JP2024063371A - Monitoring device, spot welding device, and program - Google Patents

Abstract

[Problem] The present invention aims to provide a monitoring device, spot welding device, and program that can determine the quality of welding based on the timing of expulsion occurrence and improve the accuracy of quality assurance. [Solution] A monitoring device according to one aspect of the present invention is a monitoring device that monitors expulsion that occurs in metal plates that constitute a sheet assembly during spot welding to manufacture a welded joint, and includes a detection unit that detects the occurrence of expulsion during spot welding, and a judgment unit that judges the quality of the welded joint to be poor if expulsion occurs within a time period set according to the sheet assembly configuration. [Selected Figure] Figure 11

Description

The present invention relates to a monitoring device, a spot welding device, and a program.

In a manufacturing method for spot welded joints using a spot welding device, a pair of opposing electrodes sandwiches a metal plate assembly, and the pair of electrodes heats and melts the overlapping surfaces of the plate assembly to form a weld metal (nugget), thereby joining the metal plates together. Increasing the nugget diameter increases the strength of the welded joint, but attempting to increase the nugget diameter makes it more likely to cause splashing. Splashing refers to a phenomenon in which the molten metal becomes larger than the pressure-welded area of the overlapping surfaces due to pressure applied by the electrodes, and the molten metal splashes out from the area not restrained by pressure. There is a concern that the occurrence of splashing during spot welding may reduce the strength of the joint. Therefore, depending on the production line, the quality of a welded joint where splashing has occurred may be judged to be poor, and the weldability may be reduced. In addition, spot welding may not be applied to new materials because the conditions under which splashing occurs are unknown. Therefore, it is important to understand the occurrence of splashing during spot welding, and various technologies have been developed.

For example, Patent Document 1 discloses a method for detecting scattering that determines the occurrence of scattering based on the distance between a pair of electrodes.

Patent Document 2 also discloses a method for controlling resistance welding, which includes the steps of detecting interelectrode resistance or interelectrode voltage, judging whether the detected value is equal to or lower than a threshold value, judging that an expulsion phenomenon has occurred when the detected value is equal to or lower than the threshold value, and performing a current-fitting process to eliminate poor plate fit between the welded parts, and continuing welding by re-applying the welding current before the current-fitting process.

Patent Document 3 also discloses a device that distinguishes between vibrations that melt (evaporate) the plating and vibrations that cause splashing based on the time they occur, and determines whether welding is good or bad and whether splashing has occurred.

Patent document 4 also discloses a method for suppressing excessive current changes after scattering by performing constant power control until scattering occurs and then switching to constant current control when scattering occurs.

JP 2019-141851 A JP 2015-13302 A JP 2001-170777 A JP 2000-301348 A

However, the technology described in Patent Document 2 requires a period of time for construction because it requires a period of time for adaptation and then re-energization.

As will be described in detail later, the inventors have found that the main cause of the decrease in joint strength due to the occurrence of expulsion is a decrease in nugget diameter, and that even if expulsion occurs, joint strength will not decrease if the nugget diameter does not decrease. Furthermore, the inventors' studies have found that the decrease in nugget diameter due to expulsion depends on the timing of the occurrence of expulsion, and that if expulsion occurs early in the current flow, the final nugget diameter will be small and the variation in nugget diameter will also be large, but even if expulsion occurs later in the current flow, no decrease in nugget diameter is observed and the variation in nugget diameter will also be small.

On the other hand, Patent Document 3 describes that welding control is performed based on welding conditions that promote nugget formation, such as increasing the welding current when the pass/fail judgment is negative, but does not mention specific conditions. Furthermore, Patent Document 1 and Patent Document 4 make no mention of the fact that the final nugget diameter varies depending on the timing of expulsion.

The present invention was made in consideration of the above circumstances, and aims to provide a monitoring device, spot welding device, and program that can determine the quality of welding based on the timing at which expulsion occurs, thereby improving the accuracy of quality assurance.

