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.

Additive manufacturing offers a significant potential for producing metallic parts with distinctly localised microstructures and mechanical properties, commonly known as functional grading. While functional grading is generally accomplished through compositional variations or in-situ thermo-mechanical treatments, variation of process parameters during additive manufacturing can offer a promising alternative approach. Focusing on the electric arc-based additive manufacturing process, this work focuses on the functional grading of high-strength steel (S690 grade) by adjusting the travel speed and inter-pass temperature. Through a combination of thermal simulations and experimental measurements on single bead-on-plate depositions, it is shown that the microstructure and the mechanical properties of parts can be controlled through the rational adjustment of process parameters. A rectangular block was fabricated to demonstrate functional grading using a constant wire feed rate and varying travel speed. The rectangular block consisted of a low heat input (LHI) region deposited between high heat input (HHI) zones. A graded microstructure was obtained with the HHI zones composed of a mixture of polygonal ferrite, acicular ferrite, and bainite, while the LHI region was primarily composed of martensite. The hardness and profilometry-based indentation plastometry measurements indicated that the LHI region exhibited higher hardness (32%) and strength (50%), but lower uniform elongation (80%), compared to the HHI zones. The present study demonstrates the potential to achieve functional grading by adjusting process parameters in electric arc-based additive manufacturing, providing opportunities for tailor-made properties in parts.

A cellular automata (CA) model has been developed for solidification simulation considering the kinetic undercooling at the interface. The state-of-the-art model incorporates a decentered growth algorithm to suppress the grid anisotropy and a generalized height function method to calculate the curvature accurately. To develop a CA model which is independent of the mesh size, a new diffusion term is proposed to handle the diffusion between the interface cells and liquid cells. The developed CA model is employed to simulate the single-dendritic solidification of an Al–3Cu (wt pct) alloy. The simulated tip velocities agree with the prediction of the Kurz–Giovanola–Trivedi (KGT) model. Further studies show that the developed CA model converges to an equilibrium model with increasing kinetic mobility values. Moreover, it is found that the virtual liquid cell assumption which is commonly used in existing CA models may lead to a deviation in the mass balance. The mass balance error has been resolved by redistributing solutes from neighboring liquid cells in each time step. The developed CA model could be potentially used in solidification simulations with a high undercooling, which is common in welding and additive manufacturing.

Internal flow behaviour and melt-pool surface oscillations during arc welding are complex and not yet fully understood. In the present work, high-fidelity numerical simulations are employed to describe the effects of welding position, sulphur concentration (60-300 ppm) and travel speed (1.25-5 mms-1) on molten metal flow dynamics in fully-penetrated melt-pools. A wavelet transform is implemented to obtain time-resolved frequency spectra of the oscillation signals, which overcomes the shortcomings of the Fourier transform in rendering time resolution of the frequency spectra. Comparing the results of the present numerical calculations with available analytical and experimental datasets, the robustness of the proposed approach in predicting melt-pool oscillations is demonstrated. The results reveal that changes in the surface morphology of the pool resulting from a change in welding position alter the spatial distribution of arc forces and power-density applied to the molten material, and in turn affect flow patterns in the pool. Under similar welding conditions, changing the sulphur concentration affects the Marangoni flow pattern, and increasing the travel speed decreases the size of the pool and increases the offset between top and bottom melt-pool surfaces, affecting the flow structures (vortex formation) on the surface. Variations in the internal flow pattern affect the evolution of melt-pool shape and its surface oscillations.

Additive manufacturing (AM) is now recognised as a viable alternative to processes such as casting, forging, and subtractive technologies such as machining. Wire arc additive manufacturing (WAAM) has emerged as a cost-effective AM approach for component fabrication and a considerable body of literature on the subject has become available over the last 30 years. This review references the published work in a critical manner. It traces the development of WAAM, the principles of operation, materials considerations, process options, process physics, numerical simulation, process control, the current status and future research needs.

Oxide dispersion strengthened (ODS) steels are leading candidates for structural materials in nuclear fission and fusion power plants. Understanding the nature of nano-oxide particles in ODS steels is vital for a better control of the microstructure and mechanical properties to further their applications. In this study, electron microscopy and atom probe tomography (APT) have been used to investigate the nanocluster features in ODS Eurofer steel. With the addition of V and Ta in ODS Eurofer, the nanoclusters exhibit a higher number density with a decreased average diameter, indicating that V and Ta are beneficial for the formation of small clusters. Irrespective of the composition of the base material, the smaller particles have a variable stoichiometry while the larger particles are likely to have Y₂O₃ stoichiometry. The nanoclusters were found to have a core/shell structure, where Y, O and Ta are enriched in the core and Cr and V are predominant in the shell. The formation of the complex structure is possibly the result of a competing effect between Ta, Y, V and Cr binding with O. It is deduced that Ta tends to combine with O in the core (Y₂O₃) of the clusters due to a higher affinity, and pushes V and Cr to the surrounding shell during the formation of nanoclusters.

