JPS6257431B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an apparatus for performing non-consumable electrode arc welding using an AC power source.

Non-consumable electrode arc welding of aluminum, magnesium and their alloys (for simplicity)
TIG welding (TIG welding) is generally performed using an AC power source because it is necessary to perform welding while removing the strong oxide film on the surface by the cleaning action of a reverse polarity welding current. By the way, when TIG welding is performed using an AC welding power source powered by a general commercial frequency, an abnormal consumption phenomenon occurs in which the tip of the tungsten electrode, which is a non-consumable electrode, is partially destroyed at the start of welding. Small pieces of tungsten get mixed into the welded metal parts. This not only shortens the life of the tungsten electrode, but also deteriorates the mechanical properties of the weld. The reason for this is that when the temperature of the tungsten electrode is still low at the start of welding and it becomes a cathode, electron emission is mainly performed by field emission. Therefore, the electron emission ability is low,
Electrons are locally emitted from certain points of the oxide on the surface of the tungsten electrode. As a result, the current density of the electron emitting part becomes abnormally high and it heats up rapidly, resulting in partial melting, and due to minute explosions and concentration of pinch force, part of the tungsten electrode becomes fine particles and scatters, causing the molten metal to melt. It is thought that it will get mixed into the pond.
This tendency rapidly increases as the positive polarity current increases, with the tungsten electrode serving as the cathode. On the other hand, when the tungsten electrode becomes an anode and the polarity is reversed, unlike when the polarity is positive, current concentration does not occur, so the electrode is only heated evenly and the tungsten is not partially destroyed.

By the way, in consumable electrode arc welding, in order to obtain stability at the start of welding, some methods are used to start welding with a small current and then gradually or stepwise increase the current, but this method is called TIG welding. Even if applied, sufficient effects cannot be obtained. i.e. T.I.G.
In welding as well, welding
Even if you simply reduce the current value to prevent burn-through at the welding starting point using a reverse symmetrical AC power supply, the positive polarity current at this time will be low enough to eliminate the wear of the tungsten electrode at the start of welding. I can't. Moreover, the reverse polarity current, which effectively heats the tungsten electrode without being consumed, also decreases by the same amount as the positive polarity current, so the heating of the tungsten electrode is delayed by this amount, and the electrode is not sufficiently heated for thermionic emission. The time required to heat up the tungsten electrode becomes longer, and the overall amount of consumption of the tungsten electrode during startup is not improved. Furthermore, by reducing the welding current to a current value that does not cause any wear on the tungsten electrode, it is possible to prevent electrode wear and tungsten from entering the molten metal pool, but with such a small current, both the electrode and the workpiece There is no heating and therefore no good weld start-up. Furthermore, there is a method in which only reverse polarity current is used at the start of welding, that is, reverse polarity DC current is switched to positive polarity after the arc starts, but this is related to DC arc welding, and furthermore, in such a method, the electrode Although the welded object is heated, it is hardly heated, and by the time the polarity is switched to positive, the current is already high for main welding.Thermal shock to the welded object cannot be prevented, resulting in burn-through and poor shape of the weld metal at the starting edge. is not improved in any way.

To solve the above problem, the present invention reduces the proportion of positive polarity current at the start of welding in AC TIG arc welding, and increases the proportion of positive polarity current as time passes, or increases the total welding current as the proportion of positive polarity current increases. A good welding start-up is achieved by increasing the value continuously or stepwise. Furthermore, in addition to the time at the start of welding, the ratio of the positive polarity current at the end of welding, contrary to the time at the start of welding, decreases the total welding current value continuously or in stages together with the ratio of the positive polarity current. Therefore, we have proposed an apparatus that skillfully utilizes the difference in penetration shape between positive and reverse polarity currents and the state of the welding arc to effectively treat craters that occur at the weld end.

