JPS6044075B2 - Flash butt welding voltage control method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a flash pad welding voltage that generates minute flashes at as low a voltage as possible during the flash process of flash pad welding, especially in the latter stages, thereby obtaining good weld quality. Concerning control methods.

In flash pad welding, a voltage is applied between the objects to be welded, the objects to be welded are brought into contact with each other, and a large current flows between the objects to repeatedly cause welding spatter (flash effect). The flash process involves heating and melting the welded parts (referred to as end faces), and after heating and melting, strong pressure is applied to the objects to be welded.
It consists of an upset process of press-welding.

In order to obtain the best weld quality by flash pad welding, a fine flash is generated over the entire end face during the flashing process, especially in the latter stage, and this causes uniform heating and melting over the entire end face. Therefore, it is necessary to control the welding voltage to the lowest possible voltage. That is, at the beginning of the flashing process, the end face is not heated, so flashing is difficult to occur, and therefore a large voltage is required.

The large voltage causes a large flash, which causes a deep crater on the end face. As the cal-flash process progresses, the end face heats up, so flash can easily occur even at a lower voltage than at the initial stage. Therefore, in the latter half of the conventional flash process, the voltage is lowered, thereby generating a fine flash, and after correcting and smoothing the deep craters that occurred in the first half of the flash process and making the end face temperature uniform, the upset process is started. It is being migrated. However, the degree to which the voltage is lowered and the timing thereof vary in a complex manner depending on various conditions such as the cross-sectional area of the workpiece, the impedance of the welding machine, and the platen displacement speed, so it is difficult to perform welding under optimal conditions. It was extremely difficult. Therefore, in conventional flash pad welding, the platen displacement speed is changed at a preset time or when the peak value of the flash current falls below a certain value.

However, since this method requires stepwise adjustment, it is not possible to constantly perform welding under optimal conditions in response to the ever-changing end face conditions as the flashing process progresses. Further, in order to generate a fine flash in the flashing process, a low voltage is desirable as described above, but if the voltage is too low, no flash will be generated, the end faces will be short-circuited, and free zinc will occur. Conventionally, the method of releasing this freezing has been to stop or move the platen back, but with this method, the welded end faces may not separate and the freezing cannot be removed.
For this reason, welding had to be performed at a voltage higher than the lower limit voltage for safety reasons. The present invention uses the lowest possible voltage through the flash process of flash pad welding,
This is a method of generating a minute flash, and as the flashing process progresses, the voltage on the primary side of the welding machine transformer is reduced until freezing occurs, based on the fact that the limit for voltage reduction in the flashing process is the point at which freezing occurs. The welding voltage is continuously lowered in stages, and when freezing occurs, the voltage is increased to release the freezing, thereby maintaining the welding voltage at the lower limit throughout the flash process.

The following will explain the invention based on embodiments shown in the detailed description of the invention.

In FIG. 1, 1 is a DC voltage command device which outputs a voltage command signal for lowering or increasing the welding voltage.
2 is an AC to DC converter, which is composed of, for example, a static device in order to smoothly increase or decrease the welding voltage;
In order to obtain a voltage according to the signal from the DC voltage command device 1, AC (50 or 60 Hz commercial power supply) is rectified and converted into DC by, for example, controlling the firing phase of a controlled rectifier.

Reference numeral 3 is a DC-to-AC converter that converts the DC obtained by the DC-to-AC converter into an AC of a frequency suitable for flash pad welding in order to easily obtain suitable current welding voltage and welding current depending on the voltage. Convert.

Note that since the AC waveform requires a long flash time in one cycle, the circuit configuration is such that it can generate not only a normal sine wave but also a square wave. 4 is a welding machine transformer which reduces the output voltage from the DC-to-AC converter to the welding voltage.

A welding current detection circuit 5 detects a welding current during welding.

A temperature detector 6 detects three temperatures at the end face of the workpiece.

7 is a freezing determination circuit, and the current detector 5
The freezing characteristic value 4 (is calculated based on the flash current value, the flash current peak value, the number of occurrences per unit time, the end surface temperature of the welded part, etc.) using the output from the temperature detector 6 as an input signal, and The value signal is compared with the set value, the presence or absence of freezing is determined through the flushing process, and a signal for correcting the command voltage is output to the DC voltage command device 1.

