JP2012152772A - Method for controlling arc start of plasma mig welding - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress generation of black soot (smut) on surfaces of beads in arc start.SOLUTION: A welding wire is temporarily brought into contact with a base material and is fed backward to get away therefrom so that application of an initial arc current generates an initial MIG arc, and the backward feeding is continued to increase arc length to generate a plasma arc, and thereafter the feeding is switched to forward feeding and a MIG welding current is applied to shift the arc to a regular MIG arc. During a period (t3-t31) until an arc length La reaches a first reference distance Lt1, a first initial arc current Ii1 of an electrode negative polarity is applied, and during a period (t31-t4) thereafter, a second initial arc current Ii2 of an electrode positive polarity is applied. An expression of |Ii1|>Ii2 is satisfied. The first initial arc current Ii1 is applied to generate the arc to fuse the welding wire, and thereby to rapidly increase the arc length La. Therefore, since a period where only the MIG arc is generated is shortened, generation of the smut is suppressed.

Description

  The present invention relates to an arc start control method for plasma MIG welding in which a MIG arc and a plasma arc are simultaneously generated using a single welding torch for welding.

  Conventionally, a plasma MIG welding method combining a plasma welding method and a MIG welding method has been proposed. In this plasma MIG welding method, a MIG arc is generated by applying a MIG welding current between a welding wire fed through a welding torch and a base material. A hollow plasma electrode is arranged so as to surround the welding wire, and a plasma such as argon is supplied, and a plasma welding current is passed between the plasma electrode and the base material through this gas to generate plasma. Generate an arc. The MIG arc is generated between the welding wire fed through the axis of the welding torch and the base material, and a plasma arc is generated so as to surround the MIG arc. Therefore, the MIG arc is encased in a plasma arc. The welding wire functions as an electrode for generating a MIG arc, and the tip of the welding wire melts to form a droplet to assist the joining of the base materials. Therefore, the plasma MIG welding method is often used for high-efficiency welding of thick plates, high-speed welding of thin plates, and the like.

  The MIG welding current generally uses a pulse waveform in order to suppress the generation of spatter and stably supply droplets. Therefore, the MIG welding method is a general MIG pulse welding method. In the consumable electrode type arc welding method including the MIG pulse welding method, it is important to maintain the arc length during welding at an appropriate value, and therefore arc length control is performed. On the other hand, a constant direct current is often used for the plasma welding current. In the following description, when the arc length is simply described, it means the arc length of the MIG arc. Hereinafter, the plasma MIG welding method described above will be described.

  FIG. 5 is a waveform diagram in a steady state of the plasma MIG welding method in the prior art. (A) shows the MIG welding current Iwm, (B) shows the MIG welding voltage Vwm, and (C) shows the plasma welding current Iwp. The MIG welding current Iwm, the MIG welding voltage Vwm, and the plasma welding current Iwp are output with an electrode positive polarity in which the electrode serves as an anode and the base material serves as a cathode. Hereinafter, a description will be given with reference to FIG.

  In the figure, the period from time t1 to t3 represents the (n-1) th pulse period Tf (n-1), and the period from time t3 to t5 represents the nth pulse period Tf (n). The (n−1) th pulse cycle Tf (n−1) is the (n−1) th peak period Tp (n−1) at times t1 to t2 and the (n−1) th base period Tb at times t2 to t3. (n-1). The n-th pulse period Tf (n) is formed from the n-th peak period Tp (n) from time t3 to t4 and the n-th base period Tb (n) from time t4 to t5.

  As shown in FIG. 9A, in the (n-1) th pulse cycle Tf (n-1), a predetermined peak current Ip is applied during the peak period Tp (n-1) from time t1 to t2. During the base period Tb (n−1) from time t2 to t3, a predetermined base current Ib is energized. Therefore, the MIG welding current Iwm is formed from the peak current Ip and the base current Ib. Then, as shown in FIG. 5B, in the (n-1) th pulse period Tf (n-1), the peak voltage Vp (n-1) proportional to the arc length during the peak period Tp (n-1). ) Is applied between the welding wire and the base metal, and a base voltage Vb (n-1) proportional to the arc length is applied during the base period Tb (n-1). Therefore, the MIG welding voltage Vwm is formed from the peak voltage Vp and the base voltage Vb. The same applies to the nth pulse cycle Tf (n). Here, the peak current Ip and the base current Ib in the (n−1) th pulse cycle Tf (n−1) have the same values as the peak current Ip and the base current Ib in the nth pulse cycle Tf (n), respectively. Become. On the other hand, when the arc generation state is stable, the peak voltage Vp (n-1) and the base voltage Vb (n-1) in the (n-1) th pulse period Tf (n-1) are nth. The peak voltage Vp (n) and the base voltage Vb (n) in the second pulse period Tf (n) are substantially equal to each other.

  In MIG welding, arc length control is performed to maintain the arc length at an appropriate value in order to obtain good welding quality. Normally, this arc length control uses the fact that the MIG welding voltage Vwm is approximately proportional to the arc length, and the pulse period Tf is set so that the average value of the MIG welding voltage Vwm becomes equal to a predetermined voltage setting value. Be controlled. The average value of the MIG welding voltage Vwm is generated by passing the MIG welding voltage Vwm through a low-pass filter (cutoff frequency is about 1 to 10 Hz). This arc length control method is called a frequency modulation method. In this case, the peak period Tp, the peak current Ip, and the base current Ib are set to predetermined values and become pulse parameters. The peak current Ip is set to a critical value or more and is called a unit pulse condition in combination with the peak period Tp. This unit pulse condition is set so that one droplet period is one droplet transfer. The base current Ib is set to a small current value of about several tens of A that is less than the critical value. The unit pulse condition and the base current Ib are set to appropriate values according to the welding wire material, diameter, feeding speed, and the like.

  In addition to frequency modulation control, pulse width modulation control may be used as an arc length control method. In this pulse width modulation control, the pulse period Tf, the peak current Ip, and the base current Ib are set to predetermined values and become pulse parameters. Then, the peak period Tp (pulse width) is controlled so that the average value of the MIG welding voltage Vwm is equal to the voltage setting value.

