JP2007050426A - Method for ac tig welding - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for AC TIG welding for carrying out the welding work by generating an AC arc by supplying an AC current Iw, which method can greatly suppress the generation of blow holes in a welded portion by causing no large agitation and sloshing in a molten pool without mounting an exciting coil for magnetic agitation on a welding torch. <P>SOLUTION: In the method for AC TIG welding for carrying out the welding work by generating the AC arc by supplying the AC current Iw, the frequency of the AC current Iw is set within the range for causing the cavitation due to the high frequency vibration in the molten pool so as to reduce the generation of the blow holes by promoting the discharge of air bubbles from the inside of the molten pool. The range of the frequency of AC is 5 kHz to 35 kHz. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an AC TIG welding method for suppressing generation of blowholes by applying vibration to a molten pool.

  A major problem when TIG welding aluminum or an alloy thereof (hereinafter referred to as aluminum) is a blow hole generated in the welded portion. FIG. 5 is a schematic diagram showing a mechanism for generating this blow hole. As shown in FIG. 2A, an arc 3 is generated between the non-consumable electrode 1 made of tungsten and the molten pool 4. A shield gas 2 such as argon gas shields the generation part of the arc 3 from the air. The shield gas 2 contains a slight amount of moisture, and moisture in the air is slightly mixed in the shield of the shield gas 2. These moisture are decomposed by the arc 3 to generate hydrogen 5. This hydrogen 5 is absorbed by the molten pool 4. Subsequently, as shown in FIG. 5B, hydrogen 5 generates bubbles 6 in the molten pool 4. Subsequently, as shown in FIG. 3C, the bottom of the molten pool 4 starts to solidify. Since the hydrogen 5 has a lower solubility in the solid phase than in the liquid phase, the hydrogen 5 is released into the liquid phase at the solidification boundary, floats up in the molten pool, and is released to the outside. Subsequently, as shown in FIG. 2D, the bubbles 6 that have not been released in time due to the solidification of the molten pool 4 remain and become blowholes 7. Subsequently, as shown in FIG. 5E, solidification of the molten pool 4 is completed, and a blow hole 7 is formed therein.

  In order to solve the above-described problem of blowhole generation, in the prior art 1 described in Patent Document 1, the molten pool is magnetically stirred by attaching an exciting coil to the welding torch and forming a magnetic field in the molten pool. Thereby, the bubble floating and external discharge described above with reference to FIG. 5C are promoted, and the generation of blowholes can be suppressed.

  Further, in the prior art 2 described in Patent Document 2, in the pulse MIG welding method in which the arc length is periodically changed by periodically switching between the first pulse current group and the second pulse current group, the switching frequency is set to 0.5. By setting the frequency within a range of ˜25 Hz, low-frequency vibration is applied to the molten pool and the molten pool is agitated. Thereby, the bubble floating and external discharge described above with reference to FIG. 5C are promoted, and the generation of blowholes can be suppressed.

JP-A-5-148666 Japanese Patent No. 2993174

  In the prior art 1 described above, it is necessary to attach a large exciting coil to the welding torch, and the welding torch becomes large. For this reason, it often interferes with a welded structure, a jig, etc., and there is a problem that the application range is limited. Furthermore, the speed and magnitude of the convection of the molten metal rotating by magnetic stirring are greatly affected by the strength and frequency of the magnetic field. For this reason, there exists a problem that the appropriate condition range for exhibiting sufficient blowhole suppression effect is narrow. When deviating from this appropriate condition range, the molten metal may jump out of the molten pool and cause a bead appearance defect or may not exhibit a sufficient blowhole suppression effect.

  Further, in the above-described conventional technique 2, there is a problem that the bead appearance is deteriorated depending on the welding posture, joint shape, and the like because the molten metal is greatly swung and stirred.

  Then, this invention provides the alternating current TIG welding method which can suppress generation | occurrence | production of a blowhole, after solving the subject mentioned above.

In order to solve the above-described problem, the first invention is an AC TIG welding method in which an AC current is applied to generate an AC arc and welded.
The AC TIG welding is characterized in that the AC frequency of the AC current is set in a range that causes cavitation due to high-frequency vibration in the molten pool, and the generation of blowholes is reduced by promoting the release of bubbles from the molten pool. Is the way.

  The second invention is an AC TIG welding method characterized in that the range of the AC frequency described in the first invention is 5 kHz or more and 35 kHz or less.

  According to the AC TIG welding method of the present invention, by supplying an AC current having an AC frequency of 5 to 35 kHz to the arc and the molten pool, cavitation due to high-frequency vibration is generated in the molten pool, and hydrogen gas from the molten pool is generated. It is possible to promote the release of bubbles due to, and to greatly reduce the occurrence of blowholes. In the present invention, unlike the prior art 1, it is not necessary to attach an exciting coil to the welding torch, and a normal welding torch can be used. Therefore, the application location is not limited by interference. In the present invention, the welding condition range is not narrowed because the other welding conditions are substantially the same as those of normal AC TIG welding simply by setting the AC frequency to a value that causes cavitation. Further, in the present invention, since the molten pool is not agitated and shaken, the bead appearance is not deteriorated by the welding posture, the joint shape, and the like.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 is a block diagram of a welding power source for performing an AC TIG welding method according to an embodiment of the present invention. The three-phase bridge diode DP receives a commercial power supply AC such as a three-phase 200V as an input, and outputs a DC voltage by performing three-phase full-wave rectification. The smoothing capacitor C smoothes the DC voltage. The switching elements TR1 to TR4 form a full-bridge inverter circuit and output high-frequency alternating current. The Invar et al. Circuit may be a push-pull system or a half-bridge system. The free-wheeling diodes D1 to D4 are connected in antiparallel to the respective switching elements and energize the free-wheeling current from the exciting inductance of the transformer Tr when the switching elements are in the off state. The transformer Tr steps down the voltage value of the high frequency alternating current to a voltage value suitable for arc welding. The secondary output of the transformer Tr is supplied as it is between the electrode and the base material. Therefore, the alternating current frequency f of the alternating current Iw and the alternating voltage Vw becomes the same value as the carrier frequency of the inverter circuit. In a normal inverter welding power source, the secondary output of the transformer Tr is rectified and a low frequency alternating current of about several tens Hz is output by the secondary side inverter circuit. On the other hand, the inverter welding power source of the present invention outputs a high frequency alternating current of several tens of kHz. The pulse width modulation circuit PWM outputs a pulse width modulation signal Pwm for controlling the AC output. The drive circuit DV outputs a signal Sa for starting the switching elements TR1 and TR4 and a signal Sb for driving the switching elements TR2 and TR3. Therefore, the AC frequency f (kHz) of the AC current Iw and the AC voltage Vw can be set to a desired value by the pulse width modulation circuit PWM.