The gist of the present invention is as follows.
[1] A monitoring device according to one aspect of the present invention is a monitoring device that monitors expulsion that occurs in metal plates that constitute a plate assembly during spot welding to produce a welded joint, and includes a detection unit that detects the occurrence of expulsion during the spot welding, and a determination unit that determines the quality of the welded joint to be defective when expulsion occurs within a time period that is set in accordance with the configuration of the plate assembly.
[2] In the monitoring device described in [1] above, the detection unit may detect the occurrence of shattering using one or more of a voltage value, a resistance value, an electrode displacement amount, a pressure force, an amount of strain, and an appearance.
[3] The monitoring device described in [1] or [2] above may further include an alert unit that issues an alarm if scattering occurs within a time period set according to the configuration of the board assembly.
[4] In the monitoring device described in [3] above, the alert unit does not need to issue the alarm if the scattering occurs outside of a time period set in accordance with the configuration of the board assembly.

[5] In addition, a spot welding device according to another aspect of the present invention is equipped with a monitoring device described in any one of [1] to [4] above.

[6] Furthermore, a program according to yet another aspect of the present invention causes a computer to function as a detection unit that detects the occurrence of expulsion during spot welding, and as a determination unit that determines that the quality of a welded joint produced by the spot welding is defective if expulsion occurs within a time period set according to the plate assembly configuration.

The present invention provides a monitoring device, spot welding device, and program that can determine the quality of welding based on the timing at which expulsion occurs, improving the accuracy of quality assurance.

1 is a graph showing the relationship between current value and cross tension strength (CTS). 1 is a graph showing the relationship between nugget diameter and CTS. 1 is a graph showing the relationship between current value and nugget diameter. 1 is a graph showing changes over time in current, electrode distance (displacement), welding force, and voltage when spot welding is performed with a current value of 8.2 kA. 1 is a graph showing changes over time in current, electrode distance (displacement), welding force, and voltage when spot welding is performed with a current value of 8.6 kA. 1 is a graph showing a change over time in energizing current during spot welding, which was conducted to examine the relationship between the timing of expulsion occurrence and the nugget diameter. 1 is a table summarizing photographs of cross sections of welded parts after spot welding. 1 is a graph showing an example of an average nugget diameter and a variation in nugget diameter. 1 is a graph showing the relationship between the timing of expulsion and the nugget diameter. FIG. 1 is a conceptual diagram for explaining the present inventors' speculation on the relationship between the timing of expulsion occurrence and the growth of the nugget. 1 is a side view showing an appearance of a spot welding device according to an embodiment of the present invention. FIG. 2 is a functional block diagram of the spot welding device according to the embodiment.

<Background>
Prior to describing the embodiments of the present invention, the circumstances by which the present inventors came to the present invention will be described. FIG. 1 is a graph showing the relationship between current value and cross tension strength (CTS), and FIG. 2 is a graph showing the relationship between nugget diameter and CTS. FIG. 1 and FIG. 2 are graphs obtained by spot welding performed under the following conditions. That is, as shown in FIG. 1, a current value was set to 5.2 kA, 5.4 kA, 6.4 kA, or 7.4 kA, and two 980 MPa-class TRIP (Transformation Induced Plasticity) steel sheets having a thickness of 1.6 mm were stacked and spot-welded with a pressure of 4.0 kN, a current flow time of 18 cycles (50 Hz), and a holding time of 10 cycles (50 Hz) to prepare a test piece for CTS measurement. CTS was measured in accordance with JIS Z 3137:1999.

As shown in Figure 1, it was found that when the current value is the same, the occurrence of expulsion reduces the CTS. In addition, as shown in Figure 2, when comparing at the same current value, it was found that the occurrence of expulsion tends to result in a smaller nugget diameter.

Figure 3 shows the relationship between current value and nugget diameter. Figure 3 is a graph obtained by spot welding performed under the following conditions: current values were constant from 3.5 kA to 9.5 kA, two 980 MPa-class TRIP steel sheets with a thickness of 1.6 mm were stacked, the pressure was 4.0 kN, the current flow time was 18 cycles (50 Hz), and the hold time was 10 cycles (50 Hz).

As shown in Figure 3, it was found that when expulsion occurs, the nugget diameter may become smaller even if the current value is large. In other words, even if the current value is increased to increase the nugget diameter and therefore the strength of the welded joint, if expulsion occurs, the nugget diameter does not increase, and the strength of the welded joint may not be increased.

