JP7371664B2 - Resistance spot welding method, method for manufacturing welded parts, and welding equipment - Google Patents

Description

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

Generally, a resistance spot welding method, which is a type of lap resistance welding method, is used to join stacked steel plates together.
This welding method involves sandwiching two or more stacked steel plates, applying pressure from above and below with a pair of electrodes, and applying a high welding current between the top and bottom electrodes for a short period of time to join them. Point-shaped welds can be obtained by utilizing the resistance heat generation generated by applying a welding current of . These dot-shaped welds are called nuggets, and are the parts where when an electric current is applied to the stacked steel plates, the two steel plates melt and solidify at the point where they come into contact. These nuggets join the steel plates together in a dotted manner.

In order to obtain good weld quality, it is important that the nugget diameter be formed within an appropriate range. The nugget diameter is determined by welding conditions such as welding current, current application time, electrode shape, and pressing force. Therefore, in order to form an appropriate nugget diameter, it is necessary to appropriately set the above-mentioned welding conditions according to the conditions of the material to be welded, such as the material, plate thickness, and number of stacked sheets.

For example, when manufacturing automobiles, thousands of spot welds are performed per automobile, and it is necessary to weld a continuous flow of materials (workpieces). At this time, if the conditions of the materials to be welded, such as the material, plate thickness, and number of stacked sheets, are the same at each welding location, the welding conditions such as welding current, energization time, and pressing force are also the same. The nugget diameter can be obtained. However, in continuous welding, the contact surface of the electrode to the welded material gradually wears out, and the contact area gradually becomes wider than the initial state. If the same value of welding current as in the initial state is applied with the contact area increased in this way, the current density in the welded material will decrease, the temperature rise in the weld will be lower, and the nugget diameter will become smaller. . For this reason, the electrode is polished or replaced every time several hundred to several thousand points are welded to prevent the tip diameter of the electrode from expanding too much.

In addition, resistance welding equipment that has a function (stepper function) that increases the welding current value after performing welding a predetermined number of times to compensate for the decrease in current density due to electrode wear has been used. . To use this stepper function, it is necessary to properly set the welding current change pattern described above in advance. However, for this reason, it takes a lot of time and cost to derive welding current change patterns corresponding to a large number of welding conditions and welding material conditions through tests or the like. Furthermore, in actual construction, there are variations in the progress of electrode wear, so it cannot be said that a predetermined welding current change pattern is always appropriate.

Furthermore, if there is a disturbance during welding, for example, if there is a previously welded point (already welded point) near the welding point, or if the material to be welded has a large surface unevenness near the welding point, If a contact point exists, current is shunted to the already welded point or the contact point during welding. In such a state, even if welding is performed under predetermined conditions, the current density at the desired welding position directly under the electrode will decrease, so that a nugget with the required diameter will still not be obtained. In order to compensate for this lack of heat generation and obtain a nugget of the required diameter, it is necessary to set a high welding current in advance.

In addition, if the area around the welding point is strongly constrained due to surface irregularities or the shape of the parts, or if foreign objects are caught between the steel plates around the welding point, the gap between the steel plates will increase. This may narrow the contact diameter between the steel plates, making it easier for them to splinter.

The following techniques have been proposed to solve the above problems.
For example, Patent Document 1 describes a first step in which nuggets are generated by gradually increasing the current applied to a high-tensile steel plate, a second step in which the current is decreased after the first step, and a second step in which the current is lowered after the first step. Spot welding is performed through a process that includes main welding by increasing the current after the step, and a third step in which the applied current is gradually decreased, thereby attempting to suppress splintering caused by poor familiarization at the initial stage of energization. A method for spot welding high-strength steel plates is described.

Patent Document 2 discloses that the surface of the workpiece is softened by maintaining the current value at a level that can suppress the generation of spatter for a predetermined period of time at the beginning of the energization time, and then maintaining the current value high for a predetermined period of time to generate spatter. A spot welding energization control method is described that allows nugget growth while suppressing nugget growth.

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

Patent Document 4 attempts to perform good welding by detecting welding current and tip-to-tip voltage, simulating the welding part by heat conduction calculation, and estimating the state of nugget formation in the welding part during welding. A method of controlling welding conditions for a resistance welding machine is described.

