Resistance spot welding method and method for manufacturing a welded member

Abstract

To provide a resistance spot welding method that can stably form a nugget with a desired nugget diameter, even if there is disturbance.SOLUTION: In a resistance spot welding method, materials to be welded having a plurality of metal plates 1 and 2 overlapped with each other is pinched by a pair of electrodes 4 and 5, so that the plates are jointed to each other while being pressurized and energized. In a pressurizing step that is performed before welding, a time T elapsing from a time point when pressurizing force from the metal plates 1 and 2 to the electrodes 3 and 4 is detected until a time point when variations in shapes of the metal plates 1 and 2 become sufficiently small is measured, a target cumulative amount of heat generation is set based on the measured time T, and amounts of energization are controlled in accordance with the target cumulative amounts of heat generation, so as to join the materials to be welded.SELECTED DRAWING: Figure 4

Description

Technical Field

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

[0002] Generally, for joining overlapping metal plates, for example, steel plates, a resistance spot welding method, which is a type of resistance seam welding method, is used. This welding method is a method of sandwiching two or more overlapping metal plates and applying pressure from above and below with a pair of electrodes, while passing a high-current welding current between the upper and lower electrodes for a short time to join them. By utilizing the resistance heat generated by flowing a high-current welding current, a dot-shaped welded part is obtained. This dot-shaped welded part is called a "nugget" and is the part where both metal plates are melted and solidified at the contact location of the metal plates when an electric current is passed through the overlapping metal plates. Due to this nugget, the metal plates are joined point by point.

[0003] In order to obtain good welding part quality, it is important to adjust the diameter of the nugget within an appropriate range. The nugget diameter is determined by welding conditions such as welding current, energization time, electrode shape, and pressing force. Therefore, in order to obtain an appropriate nugget diameter, it is necessary to appropriately set the above welding conditions according to the conditions of the material to be welded, such as the material, plate thickness, and number of stacked sheets of the material to be welded.

[0004] For example, in the manufacture of automobiles, thousands of spot welds are performed per vehicle, and it is necessary to weld the workpieces (materials to be welded) that flow continuously. At this time, if the state of the material to be welded, such as the material, plate thickness, and number of stacked sheets of the material to be welded, is the same at each welding location, the same welding conditions such as welding current, energization time, and pressing force can be used to obtain the same nugget diameter.

[0005] However, in continuous welding, the contact area of the electrode with the material to be welded gradually wears, and the contact area becomes gradually wider than the initial state. When a welding current of the same value as the initial state is passed in a state where the contact area of the electrode has become wider in this way, the current density in the material to be welded decreases, and the temperature rise of the welded part becomes lower, so the nugget diameter becomes smaller. For this reason, for every several hundred to several thousand welds, the electrode is polished or replaced so that the tip diameter of the electrode does not become too large.

[0006] In addition, a resistance welding apparatus having a function (stepper function) of increasing the welding current value when welding is performed a predetermined number of times to compensate for the decrease in current density due to electrode wear has been conventionally used. In order to use this stepper function, it is necessary to appropriately set the above-described welding current change pattern in advance.

[0007] However, for this purpose, it takes a lot of time and cost to derive a welding current change pattern corresponding to a large number of welding conditions and material-to-be-welded conditions by tests or the like. Also, in actual construction, since there are variations in the progress state of electrode wear, the predetermined welding current change pattern is not always appropriate.

[0008] Furthermore, when there is a disturbance during welding, for example, when there is a point that has already been welded (already welded point) near the point to be welded, or when 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, current is shunted to the already welded point and the contact point during welding. In such a state, even if welding is performed under predetermined conditions, the current density at the position where welding is desired directly under the electrode decreases, so that a nugget of the required diameter cannot be obtained. In order to compensate for this insufficient heat generation amount and obtain a nugget of the required diameter, it is necessary to set a high welding current in advance.

