Influence of Interlayer Thermal Cycling on Microstructural Evolution in WAAM Processed Carbon Steel

Influence of Interlayer Thermal Cycling on Microstructural Evolution in WAAM Processed Carbon Steel | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Influence of Interlayer Thermal Cycling on Microstructural Evolution in WAAM Processed Carbon Steel Andres Fernando Gil Plazas, Theylor Andres Amaya Villabón, David Alberto Ramírez Vargas, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7394849/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 5 You are reading this latest preprint version Abstract Wire Arc Additive Manufacturing (WAAM) has emerged as a cost effective and scalable process for fabricating metallic components. In carbon steel, the repeated thermal cycles during deposition significantly influence grain morphology and mechanical properties. This study investigates the evolution of grain size across ten sequentially deposited layers using GMAW based WAAM. An analysis was conducted through metallographic preparation and linear reconstruction of the deposited volume, allowing quantification of grain size at each level. The results show that initial layers exhibit columnar grains with acicular ferrite, and with continued deposition, thermal cycling promotes grain coarsening and recrystallization. Grain size increased from ~ 2 µm in the first layer to ~ 10 µm by the subsequent layers, indicating the onset of recrystallization-induced equiaxiality. The upper layers showed a higher presence of allotriomorphic ferrite, while the lower layers developed equiaxed ferrite due to repeated reheating. These findings confirm that thermal cycling during WAAM leads to microstructural homogenization, which is essential for achieving consistent mechanical behavior across the build height. WAAM Grain Growth Ferrite Recrystallization Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Introduction According to ASTM 52900, Additive Manufacturing (AM) is categorized into seven process families. Among these, Direct Energy Deposition (DED) is the most widely applied in metal-based applications, with Wire Arc Additive Manufacturing (WAAM) being one of its prominent variants. Over the past two decades, AM has gained significant attention as a promising technology for producing complex components in low-volume production [ 1 ]. However, its full industrial implementation remains limited, largely due to gaps in the understanding of the mechanical and microstructural behavior of AM produced parts [ 2 ]. Compared to conventional manufacturing methods such as machining, AM offers notable advantages, particularly in reducing raw material consumption [ 3 ]. WAAM, like other additive manufacturing techniques, relies on the layer-by-layer deposition principle. However, it distinguishes itself by using an electric arc to melt metal wire, allowing the creation of fully metallic parts [ 4 ]. WAAM is fundamentally based on welding processes; as such, several parameters critically influence metal deposition. These include current, voltage, deposition rate, wire feed speed, travel speed, and shielding gas composition, among others [ 5 ]. The proper selection of process parameters plays a critical role in the ability of WAAM to successfully fabricate various structural components, such as columns, trusses, structural tubes, lattice structures, bridges, facades, and staircase [ 6 ], [ 7 ], [ 8 ]. In addition to its structural capabilities, WAAM also contributes to reducing greenhouse gas emissions by eliminating the need for molds and minimizing material waste. This versatility has led to the growing adoption of WAAM across multiple fields including civil engineering, art, materials science, and mechanical engineering. Numerous research institutions are actively investigating WAAM due to its ability to process a wide range of materials, including both ferrous and non-ferrous metals [ 9 ]. WAAM faces significant challenges, as the mechanical properties of the fabricated parts are intrinsically linked to microstructural evolution. The layer-by-layer heat input directly influences the temperature gradient in the liquid (G L ) and the solidification growth rate (R), both of which determine the solidification mode. Depending on these conditions, the microstructure can develop as planar, cellular, dendritic, columnar, or equiaxed. Since WAAM is based on welding processes, it typically produces cellular or cellular-dendritic solidification structures [ 10 ]. However, from a mechanical performance standpoint, equiaxed grain structures are preferred due to their improved isotropy and strength [ 11 ]. To achieve this, the thermal cycling introduced by successive layers can provide sufficient heat to promote recrystallization, acting similarly to the “Temper Weld Process” [ 12 ]. WAAM operates through successive thermal cycles, the interaction between layers becomes a factor in determining the final microstructure and the mechanical performance of the component [ 13 ]. Each new deposition affects the underlying material, potentially refining grains or introducing defects [ 5 ]. Therefore, understanding the interlayer behavior is essential for optimizing process parameters and achieving consistent, high-quality parts. This study aims to explore microstructural evolution due to the thermal cycles involved between layers in WAAM, emphasizing their influence on the grain growing. Although numerous studies have examined microstructural evolution in WAAM, most of them rely on localized analyses that divide the build into three representative regions: bottom, middle, and top [ 14 ], [ 15 ], [ 16 ], [ 17 ]. While this approach provides a general overview of the influence of heat accumulation and thermal cycling, it fails to capture the progressive and successive nature of microstructural transformations along the entire Z-axis of the component [ 18 ], [ 19 ]. In reality, each deposited layer undergoes a unique thermal history, as it is subjected not only to its own solidification dynamics but also to repeated reheating from subsequent layers [ 20 ], [ 21 ], [ 22 ]. This continuous thermal interaction suggests that the grain structure may evolve gradually from one layer to the next, potentially following trends of refinement, coarsening, or recrystallization that cannot be fully described by the conventional bottom, middle and top segmentation [ 23 ]. Consequently, there remains a significant research gap regarding the detailed layer-by-layer characterization of grain size evolution throughout the build height [ 24 ]. Understanding this progression is essential for establishing robust correlations between deposition strategy, thermal management, and the resulting mechanical performance, ultimately enabling the design of WAAM components with predictable and homogeneous properties across their entire section [ 25 ], [ 26 ]. In this study, ER70S-6 carbon steel was deposited using the WAAM process to investigate the progressive microstructural evolution along the build height. A series of samples were fabricated with varying layer numbers, enabling a detailed, layer-by-layer grain size analysis across the Z-axis. This approach allows for the identification of microstructural trends, such as grain coarsening, refinement, or recrystallization, that emerge under successive thermal cycles. By focusing on the cumulative effects of interlayer reheating, the study seeks to provide a deeper understanding of how thermal history influences grain morphology and homogeneity, thereby offering insights for optimizing process parameters and improving the reliability of WAAM-produced components. Materials and Methods A. Raw materials In this study, a ER70S-6 steel wire with a diameter of 0.035 