Layer-Resolved Grain Morphology and Recrystallized Crystal Evolution in FSP-Assisted Wire Arc Additive Manufacturing of Aluminum Alloy 4043

Wire arc additive manufacturing of aluminum generates coarse, anisotropic solidification microstructures that limit mechanical performance, and interlayer friction stir processing (FSP) is increasingly applied to refine them. This study reports the layer-resolved grain morphology and the recrystallized crystal evolution in MIG + FSP-fabricated aluminum alloy 4043 walls, pairing the FSP spindle torque recorded from the CNC controller with multi-descriptor grain morphology in a coupling that, to the authors’ knowledge, has not been previously reported in the WAAM + FSP literature. Methodologically, two four-bead, three-layer walls were co-fabricated under identical deposition conditions on a HAAS VF-3 CNC platform, one by MIG deposition alone and one by the complete MIG + FSP route; the FSP spindle torque was measured at three positions per layer (118 ± 6 N·m at 600 RPM for L1, and 19.1 ± 1.0 and 26.6 ± 1.3 N·m at 1200 RPM for L2 and L3), and quantitative image analysis of 10,091 grains provided the layer-resolved mean grain area, equivalent diameter, aspect ratio, perimeter-to-area ratio, and circularity. The results show that the mean grain area increased from 8.55 μm2 (L1) to 12.96 μm2 (L3) while the aspect ratio decreased monotonically (1.389 to 1.323), indicating progressive grain equiaxiality with build height; the P/A ratio followed a non-monotonic layer dependence (2.54 to 2.11 to 2.50 μm−1), with the L2 minimum consistent with reduced boundary line density under the combined thermal influence of two adjacent FSP events. The MIG + FSP route produced grain areas 29–48× smaller per layer than the MIG wall and a 45.8% higher hardness (75.8 ± 7.7 versus 52.0 ± 1.3 HV; n = 6; p = 0.0027). In conclusion, the L3 torque exceeds the L2 torque at equal 1200 RPM, qualitatively consistent with the d−p term in the grain-size-explicit creep framework γ. = C·(τn/dp)·exp(−Q/RT), although temperature, strain rate, and grain size cannot be fully decoupled from the present three-layer dataset. The morphology and the distributional evidence are consistent with dynamic recrystallization (DRX); discrimination between continuous and discontinuous DRX requires EBSD.