  On the other hand, as shown in FIG. 5C, the plasma welding current Iwp is constant-current controlled and becomes a DC waveform having a predetermined constant value. Therefore, the plasma arc is generated by energizing the plasma welding current Iwp with a constant value.

  Since the plasma MIG welding method described above is often used for high quality welding, in addition to arc stability in a steady state, it is also required that arc starting performance is good. For this purpose, a method is employed in which a mig arc is generated by a retract start method, which will be described later, and a plasma arc is induced by the mig arc (see, for example, Patent Documents 1 and 2). Hereinafter, the arc start control method of the plasma MIG welding of this prior art is demonstrated.

  FIG. 6 is a timing chart showing a plasma MIG welding arc start control method in the prior art. (A) shows the welding start signal St, (B) shows the welding wire feed speed Fw, (C) shows the MIG welding voltage Vwm, and (D) shows MIG welding. (E) shows the plasma welding voltage Vwp, (F) shows the plasma welding current Iwp, and (G1) to (G5) are schematic diagrams showing the arc generation state at each time. FIG. As described above, the MIG welding voltage Vwm, the MIG welding current Iwm, the plasma welding voltage Vwp, and the plasma welding current Iwp are output as positive waveforms because they are output with positive polarity. Hereinafter, a description will be given with reference to FIG.

(1) Forward feeding period (slow-down feeding period) at times t1 to t2
At time t1, as shown in FIG. 6A, when the welding start signal St from the outside becomes a high level (welding start), as shown in FIGS. The value becomes a small positive value, and the welding wire 1a starts forward feeding at a slow slow-down feeding speed Fi of about 1 to 2 m / min. Here, when the feeding speed Fw is a positive value, the welding wire 1a is fed forward in a direction approaching the base material 2, and when the feeding speed Fw is a negative value, the welding wire 1a is a direction away from the base material 2. It shows that it is repatriated to. As shown in FIG. 2C, a no-load voltage is applied between the welding wire 1a and the base material 2, and as shown in FIG. Another no-load voltage is applied.

(2) Reverse feed short circuit period at time t2 to t3 At time t2, as shown in FIG. (G2), when the welding wire 1a contacts (short circuit) with the base material 2 by the above forward feed, As shown in C), the MIG welding voltage Vwm decreases from a no-load voltage to a short-circuit voltage value of about several volts. When a short circuit is determined by this voltage drop, the feed speed Fw is switched to a negative reverse feed speed Fb, and the welding wire 1a moves backward in the direction away from the base material 2, as shown in FIG. Be sent. Further, as shown in FIG. 4D, the MIG welding current Iwm is an initial short-circuit current Is that is a constant DC current of about 20 to 80 A, which hardly melts the welding wire by Joule heat.

(3) Reverse feed arc period from time t3 to t4 At time t3, as shown in FIG. 3G3, when the wire tip is separated from the base material 2 by the above reverse feed, an initial mig arc 31a is generated, As shown in FIG. 4D, the MIG welding current Iwm is an initial arc current Ii that is a constant direct current of about 20 to 80 A, which hardly melts the welding wire with arc heat. Here, it is a case where Is = Ii. The initial MIG arc 31a is generated by the reverse feed, and the current value (initial short circuit current value Is) immediately before the arc is small is small, so that almost no spatter is generated and a reliable arc start can be realized. When the initial MIG arc 31a is generated, the MIG welding voltage Vwm rapidly increases from the short-circuit voltage value to an arc voltage value of about several tens of volts as shown in FIG. Since the reverse feed is continued at the time t3, the arc length of the initial MIG arc 31a is gradually increased as shown in FIG. Since the inside of the initial MIG arc 31a generated between the tip of the welding wire 1a and the base material 2 is a plasma atmosphere, as the arc length increases, the gap between the plasma electrode 1b and the base material 2 is increased. A plasma atmosphere is also formed in the space.

(4) Generation period of mig arc 3a and plasma arc 3b at time t4 At time t4, as shown in FIG. 5E, no-load voltage has already been applied between the plasma electrode 1b and the base material 2, Further, as described above, since the space between the plasma electrode 1b and the base material 2 is a plasma atmosphere, the plasma arc 3b is induced and generated as shown in FIG. When the plasma arc 3b is generated, the plasma welding voltage Vwp decreases from the no-load voltage to the arc voltage value as shown in FIG. 5E, and the plasma welding current Iwp is determined in advance as shown in FIG. The steady-state plasma welding current value Iwpc is obtained. At the same time, when the plasma arc 3b is generated at time t4, the feed speed Fw is switched from the negative reverse feed speed Fb to the positive steady feed speed Fwc, as shown in FIG. The welding wire 1a is fed forward again. As a result, as shown in FIG. 4D, the MIG welding current Iwm becomes a pulse waveform formed from the peak current Ip and the base current Ib as described above with reference to FIG. 5, and as shown in FIG. The MIG welding voltage Vwm has a pulse waveform formed from the peak voltage Vp and the base voltage Vb as described above with reference to FIG. The average value of the MIG welding current Iwm at time t4 is substantially determined by the steady feeding speed Fwc. Further, the average value of the MIG welding voltage Vwm at time t4 is controlled so as to be substantially equal to the voltage setting value as described above with reference to FIG. 5, and is feedback controlled so that the arc length becomes an appropriate value. Then, after 100 to 700 ms from time t4, the arc length of the MIG arc 3a converges to a steady value and enters a steady state. Thereby, the arc start of plasma MIG welding is completed. The plasma arc 3b is generated because the space between the plasma electrode 1b and the base material 2 becomes a plasma atmosphere by the initial MIG arc 31a in a state in which no load voltage is applied. There is variation in the range. Therefore, the reverse arc period from time t3 to t4 has some variation.