inches was used. The protective gas, composed of 98% Ar and 2% O 2 (Linde PLC), was utilized during the Gas Metal Arc Welding – Additive Manufacturing process (GMAW-AM) with a flow of 35 CFH. B. WAAM Equipment The Wire Arc Additive Manufacturing (WAAM) setup employed in this study integrated a Gas Metal Arc Welding (GMAW) power source, a three-axis CNC router, dedicated control software, and a series of custom software tools for controlling the welding torch and a forced-convection fan between passes. C. GMAW Power Source and Deposition Setup A SKY MIG 4060 TX (SWEISS) GMAW unit served as the primary deposition source. The machine was configured for short-circuit transfer under constant voltage conditions, stick out 10 mm operating at 20 V, with a travel speed of 250 mm/min and a wire feed speed of 6 m/min. The previously specified filler metal and shielding gas were delivered through a standard GMAW torch mounted on the CNC router’s Z-axis head assembly. D. Characterization Microstructural evolution was assessed through metallographic analysis (Zeiss Axio Observer Z1.m) following ASTM E3 guidelines. After the preparation of the sample, a linear reconstruction of the microstructure was performed from the base to the top region of the WAAM-deposited material. This reconstruction involved stitching multiple micrographic images captured at 500× magnification to create a continuous tile. Subsequently, the reconstructed microstructural image underwent an automated segmentation process to detect individual grain boundaries. This segmentation was achieved using a computational methodology employing FFT filtering, thresholding, and watershed segmentation techniques [ 27 ]. Once segmented, each grain was automatically identified, allowing precise quantitative analysis of grain size distribution across the deposition layers. This detailed segmentation enabled the assignment of distinct colors to grains based on their respective sizes, providing an intuitive visual representation of grain size distribution. Such color-coded mapping significantly enhances the visualization of microstructural variations, clearly revealing the effects of repeated thermal treatments on grain evolution in the successive upper layers. Figure 1 illustrates the sequential steps of this methodology. Part (a) displays the tiled reconstruction composed of three micrographic images captured at 500× magnification, showing the continuous grain morphology across the analyzed area. Part (b) depicts the segmented microstructure, with clearly defined grain boundaries marked. Finally, part (c) presents the segmented image with grains colored according to size, highlighting the grain size distribution and allowing immediate visual assessment of microstructural changes induced by interlayer thermal cycling. Results and Discussion Ten samples were fabricated using the WAAM process, with the first sample consisting of a single layer, the second with two successive layers, and so on up to the tenth sample with ten layers. This incremental approach was designed to observe the microstructural evolution as a function of the number of deposited layers. Once the samples were fabricated, a metallographic specimen was prepared from each. Figure 2 illustrates the ten fabricated samples, cross-sectioned samples, and the resulting metallographic specimens. With the selected parameters voltage, travel speed, and wire feed rate the depositions remained stable and did not exhibit the formation of humps or valleys [ 28 ]. Figure 3 to 7 presents a microstructural characterization of a WAAM-deposited sample, focusing on grain size distribution across the build height. The upper left section shows a cross-sectional view of a metallographically prepared sample, where four distinct horizontal zones (labeled A to D) were selected for analysis along the build direction (BD). These regions span from the top to the bottom of the deposit and reflect different thermal histories due to successive heat input during the WAAM process. To the right of each Figure (3 to 7), a grain size area distribution graph is presented. It shows a 100% stacked area bar chart that quantifies the percentage of grains in each region according to their equivalent circular diameter, divided into bins. At the first layer (Fig. 3 ), the material shows macroscopic columnar grain growth aligned with the original austenitic grain boundaries. Additionally, intergranular growth reveals the presence of acicular ferrite with a fine grain size of approximately 3 µm, attributed to the rapid solidification conditions, Fig. 3 - Zone A, B, C and D. When the third layer was deposited, Fig. 4 , the thermal input affected the underlying material in the first layer, promoting grain growth due to recrystallization, Fig. 4 - Zone C and D. As a result, the grain size increased from approximately 3 µm to between 5 and 10 µm. In the newly deposited third layer, the grains exhibited even greater growth compared to those observed in the first layer, indicating a cumulative thermal effect with each additional deposition, Fig. 4 – Zone A. By the fifth layer, Fig. 5 , the thermal cycling effect becomes more pronounced. The underlying layers, particularly the first and second, exhibit further grain coarsening, with grains surpassing 10 µm in some regions. This continued grain growth is attributed to repeated exposure to the heat generated by subsequent depositions, Fig. 5 – Zone C and D. In the fifth layer itself, the microstructure maintains a predominantly columnar morphology, but with visibly wider grains compared to earlier layers, Fig. 5 – Zone A. At the seventh layer, Fig. 6 , the influence of accumulated thermal cycles becomes even more evident. The microstructure in the lower layers (1–5) shows significant grain coarsening, with some grains exceeding 15 µm, Fig. 6 – Zone C. In the seventh layer, grain morphology remains predominantly columnar, yet the grains are noticeably longer and wider compared to those in the fifth layer, Fig. 6 – Zone A. The presence of equiaxed grains becomes more frequent, indicating that localized thermal saturation begins to alter the solidification behavior and promote grain refinement in the lowest layers, Fig. 6 – Zone C and D. By the tenth layer, Fig. 7 , the microstructural evolution reaches a critical point where the cumulative heat input leads to a more complex grain distribution throughout the build. The lower layers show extensive grain growth and loss of the initial columnar orientation, Fig. 7 – Zone C and D. In the topmost layer (layer 10), grains are considerably larger, with increased presence of allotriomorphic ferrite, this ferrite tends to appear ever in the top layer after the first layer, Fig. 7 – Zone A. In contrast, the lower layers present ferrite equiaxed grains, formed by recrystallization induced by repeated thermal cycling, Fig. 7 – Zone B, C and D. During solidification in WAAM, the material initially forms an austenitic structure. As the temperature decreases, the steel transitions into the austenite to ferrite transformation region [ 29 ]. Within this range, various ferrite morphologies can form depending on the thermal conditions. Allotriomorphic ferrite generally nucleates along prior austenite grain boundaries, growing without a faceted morphology [ 30 ]. At lower transformation temperatures, the mobility of these boundaries decreases, promoting the formation of Widmanstätten side plates. These side plates grow rapidly, aided by efficient carbon diffusion away from the growing tips, and occur without significant substitutional atom diffusion [ 31 ]. Once grain boundary nucleation sites are saturated, the transformation may proceed within the grain interiors, where inclusions act as nucleation