Laser shock processing influence on constitutive behaviour of graded structures produced by laser welding of ferritic to austenitic stainless steel

The main objective of the present investigation was to assess the constitutive behaviour of the graded structures produced by laser shock processing (LSP) on laser welded joints made between thin sheets of ferritic and austenitic stainless steel. First, the evolution of surface strain patterns during uniaxial loading of the as welded and LSP treated joints was experimentally obtained by using a video-image correlation technique. Overall and local constitutive behaviour across the weld, heat affected zone and parent metals resulted by coupling the applied load level with the corresponding strain average of each zone. The differences found in mechanical resistance of the LSP treated zones when comparing with the as welded ones were due to the processing features, namely the generation of compressive stresses and the development of graded surface structures. Special attention was paid on changes occurred in the ferritic steel side of the joint, which acts as a mechanical cut-out fuse in structures working in energy related applications. Then, by extrapolating the fatigue fracture behaviour of macroscopic notches to the surface cavities and irregularities from which surface topography results it was found that LSP improves the ferritic steel side fatigue resistance. Additionally, the growth of eventual surface flaws is delayed by the compressive stress state induced by LSP, as indicated by in-depth X-ray measurements.