In this study, three-dimensional functionally graded NiTi bulk materials were fabricated using laser powder bed fusion (LPBF) by in-situ adding Ni powder into equiatomic NiTi powder. The gradient zone exhibited a Ni composition ranging from approximately 49.6 to 52.4 at.% over a distance of about 2.75 mm. The functionalities along the compositional gradient were examined through differential scanning calorimetry analysis and spherical indentation. This unique gradient resulted in location-specific functionalities, including superelasticity characterized by wide and narrow hysteresis loops, shape memory effect, and various phase transformation temperatures. The rapid cooling rate during fabrication led to the presence of excess Ni in the solid-solute state within NiTi. This unique solid-solute compositional gradient in NiTi resulted in varying lattice parameters, influencing the compatibility between martensite and austenite and allowing for tailored hysteresis. This discovery presents new avenues for designing multifunctional materials through in-situ additive manufacturing.

In the present study, the crystallography aspects of the liquid metal embrittlement (LME) phenomenon are investigated in a bi-metallic bronze-stainless steel structure, produced using wire arc additive manufacturing. Most of the LME cracks were found to be propagated along high-angle grain boundaries of the austenitic structure. Surprisingly, it was observed that in some cases, LME cracks propagated transgranularly in austenite grains, which is a rare phenomenon in LME of steels. The Ʃ3-coincidence site lattice (CLS) boundaries showed the highest resistance to LME compared to other CLS types. It was also found that the presence of elongated grains in additively manufactured microstructures can accelerate the LME phenomenon.

Superelastic metamaterials have attracted significant attention recently, but achieving such functionality remains challenging due to partial superelasticity and premature fracture in additively manufactured components. To address these issues, this study investigates the premature fracture in Ni-rich NiTi metamaterials fabricated by laser powder bed fusion. A comparative analysis of two structures (Gyroid network and Diamond shell) reveals that the structural stability of bending- and stretching-dominated structures is reversed compared to typical elastic-plastic response, due to the tension-compression asymmetry of base NiTi. The premature fracture and partial superelasticity of these as-fabricated samples are attributed to low deformation ability for accommodating tensile stress. Based on these findings, a heat treatment introducing Ni₄Ti₃ precipitates was employed, successfully achieving macroscopic superelasticity in the NiTi metamaterials, with consistency between model prediction and experiments.

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.

Microstructure features including grain morphology and texture are key factors in determining the properties of laser additively manufactured metallic components. Beyond the traditional trial-and-error approach, which is costly and time-consuming, microstructure control increasingly relies on predictions from mechanistic models. However, existing mechanistic models to predict microstructure and texture are computationally expensive. Here, we present a cellular automata solidification model, which is up to two orders-of-magnitude faster than traditional models. By analytically calculating growth length and utilizing a multi-level capture algorithm, a large time step can be employed without compromising simulation accuracy. The model is validated through simulations of 316L steel and three NiTi cases, showing good agreement with experimental results. Our findings reveal that preferential orientations are selected by the vertical and the inclined temperature gradients from multi-pass temperature profiles, leading to different microstructures and textures. Three-dimensional additive manufacturing simulations demonstrate that orientation-dependent growth patterns govern grain growth, leading to columnar, planar and spiral shaped grains. This approach offers a significant reduction in computational cost while maintaining accuracy, contributing to the practical application of microstructure control in additive manufacturing. The insights gained on grain and texture evolution pave the way for customized microstructure design through additive manufacturing.

