Arc welding method

Abstract

To provide an arc welding method capable of improving fatigue strength of a welded portion of a steel plate.SOLUTION: An arc welding method includes a jointing step of forming a weld metal 14 using a laser 15 preheated while moved along a root portion 13, and a wire 17 for generating an arc 16 while following the laser 15. When the direction of separating from a second steel plate 12 along an upper surface of a first steel plate 11 is a + direction and the direction of overlaying the first steel plate 11 and the second steel plate 12 is a - direction, the position of an end of the + direction in a region irradiated with the laser 15 in the jointing step is W1 (mm) and the position of an end of the - direction is W2 (mm), and the average thickness of the first steel plate 11 and the second steel plate 12 is t (mm), the wire aiming position of the wire 17 is located in the - direction relative to W1, a value A1, calculated by formula (1): A1=W1 / t1 / 4, is 1.0 or more and 5.2 or less, and a value A2, calculated by formula (2): A2=W2 / t1 / 4, is -5.5 or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] In recent years, progress has been made in reducing CO2 emissions by reducing the weight of automotive parts. One way to improve the strength of steel plates used in automotive parts and reduce their thickness is to reduce their weight while maintaining crashworthiness. However, even if the strength of steel plates is improved, it is difficult to improve the fatigue strength of welded parts (arc welded parts). Therefore, high-strength thinning has not been fully achieved in parts that require fatigue strength, such as suspension parts.

[0003] To improve fatigue strength, post-weld grinding and peening processes have been put into practical use. These processes can change the shape and stress state of the weld, leading to improved fatigue strength, but they require additional processing steps, which reduces efficiency. On the other hand, a practical method for changing the penetration shape of the weld without requiring additional processing steps is to use two electrodes, a leading electrode and a trailing electrode, for welding.

[0004] As a method of welding using two electrodes, for example, Patent Document 1 proposes a plasma arc hybrid welding method in which gas-shielded arc welding is used as the leading electrode and plasma arc welding is used as the trailing electrode to manufacture automobile undercarriage parts by lap fillet welding. Patent Document 1 specifies the distance in the weld line direction between the gas-shielded arc welding electrode and the plasma welding electrode, the flank angle of the toe of the weld bead on the lower plate side, the radius of curvature, and the plate thickness of the upper plate, and describes that this can improve the joint fatigue strength of the undercarriage part.

[0005] Furthermore, Patent Document 2 discloses a method for manufacturing a welded joint that uses laser-arc hybrid welding, controls the welding conditions, and provides an unwelded portion at the end of the lower plate at the overlapping surface between the upper and lower plates, thereby suppressing the occurrence of fatigue cracks. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-161899 [Patent Document 2] Japanese Patent Publication No. 2022-86780 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the above-mentioned Patent Document 1 has the following problems: (1) magnetic blow and (2) limitations on the range of application.

[0008] (1) According to the method described in Patent Document 1, the two arcs, the gas shielded arc and the plasma arc, interfere with each other, which can cause a phenomenon known as "magnetic arc blow." This can lead to arc disturbance and spatter, a lack of arc stability, and variations in the shape of the weld metal toe.

[0009] (2) When welding using a robot to manufacture automobile parts, etc., both welding torches are generally held by the same robot arm. As described in Patent Document 1, when gas-shielded arc welding and plasma arc welding are used together, the relative positions of the two welding torches are fixed, making it difficult to adapt to changes in welding conditions, such as changes in the thickness of the steel plate. Controlling each welding torch separately requires a new movable mechanism for moving the plasma arc welding torch, separate from the gas-shielded arc welding torch device. Furthermore, automating welding requires electrical and system connections, complicating the design. Furthermore, because both the gas-shielded arc welding torch and the plasma arc welding torch have large tips, the degree of freedom in the distance between the electrodes is limited.

[0010] Furthermore, Patent Document 2 examines fatigue at the root and weld toe, but lists only one method for smoothing the weld toe shape: setting the torch tilt angle in gas-shielded arc welding to 70° or less. However, no matter what torch tilt angle is set to below 70°, it cannot be said that the weld toe shape will be sufficiently smooth, and it is difficult to sufficiently suppress the decrease in fatigue strength due to the toe shape.

[0011] The present invention has been made in view of the above circumstances, and has an object to provide an arc welding method that can improve the fatigue strength of a welded portion of a steel plate. [Means for solving the problem]

[0012] The above object of the present invention is achieved by the following configuration [1] relating to an arc welding method.

