Formation mechanism of intermetallic compound in liquid steel and liquid aluminum by laser spiral fusion welding

Laser spiral fusion welding was employed to connect immiscible aluminum and steel. The microstructural evolution at the interface between liquid aluminum and liquid steel was analyzed using quantitative microstructural characterization, with particular attention to the influence of fluid flow. It was revealed that the diffusion of reactive elements is predominantly influenced by Marangoni convection mechanisms. An increase in aluminum content reduces the free energy difference between S-ferrite and gamma-austenite, thereby stabilizing the S-ferrite phase. On the steel side of the weld, a microstructure consisted of lath martensite, ferrite, and S-ferrite. The continuous growth of interfacial intermetallic compounds (IMCs), specifically q-Fe2Al5 and 0-Fe4Al13 were successfully suppressed by the presence of numerous iron-rich 'peninsula structure 'at the interface. The formation of a single Fe4Al13 layer (the thinnest, 0.9 mu m), a double layer comprising Fe2Al5 and Fe4Al13, and multilayer 'sandwich' structure of Fe4Al13-Fe2Al5-Fe4Al13 was attributed to convective effects. The liquid aluminum, delayed solidification due to heat release during solidification of the molten pool of steel, can effectively make up the microcracks generated by the volume expansion of Fe4Al13 during the phase transformation process. The strength of the interface was significantly enhanced, with an average shear tensile force of 5022.41 N and a maximum peel force of 204.05 N for the joint.