Inhomogeneous mechanical properties in flash butt welded high-strength low-alloy steel joints via nanoindentation and reverse analysis

This study investigates the inhomogeneous mechanical properties of flash butt welded (FBW) S590LW high-strength low-alloy steel joints in wheel rims and their impact on the flaring process. Nanoindentation tests, complemented by microhardness measurements and microstructural analysis, were conducted to divide the weld joint into four distinct zones: interface zone (IZ), coarse grain zone (CZ), normalizing zone (NZ), and partial normalizing zone (PZ). Each zone exhibited distinct load–penetration depth behavior, microstructure, and microhardness characteristics. True stress–strain relationships for each zone were determined through finite element (FE) modeling combined with reverse analysis of nanoindentation data. The results revealed significant gradients in yield strength and strain hardening exponent across the weld joint, with IZ showing the highest strength and lowest strain hardening. A three-dimensional FE simulation of the flaring process was developed using the localized mechanical properties of each weld zone. The model revealed nonuniform stress and strain distributions, with IZ experiencing high Von Mises stress, indicating its vulnerability to fracture. These findings underscore the importance of accurately characterizing local mechanical property gradients in welded structures, providing a foundation for improved predictive accuracy and enhanced quality control in rim manufacturing processes.