Double shield tig welding method

Abstract

To provide a double shield tig welding method which suppresses occurrence of a blow hole.SOLUTION: In a double shield tig welding method using a welding torch having an inner nozzle jetting an inner gas and an outer nozzle jetting an outer gas which performs a pre-flow (times t1 to t3) of the inner gas Fi and the outer gas Fo at the start of welding, then generates arc, energizes a welding current Iav, and performs welding, helium is used as the inner gas, argon is used as the outer gas, and the flow rate Fi of the inner gas is set so that the flow rate of the inner gas is within ±20% of the flow rate of the outer gas during an initial period (times t3 to t4) from when the arc is generated at the time t3.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a double-shielded tig welding method.

Background Art

[0002] A double-shielded tig welding method is commonly used in which a welding torch having an inner nozzle for ejecting an inner gas and an outer nozzle for ejecting an outer gas is used, and a welding current is passed to perform welding (see, for example, Patent Document 1). As the inner gas and the outer gas, an inert gas such as argon or helium is used.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the double-shielded tig welding method, when the arc generation state is in a transient state at the start of welding, there is a problem that blowholes are likely to occur due to the disturbance of the gas flow. In particular, blowholes are likely to occur when welding aluminum, aluminum alloys, magnesium, etc.

[0005] Therefore, an object of the present invention is to provide a double-shielded tig welding method capable of suppressing the occurrence of blowholes.

Means for Solving the Problems

[0006] In order to solve the above-described problems, the invention of claim 1 is using a welding torch having an inner nozzle for ejecting an inner gas and an outer nozzle for ejecting an outer gas, In a double-shielded TIG welding method in which, after performing a preflow of the inner gas and the outer gas at the start of welding, an arc is generated and a welding current is passed to perform welding, helium is used for the inner gas and argon is used for the outer gas, the flow rate of the inner gas is set such that the flow velocity of the inner gas is within ±20% of the flow velocity of the outer gas during an initial period from the time when the arc is generated, which is a double-shielded TIG welding method characterized by this.

[0007] The invention according to claim 2 is maintaining the value of the flow rate of the inner gas as it is even after the end of the initial period, which is the double-shielded TIG welding method according to claim 1, characterized by this.

[0008] The invention according to claim 3 is setting the initial period to a value at which blowholes do not occur at the welding start portion, which is the double-shielded TIG welding method according to claim 1, characterized by this.

[0009] The invention according to claim 4 is the welding current is formed from an electrode negative polarity current during the electrode negative polarity period and an electrode positive polarity current during the electrode positive polarity period, the electrode positive polarity current is formed from a first electrode positive polarity current during a first electrode positive polarity period and a second electrode positive polarity current during a second electrode positive polarity period, the value of the first electrode positive polarity current is 20% to 60% of the peak value of the second electrode positive polarity current, which is the double-shielded TIG welding method according to any one of claims 1 to 3, characterized by this.

Advantages of the Invention

[0010] According to the double-shielded TIG welding method according to the present invention, the generation of blowholes can be suppressed and good welding quality can be obtained.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Modes for Carrying Out the Invention

[0012] Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[0013] [Embodiment 1] FIG. 1 is a block diagram of a welding apparatus for carrying out a double shielded tig welding method according to Embodiment 1 of the present invention. Hereinafter, each block will be described with reference to the same figure.

[0014] The same figure shows the case of a double shielded tig welding method in which an alternating welding current Iw formed from an electrode negative polarity current during the electrode negative polarity period and an electrode positive polarity current during the electrode positive polarity period is energized. The double shielded tig welding method in which the alternating welding current Iw is energized is used for welding aluminum, aluminum alloys, magnesium, etc.

[0015] The welding torch WT mainly includes an electrode 1, an inner nozzle 4 surrounding it, and an outer nozzle 5 surrounding it. A tungsten electrode or the like is used for the electrode 1. For example, the inner diameter of the inner nozzle 4 is 5 mm, and the inner diameter of the outer nozzle 5 is 13 mm.

[0016] The welding start circuit ON outputs a welding start signal ON that becomes high level when starting welding. This welding start circuit ON is a torch switch provided on the welding torch WT. Also, the welding start circuit ON may be provided within the robot control device.

[0017] The preflow period circuit STP takes the above welding start signal On as an input and outputs a preflow period signal Stp that becomes high level from the point in time when the welding start signal On changes to high level until a predetermined preflow period elapses.

[0018] The inner gas ejection start circuit SI takes the above preflow period signal Stp as an input, becomes high level at the point in time when a predetermined delay time Td has elapsed from the point in time when the preflow period signal Stp changes to high level, and outputs an inner gas ejection start signal Si that becomes low level when the preflow period signal Stp becomes low level. The delay time Td is a time shorter than the preflow period.

