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 when a plurality of overlapping plate-shaped welding materials are resistance spot welded. [Background technology]

[0002] In resistance spot welding, a welded portion of multiple stacked plate-shaped welding materials is sandwiched between a pair of electrodes and pressure is applied while current is passed through the pair of electrodes to perform resistance welding. Excessive heat generated by this resistance welding can cause spatter, which is scattering of parts of the weld. Spatter can lead to poor welding at or around the weld. Patent Document 1 therefore discloses a method for detecting spatter occurrence during resistance spot welding by detecting changes in electrode displacement, interelectrode voltage, or interelectrode resistance.

[0003] Patent Document 1 discloses a resistance welding method in which a welding current is passed through the contact portion of joined members to heat the contact portion by resistance heating and apply pressure to weld the contact portion, 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 members 1 and 2 to be welded until a preset current flow time T expires. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-105041 Summary of the Invention [Problem to be solved by the invention]

[0005] One possible method for detecting the occurrence of sputtering is to determine that sputtering has occurred when a change in a characteristic such as the amount of electrode displacement, interelectrode voltage, or interelectrode resistance falls below (or exceeds) a judgment threshold (a predetermined value). However, when determining whether or not sputtering has occurred using a single judgment threshold, there is a problem in that if 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 an erroneous determination of whether or not sputtering has occurred.

[0006] The present disclosure has been made to solve such problems, and aims to provide a spatter detection method that accurately determines whether or not spatter has occurred during 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 together, and includes a welding process in which the multiple plate-shaped welding materials are welded together by sandwiching the welding 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 in which an expansion amount of the welded portion 109 is calculated based on the applied pressure and stroke between the pair of electrodes; and a determination process in which, if the magnitude of the slope of an expansion amount waveform indicating the change in the expansion amount over time falls below a predetermined determination threshold, a different determination threshold is applied to each of the sections obtained by dividing a target period for 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. [Effects of the Invention]

[0008] The present disclosure provides a spatter detection method that accurately determines whether or not spatter occurs during resistance spot welding. [Brief explanation of the drawings]

[0009] [Figure 1]FIG. 1 is a schematic diagram of a resistance spot welding system. [Figure 2] FIG. 10 is a diagram showing the change over time in the expansion amount and the slope of the expansion amount waveform. [Figure 3] 10A and 10B are diagrams illustrating the amount of expansion and the change over time in the slope of the expansion amount waveform, for explaining the spatter detection method according to the embodiment. [Figure 4] 10 is a diagram showing the change over time in the amount of expansion and the slope of the expansion amount waveform, for explaining the spatter detection method of Comparative Example 1. FIG. [Figure 5] 10 is a diagram showing the change over time in the amount of expansion and the slope of the expansion amount waveform, for explaining the spatter detection method of Comparative Example 2. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0010] Embodiment 1 Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. However, the present disclosure is not limited to the following embodiments. Furthermore, for clarity of explanation, the following description and drawings have been simplified as appropriate. In the following description, identical or equivalent elements are designated by the same reference numerals, and redundant explanations will be 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 arranged in an overlapping relationship 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 stacked one on top of the other.

[0012] The upper movable electrode 101 moves up and down as shown by the arrows, thereby applying 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 is measured by the lower fixed electrode 103. A processing device such as a computer (not shown) uses these measurements 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 shows the change in the amount of expansion over time.

[0014] Fig. 2 is a diagram showing the change over time in the expansion amount and the slope of the expansion amount waveform. The left side of Fig. 2 is a graph showing the change over time in the expansion amount and the slope of the expansion amount waveform when spatter occurs in the first half of the current application time and when no spatter occurs. The right side of Fig. 2 is a graph showing the change over time in the expansion amount and the slope of the expansion amount waveform when spatter occurs near the end of the current application time and when no spatter occurs. With reference to Fig. 2, the trends in the expansion amount and the expansion amount waveform when spatter occurs and when no spatter occurs will be described.

[0015] In each graph in Figure 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 Figure 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, the vertical axis represents 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 applied pressure into strain, and can be set to any constant.

[0017] In this way, by using the stroke measured by the upper movable electrode 101 and the pressure force 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 Figure 2, when current begins to flow, spatter may occur in the section from time t0 to time t1. The amount of expansion changes (decreases) abruptly when spatter occurs. Therefore, spatter can be detected from the slope of the expansion amount waveform. To detect spatter, for example, a method can be considered in which spatter is determined to have occurred when the magnitude of the slope of the expansion amount waveform falls below a preset determination threshold.