sites for acicular ferrite. This structure consists of interlocking ferrite laths that grow in multiple directions, resulting in a refined microstructure with high-angle grain boundaries and fine microphases (e.g., bainite) between laths, which enhance toughness and isotropy [ 23 ]. Additionally, the repeated thermal cycling inherent to the WAAM process leads to partial or full recrystallization in the underlying layers. Each new layer introduces heat that may raise the material above the critical transformation temperatures AC 1 and even AC 3 allowing the austenitic phase to reappear locally [ 11 ]. Upon cooling, the microstructure transforms again, potentially refining grains or altering their morphology. This recurrent heating and cooling promote dynamic recrystallization and grain refinement, especially in the lower layers, which experience more cumulative thermal exposure [ 15 ]. Figure 8 illustrates the mean evolution of grain size along the build height and across different deposition layers in samples fabricated using WAAM process. The upper section displays the macrographs of ten samples, each corresponding to a different number of deposited layers (from 1 to 10). These samples were sectioned at specific heights (labeled A through J) to investigate the microstructural response to thermal cycling. The lower part of the figure presents a graph that quantifies the average grain size (in µm) at each sectional position for every sample. The analysis of grain size evolution across zones A to J reveals the progressive impact of thermal cycling and recrystallization during the WAAM process. Zone A, corresponding to the first layer, shows the smallest median grain size (~ 4.5 µm), reflecting rapid solidification with limited thermal exposure [ 24 ]. As new layers are deposited, zones B and C begin to exhibit moderate increases in grain size (around 6.0–6.2 µm), suggesting the initial influence of reheating from subsequent passes [ 32 ]. In zone D, a slight decrease in grain size (~ 5.8 µm) may indicate localized refinement or incomplete recrystallization. From zone E onward, a clear grain growth trend emerges, with median sizes rising to approximately 7.8 µm in zone E and continuing through zones F and G (reaching ~ 8.5–9.0 µm). This reflects the cumulative effect of heat input, promoting recrystallization and grain coarsening [ 25 ]. By zones H to J, the grain size distribution stabilizes, and differences between layers diminish. This stabilization suggests that, after sufficient thermal cycling, the material reaches a steady-state microstructural condition. As a result, the overall grain morphology becomes more equiaxed, indicating that dynamic recrystallization has homogenized the microstructure throughout the height of the sample [ 12 ]. Finally, Fig. 9 illustrates the thermal behavior experienced by each layer during the WAAM deposition process and its correlation with microstructural transformations. The upper section shows the physical samples, labeled A to J, corresponding to successive layers of material deposited. Each sample is segmented by color-coded horizontal lines representing the deposition levels (layer 1 to layer 10), where thermal analysis was performed. The lower part of the figure presents a classic Fe-C phase diagram (left) and a simulated thermal profile (right) for each layer across all zones. The simulation results clearly indicate the evolution of temperature in each layer, highlighting the repeated thermal cycling undergone by the lower regions of the build. The red lines in the diagram correspond to the critical transformation temperatures: AC1 (~ 727°C) and AC3 (~ 910°C). These thresholds define the boundaries for austenite formation and full transformation, respectively [ 29 ], [ 30 ]. The thermal profiles show that with each new deposition, underlying layers are re-exposed to significant heat input, often surpassing the AC1 and AC3 lines multiple times. In early zones (e.g., A to D), this repeated heating promotes phase transformation from ferrite to austenite and back, favoring recrystallization and subsequent grain growth [ 16 ]. The grain refinement or coarsening depends on the degree and frequency of these thermal cycles, which are clearly more intense in the lower sections of the sample due to the accumulation of heat from the successive layers above [ 14 ]. In contrast, the upper layers (zones H to J) show less thermal cycling and lower overall heat accumulation. These layers are closer to ambient and lose heat more readily to the surroundings, resulting in less opportunity for recrystallization. As a result, the topmost deposited layers retain microstructures more directly associated with solidification, such as columnar grains and allotriomorphic ferrite (Fig. 10 ). These ferrite morphologies tend to nucleate along the prior austenite grain boundaries without regular faceted shapes and grow under limited diffusion conditions [ 30 ]. The simulation confirms the microstructural observations described previously: a transition from acicular or equiaxed ferrite in the thermally cycled lower regions to coarse, irregular ferritic structures in the upper layers (Fig. 10 ). Therefore, the evolution of thermal cycles across the build height plays a fundamental role in defining the local microstructure and, ultimately, the mechanical behavior of WAAM components. Conclusions An ER70S-6 steel wire was deposited using the WAAM process, and a grain size evolution study was conducted to understand how the process promotes microstructural homogeneity. A customized methodology was applied to evaluate the impact of thermal cycles on grain development, providing a basis for future research and highlighting the importance of process parameters on material behavior. The main findings of this study include the following: Continuous heat input promotes recrystallization, which favors the formation of equiaxed ferrite. Thermal cycles during the WAAM process contribute to the development of a homogeneous microstructure throughout the deposited volume. Grain size analysis revealed a clear tendency toward equiaxed grain morphology from the intermediate layers onward, driven by repeated thermal cycling and recrystallization. The findings suggest that the uppermost region, approximately the last 5 mm exhibits greater microstructural variability and could be excluded or machined without compromising the overall integrity of the component. In other words, if a specific final height is required for a component, depositing at least two additional layers beyond that height would ensure sufficient recrystallization of the desired volume, resulting in a more homogeneous and stable microstructure. Declarations COMPETING INTERESTS The authors declare no competing interests. FUNDING Open access funding provided by la Universidad Nacional de Colombia – Sede Bogotá. AUTHORS’ CONTRIBUTION Andres Fernando Gil Plazas: Conceptualization, Research, Methodology, Writing -Original draft, Writing – Review & editing. Theylor Andres Amaya Villabón: Conceptualization, Research, Simulation, Methodology, Writing - Original draft, Writing – Review & editing. David Alberto Ramírez Vargas: Conceptualization, Methodology, Visualization. Julián David Rubiano Buitrago: Conceptualization, Methodology, Visualization. Liz Karen Herrera Quintero: Conceptualization, Supervision. Acknowledgments The authors would like to thank Linde PLC and the Centro de Materiales y Ensayos of SENA, Regional Distrito Capital with the Plan de Acción PA_2025_13_69, for their logistical, technical, human, and financial support of the project. Special thanks are extended to the Escuela de Diseño – Facultad de Artes at the Universidad Nacional de Colombia, Sede Bogotá, for their support of the doctoral studies through the award of a Teaching Assistantship." 