JP2021112773A - Resistance spot welding method, method for manufacturing welding member, and welding apparatus - Google Patents

Abstract

To provide a resistance spot welding method by which a desired nugget diameter may be stably obtained without occurrence of dispersion even if there is disturbance effect, particularly, a large clearance between metal plates constituting an object to be welded.SOLUTION: Before starting energization for regular welding, an objected to be welded is pressurized until reaching an initial setting pressurizing force. Subsequently, a pressurizing force during the energization for the regular welding is set using a parameter that becomes an index of a pressurizing force obtained until reaching the initial setting pressurizing force from a pressurization start point before staring the energization for the regular welding.SELECTED DRAWING: Figure 1

Description

The present invention relates to a resistance spot welding method, and aims to make it possible to stably secure a desired nugget diameter without causing scattering even when the influence of disturbance is particularly large.

Generally, a resistance spot welding method, which is a kind of lap resistance welding method, is used for joining the stacked steel plates.
This welding method is a method in which two or more stacked steel plates are sandwiched and pressed by a pair of electrodes from above and below, and a high current welding current is applied between the upper and lower electrodes for a short time to join them. A point-shaped welded portion can be obtained by utilizing the resistance heat generated by passing the welding current of. This point-shaped welded portion is called a nugget, and is a portion where both steel plates are melted and solidified at a contact point between the steel plates when an electric current is passed through the stacked steel plates. With this nugget, the steel plates are joined in dots.

In order to obtain good weld quality, it is important that the nugget diameter is formed in 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 form 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 of the material to be welded, the plate thickness, and the number of stacked sheets.

For example, in the manufacture of automobiles, thousands of spot welds are performed on each vehicle, and it is necessary to weld the materials (workpieces) to be treated that flow one after another. At this time, if the state of the material to be welded such as the material, plate thickness and the number of layers of the material to be welded at each welding location is the same, the welding conditions such as the welding current, energization time and pressing force are also the same under the same conditions. You can get the nugget diameter. However, in continuous welding, the contact surface of the electrode to be welded is gradually worn and the contact area is gradually wider than in the initial state. If a welding current of the same value as the initial state is applied in a state where the contact area is widened in this way, the current density in the material to be welded decreases and the temperature rise of the welded portion decreases, so that the nugget diameter becomes smaller. .. Therefore, the electrode is polished or replaced every several hundred to several thousand points of welding so that the tip diameter of the electrode does not expand too much.

In addition, a resistance welding device having a function (stepper function) of increasing the welding current value when welding is performed a predetermined number of times to compensate for a decrease in current density due to electrode wear has been conventionally used. .. In order to use this stepper function, it is necessary to properly set the above-mentioned welding current change pattern in advance. However, for this reason, it takes a lot of time and cost to derive a welding current change pattern corresponding to a large number of welding conditions and welding material conditions by a test or the like. Further, in actual construction, since the progress state of electrode wear varies, it cannot be said that the predetermined welding current change pattern is always appropriate.

Further, when there is a disturbance during welding, for example, when there is an already welded point (already welded point) near the welding point, or when the surface unevenness of the material to be welded is large, the material to be welded is near the point where it is welded. If there is a contact point, a current is diverted to the existing weld point or contact point during welding. In such a state, even if welding is performed under predetermined conditions, the current density at the position to be welded immediately below the electrode decreases, so that a nugget having a required diameter cannot be obtained. In order to compensate for this insufficient calorific value and obtain a nugget with a required diameter, it is necessary to set a high welding current in advance.

In addition, when the circumference of the welding point is strongly restrained by the surface unevenness or the shape of the member, or when foreign matter is caught between the steel plates around the welding point, the plate gap between the steel plates becomes large. In some cases, the contact diameter between the steel sheets is narrowed, and scattering is likely to occur.

The following techniques have been proposed to solve the above problems.
For example, Patent Document 1 describes a first step of generating a nugget by gradually increasing the energizing current of a high-strength steel plate, a second step of lowering the current after the first step, and the second step. After the step, the current is increased for main welding, and spot welding is performed by a step including a third step of gradually decreasing the energizing current in an attempt to suppress scattering due to poor familiarity at the initial stage of energization. A method of spot welding a high-strength steel plate is described.

