JP7245589B2 - How to set welding conditions for spot welding - Google Patents

Description

The present invention relates to a method for setting welding conditions (current value and applied pressure) for spot welding.

For example, in the assembly process of an automobile body, spot welding is performed in which a pair of electrodes are brought into contact with a plurality of steel plates and an electric current is passed through them, so that the contact portions of the steel plates are melted by resistance heat generation to form a nugget.

For spot welding, various techniques have been proposed for forming a suitable nugget between multiple metal plates. For example, Japanese Unexamined Patent Application Publication No. 2002-200003 discloses an energization pattern shown in FIG. 4 . In this energization pattern, the current value is increased while alternately repeating a time period in which a high current value is maintained to grow nuggets and a time period in which a low current value is maintained to soften the steel sheet without generating spatter. gradually increasing. It is described that, as a result, the rapid growth of the nugget can be suppressed and the generation of spatter can be suppressed, so that the quality of the welded portion can be ensured and the spot resistance welding can be performed efficiently.

Further, Patent Document 2 below discloses an indirect spot welding method that is performed while controlling the current value and the applied pressure. Specifically, as shown in FIG. 5, the energization time is divided into two time periods t1 and t2, and in the first time period t1, high pressure f1 is applied while low current value c1 is applied. Thereby, the contact area between the electrode and the metal plate is secured, the current density is suppressed, and the melt scattering on the metal plate surface is prevented. In the next time period t2, the high current value c2 is applied while applying the low pressure f2. As a result, it is possible to suppress the sinking of the tip of the electrode into the metal plate and sufficiently increase the current density, so that sufficient heat can be obtained to grow the nugget and the nugget can be stably obtained. It is

In addition, the present inventors have proposed a pressurization current pattern for indirect spot welding as shown in FIG. 6 in Japanese Patent Application No. 2018-102540. Specifically, in a first step S1, a low current value C1 is applied between the welding electrode and the ground electrode while applying a high pressing force F1 to the overlapped portion of the metal plate with the welding electrode. In the subsequent second step S2, while decreasing the pressure applied by the welding electrode from the high pressure F1 to the low pressure F2, a current value C2 lower than the current value C1 in the first step S1 is applied between both electrodes. do. Thereafter, while maintaining the low pressing force F2, the current value is increased stepwise to C4 and C5 in the third and fourth steps S3 and S4, and the low current value C5 is applied in the fifth step S5.

JP 2006-181621 A JP 2010-194609 A

However, even when the above method is adopted, there are cases where an appropriate nugget is not formed between the metal plates. For example, when joining mild steel plates by spot welding, the contact portion between the mild steel plates is less likely to generate heat because the specific resistance value of the mild steel plate itself is low. It is difficult to let When joining such difficult-to-weld plate assemblies by spot welding, there is no choice but to adjust the current value and pressure by repeating trial and error based on experience, etc., so the optimum welding conditions (such as the current pattern ) is very time-consuming to set up.

Therefore, the problem to be solved by the present invention is to enable the optimum welding conditions to be easily set even for a plate combination that is difficult to weld.

Spot welding (resistance welding) is a method of melting the contact portion by causing the contact portion to generate resistance heat according to the relationship between the current density (current value I/contact area S) and the resistance value R at the contact portion between metal plates. This is a method of forming a nugget. The present inventors have investigated several factors that affect the heat generation state of the contact portion between metal plates (specifically, the current value I flowing through the contact portion and the resistance value R of the contact portion that changes depending on the temperature In addition, the inventors considered unifying the contact area S), which varies depending on the hardness of the metal plate and the pressing force of the electrode. As a result, the heat generation density D, which is the value obtained by dividing the heat energy J (=I V = I ² R) input to the contact area between the metal plates by the contact area S between the metal plates, It was found that the exothermic state can be monitored.

The present invention, which has been made based on the above findings, is a method for joining a plurality of metal plates by applying an electric current between the pair of electrodes in a state in which the pair of electrodes are in contact with the plurality of superimposed metal plates. A method for setting welding conditions, wherein D is the current value flowing between the pair of electrodes, V is the voltage between the pair of electrodes, and S is the contact area between the plurality of metal plates. The energization pattern is set based on the heat generation density D expressed by =I·V/S.

Specifically, for example, through a step of acquiring a heat generation density D when welding is performed according to a temporarily set energization pattern, and a step of adjusting the temporarily set energization pattern based on the heat generation density D, the optimum energization pattern can be set.

As described above, by setting the energization pattern based on the heat generation density D, it is possible to easily set the optimum welding conditions that sufficiently generate heat at the contact portions of the metal plates even in a plate combination that is difficult to weld. can be done.

