AUTOMATIC ARC-WELDING METHOD

Abstract: An automatic arc-welding method, which comprises: when welding two objects to be welded together by means of a welding wire along a groove formed between the objects to be welded, continuously determining by cal­ culation a target welding current, with which a depth of penetration agrees with a target value thereof, on the basis of the root gap, the material of the welding wire and the like; continuously determining by calcula­ tion a target feeding rate of the welding wire, at which an extension length of the welding wire agrees with a target value thereof, on the basis of the welding current, the material of the welding wire and the like; continuously determining by calculation a target welding voltage, with which a length of the arc agrees with a target value thereof, on the basis of the welding current, the feeding rate of the welding wire, the material of the welding wire and the like; and automatically controlling the welding current, the feeding rate of the welding wire and the welding voltage on the basis of the respective target values thus determined.

Description: FIELD OF THE INVENTION The present invention relates to an automatic arc-welding method using a welding wire as a consumable welding electrode and more particularly an automatic arc-welding method which permits, when welding two objects to be welded together along the longitudinal direction of a groove formed in between, prevention of a burn-through even upon change in the distance of a root gap, and stabilization of an arc. VDI-Z, Vol. 128, No. 4, February 1986, VDI-Verlag Düsseldorf (DE), P. DREWS et al., "Automatisches Schweißsystem ASTRA", pages 123 to 127, discloses an automatic arc-welding method, which comprises continuously supplying a welding wire as a consumable welding electrode at a prescribed feeding rate substantially vertically through a nozzle toward a groove formed between the objects to be welded. The nozzle is continuously moved together with the welding wire along the longitudinal direction of the groove. A welding current having a defined welding voltage is supplied to the welding wire to produce an arc between the tip of the welding wire and said groove, thereby welding the objects to be welded together along the groove by means of the arc heat. According to this document, the distance of a root gap of the groove is continuously measured in the downstream of said nozzle in the travelling direction thereof. Further, dependent on the distance of the actual root gap, the welding parameters such as the welding current and the target welding speed are continuously adjusted. However, this prior art teaches that the welding parameters are controlled as follows: First of all, the root gap widths are classified in accordance with Richter's results, which determines allowable welding parameter tolerances for various gap widths in the thin sheet metal region. Then, there is prepared a table in which each value of the welding parameters applicable to the thus classified root gap widths is determined. Then, the welding parameters, i.e., the welding current, the feeding rate of the welding wire, the welding voltage and the like are selected by means of the table thus prepared in accordance with the actual root gap width. US-A-4 532 404 discloses an automatic arc-welding method, which comprises continuously supplying a welding wire as a consumable welding electrode at a prescribed feeding rate substantially vertically through a nozzle toward a groove formed between two objects to be welded. The nozzle is continuously moved together with the welding wire along the longitudinal direction of said groove. A welding current having a defined welding voltage is supplied to the welding wire to produce an arc between the tip of the welding wire and the groove, thereby welding the objects to be welded together along the groove by means of the arc heat. According to this document, when welding the objects along the groove, a distance of a root gap of the groove in the downstream of the nozzle in the travelling direction thereof is continuously measured, the thus measured distance of the root gap is used to control the electrode position control circuit, and to control the travelling speed of the welding electrode and the welding wire feed rate, and the above-mentioned distance of the root gap is further used to control the welding current. However, this document teaches that the welding parameters are controlled as follows: The electrode position, the travelling speed of the electrode and the feeding rate of the welding wire and the like are controlled in real time so as to maintain a constant thermal radiation profile in the vicinity of the root gap area, which is collected by the sensor, even if the actual root ghap width changes, taking into account the phenomenon that a thermal radiation profile in the vicinity of the root gap area changes according to changes in the actual root gap widths, heat dissipating inhibiting elements, and heat sink elements. BACKGROUND OF THE INVENTION When arc-welding two objects to be welded together along the longitudinal