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Henke, “The Impact of Multiple Thermal Cycles Using CMT® on Microstructure Evolution in WAAM of Thin Walls Made of AlMg5,” Metals (Basel). , vol. 14, no. 6, p. 717, Jun. 2024, doi: 10.3390/met14060717. Cite Share Download PDF Status: Under Review Version 1 posted Reviewers agreed at journal 02 Sep, 2025 Reviewers invited by journal 02 Sep, 2025 Editor invited by journal 02 Sep, 2025 Editor assigned by journal 22 Aug, 2025 First submitted to journal 20 Aug, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7394849","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":508958591,"identity":"1e31d870-a3c9-4ce3-a208-3dbc3363accc","order_by":0,"name":"Andres Fernando Gil Plazas","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA0klEQVRIiWNgGAWjYFACxsYDDBVAmh2IeYjU0nCA4QyQZiZeCwPDAcY2UrTw9y9uOFw473AefzMD44O3bQzy/IS0SNx42HB45rbDxRKHGZgN57YxGM5sIKTnxsGGw7zbDic2HGZgk+ZtY0gwOEBAhzxYy5zDifMPM7D/JkqLwflGoJaGw4kbgLYwE6XF8AZjw+EZx9ITNx5mbJacc06CsF/kzh9/+Ligxjpx3vHmgx/elNkQDjEGiQRwjDCA4hTEJagBGDEHYFpGwSgYBaNgFOAAAMjKRLQyxgqNAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0001-6585-9121","institution":"Universidad Nacional de Colombia - Sede Bogotá: Universidad Nacional de Colombia","correspondingAuthor":true,"prefix":"","firstName":"Andres","middleName":"Fernando Gil","lastName":"Plazas","suffix":""},{"id":508958592,"identity":"16813085-1dde-46d2-88f9-95b141904cb7","order_by":1,"name":"Theylor Andres Amaya Villabón","email":"","orcid":"","institution":"Universidad Nacional de Colombia - Sede Bogotá: Universidad Nacional de Colombia","correspondingAuthor":false,"prefix":"","firstName":"Theylor","middleName":"Andres Amaya","lastName":"Villabón","suffix":""},{"id":508958593,"identity":"3d312f62-b1fa-4e32-87bb-06f7907e0bfb","order_by":2,"name":"David Alberto Ramírez Vargas","email":"","orcid":"","institution":"Linde Plc","correspondingAuthor":false,"prefix":"","firstName":"David","middleName":"Alberto Ramírez","lastName":"Vargas","suffix":""},{"id":508958594,"identity":"60688922-ea86-4e4a-a956-7e8c245b5c0e","order_by":3,"name":"Julián David Rubiano Buitrago","email":"","orcid":"","institution":"Universidad Nacional de Colombia - Sede Bogotá: Universidad Nacional de Colombia","correspondingAuthor":false,"prefix":"","firstName":"Julián","middleName":"David Rubiano","lastName":"Buitrago","suffix":""},{"id":508958600,"identity":"3e352d47-2e51-434f-b644-7213cbd1b4a1","order_by":4,"name":"Liz Karen Herrera Quintero","email":"","orcid":"","institution":"Universidad Nacional de Colombia - Sede Bogotá: Universidad Nacional de Colombia","correspondingAuthor":false,"prefix":"","firstName":"Liz","middleName":"Karen Herrera","lastName":"Quintero","suffix":""}],"badges":[],"createdAt":"2025-08-18 01:34:21","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7394849/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7394849/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":90918393,"identity":"4639e227-98fa-4c8e-a9c1-6f92821c3f4a","added_by":"auto","created_at":"2025-09-09 14:27:16","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":290604,"visible":true,"origin":"","legend":"\u003cp\u003eMethodology for microstructural analysis: (a) reconstructed microstructure, (b) segmented grain boundaries, and (c) grain size distribution visualization.\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7394849/v1/107535427855bd61e78162b0.jpg"},{"id":90918391,"identity":"3a9ccea6-bd5b-4e7a-ac82-101838c406fe","added_by":"auto","created_at":"2025-09-09 14:27:16","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":386485,"visible":true,"origin":"","legend":"\u003cp\u003eSamples obtained using the WAAM process. (a) As-welded samples, (b) Cross-sectioned samples, (c) Metallographic specimens extracted from each layer level (1–10).\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7394849/v1/ba414ba34aef8dbd8ed76016.jpg"},{"id":90918908,"identity":"86328018-901a-4533-b107-15893f56bd0a","added_by":"auto","created_at":"2025-09-09 14:35:16","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":360089,"visible":true,"origin":"","legend":"\u003cp\u003eGrain growth characterization for the first layer WAAM deposition.\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7394849/v1/5223f36b6013e4eb43c89f30.jpg"},{"id":90918395,"identity":"b8e9f130-00ff-4611-ad7d-010c163bef33","added_by":"auto","created_at":"2025-09-09 14:27:16","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":331631,"visible":true,"origin":"","legend":"\u003cp\u003eGrain growth characterization for the third layer WAAM deposition.\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7394849/v1/e550a9b21dbfb57200f048f7.jpg"},{"id":90918910,"identity":"05d639fb-0553-427e-9dcf-a8942372435a","added_by":"auto","created_at":"2025-09-09 14:35:16","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":325150,"visible":true,"origin":"","legend":"\u003cp\u003eGrain growth characterization for the fifth layer WAAM deposition.\u003c/p\u003e","description":"","filename":"5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7394849/v1/8d32341d2106548487266f12.jpg"},{"id":90918914,"identity":"9f15a62d-012c-40fa-9884-b54cf714ab41","added_by":"auto","created_at":"2025-09-09 14:35:16","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":357184,"visible":true,"origin":"","legend":"\u003cp\u003eGrain growth characterization for the seventh layer WAAM deposition.\u003c/p\u003e","description":"","filename":"6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7394849/v1/f100a05bf924584c3bdaaa80.jpg"},{"id":90920137,"identity":"149ff658-0eb2-43df-ae35-cca3c812fd29","added_by":"auto","created_at":"2025-09-09 14:43:16","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":319009,"visible":true,"origin":"","legend":"\u003cp\u003eGrain growth characterization for the tenth layer WAAM deposition.\u003c/p\u003e","description":"","filename":"7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7394849/v1/04b75fa1be9cdfebaf854308.jpg"},{"id":90918912,"identity":"4c56cfed-b7a7-4f07-b779-0e06fedbed5d","added_by":"auto","created_at":"2025-09-09 14:35:16","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":180157,"visible":true,"origin":"","legend":"\u003cp\u003eGrain size evolution along the build height in WAAM-deposited ER70S-6 steel.\u003c/p\u003e","description":"","filename":"8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7394849/v1/d0afd060a91a72ff59e4708e.jpg"},{"id":90918915,"identity":"aa59af1f-c16b-4460-8845-49230f778575","added_by":"auto","created_at":"2025-09-09 14:35:16","extension":"jpg","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":241926,"visible":true,"origin":"","legend":"\u003cp\u003eThermal simulation history of each analyzed zone and its relationship to the Fe-C phase diagram.\u003c/p\u003e","description":"","filename":"9.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7394849/v1/260e28e3687276659c04d978.jpg"},{"id":90918405,"identity":"3b253827-a360-44cf-8893-5b25d4bf4449","added_by":"auto","created_at":"2025-09-09 14:27:16","extension":"jpg","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":271600,"visible":true,"origin":"","legend":"\u003cp\u003eMicrostructural features of the WAAM-deposited layers: a) Lower region, b) Middle zone, c) Upper layer.\u003c/p\u003e","description":"","filename":"10.