[0009] Also, when the periphery of the point to be welded is strongly constrained due to surface unevenness or the shape of the member, or when foreign matter is sandwiched between the metal plates around the welded point, the gap between the metal plates (hereinafter also referred to as "plate gap") becomes large, narrowing the contact diameter between the metal plates and making it easier for scattering to occur.

[0010] In order to solve such welding instability, so-called adaptive control welding has been proposed. In adaptive control welding, changes in the welding phenomenon due to electrode wear and disturbances described above are directly measured or calculated as electrical signals from changes in current, voltage, resistance, and heat generation amount during welding, and input parameters such as welding current and voltage are controlled based on those values.

[0011] Patent Document 1 describes a welding condition control method for a resistance welder that attempts to perform good welding by detecting welding current and voltage between tips, performing simulation of the welded part by heat conduction calculation, and estimating the formation state of nuggets in the welded part during welding.

[0012] Patent Document 2 describes a resistance welding system that attempts to perform good welding regardless of the type of workpiece and the wear state of the electrode by calculating the cumulative heat generation amount per unit volume that can weld the workpiece well from the plate thickness and energization time of the workpiece, and performing a process of adjusting to a welding current or voltage that generates the calculated heat generation amount per unit volume and per unit time.

[0013] Patent Document 3 describes a resistance spot welding method that divides the welding pattern into two steps: a step for securing a current path directly under the electrode and a subsequent step for forming a nugget of a predetermined diameter. After storing the time change 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 electrodes when forming an appropriate nugget by energization under constant current control in a test weld, as target values respectively, the energization amount is controlled so that the cumulative heat generation amount of this weld matches the cumulative heat generation amount obtained in advance in the test weld, thereby obtaining a nugget diameter of a certain size or more.

[0014] In Patent Document 4, when the cumulative heat generation amount is memorized by test welding, after memorizing by simulating a state with disturbance, the energization amount is controlled so that the cumulative heat generation amount of the present welding matches the cumulative heat generation amount previously obtained by test welding, whereby a nugget diameter of a certain size or more can be obtained, and a resistance spot welding method is described.

[0015] In Patent Document 5, before the energization of the present welding starts, pressure is applied until the initial set pressure is reached, the pressure is also measured during the present welding, and the pressure during energization is set using the parameter of the pressure index obtained from the start point of the pressure application until before the initial set pressure is reached, whereby a resistance spot welding method capable of stably obtaining a desired nugget diameter is described.

Prior Art Documents

Patent Documents

[0016]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0017] However, in the technique described in Patent Document 1, in order to estimate the temperature of the nugget based on a heat conduction model (heat conduction simulation) or the like, complicated calculation processing is required, not only does the configuration of the welding control device become complicated, but also the welding control device itself becomes expensive.

[0018] Also, in the technology described in Patent Document 2, it is considered that good welding can be performed by controlling the cumulative heat generation amount to a target value, even if the electrode has worn by a certain amount. However, when the set conditions of the material to be welded are significantly different from the actual conditions of the material to be welded, for example, when there is a large gap between the metal plates to be welded, even if the final cumulative heat generation amount can be adjusted to the target value, the form of heat generation, that is, the temporal change in the temperature distribution of the welded part, may deviate from the heat amount pattern that can obtain a good welded part as the target, and the required nugget diameter may not be obtained or scattering may occur.

[0019] Furthermore, in the technologies described in Patent Documents 3 and 4, the cumulative heat generation amount is stored regardless of the presence or absence of disturbance, but there is a problem that the weldability becomes unstable with respect to unknown disturbances.

[0020] Furthermore, in the technology described in Patent Document 5, only the increase in the pressurization time due to the plate gap is dealt with, but there is a problem that it may not be said to be an optimal solution for fluctuations caused by multiple disturbances, such as when the included angle is combined with the plate gap.

[0021] The present invention has been made in view of the above problems, and an object thereof is to provide a resistance spot welding method capable of stably forming a nugget having a desired nugget diameter even when disturbances exist.

