Laser-assisted MAG high-speed welding process for galvanized sheet

Abstract

The invention discloses a laser-assisted MAG high-speed welding process for galvanized sheet, which uses galvanized sheet as base material, carbon steel argon arc welding material as welding wire, selects laser-assisted MAG arc as composite heat source, and adopts a paraxial composite method of laser in front and MAG arc in the back for welding. The laser-assisted MAG high-speed welding process for galvanized sheet of the invention can ensure high-speed welding of automotive galvanized sheet while avoiding the phenomenon of galvanized layer being damaged and galvanized layer being burned, and can avoid weld defects such as humps caused by excessively high welding speed when conventional automotive galvanized sheet is welded.

Description

Technical Field

[0001] The invention relates to the technical field of galvanized sheet welding, and more particularly to a laser-assisted MAG high-speed welding process for galvanized sheets. Background Art

[0002] In recent years, with the rapid development of the automobile manufacturing industry, requirements have been put forward for lightweight and service safety of automobile bodies. As an important manufacturing process, welding has a significant impact on enterprises in terms of efficiency and quality. The manufacturing efficiency of automobile body-in-white reflects the level of automation in the production process on the one hand, and the production efficiency of the welding process on the other hand. The main way to improve welding efficiency is to increase the welding speed. However, when the welding speed reaches a certain range, both traditional arc welding and high-energy beam welding will have weld defects, such as weld undercut, hump, discontinuity, etc. This is a problem faced by all high-speed welding methods.

[0003] It is generally believed that welding speeds above 100cm / min can be considered high-speed welding. In traditional MAG welding methods, when the welding speed is increased to a certain range, a hump will appear in the weld. According to research, during high-speed MAG welding, the "backward liquid flow" formed in the front of the molten pool causes the liquid metal to gather at the tail of the molten pool; when there is a large surface tension at this point, the liquid metal "bulges" and cannot be backfilled to the front of the molten pool, resulting in a hump weld. Therefore, how to achieve high-speed welding while avoiding the appearance of a hump weld is a problem that needs to be solved in the current automotive body manufacturing field. In addition, for galvanized sheets commonly used in the automotive manufacturing industry, another important issue that needs to be considered during high-speed welding is to avoid damage to the zinc layer and minimize zinc burn-off. Summary of the invention

[0004] To this end, the technical problem to be solved by the present invention is to provide a laser-assisted MAG high-speed welding process for galvanized sheets, so as to solve the problems of zinc layer damage and galvanizing burnout that are prone to occur during high-speed welding of automotive galvanized thin sheets, as well as unstable welding process, large spatter, and weld hump when the welding speed is too high.

[0005] In order to solve the above technical problems, the present invention provides the following technical solutions:

[0006] The laser-assisted MAG high-speed welding process of galvanized sheet uses galvanized sheet as the base material, carbon steel argon arc welding material as the welding wire, and laser-assisted MAG arc as the composite heat source. The laser is in front and MAG arc is in the back. The parallel-axis composite method of welding is adopted. Under high-speed welding conditions, the laser as an auxiliary heat source generates charged particles to the arc on the one hand, and preheats and partially melts the base material to be welded on the other hand. Therefore, high-speed welding can be achieved with the laser in front and the arc in the back. If the arc is in front and the laser is in the back, although the laser can provide charged ions for the arc, it is difficult to achieve high-speed welding due to the lack of preheating and partial melting of the base material to be welded.

[0007] In the laser-assisted MAG high-speed welding process of the above-mentioned galvanized sheet, the galvanized sheet is a HC340LAD+Z galvanized sheet with a thickness of 1.05 mm, the carbon steel argon arc welding material is a welding wire with a diameter of 1.0 mm, and the welding wire is ER70S-6 welding wire, ER49-1 welding wire or ER50-G welding wire.

[0008] In the laser-assisted MAG high-speed welding process of the galvanized sheet, in the HC340LAD+Z galvanized sheet: the carbon content is ≤0.11wt%, the manganese content is ≤1.00wt%, the silicon content is ≤0.5wt%, the phosphorus content is ≤0.03wt%, the sulfur content is ≤0.025wt%, and the HC340LAD+Z galvanized sheet does not contain chromium, nickel and copper.

[0009] In the above-mentioned laser-assisted MAG high-speed welding process of galvanized sheet, the carbon steel argon arc welding material is ER70S-6 welding wire; in the ER70S-6 steel welding wire: the carbon content is 0.06-0.15wt%, the manganese content is 1.40-1.85wt%, the silicon content is 0.80-0.90wt%, the chromium content is ≤0.20wt%, the nickel content is ≤0.30wt%, the copper content is ≤0.20wt%, the phosphorus content is ≤0.025wt%, and the sulfur content is ≤0.025wt%.

