HORIZONTAL WELDING METHOD AND WELDING EQUIPMENT

Abstract: In order to enable highly efficient welding without the generation of weld defect such as undercut, overlap, and defective meeting angle, a plurality of additive wires (5a, 5b) are inserted into a melting pool (12) during horizontal welding, a unidirectional current is made to flow between the wires, and a magnetic field (9) is used to provide a magnetic field to the melting pool substantially in a direction perpendicular to a surface of a parent metal so as to cause the melting pool (12) to generate upward (antigravitational direction) Lorentz's force (11), whereby a molten metal is lifted upward to prevent dripping of the molten metal and to improve its bead shape.

Description: TECHNICAL FIELD This invention concerns a method of welding in the horizontal position, in which the molten pool is liable to experience deformation under the influence of gravity, and a welding apparatus for that purpose. More specifically, it concerns a welding method in the horizontal position to be used in the production of large structures such as smokestacks, bridges, pressure vessels and the like, and a welding apparatus for that purpose. It also concerns an apparatus for performing the method. BACKGROUND OF THE TECHNOLOGY Welding has been an indispensable process in the manufacture of large steel structures such as smokestacks, bridges and pressure vessels. Because such large structures cannot easily be inverted during production, some portions must be welded in the horizontal position. In butt welding, four welding positions may be used: the flat position, the horizontal position, the vertical position and the overhead position. In fillet welding, four welding positions also may be used: the flat position, the horizontal position, the vertical position and the overhead position. Generally, "welding in the overhead position" refers to welding a joint from below with the axis of the weld approximately horizontal. "Welding in the flat position" refers to welding a joint from above with the axis of the weld approximately horizontal. "Welding in the horizontal position" refers to welding a joint from the side with the axis of the weld approximately horizontal. "Welding in the vertical position" refers to welding a joint with a vertical axis of the weld lengthwise along the vertical. In the disclosures which follow, the phrase "welding in the horizontal position" will refer to welds such that the angle of the parent material with respect to a horizontal surface is either approximately vertical or within the range of 45° to 90°; and the weld line is either approximately horizontal or its angle with respect to a horizontal surface is within the range of 0° to 45°. In other words, the present invention is also applicable in a range of welding positions which might be called intermediate between the common overhead, flat and horizontal positions. In this discussion we shall use the phrase "horizontal position" in the wider meaning specified above. When a joint is welded in the horizontal position, as shown in the sketch in Figure 9, the weight of the molten metal gives weld bead 7A a tendency to sag, with the result that the upper portion of the open end of the bead is undercut (A in Figure 9) while the lower portion has an overlap (B in Figure 9). This may lead to welding defects such as inadequate fusion when the subsequent layer is welded. This is why such welding is usually executed by a highly-skilled welder using a semi-automated process. Even if this welding process is automated, the welding apparatus must have the same level of capability as a skilled welder mentioned above. A number of weaving mechanisms will be necessary. And since such mechanisms are still not enough, the process must be conducted at a low temperature with a resulting low efficiency. This stands in the way of making the welding process more efficient. With the techniques used in the prior art, then, welding defects such as an undercut of the upper portion of the open end of the bead or an overlap of the lower portion were likely to occur when a joint was welded in the horizontal position. JP-A-63-119980, which represents the closest prior art, discloses a welding method which uses a coil to produce a magnetic force to push the molten welding pool up against the force of gravity. DESCRIPTION OF THE INVENTION It is an object of the invention to provide a method and an apparatus suitable for welding in horizontal position, which are efficient and prevent defective welds caused by undercuts or overlaps. This object is solved by the method set forth in claim 1 and by the apparatus for carrying out this method, as set forth in claim 4. The subclaims are directed to preferred embodiments of the invention. A preferred embodiment of the present invention is designed as follows. It concerns a method of welding in the horizontal position, in which a heat source such as an arc, a laser or an electron beam is used to melt the parent material and form a molten pool. Because of the position of the weld, gravity tends to cause the bead to deform. A current is made to flow in the molten pool in approximately the same direction as the weld line, and a magnetic field is induced in the molten pool orthogonal to the direction of the current. The joint is welded while an upward Lorentz force (opposite the direction of gravity) is generated in the molten pool. The means by which a current is made to flow in the direction of the weld line in the molten pool, comprise a plurality of auxiliary wires. These wires are inserted into the molten pool and a current