JPS62127168A - Method and device for ac square wave plasma arc welding - Google Patents

Abstract

PURPOSE:To execute exactly an arc re-ignition at the time of converting the polarity of one set of power source, and to improve the welding operability by executing the welding by using an AC square wave voltage which can adjust optionally a frequency and a time ratio of a positive electrode component and a negative electrode component, as a welding power source. CONSTITUTION:A square wave signal which has been obtained by an oscillating circuit 6 is set to a desired optional ratio by a straight polarity/opposite polarity time ratio adjusting circuit 5. This signal is amplified to a value which can drive an inverter, by a pre-amplifying circuit 4 for an inverter circuit 3, and applied to each transistor 11-14 in the circuit 3. In this state, when one pair 11, 14 or 12, 13 of them has been cut off, a current flows by a small charge which has remained in the transistors. A short circuit between each transistor at that time is prevented by a control circuit 7. Thereafter, the other pair of transistors are conducted electrically at the same time, cut off after a prescribed time, the electric conduction is repeated alternately without being short-circuited as mentioned above, and a DC which has been obtained from a transformer 1 having a high no-load voltage and a rectifying circuit 2 is outputted as an AC square wave. This square wave becomes a plasma arc flame between an electrode 8 in a nozzle 9 and a base material 10.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a plasma arc welding method and apparatus,
In particular, the present invention relates to an AC rectangular wave plasma arc welding method and an apparatus therefor, which is performed by converting DC to AC using an inverter.

[Conventional technology]

In general, in plasma arc welding, it is known that as a result of being significantly cooled by argon gas flowing within a narrow nozzle, the plasma arc causes a thermal pinch effect in which the surface area shrinks and narrows in order to cope with the increased heat dissipation. . Therefore, when the plasma arc contracts, the heat is highly concentrated in the base material and heated locally, making it possible to keep the heat-affected zone within a minimum range. In addition, by ejecting plasma gas at high speed, a hole called a keyhole can be created by penetrating the base material, so 100%
It is possible to obtain a sufficient amount of welding and a sufficient Uranami bead.

However, in the case of TIG welding, the arc shape is conical and the apex angle is about 45 degrees, and when the arc length is changed, the spread of the arc on the base metal surface is
The amount of heat input to the base metal per unit area changes significantly, causing melt drop and poor fusion. Therefore, in the case of TIG welding, the arc length must be shortened to allow heat concentration, and it must be controlled so that there is almost no fluctuation, and the welding workability is
It becomes extremely difficult. Therefore, plasma arc welding, which requires heat to be concentrated in a minute part of the weld, requires TI
It is known that it is possible to weld extremely thin plates, which is impossible with G welding. Furthermore, in plasma arc welding or TIG welding, the characteristics differ depending on the polarity connection method. Positive polarity connection, in which the base metal is the anode and the electrode is the cathode, provides deep penetration; Although the weld penetration is shallow in reverse polarity connection using an anode, it is known that it has a cleaning effect by removing the oxide film on the surface of the base material. In particular, when welding materials such as aluminum or aluminum alloys whose base metal surfaces are covered with an oxide film with a high melting temperature,
Welding is performed using alternating current because a cleaning effect is required. However, not only plasma welding but also AC arc welding using a normal commercial frequency is a sinusoidal alternating current as shown in FIG. 4, in which the positive and negative poles alternate at a rate of 50 to 60 times per second. Therefore, as shown in the figure, the no-load voltage 14 always passes through 0 volts, gradually increases, reaches the maximum value, gradually decreases, approaches O volts, and reaches 0 volts.

The voltage that has passed 0 volts repeats rising and falling on the negative electrode side as well. In this way, in the sine wave AC, the polarity change is slow near the boundary between the positive and negative electrodes where the voltage becomes O volts.