[0010] In the laser-assisted MAG high-speed welding process of the galvanized sheet mentioned above, the laser beam in the composite heat source is provided by a paraxial fiber laser PLW3000, with a maximum output power of 3KW, a laser wavelength of 1070μm in continuous output mode, and a focal spot diameter of 300μm; a water chiller is used to cool the laser.

[0011] In the laser-assisted MAG high-speed welding process of the galvanized sheet mentioned above, during welding, the working power of the laser is 560-1360W, the angle β between the laser beam and the surface of the galvanized sheet is 110-120°, and the laser defocus is 0; the galvanized sheet is welded by lap or butt welding, and the tilt angle is 22° when the galvanized sheet is lap welded [when the tilt angle is large, assembly defects are prone to occur, such as welding gun bumps, and when the tilt angle is small, the arc heat source is easy to act on the lower plate parent material, and reduce the melting amount of the upper plate parent material, thereby affecting the welding quality]. For the welding of 1.05mm thick galvanized sheet, the laser is only used as an auxiliary heat source to stabilize the arc; due to the short-circuit transition of the arc, when the laser is in positive and negative defocus, the energy of the light spot acting on the workpiece surface is less than the actual value, so the laser power needs to be increased, which does not meet the characteristics of laser assistance.

[0012] In the laser-assisted MAG high-speed welding process of the galvanized sheet mentioned above, the arc welding power source is a TPS2700 welding machine, the shielding gas in the MAG welding gun is 100% carbon dioxide gas, and the flow rate of the shielding gas is 17L / min. Pure carbon dioxide as a shielding gas can greatly improve the fluidity of the metal, thereby achieving molten metal backfilling the weld during high-speed welding; if a mixed gas is selected, the metal fluidity will be reduced and welding defects will occur. When the shielding gas flow rate is less than 17L / min, for high-speed welding, the protection effect is poor due to the influence of gas flow; when the shielding gas is greater than 17L / min, the arc stability is reduced due to the influence of gas flow.

[0013] In the above-mentioned laser-assisted MAG high-speed welding process of galvanized sheet, the angle between the MAG welding gun and the surface of the galvanized sheet is 65° [the welding gun is a gooseneck welding gun. When the welding gun angle is less than 65°, the angle between the welding wire and the workpiece becomes smaller, and the molten drop formed by the short-circuit transition is offset due to the influence of gravity; when the welding gun angle is greater than 65°, it seriously affects the assembly accuracy of the welding gun and the laser during welding, making the composite process unstable], the distance between the intersection of the welding wire on the MAG welding gun and the center of the laser beam spot on the surface of the galvanized sheet (i.e., the filament spacing) is 2mm, the diameter of the MAG welding wire is 1mm, and a small current short-circuit transition is adopted. When the filament spacing is less than 2mm, the metal vapor generated by the laser beam will affect the molten droplet transition; when the filament spacing is greater than 2mm, the compression and attraction of the laser on the arc are weakened; the elongation of the welding wire between the conductive nozzle of the MAG welding gun and the surface of the galvanized sheet is 10mm. When the wire elongation is greater than 10mm, the wire may melt due to being too long. When it is less than 10mm, the distance between the MAG gun nozzle and the workpiece is too close. On the one hand, the tooling is limited, and on the other hand, the shielding gas space is reduced, and the gas flow is likely to interfere with the arc stability.

[0014] In the laser-assisted MAG high-speed welding process of the galvanized sheet, during welding, the arc current of the MAG welding gun is 70 to 90 A, and the welding speed is 150 to 240 cm / min.

[0015] In the above-mentioned laser-assisted MAG high-speed welding process of galvanized sheet, when the welding current is 70A-90A, the laser power is 500W-1360W, and the energy ratio of the MAG arc heat source to the laser heat source in the composite heat source is 1.05-2.94, the auxiliary effect of the laser can better and fully improve the stability of the MAG arc, thereby realizing high-speed and efficient welding under small current specifications, so that the galvanized sheet can be overlapped at a welding speed in the range of 150cm / min-240cm / min.

[0016] The technical solution of the present invention achieves the following beneficial technical effects:

[0017] 1. The present invention uses galvanized sheet as base material, carbon steel argon arc welding material as welding wire, selects laser-assisted MAG arc as composite heat source, and adopts a paraxial composite method of laser in front and MAG arc in the back for welding. It can ensure high-speed welding of automotive galvanized sheet while avoiding the phenomenon of zinc coating being damaged and zinc coating being burned, and can avoid weld defects such as humps caused by excessively high welding speed during conventional automotive galvanized sheet welding.

