JPS6257438B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention is directed to a consumable electrode method which is carried out in an inert gas such as argon or helium, or in a shield gas in which oxygen, carbon dioxide, etc. are added to the inert gas. This invention relates to a DC arc welding method.

[Prior art] Generally, conventional welding is performed in a gas atmosphere containing an inert gas as a main component using a consumable electrode wire made of steel, stainless steel, etc. with opposite polarity (i.e., the electrode is used as an anode and the workpiece is used as a cathode). In the DC arc welding method, there is a phenomenon in which transferred droplets become finer when the current value exceeds a certain value determined by the electrode material, electrode diameter, and shielding gas components. This current value is called a critical current, and in arcs where the current value is higher than the critical current value, the transfer of droplets is usually carried out in the form of a spray at a rate of 100 to 200 times/second. In this arc form, there is almost no variation in the macroscopic arc length (distance between the electrode tip and the molten pool surface), and the arc is stable.
Spray transfer is widely used in actual welding. However, in recent years, with the diversification of materials to be welded, the conventional welding method using a nearly smooth DC power source may not be able to meet the requirements in terms of stabilizing the arc and improving the bead shape. It has become extremely popular.

FIG. 1a shows an arc current waveform of a consumable electrode type DC arc using a conventionally used general DC welding power source. The ripples seen in the figure are due to phase control by a magnetic amplifier or thyristor element, and correspond to the primary power supply frequency.
The frequency is 50 to 60 Hz or its 2nd, 3rd, or 6th harmonic, and the ripple peak value is usually as small as possible.

[Problems to be solved by the invention] How to improve the droplet transfer form when welding is performed using a welding power source with such an output current waveform and setting the average current value Iav larger than the critical current value Ic. When observed with velocity photographs, the droplets migrate as shown in Figure 1b, and the average diameter of the droplets is about the same as the wire diameter, resulting in a spray-like shape, but the timing of migration is different from the current ripple cycle. It's not fully synchronized. In such a transition mode, the heat input to the workpiece, which is mainly determined by the welding current and welding speed, is constant,
Moreover, it is virtually impossible to significantly change the penetration depth by ensuring sufficient melting of the workpiece.

In the conventional MIG arc welding method, when a steel-based consumable electrode is used, as the arc length is shortened, spray-like droplets suddenly change to large droplets, making the bead discontinuous and impractical. This is a welding process.
This phenomenon also occurs in high-grade steels such as stainless steel and nickel steel. Further, this phenomenon occurs when the arc length is 3 to 5 mm or less in an inert gas of argon, and occurs when the arc length is about 7 to 10 mm in a helium atmosphere. For this reason, in the conventional MIG welding method using a steel wire as a consumable electrode, welding is performed using a shielding gas prepared by adding 1 to 5% oxygen or carbon dioxide gas to an inert gas. If such a shielding gas is used, it will form a stable spray-like droplet and the arc will be stabilized, but on the other hand, problems such as oxidation of the metal during welding, deterioration of the properties of the weld metal, and mill scale on the weld bead may occur. Become.

Figure 2 shows the state of droplet transfer using a stainless steel consumable electrode in an inert gas atmosphere above the critical current value, and Figure a shows the case where the arc length is long;
This figure shows that the droplets are becoming finer. However, if the arc length is particularly short, or even if the arc length is not particularly short but in helium air, the droplets will turn into large droplets as shown in Figure b. Note that 7 is a consumable electrode, and 9 is an object to be welded.

In general, if the arc length is kept long as shown in Figure a, problems arise such as the actual target position of welding becoming unclear and poor penetration occurring. Furthermore, if the arc length is kept long, the cleaning width becomes wider, which also causes the problem of meandering beads.

Furthermore, if oxygen or carbon dioxide is added to the inert gas to stabilize the droplet transfer continuously with a short arc length, the droplet transfer will continue, but in this case scale will adhere to the bead surface. Since this scale causes welding defects such as slag entrainment during multilayer welding, it must be completely removed, which requires labor such as grinding.

[Means for Solving the Problems] The present invention is applicable to the above-mentioned case, that is, in a consumable electrode type DC arc welding method in which welding is performed with reverse polarity in a shielding gas atmosphere containing an inert gas as a main component. When welding at a critical current value or higher determined by the electrode material, electrode diameter, shielding gas composition, etc.
This paper proposes a DC arc welding method that aims to improve the bead shape while ensuring the stability of the arc and droplet transfer.

As shown in FIG. 3, the present invention provides an average current value
If a periodic pulsating current with a waveform such that Iav is greater than or equal to the critical current value Ic and the minimum value Il does not exceed the critical current value Ic and the frequency of the pulsating current is set in the range of 250Hz to 1000Hz, pure shielding gas can be used. Using inert gas and smoothing the transfer of droplets,
Since the arc can be stabilized, the above-mentioned problems caused by adding oxygen or carbon dioxide to the shielding gas can be solved. In this case, the shielding gas is 5~
It is appropriate to use a gas mixed with 50% helium.