8 and 8'' are electrodes, and 9 and 9'' are objects to be welded.

Welding voltage control by this device is performed as follows.

That is, a predetermined voltage command signal necessary for generating an initial flash is input from the DC voltage command device 1 to the AC-to-DC converter 2, and the current from the AC power source is converted to DC and applied to the DC-to-AC converter 3. The device converts direct current to alternating current, enters the welding machine transformer 4, reduces the voltage to a predetermined welding voltage in the welding machine transformer 4, and applies the voltage to the workpieces 9, 9 through the electrodes 8, 8''. '' to start welding. Then, by the voltage command signal from the DC voltage command device 1, the welding machine transformer primary voltage (t) is changed to, for example, the following (
1) It is continuously reduced according to the formula. t: The time when welding starts is t=o, and the elapsed time A, b
, c, n; Secondary impedance of the welding machine,
Constants determined by the platen moving speed, the cross-sectional area of the workpiece, etc., and the detection signals from the temperature detector 6 and current detector 5 are input to the freezing determination circuit 7, and the welding state signal and freezing value signal based on the signals are input. are compared.

When freezing does not occur, the freezing determination circuit 7 does not output a specific signal. On the other hand, when freezing occurs, the welding state signal becomes equal to or less than the set value, and the freezing determination circuit 7 obtains, for example, a signal of voltage v (Tk + ΔTk) according to the following equation (2),
This signal is output to the DC voltage command device 1, and the command voltage is transmitted from the welding machine transformer 4 to the electrodes 8, 8, the objects to be welded 9, 9'' via the AC-DC converter 2 and the DC-AC converter 3.
The welding voltage is increased, and freezing is thereby released. v(Tk); Welding machine primary Ii'1゛K ; Welding machine secondary impedance, platen displacement speed, welded object
The constant determined by the cross-sectional area of the object, etc. Figure 2 is the voltage (t)
It shows the time change of , and the point at which freezing occurs, ■ (Tk)
, (Tk') and (Tk), KV (Tk'') represent the primary voltage of the welding geometry at that time and the same voltage increased by equation (2).

Freezing detection involves inputting the welding current to a threshold circuit to identify whether it is a flash current or a freezing current.If the welding current is determined to be a freezing current, the duration of the current is measured using a time measurement circuit, and based on the result, the freezing current is detected. It is determined whether the state is the same or not. That is, the values of the flash current and the freezing current are determined by the steel type, size, temperature, secondary impedance, working voltage, etc. of the welding material, and for example, the latter is three times or more of the former. Therefore, if a suitable voltage corresponding to the flash current value or the like is set as the threshold value of the threshold circuit, freezing current can be reliably identified. Furthermore, in reality, even if freezing occurs, it may disappear naturally within a certain period of time, for example, 0.6 to 1.0 seconds. Therefore, it is preferable to make the freezing determination after the above-mentioned time period during which natural release is impossible. The time measurement described above is for this purpose. Table 1 shows an example according to the present invention, in which a common steel billet with a cross-sectional area of 6400Tn1t is welded from cold pieces, a welding method to which the present invention is applied and a conventional method to which the present invention is not applied. Show the results. Welding conditions Welding machine secondary impedance Ni 150 μΩ Platen displacement speed = 2.12 cutting 0-7t 4.14 (Wn)
*Reduced voltage on the primary side of the welding machine During freezing voltage rise time ΔT, = 0.1 (Sec) K = 1.3 As shown in Table 1, according to the present invention, the voltage just before the peak set is 67% compared to the conventional method. % lower, thus resulting in high quality welds and further reducing the welding time by 23%.

Therefore, the method of the present invention is very effective for welding billets, plates, vibs, etc. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing an example of the apparatus of the present invention, and FIG. 2 is a graph showing an example of changes in welding voltage.

1: DC voltage command device, 2: AC-DC converter, 3:
DC-AC converter, 4: welding machine transformer, 5: welding current detection circuit, 6: temperature detector, 7: freezing determination circuit, 8, 8'': electrode, 9, 9'': object to be welded.

Claims (1)

[Claims] 1. In flash butt welding, as the flash process progresses, the primary side voltage of the welding machine transformer is continuously lowered until free zinc occurs, and when free zinc is detected, the voltage is increased. Butt voltage control method.