[0018] 2. The present invention selects a suitable base material and welding wire, controls the angle between the laser beam and the surface of the galvanized sheet, the angle between the MAG welding gun and the surface of the galvanized sheet, the laser defocus, the light wire spacing, the wire elongation, the shielding gas composition and flow rate, and adjusts the energy ratio of the laser heat source and the MAG arc heat source in the composite heat source by controlling the welding current and the laser power range, so that the cathode spot is stabilized at the same position, thereby improving the stability of the composite heat source; by controlling the above welding parameters, the recoil pressure of the metal vapor, the surface tension of the molten metal, and the impact force during the molten droplet transition are further reduced so that low spatter can be achieved while effectively increasing the welding speed, reducing the burning loss of the galvanized layer and effectively suppressing defects such as weld undercut and hump, thereby obtaining a smooth, uniform weld without pores. The galvanized sheet laser-MAG composite heat source welding process of the present invention can achieve a welding speed of 240 cm / min. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 A schematic diagram of the connection of a laser-assisted MAG arc composite heat source welding system in an embodiment of the present invention;

[0020] Figure 2 Schematic diagram of laser-assisted MAG arc composite heat source welding lap joint in an embodiment of the present invention;

[0021] Figure 3a The weld formed when the wire feeding speed is 2.2 m / min and the welding speed is 48 cm / min with a single MAG heat source in the embodiment of the present invention (the left side is the front of the weld and the right side is the back of the weld);

[0022] Figure 3b The weld formed when the laser-assisted MAG composite heat source in the embodiment of the present invention has a wire feeding speed of 2.4 m / min, a laser power of 560 W, and a welding speed of 120 cm / min (the left side is the front of the weld, and the right side is the back of the weld);

[0023] Figure 4a The weld formed when a single MAG heat source, a welding current of 97A and a welding speed of 150cm / min are used in the embodiment of the present invention (the left side is the front of the weld and the right side is the back of the weld);

[0024] Figure 4b The weld formed when the laser-assisted MAG composite heat source in the embodiment of the present invention has a welding current of 70A, a laser power of 560W, and a welding speed of 150cm / min (the left side is the front of the weld, and the right side is the back of the weld);

[0025] Figure 5 Schematic diagram of laser incident angle in an embodiment of the present invention;

[0026] Figure 6a In the embodiment of the present invention, laser-assisted MAG composite heat source welding, welding current 80A, laser power 960W, welding speed 240cm / min, laser defocus 0, filament spacing 2mm, and laser incident angle 90°, actual shot of weld formation (the left side is the front of the weld, and the right side is the back of the weld);

[0027] Figure 6b In the embodiment of the present invention, laser-assisted MAG composite heat source welding, welding current 80A, laser power 960W, welding speed 240cm / min, laser defocus 0, filament spacing 2mm, and laser incident angle 100°, actual shot of weld formation (the left side is the front of the weld, and the right side is the back of the weld);

[0028] Figure 6c In the embodiment of the present invention, laser-assisted MAG composite heat source welding, welding current 80A, laser power 960W, welding speed 240cm / min, laser defocus 0, filament spacing 2mm, and laser incident angle 110°, actual shot of weld formation (the left side is the front of the weld, and the right side is the back of the weld);

[0029] Figure 6d Laser-assisted MAG composite heat source welding in the embodiment of the present invention, welding current 80A, laser power 960W, welding speed 240cm / min, laser defocus 0, filament spacing 2mm, and laser incident angle 120°, actual shot of weld formation (the left side is the front of the weld, and the right side is the back of the weld);

[0030] Figure 7aLaser-assisted MAG composite heat source welding in the embodiment of the present invention, A1 group (welding current 70A, laser power 560W, welding speed 150cm / min, laser defocus 0, filament spacing 2mm, and laser incident angle 110°) actual shot of weld formation (the left side is the front of the weld, and the right side is the back of the weld);

[0031] Figure 7b Laser-assisted MAG composite heat source welding in the embodiment of the present invention, A2 group (welding current 80A, laser power 560W, welding speed 170cm / min, laser defocus 0, filament spacing 2mm, and laser incident angle 110°) actual shot of weld formation (the left side is the front of the weld, and the right side is the back of the weld);

[0032] Figure 7c Laser-assisted MAG composite heat source welding in the embodiment of the present invention, A3 group (welding current 90A, laser power 560W, welding speed 190cm / min, laser defocus 0, filament spacing 2mm, and laser incident angle 110°) actual shot of weld formation (the left side is the front of the weld, and the right side is the back of the weld);