Patent Document 5 discloses that the cumulative calorific value per unit volume that can satisfactorily weld the welded object is calculated from the plate thickness of the welded object and the energization time, and the calculated amount of heat per unit volume/unit time is calculated. A resistance welding system is described that attempts to perform good welding regardless of the type of workpiece or the wear state of the electrode by using a welding system that adjusts the welding current or voltage to generate a calorific value of There is.

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

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

In addition, the techniques described in Patent Documents 3 and 4 require complex calculation processing to estimate the nugget temperature based on a heat conduction model (heat conduction simulation), etc., and the configuration of the welding control device becomes complicated. In addition, there was a problem that the welding control device itself became expensive.

Furthermore, with the technique described in Patent Document 5, it is considered that by controlling the cumulative heat generation amount to a target value, it is possible to perform good welding even if the electrode is worn a certain amount.
However, if the set material conditions to be welded and the actual material conditions to be welded are significantly different, for example, if there is a large gap between the metal plates that are to be welded, the final cumulative calorific value cannot be calculated. Even if the target value can be met, the form of heat generation, that is, the time change in the temperature distribution of the weld zone, deviates from the heat amount pattern that would yield a good weld zone, and the required nugget diameter may not be obtained. , scattering may occur.

The present invention has been developed in view of the above-mentioned current situation, and even when the influence of external disturbances, especially the gap between the metal plates constituting the material to be welded (hereinafter also referred to as "plate gap") is large, An object of the present invention is to provide a resistance spot welding method that can stably obtain a desired nugget diameter without causing expulsion.
Another object of the present invention is to provide a method for manufacturing a welded member, in which a plurality of stacked metal plates are joined by the above-described resistance spot welding method.

Now, in order to achieve the above object, the inventors have made extensive studies and have obtained the following knowledge.
(1) If 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 time of starting energization, so expulsion is likely to occur. Further, if the metal plate is excessively pressurized with an electrode when the plate gap is large, the metal plate will warp significantly. As a result, the contact area between the metal plate and the electrode increases excessively, promoting heat removal to the electrode, and as a result, the nugget diameter and nugget thickness may become smaller.
(2) To alleviate the effect of such a plate gap, set the pressure force when energizing according to the size of the plate gap to ensure that the space between the metal plates is appropriate when energizing, especially at the beginning of energizing. It is effective to ensure a large contact area.

Therefore, based on the above knowledge, the inventors further studied the method of setting the pressing force during energization according to the size of the plate gap, and obtained the following knowledge.
(3) The influence of the plate gap is reflected in the parameter that is an index of the pressurizing force from the start of pressurization until reaching the predetermined set pressurizing force.
For example, if there is no gap between the metal plates and if there is a gap between the metal plates, if the metal plates are pressurized under the same conditions, the predetermined set pressure will be reached after the pressurization starts. The time it takes to reach this point and the behavior of the servo motor will be different for each.
(4) Therefore, first, before starting energization, pressurize the material to be welded until it reaches a predetermined set pressure (hereinafter also referred to as initial set pressure),
Then, as shown in Fig. 1, using parameters that serve as an index of the pressurizing force obtained from the time when pressurizing starts before starting energization until reaching the initial set pressurizing force, and taking into account the effect of the plate gap, (initial Separately from the set pressure force: F ₀ ), set the pressure force when energizing: F _A and energize. In particular, set the pressure force during energization within a range that satisfies the following formula (1) and energize it. I do,
By this,
It is possible to secure an appropriate contact area between the metal plates at the start of energization, and as a result, it is possible to stably obtain the desired nugget diameter without the influence of plate gaps and without the occurrence of expulsion. .
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 : Pressure force during current welding
F ₀ : Initial setting force for main welding
T _A : Time from the start of pressurization before starting energization to reaching the initial set pressurization force during main welding
T ₀ : When there is no gap between the metal plates constituting the material to be welded, it is the time from the time when pressurization starts before starting energization until the initial set pressurizing force is reached.
The present invention was completed based on the above findings and further studies.

