Resistance spot welding method and method for manufacturing welded joint

Abstract

An objective of the present invention is to provide resistance spot welding capable of obtaining a stable nugget diameter even in the presence of disturbance. A resistance spot welding method characterized in that: main welding and test welding prior to the main welding are performed; during the test welding, a time change curve of an instantaneous heat generation amount per unit volume and a cumulative heat generation amount per unit volume are stored that are calculated from electric characteristics between electrodes when a narrow nugget diameter is formed by energizing using constant current control; in the main welding, the time change curve of the instantaneous heat generation amount per unit volume and the accumulated heat generation amount per unit volume stored during the energization of the test welding are set as the target values for the energization; adaptive control welding is performed according to the target value to control the energization amount; and in the main welding, the current is provided with a fixed lower limit set value.

Description

[Technical field]

[0001] The present invention relates to a resistance spot welding method and a method for manufacturing a welded joint. [Background technology]

[0002] Generally, resistance spot welding, a type of lap resistance welding method, is used to join overlapping steel sheets. This welding method is a method in which two or more overlapping steel sheets are sandwiched between a pair of electrodes from above and below and pressure is applied while a high welding current is passed between the upper and lower electrodes for a short period of time to join them. The contact points of the steel sheets melt due to the resistance heat generated by passing a high welding current, resulting in a spot-like weld. This spot-like weld is called a nugget, and is the part where the two steel sheets melt and solidify at the contact point when a current is passed through the overlapping steel sheets. This nugget joins the steel sheets together in a spot-like manner.

[0003] In order to obtain good weld quality, it is important that the nugget diameter is formed within an appropriate range. The nugget diameter is determined by welding conditions such as welding current, current application time, electrode shape, and pressure. Therefore, in order to form an appropriate nugget diameter, it is necessary to set the above welding conditions appropriately according to the conditions of the workpieces, such as the material, plate thickness, and number of overlapping sheets of the workpieces.

[0004] For example, when manufacturing an automobile, several thousand spot welds are applied to each vehicle, and it is necessary to weld the workpieces (workpieces) that flow in one after another. In this case, if the condition of the workpieces at each welding point, such as the material, plate thickness, and number of overlapping sheets, is the same, the same nugget diameter can be obtained under the same welding conditions, such as the welding current, current flow time, and pressure.

[0005] However, if there is a disturbance during welding, for example, if there is a previously welded point (previously welded point) near the point to be welded, or if the surface unevenness of the material to be welded is large and there is a contact point of the material to be welded near the point to be welded, the current will be diverted to the previously welded point or the contact point during welding. In such a state, even if welding is performed under specified conditions, the current density at the position to be welded directly below the electrode will be low, and a nugget of the required diameter will not be obtained. To compensate for this lack of heat generation and obtain a nugget of the required diameter, it is necessary to set a high welding current in advance.

[0006] In addition, if the area around the welding point is tightly constrained due to surface unevenness or the shape of the parts, or if a foreign object is trapped between the steel sheets around the welding point, the gap between the steel sheets will become larger, narrowing the contact diameter between the steel sheets and making spatter more likely to occur.

[0007] To solve this type of welding instability, so-called adaptive control welding has been proposed. In adaptive control welding, changes in the welding phenomenon caused by the electrode wear and disturbances described above are directly measured or calculated as electrical signals in the form of changes in current, voltage, resistance and heat generation during welding, and input parameters such as the welding current and voltage are controlled based on these values.

[0008] Patent Document 1 describes a method for controlling welding conditions of a resistance welding machine that aims to perform good welding by detecting the welding current and the voltage between the tips, simulating the welded part by heat conduction calculations, and estimating the nugget formation state of the welded part during welding.

[0009] Patent Document 2 describes a resistance welding system that calculates the cumulative heat generation amount per unit volume that can satisfactorily weld the workpieces from the plate thickness and current flow time of the workpieces, and adjusts the welding current or voltage to generate the calculated amount of heat generation per unit volume and unit time. This allows for good welding regardless of the type of workpieces or the wear state of the electrodes.