[0013] [1] An arc welding method in which a first steel plate and a second steel plate are arranged in an overlapping state so that an end face of the second steel plate is arranged on an upper surface of the first steel plate, and a weld metal is formed at a root portion between the upper surface of the first steel plate and the end face of the second steel plate to join the first steel plate and the second steel plate, a joining step of forming a weld metal at the root portion using a heat source that preheats at least a part of the upper surface of the first steel plate while moving along the root portion, and an electrode that generates an arc while moving along the root portion while following the heat source, When the root portion is defined as a reference position 0 (mm), a direction perpendicular to the direction in which the root portion extends and along the upper surface of the first steel plate away from the second steel plate is defined as a + direction, and a direction in which the first steel plate and the second steel plate are overlapped is defined as a - direction, the position of the end of the + direction in the region to which the heat source is irradiated in the joining process is defined as W1 (mm), the position of the end of the - direction in the region to which the heat source is irradiated is defined as W2 (mm), and the average plate thickness of the first steel plate and the second steel plate is defined as t (mm), The target position of the electrode is set to a position in the negative direction from W1, The value A1 calculated by the following formula (1) is 1.0 or more and 5.2 or less, An arc welding method, characterized in that a value A2 calculated by the following formula (2) is set to be −5.5 or more. Formula (1): A1=W1 / t 1 / 4 Formula (2): A2=W2 / t 1 / 4

[0014] Furthermore, preferred embodiments of the present invention relating to the arc welding method relate to the following [2] to [9].

[0015] [2] The arc welding method according to [1], characterized in that in the joining step, the target position of the electrode is set so that the end of the weld metal formed by the electrode in the positive direction is included in the area irradiated with the heat source.

[0016] [3] The arc welding method according to [1] or [2], characterized in that in the joining step, at least a portion of the area on the upper surface of the first steel plate irradiated with the heat source melts.

[0017] [4] The arc welding method according to [1] or [2], characterized in that in the joining step, the area irradiated with the heat source melts.

[0018] [5] The arc welding method according to any one of [1] to [4], wherein in the joining step, the width W of the region to which the heat source is irradiated is 1.2 (mm) or more.

[0019] [6] The arc welding method according to any one of [1] to [5], wherein the heat source is a laser.

[0020] [7] The arc welding method according to [6], characterized in that the output of the laser is 0.5 (kW) or more and 6.0 (kW) or less.

[0021] [8] The arc welding method according to any one of [1] to [7], wherein in the joining step, the heat source is moved along the root portion while being oscillated.

[0022] [9] When the welding speed in the joining process is v (m / min), The arc welding method according to any one of [1] to [8], characterized in that the distance between the target position P1 of irradiation by the heat source and the target position P2 of the electrode is 8v (mm) or less. [Effects of the Invention]

[0023] According to the present invention, it is possible to provide an arc welding method that can improve the fatigue strength of the welded portion of steel plates, thereby making it possible to reduce the thickness and weight of the welded joint. [Brief explanation of the drawings]

[0024] [Figure 1] FIG. 1 is a schematic diagram showing an arc welding method according to an embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram showing an arc welding method according to another embodiment of the present invention. [Figure 3] FIG. 3 is a schematic diagram for explaining an area to be irradiated with a laser in the arc welding method according to the present embodiment. [Figure 4] FIG. 4 is a schematic diagram showing an example of laser oscillation. [Figure 5] FIG. 5 is a schematic diagram showing the sweepback angle of the laser and the sweepback angle of the wire. [Figure 6] FIG. 6 is a schematic diagram showing the laser irradiation angle and the arc torch angle. [Figure 7] FIG. 7 is a schematic diagram for explaining a method for evaluating the shape of the weld metal. [Figure 8] FIG. 8 is a photograph showing a cross section of the welded portion of Example No. 2 and Comparative Examples Nos. 1 and 2. DETAILED DESCRIPTION OF THE INVENTION

[0025] As a result of extensive research, the present inventors have found that using a heat source for preheating the region to be welded and arc welding, and controlling the relationship between the preheated region and the region where the arc is generated, taking into account the region where the weld metal will be formed after welding, is an effective way to solve the above problems.The present inventors have also found that by controlling the parameters for the preheating range within an appropriate range, the preheated region can be easily set and the fatigue strength of the weld can be improved.The present invention was made based on the above findings.

[0026] Hereinafter, embodiments for carrying out the present invention will be described in detail. Note that the present invention is not limited to the embodiments described below, and can be carried out with any modifications within the scope of the gist of the present invention.