direction of a groove formed in between by means of a welding wire as a consumable welding electrode, welding is usually carried out with a distance of a root gap of the groove of zero, i.e., with the root faces kept in contact with each other. Particularly when conducting automatic arc-welding, it is the usual practice to strictly control the distance of the root gap so that the distance of the root gap is kept at 1 mm on the maximum. A large distance of the root gap leads to an incomplete penetration and a burn-through. Actually, however, variations in the distance of the root gap in the longitudinal direction of the groove are inevitable because of the limit in accuracy of machining. It is thus the conventional practice, before welding objects to be welded together along a groove, to modify the distance of the root gap in advance within allowable limits over the entire length of the groove by filling up portions with a large distance of the root gap by a manual welding, for example. In addition, it is necessary to increase a height of the root face so as not to cause a burn-through. However, such a manual operation to fill up the root gap carried out before the full-scale welding leads to a decreased efficiency of welding operations as a whole. As described above, furthermore, it is necessary to increase the height of the root face of the groove. As a result, when double-welding the objects to be welded from both of the first and second sides thereof, after welding of a first groove formed on the first side and before welding of a second groove formed on the second side, it is necessary to chip the second groove by gouging for example, until the bottom surface of the second groove reaches the weld bead formed in the first groove, in order to ensure a satisfactory penetration. Under such circumstances, there is a demand for development of an automatic arc-welding method, which ensures a constant depth of penetration, even upon change in the distance of a root gap in the longitudinal direction of the groove, permits prevention of a burn-through without increasing a height of the root face, and stabilization of an arc, but such an automatic arc-welding method has not as yet been proposed. SUMMARY OF THE INVENTION An object of the present invention is therefore to provide an automatic arc-welding method, which ensures a constant depth of penetration, even upon change in the distance of a root gap in the longitudinal direction of the groove, permits prevention of a burn-through without increasing a height of the root face, and stabilization of an arc. In accordance with one of the features of the present invention, there is provided, an automatic arc-welding method as claimed in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram illustrating a first embodiment of the automatic arc-welding method of the present invention; Fig. 2 is a block diagram illustrating a second embodiment and a third embodiment of the automatic arc-welding method of the present invention; Fig. 3 is a graph illustrating the relationship between a distance of the root gap of the groove and a welding current, with a depth of penetration as the parameter; Fig. 4 is a cross-sectional view illustrating a typical example of the groove; and Fig. 5 is a graph illustrating the relationship between a welding current, a feeding rate of the welding wire, a welding voltage and a welding speed. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS From the above-mentioned point of view, extensive studies were carried out for the purpose of developing an automatic arc-welding method, which ensures a constant depth of penetration, even upon change in the distance of a root gap in the longitudinal direction of the groove, permits prevention of a burn-through without increasing a height of the root face, and stabilization of an arc. As a result, the following finding was obtained: (1) It is possible to determine by calculation a target welding current, with which a depth of penetration agrees with a target value thereof, by measuring a distance of a root gap of the groove, if there is found out a certain relationship between the welding current and the distance of the root gap, in respect of the depth of penetration. (2) It is possible to determine by calculation a target feeding rate of the welding wire under the above-mentioned target welding current, at which an extension length of the welding wire from the tip of the nozzle agrees with a target value thereof, if there is found out a certain relationship between the welding current and the feeding rate of the welding wire, in respect of the extension length of the welding wire. (3) It is possible to determine by calculation a target welding voltage under the above-mentioned target welding current and the target feeding rate of the welding wire, with which a length of the arc agrees with a target value thereof, if there is found out a certain relationship between the welding voltage, welding current and the feeding rate of the welding wire, in respect of the length of the arc. The present invention was made on the basis of the above-mentioned