JP2020116630A - Indirect spot welding method - Google Patents

Abstract

To provide an indirect spot welding method, in which quality of each welding during actual welding is quantitatively evaluated.SOLUTION: An indirect spot welding method, in which an overlapped part P between a first metal plate 1 and a third metal plate 3 is pressurized by a welding electrode 10 and an earth electrode 20 is contacted with a site different from the overlapped part P so as to carry currents between both electrodes, includes a step of carrying currents between both electrodes without hardly pressurizing the overlapped part P by the welding electrode 10 at the early stage of welding.SELECTED DRAWING: Figure 2

Description

The present invention relates to an indirect spot welding method.

In a vehicle assembly process, a vehicle body is assembled by joining a plurality of metal plates by spot welding. As spot welding, direct spot welding in which a plurality of metal plates are sandwiched between a pair of electrodes and electricity is applied is often used. However, depending on the shape of the component, it may not be possible to sandwich a plurality of metal plates with a pair of electrodes, and direct spot welding may not be applicable. In this case, indirect spot welding is applied in which the overlapped portion of a plurality of metal plates is pressed by a welding electrode, and welding is performed by energizing between the electrodes in a state where the earth electrode is in contact with a portion different from the overlapped portion. It

However, in indirect spot welding, the welding electrode and the ground electrode are often arranged apart from each other, and a current flowing through a contact portion (for example, a welding point previously welded) between metal plates other than the overlapping portion. Since (reactive current) is likely to occur, it is a problem that it is difficult to form a good nugget.

For example, in Patent Document 1 below, a seating surface is provided in advance on a metal plate, and the seating surface is pressed while being crushed by a welding electrode to reduce the contact area between the metal plates and increase the current density. A method for facilitating the formation of nuggets is shown.

Further, Patent Document 2 below discloses an indirect spot welding method capable of stably obtaining a nugget by controlling a pressing force and a current value.

JP, 2002-239742, A JP, 2010-194609, A

However, since there was no method for quantitatively evaluating the ease of forming a nugget at the welding point, trial and error was repeated while trying the above-mentioned method to find out the welding conditions for forming an appropriate nugget.

Therefore, the present inventors individually measure the resistance value of the active current path that contributes to welding and the resistance value of the reactive current path that does not contribute to welding, and set the active current set based on these resistance values. An attempt was made to evaluate the easiness of the nugget at the welding point based on the rate.

Specifically, as shown in FIG. 4(A), a first metal plate 1, a second metal plate 2 having a hat-shaped cross section, a first metal plate 1 and a second metal plate 2 are formed. For the component 100 constituted by the third metal plate 3 having a hat-shaped cross section and arranged in the hollow portion constituted by, the top plate portion 3b of the first metal plate 1 and the third metal plate 3 An evaluation method when indirect spot welding is performed on the overlapped portion P with will be described. In addition, already welded points Q1 and Q2 are provided between the respective metal plates.

First, the resistance value R _B of the reactive current path that does not contribute to welding is measured. Specifically, one terminal 31 of the resistance measuring device 30 is brought into contact with the overlapping portion P of the component 100, which is a portion with which the welding electrode is brought into contact during welding, from above. Further, the other terminal 32 of the resistance measuring device 30 is brought into contact with one already welded point Q2 of the component 100, which is a portion with which the ground electrode is brought into contact during welding, from below. In this state, the resistance value of the current path between the terminals 31 and 32 is measured without pressing the overlapping portion P of the component 100. Since the overlapped portion P is not pressed, the overlapped portions P of both metal plates 1 and 3 are not substantially in contact with each other, and almost no current flows through the overlapped portion P without insulation. Therefore, one terminal 31→first metal plate 1→welded point Q1→second metal plate 2→the other terminal 32 is a current path that does not pass through the overlapping portion P (interface between both metal plates 1 and 3). C1 is formed, and this current path C1 can be regarded as a current path of a reactive current that does not contribute to welding.