[0019] The current detection circuit ID detects the welding current Iw, converts it to an absolute value, and outputs a current detection signal Id. The arc generation discrimination circuit AD takes the above current detection signal Id as an input, and when the value of the current detection signal Id is equal to or greater than the energization discrimination value (about 5 A), it discriminates that an arc has occurred and outputs an arc generation discrimination signal Ad that becomes high level.

[0020] The initial period circuit STI takes the above preflow period signal Stp and arc generation discrimination signal Ad as inputs, becomes high level when the preflow period signal Stp becomes low level, and then outputs an initial period signal Sti that becomes high level until a predetermined initial period elapses from the point in time when the arc generation discrimination signal Ad changes to high level. The initial period is set to a value at which blowholes do not occur at the welding start portion by test welding.

[0021] The first - electrode negative - polarity current setting circuit IN1R outputs a first - electrode negative - polarity current setting signal In1r with a predetermined positive value. The second - electrode negative - polarity current setting circuit IN2R outputs a second - electrode negative - polarity current setting signal In2r with a predetermined positive value. Here, In1r < In2r.

[0022] The first - electrode positive - polarity current setting circuit IP1R outputs a first - electrode positive - polarity current setting signal Ip1r with a predetermined positive value. The second - electrode positive - polarity current setting circuit IP2R outputs a second - electrode positive - polarity current setting signal Ip2r with a predetermined positive value. Here, Ip1r < Ip2r.

[0023] The steady - state inner - gas flow rate setting circuit FICR takes the above - mentioned second - electrode negative - polarity current setting signal In2r as an input, inputs the second - electrode negative - polarity current setting signal In2r [A] into the following predetermined steady - state inner - gas flow rate setting function, and outputs the calculated value as the steady - state inner - gas flow rate setting signal Ficr [l / min]. An example of the steady - state inner - gas flow rate setting function is shown below. Ficr=(In2r - 75) / 50 + 3.5 (Equation (1)) However, it is in the range of 75 ≤ In2r ≤ 150. When In2r < 75, it has the same value as In2r = 75. When In2r > 150, it has the same value as In2r = 150. Thus, the larger the value of the second - electrode negative - polarity current setting signal In2r, the larger the value of the inner - gas flow rate setting signal Fir.

[0024] The steady - state outer - gas flow rate setting circuit FOCR takes the above - mentioned second - electrode negative - polarity current setting signal In2r as an input, inputs the second - electrode negative - polarity current setting signal In2r [A] into the following predetermined steady - state outer - gas flow rate setting function, and outputs the calculated value as the steady - state outer - gas flow rate setting signal Focr [l / min]. An example of the steady - state outer - gas setting function is shown below. Focr=(In2r - 75) / 50 + 5.5 (Equation (2)) However, it is in the range of 75 ≦ In2r ≦ 150. When In2r < 75, it has the same value as In2r = 75, and when In2r > 150, it has the same value as In2r = 150. Thus, the larger the value of the second electrode minus-polarity current setting signal In2r, the larger the value of the outer gas flow rate setting signal For.

[0025] The initial inner gas flow rate setting circuit FIIR takes the above-mentioned steady outer gas flow rate setting signal Focr as an input, performs the following calculation, and outputs the initial inner gas flow rate setting signal Fiir. The calculation calculates the inner gas flow rate such that the flow velocity of the inner gas during the initial period is within ±20% of the flow velocity of the outer gas. Fiir = Focr × R × K (Equation (3)) However, R = (cross-sectional area of the flow path of the inner gas 7 in the inner nozzle 4) / (cross-sectional area of the flow path of the outer gas 9 in the outer nozzle 5), and K is a constant in the range of 0.8 to 1.2 The above equation will be explained with numerical examples. Assuming that the inner diameter of the inner nozzle 4 = 5 mm and the inner diameter of the outer nozzle 5 = 13 mm, (cross-sectional area of the flow path of the inner gas 7 in the inner nozzle 4) = 3.14 × 2.5 × 2.5 (cross-sectional area of the flow path of the outer gas 9 in the outer nozzle 5) = 3.14 × (6.5 × 6.5 - 2.5 × 2.5) As a result, R = 0.36. (1) When the second electrode minus-polarity current In2 = 75 A Steady outer gas flow rate Focr = 5.5 l / min, steady inner gas flow rate Ficr = 3.5 l / min K = 0.8 → Initial inner gas flow rate Fiir = 1.59 K = 1.0 → Initial inner gas flow rate Fiir = 1.99 K = 1.2 → Initial inner gas flow rate Fiir = 2.39 (2) When the second electrode minus-polarity current In2 = 150 A Steady outer gas flow rate Focr = 7 l / min, steady inner gas flow rate Ficr = 5 l / min K = 0.8 → Initial inner gas flow rate Fiir = 2.03 K = 1.0 → Initial inner gas flow rate Fiir = 2.54 K = 1.2 → Initial inner gas flow rate Fiir = 3.04

[0026] The preflow inner gas flow rate setting circuit FIPR outputs a predetermined preflow inner gas flow rate setting signal Fipr. Here, Fipr < Ficr, and it may be set that Fipr = Fiir.