Wire arc additive manufacturing (WAAM) is a significant area of interest within the field of additive manufacturing (AM). In the present research, WAAM technology was employed to deposit a Ni-based alloy on a ductile cast iron substrate to fabricate a bimetallic structure of Ni-45 %Fe alloy and ductile cast iron. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), electron back scattered diffraction (EBSD) and X-ray diffraction (XRD) were used to study phase transformations, microstructure and crystallographic texture development in interfacial regions as well as deposited material. The mechanical properties were also studied using micro-hardness and profilometry-based indentation plastometry (PIP) measurements. The results showed that a wide variety of phases are generated within the heat-affected zone (HAZ) and partially melted zone (PMZ). These phases form complex microstructures with single and double shell morphology. The deposited alloy has a face-centred cubic (FCC) structure, with some carbides and graphite that are formed during the solidification of the first deposited layer. The compositional changes were also observed across the interface. The texture of the deposited alloy showed around 30° deviation from 〈100〉 II building direction due to the shape and overlap of the melt pools. The present results provide a better understanding of interface development mechanisms during WAAM of bimetallic structures. The peak of the hardness across the interface was observed in PMZ because of the formation of a martensitic matrix. The PIP measurements showed that the σ_y and the UTS of deposited alloy are lower than the cast iron base metal.

The sensitivity of the single-phase, low thermal expansion (LTE) alloy 36 (Fe-36Ni) to intergranular cracking hinders its processability during additive manufacturing. This study investigates the effect of accelerated cooling via a CO₂ jet and the addition of TiC particles on the cracking susceptibility of the LTE36 alloy during wire-arc additive manufacturing (WAAM). Results show that accelerated cooling reduces inter-pass deposition times and the susceptibility to cracking due to increased heat dissipation. A crack-free microstructure was achieved only with the addition of TiC particles, which pinned the high-angle grain boundaries and induced tortuosity, thereby limiting grain growth and mitigating intergranular cracking. Mechanical performance was restored compared to the cracked condition, and the critical LTE property of the as-deposited LTE36 alloy was improved due to the enhanced ferromagnetic character of the alloy. Therefore, the combined approach effectively mitigated intergranular cracking while retaining the LTE behaviour during WAAM of the LTE36 alloy.

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.

A multi-scale multi-physics modeling framework has been developed to predict solidification cracking susceptibility (SCS) during welding. The framework integrates a thermo-mechanical finite element model to simulate temperature and strain rate profiles during welding, a cellular automata model to simulate the solidified microstructure in the weld pool, and a granular model to calculate the pressure drop in the mushy zone. Verification was achieved by comparing the model’s predictions with welding experiments on two steels, demonstrating its capability to accurately capture the effects of process parameters, grain refinement, and alloy composition on SCS. Results indicate that increasing welding velocity, while maintaining a constant power-to-velocity ratio, extends the size of the mushy zone and increases the maximum pressure drop in the mushy zone, leading to higher SCS. Grain refinement decreases separation velocities and the permeability of liquid channels, which increases SCS, but it also raises the coalescence temperature, resulting in an overall reduction in SCS. Alloy composition impacts SCS through thermal diffusivity and segregation. Lower thermal diffusivity or stronger segregation tends to elongate the mushy zone, resulting in an increase in SCS. This framework provides a robust tool for understanding the mechanisms of solidification cracking, optimizing welding parameters to prevent its occurrence, and comparing SCS of different compositions during alloy design.

Copper-steel functionally graded materials combine the thermal conductivity of copper with the mechanical strength of steel. This study examines the microstructural, mechanical, and thermophysical properties of the constitutive layers of copper-4130 steel functionally graded material fabricated via laser directed energy deposition, considering four intermediate compositions: 100% 4130, 75% 4130 – 25% Cu, 50% 4130 – 50% Cu, and 25% 4130 – 75% Cu. It was observed that the amount of Cu-rich terminal liquid governs crack formation and backfilling during solidification, while Cu-Fe liquid phase separation and Marangoni convection within the melt pool generate macrostructures composed of alternating Cu- and Fe-rich phases. Increasing Cu content progressively enhances thermal diffusivity due to the formation of interconnected copper regions. The application of quenching and tempering treatments induced softening of Cu-containing samples due to Cu recrystallization and diffusion from supersaturated Fe-rich phases. Although solidification cracking was only observed in 75% 4130–25% individual samples, the analysis of a complete multilayer structure revealed that interlayer mixing causes local compositional variations, extending cracking susceptibility beyond this region. These findings provide insights into the key factors governing laser directed energy deposition of copper-steel functionally graded materials, supporting process optimization and predictive model development to enhance manufacturability.

