Functional grading of steel structures via wire arc additive manufacturing

Abstract

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.