Investigation on microstructure and properties of U71Mn steel joints by Flash-Friction hybrid welding

Flash butt welding, when applied to rail welding, is prone to the formation of oxidized inclusion defects and softening layers at the weld, significantly impairing the mechanical properties of the joints. To address these issues, this study proposed a novel flash-friction hybrid welding(F-FHW) method and explored the welding process for U71Mn steel based on varying upsetting distances. The results revealed that the welded joint was well formed and the joint structure was mainly divided into base material (BM), thermo-mechanically affected zone (TMAZ), heat-affected zone (HAZ) and weld zone (WZ). Notably, the grains in the TMAZ and WZ were refined under the combined influence of welding heat input and friction upsetting. Additionally, under the action of linear friction, the softening layer with elevated ferrite did not appear in the WZ. The microhardness of the joints displayed an M-shaped distribution, with the highest hardness located at TMAZ. The tensile strength of the joint initially increased and then decreased as the upsetting distance extended. The maximum tensile strength of the joints was approximately 900.3 MPa, achieving 89.1% of the BM, and its fracture form is quasi-cleavage fracture.