JP 2007-144509 A JP 2008-229704 A

  By adopting the retract start method for arc start as in the prior art described above, it is possible to perform reliable arc start with very little spatter generation. However, the retract start method has the following problems. That is, there is a problem that during the period when only the MIG arc is generated at the time of arc start (period from time t3 to t4 in FIG. 6), black wrinkles (called smut) adhere to the bead surface and the bead appearance is deteriorated. . Furthermore, when the smut is generated, there is a problem that the adhesion is deteriorated in the surface treatment such as painting. When the mig arc is wrapped with the plasma arc, this smut does not occur.

  In order to solve the above-mentioned problem, it is only necessary to shorten the backward feeding period, so a countermeasure for increasing the backward feeding speed can be considered. However, if the reverse feed speed is increased, the arc length of the initial MIG arc rapidly increases, so that the arc length at the time when the plasma arc is induced (time t4 in FIG. 6) becomes too long and is not good. It may become stable. Furthermore, when switching from reverse feed to re-forward feed, it takes time to reverse the feed direction, and the arc length overshoots and becomes long, resulting in instability.

  Therefore, an object of the present invention is to provide a plasma MIG welding arc start control method capable of suppressing the occurrence of smut at the time of arc start and suppressing the arc length of the initial MIG arc from becoming too long. To do.

In order to solve the above-mentioned problem, the invention of claim 1 is characterized in that a peak current during a peak period and a base current during a base period are defined as one pulse period between a welding wire and a base material fed through a welding torch. A plasma MIG that generates a MIG arc by energizing a MIG welding current to be generated and generates a plasma arc by energizing a plasma welding current between the plasma electrode disposed so as to surround the welding wire and the base material. In welding,
At the start of welding, a no-load voltage is applied between the plasma electrode and the base material, and the welding wire is fed forward, once brought into contact with the base material, and then retracted and pulled away. An initial MIG arc that is energized by an initial arc current is generated, the backward feeding is continued, and the arc length of the initial MIG arc is gradually increased to form a plasma atmosphere in the space between the plasma electrode and the base material. Thus, the plasma arc to be energized by the plasma welding current is generated, and when the plasma arc is generated, the welding wire is switched to re-forward feeding and the MIG welding current formed from the peak current and the base current is energized. In the arc MIG welding arc start control method to shift to steady MIG arc,
During the period from when the initial MIG arc is generated to when the arc length reaches a predetermined first reference distance, the first initial arc current having an electrode negative polarity is applied, and thereafter the period from when the plasma arc is generated. Energizes a second initial arc current of electrode positive polarity and sets the absolute value of the first initial arc current to a value greater than the absolute value of the second initial arc current;
This is an arc start control method for plasma MIG welding.

The invention of claim 2 determines that the arc length of the initial MIG arc has reached the first reference distance by the fact that the absolute value of the MIG welding voltage has reached a predetermined reference voltage value.
The arc start control method of plasma MIG welding according to claim 1.

The invention of claim 3 sets the first reference distance as a value obtained by multiplying the distance between the plasma electrode and the base material by a predetermined reference ratio.
The arc start control method of plasma MIG welding according to any one of claims 1 and 2.

The invention of claim 4 sets the first reference distance as a value obtained by subtracting a predetermined second reference distance from the distance between the plasma electrode and the base material.
The arc start control method of plasma MIG welding according to any one of claims 1 and 2.

  According to the present invention, a first initial arc current having an electrode negative polarity is applied during a period from when the initial MIG arc occurs until the arc length reaches a predetermined first reference distance, and a plasma arc is generated thereafter. During this period, the second initial arc current having a positive electrode polarity is applied, and the absolute value of the first initial arc current is set to a value larger than the absolute value of the second initial arc current. Since the first initial arc current has an electrode negative polarity and its value is large, the welding wire melting rate increases. Since the backward feeding of the welding wire is added to this, the arc length of the initial MIG arc is rapidly increased to reach the first reference distance. For this reason, since the period during which the initial mig arc is independently generated can be shortened, the occurrence of smut can be suppressed. Furthermore, when the arc length of the initial MIG arc becomes longer than the first reference distance and approaches the plasma electrode, the initial arc current becomes the positive polarity of the electrode, and the value is also switched to a small value, so that the welding wire is hardly melted. Disappear. For this reason, the arc length of the initial MIG arc is gradually increased because it is fed backward at a relatively low backward feeding speed. As a result, since the arc length of the MIG arc does not become too long when the plasma arc is generated and when switching from backward feed to re-forward feed, the arc state does not become unstable.

It is a timing chart which shows the arc start control method of plasma MIG welding concerning an embodiment of the invention. It is a lineblock diagram of a welding device for enforcing the arc start control method of plasma MIG welding concerning an embodiment of the invention. It is a block diagram of the MIG welding power supply PSM which comprises the welding apparatus of FIG. It is a block diagram of the plasma welding power supply PSP which comprises the welding apparatus of FIG. It is a wave form diagram in the steady state of the plasma MIG welding method in a prior art. It is a timing chart which shows the arc start control method of plasma MIG welding in a prior art.

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

  In the invention according to the embodiment of the present invention, a first initial arc current having an electrode negative polarity is applied during a period from when the initial MIG arc occurs until the arc length reaches a predetermined first reference distance. A second initial arc current having a positive electrode polarity is applied, and the absolute value of the first initial arc current is set to a value larger than the absolute value of the second initial arc current. In this specification and the like, when the initial arc current is a positive value, it indicates that the electrode has a positive polarity, and when the initial arc current is a negative value, it indicates that the electrode has a negative polarity. The magnitude of the initial arc current value represents the magnitude of the absolute value. Hereinafter, this embodiment will be described.