is passed through them. The current, ideally a unidirectional current, is made to flow in the molten pool in the direction of the weld line. It would be also possible that, when an arc is used as the heat source for welding, a portion of the welding current is split off and passed through the auxiliary wires. A welding apparatus which is advantageous to implement the welding method comprises: a heat source for welding comprising an arc, a laser or an electron beam; a plurality of auxiliary wires placed so as to face the molten pool formed by the heat source; a power supply for generating a current to run through the wires; and a magnetic field inducing means to induce a magnetic field in the molten pool which intersects the surface of the parent material. If a welding apparatus is used which employs an arc as the heat source for welding, an apparatus can be provided to pass a portion of the welding current into the auxiliary wires. The operation of this invention will be discussed with reference to the circuit connection diagrams in Figures 1 and 2 and the drawing in Figure 3 showing how the molten metal is upheld in a preferred embodiment. In Figures 1 and 2, two auxiliary wires 5a and 5b are used. As is shown in Figure 1, the minus and plus sides of power supply 2 are connected to the respective wires. In addition to this, or instead of power supply 2, a portion of the welding current can be passed to auxiliary wire 5b, as shown in Figure 2. When a current is conducted between auxiliary wires 5a and 5b, a unidirectional current A is created in molten pool 12. When magnetic field 9 is induced in a vertical direction in the drawing, a Lorentz force 11 is activated in the direction which allows it to support the molten pool. This prevents the weld bead from sagging. When a current is conducted through auxiliary wires 5a and 5b, these wires are heated by the Joule effect, and their deposition rate is enhanced. If a number of auxiliary wires are used, the surface area of the wires which can absorb the radiant heat of the arc will be larger. Thus the melting capability of the wires will increase. The TIG hot-wire welding method is similar to the method according to this invention. In the TIG method, as shown in Figure 10, heating power supply 102, which passes a current into consumable wire 101, is separate from TIG arc power supply 100. Current is supplied to consumable wire 101 from a point approximately 10 cm away from parent material 7, and the resistance heating heats wire 101. Since wire 101 is sent to the molten pool in a half-melted state, the deposition rate is increased by up to three times that of the usual method. The TIG hot-wire welding method and that according to the present embodiment have in common that they both involve supplying current to a consumable wire. However, the object of the former method is to heat the wire by resistance and send it to the molten pool in a half-melted state. That of the present embodiment is to pass a current through multiple melting wires in the direction of the weld line, and so induce a magnetic field in the molten pool 12 which is roughly orthogonal to the current. In this way a Lorentz force 11 will be activated in an upward direction in the molten pool 12. The basic concepts underlying the two methods are different, as are the necessary configurations: the former uses a single consumable wire, while the present embodiment uses two auxiliary wires, 5a and 5b, to pass a current in molten pool 12 in the direction of the weld line. Auxiliary wires 5a and 5b need not be consumable wires. As the apparatus to apply a magnetic field, an apparatus may be provided, as shown in Figure 7, which concentrates the heat source and applies a magnetic field. The present invention is particularly advantageously applied when the weld line is approximately horizontal and the surface of the parent material is approximately vertical. However, the effects of this invention are readily achieved throughout the range of horizontal position welding defined above, so long as a Lorentz force is generated in an upward direction (against the force of gravity) in the molten pool while the joint is welded. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows the circuit connections in a welding apparatus in the horizontal position which is a first preferred embodiment of this invention. Figure 2 shows the circuit connections in a welding apparatus in the horizontal position which is a second preferred embodiment of this invention. Figure 3 is a drawing to illustrate how the molten metal is supported in a method of welding in the horizontal position according to an embodiment of this invention. Figure 4 shows cross sections of the bead in horizontal position welding. (A) shows the appearance of the bead without the application of a magnetic field. (B) is a cross section of a bead produced by an embodiment of this invention. Figure 5 shows the circuit connections in a welding apparatus in the horizontal position which is a third preferred embodiment of this invention. Figure 6 shows the circuit connections in a welding apparatus in the horizontal position which is a fourth preferred embodiment of this invention. Figure 7 shows the circuit connections in a welding apparatus in the horizontal position which is a fifth preferred embodiment of this invention. In this figure, (A) is an enlarged perspective drawing of the magnetic coil. Figure 8 shows the circuit connections in a welding apparatus in the horizontal position which is a sixth preferred embodiment of this invention. Figure 9 is a cross section of the welding area when