[0010] In Patent Document 3, the welding pattern is divided into two steps: a step for securing a current path directly under the electrodes, and a subsequent step for forming a nugget of a specified diameter. The time change in instantaneous heat generation per unit volume and the cumulative heat generation per unit volume, which are calculated from the electrical characteristics between the electrodes when a proper nugget is formed by passing current under constant current control in test welding, are stored as target values, and the current flow is adaptively controlled so that the cumulative heat generation in the actual welding matches the cumulative heat generation previously determined in the test welding. This describes a resistance spot welding method that can obtain a nugget diameter of a certain size or more.

[0011] In Patent Document 4, when storing the cumulative heat generation amount in test welding, a disturbance state is simulated and stored, and then the current flow is adaptively controlled so that the cumulative heat generation amount in actual welding matches the cumulative heat generation amount previously obtained in test welding. This describes a resistance spot welding method that can obtain a nugget diameter of a certain size or more.

[0012] In Patent Document 5, a resistance spot welding method is described in which, before starting energization for main welding, pressure is applied until an initial initial pressurizing force is reached, and the pressurizing force is similarly measured during main welding, and the pressurizing force during energization is set using a parameter of the pressurizing force index obtained from the start of energization until the initial pressurizing force is reached, thereby obtaining a desired nugget diameter. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] Japanese Patent Application Publication No. 10-94883 [Patent Document 2] Japanese Patent Application Publication No. 11-33743 [Patent Document 3] WO2015 / 049998 [Patent Document 4] JP 2019-034341 A [Patent Document 5] WO2020 / 095847 Summary of the Invention [Problem to be solved by the invention]

[0014] However, in the resistance spot welding technique described in Patent Document 1, the nugget temperature is estimated based on a heat conduction model (heat conduction simulation), etc., which requires complex calculation processing. This not only complicates the configuration of the welding control device, but also makes the welding control device itself expensive.

[0015] In the resistance spot welding technique described in Patent Document 2, it is believed that good welding can be performed even if the electrode is worn to a certain extent by controlling the cumulative heat generation to a target value. However, when the set conditions of the welded material and the actual conditions of the welded material are significantly different, for example, when there is a large gap between the metal plates to be welded, even if the final cumulative heat generation can be matched to the target value, the form of heat generation, that is, the change over time in the temperature distribution of the welded part, deviates from the heat pattern that results in the desired good welded part, and the required nugget diameter cannot be obtained or expulsion occurs.

[0016] Furthermore, in the resistance spot welding techniques described in Patent Documents 3 to 5, the accumulated heat value is stored, but there are cases where it is difficult to stably form a nugget in the presence of a strong disturbance. [Means for solving the problem]

[0017] The present invention has been made in consideration of the above circumstances, and has an object to provide a resistance spot welding method and a method for manufacturing a welded joint that are capable of obtaining a stable nugget diameter even when an external disturbance is present. The inventors of the present invention have conducted extensive experimental studies on these points.