findings. Now, a first embodiment of the automatic arc-welding method of the present invention is described in detail with reference to drawings. Fig. 1 is a block diagram illustrating a first embodiment of the automatic arc-welding method of the present invention. As shown in Fig. 1, two objects to be welded 1A and 1B are welded together along a groove 2 formed in between. A nozzle 3 continuously travels at a prescribed speed along the groove 2 by the action of a transfer unit 4. The nozzle 3 is rotatable at a high speed around the center axis thereof in the direction indicated by A in Fig. 1. A welding wire 6 as a consumable welding electrode is inserted into the nozzle 3 eccentrically from the center axis thereof. The welding wire 6 is continuously supplied by a welding wire feeder 7 toward the groove 2 substantially vertically through the nozzle 3. A welding current, with which a depth of penetration agrees with a target value thereof, as described later, is supplied between the objects to be welded 1A and 1B and the welding wire 6 from a welding power source 10 which is controlled by a first controller 5, whereby an arc is produced between the tip of the welding wire 6 and the groove 2, and the objects to be welded 1A and 1B are welded together along the groove 2 by the arc heat. The groove 2 is shielded from the open air by a shielding gas ejected from a nozzle (not shown) provided in the vicinity of the tip of the nozzle 3. It is needless to mention that application of the present invention is not limited to the above-mentioned method known as the rotating arc-welding method. A television camera 11 continuously shoots the groove 2 in the downstream in the travelling direction of the nozzle 3 (the direction indicated by B in Fig. 1). The television camera 11 is secured to the transfer unit 4. An image processor 12 continuously measures a distance of a root gap of the groove 2 in response to a shooting signal from the television camera 11. The data on the distance of the root gap measured by the image processor 12 are entered into a computer described later. The following formula (1) for use when welding the objects to be welded 1A and 1B together along the groove 2 is entered into the computer 13 for the calculation of a target welding current, with which a depth of penetration agrees with the target value thereof: I = I o - KG (1) where, I : target welding current, I o : welding current when the distance of root gap is zero, K : constant dependent on the depth of penetration, the kind of shielding gas, the material of welding wire and the like, and G : distance of root gap. The above-mentioned "I o " and "K" are previously entered from an input unit 14 into the computer 13. The computer 13 provides the first controller 5 for controlling the welding power source 10 with the result of calculation, i.e., a target welding current, with which the depth of penetration agrees with the target value thereof. Fig. 3 is a graph illustrating the relationship between a distance of the root gap of the groove and a welding current, with a depth of penetration as the parameter. More specifically, the graph of Fig. 3 clearly shows that there exists a certain relationship between the actual distance as measured of the root gap and the actual welding current as applied when accomplishing the respective depth of penetration of 1mm, 2mm and 3mm, and therefore demonstrates that Formula (1) above is valid. Now, the case in which the objects to be welded 1A and 1B are welded together along the groove 2 in accordance with the first embodiment of the present invention, will be described. The nozzle 3 continuously travels along the groove 2 by the action of the transfer unit 4 while rotating at a high speed around the center axis thereof. A welding current is supplied to the welding wire 6 inserted into the nozzle 3 from the welding power source 10 which is controlled by the first controller 5, whereby an arc is produced between the tip of the welding wire 6 and the groove 2, and the objects to be welded 1A and 1B are welded together along the groove 2 by the arc heat. The groove 2 is shot by the television camera 11 in the downstream in the travelling direction of the nozzle 3. A shooting signal from the television camera 11 is entered into the image processor 12. The image processor 12 continuously measures the distance (G) of the root gap of the groove 2 in response to the shooting signal. The data on the thus measured distance of the root gap is entered into the computer 13. The computer 13 calculates a target welding current (I), with which a depth of penetration agrees with a target value thereof, in accordance with the above-mentioned formula (1) entered previously into the computer 13. The computer 13 provides the first controller 5 with the thus calculated target welding current. The first controller 5 controls the welding power source 10 so that a welding current which agrees with the target welding current calculated as described above is supplied to the welding wire 6, whereby the depth of penetration of the groove 2 always agrees with the target value thereof, irrespective of