Spatter detection method

Abstract

To provide a spatter detection method which can accurately determine whether spatter is generated or not when a resistance spot weld is performed.SOLUTION: A spatter detection method detecting spatter generated when performing a resistance spot weld on a plurality of overlapped tabular weld materials comprises steps of: welding the plurality of tabular weld materials through sandwiching and pressing a weld zone of the weld materials between a pair of electrodes and energizing the pair of electrodes; calculating expansion amount of the weld zone based on applied pressure and stroke between the pair of electrodes; and determining generation of the spatter when magnitude of the inclination of the expansion amount waveform representing time change of the expansion amount is less than a determination threshold value set in advance. The determination step includes applying the determination threshold values different for each of separate sections where a target period to be determined is divided into at least two sections, and comparing the determination threshold value with the magnitude of the inclination of the expansion amount waveform corresponding to each of the sections.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present disclosure relates to a spatter detection method, and more particularly to a spatter detection method for detecting spatter generated during resistance spot welding of a plurality of overlapping plate-shaped welding materials. [Background technology]

[0002] In resistance spot welding, the welded portion of a plurality of overlapping plate-like welding materials is sandwiched between a pair of electrodes and pressure is applied to the pair of electrodes, thereby performing resistance welding. If the heat generated by this resistance welding becomes excessive, spattering may occur, causing parts of the welded portion to fly off. If spattering occurs, there is a risk of poor welding at the welded portion or its surroundings. Therefore, Patent Document 1 discloses a method for detecting the occurrence of spattering by detecting changes in the amount of electrode displacement, interelectrode voltage, or interelectrode resistance in resistance spot welding.

[0003] Patent Document 1 discloses a resistance welding method in which a welding current is passed through the contact part of joined parts to resistively heat the contact part and to apply pressure to weld the contact part, and if the generation of spatter is detected during current flow, a current (Iw+Iα) obtained by adding a predetermined amount of current Iα to a preset welding current Iw is reset as the welding current, and this current (Iw+Iα) is passed through the parts to be welded 1, 2 until a preset current flow time T expires. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2008-105041 A Summary of the Invention [Problem to be solved by the invention]

[0005] As a method for detecting the occurrence of sputtering, for example, it is considered to determine that sputtering has occurred when a change in a characteristic such as an amount of electrode displacement, an interelectrode voltage, or an interelectrode resistance falls below (or exceeds) a judgment threshold (a predetermined value). However, when a single judgment threshold is used to determine whether or not sputtering has occurred, there is a problem that when a change in a characteristic resulting from the occurrence of sputtering is superimposed on a change in a characteristic resulting from a factor other than the occurrence of sputtering, the amount of change may become large, leading to a risk of erroneously determining whether or not sputtering has occurred.

[0006] The present disclosure has been made to solve such problems, and has an object to provide a spatter detection method that accurately determines whether or not spatter has occurred when performing resistance spot welding. [Means for solving the problem]

[0007] A spatter detection method for resistance spot welding according to one embodiment is a spatter detection method for detecting spatter generated when multiple stacked plate-shaped welding materials are resistance spot welded, and includes a welding process for welding the multiple plate-shaped welding materials by sandwiching the welded portion of the welding material between a pair of electrodes and applying pressure to the pair of electrodes while passing current through the pair of electrodes, a calculation process for calculating an expansion amount of the welded portion 109 based on the pressing force and stroke between the pair of electrodes, and a determination process for determining that spatter has occurred when the magnitude of the slope of an expansion amount waveform, which indicates the time change in the expansion amount, falls below a predetermined determination threshold, and in the determination process, a different determination threshold is applied to each section obtained by dividing a period subject to determination into at least two sections, and the magnitude of the slope of the expansion amount waveform corresponding to each section is compared with the determination threshold corresponding to each section. Effect of the Invention

[0008] The present disclosure provides a spatter detection method that accurately determines whether or not spatter has occurred when performing resistance spot welding. [Brief description of the drawings]

