| At present,MIG welding is the most widely used method in thick plate joining,which shows low welding efficiency and poor joint performances on account of the reality that multi-layer and multi-pass welding are generally required during thick plate welding.By comparison,hybrid laser-MIG welding is a very suitable method for thick plate welding,which enjoys advantages of high efficiency and sound bead appearance coming from the high energy density laser beam and stable MIG welding process,respectively.In order to lay the foundation of hybrid laser-MIG welding,MIG welding and tandem MIG welding of 30 mm thick aluminum alloy are investigated first.Results verified that six passes and five passes were needed for MIG process and tandem process,respectively.The joints obtained by the two processes showed good bead appearance,meaning the stable procedures.On this basis,hybrid laser-MIG welding of 30 mm thick aluminum alloy was studied.To improve the welding efficiency to the most extent,the first step was to obtain the maximum penetration.Considering both bead appearance and weld penetration,results pointed out that laser leading mode was beneficial,and the distance between laser and arc 2 mm and defocusing distance 0 mm were finalized.The regression model established based on response surface methodology provided guidance for selecting the appropriate experimental parameters.The maximum penetration,18 mm,was acquired under the laser power 10 kW,welding current 136 A and welding speed 0.6 m/min,which signified the plate with thickness 30 mm could be jointed by merely two passes.Therefore,the innovative double-side hybrid laser-MIG welding was design and employed to the 30 mm thick plate.The welding process was optimized based on the analysis of the mechanism that defect formed and its distribution,i.e.,through overlaying a MIG welding upon the weld,all porosities were wiped out,and weld undercut and underfill were all eliminated.A perfect weld with sound bead appearance was obtained by the optimized process.Analysis on microstructure indicated that the weld existed columnar grain,equiaxed grain and feathery grain.For MIG zone,on the fusion line,there existed coarse columnar grains;in the center part,some feathery grains appeared;and other areas were equiaxed grains.For laser zone,the whole weld were equiaxed grains.It was noteworthy that grain size showed great difference for different zone of the weld,for example,the grain size of MIG zone was 69.8 ?m,the laser zone,however,was only 26.3 ?m,which was attributed to the specificity of laser.The laser brought about deep penetration,the heat in the laser tip was conducted out rapidly because of the large amount of base metal around.So the cooling rate in the laser tip was such fast that grains did not have enough time to growth.Along the longitudinal direction of the weld,from two fringes to the center,micro-hardness,yield strength,tensile strength and ductility increased gradually,which was caused by the finer and finer grains from fringes to the center of the weld.The decreased grain size led to the increase of grain boundary density,its obstruction to the slips of dislocations increased the mechanical properties of the joint.A comprehensive comparison among MIG welding,tandem MIG welding and hybrid laser-MIG welding was made to reveal the superiorities of the latter For hybrid laser-MIG welding,groove machining and back gouging were not required,increasing the welding efficiency to three times higher than MIG and tandem MIG welding.Besides the heat input and weld area were obviously small.The tensile strength was the highest,reached 311 MPa.Conclusion could be made that the double-side hybrid laser-MIG welding process established in this research was promising in industrial application. |
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