In Patent Document 2, the surface of the workpiece is softened by maintaining a current value that can suppress the occurrence of spatter at the initial stage of the energization time for a predetermined time, and then the current value is maintained high for a predetermined time to generate spatter. The energization control method of spot welding that grows the nugget while suppressing the pressure is described.

Patent Document 3 describes a control device for a resistance welder that attempts to obtain a set nugget diameter by controlling the output of the welder by comparing the estimated temperature distribution of the welded portion with the target nugget. ..

Patent Document 4 attempts to perform good welding by detecting the welding current and the inter-chip voltage, simulating the welded portion by heat conduction calculation, and estimating the nugget formation state of the welded portion during welding. The welding condition control method of the resistance welding machine is described.

In Patent Document 5, the cumulative calorific value per unit volume at which the object to be welded can be welded satisfactorily is calculated from the plate thickness of the object to be welded and the energization time, and the calculated unit volume and per unit time. A resistance welding system that attempts to perform good welding regardless of the type of object to be welded or the wear state of the electrodes is described by using a welding system that adjusts to the welding current or voltage that generates the calorific value of the above. There is.

Japanese Unexamined Patent Publication No. 2003-236674 Japanese Unexamined Patent Publication No. 2006-43731 Japanese Unexamined Patent Publication No. 9-216071 Japanese Unexamined Patent Publication No. 10-94883 Japanese Unexamined Patent Publication No. 11-33743

However, in the techniques described in Patent Documents 1 and 2, it is considered that the appropriate welding conditions change depending on the presence or absence of disturbance and the magnitude of the disturbance. When this occurs, there is a problem that a desired nugget diameter cannot be secured without causing scattering.

Further, in the techniques described in Patent Documents 3 and 4, since the temperature of the nugget is estimated based on a heat conduction model (heat conduction simulation) or the like, complicated calculation processing is required, and the configuration of the welding control device becomes complicated. Not only that, there is a problem that the welding control device itself becomes expensive.

Further, in the technique described in Patent Document 5, it is considered that by controlling the cumulative calorific value to a target value, good welding can be performed even if the electrodes are worn by a certain amount.
However, when the set conditions for the material to be welded and the actual conditions for the material to be welded are significantly different, for example, when there is a large gap between the metal plates to be the material to be welded, the final cumulative calorific value is calculated. Even if the target value can be met, the form of heat generation, that is, the time change of the temperature distribution of the weld, deviates from the calorific value pattern that can obtain the target good weld, and the required nugget diameter cannot be obtained. , Scattering occurs.

The present invention has been developed in view of the above situation, and even when the influence of disturbance, particularly the gap between the metal plates constituting the material to be welded (hereinafter, also referred to as "plate gap") is large. It is an object of the present invention to provide a resistance spot welding method capable of stably obtaining a desired nugget diameter without the occurrence of scattering.
Another object of the present invention is to provide a method for manufacturing a welded member, which joins a plurality of overlapping metal plates by the above-mentioned resistance spot welding method.

By the way, the inventors have made extensive studies in order to achieve the above object, and obtained the following findings.
(1) When the plate gap is large, the contact area between the metal plates (hereinafter, also simply referred to as “metal plates”) constituting the material to be welded is small at the start of energization, so that scattering is likely to occur. Further, when the plate gap is large, if the metal plate is excessively pressed by the electrodes, the metal plate warps significantly. As a result, the contact area between the metal plate and the electrode is excessively increased to promote heat removal to the electrode, and as a result, the nugget diameter and the nugget thickness may be reduced.
(2) In order to mitigate the influence of such a plate gap, the pressing force at the time of energization is set according to the size of the plate gap, and it is appropriate between the metal plates at the time of energization, especially at the start of energization. It is effective to secure a good contact area.

