Arc welding method

Abstract

According to the present invention, an arc period includes a first period and a second period that comes after the first period. In the first period, a welding current is changed such that the welding current becomes larger than a virtual straight line connecting a first change point P1 and a second change point P2. In the second period, the welding current is changed in a straight line connecting the second change point P2 and a third change point P3.

Description

Arc welding method

[0001] The present invention relates to an arc welding method.

[0002] Patent Document 1 discloses an arc welding machine that, after a predetermined time has elapsed since the short circuit was released, performs constant current control for a predetermined period at a current value higher than the current value output by conventional constant voltage control, thereby suppressing the occurrence of a short circuit immediately after the short circuit is released and suppressing the occurrence of spatter.

[0003] Japanese Patent Application Publication No. 10-109163

[0004] In arc welding, in order to ensure a sufficient bead width and penetration, it is necessary to increase the amount of molten metal.

[0005] However, if the welding current is increased at a steep rate until it reaches a peak current in order to increase the amount of molten metal, there is a problem in that spatter is more likely to occur.

[0006] The aspects of the present disclosure have been made in consideration of the above points, and have an object to suppress the generation of spatter by appropriately changing the welding current during the arc period.

[0007] A first aspect is an arc welding method for welding by cyclically repeating forward and reverse feed of a welding wire, which is a consumable electrode, and alternating between short circuit periods in a short circuit state and arc periods in an arc state, the arc period including a first period and a second period subsequent to the first period, a point in time at which a change in welding current begins in the first period being a first change point, a point in time at which the change in the welding current begins in the second period being a second change point, and a point in time at which the welding current reaches a peak current in the second period being a third change point, the method comprising: a first step of changing the welding current in the first period so that the welding current becomes larger than a virtual straight line connecting the first change point and the second change point; and a second step of changing the welding current in the second period along a straight line connecting the second change point and the third change point.

[0008] In the first aspect, the welding current is changed so that it is greater than the imaginary line connecting the first change point and the second change point during the first period, thereby ensuring the heat input to the welding wire. This allows stable formation of a droplet at the tip of the welding wire in the initial state of the arc period.

[0009] In addition, in the second period, the welding current is changed in a linear fashion connecting the second change point and the third change point, thereby ensuring a constant rate of increase in the heat input to the welding wire.

[0010] This allows droplets to be constantly held at the tip of the welding wire from the initial stage of droplet formation through to the growth stage. Furthermore, droplet growth is stably carried out until the peak current is reached, allowing droplets of a desired size to be formed.

[0011] Furthermore, the molten metal droplet held at the tip of the welding wire can be stably transferred toward the molten pool, which makes it easier to form a wide bead and suppresses the generation of spatter due to micro-short circuits.

[0012] In a second aspect, in the arc welding method of the first aspect, in the first step, the welding current is changed along an arc-shaped curve in which the welding current is greater than the virtual straight line.

[0013] In the second aspect, the welding current is changed along an arc-shaped curve in the first period to ensure the heat input to the welding wire, thereby enabling stable formation of a droplet at the tip of the welding wire in the initial state of the arc period.

[0014] In a third aspect, in the arc welding method of the first aspect, a predetermined point between the first change point and the second change point is set as a fourth change point, and in the first step, the welding current is changed in a linear manner connecting the first change point and the fourth change point, and then the welding current is changed in a linear manner connecting the fourth change point and the second change point.

[0015] In the third aspect, in the first period, the welding current is changed in a linear fashion connecting the first change point and the fourth change point, and then the welding current is changed in a linear fashion connecting the fourth change point and the second change point, thereby ensuring the heat input to the welding wire. This allows stable formation of a droplet at the tip of the welding wire in the initial state of the arc period.

[0016] In a fourth aspect, in the arc welding method of any one of the first to third aspects, a gradient of a line connecting the second change point and the third change point is 200 A / msec or more and 1500 A / msec or less.

[0017] In the fourth aspect, by appropriately setting the gradient of the line connecting the second change point and the third change point, it is possible to suppress the generation of spatter while ensuring the heat input to the welding wire.

[0018] Specifically, when the gradient of the line connecting the second and third change points is smaller than 200 A / msec, the heat input to the welding wire is small, and droplet growth at the tip of the welding wire becomes unstable, which disrupts the periodicity of the short circuit period and the arc period, potentially resulting in unstable welding.