any change in the distance (G) of the root gap. Now, a second embodiment of the automatic arc-welding method of the present invention is described with reference to Fig. 2. In the second embodiment of the automatic arc-welding method of the present invention, as in the above-mentioned first embodiment of the present invention, a target welding current, with which a depth of penetration agrees with a target value thereof, is calculated, and not only the welding current is controlled so as to agree with the thus calculated target welding current, but also a target feeding rate of the welding wire, at which an extension length of the welding wire 6 from the tip of the nozzle 3 agrees with a target value thereof, is calculated, and the feeding rate of the welding wire 6 is controlled so as to agree with the thus calculated target feeding rate of the welding wire 6. More particularly, the method according to the second embodiment of the present invention is the same as the method according to the above-mentioned first embodiment of the present invention, except that the feeding rate of the welding wire 6 by the welding wire feeder 7 is controlled by the second controller 8 so that an extension length of the welding wire 6 from the tip of the nozzle 3 agrees with a target value thereof. In the second embodiment of the present invention, not only the formula (1) used in the first embodiment of the present invention, but also the following formula (2) for calculating a target feeding rate of the welding wire 6, at which an extension length of the welding wire 6 from the tip of the nozzle 3 agrees with a target value thereof, are previously entered into the computer 13: V f = A·I + B·ℓ·I² (2) where, V f : target feeding rate of welding wire, I : welding current, ℓ : extension length of welding wire, and A, B : constants dependent on the kind of shielding gas, the material of the welding wire and the like. The above-mentioned "ℓ", "A" and "B" are previously entered from the input unit 14 into the computer 13. Fig. 5 is a graph illustrating the relationship between a welding current, a feeding rate of the welding wire, a welding voltage and a welding speed. In Fig. 5, the line representing the feeding rate of the welding wire clearly shows that there exists a certain relationship between the actual welding current as applied and the actual feeding rate as applied of the welding wire, and therefore demonstrates that Formula (2) is valid. The computer, into which the above-mentioned formulae (1) and (2) are previously entered, provides the first controller 5 for controlling the welding power source 10 with the result of calculation according to the formula (1), i.e., a target welding current, with which a depth of penetration agrees with the target value thereof, and on the other hand, provides the second controller 8 for controlling the welding wire feeder 7 with the result of calculation according to the formula (2), i.e., a target feeding rate of the welding wire 6, at which an extension length of the welding wire 6 from the tip of the nozzle 3 agrees with a target value thereof. The feeding rate of the welding wire 6 is controlled so that the extension length of the welding wire 6 agrees with the target value thereof to ensure stabilization of the arc. Now, the case in which the objects to be welded 1A and 1B are welded together along the groove 2 in accordance with the second embodiment of the present invention, will be described. The nozzle 3 continuously travels along the groove 2 by the action of the transfer unit 4 while rotating at a high speed around the center axis thereof. A welding current is supplied to the welding wire 6 inserted into the nozzle 3 from the welding power source 10 which is controlled by the first controller 5, whereby an arc is produced between the tip of the welding wire 6 and the groove 2, and the objects to be welded 1A and 1B are welded together along the groove 2 by the arc heat. The groove 2 is shot by the television camera 11 in the downstream in the travelling direction of the nozzle 3. A shooting signal from the television camera 11 is entered into the image processor 12. The image processor 12 continuously measures the distance (G) of the root gap of the groove 2 in response to the shooting signal. The data on the thus measured distance of the root gap is entered into the computer 13. The computer 13 calculates a target welding current (I), with which a depth of penetration agrees with a target value thereof, in accordance with the above-mentioned formula (1) entered previously into the computer 13. The computer 13 provides the first controller 5 with the calculated target welding current. The first controller 5 controls the welding power source 10 so that a welding current which agrees with the target welding current calculated as described above is supplied to the welding wire 6, whereby the depth of penetration of the groove 2 always agrees with the target value thereof, irrespective of any change in the distance (G) of the root gap. The computer 13 calculates also a target feeding rate (V f ) of the welding wire 6 under the