Double-shielded TIG welding method and double-shielded TIG welding apparatus

Abstract

A double-shielded TIG welding method that suppresses grain coarsening of the weld metal and obtains good weld quality. [Solution] A double-shielded TIG welding method is used, which involves using a welding torch WT equipped with an inner nozzle 4 for ejecting inner gas 7 and an outer nozzle 5 for ejecting outer gas 9, and applying a welding current to generate an arc 3 for welding, wherein the flow rate Fi of the inner gas 7 is periodically changed during the steady-state welding period after arc generation.

Description

Technical Field

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

Background Art

[0002] A double-shielded arc welding method is commonly used, which uses a welding torch equipped with an inner nozzle for ejecting inner gas and an outer nozzle for ejecting outer gas, and generates an arc by applying a welding current for welding (see, for example, Patent Document 1). As the inner gas and the outer gas, inert gases such as argon and helium are 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, the crystal grains of the welded metal may become coarser. Such coarsening of the crystal grains leads to a deterioration of the welding quality (e.g., strength, toughness, etc.).

[0005] Therefore, an object of the present invention is to provide a double-shielded tig welding method and a double-shielded tig welding apparatus that can suppress coarsening of crystal grains of the welded metal and obtain good welding quality.

Means for Solving the Problems

[0006] A double-shielded TIG welding method provided by a first aspect of the present invention is a double-shielded TIG welding method that uses a welding torch equipped with an inner nozzle for ejecting an inner gas and an outer nozzle for ejecting an outer gas, and generates an arc by applying a welding current to perform welding, characterized in that the flow rate of the inner gas is periodically changed during the steady-state welding period after the arc is generated.

[0007] As an example, the double-shielded TIG welding method of the present invention is characterized in that the flow rate of the inner gas during the steady-state welding period includes a first flow rate and a second flow rate that are different from each other, and the first flow rate and the second flow rate are changed in a pulsed manner.

[0008] As an example, the double-shielded TIG welding method of the present invention is characterized in that the second flow rate is smaller than the first flow rate, and the first flow rate is set such that the flow velocity of the inner gas is 2.5 times or more and 5 times or less than the flow velocity of the outer gas.

[0009] As an example, the double-shielded TIG welding method of the present invention is characterized by changing the first flow rate and the second flow rate at a period in which the frequency is 0.5 Hz or more and 5 Hz or less.

[0010] A double-shielded TIG welding apparatus provided by a second aspect of the present invention is a double-shielded TIG welding apparatus that uses a welding torch equipped with an inner nozzle for ejecting inner gas and an outer nozzle for ejecting outer gas, and generates an arc by applying a welding current to perform welding, characterized in that the flow rate of the inner gas changes periodically during the steady-state welding period after arc generation. [Effects of the Invention]

[0011] According to the above configuration, for example, with regard to a double-shielded TIG welding method and a double-shielded TIG welding apparatus, it is possible to suppress the coarsening of the crystal grains of the weld metal and obtain good welding quality. [Brief explanation of the drawing]

[0012] [Figure 1] This is a block diagram of a double-shielded TIG welding apparatus for implementing a double-shielded TIG welding method according to an embodiment of the present invention. [Figure 2] This is a current and voltage waveform diagram for a double-shielded TIG welding method according to an embodiment of the present invention. [Figure 3] Figure 1 shows a timing chart of each signal in a double-shielded TIG welding method according to an embodiment of the present invention. [Modes for carrying out the invention]

[0013] Embodiments of the present invention will be described below with reference to the drawings.

[0014] Figure 1 is a block diagram of a double-shielded TIG welding apparatus for implementing a double-shielded TIG welding method according to an embodiment of the present invention. Each block will be described below with reference to this figure.

[0015] The figure shows a double-shielded TIG welding method in which an alternating current Iw is applied, formed from the negative electrode current during the negative electrode polarity period and the positive electrode current during the positive electrode polarity period. This double-shielded TIG welding method, using an alternating current Iw, is used for welding aluminum, aluminum alloys, magnesium, and the like.

[0016] A welding torch (WT) primarily comprises an electrode 1, an inner nozzle 4 surrounding it, and an outer nozzle 5 surrounding the inner nozzle 4. A tungsten electrode or the like is used for 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. However, the inner diameters of the inner nozzle 4 and outer nozzle 5 are not limited to these.

[0017] The torch switch circuit ON is a torch switch mounted on the welding torch WT. When the welder turns it on, it outputs a torch switch signal On that becomes High level, and when turned off, it becomes Low level.

[0018] 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. When the value of the current detection signal Id is equal to or greater than the energization discrimination value (about 5A), it discriminates that the arc 3 is generated and outputs an arc generation discrimination signal Ad that becomes High level.

[0019] The period discrimination circuit TP takes the above torch switch signal On and the above arc generation discrimination signal Ad as inputs, performs the following processing, and outputs a period discrimination signal Tp. When welding is finished, the period discrimination signal Tp = 0. 1) When the torch switch signal On changes to High level, it outputs a period discrimination signal Tp = 1 (preflow period start). 2) At the time when a predetermined delay time Td has elapsed since the time when Tp changed to 1, it outputs a period discrimination signal Tp = 2 (inner gas ejection start). 3) When a predetermined preflow period has elapsed since the time when Tp changed to 1, and the arc generation discrimination signal Ad changes to High level, it outputs a period discrimination signal Tp = 3 (initial period start). 4) At the time when a predetermined initial period has elapsed since the time when Tp changed to 3, it outputs Tp = 4 (steady welding period start). 5) Then, when the torch switch signal On changes to Low level and then changes to High level again, it outputs Tp = 5 (crater treatment period start). 6) Then, when the torch switch signal On changes to Low level again, it outputs a period discrimination signal Tp = 6 (afterflow period start). 7) When a predetermined afterflow period has elapsed, it outputs a period discrimination signal Tp = 0 (welding end state).