Means for Solving the Problems

[0022] Now, regarding the condition setting method for solving the above problems and stably forming a nugget having a desired nugget diameter even when disturbances exist, the inventors have intensively conducted experimental studies and obtained the following findings.

[0023] (1) When the plate gap is large, at the start of energization, the contact area between the metal plates (hereinafter also simply referred to as "metal plates") constituting the workpieces to be welded is small, so the resistance is detected as high, and scattering is likely to occur. Also, when the metal plates are overly pressurized with a large plate gap, the metal plates will warp significantly. As a result, the contact area between the metal plates and the electrodes increases excessively, reducing the contact resistance in that area, and the electrical characteristics of the plate assembly change significantly. Additionally, heat extraction from the electrodes is promoted, and the nugget diameter and nugget thickness become smaller. (2) In order to mitigate the influence of such a plate gap, it is effective to set the pressure during energization according to the size of the plate gap and to apply a cumulative heat generation amount corresponding to the contact state between the metal plates during energization, particularly at the start of energization. (3) The influence of the plate gap is reflected in a parameter that serves as an index of the electrode position from the start of pressurization until a predetermined set pressure (initial set pressure) is reached. For example, when pressurizing the metal plates under the same conditions with and without a gap between the metal plates, the change in pressure is the same after the plates come into contact in both cases. (4) At this time, when there is a plate gap, the pressure does not directly transmit to the metal plates, so until the plate gap disappears, the rate of increase in pressure is slower than the setting. (5) However, relying solely on the pressure parameter, variations can become large due to vibrations of the metal plates, etc., and it may not be possible to accurately capture the variation in the plate gap.

[0024] Therefore, based on the above findings, the inventors further studied the method of setting the cumulative heat generation amount during energization according to the size of the plate gap and obtained the following findings. (6) To accurately determine the plate gap state, it is necessary to use one or more parameters in addition to the pressure. As this parameter, an external sensor that captures the change in the shape of the metal plate can be used, and by adding the measured shape obtained thereby, higher accuracy can be achieved. (7) As this mechanism, since the shape change is minimized when the metal plates come into contact with each other, it is considered reasonable to capture the shape change. Therefore, the contact between the metal plate and the electrode is detected by the pressing force parameter, and the shape parameter captures external shape changes. By measuring the time T from the moment when the measured pressing force is first detected until the shape change is minimized, it becomes possible to highly accurately detect the plate gap, which has been difficult in the past. And by setting the target cumulative heat generation amount based on this detection, it becomes possible to stably obtain a nugget with a desired nugget diameter without the occurrence of scattering, regardless of the influence of the plate gap. As a result, it has been found that by capturing the shape change near the electrode, the pressing state and the contact state in the presence of disturbances can be measured more precisely.

[0025] Specifically, it is a resistance spot welding method in which a weldment obtained by stacking a plurality of metal plates is sandwiched between a pair of electrodes, energized while being pressed, and joined. In the pressurizing step before welding, from the time when the pressing force from the metal plate to the electrode is detected until the time when the change in the shape of the metal plate becomes sufficiently small, that is, until it is minimized, the time T is measured. Based on the measured time T, the target cumulative heat generation amount is set, and by controlling the energization amount according to the target cumulative heat generation amount, it has been found that a nugget with a desired nugget diameter can be stably formed even when there are disturbances, and the present invention has been completed.

[0026] That is, the present invention for solving the above problems is as follows. [1] A resistance spot welding method in which a weldment obtained by stacking a plurality of metal plates is sandwiched between a pair of electrodes, energized while being pressed, and joined, in the pressurizing step before welding, measuring the time T from the time when the pressing force from the metal plate to the electrode is detected until the time when the change in the shape of the metal plate becomes sufficiently small, setting a target cumulative heat generation amount based on the measured time T, and controlling the energization amount according to the target cumulative heat generation amount to join the weldment to be joined, which is a resistance spot welding method characterized by this.