[0027] [Arc welding method] Fig. 1 is a schematic diagram showing an arc welding method according to an embodiment of the present invention. Fig. 2 is a schematic diagram showing an arc welding method according to another embodiment of the present invention. In Fig. 2, the same components as in Fig. 1 are given the same reference numerals, and detailed description of Fig. 2 will be omitted. First, the arc welding method according to this embodiment will be described with reference to Fig. 1.

[0028] First, the first steel plate 11 and the second steel plate 12, which will serve as the upper plate, are arranged in an overlapping state so that the end surface 12b of the second steel plate 12, which will serve as the upper plate, is placed on the top surface 11a of the first steel plate 11, which will serve as the lower plate. In this embodiment, there is a joining step in which weld metal 14 is formed in a root portion 13 between the top surface 11a of the first steel plate 11 and the end surface 12b of the second steel plate 12, thereby joining the first steel plate 11 and the second steel plate 12. The joining step will be described in detail below.

[0029] <Joining process> 1, in the joining process, a weld metal 14 is formed on the root portion 13 using a laser (heat source) 15 that preheats at least a portion of the upper surface 11a of the first steel plate 11, and a wire (electrode) 17 that generates an arc 16 while moving along the root portion 13 while following the laser 15. Specifically, a leading laser oscillator (not shown) is moved along the root portion 13, while the laser 15 is irradiated onto a predetermined range (described later). In addition, a predetermined region (described later) is set as the target position for the wire 17, and a trailing welding torch 18 is moved while following the movement of the laser oscillator, to perform arc welding.

[0030] In this embodiment, the relationship between the preheating region by the laser 15 and the region where the arc is generated (the target position of the electrode) is controlled to maintain a good weld toe shape. If the conditions described below are satisfied, for example, as shown in Fig. 1, a good weld toe shape can be obtained even when the width of the preheating region 21a in the direction perpendicular to the extension direction of the root 13 is set to be narrow. Also, as shown in Fig. 2, a good weld toe shape can be obtained and fatigue strength can be improved even when the laser 15 is moved back and forth along the root 13 in the direction perpendicular to the extension direction of the root 13, widening the range of the preheating region 21b.

[0031] In the joining process, if at least a portion of the region irradiated with the laser 15 on the top surface of the first steel plate 11 is melted, sufficient preheating is provided, and an even better weld toe shape can be obtained. Furthermore, if the entire region irradiated with the laser 15 is melted, the overall temperature distribution of the surface becomes higher, making it easier to wet (easier to smooth the shape), and an even better weld toe shape can be obtained.

[0032] In the above arc welding method, a laser 15 is used as the preceding heat source, but the heat source in the present invention is not limited to a laser and may be any heat source that can preheat the irradiated area. However, when a laser 15 is used, the distance between the heat source and the electrode, the width and direction of the heat source irradiation, etc. can be adjusted simply by adjusting the laser mirror, etc., while the laser oscillator and welding torch are held by the same driving device. Therefore, it is preferable to use a laser as the heat source.

[0033] Next, the region to be irradiated with the heat source and the target position of the electrode in the arc welding method according to this embodiment will be described in detail.

[0034] <Value A1 calculated by formula (1): 1.0 or more and 5.2 or less> <Value A2 calculated by formula (2): -5.5 or more> Fig. 3 is a schematic diagram illustrating a region to be irradiated with a laser in the arc welding method according to the present embodiment. As shown in Fig. 3, the root portion 13 is defined as the reference position 0 (mm), the direction perpendicular to the direction in which the root portion 13 extends and along the upper surface 11a of the first steel sheet 11 away from the second steel sheet 12 is defined as the + direction, and the direction in which the first steel sheet 11 and the second steel sheet 12 are overlapped is defined as the - direction. Furthermore, the position of the end R1 in the + direction in the region R to be irradiated with the laser 15 in the joining step is defined as W1 (mm), the position of the end R2 in the - direction in the region R to be irradiated with the laser 15 is defined as W2 (mm), and the average thickness of the first steel sheet 11 and the second steel sheet 12 is defined as t (mm).

[0035] In this embodiment, the region to be irradiated with the laser 15 is defined using the following formulas (1) and (2). Specifically, it is preferable to preheat the region that will become the weld toe after welding with the laser 15 or the like. The region that will become the weld toe is affected by the plate thickness, but is not simply proportional to the plate thickness; the thinner the plate thickness, the wider the range to be irradiated with the laser 15 can be. Therefore, the inventors have found the following formulas (1) and (2) for the region that will become the weld toe, using the average plate thickness t of the first steel plate 11 and the second steel plate 12, and have set the region to be irradiated with the laser 15 by defining the values ​​A1 and A2 calculated by these formulas.