[0020] The first electrode minus-polarity current setting circuit INIR outputs a first electrode minus-polarity current setting signal In1r with a predetermined positive value. The second electrode minus-polarity current setting circuit IN2R outputs a second electrode minus-polarity current setting signal In2r with a predetermined positive value. Here, In1r < In2r.

[0021] The first electrode plus-polarity current setting circuit IP1R outputs a first electrode plus-polarity current setting signal Ip1r with a predetermined positive value. The second electrode plus-polarity current setting circuit IP2R outputs a second electrode plus-polarity current setting signal Ip2r with a predetermined positive value. Here, Ip1r < Ip2r.

[0022] The steady-state inner gas flow rate setting circuit FICR outputs a steady-state inner gas flow rate setting signal Ficr [L / min] that periodically changes the flow rate Fi of the inner gas 7. The steady-state inner gas flow rate setting signal Ficr [L / min] is, for example, a pulsed signal in which a level corresponding to the first flow rate and a level corresponding to the second flow rate alternate at a predetermined frequency. The second flow rate is a smaller value than the first flow rate. For example, the second flow rate may be a value obtained by calculating a predetermined percentage of the first flow rate (for example, about 20-40%), or it may be a value obtained by subtracting a predetermined value (for example, 3-6 L / min) from the first flow rate. The first and second flow rates are not limited in any way, but for example, the first flow rate may be 8 L / min and the second flow rate may be 3 L / min. The first flow rate may be set so that the flow velocity of the inner gas 7 is about 2.5 to 5 times (preferably about 4 times) faster than the flow velocity of the outer gas 9. Furthermore, the predetermined frequency is not limited in any way, but is, for example, 0.5Hz to 5Hz (period of 0.2s to 2s), and preferably 1Hz to 3Hz. The steady-state inner gas flow rate setting signal Ficr [L / min] is, for example, a square wave, but may also be a sine wave or a triangular wave. In the example where the steady-state inner gas flow rate setting signal Ficr [L / min] is a square wave, the ratio of the level corresponding to the first flow rate in one period, i.e., the duty cycle, is, for example, 50% (i.e., the length of the level corresponding to the first flow rate and the length of the level corresponding to the second flow rate in one period are 1:1), but is not limited to this. The steady-state inner gas flow rate setting circuit FICR may output a steady-state inner gas flow rate setting signal Ficr [L / min] in which three or more levels, each individually corresponding to three or more flow rates, are sequentially switched at a period of the predetermined frequency described above.

[0023] Unlike this example, the steady-state inner gas flow rate setting circuit FICR may generate (output) the steady-state inner gas flow rate setting signal Ficr[L / min] as follows. For example, the steady-state inner gas flow rate setting circuit FICR takes the above-mentioned second electrode negative polarity current setting signal In2r as input and calculates the value obtained by inputting the second electrode negative polarity current setting signal In2r[A] into the following predetermined steady-state inner gas flow rate setting function as the first flow rate. An example of a steady-state inner gas flow rate setting function is shown below. Ficr=(In2r-75) / 50+3.5 ···(1) formula However, this is within the range of 75 ≤ In2r ≤ 150, and if In2r < 75, it is the same value as In2r = 75, and if In2r > 150, it is the same value as In2r = 150. As a result, the larger the value of the second electrode negative polarity current setting signal In2r, the larger the value of the first flow rate. The steady-state inner gas flow rate setting circuit FICR calculates the first flow rate, and then calculates the second flow rate (< first flow rate). The second flow rate is calculated from the first flow rate, and for example, the second flow rate may be calculated as a value obtained by finding a predetermined percentage of the first flow rate (for example, around 20% to 40%), or the second flow rate may be calculated as a value obtained by subtracting a predetermined value (for example, 3 to 6 L / min) from the first flow rate. The steady-state inner gas flow rate setting circuit FICR, once it has determined (calculated) the first and second flow rates, generates and outputs a predetermined pulse-like steady-state inner gas flow rate setting signal Ficr [L / min] (in this embodiment, a square wave with a frequency of 0.5 Hz to 5 Hz and a duty cycle of 50%) in which the level corresponding to the first flow rate and the level corresponding to the second flow rate alternately. In this example, however, the method for calculating the first and second flow rates by the steady-state inner gas flow rate setting circuit FICR is not limited to the example described above. For example, the steady-state inner gas flow rate setting circuit FICR may calculate the value obtained by calculating equation (1) above as the second flow rate and determine the first flow rate from the two calculated flow rates, or it may determine the first and second flow rates such that the value obtained by calculating equation (1) above is an intermediate value between the first and second flow rates. In these cases as well, it is sufficient that the relative value of the second flow rate to the first flow rate is a predetermined ratio or predetermined value. Alternatively, the second flow rate, or an intermediate value between the first and second flow rates, may be set so that the flow velocity of the inner gas 7 is approximately 2.5 to 5 times faster than the flow velocity of the outer gas 9.

[0024] The steady-state outer gas flow rate setting circuit FOCR outputs a predetermined steady-state outer gas flow rate setting signal Focr. For example, the steady-state outer gas flow rate setting signal Focr is set to 8 L / min. In contrast to this example, the steady-state outer gas flow rate setting circuit FOCR may also take the above-mentioned second electrode negative polarity current setting signal In2r as input, input the second electrode negative polarity current setting signal In2r[A] into the following predetermined steady-state outer gas flow rate setting function, and output the calculated value as the steady-state outer gas flow rate setting signal Focr[L / min]. An example of a steady-state outer gas setting function is shown below. Focr=(In2r-75) / 50+5.5 ···(2) formula However, this is within the range of 75 ≤ In2r ≤ 150, and if In2r < 75, it is the same value as In2r = 75, and if In2r > 150, it is the same value as In2r = 150. As a result, the value of the steady-state outer gas flow rate setting signal Focr increases as the value of the second electrode negative polarity current setting signal In2r increases.

