One-side spot welding method and control device

Abstract

To form a preferable nugget regardless of presence or absence of a plate gap even when an upper plate is thick in one-side spot welding.SOLUTION: A one-side spot welding method comprises: a first step S1 of performing energization across electrodes 10, 20 while pressurizing, by the welding electrode 10, a bonding planned part P1 from one side in a thickness direction; and a second step S2 of performing, after the first step S1, energization across the electrodes 10, 20 while pressurizing, by the welding electrode 10, the bonding planned part P1 from one side in the thickness direction with a pressurization force F2 larger than a pressurization force F1 in the first step S1.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method and control device for single-sided spot welding. [Background technology]

[0002] Direct spot welding, in which a pair of electrodes sandwiches a metal assembly consisting of multiple overlapping metal sheets and current is applied, is commonly used. However, depending on the shape of the part, direct spot welding may not be applicable because the electrode cannot sandwich the joint between the electrodes. In such cases, "one-sided spot welding" may be used, in which the electrode is pressed against the joint from only one side of the thickness direction, without supporting the joint from the other side. Known one-sided spot welding methods include indirect spot welding (see Figure 1), in which the welding electrode presses the joint from one side of the thickness direction while a ground electrode is placed in contact with a location on the plate assembly other than the joint, and current is applied between the two electrodes. Also known is series spot welding (see Figure 5), in which a pair of electrodes is pressed against the plate assembly from one side of the thickness direction and current is applied to simultaneously weld two locations.

[0003] In a sheet assembly to be welded, there are areas where the metal sheets are in contact with each other and areas where there is a small gap (gap) between the metal sheets. With direct spot welding, even if there is a gap in the area to be joined, the sheet assembly can be sandwiched between a pair of electrodes to ensure that the metal sheets are in contact with each other, so the presence or absence of a gap has almost no effect on the weld quality.

[0004] On the other hand, in single-side spot welding, the electrode presses the sheet assembly from only one side in the thickness direction, making the sheet assembly prone to deformation. Therefore, the electrode pressure in single-side spot welding must be smaller than the pressure in direct spot welding. Because the electrode pressure is small in single-side spot welding, if a gap exists in the planned joining area of ​​the sheet assembly, the electrode pressure may not be enough to close the gap, preventing the metal sheets from coming into contact with each other. Therefore, in single-side spot welding, the presence or size of a gap is likely to affect the quality of the weld.

[0005] For example, Patent Document 1 listed below discloses a current-pressure pattern for indirect spot welding in which a high pressure and a low current are applied in the first half of the current application period, and a low pressure and a high current are applied in the second half of the current application period. In this way, applying a high pressure and a low current in the first half of the current application period increases the contact area between the metal sheets while preventing spatter. With the contact area between the metal sheets secured in this way, lowering the pressure and increasing the current in the second half of the current application period is said to reliably form a nugget regardless of whether a gap exists. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2019-206024 Summary of the Invention [Problem to be solved by the invention]

[0007] However, it has become clear that the pressure-current pattern disclosed in Patent Document 1 has difficulty in dealing with variations in the gap, especially when the metal plate (hereinafter referred to as the "upper plate") in contact with the welding electrode is thick. Specifically, when the upper plate is thick, it is difficult to deform. Therefore, when a gap exists at the joint of the plate assembly, applying pressure to the upper plate with the electrode does not close the gap. As a result, the contact area between the metal plates becomes too small, the current density becomes too high, and burn-through occurs. In this case, increasing the electrode pressure during the first half of the current application period can close the gap and ensure the contact area between the metal plates. However, increasing the electrode pressure results in an excessively large contact area when the gap is zero, making it difficult to increase the current density and form a nugget. In this case, even if the current amount is increased during the second half of the current application period, the excessive contact area makes it difficult to form a sufficiently large nugget.

[0008] As described above, when the upper plate is thick, the difference in contact area between the metal plates when there is a gap between them becomes large compared to when there is a gap, and therefore, adjusting the current value and pressure alone is not enough to fill the gap in the contact area, making it easy for poor welding to occur.

[0009] Therefore, an object of the present invention is to form a good nugget in one-side spot welding, even when the upper plate is thick, regardless of whether a plate gap exists or not. [Means for solving the problem]

[0010] Generally, spot welding is performed through the steps of bringing metal sheets into contact with each other during the first half of the current-carrying period, and then forming and growing a nugget at the contact point between the metal sheets during the second half of the current-carrying period. In this case, whether or not there is a gap between the sheets, the nugget is mainly formed during the second half of the current-carrying period. The inventors have found that by varying the timing of nugget formation depending on whether or not there is a gap between the sheets, a good nugget can be formed in either case.