Therefore, based on the above findings, the inventors further studied a method of setting the pressing force at the time of energization according to the size of the plate gap, and obtained the following findings.
(3) The influence of the plate gap is reflected in the parameter that is an index of the pressing force from the start of the pressurization to the arrival of the predetermined set pressing force.
For example, when there is no gap between the metal plates and when there is a gap between the metal plates, if the metal plates are pressurized under the same conditions, the pressurization starts and then the predetermined set pressing force is reached. The time to time and the behavior of the servo motor will be different.
(4) Therefore, first, before the start of energization, the material to be welded is pressurized until it reaches a predetermined set pressing force (hereinafter, also referred to as initial set pressing force).
Then, using a parameter that is an index of the pressing force obtained from the start of pressurization before the start of energization until the initial setting pressing force is reached, as shown in FIG. 1, in consideration of the influence of the plate gap (initially). Set pressing force: _{Separately from F 0} ) Energizing pressure: _{Set F A} to energize, especially set the energizing pressure within the range that satisfies the following formula (1) and energize. I do,
By
At the start of energization, an appropriate contact area can be secured between the metal plates, and as a result, a desired nugget diameter can be stably obtained regardless of the influence of the plate gap and without the occurrence of scattering. ..
F ₀ × (1 + 0.1 × (T _A -T ₀ ) / T ₀ ) ≤ F _A ≤ F ₀ × (1 + 3.0 × (T _A -T ₀ ) / T ₀ ) ・ ・ ・ (1)
here,
F _A : Pressurizing pressure when energized in main welding
F ₀ : Initial setting pressing force in main welding
T _A : Time from the start of pressurization before the start of energization in main welding to the arrival of the initial set pressing force
T ₀ : The time from the start of pressurization before the start of energization to the arrival of the initial set pressing force when there is no gap between the metal plates constituting the material to be welded.
The present invention has been completed with further studies based on the above findings.

That is, the gist structure of the present invention is as follows.
1. 1. This is a resistance spot welding method in which a material to be welded, which is a stack of multiple metal plates, is sandwiched between a pair of electrodes and energized while being pressurized.
Before the start of energization of the main welding, the material to be welded is pressurized until the initial set pressing force is reached.
Then, the pressing force at the time of energization of the main welding is set by using the parameter which is an index of the pressing force obtained from the start of pressurization before the start of energization of the main welding until the initial setting pressing force is reached. ,
Resistance spot welding method.

2. The resistance spot welding method according to 1 above, wherein the pressing force at the time of energization of the main welding is set within a range satisfying the following equation (1).
F ₀ × (1 + 0.1 × (T _A -T ₀ ) / T ₀ ) ≤ F _A ≤ F ₀ × (1 + 3.0 × (T _A -T ₀ ) / T ₀ ) ・ ・ ・ (1)
here,
F _A : Pressurizing pressure when energized in main welding
F ₀ : Initial setting pressing force in main welding
T _A : Time from the start of pressurization before the start of energization in main welding to the arrival of the initial set pressing force
T ₀ : The time from the start of pressurization before the start of energization to the arrival of the initial set pressing force when there is no gap between the metal plates constituting the material to be welded.

3. 3. Prior to the main welding, test welding shall be performed.
In the test welding, the time change curve of the instantaneous calorific value per unit volume and the cumulative calorific value per unit volume calculated from the electrical characteristics between the electrodes when energized by constant current control to form an appropriate nugget are obtained. Remember,
Further, in the energization of the main welding, the time change curve of the instantaneous calorific value per unit volume and the cumulative calorific value per unit volume in the test welding are set as target values, and the energization amount is controlled according to the target value. The resistance spot welding method according to 1 or 2.

4. A method for manufacturing a welded member, in which a plurality of stacked metal plates are joined by the resistance spot welding method according to any one of 1 to 3 above.

According to the present invention, a good nugget can be obtained without the occurrence of scattering even when the influence of disturbance, particularly when the plate gap is large.
Further, according to the present invention, it is a case where the materials to be treated that flow one after another in actual work such as manufacturing of an automobile are continuously welded (the state of disturbance varies depending on the welding position and the material to be treated). However, it is possible to stably secure a desired nugget diameter in response to fluctuations in the state of disturbance, and as a result, it is extremely advantageous in terms of improving work efficiency and yield.

It is a figure which shows the relationship between the pressing force and time, and the welding current and time of the resistance spot welding method which concerns on one Embodiment of this invention. When (a) there is no gap between the metal plates to be welded and (b) when there is a gap between the metal plates, the metal plate to be welded is pressurized under the same conditions. , The relationship between the pressing force and the time, and the welding current and the time. It is a figure which shows typically an example of the case where (a) a two-ply plate assembly and (b) a three-ply plate assembly are welded without a plate gap (disturbance). It is a figure which shows typically an example of the case where welding is performed on (a) the two-ply board assembly with a plate gap, and (b) the three-ply plate assembly with a plate gap. It is a figure which shows typically an example of 1-step energization in test welding. It is a figure which shows typically an example of two-step energization in test welding.

