Inspection method

Abstract

Provided is an inspection method capable of discriminating defective products with high accuracy even in the presence of interference. The inspection method according to the present disclosure is an inspection method for spot welding in which an electrode sandwiching a steel plate is energized, the amount of contraction of the steel plate is calculated at the electrode on the basis of the amount of displacement between the electrodes and the pressure after energization is completed, and the spot welding is determined to be defective when the amount of contraction is equal to or less than a determination threshold value. Furthermore, in the inspection method according to the present disclosure, the amount of swelling of the steel sheet is calculated at the electrodes on the basis of the pressing force from the start of energization to the end of energization and the time change in the amount of displacement between the electrodes, and the amount of shrinkage is calculated on the basis of the difference between the maximum value and the minimum value of the swelling amount.

Description

Technical Field

[0001] The present disclosure relates to an inspection method, and in particular to an inspection method for spot welding. Background Art

[0002] Patent Document 1 (International Publication No. 2016 / 174842) discloses a welding method for joining overlapping steel plates using spot welding. Summary of the invention

[0003] Patent Document 1 discloses that, in joining using spot welding, welding is performed based on a time variation curve of instantaneous calorific value per unit volume and a cumulative calorific value stored as target values ​​in test welding.

[0004] In addition, Patent Document 1 discloses that in any step, when the time variation of the instantaneous calorific value per unit volume deviates from the time variation curve serving as a reference due to the generation of spatter, the deviation is compensated for during the remaining energization time of the step. In addition, Patent Document 1 discloses that the amount of energization is controlled so that the cumulative calorific value in the actual welding is consistent with the cumulative calorific value in the test welding, and when the generation of spatter is detected in any step, the target value of the subsequent cumulative calorific value is lowered.

[0005] Patent Document 1 discloses a method of setting a threshold value for determining an abnormal value of the cumulative heat amount after spattering occurs to perform correction control, which is also used in an inspection method for determining poor welding. On the other hand, in a sample with large external interference, i.e., large disturbance, with respect to the operation of the manufacturing equipment, the expansion amount during power-on varies depending on the intensity of the disturbance, and therefore it is difficult to set a threshold value for determining an abnormal value.

[0006] The present disclosure has been made to solve such a problem, and an object of the present disclosure is to provide an inspection method capable of discriminating defective products with high accuracy even in the presence of disturbance.

[0007] The inspection method disclosed herein is a method for inspecting spot welding by energizing electrodes sandwiching steel plates, wherein the shrinkage of the steel plates is calculated based on the applied force after the energization and the displacement between the electrodes, and when the shrinkage is below a discrimination threshold, the spot welding is judged to be defective. Thus, an inspection method capable of discriminating defective products with high accuracy can be provided.

[0008] Alternatively, at the electrodes, the expansion of the steel sheet may be calculated based on the time variation of the applied force and the inter-electrode displacement from the start of energization to the end of energization, and the shrinkage may be calculated based on the difference between the maximum and minimum values ​​of the expansion. Thus, an inspection method capable of distinguishing defective products even in the presence of interference can be provided.

[0009] According to the present disclosure, it is possible to provide an inspection method capable of discriminating defective products with high accuracy even in the presence of disturbance.

[0010] The above and other objects, features and advantages of the present disclosure will be more fully understood from the detailed description given below and the accompanying drawings which are given by way of example only and thus should not be considered as limiting the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1A This is a schematic diagram of spot welding according to this embodiment.

[0012] Figure 1B This is a schematic diagram of spot welding according to this embodiment.

[0013] Figure 1C This is a schematic diagram of spot welding according to this embodiment.

[0014] Figure 2A It is a diagram showing the expansion / contraction waveform of this embodiment.

[0015] Figure 2B is a cross-sectional view of a spot-welded sample.

[0016] Figure 3 It is a diagram for explaining failure determination in this embodiment.

[0017] Figure 4A It is a diagram for explaining spot welding in the presence of interference according to the present embodiment.

[0018] Figure 4B It is a diagram for explaining spot welding in the presence of interference according to the present embodiment.

[0019] Figure 4C It is a diagram for explaining spot welding in the presence of interference according to the present embodiment.

[0020] Figure 5A It is a diagram for explaining failure determination in the presence of disturbance according to the present embodiment.

[0021] Figure 5B It is a diagram for explaining failure determination in the presence of disturbance according to the present embodiment. DETAILED DESCRIPTION

[0022] Hereinafter, the present disclosure will be described with reference to the accompanying drawings. Figure 1A ~C is a schematic diagram of spot welding of the present disclosure.

[0023] Spot welding is performed by sandwiching the overlapping steel plates 11 and 12 by a pair of electrodes 21 and 22 (see Figure 1A ) and conduct electricity while applying pressure, thereby forming a nugget 30 and performing joining (refer to Figure 1B ). When the power is turned on, the nugget 30 starts to expand, resulting in a temperature rise and an expansion of the current-carrying area due to an increase in resistance. When the power is turned off, the nugget 30 shrinks, the resistance decreases, and due to the shrinkage, a portion of the electrodes 21 and 22 sinks into the steel plates 11 and 12 (see Figure 1C ).

[0024] Figure 2A The expansion / contraction amount of the steel plates 11 and 12, especially the expansion / contraction amount of the nugget 30, during the spot welding, i.e., from the start to the end of the energization, is obtained based on the measured values ​​of the applied force and the displacement between the electrodes 21 and 22, and the time variation thereof is plotted. Figure 2B Each cross-sectional view showing a good / bad sample subjected to spot welding. In the expansion section when power is applied, the nugget 30 expands between the steel plates 11 and 12, and in the contraction period when power is applied and the temperature drops, the nugget 30 contracts. At this time, the sample in which the nugget 30 expands appropriately is judged as a good product. On the other hand, the sample in which the nugget 30 hardly expands or the nugget 30 is not formed is judged as a bad product. In this way, the good product has the characteristic that the nugget 30 expands significantly compared with the bad product.

