Inspection method

Abstract

To provide an inspection method capable of accurately discriminating a defective article even under the presence of disturbance.SOLUTION: According to the present disclosure, an inspection method for spot welding of energizing electrodes 21, 22 sandwiching steel plates 11, 12 includes: calculating a contraction amount of the steel plates at the electrodes from applied force and an inter-electrode displacement amount after an end of the energization; and determining that the spot welding is defective when the contraction amount is equal to or less than a determination threshold value. Furthermore, the inspection method includes: calculating an expansion amount of the steel plates at the electrodes from temporal change of the applied pressure and the inter-electrode displacement amount from the start of energization to the end of energization; and calculating a contraction amount from a difference between a maximal value and a minimal value of the expansion amount.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to inspection methods, and more particularly to inspection methods for spot welds. [Background technology]

[0002] Patent Document 1 discloses a welding method for joining overlapping steel plates by spot welding. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2016 / 174842 Summary of the Invention [Problem to be solved by the invention]

[0004] Patent Document 1 discloses that in joining using spot welding, welding is performed based on the time change curve of the instantaneous heat generation per unit volume for each step and the accumulated heat generation amount stored as target values ​​in test welding.

[0005] Patent Document 1 also discloses that, when the time change in the instantaneous heat generation amount per unit volume deviates from a reference time change curve due to the occurrence of expulsion in any step, the deviation is compensated for within the remaining current flow time of the step. Patent Document 1 further discloses that the current flow is controlled so that the accumulated heat generation amount in main welding matches the accumulated heat generation amount in test welding, and that when the occurrence of expulsion is detected in any step, the target value of the accumulated heat generation amount thereafter is reduced.

[0006] In Patent Document 1, a threshold value is set for determining whether the cumulative heat generation amount is an abnormal value after the occurrence of expulsion, and control is performed to correct the value, and this method is also used in an inspection method for determining whether welding is defective. However, in samples where the operation of the manufacturing equipment is subject to large external interference, that is, where there is a large disturbance, the amount of expansion during current flow varies depending on the strength of the disturbance, making it difficult to set a threshold value for determining whether the abnormal value is a defective value.

[0007] The present disclosure has been made to solve such problems, and has an object to provide an inspection method capable of accurately identifying defective products even in the presence of disturbances. [Means for solving the problem]

[0008] The inspection method according to the present disclosure is a method for inspecting spot welding in which a current is passed through electrodes that sandwich a steel sheet, the method calculates the amount of shrinkage of the steel sheet from the applied pressure and the amount of electrode displacement after the current is stopped in the electrodes, and determines that the spot welding is defective if the amount of shrinkage is equal to or less than a discrimination threshold. In this way, it is possible to provide an inspection method that can accurately determine whether a product is defective.

[0009] In addition, the amount of expansion of the steel sheet may be calculated from the change in the pressure and the inter-electrode displacement over time from the start of energization to the end of energization, and the amount of contraction may be calculated from the difference between the maximum and minimum values ​​of the amount of expansion. In this way, it is possible to provide an inspection method that can identify defective products even in the presence of disturbances. Effect of the Invention

[0010] The present disclosure makes it possible to provide an inspection method that can accurately identify defective products even in the presence of disturbances. [Brief description of the drawings]

[0011] [Figure 1] 1 is a schematic diagram of spot welding according to an embodiment of the present invention; [Diagram 2] FIG. 4 is a diagram showing an expansion / contraction waveform according to the present embodiment. [Diagram 3] 10A and 10B are diagrams illustrating a defect determination according to the present embodiment. [Figure 4] 1 is a diagram illustrating spot welding in the presence of a disturbance according to an embodiment of the present invention; [Diagram 5] 10A and 10B are diagrams illustrating a defect determination in the presence of a disturbance according to the present embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a schematic diagram of spot welding according to an embodiment.

[0013] In spot welding, overlapping steel sheets 11, 12 are sandwiched between a pair of electrodes 21, 22 (see FIG. 1(a)), and a nugget 30 is formed by passing an electric current while applying pressure, and then the sheets are joined (see FIG. 1(b)). When the electric current starts to pass, the nugget 30 starts to expand, causing an increase in resistance, which leads to a rise in temperature and an expansion of the current-carrying area, and when the electric current stops passing, the nugget 30 shrinks, causing a decrease in resistance, and the shrinkage causes parts of the electrodes 21, 22 to sink into the steel sheets 11, 12 (see FIG. 1(c)).

[0014] FIG. 2 shows the amount of expansion / contraction of the steel sheets 11 and 12, particularly the nugget 30, during spot welding, that is, from the start of current application to the end of current application, which is obtained from the measured values ​​of the pressing force and the inter-electrode displacement amount at the electrodes 21 and 22, and plots the time change. Furthermore, FIG. 2 shows each cross-sectional view of a good / bad sample in which spot welding has been performed. In the expansion section in which current is applied, the nugget 30 expands between the steel sheets 11 and 12, and in the contraction period in which current application is terminated and the temperature drops, the nugget 30 contracts. At this time, a sample in which the nugget 30 expands appropriately is judged to be a good product. On the other hand, a sample in which the nugget 30 hardly expands or is not formed is judged to be a bad product. In this way, a good product has a characteristic in which the nugget 30 expands more than a bad product.

