WO2016013212A1

WO2016013212A1 - Resistance spot welding method

Info

Publication number: WO2016013212A1
Application number: PCT/JP2015/003657
Authority: WO
Inventors: 公一谷口; 央海澤西; 泰明沖田; 池田　倫正
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2014-07-23
Filing date: 2015-07-21
Publication date: 2016-01-28
Also published as: JP5991444B2; JPWO2016013212A1

Abstract

Provided is a resistance spot welding method whereby a nugget can be quickly and stably formed even when the distance between welding points is small. After brief preliminary application of a high current, main current application for forming a desired nugget is carried out at one electrode pair (electrodes A, B), and current application is initiated at the other electrode pair (electrodes C, D) after the start of the preliminary current application and before the end of the main current application.

Description

Resistance spot welding method

The present invention relates to a resistance spot welding method in which two or more steel plates are overlapped.

Resistance spot welding is widely used to assemble car bodies such as automobiles. Resistance spot welding of thousands of points is performed with one car body.

FIG. 1 is a diagram showing an example of general resistance spot welding. In resistance spot welding, two or more steel plates are overlapped (steel plates 1 and 2 in FIG. 1), sandwiched between a pair of upper and

lower welding electrodes

4 and 5, and energized while being pressed, thereby welding the steel plates. A nugget 6 having a predetermined size is formed at (welding point) to obtain a welded joint.

In recent years, in order to balance the weight reduction of the car body and the safety of passengers, in addition to adopting high-strength steel sheets and increasing the rigidity of the car body structure, studies on increasing the rigidity by increasing the number of spot welding points have been made. ing.

For example, Patent Document 1 discloses a technique for spot welding a plurality of welding points simultaneously. This is because multiple welding shanks for attaching electrodes are installed at the end of the main body of one welding machine, and multiple electrodes (such as two-forked electrodes) are energized while being in pressure contact with each other. Spot welding is performed simultaneously at points.

Further, in Patent Document 2, a step of performing series spot welding with a plurality of resistance welders and a welding electrode of one resistance welder among the plurality of resistance welders is used as an indirect welding electrode. A resistance electrode having a step of performing indirect spot welding by using a welding electrode of another resistance welding machine as a ground electrode by bringing the welding electrode into contact with a spot previously welded by series spot welding with the welding electrode. A welding method is disclosed.

JP 2010-179318 A Japanese Patent No. 4836515

However, when the number of welding points is increased as described above, the distance between the welding points is shortened, and a shunt phenomenon occurs in which the current branches to a point where welding has already been performed (already welded point). As a result, there arises a problem that a sufficient current does not flow at a newly welded welding point and a predetermined nugget diameter cannot be secured.

In response to such a problem, the present inventors examined influential factors for the diversion phenomenon to the already welded point. When there is an already welded point, when a current is applied in resistance spot welding, an electric circuit in parallel with the already welded point and the weld point to be newly welded is formed. The current value to be diverted to the already welded point is determined by the relationship between the path to the already welded point and the resistance value of the already welded point itself and the resistance value of the welded point to be newly welded. Therefore, it is considered that shunt current can be suppressed by increasing the resistance value of the welded point.

On the other hand, it is widely known that when the temperature of the steel plate rises, the resistance value of the steel plate increases. That is, if the temperature of the already welded point can be raised, it is considered that the shunt phenomenon can be suppressed and the nugget can be stably formed.

On the other hand, as in Patent Document 1, the method of spot-welding two welding points at the same time using a two-pronged electrode simultaneously raises the resistance value by simultaneously raising the temperatures of the two welding points. Thereby, the influence by the mutual shunting of the two welding points can be suppressed. However, since the temperature does not rise in the initial stage of welding, the influence of the diversion to the already welded point is inevitable, and the nugget formation may become unstable at the weld point near the already welded point.

Moreover, in Patent Document 1, since a specially shaped electrode (bifurcated electrode) is used, it is necessary to use a special apparatus for electrode dressing performed in normal construction. There is a problem that management becomes difficult.

On the other hand, in the cited document 2, there is a problem that a process of bringing the ground electrode into contact with the already-welded point is required, which increases the number of work steps and the work time.

The present invention has been made for such a problem, and provides a resistance spot welding method capable of stably forming a nugget in a short time even when the distance between welding points is short. Objective.