According to experiments conducted by the present inventors, there is a correlation between the consumption of the tungsten electrode at the start of welding and its contamination into the molten metal pool with respect to the total welding current value and the ratio of the positive polarity current value to the total welding current value. I understand. Figure 1 shows the results of an X-ray examination of the presence or absence of contamination of the tungsten electrode into the deposited metal at the welding starting end, with the vertical axis representing the total welding current Iw and the horizontal axis representing the ratio β of the positive polarity current value. It is. The other conditions used for the measurements shown in the figure are as follows.

Electrode...Pure tungsten 4.8mmφ Shielding gas...Pure argon 15/min Workpiece...Aluminum 5083 6mmt In the figure, the area below the boundary line in A is the area without any tungsten mixed in, and B is the area where no tungsten is mixed. This is within the recognized range of contamination. From the same figure, the smaller the ratio β of the positive polarity current and the smaller the welding current 1W, the lower the possibility of tungsten being mixed in. This means that the consumption of tungsten at the start of welding and the mixing into the weld metal are more likely to occur as described above. The ratio β of positive polarity current and the magnitude of the total current value are as follows:
It was confirmed that this was caused by Iw.
Furthermore, when welding under the same conditions as shown in Figure 1, we observed using high-speed photography the period during which tungsten electrodes were mixed in at the start of welding, and found that tungsten scattering and mixing occurred within 0.2 seconds to 2 seconds from the start of welding. It was found that this phenomenon occurs only within a very short period of time, about seconds, and is rarely observed thereafter. Of course, this time will vary depending on the diameter of the tungsten electrode used and the welding current, but once these other factors are determined, the period during which tungsten contamination occurs will also be determined, and after this period the electrode will become red hot and thermionic emission will be sufficient. It is thought that local damage to tungsten will no longer occur. Therefore, if the ratio β of the positive polarity current to the welding current is small for a certain period of time from the start of welding, then
Even if the total welding current is relatively large, tungsten electrode scattering does not occur, and by making the welding current relatively large, the time to heat the tungsten is shortened, and the time to reduce the ratio β of the positive polarity current is also shortened. I'll try it. It is desirable that this ratio β be 50% or less, and it may be switched to the normal ratio after a certain period of time (for example, about 2 to 3 seconds) after starting welding, but the temperature of the electrode will rise continuously. In addition, it is ideal if the ratio β is increased continuously in order to gradually heat the workpiece to be welded.
Furthermore, it is convenient if the total welding current is gradually increased from a small value, since the thermal shock to the tungsten electrode and the workpiece to be welded will be gentle. At this time, the increase in the ratio β of the positive polarity current does not necessarily have to match the increase rate of the total welding current, and if they are made independent, it is possible to set the optimal change rate for both. Become.