  FIG. 1 is a timing chart showing an arc start control method of plasma MIG welding according to an embodiment of the present invention. FIG. 4A shows the change over time of the welding wire feeding speed Fw, FIG. 4B shows the change over time of the MIG welding current Iwm, and FIG. 4C shows the change over time of the plasma welding current Iwp. FIG. 4D shows the time change of the arc length La of the MIG arc. Regarding the MIG welding current Iwm and the plasma welding current Iwp, positive values indicate that the electrode has positive polarity, and negative values indicate that the electrode has negative polarity. This figure shows the change of each signal during the period from time t2 to t4, which is the period during which the welding wire is retracted in FIG. As shown in FIG. 5B, when the polarity of the MIG welding current Iwm is switched (time t3 and t31), a high voltage of several hundred volts is applied between the welding wire and the base material in order to prevent arc breakage. Is applied instantaneously. Since the operation up to the time t2 and the operation after the time t4 are the same as those in FIG. 6, these periods are omitted in order to explain the operation in the period characteristic of the present invention in detail. Hereinafter, a description will be given with reference to FIG.

(21) Reverse feed short circuit period from time t2 to t3 At time t2, the welding wire is in a short circuit state with the base material, and neither a MIG arc nor a plasma arc is generated. Since the welding wire is reversely fed, the feed speed Fw is a negative value of the reverse feed speed Fb as shown in FIG. Here, the sign indicates the feeding direction, a negative value indicates a backward feeding state, and a positive value indicates a forward feeding state. The reverse feed speed Fb is switched from reverse feed to re-forward feed so that the arc length La when the plasma arc is generated does not become too long, similar to the reverse feed speed in FIG. Sometimes the arc length La is set to a relatively low speed so as not to overshoot. The reverse feed speed Fb is set to about 1 to 5 m / min, for example. As shown in FIG. 6B, the MIG welding current Iwm is the initial short-circuit current Is having an electrode plus polarity, as in FIG. As shown in FIG. 5C, the plasma welding current Iwp is 0 A because no current is supplied. As shown in FIG. 4D, the arc length La is 0 mm because no MIG arc is generated and the welding wire is short-circuited to the base material. Here, in FIG. 4D, the broken line drawn parallel to the horizontal axis indicates the distance Lp between the plasma electrodes and the base material, and the alternate long and short dash line indicates the predetermined first reference distance Lt1. Lp> Lt1.

(31) Reverse feed arc period from time t3 to t4 When the tip of the welding wire is separated from the base material at time t3 by reverse feed, an initial mig arc is generated. As shown in FIG. 5A, the feeding speed Fw remains the above backward feeding speed Fb. When the initial mig arc is generated, the polarity is switched and the first initial arc current Ii1 having a negative electrode polarity having a negative value is applied as shown in FIG. The first initial arc current Ii1 is set to a value that can melt the wire tip by the arc, and is set to a value that is larger than the initial arc current Ii of FIG. 6 and a second initial arc current Ii2 described later. Further, since the first initial arc current Ii1 has an electrode negative polarity, the melting rate of the welding wire is about 1.5 times faster than the electrode positive polarity even with the same current value. The first initial arc current Ii1 is set to about 100 to 200 A, for example. The first initial arc current Ii1 is set to an appropriate value by experiment according to these parameters because the melting rate changes when the material, diameter, type of shield gas, etc. of the welding wire change. In the figure, the case where the polarity of the MIG welding current Iwm is switched immediately after the initial MIG arc is generated is illustrated, but a predetermined delay time may be provided, and the polarity may be switched after the delay time has elapsed. . The delay time is, for example, about 0.5 to 5 ms. On the other hand, since the plasma arc has not yet occurred, the plasma welding current Iwp remains 0 A as shown in FIG. The welding wire is fed backward at the backward feed speed Fb and the wire tip is melted by the arc supplied with the first initial arc current Ii1, so that the arc length La of the initial MIG arc is shown in FIG. Grows rapidly. At time t31, when the arc length La reaches the first reference distance Lt1 indicated by the alternate long and short dash line as shown in (D), the polarity is switched again as shown in (B). A second initial arc current Ii2 having a positive value and an electrode positive polarity is applied. Here, Ii2 <| Ii1 |. The second initial arc current Ii2 is set in a range of about 20 to 80 A so that the tip of the wire is hardly melted by the arc, like the initial arc current value Ii of FIG. As shown in FIG. 5A, the feed speed Fw remains the reverse feed speed Fb. As a result, the arc length La gradually increases as shown in FIG. At time t4, a plasma arc is induced and generated. As described above, since the tip of the wire is melted and added to the reverse feed speed Fb by the arc in which the polarity is electrode negative polarity and the first initial arc current Ii1 having a large current value is energized, the arc length La Is rapidly increased, the period of time t3 to t31 is shortened. For this reason, the reverse feed arc period from time t3 to t4, which is a period in which the MIG arc is generated independently, is shortened as a whole. As shown in FIG. 4D, when the arc length La reaches the first reference distance, the arc length La gradually increases. That is, when the arc length La approaches the plasma electrode to some extent, the arc length La is gradually increased. Since the plasma atmosphere is present during the initial MIG arc, as the arc length La gradually increases, the plasma atmosphere also gradually increases to fill the space under the plasma electrode. As a result, when the plasma arc is induced and generated, the arc length La of the initial MIG arc does not become too long, so that the arc state does not become unstable.

(41) Mig Arc and Plasma Arc Generation Period Transitioned to Time t4 At time t4, no-load voltage has already been applied between the plasma electrode and the base material, and as described above, the plasma electrode and the base material 2 Since the space between them is a plasma atmosphere, a plasma arc is induced and generated. When the plasma arc is generated, the plasma welding current Iwp becomes a predetermined steady plasma welding current value Iwpc as in FIG. 6, as shown in FIG. At the same time, as shown in FIG. 6A, the feeding speed Fw is switched from the negative value backward feeding speed Fb to the positive value steady feeding speed Fwc, and the welding wire is fed forward again. Is done. As a result, since the initial MIG arc shifts to the steady MIG arc, as shown in FIG. 5B, the MIG welding current Iwm is a pulse waveform formed from the peak current Ip and the base current Ib as described above with reference to FIG. It becomes. During the transition to time t4, as described above, arc length control by frequency modulation control is performed so that the average value of the MIG welding voltage is equal to the voltage set value. As shown in FIG. 4D, the arc length La slightly overshoots at the time t4 when the feed direction is reversed, and then gradually becomes shorter by the arc length control and converges to the steady arc length. This completes the arc start. The overshoot of the arc length La at the time of reversing the feeding direction becomes only a little because the reverse feeding speed Fb is low, and the arc state does not become unstable.