[0025] The flow velocity reduction inner gas flow rate setting circuit FIIR takes the above steady-state outer gas flow rate setting signal Focr as input, performs the following calculation, and outputs the flow velocity reduction inner gas flow rate setting signal Fiir [L / min]. The calculation determines the inner gas flow rate Fi such that the flow velocity of inner gas 7 during the initial period is within ±20% of the flow velocity of outer gas 9. Fiir = Focr × R × K ... (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 between 0.8 and 1.2. The above formula will be explained with numerical examples. Assuming the inner diameter of the inner nozzle 4 is 5 mm and the inner diameter of the outer nozzle 5 is 13 mm, then (cross-sectional area of the flow path of the inner gas 7 in the inner nozzle 4) = 3.14 × 2.5 × 2.5, and (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 = 0.17. Therefore, in the example where the steady outer gas flow rate setting signal Focr is 8 L / min, when K = 0.8, the flow rate reduction inner gas flow rate setting signal Fiir is a value corresponding to 1.088 L / min (= 8 × 0.17 × 0.8), and when K = 1.2, the flow rate reduction inner gas flow rate setting signal Fiir is a value corresponding to 1.632 L / min (= 8 × 0.17 × 1.2). Note that the flow rate reduction inner gas flow rate setting signal Fiir is not limited to the configuration calculated in the above example and may be a preset value.

[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 is also possible that Fipr = Fiir.

[0027] The inner gas flow rate setting circuit FIR takes the above period discrimination signal Tp, the above preflow inner gas flow rate setting signal Fipr, the above flow rate reduction inner gas flow rate setting signal Fiir, and the above steady inner gas flow rate setting signal Ficr as inputs, performs the following processing, and outputs an inner gas flow rate setting signal Fir. 1) When the period discrimination signal Tp = 2 (preflow period), 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 period discrimination signal Tp = 3 (initial period), the value of the flow rate reduction inner gas flow rate setting signal Fiir is output as the inner gas flow rate setting signal Fir. 3) When the period discrimination signal Tp=4 (steady-state welding period), the value of the steady-state inner gas flow rate setting signal Ficr increases over time (either a level corresponding to the first flow rate or a level corresponding to the second flow rate), and then the value of the steady-state inner gas flow rate setting signal Ficr is output as the inner gas flow rate setting signal Fir. Here, it is also possible to switch to the value of the steady-state inner gas flow rate setting signal Ficr (either a level corresponding to the first flow rate or a level corresponding to the second flow rate) when the initial period ends. 4) When the period discrimination signal Tp=5 (crater processing period), the value of the flow velocity reduction inner gas flow rate setting signal Fiir is output as the inner gas flow rate setting signal Fir. 5) When the period discrimination signal Tp=6 (after-flow period), the value of the flow velocity reduction inner gas flow rate setting signal Fiir is output as the inner gas flow rate setting signal Fir.

[0028] The inner gas flow regulator CI is a known mass flow controller that takes the above-mentioned period discrimination signal Tp and the above-mentioned inner gas flow rate setting signal Fir as inputs and adjusts the flow rate Fi of the inner gas 7 from the inner gas cylinder 6 to a value determined by the inner gas flow rate setting signal Fir during the period discrimination signal Tp = 2 to 6, and then ejects it. Therefore, the ejection of the inner gas 7 begins when the torch switch is turned ON and the pre-flow period begins, after a delay time Td has elapsed, and the ejection stops when the after-flow period ends.

[0029] The outer gas flow rate setting circuit FOR takes the above-mentioned period discrimination signal Tp and the above-mentioned steady-state outer gas flow rate setting signal Focr as inputs and outputs an outer gas flow rate setting signal For which the pre-flow outer gas flow rate value is predetermined when the period discrimination signal Tp = 1 to 2 (pre-flow period), and the value of the steady-state outer gas flow rate setting signal Focr is obtained when the period discrimination signal Tp = 3 to 6. Here, it is desirable that the pre-flow outer gas flow rate value be set to a value greater than the value of the steady-state outer gas flow rate setting signal Focr. As a result, the outer gas flow rate Fo is the pre-flow outer gas flow rate value during the pre-flow period, and the value of the steady-state outer gas flow rate setting signal Focr is obtained during the initial period, steady-state welding period, crater processing period, and after-flow period.

[0030] The outer gas flow regulator CO is a known mass flow controller that takes the above-mentioned period determination signal Tp and the above-mentioned outer gas flow rate setting signal For as inputs, and adjusts the flow rate Fo of the outer gas 9 from the outer gas cylinder 8 to a value determined by the outer gas flow rate setting signal For during the period determination signal Tp = 1 to 6, and then ejects it. Therefore, the outer gas 9 is ejected during the pre-flow period, initial period, steady-state welding period, crater processing period, and after-flow period.

[0031] Inner gas 7 flows through the passage inside inner nozzle 4. Outer gas 9 flows through the passage outside inner nozzle 4 and inside outer nozzle 5. Inert gases such as argon and helium are used for inner gas 7 and 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 input, measures the period until the fluctuation 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-mentioned voltage detection signal Vd as input, measures the period until the fluctuation 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-mentioned first electrode negative polarity period setting signal Tn1r, second electrode negative polarity period setting signal Tn2r, first electrode positive polarity period setting signal Tp1r, second electrode positive polarity period setting signal Tp2r, first electrode negative polarity current setting signal In1r, second electrode negative polarity current setting signal In2r, first electrode positive polarity current setting signal Ip1r, second electrode positive polarity current setting signal Ip2r, and current detection signal Id as input, 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, which is 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, which is 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) Next, 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, which is 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, which is 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) Next, a current setting signal Ir for 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) Repeat steps 1) to 6) above.