Therefore, no arc discharge occurs during the period 19 in FIG. 4 when the no-load voltage reaches the arc discharge starting voltage. 20-a and 2 in the same figure until the no-load voltage is reversed and the arc is re-ignited.
Between 0 and b, it is an unstable state that is maintained only by the self-holding property of the arc or the circuit accelerator. Therefore, in an AC arc, the direction of flow is reversed every half wave of current, and each time the arc must be extinguished and then re-ignited in the opposite direction, and if the re-ignition fails, it will be re-ignited. Ignition is no longer possible and the arc is extinguished. Once the arc is extinguished during welding, welding defects such as poor fusion or slag entrainment are induced, welding workability deteriorates, and it becomes extremely difficult to create a sound welded joint. The reason that AC arcs are more difficult to maintain than DC arcs is the difficulty of restriking. Therefore, in the conventional technology,
As shown in Publication No. 5-126384, in order to assist re-ignition, a spike-like high voltage is applied immediately after the voltage becomes 0 volts, or a high-voltage high-frequency voltage is applied in TIG welding, plasma arc welding, etc. It is known that by superimposing the arc, the arc is forced to restart and the arc is maintained stably. However, in the above method, in order to control the penetration depth, it is necessary to control the welding voltage, welding current, or welding speed. Furthermore, in the method using high-voltage, high-frequency waves, since the high-frequency waves are generated by spark discharge, there is a problem in that electromagnetic waves are radiated into the air and this electromagnetic waves can cause interference with other devices such as televisions, radios, computers, etc. There is. Also T.I.G.
In non-consumable electrode welding methods such as welding or plasma arc welding, the amount of heat generated by the electrode during reverse polarity is greater than the amount of heat generated during positive electrode because electrons generated from the base material collide with the electrode. Much larger. Therefore, with the same electrode diameter, the current range that can be passed is approximately 10 times lower in the case of DC positive polarity than in the case of DC reverse polarity.The same is true for AC welding, In welding using wave alternating current, the positive/reverse polarity ratio is l:l, which requires approximately 5 times the heat capacity of the electrode, so there are restrictions such as the 7r1 pole diameter and the size of the welding torch itself, so +8 contact Considering the workability, it becomes large and cannot be said to be preferable.

Therefore, the present inventors have already published Japanese Patent Application Laid-Open No. 59-921760
I tried plasma arc welding using the device according to the official publication, but
At the time of polarity change, smooth and reliable arc-to-arc ignition could not be performed and the plasma arc would disappear, making it difficult to create a sound welded joint.

In addition, in order to improve these problems of the conventional technology, welding journal in plasma arc welding.

FEBRUARY 19g4. (pages 25 to 35), by using two power supplies for positive polarity and reverse polarity, and by switching these alternately, the voltage can be instantly switched, and the above-mentioned re-ignition or It has been reported that alternating current plasma welding can be used to solve various problems such as heat capacity.

However, using the two power sources mentioned above increases the size of the welding device and complicates the operation of the device such as setting welding conditions, resulting in poor welding workability (it cannot be said to be an on-site welding device).

[Problem that the invention seeks to solve]

The present invention was made in view of these points, and uses a single high no-load voltage welding power source and square wave alternating current to reliably and smoothly re-ignite the arc at the time of polarity change. By making it possible to arbitrarily adjust the period and also arbitrarily adjusting the ratio of time between positive polarity and reverse polarity, it is possible to weld, for example, materials whose surfaces are covered with an oxide film with a high melting temperature. The object of the present invention is to provide a method and apparatus for producing a sound and simple welded joint.

[Means for solving problems]

The present invention relates to an AC rectangular wave plasma arc welding method characterized in that welding is performed using an AC rectangular wave voltage as a welding power source in which the frequency and time ratio of positive polarity component and negative polarity component can be arbitrarily adjusted. And transformers with high no-load voltage, converting alternating current to direct current! ! a current circuit, an inverter circuit consisting of two pairs of switching elements that converts the direct current into an alternating current rectangular wave, a preamplifier circuit for the inverter circuit to drive the inverter circuit, an output rectangular wave frequency and positive polarity component of the inverter, and a negative polarity. This welding device includes a control circuit for arbitrarily adjusting the time ratio of the components, and a control circuit for preventing phase-to-phase short circuit between two pairs of switching elements of the inverter.

The present invention will be explained in detail below with reference to the drawings.

[Effect]

FIG. 1 is a block diagram showing the configuration of a plasma arc welding apparatus that implements one embodiment of the present invention. This is the transformer 1, rectifier circuit 2, which has a high no-load voltage in the figure. Inverter circuit 3. It consists of an inverter circuit preamplifier circuit 4, a positive/reverse polarity time ratio adjustment circuit 5, an oscillation circuit 6, and an interphase short circuit prevention circuit 7.

