Study On The Technology And Blowhole Defects Of Paraxial Laser-MIG Hybrid Welding For Aluminum-alloy Thin-plate

Lightweight design is the main measure to achieve energy conservation and emissionreduction in cars, trains, aerospace and other industry fields. Aluminum alloys are widelyused for its advantages of high specific strength. It is expected that laser-arc hybrid weldingwill play an important role in cars, trains, aerospace and other basic industries concerningnational welfare and people’s livelihood, owing to its unique advantages in weldingaluminum alloy. In abroad, some research institutions have made some significant progressin laser-arc hybrid welding aluminum alloy, and gradually begun to apply it to someindustry fields. However, researches on this field have seldom reported in China. Comparedwith developed countries such as Germany, Japan and so on, a certain gap occurs becauseof this reason. In this study, we described a process experiment of aluminum alloy inlaser-MIG hybrid welding by means of fiber laser and self-built hybrid welding platformand investigated its process characteristics. The mechanism of producing pore defects andformation features in the weld were also analyzed. In addition, we further studied themicrostructure and properties of welded joints in5754aluminum alloy with an overlap and6005A aluminum alloy with a butt.The experimental results demonstrated that the weld with a deeper penetration depth andsmaller bead width was achieved using a method of the arc in front of laser when laser-archybrid welding aluminum alloy. When the welding speed kept constant, the weldpenetration depth was mainly affected by laser power, but the bead width was mainlyinfluenced by the welding current. A certain process range existed under the correspondingprocess parameters, heat source interval of which is characteristic of space between thelight and wire. With other welding parameters fixed, the distance between laser and wiremust be controlled in. It also affected penetration depth and the bead width of the weld.Hydrogen porosity, process porosity and interlayer porosity were found in the weld usinglaser-MIG hybrid welding. Hydrogen porosity with a regular spherical shape, smooth interwall and small size, which was caused by much hydrogen precipitating in the process of the molten pool solidification. Process porosity was as a result of the instantaneous instabilityof the keyhole during the welding. These porosities had irregular shape, stepped inner walland different sizes. The formation of the interlayer porosity can be attributed to therefractory oxide film. These porosities possessed irregular shape, large sizes, acute angleand oxides in the wall. Hydrogen porosity and interlayer porosity can be effectivelyinhibited via removing the oxide film and using inert gas to protect welding pool.Meantime, process porosity in the weld can be reduced by using appropriate weldingparameters, such as lower welding speed, bigger defocus, preset clearance and penetration.Characteristics of a wide upside and narrow downside appeared in the hybrid weldingjoint. The upper part showed some characteristics of the arc welding, while the lower partwas close to laser welding. The microstructure near the fusion line was composed ofparallel fine columnar grains which are vertical in this line. In the center weld, themicrostructure covered equiaxed dendrites. The lower grain sizes were smaller than that ofthe upper part. Alloy composition of weld seam between the upper and the lower was verysimilar under the action of arc stirring and weld pool flowing. At the same time, the loss ofMg element was supplemented by addition of the filler wire.The hardness of the weld using hybrid welding was slightly lower than that of base metal,and a higher hardness appeared in the bottom instead of the top of the weld. The occurrenceof low hardness in the heat affected zone is as a result of joint softening. Properties ofwelded joints can be affected greatly by the porosity in the weld. When little porositiesoccurred in the weld, the shear strength of lap joints reached219MPa in5754aluminumalloy and butt joints of6005A aluminum alloy exhibited good tensile strength and bendingperformance.