JPS619982A - Resistance welding method - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a resistance welding method in which welding quality is guaranteed by detecting the interelectrode voltage during welding and controlling the welding current, the current application time, or both.

It has been widely accepted that the change in interelectrode resistance during welding has a good correlation with the growth of nuggets in the weld zone, and various adaptive control methods have been proposed to utilize this fact to guarantee welding quality. Figure 1, which has been used in practice, shows a typical example of the change in interelectrode resistance when current is applied at an appropriate constant current value to a bare steel plate without surface treatment in general single-phase AC welding. This is what is shown. A in FIG. 1 is an unstable region immediately after the start of energization, and the behavior of the interelectrode resistance during this period depends on the degree of heating of the material to be welded, the state of contamination on the surface, etc. This surface contact resistance is 1 after the start of energization.
The resistance between the electrodes disappears after ~2 cycles, and the interelectrode resistance rapidly decreases.Next, in Figure 1b, the temperature rise in the weld zone increases the specific resistance of the material to be welded, and the weld zone softens and collapses, resulting in an expansion of the current-carrying path area. proceed at the same time. During this period, the increase in weld resistance due to the increase in specific resistance outweighs the decrease in resistance due to the expansion of the current carrying path area, and as a result, the inter-electrode resistance increases, reaching a maximum value near the end of this process. During this period, nuggets begin to form and grow. Thereafter, as shown in C in FIG. 1, the current carrying path area continues to expand with the growth of the nugget, but the resistivity reaches a saturation value and tl becomes constant, so the interelectrode resistance decreases.

In the conventional adaptive control method, taking the above fact into account, and from the viewpoint that the amount of decrease from the maximum point of the interelectrode resistance or its integral value has a deep correlation with the nugget diameter, a predetermined current is set in advance. The voltage between the electrodes is detected every moment during welding, and the transition of the resistance between the electrodes is determined by dividing this by the current value. The welding quality was guaranteed by cutting off the current when the value corresponding to was reached. Alternatively, in addition to this, the amount of heat input to the welding part was sometimes corrected by controlling the current value so that the transition of the interelectrode voltage followed a pre-measured reference waveform that yielded an appropriate nugget. .

However, while the adaptive control method described above is effective for bare steel sheets without surface treatment, when welding surface-treated steel sheets such as galvanized steel sheets and synchro metal sheets, which have been in high demand in recent years, The deposits have a large effect on the behavior of the interelectrode resistance, and generally interfere with the displacement it, resulting in a decrease in the accuracy of the detected amount, which in turn leads to a decrease in welding quality. .

As an example, FIG. 2 shows a typical example of the change in interelectrode resistance of an alloyed galvanized steel sheet. In the initial unstable region a of Fig. 2, the electrical resistance of surface zinc is lower than that of iron, so the interelectrode resistance is slightly lower than that of a bare steel plate. At the start of nugget formation and initial growth as shown in Fig. 2b, the zinc around the weld zone melts and fills the gaps in the welding material, expanding the area of the current-carrying path, as shown in Fig. 3. The increase in resistance is slowed down, and the maximum value of the material itself is also low. In the nugget growth process shown at C in Figure 2 after i, the current carrying path area has already been expanded by molten zinc, so even if the nugget diameter increases, the resulting decrease in interelectrode resistance is quite noticeable in the case of a bare steel plate. Not.

Therefore, because the occurrence of the maximum point is not clear and the rate of decrease in interelectrode resistance is slow, it has been extremely difficult to accurately detect the amount of decrease or its integral value.

The present invention was made to solve this problem, and the welding current, which was conventionally applied to the welding medium at a constant rate or with a waveform such as a simple ramp-up, is changed depending on the type of material to be welded, i.e. 1 i! that corresponds to the surface treatment situation, etc. is adjusted to an appropriate value every half cycle or every two cycles, thereby promoting the growth of nuggets, preventing the narrowing of the interelectrode resistance transition, increasing its detection accuracy, and improving the stability of control. , which guarantees uniform welding quality.

FIG. 4 shows an embodiment in which the present invention is applied to welding of alloyed galvanized steel sheets, and will be described below with reference to the drawings. In FIG. 4, waveform 1 indicates that a constant current is applied according to the conventional adaptive control method, and waveform 2 shows the change in interelectrode resistance in that case, and is the same as the waveform in FIG. Waveform 3#'
The appropriate energization waveform for the welding material determined by the i-experiment takes into consideration improvements in weldability and improvement in detection accuracy of waveform 4, that is, the transition of interelectrode resistance at that time.

Compared to the conventional adaptive control method, the current value is gradually increased during the initial stage of nugget generation and growth to compensate for the decrease in current density due to the expansion of the current carrying path area caused by the molten zinc in FIG. As a result, the temperature of the weld zone increases and the specific resistance also increases, resulting in an increase in the interelectrode resistance and a clear maximum point. At this point, nuggets have already been generated and have started to grow. From now on, in order to prevent scattering due to excessive current, the current value is completely reduced and the nuggets are allowed to grow somewhat slowly. If current is applied as shown in waveform 3 as described above, the interelectrode resistance will change as shown in waveform 4. For example, if the amount of decline from the maximum point or its integral value is to be detected, in the case of waveform 2 It is clear that this will greatly improve precision. Therefore, fluctuations in the growth rate of nuggets due to various disturbances can be detected more sensitively, good reproducibility can be obtained, and uniform welding quality can be guaranteed.

) Welding experiments were conducted in advance for each type of N welding material in order to find the appropriate energization waveform, and good welding quality was achieved.
Furthermore, the current value for every half cycle or every two cycles is determined while taking into account that the amount of displacement in the transition of the interelectrode resistance becomes large. In the past, it was difficult to store a large number of set values in the control device for one energization, but recent advancements in microcomputer technology have made it possible to do so. This fact is the background of the present invention.

[Brief explanation of drawings]

The waveform diagram in FIG. 1 is a typical example of the change in interelectrode resistance when current is applied to a bare steel plate without surface treatment at an appropriate constant current value. Moreover, the waveform diagram in FIG. 2 is obtained by performing this on an alloyed galvanized steel sheet. Figure 3 ####(d) This is a cross-sectional view near the weld of an alloyed galvanized steel sheet, showing the situation where molten zinc suppresses sort separation and expands the current carrying path area. The waveform diagram is for comparing the current waveform and the change in interelectrode resistance between the welding method of the present invention and the conventional welding method.
Waveform 4 and waveform 4 are the changes in interelectrode resistance during welding, and correspond to waveform 1.3, respectively. Fig. 1 Energizing time (cycle) Fig. 2 Fig. 3 0 5 10 Energizing time ("J cycle") Fig. 4

Claims (1)

[Claims] In the resistance welding method, which detects the interelectrode voltage during welding, calculates the change in interelectrode resistance from this, and controls the welding current, energization time, or both, the welding current is set in advance every half cycle or every two cycles. A resistance welding method that applies electricity with a predetermined waveform to promote nugget growth and allows for highly accurate detection of changes in weld resistance during welding, thereby ensuring uniform welding quality.