[0009] [Figure 1]FIG. 1 is a schematic diagram of a resistance spot welding system. [Diagram 2] FIG. 11 is a diagram showing changes over time in the expansion amount and the slope of the expansion amount waveform. [Diagram 3] 5A and 5B are diagrams illustrating the amount of expansion and the change in the slope of the expansion amount waveform over time, for explaining the sputter detection method according to the embodiment. [Figure 4] 13 is a diagram showing the amount of expansion and the change over time in the slope of the expansion amount waveform, for illustrating the sputter detection method of Comparative Example 1. FIG. [Diagram 5] 13 is a diagram showing the amount of expansion and the change over time in the slope of the expansion amount waveform, illustrating the sputter detection method of Comparative Example 2. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] First embodiment Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings. However, the present disclosure is not limited to the following embodiment. In addition, in order to clarify the description, the following description and drawings are appropriately simplified. In the following description, the same or equivalent elements are given the same reference numerals, and duplicate descriptions are omitted.

[0011] FIG. 1 is a schematic diagram of a resistance spot welding system. The resistance spot welding system 100 will be described with reference to FIG. 1. The resistance spot welding system 100 shown in FIG. 1 includes a pair of electrodes, an upper movable electrode 101 and a lower fixed electrode. Metal plates 105 and 107 to be welded have welded portions 109 that are overlapped and disposed between the upper movable electrode 101 and the lower fixed electrode 103. The movable electrode and the fixed electrode may be upside down. The metal plates 105 and 107 are multiple plate-shaped welding materials that are stacked one on top of the other.

[0012] The upper movable electrode 101 moves up and down as shown by the arrow to apply pressure to the welded portion 109. The upper movable electrode 101 and the lower fixed electrode 103 sandwich the welded portion 109 from the direction in which the metal plates 105 and 107 are stacked. The upper movable electrode 101 and the lower fixed electrode 103 apply pressure to the welded portion 109 while passing a current. The metal plates 105 and 107 are welded by resistance heat generated in the welded portion 109 sandwiched between the upper movable electrode 101 and the lower fixed electrode 103. The welded portion 109 is melted by the resistance heat and then solidifies to form a nugget. Although two metal plates are shown in FIG. 1, three or more metal plates may be used.

[0013] The amount by which the upper movable electrode 101 is pressed into the metal plate 105 after coming into contact with it is called the stroke. The stroke is measured by the upper movable electrode 101. The pressure force is measured by the lower fixed electrode 103. A processing device such as a computer (not shown) uses these measured values ​​to calculate the amount of expansion of the welded portion 109. Then, based on the calculated amount of expansion, the processing device generates an expansion amount waveform that indicates the change in the amount of expansion over time.

[0014] FIG. 2 is a diagram showing the time change of the expansion amount and the slope of the expansion amount waveform. The left side of FIG. 2 is a graph showing the time change of the expansion amount and the slope of the expansion amount waveform for each of the cases where sputtering occurs in the first half of the current flow time and where sputtering does not occur. The right side of FIG. 2 is a graph showing the time change of the expansion amount and the slope of the expansion amount waveform for each of the cases where sputtering occurs near the end of the current flow time and where sputtering does not occur. The tendency of the expansion amount and the expansion amount waveform for each of the cases where sputtering occurs and where sputtering does not occur will be described with reference to FIG. 2.

[0015] In each graph in Fig. 2, the amount of expansion and the slope of the expansion waveform over time when spattering occurs are shown by a solid black line. In each graph in Fig. 2, the amount of expansion and the slope of the expansion waveform over time when spattering does not occur are shown by a solid gray line.

[0016] 2 indicates the amount of expansion of the welded portion 109 calculated from the measured values ​​of the stroke and pressure. The amount of expansion of the welded portion 109 is calculated by the following formula (1). E = S + a × F (1) Here, E is the amount of expansion, S is the stroke, a is a strain conversion coefficient, and F is the applied pressure. The strain conversion coefficient a is a coefficient for converting the applied pressure into a strain amount, and can be an arbitrary constant.

[0017] In this way, by using the stroke measured by the upper movable electrode 101 and the pressure measured by the lower fixed electrode 103, the expansion amount can be calculated more accurately than by calculating each independently. The slope of the expansion amount waveform, which shows the change in the expansion amount over time, is the amount of change in the expansion amount per unit time.

[0018] Time t0 indicates the start of the energization time. Time t1 indicates the end of the energization time. Time t1 also indicates the start of the retention time during which the upper movable electrode 101 and the lower fixed electrode 103 are held until they are released. Time t2 indicates the end of the retention time during which the upper movable electrode 101 and the lower fixed electrode 103 are held until they are released.