The present invention will be described based on the following embodiments.
One embodiment of the present invention is a resistance spot welding method in which a material to be welded in which a plurality of metal plates are superposed is sandwiched between a pair of electrodes and energized while being pressurized.
Before the start of energization of the main welding, the material to be welded is pressurized until the initial set pressing force is reached.
Then, the pressing force at the time of energization of the main welding is set by using the parameter which is an index of the pressing force obtained from the start of pressurization before the start of energization of the main welding until the initial setting pressing force is reached. ,.
The main welding means a process of actually welding the target material to be welded, and is used to distinguish it from the test welding described later.

The welding apparatus that can be used in the resistance spot welding method according to the embodiment of the present invention may be provided as long as it is provided with a pair of upper and lower electrodes and can arbitrarily control the pressing force and the welding current during welding. (Fixed type, robot gun, etc.), electrode shape, etc. are not particularly limited.

Hereinafter, the main welding of the resistance spot welding method according to the embodiment of the present invention will be described.

(1) Main welding As described above, in order to mitigate the influence of the plate gap, the pressing force at the time of energization is set according to the size of the plate gap, and the metal is energized, especially at the start of energization. It is effective to secure an appropriate contact area between the boards.
Further, the influence of the plate gap is reflected in the parameter which is an index of the pressing force from the start of the pressurization to the arrival of the predetermined set pressing force.
Therefore, before the start of energization of the main welding, the material to be welded is pressurized until it reaches the initial set pressing force, and then it is obtained from the time of the start of pressurization before the start of energization of the main welding until the initial set pressing force is reached. It is important to set the pressing force when the main welding is energized by using the parameter that is the index of the pressing force.

For example, as shown in FIG. 2, when the metal plates are pressurized under the same conditions when there is no gap between the metal plates and when there is a gap between the metal plates, the pressure is determined after the pressurization is started. Time to reach the set pressing force of: T _A will be different for each.
That is, when there is no gap between the metal plates, the initial set pressing force is reached in a relatively short time after the start of pressurization by the electrodes. On the other hand, if there is a plate gap between the metal plates, the metal plate is deformed and the plate gap is crushed (contact between the metal plates) in the initial stage of pressurization, so until the initial set pressing force is reached. The time will be longer.
Therefore, time to reach the set pressure from the start pressure: using a parameter indicative of the obtained pressure until the pressure beginning before the energization start such T _A to reach the initial set pressure , As shown in FIG. 1, in consideration of the influence of the plate gap, the pressing force at the time of energization: F _{A is set (in addition to the initial setting pressing force: F 0} ) to energize, especially at the time of energizing. By setting the pressing force of the above within a range satisfying the following formula (1) and energizing, an appropriate contact area can be secured between the metal plates at the start of energization, and as a result, the plate gap can be secured. Regardless of the influence, it is possible to stably obtain a desired nugget diameter without the occurrence of scattering.
F ₀ × (1 + 0.1 × (T _A -T ₀ ) / T ₀ ) ≤ F _A ≤ F ₀ × (1 + 3.0 × (T _A -T ₀ ) / T ₀ ) ・ ・ ・ (1)
here,
F _A : Pressurizing pressure when energized in main welding
F ₀ : Initial setting pressing force in main welding
T _A : Time from the start of pressurization before the start of energization in main welding to the arrival of the initial set pressing force
T ₀ : The time from the start of pressurization before the start of energization to the arrival of the initial set pressing force when there is no gap between the metal plates constituting the material to be welded.