[0036] The welding power supply PS takes the above-mentioned period determination signal Tp, current setting signal Ir, current detection signal Id, and polarity switching signal Snp as inputs. When the period determination signal Tp is 3 to 5 (initial period, steady welding period, and crater processing period), it applies a high-frequency high voltage between electrode 1 and base material 2. When arc 3 is generated, it outputs a welding current Iw and welding voltage Vw with the current value set by the current setting signal Ir and the power supply polarity set by the polarity switching signal Snp. When the period determination signal Tp is 6 (after-flow period), it stops outputting. The welding power supply PS, although not shown in the diagram, is connected to a commercial AC power supply such as 3-phase 200V and includes a primary rectifier circuit that rectifies the commercial AC power supply to DC, a capacitor that smooths the rectified DC, a primary 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 suitable for arc welding, a secondary rectifier circuit that rectifies the stepped-down high-frequency AC to DC, a reactor that smooths the rectified DC, a secondary inverter circuit that switches the smoothed DC to electrode negative polarity EN or electrode positive polarity EP according to a 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 are equal, and a drive circuit that drives the primary inverter circuit based on the output of the modulation circuit.

[0037] Figure 2 is a current-voltage waveform diagram in a double-shielded TIG welding method according to an embodiment of the present invention. Figure (A) shows the time variation of the welding current Iw, Figure (B) shows the time variation of the welding voltage Vw, and Figure (C) shows the time variation of the polarity switching signal Snp. The operation during the steady-state welding period will be described below with reference to the figure.

[0038] In Figure (A), the welding current Iw, and in Figure (B), the welding voltage Vw, above 0 represent the waveform for a negative electrode polarity (EN), and below 0 represent the waveform for a positive electrode polarity (EP). In the following, the magnitude of the welding current Iw and welding voltage Vw is described in terms of their absolute values, regardless of whether they represent a negative electrode polarity (EN) or a positive electrode polarity (EP).

[0039] An inner gas 7 and an outer gas 9 (not shown) are injected into the arc generation section. The flow rate Fi of the inner gas 7 is a value corresponding to the steady-state inner gas flow rate setting signal Ficr described above. The flow rate Fo of the outer gas 9 is a value corresponding to the steady-state outer gas flow rate setting signal Focr described above.

[0040] (1) Explanation of operation of the negative electrode period Ten Just before time t11, as shown in Figure (A), the welding current Iw decreases from a negative value, the second electrode positive polarity current value Ip2, to a predetermined negative polarity switching current value. At time t11, as shown in Figure (A), when the welding current Iw becomes equal to the polarity switching current value, as shown in Figure (C), the polarity switching signal Snp changes from a low level to a high level, transitioning to the electrode negative polarity period Ten. In response to this, as shown in Figure (A), the welding current Iw changes sharply from a negative value, the polarity switching current value, to a predetermined positive value, the first electrode negative polarity current value In1. As shown in Figure (B), the welding voltage Vw takes on 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 t11 to t12, as shown in Figure (A), the welding current Iw becomes the first electrode negative polarity current value In1. As shown in Figure (B), the welding voltage Vw fluctuates during this period, and the fluctuation converges just before time t12. This fluctuation in welding voltage Vw is because the arc generation state during polarity switching is in a transient state. In the double-shielded TIG welding method, which uses both inner gas 7 and outer gas 9, turbulence is more likely to occur compared to the normal TIG welding method which uses only shielding gas, due to the difference in flow velocity between the two gases. Turbulence is more likely to occur when the arc generation state is in a transient state because the fluctuations are large. Therefore, by setting the first electrode negative polarity current value In1 to a small value, fluctuations in the arc generation state are suppressed, and the generation of turbulence is prevented. As a result, it is possible to suppress the occurrence of blowholes, which can result from incomplete shielding of the arc 3 due to the generation of turbulence. 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 is set to 50A. The reason for reducing the welding current Iw to the polarity switching current value and switching the polarity is to prevent the secondary inverter circuit in the welding power supply PS in Figure 1 from failing due to the surge voltage during switching.

[0042] During the predetermined second electrode negative polarity period Tn2 from time t12 to t13, as shown in Figure (A), the welding current Iw increases to the second electrode negative polarity current value In2. As shown in Figure (B), the welding voltage Vw becomes larger than during the first electrode negative polarity period Tn1. Since the melting of the base material 2 is accelerated during this period, it becomes the main welding period.

[0043] At time t13, when the second electrode negative polarity period Tn2 ends, the welding current Iw decreases in a slope, as shown in Figure (A), and becomes the polarity switching current value at time t14. The slope is determined by the inductance value of the current path of the welding current Iw. As shown in Figure (B), the welding voltage Vw also decreases.