[0036] Formula (1): A1=W1 / t 1 / 4 Formula (2): A2=W2 / t 1 / 4

[0037] If the value A1 calculated by the above formula (1) is less than 1.0, the preheating of the portion that will become the weld toe will be insufficient. Therefore, the value A1 calculated by the above formula (1) should be 1.0 or more, and preferably 2.0 or more. Furthermore, if the value A1 calculated by the above formula (1) exceeds 5.2, the inflow of molten metal toward the + direction shown in Figure 3 will increase, causing undercutting of the second steel plate 12 and reducing fatigue strength. Therefore, the value A1 calculated by the above formula (1) should be 5.2 or less.

[0038] Furthermore, if the value A2 calculated by the above formula (2) is less than -5.5, the molten metal will flow too far toward the negative side shown in Figure 3, resulting in insufficient reinforcement on the first steel plate 11 side, causing undercut and reducing fatigue strength. Therefore, the value A2 calculated by the above formula (2) is set to be -5.5 or greater. There is no particular upper limit to the value A2 calculated by the above formula (2), as long as it is located in the negative direction relative to value A1.

[0039] <Target position of electrode: Position in the negative direction from W1> If the target position of the wire (electrode) 17 is in a position in the positive direction from W1, the position that will become the weld toe will also be in a position in the positive direction from W1, making it impossible to obtain an excellent weld toe shape. Therefore, the target position of the wire 17 is set to a position in the negative direction from W1.

[0040] <Laser irradiation width: 1.2 mm or more> The width W of irradiation with the laser 15 (laser irradiation width W) is not particularly limited, but if the irradiation width W is too small, the preheating area becomes narrow and it becomes difficult to control the target position of the wire 17. Therefore, the irradiation width W of the laser 15 is preferably 1.2 mm or more, and more preferably 2.0 mm or more. As shown in FIG. 3, the irradiation width W of the laser 15 is a value (W1-W2) obtained by subtracting the position W2 of the end R2 in the negative direction in the region R from the position W1 of the end R1 in the positive direction in the region R.

[0041] <How to move the heat source> As long as the preheating range by the heat source is within the scope of the present invention, there is no particular limitation on the method of moving laser (heat source) 15. Laser (heat source) 15 may be moved linearly along root portion 13, or may be moved along root portion 13 while oscillating in a direction at an angle relative to root portion 13.

[0042] 4 is a schematic diagram showing examples of laser oscillation. Oscillation example 22a shows an example in which laser 15 is reciprocated in a straight line in a direction perpendicular to root portion 13, and oscillation example 22b shows an example in which laser 15 is moved along root portion 13 in the state of oscillation example 22a. Oscillation example 22c shows an example in which laser 15 is moved so as to rotate in a circle around a point on or near root portion 13, and oscillation example 22d shows an example in which laser 15 is moved along root portion 13 in the state of oscillation example 22c. Either shape can achieve the same preheating effect, and an ellipse or any other arbitrary shape can be applied even when moving while rotating.

[0043] Furthermore, preferred welding conditions for the base metal, heat source, electrodes, etc. in the arc welding method according to this embodiment will be described below.

[0044] <Base material> In the arc welding method according to the present embodiment, the base material is not particularly limited as long as it is a steel sheet. For example, steel sheets with various tensile strengths, such as 440 MPa-class unplated steel sheets and 1180 MPa-class unplated steel sheets, can be used, and various plated steel sheets can also be used.

[0045] (Base material thickness: 0.8mm to 6.0mm) The thickness of the base metal (first steel plate 11, second steel plate 12) is not particularly limited as long as it allows the desired weld metal to be formed in one pass during arc welding of a fillet joint. This is based on the idea of ​​using a laser to widen the area that will become the weld toe. Even when weld metal is formed through multiple passes, the thickness of the base metal is not particularly limited, and preheating in the final pass can sufficiently smooth the weld toe shape and improve fatigue strength. Strictly speaking, preheating does not necessarily have to be performed in the final pass. For example, if the finally formed weld metal is considered to be one weld metal, preheating in the pass that forms the portion that will become the weld toe can smooth the weld toe shape. Even if preheating is performed in other passes, the area preheated in the subsequent passes will not be the weld toe, and the effect of preheating cannot be obtained.