On the other hand, at the end of welding, it is necessary to carry out so-called crater treatment to fill the crater dug down by the arc force during welding, but in the past, the current or current and voltage were simply switched to small values. Therefore, the arc force was lowered, and the molten metal that had been dug down by the strong arc force during welding was returned to the crater by surface tension and gravity to fill it. However, when the current or current and voltage are simply reduced in this way, the arc force is certainly small and it is possible to treat some cratering, but in order to maintain the arc stably, the temperature of the tungsten electrode must be raised to a level where thermionic emission is sufficient. It is necessary to maintain the current at or above a certain value for the welding process, and for this reason, the current cannot be too low, but rather a relatively large current value (for example, about 70% of the welding time). Craters corresponding to this relatively large current still remained, and complete crater treatment was not achieved. As shown in FIG. 2, there is a wide difference in the shape of penetration between the positive and reverse polarities of the welding current. In other words, when welding is performed with a positive polarity current of 100% (DC), the fusion width becomes relatively narrow and the penetration is deep, as shown in Figure a, whereas when welding is performed with a positive polarity current of 100% (DC), the welding width is relatively narrow and the penetration is deep.
% (DC) welding, the fusion width is wide and the depth of penetration is extremely shallow, as shown in Figure c.
When welding is performed using an alternating current that includes both forward and reverse polarities, the shape will be intermediate between the two, as shown in Figure b, depending on the ratio of the forward and reverse currents.
Figure 3 shows the results confirmed by the inventors through experiments, in which the fusion width _W
and changes in penetration depth P are measured. As is clear from the figure, it can be seen that the penetration depth P changes widely as the ratio β occupied by the positive polarity current changes. In addition to the above-mentioned processing of the welding start end, the present invention takes advantage of the phenomenon caused by the difference in polarity to gradually or stepwise reduce the ratio β of the positive polarity current at the end of welding to change the penetration shape to a second shape. The purpose is to effectively perform crater treatment by bringing the state from the state shown in Figures a or b to the state shown in c. By doing this, the arc force that is excessively concentrated during the main welding current, in which the proportion of the positive polarity current is normally relatively large, is dispersed over a wide range, and the penetration is shallower to prevent craters. It is possible to fill it. In addition, by doing this, when adding filler metal (filler wire) to make up for the molten metal that is insufficient during crater treatment, only the filler metal can be effectively melted without digging deeply into the workpiece. The crater can be filled by Furthermore, in addition to lowering the ratio of positive polarity current, more complete crater treatment can be achieved by simultaneously lowering the value of all welding currents, as is done in general crater treatment methods. Especially when doing this, even if the positive polarity current is sufficiently small, if the reverse polarity current is maintained at a certain value, the temperature of the tungsten electrode can be maintained above the required value, so the arc can be maintained stably, and therefore the melting The positive polarity current can be reduced to zero so that the depth is minimized, allowing complete crater treatment.

FIG. 4 is a waveform diagram showing changes in welding current in the welding apparatus of the present invention. In the same figure, a
and b show how the welding current changes over time at the start of welding in the device of the present invention, and a shows the change in the welding current over time when the reverse polarity current is constant and the positive polarity current is changed stepwise. An example is shown in which the proportion occupied by the positive polarity current is increased stepwise, and Figure b shows an example where the proportion occupied by the positive polarity current is continuously increased by continuously changing the positive polarity current while keeping the reverse polarity current constant. Here is an example. Figure c shows how the welding current changes at the start and end of welding, in which both the total welding current value and the ratio occupied by the positive polarity current are changed in stages. In Figure d, the change in the reverse polarity current is gradual, and the change in the positive current, which has a large influence, is continuous. In addition, in the figure e, the positive polarity current and the reverse polarity current have the same peak value and their conduction periods are changed.

A method that changes only the ratio of forward and reverse currents, as shown in a and b in the same figure, is suitable when the welding current is relatively small, and in this case, the reverse polarity current I _RP is determined by the temperature of the tungsten electrode and the required cleaning width. If it is set to the minimum value that can be maintained, the positive polarity current I
Since the penetration shape is almost determined by _SP , it is convenient for setting welding conditions. Figures c and d are suitable for welding using a relatively large current, and can prevent thermal shock at the start of welding and completely remove craters. In particular, in Figure d, the reverse polarity current I RP is changed to increase before the positive polarity current I _SP increases at the start of welding, and the positive polarity current I _RP increases before the reverse polarity current I _RP decreases at the end of welding. Since _{the SP} is changed to decrease, the welding arc can be made more stable. In addition, in Figure e, instead of changing the peak value, the ratio of the forward and reverse periods is changed, allowing fine control in each period, reducing the pulsation of the arc force, stabilizing the arc, and improving the weld metal surface. This prevents waving.