[0027] [2] The point in time when the change in the shape of the metal plate becomes sufficiently small means the point in time when the change in the shape of the metal plate is the smaller of either 3% or less and 0.1 mm or less with respect to the total thickness of the plurality of metal plates. The resistance spot welding method according to [1] above. Here, the "change in the shape of the metal plate" refers to the difference between the distance between the outermost surface and the re-innermost surface of the plurality of metal plates and the total thickness of the plurality of metal plates.

[0028] [3] Prior to this welding, under conditions without disturbance, a test weldment in which a plurality of metal plates are stacked is clamped by a pair of electrodes, and from the point in time when the pressing force from the metal plate to the electrode is detected, the time Ts from the point in time when the change in the shape of the metal plate becomes sufficiently small is measured. Based on the time T and Ts, the target cumulative heat generation amount is set. The resistance spot welding method according to [1] or [2] above.

[0029] [4] Prior to this welding, test welding is performed. In the test welding, the cumulative heat generation amount per unit volume calculated from the electrical characteristics between the pair of electrodes when energizing is performed under constant current control to form a nugget with the predetermined nugget diameter is obtained. In the energization of this welding, the cumulative heat generation amount per unit volume obtained in the test welding is set as the target cumulative heat generation amount, and the energization amount is controlled according to the target cumulative heat generation amount. The resistance spot welding method according to any one of [1] to [3] above.

[0030] [5] A method for manufacturing a welded member, in which a welded member is obtained by joining a weldment in which a plurality of metal plates are stacked by the resistance spot welding method according to any one of [1] to [4] above.

Effect of the Invention

[0031] According to the present invention, even when there is disturbance, a nugget with a desired nugget diameter can be stably formed.

Brief Description of the Drawings

[0032]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0033] Hereinafter, embodiments of the present invention will be described with reference to the drawings. As shown in FIG. 1, the resistance spot welding method according to the present invention is a resistance spot welding method in which a workpiece 3 obtained by overlapping a plurality of metal plates 1 and 2 is sandwiched between a pair of electrodes 4 and 5 and energized while being pressurized to join them. Due to the resistance heat generated by the above energization, a dot-shaped weld portion 6 is formed. This dot-shaped weld portion 6 is called a "nugget", and when a welding current is passed through the overlapped metal plates 1 and 2, it is a portion where both metal plates 1 and 2 are melted and solidified at the contact portion of the metal plates 1 and 2, whereby the metal plates 1 and 2 are joined pointwise.

[0034] In the resistance spot welding method according to the present invention, in the pressurization step before welding, the time T from the time when the pressure from the metal plate to the electrode is detected until the change in the shape of the metal plate becomes sufficiently small is measured, and a target cumulative heat generation amount is set based on the measured time T, and the energization amount is controlled according to the target cumulative heat generation amount to join the workpiece to be joined.

[0035] As described above, in order to mitigate the influence of disturbances such as plate gaps, it is effective to set a target cumulative heat generation amount according to the magnitude of the disturbance. Further, the influence of the disturbance is reflected in a parameter that is an index of the electrode position (electrode movement amount) from the start to the end of pressurization. Therefore, precisely detecting the influence of this disturbance is a necessary requirement for obtaining a nugget with a stable nugget diameter regardless of the disturbance.

[0036] For example, when there is no gap between metal plates 1 and 2 as shown in Fig. 2(a), and when there is a gap between metal plates 1 and 2 as shown in Fig. 2(b), when the metal plates 1 and 2 are pressurized under the same conditions respectively, the movement amount (i.e., the distance between the electrodes) L of the electrodes 4 and 5 from the start of pressurization to reaching the initial set pressure will be different.

[0037] That is, when there is no gap between metal plates 1 and 2, after the start of pressurization by the electrodes 4 and 5, the initial set pressure is reached with a relatively small movement amount of the electrodes 4 and 5. On the other hand, when there is a gap between metal plates 1 and 2, in the initial stage of pressurization, the metal plates 1 and 2 will be deformed to bring them into contact. Therefore, the increasing rate of pressurization is different before and after contact, and the movement amount of the electrodes 4 and 5 until the initial set pressure is reached increases.