[0044] (2) Operation of the positive electrode period Tep At time t14, as shown in Figure (A), when the welding current Iw becomes equal to the polarity switching current value, as shown in Figure (C), the polarity switching signal Snp changes to a low level, and the electrode transitions to the positive polarity period Tep. In response to this, as shown in Figure (A), the welding current Iw changes abruptly from the positive polarity switching current value to a predetermined negative first electrode positive polarity current value Ip1. As shown in Figure (B), the welding voltage Vw takes on 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 t14 to t15, as shown in Figure (A), the welding current Iw becomes the first electrode positive polarity current value Ip1. As shown in Figure (B), the welding voltage Vw fluctuates during this period, and the fluctuation converges just before time t15. This fluctuation in welding voltage Vw is because the formation state of the cathode point, which is formed to obtain the oxide film, is in a transient state. In the double-shielded TIG welding method, which uses both inner gas 7 and outer gas 9, turbulence is more likely to occur compared to the normal TIG welding method which uses only shielding gas, due to the difference in flow velocity between the two gases. Turbulence is more likely to occur when the cathode point formation state is in a transient state because the fluctuations are large. This turbulence is more intense when switching to electrode positive polarity EP than when switching to electrode negative polarity EN. Therefore, by setting the first electrode positive polarity current value Ip1 to a small value, fluctuations in the arc generation state are suppressed, and the generation of turbulence is prevented. As a result, the occurrence of turbulence can suppress the incomplete shielding of arc 3 and the resulting blowhole formation. Therefore, the first electrode positive polarity period Tp1 is set to the period during which the fluctuations in the welding voltage Vw converge.

[0046] During the predetermined second electrode positive polarity period Tp2 from time t15 to t16, as shown in Figure (A), the welding current Iw increases to the second electrode positive polarity current value Ip2. As shown in Figure (B), the welding voltage Vw becomes greater than during the first electrode positive polarity period Tp1. The oxide film is removed mainly by the cleaning action during this period.

[0047] At time t16, when the second electrode positive polarity period Tp2 ends, as shown in Fig. (A) of the same figure, the welding current Iw decreases with a slope and becomes the polarity switching current value at time t17. The slope is determined by the inductance value of the current path of the welding current Iw. As shown in Fig. (B) of the same figure, the welding voltage Vw also decreases. After this, the operation returns to that at time t11.

[0048] The figure shows the case of the balanced waveform of In2 = Ip2, but there may also be a case of an unbalanced waveform of In2 < Ip2. Also, the figure shows the case where the current waveform is a substantially rectangular wave, but 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 = 4ms, Tp1 = 0.3ms

[0050] Fig. 3 is a timing chart of each signal in the double - shielded tig welding apparatus of Fig. 1 showing the double - shielded tig welding method according to an embodiment of the present invention. Fig. (A) of the same figure shows the time change of the torch switch signal On, Fig. (B) shows the time change of the outer gas flow rate Fo [L / min], Fig. (C) shows the time change of the inner gas flow rate Fi [L / min], Fig. (D) shows the time change of the average value of the absolute value of the welding current Iw shown in Fig. 2, which is the average welding current value Iav, and Fig. (E) shows the time change of the arc generation discrimination signal Ad. Hereinafter, the operations of each signal will be described with reference to the same figure.

[0051] [Operation during the pre - flow period] At time t1, when the welder turns on the torch switch on the welding torch WT in Figure 1, the torch switch signal On changes to a High level, as shown in Figure (A). In response, the period discrimination signal Tp in Figure 1 changes to 1, and the system transitions to the pre-flow period. Simultaneously, the outer gas flow regulator CO in Figure 1 starts ejecting the outer gas 9. As shown in Figure (B), the outer gas flow rate Fo becomes a predetermined pre-flow outer gas flow rate value determined by the outer gas flow rate setting signal For in Figure 1. It is desirable that the pre-flow outer gas flow rate value be set to a value greater than the value of the steady-state outer gas flow rate setting signal Focr in Figure 1.

[0052] At time t2, after a predetermined delay time Td has elapsed from time t1, the period discrimination signal Tp in Figure 1 changes to 2. In response to this, the inner gas flow regulator CI in Figure 1 starts ejecting the inner gas 7. As shown in Figure (C), the inner gas flow rate Fi is determined by the pre-flow inner gas flow rate setting signal Fipr in Figure 1. The value of the pre-flow inner gas flow rate setting signal Fipr is preferably smaller than the value of the steady-state inner gas flow rate setting signal Ficr in Figure 1 (for example, the second flow rate), and may be equal to the value of the velocity reduction inner gas flow rate setting signal Fiir in Figure 1.

[0053] During the pre-flow period, the inner gas 7 is ejected after the outer gas 9. In this way, the ejection of the inner gas 7 begins with the surrounding area shielded by the ejection of the outer gas 9, so the inner gas 7 does not entrain the surrounding air, and the system can converge to a steady state quickly. For this reason, in this embodiment, the pre-flow time can be shortened to about 50% compared to when the ejection of the outer gas 9 and inner gas 7 is started simultaneously. Therefore, in this embodiment, sufficient shielding can be ensured even if the pre-flow time at the start of welding is set short, so work efficiency can be increased and the consumption of expensive inert gas can be reduced. For example, in the conventional technology, the pre-flow period had to be set to about 6 seconds, but in this embodiment it can be set to about 3 seconds.

[0054] [Initial operation] At time t3, when the preflow period ends and the period discrimination signal Tp=3 in Figure 1 changes, the welding power supply PS in Figure 1 applies a high-frequency high voltage between the electrode 1 and the base material 2 in Figure 1, generating arc 3 in Figure 1, and the average welding current Iav is applied, as shown in Figure (D). The waveforms of the welding current Iw and welding voltage Vw are the waveforms shown in Figure 2 above. At time t3, when the generation of arc 3 is determined by the application of the welding current Iw, the arc generation discrimination signal Ad changes to a high level, as shown in Figure (E). At time t3, as shown in Figure (B), the outer gas flow rate Fo becomes the value determined by the steady-state outer gas flow rate setting signal Focr in Figure 1. Then, welding starts from time t3.