Additively manufactured Nitinol (NiTi) architectured materials, designed with unit cell architectures, hold promise for customisable applications. However, the common assumption of homogeneity in modeling and additive manufacturing of these architectured materials needs further investigation because geometric-dependent melt pool behaviour results in inhomogeneous microstructure and thermomechanical properties. This study shows that property inhomogeneity at the mesoscale is one reason for pseudo-linear response and partial superelasticity of the fabricated NiTi body-centered cubic (BCC) architectured materials. We modeled using a phenomenological constitutive relation and additively manufactured NiTi architectured materials with varying relative densities. These fabricated samples showed distinct microstructural textures and compositions that affected their local recoverability. The edge effects and laser turn regions were identified as the causes underlying the observed microstructural inhomogeneity. The dimensionless Fourier number is used to describe the transition of printing modes. This study provides valuable information on rigorous experimental/computational consistency in future work.

The binary Fe–Ni system offers alloys with notably low linear coefficients of thermal expansion (CTE), contingent upon their Ni content. In this respect twin-wire arc additive manufacturing (T-WAAM) presents the opportunity of in-situ alloying through the simultaneous feeding of two metal wires into a weld pool to obtain desired alloy compositions. This study aims to deposit a graded wall with Ni contents targeted at 42, 46, and 52 wt% in the building direction of the wall, along with a block comprising of 46 wt% Ni, employing the T-WAAM approach. The results show effective incorporation of additional Ni into the weld pool and geometrically stable weld beads in the continuous metal transfer mode during the T-WAAM process. This mode led to the defect-free and chemically stable deposition of the graded wall and the block with average Ni contents of 42.3 ± 1.1, 45.8 ± 1.4, 52.6 ± 0.8 wt%, and 46.4 ± 0.9 wt%, respectively. The measured Curie temperatures of the as-deposited alloys and the mean CTE values of alloy 46 were found to be comparable to the commercial alloys. In summary, this study validates the feasibility of in-situ deposition of low thermal expansion alloy compositions, thereby enabling the possibility of on-demand thermal expansion properties.

Boron doped MoSi₂ particles have been envisioned as sacrificial particles for self-healing thermal barrier coatings (TBCs) but their oxidation behaviour is yet not well understood. In this work, oxidation of MoSi₂ based particle is studied in the temperature range of 1050–1200 °C. The oxidation proceeds from a transient to a steady-state oxidation stage. The kinetics during steady-state oxidation is captured with a thermal diffusion-based model. As compared to the oxidation of pure MoSi₂ particles, the addition of boron strongly enhances the silica formation. Also, a finer dispersion of MoB_x in the MoSi₂ matrix accelerates the formation of silica. The oxide growth rate constant increases proportional with the boron content of the MoSi₂ particles. This enhanced oxidation is related to the microstructure of the oxide scale. Upon oxidation, boron yields B₂O₃, which promptly merge with SiO₂ to form amorphous borosilicate, hindering the formation of crystalline SiO₂. Consequently, the migration of oxygen in the borosilicate oxide scale is faster than in the silica oxide scale on pure MoSi₂ particles.

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.

Superelastic metallic materials possessing large recoverable strains are widely used in automotive, aerospace and energy conversion industries. Superelastic materials working at high temperatures and with a wide temperature range are increasingly required for demanding applications. Until recently, high-temperature superelasticity has only been achievable with multicomponent alloys fabricated by complex processes. In this study, a novel framework of multi-scale models enabling texture and microstructure design is proposed for high-performance NiTi fabrication via laser powder bed fusion. Based on the developed framework, a Ni-lean Ni(49.4 at.%)-Ti alloy is, for the first time, endowed with a 4% high-temperature compressive superelasticity. A 001 texture, unfavorable for plastic slip, is created to realize enhanced functionality. The unprecedented superelasticity can be maintained up to 453 K, which is comparable with but has a wider superelastic temperature range (∼110 K) than rare earth alloyed NiTi alloys, previously only realizable with grain refinement, and other complicated post-processing operations. At the same time, its shape memory stability is also improved due to existing textured 100 martensite and intergranular precipitation of Ti₂NiOx. This discovery reframes the way that we design superior performance NiTi based alloys through directly tailoring crystallographic orientations during additive manufacturing.