[0019] As shown on the left side of FIG. 2, when current begins to flow, spatters may occur in the section from time t0 to time t1. When spatters occur, the amount of expansion changes (decreases) suddenly. Therefore, it is possible to detect spatters from the slope of the expansion amount waveform. To detect spatters, for example, a method can be considered in which it is determined that spatters have occurred when the magnitude of the slope of the expansion amount waveform falls below a preset determination threshold value.

[0020] As shown on the right side of FIG. 2, the welded portion 109 of the welding material shrinks with the end of the current flow, and a change (decrease) in the amount of expansion due to the shrinkage of the welding material occurs immediately after the end of the current flow time. Therefore, the amount of expansion and the slope of the expansion amount waveform decrease from around time t1, as shown by the gray solid line. Here, if spatter occurs just before the end of the current flow time, the change in the amount of expansion due to the shrinkage of the welding material is superimposed on the change in the amount of expansion due to the generation of spatter. When these two changes in the amount of expansion are superimposed, the amount of expansion and the slope of the expansion amount waveform change (decrease) suddenly from time t1, as shown by the black solid line. The minimum value of the slope of the expansion amount waveform in the section from time t1 to time t2 is significantly smaller than the minimum value of the slope of the expansion amount waveform in the section from time t0 to time t1.

[0021] Therefore, when one cycle of the expansion amount waveform including the period from time t0 to time t2 is set as the target period for judgment, if a single judgment threshold is applied to the target period to judge whether or not spatters have occurred, there is a problem that the detection accuracy of spatters is reduced. As an example of the occurrence of such a problem, the spatter detection methods of Comparative Examples 1 and 2, in which a single judgment threshold is applied to the target period, will be specifically described.

[0022] FIG. 4 is a diagram showing the change over time in the amount of expansion and the slope of the expansion amount waveform to explain the spatter detection method of Comparative Example 1. In the spatter detection method of Comparative Example 1 shown in FIG. 4, a preset judgment threshold of -0.006 mm / ms is applied to the target period. The spatter detection method of Comparative Example 1 can judge that spatter has occurred when the magnitude of the slope of the expansion amount waveform falls below -0.006 mm / ms. The judgment threshold of -0.006 mm / ms is a value that is a predetermined percentage larger than the minimum value of the slope of the expansion amount waveform when the change in the expansion amount due to the occurrence of spatter and the change in the expansion amount due to the contraction of the welding material are superimposed.

[0023] As shown by the black solid line on the left side of Fig. 4, when spatter detection is performed by the spatter detection method of Comparative Example 1, the slope of the expansion amount waveform showing the change in the expansion amount due to spatter generated in the first half of the current flow time in the section from time t0 to time t1 is equal to or greater than the judgment threshold. Also, as shown by the gray solid line on the left side of Fig. 4, the slope of the expansion amount waveform showing the change in the expansion amount due to the contraction of the welding material generated in the section from time t1 to time t2 is also equal to or greater than the judgment threshold. Therefore, the spatter detection method of Comparative Example 1 is able to suppress erroneous determination of the contraction of the welding material generated immediately after the end of the current flow time as spatter, but has the problem of being unable to detect spatter generated in the first half of the current flow time.

[0024] On the other hand, as shown by the black solid line on the right side of Fig. 4, the change in the amount of expansion caused by the spatters generated near the end of the current flow time in the section from time t0 to time t1 is superimposed on the change in the amount of expansion caused by the contraction of the welding material. Therefore, when spatters are detected using the spatter detection method of Comparative Example 1, the slope of the expansion amount waveform in which these two changes in the amount of expansion are superimposed in the section from time t1 to time t2 is below the judgment threshold. Therefore, the spatter detection method of Comparative Example 1 can detect spatters generated near the end of the current flow time.

[0025] Next, FIG. 5 is a diagram showing the change over time in the amount of expansion and the slope of the expansion amount waveform to explain the spatter detection method of Comparative Example 2. In the spatter detection method of Comparative Example 2 shown in FIG. 5, a preset judgment threshold of -0.002 mm / ms is applied to the target period. The spatter detection method of Comparative Example 2 can judge that spatter has occurred when the slope of the expansion amount waveform falls below -0.002 mm / ms. The judgment threshold of -0.002 mm / ms is a value that is a predetermined percentage larger than the minimum value of the slope of the expansion amount waveform when a change in the expansion amount due to the occurrence of spatter occurs.