In order to solve the above-mentioned problems, the present inventors do not use a welding machine equipped with a special electrode as in Patent Document 1, but use two welding machines equipped with normal electrodes. The study was conducted on the assumption that the welding machine of the same time would be controlled simultaneously.

As a result, when spot welding of two welding points is performed using two welding machines, welding is performed in advance by providing a difference in the welding start time of each welding point by the two welding machines. The temperature of the starting welding point (preceding welding point) is raised, and the shunting from the welding point (following welding point) that has started welding after that is suppressed to the preceding welding point, thereby the subsequent welding point By rapidly raising the temperature of the welding point, it is possible to weld both the preceding and subsequent welding points while minimizing the increase in welding time by suppressing the influence of the mutual splitting of the two welding points. I thought.

However, even with the above-described time-difference welding method, a shunt to the welded point occurs in the preceding welding, which becomes a variable factor of nugget diameter formation. In particular, when the preceding welding point is close to the existing welding point (for example, 50 mm or less), the influence is significant, and even if the above welding method is used, the nugget diameter of the preceding welding point may not be stable. It was.

Therefore, the present inventors changed the way of thinking and considered a method of utilizing the diversion from the preceding welding point to the existing welding point. Specifically, in addition to the above-described time difference welding method, preliminary energization is performed at the preceding welding point, and the existing welding point is heated by a shunt from the preceding welding point to the existing welding point. This is a method of performing the main energization of the preceding welding point after increasing the resistance value.

The present invention is based on the above idea and has the following features.

[1] When two or more steel plates are overlapped and resistance spot welding is performed, a difference is provided in the welding start time at two adjacent welding points, and a target nugget is set at a welding point where welding is started in advance. At the welding point where the main energization is performed before the main energization to be obtained and then the main energization is performed and welding is started later, the preliminary energization at the welding point where the welding is started in advance and the end of the main energization are completed. Resistance spot welding method that starts energization in between.

[2] The resistance spot welding method according to [1], wherein the distance between the welding points is 10 mm or more and 50 mm or less.

[3] The resistance spot welding method according to [1] or [2], wherein the energization time of the preliminary energization is 10 ms or more and 100 ms or less.

[4] The resistance spot welding method according to any one of [1] to [3], wherein the current value of the preliminary energization is equal to or greater than the current value of the main energization.

[5] When there is an already-welded point previously welded when starting the preceding welding, the welding point closer to the already-welded point is used as the welding point for starting welding in advance, The resistance spot welding method according to any one of [1] to [4], wherein a welding point far from the welding point is a welding point at which welding is started later.

According to the present invention, a nugget can be stably formed in a short time even when the distance between welding points is short.

FIG. 1 is a diagram illustrating an example of general resistance spot welding. FIG. 2 is a diagram illustrating an example of resistance spot welding according to an embodiment of the present invention. FIG. 3 is a diagram illustrating an example of resistance spot welding according to an embodiment of the present invention. FIG. 4 is a diagram showing energization conditions in one embodiment of the present invention.

An embodiment of the present invention will be described with reference to the drawings.

FIG. 2 is a diagram showing a basic welding method in one embodiment of the present invention, and FIG. 3 is a diagram showing a welding state including already welded points in one embodiment of the present invention. FIG. 4 is a diagram showing energization conditions in one embodiment of the present invention.

As shown in FIG. 2, the basic welding method in this embodiment is as follows.

First, a steel plate 1 disposed below (hereinafter referred to as a lower steel plate) and a steel plate 2 disposed below (hereinafter referred to as an upper steel plate) are overlapped.

Next, the superposed steel plates 1 and 2 are sandwiched between an electrode pair composed of the upper electrode A and the lower electrode B and an electrode pair composed of the upper electrode C and the lower electrode D, and are energized while being pressurized. At that time, one electrode pair (electrodes A and B) conducts two-stage energization and starts energization ahead of the other electrode pair (electrodes C and D). Specifically, in one electrode pair (electrodes A and B), after performing a high current preliminary energization in a short time, a main energization for obtaining a target nugget is performed, and the other electrode pair ( In the electrodes C and D), energization is started from the start of preliminary energization of one electrode pair (electrodes A and B) to the end of the main energization.

