The increasing demand for efficient manufacturing of specialized large-scale metallic components in critical sectors such as maritime, energy, and heavy industry underscores the limitations of traditional fabrication methods, as they are often constrained by cost, low production volumes, and complex logistics. Additive manufacturing (AM) presents a promising alternative, offering geometric flexibility, material efficiency, and supply chain simplification. However, many AM technologies remain unsuitable for large-scale applications due to low deposition rates, high material costs, and build size limitations. Wire Arc Additive Manufacturing (WAAM), by contrast, leverages arc welding technologies to produce near-net-shape, fully dense components at high deposition rates, without inherent size restrictions. This makes WAAM a leading candidate for large-scale metal part production.

A key innovation enabled by WAAM is functional grading: the ability to spatially tailor microstructural or compositional features across a part. This work focuses specifically on compositional grading between High-Strength Low-Alloy (HSLA) steel and austenitic stainless (AS) steel, a material pairing with industrial relevance due to their complementary properties. Despite growing interest in such bi-material systems, significant challenges persist in controlling the resulting microstructure, especially at the chemically graded interface, where microstructural gradients can lead to unpredictable mechanical performance. This research addresses these challenges through a systematic investigation of interface microstructure and mechanical response, guided by three primary objectives: (1) to understand microstructural evolution at the interface, (2) to assess deformation and fracture behaviour, and (3) to evaluate fatigue performance of WAAM-fabricated AS steel– HSLA steel bi-metal components.

To achieve these aims, a comprehensive experimental programme was implemented. Representative mono-material and bi-material blocks were fabricated using Cold Metal Transfer (CMT)-assisted WAAM, with consistent deposition parameters to ensure comparability. The bi-material blocks featured AS steel deposited onto a previouslydeposited HSLA steel material volume, creating a chemically diluted interface. Microstructural characterization was conducted across multiple length scales using optical microscopy, SEM, EBSD, EPMA, TEM, and TKD. These techniques enabled detailed investigation of solidification features, phase transformations, and chemical segregation. The insights drawn were further supported by CALIPHAD simulations of microstructural development. Mechanical testing included quasi-static tensile tests supported by DIC, fracture toughness evaluation using custom J-integral formulations via FEA modelling, and fatigue crack propagation studies under constant-load and constant-ΔK conditions.

The results reveal that the interface formed during WAAM is highly heterogeneous, shaped by partial mixing and thermal gradients. Multiscale characterization identified a complex structure involving intrusions of unmixed material, phase segregation, and metastable austenite capable of transforming into ε- and α′-martensite. Under quasi-static loading, the interface layer exhibited enhanced work hardening and delayed failure due to the activation of a Transformation Induced Plasticity (TRIP) mechanism. While this effect contributed positively to tensile performance, fracture toughness testing revealed variability due to brittle martensitic regions that act as crack initiators. Fatigue testing further confirmed the dual nature of the interface: TRIP initially slowed crack propagation, but once transformation was exhausted, propagation rates increased sharply in regions containing pre-existing allotropic martensite. These results highlight the importance of local microstructural control in determining the mechanical reliability of bi-material WAAM structures.

This thesis makes several key contributions to understand the process-microstructure-property relationship of additively-manufactured, functionally graded components. It demonstrates that controlled heterogeneity can be beneficial in functionally graded steels, provided that metastable phases are stabilized and effectively leveraged. It emphasizes the critical role of local thermal conditions during deposition and the need to design process parameters that account for heat flow and dilution effects. Beyond the specific AS steel-HSLA steel system, these findings support broader efforts to establish qualification pathways for functionally graded materials in additive manufacturing. They underscore the importance of localized property assessment, microstructural validation, and real-time process monitoring as foundational elements for reliable design and certification. By offering a comprehensive case study of dissimilar metal WAAM, this work contributes both fundamental insights and practical guidance for the development and qualification of advanced graded structures in industrial settings.

This study concentrates on the fatigue crack propagation behaviour of a high-strength low-alloy (HSLA) steel and austenitic stainless (AS) steel bi-material part, as obtained by wire arc additive manufacturing (WAAM). Due to partial mixing in the weld pool, the first layer of AS steel laid onto the previously deposited HSLA steel results in a diluted interface layer of distinct chemical and microstructural characteristics. Average Paris parameters are obtained for the interface layer along transverse and longitudinal planes to the deposition direction (BD-LD plane: m = 2.79, log₁₀(C) = –7.83 log₁₀(da/dN)) (BD-TD plane: m = 3.47, log₁₀(C) = –8.39 log₁₀(da/dN)). However, it is observed that this interface layer manifests an intriguing crack propagation behaviour. FCGR consistently drop as the crack front transitions from undiluted AS steel to the interface. At ΔK = 20 MPa⋅m^0.5, the greatest Δ is −0.77 log₁₀ steps (R = 0.1). As cracks near the HSLA fusion line, rates re-accelerate up to + 0.75 log₁₀ steps (R = 0.5). The phenomenon is attributed to the interplay between deformation-induced martensitic transformation and pre-existing allotropic martensite. Our findings, derived from a series of fatigue tests in correlation with multiscale microstructural and fracture characterization, offer insights into the damage-tolerant behaviour of these bi-material structures.