The inventors further investigated the relationship between the occurrence of expulsion and nugget diameter. Figure 4 is a graph showing the changes over time in current, electrode distance (displacement), pressure, and voltage when spot welding was performed with a current value of 8.2 kA. Figure 5 is a graph showing the changes over time in current, electrode distance (displacement), pressure, and voltage when spot welding was performed with a current value of 8.6 kA. In the spot welding from which the graph in Figure 4 was obtained, expulsion occurred in the later stages of current flow, and in the spot welding from which the graph in Figure 5 was obtained, expulsion occurred in the early stages of current flow.

It is known that when expulsion occurs during current flow, the distance between the electrodes becomes shorter and the pressure and voltage decrease. In the example shown in Figure 4 where expulsion occurred in the later stages of current flow, the nugget diameter was 6.6 mm, and in the example shown in Figure 5 where expulsion occurred in the early stages of current flow, the nugget diameter was 4.8 mm. As can be seen from Figures 4 and 5, when expulsion occurs in the early stages of current flow, the nugget diameter becomes smaller.

The inventors further conducted the following experiment to investigate in detail the relationship between the timing of expulsion and the nugget diameter. Using a single-phase AC spot welding machine, the plate assembly was welded while controlling the timing of expulsion, and the nugget diameter was measured. Two 980 MPa-class TRIP steel plates with a plate thickness of 1.6 mm were stacked, and the pressure was 4.0 kN, the current time was 18 cycles (50 Hz), and the holding time was 10 cycles (50 Hz). As an example of No. 1, as shown in (A) of FIG. 6, the current was applied at 7.4 kA for all 18 cycles. As an example of No. 2, as shown in (B) of FIG. 6, the current value was 7.4 kA for 2 cycles after the start of current application, the current value was 10 kA for 3 to 4 cycles, and the current value was 7.4 kA for 5 to 18 cycles. As an example of No. 3, as shown in FIG. 6(C), the current value was set to 7.4 kA for the 16th cycle after the start of current flow, and to 10 kA for the 17th and 18th cycles. There were 10 samples for each condition. In examples No. 2 and No. 3, spattering occurred when a current of 10 kA was passed.

Figure 7 is a table summarizing photographs of the cross section of the weld after spot welding under each condition, and Figure 8 is a graph showing the average nugget diameter and the variation in nugget diameter under each condition. As shown in Figures 7 and 8, it was found that the nugget diameter was small and the variation was large in Example No. 2. Furthermore, when comparing Example No. 1 and Example No. 3, it was found that the nugget diameters were about the same and the variation was also small. In other words, it was found that the earlier the expulsion occurred, the smaller the nugget diameter was and the greater the variation.

The inventors further performed spot welding tests by changing the energizing current conditions. The energizing conditions were as shown in Table 1, and the conditions other than the energizing current were the same as in the above experiment. The number of samples for each condition was 5.

Figure 9 is a graph showing the nugget diameter for each test example. The open plots in Figure 9 show the nugget diameter when a single current of 7.4 kA was applied and the current was stopped before expulsion occurred. As shown in Figure 9, it was found that the earlier the current was increased, in other words, the earlier the expulsion occurred, the greater the variation in nugget diameter.

The inventors speculate as follows about the reason why the nugget diameter becomes smaller as the timing of the occurrence of expulsion becomes earlier. That is, when the welding conditions are appropriate, as the current and pressure are applied, the metal melts inside the pressure-welded area, and the nugget is formed and grows. As shown in the upper part of FIG. 10, even if expulsion occurs in the later period of current application, the nugget grows sufficiently. When the welding conditions are not appropriate, as shown in the lower part of FIG. 10, the metal melts faster and the metal melts even outside the pressure-welded area. Outside the pressure-welded area, the molten metal is scattered because no pressure force acts to restrain the molten metal. In other words, expulsion occurs. When expulsion occurs, the volume of the molten part decreases and the amount of electrode pressing increases. This increases the contact area at the interface between the metal plates, which reduces the current density after expulsion occurs and reduces the amount of heat generated. As a result, the nugget does not grow.

