JPH0790384B2 - Resistance spot welding method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a resistance spot welding method.

(Prior Art and Problems Thereof) In order to guarantee the quality of the resistance spot welded portion, conventionally, only the size of the nugget diameter has been guaranteed. Since the deterioration of the welding quality (reduction of the nugget diameter) is not observed even when the bare steel plate or the like is spattered, this spot has been used as a quality monitor and as a means of quality assurance on site. However, in the case of plated steel sheets which have been used more and more recently, the nugget diameter is rather reduced due to the occurrence of the scattering. In this case, dust is not a means of ensuring quality. In addition, with the introduction of welding robots, the deterioration of the work environment due to the occurrence of this scattering has become a major problem. For this reason, in recent years, it has been desired to realize welding in which scattering does not occur and quality can be secured (nugget diameter is secured).

To achieve this goal, feedback control is one solution. However, an accurate monitoring method has not been established at present. Another idea is to adjust the power supply characteristics to increase the current range in which no scattering occurs and a certain size of the nugget diameter can be obtained, that is, the so-called welding condition margin. As part of this idea, the influence of the current waveform and the influence of the power source characteristic have already been made, and it has been clarified that the effect of widening the welding condition margin can be obtained as a result. However, in the power conversion of the power supply characteristics using the DC power supply (three-phase rectification power supply), which is judged to be most effective, there remains a problem that the adjustment effect does not appear on the short time side. This is assumed to be due to the delay in nugget growth.

The present invention has been made to solve the above-mentioned conventional drawbacks, and its purpose is to provide a short-time energization condition.
An object of the present invention is to provide a resistance spot welding method capable of expanding the welding condition tolerance as compared with the conventional method.

(Means for Solving Problems) Therefore, the resistance spot welding method of the present invention supplies the maximum current in the initial stage before the start of nugget formation, reduces the current value at the nugget formation start time, and then gradually increases the current value. It is characterized by increasing.

(Operation) According to the resistance spot welding method described above, as shown by the broken line in FIG. 1, the nugget formation start time is earlier than in the conventional case (solid line in the figure) to eliminate the delay in nugget growth, and to make contact with the nugget diameter. It is difficult for the diameters to approach each other, and the effect of increasing the welding condition tolerance can be obtained even under a short-time energization condition.

(Example) Next, a specific example of the resistance spot welding method of the present invention will be described in detail with reference to the drawings.

First, FIG. 2 shows a configuration diagram of a computer-controlled inverter power supply. In the figure, 1 is a computer, 2 is a GT
O control device, 3 is an electrode, 4 is a pressurizing cylinder, and 5 is a shunt. In this case, the oscillation frequency is 60
I set it to 0Hz. The detected welding current and tip-to-tip voltage are taken into a personal computer via an A / D converter and recorded on a floppy disk.

The welding current value was the effective current value during the entire welding period, as shown in FIG. This is because as shown in FIG. 3 (b), a good relationship with the nugget diameter was established regardless of the welding current waveform.

An R-shaped electrode with a tip radius of 50 mm was adopted as the electrode. This is CF
This is to eliminate the limitation of the maximum contact diameter due to the shape of the electrode such as the shape.

The pressurizing part of the welding machine is a direct pressurizing type. The pressure cylinder is a pneumatic one with low friction characteristics, and a centering mechanism is incorporated in the lower electrode so that the center axes of the upper and lower electrodes do not shift.

Welding conditions were based on the following RWMA recommended Class A conditions, and a 60 × 70 mm large mild steel flat plate was used as the test piece.

Plate thickness (mm) Welding time (ms) Pressurizing force (kN) 0.4 83 1.2 0.8 133 1.9 1.2 200 2.7 In order to control the current waveform computer with this inverter power supply, it is necessary to give an input signal waveform that determines the set voltage of the GTO. There is. Therefore, the GTO control circuit was modeled as the input / output characteristics of the LRC circuit, which is a simple secondary system, and compared with the step response waveform of the known LRC circuit shown in Fig. 4, and the GTO
The coefficient of the differential equation of the following equation showing the input / output characteristics of the control circuit was determined.

However, ω _n : natural frequency ξ: damping coefficient and a method of determining the input signal waveform from the current waveform to be output by solving by differentializing the differential equation and using it.

The current waveform is observed by averaging the DC waveform on which the pulsating component is superimposed to make it easier to see the change.

The relationship between each current / voltage waveform and the nugget formation process is shown in FIG. The nugget diameter and corona bond diameter were measured with a magnifying glass after twisting and breaking the spot weld. The figure (a) is a conventional constant current type, the figure (b) is a conventional constant power type, and the figure (C) is an example of the initial high current type introduced this time. (A)
In the constant current type, since the heat input is high during the nugget growth period, the width between the nugget diameter and the corona bond diameter (contact diameter) becomes narrow before the nugget diameter saturates. In the constant power type of (b), since the heat input in the latter period is relatively high, the nugget grows mainly in the middle period and thereafter, and the nugget diameter and the corona bond diameter become narrow in the latter period. on the other hand,
In the initial high-current type of (C), although the nugget grows until the latter half, the nugget diameter and the corona bond diameter are close to each other, and the nugget formation start time is early. When the diameter of the nugget and the diameter of the corona band are close to each other, scattering easily occurs. Therefore, in the initial high-current type (C), it is expected that the nugget growth will not be delayed and the scattering will be suppressed.