[0046] The thickness of the upper plate (second steel plate 12) and the thickness of the lower plate (first steel plate 11) may be the same or different. Considering application to parts other than automobiles, the thickness of the first steel plate 11 and the thickness of the second steel plate 12 are each preferably 0.8 mm or more, and more preferably 1.0 mm or more. On the other hand, if the plate thickness is too large, it becomes difficult to weld in one pass. Therefore, the thickness of the first steel plate 11 and the thickness of the second steel plate 12 are each preferably 6.0 mm or less, more preferably 4.0 mm or less, and even more preferably 3.5 mm or less. Furthermore, the thickness of the first steel plate 11 and the thickness of the second steel plate 12 are each more preferably 3.0 mm or less, and particularly preferably 2.6 mm or less.

[0047] <Laser (heat source)> (Laser sweepback angle θb: -15° to 60°) In this embodiment, there are no particular limitations on the sweep back angle θb of the laser 15. Fig. 5 is a schematic diagram showing the sweep back angle of the laser 15 and the sweep back angle of the wire 17. The sweep back angle θb of the laser 15 represents the angle at which the laser 15 is tilted in the welding direction indicated by the arrow in Fig. 5 with respect to the direction perpendicular to the upper surface 11a of the first steel plate 11. In other words, when the laser 15 is tilted in the direction opposite to the welding direction, it is represented by - (minus).

[0048] In this embodiment, laser 15 is advanced and wire 17 follows laser 15, so that in consideration of interference with the welding torch or arc weld, the sweepback angle θb of laser 15 is preferably −15° or more, and more preferably −5° or more. Furthermore, in order to prevent a decrease in absorptivity due to light reflection, the sweepback angle θb of laser 15 is preferably 60° or less, and more preferably 50° or less.

[0049] (Laser irradiation angle θr: -15° to 60°) In this embodiment, there is no particular limitation on the irradiation angle θr of the laser 15. Fig. 6 is a schematic diagram showing the irradiation angle of the laser 15 and the torch angle of the arc. The irradiation angle θr of the laser 15 represents the angle at which the laser 15 is tilted in the + direction shown in Fig. 6 with respect to the direction perpendicular to the upper surface 11a of the first steel plate 11.

[0050] If the irradiation angle θr of the laser 15 is tilted too far in the negative direction shown in FIG. 6, it may be difficult to irradiate the desired area with the laser 15. Therefore, the irradiation angle θr of the laser 15 is preferably set to -15° or more, and more preferably set to 0° or more. Furthermore, by maintaining the irradiation angle θr of the laser 15 appropriately, it is possible to stabilize the targeting of the irradiation area and prevent a decrease in absorptance due to light reflection. Therefore, the irradiation angle θr of the laser 15 is preferably set to 60° or less, and more preferably set to 45° or less.

[0051] (Laser output: 0.5 kW or more, 6.0 kW or less) In this embodiment, the output of the laser 15 is not particularly limited. However, setting the output of the laser 15 to 0.5 kW or more prevents preheating from becoming too weak and improves the shape of the weld toe. Therefore, the output of the laser 15 is preferably 0.5 kW or more, and more preferably 1.0 kW or more. On the other hand, setting the output of the laser 15 to 6.0 kW or less reduces the energy used for preheating and prevents burn-through and spattering due to excessive heat input, while improving the shape of the weld toe. Therefore, the output of the laser 15 is preferably 6.0 kW or less, and more preferably 5.0 kW or less. The laser output can be freely set based on the laser spot diameter (power density) and the surface melting state.

[0052] (laser type, wavelength) When using laser 15 as the heat source, there are no particular limitations on the type or wavelength of the laser. For example, a CO laser, a YAG (Yttrium Aluminum Garnet) laser, a fiber laser, a disk laser, a semiconductor laser, an excimer laser, etc. can be used.

[0053] (Laser profile and beam diameter) There are no particular limitations on the intensity distribution (profile) of the laser 15, and a general Gaussian beam can be applied. There are also no particular limitations on the beam diameter of the laser 15, and it is preferable to set it to, for example, about 0.1 mm or more and 3.0 mm or less.

[0054] (laser oscillation frequency) In this embodiment, the oscillation frequency of the laser 15 is not particularly limited, but if the frequency is too low, uneven heating may occur in the direction of the weld line. Therefore, when the welding speed is v (m / min), the laser oscillation frequency f is preferably 5v or higher. The upper limit of the laser oscillation frequency f depends on the limitations of the device.