Next, the resistance value _RA of the effective current path that contributes to welding is measured. Specifically, as shown in FIG. 4(B), one terminal 31 of the resistance measuring device 30 is inserted into the slit S provided in the first metal plate 1 so that the third metal plate 3 has The top plate portion 3b is brought into contact with the vicinity of the overlapping portion P from above. Further, the other terminal 32 of the resistance measuring device 30 is brought into contact with one already welded point Q2 from below. As a result, a current path C2 of one terminal 31→the third metal plate 3→the already-welded point Q2→the second metal plate 2→the other terminal 32 is formed, and the resistance value of this current path C2 is measured.

Based on the resistance values R _B and R _A measured as described above, the ease with which the nugget can be formed at the welding point can be evaluated. For example, calculated as a resistance value of the active current path R _A and the total resistance value is a combined resistance of the R _B reactive current path resistance _{R T {= R A · R} B / (R A + R B)}, the effective current ratio K is determined as the ratio of the resistance value R _B of the reactive current path to the total resistance R _T (K=R _B /R _T ). The effective current rate K is an index representing the ease with which an active current flows, and the ease with which a nugget can be formed in the overlapping portion P can be evaluated based on the value of the effective current rate K. Then, by determining the welding conditions such as the welding position by using the effective current rate K as an evaluation index, it becomes possible to set the ideal welding conditions for obtaining a good product.

However, at the time of actual welding, for example, in a mass production site such as an automobile production line, there are variations in the welding position, the gap between the plates, and the like for each welding, so that the value of the effective current ratio K also has an error. .. Therefore, even if the ideal welding conditions are set based on the effective current rate K calculated at the trial production stage, the actual current welding will cause the effective current rate K to deviate from the non-defective product range due to variations in individual welding. There was a risk of causing defects. Therefore, in an automobile production line or the like, it has been necessary to calculate the effective current rate K for each welding, judge the quality of the welding, and prevent the defective product from flowing to the subsequent process.

However, in the above measuring method, as shown in FIG. 4A, the resistance value R _B is measured under the condition that the current flows only in the reactive current path C1 and the effective current is measured as shown in FIG. 4B. It is necessary to measure each resistance value under the condition that the current flows through only one of the paths, such as the measurement of the resistance value R _{A under} the condition that the current flows through the path C2 only. However, at the time of actual welding, the welding electrode pressurizes the overlapping portion P and the first metal plate 1 and the top plate portion 3b come into contact with each other, so that a current flows in both the reactive current path C1 and the active current path C2. Therefore, the measuring method as described above cannot be adopted, and it has been difficult to calculate the effective current rate K in a series of welding processes. Therefore, in an environment where welding is continuously performed on parts that constantly flow on the line, such as in a welding process of an automobile production line, the effective current rate K cannot be calculated for all of them, and the quality of the welding cannot be determined. There was a problem that it could not be judged.

In order to solve the above-mentioned problems, the present invention applies indirect spot welding in which a superposed portion of a plurality of workpieces is pressed by a welding electrode, and a ground electrode is brought into contact with a portion different from the superposed portion to energize between the electrodes. In the method, at the initial stage of welding, a step of bringing the welding electrode into contact with the overlapping portion and energizing the electrodes so that a current flows only in a reactive current path that does not contribute to welding, To do.

According to the present invention, in the initial stage of welding, by providing a step of energizing in a state in which a current flows only in the reactive current path, that is, a step of hardly performing pressurization by the welding electrode, the workpieces almost come into contact with each other in the overlapping portion. It is possible to carry out the measurement in the state where it is not. Therefore, the resistance value of the reactive current path (corresponding to the resistance value R _B described above) can be measured. In addition to this, the measurement of the resistance value in the subsequent step, that is, the overlapped portion is pressed by the welding electrode with a considerable pressing force, and the measurement is performed while the workpieces are sufficiently abutted, The total resistance value (corresponding to the total resistance R _T described above) can be measured. Based on these results, the effective current rate K can be calculated. That is, according to the present invention, it is possible to quantitatively evaluate the quality of welding by calculating the effective current rate K in the flow of a series of welding processes. Therefore, the quality of welding can be judged based on the effective current rate K calculated for each individual welding for all the parts that actually flow in the production line, and the outflow of defective products to the subsequent process can be prevented, or the calculated effective It is possible to review the welding conditions based on the current rate K.

