Resistance spot welding method and method for manufacturing welded joint

Abstract

The purpose of the present invention is to provide resistance spot welding with which it is possible to obtain a stable nugget diameter even in the presence of interference. The resistance spot welding method according to the present invention is characterized by performing a main welding and a test welding prior to the main welding, a time change curve of instantaneous heat generation per unit volume and a cumulative heat generation per unit volume, which are calculated on the basis of electrical characteristics between electrodes when a target nugget diameter is formed by constant current control energization, are stored. The time change curve of the instantaneous heat generation amount per unit volume and the cumulative heat generation amount per unit volume, which are stored during the energization of the test welding, are set as target values for the energization during the formal welding, and adaptive control welding is performed in which the amount of energization is controlled on the basis of the target values. And a certain lower limit set value is set for the current.

Description

Technical Field

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

[0002] Resistance spot welding, a type of overlapping resistance welding method, is typically used to join overlapping steel plates. This welding method involves clamping two or more overlapping steel plates with a pair of electrodes from above and below, applying pressure, and briefly passing a high-current welding current between the electrodes to join them. The high-current welding current generates resistance heat, melting the contact areas of the steel plates, forming a point-shaped weld. This point-shaped weld is called a nugget, and is the portion of the steel plates that melts and solidifies at the contact point when current is passed through the overlapping steel plates. This nugget forms a point-shaped joint between the steel plates.

[0003] To achieve good weld quality, it is crucial to maintain a nugget diameter within an appropriate range. This nugget diameter is determined by welding conditions such as welding current, current-on time, electrode shape, and pressure. Therefore, to achieve an appropriate nugget diameter, it is necessary to appropriately set these welding conditions based on the weld material, including its material, thickness, and number of overlaps.

[0004] For example, in automobile manufacturing, thousands of spot welds are performed on each vehicle, and the workpieces (or workpieces) that flow through the vehicle are welded one by one. In this case, if the weld conditions, such as the material, thickness, and number of overlaps, are the same at each weld point, then the same nugget diameter can be achieved under the same welding conditions, such as welding current, duration, and pressure.

[0005] However, if there are interferences during welding, such as when there are already welded points (existing welded points) near the point to be welded, or when the surface of the materials being welded has significant unevenness and there are contact points between the materials being welded and the point to be welded, the current can be diverted to the existing welded points or contact points during welding. In such situations, even if welding is performed under the specified conditions, the current density at the intended weld point directly below the electrode decreases, making it impossible to obtain a nugget of the desired diameter. To compensate for this insufficient heat generation to achieve the desired nugget diameter, a high welding current must be set.

[0006] In addition, when the area around the point to be welded is severely restricted due to surface irregularities, the shape of the parts, etc., or when foreign matter is sandwiched between the steel plates around the welding point, the plate gap between the steel plates becomes larger and the contact diameter between the steel plates becomes narrower, which sometimes easily causes splashing (sputtering).

[0007] To address this welding instability, so-called adaptive control welding has been proposed. In adaptive control welding, changes in welding current, voltage, resistance, and heat generation are converted into electrical signals. Changes in welding phenomena caused by electrode wear and interference are directly measured or calculated, and input parameters such as welding current and voltage are controlled based on these values.

[0008] Patent Document 1 describes a method for controlling welding conditions for a resistance welding machine, wherein the welding current and inter-chip voltage are detected, the weld is simulated by heat conduction calculation, and the nugget formation state of the weld is estimated during welding, thereby achieving good welding.

[0009] Patent Document 2 describes a welding system that calculates the cumulative heat generation per unit volume of the workpiece, based on its thickness and the duration of current flow, to ensure a satisfactory weld. The welding current and voltage are then adjusted to produce the calculated heat generation per unit volume and per unit time. This system allows for excellent welding regardless of the type of workpiece or electrode wear. This describes a resistance welding system.