[0027] The inner gas flow rate setting circuit FIR takes the above preflow period signal Stp, the above initial period signal Sti, the above preflow inner gas flow rate setting signal Fipr, the above initial inner gas flow rate setting signal Fiir, and the above steady inner gas flow rate setting signal Ficr as inputs, performs the following processes, and outputs an inner gas flow rate setting signal Fir. 1) When the preflow period signal Stp is at the High level, the value of the preflow inner gas flow rate setting signal Fipr is output as the inner gas flow rate setting signal Fir. 2) When the initial period signal Sti is at the High level, the value of the initial inner gas flow rate setting signal Fiir is output as the inner gas flow rate setting signal Fir. 3) When the initial period signal Sti changes to the Low level and transitions to the steady welding period, an inner gas flow rate setting signal Fir that increases over time to the value of the steady inner gas flow rate setting signal Ficr is output. Here, it may be switched to the value of the steady inner gas flow rate setting signal Ficr at the time when the initial period ends.

[0028] The inner gas flow rate regulator CI is a known mass flow controller. Taking the above welding start signal On, the above inner gas ejection start signal Si, and the above inner gas flow rate setting signal Fir as inputs, during the period from the time when the inner gas ejection start signal Si changes to the High level until the welding start signal On changes to the Low level and a predetermined afterflow time has elapsed, the flow rate Fi of the inner gas 7 from the inner gas cylinder 6 is adjusted to the value determined by the inner gas flow rate setting signal Fir and ejected.

[0029] The outer gas flow rate setting circuit FOR takes the above-mentioned preflow period signal Stp and the above-mentioned steady outer gas flow rate setting signal Focr as inputs, and outputs an outer gas flow rate setting signal For that becomes a predetermined preflow outer gas flow rate value when the preflow period signal Stp is at a high level and becomes the value of the steady outer gas flow rate setting signal Focr when it is at a low level. Here, it is desirable that the preflow outer gas flow rate value be set to a value larger than the value of the steady outer gas flow rate setting signal Focr.

[0030] The outer gas flow regulator CO is a known mass flow controller. Taking the above-mentioned welding start signal On and the above-mentioned outer gas flow rate setting signal For as inputs, during the period from when the welding start signal On changes to a high level until it changes to a low level, it adjusts the flow rate Fo of the outer gas 9 from the outer gas cylinder 8 to the value determined by the outer gas flow rate setting signal For and ejects it.

[0031] The inner gas 7 flows through the passage inside the inner nozzle 4. Also, the outer gas 9 flows through the passage between the outside of the inner nozzle 4 and the inside of the outer nozzle 5. Helium is used for the inner gas 7, and argon is used for the outer gas 9.

[0032] The voltage detection circuit VD detects the welding voltage Vw, converts it to an absolute value, and outputs a voltage detection signal Vd.

[0033] The first electrode negative polarity period setting circuit TN1R takes the above-mentioned voltage detection signal Vd as an input, measures the period until the variation of the voltage detection signal Vd during this period converges, and outputs a predetermined first electrode negative polarity period setting signal Tn1r. The second electrode negative polarity period setting circuit TN2R outputs a predetermined second electrode negative polarity period setting signal Tn2r.

[0034] The first electrode positive polarity period setting circuit TP1R takes the above voltage detection signal Vd as an input, measures the period until the variation of the voltage detection signal Vd during this period converges, and outputs a predetermined first electrode positive polarity period setting signal Tp1r. The second electrode positive polarity period setting circuit TP2R outputs a predetermined second electrode positive polarity period setting signal Tp2r.

[0035] The current setting circuit IR takes the above first electrode negative polarity period setting signal Tn1r, the above second electrode negative polarity period setting signal Tn2r, the above first electrode positive polarity period setting signal Tp1r, the above second electrode positive polarity period setting signal Tp2r, the above first electrode negative polarity current setting signal In1r, the above second electrode negative polarity current setting signal In2r, the above first electrode positive polarity current setting signal Ip1r, the above second electrode positive polarity current setting signal Ip2r, and the above current detection signal Id as inputs, performs the following processing, and outputs a current setting signal Ir and a polarity switching signal Snp. 1) During the first electrode negative polarity period Tn1 determined by the first electrode negative polarity period setting signal Tn1r, the first electrode negative polarity current setting signal In1r is output as the current setting signal Ir. During this period, a high-level polarity switching signal Snp is output. 2) Subsequently, during the second electrode negative polarity period Tn2 determined by the second electrode negative polarity period setting signal Tn2r, the second electrode negative polarity current setting signal In2r is output as the current setting signal Ir. During this period, a high-level polarity switching signal Snp is output. 3) Subsequently, a current setting signal Ir with a predetermined polarity switching current value is output and maintained until the value of the current detection signal Id drops to the polarity switching current value. During this period, a high-level polarity switching signal Snp is output. 4) Subsequently, during the first electrode positive polarity period Tp1 determined by the first electrode positive polarity period setting signal Tp1r, the first electrode positive polarity current setting signal Ip1r is output as the current setting signal Ir. During this period, a low-level polarity switching signal Snp is output. 5) Subsequently, during the second electrode positive polarity period Tp2 determined by the second electrode positive polarity period setting signal Tp2r, the second electrode positive polarity current setting signal Ip2r is output as the current setting signal Ir. During this period, a low-level polarity switching signal Snp is output. 6) Subsequently, the current setting signal Ir of the polarity switching current value is output and maintained until the value of the current detection signal Id drops to the polarity switching current value. During this period, a low-level polarity switching signal Snp is output. 7) The processes of 1) to 6) above are repeated.