[0033] Figure 7d Laser-assisted MAG composite heat source welding in the embodiment of the present invention, actual shot of weld formation in group B2 (welding current 80A, laser power 960W, welding speed 200cm / min, laser defocus 0, filament spacing 2mm, and laser incident angle 110°) (the left side is the front of the weld, and the right side is the back of the weld);

[0034] Figure 7e Laser-assisted MAG composite heat source welding in the embodiment of the present invention, B3 group (welding current 90A, laser power 960W, welding speed 220cm / min, laser defocus 0, filament spacing 2mm, and laser incident angle 110°) actual shot of weld formation (the left side is the front of the weld, and the right side is the back of the weld);

[0035] Figure 7f Laser-assisted MAG composite heat source welding in the embodiment of the present invention, C3 group (welding current 90A, laser power 1360W, welding speed 252cm / min, laser defocus 0, filament spacing 2mm, and laser incident angle 110°) actual shot of weld formation (the left side is the front of the weld, and the right side is the back of the weld);

[0036] Figure 8aIn the embodiment of the present invention, laser-assisted MAG composite heat source welding, welding current 80A, laser power 960W, welding speed 120cm / min, laser defocus 0, filament spacing 2mm, and laser incident angle 110°, actual shots of weld formation (from left to right, weld cross section 1mm, weld front 10mm, weld back 10mm);

[0037] Figure 8b In the embodiment of the present invention, laser-assisted MAG composite heat source welding, welding current 80A, laser power 960W, welding speed 150cm / min, laser defocus 0, filament spacing 2mm, and laser incident angle 110°, actual shots of weld formation (from left to right, weld cross section 1mm, weld front 10mm, weld back 10mm);

[0038] Figure 8c In the embodiment of the present invention, laser-assisted MAG composite heat source welding, welding current 80A, laser power 960W, welding speed 180cm / min, laser defocus 0, filament spacing 2mm, and laser incident angle 110°, actual shots of weld formation (from left to right, weld cross section 1mm, weld front 10mm, weld back 10mm);

[0039] Figure 8d In the embodiment of the present invention, laser-assisted MAG composite heat source welding, welding current 80A, laser power 960W, welding speed 210cm / min, laser defocus 0, filament spacing 2mm, and laser incident angle 110°, actual shots of weld formation (from left to right, weld cross section 1mm, weld front 10mm, weld back 10mm);

[0040] Figure 8e In the embodiment of the present invention, laser-assisted MAG composite heat source welding, welding current 80A, laser power 960W, welding speed 240cm / min, laser defocus 0, filament spacing 2mm, and laser incident angle 110°, actual shots of weld formation (from left to right, weld cross section, front side 1mm, front side 10mm, back side 10mm);

[0041] Fig. 9 A graph showing the relationship between different welding speeds and penetration depth and weld width in an embodiment of the present invention;

[0042] Fig.10 A graph showing the relationship between the weld formed at different welding speeds and the maximum force in an embodiment of the present invention;

[0043] Fig.11 Arc behavior diagram of laser-assisted MAG composite heat source welding in an embodiment of the present invention. DETAILED DESCRIPTION

[0044] 1 Experimental materials and equipment

[0045] The test material of this embodiment is HC340LAD+Z galvanized sheet with a thickness of 1.05 mm, the size of the galvanized sheet is 160 mm×80 mm×1.05 mm, and the filling material is ER70S-6 steel welding wire with a diameter of 1.0 mm; the chemical compositions of the base material and welding wire used in this embodiment are shown in Table 1.

[0046] Table 1 Chemical composition of base metal and welding wire (ω is mass fraction wt%)

[0047]

[0048] This embodiment uses a composite heat source constructed by a paraxial fiber laser and a MAG welder. The laser is a PLW3000 laser produced by Tianjin Shangke CNC Technology Co., Ltd. The laser parameters are as follows: the maximum output power is 3KW, the laser wavelength is 1070μm in continuous output mode, and the focal spot diameter is 300μm; the arc welding power source is a TPS2700 welder produced by Fronius, Austria; the three-dimensional welding platform and control terminal used in this embodiment to build a composite lap welding system such as Figure 1 When the laser is working, a water cooler is used to cool the laser to ensure stable output of the laser beam. During welding, the protective gas in the MAG welding gun is 100% CO2, and its flow rate is 17L / min.

[0049] In this embodiment, the MAG welding gun acts on the surface of the galvanized sheet test piece at a downward angle of 65°, and the welding is performed in a paraxial composite mode with the laser in front and the arc in the back. The focal distance (wire spacing) between the MAG welding wire and the laser beam on the test piece surface is 2 mm, and the laser defocus is 0; the wire dry extension length between the MAG welding gun conductive tip and the weld surface is 10 mm; other main welding test process parameters are shown in Table 2.