  As described above, in the present embodiment, since the backward feeding arc period from time t3 to t4 is shortened, the period during which the mig arc is generated alone can be shortened to suppress the occurrence of smut. Further, when the plasma arc is generated and when switching from backward feed to re-forward feed, the arc length of the MIG arc does not become too long, so that the arc state does not become unstable.

  In the figure, it can be determined as follows that the arc length La of the initial MIG arc has reached the first reference distance Lt1. That is, it is determined that the arc length La has reached the first reference distance Lt1 by the fact that the absolute value of the MIG welding voltage Vwm has reached a predetermined reference voltage value Vt1. The arc length La is substantially proportional to the absolute value of the MIG welding voltage Vwm. The reference voltage value Vt1 is set to a value corresponding to the first reference distance Lt1. Then, the time when the absolute value of the MIG welding voltage Vwm reaches the reference voltage value Vt1 is the time when the arc length La reaches the first reference distance Lt1.

Next, a method for setting the first reference distance Lt1 will be described.
a) The first reference distance Lt1 is set as a value obtained by multiplying the plasma electrode / base material distance Lp by a predetermined reference ratio α. That is, Lt1 = Lp × α. α is set to about 0.6 to 0.8. In this way, even if the distance Lp between the plasma electrode and the base material changes, the appropriate first reference distance Lt1 can be set. Since the distance Lp between the plasma electrode and the base material is usually a constant value during welding, it may be measured before welding is started. It is often used at 20 mm ≦ Lp ≦ 30 mm. Here, when α = 0.7, 14 mm ≦ Lt1 ≦ 21 mm corresponding to the change in Lp.
b) The first reference distance Lt1 is set as a value obtained by subtracting a predetermined second reference distance Lt2 from the plasma electrode-base material distance Lp. That is, Lt1 = Lp−Lt2. The second reference distance Lt2 is set to about 5 to 10 mm. By setting the second reference distance Lt2 to a constant value even if the plasma electrode-base material distance Lp changes, it is possible to suppress the arc length La from becoming too long, and then set the above-described reverse feed arc period to It can be shortened as much as possible. This second reference distance Lt2 is set to an appropriate value according to the response of the feed motor, the material of the welding wire, the diameter, and the like.

  FIG. 2 is a configuration diagram of a welding apparatus for performing the arc start control method of plasma MIG welding according to the embodiment of the present invention described above with reference to FIG. Hereinafter, each component will be described with reference to FIG.

  This welding apparatus includes a welding torch WT, a MIG welding power source PSM, and a plasma welding power source PSP surrounded by a broken line. The welding torch WT has a structure in which a plasma nozzle 51, a plasma electrode 1b, and a power feed tip 4 are arranged on a concentric axis in a shield gas nozzle 52. From the gap between the shield gas nozzle 52 and the plasma nozzle 51, for example, a shield gas 63 such as argon gas or a mixed gas of argon gas and carbon dioxide gas is supplied. A plasma gas 62 such as argon gas or a mixed gas of argon gas and carbon dioxide gas is supplied between the plasma nozzle 51 and the plasma electrode 1b. A center gas 61 such as argon gas or a mixed gas of argon gas and carbon dioxide gas is supplied between the plasma electrode 1 b and the power feed tip 4. These three systems of gas may be collectively expressed simply as gas.

  A welding wire 1 a is fed from a through hole provided in the power feed tip 4. The power feed tip 4 is in contact with the welding wire 1a and supplies power for generating a mig arc. The welding wire 1a is fed by the rotation of the feed roll 7 using the feed motor WM as a drive source. The plasma electrode 1b is made of, for example, copper or a copper alloy, and is indirectly water-cooled by cooling water passing through a path outside the figure. The plasma electrode 1b is insulated from the welding wire 1a and the power feed tip 4. The plasma nozzle 51 is made of, for example, copper or a copper alloy, and is directly cooled by forming a flow path through which cooling water passes. The welding torch WT is moved relative to the base material 2 while being held by a normal robot (not shown). A mig arc 3 a is generated between the tip of the welding wire 1 a and the base material 2. Between the plasma electrode 1 b and the base material 2, a plasma arc 3 b thermally generated by the plasma gas 62 is generated. Therefore, the MIG arc 3a is in a state of being surrounded by the plasma arc 3b. For this reason, the plasma arc 3b has the effect | action which restrains that the shape of the mig arc 3a spreads. Moreover, the welding wire 1a receives heat radiation from the plasma arc 3b.

  The MIG welding power source PSM is a power source for energizing the MIG welding current Iwm by applying the MIG welding voltage Vwm between the welding wire 1 a and the base material 2 via the power supply tip 4. As shown in FIG. 5A, this MIG welding current Iwm is formed from a peak current during the peak period and a base current during the base period. A feed control signal Fc is sent from the MIG welding power source PSM to the feed motor WM, and the feed direction and feed speed Fw of the welding wire 1a are controlled. When the MIG welding voltage Vwm is applied from the MIG welding power source PSM, the welding wire 1a is set to the + side except for a partial period at the time of arc start (period from time t3 to t31 in FIG. 1). The MIG welding power source PSM is a power source having a constant voltage characteristic except for a certain period at the time of arc start as will be described later, and the MIG welding voltage Vwm is equal to a predetermined voltage setting signal Vr (not shown). To be controlled. The average value of the MIG welding current Iwm is determined by the steady feeding speed of the welding wire 1a. Further, as will be described later with reference to FIG. 3, the MIG welding power source PSM receives a plasma arc generation determination signal Ad that becomes a high level when the plasma arc 3b is generated at the time of arc start (time t4 in FIG. 1). Input from PSP.

