Research On The Microstructure And Process Of Thick 2A12 Aluminum Alloy Electron Beam Welded Joints

2A12 aluminum alloy can be strengthened by heat treatment,which belongs to moderate and high strength aluminum alloy.It has been widely used in aircraft skin sheet,frames,wing ribs,wing beam and many other different fields.High quality aluminum alloy welding joint can be obtained by electron beam welding.In this paper,aluminum alloy with thickness of 25 mm was welded by electron beam.Joint forming,microstructure and mechanical properties under different proces s were investigated.Finally,a better welding process was obtained.During the welding process,the alloy elements in 2A12 aluminum alloy were evaporated violently,which made the welding process very unstable and the surface of welded joint seriously concave.The welding surface forming and the quality of joint can be improved significantly by using scanning process and welding modification.In general,weld defects such as shrinkage,cold separation and nail tip are easy to occur at the bottom of aluminum alloy welding seam,which can be lead to the bottom.The main strengthening phase of 2A12 aluminum alloy base metal is Al 2Cu Mg,while the main strengthening phases of weld metal are Al2 Cu Mg and Al2 Cu.During the non-equilibrium solidification of molten pool metal,solute elements Cu,Mg and impurity elements aggregated at the grain boundaries,resulting in the formation of primary secondary phases and eutectic phases at grain boundaries.With the increase of welding speed,the solute content in the solidified matrix was increased,so that solute elements and the eutectic phase in grain boundary were reduced.The grain crystallization process in the weld has certain preferential growth without scanning process.However,the scanning process makes the grain distribution in the weld uniform and without obvious preferred orientation.In addition,the proportion of small and medium angle grain boundaries in the weld increased obviously,which was beneficial to precipitate in the semi coherent transition phase.It was found that increasing welding speed and adding scanning process within certain range can remarkably improve the microstructure uniformity,grain refinement,grain boundary eutectic structure and semi coherent transition phase.Mechanical properties of the joints were carried out that maximum tensile strength exceeded 383 MPa.Fracture occured through the fusion zone,which p resented brittle intergranular-ductile dimple mixed fracture characteristics.In the welding process,as welding speed increased,the number of pores in the weld were gradually reduced.Based on the above phenomena,a three-dimensional transient model of electron beam welding of aluminum alloy was established,and the stability of keyhole and flow analysis of molten pool were analyze d.The calculation results showed that with the increase of welding speed,the slope of the rear wall of the bath increased and the keyhole becomes more stable,thus reducing the metal vapor cavity.The metal at the edge of the molten pool flowed to the vicinity of the keyhole,converging with the liquid metal near the keyhole wall and flowed upward together under the action of metal vapor,so the gas bubble is favorable to escape and the blowhole defect was reduced.