[0026] As shown by the black solid line on the left side of Fig. 5, when spatter detection is performed using the spatter detection method of Comparative Example 2, the slope of the expansion amount waveform showing the change in the expansion amount due to spatter generated in the first half of the current flow time in the section from time t0 to time t1 is below the judgment threshold. Also, as shown by the gray solid line on the left side of Fig. 5, the slope of the expansion amount waveform showing the change in the expansion amount due to the contraction of the welding material generated in the section from time t1 to time t2 is also below the judgment threshold. Therefore, the spatter detection method of Comparative Example 2 can detect spatter generated in the first half of the current flow time, but has a problem in that it erroneously determines that the contraction of the welding material generated immediately after the end of the current flow time is spatter.

[0027] On the other hand, as shown by the black solid line on the right side of Fig. 5, when spatter detection is performed using the spatter detection method of Comparative Example 2, the slope of the expansion amount waveform in which the change in the expansion amount due to spatter generated near the end of the current flow time in the section from time t0 to time t1 and the change in the expansion amount due to the contraction of the welding material generated in the section from time t1 to time t2 are superimposed is also below the judgment threshold. Therefore, the spatter detection method of Comparative Example 2 can detect spatter generated near the end of the current flow time.

[0028] In contrast, the spatter detection method according to the present embodiment is a spatter detection method for detecting spatter generated when multiple stacked plate-shaped welding materials are resistance spot welded, and includes the following welding process, calculation process, and judgment process.

[0029] In the welding process, a plurality of plate-shaped welding materials are welded by sandwiching a welded portion 109 of the welding material between a pair of electrodes and applying pressure to the pair of electrodes while passing current through the pair of electrodes. In the calculation process, the amount of expansion of the welded portion 109 is calculated based on the pressure and stroke between the pair of electrodes. In the determination process, when the magnitude of the slope of an expansion amount waveform showing the time change of the expansion amount falls below a preset determination threshold, it is determined that spattering has occurred. In the determination process, different determination thresholds are applied to each of sections obtained by dividing the target period of determination into at least two sections, and the magnitude of the slope of the expansion amount waveform is compared with the determination threshold corresponding to each section.

[0030] First, in the welding process, the welded portion 109 of the metal plates 105 and 107 is pressed and sandwiched between the upper movable electrode 101 and the lower fixed electrode 103 of the resistance spot welding system 100 described above, and current is passed through the upper movable electrode 101 and the lower fixed electrode 103 to weld the metal plates 105 and 107. In the welding process, when current is passed, the welded portion 109 of the welding material sandwiched between the upper movable electrode 101 and the lower fixed electrode 103 expands. Then, after current is passed, the welded portion 109 of the welding material contracts while the upper movable electrode 101 and the lower fixed electrode 103 are held until they are released. In the welding process, the welded portion 109 solidifies to form a nugget between the metal plates 105 and 107, and the metal plates 105 and 107 are welded.

[0031] Next, in the calculation step, an expansion amount waveform showing a time change in the expansion amount of the welded portion 109 is calculated from the measured values ​​of the stroke and the pressing force. The expansion amount of the welded portion 109 may be obtained by the above formula (1) using the stroke measured at the upper movable electrode 101 and the pressing force measured at the lower fixed electrode 103. Fig. 3 is a diagram showing the time change in the expansion amount and the slope of the expansion amount waveform for explaining the spatter detection method according to the embodiment.

[0032] The determination step will be specifically described with reference to Fig. 3. As shown in Fig. 3, in the determination step, it is preferable to divide the target period into two sections: a first section from the start to the end of the current-flow time, and a second section from the end of the current-flow time to the end of a holding time in which the upper movable electrode 101 and the lower fixed electrode 103 are held until they are released. That is, the target period is the period from time t0 to time t2. The first section is preferably the section from time t0 to time t1, and the second section is preferably the section from time t1 to time t2. This makes it possible to detect with high accuracy not only sputters that occur in the first half of the current-flow time, but also sputters that occur just before the end of the current-flow time.