  The plasma welding power source PSP is a power source for energizing the plasma welding current Iwp by applying a plasma welding voltage Vwp between the plasma electrode 1b and the base material 2. When the plasma welding voltage Vwp is applied from the plasma welding power source PSP, the plasma electrode 1b is set to the + side. The plasma welding power source PSP is a power source having a constant current characteristic, and is controlled so that the plasma welding current Iwp becomes a predetermined value. As will be described later with reference to FIG. 4, the plasma welding power source PSP is provided with a circuit for determining the generation of the plasma arc 3b, and outputs the plasma arc generation determination signal Ad to the MIG welding power source PSM.

  The welding start circuit ST installed outside both power supplies outputs a welding start signal St to the MIG welding power source PSM and the plasma welding power source PSP. When this welding start signal St is input, both welding power sources are activated. The welding start circuit ST is provided in the robot controller for robot welding.

  FIG. 3 is a block diagram of the MIG welding power source PSM constituting the above-described welding apparatus of FIG. Hereinafter, each block will be described with reference to FIG.

  The inverter circuit INV performs output control such as inverter control on a DC voltage obtained by rectifying and smoothing a commercial power source (not shown) such as a three-phase 200 V, and outputs high-frequency AC. The inverter transformer INT steps down the high frequency AC voltage to a voltage value suitable for arc welding. Secondary rectifiers D2a to D2d rectify the stepped-down high-frequency alternating current into direct current. The electrode plus polarity transistor PTR is turned on by an electrode plus polarity drive signal Pd described later, and at that time, the output of the welding power source becomes the electrode plus polarity. The electrode negative polarity transistor NTR is turned on by an electrode negative polarity drive signal Nd described later, and at that time, the output of the welding power source has an electrode negative polarity. The reactor WL smooths the rippled output. By these circuits, the MIG welding voltage Vwm is applied between the welding wire 1a and the base material 2, and the MIG welding current Iwm is energized. The welding wire 1 a is fed through the power feed tip 4 by a feed roll 7 coupled to a feed motor WM, and a mig arc 3 a is generated between the welding wire 1 a and the base material 2. The structure of the welding torch is as shown in FIG. 2 and is shown here in a simplified manner.

  The voltage detection circuit VD detects the absolute value of the MIG welding voltage Vwm and outputs a voltage detection signal Vd. The voltage average value calculation circuit VAV calculates an average value of the voltage detection signal Vd and outputs a voltage average value signal Vav. The average value is calculated by passing through a low-pass filter as described above. The voltage setting circuit VR outputs a predetermined voltage setting signal Vr. The voltage error amplification circuit EV amplifies an error between the voltage setting signal Vr and the voltage average value signal Vav, and outputs a voltage error amplification signal Ev. The voltage / frequency conversion circuit VF outputs a pulse period signal Tf having a frequency corresponding to the value of the voltage error amplification signal Ev. The pulse period signal Tf is a trigger signal that becomes High level for a short time every pulse period.

  The peak period setting circuit TPR outputs a predetermined peak period setting signal Tpr. The peak period timer circuit TP outputs a peak period signal Tp that is at a high level only during a period determined by the value of the peak period setting signal Tpr when the pulse period signal Tf is at a high level. The peak period is the peak period when the peak period signal Tp is at the high level, and the base period is when the peak period signal Tp is at the low level.

  The base current setting circuit IBR outputs a predetermined base current setting signal Ibr. The peak current setting circuit IPR outputs a predetermined peak current setting signal Ipr. The pulse current setting switching circuit SWP outputs the base current setting signal Ibr as the pulse current setting signal Irc when the peak period signal Tp is at the low level, and outputs the peak current setting signal Ipr when the peak period signal Tp is at the high level. It is output as a pulse current setting signal Irc.

  The short circuit determination circuit SD receives the voltage detection signal Vd as described above, and outputs a short circuit determination signal Sd that is determined to be in a short circuit state when the value is equal to or less than the short circuit reference value and becomes High level. The MIG arc generation determination circuit MD receives the voltage detection signal Vd as described above, and outputs a MIG arc generation determination signal Md that is determined to be in the MIG arc generation state when the value is within a predetermined range and becomes High level. The initial short circuit current setting circuit ISR outputs a predetermined initial short circuit current setting signal Isr. The first initial arc current setting circuit II1R outputs a predetermined first initial arc current setting signal Ii1r. The second initial arc current setting circuit II2R outputs a predetermined second initial arc current setting signal Ii2r. The first reference distance setting circuit LT1R outputs the first reference distance setting signal Lt1r by the method described above in the items a) and b). That is, the value of the first reference distance setting signal Lt1r is set using the plasma electrode-base material distance Lp, the reference ratio α, and the second reference distance Lt2. The first reference distance arrival determination circuit LD receives the first reference distance setting signal Lt1r, the voltage detection signal Vd and the Mig arc generation determination signal Md as input, and from the time when the Mig arc generation determination signal Md changes to the High level. When the value of the voltage detection signal Vd reaches a predetermined reference voltage value corresponding to the value of the first reference distance setting signal Lt1r, a first reference distance arrival determination signal Ld that changes to a high level is output. The initial current setting switching circuit SWS includes the initial short circuit current setting signal Isr, the first initial arc current setting signal Ii1r, the second initial arc current setting signal Ii2r, and a welding start signal from an external welding start circuit ST. When the St, Mig arc occurrence determination signal Md and the first reference distance arrival determination signal Ld are input, and the welding start signal St becomes High level (welding start), the initial short-circuit current setting signal Isr is set as the initial current setting signal Iir. The first initial arc current setting signal Ii1r is output as the initial current setting signal Iir when the mig arc occurrence determination signal Md becomes High level, and then the second reference distance arrival determination signal Ld becomes the second level when the first reference distance arrival determination signal Ld becomes High level. The initial arc current setting signal Ii2r is output as the initial current setting signal Iir. Therefore, as shown in FIG. 1B, the initial current setting signal Iir becomes the initial short-circuit current setting signal Isr during the period before time t3, and the first initial arc current setting signal Ii1r during the period from time t3 to t31. Thus, during the period after time t31, the second initial arc current setting signal Ii2r is obtained.