[0010] Patent Document 3 describes a welding mode consisting of two steps: a step for securing a current path directly below the electrode and a step for subsequently forming a nugget of a specified diameter. The temporal variation in the instantaneous calorific value per unit volume and the cumulative calorific value per unit volume, calculated from the electrical characteristics between the electrodes during test welding, when energization is controlled by a constant current to form an appropriate nugget, are stored as target values. The energization amount is then adaptively controlled to ensure that the cumulative calorific value during the actual welding matches the cumulative calorific value calculated in advance during the test welding. This method achieves a nugget diameter of a certain size or greater. This describes a resistance spot welding method.

[0011] Patent Document 4 describes a resistance spot welding method that simulates and stores the cumulative heat value during test welding. The method also adaptively controls the amount of current flow to ensure that the cumulative heat value during the actual welding matches the cumulative heat value previously calculated during the test welding. This method achieves a nugget diameter of at least a certain value.

[0012] Patent Document 5 describes a resistance spot welding method in which pressure is applied until an initial pressure setting is reached before the start of welding. The pressure is similarly measured during welding. The pressure index parameter obtained from the start of pressure application until the initial pressure setting is reached is used to set the pressure during power-on. This method allows the nugget diameter to be determined.

[0013] Patent Document 1: Japanese Patent Application Laid-Open No. 10-94883

[0014] Patent Document 2: Japanese Patent Application Laid-Open No. 11-33743

[0015] Patent Document 3: WO2015 / 049998

[0016] Patent Document 4: Japanese Patent Application Publication No. 2019-034341

[0017] Patent Document 5: WO2020 / 095847

[0018] However, the resistance spot welding technology described in Patent Document 1 estimates the nugget temperature based on a thermal conduction model (thermal conduction simulation) or the like, requiring complex calculations. This not only complicates the structure of the welding control device but also increases the cost of the welding control device itself.

[0019] The resistance spot welding technology described in Patent Document 2 assumes that by controlling the cumulative heating value to a target value, good welding can be achieved even if the electrode wear is moderate. However, if the conditions of the materials to be welded differ significantly from the conditions of the materials to be welded, for example, if there is a large gap between the metal plates being welded, even if the final cumulative heating value can be matched to the target value, the heat generation pattern, that is, the temporal variation of the temperature distribution in the weld, will deviate from the target heat pattern for a good weld, making it impossible to achieve the desired nugget diameter or causing spattering.

[0020] Furthermore, in the resistance spot welding techniques described in Patent Documents 3 to 5, although the accumulated heat amount is stored in advance, it may be difficult to form a stable nugget against strong interference. Summary of the Invention

[0021] Therefore, the present invention has been made in view of the above circumstances, and an object of the present invention is to provide a resistance spot welding method and a method for manufacturing a welded joint that can obtain a stable nugget diameter even in the presence of disturbances.

[0022] As a result, it was found that, in resistance spot welding, by setting an appropriate lower limit for the current, an excessive current drop and a decrease in the nugget diameter were effectively suppressed even in the presence of interference. This current reduction is believed to be due to, for example, the reduction in contact between the plates, which reduces the contact area S in equation (2) described later, increasing the resistance R and the calorific value q in equation (3). This control, which reduces the calorific value, results in a current reduction.

[0023] The present invention has been accomplished based on the above findings, and the gist of the present invention for solving the problems is as follows.

[0024] [1] A resistance spot welding method is a method of joining a plurality of metal plates to be welded by clamping them between a pair of electrodes while applying pressure and electricity, wherein:

[0025] Perform formal welding and test welding before formal welding.

[0026] In the test welding, a time variation curve of the instantaneous calorific value per unit volume and a cumulative calorific value per unit volume calculated from the electrical characteristics between the electrodes when the target nugget diameter is formed by energization under constant current control are stored.

[0027] In the main welding, the time variation curve of the instantaneous heat generation per unit volume and the cumulative heat generation per unit volume stored during the test welding are set as target values ​​for the main welding, and adaptive control welding is performed in which the amount of energization is controlled according to the target values.

[0028] In the above-mentioned actual welding, a certain lower limit setting value is set for the current.