[0020] As shown on the right side of FIG. 2, the welded portion 109 of the welding material contracts as the current flow ends, causing a change (decrease) in the amount of expansion due to the contraction of the welding material immediately after the end of the current flow time. Therefore, the expansion amount and the slope of the expansion amount waveform decrease from around time t1, as shown by the gray solid line. If spatter occurs just before the end of the current flow time, the change in the expansion amount due to the contraction of the welding material is superimposed on the change in the expansion amount due to the generation of spatter. When these two changes in expansion amount are superimposed, the expansion amount 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 spatter has occurred, there is a problem that the accuracy of spatter detection decreases. As an example of when such a problem occurs, 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 illustrating the change over time in the amount of expansion and the slope of the expansion amount waveform, illustrating 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 determine 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 generation 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 using the spatter detection method of Comparative Example 1, the slope of the expansion amount waveform, which shows the change in the expansion amount due to spatter generated in the first half of the current flow period 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, which shows the change in the expansion amount due to contraction of the welding material generated in the period 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 prevent the contraction of the welding material generated immediately after the end of the current flow period from being erroneously determined to be spatter, but it has the problem of being unable to detect spatter generated in the first half of the current flow period.

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

[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, illustrating 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 determine 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, which shows the change in the expansion amount due to spatter generated in the first half of the current flow period 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, which shows the change in the expansion amount due to contraction of the welding material generated in the period from time t1 to time t2, is also below the judgment threshold. Therefore, while the spatter detection method of Comparative Example 2 can detect spatter generated in the first half of the current flow period, there is a problem in that it erroneously determines that contraction of the welding material generated immediately after the end of the current flow period 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, which is a superposition of the change in expansion amount due to spatter generated just before the end of the welding time in the section from time t0 to time t1 and the change in expansion amount due to 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 just before the end of the welding 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 determination process.

[0029] In the welding process, a plurality of plate-shaped welding materials are welded by sandwiching a welding 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 welding portion 109 is calculated based on the applied pressure and stroke between the pair of electrodes. In the determination process, it is determined 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. In the determination process, a period of time to be determined is divided into at least two sections, and different determination thresholds are applied to each section, 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, a weld 109 of the metal plates 105 and 107 is clamped and pressed between the upper movable electrode 101 and the lower fixed electrode 103 of the resistance spot welding system 100, 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 weld 109 of the welding material sandwiched between the upper movable electrode 101 and the lower fixed electrode 103 expands. After current is passed, the weld 109 of the welding material contracts while the upper movable electrode 101 and the lower fixed electrode 103 are held in place until they are released. In the welding process, the weld 109 solidifies, forming a nugget between the metal plates 105 and 107, and the metal plates 105 and 107 are welded together.

[0031] Next, in the calculation step, an expansion amount waveform showing the time change in the expansion amount of the welded portion 109 is calculated from the measured values ​​of the stroke and the applied pressure. The expansion amount of the welded portion 109 can be calculated by the above formula (1) using the stroke measured at the upper movable electrode 101 and the applied pressure 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 to explain the spatter detection method according to the embodiment.

[0032] The determination step will be described in detail 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 energization time, and a second section from the end of the energization time to the end of a holding time during 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 energization time, but also sputters that occur just before the end of the energization time.

[0033] In the determination step, different first and second determination thresholds are applied as determination thresholds to the first and second sections. Specifically, in the determination step, the first determination threshold is applied to the first section, and the second determination threshold, which 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, if the magnitude of the slope of the expansion amount waveform falls below −0.002 mm / ms in the first section, it is determined that spatter has occurred. 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 spatter occurs.

[0035] In the determination step, it is determined that spatter has 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 generation of spatter 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 of this embodiment, the slope of the expansion amount waveform, which shows 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, which shows the change in the expansion amount due to 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 of this embodiment, it is possible to detect spatter generated in the first half of the current flow time while suppressing the erroneous determination that 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 of this embodiment, the slope of the expansion amount waveform, which is a superposition of the change in the expansion amount due to spatter generated just before the end of the current flow time in the first section and the change in the expansion amount due to contraction of the welding material generated in the second section, is below the second judgment threshold. Therefore, the spatter detection method of this embodiment can also detect spatter generated just before the end of the current flow time.

[0038] As described above, the spatter detection method according to this embodiment determines whether spatter has occurred by comparing the slope of the expansion amount waveform with a plurality of determination thresholds that are set to different values ​​depending on the timing of spatter occurrence. Therefore, the spatter detection method according to this embodiment can accurately determine whether spatter has occurred during resistance spot welding.

[0039] The present disclosure is not limited to the above-described embodiment, and can be modified as appropriate within the 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 pressed while passing current through 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; a determination step of determining that the sputtering has occurred when the magnitude of the slope of the expansion amount waveform, which indicates the time change of the expansion amount, is below a predetermined determination threshold value, In the determination step, a different determination threshold is applied to each of the intervals obtained by dividing the period to be determined into at least two intervals, and the magnitude of the slope of the expansion amount waveform is compared with the determination threshold corresponding to each interval.

2. In the determination step, Dividing the target period into two sections, a first section from the start to the end of a current-carrying time and a second section from the end of the current-carrying 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 second section is applied with a decision threshold that is smaller than the decision threshold applied to the first section.

3. The expansion amount is E=S+a×F...(1) 2. The spatter 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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