Wire arc additive manufacturing (WAAM) offers a novel approach to fabricate functionally graded components. By changing the wire consumable between layers, chemical grading can be used to obtain specific properties across a part's volume. This is an interesting approach to design large metal components that achieve unconventional performance in demanding engineering applications, such as sulphide-resistant pressure vessels or sea ballast piping with extended lifetime. However, challenges derived from dissimilar material combinations draw the need to study the effect of compositional grading on the mechanical properties. This study focuses on the deformation and fracture toughness behaviour of WAAM-fabricated high-strength low-alloy (HSLA) and austenitic stainless (AS) steel bi-material specimens, particularly examining the diluted interface layer obtained during deposition. Tensile testing results indicate that the elastic modulus at the interface matches that of un-diluted AS steel (157 ±17 GPa) along the build direction. Fracture toughness showed a lower J_IC (180 kJ/m²) when compared to the undiluted AS steel (459 ±69 kJ/m²) and HSLA steel (408 ±25 kJ/m²). Scanning electron microscopy and electron backscatter diffraction are used to establish a connection between the microstructure at the interface and the observed mechanical properties. It is concluded that deformation at the interface is in large controlled by the deformation-induced martensitic transformation of metastable austenite. These results underline the influence of chemical dilution on the deformation mechanisms and fracture behaviour of HSLA and AS steel bi-material parts, which needs to be accounted for in the design of parts composed by this bi-metal couple.

Wire arc additive manufacturing (WAAM) of high-strength steel (HSS) has gained significant attention for structural applications. Achieving precise control over the manufacturing process and understanding the relationship between process parameters and the resulting material characteristics is crucial for optimizing the performance of these steel walls to achieve tailored properties. The present study was performed to comprehend the influence of process parameters on the microstructure and properties of wire arc additively manufactured (WAAM) high-strength steel (HSS) thin-wall structures. Multi-layer thin walls of ER110S-G high-strength steel comprising 30 layers were deposited bidirectionally and were fabricated with different travel speeds and wire-feed rates. Geometrical analysis conducted on samples indicates that achieving minimal surface waviness for single-bead thin walls depends on adjusting wire feed rates and travel speeds. Specifically, lower wire feed rates are found to be more effective in minimizing waviness when dealing with single-bead thin walls (thickness < 5 mm). Conversely, lower travel speeds are preferred for reducing surface irregularities in walls fabricated at high deposition rates for thicker single-bead walls (thickness > 8 mm). Cooling rate analysis from midpoints of the 5th, 15th and 25th layers of each sample indicates high cooling rates for low heat input (HI=178 J/mm) samples even for the 25th layer. Microstructural characterization of the samples suggests an increase in acicular ferrite and martensite volume fraction with lower heat input. Additionally, microstructural quantification with EBSD reveals smaller grain sizes and higher Kernel average misorientation for low heat input deposits. Mechanical properties like hardness and tensile strength display an increasing trend with decreasing heat input while elongation to fracture is reduced under the same conditions. Furthermore, anisotropic behaviour is observed in tensile strength and elongation to fracture between building and deposition directions due to the presence of microstructural inhomogeneities.

This study examines the interface layer between a high-strength low-alloy steel and an overlaying austenitic stainless steel as deposited through wire arc additive manufacturing in a bi-metal block. By utilizing optical and electron microscopy techniques, and accompanied by phenomenological and thermodynamic modeling, the work elucidates on the nature of the distinct microstructural features at a new level of detail. Results showcase martensite in the form of a band along the fusion line of the first dissimilar layer, as well as in segregated islands. Within the same bead, yet away from the fusion line, an austenite matrix is identified alongside a large phase fraction of primary ferrite and sparse bainite. These findings enhance our understanding of the nature of the heterogeneous microstructure at the interface of a bi-metal build and establish empirical evidence for future modeling of microstructural development. Supplementary characterization reveals the impact of these microstructural heterogeneities on bulk mechanical performance. Hardness indents exhibit varied results along the interface, peaking at martensite islands with values up to 370HV_0.2, surpassing the neighboring matrix by 50%. Under quasi-static tensile loading, bi-metallic specimens display strain partitioning along the fusion boundary, as confirmed by Digital Image Correlation. When compared to the adjoining stainless steel, the diluted interface layer exhibits superior strength (σ_y: 411 MPa) and comparable ductility (24%), leading to necking and failure away from this region. These results help predict the structural performance of bi-metal parts, and build a base for further research in more intricate loading scenarios, such as crack propagation processes.