[0038] However, it may be difficult to accurately detect the distance between the electrodes (the movement amount of the electrodes). In particular, when the metal plates 1 and 2 come into contact, an impact occurs, which may cause an error in the measurement of the pressure. Also, the plastic deformation of the metal plates 1 and 2 is also a cause of the measurement position error. In contrast, compared with the contact time point between the metal plates 1 and 2, the contact between the electrodes 4 and 5 and the metal plates 1 and 2 is at the timing when the pressure applied to the electrodes 4 and 5 switches from zero to a significant value, so it is easier to detect.

[0039] FIG. 3 shows the relationship between time and the shape changes of metal plates 1 and 2 and the measured pressing force of electrodes 4 and 5. (a) shows the relationship between time and the shape changes of metal plates 1 and 2, and (b) shows the relationship between time and the measured pressing force of electrodes 4 and 5, respectively. Note that the "shape change" in FIG. 3(a) indicates the position of the measurement unit of the shape measuring machine, and the change in the position of the measurement unit indicates the shape change of metal plates 1 and 2. As shown in FIG. 3(a), before the electrodes 4 and 5 contact the metal plates 1 and 2 (before time 0), there is no shape change in the metal plates 1 and 2. However, when the electrodes 4 and 5 contact the metal plates 1 and 2 and a pressing force is applied to the metal plates 1 and 2, the shape of the metal plates 1 and 2 changes. Then, when the gap between the metal plates 1 and 2 disappears (after time T), the shape change of the metal plates 1 and 2 disappears (i.e., is minimized). On the other hand, as shown in FIG. 3(b), when the electrodes 4 and 5 contact the metal plates 1 and 2 (after time 0), the pressing force gradually increases from zero, and when the gap between the metal plates 1 and 2 disappears (after time T), it becomes a constant value.

[0040] Therefore, in the present invention, as shown in FIG. 4, the shape measuring machine 7 is configured to be able to measure the pressing force transmitted from the metal plates 1 and 2 to the electrodes 4 and 5 and the shape changes of the metal plates 1 and 2 during welding. In the pressurizing process before welding, the shape changes of the metal plates 1 and 2 are configured to be measured over time. By measuring the time T from the point when the pressing force from the metal plates 1 and 2 to the electrodes 4 and 5 is detected to the point when the shape change of the metal plates 1 and 2 is minimized, an accurate measurement of the plate gap distance (i.e., the size of the gap) is performed.

[0041] Examples of the parameters serving as indicators of the above electrode positions include · The amount of electrode displacement, electrode displacement speed, and pressing force directly measured by an external sensor · The amount of electrode displacement and pressing force indirectly measured by the strain of the gun or housing · In the case of a servo pressurizing mechanism, the reaction force detected by the mechanism, the torque value, rotation speed, and number of rotations of the servo motor · The above variation time and so on.

[0042] Examples of the parameters for measuring the shape change include ·Contact measuring instrument ·Non-contact measuring instrument such as a laser sensor Either of them may be used, ·Distance variation at a specific position ·Distribution of shapes on a straight line or a curve ·Three-dimensional surface shape Either of them may be used. It is desirable to detect the plate gap using these parameters.

[0043] Also, regarding the threshold value when it is determined that the shape changes of the metal plates 1 and 2 are minimized, although it should be determined industrially, for example, considering the deformation and response control of the metal plates 1 and 2, when it becomes the smaller value of either 3% or less of the total plate thickness and 0.1 mm or less, it is desirable to determine that the shape changes of the metal plates 1 and 2 are minimized. The above-mentioned "shape changes of the metal plates 1 and 2" are preferably measured in a distance range of 100 mm or less from the tips of the electrodes 4 and 5 in terms of reducing the measurement error of the shape changes. Here, the "shape changes of the metal plate" refers to the difference between the distance between the outermost surface and the rearmost surface of the plurality of metal plates and the total thickness of the plurality of metal plates.