the prior art welding method is used. Figure 10 shows an apparatus employing the TIG hot-wire welding method, a method belonging to the prior art. PREFERRED EMBODIMENTS We shall now discuss in detail several preferred embodiments of this invention, with reference to the drawings. To the extent that the dimensions, material, shape and relative positions of the constituent components are not specifically disclosed in these embodiments, the scope of the present invention is not limited only to those in the embodiments, which serve merely as illustrative examples. Figures 1, 3 and 4 show a first preferred embodiment of this invention. This method, and the apparatus to make use of it, are used in TIG welding in the horizontal position. It employs an arc for heat source 10 and a nonconsumable electrode (a tungsten electrode) for electrode 13. In Figure 1, 1 is the welding power supply. A TIG welding power supply with DC constant current characteristics is used. An arc (the heat source) is generated between electrode 13, which protrudes from welding torch 4, and parent material 7. The heat of the arc melts the parent material. As can be seen in Figures 1 and 3, auxiliary (melting) wires 5a and 5b are placed on molten pool 12 on either side of arc heat source 10. The wires 5a and 5b are connected, respectively, to the minus and plus sides of power supply 2. In this way a current is made to flow horizontally parallel to the weld line. The power supply 2 supplies a direct current and generates a constant electric field. In addition to or instead of power supply 2, a current splitting device 8 shown in Figure 2 can be used to pass a portion of the welding current to wire 5b. Current splitting device 8 may comprise two variable resistors so that the current supplied to auxiliary wire 5b may be varied. The welding torch 4 is surrounded by magnetic coil 6, which is connected to power supply 3. Magnetic field 9 can be induced in the direction which appears to be upward in Figure 1 (horizontally and orthogonal to the weld line). With this configuration, as can be seen in Figure 3, the current which flows in molten pool 12 between the auxiliary wires 5a and 5b generates a unidirectional current A along a horizontal path parallel to the weld line. When a magnetic field 9 is induced which is orthogonal to the weld line and to the unidirectional current A, a Lorentz force 11 is generated which operates so as to uplift molten pool 12 (i.e., it operates against the force of gravity). This produces a weld bead which does not sag. When a current is passed through auxiliary wires 5a and 5b, they are heated by the Joule effect just as in TIG hot-wire welding methods belonging to the prior art, and their deposition rate increases. However, the present invention makes use of multiple wires, 5a and 5b, so there is a greater surface area of these wires to absorb the radiant heat of arc heat source 10. As a result, the melting capacity of wires 5a and 5b is increased. The effect of this embodiment is verified from the shape of the bead obtained in a bead-on-plate welding test. The conditions of welding are a welding current of DC:500 A and a current to the wires of DC:120 A. The deposition rate of wires 5a and 5b is 100 g/min. The welding speed is 50 cm/min. The strength of the magnetic field 9 applied to molten pool 12 can be varied. The magnetic density is varied by changing the strength of the current from power supply 3 to magnetic coil 6, which is wound around TIG welding torch 4 as shown in Figure 1. Varying this current during welding will cause the magnetic flux density to vary. Figure 4 shows two cross sections of the welding location. In the one on the left, no magnetic field is applied. In the one on the right, a field with a magnetic flux density of 0.01 T (tesla) is applied. (The current from power supply 3 which induces the field is DC. The welding is done in a bead-on-plate test.) If we compare the two, we see clearly that the welding method according to this invention produces a bead which is buoyed up against the force of gravity. Just as in the TIG hot-wire welding method, passing a current through auxiliary wires 5a and 5b causes them to heat up due to the Joule effect. As a result, the welding speed is increased. With this configuration, the welding speed will be two to three times that achieved if a current is not passed through wires 5a and 5b. Figure 2 illustrates the circuit connections in a welding apparatus in the horizontal position which is a second preferred embodiment of this invention. In the embodiment shown in this figure, current splitting device 8 is placed in a location where the currents from power supplies 1 and 2 are confluent. A portion of the welding current supplied to parent material 7 is split off and supplied to auxiliary wire 5b. Current splitting device 8 comprises two variable resistors so that the current supplied to auxiliary wire 5b may be varied. Thus a circuit configuration such that a portion of the welding current is supplied to one of the auxiliary wires will yield the same result as did the configuration used in the first embodiment. Figure 5 illustrates the circuit connections in a welding apparatus in the horizontal position which is a third preferred embodiment of this invention. In this figure, the same TIG welding method is used as in the first embodiment. Auxiliary wires 5a and 5b are on either side of arc heat source 10, but in this embodiment four wires are used, two on each side. To be more specific, one pair of auxiliary wires 5a and 5b is above heat source 10 and one pair is below it, parallel to the first