JPS6043834B2 - Welding method of resistance welding - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a welding method for resistance welding a workpiece having a low surface resistance and having a surface treatment or the like.

Conventionally, a common method for monitoring resistance welding is to monitor the resistance between the electrodes during welding and stop or reduce the welding current when this resistance decreases by a preset value from the initial maximum value. A method was adopted to control the quality of resistance welding. However, with this conventional method, when resistance welding steel sheets that have been surface-treated such as galvanized, the electrical resistance is lower than before welding, so the maximum value of the initial interelectrode resistance during welding does not clearly occur. Therefore, there was a drawback that it was sometimes difficult to control the quality of resistance welding based on the amount of voltage drop from the maximum value.

In addition, when resistance welding steel sheets that have been surface-treated such as galvanized, the electrical resistance of the resistance welding surface is low, so the welding current must be increased to obtain the necessary amount of heat for welding. . Therefore, there was a drawback that splash occurred in the latter half of the welding period. SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a resistance welding method that does not cause the above-mentioned drawbacks even when resistance welding objects having low surface resistance.

Embodiments of the invention will be described based on the drawings. The resistance welding control device used in the embodiment will be described based on Fig. 1.2? 24 is a welding control device; 24 is a welding transformer; 21 and 22 are primary lead wires of the welding transformer; 25 and 26 are secondary lead wires of the welding transformer; and 27 is a current detection device for the secondary lead wire of the welding transformer. 3 and 4 are electrodes for performing resistance welding; 2 is the object to be welded; 31 is a constant current circuit that removes fluctuations in the welding current detected by the current transformer; this constant current circuit 31 is the current transformer 27; The response speed changes the detected current fluctuation to a constant current within half a cycle. 23 is a thyristor that is arranged on the primary lead wire 22 of the welding transformer and is driven by a constant current circuit 31 to control the welding current at a constant level; 32 and 33 are leads that detect the voltage between the electrodes; 34 is an input adjustment circuit, 35 is an induced electromotive force removal circuit that removes the induced electromotive force caused by the welding current generated in the lead wires 32 and 33 that detects the voltage between the electrodes, and 36 is an absolute value conversion circuit by full-wave rectification. Circuits 37 and 38 are sample and hold circuits, and sample and hold circuits 37 and 38 are constant current circuits 3
1, sample signals are alternately applied every half cycle immediately after half cycle energization.

39 is a sample hold circuit 37 which alternately sends sample signals.
, 38; 40 is a comparison circuit that outputs the larger voltage signal among the voltage signals applied from sample and hold circuits 37 and 38; 41 is a peak value hold that stores the maximum value of the voltage signal. 42 is a differential arithmetic circuit that compares the peak value of the voltage signal with the detected voltage signal and detects the amount of change ΔV between the peak value and the subsequent voltage signal; 43 is an output signal of the differential arithmetic circuit; A control circuit detects that the voltage has become lower than the peak value and drives the welding control device 20 and constant current circuit 31 to reduce the welding current by 10%, and 44 is a voltage drop amount Δ that is used to terminate resistance welding in advance.
The variable resistor 45 is a comparison circuit that compares the voltage drop amount Δ■ with the set voltage set by the variable resistor. When the set value is reached, the output signal from the comparison circuit 45 controls the welding control device to stop the welding current.

Next, the waveform of the voltage between the electrodes in Figure 2A is the waveform when two galvanized steel plates of 1 thickness are resistance welded at a welding current of 8.3KA'80.75TIrII over the entire welding period, and the maximum value The amount of voltage drop is small, and splash occurs in the latter half of the welding period.

Next, FIG. 2B shows the waveform of the interelectrode voltage according to this example, which is obtained after the maximum value of the welding period when resistance welding two galvanized steel plates with a thickness of 0.75 mm at a welding current of 8.5 KA. In the waveform obtained by reducing the welding current by 10% to 7.65 KA, the amount of voltage drop from the maximum value was large, and no splash occurred even in the latter half of the welding period.

Next, Figure 3 shows the relationship between welding current and welding strength.
The curve shown by the solid line represents the case where the welding current is lowered by 10% in the latter half of the welding period, and in this case, the welding current and welding strength are approximately proportional, which has a negative effect on the welding strength. There was no problem, and no splash occurred up to a welding current of about 00A.

On the other hand, the curve shown by the broken line shows the case where welding is performed with a uniform welding current over the entire welding period, and the welding current and welding strength are not in a proportional relationship. Although the welding current must be greater than that, splash occurs when the welding current exceeds 8000 A.Since this invention is constructed as claimed in the claims based on the embodiments described above, the electrical resistance of the surface-treated When resistance welding a workpiece with a low resistance to welding and monitoring its quality, the maximum value of the initial interelectrode resistance during welding clearly occurs, making it easier to control the quality of resistance welding. When resistance welding objects with low electrical resistance, we were able to suppress the occurrence of splash even when welding strength was increased.

[Brief explanation of drawings]

Figure 1 is a block diagram of the control device for resistance welding, Figure 2A is the waveform of the voltage between the electrodes when resistance welding is performed with a uniform welding current over the entire welding period, and Figure 2B is the waveform of the voltage between the electrodes in the latter half of the welding period. The waveform of the voltage between the electrodes in the case of resistance welding in which the welding current is reduced by 10%, and FIG. 3 is a graph showing the relationship between the welding current and the welding strength.

Claims (1)

[Scope of Claims] 1. A welding method for resistance welding in which the welding current is lowered in the latter half of the welding period when resistance welding a welded object with a surface treatment with low surface resistance using a constant current. . 2. The resistance welding method according to claim 1, wherein the welding current is lowered by about 10% in the latter half of the welding period.