predetermined value) is determined. In this embodiment, the separation gap must be 0.65 mm or less. If the separation gap δ of the model calculated by the simulation shown in FIGS. 1 and 2 is 0.65 mm or less, it is determined that the model has sufficient rigidity and a good weld can be obtained. On the other hand, if the separation gap δ is greater than 0.65 mm, it is determined that the model lacks rigidity and the weld may be defective.

[0022] As described above, it is possible to determine whether the model has sufficient rigidity based on the separation gap δ when the overlapping portion 3 of the upper plate model 1 and the lower plate model 2 is pressed (when no current is applied). Therefore, by performing a simulation while changing the plate thickness, etc. of the upper plate model 1 and the lower plate model 2, it is possible to obtain the minimum structure (plate thickness, bead shape, etc.) required to obtain sufficient rigidity.

[0023] The present invention is not limited to the above-described embodiment. Other embodiments of the present invention will be described below, but redundant description of points similar to those of the above-described embodiment will be omitted.

[0024] A workpiece structure evaluation method according to a second embodiment of the present invention differs from the above-described embodiments in that, as shown in FIG. 3 , instead of pressing an upper plate model 1, a lower plate model 2 is pressed with a predetermined pressure force F. The lower plate model 2 has higher rigidity than the lower plate 102 of the actual workpiece. Specifically, the lower plate model 2 has the same shape as the lower plate 102, and is fixed to the hat-shaped plate model at the same locations as the lower plate 102. The lower plate model 2 has a rigidity that is the sum of the rigidity of the lower plate 102 and the rigidity of the upper plate 101. In the illustrated example, the lower plate model 2 has the same tensile strength as the lower plate 1 (i.e., the material is the same), and the plate thickness t of the lower plate model 2 is equal to the sum of the plate thickness t1 of the upper plate 101 and the plate thickness t2 of the lower plate 102 (t = t1 + t2).

[0025] When a portion of this lower plate model 2 corresponding to the to-be-joined portion P is pressed from above with a predetermined pressure F, the lower plate model 2 is pressed down as shown in FIG. 4. The amount of deformation of the lower plate model 2 at this time is obtained. In this embodiment, the amount of displacement in the pressure direction of the lower plate model 2 (see the solid line in FIG. 4) that has been pressed and curved with the predetermined pressure F, relative to the lower plate model 2 before pressure application (see the dotted line in FIG. 4), is calculated. More specifically, the maximum amount of displacement within a cylindrical region R that is centered on a straight line L in the pressure direction that passes through the to-be-joined portion P and has the same diameter D as the electrode 130, specifically, the amount of displacement in the pressure direction of the lower plate model 2 at the point in the cylindrical region R that is closest to the end 2a of the lower plate model 2 (the right end in FIG. 4), is obtained as the separation gap amount δ.

[0026] Next, it is determined whether the obtained separation gap δ is within a predetermined range. If the separation gap δ is within the predetermined range, it is determined that the model has sufficient rigidity and a good weld will be obtained. On the other hand, if the separation gap δ is outside the predetermined range, it is determined that the model does not have enough rigidity and there is a possibility that the weld will be defective.

[0027] As described above, in the structural analysis of a workpiece by simulation, by omitting the upper plate model 1 and performing an evaluation based on the amount of deformation when pressure is applied only to the lower plate model 2, it is not necessary to consider contact between the upper plate model 1 and the lower plate model 2, which simplifies the simulation and reduces calculation time. Furthermore, by having the lower plate model 2 have a rigidity that is the sum of the rigidity of the upper plate 101 and the rigidity of the lower plate 102, it is possible to calculate the amount of deformation of the lower plate model 2 taking into account the rigidity of the upper plate 101, which reduces the deviation from the actual deformation state and improves the reliability of the evaluation results.

[0028] 3 and 4, the plate thickness t of the lower plate model 2 is made thicker than the plate thickness t2 of the lower plate 102 of the actual workpiece, thereby imparting higher rigidity to the lower plate model 2 than the lower plate 102, but this is not limited to this. For example, the tensile strength of the lower plate model 2 may be made higher than the tensile strength of the lower plate 102 of the actual workpiece, thereby imparting higher rigidity to the lower plate model 2 than the lower plate 102. Alternatively, the plate thickness and tensile strength of the lower plate model 2 may both be made greater than the plate thickness and tensile strength of the lower plate 102 of the actual workpiece, thereby imparting higher rigidity to the lower plate model 2 than the lower plate 102.

[0029] Furthermore, in the above embodiment, the upper plate 101 and the lower plate 102 are made of the same material, but they may be made of different materials. In this case, if the difference in material can be ignored, similar to the above embodiment, the deformation amount can be obtained using a lower plate model 2 that has a plate thickness equal to the sum of the plate thickness of the upper plate 101 and the plate thickness of the lower plate 102 and has the same tensile strength as either the upper plate 101 or the lower plate 102. Furthermore, if the difference in material cannot be ignored, for example, the lower plate model 2 may be made of a single plate in which the upper plate 101 and the lower plate 102 are joined in the thickness direction (a single plate with different tensile strengths in the thickness direction).

[0030] The present invention is not limited to structural evaluation of workpieces subjected to indirect spot welding, but can also be applied to structural evaluation of workpieces subjected to series spot welding, in which current is passed through a pair of welding electrodes while they are pressed against an overlapping portion of metal plates from only one side in the thickness direction with a predetermined pressure. [Explanation of symbols]

[0031] 1 Top plate model 2 Lower plate model 3 Polymerization section 101 Upper Plate 102 Lower plate 103 Polymerization section 130 Welding electrodes 140 Earth electrode P Joint area δ Separation gap

Claims

1. A method for evaluating a structure of a workpiece including an upper plate and a lower plate by simulation, in which one-side spot welding is performed by applying current to an overlapping portion between an upper plate arranged on one side in a thickness direction and a lower plate arranged on the other side in the thickness direction while pressing the overlapping portion with a predetermined pressure from only one side in the thickness direction with an electrode, a step of obtaining, by simulation, the size of a gap formed between the upper plate and the lower plate when the overlapping portion between the upper plate and the lower plate is pressed with a predetermined pressure from only one side in the thickness direction; and evaluating the structure of the workpiece based on the size of the gap.

2. A method for evaluating a structure of a workpiece including an upper plate and a lower plate by simulation, in which one-side spot welding is performed by applying current to an overlapping portion between an upper plate arranged on one side in a thickness direction and a lower plate arranged on the other side in the thickness direction while pressing the overlapping portion with a predetermined pressure from only one side in the thickness direction with an electrode, a step of creating a lower plate model having higher rigidity than the lower plate by simulation, and acquiring a deformation amount of the lower plate model when the lower plate model is pressed from one side in a thickness direction with the predetermined pressure force; and evaluating the structure of the workpiece based on the deformation amount of the lower plate model.

3. 3. The method for evaluating a workpiece structure subjected to one-side spot welding according to claim 2, wherein the thickness of the lower plate model is the sum of the thickness of the lower plate and the thickness of the upper plate.

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