Therefore, the earlier the expulsion occurs, the smaller the nugget diameter will be, which will have a greater impact on CTS, but if expulsion occurs late, a relatively large nugget will be formed and the impact on CTS will not be significant. Therefore, by monitoring the timing of expulsion and determining that the quality is poor when expulsion occurs early in the current flow, the accuracy of quality assurance can be improved.

The following describes the spot welding device, monitoring device, and program according to an embodiment of the present invention.

<Spot welding device>
A spot welding device according to an embodiment of the present invention will be described with reference to Figures 11 and 12. Figure 11 is a side view showing the appearance of a spot welding device 10 according to an embodiment of the present invention, and Figure 12 is a functional block diagram of the spot welding device 10.

The spot welding device 10 is a device for spot welding two or more overlapping steel plates (for example, metal plates 100a and 100b shown in FIG. 11), and includes a control unit 11, a memory unit 12, a drive unit 13 (electrode drive unit), a current supply unit 14, a movable electrode 15a, a fixed electrode 15b, an input unit 16, and a monitoring unit 17. The spot welding device 10 includes a CPU, a ROM, a RAM, a splash detection device (for example, a resistor for measuring the resistance of the plate assembly during spot welding, a load cell for measuring the pressure applied by the electrode to the plate assembly, a strain sensor, or a high-speed camera), a drive unit, a current supply unit, electrodes and an electrode drive mechanism, and an input device (keyboard, input switch, etc.). Using this hardware configuration, the spot welding device 10 realizes the above-mentioned control unit 11, memory unit 12, drive unit 13, current supply unit 14, movable electrode 15a, fixed electrode 15b, input unit 16, and monitoring unit 17.

The control unit 11 controls each component of the spot welding device 10. For example, the control unit 11 controls the drive unit 13 so that the movable electrode 15a and the fixed electrode 15b press the metal plates 100a and 100b with a constant pressure. Specifically, the control unit 11 outputs drive information to the drive unit 13. The control unit 11 also controls the current supply unit 14, for example, with a preset current supply period and current value. Specifically, during the current supply period, the control unit 11 outputs current supply start information to the current supply unit 14.

The memory unit 12 stores data necessary for the processing performed by the spot welding device 10 and various information for detecting the occurrence of expulsion. Examples of data necessary for the processing performed by the spot welding device 10 include programs necessary for the operation of the control unit 11. Information for detecting the occurrence of expulsion is, for example, various data at each time during spot welding, specifically, the voltage value, resistance value, electrode distance, applied pressure, amount of strain, and external appearance photographs for each time.

The driving unit 13 drives the movable electrode 15a based on the driving information output by the control unit 11. This causes the movable electrode 15a and the fixed electrode 15b to be pressed into the metal plates 100a and 100b. The driving unit 13 may be, for example, a servo motor or an air cylinder.

Based on the current start information output by the control unit 11, the current supply unit 14 starts current supply to the movable electrode 15a and the fixed electrode 15b according to the preset current supply conditions.

The driving unit 13 drives the movable electrode 15a based on the driving information output by the control unit 11, and the metal plates 100a, 100b are pressurized by the movable electrode 15a and the fixed electrode 15b. The current supply unit 14 starts current supply to the movable electrode 15a and the fixed electrode 15b according to the preset current supply conditions based on the current supply start information output by the control unit 11. As a result, the movable electrode 15a and the fixed electrode 15b sandwich two or more overlapping metal plates (for example, the metal plates 100a, 100b shown in FIG. 11), and current is passed through the overlapping surfaces to heat and melt the overlapping surfaces.

The input unit 16 can be operated by an operator, and the operator uses the input unit 16 to input various information, such as a current value and a pressure force. The input unit 16 may also be used to start and stop the flow of electricity and the movement of the electrodes.

The monitoring unit 17 monitors the splashing that occurs in the metal plates that make up the plate assembly during spot welding to produce a welded joint. The monitoring unit 17 includes a detection unit 171 and a determination unit 172.