Pulsed laser beam welding was used successfully to join the oxide dispersion-strengthened (ODS) Eurofer steel. The joining was conducted with a laser power of 2500 W and a pulsed duration of 4 ms. With the filler material being used, a minor material loss and microvoids were observed in the joint. The microstructure of the fusion zone consists of dual phase elongated structures. The heat-affected zone has a width of around 0.06 mm with finer grains. The transmission electron microscopy observation reveals that nanoprecipitates are finely distributed in the fusion zone. The tensile strength, yield strength and elongation of the joint are slightly inferior to the base material. The fractography results reveal a typical ductile fracture. The experimental results indicate a reasonable joint from the perspective of both the microstructure and mechanical behaviour.

Oxide dispersion strengthened (ODS) steels are considered to be one of the candidate structural materials for advanced nuclear applications due to their high elevated-temperature strength, corrosion resistance, and radiation tolerance. Joining of ODS steels by traditional fusion joining techniques is not applicable, because the melting process results in the coarsening of fine grains and agglomeration of nanosized oxide particles, and consequently a significant loss of strength. Spark plasma sintering (SPS) has recently been employed as a novel joining technique, which could be beneficial for joining ODS steels considering the solid state characteristic. A powder metallurgy prepared ODS Eurofer steel was successfully joined using SPS. The microstructure and mechanical properties of the joints were investigated. An almost defect-free joint was obtained at the selected processing condition. The tensile properties of the joints are comparable to the base material. Fracture analysis shows an intergranular fracture in the as-joined sample, while a ductile fracture with well-defined dimples is found in the tempered sample.

The present work deals with oxide dispersion strengthened (ODS) Eurofer steel fabricated by powder metallurgy involving mechanical alloying and spark plasma sintering. A heat treatment route including normalising and tempering was applied to the as-produced steel, based on differential scanning calorimetry (DSC) measurement. The microstructure was characterised by scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), electrolytic extraction, X-ray diffraction (XRD) and transmission electron microscopy (TEM). Thermodynamic calculations conducted using Thermo-Calc software were used to determine the precipitation conditions. The results show that the Vickers microhardness of the sample after the designed heat treatment is more uniform compared to the as-produced condition. A dual phase and bimodal microstructure is formed in the as-produced and tempered steels. M₂₃C₆ and M₆C carbides were found in the as-produced sample while only M₂₃C₆ carbides were observed in the tempered sample. The carbides dissolve and reprecipitate during the heat treatment, preferably at the grain boundaries. Nanosized Y₂O₃ particles were found to be homogenously distributed in the steel matrix, which is crucial for the mechanical properties. The dislocation density in the material is decreased significantly after the normalising and tempering treatment. A yield strength model was developed that includes the strengthening contributions of solid solutes, grain size, dislocation density and nanoparticles. Good agreement is obtained between the experimentally measured and theoretically calculated strength of the as-produced and tempered steels.

Solidification cracking susceptibility during laser welding was studied experimentally and numerically in advanced high strength steel sheets, namely transformation-induced plasticity (TRIP) and dual phase (DP) steel. Using the same heat input, laser bead-on-plate welding was carried out on single sided clamped specimens at various starting distances from the free edge. It was observed that TRIP steel with high phosphorus is susceptible to cracking while in DP steel with low phosphorus, solidification cracking was not observed. The metallurgical factors affecting the solidification cracking were studied and it was found that solidification morphology, phosphorus segregation at the prior austenite grain boundaries, inclusions, interface growth rate and interdendritic liquid feeding have a prominent effect on the strength of the mushy zone. These results are discussed pertaining to the cracking mechanism. For the same welding parameters, a difference in the weld pool shape was observed in both the steels, which is attributed to the high temperature thermophysical properties. Weld pool shape affects the strain distribution in the mushy region and thus the cracking behaviour. The cracking phenomenon is further described using hot ductility curves.

Oxide dispersion strengthened (ODS)Eurofer steel was prepared via mechanical alloying (MA)and spark plasma sintering (SPS). Different combinations of MA and SPS parameters were adopted in order to optimise the fabrication process. The experimental results show that the sample milled for 30 h, sintered at 1373 K at a pressure of 60 MPa has the highest density and microhardness among all the results obtained. As-produced ODS Eurofer shows a bimodal microstructure with homogeneously dispersed nanoscale Y ₂ O ₃ , that is beneficial for the mechanical properties. The yield and tensile strengths are higher while the elongation is lower in the top and bottom surfaces compared to the middle area of the sample. This is due to the presence of a larger number of M ₂₃ C ₆ carbides, resulting from carbon diffusion from the mould material. As-produced samples were also subjected to a heat treatment. A good balance is achieved between the strength and ductility of the heat-treated material. Yielding properties comparable to hot isostatic pressing or hot extrusion as reported in the literature, the processing route presented by this study shows potential to produce high performance ODS Eurofer.