According to the present invention, the quality of each welding can be quantitatively evaluated in a series of welding processes.

It is sectional drawing which shows a mode that indirect spot welding is performed. It is a graph which shows the current value, pressurizing force, and resistance value during welding of the said indirect spot welding. It is a figure which shows the path|route of the electric current which flows during the said indirect spot welding, (A) figure is a figure in a 1st step, and (B) figure is a figure in a subsequent step. It is a figure which shows the measuring method of the resistance value of each path|route using a probe.

Embodiments of the present invention will be described below with reference to the drawings.

In this embodiment, an indirect spot welding method performed in an assembling process of a vehicle body of an automobile is shown, and specifically, a case of welding a skeleton component 100 of the vehicle body as shown in FIG. 1 is shown. The skeleton component 100 is a frame-shaped component that extends in the direction orthogonal to the paper surface, and includes a first metal plate (work) 1 having a substantially flat plate shape, a second metal plate 2 having a hat-shaped cross section, and a first metal plate. And a third metal plate (work) 3 having a hat-shaped cross section, which is arranged in a hollow portion formed by the metal plate 1 and the second metal plate 2. As the metal plates 1 to 3, for example, steel plates are used, and specifically, mild steel plates, high-tensile steel plates (tensile strength of 490 MPa or more), ultra-high-tensile steel plates (tensile strength of 980 MPa or more) and the like are used.

The 1st metal plate 1 and the flange part 2a of the 2nd metal plate 2 are joined via the already joined point Q1 previously welded by direct spot welding. The bottom portion 2b of the second metal plate 2 and the flange portion 3a of the third metal plate 3 are joined via the already joined point Q2 that has been welded in advance by direct spot welding.

Then, the overlapping portion P of the first metal plate 1 and the top plate portion 3b of the third metal plate 3 is joined by indirect spot welding. The indirect spot welding apparatus includes a welding electrode 10 and a ground electrode 20, a pressurizing unit (an air cylinder, an electric cylinder, or the like) that drives the welding electrode 10 in the axial direction to pressurize a metal plate, and a welding electrode using the pressing unit. A control unit (not shown) that controls the pressing force of 10 and the current value between both electrodes 10 and 20 is provided.

The indirect spot welding on the overlap portion P is performed by the following procedure. First, the ground electrode 20 is brought into contact with a portion of the skeleton component 100 different from the overlapping portion P. In the illustrated example, the ground electrode 20 is applied from below to the bottom portion 2b of the second metal plate 2, particularly the already-joined point Q2 between the bottom portion 2b of the second metal plate 2 and the flange portion 3a of the third metal plate 3. Abutting. In this state, while pressing the overlapping portion P of the first metal plate 1 and the top plate portion 3b of the third metal plate 3 from one side in the thickness direction (the upper side in the figure) with the welding electrode 10, both electrodes 10 , 20 to weld the overlapping portion P.

In the present embodiment, welding is performed while changing one or both of the pressure applied by the welding electrode 10 and the current value between the electrodes 10 and 20. Specifically, welding is performed in accordance with the pressurizing and energizing pattern shown in FIG.

As shown in FIG. 2, first, in step S1, the overlapping portion P is pressed by the welding electrode 10 with an extremely small pressing force F1, and a minimum current value I1 is applied between the electrodes 10 and 20. This step is a step provided for calculating an effective current ratio K described later, and is a step which does not directly contribute to welding of the overlap portion P.