[0015] In Fig. 3, the integral value of the amount of expansion in the expansion section was calculated for many samples, and the integral value is plotted on the vertical axis (unit: mm s / 1000) against the amount of shrinkage, which is the difference between the maximum and minimum amount of expansion in the calculated section, on the horizontal axis (unit: √mm / 1000). Note that in Fig. 3, the integral value and the amount of shrinkage are divided by 1000 to adjust the number of digits for sample management purposes, and is not a required process. There is a clear difference in the integral value of the amount of expansion between samples that are good products in which the nugget 30 has expanded appropriately and samples that are defective products in which there has been almost no expansion, and it can be seen that the samples that have been determined to be good products have little variation in the integral value of the amount of expansion.

[0016] Therefore, by using the integral value of the expansion amount as a discrimination parameter and setting a discrimination threshold for the discrimination parameter, it becomes possible to discriminate defective products. The discrimination threshold is set using the standard deviation σ of the variation of non-defective products. For example, in Figure 3, the discrimination threshold is set to about 0.060, which is 5σ, and samples below this discrimination threshold are judged to be defective.

[0017] This inspection method, which uses an abnormal value of the integral value of the expansion amount as a discrimination parameter to perform discrimination, is particularly useful because it makes it possible to inspect for welding defects in spot welding during the manufacturing process, i.e., in-process.

[0018] Next, as an example of a disturbance occurring during the spot welding process, a case where a hard gap occurs between steel plates will be described with reference to Fig. 4. Fig. 4(a) shows, as an example, spot welding in a state where multiple gaps 40 have occurred between three steel plates 11, 12, and 13.

[0019] If the gap 40 is soft, it is possible to close the gap when the steel sheets 11, 12, and 13 are overlapped, but if a hard gap 40 is present, the gap 40 cannot be completely closed, and current shunting occurs around the electrodes, the steel sheets 11, 12, and 13 soften, and then the gap 40 is closed. When the expansion / contraction amount of the nugget of such a sample is plotted, as shown by the solid line in Figure 4(b), the expansion amount first decreases immediately after current is applied and then expansion begins, resulting in a waveform in the expansion section that has changed significantly, such as having a negative expansion amount compared to the normal state.

[0020] In addition, the degree of decrease in the amount of expansion immediately after current is applied varies depending on the number of gaps 40 and the degree of crushing, so plots of the amount of expansion of samples containing gaps 40 will vary greatly compared to plots of the amount of expansion of samples without gaps 40.

[0021] FIG. 4(c) shows a plot of the integral value of the expansion amount in the expansion section at this time on the vertical axis and the contraction amount, which is the difference between the maximum and minimum values ​​of the calculated expansion amount in the section, on the horizontal axis. The calculation and processing of the integral value of the expansion amount and the contraction amount are the same as in FIG. 3. The plot of the expansion amount of the sample into which the plate gap 40 has entered has a large variation, so the integral value of the expansion amount also has a large variation. In addition, since there is no clear difference in the integral value of the expansion amount between good products and defective products, it becomes difficult to distinguish defective products by setting the discrimination threshold value based on the standard deviation σ of the variation of good products using the integral value of the expansion amount as a discrimination parameter. In this way, the waveform of the expansion section shown in FIG. 4(b) is greatly affected by disturbance. On the other hand, the inventors have found that the influence of disturbance on the contraction period after the end of energization, that is, after the end of energization, is small.

[0022] Taking advantage of this, as shown in Fig. 5(a), the difference between the maximum and minimum values ​​in the contraction period is taken as the contraction amount, and this contraction amount is used as the discrimination parameter. Fig. 5(b) shows the contraction amount in the expansion section calculated for many samples, and the integral value is plotted on the vertical axis, and the contraction amount, which is the difference between the maximum and minimum values ​​of the expansion amount in the calculated section, is plotted on the horizontal axis. The calculation and processing of the integral value of the expansion amount and the contraction amount are the same as in Fig. 3. Even in the presence of a disturbance such as the intrusion of the plate gap 40, there is a clear difference in the contraction amount between samples that are good and samples that are defective, and it can be seen that samples that are judged to be good have little variation in the contraction amount. Therefore, the discrimination threshold can be set to the standard deviation σ of the variation of good products to clearly discriminate between defective products.

[0023] In this way, it is possible to provide an inspection method that can accurately identify defective products even in the presence of external disturbances.

[0024] 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]

[0025] 11, 12, 13 Steel plate 21, 22 electrodes 30 Nuggets 40 Gap

Claims

1. A method for inspecting spot welding in which a current is passed through electrodes sandwiching a steel plate, comprising the steps of: The amount of shrinkage of the steel sheet is calculated from the applied pressure and the amount of electrode displacement after the end of energization of the electrodes. If the shrinkage amount is equal to or less than a discrimination threshold, the spot weld is determined to be defective. Testing method.

2. Calculating an amount of expansion of the steel sheet from a time change in the pressing force and the amount of displacement between the electrodes from a time point when current is applied to the electrodes until the time point when current is applied to the electrodes is ended; The shrinkage amount is calculated from the difference between the maximum and minimum values ​​of the expansion amount. The inspection method according to claim 1 .

Images