[0011] Specifically, the present invention relates to a one-side spot welding method in which one electrode presses a joint to be formed of a plurality of overlapping metal plates from one side in the thickness direction, and the other electrode is brought into contact with an area of ​​the plate assembly other than the joint to be formed, and current is passed between the two electrodes to perform welding in a state in which the joint to be formed is not supported from the other side in the thickness direction, a first step of applying current between the electrodes while pressing the portion to be joined from one side in a thickness direction with the one electrode; The method is characterized in that a second step is carried out in which one of the electrodes applies pressure to the intended joining portion from one side in the thickness direction with a pressure force greater than that of the first step, while current is passed between both electrodes.

[0012] Thus, in the single-side spot welding method according to the present invention, a low pressure is applied by one electrode in the first step, followed by a high pressure in the subsequent second step. When the gap is substantially zero (when the metal sheets are in contact with each other or a negligibly small gap is formed), the metal sheets contact each other even with the low pressure in the first step, and a nugget is formed by passing current through this contact area. Even if the pressure is subsequently increased in the second step, the nugget is already formed, ensuring a sufficient contact area between the metal sheets, so the current density does not increase and the condition of the nugget remains almost unchanged. Therefore, the quality (size) of the nugget formed in the first step is maintained. On the other hand, when there is a gap, particularly if the upper sheet is thick, the upper sheet is barely deformed by the pressure of one electrode in the first step, and the metal sheets are not in contact with each other, resulting in no current passing through the to-be-joined portion. Then, in the second step, the metal plates are brought into contact with each other by applying a high pressure, and electricity is passed through this contact area (area to be joined) to generate heat and melt the contact area, thereby forming a nugget.

[0013] In the above-described single-side spot welding method, it is preferable to set the maximum current value in the second step to be greater than the maximum current value in the first step. In this way, by reducing the current value in the first step, which uses a low welding force, it is possible to reduce the occurrence of spatter when forming a nugget in the first step (when the sheet gap is substantially zero). Furthermore, by increasing the current value in the second step, the current density at the contact portion between the metal sheets when forming a nugget in the second step (when there is a sheet gap) increases, allowing the nugget to grow sufficiently.

[0014] In the above single-side spot welding method, it is preferable that the current value be gradually increased from the start of current application in the first step, thereby increasing the current density at the contact portion between the metal sheets and reliably forming the nugget while preventing the generation of spatter when forming the nugget in the first step.

[0015] In the single-side spot welding method, it is preferable that the energization time in the first step is longer than the energization time in the second step. By ensuring a sufficient energization time in the first step, a sufficiently large nugget can be formed in the first step when the sheet gap is substantially zero.

[0016] In the above-described single-side spot welding method, it is preferable to provide an intermediate step between the first and second steps, in which current is applied at a value lower than the current value at the end of the first step. When the gap is substantially zero, the nugget formed in the first step is cooled once in the intermediate step at a low current, which suppresses expansion of the nugget in the subsequent second step, making it easier to maintain the state (size) of the nugget. On the other hand, when there is a gap, the nugget is not formed in the first step, so the situation remains almost unchanged in the intermediate step, and the nugget can be formed in the subsequent second step by applying current while applying a high pressure. [Effects of the Invention]

[0017] As described above, in the one-side spot welding method of the present invention, a nugget is formed in the first step if the sheet gap is substantially zero, and in the second step if there is a sheet gap. This makes it possible to form a good nugget even when the upper sheet is thick, regardless of whether there is a sheet gap or not. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a cross-sectional view showing indirect spot welding. [Figure 2] 1 is a graph showing changes in current value and welding force over time (pressure and current pattern) during indirect spot welding according to one embodiment of the present invention. [Figure 3] FIG. 10 is a cross-sectional view showing the state of a plate assembly that has been welded using the above pressure and current pattern. [Figure 4] 10 is a graph showing a pressure / current application pattern according to another embodiment. [Figure 5] FIG. 1 is a cross-sectional view showing how series spot welding is performed. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0020] In this embodiment, as shown in FIG. 1 , a joint portion P1 of a plate assembly 3 consisting of overlapping upper and lower plates 1 and 2 is joined by indirect spot welding, which is a type of one-side spot welding. Both the upper and lower plates 1 and 2 are metal plates, such as steel plates. In this embodiment, the upper and lower plates 1 and 2 are mild steel plates (steel plates with a tensile strength of 340 MPa or less), specifically, zinc-plated mild steel plates. Alternatively, one or both of the upper and lower plates 1 and 2 may be high-tensile steel plates (steel plates with a tensile strength of 490 MPa or more) or ultra-high-tensile steel plates (steel plates with a tensile strength of 980 MPa or more). In the illustrated example, the upper plate 1 is thicker than the lower plate 2; that is, the ratio t1 / t2 of the thickness t1 of the upper plate 1 to the thickness t2 of the lower plate 2 is greater than 1. The thickness ratio t1 / t2 of the plates 1 and 2 may be 1.2 or greater, or even 1.4 or greater.

