G. Padmanabham, S. B, International Advanced Research Centre for Powder Metallurgy & New Materials (ARCI), Hyderabad, India
High reflectivity, porosity and hot cracking are some of the problems requiring attention in successful laser welding of aluminium and its alloys. Nd:YAG laser is preferred over CO2 laser, due to better absorptivity. But, CO2 lasers have advantages like better wall plug efficiency, lesser safety issues and lower maintenance costs, provided the chosen laser has the requisite power and beam quality to produce an intensity required for coupling with aluminium. Successful CO2 laser welding of aluminium alloy could be of significance to industrial users. In this work, experimental results of laser welding aluminium alloy 6061 (2 mm thick sheet) using a high beam quality 3.5 kW slab laser are reported. Bead-on-plate experiments were performed to identify the threshold power density for coupling of the beam, shielding gas & flow rate and welding speed. Stable keyhole, smooth weld surface and minimum excess penetration were targeted. Helium as shielding gas yielded better and repeatable welding conditions. It was also observed that welding conditions were more stable in partial penetration welds compared to full penetration welds. Having identified usable welding parameters, improved weld strength is also aimed at. Laying a laser weld bead on an existing bead at higher speed than the original weld can result in finer fusion microstructure due to higher cooling rates. This bead-on-bead could also close some of the porosities formed in the first pass. As the laser beam will be passed over an existing bead which has a surface rougher than the shiny base metal, coupling also may be more efficient. In this backdrop, bead-on-bead laser welds were made and characterised. A marginal decrease in the threshold power density to couple, reduction in porosity and refinement of fusion zone microstructure was observed.
Summary: Results of welding 6061 aluminium alloy with high beam quality CO2 laser in bead-on-plate, butt joint and bead-on-bead procedures are reported. CO2 laser could be successfully used to produce improved welds in aluminium alloy 6061 in terms of weld quality and microstructure and consequent mechanical properties.
