Fatigue cracking characteristics of fiber Laser-VPTIG hybrid butt welded 7N01P-T4 aluminum alloy

Abstract In this paper, 4 mm-thick AA7N01P-T4 was welded using fiber laser-variable polarity tungsten inert gas (VPTIG) hybrid welding with filler wire. After natural aging of 60 days, microstructures and fatigue properties of hybrid welding joints were investigated by Optical Microscope (OM), Scanning Electron Microscope (SEM) equipped with Energy-Dispersive Spectroscopy (EDS), Transmission Electron Microscope (TEM), Electron Back Scattered Diffraction (EBSD) and high-cycle fatigue & fatigue cracking growth rate testing. The effect of reinforcement on the fatigue behavior was also investigated. The experimental results shown that natural aging induced the amount of precipitates enriched with Zn and Mg and the dislocation density in the joint to increase, and the precipitates distributing mainly along to the grain boundaries. Welding reinforcement has significant effect on the fatigue limit. The fatigue limit of joint with reinforcement is 115 MPa and the fatigue crack is initiated along to the weld toe. Whereas, joints without reinforcement show the fatigue limit of 160 MPa and the fatigue crack initiation and final fracture both in weld center. For the same aging weld without reinforcement, natural aging caused the fatigue crack growth rate to slow down compared to as-welded joint, which be equivalent to that of heat-affected zone (HAZ) and BM.