Microstructures And Mechanical Properties Of Laser-VPTIG Hybrid Welding Of A7N01 Aluminum Alloy With Filler Wire

The weight of the train vehicle can be reduced by replacing traditional steel materials with aluminum. The A7N01 aluminum alloy, a new kind of material specifically used in high-speed train, is applied in the manufacture of aluminum alloy body for high-speed train. At present, conventional arc welding is still the main welding method for the production of high-speed trains. But large welding distortion and softening joints are serious defects after welding. Laser welding has the advantages of a high energy density, high welding speed, narrow heat-affected zone after welding, and the problems of welding distortion and softening joints can be overcome. But the bad weld surface deformation and porosity are the mainly issues in A7N01 aluminum alloy laser welding. Laser-variable polarity TIG(VPTIG) hybrid welding has the advantages of both laser welding and arc welding. Cathodic cleaning can be satisfied by variable polarity TIG, it also can decrease the deterioration of tungsten electrode furthest. The defects of bad weld surface deformation and porosity can be effectively improved in the aluminum alloy welding by laser-VPTIG. This method has significant technological advantages.A7N01 aluminum alloy with the thickness of 4mm is welded by using fiber laser-VPTIG hybrid welding with filler wire. The effects of welding process parameters on weld formation and internal defects are discussed. The types and formation mechanism of porosity are analyzed. The method of porosity suppression is achieved. The joints with a good formation and few welding defects are obtained by optimization of process parameters.The experimental results show that, the weld formation is better by using laser in the front welding direction, and welding process is stable at 2mm heat source distance. With the increase of the welding current, the top weld width increases, and the change of bottom weld width is minor. With the welding speed decreasing, both the top and the bottom width increase. The results also show that the porosities are the main defects in welds. The type of porosity includes hydrogen porosity and process porosity. Hydrogen porosity can be suppressed by removing a certain thickness of the clad layer on base metal surface, and the process porosity is closely related to welding parameters. With the welding speed decreasing, process porosity decreases. While wire feed rate increasing, the process porosity increases. Porosity in the welds can be reduced at 1mm distance of laser and wire. Porosity can be effectively suppressed by optimizing welding parameters. The joint with good formation and few welding defects is obtained under the condition of 6kW laser power,180 A welding current, 4m/min welding speed and 4m/min wire feed rate.The microstructure of joints analysis shows that, the microstructure of the joint is composed of equiaxed grain zone(EQZ), columnar zone and dendrite zone from the fusion line to the center of weld. The size of EQZ around the fusion line is gradually reduced from the top to the bottom. The sizes of dendrites from the top to the bottom are gradually reduced along the weld centerline. The fusion zone hardness is lower than that of base metal, which indicates that the soften zone is occurred in the joints.Tensile tests show that, the average tensile strength of welded joints without natural aging is 325.25 MPa, which is approximately 73.5% of base metal, and the elongation is about 3.1%. After one month of natural aging, the average tensile strength of joints are significantly improved to 363.78 MPa, reaching 82.2% of base metal. And the elongation is approximately 4.65%.The tensile fracture occurs in the weld zone, and the fracture morphology is distinctly ductile characteristics.