[0029] [2] The resistance spot welding method according to [1], wherein, in the above-mentioned adaptive control welding, welding is performed based on the time variation curve of the instantaneous heat value per unit volume set as the above-mentioned target value and the cumulative heat value per unit volume. When the time variation of the instantaneous heat value per unit volume deviates from the time variation curve serving as the reference, in order to compensate for the deviation during the remaining power-on time, the power supply amount is controlled so that the cumulative heat value per unit volume during the power-on of the above-mentioned formal welding is consistent with the cumulative heat value per unit volume set as the above-mentioned target value.

[0030] [3] According to the resistance spot welding method described in [1], the energization in the above-mentioned test welding and formal welding includes formal energization and pre-energization before the formal energization, and the time variation curve of the instantaneous heat value per unit volume and the cumulative heat value per unit volume stored in the formal energization of the above-mentioned test welding are set as the target value of the formal energization in the formal welding, and as the formal energization in the formal welding, adaptive control welding is performed to control the above-mentioned energization amount according to the target value.

[0031] [4] According to the resistance spot welding method described in [2], the power-on in the test welding and the formal welding includes formal power-on and pre-power-on before the formal power-on, and the time variation curve of the instantaneous heat value per unit volume and the cumulative heat value per unit volume stored in the formal power-on of the test welding are set as the target value of the formal power-on in the formal welding, and as the formal power-on in the formal welding, adaptive control welding is performed to control the power-on amount according to the target value.

[0032] [5] The resistance spot welding method according to [1], wherein, in the adaptive control welding, the lower limit setting value of the current is set to 60% or more of the set current in the test welding.

[0033] [6] The resistance spot welding method according to [2], wherein, in the adaptive control welding, the lower limit setting value of the current is set to 60% or more of the set current in the test welding.

[0034] [7] The resistance spot welding method according to [3], wherein, in the adaptive control welding, the lower limit setting value of the current is set to 60% or more of the set current in the test welding.

[0035] [8] The resistance spot welding method according to [4], wherein, in the adaptive control welding, the lower limit setting value of the current is set to 60% or more of the set current in the test welding.

[0036] [9] The resistance spot welding method according to [5], wherein the welded material, which is a plurality of overlapping metal plates, has gaps between the plates as interference.

[0037]

[10] The resistance spot welding method according to [6], wherein the welded material, which is a plurality of overlapping metal plates, has gaps between the plates as interference.

[0038]

[11] The resistance spot welding method according to [7], wherein the welded material, which is a plurality of overlapping metal plates, has gaps between the plates as interference.

[0039]

[12] The resistance spot welding method according to [8], wherein the welded material, which is a plurality of overlapping metal plates, has gaps between the plates as interference.

[0040]

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

[12] is used.

[0041] The present invention can achieve resistance spot welding in which a stable nugget diameter can be obtained even in the presence of strong interference by setting a lower limit set value for the current value in adaptive control welding. DETAILED DESCRIPTION

[0042] The resistance spot welding method and the method for manufacturing a welded joint according to the present invention will be described based on the following embodiments.

[0043] One embodiment of the present invention is a resistance spot welding method for joining a plurality of overlapping steel plates by clamping them between a pair of electrodes while applying pressure and current. The method includes performing both main welding and test welding prior to the main welding. During the test welding, a time-varying curve of the instantaneous heat generation per unit volume and the cumulative heat generation per unit volume, calculated based on the electrical characteristics between the electrodes when the current is controlled by a constant current to form a target nugget diameter, are stored. During the main welding, the time-varying curve of the instantaneous heat generation per unit volume and the cumulative heat generation per unit volume stored during the test welding are set as target current values ​​for the main welding. Adaptive control welding is performed in which the current is controlled based on these target values, and a fixed lower limit set value is set for the current. Furthermore, metal plates other than steel plates may be used instead of steel plates as the weld materials.

[0044] Hereinafter, the details of the resistance spot welding of the present invention will be described in sequence.

[0045] The welding device that can be used in the resistance spot welding method according to one embodiment of the present invention is not particularly limited as long as it includes a pair of upper and lower electrodes and can arbitrarily control the pressure and welding current during welding. The pressure mechanism (cylinder, servo motor, etc.), type (stationary, welding robot, etc.), and electrode shape are not particularly limited. The electrical characteristics between the electrodes refer to the inter-electrode resistance or inter-electrode voltage.