above-mentioned calculated target welding current, at which an extension length of the welding wire 6 from the tip of the nozzle 3 agrees with a target value thereof, in accordance with the above-mentioned formula (2) entered previously into the computer 13. The computer 13 provides the second controller 8 with the thus calculated target feeding rate of the welding wire 6. The second controller 8 controls the welding wire feeder 7 so that the welding wire 6 is continuously supplied toward the groove 2 at a feeding rate of the welding wire 6, which agrees with the thus calculated target feeding rate of the welding wire 6, whereby the extension length of the welding wire 6 always agrees with the target value thereof, irrespective of any change in the welding current, thus permitting stabilization of the arc. Now, a third embodiment of the automatic arc-welding method of the present invention is described with reference to Fig. 2. In the third embodiment of the automatic arc-welding method of the present invention, as in the above-mentioned second embodiment of the present invention, a target welding current, with which a depth of penetration agrees with a target value thereof, is calculated, and the welding current is controlled so as to agree with the thus calculated target welding current. In addition, not only a target feeding rate of the welding wire 6, at which an extension length of the welding wire 6 from the tip of the nozzle 3 agrees with a target value thereof, is calculated, and the feeding rate of the welding wire 6 is controlled so as to agree with the thus calculated target feeding rate of the welding wire 6, but also a target welding voltage, with which a length of the arc agrees with a target value thereof, is calculated, and the welding voltage is controlled so as to agree with the thus calculated target welding voltage. More particularly, the method according to the third embodiment of the present invention is the same as the method according to the above-mentioned second embodiment of the present invention, except that the welding voltage is controlled to ensure further stabilization of the arc. In the third embodiment of the present invention, not only the formulae (1) and (2) used in the second embodiment of the present invention, but also the following formula (3) for calculating a target welding voltage, with which a length of the arc agrees with the target value thereof, are previously entered into the computer 13: E t = E ℓ + E a + E r (3) where, E t : target welding voltage, E ℓ : voltage drop caused by the extension length of the welding wire, E a : arc voltage, and E r : voltage drop caused by resistance equal to the sum of resistance between an output end of the welding power source and the nozzle and resistance between the other output end of the welding power source and the objects to be welded, said E ℓ , said E a and said E r being expressed respectively by the following formulae (4) to (6): E ℓ = a·ℓ·I - b·V f /I (4) E a = E o (I) + χ · ℓ a (5) E r = R·I (6) where, ℓ : extension length of the welding wire, I : welding current, V f : feeding rate of welding wire, a, b : constants dependent on the kind of shielding gas, the material of the welding wire and the like, E o (I) : voltage drop equal to the sum of voltage drop of the negative pole and voltage drop of the positive pole, χ : potential gradient of arc column, ℓa : arc length, and R : resistance equal to the sum of resistance between an output end of the welding power source and the nozzle and resistance between the other output end of the welding power source and the objects to be welded. The above-mentioned " ℓ ", "a", "b", "E o (I)", "χ", " ℓ a " and "R" are previously entered from the input unit 14 into the computer 13. In Fig. 5, the line representing the welding voltage clearly shows that there exists a certain relationship between the actual welding current as applied and the actual welding voltage as applied, and therefore demonstrates that Formula (3) above is valid. The computer 13, into which the above-mentioned formulae (1) to (3) are previously entered, provides the first controller 5 for controlling the welding power source 10 with the result of calculation according to the formula (1), i.e., a target welding current, with which a depth of penetration agrees with the target value thereof, provides the second controller 8 for controlling the welding wire feeder 7 with the result of calculation according to the formula (2), i.e., a target feeding rate of the welding wire 6, at which an extension length of the welding wire 6 from the tip of the nozzle 3 agrees with the target value thereof, and furthermore, provides the first controller 5 for controlling the welding power source 10 with the result of calculation according to the formula (3), i.e., a target welding voltage, with which a length of the arc agrees with the target value thereof. Now, the case in which the objects to be welded 1A and 1B are welded together along the groove 2 in accordance with the third embodiment of the present invention, will be described. The nozzle 3 continuously travels along the groove 