Post-Peak Cooling Rate Controls Porosity Evolution in Hybrid WAAM–FSP Al 4043 Multi-Layer Walls

In hybrid Wire Arc Additive Manufacturing with interlayer Friction Stir Processing (UAMFSP), refined microstructures are produced in aluminum alloy builds; however, the thermal parameters governing layer-resolved defect evolution remain poorly understood. In this study, a first mechanistic framework is presented, identifying post-peak cooling rate as a governing parameter for porosity evolution in UAMFSP Al 4043 three-layer walls. In this study, a comprehensive multi-scale characterization of three-layer Al 4043 UAMFSP walls is presented, employing infrared thermography, quantitative optical grain morphology analysis (N = 10,346 grains, Layers 1–3), scanning electron microscopy from 250× to 35,000×, and image-based porosity quantification from calibrated SEM fields. A counterintuitive layer-dependent porosity gradient is reported, wherein the upper layer (L3) exhibited 80% higher porosity (2.90 ± 1.18%) and 107% higher pore density (4,283 ± 900 pores/mm²) than the bottom layer (L1), despite recording a 26% lower peak FSP surface temperature (195.1 vs. 263.2°C) (n = 3 fields per layer; Cohen’s d ≈1.7). Based on these results, the post-peak cooling rate, rather than peak temperature, is identified as a dominant controlling parameter for void consolidation quality, as evidenced by the observation that L3 cools at −12.3 °C/s versus −16.2 °C/s for L1, which is consistent with prolonged high-temperature dwell and reduced plastic-flow-assisted pore closure in the upper layer. It should be noted that the anomalously rapid cooling of L2 (−46.9 °C/s), attributed to a bilateral thermal gradient between the substrate and the air-cooled free surface, places it in a thermally distinct regime; accordingly, L2 is utilized exclusively for high-magnification SEM characterization in this study. High-magnification SEM imaging (12,000×–35,000×) revealed a frequent spatial co-location of sub-micron pores with fragmented Al–Si eutectic particles, which is consistent with preferential void persistence near particle–matrix interfaces. Furthermore, grain morphology exhibited evolve non-monotonically with build height, with mean circularity following the order L3 (0.645) > L1 (0.621) > L2 (0.569), and the equiaxed grain fraction ranging from 25.5% (L2) to 36.1% (L3) (ANOVA: F = 56.2, p = 5.15 × 10⁻²⁵), while the mean equivalent grain diameter remained below 3.4 μm across all layers. In summary, the outcomes of this study establish post-peak cooling rate, rather than peak temperature, as a governing parameter for void consolidation quality in UAMFSP builds. These outcomes are presented as a first mechanistic framework for this class of hybrid process, and are intended to motivate targeted controlled experiments, subsurface thermal characterization, and expanded porosity sampling in future investigations of multi-layer additive–deformation manufacturing of Al-based alloys.