[0050] Table 2 Experimental process parameters

[0051]

[0052] After welding, observe the weld morphology on the front and back of the lap joint, and cut the sample perpendicular to the weld direction. After grinding, polishing and etching, observe the cross-sectional shape of the weld with a stereo microscope. The heat input Q of different welding methods is:

[0053] Q=(η M p M +η L p L ) / V

[0054] In the formula, p MThe MAG heat source power is obtained from the welding voltage U and the welding current; η M is the MAG heat source efficiency; p L is the laser beam power; η L is the heat source efficiency of the laser beam; V is the welding speed.

[0055] During laser-assisted MAG composite heat source high-speed welding, the laser generates plasma to increase arc stability, and the arc acts on the workpiece, increasing the laser absorption rate. Therefore, the laser beam heat source efficiency η L Take 0.7, MAG heat source efficiency η M Take 0.8.

[0056] 2. Analysis and discussion of test results

[0057] 2.1 Influence of lap joint form on weld formation

[0058] Based on the diversity of the welding structure of the automobile body in white, the welding process is formulated for the lap joints; Figure 2 Schematic diagram of lap joint.

[0059] During the lap joint welding process, a single MAG heat source was used for welding. The weld formed when the wire feeding speed was 2.2m / min and the welding speed was 48cm / min was as follows: Figure 3a As shown in the figure, the weld width and excess height on the front side of the weld are relatively large, and the galvanized layer on the back side is severely burned. This is because the overlap structure has a greater impact on the MAG heat source. To achieve overlap welds using only a single MAG heat source, it is necessary to rely on the uniform release of charged ions between the upper and lower plates under low-speed conditions so that the heat source can act stably at point a. However, due to the long action time of the MAG heat source, the galvanized layer on the back side of the weld will be severely burned. When the laser-assisted MAG arc composite heat source is used, the weld formed when the wire feeding speed is 2.4m / min, the laser power is 560W, and the welding speed is 120cm / min is as shown in the figure. Figure 3b As shown in the figure, the weld width and excess height on the front side of the weld are small, and the galvanized layer on the back side is burned. This is because the laser and the arc form a heat source through plasma coupling, which reduces the influence of the overlap structure on the arc stability.

[0060] Based on the results of the above tests, the lap weld formation between single MAG heat source and laser-assisted MAG heat source under the same power conditions was compared. Figure 4a The figure shows the weld formation under single MAG welding parameters (welding current of 97A, welding speed of 150cm / min). It can be found that there is a serious hump on the front of the weld, and the back is not fully melted and uneven. Figure 4bThe figure shows the weld formation under the laser-assisted MAG welding parameters (welding current of 70A, laser power of 560W, and welding speed of 150cm / min). It can be found that the front of the weld is continuous and uniform as a whole, and there is no galvanized layer burn on the back. This is because under the same heat input conditions, the energy of the laser heat source presents a Gaussian distribution, while the energy of the arc heat source is relatively dispersed. In the laser-assisted MAG welding method, the laser heat source preheats the base material, cleans the galvanized layer, and provides a plasma with lower resistance for the arc, which is beneficial to the stability of the arc; due to the single MAG heat source, the heat accumulation of the MAG arc on the base material is insufficient during high-speed welding, and the heat emission is insufficient, resulting in an unstable arc and poor fluidity of the liquid metal in the molten pool. Therefore, the use of laser-assisted MAG composite heat source for the lap joints of the body in white is an effective method to improve welding efficiency and quality.

[0061] 2.2 Effect of laser beam angle on seam forming

[0062] In laser-assisted MAG arc welding, when the MAG welding gun angle is constant, the laser beam incident angle will have a significant impact on the welding effect. The spatial position of the welding wire and the heat source of the laser beam, where β is the laser beam incident angle, such as Figure 5 shown.

[0063] In order to explore the effect of laser beam incident angle on weld formation, the optimized parameters (welding current I = 80A, laser power P = 960W, welding speed v = 240cm / min, laser defocusing amount is 0, and filament spacing is 2mm) are selected for further analysis at different angles. The weld formation obtained when the parameters are 90°, 100°, 110°, and 120° are as follows: Figures 6a to 6d shown.