2 by the action of the transfer unit 4 while rotating at a high speed around the center axis thereof. A welding current is supplied to the welding wire 6 inserted into the nozzle 3 from the welding power source 10 which is controlled by the first controller 5, whereby an arc is produced between the tip of the welding wire 6 and the groove 2, and the objects to be welded 1A and 1B are welded together along the groove 2 by the arc heat. The groove 2 is shot by the television camera 11 in the downstream in the travelling direction of the nozzle 3. A shooting signal from the television camera 11 is entered into the image processor 12. The image processor 12 continuously measures the distance (G) of the root gap of the groove 2 in response to the shooting signal. The data on the thus measured distance of the root gap is entered into the computer 13. The computer 13 calculates a target welding current (I), with which a depth of penetration agrees with a target value thereof, in accordance with the above-mentioned formula (1) entered previously into the computer 13. The computer 13 provides the first controller 5 with the calculated target welding current. The first controller 5 controls the welding power source 10 so that a welding current which agrees with the target welding current calculated as described above is supplied to the welding wire 6, whereby the depth of penetration always agrees with the target value thereof, irrespective of any change in the distance (G) of the root gap. The computer 13 calculates also a target feeding rate (V f ) of the welding wire 6 under the above-mentioned calculated target welding current, at which an extension length of the welding wire 6 from the tip of the nozzle 3 agrees with a target value thereof, in accordance with the above-mentioned formula (2) entered previously into the computer 13. The computer 13 provides the second controller 8 with the thus calculated target feeding rate of the welding wire 6. The second controller 8 controls the welding wire feeder 7 so that the welding wire 6 is continuously supplied toward the groove 2 at a feeding rate of the welding wire 6, which agrees with the thus calculated target feeding rate of the welding wire 6, whereby the extension length of the welding wire 6 always agrees with the target value thereof, irrespective of any change in the welding current, thus permitting stabilization of the arc. In addition, the computer 13 calculates a target welding voltage (E t ) under the above-mentioned calculated target welding current and the above-mentioned calculated target feeding rate of the welding wire 6, with which a length of the arc agrees with a target value thereof, in accordance with the above-mentioned formula (3) entered previously into the computer (13). The computer 13 provides the first controller 5 with the thus calculated target welding voltage. The first controller 5 controls the welding power source 10 so that a welding voltage equal to the target welding voltage, with which the arc length agrees with the target value thereof, is applied between the welding wire 6 and the objects to be welded 1A and 1B, thus ensuring further stabilization of the arc. In the present invention, the travelling speed of the transfer unit 4, i.e., the welding speed may be controlled so that a height of the weld bead agrees with a target value thereof by providing a third controller 9 as shown in Fig. 2 by the broken line. In this case, the following formula (7) for calculating a target welding speed, at which a height of the weld bead agrees with a target value thereof, is previously entered into the computer 13: V = V f (V fo /V o + d k · G) (7) where, V : target welding speed, V f : feeding rate of welding wire, V fo : feeding rate of welding wire when the distance of root gap is zero, V o : welding speed when the distance of root gap is zero, d : distance between the lowermost point of penetration and the uppermost point of weld bead, k : product of multiplication of the cross-sectional area of the welding wire and the deposition efficiency, and G : distance of root gap. The above-mentioned "V fo ", "V", "d" and "k" are previously entered from the input unit 14 into the computer 13. In Fig. 5, the line representing the welding speed clearly shows that there exists a certain relationship between the actual welding current as applied and the actual welding speed as applied, and therefore demonstrates that Formula (7) is valid. The computer 13 provides the third controller 9 for controlling the transfer unit 4 with the result of calculation according to the formula (7), i.e., the target welding speed, at which the height of the weld bead agrees with the target value thereof. Now, examples of the automatic arc-welding method of the present invention are described. EXAMPLE 1 Two stainless steel plates (SUS 304) having a thickness of 10 mm were prepared as the objects to be welded 1A and 1B, and a groove 2 of a shape as shown in Fig. 4 was formed in between. Then, the objects to be welded 1A and 1B were welded together along the groove 2 by means of an automatic arc-welding machine as shown in