1 is a schematic diagram of a direct spot welding device; FIG. 7 is a graph showing a temporarily set pressurizing energization pattern and heat generation density; It is a graph which shows the pressurization energization pattern and heat generation density after adjustment. This is the energization pattern described in Patent Literature 1. FIG. This is a pressurizing energization pattern described in Patent Document 2. FIG. This is a pressurization energization pattern described in Japanese Patent Application No. 2018-102540.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a diagram schematically showing a direct spot welding apparatus in which a pair of electrodes 3 and 4 are held between a pair of electrodes 3 and 4 and current is passed through a combination of two metal plates 1 and 2. As shown in FIG. The metal plates 1 and 2 of this embodiment are steel plates, for example, mild steel plates having a tensile strength of 300 MPa or less and a plate thickness of 1.2 mm or less. The electrodes 3 and 4 are connected to a transformer 5 as a current supply section, and a current measuring device 6 is provided on this current path. The electrode 3 is attached to a pressure means 7 (eg a cylinder). A predetermined pressure is applied to the metal plates 1 and 2 by driving the electrode 3 with the pressure means 7 and pressing it against the metal plate 1 .

A method for setting welding conditions according to the present embodiment is performed in the following procedure.
(1) A sample is prepared by welding the metal plates 1 and 2 under temporarily set welding conditions.
(2) Measure the contact area between the metal plates 1 and 2 from the cross section of the sample.
(3) Calculate the heat generation density of the sample.
(4) Adjust the welding conditions based on the heat generation density of the sample.
Each procedure will be described in detail below.

(1) Preparation of sample Fig. 2 shows temporarily set welding conditions (pressurization current pattern). This pressurizing energization pattern is generally the same as that shown in FIG. Specifically, first, a low current value C1 is applied while a high pressure force F1 is applied (step S1). After that, while decreasing the pressing force from F1 to F2, current is applied at a current value C2 lower than the current value C1 (step S2). Then, while maintaining the applied force at F2, a current value C3 higher than the current value C1 (step S3), a current value C4 higher than the current value C3 (step S4), and a current value C5 lower than the current value C4 (step S5) turns on electricity. Among these steps, steps S3 to S5 in which relatively high current values C3 to C5 are applied while applying a low pressure force F2 constitute a nugget growth period for growing nuggets.

The metal plates 1 and 2 are welded in this energization pattern, and a plurality of samples are produced by stopping the energization at a plurality of times from the start of the energization. For example, the first sample where the energization is stopped within 1 cycle (= 1/60 second) from the start of energization, the second sample where the energization is stopped within 2 cycles after the start of energization, and so on. is lengthened for each cycle to create a plurality of samples. The changes in current value I and voltage V over time during welding of each sample are recorded. Specifically, the current value I flowing between the electrodes 3 and 4, that is, the current value I of the current path passing through the contact portion of the metal plates 1 and 2 is measured by the current measuring device 6, and the change over time is recorded. A voltage V is the voltage of the secondary coil of the transformer 5 . This voltage V is also affected by the resistance value (voltage drop) of the cable connecting the transformer 5 and the electrodes 3 and 4, but since the resistance value of these cables is very small, the voltage V is substantially equal to that of the pair of electrodes. It can be regarded as a voltage between 3 and 4.

(2) Measurement of contact area Each sample is cut, and the contact area between the metal plates 1 and 2 is measured in each sample. Specifically, the diameter of the contact portion between the metal plates 1 and 2 is measured from the cross section of each sample, and the area of the contact portion is calculated from this diameter. The contact portion at this time means not only the region where the metal plates 1 and 2 are actually in contact with each other (for example, the nugget forming portion) in the cross section of each sample, but also Including areas with traces of The contact area between the metal plates 1 and 2 in each sample is the contact area between the metal plates 1 and 2 at a plurality of times in the pressurizing current pattern shown in FIG.

(3) Calculation of heat generation density D Based on the contact area S between the metal plates 1 and 2 at a plurality of times measured in (2) above, and the current value I and voltage V at that time, heat generation at the contact portion at each time Calculate the density D (=I·V/S=I ² ·R/S). Considering the principle of resistance welding, the heat generation density D is determined by a plurality of dynamic factors (specifically, the current value I , the resistance value R of the contact portion, which varies depending on the temperature, and the contact area S, which varies depending on the hardness of the metal plate and the pressing force of the electrode.

The change over time of the heat generation density D calculated above is indicated by a dotted line in the graph of FIG. As can be seen from this graph, the heat generation density D increases at the beginning of step S3, but decreases after time t1. Similarly, the heat generation density D increases at the beginning of step S4, but decreases after time t2. Furthermore, in step S5, the heat generation density D decreases throughout the period.

(4) Adjustment of energization pattern Based on the heat generation density D obtained in (3) above, the temporarily set energization pattern shown in FIG. 2, especially the energization pattern during the nugget growth period (steps S3 to S5), is adjusted. Specifically, the current value and the energization time are adjusted so that the heat generation density D does not decrease as much as possible during the nugget growth period. Specifically, a reference value D1 of the heat generation density D required to grow the nugget is set, and the energization pattern is adjusted so that the heat generation density D does not exceed the reference value D1 at an early stage and then fall below the reference value D1. adjust.