[0036] The welding power source PS takes the above welding start signal On, the above current setting signal Ir, the above current detection signal Id, and the above polarity switching signal Snp as inputs. When the welding start signal On becomes high level, a high-frequency high voltage is applied between the electrode 1 and the base material 2, and when the arc 3 is generated, the output of the welding current Iw and the welding voltage Vw of the current value set by the current setting signal Ir and the power source polarity set by the polarity switching signal Snp is started, and when the welding start signal On becomes low level, the output is stopped. Although not shown in the figure, the welding power source PS is connected to a commercial AC power source such as three-phase 200V, and includes a primary rectification circuit that rectifies the commercial AC power to DC, a capacitor that smoothes the rectified DC, a primary side inverter circuit that converts the smoothed DC to high-frequency AC, a high-frequency transformer that steps down the high-frequency AC to a voltage value suitable for arc welding, a secondary rectification circuit that rectifies the stepped-down high-frequency AC to DC, a reactor that smoothes the rectified DC, a secondary side inverter circuit that switches the smoothed DC to the electrode negative polarity EN or the electrode positive polarity EP according to the polarity switching signal Snp, a modulation circuit that performs pulse width modulation control so that the current setting signal Ir and the current detection signal Id become equal, and a drive circuit that drives the primary side inverter circuit based on the output of the modulation circuit.

[0037] Figure 2 is a current / voltage waveform diagram in the double shielded tig welding method according to Embodiment 1 of the present invention. In the figure, (A) shows the time change of the welding current Iw, (B) shows the time change of the welding voltage Vw, and (C) shows the time change of the polarity switching signal Snp. Hereinafter, the operation during the steady welding period will be described with reference to this figure.

[0038] The welding current Iw shown in FIG. (A) and the welding voltage Vw shown in FIG. (B) show waveforms with the upper side being the electrode negative polarity EN and the lower side being the electrode positive polarity EP, where values greater than 0 are considered. In the following, the magnitudes of the values of the welding current Iw and the welding voltage Vw are described in terms of their absolute values regardless of the electrode negative polarity EN and the electrode positive polarity EP.

[0039] Inner gas and outer gas (not shown) are ejected into the arc generation part. The flow rate FI of the inner gas is a value calculated by inputting the second electrode negative polarity current In2 into the above-mentioned formula (1). The flow rate Fo of the outer gas is a value calculated by inputting the second electrode negative polarity current In2 into the above-mentioned formula (2).

[0040] (1) Explanation of the operation during the electrode negative polarity period Ten Immediately before time t1, as shown in FIG. (A), the welding current Iw decreases from the negative-valued second electrode positive polarity current Ip2 to a predetermined polarity switching current value with a negative value. At time t1, when the welding current Iw becomes equal to the polarity switching current value as shown in FIG. (A), as shown in FIG. (C), the polarity switching signal Snp changes from the Low level to the High level and shifts to the electrode negative polarity period Ten. In response to this, as shown in FIG. (A), the welding current Iw sharply changes from the negative-valued polarity switching current value to the positive-valued predetermined first electrode negative polarity current value In1. As shown in FIG. (B), the welding voltage Vw has a waveform similar to the current waveform and changes from a negative voltage value to a positive voltage value.