Wire and Arc Additive Manufacturing (WAAM) emerged as a manufacturing process for large scale structures with extensive form and design freedom. WAAM can be fully exploited once the relation between the transient thermal history and its relation to microstructure development and resultant mechanical properties is established. This relation can be further used for computational design tools such as Topology Optimization. This paper presents a model to predict the relation between the thermal history and solid-state phase transformations in a widely applicable High Strength Low Alloy steel ER110S-G. The transient thermal history of parts manufactured by WAAM is modelled using finite element analysis. The modelled thermal history is validated with thermocouple measurements. Our results show that a critical cooling cycle is responsible for the solid-state phase transformation in an AM part. The cooling rate of this particular cooling cycle is superimposed onto an experimentally constructed Continuous Cooling Transformation (CCT) diagram to determine the local solid-state phase fractions. The predicted phase fractions in three wall samples with different design and processing conditions of AM parts are used to predict the hardness. The predicted hardness is 10% higher than the measured hardness of AM samples. The effect of tempering is also considered in the model through JMAK equation. The results show that the tempering is caused in regions with high martensite content and it lowers the hardness by 8 − 10%. The micrographs of the AM parts show that the microstructural features are same for the AM parts with similar critical cooling rates.

Additive manufacturing of NiTi shape memory alloys has attracted attention in recent years, due to design flexibility and feasibility to achieve four-dimensional (4D) function response. To obtain customized 4D functional responses in NiTi structures, tailorable phase transformation temperatures and stress windows as well as one-way or two-way shape memory properties are required. To achieve this goal, various heat treatments, including direct aging, annealing and annealing followed by aging, were optimized for the Ti-rich NiTi (Ni49.6Ti (at. %)) fabricated by laser powder bed fusion (L-PBF). Microstructural evolution, phase transformation, precipitation and shape memory behaviour were systematically investigated by multiscale correlative microstructural, differential scanning calorimetry analysis and thermomechanical analysis. Based on optimized heat treatments, ∼25 K phase transformation temperature windows and ∼90 MPa stress windows were achieved for the one-way shape memory effect. Solutionized annealing was found to be the most effective way to improve one-way shape memory degradation resistance, due to the reduction of defects and solid solution strengthening. One of the main findings of this study is that the heterogonous microstructures between hard intergranular Ti₂NiO_x and soft NiTi matrix, induced by solutionized annealing with subsequent aging, result in strain partitioning and enclosing the internal stress state, which was found to promote a pronounced two-way shape memory effect response. The results of this work provide in-depth knowledge on tailoring and designing functional shape memory characteristics via heat treatments, which contributes to expanding L-PBF NiTi application fields, such as biomedical implants, aerospace components, and other advanced engineering applications.

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.

Laser butt welding of thin metal sheets is a widely used fusion-based joining technique in industrial manufacturing. A comprehensive understanding of the complex transport phenomena during the welding process is essential for achieving high-quality welds. In the present work, high-fidelity numerical simulations are employed to investigate the influence of various symmetric and asymmetric welding configurations on the melt-pool behaviour in conduction-mode laser butt welding of stainless steel sheets. The analysis focuses on the effects of laser power density, heat source misplacement and different welding scenarios, including plates with a root gap, high-low mismatches, and dissimilar thicknesses, on the molten metal flow and heat transfer. The results show that advection is the dominant mechanism for energy transfer in the melt pool, and its contribution increases with higher laser power. The non-uniform temperature distribution over the melt-pool surface induces Marangoni shear forces, driving the flow of molten metal and leading to the formation of vortices and periodic flow oscillations within the pool. The effects of various types of asymmetries on the thermal and molten metal flow fields, as well as the process stability, are thoroughly examined and compared with symmetrical welding configurations. These comprehensive simulations provide valuable insights into the fluid flow dynamics and thermal field evolution during laser butt welding of thin metal sheets. The knowledge gained from this study can facilitate process optimisation and guide the improvement of weld quality in practical applications.