Influence of surface preparation and process parameters on the porosity generation in aluminum alloys

The 4 kW cw Nd:yttrium–aluminum–garnet laser welding of solid solution hardened A5083 (5% Mg) and precipitation-hardened A356 (7% Si) alloys, sensitive to porosity formation was carried out at different welding speeds, under He gas shielding, and for partial penetration conditions, on bead on plate configuration. The porosity formation was examined through observation and evaluation of weld beads, based on x-ray radiography inspection coupled with image analysis, and quantification of gas volume percentage by densitometry. The results show that both surface preparation and dual-beam configuration can have a beneficial effect on the porosity content: (1) After polishing, laser cleaning, sand blasting, or degreasing of the surface, hydrogen and oxygen sources are decreased or removed. The resulting effect is a reduction from 15% to less than 2% of the porosity rate on A356 alloy, with better results, in the case of laser cleaning. (2) The use of dual beam conditions ensures a larger weld pool, facilitates the porosity’s ejection, and stabilizes the keyhole as compared with single beam configurations. For instance, a longitudinal bi-spot configuration reduces the porosity rate from 15% to less than 0.5% on A356 alloy. In A5083 welds, most of the macroporosities remain in the beads, due to an unstable keyhole (Mg vapors) which can collapse and entrap millimeter-like blow holes.