[0041] During the first electrode negative polarity period Tn1 from time t1 to t2, as shown in Fig. (A), the welding current Iw becomes the first electrode negative polarity current value In1. As shown in Fig. (B), the welding voltage Vw fluctuates during this period, and the fluctuation converges immediately before time t2. This fluctuation of the welding voltage Vw is because the arc generation state at the time of polarity switching is in a transient state. In the double shielded TIG welding method in which inner gas and outer gas are flowed, compared with the normal TIG welding method in which only shielding gas is flowed, turbulent flow is likely to occur due to the difference in the flow rates of both gases. When the arc generation state is in a transient state, the fluctuation is large, so turbulent flow is likely to occur. Therefore, by setting the first electrode negative polarity current value In1 to a small value, the fluctuation of the arc generation state is suppressed, and the generation of turbulent flow is prevented. As a result, it is possible to suppress the occurrence of blowholes due to the incomplete shielding state of the arc caused by the generation of turbulent flow. Therefore, the first electrode negative polarity period Tn1 is set to the period until the fluctuation of the welding voltage Vw converges during this period. For example, the absolute value of the polarity switching current value is set to 50 A. The reason for decreasing the welding current Iw to the polarity switching current value and switching the polarity is to prevent the secondary side inverter circuit in the welding power source PS in Fig. 1 from malfunctioning due to the surge voltage at the time of switching.

[0042] During the predetermined second electrode negative polarity period Tn2 from time t2 to t3, as shown in Fig. (A), the welding current Iw increases to the second electrode negative polarity current value In2. As shown in Fig. (B), the welding voltage Vw becomes a larger value than during the first electrode negative polarity period Tn1. Since the melting of the base material is promoted during this period, it becomes the main period of welding.

[0043] At time t3, when the second electrode negative polarity period Tn2 ends, as shown in Fig. (A), the welding current Iw decreases with a slope and becomes the polarity switching current value at time t4. The slope is determined by the inductance value of the current path of the welding current Iw. As shown in Fig. (B), the welding voltage Vw also decreases.

[0044] (2) Explanation of the operation during the electrode positive polarity period Tep At time t4, as shown in Fig. (A), when the welding current Iw becomes equal to the polarity switching current value, as shown in Fig. (C), the polarity switching signal Snp changes to the Low level and transitions to the electrode positive polarity period Tep. In response to this, as shown in Fig. (A), the welding current Iw sharply changes from the positive polarity switching current value to the predetermined first electrode positive polarity current value Ip1 of a negative value. As shown in Fig. (B), the welding voltage Vw has a waveform similar to the current waveform and changes from a positive voltage value to a negative voltage value.

[0045] During the first electrode positive polarity period Tp1 from time t4 to t5, as shown in Fig. (A), the welding current Iw becomes the first electrode positive polarity current value Ip1. As shown in Fig. (B), the welding voltage Vw fluctuates during this period, and the fluctuation converges immediately before time t5. This fluctuation of the welding voltage Vw is because the formation state of the cathode spots formed to obtain the oxide film is in a transient state. In the double shield TIG welding method in which inner gas and outer gas are flowed, compared with the normal TIG welding method in which only shield gas is flowed, turbulent flow is likely to occur due to the difference in the flow rates of both gases. When the formation state of the cathode spots is in a transient state, the fluctuation is large, so turbulent flow is likely to occur. The generation state of this turbulent flow becomes more intense when switching to the electrode positive polarity EP than when switching the above-described polarity to the electrode negative polarity EN. Therefore, by making the first electrode positive polarity current value Ip1 a small value, the fluctuation of the arc generation state is suppressed and the generation of turbulent flow is prevented. As a result, it is possible to suppress the occurrence of blowholes due to the incomplete shielding state of the arc caused by the generation of turbulent flow. Therefore, the first electrode negative polarity period Tp1 is set to the period until the fluctuation of the welding voltage Vw converges during this period.

[0046] During the predetermined second electrode positive polarity period Tp2 from time t5 to t6, as shown in Fig. (A), the welding current Iw increases to the second electrode positive polarity current value Ip2. As shown in Fig. (B), the welding voltage Vw becomes a larger value than during the first electrode positive polarity period Tp1. The oxide film is mainly removed by the cleaning action during this period.

[0047] At time t6, when the second electrode positive period Tp2 ends, as shown in Fig. (A), the welding current Iw decreases with a slope and becomes the polarity switching current value at time t7. The slope is determined by the inductance value of the current path of the welding current Iw. As shown in Fig. (B), the welding voltage Vw also decreases. Thereafter, the operation returns to that at time t1.

[0048] Although the figure shows the case of the balanced waveform where In2 = Ip2, there may also be a case of an unbalanced waveform where In2 < Ip2. Also, although the figure shows the case where the current waveform is a substantially rectangular wave, there may also be a case of a sine wave.

[0049] Numerical examples of each parameter are shown below. In1 = 60A, In2 = 150A Ten = 10ms, Tn1 = 0.5ms Ip1 = 60A, Ip2 = 150A Tep = 3ms, Tp1 = 0.3ms

[0050] Fig. 3 is a timing chart of each signal at the start of welding and during the steady welding period in the welding apparatus of Fig. 1 in the double - shielded TIG welding method according to Embodiment 1 of the present invention. Fig. (A) shows the time change of the welding start signal On, Fig. (B) shows the time change of the pre - flow period signal Stp, Fig. (C) shows the time change of the inner gas ejection start signal Si, Fig. (D) shows the time change of the outer gas flow rate Fo (l / min), Fig. (E) shows the time change of the inner gas flow rate Fi (l / min), Fig. (F) shows the time change of the average value of the absolute value of the welding current Iw shown in Fig. 2, i.e., the average welding current value Iav, and Fig. (G) shows the time change of the arc generation discrimination signal Ad. Hereinafter, with reference to this figure, the operations of each signal will be described.