[0018] As a result, it was found that in resistance spot welding, setting an appropriate lower limit of the current is effective in preventing excessive current drop and preventing a decrease in the nugget diameter, even when disturbances are present. This current drop is believed to be caused by, for example, when the contact between sheets is reduced, the contact area S in equation (2) described below becomes smaller, which increases the resistance R, and the heat generation amount q in equation (3) increases, causing control to be performed to reduce the heat generation amount, resulting in a drop in the current. The present invention is based on the above findings, and the gist of the present invention for solving the problems is as follows. [1] A resistance spot welding method in which a workpiece made of overlapping multiple metal sheets is sandwiched between a pair of electrodes and joined by passing electricity through the electrodes while applying pressure, The actual welding and the test welding prior to the actual welding shall be performed. In the test welding, a time change curve of instantaneous heat generation amount per unit volume and a cumulative heat generation amount per unit volume, which are calculated from the electrical characteristics between the electrodes when a current is passed under constant current control to form a target nugget diameter, are stored; In the main welding, the time change curve of the instantaneous heat generation amount per unit volume and the accumulated heat generation amount per unit volume stored in the current supply of the test welding are set as target values ​​of the current supply in the main welding, and adaptive control welding is performed in which the current supply amount is controlled in accordance with the target values. The resistance spot welding method according to claim 1, wherein a lower limit value is set for the current during the main welding. [2] The resistance spot welding method of [1], in which, in the adaptive control welding, welding is performed based on the time change curve of the instantaneous heat generation per unit volume and the cumulative heat generation per unit volume set as the target values, and if the time change curve of the instantaneous heat generation per unit volume deviates from the reference time change curve, the amount of current flow is controlled so that the cumulative heat generation per unit volume in the main welding current matches the cumulative heat generation per unit volume set as the target value, in order to compensate for the deviation within the remaining current flow time. [3] The resistance spot welding method according to [1], wherein the current flow in the test welding and main welding includes a main current flow and a pre-current flow prior to the main current flow, the time change curve of the instantaneous heat amount per unit volume and the cumulative heat amount per unit volume stored in the main current flow in the test welding are set as target values ​​for the main current flow in the main welding, and adaptive control welding is performed to control the amount of current flow in accordance with the target values ​​for the main current flow in the main welding. [4] The resistance spot welding method according to [2], wherein the current flow in the test welding and the main welding includes a main current flow and a pre-current flow prior to the main current flow, the time change curve of the instantaneous heat amount per unit volume and the cumulative heat amount per unit volume stored in the main current flow in the test welding are set as target values ​​for the main current flow in the main welding, and adaptive control welding is performed in which the amount of current flow is controlled in accordance with the target values ​​for the main current flow in the main welding. [5] The resistance spot welding method according to [1], wherein, in the adaptive control welding, the lower limit current setting value is set to 60% or more of the set current in the test welding. [6] The resistance spot welding method according to [2], wherein, in the adaptive control welding, the lower limit current setting value is set to 60% or more of the set current in the test welding. [7] The resistance spot welding method according to [3], wherein, in the adaptive control welding, the lower limit current setting value is set to 60% or more of the set current in the test welding. [8] The resistance spot welding method according to [4], wherein, in the adaptive control welding, the lower limit current setting value is set to 60% or more of the set current in the test welding. [9] The resistance spot welding method according to [5], wherein the workpieces, which are made of multiple overlapping metal plates, have gaps between the plates as an external disturbance.

[10] The resistance spot welding method according to [6], wherein the workpieces, which are made of multiple overlapping metal plates, have gaps between the plates as an external disturbance.

[11] The resistance spot welding method according to [7], wherein the workpieces, which are made of multiple overlapping metal plates, have gaps between the plates as an external disturbance.

[12] The resistance spot welding method according to [8], wherein the workpieces, which are made of overlapping metal plates, have gaps between the plates as an external disturbance.

[13] A method for manufacturing a welded joint using the spot welding method described in any one of [1] to

[12] . Effect of the Invention

[0019] The present invention enables resistance spot welding that can obtain a stable nugget diameter even when strong disturbances are present, by setting a lower limit for the current value in adaptive control welding. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A resistance spot welding method and a method for manufacturing a welded joint according to the present invention will be described based on the following embodiments.

[0021] One embodiment of the present invention is a resistance spot welding method in which a workpiece made of overlapping steel sheets is sandwiched between a pair of electrodes and joined by applying current while applying pressure, and a main welding and a test welding prior to the main welding are performed. In the test welding, a time change curve of the instantaneous heat generation amount per unit volume and a cumulative heat generation amount per unit volume, which are calculated from the electrical characteristics between the electrodes when a target nugget diameter is formed by applying current under constant current control, are stored. In the main welding, the time change curve of the instantaneous heat generation amount per unit volume and the cumulative heat generation amount per unit volume stored by applying current in the test welding are set as target values ​​for current application in the main welding, and adaptive control welding is performed to control the current application amount according to the target values, and a constant lower limit setting value is set for the current in this adaptive control welding. Note that instead of the steel plate used for the workpiece, a metal plate other than a steel plate may be used.

[0022] The resistance spot welding of the present invention will be described below in detail.

[0023] As a welding apparatus that can be used in the resistance spot welding method according to an embodiment of the present invention, it is sufficient to include a pair of upper and lower electrodes and be able to arbitrarily control the pressing force and welding current during welding. The pressing mechanism (such as an air cylinder or a servo motor), the type (stationary type, robot gun, etc.), the electrode shape, etc. are not particularly limited. Further, the electrical characteristics between the electrodes mean the resistance between the electrodes or the voltage between the electrodes.

