Granularization of the elongated rod-like δ phase and the strengthening mechanism arising from the synergy between granular δ phase and the thermo-mechanical coupling effect during friction stir processing of WAAM IN718 alloy

The characterization results show that in the stir zone of the HT - FSP processed WAAM IN718 alloy, the Laves phase, δ phase, γ' and γ'' were completely dissolved, whereas in the interface transition zone γ' and γ'' were dissolved but the Laves phase and δ phase were retained and fragmented into a granular morphology, distributing uniformly in the γ matrix. After a second double-aging treatment, γ' and γ'' re-precipitated in the stir zone, and the corresponding tensile properties were significantly improved.

Compared with the as-built state, the elongation decreased by only 30.3%, while the ultimate tensile strength and yield strength increased by 119.6% and 261.8%, respectively. In addition, during FSP cooling, the granular δ phase pinned dislocations and hindered grain boundary migration, which suppressed growth of recrystallized grains and induced the formation of serrated grain boundaries with a higher dislocation storage capacity.

Consequently, the hardness and strength of the interface transition zone, which contained abundant granular δ phase, were both significantly higher than those of the stir zone. This study confirms the feasibility of using FSP to actively fragment the elongated rod-like δ phase and verifies the synergistic effect between the granular δ phase and the thermo-mechanical coupling effect during FSP in improving the microstructure and properties of the alloy.

It therefore lays the foundation for the subsequent introduction of a specific cooling process to prevent the dissolution of the granular δ phase in the stir zone, thereby comprehensively enhancing the mechanical properties of FSP joints, and provides new insights for the design of post-processing routes for WAAM IN718 alloy.