Fundamental Study On Hybrid Laser Beam-Resistance Seam Welding(LB-RSW)

Attributing to the characteristics of high-speed, low-distortion and flexibility, laser beam welding is considered to be one of the most potential welding methods in 21st century. However, the application range in engineering of laser beam welding is restricted because of several problems, such as power rank of laser, assembly condition (assembly clearance, misalignment, different thickness, etc.), process control (focus fluctuation and laser beam aligning), high reflectivity of materials (aluminum, magnesium and copper) and so on. Therefore, hybrid welding technologies, which remedy the drawbacks of laser beam welding by utilizing the speciality of other welding methods, become the research emphasis. Based on synchronized pressing, preheating and slow-cooling effect owned by resistance seam welding (RSW), Prof. Zhao Xihua firstly puts forward the new concept of LB-RSW (Laser Beam-Resistance Seam Welding) in 2003, which has been applied international invention patent (No. PCT/WO2007008363A2), and has carryed out principium simulation experiment of asynchronous LB-RSW of mild steel and aluminum alloy workpieces, and accomplished primary results. So, it is indispensable to study several basic problems of the new hybrid welding method of LB-RSW systematically, and to take advantage of laser beam welding and resistance seam welding, and to acquire an effective welding effect and high quality joint finally.The article engages in fundamental study on LB-RSW hybrid welding meyhod and system, supported by national natural science foundation (No.50575091,2006.01～2008.12).The effect of preheating on microstructure and mechanical properties of aluminum alloy laser lap welded joint were studied. The results indicated that proper preheating could improve aluminum alloy laser welding quality and increase the efficiency of laser lap welding of aluminum alloy, which laid a foundation of subsequent mechanism study of LB-RSW hybrid welding method.With the preheating temperature Tθincreasing, the weld width and penetration depth increased by a small margin when Tθ<250℃, and meanwhile, the weld width and penetration depth would increase rapidly as Tθ≥250℃. Preheating temperature influenced microstructure of aluminum alloy laser lap weld greatly. When the preheating temperature was higer than 300℃, the weld microstructure became coarse as the preheating temperature rising obviously. The microstructure of welds, consisting of both finer equiaxed dendritic grains and columnar dendritic grains in room temperature, tranformed to coarse dendritic grains in 400℃. In addition, compared with laser welding without preheating, the weld microstructure also consisted of both equiaxed dendritic grains and columnar dendritic grains with the preheating temperature of 300℃, and the microstructure near fusion line was similar.There existed remarkable effects for preheating temperature on mechanical properties of aluminum alloy laser lap welded joints. With the increase of preheating temperature, microhardness of weld, HAZ and base metal decreased to different extents: quite little in the weld, HAZ and base metal as the Tθwas lower than 100℃, 200℃, 300℃, respectively. The preheating to 300℃could increase welding speed greatly for 71%, that was from 1.4m·min-1 to 2.4m·min-1, as weld width and penetration, heat affected zone width and tensile shear strength did not change obviously.Based on the fundamental concept of hybrid laser beam and resistance seam welding (LB-RSW), considering the characteristics of laser beam welding and resistance seam welding seperately, LB-RSW hybrid testing system made up of multifunctional resistance seam welding machine and laser welding robot was designed and manufactured. Two hybrid modes were presented in LB-RSW hybrid testing system. The one mode was that two wheels positioned around two sides of the laser beam on one side of the workpiece, tilting to a certain degrees according to the laser beam shape after focusing. The other mode was that two wheels positioned on two sides of the workpiece, tilting to a certain degrees according to the laser beam shape after focusing, and laser beam positioned in one side of the wheels. By means of the two hybrid modes, both one-side and double-side hybrid welding of laser beam and RSW could be achieved. The testing results of the hybrid welding system indicated that main technological indexes met the demands of design and experiment, such as welding speed fluctuation was less than±1.5%, defocusing distance fluctuation was less than±0.03mm, resistance power capacity was 100kVA, and so on. The debugging of LB-RSW hybrid testing system was carried out and the hybrid welding scheduling was set, which established a foundation for the realization of hybrid LB-RSW.The thermo-elastoplastic and thermo-electrical circular order analyses of resistance seam welding (RSW) process with both crossflow and longitudinal flow current in LB-RSW were carried out by means of Ansys software. The current distribution and the influences of RSW current, welding speed and spacing between two RSW wheels on temperature field and thermo-cycle of weld zone were studied. In addition, the reliability of numerical simulation of aluminum alloy RSW temperature field was verified by means of infrared thermal imaging technology, which demonstrated the dependability of the simulation method adopted in the article.During RSW with crossflow current, The density of current which flowed through upper aluminum alloy sheet was high in the central zone between two wheel electrodes, while the density of current which flowed through lower aluminum alloy sheet and copper backing plate was less relatively. Comparatively, in RSW with longitudinal flow current, the current flowed through two wheel electrodes, weld