Synergistic improvement of strength and ductility through grain ultra-refinement and self-annealing effect by follow-up hammering-assisted wire arc additive manufacturing

Plastic deformation-assisted additive manufacturing technology can significantly improve the mechanical properties of the additive layer, but existing a common problem of strength-ductility trade-off. Based on the innovatively designed follow-up hammering-assisted (FH) wire arc additive manufacturing (WAAM) process, a new approach and the action mechanism that can address the problem are deeply studied. A thin-walled structure is fabricated by multi-layer deposition assisted with the FH process using the optimized parameters (lower than the austenite recrystallization temperature with deformation about 40%). The ultrafine ferrite grains with size less than 5 μm are obtained, the average grain size of coarse ferrite columnar grains is refined from 125 μm to 22 μm, and the grains arrangement and distribution are more regular and uniform compared to the single-layer deposition. Meanwhile, the texture strength of the deposited layers is weakened from 8.4 to 1.6. The law of grain evolution and ultra-refinement mechanism are revealed by using Optical Microscope (OM), Non-destructive Testing (NDT), Electron Back-Scattered Diffraction (EBSD) and Scanning Electron Microscope (SEM). The grain ultra-refinement mechanism can be attributed to three aspects in chronological order. Firstly, the deformation induced ferrite transformation in the current deposited layer and the deposited layer next to it during the implementation of the FH process. Secondly, the hammering-deformed austenite promoting the ferrite transformation in the process of the high-temperature austenite cooling and transforming into ferrite. Thirdly, the refined ferrite grains in the previous deposited layers undergo repeated deformation and recrystallization under the effect of the cyclic heat treatments of subsequent deposited layers and the corresponding FH processes. Ultimately, addition to the control of dislocation density through the self-annealing effect of multi-layer deposition, the yield strength and elongation of deposited layers are synchronously improved by 21.2% and 7.6% respectively. This study proposes the strategy of self-annealing through multi-layer deposition to solve the problem of strong-ductility trade-off of plastic deformation-assisted additive manufacturing technology without post-heat treatment.