Here, when the pressing force at the time of energization in the main welding: F _A is _{less than F 0} × (1 + 0.1 × (T _A -T ₀ ) / T ₀ ), sufficient contact between the metal plates is made at the start of energization. It becomes difficult to secure the area. On the other hand, when F _A becomes F ₀ × (1 + 3.0 × (T _A -T ₀ ) / T ₀ ), the contact area between the metal plate and the electrode is excessively increased, and heat removal to the electrode is promoted, resulting in this. Therefore, the nugget diameter and the nugget thickness may be reduced.
Therefore, when the main welding is energized, the influence of the plate gap is taken into consideration by using the parameter that is an index of the pressing force obtained from the start of pressurization before the start of energization of the main welding until the initial set pressing force is reached. It is important to set the pressing force and energize, and in particular, it is preferable to set the pressing force at the time of energizing the main welding within a range satisfying the above formula (1) and energize. Regarding the above formula (1), more preferably, F ₀ × (1 + 0.2 × (T _A -T ₀ ) / T ₀ ) or more, F ₀ × (1 + 2.0 × (T _A -T ₀ ) / T). ₀ ) It is less than or equal to.

In addition, as a parameter that is an index of pressing force,
・ The time from the start of pressurization before the start of energization in the main welding to the arrival of the initial set pressing force,
・ Time from the start of pressurization before the start of energization in the main welding to the start of energization,
・ Torque of the servo motor of the welding gun from the start of pressurization before the start of energization in the main welding until the initial set pressing force is reached.
・ Rotation speed of the servo motor of the welding gun from the start of pressurization before the start of energization in the main welding to the arrival of the initial set pressing force,
・ Strain of the welding gun from the start of pressurization before the start of energization in the main welding until the initial set pressing force is reached,
・ The amount of electrode displacement from the start of pressurization before the start of energization in the main welding to the arrival of the initial set pressing force, and ・ The electrode from the start of pressurization before the start of energization in the main welding to the arrival of the initial set pressing force Displacement speed,
It is preferable that the pressing force at the time of energization in the main welding is set within a range satisfying the above formula (1) by using these parameters, and the energization is performed.

For example, the time from the start of pressurization before the start of energization in the main welding to the start of energization is [the time from the start of pressurization before the start of energization in the main welding to the arrival of the initial set pressing force] + [main welding. It can be expressed by the time from the time when the initial setting pressure is reached to the time when the energization starts.
Therefore, if the time from the start of pressurization before the start of energization in the main welding to the start of energization is used as a parameter to be an index of the pressing force, [Energization starts from the start of pressurization before the start of energization in the main welding. Satisfy the above formula (1) for the pressing force at the time of energization in the main welding by subtracting [the time from the time when the initial setting pressing force in the main welding is reached to the time when the energization starts] from]. It is possible to set within the range to be welded.
Further, when the torque of the servomotor of the welding gun (hereinafter, also simply referred to as torque) from the start of pressurization before the start of energization in the main welding to the arrival of the initial set pressurization is used as a parameter as an index of pressurization. When the electrode comes into contact with the metal plate to be welded, the torque begins to increase rapidly, and then when sufficient pressure is applied to the steel plate, the torque reaches a stable value and saturates.
Therefore, the time from the start of torque increase to saturation is the time from the start of pressurization to the arrival of the initial set pressing force (in other words, the start point of torque increase is the start point of pressurization, and the increase in torque is By (determining that the time when the torque becomes constant after saturation is the time when the initial setting pressure is reached), the pressure at the time of energization in the main welding is set within the range that satisfies the above formula (1). It is possible.
Further, the rotation speed (hereinafter, simply referred to as the rotation speed) of the servomotor of the welding gun from the start of pressurization before the start of energization in the main welding to the arrival of the initial set pressurization is used as a parameter as an index of pressurization. In this case, when the electrode comes into contact with the metal plate to be welded, the rotation speed becomes unstable and then gradually decreases. Then, when the initial setting pressing force is reached, the electrodes do not move, so that the rotation speed becomes zero.
Therefore, the time from the start of pressurization to reaching 0 after the rotation speed becomes unstable and starts to decrease is defined as the time from the start of pressurization to the arrival of the initial set pressing force (in other words, the rotation speed is unsatisfactory). The time when the pressure becomes stable is determined to be the time when the pressurization starts, and the time when the rotation speed becomes 0 is determined to be the time when the initial set pressure is reached). ) Can be set within a satisfactory range.
In addition, by combining parameters that are indicators of multiple pressurization, such as determining the start point of torque increase as the start point of pressurization and the time point when the rotation speed becomes 0 as the time point of reaching the initial set pressurization, this The pressing force at the time of energization in welding may be set within a range satisfying the above formula (1).

