Microstructure evolution and parameters optimization of follow-up hammering-assisted hybrid wire arc additive manufacturing

Coarse columnar grains are the typical microstructure of wire arc additive manufacturing, resulting in the poor mechanical properties. A follow-up hammering-assisted (FH) hybrid wire arc additive manufacturing process is proposed to improve the microstructure of deposited weld bead. The effects of different hammering temperatures (higher than Ae3 line) and different deformation amount on the microstructure are systematically studied, and the effect on mechanical properties is investigated correspondingly by fabricating a deposited block. The Electron Back Scatter Diffraction (EBSD), Scanning Electron Microscope (SEM), and Optical Microscopy (OM) are used for characterization. The results show that the FH process can significantly refine the grain size of deposited weld bead, and meanwhile weaken the texture strength. The grain refinement mechanism for hammering higher than the austenite recrystallization temperature is austenite dynamic recrystallization. Instead, for hammering lower than the austenite recrystallization temperature, the more significant grain refinement is attributed to the high-temperature deformation increasing the ferrite phase transformation nucleation site, nucleation rate and the deformation-induced ferrite phase transformation. Hammering temperature 850 °C and hammering deformation 40% are determined to be the optimal process parameters to refine the grains. Using the optimal process parameters, the strength and hardness of the deposited layers are greatly promoted, and the anisotropy is also improved, while the ductility is reduced to some degree, but improved obviously compared to the low-temperature hammering.