This paper addresses several peculiar properties of ferritic/austenitic welded joints (Black and White Joints-BWJ) of thin sheets, which are emerging nowadays due to cost saving and satisfactory service performance. Starting from 3D thermal field modelling approached by considering Goldak’s double ellipsoidal heat source, together with a contribution of the authors, a 2D FEM-based model was used for arc welding of low carbon steel (CS) and stainless steel (SS) of thin sheets (3 mm). Conclusion helped to conduct experiments of BWJ arc welding (MMA and TIG), and laser-GMA hybrid welding, whilst infrared thermographic measurements were compared with the FEM results. Besides microstructural examination and mechanical characterization (hardness and tensile), fracture toughness and stress corrosion tests were performed on the BWJ laser hybrid welded. Whilst the model has offered important qualitative information about the asymmetric thermal field and the heat apportionment on the molten metal pool formation, the experiments have shown inadequate microstructures and constitutions of the welds for MMA and TIG welding, with respect to laser-GMAW hybrid joints. The description of the experimental program focused on B&W laser-GMA hybrid welding of thin plates aiming a characterization procedure using fracture mechanics methods is briefly presented.