[0046] Test welding

[0047] Test welding is performed on the welded material for test welding under constant current control, and a time variation curve of the instantaneous heat generation per unit volume and the cumulative heat generation per unit volume calculated based on the electrical characteristics between the electrodes when the target weld nugget is formed are stored. The welding device used in the welding method of the present invention is equipped with a storage device for storing the above-mentioned instantaneous heat generation and cumulative heat generation. In addition, the target weld nugget diameter is appropriately set according to the steel type, the environment in which the weld joint is used, etc., and is preferably greater than 2√t1, more preferably greater than 3√t1, and even more preferably greater than 4√t1. Here, t1 is the plate thickness (mm). In the case of a plate group with different plate thicknesses, t1 is set to the thickness of the thinner plate.

[0048] The test welding is energized by setting the current, voltage, and pressure to obtain the target nugget diameter. Before such energization for obtaining the target nugget diameter (hereinafter referred to as "formal energization"), pre-energization can also be performed. Regarding the welding current I1 under constant current control during pre-energization and the welding current I2 under constant current control during formal energization, the pre-energization during test welding preferably satisfies the relationship I1<I2. In addition, a power-on rest time can also be set between pre-energization and formal energization. In addition, the welding time of pre-energization is not particularly limited, but is generally 20 to 300 ms.

[0049] Formal welding

[0050] After the test welding described above, main welding is performed on the same plate assembly used for the main welding. If pre-energization and main energization were performed during the test welding, pre-energization and main energization are also performed during the main welding. If a power-off period was set between pre-energization and main energization during the test welding, a power-off period of the same length is also set during the main welding. During the main welding, adaptive control welding is performed based on the time variation curve of the instantaneous calorific value per unit volume and the cumulative calorific value stored as target values ​​during the test welding. When the time variation of the instantaneous calorific value per unit volume meets the time variation curve used as the reference, welding is continued and terminated.

[0051] However, if the temporal variation of the instantaneous calorific value per unit volume during the pre-energization or the main energization deviates from the reference temporal variation curve, the amount of energization is controlled so that the cumulative calorific value per unit volume during the pre-energization or the main energization coincides with the cumulative calorific value per unit volume previously calculated during the pre-energization or the main energization for the test welding, respectively, in order to compensate for this deviation during the remaining energization time of the pre-energization or the main energization. Specifically, if the temporal variation of the instantaneous calorific value per unit volume during the pre-energization deviates from the reference temporal variation curve during the pre-energization, the amount of energization is controlled so that the cumulative calorific value per unit volume during the pre-energization coincides with the cumulative calorific value per unit volume previously calculated during the test welding, respectively, in order to compensate for this deviation during the remaining energization time of the pre-energization. Furthermore, if the temporal variation in the instantaneous calorific value per unit volume deviates from the reference temporal variation curve during the main energization, the energization amount is controlled to compensate for this deviation during the remaining energization time period so that the cumulative calorific value per unit volume during the main energization matches the cumulative calorific value per unit volume previously calculated during the main energization of the test weld. In particular, adaptive control welding, in which the energization amount is controlled, is preferably performed during the main energization to actually form the weld. On the other hand, when performing preliminary energization under conditions where at least no weld is to be formed, there is no need to control the energization amount.

[0052] The present invention is characterized by setting a lower limit for the current value during actual energization. The reason for this lower limit is explained in detail below, using the example of a gap between plates (hereinafter referred to as "plate gap") as an example of interference. Causes of this plate gap include dimensional accuracy between components and plate deformation caused by other welds.

[0053] When the gap between the plates is large, the steel plates are pressed against each other, causing them to warp significantly along the shape of the electrodes. This reduces the contact area between the plates compared to when there is no gap. This increases the contact resistance between the plates and the voltage between the electrodes. Therefore, recognizing the increase in heat generation, adaptive control welding uses a control method to reduce the amount of current flowing, which, in turn, suppresses the growth of the nugget. To address this issue, the inventors discovered that by setting a lower limit on the current to prevent excessive reductions in the amount of current flowing, it is possible to suppress the reduction in the nugget diameter.