The rectangular wave signal obtained by the oscillation circuit 6 is set to an arbitrary ratio desired by the welder by the positive/reverse polarity time ratio adjusting circuit 5. Next, in order to prevent phase-to-phase short circuits of the inverter transistors, the phase-to-phase short circuit prevention circuit 7 prevents the short-circuiting of the inverter transistors from causing damage due to the short circuit of the inverter transistors. Set to prevent.

Figure 3a shows that the positive polarity to negative polarity time ratio is 5 in the present invention.
Figure 3b is an output voltage waveform diagram representing a case where the positive polarity time ratio is much larger than the negative polarity time ratio, and in the figure, 15-a, 15-b, 1
6-a and 16-b indicate the interphase short circuit prevention time described above. The rectangular wave signal provided with the set time is amplified by the inverter preamplifier circuit 4 to a reference value that can sufficiently drive the inverter transistor.

FIG. 2 is a circuit diagram showing a schematic configuration of the inverter circuit 3.
The amplified signal is applied to each transistor 11, 12, 13, 14 shown in the figure in the inverter circuit 3.

When either inverter 11.14 or 12.13 of the inverter pair is cut off by the applied signal, due to the characteristics of the transistor, due to the charge accumulation effect of the transistor, the small amount remaining in the transistor is The electric charge causes a phenomenon in which current flows.

Then the other pair of transistors 12.13 or 11.
14 conducts, the mutual transistors do not pass through the load and are short-circuited. Therefore, the time for which the other pair of transistors becomes conductive is slightly delayed, and a time is established in which the transistors 11, 12, 13, 14 of any pair are not simultaneously conductive and short-circuited, that is, the time set by the phase-to-phase short circuit prevention circuit 7. It will be done. This time is longer than the charge residual time described above.

Also, it is set shorter than the time required for the plasma arc to disappear without being re-ignited. Thereafter, the other pair of transistors 12, 13 or 11, 14 in the figure are made conductive at the same time, and are cut off after a certain period of time. By providing a phase-to-phase short-circuit prevention time in the same way as described above, the transistors forming the pair can be alternately connected without short-circuiting. A transformer that repeatedly conducts and has a high no-load voltage
And the direct current obtained from the rectifier circuit 2 is output as an alternating current rectangular wave.

Here, in the transformer with a high no-load voltage, when a plasma arc is generated or re-ignited, the arc discharge start points 23-a and 23-b at the positive electrode shown in FIG.

As shown at the negative polarity arc discharge starting point 24, a higher voltage is required than when the arc is sustained.
The higher the no-load voltage is than the discharge voltage, the smoother and more reliable the plasma arc start or re-ignition becomes.

The rectangular wave thus output becomes a high-temperature plasma arc flame between the electrode 8 in the nozzle 9 and the base material 10 in FIG. 1, and welding is performed. At this time, the positive electrode component of the alternating current rectangular wave ensures penetration of the base material, and the opposite polarity component cleans the oxide film formed on the surface of the base material. Therefore, the penetration depth due to the change in plate thickness is 18'! 1
Alternatively, it is possible to arbitrarily control the welding person's desired adjustment of the weld penetration depth and cleaning effect area by varying the ratio of positive polarity and reverse polarity components, for example, as shown in Figure 3b. . Further, by varying the frequency, welding workability can be improved to the best condition under each welding condition. Needless to say, the plasma arc can be re-ignited without extinction due to the high no-load voltage and instantaneous polarity change using a square wave, regardless of any frequency or the ratio of positive to reverse polarity time. It becomes possible to create a sound welded joint.

Here, each of the transistors forming a pair in the inverter circuit 3 in FIG. 1 may be a single transistor, a plurality of transistors, or a composite element such as a module. Also, by using field effect transistors as inverter elements,
This element is a voltage controlled type element that requires almost no driving power, and has no charge accumulation effect, so that it is possible to simplify the inverter circuit preamplifier circuit 4 or the phase-to-phase short circuit prevention circuit 7 in Fig. 1. This makes it possible to simplify the device or improve its performance.

Note that the method and apparatus of the present invention can be applied to welding not only aluminum and aluminum alloys but also titanium, titanium alloys, stainless steel, and the like.

Hereinafter, an embodiment of a transfer type plasma arc welding method using an AC rectangular wave will be described based on the present invention.