The detection unit 171 detects the occurrence of splashing in spot welding. The method of detecting the occurrence of splashing is not particularly limited, but for example, splashing can be detected using one or more of the voltage value, resistance value, electrode displacement amount, pressure force, strain amount, and appearance as an index. For example, when spot welding is performed with a constant current value, splashing occurs, the plate thickness of the welded area decreases rapidly and the resistivity value decreases significantly. When the resistivity value decreases, the voltage decreases significantly in order to keep the current value constant. In addition, when splashing occurs, the molten metal inside the metal plate at the pressure welding part decreases significantly, so the amount of pressing of the steel plate surface by the electrode increases. In other words, when splashing occurs, the electrode displacement amount increases rapidly. In addition, even if spot welding is performed so that the pressure force does not change significantly, the pressure force decreases significantly when splashing occurs. The amount of strain also changes significantly due to the occurrence of splashing. In addition, since splashing is a phenomenon in which molten metal is scattered, the occurrence of splashing can also be monitored by observing the appearance of the welded part during welding. Therefore, the detection unit 171 can detect the occurrence of expulsion using, for example, any one or more of the voltage value, resistance value, electrode displacement amount, applied pressure, strain amount, and appearance. There is no need to provide a complex configuration to the spot welding device to measure the resistance value. In addition, the voltage value can be measured by a voltmeter provided in advance in the spot welding device, or a voltmeter attached to the electrode from the outside. Therefore, if a voltmeter is provided in advance in the spot welding device, there is no need to provide a complex configuration, and the device configuration can be relatively simple. Therefore, it is preferable to use the voltage value or resistance value to detect expulsion. In addition, by using multiple indicators from among the voltage value, resistance value, electrode distance, applied pressure, strain amount, and appearance, the occurrence of expulsion that causes a decrease in the strength of the welded joint can be detected more accurately.

The thresholds of the voltage value, resistance value, electrode displacement amount, pressure, or strain amount when determining that expulsion has occurred may be determined according to the spot welding conditions, and it is preferable that the thresholds are determined in advance by performing test welding. The spot welding conditions referred to here are, for example, the strength of the metal plate, the plate thickness, the shape of the metal plate at the spot welded portion, the current value, the pressure, the current flow time, the holding time, the electrode shape, etc. For example, when two metal plates with a steel plate strength of 980 MPa and a plate thickness of 1.6 mm are stacked in the plate thickness direction, the current value is 8.5 kA, the pressure is 4.0 kN, the current flow time is 18 cycles (50 Hz), the holding time is 10 cycles (50 Hz), and spot welding is performed using a CrCu electrode with a tip diameter of φ6 mm, expulsion is determined to have occurred when the voltage drop is 20 V/sec or more, and expulsion can be detected.

The electrode displacement amount may be measured or calculated by a known method. For example, if the drive unit 14 is configured with a servo motor, it can be calculated based on the amount of rotation of the servo motor that drives the electrode of the spot welding device. The electrode displacement amount may also be measured using a laser displacement meter.

The pressure force is calculated based on information related to the drive of the drive unit 14. For example, if the drive unit 14 is configured as a servo motor, the pressure force is calculated based on the torque of the servo motor. Also, if the drive unit 14 is configured as an air cylinder, the pressure force is calculated based on the pressure that the air cylinder applies to the plate assembly using a load cell.

The amount of strain can be calculated using a strain gauge or strain sensor attached to the support that supports electrodes 15a and 15b.

The appearance may be observed, for example, with a known high-speed camera. In order to observe the scattering of molten metal more accurately, it is preferable to use a high-speed camera equipped with a thermo viewer.

The judgment unit 172 judges the quality of the welded joint to be poor if expulsion occurs within a time period set according to the plate assembly configuration. The time period set according to the plate assembly configuration for judging the quality of the welded joint to be poor may be determined according to the spot welding conditions described above. In the following, expulsion that occurs within a time period set according to the plate assembly configuration may be referred to as initial expulsion.

For example, when two metal plates with a steel sheet strength of 980 MPa and a sheet thickness of 1.6 mm are stacked in the sheet thickness direction, and spot welding is performed under the following conditions: current value is 8.5 kA, pressure is 4.0 kN, current flow time is 18 cycles (50 Hz), holding time is 10 cycles (50 Hz), and a CrCu electrode with a tip diameter of φ6 mm is used, if expulsion occurs between the start of spot welding and (average value of the two sheet sets) x 20 msec or more and (average value of the two sheet sets) x 120 msec or less, the quality of the welded joint is judged to be poor.

