Porosity Characteristics And Their Effects On Mechanical Properties Of Laser-MIG Hybrid Welded Of Aluminum Alloy Joints

Safety,comfort,and high-speed travel are the real needs of rail transit vehicles.It is an inevitable trend to use lightweight materials to realize lightweight manufacturing of vehicle bodies.Aluminum alloy has become the main material for high-speed train body manufacturing due to its low density,high specific strength,strong corrosion-resistance,and good formability.Laser-MIG hybrid welding with low heat input,fast welding speed and high process stability is an ideal technology for low deformation,high efficiency,and high-quality welding of high-speed train aluminum alloy body.In this paper,laser-MIG hybrid welding was used to do the butt test of 6mm thick 6082-T6 aluminum alloy for the high-speed trains.The influence of hybrid welding process parameters on the weld formation and the influence of porosity content,size and distribution position were studied.The microstructure characteristics and hardness distribution of the joint were analyzed.The influence of porosity distribution characteristics on the tensile properties and fatigue properties of the joint was analyzed.The research results can lay a theoretical foundation and provide technical support for the high efficiency,high quality,and low-cost welding manufacturing of rail vehicle bodies.The results show that good single-side welding and double-side forming weld can be obtained under the optimized composite welding process parameters and groove form.The joint microstructure includes the equiaxed crystal zone,columnar crystal zone,semi-melting zone,over-aging zone,and base metal zone.Compared with the main arc action zone,the equiaxed crystal size of the main laser action zone is slightly smaller and the microstructure is dense,while the widths of the semi-melting zone and the over-aging zone are slightly wider.The hardness of the weld and heat affected zone is lower than that of base metal.There are two softening zones in the heat affected zone,which are located near the fusion line and 6-7 mm away from the fusion line,and the latter is the weakest position of the joint.The pores mainly exist in the weld center of the laser main action zone,but when the arc current is greater than 150 A,the pores are mainly distributed near the fusion line of the arc main action zone.With the increase in welding speed,porosity content increased,but the size changed little.The pores are distributed near the fusion line when the heat source spacing is small.Laser power has little effect on porosity in full penetration welding.Protective airflow is too large to produce large pores.Negative defocusing and a smaller assembly gap can effectively reduce the porosity.The average tensile strength of the joint without pores was 255.1MPa,reaching 82.3% of base metal.When the porosity content increased to 5.1% and 8.9%,the average tensile strength of the joint decreased to 223.6MPa and 202.1MPa.When the pore distribution position is the same,tensile strength of the joint decreases significantly with the increase of pore content or size.When the porosity content and size are similar,tensile strength of the joint with porosity distribution in the weld center is smaller than that of the joint with porosity distribution near the fusion line.With the increase of porosity,the fracture position changed from heat affected zone to weld center,and the fracture characteristics changed from ductile fracture to ductile-brittle mixed fracture.When the porosity content increased from 0% to 8.9%,fatigue strength of the joint decreased from 112.5MPa to 56 MPa.Under the same stress range and stress ratio,when the pore content and size are similar,the fatigue life of the joint with pore distribution near the fusion line is slightly higher than that of the joint with pore distribution near the weld center.The crack of the joint without pores initiates at the loose tissue on the surface of the weld,and the crack of the joint with pores preferentially initiates at the position of the pores inside the weld.When the pores are dense,the crack source contains multiple pores,and secondary cracks appear in the crack propagation zone.