Microstructure evolution and porosity suppression in high-power laser-arc hybrid welded thick aluminum alloy T-joints through high-frequency beam oscillation

The mechanical properties of aluminum alloy T-joints are critically influenced by the microstructural characteristics and porosity distribution. This study systematically investigated the effects of the laser oscillation frequency from 100 to 300 Hz on weld formation mechanisms during high-power laser-arc hybrid welding of 8-mm-thick aluminum alloy T-joints. The results showed that increasing the oscillation frequency from 100 to 300 Hz not only reduced the depth of both the weld penetration and the keyhole, but also increased the width of transition zone and transition angle between the arc-melted zone and laser-melted zone accordingly. Furthermore, the molten pool flow transitioned from chaotic turbulence to ordered vortex circulation. These synergistic effects promoted dendritic grain refinement in the arc-melted zones, reducing the average grain size from 209.8 to 178.0 μm. Consequently, the weld porosity decreased dramatically from 6.34 % to 1.43 %, and the mechanical properties increased 26 % to 208.1 MPa. The frequency-dependent laser oscillation mechanisms were discussed based on the qualitative relationship between the keyhole dynamic behavior and the solidification microstructure, which provided a new insight into the porosity suppression in high-performance aluminum alloy welding.