Then, the welding electrode 10 is energized at a low current value I2 while pressurizing the overlapping portion P with a high pressing force F2 (step S2). Then, while the applied pressure is reduced from F2 to F3, energization is performed with a current value I3 lower than the current value I2 (step S3). Then, while maintaining the applied pressure at F3, the current value is gradually increased. Specifically, the current value I4 (step S4), the current value I5 (step S5), the current value I6 (step S6), and the current value I7 (step S7) are gradually increased.

Of these steps, steps S4 to S7 of energizing with a relatively high current value I4 to I7 while pressurizing with a low pressurizing force F3 are the nugget growth period for growing the nugget.

With the energization pattern as described above, the overlapping portion P can be maintained at a high temperature to promote the growth of the nugget, and the overlapping portion P between the metal plate 1 and the top plate portion 3b of the metal plate 3 has a desired size and It is possible to form a nugget having a shape as a joining point and join the metal plates 1 and 3 through the nugget.

Next, a method for measuring the resistance value of each path and calculating the active current rate K during the above welding will be described with reference to FIGS. 3(A) and 3(B).

As shown in FIG. 3(A), the welding electrode 10 and the ground electrode 20 are connected to a transformer 4 as a current supply unit, and a current measuring device 5 is provided on this energizing path. The resistance value can be calculated from the current value I measured by the current measuring device 5 and the voltage value V of the secondary coil of the transformer 4. This voltage value V is also affected by the resistance value (voltage drop) of the cable or the like connecting the transformer 4 to the welding electrode 10 and the ground electrode 20, but since the resistance value of these cables and the like is very small, it is substantially It can be regarded as a voltage value between the pair of electrodes 10 and 20.

First, in step S1, the resistance value of the reactive current path is measured. In step S1, since the welding electrode 10 has a small pressure, the first metal plate 1 and the third metal plate 3 are not substantially in contact with each other in the overlapping portion P. Therefore, the current flows only in the reactive current path C1 that does not contribute to the welding of the overlapping portion P. Therefore, the resistance value R _B of the reactive current path C1 can be calculated from the current value I and the voltage value V.

Then, in a subsequent step (for example, step S2), the welding electrode 10 presses the overlapping portion P with a relatively large pressing force, so that the first metal plate 1 and the third metal plate 3 are connected to each other as described above. Contact at the superposition section P. As a result, as shown in FIG. 3B, in addition to the reactive current path C1, the current also flows through the effective current path C2 via the overlapping portion P. From the current value I and the voltage value V measured in this state, it is possible to calculate the total resistance value R _T which is the combined resistance of the reactive current path C1 and the active current path C2.

Based on the resistance value measured as described above, it is possible to calculate the active current ratio _{K (K = R B / R} T). The effective current ratio K is an index showing the ratio of the current flowing through the overlapping portion P, that is, the ratio of the current contributing to welding, among the current flowing through the entire circuit. The larger this value is, the lower the ratio of the current flowing through the overlapping portion P is. growing.

In the welding method of the present embodiment, at the start of welding, by intentionally providing step S1 in which the overlapping portion P is hardly pressed, a state in which current flows only in the reactive current path C1 is created, and the resistance value of the reactive current path C1 is created. It becomes possible to measure R _B. Further, as described above, the total resistance R _T can be calculated by measuring the current I and the voltage V when the overlapping portion P is sufficiently pressurized (see the resistance value in FIG. 2). Further, from these resistance value R _B and total resistance R _T, can be calculated resistance value R _A of the active current path C2. Thus, by performing the measurement as described above at different steps, the resistance value R _T, R _A, it is possible to calculate all of the resistance value of R _B.

Then, the effective current rate K can be calculated from these resistance values, and the quality of welding of the overlapping portion P can be determined based on the effective current rate K. That is, if the effective current ratio K is too small, a sufficient current does not flow in the overlapping portion P and the nugget does not grow, resulting in poor welding. On the other hand, if the effective current ratio K is too high, an excessive current flows in the overlapping portion P, which may cause burn-through or cracking of the first metal plate 1 or the third metal plate 3 in the overlapping portion P. I will end up. Therefore, the quality of welding can be quantitatively evaluated depending on whether the effective current rate K is within a predetermined range.