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7394849/v1/06633ee4f0e280b03e28a1b0.jpg"},{"id":91148797,"identity":"33499de9-8978-4cf6-830f-b039ba7da5f5","added_by":"auto","created_at":"2025-09-12 06:45:39","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3549127,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7394849/v1/1ce19da7-99ec-40b1-ab81-39fac5aca673.pdf"}],"financialInterests":"","formattedTitle":"Influence of Interlayer Thermal Cycling on Microstructural Evolution in WAAM Processed Carbon Steel","fulltext":[{"header":"Introduction","content":"\u003cp\u003eAccording to ASTM 52900, Additive Manufacturing (AM) is categorized into seven process families. Among these, Direct Energy Deposition (DED) is the most widely applied in metal-based applications, with Wire Arc Additive Manufacturing (WAAM) being one of its prominent variants. Over the past two decades, AM has gained significant attention as a promising technology for producing complex components in low-volume production [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. However, its full industrial implementation remains limited, largely due to gaps in the understanding of the mechanical and microstructural behavior of AM produced parts [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Compared to conventional manufacturing methods such as machining, AM offers notable advantages, particularly in reducing raw material consumption [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eWAAM, like other additive manufacturing techniques, relies on the layer-by-layer deposition principle. However, it distinguishes itself by using an electric arc to melt metal wire, allowing the creation of fully metallic parts [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. WAAM is fundamentally based on welding processes; as such, several parameters critically influence metal deposition. These include current, voltage, deposition rate, wire feed speed, travel speed, and shielding gas composition, among others [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. The proper selection of process parameters plays a critical role in the ability of WAAM to successfully fabricate various structural components, such as columns, trusses, structural tubes, lattice structures, bridges, facades, and staircase [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e], [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e], [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eIn addition to its structural capabilities, WAAM also contributes to reducing greenhouse gas emissions by eliminating the need for molds and minimizing material waste. This versatility has led to the growing adoption of WAAM across multiple fields including civil engineering, art, materials science, and mechanical engineering. Numerous research institutions are actively investigating WAAM due to its ability to process a wide range of materials, including both ferrous and non-ferrous metals [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eWAAM faces significant challenges, as the mechanical properties of the fabricated parts are intrinsically linked to microstructural evolution. The layer-by-layer heat input directly influences the temperature gradient in the liquid (G\u003csub\u003eL\u003c/sub\u003e) and the solidification growth rate (R), both of which determine the solidification mode. Depending on these conditions, the microstructure can develop as planar, cellular, dendritic, columnar, or equiaxed. Since WAAM is based on welding processes, it typically produces cellular or cellular-dendritic solidification structures [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. However, from a mechanical performance standpoint, equiaxed grain structures are preferred due to their improved isotropy and strength [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. To achieve this, the thermal cycling introduced by successive layers can provide sufficient heat to promote recrystallization, acting similarly to the \u0026ldquo;Temper Weld Process\u0026rdquo; [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eWAAM operates through successive thermal cycles, the interaction between layers becomes a factor in determining the final microstructure and the mechanical performance of the component [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Each new deposition affects the underlying material, potentially refining grains or introducing defects [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Therefore, understanding the interlayer behavior is essential for optimizing process parameters and achieving consistent, high-quality parts. This study aims to explore microstructural evolution due to the thermal cycles involved between layers in WAAM, emphasizing their influence on the grain growing.\u003c/p\u003e\u003cp\u003eAlthough numerous studies have examined microstructural evolution in WAAM, most of them rely on localized analyses that divide the build into three representative regions: bottom, middle, and top [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e], [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e], [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e], [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. While this approach provides a general overview of the influence of heat accumulation and thermal cycling, it fails to capture the progressive and successive nature of microstructural transformations along the entire Z-axis of the component [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e], [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. In reality, each deposited layer undergoes a unique thermal history, as it is subjected not only to its own solidification dynamics but also to repeated reheating from subsequent layers [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e], [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e], [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. This continuous thermal interaction suggests that the grain structure may evolve gradually from one layer to the next, potentially following trends of refinement, coarsening, or recrystallization that cannot be fully described by the conventional bottom, middle and top segmentation [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. Consequently, there remains a significant research gap regarding the detailed layer-by-layer characterization of grain size evolution throughout the build height [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. Understanding this progression is essential for establishing robust correlations between deposition strategy, thermal management, and the resulting mechanical performance, ultimately enabling the design of WAAM components with predictable and homogeneous properties across their entire section [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e], [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eIn this study, ER70S-6 carbon steel was deposited using the WAAM process to investigate the progressive microstructural evolution along the build height. A series of samples were fabricated with varying layer numbers, enabling a detailed, layer-by-layer grain size analysis across the Z-axis. This approach allows for the identification of microstructural trends, such as grain coarsening, refinement, or recrystallization, that emerge under successive thermal cycles. By focusing on the cumulative effects of interlayer reheating, the study seeks to provide a deeper understanding of how thermal history influences grain morphology and homogeneity, thereby offering insights for optimizing process parameters and improving the reliability of WAAM-produced components.\u003c/p\u003e"},{"header":"Materials and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003eA. Raw materials\u003c/h2\u003e\u003cp\u003eIn this study, a ER70S-6 steel wire with a diameter of 0.035 inches was used. The protective gas, composed of 98% Ar and 2% O\u003csub\u003e2\u003c/sub\u003e (Linde PLC), was utilized during the Gas Metal Arc Welding \u0026ndash; Additive Manufacturing process (GMAW-AM) with a flow of 35 CFH.