[0051] At time t1, when the welder turns on the torch switch provided on the welding torch WT in Fig. 1, as shown in Fig. (A) of the same figure, the welding start signal On changes to the High level. In response to this, as shown in Fig. (B) of the same figure, the preflow period signal Stp becomes the High level. At the same time, the outer gas ejection is started by the outer gas flow regulator CO in Fig. 1. As shown in Fig. (D) of the same figure, the outer gas flow Fo becomes a predetermined preflow outer gas flow value determined by the outer gas flow setting signal For in Fig. 1. Argon is used as the outer gas.

[0052] At time t2 when a predetermined delay time Td has elapsed from time t1, as shown in Fig. (C) of the same figure, the inner gas ejection start signal Si becomes the High level. In response to this, the inner gas ejection is started by the inner gas flow regulator CI in Fig. 1. As shown in Fig. (E) of the same figure, the inner gas flow Fi becomes a value determined by the preflow inner gas flow setting signal Fipr in Fig. 1. Helium is used as the inner gas.

[0053] During the preflow period, the inner gas is ejected after the outer gas is ejected. By doing so, since the inner gas ejection is started in a state where the surroundings are shielded by the outer gas ejection, the inner gas does not entrain the surrounding air, and it is possible to converge to the steady state earlier. For this reason, in the present embodiment, compared with the prior art in which the outer gas and the inner gas are ejected simultaneously, the preflow time can be shortened to about 50%. Therefore, in the present embodiment, even if the preflow time at the start of welding is set short, sufficient shielding performance can be ensured, so that the working efficiency can be increased and the consumption of expensive inert gas can be reduced. For example, in the prior art, it was necessary to set the preflow period to about 6 seconds, but in the present embodiment, it can be set to about 3 seconds.

[0054] At time t3, as shown in Fig. (B), when the preflow period signal Stp changes to the Low level, the welding power source PS in Fig. 1 applies a high-frequency high voltage between the electrode 1 and the work 2 in Fig. 1 to generate the arc 3 in Fig. 1, and as shown in Fig. (F), the energization of the average welding current value Iav is started. The waveforms of the welding current Iw and the welding voltage Vw are the same as those in Fig. 2 described above. At time t3, when the generation of the arc is determined by the energization of the welding current Iw, as shown in Fig. (G), the arc generation determination signal Ad changes to the High level. At the same time, as shown in Fig. (D), the outer gas flow rate Fo becomes a value determined by the steady outer gas flow rate setting signal Focr in Fig. 1. Then, welding is started from time t3. The value of the steady outer gas flow rate setting signal Focr is a value calculated by substituting the value of the second electrode negative polarity current setting signal In2r in Fig. 1 into the above-mentioned formula (2).

[0055] As shown in Fig. (G), when the arc generation determination signal Ad changes to the High level, it shifts to a predetermined initial period from time t3 to t4. As shown in Fig. (E), the inner gas flow rate Fi becomes a value determined by the initial inner gas flow rate setting signal Fiir in Fig. 1. This initial inner gas flow rate setting signal Fiir is a value calculated by substituting the value of the steady outer gas flow rate setting signal Focr in Fig. 1 into the above-mentioned formula (3). That is, the inner gas flow rate Fi during the initial period is set so that the flow velocity of the inner gas is within ±20% of the flow velocity of the outer gas. Since the flow velocity of the inner gas during the steady welding period is about 1.7 to 2.0 times the flow velocity of the outer gas, the inner gas flow rate Fi during the initial period is decreased to slow down the flow velocity. For example, the initial period is set to 500 ms.

[0056] When the initial period ends at time t4, the process shifts to the steady welding period. As shown in Fig. (E) of the same figure, the inner gas flow rate Fi increases over time to a value determined by the steady inner gas flow rate setting signal Ficr in Fig. 1. The value of this steady inner gas flow rate setting signal Ficr is a value calculated by substituting the value of the second electrode minus-polarity current setting signal In2r in Fig. 1 into the above-mentioned equation (1). As described above, during the steady welding period, the flow velocity of the inner gas is about 1.7 to 2.0 times higher than that of the outer gas. By ejecting high-speed inner gas, the rigidity of the arc can be increased, so that the penetration can be deepened and the welding speed can also be increased. For example, the period during which the inner gas flow rate Fi increases over time is set to 200 ms.

[0057] [Embodiment 2] In Embodiment 2, it is different from Embodiment 1 in that the value of the inner gas flow rate is maintained as it is even after the end of the initial period.