[0054] In the above-described main welding process of the present invention, even if the current value during conventional adaptive control welding is less than the lower limit setting value, the current value is fixed at the lower limit setting value and controlled so that the current does not decrease further. The lower limit setting value of the current is set within a range below the set current during the test welding.

[0055] To minimize the reduction in nugget diameter, the lower limit setting value of the current is preferably 60% or greater of the set current used in the test welding. Typically, it is preferably 80% or greater, and more preferably 85% or greater. On the other hand, to ensure the current control range required for adaptive control, the lower limit setting value of the current is preferably 96% or less, and more preferably 95% or less, of the set current used in the test welding.

[0056] In the present invention, even if the current value exceeds the lower limit setting and decreases during conventional adaptive control welding, control is performed to maintain the current value at the lower limit setting. Therefore, while the heat output deviates from the target value of adaptive control welding, the reduction in the nugget diameter can be suppressed.

[0057] Furthermore, in the test welding, when constant current welding is performed on a plate set in a non-interference state, a current value that yields a target nugget diameter is set as the set current.

[0058] The above description uses the plate gap as an example, but the present invention does not necessarily require interference and can also be used in situations without interference. In addition, as interference, in addition to the plate gap, it can also be used in situations where one electrode contacts the metal plate first (gap), where the centers of the upper and lower electrodes are offset (eccentricity), where the electrodes are angled (electrode angle), where there is a weld point nearby (welded point), and where the electrode shape changes due to continuous use (electrode shape change).

[0059] The present invention achieves greater effects in interference welding, particularly when the gap between the plates is large. This is because a larger gap causes the steel plates to bend more significantly when pressure is applied by the electrodes, increasing contact resistance between the plates. This leads to a greater reduction in welding current in conventional adaptive control welding, resulting in a smaller nugget diameter. The effect is particularly pronounced when the gap is 1 mm or larger.

[0060] The method for calculating the calorific value is not particularly limited, but an example thereof is disclosed in Patent Document 2, which can also be adopted in the present invention. The calculation method of the calorific value per unit volume and per unit time q and the cumulative calorific value per unit volume Q based on this method is as follows.

[0061] Let the total thickness of the materials to be welded be t, the resistivity of the materials to be welded be r, the voltage between the electrodes be V, the welding current be I, and the area of ​​contact between the electrodes and the materials to be welded be 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 heating. The amount of heat generated per unit volume and per unit time, q, in this columnar portion can be calculated using the following equation (1).

[0062] q=(V·I) / (S·t)---(1)

[0063] In addition, the resistance R of the columnar portion can be obtained by the following formula (2).

[0064] R=(r·t) / S---(2)

[0065] If S is obtained using formula (2) and substituted into formula (1), the calorific value q is as follows (3):

[0066] q=(V·I·R) / (r·t 2 )

[0067] =(V 2 ) / (r·t 2 )---(3).

[0068] As can be seen from the above formula (3), the calorific value per unit volume and per unit time q can be calculated based on the inter-electrode voltage V, the total thickness t of the workpieces to be welded, and the resistivity r of the workpieces to be welded, and is not affected by the contact area S between the electrode and the workpiece. In addition, although formula (3) calculates the calorific value based on the inter-electrode voltage V, the calorific value q can also be calculated based on the inter-electrode current I. In this case, the contact area S between the electrode and the workpiece does not need to be used. Moreover, if the calorific value per unit volume and per unit time q is accumulated during the energization period, the cumulative calorific value Q per unit volume applied to the weld can be obtained. As can be seen from formula (3), this cumulative calorific value Q per unit volume can also be calculated without using the contact area S between the electrode and the workpiece.

[0069] In the above, the case where the cumulative calorific value Q is calculated by the method described in Patent Document 2 has been described, but other calculation formulas may also be used.