In addition, the initial setting pressing force in the main welding may be appropriately set according to the material and thickness of the metal plate constituting the material to be welded.
For example, when using a plate set of two 270 to 2000 MPa class steel plates with a thickness of 1.6 mm and a material: no plating or plating containing Zn on the surface as the material to be welded, the initial set pressing force is 1.0 to. It is preferably 7.0 kN.
In addition, when using a plate set of three 270 to 2000 MPa class steel plates with a thickness of 1.6 mm and a material: no plating or plating containing Zn on the surface as the material to be welded, the initial set pressing force is 2.0 kN. It is preferably ~ 10.0 kN.

Further, when there is no gap between the metal plates constituting the material to be welded, particularly when there is no disturbance, the time from the start of pressurization before the start of energization to the arrival of the initial set pressing force: T ₀ is, for example. By separately preparing a material to be welded, which is composed of metal plates of the same thickness and material as the main welding, with no gaps between the metal plates, and pressurizing the material to be welded under the same pressure conditions as the main welding. , Just ask.
In addition, when performing the test welding described later, the set pressing force at the time of the test welding may be used as it is as the initial set pressing force of the main welding.
In a general resistance spot welding device, the control method is switched when a certain pressing force (hereinafter, also referred to as a switching set value) between the start of pressurization and the arrival of the set pressing force is reached. (Position control → torque control) is performed, and normally, if the same pressing means is used and the switching set value is the same, it can be said that the pressure is applied under the same conditions.

Further, the energization in the main welding is not particularly limited, and may be performed by constant current control, or after performing the test welding described later, adaptive control for controlling the amount of energization according to the target value set in the test welding. Welding may be performed.

For example, in the case of constant current control, the welding current and the energization time may be appropriately set according to the material and thickness of the metal plate constituting the material to be welded. When using a plate assembly, it is preferable that the welding current is 3.0 to 14.0 kA and the energization time is 100 to 1000 ms.
Further, the energization of the main welding may be divided into two or more steps, and for example, pre-energization for stabilizing the contact diameter may be performed before the energization for forming the nugget (main energization). Then, post-energization for post-heat treatment may be performed. These pre-energization and post-energization may be performed by constant current control, or may be an upslope-shaped or downslope-shaped energization pattern. Further, an energization suspension time may be provided between each energization.

In the case of adaptive control welding, welding is performed based on the target values (time change curve of instantaneous calorific value per unit volume and cumulative calorific value) obtained by test welding described later, and the time of instantaneous calorific value per unit volume. If the amount of change follows the reference time change curve, welding is performed as it is and welding is completed. However, if the time change amount of the instantaneous calorific value per unit volume deviates from the standard time change curve, the cumulative amount per unit volume in the main welding is to compensate for the deviation amount within the remaining energizing time. The energization amount is controlled so that the calorific value matches the cumulative calorific value per unit volume set as the target value.

Further, in the test welding described later, when the energization is divided into two or more multi-step steps and the target value of the adaptive control welding is set based on the welding conditions divided into the multi-step steps, the adaptive control welding of the main welding is also performed. Similar to the welding conditions divided into multi-step steps in the test welding, it is preferable to perform adaptive control welding for each step, which is performed by dividing into multi-step steps.

Here, in the adaptive control welding for each step, if the time change amount of the instantaneous calorific value per unit volume deviates from the reference time change curve in any step, the deviation amount is the rest of the step. In order to compensate within the energization time of, the energization amount is controlled so that the cumulative calorific value per unit volume in the step matches the cumulative calorific value per unit volume in the step of the test welding.

The method for calculating the calorific value is not particularly limited, but an example thereof is disclosed in Patent Document 5, and this method can also be adopted in the present invention. The procedure for calculating the calorific value q per unit volume / unit time and the cumulative calorific value Q per unit volume by this method is as follows.
Let t be the total thickness of the material to be welded, r be the electrical resistivity of the material to be welded, V be the voltage between the electrodes, I be the welding current, and S be the area where the electrodes and the material to be welded come into contact. In this case, the welding current has a cross-sectional area of S and passes through a columnar portion having a thickness t to generate resistance heat. The calorific value q per unit volume and unit time in this columnar portion is calculated by the following equation (2).
q = (VI) / (ST) --- (2)
Further, the electric resistance R of this columnar portion is obtained by the following equation (3).
R = (rt) / S --- (3)
When equation (3) is solved for S and this is substituted into equation (2), the calorific value q is calculated by the following equation (4).
q = (V ・ I ・ R) / (r ・ t ² )
= (V ² ) / (r · t ² ) --- (4)
Will be.