[0044] In the present invention, it is necessary to measure the time when the metal plates 1 and 2 come into contact with the electrodes 4 and 5. For calculating this, three or more methods may be combined. For example, the positions of the electrodes 4 and 5 are determined using the average of both the electrode sensor and the pressing force, and the time when the metal plates 1 and 2 come into contact with the electrodes 4 and 5 is measured using the shape changes of the metal plates 1 and 2, etc.

[0045] Thus, the present invention is characterized by a method for setting the target cumulative heat generation amount during the main welding, and other configurations are not particularly limited.

[0046] As a welding apparatus that can be used in the resistance spot welding method according to the present invention, it suffices to be provided with a pair of upper and lower electrodes and to be able to arbitrarily control the pressing force and the welding current during welding. The form (stationary type, robot gun, etc.), the electrode shape, etc. are not particularly limited.

[0047] The plurality of metal plates 1 and 2 constituting the workpieces to be joined can be, for example, steel plates. The steel plates may be steel plates without surface treatment. Further, the steel plates may be surface-treated steel plates with surface treatment such as plating, or two or more of the plurality of metal plates 1 and 2 may be steel plates having a plating layer. Further, all of the plurality of steel plates may be steel plates having a plating layer.

[0048] Examples of the steel plate having the plating layer include zinc-based plated steel plates. The zinc-based plated steel plate is a steel plate in which a zinc-based plating is coated on the steel plate by various methods such as a hot-dip plating method, an electroplating method, a vapor deposition plating method, and a spraying method. Examples of the zinc-based plating include hot-dip zinc plating, alloyed hot-dip zinc plating, hot-dip zinc-aluminum alloy plating, hot-dip zinc-aluminum-magnesium alloy plating, electro-zinc plating, electro-zinc-nickel alloy plating, etc., but are not limited thereto, and all known zinc-based platings containing zinc are applicable. Examples of such zinc-based plated steel plates include hot-dip zinc-plated steel plates (GI) without alloying treatment and alloyed hot-dip zinc-plated steel plates (GA) with alloying treatment.

[0049] In the present invention, prior to this welding, under conditions without disturbance, a test workpiece in which a plurality of metal plates 1 and 2 are overlapped is sandwiched between a pair of electrodes, and from the time when the pressing force from the metal plates 1 and 2 to the electrodes is detected, it is preferable to measure the time Ts until the change in the shape of the metal plates 1 and 2 becomes sufficiently small, and based on the time T and Ts, set the target cumulative heat generation amount. Thereby, welding can be stabilized even when the measurement time fluctuates due to various disturbances.

[0050] Further, in the present invention, prior to this welding, test welding is performed under conditions without disturbance. In the test welding, when energizing by constant current control to form a nugget with a predetermined nugget diameter, the cumulative heat generation amount per unit volume calculated from the electrical characteristics between a pair of electrodes is obtained. In the energization of this welding, it is preferable to set the cumulative heat generation amount per unit volume obtained in the test welding as the target cumulative heat generation amount, and control the energization amount according to the target cumulative heat generation amount. Thereby, it can be controlled based on the heat generation phenomenon, and the welding can be stabilized.

[0051] Here, the method for calculating the heat generation amount is not particularly limited, but an example is disclosed in Patent Document 2, and the above method can also be adopted in the present invention. The calculation procedures for the heat generation amount q per unit volume and unit time and the cumulative heat generation amount Q per unit volume by the method described in Patent Document 2 are as follows.