That is, the gist of the present invention is as follows.
1. A resistance spot welding method in which a material to be welded, which is made up of multiple overlapping metal plates, is sandwiched between a pair of electrodes and joined by applying electricity while applying pressure,
Before starting energization for main welding, pressurize the above-mentioned material to be welded until it reaches the initial setting pressure,
Next, the pressurizing force at the time of energization for the main welding is set using a parameter that is an index of the pressurizing force obtained from the time when pressurization is started before the start of energization for the main welding to the time when the initial setting pressurizing force is reached. ,
Resistance spot welding method.

2. 2. The resistance spot welding method according to item 1, wherein the applied force during energization in the main welding is set within a range that satisfies the following formula (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 : Pressure force during current welding
F ₀ : Initial setting force for main welding
T _A : Time from the start of pressurization before starting energization to reaching the initial set pressurization force during main welding
T ₀ : When there is no gap between the metal plates constituting the material to be welded, it is the time from the time when pressurization starts before starting energization until the initial set pressurizing force is reached.

3. Prior to the main welding, test welding shall be performed,
In this test welding, we calculated the time change curve of the instantaneous heat generation amount per unit volume and the cumulative heat generation amount per unit volume, which are calculated from the electrical characteristics between the electrodes when applying current using constant current control to form a proper nugget. let me remember,
Further, in the energization of the main welding, the time change curve of the instantaneous heat generation amount per unit volume and the cumulative heat generation amount per unit volume in the test welding are set as target values, and the amount of energization is controlled according to the target values. 2. The resistance spot welding method according to 1 or 2.

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

According to the present invention, good nuggets can be obtained without scattering even under the influence of disturbances, especially when the plate gap is large.
Further, according to the present invention, in actual work such as automobile manufacturing, when materials to be processed are continuously welded one after another (the state of disturbance varies depending on the welding position and each material to be processed), Also, it becomes possible to stably secure a desired nugget diameter by effectively responding 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 showing the relationship between pressing force and time, and welding current and time of the resistance spot welding method concerning one embodiment of the present invention. (a) When there is no gap between the metal plates to be welded, and (b) When there is a gap between the metal plates, the pressure is applied to the metal plates to be welded under the same conditions. FIG. 2 is a diagram showing the relationship between pressing force and time, and welding current and time. FIG. 6 is a diagram schematically showing an example of a case where welding is performed in a state where there is no plate gap (disturbance) for (a) a two-ply plate assembly and (b) a three-ply plate assembly. FIG. 3 is a diagram schematically showing an example of welding for (a) a two-ply plate assembly with a plate gap, and (b) a three-ply plate assembly with a plate gap; It is a figure which shows typically an example of 1 stage energization in test welding. FIG. 3 is a diagram schematically showing an example of two-stage energization in test welding.

The present invention will be explained based on the following embodiments.
One embodiment of the present invention is a resistance spot welding method in which a material to be welded, which is a plurality of overlapping metal plates, is sandwiched between a pair of electrodes and joined by applying current while applying pressure.
Before starting energization for main welding, pressurize the above-mentioned material to be welded until it reaches the initial setting pressure,
Next, the pressurizing force at the time of energization for the main welding is set using a parameter that is an index of the pressurizing force obtained from the time when pressurization is started before the start of energization for the main welding to the time when the initial setting pressurizing force is reached. .
Note that main welding refers to the process of actually welding the target materials to be welded, and is used to distinguish it from test welding, which will be described later.

Note that the welding device that can be used in the resistance spot welding method according to the embodiment of the present invention may be any type as long as it is equipped with a pair of upper and lower electrodes and can arbitrarily control the pressurizing force and welding current during welding. (stationary type, robot gun, etc.), and the shape of the electrode is not particularly limited.

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

(1) Main welding As mentioned above, in order to alleviate the effect of the plate gap, the pressure force at the time of energization is set according to the size of the plate gap, and the welding It is effective to ensure an appropriate contact area between the plates.
Further, the influence of the plate gap is reflected in a parameter that is an index of the pressurizing force from the start of pressurization until reaching a predetermined set pressurizing force.
Therefore, before the start of energization for main welding, the material to be welded is pressurized until the initial set pressure is reached, and then the pressure obtained from the time when pressure starts before the start of energization for main welding until the initial set pressure is reached. It is important to use a parameter that serves as an index of the pressurizing force to set the pressurizing force during energization for main welding.