[0033] In the determination step, a first determination threshold and a second determination threshold that are different from each other are applied as determination thresholds to the first and second sections. In detail, in the determination step, the first determination threshold is applied to the first section, and the second determination threshold that is smaller than the first determination threshold is applied to the second section. In the example shown in FIG. 3, the first determination threshold is −0.002 mm / ms, and the second determination threshold is −0.006 mm / ms.

[0034] In the determination step, it is determined that sputtering has occurred when the magnitude of the slope of the expansion amount waveform falls below -0.002 mm / ms in the first section. Note that the first determination threshold value of -0.002 mm / ms is, for example, a value that is a predetermined percentage larger than the minimum value of the slope of the expansion amount waveform when a change in the expansion amount due to the occurrence of sputtering occurs.

[0035] In the determination step, it is determined that spatters have occurred when the magnitude of the slope of the expansion amount waveform falls below -0.006 mm / ms in the second section. Note that the second determination threshold value of -0.006 mm / ms is, for example, a value that is a predetermined percentage larger than the minimum value of the slope of the expansion amount waveform when the change in the expansion amount due to the occurrence of spatters and the change in the expansion amount due to the contraction of the welding material are superimposed.

[0036] As shown by the black solid line on the left side of Fig. 3, when spatter detection is performed using the spatter detection method according to this embodiment, the slope of the expansion amount waveform showing the change in the expansion amount due to spatter generated in the first half of the current flow time in the first section is below the first judgment threshold. On the other hand, as shown by the gray solid line on the left side of Fig. 3, the slope of the expansion amount waveform showing the change in the expansion amount due to the contraction of the welding material generated in the second section is equal to or greater than the second judgment threshold. Therefore, according to the spatter detection method according to this embodiment, it is possible to detect spatter generated in the first half of the current flow time while suppressing the erroneous determination that the contraction of the welding material generated immediately after the end of the current flow time is spatter.

[0037] Furthermore, as shown by the black solid line on the right side of Fig. 3, when spatter detection is performed using the spatter detection method according to this embodiment, the slope of the expansion amount waveform in which the change in the expansion amount due to the spatter generated near the end of the current flow time in the first section and the change in the expansion amount due to the contraction of the welding material generated in the second section are superimposed is below the second judgment threshold value. Therefore, the spatter detection method according to this embodiment can detect spatter generated near the end of the current flow time.

[0038] As described above, in the spatter detection method according to the present embodiment, whether or not spatter has occurred is determined based on a comparison between a plurality of determination thresholds that are set to different values ​​according to the timing at which spatter occurs and the slope of the expansion amount waveform. Therefore, the spatter detection method according to the present embodiment makes it possible to accurately determine whether or not spatter has occurred when performing resistance spot welding.

[0039] The present disclosure is not limited to the above-described embodiment, and can be modified as appropriate without departing from the spirit and scope of the present disclosure. [Explanation of symbols]

[0040] 100 Resistance Spot Welding System 101 Upper movable electrode 103 Lower fixed electrode 105 Metal plate 107 Metal plate 109 Welded Parts

Claims

1. A spatter detection method for detecting spatter generated when resistance spot welding a plurality of stacked plate-shaped welding materials, comprising: a welding process in which a welded portion of the welding material is sandwiched between a pair of electrodes and pressure is applied to the pair of electrodes while current is applied to the pair of electrodes to weld the plurality of plate-shaped welding materials; a calculation step of calculating an expansion amount of the welded portion based on a pressure and a stroke between the pair of electrodes; and a determination step of determining that the sputtering has occurred when a magnitude of a slope of an expansion amount waveform showing a time change of the expansion amount falls below a preset determination threshold value, In the judgment process, a different judgment threshold is applied to each of the intervals obtained by dividing the period to be judged into at least two, and the judgment threshold corresponding to each interval is compared with the magnitude of the slope of the expansion amount waveform.

2. In the determination step, Dividing the target period into two sections, a first section from the start to the end of a current-flow time and a second section from the end of the current-flow time to the end of a holding time in which the pair of electrodes are held until they are released, The sputter detection method according to claim 1 , wherein the decision threshold applied to the second section is smaller than the decision threshold applied to the first section.

3. The expansion amount is E=S+a×F...(1) 2. The sputter detection method according to claim 1, wherein E is the expansion amount, S is the stroke, a is a distortion amount conversion coefficient, and F is the applied pressure.

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