Fig. 1. A flux-cored wire having a diameter of 1.6 mm was used as the welding wire, and CO₂ gas was used as the shielding gas. The following data were previously entered into the computer 13 when welding the objects to be welded 1A and 1B together along the groove 2: For the calculation of the target welding current (I): P₁ : 2 mm, I o : 360A, and K : 37, where, P₁ : target depth of penetration of the groove, I o : welding current when the distance of root gap is zero, and K : constant dependent on the depth of penetration, the kind of shielding gas, the material of the welding wire and the like. Investigation of the depth of penetration of the groove 2 of the objects to be welded 1A and 1B thus welded together showed that, in spite of changes in the distance (G) of the root gap, the depth of penetration in the groove 2 agreed with the target value thereof, thus giving a satisfactory weld bead free of defects. EXAMPLE 2 Two stainless steel plates (SUS 304) having a thickness of 10 mm were prepared as the objects to be welded 1A and 1B, and a groove 2 of a shape as shown in Fig. 4 was formed in between. Then, the objects to be welded 1A and 1B were welded together along the groove 2 by means of an automatic arc-welding machine as shown in Fig. 2. A flux-cored wire having a diameter of 1.6 mm was used as the welding wire, and CO₂ gas was used as the shielding gas. The following data were previously entered into the computer 13 when welding the objects to be welded 1A and 1B together along the groove 2: For the calculation of the target welding current (I): P₁ : 2 mm, I o : 360A, and K : 37, where, P₁ : target depth of penetration of the groove, I o : welding current when the distance of root gap is zero, and K : constant dependent on the depth of penetration, the kind of shielding gas, the material of the welding wire and the like; For the calculation of the target feeding rate (V f ) of welding wire: ℓ : 15 mm, A : 0.2, and B : 4.59 x 10⁻⁵, where, ℓ : extension length of welding wire, and A, B : constants dependent on the kind of shielding gas, the material of the welding wire and the like. During welding of the objects to be welded 1A and 1B, a stable state of the arc was achieved. Investigation of the depth of penetration of the groove 2 of the objects to be welded 1A and 1B thus welded together showed that, in spite of changes in the distance (G) of the root gap, the depth of penetration in the groove 2 agreed with the target value thereof, thus giving a satisfactory weld bead free of defects. EXAMPLE 3 Two stainless steel plates (SUS 304) having a thickness of 10 mm were prepared as the objects to be welded 1A and 1B, and a groove 2 of a shape as shown in Fig. 4 was formed in between. Then, the objects to be welded 1A and 1B were welded together along the groove 2 by means of an automatic arc-welding machine as shown in Fig. 2. A flux-cored wire having a diameter of 1.6 mm was used as the welding wire, and CO₂ gas was used as the shielding gas. The following data were previously entered into the computer 13 when welding the objects to be welded 1A and 1B together along the groove 2: For the calculation of the target welding current (I): P₁ : 2 mm, I o : 360A, and K : 37, where, P₁ : target depth of penetration of the groove, I o : welding current when the distance of root gap is zero, and K : constant dependent on the depth of penetration, the kind of shielding gas, the material of the welding wire and the like; For the calculation of the target feeding rate (V f ) of welding wire: ℓ : 15 mm, A : 0.2, and B : 4.59 x 10⁻⁵, where, ℓ : extension length of welding wire, and A, B : constants dependent on the kind of shielding gas, the material of the welding wire and the like; For the calculation of the target welding voltage (E t ): a : 1.12 x 10⁻³, b : 2.19, E o (I) : 0.025 I x 16.4 (V), χ : 2.4 (V/mm), ℓ a : 1.5 mm, and R : 0.1Ω, where, a, b : constants dependent on the kind of shielding gas, the material of the welding wire and the like, E o (I) : voltage drop equal to the sum of voltage drop of the negative pole and voltage drop of the positive pole, χ : potential gradient of arc column, ℓ a : arc length, and R : resistance equal to the sum of resistance between an output end of the welding power source and the nozzle and resistance between the other output end of the welding power source and the objects to be welded. During welding of the objects to be welded 1A and 1B, a further stable state of the arc was achieved. Investigation of the depth of penetration of the groove 2 of the objects to be welded 1A and 1B thus welded together showed that, in spite of changes in the distance (G) of the root gap, the depth of penetration in the groove 2 agreed with the target value thereof, thus giving a satisfactory weld bead free of defects. According to the present invention, as described above, a constant depth of penetration is available, irrespective of any change in the distance of the root gap, thereby permitting prevention of a burn-through without increasing the height of the root faces, and stabilization of the arc, thus providing industrially useful effects.