For example, as shown in Figure 2, if 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 predetermined amount will be The time it takes to reach the set pressure: T _A will be different for each.
That is, if there is no gap between the metal plates, the initial set pressure is reached in a relatively short time after the electrodes start applying pressure. On the other hand, if there is a gap between the metal plates, in the initial stage of pressurization the metal plates are deformed to collapse the gap (bringing the metal plates into contact), so it takes a long time until the initial set pressure is reached. It takes longer.
Therefore, the time from the start of pressurization until reaching the set pressurizing force: A parameter that is an index of the pressurizing force obtained from the time when pressurizing starts before the start of energization to reaching the above-mentioned initial set pressurizing force, such as T _A , is used. , as shown in Figure 1, taking into account the influence of the plate gap, set the pressing force when energizing: F _A (separately from the initial setting pressing force: F ₀ ) and energizing. In particular, when energizing By setting the pressing force within the range that satisfies the following formula (1) and energizing, it is possible to secure an appropriate contact area between the metal plates at the start of energization, and as a result, the gap between the plates is reduced. Regardless of the influence, it becomes possible to stably obtain a desired nugget diameter without the occurrence of expulsion.
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 : Pressure force during current welding
F ₀ : Initial setting force for main welding
T _A : Time from the start of pressurization before starting energization to reaching the initial set pressurization force during main welding
T ₀ : When there is no gap between the metal plates constituting the material to be welded, it is the time from the time when pressurization is started before starting energization until the initial set pressurizing force is reached.

Here, if the pressurizing force during _energization in main welding _: F _A becomes less than F ₀ × (1+ _0.1 It becomes difficult to secure the area. On the other _{hand ,} when F _A becomes F ₀ × (1+ _3.0 In other words, the nugget diameter and nugget thickness may become smaller.
Therefore, using parameters that serve as an index of the pressurization force obtained from the time when pressurization starts before the start of current application in main welding until the initial setting pressure force is reached, and taking into account the influence of the plate gap, It is important to set the pressing force and conduct energization, and in particular, it is preferable to set the pressing force during energization during main welding within a range that satisfies formula (1) above. 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 ) or less.

In addition, the parameters that serve as indicators of pressurizing force are:
・The time from the start of pressurization before the start of energization until the initial setting pressure is reached during main welding,
・Time from the start of pressurization before the start of energization to the start of energization in main welding,
・The torque of the welding gun's servo motor from the time when pressurization starts before the start of energization until the initial setting pressure is reached during main welding,
・The rotation speed of the servo motor of the welding gun from the time when pressurization starts before the start of energization in main welding until the initial setting pressurization force is reached,
・Distortion of the welding gun from the time of starting pressurization before starting energization until reaching the initial setting pressurizing force during main welding,
・Amount of electrode displacement from the time of starting pressurization before starting energization in main welding until reaching the initial setting pressure force, and ・Electrode displacement from the time of starting pressure before starting energization until reaching the initial setting pressure force in main welding displacement speed,
It is preferable to use these parameters to set the pressure force during energization in main welding within a range that satisfies the above formula (1), and then energize.

