Research On Characteristics And Control Of Microstructure And Properties Of Narrow-gap Laser Weldind With Filler Wires For Thick 7A52 Aluminum Alloy Plate

7A52 aluminum alloy is a Al-Zn-Mg system high strength aluminum alloy which is widely used in armoured vehicle and other weapons industry.Thick plate welding is often needed in practical applications.At present,the welding process is mainly multi-layer multi-channel arc welding technology,and there are a series of problems such as large crack tendency of welding joint,serious softening of joint and large deformation.Narrow gap laser welding is a very suitable method for thick plate welding,which combines the advantages of narrow gap welding and narrow laser welding,and the high concentration efficiency of laser welding.At present,this method has not been applied to the Practical Research Report of 7 series aluminum alloy.Therefore,this paper will carry out 7 dimensional aluminum alloy thick plate narrow gap laser filling wire welding process characteristics and structure performance control research,explore the feasibility of process implementation,lay the foundation for practical engineering application.In this paper,the laser bottom welding process of 7A52 aluminum alloy is studied.Two modes and control parameters of thermal conductive welding and deep penetration welding for aluminum alloy laser welding are studied.The effective welding mode of deep penetration welding as the laser base welding of aluminum alloy is determined,and reliable aluminum alloy welded joints can be obtained.Through the study of plate surfacing,the influence rules of welding power,speed and wire feeding speed on the weld formation are obtained.It is found that the main factors affecting weld penetration and weld width are welding power and welding speed.The speed of wire feeding is the main factor for the high weld seam.It has laid the technological foundation for the study of the follow-up thick plate single channel laser welding.The finite element simulation of 7A52 aluminum alloy thick plate laser welding with narrow gap was carried out.The temperature field and stress field evolution model of single pass four layer narrow gap laser welding process were obtained.It is found that there is a large longitudinal residual stress in the welded joint after welding,and the maximum principal stress and Von Mises stress are distributed near the weld and the heat affected zone.At the same time,it is found that the welding residual stress level is greatly reduced in the case of reducing the input of welding heat.The study of stress field simulation shows that the weak parts of the welded joint are between the joint of the weld metal and the weld layer.Based on the simulation of the temperature field,thermodynamic and dynamic analysis is carried out.The results show that the microstructure will be obviously different due to the existence of thermal cycling among different welding layers,and the hardness of the lower welding layer will be higher than that of the high level welding layer.Based on the above research,this paper preliminarily designs and implements a narrow gap laser filler welding process for thick plate aluminum alloy.The research results show that the 4mm narrow gap UV combined with the groove can make the laser spot simultaneously act on the side wall and bottom of the UV welded joint,and form the pool of the welding wire and the welding groove by the form of thermal conductive welding,so that the effective welding of the thick aluminum alloy plate can be realized.The welding defects were analyzed.The defects were mainly three kinds of non fusion,blowholes and cracks.The orthogonal test method was applied to optimize the welding parameters.The best process parameters are obtained: the welding power is 3200W,the welding speed is 0.6mm/min,and the wire feeding speed is 14 m/min.The microstructure and properties of welded joints filled with different wire were further studied.The microstructure and second phase particles of the welded joint were studied by optical metallographic microscope and scanning electron microscope.It is found that the weld grain with Er wire adding rare earth element is smaller than that of ER5183 conventional wire,and the second phase particle distribution is more uniform.The microstructure of the welded joint was analyzed by EBSD.It was found that the mechanism of the joint crystallization in the fusion zone of the welded joint with rare earth element Er was interrupted by some minor equiaxed crystals.Further research shows that using conventional ER5183 welding wire,Al₃Mg₂ and aluminum weld produced in non coherent relation,is not conducive to the formation of second phase particles and the interface stability decreased;with the addition of rare earth elements in welding wire,the interaction of rare earth elements Er and transition metal Zr changes the precipitation behavior of the second phase,the formation of the new nucleation particles,refine the grains in the weld metal.The properties of different filler wire joints were studied by tensile and hardness tests at room temperature.It is found that the welded joint with rare earth element Er is better than that of ER5183 welding wire.The strength is 356 MPa,the elongation is 10%,and the hardness is also greatly improved.Meanwhile,it is found that the hardness of the weld bottom is higher than that of the surface hardness.Combined with the welding test of 10 mm thick aluminum alloy fillet welds,the two welding processes of narrow gap laser filled wire welding and inert gas shielded arc welding were compared.