[0024] ·Test welding Test welding is performed on the weldment for test welding by constant current control, and the time change curve of the instantaneous heat generation amount per unit volume and the cumulative heat generation amount per unit volume, which are calculated from the electrical characteristics between the electrodes when forming the target nugget, are stored. The welding apparatus used in the welding method of the present invention is equipped with a storage device for storing the above-mentioned instantaneous heat generation amount and cumulative heat generation amount. Note that the target nugget diameter is appropriately set according to the steel type, the environment in which the weld joint is used, etc., and is preferably 2√t1 or more, more preferably 3√t1 or more, and even more preferably 4√t1 or more. Here, t1 is the plate thickness (mm), and in the case of a plate set with different plate thicknesses, the thinner plate thickness is used.

[0025] The energization during test welding is performed by setting the current, voltage, and pressing force at which the target nugget diameter can be obtained. Prior to such energization (hereinafter referred to as "main energization") for obtaining the target nugget diameter, it is also possible to perform pre-energization. Regarding the welding current I1 under constant current control during pre-energization and the welding current I2 under constant current control during main energization during test welding, it is preferable to satisfy the relationship I1 < I2. Further, a current interruption time may be provided between the pre-energization and the main energization. Note that the welding time during pre-energization is not particularly limited, but is usually 20 to 300 ms.

[0026] ·Main welding After the above test welding, the actual welding is performed on the workpiece for the actual welding of the same plate combination as that used for the test welding. If a pre-current and a main current are applied in the test welding, then a pre-current and a main current are also applied in the actual welding. If a current rest period is provided between the pre-current and the main current in the test welding, then a current rest period of the same length is also provided in the actual welding. In the actual welding, adaptive control welding is performed based on the time change curve of the instantaneous heat generation amount per unit volume and the accumulated heat generation amount stored as target values ​​in the test welding, and if the time change amount of the instantaneous heat generation amount per unit volume follows the time change curve that is the reference, the welding is continued as is and the welding is terminated.

[0027] However, if the time change in the instantaneous heat generation amount per unit volume during the pre-energization or main energization deviates from the reference time change curve, the amount of energization is controlled so that the cumulative heat generation amount per unit volume during the pre-energization or main energization matches the cumulative heat generation amount per unit volume previously determined during the pre-energization or main energization of the test welding, in order to compensate for the deviation within the remaining energization time of the pre-energization or main energization. In other words, if the time change in the instantaneous heat generation amount per unit volume during the pre-energization deviates from the reference time change curve, the amount of energization is controlled so that the cumulative heat generation amount per unit volume during the pre-energization matches the cumulative heat generation amount per unit volume previously determined during the test welding, in order to compensate for the deviation within the remaining energization time of the pre-energization. Furthermore, if the time change in the instantaneous heat generation amount per unit volume during main current application deviates from the reference time change curve, the amount of current application is controlled so that the deviation is compensated for within the remaining current application time of the main current application, so that the cumulative heat generation amount per unit volume during the main current application matches the cumulative heat generation amount per unit volume previously determined during the main current application of the test welding. In particular, it is preferable to perform adaptive control welding in which the amount of current application is controlled during main current application to actually form a weld. On the other hand, when pre-current application is performed under conditions where a weld is not formed, it is not essential to control the amount of current application.

[0028] Here, the present invention is characterized in that a lower limit is set for the current value of the main current. The reason for setting the lower limit will be explained in detail below, taking as an example a case where a gap exists between sheets (hereinafter referred to as a "sheet gap") as a disturbance. Causes of the sheet gap include dimensional accuracy between the parts and sheet deformation due to other welds.

[0029] When the gap between the plates is large, the steel plate is significantly warped to conform to the shape of the electrode due to pressure, and the contact area between the plates is smaller than when there is no gap. This increases the contact resistance between the plates and increases the interelectrode voltage. This causes the amount of heat generated to be recognized as increased, and in adaptive control welding, control is performed to reduce the amount of current, resulting in suppression of nugget growth. In response to this problem, the inventors have found that the reduction in the nugget diameter can be suppressed by setting a lower limit for the current so that the amount of current does not decrease excessively.