[0019] On the other hand, if the slope of the line connecting the second and third change points is greater than 1500 A / msec, the heat input to the welding wire becomes excessive, increasing the arc pressure applied to the molten pool. This increased arc pressure can cause the molten pool to vibrate, potentially resulting in spatter. Furthermore, if the heat input to the welding wire is excessive, the tip of the welding wire may rise significantly from the droplet, resulting in unstable droplet transfer to the molten pool and unstable bead formation.

[0020] Therefore, in the aspect of the present disclosure, the gradient of the line connecting the second change point and the third change point is set so as to satisfy the above-mentioned condition.

[0021] In a fifth aspect, in the arc welding method of the fourth aspect, a gradient of a line connecting the second change point and the third change point is 1200 A / msec.

[0022] In the fifth aspect, by appropriately setting the gradient of the line connecting the second change point and the third change point, it is possible to suppress the generation of spatter while ensuring the heat input to the welding wire.

[0023] According to an aspect of the present disclosure, the occurrence of spatter can be suppressed by appropriately changing the welding current during the arc period.

[0024] FIG. 1 is a diagram showing a schematic configuration of an arc welding apparatus according to the first embodiment. FIG. 2 is a graph showing the time waveforms of a welding wire feed speed and a welding current. FIG. 3 is a graph showing the time waveforms of a welding wire feed speed and a welding current of Comparative Example 1. FIG. 4 is a graph showing the time waveforms of a welding wire feed speed and a welding current of Comparative Example 2. FIG. 5 is a graph showing the time waveforms of a welding wire feed speed and a welding current of Comparative Example 3. FIG. 6 is a graph showing the time waveforms of a welding wire feed speed and a welding current of Comparative Example 4. FIG. 7 is a graph showing the time waveforms of a welding wire feed speed and a welding current of Comparative Example 5. FIG. 8 is a graph showing the time waveforms of a welding wire feed speed and a welding current of Comparative Example 6. FIG. 9 is a graph showing the time waveforms of a welding wire feed speed and a welding current of Comparative Example 7. FIG. 10 is a graph showing the time waveforms of a welding wire feed speed and a welding current of Comparative Example 8. FIG. 11 is a graph showing the time waveforms of a welding wire feed speed and a welding current of the second embodiment. FIG. 12 is a graph showing the time waveforms of the welding wire feed rate and the welding current according to the third embodiment.

[0025] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that the following description of the preferred embodiments is merely exemplary in nature and is not intended to limit the present invention, its applications, or its uses.

[0026] First Embodiment As shown in FIG. 1 , an arc welding device 1 periodically repeats forward and reverse feeding of a welding wire 15, and alternates between a short-circuit period in a short-circuit state and an arc period in an arc state between the welding wire 15 and a workpiece W to generate an arc 16, thereby welding the workpiece W.

[0027] Arc welding apparatus 1 includes welding unit 10 and control unit 30. Welding unit 10 includes welding torch 11, feed motor 12, and power conversion unit 20. Feed motor 12 feeds welding wire 15 to welding torch 11 at a predetermined feed speed.

[0028] The power conversion unit 20 includes a primary side rectifier 21 , a switching unit 22 , a main transformer 23 , a secondary side rectifier 24 , a reactor 25 , a voltage detection unit 26 , and a current detection unit 27 .

[0029] Primary side rectifier 21 rectifies and outputs the output of input power source 5. Switching unit 22 converts the DC output from primary side rectifier 21 into AC. Switching unit 22 controls the welding output, which is made up of a welding current and a welding voltage.

[0030] The main transformer 23 converts the AC voltage output by the switching unit 22. The output of the main transformer 23 is output as a welding output via the secondary side rectifier 24 and the reactor 25. The secondary side rectifier 24 rectifies the secondary side output of the main transformer 23. The voltage detector 26 detects the welding voltage. The current detector 27 detects the welding current.

[0031] The control unit 30 has a drive unit 31, a state detection unit 32, a short circuit control unit 33, an arc control unit 34, a set current setting unit 35, a basic frequency setting unit 36, a basic speed amplitude setting unit 37, and an average feed speed setting unit 38. The drive unit 31 controls the switching unit 22.

[0032] The state detection unit 32 detects whether the state is a short circuit state or an arc state based on the detection result from the voltage detection unit 26. Specifically, the state detection unit 32 determines whether the welding output voltage is equal to or greater than a certain value or less than a certain value based on the signal from the voltage detection unit 26. Based on this determination result, the state detection unit 32 determines whether the state is a short circuit state in which the welding wire 15 is in contact with the workpiece W and short-circuited, or an arc state in which a welding arc is generated in a non-contact state. A determination signal indicating the determination result from the state detection unit 32 is output to the short circuit control unit 33 and the arc control unit 34.