FIG. 6 shows the welding condition tolerance in each current waveform. The welding time is the short-time energization condition (RWMA A-class recommended energization time condition) where the effect of constant power supply characteristics disappears. As is clear by comparing the welding condition tolerance of each current waveform,
In the initial high current type, the scattered current value is the highest. This can be explained and understood by the difference in the nugget formation process described above. This also shows the effect of expanding the welding condition margin of the initial high current waveform under the short-time energization condition. 4√h (h:
The current value at which the nugget diameter (plate thickness) is obtained does not differ so much in any current waveform. This is the third
This is because the nugget diameter can be arranged by the effective current value during the entire welding period regardless of the current waveform, as shown in FIG.

Such characteristics are similarly confirmed for other thick plates.

Next, the relationship between the current waveform and the nugget formation characteristics was examined centering on the initial high current type. FIG. 7 shows the results of observing changes in the nugget formation process when the current waveform after the middle period in the initial high current waveform was changed. The figure (a) shows the case where the current is constant after the middle period, the figure (b) shows the case where the current is raised to a certain point and then becomes constant, and the figure (C) shows that the current after the middle period is gradually increased until the end of welding. It shows each case. Comparing these, when the current is gradually increased as shown in FIG. 7C, the nugget grows to the latter stage. Therefore, when trying to form a large-sized nugget, it can be said that it is preferable to gradually increase the current after the middle period.

Next, FIG. 8 shows the relationship between each initial high current waveform and the nugget formation process. From the results shown in FIG. 7, the waveforms that gradually increase the current after the middle period are adopted.

First, the current waveforms of FIGS. 8A, 8B, and 8C and the nugget formation process will be compared, focusing on the time when the current is reduced. In the current waveform of FIG. 10A, the initial high current period is long, so the current value is high even at the nugget formation start time. Therefore, the nugget grows rapidly in the initial stage, and the nugget diameter and the corona bond diameter (contact diameter) approach each other. The same figure (b)
In the current waveform, the initial high current period is almost up to the nugget formation start time, and the current gradually decreases near the nugget formation start time. In the nugget forming process in this case, the nugget diameter and the corona bond diameter are not close to each other, and the pattern is appropriate for expanding the so-called welding condition tolerance. On the other hand, in the current waveform of FIG. 6C, the current value is already in the lowest state before the nugget formation start time, so that the nugget does not grow much in the initial stage and rapidly grows in the middle period. Grow up,
The nugget diameter is close to the corona bond diameter. From this, it can be said that the proper initial high current period is until the time when the nugget formation is started, and there is a time when the nugget diameter and the corona bond shape are longer and much shorter than that.

Next, let us consider the case where the timing of raising the current is changed. FIG. 9 (a) shows the above-mentioned FIG. 8 (c) again. As shown in the current waveform of FIG. 9A, FIG. 9B shows that the initial high current period is shortened and the current is increased faster by that amount. Although the nugget diameter and the corona bond diameter are closer to each other in the case of FIG. 11B than in the case of FIG. 9A, the approach in the middle period has not been resolved, and an appropriate nugget formation process is determined. Is hard to say. From this, it can be said that it is not appropriate to increase the time of gradually increasing the current in order to accelerate the initial nugget growth because it tends to cause the nugget diameter and the corona bond diameter to approach each other in the middle period.

Figure 10 shows the relationship between each current waveform and the welding condition tolerance. First, FIG. 7A shows the welding condition tolerance in each current waveform shown in FIG. As is clear from that, FIG. 7 (c)
In the waveform in which the current is gradually increased from the middle period onward, the dispersion limit current value is the highest. This suggests that the electric current is gradually increased after the middle period, and that a large-sized nugget can be easily obtained without relatively causing scattering. Next, FIG. 8B shows welding condition tolerances in the current waveforms shown in FIGS. 8A and 8B and FIG. 9B. FIG. 8 (a) represents the initial case, and FIG. 9 (b) represents the case where the nugget diameter and the corona bond diameter are close to each other after the middle period. As is clear from this figure, when trying to expand the welding condition tolerance by the initial high current waveform, pay particular attention to the initial high current period (time to decrease the current) and the current increase after the middle period It can be said that the nugget formation process should be such that the diameter and the corona bond diameter do not approach each other.

(Effects of the Invention) As described above, in the resistance spot welding method of the present invention, the maximum current is supplied in the initial stage before the nugget formation is started, the current value at the nugget formation start time is decreased, and then the current value is increased after the middle period. However, by carrying out such control, it becomes easy to obtain a large-sized nugget without causing scattering relatively.

[Brief description of drawings]

FIG. 1 is a graph schematically showing the relationship between energization time, nugget diameter, and contact diameter, FIG. 2 is a schematic diagram showing a schematic configuration of a welding apparatus for carrying out the method of the present invention, and FIG. FIG. 3 (b) is a graph showing the waveform, FIG. 3 (b) is a graph showing the relationship between the effective current and the nugget diameter, FIG. 4 is a graph showing the step response waveform of the LRC circuit, and FIG. 5 (a) (b) (c). ) Is a graph showing the relationship between the welding time, the nugget diameter, and the corona bond diameter in correspondence with the welding current and the welding voltage waveform, and FIG. 6 is a graph showing the relationship between each current waveform and the welding condition margin. 7 (a), (b) and (c) are graphs showing the relationship between welding time and nugget diameter, corona bond diameter in correspondence with welding current and welding voltage waveforms, and FIGS. 8 (a) and (b). (C) and 9th
Figures (a) and (b) are graphs showing the relationship between welding time, nugget diameter, and corona bond diameter in correspondence with welding current and welding voltage waveforms, and Figs. 10 (a) and (b) are current waveforms and welding. It is a graph which shows a relationship with conditional margin correspondingly.

Claims (1)

[Claims] 1. A resistance spot welding method characterized in that a maximum current is supplied in the initial stage before the start of nugget formation, the current value is lowered at the start of nugget formation, and thereafter the current value is gradually increased.