[0030] Here, in the main welding of the present invention, even if the current value falls below the lower limit setting in conventional adaptive control welding, the current value is fixed at the lower limit setting and controlled so as not to decrease further. The lower limit setting of the current is set within a range equal to or less than the set current in test welding. In order to suppress the reduction of the nugget diameter, the lower limit of the current is preferably set to 60% or more of the set current in the test welding. Normally, it is preferable to set it to 80% or more, and more preferably to set it to 85% or more. On the other hand, in order to secure the current control range required for adaptive control, the lower limit of the current is preferably set to 96% or less of the set current in the test welding, and more preferably to set it to 95% or less. In the present invention, even when the current value falls below the lower limit setting value in conventional adaptive control welding, the current value is controlled to be fixed at the lower limit setting value. Therefore, although there occurs a deviation from the target heat amount of adaptive control welding, it is possible to suppress the reduction of the nugget diameter.

[0031] In the test welding, the set current was the current value at which the target nugget diameter was obtained when constant current welding was performed on a plate assembly without any disturbance.

[0032] Although the above description has been given using a plate gap as an example, the presence of a disturbance is not essential for the present invention, and the present invention can be applied even when there is no disturbance. In addition, as disturbances other than a plate gap, the present invention can also be applied to cases where one electrode comes into contact with the metal plate first (clearance), where the centers of the upper and lower electrodes are misaligned (misalignment), where the electrode is angled (strike angle), where there is a welded point nearby (existing welded point), where the electrode shape has changed due to continuous use (change in electrode shape), etc.

[0033] The present invention is particularly effective when the gap is large, especially when the gap is large. When the gap is large, the steel sheet is curved more when pressed by the electrode, which increases the contact resistance between the sheets. In conventional adaptive control welding, the welding current is reduced more, which causes a reduction in the nugget diameter. In particular, the present invention is effective when the gap is 1 mm or more.

[0034] There is no particular restriction on the method for calculating the calorific value, but an example is disclosed in Patent Document 2, and this method can also be adopted in the present invention. The calculation method for the calorific value q per unit volume / unit time and the cumulative calorific value Q per unit volume using this method is as follows. The total thickness of the materials to be welded is t, the electrical resistivity of the materials to be welded is r, the voltage between the electrodes is V, the welding current is I, and the area of ​​contact between the electrodes and the materials to be welded is S. In this case, the welding current passes through a columnar portion with a cross-sectional area of ​​S and a thickness of t, generating resistance heat. The amount of heat q per unit volume and unit time in this columnar portion can be calculated using the following equation (1). q=(V·I) / (S·t) ---(1) Moreover, the electrical resistance R of this columnar portion is calculated by the following formula (2). R=(r·t) / S ---(2) By solving equation (2) for S and substituting it into equation (1), the heat generation amount q is given by the following equation (3). q=(V I R) / (r t 2 ) =(V 2 ) / (r t 2 ) ---(3) It becomes.

[0035] As is clear from the above formula (3), the amount of heat generated per unit volume and unit time q can be calculated from the interelectrode voltage V, the total thickness t of the workpieces, and the electrical resistivity r of the workpieces, and is not affected by the contact area S between the electrode and the workpieces. Note that while formula (3) calculates the amount of heat generated from the interelectrode voltage V, it is also possible to calculate the amount of heat generated q from the interelectrode current I. In this case, too, it is not necessary to use the contact area S between the electrode and the workpieces. And by accumulating the amount of heat generated q per unit volume and unit time over the current application period, the accumulated amount of heat generated per unit volume Q applied to the welding can be obtained. As is clear from formula (3), this accumulated amount of heat generated per unit volume Q can also be calculated without using the contact area S between the electrode and the workpieces. Although the case where the cumulative heat generation amount Q is calculated by the method described in Patent Document 2 has been described above, other calculation formulas may also be used.

[0036] In addition, the conditions other than the welding current in the actual welding (the energization time and the set welding pressure in the preliminary energization and the actual energization) may be the same as those in the test welding.

[0037] The steel sheet in this application includes plated steel sheet, and both sides of the steel sheet may be plated. In addition, the resistance spot welding method of the present invention is not particularly limited to the material to be welded, so long as it is a metal sheet that generates heat by resistance. It can be applied to welding not only steel sheets but also light metal sheets such as aluminum alloys. Furthermore, it can be applied to sheet assemblies in which three or more metal sheets are stacked.