[0055] When the arc generation discrimination signal Ad changes to a high level, the system transitions to a predetermined initial period from time t3 to t4. As shown in Figure (C), the inner gas flow rate Fi is determined by the velocity-reducing inner gas flow rate setting signal Fiir in Figure 1. This velocity-reducing inner gas flow rate setting signal Fiir is calculated by substituting the value of the steady-state outer gas flow rate setting signal Focr in Figure 1 into equation (3) above. In other words, the inner gas flow rate Fi during the initial period is set so that the flow velocity of the inner gas 7 is within ±20% of the flow velocity of the outer gas 9. Since the flow velocity of the inner gas 7 during the steady-state welding period is about 2.5 to 5 times that of the outer gas 9, the inner gas flow rate Fi during the initial period is reduced to slow down the flow velocity. For example, the initial period is set to 500 ms.

[0056] [Operation during steady-state welding period] When the initial period ends at time t4, the period determination signal Tp=4 in Figure 1 changes, and the system transitions to the steady-state welding period. As shown in Figure 3(C), the inner gas flow rate Fi increases over time to a value determined by the steady-state inner gas flow rate setting signal Ficr in Figure 1. In this embodiment, the steady-state inner gas flow rate setting signal Ficr in Figure 1 is a pulsed signal (square wave in this embodiment) in which level Ficr1 corresponding to the first flow rate and level Ficr2 corresponding to the second flow rate switch alternately periodically (with a frequency of 0.5Hz to 5Hz in this embodiment), and the inner gas flow rate Fi increases over time to level Ficr1 corresponding to the first flow rate or level Ficr2 corresponding to the second flow rate (up to level Ficr1 corresponding to the first flow rate in the example shown in Figure 3(C)). The switching between level Ficr1 corresponding to the first flow rate and level Ficr2 corresponding to the second flow rate is set, for example, with a duty cycle of 50%. The second flow rate is a smaller value than the first flow rate, for example, a value calculated based on the first flow rate (a predetermined percentage of the first flow rate, or a predetermined value lower than the first flow rate). As described above, the flow velocity of the inner gas 7 during steady-state welding is about 2.5 to 5 times faster than the flow velocity of the outer gas 9. By ejecting the high-speed inner gas 7, the rigidity of the arc 3 can be increased, allowing for deeper penetration and faster welding speeds. For example, the period during which the inner gas flow rate Fi increases over time is set to 200 ms.

[0057] If the welder turns off the torch switch at time t41, which is in the middle of the steady welding period from time t4 to t5, the torch switch signal On changes to a Low level, as shown in Figure (A), but the steady welding period described above is maintained.

[0058] [Operation during crater processing period] When the welder turns the torch switch back on at time t5, the torch switch signal On changes to a High level, as shown in Figure (A). In response, the period discrimination signal Tp=5 in Figure 1 changes, and the crater processing period begins. As shown in Figure (B), the outer gas flow rate Fo is the same as during the steady-state welding period. As shown in Figure (C), the inner gas flow rate Fi is determined by the flow rate reduction inner gas flow rate setting signal Fiir in Figure 1. Therefore, the inner gas flow rate Fi during the crater processing period is set so that the flow rate of the inner gas 7 is within ±20% of the flow rate of the outer gas 9. Since the flow rate of the inner gas 7 during the steady-state welding period is about 2.5 to 5 times that of the outer gas 9, the inner gas flow rate Fi is reduced during the crater processing period to slow down the flow rate. As shown in Figure (D), the average welding current Iav is smaller than during the steady-state welding period.

[0059] [Operation during the after-flow period] When the welder turns the torch switch off again at time t6, the torch switch signal On changes to a Low level, as shown in Figure (A), and the period discrimination signal Tp=6 in Figure 1 changes, transitioning to the after-flow period. In response, the welding power supply PS in Figure 1 stops outputting, so the arc 3 is extinguished, and the average welding current Iav becomes 0A, as shown in Figure (D). As shown in Figure (B), the outer gas flow rate Fo is the same value as during the steady-state welding period. As shown in Figure (C), the inner gas flow rate Fi is a value determined by the flow rate reduction inner gas flow rate setting signal Fiir in Figure 1. Therefore, the inner gas flow rate Fi during the after-flow period is set so that the flow rate of the inner gas 7 is within ±20% of the flow rate of the outer gas 9. Since the flow rate of the inner gas 7 during the steady-state welding period is about 2.5 to 5 times that of the flow rate of the outer gas 9, the inner gas flow rate Fi during the after-flow period is reduced to slow down the flow rate.

[0060] [Welding complete] At time t7, when the after-flow period ends, the period discrimination signal Tp in Figure 1 changes to 0. In response, as shown in Figure (B), the outer gas flow rate Fo becomes 0 and ejection stops. Also, as shown in Figure (C), the inner gas flow rate Fi becomes 0 and ejection stops. This completes the welding process.

[0061] In the embodiments described above, a case where a crater treatment period is provided after the steady-state welding period was explained, but the crater treatment period may be omitted. Also, in the embodiments described above, the case where the welding current Iw is AC was explained, but it can also be applied to DC current and pulsed current.

[0062] The effects of this embodiment will now be described. According to this embodiment, in a double-shielded TIG welding method in which a welding torch WT equipped with an inner nozzle 4 for ejecting inner gas 7 and an outer nozzle 5 for ejecting outer gas 9 is used, and a welding current Iw is applied to generate an arc 3 for welding, the flow rate Fi of the inner gas 7 is periodically changed during the steady-state welding period after the generation of the arc 3. In conventional double-shielded TIG welding methods, the crystal grains of the weld metal may become coarse. If the flow rate Fi of the inner gas 7 is periodically changed during the steady-state welding period, the pressure of the arc 3 will fluctuate. For this reason, in this embodiment, convection can be generated in the molten pool by varying the pressure of the arc 3, thereby stirring the molten pool. This makes it possible to refine the crystal grains of the molten metal. In other words, in this embodiment, the coarsening of the crystal grains of the molten metal can be suppressed, and good welding quality can be obtained.