[0058] The block diagram of the welding apparatus for implementing the double-shielded TIG welding method according to Embodiment 2 is the same as Fig. 1 described above. However, the operation of the steady inner gas flow rate setting circuit FICR is as follows and is different from Embodiment 1. The steady inner gas flow rate setting circuit FICR outputs the value of the above-mentioned initial inner gas flow rate setting signal Fiir as the steady inner gas flow rate setting signal Ficr as it is. Furthermore, the current-voltage waveform in the double-shielded TIG welding method according to Embodiment 2 is the same as Fig. 2 described above.

[0059] Figure 4 is a timing chart of each signal at the start of welding and during the steady welding period in the welding apparatus of FIG. 1 in the double shield tig welding method according to Embodiment 2 of the present invention. FIG. (A) shows the time change of the welding start signal On, FIG. (B) shows the time change of the preflow period signal Stp, FIG. (C) shows the time change of the inner gas ejection start signal Si, FIG. (D) shows the time change of the outer gas flow rate Fo (l / min), FIG. (E) shows the time change of the inner gas flow rate Fi (l / min), FIG. (F) shows the time change of the welding current average value Iav which is the average value of the absolute value of the welding current Iw shown in FIG. 2, and FIG. (G) shows the time change of the arc generation discrimination signal Ad. Hereinafter, with reference to the figure, the operation of each signal will be described.

[0060] The operation during the period from time t1 to t4 is the same as that in FIG. 3, and thus the description thereof will not be repeated.

[0061] When the initial period ends at time t4, the process proceeds to the steady welding period. As shown in FIG. (E), the inner gas flow rate Fi becomes a value determined by the steady inner gas flow rate setting signal Ficr in FIG. 1. In Embodiment 2, the value of the steady inner gas flow rate setting signal Ficr is equal to the value of the initial inner gas flow rate setting signal Fiir. Therefore, the value of the inner gas flow rate Fi during the steady welding period after time t4 maintains the same value even after the end of the initial period.

[0062] Hereinafter, the effects of the present embodiment will be described. According to this embodiment, helium is used as the inner gas and argon is used as the outer gas, and the flow rate of the inner gas is set such that the flow velocity of the inner gas is within ±20% of the flow velocity of the outer gas during the initial period from the time when the arc is generated. In the prior art, after the arc is generated, the flow rate of the inner gas is set so that the flow velocity of the inner gas becomes about 1.7 to 2.0 times higher than the flow velocity of the outer gas. As a result, the high-speed ejection of the inner gas can increase the stiffness of the arc, deepen the penetration, and increase the welding speed. However, in the double-shielded TIG welding method, there is a problem that blowholes are likely to occur due to the disturbance of the gas flow when the arc generation state is in a transient state at the start of welding. This is because when the arc generation state is in a transient state, the flow velocities of the inner gas and the outer gas are greatly different, resulting in the generation of turbulent flow and incomplete shielding of the arc. In this embodiment, during the initial period after the arc is generated, the flow rate of the inner gas is set so that the flow velocity of the inner gas is within ±20% of the flow velocity of the outer gas, thereby suppressing the generation of turbulent flow. As a result, in this embodiment, the occurrence of blowholes at the welding start portion can be suppressed. Further, in this embodiment, helium is used as the inner gas. Therefore, even if the flow velocity of the inner gas during the initial period is reduced, the stiffness of the arc can be increased compared to when argon is used. As a result, good welding quality can be obtained during the initial period as well as during the steady welding period.

[0063] More preferably, according to this embodiment, the value of the flow rate of the inner gas is maintained as it is even after the end of the initial period. By doing so, even if the arc generation state becomes temporarily unstable during the steady welding period, the difference in the flow velocities of the two gases is small, so the generation of turbulent flow can be suppressed and the occurrence of blowholes can be suppressed. In this embodiment, helium is used as the inner gas. Therefore, even if the flow velocity of the inner gas during the steady welding period after the end of the initial period is reduced, the stiffness of the arc can be increased compared to when argon is used. As a result, the welding quality during the steady welding period can be improved.

[0064] More preferably, according to the present embodiment, the initial period is set to a value at which blowholes do not occur at the welding start portion. By doing so, the initial period can be set to the minimum necessary value, so that the above-described characteristics of the double-shielded TIG welding method can be exhibited at an early stage.

