Autogenous laser keyhole welding of aluminum alloy 2024

Aluminum alloy (AA) 2024 is one of the heat treatable aluminum alloys regarded as difficult to fusion weld using autogenous techniques. In this study the weldability of AA 2024 sheet with autogenous laser keyhole welding was examined as part of a systematic study on the welding parameters in order to obtain the optimal welding conditions. A high-speed video camera was employed to capture the keyhole dynamics. Results from the high-speed video show that a rough weld bead surface and porosity formation are due to severe keyhole instability. A high heat input (combination of low laser energy density and low welding speed) forms a relatively large weld pool and the keyhole root locates in the center of weld pool. The welding process in this case is relatively stable and the welds contain no severe porosity. In comparison, with a low heat input (combination of high laser energy density with high welding speed) a small weld pool is formed and the keyhole root locates very close to the front edge of the weld pool; the process is less stable and significant porosity is produced in the welds. It was found that a higher shielding gas pressure could remedy the surface roughness by depressing the liquid in the weld pool. Microstructural examinations show that the fast heating and cooling cycle produced by the laser welding process leads to a narrow partially molten zone and heat affected zone. In the weld fusion zone an equiaxed dendritic structure in the central part of weld and a columnar dendritic structure adjacent to the fusion line was formed.