[0033] The short circuit control unit 33 controls the short circuit current during the short circuit period in response to a short circuit signal from the state detection unit 32. The short circuit control unit 33 outputs a command to the drive unit 31 based on the set current so that the short circuit current has a predetermined waveform.

[0034] The arc control unit 34 receives the arc signal from the state detection unit 32 and controls the arc voltage during the arc period in the arc state.

[0035] The arc control unit 34 outputs a command to the drive unit 31 to output the peak current IP and the base current IB for a predetermined time during the arc period. In this embodiment, the transition period from the peak current IP to the base current IB during the arc period is controlled by current control.

[0036] The set current setting unit 35 sets a set current. A signal indicating the set current is output to the short circuit control unit 33 and the arc control unit 34.

[0037] The basic frequency setting unit 36 ​​determines a wire feeding frequency appropriate for each set current based on the set current. Specifically, the basic frequency setting unit 36 ​​has a table or a relational expression that correlates the set current with the frequency, and the frequency is determined from this table based on the set current. A signal indicating the frequency is output to the feed motor 12.

[0038] The speed amplitude basic setting unit 37 determines a wire feeding speed amplitude appropriate for each set current based on the set current. Specifically, the speed amplitude basic setting unit 37 has a table or a relational expression that associates the set current with the speed amplitude, and determines the speed amplitude from this table based on the set current. A signal indicating the speed amplitude is output to the feed motor 12.

[0039] The average feed rate setting unit 38 determines a welding wire feed rate, which is an average wire feed rate appropriate for each set current, based on the set current. Specifically, the average feed rate setting unit 38 has a table or a relational expression that associates the set current with the average feed rate, and determines the average feed rate from this table or the like based on the set current. A signal indicating the welding wire feed rate is output to the feed motor 12.

[0040] The feed motor 12 controls the feeding of the welding wire 15 in a cyclically repeated forward and reverse directions based on the frequency, speed amplitude, and welding wire feed speed. The wire feed speed command is a command to repeat the forward and reverse feed in a sinusoidal manner using the frequency and speed amplitude in accordance with the average wire feed speed appropriate for the current value of the set current.

[0041] <Regarding the Time Waveform of the Welding Current> In Fig. 2, time t1 indicates the time when the short circuit starts. During the short circuit period from time t1 to time t2, an initial short circuit current is output for a predetermined time from time t1, and then the welding current is gradually increased.

[0042] Between time t1 and time t2, a constriction of the droplet is detected between the molten pool and the tip of the welding wire 15. When the constriction of the droplet is detected, the welding current is instantly shifted to a low current, and the short-circuit state is terminated.

[0043] Time t2 indicates the time when the short circuit state ends and an arc state occurs. The period from time t2 to time t3 is an arc period during which an arc 16 is generated between the welding wire 15 and the workpiece W. During this arc period, the arc 16 is generated between the welding wire 15 and the workpiece W, and the heat of the arc 16 forms a molten droplet at the tip of the welding wire 15 and melts a portion of the workpiece W.

[0044] During the arc period from time t2 to time t3, the welding current is increased from base current IB to peak current IP. If the welding current is increased at a steep slope from base current IB to peak current IP, spatter is more likely to occur.

[0045] Therefore, in this embodiment, the welding current is appropriately changed during the arc period to suppress the occurrence of spatter.

[0046] Specifically, the arc period includes a first period and a second period that follows the first period.

[0047] The point at which the welding current starts to change in the first period is designated as the first change point P1, the point at which the welding current starts to change in the second period is designated as the second change point P2, and the point at which the welding current reaches the peak current IP in the second period is designated as the third change point P3.

[0048] In the first period, the welding current is changed so that it is greater than the imaginary line connecting the first change point P1 and the second change point P2. In the example shown in Fig. 2, the welding current is changed along an arc-shaped curve where the welding current is greater than the imaginary line.

[0049] This ensures a sufficient amount of heat input to the welding wire 15, and allows droplets to be stably formed at the tip of the welding wire 15 in the initial state of the arc period.

[0050] Next, in the second period, the welding current is changed in a linear fashion connecting the second change point P2 and the third change point P3.