The spot welding device 10 described above may include an alert unit 173 that issues an alarm when initial expulsion occurs. The alarm issued by the alert unit 173 is not particularly limited, and may be, for example, a display indicating that initial expulsion has occurred on a display (not shown), a warning sound by a buzzer (not shown), or the lighting of a warning light (not shown). When an alarm is issued by the alert unit 173, the spot welding conditions can be changed to perform spot welding under conditions in which initial expulsion does not occur. For example, the control unit 11 that receives the alarm may change the current value or pressure, or the operator may change the spot welding conditions by operating the input unit 16. Since the spot welding conditions can be changed in response to the output from the alert unit 173, the occurrence of poor quality welded joints can be suppressed.

The alert unit 173 does not need to issue an alarm if scattering occurs outside the time period set according to the board assembly configuration.

<Monitoring equipment>
The monitoring device according to the embodiment of the present invention has the functions of the monitoring unit described above. That is, the monitoring device according to the present embodiment is a monitoring device that monitors expulsion that occurs in metal plates that constitute a sheet assembly during spot welding to manufacture a welded joint, and includes a detection unit that detects the occurrence of expulsion during spot welding, and a determination unit that determines the quality of the welded joint to be defective when expulsion occurs within a time period set according to the sheet assembly configuration.

<Program>
The above-mentioned detection unit and judgment unit judge whether welding is good or bad depending on the timing of expulsion occurrence, thereby improving the accuracy of quality assurance. Therefore, another embodiment of the present invention can be said to be a program that causes a computer to function as a detection unit that detects the occurrence of expulsion in spot welding, and as a judgment unit that judges the quality of a welded joint manufactured by spot welding to be defective when expulsion occurs within a time period set according to the configuration of the plate assembly.

So far, the spot welding device, monitoring device, and program according to the embodiment of the present invention have been described. However, the technical scope of the present invention is not limited to the above embodiment, and various modifications can be made without departing from the spirit of the present invention.

For example, in spot welding, multi-stage current flow may be performed in which the current value is changed during constant current welding, which is performed at a constant current value. In multi-stage current flow, the welding conditions (current value) are changed during current flow, so the determination of expulsion in multi-stage current flow may be performed separately before and after the change in the current value. In addition, the time set for determining that the quality of the welded joint is poor may also be set separately before and after the change in the current value.

In spot welding, the pressure may be changed during current flow. In such cases, as with multi-stage current flow, the time may be set to determine whether expulsion has occurred and whether the quality of the welded joint is poor, dividing the time into before and after the change in pressure.

Even if the multi-stage current and pressure are changed, it is sufficient to carry out test welding in advance and determine the threshold voltage value, resistance value, electrode displacement amount, pressure force, or strain amount when it is determined that expulsion has occurred, and the time to define initial expulsion.

The duration of current flow and the current value are preset as welding conditions, but the current value may be changed successively by feedback control during current flow. In this case, too, test welding may be performed in advance to determine the threshold voltage value, resistance value, electrode displacement amount, pressure force, or strain amount when it is determined that expulsion has occurred, and the time defined as initial expulsion.

Next, examples of the present invention will be described. However, the technical scope of the present invention is not limited to the following examples.

Two TRIP steel sheets with a tensile strength of 980 MPa and a plate thickness of 1.6 mm were stacked and spot welded under the conditions shown in Table 2. Specifically, the current value shown in "Current 1" was set for the time shown in "Current Time 1" in Table 2 from the start of current flow, then the current value shown in "Current 2" was set for the time shown in "Current Time 2", and then the current value shown in "Current 3" was set for the time shown in "Current Time 3". In each test example, the current was set to 10 kA for the time shown in Table 2 to intentionally cause expulsion. The target nugget diameter for each test example is shown in Table 2. Each test was performed three times, and the average nugget diameter was calculated.

The nugget diameter was calculated using the following method: the welded test piece was cut at the center of the weld, the cut surface was mirror-polished, and the weld was observed and measured using an etching solution.