FIG. 5 is a connection diagram showing an embodiment of the device of the present invention. In the figure, 1 is an AC power source, 2 is a switch that closes at the start of welding and opens at the end of welding, 3 is a welding transformer, 4 is a tungsten electrode, and 5 is an object to be welded. Reference numerals 6 and 7 indicate current limiting elements for limiting positive and reverse polarity currents, such as power transistors connected in parallel. 8 is an output current detector; 9 and 10 are diodes for determining positive polarity current and reverse polarity current detection signals; 11;
and 12 are amplifiers, and 13 and 14 are comparators. Variable resistor 15 and power supply 16 are output current setters for setting reverse polarity current, and variable resistors 18, 19 and power supply 17 are another output current setter for setting positive polarity current. Furthermore, the relay contacts 20a and 20b are contacts for switching the positive polarity current setting signal, and the normally open contact 20a is closed from a certain time after the start of welding and is opened at a certain time before the end of welding, and the normally closed contact 20b is It performs the opposite operation to the normally open contact 20a. Adjust the setting of variable resistor 18 to variable resistors 15 and 19.
The setting of the variable resistor 19 and the setting of the variable resistor 15 are appropriately selected depending on the penetration shape required during welding. When the switch 2 is closed at the start of welding, the current limiting element 6 becomes conductive in response to the low value set by the variable resistor 18, and a small positive current flows. Current limiting element 7
conducts at a value set by the variable resistor 15, and a predetermined reverse polarity current flows. At this time, as described above, since the variable resistor 18 is set to a lower value than the variable resistor 15, the positive polarity current has a smaller value than the reverse polarity current. At a certain time after the start of welding, the contact 20a closes and the contact 20b opens, so that the positive polarity current is switched to the regular welding current value set by the variable resistor 19. At the end of welding, the contact 20a opens and the contact 20b closes prior to the opening of the switch 2, so that the positive polarity current becomes the small value set by the variable resistor 18 again and the crater treatment is performed. Contact point 20
After a predetermined time after switching between a and 20b, switch 2
Finish welding by opening. As shown by the dotted line in FIG. 5, if a capacitor C is inserted into the output terminals of the variable resistors 18 and 19 for setting the positive polarity current, and the power source 17 is made into a DC power source, the relay contact 20a, Comparator 1 when switching 20b
The reference input No. 3 does not change stepwise, but can be made to change continuously at a rising or falling speed along the charging/discharging time constant of the capacitor C.

In Fig. 5, two sets of current limiting elements are provided so as to adjust both the positive and reverse polarity output currents, but it is also possible to fix the conduction amount of one and adjust only the other, or the output current detector 8,
9, 10, amplifiers 11, 12, comparators 13, 14
The object of the present invention can also be achieved by omitting the structure without feedback control. In this case, the welding transformer 3 may have constant current characteristics or drooping characteristics. For example, a known AC arc welding machine for covered arc welding using a movable core may be used, and feedback control as shown in Fig. 5 may be used. Even when welding, if the welding transformer 3 is a tap-switching type variable output transformer, coarse output adjustment can be performed by tap-switching, and the power consumption of the current limiting element is reduced, so the capacity can be reduced.

FIG. 6 is a connection diagram showing an embodiment of the device for obtaining the current change shown in FIG. 4e, and parts having the same functions as those in FIG. 5 are given the same reference numerals. Thyristors 21a and 21b are connected in series with reactor windings 23a and 23b, respectively, and then connected in antiparallel to each other to form a current limiting element. In addition, the reactor windings 23a and 23b are wound around a common iron core, and their polarity is determined by the thyristor 2 connected in series as shown in the figure.
The polarity is such that magnetic flux in the same direction is generated by conduction between 1a and 21b. Also, reactors 23a, 2
It is assumed that 3b has a sufficiently large inductance.