〔Example〕

The welding test piece used was 1100n in width and 500+n in length.
m, using a JISA 1050 aluminum plate with a temporary J-bu 5m,
The welding power source has a no-load voltage of 135■ and a current capacity of 30O.
This was carried out using the power source for plasma arc welding shown in A. The welding current at that time was 100A and the plasma arc voltage was 30V. The frequency of the rectangular wave alternating current was 1 KHz, the phase-to-phase short circuit prevention time was 0.10 msec, the ratio of positive and reverse polarity was set to 2.5:1, and the welding speed was 15 cm.
A bead-on-plate weld was performed on the specimen at 1/sec.

Note that the pilot gas and shield gas used at this time were 100% argon.

As a result, the bead appearance was 7.8 mm on average, and the average width was 1.5 wn, showing the cleaning effect, and a beautiful bead appearance with no uniform surface defects was obtained, and the penetration depth was 2. . Obtained 4 cabinets. After that, an X-ray transmission test was conducted, but
It was possible to produce a sound weld bead with no internal defects such as blowholes or cracks.

〔Effect of the invention〕

According to the present invention, compared to the conventional AC plasma welding method using a commercial frequency, by varying the ratio of the positive electrode component and the negative electrode component, the penetration depth of the base metal or the width of the cleaning effect area can be adjusted arbitrarily. Done, welding 1'l! Regardless of the size of the flow, a stable AC rectangular wave plasma arc can be maintained at all times. Also, is it possible to increase the ratio of positive and negative electrode components? l! The pole diameter can be made thinner, and therefore the welding parts 1--1 can be made smaller and lighter.

Therefore, the load on the welder can be reduced, and furthermore, stable and sound welded joints can be created at all times, and welding quality can be improved and made more uniform.

Further, by combining the present invention with a welding robot, unsupervised welding and even highly efficient welding work such as unmanned operation becomes possible. Alternatively, since the method according to the present invention uses AC rectangular waves, the influence of magnetic blowing is small, so by using multiple electrodes and adding filler metal, l
It is also possible to increase the amount of used clothing per pass, making welding much more efficient than conventional methods.

In this manner, the present invention provides a welding method and apparatus of extremely high industrial value.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the configuration of a plasma arc welding apparatus that embodies the present invention in one embodiment, and FIG. 2 is an electric circuit diagram showing the outline of the configuration of the inverter circuit 3 shown in FIG. 1. FIG. 3a is a graph showing the voltage waveform applied to the welding electrode in the present invention, in which the time ratio of positive polarity to negative polarity is 5:
FIG. 3b is a graph showing the case where the positive polarity time ratio is much longer than the negative polarity time ratio. FIG. 4 is a diagram schematically showing a voltage waveform applied to a welding electrode in conventional general AC arc welding. 1... Transformer with high no-load voltage 2... Rectifier circuit 3
... Inverter circuit 4 ... Inverter circuit preamplifier circuit 5 ... Positive and reverse polarity time ratio adjustment circuit 3 ... Oscillation circuit 7 ... Phase-to-phase short circuit prevention circuit 8 ...
・Electrode 9... Nozzle IO... Base material 11.1
2, 13.14... Transistors 15-a, 15-b forming an inverter... Inter-phase short circuit prevention time when changing from positive polarity to negative polarity 16-a, 16-b... From negative polarity Phase-to-phase short circuit prevention time when changing to positive polarity 17-a, 17-b...Positive polarity components 18-a, 1g-b-Negative polarity component 19...Time from start of welding to arc discharge 20-
a, 20-b...Time from no-load voltage of 0 volts until the arc re-ignites

Claims (2)

[Claims] (1) An alternating current rectangular wave plasma arc welding method characterized in that welding is performed using an alternating current rectangular wave voltage as a welding power source in which the frequency and time ratio of positive electrode component and negative electrode component can be arbitrarily adjusted. (2) In a plasma arc welding device, an inverter circuit consisting of a transformer with a high no-load voltage, a circuit for rectifying alternating current into direct current, two pairs of switching elements for converting the direct current into alternating current rectangular waves, and for driving the inverter. a preamplifier circuit for the inverter circuit, a control circuit for arbitrarily adjusting the output rectangular wave frequency and the time ratio of the positive polarity component and the negative polarity component of the inverter, and a phase-to-phase short circuit between the two pairs of switching elements of the inverter. An AC rectangular wave plasma arc welding device characterized by comprising a control circuit to prevent