JPS5847581A - Spot welding method and controller for steel plates having insulating film - Google Patents

Abstract

PURPOSE:To eliminate the expulsion and surface flash by melting at the preceding ends of electrode tips and the surfaces of the work and to improve welding quality and workability by breaking down insulating films with the welding current of low voltage then sloping up the voltage gradually and connecting the same to control of constant current. CONSTITUTION:When a controller 1 for conduction of electricity commands the start of conduction of electricity, a controller 2 for switching the conduction of electricity is set in the voltage mode for the initial conduction of electricity and transmits the signal from a controller 3 for sloping up of voltage to a controller 4 for firing phases, so that the output of a welding transformer 6 is sloped up gradually from an extremely low voltage via a thyristor contactor 5. Then, the insulating films on the surface of the work sandwiched by electrode tips 7 are broken, melted and removed gradually, by which a main passage for conduction of electricity is formed. Then, the mode is changed over to a constant current mode by the signal of the controller 2 and the feedback control is effected in such a way that the welding current is maintained constant with a controller 9 for constant current by the signal of the current value detected by the detector 8.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is applicable to surface-treated steel sheets having a thin insulating film, particularly those coated with zinc-rich paint, which have recently been frequently used for automobile bodies.

Generally, this kind of surface-treated steel sheet is spot welded.
・In addition to controlling the shape of the tip of the electrode tip, it is also necessary to ensure that the welding current is sufficiently stabilized against fluctuations in power supply voltage and load.

However, in the case of surface-treated steel sheets having a dynamic coating, there are problems in applying this constant current control system.

In other words, even if an attempt was made to gradually increase the welding current by performing so-called current slope-up control, the very early current control system, which conducts current through the insulating cover, was a welding transformer. Attempts are made to increase the output voltage and force current to flow through the encapsulation.

If the insulating film is destroyed in this state, a large current will suddenly flow, and together with the high contact resistance, the workpiece will be locally overheated, resulting in a momentary explosion. The tip of the electrode tip and the surface of the workpiece are melted, and the contact quality is /!几t4, it will also cause a big hindrance to welding work.

The present invention provides a solution to these problems.Firstly, at the start of welding current, constant current control is not performed, and the output voltage of the welding transformer is kept at a very small value of a few percent or less of the main welding current when flowing. By gradually increasing the voltage from that value, the breakdown current of the insulating film on the surface of the first node is suppressed to an extremely small value, and then it is gradually heated and melted, so that the resistance between the electrode tips becomes sufficiently low. The purpose is to stabilize and distribute the large welding flow required for nugget formation only after the welding process.

In addition, the present invention uses a voltage control mode in which the welding current is energized in a voltage control mode in which the slope is controlled by 1゛ in the initial stage, and in a constant current control mode in which the current value is kept constant in the latter stage.
Although it is proposed to control the t-contact current, the following method can be considered for the switching timing between the initial energization and the latter energization. -“(1) !!Method of switching based on the initial energization time obtained empirically.

(2) A method of switching by detecting that the initial current has increased until it matches the welding current setting value in the latter period.

(3) A method of switching by detecting that the voltage between the electrode tips gradually decreases and reaches a predetermined value or a steady state.

(4) The resistance between the electrode tips gradually decreases, and
□ is a method of switching when the steady state state has been reached.

It should be noted that the present invention is naturally effective in both of these methods, and that the present invention is not limited by these switching methods.

Figure 2 shows an electrical block diagram in one embodiment of the present invention.
FIG. 2 shows a waveform diagram of welding current.

Fig. 1 shows the case where a thyristor contactor is connected to a single-phase AC spot welding machine, and the welding current is detected on the primary side of the welding transformer. Even if the current is detected from the spot or from the secondary side of the welding transformer, the effectiveness of the present invention is not overridden.

In Figure 5Is1, the energization switch (2) first selects the initial energization voltage mode in response to the energization start command from the energization controller (1), and controls the ignition phase with the signal from the voltage slope up controller (3). The signal is transmitted to the container (4).

This signal causes the thyristor contactor (5) to start from a very late firing phase and gradually advance its firing phase, and as a result, the welding transformer (6)
The output voltage of increases gradually.

As a result, the thin insulating coating on the surface of the workpiece held between the electrode tips (7) is broken down by an extremely small current, and the coating is heated and melted by the gradually increasing current, and the pressure from the electrode tip causes the coating to be removed from the tip of the tip. It is pushed to the periphery to form the main current-carrying path.

When this initial energization is completed, the energization switch (2) is automatically switched to the constant current upper mode for later energization by a signal from the device according to the switching method described above.

In this constant current mode, the welding current is detected by the detector (8).
The constant current controller (9) controls the ignition phase controller (4) to keep the welding current constant against power supply voltage fluctuations, secondary circuit impedance fluctuations of the welding transformer (6), etc. This will adjust the firing phase.

This late energization itself can be handled in the same way as spot welding of general steel plates without an insulating coating.

FIG. 2 shows an example of the welding current waveform at this time.

In the figure, at the start of initial energization, a state in which current does not flow may occur in some cases.

The initial energization period in the figure is the voltage control mode of the slope-up control material, and the latter energization period is the so-called constant current control mode, as described above, but the current waveform at this time of switching is It goes without saying that it is preferable for welding to be as smooth as possible.

As described above, according to the present invention, when spot welding strips with insulating properties ima, at the initial stage of welding energization, the insulation ima, - is destroyed with a small current in the voltage up/lowe mode,
In the latter stage of welding, a stable 1 hV2 current necessary for welding nugget generation can be applied in constant current mode, which eliminates the melting of the tip of the electrode tip and the surface of the workpiece, which occurs conventionally, and improves the welding process. Quality and workability can be significantly improved.

[Brief explanation of the drawing]

FIG. 1 shows an embodiment of the present invention.Electrical block FIG. 2 is a graph showing welding current waveforms during initial energization and late energization. 1... Energization controller 2... Energization switching controller 3... Voltage slope up controller 4... Ignition phase controller 5... Thyristor contactor 6... Welding transformer ' 1...・Electrode tip 8・
...Detector 9...Constant current controller inventor Masahiro Kato

Claims (1)

[Claims] 1. During resistance spot welding of steel plates with an insulating coating, the welding transformer output voltage is gradually ramped up from a very low value to allow the welding current to flow and the insulating coating is destroyed and removed smoothly. However, in the latter stage of welding energization, constant current control is performed to maintain the welding current at a predetermined constant value, and the energization is performed to obtain a predetermined weld nugget. 2. An AC electronic switch installed on the input side of the welding transformer. In a resistance spot welding machine control device that controls the output voltage of a dissipative transformer by controlling the conduction start phase in each half cycle of It has an up control circuit, a constant current control circuit that feeds back the welding current and keeps it constant, and a switching circuit that switches the two control circuits at a predetermined time. A spot welding control device characterized by switching between a voltage slope-up control circuit and a constant current control circuit in the latter stage.