In addition, the structure which pressurizes using the electrodes A and B and the electrodes C and D, and the structure which controls the applied pressure are not particularly limited, and a conventionally known device such as an air cylinder or a servo motor is used. Can do. The configuration for supplying current when energizing and controlling the current value is not particularly limited, and conventionally known devices can be used. The current during energization may be direct current or alternating current.

If the axes of the electrodes A and B are P1 (corresponding to the welding center of the preceding welding point) and the axes of the C and D are P2 (corresponding to the welding center of the subsequent welding point), the axes P1 and P2 It is desirable that the distance L to be between 10 mm and 50 mm. If the distance L is larger than 50 mm, the influence of the diversion due to the presence of the preceding welding point becomes relatively small. However, even if the distance L is 50 mm or more, the effect of the present invention can be obtained. An effect can be obtained if the distance L is set to be larger than the nugget radius targeted by the preceding welding point. However, in consideration of mechanical limitations, it is desirable that the distance L is practically 10 mm or more.

FIG. 3 is a diagram showing a welding state including already-welded points in this embodiment.

As shown in FIG. 3, the electrodes A and B for performing the pre-welding are arranged on the side close to the existing welding point, and the electrodes C and D for performing the subsequent welding are arranged on the side far from the existing welding point. This is because it is important to suppress the diversion to the already-welded point by heating the already-welded point by preliminary energization in the preceding welding.

FIG. 4 is a diagram showing energization conditions in this embodiment.

As shown in FIG. 4, the current value of the preliminary energization in the pre-welding is Ip, the energization time is Tp, the current value of the main energization is Ia, and the energization time is Ta. The cooling time between the preliminary energization and the main energization is Tc. On the other hand, the current value in the subsequent welding is Ib, and the energization time is Tb. The time difference between the welding start time of the pre-welding (starting time of preliminary energization) and the welding start time of the subsequent welding is Ts. Further, the time difference between the welding end time of the preceding welding and the welding end time of the subsequent welding is Td.

First, the time difference Ts between the welding start time of the preceding welding and the welding start time of the subsequent welding is desirably 10 ms or more and 200 ms or less from the viewpoint of workability. When the time difference Ts is less than 10 ms, the effect of the present invention that the already welded point is heated by the preliminary energization in the pre-welding to suppress the diversion to the already welded point cannot be sufficiently obtained. When the time difference Ts is larger than 200 ms, the construction time becomes longer and the productivity becomes worse.

The pre-energization time Tp in the pre-welding is desirably a short time of 10 ms or more and 100 ms or less. When the preliminary energization time Tp is less than 10 ms, a sufficient temperature rise at the welded point cannot be obtained, and a shunt current is generated at the welded point. The temperature rise at the welded point due to the preliminary energization is most greatly obtained in the initial stage of the preliminary energization in which the contact resistance between the two steel plates is high. However, when the pre-energization time Tp is greater than 100 ms, This is because energization takes a long time and the preceding welding point is heated more than necessary, causing excessive heat generation and scattering (expulsion and surface flash) in the subsequent energization.

In addition, the current value Ip of the preliminary energization in the pre-welding is preferably set to be equal to or greater than the current value Ia of the main energization. This is because if the current value Ip of the preliminary energization is lower than the current value Ia of the main energization, the effect of increasing the temperature at the welded point by the preliminary energization is small. However, if the current value Ip for the preliminary energization is too higher than the current value Ia for the main energization, it may cause scattering. Therefore, it is desirable that the current value Ip of the preliminary energization is in the range of the following formula (1).

1.0 × Ia ≦ Ip ≦ 3.0 × Ia (1)

In addition, what is necessary is just to use the average value, when the electric current value Ia changes with time. The same applies to the current value Ip.

Also, in the prior welding, when a cooling time is provided between the preliminary energization and the main energization, the cooling time Tc is desirably 50 ms or less. This is because if the cooling time Tc is too long, the temperature of the already-welded point is lowered, and the effect of suppressing the diversion to the already-welded point is reduced.

Further, it is desirable that the time difference Td between the welding end time of the preceding welding and the welding end time of the subsequent welding is 0 ms or more and not more than the time difference Ts of the welding start time. Either the end time of the pre-welding or the end time of the post-welding may be first, but from the viewpoint of obtaining a sufficient nugget diameter by securing a sufficient welding time, the post-welding is the pre-welding. It is desirable to finish later.

