Improvement of microstructure and mechanical properties of C44300 tin brass subjected to double-pass rapid cooling friction stir welding

In this work, two-mm-thick C44300 tin brass plates were successfully joined by double-pass rapid cooling friction stir welding (RCFSW). The microstructure evolution of the welded joint was carefully studied by high resolution electron backscatter diffraction and transmission electron microscopy. The results showed that the heat-affected zone, which usually deteriorates the mechanical properties of the weld, was completely eliminated by using liquid CO2 for cooling. For single-pass RCFSW, the grain structure in the stir zone was remarkably refined, but the grain size was not uniform. The grain refinement mechanism consisted of continuous and discontinuous dynamic recrystallization. Massive dislocations were maintained in the grain interior, indicating that partial recrystallization occurred in the stir zone. The stir zone showed a simple shear texture of {111} . After double-pass RCFSW, due to the accumulation of strain and heat, the grain structure was further refined and became more uniform. The recrystallization almost finished, since the grain refinement mechanism changed to discontinuous dynamic recrystallization dominant, and thus the stir zone showed a fiber texture of {hkl} . Moreover, the dislocation density decreased, and many twins were induced in the grain interior. Due to the microstructure modification, the ultimate tensile strength and the total elongation of the stir zone were 5% and 17% higher than that of single-pass RCFSW, respectively, indicating that the strength and the plasticity were simultaneously improved. This work provides an effective method for balancing the strength and the plasticity of the FSW C44300 tin brass joint. This method can also be extended to other face-centered cubic materials, such as austenite stainless steel, nickel alloys, and high-entropy alloys. [Display omitted] •2-mm-thick C44300 tin brass plates were successfully joined by 2-pass FSW with liquid CO2 cooling.•HAZ was not appeared in the joint for the significantly improved thermal cycle.•Ultrafine grains with low dislocation density and abundant twins were produced in the SZ.•Twin boundaries play an important role in improving dislocation storage capacity.•We provide an effective strategy to enhance the strength of FSW tin brass joint without ductility loss.