[0064] right Figures 6a to 6d Analysis shows that when the welding gun angle is fixed, adjusting the appropriate laser incident angle can obtain good weld formation. When the laser beam incident angle is 90°, the laser spot area is 0.2826mm 2 The circular area of ​​the weld surface is intermittent and uneven, and burn-through occurs; when the laser beam angle is 100°, the laser spot area is 0.2870mm 2 In the elliptical area, the weld surface formation is improved, but it is still accompanied by burn-through; when the laser beam angle is 110°, the laser spot area is 0.3007mm 2 In the elliptical area, the weld surface is continuous, smooth, and has no external pores, and there is no burn-through on the back; when the laser angle is 120°, the laser spot area is 0.3263mm 2 In the oval area, the weld is well formed and there is almost no zinc coating burnt on the back.

[0065] It can be found that the laser beam incident angle increases, and the laser beam heats the longitudinal weld area. Therefore, in the hybrid welding process, when the laser angle is between 110° and 120°, the weld penetration and width are just in the optimal range.

[0066] 2.3 Effect of laser energy and welding current matching on weld formation

[0067] Laser-assisted MAG arc composite heat source welding is adopted. The stability of lap weld formation becomes an important factor affecting the performance of welded joints, which is mainly affected by the energy ratio of composite heat source.

[0068] In order to study the influence of the ratio of laser energy to arc energy on the weld, the weld formation was compared under the same heat input conditions. The welding process numbers (group A, group B, group C correspond to laser powers of 560W, 960W, and 1360W; 1, 2, and 3 correspond to welding currents of 70A, 80A, and 90A) were used for a total of six groups of tests (A1, A2, A3, B2, B3, and C3). The test results showed that under certain heat input conditions, considering the amount of metal filling, when the welding speed is increased, within the above energy range, the ratio of arc energy to laser energy is in the range of 1.05 to 2.94 (this value can be expressed as the proportion of increased laser energy), which can ensure the stability of the MAG arc when the laser-assisted MAG galvanized sheet is lapped at high speed, and achieve lap welds under certain heat input.

[0069] Figures 7a to 7f It can be found that when the laser power of group A is set to 560W, as the welding current increases from 70A to 90A, some pores gradually appear on the front of the weld, and even discontinuity occurs, and there is no burn-in of the galvanized layer on the back. The reason for this phenomenon is that when the welding speed increases, although the energy of the MAG heat source increases and the total energy increases, the energy added by the MAG heat source cannot be effectively concentrated in the welding area, so that under the same heat input conditions, not only does it not improve the welding joint formation, but it reduces the joint formation. In other words, under this heat input condition, after the MAG welding current reaches 90A, the laser beam with an energy of 560W does not fully evaporate the galvanized layer, resulting in poor electron emission ability of the matrix iron, thereby reducing the concentration of plasma near the MAG arc and causing the arc to become unstable. Therefore, when the heat input is constant, it is impossible to significantly increase the welding speed by simply increasing the welding current using the laser-assisted MAG welding method.

[0070] Based on the test results of group A, the laser power in group B was increased to 960W, and the welding currents were tested at 80A and 90A respectively. It can be observed that after increasing the energy of the laser beam, the weld formation was significantly improved, but slight surface pores still appeared when the welding current was 90A. Under certain conditions of heat input, after further increasing the welding speed, the laser beam with an energy of 960W can evaporate the local galvanized layer and provide a certain amount of plasma for the arc, but the stability of the arc is still limited by the welding speed. In the above test, under certain conditions of heat input, the laser-assisted MAG welding method can greatly increase the welding speed by increasing the laser power.

[0071] Based on the test results of Group A and Group B, the laser power in Group C was increased to 1360W and the welding current was tested at 90A. Figure 7f As shown in the figure, the weld surface is uniform, without external pores and undercuts, and the back galvanized layer has only pitting ablation. At this time, the welding speed for the lap joint has reached 252cm / min.

[0072] Through the above six test results, it can be obtained that under the condition of heat input of 54J / mm. When the laser energy is constant, with the increase of welding current, the arc pressure and the impact force of the molten droplet also increase. In the high-speed welding process, the laser-assisted MAG composite heat source effectively increases the molten pool area. Since the laser provides low-resistance charged ions for the arc, the arc moves toward the laser, and the arc coverage area increases, which is more conducive to the flow of liquid molten metal in the molten pool toward the laser. Therefore, in the laser-assisted MAG arc high-speed welding process, the auxiliary laser weakens the tendency of the liquid metal to flow backward from the two aspects of plasma ionization and reserved weld groove, thereby suppressing the phenomenon of hump and undercut during high-speed welding.