\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eB. WAAM Equipment\u003c/h3\u003e\n\u003cp\u003eThe Wire Arc Additive Manufacturing (WAAM) setup employed in this study integrated a Gas Metal Arc Welding (GMAW) power source, a three-axis CNC router, dedicated control software, and a series of custom software tools for controlling the welding torch and a forced-convection fan between passes.\u003c/p\u003e\n\u003ch3\u003eC. GMAW Power Source and Deposition Setup\u003c/h3\u003e\n\u003cp\u003eA SKY MIG 4060 TX (SWEISS) GMAW unit served as the primary deposition source. The machine was configured for short-circuit transfer under constant voltage conditions, stick out 10 mm operating at 20 V, with a travel speed of 250 mm/min and a wire feed speed of 6 m/min. The previously specified filler metal and shielding gas were delivered through a standard GMAW torch mounted on the CNC router\u0026rsquo;s Z-axis head assembly.\u003c/p\u003e\n\u003ch3\u003eD. Characterization\u003c/h3\u003e\n\u003cp\u003eMicrostructural evolution was assessed through metallographic analysis (Zeiss Axio Observer Z1.m) following ASTM E3 guidelines. After the preparation of the sample, a linear reconstruction of the microstructure was performed from the base to the top region of the WAAM-deposited material. This reconstruction involved stitching multiple micrographic images captured at 500\u0026times; magnification to create a continuous tile. Subsequently, the reconstructed microstructural image underwent an automated segmentation process to detect individual grain boundaries. This segmentation was achieved using a computational methodology employing FFT filtering, thresholding, and watershed segmentation techniques [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eOnce segmented, each grain was automatically identified, allowing precise quantitative analysis of grain size distribution across the deposition layers. This detailed segmentation enabled the assignment of distinct colors to grains based on their respective sizes, providing an intuitive visual representation of grain size distribution. Such color-coded mapping significantly enhances the visualization of microstructural variations, clearly revealing the effects of repeated thermal treatments on grain evolution in the successive upper layers.\u003c/p\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e illustrates the sequential steps of this methodology. Part (a) displays the tiled reconstruction composed of three micrographic images captured at 500\u0026times; magnification, showing the continuous grain morphology across the analyzed area. Part (b) depicts the segmented microstructure, with clearly defined grain boundaries marked. Finally, part (c) presents the segmented image with grains colored according to size, highlighting the grain size distribution and allowing immediate visual assessment of microstructural changes induced by interlayer thermal cycling.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cp\u003eTen samples were fabricated using the WAAM process, with the first sample consisting of a single layer, the second with two successive layers, and so on up to the tenth sample with ten layers. This incremental approach was designed to observe the microstructural evolution as a function of the number of deposited layers. Once the samples were fabricated, a metallographic specimen was prepared from each. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e illustrates the ten fabricated samples, cross-sectioned samples, and the resulting metallographic specimens. With the selected parameters voltage, travel speed, and wire feed rate the depositions remained stable and did not exhibit the formation of humps or valleys [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e to \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e presents a microstructural characterization of a WAAM-deposited sample, focusing on grain size distribution across the build height. The upper left section shows a cross-sectional view of a metallographically prepared sample, where four distinct horizontal zones (labeled A to D) were selected for analysis along the build direction (BD). These regions span from the top to the bottom of the deposit and reflect different thermal histories due to successive heat input during the WAAM process.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eTo the right of each Figure (3 to 7), a grain size area distribution graph is presented. It shows a 100% stacked area bar chart that quantifies the percentage of grains in each region according to their equivalent circular diameter, divided into bins.\u003c/p\u003e\u003cp\u003eAt the first layer (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), the material shows macroscopic columnar grain growth aligned with the original austenitic grain boundaries. Additionally, intergranular growth reveals the presence of acicular ferrite with a fine grain size of approximately 3 \u0026micro;m, attributed to the rapid solidification conditions, Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e - Zone A, B, C and D.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eWhen the third layer was deposited, Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, the thermal input affected the underlying material in the first layer, promoting grain growth due to recrystallization, Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e - Zone C and D. As a result, the grain size increased from approximately 3 \u0026micro;m to between 5 and 10 \u0026micro;m. In the newly deposited third layer, the grains exhibited even greater growth compared to those observed in the first layer, indicating a cumulative thermal effect with each additional deposition, Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e \u0026ndash; Zone A.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eBy the fifth layer, Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, the thermal cycling effect becomes more pronounced. The underlying layers, particularly the first and second, exhibit further grain coarsening, with grains surpassing 10 \u0026micro;m in some regions. This continued grain growth is attributed to repeated exposure to the heat generated by subsequent depositions, Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e \u0026ndash; Zone C and D. In the fifth layer itself, the microstructure maintains a predominantly columnar morphology, but with visibly wider grains compared to earlier layers, Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e \u0026ndash; Zone A.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eAt the seventh layer, Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, the influence of accumulated thermal cycles becomes even more evident. The microstructure in the lower layers (1\u0026ndash;5) shows significant grain coarsening, with some grains exceeding 15 \u0026micro;m, Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e \u0026ndash; Zone C. In the seventh layer, grain morphology remains predominantly columnar, yet the grains are noticeably longer and wider compared to those in the fifth layer, Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e \u0026ndash; Zone A. The presence of equiaxed grains becomes more frequent, indicating that localized thermal saturation begins to alter the solidification behavior and promote grain refinement in the lowest layers, Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e \u0026ndash; Zone C and D.