Effect of gas shielding and heat input on autogenous welding of duplex stainless steel

Stainless steel is increasingly being used in various applications where the laser process may be the perfect tool for welding autogenously. Use of the keyhole welding mode typically assures high weld quality and productivity. Heat tint and discoloration on the surface may decrease the corrosion resistance and have to be removed or minimized. With laser welding, it is possible to limit the weld bead and root oxidization and thereby decrease the need for post-weld cleaning. The main drawback of using a keyhole when welding duplex stainless steels is the high cooling rate, which can cause unfavorable phase balance in weld metal. Thus addition of over-alloyed filler metal or even post-weld annealing is required. Another solution is to use laser hybrid welding to increase the heat input and add filler metal. This is, however, not an attractive choice for welding thin materials and in continuous production. The excellent beam quality of modern high-brightness lasers can, together with high power provide a great opportunity to shorten the total welding time, but the difficulty in obtaining suitable phase balance remains or is even enhanced. This study concentrates on testing the effect of dynamic beam forming on the laser beam absorptivity and resulting weld microstructure in order to define the usability of this technique for welding of thin duplex stainless steel sheets. The differences between various focal spot configurations were analyzed together with the effect of gas shielding technique. The focal spot size and shape had a significant effect on the absorption and heat input and thus affected the weld metal microstructure.