[0055] <Wire (electrode)> (Wire (electrode) advance angle θf: -30° to 30°) In this embodiment, there is no particular limitation on the push angle θf of the wire 17. As shown in Fig. 5, the push angle θf of the wire 17 represents the angle at which the wire 17 is inclined in the opposite direction to the welding direction indicated by the arrow in Fig. 5, with respect to a direction perpendicular to the upper surface 11a of the first steel plate 11. In other words, when the wire 17 is inclined in the welding direction, it is represented as - (minus).

[0056] The lead angle θf of the wire 17 can be set within the range employed in a typical arc welding method. Therefore, the lead angle θf of the wire 17 is preferably −30° or more, more preferably −15° or more, even more preferably −5° or more, and particularly preferably 0° or more. The lead angle θf of the wire is preferably 30° or less, more preferably 25° or less, and even more preferably 20° or less.

[0057] (Torch angle θt: 10° to 55°) In this embodiment, the torch angle θt of the arc is not particularly limited. The torch angle θt represents the angle at which the electrode is tilted in the + direction shown in FIG. 6 with respect to the direction perpendicular to the upper surface 11a of the first steel plate 11. The torch angle θt can be set within the range adopted in ordinary arc welding methods. Specifically, the torch angle θt of the arc is preferably set to approximately 10° or more and 55° or less.

[0058] (Wire type) The type of wire is not particularly limited, and ordinary wire can be used. It is also possible to use wire that can be expected to impart compressive residual stress, such as low transformation temperature (LTT) welding material, or wire designed to improve fatigue strength. The wire may be either solid wire or flux-cored wire.

[0059] (welding speed v) The welding speed v is not particularly limited, but can be set to a speed faster than that used in normal arc welding to achieve high efficiency. Specifically, the welding speed v can be set to 0.6 m / min or more, and preferably 1 m / min or more. Furthermore, the welding speed v is preferably set to 6 m / min or less.

[0060] (Laser-arc distance d: 8v (mm) or less) As shown in Figure 5, in this embodiment, by appropriately adjusting the distance P1 (laser-arc distance d) between the target position P1 of the laser 15 and the target position P2 of the wire 17, a weld toe with a better shape can be obtained. If the distance P1 and P2 of the wire 17 is too large, the temperature of the preheated area will drop, and the preheating effect will not be fully achieved. However, if the welding speed v is high, the arc 16 will arrive before the temperature of the preheated area by the laser 15 drops, so the preferable range of the laser-arc distance d is affected by the welding speed v. For this reason, the laser-arc distance d is preferably 8v (mm) or less, and more preferably 6v (mm) or less.

[0061] On the other hand, when d / v is less than 0, i.e., when the laser 15 passes after the wire 17 has passed, there are concerns about effects on the arc welding plasma and droplets, and increased spatter due to excessive heat input to the molten pool. Therefore, it is preferable that the laser-arc distance d is 0 (mm) or greater. In this embodiment, since the laser 15 may be oscillated, the laser-arc distance d is defined as the distance when the target position P2 of the wire 17 is closest to the target position P1 of irradiation by the laser 15.

[0062] (shielding gas) In arc welding, a shielding gas of 80% Ar-20% CO2 is typically used, but this embodiment is not limited to this ratio. For example, by increasing the Ar gas content, it is possible to obtain the effect of improving paintability by reducing slag. Furthermore, by increasing the CO2 gas content, it is possible to reduce costs. Note that, although increasing the CO2 gas content causes the weld toe to become convex, even when 100% CO2 gas is used as the shielding gas, an improvement in the toe shape can be obtained compared to conventional welding methods. [Example]

[0063] The effects of the present invention will be specifically explained below with reference to examples and comparative examples, but the present invention is not limited thereto.

[0064] [Arc welding] As shown in Figure 1, a first steel plate 11 and a second steel plate 12 were placed one on top of the other so that the end surface 12b of the second steel plate (upper plate) 12 was positioned on the upper surface 11a of the first steel plate (lower plate) 11. Next, using various welding conditions, a laser 15 was moved along the root portion 13 to preheat the upper surface 11a of the first steel plate 11, and an arc 16 was generated following the laser 15 while a wire 17 was moved along the root portion 13. As a result, a weld metal 14 was formed at the root portion 13, and a welded joint was obtained. Common welding conditions are shown below.