[0051] This ensures that the heat input to the welding wire 15 increases at a constant rate, and droplet growth is stably carried out until the peak current IP is reached, thereby making it possible to form droplets of a desired size.

[0052] Furthermore, the molten metal droplet held at the tip of the welding wire 15 can be stably transferred toward the molten pool. As a result, a wide bead can be easily formed, and the generation of spatter due to micro-short circuits can be suppressed.

[0053] Here, if the gradient of the line connecting the second change point P2 and the third change point P3 is smaller than 200 A / msec, the heat input to the welding wire 15 is small, and droplet growth becomes unstable at the tip of the welding wire 15. As a result, the periodicity between the short circuit period and the arc period is disrupted, which may result in unstable welding.

[0054] On the other hand, if the gradient of the line connecting the second change point P2 and the third change point P3 is greater than 1500 A / msec, the heat input to the welding wire 15 becomes excessive, increasing the arc pressure applied to the molten pool. The increased arc pressure may then vibrate the molten pool, potentially causing spatter.

[0055] Furthermore, if the heat input to the welding wire 15 is excessive, the tip of the welding wire 15 will rise significantly from the droplet, which may cause unstable droplet transfer to the molten pool and unstable bead formation.

[0056] Therefore, in this embodiment, the slope of the line connecting the second change point P2 and the third change point P3 is set to 200 A / msec or more and 1500 A / msec or less. Preferably, the slope of the line connecting the second change point P2 and the third change point P3 is set to 1200 A / msec.

[0057] In this way, by appropriately setting the gradient of the line connecting the second change point P2 and the third change point P3, it is possible to ensure the heat input to the welding wire 15 while suppressing the occurrence of spatter.

[0058] During the arc period from time t2 to time t3, the distance between the tip of the welding wire 15 and the molten pool of the workpiece W transitions from T1 (first arc growth), to T2 (second arc growth), to T3 (third arc growth), to T4 (securing arc length), where T1<T2<T3<T4.

[0059] This allows the droplets, from the initial state of droplet formation to the process of growth, to be constantly held at the tip of the welding wire 15. Furthermore, by stably growing the droplets until the peak current IP is reached, it is possible to form droplets of a desired size.

[0060] Time t3 indicates the time when the next short circuit occurs, and is in the same state as time t1. When the welding wire 15 and the workpiece W come into contact and short-circuit, the molten metal droplet formed at the tip of the welding wire 15 during the arcing period is transferred to the workpiece W by short-circuiting, forming a molten pool and performing short-circuit welding.

[0061] 2, welding wire feed control is performed by periodically repeating forward and reverse feed alternately multiple times in a sinusoidal waveform with a predetermined frequency and a predetermined speed amplitude. At the peak of the forward feed, a short circuit occurs around time t1, and at the peak of the reverse feed, an arc 16 occurs around time t2. At the peak of the forward feed after time t2, a next short circuit occurs around time t3.

[0062] As described above, the period from time t1 to time t3 is defined as one control cycle, and welding is performed by repeating this cycle.

[0063] Comparative Example 1: Hereinafter, a description will be given of a time waveform when the welding current is changed during the arc period by a method different from that of the arc welding method according to the present embodiment. Note that in the drawings illustrating the following comparative example, the change in the welding current according to the present embodiment is shown by a virtual line to facilitate comparison with the change in the welding current according to the comparative example.

[0064] 3, the welding current is changed in a linear fashion connecting the first change point P1 and the second change point P2 during the first period, and then, in the second period, the welding current is changed in a linear fashion connecting the second change point P2 and the third change point P3.

[0065] In this case, during the period from the first change point P1 to the second change point P2, the welding current applied to the welding wire 15 is small, resulting in insufficient heat input, and therefore the growth of the droplet at the tip of the welding wire 15 becomes relatively small, which may result in a narrower bead width.

[0066] 4 , in a first period, the welding current is changed along an arc-shaped curve where the welding current is greater than the imaginary line connecting the first change point P1 and the second change point P2. Then, in a second period, the welding current is changed along an arc-shaped curve where the welding current is greater than the imaginary line connecting the second change point P2 and the third change point P3.

[0067] In this case, in the period from the second change point P2 to the third change point P3, as the welding current approaches the peak current IP, the growth of the droplet slows down, and the droplets of the welding wire 15 reach the peak current IP while remaining small. As a result, the total heat input becomes insufficient, and the droplets transfer to the molten pool in a relatively small state, which may result in a narrower weld bead width.