  The polarity switching control circuit PC receives the MIG arc occurrence determination signal Md and the first reference distance arrival determination signal Ld as input, and the MIG arc occurrence determination signal Md is at a high level and the first reference distance arrival determination signal Ld. When the signal is at the low level, the polarity switching control signal Pc that is at the high level and at the low level at other times is output. When the polarity switching control signal Pc is at a high level, the output polarity of the welding power source is an electrode plus polarity, and when it is at a low level, it is an electrode minus polarity. Therefore, the polarity switching control signal Pc is at the Low level (electrode negative polarity) in FIG. 1 during the period from time t3 to t31, and is at the High level (electrode positive polarity) during the other periods. The electrode positive polarity drive circuit PD receives this polarity switching control signal Pc, and outputs the electrode positive polarity driving signal Pd when the polarity switching control signal Pc is at the high level, and does not output it when it is at the low level. The electrode negative polarity drive circuit ND receives this polarity switching control signal Pc, outputs the electrode negative polarity driving signal Nd when the polarity switching control signal Pc is at the low level, and does not output it when it is at the high level. Although not shown, when the polarity is switched (time t3 and t31 in FIG. 1), as described above, a re-ignition voltage of several hundred volts is instantaneously applied to the welding wire 1a in order to prevent arc break. A circuit that overlaps between the base material 2 and the base material 2 is provided.

  The current setting switching circuit SWI receives a welding start signal St from the external welding start circuit ST, a plasma arc occurrence determination signal Ad from the plasma welding power source PSP, the pulse current setting signal Irc and the initial current setting signal Iir. When the welding start signal St changes to the high level (welding start), the initial current setting signal Iir is output as the current setting signal Ir, and when the plasma arc generation determination signal Ad changes to the high level (arc generation), the pulse current setting signal Irc is output as the current setting signal Ir. Therefore, the current setting signal Ir becomes the initial current setting signal Iir during the period from time t1 to t4 in FIGS. 6 and 1, and after the time t4 in FIGS. 6 and 1, the pulse current setting signal Irc (peak current setting signal). Ipr or base current setting signal Ibr).

  The current detection circuit ID detects the absolute value of the MIG welding current Iwm and outputs a current detection signal Id. The current error amplification circuit EI amplifies an error between the current setting signal Ir and the current detection signal Id, and outputs a current error amplification signal Ei. The drive circuit DV receives the welding start signal St from the external welding start circuit ST and the current error amplification signal Ei described above, and performs PWM according to the current error amplification signal Ei when the welding start signal St is at a high level (welding start). Modulation control is performed and a drive signal Dv is output, and when it is at a low level (welding stop), the drive signal Dv is not output.

  The feed speed setting circuit FR receives the welding start signal St from the external welding start circuit ST, the short-circuit determination signal Sd and the plasma arc generation determination signal Ad from the plasma welding power source PSP as inputs, and starts the welding start signal St. Changes to the high level (welding start) and becomes a predetermined slow-down feed speed setting value, and when the short circuit determination signal Sd changes to the high level (short circuit), the predetermined reverse feed speed setting value becomes and the plasma arc occurrence determination When the signal Ad changes to a high level (arc generation), a feed speed setting signal Fr that becomes a predetermined steady feed speed set value is output. Therefore, the feed speed setting signal Fr becomes a slow-down feed speed set value during the period from time t1 to t2 in FIG. 6, and becomes a reverse feed speed set value during the period from time t2 to t4 in FIG. During the period after t4, the constant feed speed setting value is set. The feed control circuit FC receives the welding start signal St and the feed speed setting signal Fr as described above, and corresponds to the value of the feed speed setting signal Fr when the welding start signal St becomes a high level (welding start). A feed control signal Fc for feeding the welding wire 1a at the feed speed is output to the feed motor WM.

  With the above circuit configuration, the MIG welding current Iwm, the MIG welding voltage Vwm, and the feed speed Fw as shown in FIGS. 6 and 1 are controlled.

  FIG. 4 is a block diagram of the plasma welding power source PSP that constitutes the welding apparatus of FIG. 2 described above. Hereinafter, each block will be described with reference to FIG.

  The power supply main circuit PM receives a commercial power supply (not shown) such as three-phase 200V as input, performs output control such as inverter control according to a drive signal Dv described later, and outputs a plasma welding current Iwp and a plasma welding voltage Vwp. The plasma welding current Iwp is energized through the plasma electrode 1b, the plasma arc 3b, and the base material 2. The structure of the welding torch is as shown in FIG. 2 described above, but is simplified here.

  The plasma welding current setting circuit IWPR outputs a plasma welding current setting signal Iwpr for setting a steady plasma welding current value. The plasma welding current detection circuit IDP detects the plasma welding current Iwp and outputs a plasma welding current detection signal Idp. The current error amplification circuit EI amplifies an error between the plasma welding current setting signal Iwpr and the plasma welding current detection signal Idp and outputs a current error amplification signal Ei. The drive circuit DV receives the welding start signal St from the external welding start circuit ST and the current error amplification signal Ei described above, and performs PWM according to the current error amplification signal Ei when the welding start signal St is at a high level (welding start). Modulation control is performed and a drive signal Dv is output, and when it is at a low level (welding stop), the drive signal Dv is not output. By performing the output control of the welding power source according to the drive signal Dv, the plasma welding current Iwp as described above with reference to FIG. 1 is energized. Therefore, the plasma welding power source PSP is controlled so that the plasma welding current Iwp is equal to the value of the plasma welding current setting signal Iwpr, and thus becomes a power source with constant current characteristics.

  The plasma arc occurrence discriminating circuit AD receives the plasma welding current detection signal Idp as described above, and determines that a plasma arc has occurred when the value exceeds a threshold value and becomes a high level. Ad is output to the MIG welding power source PSM. For example, the threshold value is set to about 5A.