The detection unit judged the occurrence of expulsion based on the following criteria. In the judgment using the voltage value, it was judged that expulsion had occurred when the voltage drop in 5 msec was 20 V/sec or more. The voltage value was measured by attaching a voltmeter to the electrode. In the judgment using the electrode displacement, it was judged that expulsion had occurred when the electrode displacement in 5 msec was 10 (mm/sec) or more. The electrode displacement was calculated using a laser displacement meter. In the judgment using the pressure, it was judged that expulsion had occurred when the drop in pressure in 5 msec was 80 (kN/sec) or more. The pressure was measured by a load cell built into the spot welding device.

In addition, the initial scattering was defined as the scattering that occurred within the range of 32 to 192 msec (average thickness of the steel plate x 20 to average thickness of the steel plate x 120 msec) from the start of energization.

An alert unit is installed in the spot welding device to output an alarm if initial dispersal occurs.

If the difference between the average value of the calculated nugget diameter and the target nugget diameter was 2% or less, the evaluation result was rated as excellent (◎); if it was more than 2% and less than 5%, the evaluation result was rated as good (◯); if it was more than 5%, the evaluation result was rated as poor (×). If the evaluation result was ×, the nugget diameter was too small and the desired strength of the welded joint could not be obtained. Such evaluation was performed by the evaluation unit. The results are shown in Table 3.

As shown in Examples 1 to 12 of Tables 2 and 3, the detection unit was able to detect the occurrence of expulsion based on at least one of the voltage drop, electrode displacement, or pressure drop. In other words, the occurrence of expulsion was able to be detected based on at least one of the voltage value, electrode displacement, or pressure. In addition, the resistance value, strain amount, and appearance of the spot weld also change due to the occurrence of expulsion, so the detection unit can detect the occurrence of expulsion based on these.

In addition, in Examples 2 to 6 and 10, where the time elapsed from the start of current flow to the occurrence of expulsion was in the range of 32 to 192 msec (average thickness of steel plate x 20 to average thickness of steel plate x 120 msec), the reduction in the average nugget diameter compared to the target nugget diameter exceeded 5% (evaluation results unacceptable). In these examples, the average nugget diameter obtained was too small compared to the target nugget diameter, so the strength of the welded joint was too small.

In addition, in Examples 11 and 12, the voltage drop over 5 msec was a value indicating the occurrence of expulsion, but the electrode displacement and pressure drop did not indicate the occurrence of expulsion, and the evaluation results for Examples 11 and 12 were excellent. As shown in Examples 11 and 12, it was found that by using multiple indicators to determine the occurrence of expulsion, it is possible to detect the occurrence of expulsion that reduces the strength of the welded joint with greater accuracy. In Examples 11 and 12, the threshold value for the voltage drop was optimized, making it possible to detect the occurrence of expulsion that reduces the strength of the welded joint using only the voltage value.

As described above, it was found that by detecting initial expulsion rather than simply detecting the occurrence of expulsion, the quality of the welded joint can be determined with high accuracy.

REFERENCE SIGNS LIST 10 Spot welding device 11 Control unit 12 Memory unit 13 Driving unit 14 Current supply unit 15a Movable electrode 15b Fixed electrode 16 Input unit 17 Monitoring unit 171 Detection unit 172 Determination unit 173 Alert unit

Claims (6)

A monitoring device for monitoring splashing occurring in a metal plate constituting a plate assembly during spot welding to manufacture a welded joint,
A detection unit that detects the occurrence of expulsion during the spot welding;
a judgment unit that judges the quality of the welded joint to be defective if expulsion occurs within a time period that is set according to the configuration of the plate assembly. The monitoring device according to claim 1, wherein the detection unit detects the occurrence of shattering using one or more of the voltage value, resistance value, electrode displacement amount, applied pressure, strain amount, and appearance. The monitoring device according to claim 1 or 2 further comprises an alert unit that issues an alarm if scattering occurs within a time period set according to the board assembly configuration. The monitoring device according to claim 3, wherein the alert unit does not issue the alarm if the scattering occurs outside of a time period set according to the board assembly configuration. A spot welding device equipped with the monitoring device according to claim 1 or 2. Computer,
A detection unit that detects the occurrence of expulsion during spot welding;
The program causes the program to function as a judgment unit that judges the quality of the welded joint produced by the spot welding to be defective if expulsion occurs within a time period set according to the configuration of the plate assembly.

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