[0038] An embodiment of the present invention can also be applied to a case where, as in Patent Document 4, when storing the cumulative heat generation amount in test welding, a disturbance state is simulated and stored, and the current flow is adaptively controlled so that the cumulative heat generation amount in actual welding matches the cumulative heat generation amount in a disturbance state determined in advance in test welding. EXAMPLES

[0039] Table 1 lists the strength (MPa), sheet thickness (mm), and type of plating of the steel sheets used in each sheet combination in the examples. Sheet combination No. 1 consisted of sheets 1 and 2 stacked together and used for resistance spot welding, while sheet combination No. 2 consisted of sheets 1, 2, and 3 stacked together in that order and used for resistance spot welding. If no mention is made of the plating type, it means that an unplated metal sheet was used, and if the plating type is listed as GA, it means that a metal sheet with alloyed hot-dip galvanizing applied on both sides was used.

[0040] Test welding was performed under the conditions shown in Table 2 for the steel plate assembly shown in Table 1, and the time change curve of the instantaneous heat generation amount and the cumulative heat generation amount were stored as target values. Next, in the actual welding, when there was no plate gap or when there was a plate gap, adaptive control welding was performed in the actual welding based on the target values ​​recorded in the test welding, and a welded joint was produced. In the test welding and the actual welding, pre-current was performed before the actual current was applied under the conditions shown in Table 2. The conditions of pre-current were the same as the actual current, the pressure, the current value, and the current application time of 5 cycles. In other words, in the actual welding, adaptive control welding was not performed in pre-current. No molten zone is generated during such a low current and short pre-current. Therefore, it is not essential to perform adaptive control welding in pre-current. In addition, when there is a plate gap, the actual welding was performed under the condition that the plate gap is 0.5 mm to 2 mm. For each welded joint obtained, the welded portion was cut, the cross section was etched, and then observed with an optical microscope. The difference between the obtained nugget diameter and the target nugget diameter was evaluated as follows. The target nugget diameter is the ideal diameter of the nugget when resistance spot welding is performed under conditions without disturbance. The target nugget diameter can be set appropriately depending on the steel type and the environment in which the welded joint is used, but in the present invention, the target nugget diameter is set to 2√t1 or more. t1 is the plate thickness (mm), and in the case of a plate assembly with different plate thicknesses, the plate thickness of the thinner plate thickness is used. As described above, the target nugget diameter can be set appropriately depending on the steel type and the environment in which the welded joint is used, so it is not limited to this value.

[0041] [Table 1]

[0042] [Table 2]

[0043] The difference between the lower limit current and the set current in Table 2 is the difference between the lower limit current set by the adaptive control performed in actual welding and the current set value in test welding. The ratio of the lower limit current to the set current is the ratio of the lower limit current set by the adaptive control performed in actual welding to the current set value in test welding. The diameter variation in Table 2 is categorized as ◯ (good) when the absolute value of the difference between the target nugget diameter mentioned above and the nugget diameter obtained by actual welding is smaller than 0.45 × the square root of the plate thickness, △ (pass) when it is equal to or greater than 0.45 × the square root of the plate thickness and less than 0.5 × the square root of the plate thickness, and × (fail) when it is equal to or greater than 0.5 × the square root of the plate thickness. ○ (Good): When the absolute value of the difference between the target nugget diameter and the nugget diameter obtained by this welding is less than 0.45 × the square root of the plate thickness. △ (Pass): When the absolute value of the difference between the target nugget diameter and the nugget diameter obtained by this welding is 0.45 × the square root of the plate thickness or more and less than 0.50 × the square root of the plate thickness. × (Fail): When the absolute value of the difference between the target nugget diameter and the nugget diameter when there is disturbance is 0.50 × the square root of the plate thickness or more.

[0044] In Table 2, the time is expressed by the number of cycles of the welding current. That is, when the frequency of the welding current is 50 Hz, one cycle is 20 ms, and for example, 16 cycles in condition No. 1 (reference example) in Table 2 means 320 ms. In this embodiment, one cycle is 20 ms.