For example, at step S3 in FIG. 2, the heat density D decreases before reaching the reference value D1. Further, in step S4, the heat generation density D once exceeds the reference value D1, but then decreases to fall below the reference value D1. From these results, in the board structure of the present embodiment, each step (the time of energizing at a constant current value) is shortened in the nugget growth period, and the current value is increased before the heat generation density D starts to decrease. is considered preferable. Further, in step S5, since the heat density D continues to decrease while remaining below the reference value D1, this step S5 hardly contributes to the growth of the nugget and can be omitted.

According to the examination based on the heat generation density D as described above, the temporarily set energization pattern shown in FIG. 2 is adjusted, and the energization pattern shown in FIG. 3 is set. In this energization pattern, the current value is increased stepwise from C3' to C6' in a short period of time during the nugget growth period (steps S3' to S6'). As a result, the heat generation density D is maintained at a high value without falling below the reference value D1 after increasing to the reference value D1 or higher. By maintaining the heat generation density D at the contact portion of the metal plates 1 and 2 at a high value in this manner, the contact portion is maintained at a high temperature, so that the growth of the nugget can be promoted. In addition, as described above, in order to prevent the heat generation density D from decreasing, a high current value is applied for a short period of time, thereby shortening the energization time during the nugget growth period and shortening the spot welding cycle time.

Conventionally, a nugget is formed by welding under predetermined welding conditions (energization pattern), and if the nugget is not of the desired size and shape, the current value and energization time (cycle) of each step of the energization pattern are determined by experience. It was customary to make blind adjustments based on In order to reach the energization pattern shown in FIG. 3 from the energization pattern shown in FIG. 2 with such a method, a huge amount of trial and error is required. Therefore, as described above, the heat generation density D at a plurality of times of the temporarily set energization pattern is obtained, and the current value and the energization time are adjusted based on the heat generation density D, thereby obtaining the optimum energization pattern. Man-hours are greatly reduced.

In the above-described embodiment, the case where the contact area S between the metal plates 1 and 2 is obtained from the cross section of the sample has been shown, but the present invention is not limited to this. For example, the correlation between the contact area S between the metal plates 1 and 2 at each time and the displacement amount x of the welding electrode 3 from the reference position at that time is acquired in advance when welding a predetermined combination of plates. . Then, when setting the energization pattern for welding the same plate assembly as described above, the above correlation is used to obtain the contact area S at each time from the displacement amount x of the welding electrode 3 during welding. can be done. In this case, since it is not necessary to cut the sample and measure the contact area S, the contact area S and the heat generation density D can be easily obtained.

Further, in the above-described embodiment, the case where the energization pattern is set based on the value of the heat generation density D itself is shown, but this is not restrictive. For example, an index (actual heat generation The energization pattern may be set based on the density D'=K·D). The active current rate K is an index representing the ratio of the active current that passes through the contact portion between the metal plates 1 and 2 and contributes to the welding in the total current flowing between the electrodes 3 and 4 . Specifically, when the resistance value of the conducting path of the active current is _RA and the resistance value of the conducting path of the reactive current is _RB , the active current rate K is, for example, K= _RB /( _RA + _RB ). can be defined. Alternatively, the active current rate K may be defined as K= _RB / _RA . Alternatively, when the combined resistance value of the resistance value _RA of the active current conduction path and the resistance value _RB of the reactive current conduction path is R _TOTAL , the active current rate K is defined as K = _RB /R _TOTAL . may

In addition, in the above embodiment, the case where the energization pattern (current value and energization time) is adjusted based on the heat generation density D when welding the sample is shown, but not limited to this, based on the heat generation density D The pressurization pattern (pressurization force and pressurization time), or both the energization pattern and the pressurization pattern may be adjusted.

Further, in the above-described embodiment, the case where the heat generation density D is obtained from the current value I, the voltage V, and the contact area S when welding is actually performed has been shown. A heat generation density D may be obtained when welding is performed with a predetermined energization pattern that has been set.

Moreover, the method for setting welding conditions according to the present invention is not limited to direct spot welding as described above, but can be applied to other spot welding such as indirect spot welding and series spot welding. In addition, the plate assembly to be welded is not limited to the above, and the welding conditions according to the present invention can also be applied to spot welding of plate assembly consisting of three or more metal plates and plate assembly including high-tensile steel plate and ultra-high-tensile steel plate. A setting method can be applied.

1, 2 metal plates 3, 4 electrode 5 transformer 6 current measuring device 7 pressure means

Claims (2)

A method for setting welding conditions for joining a plurality of metal plates by energizing between the pair of electrodes in a state in which the pair of electrodes are in contact with the plurality of superimposed metal plates. hand,
When the current value flowing between the pair of electrodes is I, the voltage between the pair of electrodes is V, and the contact area between the plurality of metal plates is S, the heat generation density is expressed by D=IV/S. A method for setting welding conditions for spot welding in which, once D exceeds a reference value D1, an energization pattern is set so that it does not fall below the reference value D1 until the end of energization . The step of acquiring the heat generation density D when welding is performed according to the temporarily set energization pattern, and the step of adjusting the temporarily set energization pattern based on the heat generation density D. Spot welding according to claim 1. How to set welding conditions.

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