As is clear from the above equation (4), the calorific value q per unit volume / unit time can be calculated from the voltage V between the electrodes, the total thickness t of the work piece, and the electrical resistivity r of the work piece, and the electrodes. It is not affected by the area S in which the object to be welded and the object to be welded come into contact with each other. In Eq. (4), the calorific value is calculated from the voltage V between the electrodes, but the calorific value q can also be calculated from the current I between the electrodes. There is no need to use. Then, by accumulating the calorific value q per unit volume and unit time over the energization period, the cumulative calorific value Q per unit volume applied to welding can be obtained. As is clear from the equation (4), the cumulative calorific value Q per unit volume can also be calculated without using the area S where the electrode and the material to be welded contact.
The case where the cumulative calorific value Q is calculated by the method described in Patent Document 5 has been described above, but it goes without saying that other calculation formulas may be used.

(2) Test Welding In the above-mentioned main welding energization, when adaptive control welding is performed, test welding is performed prior to the main welding, and in the test welding, energization is performed by constant current control to form an appropriate nugget. The time-varying curve of the instantaneous calorific value per unit volume and the cumulative calorific value per unit volume calculated from the electrical characteristics between the electrodes in the case are stored.
That is, in test welding, a preliminary welding test of the same steel type and thickness as the material to be welded is performed under various conditions under constant current control without diversion to the existing welding point or plate gap, and the optimum conditions for test welding. Find out.
Then, energization is performed under the above conditions, and the time change curve of the instantaneous calorific value per unit volume and the cumulative calorific value per unit volume calculated from the electrical characteristics between the electrodes at the time of this energization are the targets in this welding. Store as a value. The electrical characteristics between the electrodes mean the resistance between the electrodes or the voltage between the electrodes.
Further, as described above, the energization in the test welding may be divided into two or more multi-step steps, and the adaptive control welding for each step may be performed in the main welding.

The material to be welded is not particularly limited, and can be applied to welding steel plates having various strengths from mild steel to ultra-high-strength steel plates and light metal plates such as plated steel plates and aluminum alloys, and three or more steel plates are stacked. It can also be applied to steel plate assembly.

Further, in the resistance spot welding method according to the embodiment of the present invention described above, the case where the material to be welded has a plate gap has been described as an example, but it is desired even when there is an already welded point in the vicinity of the welded point. The effect of stably securing the diameter of the nugget can be obtained.
That is, when there is an already welded point in the vicinity of the welded point, a slight gap is generated between the metal plates to be welded due to the influence of the sheet separation at the time of welding at the already welded point. Therefore, the distance between the welding point and the existing welding point is short, and especially when the metal plate constituting the material to be welded is a high-strength steel plate or the like, it becomes difficult to crush this gap by pressurization, and the space between the metal plates becomes difficult. It is not possible to secure a sufficient contact area, and the flow to the existing welding point is likely to occur.
According to the resistance spot welding method according to the embodiment of the present invention, even when there is a plate gap as described above, the pressing force at the start of energization is appropriately controlled, and an appropriate contact area between the metal plates is obtained. Can be secured. Further, in particular, when adaptive control welding is performed, it is possible to more effectively prevent erroneous recognition of the amount of heat generated in adaptive control welding by avoiding the diversion to the existing welding point.

Then, by joining a plurality of overlapping metal plates using the resistance spot welding method described above, various types of metal plates can be stably secured while stably responding to fluctuations in the state of disturbance. Welding members, in particular, welding members such as automobile parts can be manufactured.