For example, the time from the time when pressure starts before energization starts to the time when energization starts in main welding is [time from the time when pressure starts before energization starts in main welding until reaching the initial set pressure] + [main welding] It can be expressed as the time from the time when the initial setting pressure is reached to the time when energization starts].
Therefore, when using the time from the time when pressure starts before the start of energization in main welding to the time when energization starts as a parameter that serves as an index of the welding force, By subtracting [the time from the time when the initial setting pressure force is reached to the time when energization starts] from [the time up to the time], the pressure force at the time of energization in the main welding satisfies the above formula (1). It is possible to set it within the range.
In addition, when the torque of the welding gun's servo motor (hereinafter also simply referred to as torque) from the time when pressurization starts before the start of current application until the initial setting pressurization force is reached during main welding is used as a parameter that is an index of the pressurization force. When the electrode comes into contact with the metal plate that is the material to be welded, the torque begins to increase rapidly, and then, when a sufficient pressing force is applied to the steel plate, the torque reaches a stable value and becomes saturated.
Therefore, the time from the start of torque increase until saturation is the time from the time when pressurization starts until the initial set pressure is reached (in other words, the time when torque starts to increase is the time when pressurization starts, and the time when torque increases is (The moment when the torque becomes saturated and constant is determined to be the time when the initial setting force is reached), the force when energizing in main welding is set within a range that satisfies the above formula (1). Is possible.
Furthermore, the rotational speed of the welding gun's servo motor (hereinafter also simply referred to as rotational speed) from the time when pressurization starts before the start of energization in main welding until it reaches the initial setting pressurization force is used as a parameter that is an index of the pressurization force. In this case, when the electrode comes into contact with the metal plate that is the material to be welded, the rotation speed becomes unstable and then gradually decreases. Then, when the initial setting pressure is reached, the electrode stops moving, so the rotation speed becomes zero.
Therefore, the time from when the rotation speed becomes unstable and starts decreasing until it reaches 0 is the time from the time when pressurization starts until the initial set pressure is reached (in other words, when the rotation speed becomes unstable) The time when the rotation speed becomes stable is determined as the time to start pressurization, and the time when the rotation speed becomes 0 is determined as the time when the initial setting pressure is reached. ) can be set within a range that satisfies.
In addition, by combining parameters that serve as indicators of multiple pressurizing forces, such as determining the point at which the torque starts to increase as the point at which pressurization starts, and the point at which the rotation speed reaches 0 as the point at which the initial set pressurizing force is reached, this The pressurizing force during energization during welding may be set within a range that satisfies the above formula (1).

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

In addition, when there is no gap between the metal plates constituting the material to be welded, especially when there is no disturbance, the time from the start of pressurization before starting energization to the time when the initial setting pressure is reached: T ₀ is, for example, , by preparing a separate material to be welded consisting 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. , all you have to do is ask.
In addition, when performing test welding, which will be described later, the pressurizing force set during the test welding may be used as the initial setting pressurizing force for the main welding.
In addition, in general resistance spot welding equipment, the control method is set to be switched when a certain pressure (hereinafter referred to as the switching setting value) is reached between the time when pressure starts and the set pressure is reached. (position control→torque control), and normally, if the same pressurizing force means is used and the switching setting value is the same, it can be said that pressurization is performed under the same conditions.

Furthermore, energization in main welding is not particularly limited, and may be performed by constant current control, or by adaptive control that controls the amount of energization according to the target value set in the test welding after performing test welding, which will be described later. Welding may also be performed.

For example, in the case of constant current control, the welding current and energization time can be set appropriately depending on the material and thickness of the metal plates that make up the welded material. When using a plate assembly, it is preferable that the welding current is 3.0 to 14.0 kA and the current application time is 100 to 1000 ms.
In addition, the energization for main welding may be divided into two or more multi-stage steps. For example, before the energization to form the nugget (main energization), pre-energization may be performed to stabilize the contact diameter. However, post-energization may be performed for post-heat treatment. These pre-energization and post-energization may be performed by constant current control, or may be performed in an upslope or downslope energization pattern. Furthermore, an energization pause time may be provided between each energization.

In addition, in the case of adaptive control welding, welding is performed based on the target values (time change curve of instantaneous heat generation per unit volume and cumulative heat generation) obtained from test welding described later, and the time of instantaneous heat generation per unit volume is If the amount of change is along the standard time change curve, welding is continued and the welding is completed. However, if the amount of change over time in the instantaneous heat generation amount per unit volume deviates from the standard time change curve, in order to compensate for the deviation within the remaining energization time, the cumulative amount per unit volume during main welding is The amount of energization is controlled so that the amount of heat generated matches the cumulative amount of heat generated per unit volume set as a target value.

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

Here, in step-by-step adaptive control welding, if the amount of time change in instantaneous heat generation per unit volume deviates from the standard time change curve in any step, the amount of deviation is calculated from the rest of the step. In order to compensate within the current application time, the amount of current applied is controlled so that the cumulative amount of heat generated per unit volume in the relevant step matches the cumulative amount of heat generated per unit volume in the relevant step of test welding.