[0070] In addition, the conditions other than the welding current in the actual welding (the energization time and the set pressure in the preliminary energization and the actual energization) may be the same as those in the test welding.

[0071] The steel plates referred to in this application also include plated steel plates, and both surfaces of the steel plates may be plated. Furthermore, the resistance spot welding method of the present invention is not particularly limited as long as the materials being welded are metal plates that generate heat through resistance. The above method can be used not only for steel plates but also for welding lightweight metal plates such as aluminum alloys. Furthermore, it can be used for stacks of three or more stacked metal plates.

[0072] The embodiments of the present invention can also be used in the following situations: as described in Patent Document 4, when the cumulative heat value is stored in test welding, the disturbed state is simulated and stored, and the amount of current is adaptively controlled so that the cumulative heat value of the actual welding is consistent with the cumulative heat value of the disturbed state previously calculated in the test welding.

[0073] Example

[0074] Table 1 lists the strength (MPa), thickness (mm), and plating type of the steel plates used in each plate set in the Examples. Plate Set No. 1 uses resistance spot welding of plates 1 and 2, while Plate Set No. 2 uses resistance spot welding of plates 1, 2, and 3 in that order. If the plating type is not specified, it indicates that unplated metal plates were used. If the plating type is specified as GA, it indicates that both surfaces of the metal plates were galvanized.

[0075] For the steel plate sets shown in Table 1, test welding was performed under the conditions shown in Table 2. The temporal variation of the instantaneous heat generation and the cumulative heat generation were stored as target values. Subsequently, adaptive control welding was performed during the main welding process, either with or without a plate gap, based on the target values ​​recorded during the test welding process, to produce welded joints. Furthermore, pre-energization was performed before the main welding process under the conditions shown in Table 2 during both the test and main welding processes. The pre-energization conditions were: the same pressure as for the main welding process, a current of 2 kA, and a energization time of 5 cycles. In other words, adaptive control welding was not performed during the pre-energization process during the main welding process. This low-current, short-duration pre-energization process does not produce a molten zone. Therefore, adaptive control welding during the pre-energization process is not essential. Furthermore, in the case of a plate gap, the main welding process was performed with a plate gap of 0.5 mm to 2 mm. For each weld joint obtained, the weld portion was cut, the cross section was etched, and then observed with the aid of an optical microscope. The difference between the obtained nugget diameter and the target nugget diameter was evaluated as follows. In addition, the target nugget diameter is the diameter of the ideal nugget when resistance spot welding is performed under interference-free conditions. The target nugget diameter can be appropriately set according to the type of steel, the environment in which the weld joint is used, etc., but in the example of the present invention, the target nugget diameter is set to 2√t1 or more. t1 is the plate thickness (mm). In the case of a plate group with different plate thicknesses, t1 is set to the plate thickness with a thinner plate thickness. In addition, as described above, the target nugget diameter can be appropriately set according to the type of steel, the environment in which the weld joint is used, etc., and is therefore not limited to this value.

[0076] Table 1

[0077]

[0078]

[0079] The difference between the lower current limit and the set current in Table 2 refers to the difference between the lower current limit set by the adaptive control during the actual welding and the current set value during the test welding. The ratio of the lower current limit to the set current refers to the ratio of the lower current limit set by the adaptive control during the actual welding to the current set value during the test welding. The diameter variation in Table 2 refers to the following: when the absolute value of the difference between the target nugget diameter and the nugget diameter obtained by the actual welding is less than the square root of 0.45×the plate thickness, it is set as 0 (good); when it is greater than the square root of 0.45×the plate thickness and less than the square root of 0.5×the plate thickness, it is set as △ (acceptable); when it is greater than the square root of 0.5×the plate thickness, it is set as × (unacceptable).

[0080] ○ (good): The absolute value of the difference between the target nugget diameter and the nugget diameter obtained by actual welding is less than the square root of 0.45×the plate thickness.

[0081] Δ (acceptable): The absolute value of the difference between the target nugget diameter and the nugget diameter obtained by main welding is not less than 0.45×the square root of the plate thickness and less than 0.50×the square root of the plate thickness.