[0063] More preferably, according to this embodiment, the flow rate Fi of the inner gas 7 during the steady-state welding period includes a first flow rate and a second flow rate that are different from each other, and the first flow rate and the second flow rate are varied in a pulsed manner. In this embodiment, the pressure of the arc 3 can be varied, so that convection is generated in the molten pool and the molten pool can be stirred.

[0064] More preferably, according to this embodiment, the second flow rate is smaller than the first flow rate, and the first flow rate is set so that the flow velocity of the inner gas 7 is 2.5 times or more and 5 times or less than the flow velocity of the outer gas 9. In this embodiment, the rigidity of the arc 3 can be increased, ensuring an appropriate penetration depth in the molten pool, while also allowing for variations in the pressure of the arc 3 (i.e., stirring the molten pool). In particular, setting the first flow rate of the inner gas 7 during the steady-state welding period so that the flow velocity of the inner gas 7 is 4 times that of the outer gas 9 is preferable for increasing the rigidity of the arc 3 while also allowing for variations in the pressure of the arc 3.

[0065] More preferably, according to this embodiment, the first flow rate and the second flow rate are changed with a period of frequency between 0.5 Hz and 5 Hz. If the switching period between the first and second flow rates is too long (the switching frequency is too low), it may not be possible to create appropriate variations in the pressure of the arc 3, and there is a risk that appropriate convection will not occur in the molten pool (the molten pool will not be stirred). In contrast, in this embodiment, by changing the flow rate Fi of the inner gas 7 during the steady-state welding period with a frequency of 0.5 Hz or higher (a period of 2 seconds or less), appropriate convection can be created in the molten pool, and the stirring effect of the molten pool can be effectively obtained. On the other hand, if the switching period between the first and second flow rates is too short (the switching frequency is too high), the first and second flow rates will not switch properly, and there is a risk that appropriate variations in the pressure of the arc 3 will not be possible. For example, when switching the inner gas flow rate Fi from the first flow rate to the second flow rate, there is a risk that the steady-state inner gas flow rate setting signal Ficr may switch from level Ficr2 corresponding to the second flow rate to level Ficr1 corresponding to the first flow rate before the inner gas flow rate Fi reaches the second flow rate. In contrast, in this embodiment, the flow rate Fi of the inner gas 7 during the steady-state welding period is changed at a frequency of 5 Hz or less (period of 0.2 sec or more), thereby ensuring an appropriate switching time for the inner gas flow rate Fi. As a result, the inner gas flow rate Fi switches appropriately between the first and second flow rates, allowing for appropriate variations in the pressure of the arc 3. In other words, by changing the flow rate Fi of the inner gas 7 during the steady-state welding period at a period of 0.5 Hz to 5 Hz, appropriate variations in the pressure of the arc 3 can be generated, resulting in appropriate convection in the molten pool (appropriate stirring of the molten pool), which can lead to refinement of the crystal grains of the weld metal. In particular, by changing the flow rate Fi of the inner gas 7 during the steady-state welding period with a period of 1Hz to 3Hz, the crystal grains of the weld metal are appropriately refined, which is preferable in suppressing coarsening of the weld metal.

[0066] Furthermore, according to this embodiment, in a double-shielded TIG welding apparatus that uses a welding torch WT equipped with an inner nozzle 4 for ejecting inner gas 7 and an outer nozzle 5 for ejecting outer gas 9, and generates an arc 3 by applying a welding current Iw for welding, the flow rate Fi of the inner gas 7 changes periodically during the steady-state welding period after the generation of the arc 3. The double-shielded TIG welding apparatus according to this embodiment provides the above-mentioned effects.

[0067] More preferably, according to this embodiment, the afterflow is set so that the flow rate Fi of the inner gas 7 is within ±20% of the flow rate of the outer gas 9. During the steady-state welding period, the flow rate Fi of the inner gas 7 is set so that the flow rate of the inner gas 7 is 2.5 to 5 times faster than the flow rate of the outer gas 9. This makes it possible to increase the rigidity of the arc 3 by ejecting the high-speed inner gas 7, deepen the penetration, and increase the welding speed. In the conventional technology, the conditions of the steady-state welding period are continued even during the afterflow period. However, during the afterflow period, the welding state becomes transient because the arc 3 has been extinguished. In the double-shielded TIG welding method, when the welding state becomes transient, a problem occurs in which the electrode tip is oxidized due to turbulence in the gas flow. This is because when the generation state of the arc 3 is transient, turbulence occurs due to the large difference in flow rates between the inner gas 7 and the outer gas 9, resulting in incomplete shielding. In this embodiment, during the after-flow period, the flow rate Fi of the inner gas 7 is set so that the flow velocity of the inner gas 7 is within ±20% of the flow velocity of the outer gas 9, thereby suppressing the generation of turbulence. As a result, in this embodiment, the problem of oxidation of the tip of the electrode 1 and subsequent deterioration of welding quality can be suppressed. To further suppress the generation of turbulence, it is more preferable to set the flow rate Fi of the inner gas 7 so that the flow velocity of the inner gas 7 and the flow velocity of the outer gas 9 are equal.