Further, according to the above method, it becomes possible to calculate the effective current rate K in a series of welding processes. For example, in the welding process of the automobile production line, the effective current rate K is calculated for all of them. Therefore, it becomes possible to quantitatively evaluate the quality of individual welding. As a result, it is possible to prevent defective products from flowing out to the subsequent process, and to review the welding conditions based on the calculated effective current rate K.

The pressing force F1 in step S1 is particularly small as compared with the pressing forces F2 and F3 in the other steps, and step S1 is a step of energizing the overlapping portion P with almost no pressurization. The state in which the overlapped portion P in "step S1 is hardly pressed" is a state in which the welding electrode 10 is pressed against the third metal plate 3 and is pressed with a pressing force sufficient to allow a current to flow in the reactive current path C1. There is also a state in which the overlapping portion P is pressed with a lower pressure than when the first metal plate 1 is deformed and comes into contact with the third metal plate 3. As an example, in this embodiment, the pressing force F1 is set to 5 [kgf]. The step of energizing with such a pressing force is an initial stage of welding, that is, a stage before both metal plates 1 and 3 press the overlap portion P with a considerable pressing force by the welding electrode 10. The resistance value of the reactive current path C1 can be calculated by providing 3 at a stage where the overlapping portion P is not substantially in contact.

Further, the heat generation density D during welding may be calculated using the effective current rate K during welding calculated as described above. The heat generation density D is D ₌ , where I P is the current flowing in the overlapping portion P during welding, V is the voltage, and S is the contact area between the first metal plate 1 and the third metal plate 3 in the overlapping portion P. It can be calculated as V·I _P /S. The voltage value of the secondary coil of the transformer 4 described above can be used as the voltage V.

The current value I _P flowing in the overlapping portion P can be obtained from I _P =V/ _RA by calculating the resistance value _RA of the effective current path C2 from the total resistance _RT . The resistance value R _A can be obtained by _RA =R _T ·R _B /(R _B −R _T ).

The contact area S can be calculated using the correlation with the displacement amount x of the welding electrode 10 from the reference position. That is, the correlation between the contact area S between the metal plates 1 and 3 at each time and the displacement amount x from the reference position of the welding electrode 10 at each time when the above plate assembly is welded in advance is acquired. Then, the contact area S at each time can be obtained from the displacement amount x of the welding electrode 10 at each time during actual welding by using the above correlation.

The heat generation density D during actual welding can be calculated from the respective values calculated as described above. That is, it becomes possible to calculate the heat generation density D in a series of actual welding processes. The heat generation density D is a plurality of dynamic factors that affect the heat generation state of the contact portion between the metal plates 1 and 3 in consideration of the principle of resistance welding (specifically, the current value flowing in the overlapping portion P). It is a parameter that integrates I _p , the resistance value that changes depending on the temperature, and the contact area S) that changes depending on the hardness of the metal plate and the pressing force of the electrode. From the heat generation density D, the magnitude of heat generation energy per unit area of the overlapping portion P can be quantitatively evaluated.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and it goes without saying that various modifications can be made without departing from the scope of the present invention.

In the above description, the active current rate is set based on the resistance values of the reactive current path and the active current path, but it may be set by the current value, for example, the current value flowing through the active current path with respect to the entire resistance value. May be

1 First metal plate (work)
2 Second metal plate 3 Third metal plate (work)
10 Welding electrode 20 Earth electrode C1 Reactive current path C2 Active current path P Overlapping part Q1, Q2 Already joined point

Claims (1)

An indirect spot welding method in which a superposed portion of a plurality of works is pressed with a welding electrode, and a ground electrode is brought into contact with a portion different from the superposed portion to energize between both electrodes,
At the initial stage of welding, indirect spot welding including a step of bringing the welding electrode into contact with the overlapped portion and energizing the electrodes so that a current flows only in a reactive current path that does not contribute to welding. Method.

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