[0052] That is, let the total thickness of the material to be welded such as a steel plate be t, the electrical resistivity of the material to be welded be r, the voltage between the electrodes be V, the welding current be I, and the area where the electrode and the material to be welded contact be S. In this case, the welding current I passes through a columnar portion with a cross-sectional area of S and a thickness of t to generate resistance heat. The heat generation amount q per unit volume and unit time in this columnar portion is obtained by the following formula (1). q=(V·I) / (S·t) (1)

[0053] Also, the electrical resistance R of the above columnar portion is obtained by the following formula (2). R=(r·t) / S (2)

[0054] Solve formula (2) for S and substitute this into formula (1), then the heat generation amount q is given by the following formula (3). q=(V·I·R) / (r·t 2 )=(V 2 ) / (r·t 2 ) (3)

[0055] As is apparent from the above formula (3), the calorific value q per unit volume per unit time can be calculated from the voltage V between the electrodes, the total thickness t of the metal plates, and the electrical resistivity r of the metal plates, and is not affected by the contact area S between the electrodes and the metal plates. Although formula (3) calculates the calorific value q from the voltage V between the electrodes, the calorific value q can also be calculated from the current I between the electrodes, and in this case as well, it is not necessary to use the contact area S between the electrodes and the metal plates. Then, by accumulating the calorific value q per unit volume per unit time over the energization period, the cumulative calorific value Q per unit volume applied during welding is obtained. As is apparent from formula (3), this cumulative calorific value Q per unit volume can also be calculated without using the contact area S between the electrodes and the metal plates.

[0056] The pressing force P set in the present invention may be appropriately set according to the material and thickness of the metal plates 1 and 2 constituting the workpieces to be welded 3. For example, when using a plate assembly in which two sheets of 270 to 2000 MPa grade steel plates plated with Zn on the surface with a thickness of 1.4 mm are stacked as the workpieces to be welded, the initial set pressing force is preferably 1.0 to 7.0 kN. Further, when using a plate assembly in which three sheets of 270 to 2000 MPa grade steel plates plated with Zn on the surface with a thickness of 1.4 mm are stacked as the workpieces to be welded, the initial set pressing force is preferably 2.0 kN to 10.0 kN.

[0057] The present invention performs adaptive control welding. This means that welding is performed based on the target value of the calorific value described above. When the time change amount of the instantaneous calorific value per unit volume follows the time change curve that is the standard, welding is continued as it is until welding is completed. However, when the time change amount of the instantaneous calorific value per unit volume deviates from the time change curve that is the standard, in order to compensate for the difference in calorific value within the remaining energization time, the energization amount is controlled so that the cumulative calorific value per unit volume in this welding matches the cumulative calorific value per unit volume set as the target value.

[0058] The desired nugget diameter of the nugget formed by the resistance spot welding method according to the present invention may be any value greater than that required for production. However, of the two plates constituting the plate gap, assuming the thickness of the thinner plate is t (mm), it is desirable to aim for a nugget diameter in the plate gap of 4√t (mm) or more.

[0059] (Method for manufacturing welded member) The method for manufacturing a welded member according to the present invention is characterized by joining a plurality of stacked workpieces by the resistance spot welding method according to the present invention described above.

[0060] As described above, in the resistance spot welding method according to the present invention, in the pre-welding pressurization step, the time T from the point when the pressure applied from the metal plate to the electrode is detected until the change in the shape of the metal plate becomes sufficiently small is measured. Based on the measured time T, a target cumulative heat generation amount is set, and the energization amount is controlled according to the target cumulative heat generation amount. Thereby, even when there are disturbances, a nugget with a desired nugget diameter can be formed. Therefore, by joining a plurality of stacked metal plates as workpieces by the resistance spot welding method according to the present invention, a welded member with high welding strength having the desired nugget diameter can be manufactured.

Examples

[0061] Hereinafter, examples of the present invention will be described, but the present invention is not limited to the examples.