[0021] The indirect spot welding method of this embodiment is performed using welding equipment including an indirect spot welding device and a controller connected to the device.

[0022] The indirect spot welding device includes a welding electrode 10, a ground electrode 20, a power source 30 for applying current between the electrodes 10, 20, and a drive means (e.g., an air cylinder) for applying pressure to the welding electrode 10 in the axial direction (see the arrow in the figure). The welding electrode 10 is positioned directly above the portion to be joined P1. The ground electrode 20 is in contact with an area of ​​the plate assembly 3 other than the portion to be joined P1. In the illustrated example, the welding electrode 10 is in contact with the upper plate 1 from above, and the ground electrode 20 is in contact with the lower plate 2 from below. Alternatively, the ground electrode 20 may be in contact with the lower plate 2 from above, or both the welding electrode 10 and the ground electrode 20 may be in contact with the upper plate 1.

[0023] The control device controls the pressure applied to welding electrode 10 by the drive means and the current value between welding electrode 10 and earth electrode 20 (the voltage of power source 30 in this embodiment) according to an installed control program.

[0024] In the welding method according to this embodiment, welding is performed while changing the pressure and current values ​​over time. Fig. 2 shows a pressure and current pattern showing the changes in pressure and current values ​​over time according to this embodiment. As shown in Fig. 2, the welding method according to this embodiment has a first step S1, a second step S2, and an intermediate step Sm provided between them.

[0025] In the first step S1, the pressure applied by the welding electrode 10 is maintained at a constant pressure F1. The pressure applied in indirect welding is significantly smaller than the pressure applied in direct spot welding. The pressure applied varies depending on the material and thickness of the sheet assembly, but generally, the pressure applied in direct spot welding is approximately 1500 to 5000 N, while the pressure applied in indirect welding is approximately 200 to 1000 N. When joining a sheet assembly 3 made of mild steel sheets by indirect spot welding, as in the present embodiment, the pressure applied in the first step S1 is set to, for example, 200 to 400 N.

[0026] In this embodiment, the current flow pattern of the first step S1 has a slope portion S1a in which the current value gradually increases from the start of current flow. After the slope portion S1a, a high current portion S1b is provided in which current is flowed for a predetermined time at a current value C2 that is greater than the maximum current value C1 of the slope portion S1a. The current value C2 of this high current portion S1b is the maximum current value in the first step S1.

[0027] In the intermediate step Sm, the welding force of the welding electrode 10 is gradually increased from F1 to F2. Specifically, when a command to increase the welding force is issued to the drive means that drives the welding electrode 10, the welding force does not instantly increase to F2. However, due to the structure of the drive means, a transition period during which the welding force gradually increases to F2 is necessarily provided, and this transition period corresponds to the intermediate step Sm. In the intermediate step Sm, current is supplied at a current value C3 that is lower than the current value at the end of the first step S1 (i.e., the current value C2 of the high current portion S1b). In the illustrated example, the current value C3 in the intermediate step Sm is equal to or less than half the current value C2 at the end of the first step S1. Note that the current value in the intermediate step Sm is greater than zero, and current is supplied continuously between the first step S1 and the second step S2 without interruption.

[0028] In the second step S2, a constant pressure F2 greater than the pressure F1 in the first step S1 is maintained. In this embodiment, the pressure F2 in the second step S2 is set to, for example, 400 to 600 N. In this embodiment, the current value is increased stepwise in the second step S2, and in the illustrated example, it is increased in two steps. Specifically, after energizing for a predetermined time at a current value C4 higher than the current value C3 in the intermediate step Sm, the current value is further increased to C5. The maximum current value C5 in the second step S2 is greater than the maximum current value C2 in the first step S1. In the illustrated example, the current values ​​C4 and C5 throughout the entire second step S2 are greater than the maximum current value C2 in the first step S1.