[0065] Base material A: 440MPa class steel plate, plate thickness 2.3mm Base material B: 1180MPa class steel plate, plate thickness 1.4mm Type of wire when base material B is not used for either the first steel plate 11 or the second steel plate 12: JIS Z 3312 YGW12 equivalent solid wire, diameter 1.2 mm Wire type when base material B is used for at least one of the first steel plate 11 and the second steel plate 12: 1180 MPa class solid wire, diameter 1.2 mm Shielding gas: 82%Ar-18%CO2 Arc welding current (A): 187A (pulse) Arc welding voltage (V): 22.7V Wire advance angle θf: 15 degrees Torch angle θt: 30 degrees Target position of wire tip P2: root Distance between tip and base material L: 15 mm Laser type: Fiber laser Laser sweepback angle θb: 40 degrees Laser irradiation angle θr: 30 degrees Laser spot diameter: 1.2 mm

[0066] [Evaluation of weld metal shape] At an arbitrary position on the weld metal 14 of each obtained weld joint, the surface irregularities were measured using a profile measuring machine along a direction perpendicular to the root portion 13. Fig. 7 is a schematic diagram for explaining a method for evaluating the shape of the weld metal. The evaluation method and evaluation criteria for the toe curvature radius ρ, toe angle θ, and undercut depth Hu will be explained with reference to Fig. 7.

[0067] (Toe radius of curvature ρ) A cross-sectional view as shown in Fig. 7 was created using a profile measuring machine, and an imaginary circle 31 was created along the shape of the weld toe 14a. The radius of this imaginary circle 31 was taken as the toe curvature radius ρ. The larger the toe curvature radius ρ, the smoother the weld toe and the less stress concentration there is in the shape. In this example, a toe curvature radius ρ of 0.5 mm or more was evaluated as good, and a toe curvature radius ρ of less than 0.5 mm was evaluated as poor.

[0068] (Toe angle θ) 7, the point where the surface of the weld metal 14 intersects with an extension 12c of a line 12c that is half the thickness T1 of the second steel plate 12 is designated as an intersection 14b, and an imaginary line 32 is drawn connecting the weld toe 14a and the intersection 14b. The angle formed by the upper surface 11a of the first steel plate 11 and the imaginary line 32 is designated as the toe angle θ. The larger the toe angle θ, the smoother the weld metal 14 is, and the less stress concentration there is in the shape. In this example, a toe angle θ of less than 40° was evaluated as good, and a toe angle θ of 40° or greater was evaluated as poor.

[0069] When an undercut 14c was formed, the distance from the upper surface 12a of the second steel plate 12 to the lowest point of the undercut 14c was measured and defined as the undercut depth U2. Although not shown, when an undercut was formed on the upper surface 11a of the first steel plate 11, the distance from the upper surface 11a of the first steel plate 11 to the lowest point of the undercut was also measured and defined as the undercut depth U1. In this example, when the undercut depth U2 was 10% or more of the plate thickness T1 of the second steel plate 12, or when the undercut depth U1 was 10% or more of the plate thickness T2 of the first steel plate 11, it was determined to be defective and was marked with "X." On the other hand, when the undercut depth U2 was less than 10% of the plate thickness T1 of the second steel plate 12 and the undercut depth U1 was less than 10% of the plate thickness T2 of the first steel plate 11, it was judged to be good and marked with ○.

[0070] The welding conditions are shown in Tables 1 and 2 below, and the evaluation results are shown in Table 3 below. Note that the irradiation pattern in Table 1 below indicates whether it is a "LINE" shape or a "CIRCLE" shape as shown in Figure 4. Also, the laser irradiation width W in Table 2 indicates the sum of the laser oscillation width and the laser spot diameter (diameter: 1.2 mm).

[0071] [Table 1]

[0072] [Table 2]

[0073] [Table 3]

[0074] As shown in Tables 1 to 3 above, in Examples 1 to 18, the preheating area by the heat source and the target position by the electrode were appropriately controlled, resulting in a good weld toe shape and suppressing undercut. Note that Examples 4 and 5 were performed using different laser powers. The higher the laser power, the smoother the bead, but even a low power can improve the bead shape through the preheating effect. In Example 7, the laser oscillation pattern was rotated in a circular pattern while moving along the root, achieving results similar to those achieved when the laser was moved back and forth in a linear pattern. In Examples 17 and 18, base materials of different thicknesses were used, but like the other examples, a good weld toe shape was obtained and undercut was suppressed.

[0075] On the other hand, in Comparative Examples 1 and 6, the joints were only arc-welded and no preheating was performed with a heat source, so the toe curvature radius was 0.5 mm or less and the toe angle was 40° or more, making it impossible to obtain a smooth bead. In Comparative Example 2, no preheating with a heat source was performed and the welding speed was reduced compared to Comparative Example 1, so the toe angle improved but the toe curvature radius could not be kept within the desired range.