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eBy the tenth layer, Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e, the microstructural evolution reaches a critical point where the cumulative heat input leads to a more complex grain distribution throughout the build. The lower layers show extensive grain growth and loss of the initial columnar orientation, Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e \u0026ndash; Zone C and D. In the topmost layer (layer 10), grains are considerably larger, with increased presence of allotriomorphic ferrite, this ferrite tends to appear ever in the top layer after the first layer, Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e \u0026ndash; Zone A. In contrast, the lower layers present ferrite equiaxed grains, formed by recrystallization induced by repeated thermal cycling, Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e \u0026ndash; Zone B, C and D.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eDuring solidification in WAAM, the material initially forms an austenitic structure. As the temperature decreases, the steel transitions into the austenite to ferrite transformation region [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. Within this range, various ferrite morphologies can form depending on the thermal conditions. Allotriomorphic ferrite generally nucleates along prior austenite grain boundaries, growing without a faceted morphology [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. At lower transformation temperatures, the mobility of these boundaries decreases, promoting the formation of Widmanst\u0026auml;tten side plates. These side plates grow rapidly, aided by efficient carbon diffusion away from the growing tips, and occur without significant substitutional atom diffusion [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eOnce grain boundary nucleation sites are saturated, the transformation may proceed within the grain interiors, where inclusions act as nucleation sites for acicular ferrite. This structure consists of interlocking ferrite laths that grow in multiple directions, resulting in a refined microstructure with high-angle grain boundaries and fine microphases (e.g., bainite) between laths, which enhance toughness and isotropy [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eAdditionally, the repeated thermal cycling inherent to the WAAM process leads to partial or full recrystallization in the underlying layers. Each new layer introduces heat that may raise the material above the critical transformation temperatures AC\u003csub\u003e1\u003c/sub\u003e and even AC\u003csub\u003e3\u003c/sub\u003e allowing the austenitic phase to reappear locally [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. Upon cooling, the microstructure transforms again, potentially refining grains or altering their morphology. This recurrent heating and cooling promote dynamic recrystallization and grain refinement, especially in the lower layers, which experience more cumulative thermal exposure [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e illustrates the mean evolution of grain size along the build height and across different deposition layers in samples fabricated using WAAM process. The upper section displays the macrographs of ten samples, each corresponding to a different number of deposited layers (from 1 to 10). These samples were sectioned at specific heights (labeled A through J) to investigate the microstructural response to thermal cycling. The lower part of the figure presents a graph that quantifies the average grain size (in \u0026micro;m) at each sectional position for every sample.\u003c/p\u003e\u003cp\u003eThe analysis of grain size evolution across zones A to J reveals the progressive impact of thermal cycling and recrystallization during the WAAM process. Zone A, corresponding to the first layer, shows the smallest median grain size (~\u0026thinsp;4.5 \u0026micro;m), reflecting rapid solidification with limited thermal exposure [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. As new layers are deposited, zones B and C begin to exhibit moderate increases in grain size (around 6.0\u0026ndash;6.2 \u0026micro;m), suggesting the initial influence of reheating from subsequent passes [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. In zone D, a slight decrease in grain size (~\u0026thinsp;5.8 \u0026micro;m) may indicate localized refinement or incomplete recrystallization. From zone E onward, a clear grain growth trend emerges, with median sizes rising to approximately 7.8 \u0026micro;m in zone E and continuing through zones F and G (reaching\u0026thinsp;~\u0026thinsp;8.5\u0026ndash;9.0 \u0026micro;m). This reflects the cumulative effect of heat input, promoting recrystallization and grain coarsening [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. By zones H to J, the grain size distribution stabilizes, and differences between layers diminish. This stabilization suggests that, after sufficient thermal cycling, the material reaches a steady-state microstructural condition. As a result, the overall grain morphology becomes more equiaxed, indicating that dynamic recrystallization has homogenized the microstructure throughout the height of the sample [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFinally, Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e illustrates the thermal behavior experienced by each layer during the WAAM deposition process and its correlation with microstructural transformations. The upper section shows the physical samples, labeled A to J, corresponding to successive layers of material deposited. Each sample is segmented by color-coded horizontal lines representing the deposition levels (layer 1 to layer 10), where thermal analysis was performed.\u003c/p\u003e\u003cp\u003eThe lower part of the figure presents a classic Fe-C phase diagram (left) and a simulated thermal profile (right) for each layer across all zones. The simulation results clearly indicate the evolution of temperature in each layer, highlighting the repeated thermal cycling undergone by the lower regions of the build. The red lines in the diagram correspond to the critical transformation temperatures: AC1 (~\u0026thinsp;727\u0026deg;C) and AC3 (~\u0026thinsp;910\u0026deg;C). These thresholds define the boundaries for austenite formation and full transformation, respectively [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e], [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eThe thermal profiles show that with each new deposition, underlying layers are re-exposed to significant heat input, often surpassing the AC1 and AC3 lines multiple times. In early zones (e.g., A to D), this repeated heating promotes phase transformation from ferrite to austenite and back, favoring recrystallization and subsequent grain growth [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. The grain refinement or coarsening depends on the degree and frequency of these thermal cycles, which are clearly more intense in the lower sections of the sample due to the accumulation of heat from the successive layers above [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eIn contrast, the upper layers (zones H to J) show less thermal cycling and lower overall heat accumulation. These layers are closer to ambient and lose heat more readily to the surroundings, resulting in less opportunity for recrystallization. As a result, the topmost deposited layers retain microstructures more directly associated with solidification, such as columnar grains and allotriomorphic ferrite (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e). These ferrite morphologies tend to nucleate along the prior austenite grain boundaries without regular faceted shapes and grow under limited diffusion conditions [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eThe simulation confirms the microstructural observations described previously: a transition from acicular or equiaxed ferrite in the thermally cycled lower regions to coarse, irregular ferritic structures in the upper layers (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e). Therefore, the evolution of thermal cycles across the build height plays a fundamental role in defining the local microstructure and, ultimately, the mechanical behavior of WAAM components.