[0082] × (unacceptable): The absolute value of the difference between the target nugget diameter and the nugget diameter in the presence of interference is equal to or greater than 0.50 × the square root of the plate thickness.

[0083] In Table 2, time is expressed as the number of welding current cycles. Specifically, when the welding current frequency is 50 Hz, one cycle is 20 ms. For example, 16 cycles under Condition No. 1 (reference example) in Table 2 represent 320 ms. In this example, one cycle is 20 ms.

[0084] In the inventive example, even when interference was detected, the diameter variation was good or acceptable when the lower limit setting for the current was set. On the other hand, in the comparative example, where the lower limit setting for the current was not set, the diameter variation was unacceptable. This result is believed to be due to the fact that by setting a lower limit setting for the current, excessive current reduction is prevented, maintaining ideal heating and melting of the nugget, and achieving the desired nugget diameter. In the examples shown in Table 2, even when various interferences were intentionally set and detected, the inventive example achieved good results by setting the lower limit setting for the current and performing actual welding. In actual production sites, interferences can occur unintentionally due to various factors. Therefore, to achieve a welding method suitable for use in actual production sites, the lower limit setting for the current is set, regardless of the detection of interference, with the possibility of interference as a prerequisite, as a preventative measure to prevent undesirable results even in the event of interference.

Claims

1. A resistance spot welding method in which a plurality of metal plates to be welded are sandwiched between a pair of electrodes and pressurized while current is applied to join the metal plates, wherein: Perform formal welding and test welding before formal welding. In the test welding, a time variation curve of the instantaneous calorific value per unit volume and a cumulative calorific value per unit volume calculated based on the electrical characteristics between the electrodes when the target nugget diameter is formed by energization under constant current control are stored. In the actual welding, the time variation curve of the instantaneous heat generation per unit volume and the cumulative heat generation per unit volume stored in the test welding are set as the target values ​​of the power supply in the actual welding, and adaptive control welding is performed in which the power supply amount is controlled according to the target values. In the actual welding, a certain lower limit setting value is set for the current.

2. The resistance spot welding method according to claim 1, wherein: In the adaptive control welding, welding is performed based on the time variation curve of the instantaneous heating value per unit volume set as the target value and the cumulative heating value per unit volume. When the time variation of the instantaneous heating value per unit volume deviates from the time variation curve serving as the reference, in order to compensate for the deviation within the remaining power-on time, the power-on amount is controlled so that the cumulative heating value per unit volume during the power-on of the actual welding is consistent with the cumulative heating value per unit volume set as the target value.

3. The resistance spot welding method according to claim 1, wherein: The power-on in the test welding and formal welding includes formal power-on and pre-power-on before the formal power-on. The time variation curve of the instantaneous heat value per unit volume and the cumulative heat value per unit volume stored in the formal power-on of the test welding are set as the target value of the formal power-on in the formal welding. As the formal power-on in the formal welding, adaptive control welding is performed to control the power-on amount according to the target value.

4. The resistance spot welding method according to claim 2, wherein: The power-on in the test welding and formal welding includes formal power-on and pre-power-on before the formal power-on. The time variation curve of the instantaneous heat value per unit volume and the cumulative heat value per unit volume stored in the formal power-on of the test welding are set as the target value of the formal power-on in the formal welding. As the formal power-on in the formal welding, adaptive control welding is performed to control the power-on amount according to the target value.

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

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

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

9. The resistance spot welding method according to claim 5, wherein: The welded material composed of a plurality of stacked metal plates has gaps between the plates as interference.

10. The resistance spot welding method according to claim 6, wherein: The welded material composed of a plurality of stacked metal plates has gaps between the plates as interference.

11. The resistance spot welding method according to claim 7, wherein: The welded material composed of a plurality of stacked metal plates has gaps between the plates as interference.

12. The resistance spot welding method according to claim 8, wherein: The welded material composed of a plurality of stacked metal plates has gaps between the plates as interference.

13. A method for manufacturing a welded joint, wherein: The spot welding method according to any one of claims 1 to 12 is used.

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