Fusion Welding On Copper-steel Dissimilar Materials And Joint Microstructure Evolution

With th e rapid development of modern manufacturing industry,single materials can't satisfy the needs of industrial production anymore.Composite structures of dissimilar materials have a first priority in the equipment manufacturing industry such as aerospace,power companies,petroleum engineering,etc.During the process of producing oil-immersed ultra-high voltage power transformer,the composite structure of T2-Q345B is used as the outer shield shell of the transformer,and the composite structure of T2-SUS304 is used as the internal oil conservator of the transformer.On the one hand,the characteristics of low cost,high strength as well as high corrosion resistance of Q345B and SUS304 make them to be the appropriate structural materials for the shield shell and the oil conservator.On the other hand,using the high thermal conductivity and reverse magnetic properties of copper to derive the heat generated by the eddy current effect,reducing the stray loss inside the transfomer,thereby increasing the service life of the oil-immersed ultra-high voltage power transformer.Aiming at the composite structure of the large shield shell outside the transforamer,this paper selects the most suitable gas metal arc welding(GMAW)method for the welding experiment of T2-Q345B dissimilar materials,and conducts researches between different welding processes to detect effects on the T2-Q345B dissimilar welding joint structure.The most suitable welding process parameters were obtained:welding current 220-235A,welding voltage 23-24V,welding speed 4.15cm/min,30%He+70%Ar shielding gas with 15L/min flow rate.The results show that the copper fusion zone of T2-Q345B dissimilar welding joints forms coarse grains due to epitaxial growth,while the steel fusion zone forms a Fe-based melting retention layer(10?m)due to solute segregation.A number of dendrites and spheres a-Fe solid solution are distributed in the weld.The maximum tensile strength of the T2-Q345B dissimilar welding joint is 223 MPa,and the impact energy is 76 J,which completely meets the actual working conditions of the transformer shield shell.Based on the analysis of the T2-Q345B dissimilar material's gas metal arc welding process and its combined interface metallurgical properties,it is found that the fusion ratio of the base metal as well as the welding line energy play an important role in the comprehensive performance of the welding j oint.Therefore,for the composite structure of the internal oil conservator in the transformer,the T2-SUS304 dissimilar material is welded by the laser welding with concentrated energy density.This paper chooses the laser welding method with concentrated energy density to weld the copper-steel dissimilar materials,and controls the melting amount of copper and steel base materials by beam offset.Aim to find the best copper-steel laser welding process.The fusion ratio of the base metal is controlled by the beam offset,and different welding processes are studied.The most suitable welding process parameters were obtained:laser power 3700W,welding speed 1000mm/min,beam offset steel side 0.4mm,defocusing amount-2mm,pure Ar shielding gas with 25L/min flow rate.The research shows that when the beam is offset to the steel side,the weld microstructure was relatively uniform ?-Fe cell crystal with Cu precipitation in the grain boundary,and the copper heat affected zone(HAZ)is narrow;when the beam is offset to the copper side,liquid phase separation between Cu and Fe is observed from the weld microstructure,and the copper heat affected zone(HAZ)is wider.The maximum tensile strength of the T2-SUS304 dissimilar welding joint is 240 MPa,which completely meets the actual working conditions of the oil conservator.Combined with the metallurgical characteristics of GMAW and laser welding as well as the solidification characteristics of the molten pool,the metallurgical transition characteristics and microstructure evolution mechanism of the joint interface of T2-Q345B and T2-SUS304 dissimilar materials welding joints were analyzed in depth,and the generation of seepage cracks was clarified.The mechanism and the liquid phase separation mechanism of Cu and Fe provide effective theoretical guidance for the GMAW and laser deep-fusion welding processes of T2-Q345B and T2-SUS304 heterogeneous materials,and broaden the application of copper-steel dissimilar composite materials.The application field has promoted the large-scale engineering application of the composite material.