[0045] In the example of the invention, even when a disturbance was detected, the diameter fluctuation was good or acceptable when a lower limit current setting was set. On the other hand, in the comparative example in which a lower limit current setting was not set, the diameter fluctuation was unacceptable. It is believed that such a result was achieved because the heating and melting of the nugget part progressed ideally and a state in which a desired nugget diameter could be obtained was maintained by setting a lower limit current setting and preventing an excessive current drop. In the examples shown in Table 2, various disturbances were intentionally set, and even when a disturbance was detected, the lower limit current setting was set and the main welding was performed in the example of the invention, and good results were obtained. In an actual production site, disturbances may occur unintentionally due to various factors. Therefore, in order to make the welding method suitable for use in an actual production site, a lower limit current setting is set without the condition of disturbance detection as a preventive measure to prevent undesirable results from being obtained even when a disturbance occurs, assuming the possibility of disturbance occurrence.

Claims

1. A resistance spot welding method in which a workpiece made of overlapping metal plates is sandwiched between a pair of electrodes and joined by passing current through the workpiece while applying pressure, The actual welding and the test welding prior to the actual welding shall be performed. In the test welding, a time change curve of instantaneous heat generation amount per unit volume and a cumulative heat generation amount per unit volume, which are calculated from the electrical characteristics between the electrodes when a current is passed under constant current control to form a target nugget diameter, are stored; In the main welding, the time change curve of the instantaneous heat generation amount per unit volume and the accumulated heat generation amount per unit volume stored in the current supply of the test welding are set as target values ​​of the current supply in the main welding, and adaptive control welding is performed in which the current supply amount is controlled in accordance with the target values. The resistance spot welding method according to claim 1, wherein a constant lower limit value is set for the current during the main welding.

2. 2. The resistance spot welding method according to claim 1, wherein in the adaptive control welding, welding is performed based on a time change curve of the instantaneous heat generation amount per unit volume and a cumulative heat generation amount per unit volume set as the target values, and if the time change curve of the instantaneous heat generation amount per unit volume deviates from the reference time change curve, the amount of current flow is controlled so that the cumulative heat generation amount per unit volume in the current flow for the main welding matches the cumulative heat generation amount per unit volume set as the target value in order to compensate for the deviation within the remaining current flow time.

3. 2. The resistance spot welding method according to claim 1, wherein the current passing in the test welding and the main welding includes a main current passing and a pre-current passing prior to the main current passing, the time change curve of the instantaneous heat amount per unit volume and the accumulated heat amount per unit volume stored in the main current passing in the test welding are set as target values ​​for the main current passing in the main welding, and adaptive control welding is performed in which the current amount is controlled in accordance with the target values ​​as the main current passing in the main welding.

4. 3. The resistance spot welding method according to claim 2, wherein the current passing in the test welding and the main welding includes a main current passing and a pre-current passing prior to the main current passing, the time change curve of the instantaneous heat amount per unit volume and the accumulated heat amount per unit volume stored in the main current passing in the test welding are set as target values ​​for the main current passing in the main welding, and adaptive control welding is performed in which the current amount is controlled in accordance with the target values ​​as the main current passing in the main welding.

5. 2. The resistance spot welding method according to claim 1, wherein in the adaptive control welding, the lower limit current setting value is set to 60% or more of the set current in the test welding.

6. 3. The resistance spot welding method according to claim 2, wherein in the adaptive control welding, the lower limit current setting value is set to 60% or more of the set current in the test welding.

7. 4. The resistance spot welding method according to claim 3, wherein in the adaptive control welding, the lower limit current setting value is set to 60% or more of the set current in the test welding.

8. The resistance spot welding method according to claim 4, wherein in the adaptive control welding, the lower limit current setting value is set to 60% or more of the set current in the test welding.

9. 6. The resistance spot welding method according to claim 5, wherein the workpieces made of overlapping metal plates have gaps between the plates as external disturbances.

10. 7. The resistance spot welding method according to claim 6, wherein the workpieces, which are made up of a plurality of overlapping metal plates, have gaps between the plates as external disturbances.

11. 8. The resistance spot welding method according to claim 7, wherein the workpieces, which are made up of a plurality of overlapping metal plates, have gaps between the plates as external disturbances.

12. 9. The resistance spot welding method according to claim 8, wherein the workpieces made of overlapping multiple metal plates have gaps between the plates as external disturbances.

13. A method for manufacturing a welded joint using the spot welding method according to any one of claims 1 to 12.

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