[0076] In Comparative Examples 3, 5, and 8, the value A1 calculated by formula (1) exceeded the upper limit specified in the present invention, causing undercut. In Comparative Examples 4 and 7, the value A2 calculated by formula (2) was less than the lower limit specified in the present invention, causing undercut.

[0077] [Evaluation by fatigue strength test] Of the various joints obtained as described above, fatigue strength tests were carried out on some of the joints, and the fatigue strength was evaluated by measuring the number of cycles until failure. The test conditions for the fatigue strength test are shown below, and the evaluation results are shown in Table 4 below. Test method: Schenck plane bending fatigue test Repetition frequency: 25Hz Stress ratio: 0 (fully pulsating) Stress amplitude: 370 MPa Evaluation bead width: 22 mm

[0078] [Table 4]

[0079] Figure 8 is a photograph showing the cross-sections of welds in Example 2 and Comparative Examples 1 and 2. As mentioned above, Comparative Examples 1 and 2 were arc-welded joints only, without preheating using a heat source. In contrast, Example 2 was arc-welded after laser preheating under the conditions specified in this invention. Generally, the discontinuous shape of the weld toe results in stress concentration, making it prone to fatigue cracking. However, a larger toe radius and a smaller toe angle result in a smaller stress concentration factor, improving fatigue strength. Fatigue tests were conducted on the three joints. As shown in Tables 3 and 4 and Figure 8, Example 2 achieved approximately three times the fatigue strength of Comparative Examples 1 and 2. This indicates that a larger toe radius and a smaller toe angle actually provide superior fatigue performance. [Explanation of symbols]

[0080] 11 First steel plate 12 Second steel plate 13 Root 14 Weld metal 14a Weld toe 14c undercut 15 Laser 16 Arc 17 wires 18 Welding Torch 21a, 21b Preheating area

Claims

1. an arc welding method including: arranging a first steel plate and a second steel plate in an overlapping state such that an end surface of the second steel plate is disposed on an upper surface of the first steel plate; and forming a weld metal at a root portion between the upper surface of the first steel plate and the end surface of the second steel plate to join the first steel plate and the second steel plate; a joining step of forming a weld metal at the root portion using a heat source that preheats at least a part of the upper surface of the first steel plate while moving along the root portion, and an electrode that generates an arc while moving along the root portion while following the heat source, When the root portion is defined as a reference position 0 (mm), a direction perpendicular to the direction in which the root portion extends and along the upper surface of the first steel plate away from the second steel plate is defined as a + direction, a direction in which the first steel plate and the second steel plate are overlapped is defined as a - direction, a position of an end portion in the + direction in the region to which the heat source is irradiated in the joining process is defined as W1 (mm), a position of an end portion in the - direction in the region to which the heat source is irradiated is defined as W2 (mm), and an average plate thickness of the first steel plate and the second steel plate is defined as t (mm), The target position of the electrode is set to a position in the negative direction from W1, The value A1 calculated by the following formula (1) is 1.0 or more and 5.2 or less, An arc welding method, characterized in that the value A2 calculated by the following formula (2) is −5.5 or more. Formula (1): A1 = W1 / t １／４ Formula (2): A2 = W2 / t １／４

2. 2. The arc welding method according to claim 1, wherein, in the joining step, a target position of the electrode is set so that an end portion of the weld metal formed by the electrode in the positive direction is included in an area irradiated with the heat source.

3. The arc welding method according to claim 1, wherein in the joining step, at least a part of the region on the upper surface of the first steel plate irradiated with the heat source is melted.

4. The arc welding method according to claim 1, wherein the region irradiated with the heat source melts in the joining step.

5. 2. The arc welding method according to claim 1, wherein in the joining step, a width W of the region irradiated with the heat source is set to 1.2 (mm) or more.

6. 6. The arc welding method according to claim 1, wherein the heat source is a laser.

7. 7. The arc welding method according to claim 6, wherein the output of the laser is 0.5 (kW) or more and 6.0 (kW) or less.

8. 6. The arc welding method according to claim 1, wherein in the joining step, the heat source is moved along the root portion while being oscillated.

9. When the welding speed in the joining step is v (m / min), The arc welding method according to any one of claims 1 to 5, characterized in that the distance between the target position P1 of irradiation by the heat source and the target position P2 of the electrode is 8v (mm) or less.

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