Note that there is no particular restriction on the method of calculating the calorific value, 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 using this method is as follows.
The total thickness of the material to be welded is t, the electrical resistivity of the material to be welded is r, the voltage between the electrodes is V, the welding current is I, and the area where the electrode and the material to be welded are in contact is S. In this case, the welding current passes through a columnar portion with a cross-sectional area of S and a thickness of t, generating resistance heat generation. The calorific value q per unit volume/unit time in this columnar portion is determined by the following equation (2).
q=(V・I)/(S・t) --- (2)
Further, the electrical resistance R of this columnar portion is determined by the following equation (3).
R=(r・t)/S --- (3)
Solving equation (3) for S and substituting this into equation (2), the calorific value q is given by the following equation (4)
q=(V・I・R)/(r・t ² )
=(V ² )/(r・t ² ) --- (4)
becomes.

As is clear from the above equation (4), the calorific value q per unit volume/unit time can be calculated from the interelectrode voltage V, the total thickness t of the welded object, and the electrical resistivity r of the welded object. It is not affected by the contact area S between the welded object and the welded object. Although equation (4) calculates the amount of heat generated from the interelectrode voltage V, it is also possible to calculate the amount of heat q from the interelectrode current I, and in this case, the area S where the electrode and the workpiece are in contact There is no need to use Then, by accumulating the calorific value q per unit volume/unit time over the energization period, the cumulative calorific value Q per unit volume added to welding is obtained. As is clear from 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 are in 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 If adaptive control welding is performed in the above-mentioned main welding energization, test welding is performed prior to main welding, and in the test welding, current is applied under constant current control to form a proper nugget. The time change curve of the instantaneous heat generation amount per unit volume and the cumulative heat generation amount per unit volume, which are calculated from the electrical characteristics between the electrodes in the case, are stored.
In other words, in test welding, preliminary welding tests on the same steel type and thickness as the material to be welded are performed under various conditions with constant current control, without branching to existing welding points or gaps, to determine the optimal conditions for test welding. Find.
Then, energization is carried out under the above conditions, and the time change curve of instantaneous heat generation per unit volume and cumulative heat generation per unit volume, which are calculated from the electrical characteristics between the electrodes during energization, are calculated as the target for this welding. Store it as a value. Note that the electrical property between the electrodes means 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-steps, and the adaptive control welding may be performed for each step in the main welding.

There are no particular restrictions on the materials to be welded, and it can be applied to welding steel plates with various strengths from mild steel to ultra-high tensile strength steel plates, as well as light metal plates such as plated steel plates and aluminum alloys. It can also be applied to plate construction.

Further, in the resistance spot welding method according to the embodiment of the present invention, the case where there is a gap in the material to be welded has been explained as an example, but even when there is an existing welding point in the vicinity of the welding point, it is possible to This has the effect of stably securing a nugget diameter of .
That is, when there is an existing welding point in the vicinity of the welding point, a slight gap is created between the metal plates to be welded due to the influence of sheet separation during welding at the existing welding point. Therefore, when the distance between the welding point and the already welded point is close, especially when the metal plate constituting the material to be welded is a high-tensile steel plate, it becomes difficult to close this gap by applying pressure, and the gap between the metal plates becomes small. , sufficient contact area cannot be secured, and flow is likely to flow to the already welded point.
According to the resistance spot welding method according to an embodiment of the present invention, even when there is a gap between the metal plates as described above, the pressing force at the start of energization can be appropriately controlled to maintain an appropriate contact area between the metal plates. It becomes possible to secure the following. In addition, especially when performing adaptive control welding, by avoiding flow diversion to already welded points, it becomes possible to more effectively prevent misrecognition of the amount of heat generated in adaptive control welding.

By joining multiple overlapping metal plates using the above-mentioned resistance spot welding method, the desired nugget diameter can be stably secured while effectively responding to fluctuations in disturbance conditions. Welded parts, in particular welded parts such as automobile parts, can be manufactured.

Regarding the metal plate assembly shown in Table 1, there is no plate gap (disturbance) as shown in Figures 3 (a) and (b), or there is a plate gap as shown in Figures 4 (a) and (b). In this state, main welding was performed under the conditions shown in Table 2 to produce a welded joint. Here, No. In 1, 2, 4 to 13, the time from the start of pressurization before the start of energization in main welding until the initial setting pressure is reached is used as a parameter serving as an index of the pressure force, and the pressure force during energization in main welding is It was set. On the other hand, No. In No. 3, energization for main welding was carried out with the pressure applied during energization kept at the initial setting pressure.
In FIGS. 4A and 4B, spacers were inserted between the metal plates and clamped from above and below (not shown) to provide gaps having various thicknesses tg. Note that the distance between spacers was 40 mm in both cases.