  FIG. 2 is a relationship diagram between the AC frequency f and the number of blowholes generated. In the figure, AC TIG welding was performed on an aluminum material using the above-described welding power source of FIG. 1, and the number of blow holes generated per 50 mm weld length was measured. As shown in the figure, the number of blowholes rapidly decreases when f ≧ 5 kHz and rapidly increases again when f> 35 kHz. Therefore, when it is in the range of 5 ≦ f ≦ 35, the number of blowholes is greatly reduced.

  The reason why the number of blowholes is thus suppressed is as follows. That is, high-frequency vibration is generated in the molten pool by the application of the alternating current Iw. Due to this high frequency vibration, a large number of holes close to a vacuum state are formed inside the molten pool. This phenomenon is called cavitation. This cavitation occurs when the liquid flow becomes unable to follow the pressure change. At this time, when the AC frequency f becomes low, the pressure change is relaxed by the flow of the liquid, and the energy of vibration is consumed by the vibration of the entire molten pool such as the vibration of the liquid surface, so that cavitation occurs. It becomes difficult. It is known that the sound pressure (cavitation threshold) necessary for cavitation to occur becomes large at a predetermined frequency or higher. This is because the pressure change due to the high frequency becomes faster than the time required for generating the holes. On the other hand, the higher the AC frequency f, the greater the vibration attenuation. Therefore, cavitation is less likely to occur above a predetermined frequency. Thus, cavitation occurs when the AC frequency f is within the appropriate range. Hydrogen in the molten pool is agglomerated in the cavitation holes to form large bubbles. Large bubbles have high buoyancy, and float in the molten pool and are released to the outside. In this way, since the release of hydrogen gas is promoted, the number of blowholes generated is suppressed. As shown in the figure, when the AC frequency f is in the range of 5 to 35 kHz, cavitation occurs and the occurrence of blowholes is suppressed.

  FIG. 3 is a diagram comparing the number of blowholes generated in the normal TIG welding method and the AC TIG welding method of the present invention. In this case, pure aluminum alloy A1050 (plate thickness 5 mm) is used as the aluminum material, and TIG welding is performed at an average value of alternating current of 100 A and a welding speed of 10 cm / min. In order to clearly verify the blowhole suppression effect, a test mixed gas of 99.5% by volume argon + 0.5% by volume hydrogen was used as a shielding gas. Normal TIG welding is a welding method that uses a low-frequency AC output as described above. The AC frequency f of the present invention is set to a value at which cavitation occurs. As shown in the figure, the number of blow holes generated according to the present invention is greatly reduced as compared with ordinary TIG welding.

  FIG. 4 is a diagram comparing the blowhole suppressing effect of the T-shaped joint between the prior art 1 and the present invention. The welding conditions excluding the joint are the same as those in FIG. In prior art 1, it is necessary to increase the distance between the electrode and the base material in order to prevent the exciting coil mounted on the welding torch from interfering with the web material and flange material of the joint. For this reason, the magnetic stirring action is weakened and many blow holes are generated. On the other hand, in the present invention, since the welding torch is a normal one, it does not interfere with the joint, and an appropriate distance between the electrode and the base material can be ensured. For this reason, the number of blowholes generated is greatly reduced by high frequency vibration.

It is a block diagram of the welding power source for enforcing the alternating current TIG welding method concerning an embodiment of the invention. It is a related figure of AC frequency f in an embodiment of the invention, and the number of blowhole generation. It is a comparison figure of the number of blowhole generation of this invention and normal TIG welding. It is a comparison figure of the number of blowhole generation of the present invention and prior art 1 in T-shaped joint welding. It is a figure which shows the generation | occurrence | production mechanism of the blowhole in a prior art.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Electrode 2 Shielding gas 3 Arc 4 Molten pool 5 Hydrogen 6 Bubble 7 Blowhole AC Commercial power supply C Smoothing capacitor D1-D4 Free-wheeling diode DP Three-phase bridge diode DV Drive circuit f AC frequency Iw AC current PWM Pulse width modulation circuit Pwm Pulse width Modulation signal Sa, Sb Drive signal Tr Transformers TR1 to TR4 Switching element Vw AC voltage

Claims (2)

In the AC TIG welding method of welding by generating an AC arc by energizing an AC current,
The AC TIG welding is characterized in that the AC frequency of the AC current is set in a range that causes cavitation due to high-frequency vibration in the molten pool, and the generation of blowholes is reduced by promoting the release of bubbles from the molten pool. Method. 2. The AC TIG welding method according to claim 1, wherein the range of the AC frequency is 5 kHz or more and 35 kHz or less.

Images