[0062] For the plate sets No. 1 to 22 composed of metal plates shown in Table 1, when there was no plate gap or when there was a plate gap, this welding was carried out under the conditions shown in Table 1 to fabricate welded joints. For each of the obtained welded joints, the welded part was cut, the cross-section was etched, and then observed with an optical microscope to measure the nugget diameter. The measured nugget diameter and the variation value of the measured nugget diameter from the target nugget diameter were evaluated as follows. At that time, the target nugget diameter was based on the case where there was no plate gap, and was set in the range of 0.1 mm to 3.0 mm for the plate gap, and the variations in those cases were compared. The obtained evaluation results are shown in Table 1. Note that Es in Table 1 represents the target cumulative heat input when there was no plate gap, and E represents the target cumulative heat input set according to the present invention. Also, for the plate sets No. 9 and 10 composed of three metal plates, the plate gap was provided only between the second and the third metal plates.

[0063]

Table 1

[0064] Diameter determination: ○ (Qualified): The nugget diameter is larger than the target nugget diameter. × (Unqualified): The nugget diameter is smaller than the target nugget diameter.

[0065] Diameter variation: 〇 (Qualified): The variation of the nugget diameter is within the square root of 0.3 × (plate thickness (mm)) or less from the target nugget diameter. × (Unqualified): The variation of the nugget diameter exceeds the square root of 0.3 × (plate thickness (mm)) from the target nugget diameter. Here, the (plate thickness) refers to the plate thickness (mm) of the thinnest steel plate in the plate set.

[0066] As is clear from Table 1, in all the inventive examples, not only was a predetermined nugget diameter obtained regardless of the plate gap, but also the variation value was small. On the other hand, in the conventional examples, although a predetermined nugget diameter was obtained, the variation value was large.

Industrial Applicability

[0067] According to the present invention, even when there is disturbance, a nugget having a desired nugget diameter can be stably formed.

Explanation of Signs

[0068] 1, 2 Metal plates 3 Workpiece to be welded 4, 5 Electrodes 6 Weld zone (nugget) 7 Shape measuring machine

Claims

1. A resistance spot welding method in which a weldment obtained by stacking a plurality of metal plates is sandwiched between a pair of electrodes, energized while being pressurized, and joined, comprising: In the pressurization step before welding, the time T from the point in time when the pressure applied from the metal plate to the electrode is detected until the change in the shape of the metal plate becomes sufficiently small is measured. Based on the measured time T, a target cumulative heat generation amount per unit volume is set, and the energization amount is controlled according to the target cumulative heat generation amount to join the weldment to be joined by adaptive control welding. Prior to this welding, under conditions without disturbance, a test weldment obtained by stacking a plurality of metal plates is sandwiched between a pair of electrodes, and the time Ts from the point in time when the pressure applied from the metal plate to the electrode is detected until the change in the shape of the metal plate becomes sufficiently small is measured. Based on the times T and Ts, the target cumulative heat generation amount is set. When the time change amount of the instantaneous heat generation amount per unit volume deviates from a time change curve that is a reference, the energization amount is controlled so that the cumulative heat generation amount per unit volume in this welding matches the target cumulative heat generation amount in order to compensate for the difference between the instantaneous heat generation amount and the time change curve within the remaining energization time. A resistance spot welding method characterized by this.

2. The point in time when the change in the shape of the metal plate becomes sufficiently small is the point in time when the change in the shape of the metal plate is smaller than either 3% or less of the total thickness of the plurality of metal plates and 0.1 mm or less. The resistance spot welding method according to Claim 1.

3. Performing a test weld prior to this welding. In the test weld, the cumulative heat generation amount per unit volume calculated from the electrical characteristics between the pair of electrodes when forming a nugget with a predetermined nugget diameter by energization under constant current control is obtained. In the energization of this welding, the cumulative heat generation amount per unit volume obtained in the test weld is set as the target cumulative heat generation amount, and the energization amount is controlled according to the target cumulative heat generation amount. The resistance spot welding method according to Claim 1 or 2.

4. A method for manufacturing a welded member, comprising joining a weldment obtained by stacking a plurality of metal plates by the resistance spot welding method according to Claim 1 or 2 to obtain a welded member.

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