Regarding the plate assembly of the metal plates shown in Table 1, there is no plate gap (disturbance) as shown in FIGS. 3 (a) and 3 (b), or there is a plate gap as shown in FIGS. 4 (a) and 4 (b). In this state, main welding was performed under the conditions shown in Table 2 to prepare a welded joint. Here, No. In 1, 2, 4 to 13, the time from the start of pressurization before the start of energization in the main welding until the initial set pressurization is reached is used as a parameter as an index of the pressurization, and the pressurization during the energization of the main weld is used. It was set. On the other hand, No. In No. 3, the main welding was energized while the pressing force at the time of energization was left as the initial setting pressing force.
In addition, in FIGS. 4A and 4B, a spacer was inserted between the metal plates and clamped from above and below (not shown) to provide a plate gap having various plate gap thickness tg. The distance between spacers was set to 40 mm.

Further, in some examples, before the main welding, test welding is performed under the conditions shown in Table 2 without the plate gap shown in FIG. 3, from the start of pressurization until the initial set pressing force is reached. In addition to measuring the time of, the time change curve of the instantaneous calorific value per unit volume and the cumulative calorific value per unit volume when the test welding was energized were memorized.
It should be noted that the energization of the test welding is the one-stage energization as shown in FIG. 5 or the two-stage energization as shown in FIG. Welding was done.
Furthermore, for those that have not been test-welded, a separate material to be welded (material to be welded without disturbance) that has no gaps between the metal plates and is composed of metal plates of the same thickness and material as the main weld is prepared. Then, by pressurizing the material to be welded under the same pressurizing conditions as in the main welding, the time from the start of pressurization to reaching the initial set pressing force was measured.
Table 2 also shows the time from the start of pressurization before the start of energization to the arrival of the initial set pressing force when there is no gap between the measured metal plates constituting the material to be welded.

Each of the obtained welded joints was evaluated in the following three stages based on the nugget diameter and the presence or absence of scattering after cutting the welded portion and etching the cross section and observing with an optical microscope.
◎ (Passed, especially excellent): The nugget diameter is 4.5√t'or more (t': the thinnest metal plate thickness (mm) of the plate assembly) regardless of the plate gap, and no scattering occurs ○ ( Passed, excellent): Nugget diameter is 4.0√t'or more and no scattering occurs regardless of board gap (however, ◎ (passed, especially excellent) is excluded)
X: Depending on the plate gap, the nugget diameter is less than 4.0√t'and / or scattering occurs.

In each of the examples of the invention, a nugget diameter having a sufficiently large size was obtained regardless of the plate gap and without the occurrence of scattering. On the other hand, in the comparative example, a sufficient nugget diameter could not be obtained or scattering occurred depending on the plate gap.
The parameters that serve as indicators of the pressing force are the time from the start of pressurization before the start of energization in the main welding to the start of energization, and from the start of pressurization before the start of energization in the main welding until the initial set pressing force is reached. Welding gun servo motor torque, welding gun servo motor rotation speed from the start of pressurization before the start of energization in main welding to reaching the initial set pressing force, the start of pressurization before the start of energization in main welding Welding gun strain from to reaching the initial set pressing force, electrode displacement amount from the start of pressurization before the start of energization in main welding to reaching the initial set pressing force, and pressurization before starting energization in main welding The same result as above was obtained when the electrode displacement rate from the start point to the arrival of the initial set pressing force was used.

11, 12: Metal plate 14: Electrode 15: Spacer

Claims (4)

This is a resistance spot welding method in which a material to be welded, which is a stack of multiple metal plates, is sandwiched between a pair of electrodes and energized while being pressurized.
Before the start of energization of the main welding, the material to be welded is pressurized until the initial set pressing force is reached.
Then, the pressing force at the time of energization of the main welding is set by using the parameter which is an index of the pressing force obtained from the start of pressurization before the start of energization of the main welding until the initial setting pressing force is reached. ,
Resistance spot welding method. Prior to the main welding, test welding shall be performed.
In the test welding, the time change curve of the instantaneous calorific value per unit volume and the cumulative calorific value per unit volume calculated from the electrical characteristics between the electrodes when energized by constant current control to form an appropriate nugget are obtained. Remember,
Further, in the energization of the main welding, the time change curve of the instantaneous calorific value per unit volume and the cumulative calorific value per unit volume in the test welding are set as target values, and the energization amount is controlled according to the target value. Item 2. The resistance spot welding method according to Item 1. A method for manufacturing a welded member, which joins a plurality of stacked metal plates by the resistance spot welding method according to claim 1 or 2. A welding device used in the resistance spot welding method according to claim 1 or 2.

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