New Insight into Mechanical, Microstructural and Failure Features of Lap-Fillet Autogenous Laser-Welded Similar and Dissimilar Joints of Ultra-Thin Steel Sheets

This research work addresses the mechanical and metallurgical characterisation, as well as the failure features, of two types of lap-fillet autogenous laser-welded joints made of ultra-thin sheets by applying an appropriate welding technology for producing sound welds and flawless joints. Both welded samples, one made only of stainless steel (SS-SS) sheets, and the other made of stainless steel and carbon steel (SS-CS) plates, were subjected to tensile–shear loads that are representative of the in-service conditions. The experimental research was focused on determining, by the digital image correlation (VIC-2D) method, the strain field and the rotation angle of the welded joints that were developed during loading tests of the welded specimens. Comparing to the classical testing method applied to study the joint overall mechanical properties, the novelty of this research consists of local mechanical behaviour assessment of relevant zones from similar and dissimilar welded joints, by using the innovative technique VIC-2D. Based on the analysis of the experimental results, it was found that the maximum rotation angle is 2.5 times higher in the SS-SS similar welded joint, in comparison with the SS-CS dissimilar welded joint. Despite this finding, the SS-CS specimen failed in the CS base material, far from the weld, with the failure phenomenon being preceded by the material yielding and necking. This failure mode is consistent with the detected strength mismatch of the SS-CS joint, with respect to the CS base material. In contrast, the quasi-ductile fracture of the SS-SS welded joint occurred by plastic exhaustion at the boundary between the narrow Heat-Affected Zone (HAZ) of SS and the Fuzion Zone (FZ). These outcomes are consistent with the hardness profile, microstructural heterogeneities found in the lap-fillet welded joints, and the load versus elongation curves that are determined and discussed in this paper. This research provides new insight and original information on the materials’ response to the autogenous laser welding, which will contribute to improving the knowledge on the ultra-thin lap-fillet welded similar and dissimilar steels.