pair, with the result that two unidirectional currents A are made to flow. This configuration achieves the same result as the first embodiment, except that the unidirectional current A is distributed over a larger area. This causes an upward Lorentz force to operate throughout molten pool 12 to produce a bead with a flat, smooth surface. Figure 6 illustrates the circuit connections in a welding apparatus in the horizontal position which is a fourth preferred embodiment of this invention. In this figure, an arc is used as the heat source. Consumable electrode 14 is used to generate the arc instead of the non-consumable electrode in Figure 1. This configuration achieves the same effect as the first embodiment. Figure 7 illustrates the circuit connections in a welding apparatus in the horizontal position using a laser as the heat source. This is a fifth preferred embodiment of this invention. In this figure, a YAG or CO 2 laser is used as the heat source. (In this example, the laser output is 4.5 Kw and the welding speed is 50 cm/min.) The current which flows in molten pool 12 consists only of the current between auxiliary wires 5a and 5b; however, the same result is achieved as in the first embodiment. 16 in the drawing is a laser oscillator, and 15 is a condenser lens. Laser beam 17 is condensed onto the surface of the parent material. Magnetic coil 6 is wound around the condenser lens 15. On the top of magnetic coil 6, as is shown in (A), is a hollow member 61 formed of a magnetic metal and tapering conically. This hollow member condenses the magnetic field onto the focal point of the laser beam 17. Figure 8 illustrates the circuit connections in a welding apparatus in the horizontal position which uses an electron beam as the heat source. This is a sixth preferred embodiment of this invention. As can be seen in the drawing, an electron beam is used as the heat source. If the voltage used to accelerate the beam is 60 kV, the beam current is 10 Ma, and the welding speed is 50 cm/min, the same result is achieved as in the first embodiment. In the drawing, 19 is an electron gun and 18 is a condenser lens. Electron beam 10' is condensed onto the surface of the parent material. Magnetic coil 6 is wound around the electron beam 10' in the vicinity of parent material 7. In each of the described embodiments, the current between auxiliary wires 5a and 5b and that flowing into the electromagnet are DC. However, the same effects could be achieved with AC currents by changing the superposition of the DC components as well as their duty factors and by controlling the phase relationship between the aforementioned two currents. EFFECTS OF THE INVENTION With this invention, as has been discussed above, a current is made to flow in the molten pool in the direction of the weld line when a joint is being welded in the horizontal position. A magnetic field is induced in the pool so as to generate in that pool an upward Lorentz force (opposite to the pull of gravity). This force supports the molten metal and prevents it from dripping, resulting in a better-shaped bead. Various other effects are also achieved.

Claims: A method for welding in a horizontal position, wherein a parent material (7) is melted using a heat source (10, 10') to produce a molten pool (12) which is liable to deformation under the influence of gravity, further comprising steps of: inserting a plurality of auxiliary wires (5a-5d) into said molten pool (12), generating through said auxiliary wires (5a-5d) an electric current flow (A) in said molten pool (12) approximately along a direction of a weld line, and inducing a magnetic field (9) in said molten pool (12) approximately orthogonal to the direction of said electric current (A) to generate an upward Lorentz force (11) opposite a pull of gravity in said molten pool. A method according to claim 1, wherein said electric current flow (A) is a unidirectional current. A method according to claim 1 or 2, wherein said heat source (10, 10') is an arc, and a portion of a welding current for said arc is split off and provided to an auxiliary wire (5b). A welding apparatus for carrying out the method of any of claims 1 to 3, comprising: a heat source (10, 10') for welding which may be an arc, a laser (17) or an electron (10') beam; a magnetic field generating means (2, 5a-5d; 3, 6) to generate a magnetic field (9) in said molten pool (12) which intersects a surface of the parent material (7); and characterized by a plurality of auxiliary wires (5a-5d) provided so as to face the molten pool (12) formed by said heat source (10, 10') in the parent material (7); a power source (2) for generating an electric current through said plurality of auxiliary wires (5a-5d) and in said molten pool (12) approximately along a direction of a weld line. A welding apparatus according to claim 4, wherein said heat source is an arc, and a current splitting device (8) is provided to split off a portion of a welding current for said heat source and provides said portion to said auxiliary wires (5a-5d). A welding apparatus according to claim 4 or 5, further comprising a magnetic condensing means (61) to condense said magnetic field (9) onto said molten pool (12).

Inventors: HIROMOTO YOSHINORI, MANABE YUKIO, ZENITANI SATORU, HIROMOTO, YOSHINORI, MANABE, YUKIO, ZENITANI, SATORU, Hiromoto, Yoshinori, Mitsubishi Heavy Ind., Ltd., MANABE, Yukio, Mitsubishi Heavy Industries Ltd., ZENITANI, Satoru, Mitsubishi Heavy Ind. Ltd.
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI HEAVY INDUSTRIES, LTD.
Application year: 1997
Main classification: B23K9
IPC codes: B23K26, B23K9, B23K37