Research On The Laser-MIG Hybrid Welding Technique With Microstructures And Mechanical Properties Of Welded Joints Of A7N01

The laser-MIG hybrid welding technology of aluminum alloy has the advantages of high welding efficiency and wire-filling by MIG welding.The filler metal of MIG welding can also improve the structure and metallurgy of welded joints.It also integrates the advantages of both MIG and laser heat sources,which has been more and more widely usedA7N01 aluminum alloy has excellent physical and chemical properties,which have been extensively used in aerospace,aircraft industries,high-speed train and automobile manufacturing.Aluminum alloy profiles are widely applied to high-speed train structural materials.However,the researches had been showed that weld defect such as pores,cracks and bad weld appearance readily occurred during thick aluminum plate welding.The performance of welded joints has yet to be further improved.The laser-MIG hybrid and MIG welding experiments of A7N01 aluminum alloy were carried out by an IPG YLS-5000 with KEMPPI in connection with KUKA robot.The microstructural evolution of the joint and the properties of the joint were systematically studied.Which aims to lay the theoretical basis for the laser-MIG hybrid welding of A7N01 aluminum alloy.The detailed research results are as follows:Through continuous improvement of A7N01 aluminum alloy laser-MIG hybrid welding process,An hybrid welded joint with good forming and no weld pores and cracks was obtained.The laser-MIG hybrid welded joint of A7N01 aluminum alloy is divided into weld metal zone,fusion zone and heat affected zone.Which exhibit different microstructural features due to experienced different thermal cycles.At the top of the hybrid welded joint,the equiaxed crystal region,the transition region and the columnar crystal region are sequentially formed from the center of the weld to the edge of the weld.While at the bottom of the hybrid welded joint,There is no obvious columnar crystal region.The alloy elements in different regions of welded joints is nonuniform distribution by EDAX analysis.The microhardness of the weld zone of the laser-MIG hybrid welded joint is the lowest about 76 HV,and the microhardness of the base metal is the highest about 108 HV.the microhardness near the fusion line is fluctuation.The tensile strength of the hybrid welded joint is 309 MPa,which is 71.7% of the base metal strength.The macroscopic fracture has obvious plastic deformation,and there are a lot of dimples on the microfracture surface.Tensile specimens were broken in the weld position,so the welded joints become the weakest position.The fatigue fracture of the laser-MIG hybrid welded joint can be divided into the crack initiation,the crack stable extension region and the transient fracture region.The fatigue crack initiation usually occurs near the coarse second phase and inclusions at the sample surface,There are a lot of fatigue strip and secondary cracks in the extension zone.The transient fracture zone is characterized by brittle and ductile fracture,similar to the microstructures of the static tensile fracture.In the early stage of the fatigue cycle,the ratcheting effect is obvious,especially in the first 10 cycles,the material undergoes obvious plastic deformation.In the final stage of the fatigue cycle,the crack propagation speed increases.The microcavity in the material grows up,which results in the stress concentration inside the material and leads to the fatigue failure of the material.By observing the surface damage of the fatigue specimens,fatigue crack initiation occurred preferentially at the weld defects such as the inclusion and the pores in the weld joints.The cycle softening of the laser-MIG hybrid welded joint mainly occurs in the fatigue process,and the cyclic hardening occurs periodically.In the stress control mode,as the number of cycles increases,the plastic deformation gradually increases.The stress-strain hysteresis loops are different at different stress levels for the time to reach stable circulation.