  According to the above-described embodiment, the first negative arc current having the electrode negative polarity is applied during the period from the time when the initial MIG arc is generated until the arc length reaches the predetermined first reference distance. The polar second initial arc current is energized, and the absolute value of the first initial arc current is set to a value larger than the absolute value of the second initial arc current. Since the first initial arc current has an electrode negative polarity and its value is large, the welding wire melting rate increases. Since the backward feeding of the welding wire is added to this, the arc length of the initial MIG arc is rapidly increased to reach the first reference distance. For this reason, since the period during which the initial mig arc is independently generated can be shortened, the occurrence of smut can be suppressed. Furthermore, when the arc length of the initial MIG arc becomes longer than the first reference distance and approaches the plasma electrode, the initial arc current becomes the positive polarity of the electrode, and the value is also switched to a small value, so that the welding wire is hardly melted. Disappear. For this reason, the arc length of the initial MIG arc is gradually increased because it is fed backward at a relatively low backward feeding speed. As a result, since the arc length of the MIG arc does not become too long when the plasma arc is generated and when switching from backward feed to re-forward feed, the arc state does not become unstable. Furthermore, since the period during which the initial mig arc is generated alone can be shortened, the time required for the arc start can also be shortened.

1a Welding wire 1b Plasma electrode 2 Base material 3a MIG arc
31a Initial MIG arc 3b Plasma arc 4 Feed tip 51 Plasma nozzle 52 Shield gas nozzle 61 Center gas 62 Plasma gas 63 Shield gas 7 Feeding roll AD Plasma arc generation determination circuit Ad Plasma arc generation determination signal D2a to D2d Secondary rectifier DV Drive circuit Dv Drive signal EI Current error amplification circuit Ei Current error amplification signal EV Voltage error amplification circuit Ev Voltage error amplification signal Fb Reverse feed speed FC Feed control circuit Fc Feed control signal Fi Slow down feed speed FR Feed speed setting circuit Fr Feeding speed setting signal Fw Feeding speed Fwc Steady feeding speed Ib Base current IBR Base current setting circuit Ibr Base current setting signal ID Current detection circuit Id Current detection signal IDP Plasma welding current detection circuit Idp Plasma welding current detection signal Ii Initial arc Current Ii 1 First initial arc current value II1R First initial arc current setting circuit Ii1r First initial arc current setting signal Ii2 Second initial arc current value II2R Second initial arc current setting circuit Ii2r Second initial arc current setting signal Iir Initial current setting Signal INV Inverter circuit INT Inverter transformer Ip Peak current IPR Peak current setting circuit Ipr Peak current setting signal Ir Current setting signal Irc Pulse current setting signal Iwm Mig welding current Iwp Plasma welding current Iwpc Steady plasma welding current value IWPR Plasma welding current setting circuit Iwpr Plasma welding current setting signal La (Mig arc) arc length LD First reference distance arrival determination circuit Ld First reference distance arrival determination signal Lp Plasma electrode-base material distance Lt1 First reference distance LT1R First reference distance setting circuit Lt1r 1 reference distance setting signal Lt2 2nd reference distance D Migarc generation determination circuit Md Migarc generation determination signal ND Electrode minus polarity drive circuit Nd Electrode minus polarity drive signal NTR Electrode minus polarity transistor PC Polarity switching control circuit Pc Polarity switching control signal PD Electrode plus polarity driving circuit Pd Electrode plus polarity driving signal PSM MIG welding power source PSP Plasma welding power source PTR Electrode plus polarity transistor SD Short circuit determination circuit Sd Short circuit determination signal ST Welding start circuit St Welding start signal SWI Current setting switching circuit SWP Pulse current setting switching circuit SWS Initial current setting switching circuit Tb Base period Tf Pulse Period (signal)
TP Peak period timer circuit Tp Peak period (signal)
TPR Peak period setting circuit Tpr Peak period setting signal VAV Voltage average value calculation circuit Vav Voltage average value signal Vb Base voltage VD Voltage detection circuit Vd Voltage detection signal VF Voltage / frequency conversion circuit Vp Peak voltage VR Voltage setting circuit Vr Voltage setting signal Vt1 Reference voltage value Vwm Mig welding voltage Vwp Plasma welding voltage WL Reactor WM Feed motor WT Welding torch α Reference ratio

Claims (4)

A MIG arc is generated by passing a MIG welding current having a peak current during a peak period and a base current during a base period as one pulse period between a welding wire and a base material fed through a welding torch, and In plasma MIG welding that generates a plasma arc by energizing a plasma welding current between a plasma electrode and a base material arranged so as to surround a welding wire,
At the start of welding, a no-load voltage is applied between the plasma electrode and the base material, and the welding wire is fed forward, once brought into contact with the base material, and then retracted and pulled away. An initial MIG arc that is energized by an initial arc current is generated, the backward feeding is continued, and the arc length of the initial MIG arc is gradually increased to form a plasma atmosphere in the space between the plasma electrode and the base material. Thus, the plasma arc to be energized by the plasma welding current is generated, and when the plasma arc is generated, the welding wire is switched to re-forward feeding and the MIG welding current formed from the peak current and the base current is energized. In the arc MIG welding arc start control method to shift to steady MIG arc,
During the period from when the initial MIG arc is generated to when the arc length reaches a predetermined first reference distance, the first initial arc current having an electrode negative polarity is applied, and thereafter the period from when the plasma arc is generated. Energizes a second initial arc current of electrode positive polarity and sets the absolute value of the first initial arc current to a value greater than the absolute value of the second initial arc current;
An arc start control method of plasma MIG welding characterized by the above. The fact that the arc length of the initial MIG arc has reached the first reference distance is determined by the fact that the absolute value of the MIG welding voltage has reached a predetermined reference voltage value,
The arc start control method of plasma MIG welding according to claim 1. The first reference distance is set as a value obtained by multiplying a distance between the plasma electrode and the base material by a predetermined reference ratio;
The arc start control method of plasma MIG welding according to any one of claims 1 and 2. The first reference distance is set as a value obtained by subtracting a predetermined second reference distance from the distance between the plasma electrode and the base material;
The arc start control method of plasma MIG welding according to any one of claims 1 and 2.

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