[0068] 5 , in a first period, the welding current is changed along an arc-shaped curve such that the welding current is greater than the imaginary line connecting the first change point P1 and the second change point P2. Then, in a second period, the welding current is changed along a straight line connecting the second change point P2 and the third change point P3′. Here, the third change point P3′ is later than the third change point P3 in the above embodiment.

[0069] Therefore, the slope of the line connecting the second change point P2 and the third change point P3' is gentler than the slope of the line connecting the second change point P2 and the third change point P3 in the above embodiment.

[0070] In this case, in the period from the second change point P2 to the third change point P3′, the amount of heat input to the welding wire 15 becomes small, the growth of droplets at the tip of the welding wire 15 becomes unstable, and the periodicity between the short-circuit period and the arc period is disrupted, which may result in unstable welding.

[0071] 6 , the welding current is changed along an arc-shaped curve from the first period to the second period, where the welding current is greater than the imaginary line connecting the first change point P1′ and the third change point P3′. Here, the first change point P1′ occurs later than the first change point P1 in the above-described embodiment. Furthermore, the wire feed speed is slower than the wire feed speed in the above-described embodiment, and the peak current IP′ at the third change point P3′ is smaller than the peak current IP at the third change point P3 in the above-described embodiment.

[0072] In this case, in the period from the first change point P1′ to the third change point P3′, the welding current applied to the welding wire 15 is small, resulting in insufficient heat input, and therefore the growth of the droplet at the tip of the welding wire 15 becomes relatively small, which may result in a narrower bead width.

[0073] 7 , the welding current is changed along an arc-shaped curve from the first period to the second period, where the welding current is greater than the imaginary line connecting the first change point P1′ and the third change point P3. Here, the first change point P1′ is later than the first change point P1 in the above embodiment.

[0074] In this case, the change in the welding current becomes steeper overall from the first change point P1' to the third change point P3, and the welding current also increases during the rising period. As a result, the arc pressure applied to the molten pool increases, causing the molten pool to vibrate, which may cause spattering during the second arc growth, as shown in the figure.

[0075] 8 , the welding current is changed along an arc-shaped curve from the first period to the second period, where the welding current is greater than the imaginary line connecting the first change point P1′ and the third change point P3. Here, the first change point P1′ is later than the first change point P1 in the above embodiment. Note that the change in welding current is the same in Comparative Example 6 and Comparative Example 5, but the cause of the welding defect in Comparative Example 6 will be described, which is different from that in Comparative Example 5.

[0076] In this case, during the period from the first change point P1′ to the third change point P3, excessive heat input to the welding wire 15 causes droplets that have grown at the tip of the welding wire 15 to migrate to the side of the welding wire 15, disrupting the periodicity between the short-circuit period and the arc period and making the bead appearance unstable, which may further lead to spatter scattering due to a micro-short circuit.

[0077] 9 , the welding current is changed linearly from the first period to the second period, connecting the first change point P1′ and the third change point P3′. The first change point P1′ occurs later than the first change point P1 in the previous embodiment. The wire feed speed is slower than that in the previous embodiment, and the peak current IP′ at the third change point P3′ is smaller than the peak current IP at the third change point P3 in the previous embodiment.

[0078] In this case, in the period from the first change point P1′ to the third change point P3′, the welding current applied to the welding wire 15 is small, resulting in insufficient heat input, and therefore the growth of the droplet at the tip of the welding wire 15 becomes relatively small, which may result in a narrower bead width.

[0079] 10 , the welding current is changed linearly from the first period to the second period, connecting the first change point P1′ and the third change point P3. Here, the first change point P1′ is a point in time later than the first change point P1 in the above embodiment.

[0080] In this case, the change in the welding current becomes steeper overall from the first change point P1' to the third change point P3, and the welding current also increases during the rising period. As a result, the arc pressure applied to the molten pool increases, causing the molten pool to vibrate, which may cause spattering during the second arc growth, as shown in the figure.

[0081] -Effects of First Embodiment- As described above, according to the arc welding method of the first embodiment, the heat input to welding wire 15 can be ensured by changing the welding current in the first period so that the welding current is greater than the imaginary line connecting first change point P1 and second change point P2.

[0082] Furthermore, by changing the welding current in a linear manner connecting the second change point P2 and the third change point P3 in the second period, the heat input to the welding wire 15 can be ensured at a constant rate of increase.

