Research On The Porosity Morphology And Influence Factors In Laser-MIG Hybrid Welded Joint For 5A06 Aluminum Alloy

Laser-MIG(Melt Inert Gas)hybrid welding technology,which combines the advantages of both laser welding and arc welding,has become an efficient technology to join the aluminum alloy with the medium thickness.The 5A06 aluminum alloy has been widely used in spacecraft tank thanks to its low density,high strength and excellent corrosion resistance.Comparing with the traditional joining technologies used in spacecraft tank currently,the joining efficiency of laser-MIG hybrid welding improves significantly.However,the serious problem,i.e.porosity,is eagerly expected to solve during laser-MIG hybrid welding.In this paper,the laser-MIG hybrid welding experiments for 5A06 aluminum alloy with the thickness of 6.9mm is performed by the high-power disc laser system and MIG welding system.The porosity morphology and distribution characteristic are comprehensively studied.Moreover,the porosity formation mechanism and its influence factors are investigated.Firstly,the preliminary laser-MIG hybrid welding experiment for 6.9 mm 5A06 aluminum alloy is carried out.Based on the experiment results,a three-dimensional finite element model is established to investigate the thermal field distribution of laser-MIG hybrid welding process.The laser-MIG hybrid welding experiment with different welding heat input is designed and performed further.The macro morphology,microstructure and tensile property of welded joints are respectively researched.The results show that the macro morphology of weld seam is closely related to the welding heat input.From the fusion line to weld center,the microstructure gradually changes from columnar crystal to equiaxed dendrite.According to the morphology of welded joints under different MIG heat input,the welded joints are divided into two categories.Besides,the burning behavior of Mg element is inevitable.The welded joint with the highest tensile strength,85.14% of the base metal,is obtained under the smallest energy ratio coefficient of laser-MIG hybrid welding.Secondly,the porosity morphology inside the laser-MIG hybrid weld seam for 5A06 aluminum alloy is researched.It is resulted that the number of metallurgical porosities in the weld seam is obviously more than that of process porosities.Meanwhile,the growth of grain is significantly influenced by the formation of porosity.The process porosities are mainly distributed in the bottom half of the weld seam because the formation of process porosity is closely related to the stability of the keyhole tip.The formation of the metallurgical porosity is closely related to the generation of hydrogen.Besides,the precipitated phase,which is formed simultaneously,easily aggregates on the wall of metallurgical porosity.The metallurgical porosities with smaller size are mainly distributed in the upper region of weld seam.Finally,the effect of different welding factors on the porosity is investigated,where the welding heat input,groove form and prefabricated plate are mainly concerned.The results illustrate that the porosity of the full penetrated weld seam is lower than that of the partial penetrated weld seam.The higher MIG heat input is beneficial to reduce the porosity.When the energy ratio coefficient of laser-MIG hybrid welding is designed at 0.5 approximately,the welded joint with a lower porosity is expected to obtain.In addition,compared with the Y groove,the molten pool obtained with I groove is not beneficial to the escape of bubble during the welding process owing to the smaller volume of molten pool and the certain limitations on the value scope of MIG heat input.Although the welded joints with the better surface morphology is obtained through prefabricated plate,the process porosities are easily generated on the back of welded joint near the prefabricated plate under the higher laser heat input.