[0073] Through the above test analysis, it is found that in the laser-assisted MAG welding method, laser power is the main factor affecting the welding speed. Therefore, under different welding speed conditions, the ratio of arc energy to laser energy directly affects the weld formation. For the overlap of 1.05mm galvanized sheet, the welding current is 70A~90A, the laser power is 560W~1360W, the welding current is selected as 90A, the laser power is selected as 1360W, and when the energy ratio is 1.05, the welding speed can reach 252cm / min.

[0074] 2.4 Effect of welding speed on weld formation

[0075] Based on the characteristics of laser-assisted MAG composite heat source, the laser beam provides low-resistance plasma for the MAG arc, which increases the arc area and changes the arc force, making it easier for the molten metal in the liquid molten pool and the molten droplets to flow along the welding speed direction, avoiding the hump weld and achieving high-speed welding. The following analyzes the effect of welding speed on the formation of lap welds.

[0076] Based on the above experiments, the welding speed test was carried out on the optimized welding process parameters (laser power of 960W, welding current of 80A, laser beam angle of 110°, laser defocus of 0, and filament spacing of 2mm). The experimental results are as follows Figures 8a to 8e shown.

[0077] from Figures 8a to 8e Under the above welding process parameters, the welding speeds are set to 120cm / min, 150cm / min, 180cm / min, 210cm / min, and 240cm / min in sequence; Figures 8a to 8e It can be found that when the welding speed is in the range of 120cm / min to 240cm / min, the weld is smooth and uniform as a whole, with almost no spatter, no defects such as pores and undercuts on the surface, and the burn loss of the back galvanized layer is gradually decreasing.

[0078] Fig. 9 The changing trend of the weld depth and width with the increase of welding speed shows that the maximum change of the width is about 15.38%, and the maximum change of the depth is about 25.47%. At the same time, it can be found that under the same welding current conditions, with the increase of welding speed, the change of weld width is much lower than the change of weld depth. This means that in the process of laser-assisted MAG composite heat source high-speed welding, the action time of the laser determines the weld depth, while the action time of the MAG heat source determines the weld width.

[0079] Fig.10 The maximum force curves of the lap weld under tension to fracture at different welding speeds show that the maximum force shows a downward trend with the increase of welding speed. It is also observed through the welding cross section that with the increase of welding speed, the weld presents a narrow and shallow morphology, which reduces the overall tensile strength of the weld.

[0080] 3 Arc behavior of laser-assisted MAG welding

[0081] Fig.11 The image in the figure shows the plasma change process within one cycle of the laser-assisted MAG composite heat source. The influence of the photoplasma generated by the laser beam on the MAG arc is as follows: during laser-assisted MAG high-speed welding, the MAG short-circuit transition characteristics are divided into three parts: liquid metal bridge connection, maintenance stage, and the molten droplet entering the molten pool after unstable rupture.

[0082] In the time period from t0 to t0+1.75ms, Fig.11At t0+0.5ms, it can be found that at the moment of MAG arc burning, there will be a 0.25ms arc plasma uprush phenomenon, and then the phenomenon disappears immediately. The MAG arc is compressed and attracted to the vicinity of the laser beam action point, such as Fig.11 The reason for the above phenomenon is that in the early stage of MAG arc burning, the arc diameter increases, which will lead to increased heat loss in the arc column area and make the arc unstable. Under the action of the laser beam, the energy of the arc column area is further increased, causing the conductive channel to shrink, thereby increasing the conductivity of the conductive channel by increasing the electron density and stabilizing the arc.

[0083] In the time period from t0+2.0ms to t0+5.75ms, it can be observed that as time goes by, the overlapping part between the MAG arc and the laser beam heat source gradually separates, and the arc diameter decreases and maintains a stable state. On the one hand, under the action of the laser, zinc and iron are very easy to vaporize, but the ionization energy of zinc is 9.36eV, which is higher than the ionization energy of iron, 7.83eV. Therefore, under the action of the laser, zinc is not the main factor in accelerating the formation of plasma. At this time, iron with low ionization energy can preferentially form some electrons to supplement the electron density required in the arc space. On the other hand, the cathode is heated by the laser beam, so that zinc and part of the iron with lower melting and boiling points are vaporized, and the electron emission ability of the hot cathode exposed in the arc space is enhanced. Therefore, the arc maintains a short arc in the arc burning stage, controls the size of the molten droplet, ensures a stable short-circuit transition, and achieves the characteristics of low spatter and high-speed welding.

[0084] In summary, the following conclusions can be drawn from the experiment in this embodiment:

[0085] (1) When the high-speed lap joint process of 1.05 mm thick galvanized sheet was tested by laser energy-assisted MAG arc, it was found that the matching of laser power and welding current showed a nonlinear increasing relationship.