zone and copper backing plate. During RSW with both crossflow and longitudinal flow current, the surface temperature of weld zone and the temperature grades both in front and back of wheels increased with RSW current I increasing, and decreased when welding speed v or wheel electrodes spacing d1 rised. The rising and decreasing rates of weld zone surface temperature increased with I or v increasing, and decreased when d1 rised. The effect of RSW current I on weld zone surface temperature showed a trend of enhancing with the increase of I, while the effect of welding speed v on weld zone surface temperature showed a trend of declining with the increase of v. During RSW with crossflow current, the maximum temperature point on the Y-axis of the workpiece, positioning at 0.95mm behind wheel electrode, received little effect from I, v and d1. Comparatively, during RSW with longitudinal flow current, the maximum temperature point on the Y-axis of the workpiece, positioning at 2.86mm behind wheel electrode at v=0.8m·min-1and 4.77mm behind wheel electrode at v=1.4m·min-1, received little effect from I and d1, but much from v.Process experiments of LB-RSW with both crossflow and longitudinal flow current were performed by self-made LB-RSW hybrid testing system mentioned above, and the best style of feed was decided while weld formation, microstructure and mechanical properties of LB-RSW under the condition of the best feed style. The keyhole depth became shallow as the electromagnetic force downward received by weld pool during LB-RSW with crossflow current, and therefore the weld with large width and shallow weld depth was obtained under the auxiliary function of heating to workpieces from RSW. The feed style of secondary curcuit of multifunctional resistance seam welding machine in LB-RSW hybrid welding system was changed from crossflow current to longitudinal flow current, and the experiment of 5754 aluminum alloys with 1.0mm and 5052 aluminum alloys with 1.5mm LB-RSW with longitudinal flow current were researched seperately. The results indicated that the forced state of weld pool was changed with the change of feed style, and the weld widh, penetration depth and tensile shear strength of weld, obtained by LB-RSW with longitudinal flow current, would increase with the rising of current under the appropriate current range(2~5 kA for 5052 aluminum alloys with 1.5mm at v=0.8m·min-1, and 2～7kA for 5754 aluminum alloys with 1.0mm at v=1.5m·min-1), and that deeper weld depth and larger tensile shear stength were acquired by LB-RSW, compared with single laser beam welding. That was to say, LB-RSW with longitudinal flow current improved the machining ability of laser beam. Thereinto, welding speed could increase for 67% during LB-RSW of 5754 aluminum alloy sheet with 1.0mm, that was from 1.5m·min-1 to 2.5m·min-1, under the premise of the tensile shear load of joints keeping consistent in basically.The weld surface fomation of 5754 aluminum alloy LB-RSW was superior to that of LBW. The microstructure of weld obtained by both LB-RSW and LBW consisted of columnar dendritic grains near fusion line and equiaxed dendritic grains in the weld center. The microstructure of 5052 aluminum alloy of 1.5mm thicknesses weld obtained by LB-RSW at low welding speed was coarser than that obtained by LBW, but the coarsing trend of microstructure became less evident with the increase of welding speed. In addition, the microstructure in weld center of 5052 aluminum alloy of 1.5mm thicknesses obtained by LB-RSW was finer than that obtained by LBW.The regression equation of weld width of both upper sheet w1 and lower sheet w2, weld depth h and tensile shear load F about RSW current, welding speed and hybrid distance between laser beam and wheel electrode d2 was set up by means of quadratic regression combination design process, and the influence law by welding parameters on them was also dicussed, which laid a foundation for further study on the mechanism and application of LB-RSW with longitudinal flow current. Two main factors which influenced tensile shear load F, weld width of lower sheet w2 and weld penetration depth h significantly were welding speed v and RSW current I. Meanwhile, two main factors which influenced weld width of upper sheet w1 much were welding speed v and hybrid distance between laser beam and wheel electrode d2. When it came to interaction effect of some welding parameters, interaction effect of RSW current I and hybrid distance between laser beam and wheel electrode d2 on tensile shear load F, weld width of lower sheet w2 and weld penetration depth h was evident. Comparatively, interaction effect of welding speed v and hybrid distance between laser beam and wheel electrode d2 on tensile shear load F was evident. However, interaction effect of welding speed v and RSW current I had no influence on tensile shear load F, weld widths of both upper and lower sheets and weld penetration depth h.The optimum welding parameters corresponding to the maximum of tensile shear load F had been obtained by programming using Matlab software, which is v=0.8m·min-1, I=4.5kA and d2=6.2mm. Under the condition of the optimum welding parameters, the joint with reasonable weld shaping and no visible defects could be obtained, with tensile shear load, weld width of upper sheet, weld width of lower sheet and weld penetration depth being 4.72kN, 3.78mm, 2.33mm and 2.91mm, respectively, which coincided with the predicted values. The weld consisted of columnar dendritic grains near boundary and equiaxed dendritic grains in weld center.According to the above study, further understanding about LB-RSW hybrid welding method was achieved, and several basic problems of LB-RSW were solved, which laid a foundation for both deep research and popularization and application on the new hybrid welding method.