Influence of CO2 laser beam welding on microstructure of aluminum metal matrix composites reinforced with Al2O3 particles.

The continuous CO2 laser beam welding of two aluminum metal matrix composit es (6061 and 2618 reinforced with 20% of Al2O3) has been investigated with special attention to the influence of the base alloys, the filler material and the process parameters on the microstructure of the welding bead. In th is work square butt welding has been obtained with different CO2 laser powe r, feed rate and shielding gases. The microstructure of the welding beads h as been examined by optical and electronic microscopy. A migration of the A l2O3 reinforcement particles from the fusion zone (FZ) towards the heat aff ected zone (HAZ) has been detected and the particles agglomerate near the i nterface FZ-HAZ. An increase in both porosity and agglomerate sizes inside the welding bead has been observed as the feed rate has been reduced. The h ardness of welding beads has been higher than the unaffected composites. A reduction of both the beads hardness and the Al2O3 migration and agglomerat ion has been obtained by the use of magnesium-rich filler material in the w elding process, probably because magnesium is able to increase the reinforc ement wettability, and to reduce the rate of formation of spinel phase MgAl 2O4. The use of Ar as shielding gas was the most effective, both in order t o avoid the development of porosity and to reduce the Al2O3 agglomeration.