In this embodiment, by preliminarily energizing a high current in the preceding welding in a short time, the existing welding point is heated to increase the resistance value of the existing welding point. In addition to suppressing the diversion, the time difference welding between the preceding welding and the succeeding welding rapidly raises the temperature of the preceding welding point and the temperature of the succeeding welding point, thereby reducing the influence of the two welding points on each other. Suppress. As a result, it is possible to stably form nuggets at both the preceding welding point and the subsequent welding point while minimizing an increase in welding time.

As an example of the present invention, as shown in FIG. 3, the lower steel plate 1 and the upper steel plate 2 were overlapped and resistance spot welding was performed.

The lower steel plate 1 and the upper steel plate 2 have the same thickness and the same steel type, and the shape is a rectangle having a long side of 150 mm and a short side of 50 mm.

The nugget diameter d3 is 4 at a distance of L1 in the direction opposite to the axis P2 of the electrodes C and D starting the welding from the axis P1 of the electrodes A and B starting the welding in advance. An already-welded point that is √t (t: plate thickness) was formed. The distance L1 is the distance between the axis P1 of the electrodes A and B that start welding in advance and the center P3 of the nugget diameter of the already welded point. The distance L2 is the distance between the axis P1 of the electrodes A and B and the axis P2 of the electrodes C and D.

Electrodes A, B, C and D were all DR-type electrodes made of alumina-dispersed copper with a tip diameter of 6 mm and a curvature radius of 40 mm.

The control of the pressure applied to the upper electrodes A and B was performed by a servo motor that drives the upper electrodes A and B, and a single-phase AC power source with a frequency of 50 Hz was used during energization.

The energization conditions as shown in FIG. 4 are as shown in Table 1.

As shown in Table 1, the example of the present invention is obtained by performing spot welding based on the above-described embodiment of the present invention. On the other hand, in Comparative Example 1, only the electrodes A and B were used and spot welding was performed for each welding point individually. In Comparative Example 2, welding of electrodes A and B and welding of electrodes C and D were performed simultaneously (Ts = 0 ms, Td = 0 ms). In both Comparative Examples 1 and 2, preliminary energization with the electrodes A and B was not performed.

Then, the weld cross section of the welded joint obtained under each setting condition is cut, and the nugget diameter d1 of the welding point by the electrodes A and B and the nugget diameter d2 of the welding point by the electrodes C and D are each set as a reference nugget diameter. Compared to 0 mm. By the way, the standard nugget diameter of 5.0 mm is the nugget diameter of the welded portion in which welding is performed under the welding conditions of the electrodes C and D in the state where no existing welding points are formed and the electrodes A and B are not energized. is there.

The diameter d1 of the welded portion by the electrodes A and B and the diameter d2 of the welded portion by the electrodes C and D are less than 5% of the reduction rate compared to the reference welded portion diameter of 5.0 mm, or the reference In the case where the diameter of the welded portion is larger than 5.0 mm, it is indicated as ◯, and the case where it is not so is indicated as ×. The results are shown in Table 1.

As shown in Table 1, in the inventive examples, sufficient nugget formation was recognized as compared with Comparative Examples 1 and 2.

This confirms the effectiveness of the present invention.

1 Lower steel plate 2 Upper steel plate 4, B, D Lower electrode 5, A, C Upper electrode 6 Nugget

Claims

When two or more steel plates are overlapped and resistance spot welding is performed, there is a difference in the welding start time at two adjacent welding points, and the main energization to obtain a target nugget at the welding point where welding is started in advance. At the welding point where the main energization is performed after the preliminary energization before starting the welding and the welding is started later, the energization is performed between the start of the preliminary energization at the welding point where the welding is started in advance and the end of the main energization. To start resistance spot welding method.
The resistance spot welding method according to claim 1, wherein a distance between the welding points is 10 mm or more and 50 mm or less.
The resistance spot welding method according to claim 1 or 2, wherein the energization time of the preliminary energization is 10 ms or more and 100 ms or less.
The resistance spot welding method according to any one of claims 1 to 3, wherein the current value of the preliminary energization is equal to or greater than the current value of the main energization.
When there is an already welded point previously welded when starting welding in advance, the welding point closer to the already welded point is set as the welding point to start welding in advance, and the side far from the already welded point 5. The resistance spot welding method according to claim 1, wherein the welding point is a welding point at which welding is started later.