Study On Microstructure And Properties Of Welding Joint Of Laser Repair Of High-Strength Aluminum Alloy

As a high-strength Al-Zn-Mg heat-treatable aluminum alloy,A7N01P-T4 aluminum alloy is used in the main bearing structure of high-speed trains.Under the interaction of load?dynamic load or static load?and corrosion environment,aluminum alloy welded structures may produce cracks as a sensitive area for cracks,which are an important hidden danger of safe operation of high-speed trains.Compared with arc repair,laser repair can reduce filler metal,heat input,and deformation to repair cracked parts.In this paper,a 15-mm-thick aluminum alloy MIG welding joint was repaired by laser narrow gap welding.The parameters are P_laser=4k W,v_weld=0.48m/min,v_wire=2m/min,v_gas=30L/min.A U-shaped groove form with a width of 3mm is selected.And a copper palte was used to force weld formation at the bottom.Finally,the 15mm thick plate was repairs by7 layers and the weld shape is good.The hardness distribution,microstructure,tensile properties,residual stress distribution,and corrosion resistance of joints were analyzed before and after laser repair.Compared with that before laser repair,the hardness-softening zone of the joint does not expand and repair layer weld grain is finer after laser repair,but defects such as pores and inclusions reduce the tensile strength of the joint.Compared with the base metal,the heat input caused by MIG welding and laser repair will make the?phase?MgZn2?on the grain boundary grow and increase the continuity.The morphology and distribution of the second phase inside grains are similar.There is mainly the GP zone and a small amount of tiny?'phase and coarse?phase.Thermal input during laser repair results in an increase in the residual stress near the repair layer,but the increase is small.When loading with three-point bending,the stress corrosion cracking of A7N01P-T4aluminum alloy is not a full-thickness through-crack,and crack initiation and propagation are selective.The stress corrosion crack initiation is mainly anodic dissolution,and the expansion is mainly due to the effect of hydrogen.There are two directions of stress corrosion crack propagation,one is the L-direction crack perpendicular to the applied load stress,and the other is the T-direction crack caused by the wedge force produced by the corrosion product in the crack.The heat affected zone of the joint was the area with the maximum rate of stress corrosion crack propagation?3.0×10-7 mm·s-1?,but it was not much different from that before the repair.The exfoliation of the joint heat affected zone is the most serious after the repair.The E_corr of the heat affected zone of the joint after laser repair is the lowest.After AC impedance analysis,it was found that the corrosion rate during the pitting corrosion process of the joint heat affected zone after repair was significantly larger than that before the repair,which was the main reason for the different exfoliation morphologys of the heat affected zone before and after the repair;the magnesium alloy plate can be to some extent improve the corrosion resistance of the joint heat affected zone after repair.