[0065] More preferably, according to the present embodiment, the welding current is formed from the electrode negative polarity current during the electrode negative polarity period and the electrode positive polarity current during the electrode positive polarity period. The electrode positive polarity current is formed from the first electrode positive polarity current during the first electrode positive polarity period and the second electrode positive polarity current during the second electrode positive polarity period. The value of the first electrode positive polarity current is 20% to 60% of the peak value of the second electrode positive polarity current. More preferably, the value of the first electrode positive polarity current is 30% to 50% of the peak value of the second electrode positive polarity current. When switching to the electrode positive polarity, the formation state of the cathode spots formed for obtaining the oxide film becomes a transient state. In the double-shielded TIG welding method in which the inner gas and the outer gas flow, turbulent flow is likely to occur due to the difference in the flow rates of both gases compared to the normal TIG welding method in which only the shielding gas flows. When the formation state of the cathode spots is in a transient state, the arc generation state fluctuates greatly, so turbulent flow is likely to occur. The generation state of this turbulent flow is more intense when switching to the electrode positive polarity than when switching to the electrode negative polarity. Therefore, by making at least the first electrode positive polarity current value a small value, the fluctuation of the arc generation state is suppressed and the generation of turbulent flow is prevented. As a result, it is possible to suppress the incomplete shielding state of the arc due to the generation of turbulent flow and the occurrence of blowholes. The value of the second electrode positive polarity current is set as the peak value in order to include not only the rectangular wave but also the case of a sine wave whose value changes.

Explanation of reference numerals

[0066] 1 Electrode 2 Base material 3 Arc 4 Inner nozzle 5 Outer nozzle 6 Inner gas cylinder 7 Inner gas 8 Outer gas cylinder 9 Outer gas AD Arc generation discrimination circuit Ad Arc generation discrimination signal CI Inner gas flow regulator CO Outer gas flow regulator EN Electrode negative polarity EP Electrode positive polarity Fi Inner gas flow FICR Steady inner gas flow setting circuit Ficr Steady inner gas flow setting signal FIIR Initial inner gas flow setting circuit Fiir Initial inner gas flow setting signal FIPR Preflow inner gas flow setting circuit FiPr Preflow inner gas flow setting signal FIR Inner gas flow setting circuit Fir Inner gas flow setting signal Fo Outer gas flow FOCR Steady outer gas flow setting circuit Focr Steady outer gas flow setting signal FOR Outer gas flow setting circuit For Outer gas flow setting signal Iav Welding current average value ID Current detection circuit Id Current detection signal In1 First electrode negative polarity current IN1R First electrode negative polarity current setting circuit In1r First electrode negative polarity current setting signal In2 Second electrode negative polarity current IN2R Second electrode negative polarity current setting circuit In2r Second electrode negative polarity current setting signal Ip1 First electrode positive polarity current IP1R First electrode positive polarity current setting circuit Ip1r First electrode plus-polarity current setting signal Ip2 Second electrode plus-polarity current IP2R Second electrode plus-polarity current setting circuit Ip2r Second electrode plus-polarity current setting signal IR Current setting circuit Ir Current setting signal Iw Welding current ON Start switch On Welding start signal PS Welding power source SI Inner gas ejection start circuit Si Inner gas ejection start signal STI Initial period circuit Sti Initial period signal STP Preflow period circuit Stp Preflow period signal Ten Electrode minus-polarity period Tep Electrode plus-polarity period Tn1 First electrode minus-polarity period TN1R First electrode minus-polarity period setting circuit Tn1r First electrode minus-polarity period setting signal Tn2 Second electrode minus-polarity period TN2R Second electrode minus-polarity period setting circuit Tn2r Second electrode minus-polarity period setting signal Tp1 First electrode plus-polarity period TP1R First electrode plus-polarity period setting circuit Tp1r First electrode plus-polarity period setting signal Tp2 Second electrode plus-polarity period TP2R Second electrode plus-polarity period setting circuit Tp2r Second electrode plus-polarity period setting signal VD Voltage detection circuit Vd Voltage detection signal WT Welding torch

Claims

1. Using a welding torch equipped with an inner nozzle for ejecting inner gas and an outer nozzle for ejecting outer gas, In a double shielded TIG welding method where, after performing a preflow of the inner gas and the outer gas at the start of welding, an arc is generated and a welding current is passed to perform welding, Helium is used for the inner gas and argon is used for the outer gas, The flow rate of the inner gas is set such that during an initial period from the time when the arc is generated, the flow velocity of the inner gas is within ±20% of the flow velocity of the outer gas, A double shielded TIG welding method characterized by this.

2. Even after the end of the initial period, the value of the flow rate of the inner gas is maintained as it is, The double shielded TIG welding method according to Claim 1, characterized by this.

3. The initial period is set to a value at which blowholes do not occur at the welding start portion, The double shielded TIG welding method according to Claim 1, characterized by this.

4. The welding current is formed from an electrode negative polarity current during the electrode negative polarity period and an electrode positive polarity current during the electrode positive polarity period, The electrode positive polarity current is formed from a first electrode positive polarity current during a first electrode positive polarity period and a second electrode positive polarity current during a second electrode positive polarity period, The value of the first electrode positive polarity current is 20% to 60% of the peak value of the second electrode positive polarity current, The double shielded TIG welding method according to any one of Claims 1 to 3, characterized by this.

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