[Function] The method of the present invention uses a shielding gas mainly composed of an inert gas, and the material is steel, stainless steel,
A direct current arc welding method that performs welding with reverse polarity using a consumable electrode made of steel such as nickel alloy steel,
In the present invention, as shown in Fig. 3, the average value Iav of the welding current is set to at least the critical current value Ic, and the minimum value Il of the peak value is Critical current value Ic
Welding is performed using a welding current with a current waveform that does not exceed .

The method of the present invention will be explained below with reference to the drawings.

Figure 3a shows an example of the welding current waveform used in the welding method of the present invention, and its average value Iav
is set to the same value as in FIG. 1a, but the amplitude of its pulsation is large, and its minimum value Il is less than the critical current value Ic. Figure 2b shows the form of droplet transfer at this time.The number of transfers and the diameter of the droplet are the same as those shown in Figure 1b, but at the minimum value Il of the pulsating current, the droplet is No transition occurs, but a transition occurs reliably near the maximum value, and a synchronous transition with the pulsation frequency is observed.

In the conventional consumable electrode type DC arc welding method, which uses a shielding gas mainly composed of inert gas and passes a pulsed current, the average value Iav is less than the critical current value Ic and the maximum value Ih is the critical current value. The droplets were made finer by passing a pulsed pulsating current greater than Ic, that is, a current of Iav<Ic and Ih>Ic, to cause the droplets to move synchronously. The present invention is completely different from such conventional methods in that the average current value Iav is greater than or equal to the critical current value Ic, and the minimum value Il is less than the critical current value Ic.
The major feature is that the transfer of droplets is synchronized with the pulsating current by flowing a periodic pulsating current with a waveform that does not exceed , that is, Iav > Ic and Il < Ic.

According to the results of various experiments conducted by the present inventor,
The frequency range of the pulsating current suitable for implementing the method of the present invention is 250Hz to 1000Hz.

[Example] The results of experiments conducted by the present inventor will be described below.

In order to eliminate scale formation and improve the appearance of the bead, the inventor first conducted an experiment using a mixed gas of only an inert gas of argon and helium as a shielding gas. Some of the results are shown in the 4th section.
As shown in the figure. As shown in the figure, as the amount of helium added to the argon atmosphere (horizontal axis %) increases,
At the same current value and welding speed, the bead width BW
(vertical axis mm) tends to increase at first and then decrease. When the amount of helium added is 10% or more, the bead width BW almost stops increasing, and when the amount of helium added is up to 50%, the bead width BW becomes approximately constant. On the other hand, the cleaning width CW decreases as the amount of helium gas added increases, and when the amount of helium added exceeds 50%, the cleaning width CW decreases as the amount of helium gas added increases.
The decrease in CW becomes too large and the difference from the bead width becomes small. For this purpose, the amount of helium added is 50
%, the bead width BW starts to decrease. It has also been found that when the amount of helium gas added is 50% or more, the influence of helium gas becomes too large, and large particle migration accompanied by fume generation is likely to occur, similar to when only helium gas is used. Therefore, it has become clear that it is appropriate to set the amount of helium gas added in the range of 10 to 50%.

Next, when welding is performed using a steel-based consumable electrode using a shielding gas containing argon with 10% to 50% helium added, the inventor has determined that the frequency of the superimposed periodic pulsating current is 250 to 500 Hz. An experiment was conducted to confirm that the transfer of droplets could be stabilized by setting the droplet within a certain range. The results are shown in FIG. In this experiment, a stainless steel consumable electrode with a diameter of 1.6 mm (critical current is approximately 220 A) was used as the consumable electrode, the average current value was set to 300 A (constant), and the base current value
A rectangular pulsating current of 140A and a peak current value of 460A was applied. Then, the frequency F of the pulsating current is set to 0Hz to 1,
The relationship between the frequency F and the length Lo of the melted portion of the wire was investigated by changing the temperature up to 000Hz.

Figure 5 shows the relationship between the frequency F of the pulsating current and the magnitude of the variation in the length Lo of the wire melting portion immediately before and immediately after the droplet falls, with the frequency F plotted on the horizontal axis. The vertical axis shows the melting length Lo of the maximum and minimum consumption electrodes. Melting length of consumable electrode
The difference ΔLo between the maximum and minimum values of Lo affects the stability of droplet transfer. As is clear from FIG. 5, the frequency of the pulsating current is as large as ΔLh from 0 Hz to 250 Hz, for example at 100 Hz, so a short circuit will occur unless the arc length is increased considerably. Therefore, if the arc length is increased to prevent short circuits, the arc becomes unstable and is excluded from practical use. When the frequency of pulsating current exceeds 250Hz,
For example, at 250 Hz, the value becomes small as ΔLl and becomes a substantially constant value, so the arc is stable even if the arc length is shortened. When the frequency of the pulsating current exceeds 1000Hz, the period becomes too short and the droplet transfer at the tip of the consumable electrode cannot follow the maximum value of the pulsating current. I can't get it.