In addition, for some examples, test welding was performed under the conditions shown in Table 2 in a state without the plate gap shown in Fig. 3 before the main welding, and from the start of pressurization until the initial setting pressure was reached. In addition to measuring the time, the time change curve of the instantaneous heat generation amount per unit volume and the cumulative heat generation amount per unit volume during test welding were memorized.
The energization for test welding is either one-step energization as shown in Figure 5 or two-stage energization as shown in Figure 6.If two-step energization is performed during test welding, adaptive control is applied for each step during main welding. Performed welding.
Furthermore, for those for which test welding was not performed, separately prepare a welding material (material to be welded without disturbance) consisting of metal plates of the same thickness and material as the actual welding, with no gaps between the metal plates. Then, by pressurizing the material to be welded under the same pressurizing conditions as for the main welding, the time from the start of pressurization until reaching the initial set pressurizing force was measured.
Table 2 also shows the time taken from the start of pressurization before the start of energization until the initial set pressurizing force is reached when there is no gap between the metal plates constituting the measured materials to be welded.

For each of the obtained welded joints, the welded portion was cut, the cross section was etched, and then observed using an optical microscope, and evaluated in the following three grades based on the nugget diameter and the presence or absence of expulsion.
◎ (Passed, particularly excellent): Regardless of the plate gap, the nugget diameter is 4.5√t' or more (t': thickness of the thinnest metal plate in the plate assembly (mm)), and there is no scattering ○ ( Pass, excellent): Regardless of the plate gap, the nugget diameter is 4.0√t' or more, and there is no scattering (excluding ◎ (pass, particularly excellent))
×: Depending on the plate gap, the nugget diameter is less than 4.0√t' and/or scattering occurs.

In all of the invention examples, a sufficiently large nugget diameter was obtained without any scattering, regardless of the plate gap. On the other hand, in the comparative example, depending on the plate gap, a sufficient nugget diameter could not be obtained or scattering occurred.
In addition, the parameters that serve as indicators of the pressurizing force are the time from the start of pressurization before starting energization in main welding to the time of starting energization, and the time from the start of pressurization before starting energization in main welding until the initial setting pressure is reached. The torque of the servo motor of the welding gun, the rotational speed of the welding gun servo motor from the time of starting pressure before starting energization in main welding until the initial setting pressure is reached, the time of starting pressure before starting energization in main welding. Distortion of the welding gun from the time it reaches the initial set pressure, the amount of electrode displacement from the start of pressure before starting current welding until it reaches the initial set pressure, and the pressure before starting current welding during main welding. Similar results were obtained when the electrode displacement rate from the starting point until reaching the initial set pressure force was used.

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

Claims (4)

A resistance spot welding method in which a material to be welded, which is made up of multiple overlapping metal plates, is sandwiched between a pair of electrodes and joined by applying electricity while applying pressure,
Before starting energization for main welding, pressurize the above-mentioned material to be welded until it reaches the initial setting pressure,
Next, the pressurizing force at the time of energization for the main welding is set using the parameter that is an index of the pressurizing force obtained from the time when pressurization is started before the start of energization for the above-mentioned main welding until the above-mentioned initial setting pressurizing force is reached. ,
The parameters that serve as indicators of the above-mentioned pressurizing force are the time from the time when pressurization starts before the start of energization in the above-mentioned main welding until the initial setting pressure is reached, and the time from the time when the pressurization starts before the start of energization in the above-mentioned main welding to the time when energization starts. using at least one of the time up to the starting point ;
Resistance spot welding method. Prior to the main welding, test welding shall be performed,
In this test welding, we calculated the time change curve of the instantaneous heat generation amount per unit volume and the cumulative heat generation amount per unit volume, which are calculated from the electrical characteristics between the electrodes when applying current using constant current control to form a proper nugget. let me remember,
Further, in the energization of the main welding, the time change curve of the instantaneous heat generation amount per unit volume and the cumulative heat generation amount per unit volume in the test welding are set as target values, and the amount of energization is controlled according to the target values. Item 1. The resistance spot welding method according to item 1. A method for manufacturing a welded member, comprising joining 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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