[0086] (2) Under the same heat input conditions, when high-speed lap welding of 1.05 mm thick galvanized sheet is performed, given a welding current of 70 A to 90 A, a laser power of 500 W to 1360 W, and a ratio of arc power to laser power of 1.05 to 2.94, the stability of the MAG heat source during the welding process can be guaranteed. Improving the laser power is the key factor in achieving high-speed welding.

[0087] (3) When laser-assisted MAG composite heat source high-speed welding is performed on 1.05 mm thick galvanized sheet, while ensuring a certain energy, on the one hand, the laser beam can share the arc energy, thereby reducing the arc pressure and the impact force during droplet transfer; on the other hand, the laser can heat the galvanized sheet base material to evaporate the zinc layer, and the exposed base iron is more likely to provide charged particles for the arc, increasing the stability of the MAG arc.

[0088] (4) Comparing the laser incident inclination angles of 90°, 100°, 110°, and 120°, when the laser incident inclination angle is 110°, the weld formation of the galvanized sheet can be guaranteed and the welding speed can be effectively increased.

[0089] (5) When high-speed welding of galvanized sheet metal is performed, after adding laser, the arc recoil height at the initial stage of arc ignition by MAG heat source is reduced, and the arc ignition to arc stabilization period is shorter.

[0090] In some other embodiments, the above-mentioned process can be used to weld the galvanized sheets in a butt welding manner. Under the premise of achieving high-speed welding, the galvanized layer can be avoided from being damaged and the galvanized layer can be burned, and weld defects such as undercuts and humps caused by excessively high welding speeds can be avoided. Moreover, the strength of the butt joint after butt welding of the galvanized sheets is higher than that of the butt joint of the galvanized sheets welded using the CMT welding process.

Claims

1. Laser-assisted MAG high-speed welding process for galvanized sheet, characterized in that: Using galvanized sheet as base material, carbon steel argon arc welding material as welding wire, laser-assisted MAG arc as composite heat source, and adopting paraxial composite welding method with laser in front and MAG arc in the back; The laser beam in the composite heat source is provided by a paraxial fiber laser PLW3000, with a maximum output power of 3KW, a laser wavelength of 1070μm in continuous output mode, and a focal spot diameter of 300μm. A water chiller is used to cool the laser. During welding, the working power of the laser is 560-1360W, the angle β between the laser beam and the surface of the galvanized sheet is 110-120°, and the laser defocus is 0; the galvanized sheet is welded by lap or butt welding, and the tilt angle is 22° during lap welding of the galvanized sheet; The arc welding power source is TPS2700 welding machine, the shielding gas in the MAG welding gun is 100% carbon dioxide gas, and the flow rate of shielding gas is 17L / min; The angle between the MAG welding gun and the galvanized sheet surface is 65°, and the focal distance between the welding wire on the MAG welding gun and the laser beam on the galvanized sheet surface is 2 mm; the elongation of the welding wire between the conductive nozzle of the MAG welding gun and the galvanized sheet surface is 10 mm; During welding, the arc current of the MAG welding gun is 70-90A, and the welding speed is 150-240cm / min; When the welding current is 70A-90A and the laser power is 500W-1360W, the energy ratio of the MAG arc heat source to the laser heat source in the composite heat source is 1.05-2.

94.

2. The laser-assisted MAG high-speed welding process for galvanized sheet according to claim 1, characterized in that: The galvanized sheet is HC340LAD+Z galvanized sheet with a thickness of 1.05 mm, and the carbon steel argon arc welding material is welding wire with a diameter of 1.0 mm, and the welding wire is ER70S-6 welding wire, ER49-1 welding wire or ER50-G welding wire.

3. The laser-assisted MAG high-speed welding process for galvanized sheet according to claim 2, characterized in that: In the HC340LAD+Z galvanized sheet: the carbon content is ≤0.11wt%, the manganese content is ≤1.00wt%, the silicon content is ≤0.5wt%, the phosphorus content is ≤0.03wt%, the sulfur content is ≤0.025wt%, and the HC340LAD+Z galvanized sheet does not contain chromium, nickel and copper.

4. The laser-assisted MAG high-speed welding process for galvanized sheet according to claim 2, characterized in that: The carbon steel argon arc welding material is ER70S-6 welding wire; in the ER70S-6 steel welding wire: the carbon content is 0.06-0.15wt%, the manganese content is 1.40-1.85wt%, the silicon content is 0.80-0.90wt%, the chromium content is ≤0.20wt%, the nickel content is ≤0.30wt%, the copper content is ≤0.20wt%, the phosphorus content is ≤0.025wt%, and the sulfur content is ≤0.025wt%.

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