As mentioned above, change the frequency of the pulsating current to 250Hz or
If it is set in the 1000 Hz range, droplets can be transferred stably even when welding steel, and pure inert gas can be used as the shielding gas.

As an example, when welding was performed using a mixed gas of 75% argon and 25% helium as a shielding gas and a pulsed current frequency of 500Hz,
We were able to obtain uniform and stable welding results with no bead meandering and no scale adhesion.

Figures 6a to 6d show examples of current waveforms when implementing the present invention, where a is approximately a sine wave;
The figure b shows a substantially triangular wave, the figure c shows a rectangular wave, and the figure d shows a composite of the waveforms shown in figures b and c, but in all cases the average current value is greater than the critical current value and the minimum value of the pulsating component. is below the critical current value.

The above waveforms have different utility values depending on the purpose of welding. For example, the waveforms shown in FIGS. 6a and 6b have few harmonic components, so the arc noise is low at frequencies near 1 KHz and does not cause discomfort to the operator.
On the other hand, in c of the same figure, the arc sound is quite strong, but the waveform rate can be varied over a wide range by changing the time rate of high and low current values and the ratio of the current values, so the penetration depth can be changed. There is an advantage that the sheath bead width can be greatly controlled. Also,
Figure d gives minute vibrations to the droplet during high current and enables reliable transfer, increasing the possible frequency of synchronous transfer, and is therefore suitable for welding steel-based metals with high viscosity. ing.

As a means of realizing the above waveform example, the pulse superimposition method using switching elements such as thyristors, which has been widely used in the past, can only generate waveforms with frequencies that are integral multiples of the primary power supply frequency.
Furthermore, since it is impossible to output an arbitrary waveform, it is necessary to use a welding power source with an analog transistor amplifier as the main control element.

FIG. 7 shows an example of an apparatus for carrying out the welding method of the present invention using a welding power source using an analog transistor.

In the figure, 1 is a welding machine, 2 and 2 are input terminals of the welding machine, 3 is a welding transformer, 4 is a rectifier, 5 is a plurality of transistors connected in parallel, 6 is a current detector, and 7 is a consumable electrode. , 8 is an electrode feeding mechanism, and 9 is an object to be welded. The plurality of transistors 5 are connected to the output signal of the welding current setting device 11 and the pulse signal generator 12.
It is controlled by the output signal of the operational amplifier 13 which receives the output signal of the current detector 6 and the output signal of the current detector 6, and outputs having waveforms as shown in FIGS. The welding method of the present invention is carried out by being supplied between the workpiece 9 and the workpiece 9 to be welded.

[Effects of the Invention] As described above, according to the inert gas shielded consumable electrode DC arc welding method for welding steel-based materials of the present invention, the average value of the welding current is set to at least the critical current value or more. , and a periodic pulsating current with a frequency of 250 to 1000 Hz, using a current waveform whose minimum peak value does not exceed the critical current value, synchronizes the droplets of the consumable electrode with the maximum value of the pulsating current, and generates a strong arc. By transferring the material by force, it is more effective than conventional pulse welding, as it can ensure sufficient melting of the welded object and greatly change the penetration depth.

[Brief explanation of the drawing]

Figure 1a is a diagram showing the arc current waveform from a conventionally used general DC power supply, Figure 1b is an explanatory diagram showing the transfer form of droplets in relation to Figure 2 The figure is a diagram illustrating the state of droplet transfer at a critical current value or higher using a stainless steel consumable electrode in an inert gas, and figures a and b are explanatory diagrams showing cases where the arc length is long and short, respectively. , FIG. 3a is a diagram showing an example of the welding current waveform used in the welding method of the present invention, and FIG.
Figure 4 is an explanatory diagram showing the transfer form of droplets in relation to the MIG using stainless steel consumable electrodes.
A line diagram showing the relationship between the components of a mixed gas of argon and helium and the cleaning width CW and bead width BW in welding. Figure 5 is a line showing the relationship between the pulse frequency and the melting length Lo of the maximum and minimum consumable electrodes. Figures 6a to 6d are diagrams showing different examples of welding current waveforms used in the welding method of the present invention, and Figure 7 is a schematic circuit configuration of a welding machine that implements the welding method of the present invention. It is a diagram. 1... Welding machine, 3... Welding transformer, 4... Rectifier, 5... Transistor, 6... Current detector,
7... Consumable electrode, 9... Work to be welded, 11... Welding current setting device, 12... Pulse signal generator, 13...
...Operation amplifier.

Claims (1)

[Claims] 1. In the direct current arc welding method, which uses a shielding gas mainly composed of inert gas and performs welding with reverse polarity using a consumable electrode made of steel, stainless steel, nickel alloy steel, etc., the welding current is Set the average value to at least the critical current value, and
A DC arc welding method in which welding is performed using a periodic pulsating current with a frequency of 250 Hz or more and 1000 Hz or less, and a welding current with a current waveform such that the minimum value of the peak value does not exceed the critical current value.