[0025] Figure 3 The integral value of the expansion amount in the expansion interval is calculated for multiple samples, and the integral value is plotted on the vertical axis (unit: mm•s / 1000), and the difference between the maximum and minimum expansion amounts of the calculated interval, that is, the shrinkage amount, is plotted on the horizontal axis (unit: √mm / 1000). Figure 3 In the figure, the integral value and the shrinkage amount are divided by 1000 for adjusting the number of digits in sample management, and are not essential processing. It can be seen that there is a clear difference in the integral value of the expansion amount between the sample in which the nugget 30 expands appropriately and the sample in which the nugget 30 expands almost not and is a defective product, and the deviation of the integral value of the expansion amount of the sample determined as a good product is small.

[0026] Therefore, by setting the discrimination threshold of the discrimination parameter as the discrimination parameter of the integral value of the expansion amount, it is possible to discriminate defective products. The standard deviation σ of the deviation of good products is used to set the discrimination threshold. For example, Figure 3 In the above analysis, the discrimination threshold is set to 5σ, i.e., about 0.060, and samples below this discrimination threshold are judged as defective products.

[0027] As described above, the inspection method using the abnormal value of the integrated value of the expansion amount as a discrimination parameter for identification is very useful because it can detect welding defects such as spot welding during the manufacturing process, that is, during processing.

[0028] Next, as an example of interference in the spot welding process, Figure 4A C to illustrate the situation where the hard plate gap enters between the steel plates. As an example, Figure 4A The spot welding is shown in a state where a plurality of plate gaps 40 enter between the three steel plates 11 , 12 , and 13 .

[0029] If the plate gap 40 is soft, the plate gap can be crushed at the time when the steel plates 11, 12, and 13 are overlapped. However, when a hard plate gap 40 enters, the plate gap 40 cannot be completely crushed, and a shunt occurs around the electrode, and the steel plates 11, 12, and 13 soften, and then the plate gap 40 is crushed. When the expansion / contraction amount of the weld nugget of such a sample is plotted, as shown in FIG. Figure 4B As shown by the solid line, the expansion amount decreases immediately after power is applied, and then begins to expand. Therefore, the waveform of the expansion interval changes greatly, such as having a negative expansion amount compared to the normal state.

[0030] Furthermore, the degree of reduction in the expansion amount immediately after power-on also varies depending on the number of plate gaps 40 and the degree of crushing. Therefore, the deviation in the expansion amount plot of the sample with the plate gaps 40 is very large compared to the expansion amount plot of the sample without the plate gaps 40.

[0031] exist Figure 4C , the integral value of the expansion amount in the expansion interval at this time is plotted on the vertical axis, and the difference between the maximum and minimum values ​​of the expansion amount in the interval calculated, that is, the contraction amount, is plotted on the horizontal axis. In addition, the integral value of the expansion amount and the calculation and processing of the contraction amount are similar to those in FIG. Figure 3 The expansion amount of the sample entering the plate gap 40 has a large deviation in the plot, so the deviation in the integral value of the expansion amount also increases. In addition, since there is no clear difference in the integral value of the expansion amount between good products and defective products, it is difficult to use the integral value of the expansion amount as a discrimination parameter and to discriminate defective products by setting a discrimination threshold value based on the standard deviation σ of the deviation of good products. In this way, Figure 4B The waveform in the expansion period shown is greatly affected by the noise. On the other hand, the inventors of the present invention have found that the influence of the noise on the period after the end of the energization, that is, the contraction period, is small.

[0032] Taking advantage of this, Figure 5A As shown in FIG. 1 , the difference between the maximum value and the minimum value during the contraction period is taken as the contraction amount, and the contraction amount is used as a discrimination parameter. Figure 5BThe shrinkage amount in the expansion interval is calculated for multiple samples, and the integral value is plotted on the vertical axis, and the difference between the maximum and minimum expansion values ​​of the calculated interval, that is, the shrinkage amount, is plotted on the horizontal axis. In addition, the calculation and processing of the integral value of the expansion amount and the shrinkage amount are similar to Figure 3 It is known that even in the presence of interference such as the plate gap 40 entering, there is a clear difference in the shrinkage amount between the samples that are good products and the samples that are defective products, and it is also known that the deviation of the shrinkage amount of the samples that are judged as good products is small. Therefore, the standard deviation σ of the deviation of good products can be used as the setting of the discrimination threshold to clearly discriminate defective products.

[0033] In this way, it is possible to provide an inspection method that can accurately identify defective products even in the presence of noise.

[0034] In addition, the present disclosure is not limited to the above-described embodiments, and can be appropriately modified within a scope not departing from the gist.

Claims

1. An inspection method for spot welding in which electrodes sandwiching steel sheets are energized, wherein: At the electrodes, the shrinkage of the steel plate is calculated based on the applied pressure after the power is turned on and the displacement between the electrodes. When the shrinkage amount is equal to or less than the determination threshold, it is determined that the spot welding is defective.

2. The inspection method according to claim 1, At the electrodes, the expansion amount of the steel plate is calculated based on the time changes of the applied force and the displacement between the electrodes from the start to the end of energization, The shrinkage amount is calculated based on the difference between the maximum value and the minimum value of the expansion amount.

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