[0083] This allows the droplet held at the tip of the welding wire 15 to be stably transferred toward the molten pool, making it easier to form a wide bead and suppressing the generation of spatter due to micro-short circuits.

[0084] Furthermore, even if the feeding speed of the welding wire 15 is increased, the welding can be stabilized, and therefore the tact time can be reduced.

[0085] For example, when welding two overlapping plate materials with a thickness of 1.6 to 6.0 mm as the workpieces W, the welding speed can be set in the range of 0.6 to 1.5 m / min. Also, productivity can be improved by improving the gap tolerance.

[0086] Second Embodiment Hereinafter, the same parts as those in the first embodiment will be denoted by the same reference numerals, and only the differences will be described.

[0087] As shown in FIG. 11, the arc period includes a first period and a second period that follows the first period.

[0088] The point at which the welding current starts to change in the first period is called the first change point P1, the point at which the welding current starts to change in the second period is called the second change point P2, the point at which the welding current reaches the peak current IP in the second period is called the third change point P3, and a predetermined point between the first change point P1 and the second change point P2 is called the fourth change point P4.

[0089] During the first period, the welding current is changed so that it is greater than the imaginary line connecting the first change point P1 and the second change point P2. In the example shown in Fig. 11, the welding current is changed along a line connecting the first change point P1 and the fourth change point P4, and then along a line connecting the fourth change point P4 and the second change point P2. Thereafter, the welding current is changed along a line connecting the second change point P2 and the third change point P3.

[0090] As described above, according to the arc welding method of the second embodiment, droplets can be stably formed at the tip of the welding wire 15 in the initial state of the arc period.

[0091] Third Embodiment As shown in FIG. 12, the arc period includes a first period and a second period that follows the first period.

[0092] The point at which the welding current starts to change in the first period is called the first change point P1, the point at which the welding current starts to change in the second period is called the second change point P2, the point at which the welding current reaches the peak current IP in the second period is called the third change point P3, a predetermined point between the first change point P1 and the second change point P2 is called the fourth change point P4, and a predetermined point between the first change point P1 and the fourth change point P4 is called the fifth change point P5.

[0093] During the first period, the welding current is changed so that it is greater than the imaginary line connecting the first change point P1 and the second change point P2. In the example shown in Figure 12, the welding current is changed along a line connecting the first change point P1 and the fifth change point P5, then along a line connecting the fifth change point P5 and the fourth change point P4, and finally along a line connecting the fourth change point P4 and the second change point P2. Thereafter, the welding current is changed along a line connecting the second change point P2 and the third change point P3.

[0094] As described above, according to the arc welding method of the third embodiment, a droplet can be stably formed at the tip of the welding wire in the initial state of the arc period.

[0095] As described above, the present invention has a highly practical effect of suppressing the generation of spatter by appropriately changing the welding current during the arc period, and is therefore extremely useful and has high industrial applicability.

[0096] 1 Arc welding device 15 Welding wire IP Peak current P1 First change point P2 Second change point P3 Third change point P4 Fourth change point

Claims

1. An arc welding method for welding by cyclically repeating forward and reverse feed of a welding wire, which is a consumable electrode, and alternating between short circuit periods in a short circuit state and arc periods in an arc state, wherein the arc period includes a first period and a second period after the first period, wherein a first change point is a point in time when the welding current starts to change in the first period, a second change point is a point in time when the welding current starts to change in the second period, and a third change point is a point in time when the welding current reaches a peak current in the second period, the method comprising: a first step of changing the welding current in the first period so that the welding current becomes larger than an imaginary line connecting the first change point and the second change point; and a second step of changing the welding current in the second period along a line connecting the second change point and the third change point.

2. The arc welding method according to claim 1, wherein in the first step, the welding current is changed along an arc-shaped curve in which the welding current is greater than that on the virtual straight line.

3. The arc welding method of claim 1, wherein a predetermined point between the first change point and the second change point is defined as a fourth change point, and in the first step, the welding current is changed in a linear fashion connecting the first change point and the fourth change point, and then the welding current is changed in a linear fashion connecting the fourth change point and the second change point.

4. An arc welding method according to any one of claims 1 to 3, wherein the gradient of the line connecting the second change point and the third change point is not less than 200 A / msec and not more than 1500 A / msec.

5. The arc welding method according to claim 4, wherein the gradient of the line connecting the second change point and the third change point is 1200 A / msec.

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