With this configuration, an approximately rectangular wave-like alternating current output current can be obtained between the electrode 4 and the workpiece 5 by the action of the reactor. That is, the thyristor 22a
The positive current that flows due to the conduction of the reactor 23
Welding transformer 3 for large inductance of a
Even during a period in which the output voltage changes sinusoidally and becomes higher than the welding voltage appearing between the electrode 4 and the object to be welded 5, the value remains approximately constant, and energy is accumulated in the reactor 23a during this period. Next, during a period when the output voltage of the welding transformer 3 decreases and becomes lower than the welding voltage, the accumulated energy is released to prevent the current from decreasing and maintain a substantially constant current. Next, when the thyristor 21b becomes conductive, the magnetic flux generated in the reactor 23b due to the conduction of the thyristor 21b is in the same direction as the magnetic flux that was generated when the thyristor 21a was previously conductive. The impedance of the reactor 23a is approximately equal to the impedance of the reactor 23a, and the output current only has its polarity reversed, and a current having approximately the same value as before flows. In this way, in the device shown in FIG.
A rectangular waveform output current whose polarity is instantaneously reversed according to the conduction phase of b is obtained. Thyristor 21
The conduction phase of a and 21b is determined by the phase control circuit 2.
2a and 22b. Variable resistor 2
6, 27, the power supply 28 is an output current setting device for setting the peak value of the output current, and is variable by relay contacts 29 and 30, which switch at a predetermined time after the start of welding and return at a predetermined time before the end of welding. The output voltage of either resistor 26 or 27 is selected. 31 is a variable resistor serving as a positive current ratio setting device, and has two sliders 32a and 32b, and relay contact 3
Either one is selected by 3 and 34. These relay contacts 33 and 34 switch at a predetermined time after welding has started and return to a predetermined time before welding ends, but this may be the same as the operating time of the relay contacts 29 and 30. However, a time lag may be provided between them as necessary.

In FIG. 6, the output of the welding current detector 8 is compared with the output of the variable resistor 26 or 27 by the comparator 24, and the difference output is determined by the ratio set by the variable resistor 31 to phase the thyristors 21a and 21b. The current is dividedly supplied to the control circuits 22a and 22b and controlled so that the output current is equal to the set value. Since the relay contacts 29 and 33 are closed for a certain period of time after the start of welding and before the end of welding, the welding current has a magnitude determined by the variable resistor 26, and the proportion of the positive polarity current is The value is determined by the slider 32a of the variable resistor 31, while during welding except for the certain period of time, the variable resistor 27 is replaced by the variable resistor 26, and the slider 32a is
Instead, 32b becomes effective and a predetermined welding current and ratio are obtained. At this time, the variable resistor 26 is set to a lower value than the variable resistor 27, and the slider 3
If the output from slider 2a is set to be a lower value than the output from slider 32b, a change in welding current as shown in FIG. 4e can be obtained.

In FIG. 6, the variable resistor 31 has only one slider, and if it is moved by an electric motor or the like in the direction in which the positive polarity current ratio increases or decreases at the start and end of welding, a continuous change can be obtained. Also in FIG. 6, it is possible to omit the output current detector 8 and the comparator 24 to omit feedback control, and it is also possible to provide a tap on the welding transformer 3 to roughly adjust the output current. .

As described above, according to the present invention, it is possible to minimize the adverse effects of the positive polarity current when the temperature of the tungsten electrode is low at the start of welding. This eliminates contamination of the welded metal parts and prevents deterioration of the mechanical properties of the welded metal. Furthermore, since the tungsten electrode can be quickly heated with a reverse polarity current, the period during which the welding arc is unstable at the start of welding is shortened, and the risk of welding defects is accordingly reduced. Furthermore, even at the end of welding, by reducing the decrease in reverse polarity current and actively reducing the positive polarity current, it is possible to completely perform crater treatment while maintaining the temperature of the tungsten electrode sufficiently high, and even with a small current. It is also possible to maintain a stable arc.

[Brief explanation of the drawing]

Fig. 1 is a graph examining the range in which tungsten electrode contamination occurs when the ratio β of the positive polarity current is changed, Fig. 2 is a graph showing changes in the penetration shape due to differences in welding current polarity, Fig. 3 The figure is a graph showing the relationship between the fusion width W _B and the penetration depth P with respect to the ratio β occupied by the positive polarity current. The figure is a connection diagram showing an embodiment of the present invention. 1...Power supply, 3...Welding transformer, 4...Electrode,
5... Work to be welded, 6, 7... Current limiting element, 8...
... Output current detector, 13, 14, 24 ... Comparator, 15 ... Variable resistor for setting reverse polarity current, 1
8, 19...Positive current setting variable resistor, 2
6, 27... Variable resistor for welding current setting, 20
a, 20b, 29, 30, 33, 34...Switching relay contact, 31...Positive polarity current ratio setting variable resistor, _IW ...Total welding current, β...Ratio occupied by positive polarity current, I _SP ...Positive polarity current, _IRP ...Reverse polarity current.

