Effect of shielding gas composition on CO₂-laser welding quality

Shielding gas is a significant cost factor in CO2-laser welding, especially if helium is used. Replacing a part of the helium with some other gases, such as argon and carbon dioxide, might reduce operation costs. The effect of such a substitutive shielding gas mixture on the total laser welding quality was studied by the Ruukki Metals steel service centre in Uusikaupunki, Finland. Several shielding gas mixtures (He+Ar+CO₂) were tested. The percentage quantity of helium was set at a constant of 35% and the percentage quantities of argon varied from 40% to 55% and carbon dioxide from 10% to 25%. The test material was a wear-resistant steel strip plate Raex 450 with a thickness of 4 mm. Raex is a trademark for wear-resistant steels manufactured by Finnish steel making company Ruukki. The number 450 signifies the hardness of steel in Brinell units. Wear-resistant steel strip plates as well as other thermo-mechanically hot rolled and direct quenched strip steels produced on a modern hot rolling line integrated with direct quenching are typically used in applications under extremely demanding conditions that require steel with very high strength and other special properties, such as excellent abrasion resistance. The manufacture of these steels is very demanding because of the complex controlling of the hot rolling and the coiling process. The width of a ready strip steel coil can be insufficient for end products, such as buckets and containers and laser welding is a suitable joining process for achieving the desired size of the steel plate by joining together two or more thin steel strip plates. A 5 kW CO2 laser was first used for the laser cutting to trim the edges of plates and finally for joining plate with a butt joint. The welding speed remained invariable and was 2300 mm/min. The total weld quality was evaluated according to the standards ISO 13919-1 and ISO 15614-11.