[0029] The energization time of the first step S1 is longer than the energization time of the second step S2. In this embodiment, the energization time of the first step S1 is set to be at least twice the energization time of the second step S2. For example, the energization time of the first step S1 is set to 5 to 30 cycles, the energization time of the second step S1 is set to 5 to 10 cycles, and the energization time of the intermediate step Sm is set to 5 to 10 cycles. Note that one cycle is the reciprocal of the frequency of the AC power supply, and is, for example, 1 / 60 seconds.

[0030] Below, a case will be described in which indirect spot welding is performed on a portion to be welded P1 where the plate gap is substantially zero, using the pressure application and current application pattern described above.

[0031] First, in a first step S1, a welding electrode 10 applies pressure to a portion P1 of the sheet assembly 3 to be joined with a low pressure F1 while current is passed between the electrodes 10 and 20. At this time, the gap between the upper sheet 1 and the lower sheet 2 is substantially zero, so that by applying pressure to the portion P1 of the sheet assembly 3 to be joined with the welding electrode 10, the sheets 1 and 2 come into contact with each other even with the low pressure F1, and a current path L is formed that passes through this contact point and the electrodes 10 and 20. By passing current through this current path L, the contact point between the sheets 1 and 2 is heated and melted, and a nugget N is formed (see column (a) of FIG. 3).

[0032] In this embodiment, the current application time in the first step S1 is longer than the current application time in the second step S2, so sufficient energy can be applied to the contact portion between the sheets 1 and 2, facilitating the formation of the nugget N. In this embodiment, the current value is gradually increased from 0 at the start of current application (slope portion S1a). This makes it possible to avoid a sudden increase in current density at the contact portion between the sheets 1 and 2 in the first half of the first step S1, thereby preventing the occurrence of spatter. Then, by increasing the current value in the second half of the first step S1, the current density at the contact portion between the sheets 1 and 2 can be increased to form the nugget N. In this embodiment, the growth of the nugget N is promoted by providing a high-current portion S1b after the slope portion S1a, where current is applied at a current value C2 higher than the maximum current value C1 of the slope portion S1a.

[0033] In the intermediate step Sm, the pressing force is increased from F1 to F2 while applying a current C3 lower than the current C2 at the end of the first step S1 (see FIG. 2). In this way, by decreasing the current value to C3, the nugget N formed in the first step S1 is temporarily cooled.

[0034] In the second step S2, current is passed between the electrodes 10 and 20 while applying pressure to the portion P1 to be joined of the sheet assembly 3 with the welding electrode 10 at a pressure F2. At this time, a nugget N has already been formed at the contact portion between the upper sheet 1 and the lower sheet 2, so the contact area between the two sheets 1 and 2 is sufficiently secured. Therefore, even if current is passed while applying pressure at a high pressure F2 in the subsequent second step S2, the current density does not increase and the amount of heat generated is small. Therefore, the nugget N does not grow any further, and the size (diameter) of the nugget N formed in the first step S1 is maintained (see column (b) of Figure 3).

[0035] Next, a case will be described in which indirect spot welding is performed on a portion to be joined P1 having a gap (for example, 1 mm) using the pressure and current application pattern described above.

[0036] First, in a first step S1, current is passed between the electrodes 10 and 20 while applying pressure to the portion to be joined P1 of the sheet assembly 3 with a low pressure F1 using the welding electrode 10. At this time, because there is a gap between the upper sheet 1 and the lower sheet 2, the sheets 1 and 2 do not come into contact with each other at the low pressure F1 (see column (c) in FIG. 3). In particular, in this embodiment, the upper sheet 1 is thicker than the lower sheet 2, so the upper sheet 1 is less likely to deform, and the upper sheet 1 and the lower sheet 2 do not come into contact with each other even when pressure is applied with the low pressure F1. In this way, even if current is passed while the upper sheet 1 and the lower sheet 2 are not in contact with each other, a nugget is not formed at the portion to be joined P1.

[0037] In the intermediate step Sm, the pressure is increased from F1 to F2 while current is applied at a current value C3. In this embodiment, since the upper sheet 1 is thick, the upper sheet 1 is less likely to deform even when the pressure is increased to F2, and the upper sheet 1 and the lower sheet 2 are maintained in a non-contact state. Note that while the pressure is being increased from F1 to F2, the upper sheet 1 and the lower sheet 2 may come into contact with each other. Even in this case, by keeping the current value C3 in the intermediate step Sm low, it is possible to prevent spatter from occurring at the contact area between the sheets 1 and 2.