Claims (1)

[Scope of Claims] 1. A non-consumable electrode AC arc welding device that obtains power from an AC power source and ignites an arc between a non-consumable electrode and a workpiece, including a welding transformer and a welding transformer. a pair of adjustable current limiting elements for respectively limiting a positive polarity component and a reverse polarity component of the output current; a pair of output current setting devices for respectively setting the positive polarity and reverse polarity output currents; At the time of starting welding, the ratio of the output of the output current setting device of the pair of output current setting devices that sets the positive polarity current to the output of the reverse polarity output current setting device,
a first switching circuit for increasing the ratio continuously or stepwise to a normal value for welding with the passage of time; the second for decreasing continuously or stepwise to smaller values;
A non-consumable electrode AC arc welding device equipped with a switching circuit. 2. In a non-consumable electrode AC arc welding device that uses electric power from an AC power source to strike an arc between a non-consumable electrode and a workpiece, a welding transformer and a positive polarity component of the output current of the welding transformer are used. and a pair of adjustable current limiting elements for respectively limiting the reverse polarity components, and a pair of output current setters for respectively setting positive and reverse polarity output currents; At the time of welding, at least one of the outputs of the setting device and the ratio of the output of the output current setting device on the side that sets the positive polarity current of the pair of output current setting devices to the output of the reverse polarity output current setting device are determined at the time of welding at the start of welding. first and second switching circuits for increasing each of the outputs continuously or stepwise to a normal value for welding as time passes; and the pair of output current settings. third and fourth switching circuits for continuously or stepwise reducing at least one of the outputs of the device and the ratio during crater treatment to a smaller value than during welding, and detecting output currents of positive polarity and reverse polarity. a pair of output current detectors for comparing the output of each of the output current detectors with the output of each of the output setting devices to obtain a difference output; A non-consumable electrode AC arc welding device comprising a pair of conduction amount control circuits that determine the conduction amount of each of the current limiting elements. 3. In a non-consumable electrode AC arc welding device that uses electric power from an AC power source to strike an arc between a non-consumable electrode and a workpiece, a welding transformer and a positive polarity component of the output current of the welding transformer are used. and a pair of adjustable current limiting elements for respectively limiting the reverse polarity component, an output current setting device, and a positive polarity current ratio setting device for determining the proportion of the positive polarity current in the output current. , a first switching circuit for making the output of the positive current ratio setting device smaller at the start of welding than during welding and increasing it continuously or stepwise as time passes to a normal value for welding; A non-consumable electrode AC arc welding device comprising: a second switching circuit for continuously or stepwise reducing the output of the positive current ratio setting device during crater treatment to a smaller value than during welding. 4. In a non-consumable electrode AC arc welding device that uses electric power from an AC power source to strike an arc between a non-consumable electrode and a workpiece, a welding transformer and a positive polarity component of the output current of the welding transformer are used. and a pair of adjustable current limiting elements for respectively limiting the reverse polarity component, an output current setting device, and a positive polarity current ratio setting device for determining the proportion of the positive polarity current in the output current. , the output of the positive polarity current ratio setting device and the output of the output current setting device are made smaller at the start of welding than during welding, and are increased continuously or stepwise to the normal value for welding as time passes; first and second switching circuits, and a third switching circuit that continuously or stepwise switches the output of the positive polarity current ratio setter and the output of the output current setter during crater treatment to a value smaller than that during welding. a fourth switching circuit, an output current detector, a comparator that compares the output of the output current detector and the output of the output setter and supplies a difference output to the positive current ratio setter, and the positive polarity and a conduction amount control circuit that determines the conduction amount of the current limiting element based on the output of the current ratio setting device.