Micromechanical analysis and finite element modelling of laser-welded 5-mm-thick dissimilar joints between 316L stainless steel and low-alloyed ultra-high-strength steel

Abstract

As base metals (BMs), plates of 5-mm-thick low-alloyed ultra-high-strength carbon steel (LA-UHSS) with a tensile strength of 1.3 GPa and 5-mm-thick 316L austenitic stainless steel were laser-welded at two different energy inputs (EIs; 60 and 100 J/mm). The microstructural characteristics of the fusion zones (FZs) in the welded joints were examined using electron backscattering diffraction (EBSD) and transmission electron microscopy. The fine microstructural components, such as the prior austenite grain size (PAGS) and effective grain size of the fresh martensite promoted during welding, were analysed by processing the EBSD maps using MATLAB software. The micromechanical performance of the weldments was investigated using microindentation hardness (HIT) to display the mechanical responses of different zones. Uniaxial tensile testing was conducted to explore the joint strength and plasticity failure. The dominant phase structures promoted in the FZs at low and high EIs were similar, that is, martensite with a small fraction of austenite. The HIT values displayed a distinct variation in strength between different zones. The HIT values of 316L, LA-UHSS, and FZ were 1.95, 5.55, and 4.63 GPa, respectively. The PAGS increased from 45 to 70 μm with an increasing EI, and a finer martensitic grain structure with an average size of 2.62 μm was observed at high EIs. The mechanical tensile properties of the dissimilar joints at the studied EIs closely matched those of the BM 316L, demonstrating comparable yield and tensile strengths of 225 MPa and 650 MPa, respectively. This similarity can be attributed to the localized plastic tensile deformation occurring primarily within the relatively softer BM 316L, ultimately resulting in joint failure. The flow behaviour of the dissimilar joints under uniaxial tensile testing was analysed using finite element modelling to determine the stress and strain distributions. The plastic strain was mainly localised within the soft metal 316L owing to enhanced dislocation-mediated plasticity.