The 4 kW cw YAG laser welding of solid solution hardened A5083 (5% Mg) and precipitation-hardened Al7Si0.3Mg (7% Si) alloys, sensitive to porosity formation was carried out at different welding speeds, under He gas shielding, and for partial penetration conditions. The specificity of the Al7Si0.3Mg alloy comes from an important cast oxide layer at the surface.
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:
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 Al7Si0.3Mg alloy, with better results, in the case of laser cleaning.
The use of dual beam conditions ensures a larger weld pool, facilitates the porosity’s ejection and stabilises the key-hole 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 Al7Si0.3Mg alloy.
In A5083 welds, most of the macro porosities remain in the beads, due to an unstable keyhole (Mg vapours) which can collapse and entrap millimetre-like blow holes.
The defocalisation of the laser beam inside the plate (−2 min) induces an increase of the penetration from 2 mm to 4 mm for a 5m/min weld bead, but also an increase of the percentage of porosity in the same proportions and for the two materials (from 4% to 8%). This may due to the geometry of the bead which becomes narrower and deeper and susceptible of entrapping more bubbles.