Claims: An automatic arc-welding method, which comprises: continuously supplying a welding wire as a consumable welding electrode at a prescribed feeding rate substantially vertically through a nozzle toward a groove formed between two objects to be welded; continuously moving said nozzle together with said welding wire along the longitudinal direction of said groove; supplying a prescribed welding current having a prescribed welding voltage to said welding wire to produce an arc between the tip of said welding wire and said groove, thereby welding said objects to be welded together at a prescribed welding speed along said groove by means of the arc heat; continuously measuring a distance of a root gap of said groove in the downstream of said nozzle in the travelling direction thereof, while continuing said welding of said objects to be welded; and controlling said welding current, said feeding rate of said welding wire and said welding voltage on the basis of said distance thus measured of said root gap of said groove; characterized in that: said control of said welding current comprises calculating a target welding current (I), at which a depth of penetration agrees with a target value thereof, in accordance with the following formula (1): I = I₀ - KG (1) and causing said welding current to agree with the thus calculated target welding current; said control of said feeding rate of said welding wire comprises calculating a target feeding rate (Vf) of said welding wire, at which an extension length of said welding wire from the tip of said nozzle agrees with a target value thereof, in accordance with the following formula (2): Vf = A · I + B · ℓ · I² (2) and causing said feeding rate of said welding wire to agree with the thus calculated target feeding rate of said welding wire; and said control of said welding voltage comprises calculating a target welding voltage (E t ), at which a length of said arc agrees with a target value thereof, in accordance with the following formula (3): E t = E ℓ + E a + E r (3) said E ℓ , said E a and said E r being expressed respectively by the following formulae (4) to (6): E ℓ = a. ℓ .I - b.vf/I (4) E a = E₀(I) + x. ℓ a (5) E r = R.I (6) and causing said welding voltage to agree with the thus calculated target welding voltage; where, in said formulae (1) to (6), I : target welding current, I₀ : welding current when the distance of root gap is zero, K : constant dependent on the depth of penetration, the kind of shielding gas, and the material of the welding wire, G : distance of root gap, Vf : target feeding rate of welding wire, ℓ : extension length of welding wire, A,B : constants dependent on the kind of shielding gas, and the material of the welding wire, E t : target welding voltage, E ℓ : voltage drop caused by the extension length of the welding wire, E a : arc voltage, E r : voltage drop caused by resistance equal to the sum of resistance between an output end of the welding power source and the nozzle and resistance between the other output end of the welding power source and the objects to be welded, a,b : constants dependent on the kind of shielding gas, and the material of the welding wire, E₀(I) : voltage drop equal to the sum of voltage drop of the negative pole and voltage drop of the positive pole, x : potential gradient of arc column, ℓ a : arc length, and R : resistance equal to the sum of resistance between an output end of the welding power source and the nozzle and resistance between the other output end of the welding power source and the objects to be welded.

Inventors: NISHI, YASUHIKO C/O NKK CORPORATION, SUGITANI, YUJI C/O NKK CORPORATION
Applicants: NKK CORPORATION
Application year: 1988
Main classification: B23K9
IPC codes: B23K9