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eAn ER70S-6 steel wire was deposited using the WAAM process, and a grain size evolution study was conducted to understand how the process promotes microstructural homogeneity. A customized methodology was applied to evaluate the impact of thermal cycles on grain development, providing a basis for future research and highlighting the importance of process parameters on material behavior. The main findings of this study include the following:\u003c/p\u003e\u003cp\u003e\u003cul\u003e\u003cli\u003e\u003cp\u003eContinuous heat input promotes recrystallization, which favors the formation of equiaxed ferrite.\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eThermal cycles during the WAAM process contribute to the development of a homogeneous microstructure throughout the deposited volume.\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eGrain size analysis revealed a clear tendency toward equiaxed grain morphology from the intermediate layers onward, driven by repeated thermal cycling and recrystallization.\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eThe findings suggest that the uppermost region, approximately the last 5 mm exhibits greater microstructural variability and could be excluded or machined without compromising the overall integrity of the component. In other words, if a specific final height is required for a component, depositing at least two additional layers beyond that height would ensure sufficient recrystallization of the desired volume, resulting in a more homogeneous and stable microstructure.\u003c/p\u003e\u003c/li\u003e\u003c/ul\u003e\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003ch2\u003eCOMPETING INTERESTS\u003c/h2\u003e\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e\u003c/p\u003e\u003ch2\u003eFUNDING\u003c/h2\u003e\u003cp\u003eOpen access funding provided by la Universidad Nacional de Colombia \u0026ndash; Sede Bogot\u0026aacute;.\u003c/p\u003e\u003ch2\u003eAUTHORS\u0026rsquo; CONTRIBUTION\u003c/h2\u003e\u003cp\u003eAndres Fernando Gil Plazas: Conceptualization, Research, Methodology, Writing -Original draft, Writing \u0026ndash; Review \u0026amp; editing. Theylor Andres Amaya Villab\u0026oacute;n: Conceptualization, Research, Simulation, Methodology, Writing - Original draft, Writing \u0026ndash; Review \u0026amp; editing. David Alberto Ram\u0026iacute;rez Vargas: Conceptualization, Methodology, Visualization. Juli\u0026aacute;n David Rubiano Buitrago: Conceptualization, Methodology, Visualization. Liz Karen Herrera Quintero: Conceptualization, Supervision.\u003c/p\u003e\u003ch2\u003eAcknowledgments\u003c/h2\u003e\u003cp\u003eThe authors would like to thank Linde PLC and the Centro de Materiales y Ensayos of SENA, Regional Distrito Capital with the Plan de Acci\u0026oacute;n PA_2025_13_69, for their logistical, technical, human, and financial support of the project. Special thanks are extended to the Escuela de Dise\u0026ntilde;o \u0026ndash; Facultad de Artes at the Universidad Nacional de Colombia, Sede Bogot\u0026aacute;, for their support of the doctoral studies through the award of a Teaching Assistantship.\"\u003c/p\u003e\u003ch2\u003eDATA AVAILABILITY STATEMENT\u003c/h2\u003e\u003cp\u003eAll data that support the findings of this study are included within the article.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eA. Shah, R. Aliyev, H. Zeidler, and S. Krinke, \u0026ldquo;A Review of the Recent Developments and Challenges in Wire Arc Additive Manufacturing (WAAM) Process,\u0026rdquo; Jun. 01, 2023, \u003cem\u003eMDPI\u003c/em\u003e. doi: 10.3390/jmmp7030097.\u003c/li\u003e\n\u003cli\u003eD. S. M. Serrati, M. A. Machado, J. P. Oliveira, and T. G. 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Henke, \u0026ldquo;The Impact of Multiple Thermal Cycles Using CMT\u0026reg; on Microstructure Evolution in WAAM of Thin Walls Made of AlMg5,\u0026rdquo; \u003cem\u003eMetals (Basel).\u003c/em\u003e, vol. 14, no. 6, p. 717, Jun. 2024, doi: 10.3390/met14060717.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"welding-in-the-world","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"witw","sideBox":"Learn more about [Welding in the World](https://www.springer.com/journal/40194)","snPcode":"40194","submissionUrl":"https://www.editorialmanager.com/witw/","title":"Welding in the World","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"WAAM, Grain Growth, Ferrite, Recrystallization","lastPublishedDoi":"10.21203/rs.3.rs-7394849/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7394849/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eWire Arc Additive Manufacturing (WAAM) has emerged as a cost effective and scalable process for fabricating metallic components. In carbon steel, the repeated thermal cycles during deposition significantly influence grain morphology and mechanical properties. This study investigates the evolution of grain size across ten sequentially deposited layers using GMAW based WAAM. An analysis was conducted through metallographic preparation and linear reconstruction of the deposited volume, allowing quantification of grain size at each level. The results show that initial layers exhibit columnar grains with acicular ferrite, and with continued deposition, thermal cycling promotes grain coarsening and recrystallization. Grain size increased from ~\u0026thinsp;2 \u0026micro;m in the first layer to ~\u0026thinsp;10 \u0026micro;m by the subsequent layers, indicating the onset of recrystallization-induced equiaxiality. The upper layers showed a higher presence of allotriomorphic ferrite, while the lower layers developed equiaxed ferrite due to repeated reheating. These findings confirm that thermal cycling during WAAM leads to microstructural homogenization, which is essential for achieving consistent mechanical behavior across the build height.\u003c/p\u003e","manuscriptTitle":"Influence of Interlayer Thermal Cycling on Microstructural Evolution in WAAM Processed Carbon Steel","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-09-09 14:27:11","doi":"10.21203/rs.3.rs-7394849/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2025-09-02T08:46:58+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-09-02T08:37:33+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Welding in the World","date":"2025-09-02T07:22:42+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-08-22T06:16:25+00:00","index":"","fulltext":""},{"type":"submitted","content":"Welding in the World","date":"2025-08-20T14:00:57+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"welding-in-the-world","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"witw","sideBox":"Learn more about [Welding in the World](https://www.springer.com/journal/40194)","snPcode":"40194","submissionUrl":"https://www.editorialmanager.com/witw/","title":"Welding in the World","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"29279d80-ab3c-4f7d-a1d3-b19af717f5d0","owner":[],"postedDate":"September 9th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2025-10-24T05:26:02+00:00","versionOfRecord":[],"versionCreatedAt":"2025-09-09 14:27:11","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7394849","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7394849","identity":"rs-7394849","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}