Study On Laser Fusion Brazing Technology Of Aluminium/Copper Dissimilar Materials And The Corrosion Behavior Of The Joint

Aluminium/copper（Al/Cu）dissimilar materials composite structure is an advanced structural material that replaces Cu with Al,which have great potential for applications in the petrochemical,electronic,electrical,refrigeration and thermal fields.In this paper,the formation of Al-Cu IMCs in the Al/Cu laser fusion brazing joint was reduced by addting Zn-10%Al flux-cored wire.The optimal laser fusion brazing process matching the wire was obtained by optimizing the laser process parameters to achieve a reliable connection of Al/Cu dissimilar materials.On this basis,the impacts of interfacial compounds formation was analysis,and elucidation of the mechanism of formation of interfacial compounds.The corrosion behavior of the joint was also studied for revealing corrosion mechanism.2A16 Al alloy and T2 pure Cu were connected by used Zn-10%Al flux cored wire.The effects of laser process parameters on macro-forming,microstructure and mechanical properties have been investigated.With the increasing of the laser power or decreasing of welding speed,the welding line energy was garadually increased,and the formation of welding seam was more better.When the welding line energy is too high,the holes and undercutting defects appeared on the welding seam surface.The laser fusion welding joint was mainly composed of Al side fusion zone,welding centre zone and Cu side brazing zone.The Al side fusion zone and welding centre zone have similar microstructure and are mainly composed ofα-Al solid solution,η-Zn solid solution and Zn Al eutectic tissue,while the Cu side brazing zone was composed of interface layer(Cu Zn+Al_4.2Cu_3.2Zn_0.7)and the transition zone（Cu Zn₅+Al₄Cu₉+Al₂Cu+solid solution phase+eutectic tissue）.The optimum parameters are laser power of 2200 W,welding speed of 12 mm/s.The maximum tensile strength of the laser fusion welidng joints reaches 204 MPa at room temperature.The fracture position is adjecent to the interface of Cu side brazing zone,and performance a brittle+ductile hybrid fracture.Based on the microstructure characteristics of the interface,the evolution mechanism of the interfacial compounds in Al/Cu laser fusion welding joins were analyzed by combining the thermodynamic caculation method of phase diagram and the analysis of atomic diffusion kinetics theory.The results show that Al-Cu,Cu-Zn and Al-Cu-Zn ternary compounds have the thermodynamic conditions for formation in the temperature range of laser fusion welding process.The Al_4.2Cu_3.2Zn_0.7and Cu Zn compounds have lower Gibbs free energies,which were priority formed and growthed as interface.The formation of the Cu Zn interface in layer I has priority over the Al_4.2Cu_3.2Zn_0.7interface in layer II due to the preferential diffusion of Zn atoms to form Cu Zn nuclei and grow at interfacial defects in the copper base material.The evolution of the interface in the copper side of the brazing zone is divided into four stages:interfacial atomic diffusion,reaction formation and growth of the interface I layer,reaction formation and growth of the interface II layer,and precipitation of IMCs in the transition zone.It has been verified that the formation of the interface layer occurs in the welding heating process,while the saturation precipitation of the IMCs in the transition zone occurs in the welding cooling stage.To further evaluate the service performance of the Al/Cu laser fusion brazing joint,the electrochemical corrosion behavior and immersion corrosion behavior of the joint in 3.5 wt%Na Cl solution were investigated,and corrosion mechanism of the welds was revealed.The corrosion potential of the weld zone was reduced when a large amount Zn element addition,which makes the macroscopic galvanic coupling corrosion between the weld zone and the base material.The corrosion susceptibility of the weld zone is increased.The corrosion of Al/Cu laser fusion brazing joints starts with the anodic dissolution of theη-Zn phase in the weld dendrite gap and rapidly develops into crevice corrosion along the reticulated Zn-Al eutectic structure.The development of crevice corrosion causes the integrity of theα-Al surface passivation film to be destroyed,leading to pitting corrosion induced on theα-Al phase surface.The pitting corrosion is evolved into intergranular corrosion along the grain boundaries,and eventually developed into exfoliation corrosion.