[0068] More preferably, according to this embodiment, a crater treatment period is provided after the steady-state welding period, and during the crater treatment period, the flow rate Fi of the inner gas 7 is set so that the flow velocity of the inner gas 7 is within ±20% of the flow velocity of the outer gas 9. In the conventional technology, the conditions of the steady-state welding period are continued even during the crater treatment period. However, since the average welding current Iav changes to a smaller value during the crater treatment period, the welding state becomes transient. In the double-shielded TIG welding method, when the welding state becomes transient, there is a problem that blowholes are more likely to occur due to turbulence in the gas flow. This is because when the generation state of the arc 3 is transient, turbulence occurs because the flow velocities of the inner gas 7 and the outer gas 9 are very different, resulting in incomplete shielding. In this embodiment, during the crater treatment period, the flow rate Fi of the inner gas 7 is set so that the flow velocity of the inner gas 7 is within ±20% of the flow velocity of the outer gas 9, thereby suppressing the generation of turbulence. As a result, in this embodiment, it is possible to suppress the occurrence of blowholes and deterioration of welding quality. To further suppress the generation of turbulence, it is preferable to set the flow rate Fi of the inner gas 7 so that the flow velocity of the inner gas 7 is equal to the flow velocity of the outer gas 9.

[0069] More preferably, according to this embodiment, during the initial period from the moment arc 3 is generated, the flow rate Fi of the inner gas 7 is set so that the flow velocity of the inner gas 7 is within ±20% of the flow velocity of the outer gas 9. In the double-shielded TIG welding method, there is a problem that blowholes are likely to occur due to turbulence in the gas flow when the arc generation state is transient at the start of welding. This is because when the arc generation state of arc 3 is transient, turbulence occurs because the flow velocities of the inner gas 7 and the outer gas 9 are very different, resulting in incomplete shielding of arc 3. In this embodiment, during the initial period after arc 3 is generated, the flow rate Fi of the inner gas 7 is set so that the flow velocity of the inner gas 7 is within ±20% of the flow velocity of the outer gas 9, thereby suppressing the generation of turbulence. As a result, in this embodiment, the generation of blowholes at the start of welding can be suppressed. To further suppress the generation of turbulence, it is more preferable to set the flow rate Fi of the inner gas 7 so that the flow velocity of the inner gas 7 and the flow velocity of the outer gas 9 are equal. [Explanation of Symbols]

[0070] 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 rate, FICR: Steady-state inner gas flow rate setting circuit, Ficr: Steady-state inner gas flow rate setting signal, FIIR: Flow velocity reduction inner gas flow rate setting circuit, Fiir: Flow velocity reduction inner gas flow rate setting Constant signal, FIPR: Pre-flow inner gas flow rate setting circuit, Fipr: Pre-flow inner gas flow rate setting signal, FIR: Inner gas flow rate setting circuit, Fir: Inner gas flow rate setting signal, Fo: Outer gas flow rate, FOCR: Steady-state outer gas flow rate setting circuit, Focr: Steady-state outer gas flow rate setting signal, FOR: Outer gas flow rate setting circuit, For: Outer gas flow rate setting signal, Iav: Average welding current, 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 In2: Negative polarity current setting signal for the second electrode, IN2R: Negative polarity current setting circuit for the second electrode, In2r: Negative polarity current setting signal for the second electrode, Ip1: Positive polarity current for the first electrode, IP1R: Positive polarity current setting circuit for the first electrode, Ip1r: Positive polarity current setting signal for the first electrode, Ip2: Positive polarity current for the second electrode, IP2R: Positive polarity current setting circuit for the second electrode, Ip2r: Positive polarity current setting signal for the second electrode, IR: Current setting circuit, Ir: Current setting signal, Iw: Welding current, ON: Torch switch circuit, On: Torch switch signal, PS: Weld Power supply, Ten: Negative polarity period of the electrode, Tep: Positive polarity period of the electrode, Tn1: Negative polarity period of the first electrode, TN1R: First electrode negative polarity period setting circuit, Tn1r: First electrode negative polarity period setting signal, Tn2: Negative polarity period of the second electrode, TN2R: Second electrode negative polarity period setting circuit, Tn2r: Second electrode negative polarity period setting signal, TP: Period discrimination circuit, Tp: Period discrimination signal, Tp1: Positive polarity period of the first electrode, TP1R: First electrode positive polarity period setting circuit, Tp1r: First electrode positive polarity period setting signal, Tp2: Positive polarity period of the second electrode,TP2R: Second electrode positive polarity period setting circuit, Tp2r: Second electrode positive polarity period setting signal, VD: Voltage detection circuit, Vd: Voltage detection signal, WT: Welding torch,

Claims

1. A double-shielded TIG welding method is used, which involves using a welding torch equipped with an inner nozzle for ejecting inner gas and an outer nozzle for ejecting outer gas, and generating an arc by applying a welding current to perform welding, A double-shielded TIG welding method characterized by periodically changing the flow rate of the inner gas during the steady-state welding period after arc generation.

2. The flow rate of the inner gas during the steady-state welding period includes a first flow rate and a second flow rate, which are different from each other. The double-shielded TIG welding method according to claim 1, characterized in that the first flow rate and the second flow rate are changed in a pulsed manner.

3. The second flow rate is smaller than the first flow rate. The double-shielded TIG welding method according to claim 2, characterized in that the first flow rate is set so that the flow velocity of the inner gas is 2.5 times or more and 5 times or less than the flow velocity of the outer gas.

4. The double-shielded TIG welding method according to claim 2 or 3, characterized in that the first flow rate and the second flow rate are changed with a period of 0.5 Hz or more and 5 Hz or less.

5. A double-shielded TIG welding apparatus that uses a welding torch equipped with an inner nozzle for ejecting inner gas and an outer nozzle for ejecting outer gas, and generates an arc by applying a welding current to perform welding, A double-shielded TIG welding apparatus characterized in that the flow rate of the inner gas changes periodically during the steady-state welding period after arc generation.

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