{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,10,13]],"date-time":"2026-10-13T16:42:43Z","timestamp":1791909763402,"version":"4.3.4"},"reference-count":39,"publisher":"MDPI AG","issue":"13","license":[{"start":{"date-parts":[[2026,7,7]],"date-time":"2026-07-07T00:00:00Z","timestamp":1783382400000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Materials"],"abstract":"<jats:p>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 correlative mechanistic framework is presented in which post-peak cooling rate is identified as a plausible controlling factor for porosity evolution in UAMFSP Al 4043 three-layer walls. A multi-scale characterization is performed by employing infrared thermography, quantitative optical grain morphology analysis (N\u2009 = \u200910,346 grains, Layers 1\u20133), scanning electron microscopy from 250\u00d7 to 35,000\u00d7, and image-based porosity quantification from calibrated SEM fields. This primary quantitative comparison is established between L1 and L3 only; Layer 2 is excluded from the 250\u00d7 quantitative analysis owing to its thermally distinct cooling regime and is treated separately. A counterintuitive layer-dependent porosity gradient is reported, wherein the upper layer (L3) exhibited 80% higher porosity (2.90 \u00b1 1.18%) and 107% higher pore density (4283\u2009 \u00b1 \u2009900 pores\/mm2) than the bottom layer (L1), despite recording a 26% lower peak FSP surface temperature (195.1 vs. 263.2 \u00b0C) (n = three fields per layer; Cohen\u2019s d \u2248 1.7). Based on these results, the post-peak cooling rate, rather than peak temperature, is identified as a plausible controlling factor for void consolidation quality, as evidenced by the observation that L3 cools at \u221212.3\u2009\u00b0C\/s versus \u221216.2 \u00b0C\/s for L1, which is consistent with prolonged high-temperature dwell and reduced plastic-flow-assisted pore closure in the upper layer. The anomalously rapid cooling of L2 (\u221246.9 \u00b0C\/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\u00d7\u201335,000\u00d7) revealed a frequent spatial co-location of sub-micron pores with fragmented Al\u2013Si eutectic particles, which is consistent with preferential void persistence near particle\u2013matrix interfaces. Grain morphology also exhibits non-monotonic evolution with build height, with mean circularity following the order L3 (0.645) &gt; L1 (0.621) &gt; L2 (0.569), and the equiaxed grain fraction ranging from 25.5% (L2) to 36.1% (L3) (ANOVA: F = 56.2, p = 5.15 \u00d7 10\u221225), while the mean equivalent grain diameter remained below 3.4\u2009\u03bcm across all layers. Overall, the outcomes of this study establish post-peak cooling rate, rather than peak temperature, as a plausible controlling factor for void consolidation quality in UAMFSP builds, with the caveat that complete causal isolation requires controlled single-variable experiments. 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\u2013deformation manufacturing of Al-based alloys.<\/jats:p>","DOI":"10.3390\/ma19132922","type":"journal-article","created":{"date-parts":[[2026,7,7]],"date-time":"2026-07-07T13:19:31Z","timestamp":1783430371000},"page":"2922","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":1,"title":["Post-Peak Cooling Rate Is Strongly Associated with Layer-Resolved Porosity Evolution in Hybrid WAAM\u2013FSP Al 4043 Multi-Layer Walls"],"prefix":"10.3390","volume":"19","author":[{"ORCID":"https:\/\/orcid.org\/0009-0000-8058-3370","authenticated-orcid":false,"given":"Ahmed Nabil","family":"Elalem","sequence":"first","affiliation":[{"name":"Department of Mechanical Engineering, Wayne State University, Detroit, MI 48202, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-3466-4206","authenticated-orcid":false,"given":"Mahmood","family":"Razzaghi","sequence":"additional","affiliation":[{"name":"Department of Mechanical Engineering, University of Victoria, Victoria, BC V8P 5C2, Canada"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-2848-5604","authenticated-orcid":false,"given":"Xin","family":"Wu","sequence":"additional","affiliation":[{"name":"Department of Mechanical Engineering, Wayne State University, Detroit, MI 48202, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2026,7,7]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"13770","DOI":"10.1016\/j.jmrt.2020.09.114","article-title":"Microstructures and mechanical properties of Al\u2013Zn\u2013Mg aluminum alloy samples produced by wire + arc additive manufacturing","volume":"9","author":"Li","year":"2020","journal-title":"J. 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