[0038] In the second step S2, current is passed between the electrodes 10 and 20 while applying pressure F2 to the portion P1 to be joined of the sheet assembly 3 using the welding electrode 10. At this time, current is passed at a value C3 higher than the current C3 in the intermediate step Sm, preferably higher than the maximum current C2 in the first step S1, to soften and facilitate deformation of the upper sheet 1 in contact with the electrode 10. This closes the gap between the upper sheet 1 and the lower sheet 2, bringing the sheets 1 and 2 into contact. Current is then passed through the current path L passing through this contact area and the electrodes 10 and 20, causing the contact area between the sheets 1 and 2 to heat up and melt, forming a nugget N (see column (d) in FIG. 3 ). In particular, in this embodiment, the current is gradually increased in the second step S2, supplying a large amount of energy to the contact area between the upper sheet 1 and the lower sheet 2, thereby accelerating the growth of the nugget N.

[0039] As described above, in the indirect spot welding method of this embodiment, the timing at which the nugget N is formed is intentionally varied depending on the size of the gap. Specifically, if the gap in the portion to be joined P1 is very small (substantially zero), the nugget N is formed in the first step S1, which is provided in the first half of the energization period. However, if a certain degree of gap exists in the portion to be joined P1, the nugget N is formed in the second step S2, which is provided in the second half of the energization period. This makes it possible to form a sufficiently large nugget while avoiding burn-through, even when the upper sheet 1 is thick, regardless of whether there is a gap.

[0040] 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.

[0041] The current pattern of the first step S1 is not limited to the above, and for example, as shown in Fig. 4, the high current portion S1b may be omitted and current may be supplied at a maximum current value C1 in the slope portion S1a until the end of the first step S1. Alternatively, the current value of the high current portion S1b may be increased in stages.

[0042] The current pattern in the second step S2 is not limited to the above, and the current value may be increased in three or more stages, for example, as shown in Fig. 4. In the second step S2, the current value may be increased continuously rather than in stages.

[0043] The present invention is not limited to indirect spot welding, but is also applicable to series spot welding as shown in FIG. 5 . This series spot welding device includes welding electrodes 11, 12, a power source 30 for applying current between electrodes 11, 12, and a drive means for applying pressure to each welding electrode 11, 12 in the axial direction (see the arrows in the figure). The welding electrodes 11, 12 press against portions P1, P2 of the sheet assembly 3 to be joined from one side in the thickness direction (from above in the figure). In this state, welding is performed using the same pressure and current pattern as in the above embodiment, forming nuggets at portions P1, P2 of the sheet assembly to be joined. [Explanation of symbols]

[0044] 1 Upper plate (metal plate) 2 Lower plate (metal plate) 3. Board construction 10 Welding electrode (one electrode) 20 Earth electrode (other electrode) 30 power supply L Current path N Nugget P1, P2 planned joining area S1 First Step S1a Slope S1b High current section S2 Second step Sm Intermediate step

Claims

1. A one-side spot welding method in which one electrode presses a joint to be formed of a plurality of overlapping metal plates from one side in a thickness direction, and the other electrode is brought into contact with an area of ​​the plate assembly other than the joint to be formed, and current is passed between the two electrodes to perform welding in a state in which the joint to be formed is not supported from the other side in the thickness direction, a first step of applying a current between the electrodes while applying pressure to the portion to be joined from one side in a thickness direction with the one electrode; a second step of applying current between the electrodes while applying pressure to the portion to be joined from one side in the thickness direction with the one electrode at a pressure greater than the pressure applied in the first step and maximum throughout the entire current application period; The current value in the second step is set to be greater than the maximum current value in the first step; The one-side spot welding method, wherein the current value is increased stepwise in the second step.

2. A control device connected to a one-side spot welding device that presses a planned joining portion of a plate assembly made of a plurality of overlapping metal plates from one side in a thickness direction with one electrode, and abuts the other electrode against an area of ​​the plate assembly other than the planned joining portion, and performs welding by passing current between both electrodes in a state in which the planned joining portion is not supported from the other side in the thickness direction, a first step of applying a current between the electrodes while applying pressure to the portion to be joined from one side in a thickness direction with the one electrode; a second step in which current is applied between the electrodes while the one electrode applies pressure to the portion to be joined from one side in the thickness direction with a pressure that is greater than the pressure applied in the first step and is maximum throughout the entire current application period; and The maximum current value in the second step is set to be greater than the maximum current value in the first step; A control device that increases the current value stepwise in the second step.

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