Liquid metal embrittlement (LME) during resistance spot welding (RSW) of twinning induced plasticity (TWIP) steel is primarily driven by stress-assisted grain boundary (GB) diffusion of zinc (Zn). Although GB diffusion is widely recognized as the dominant LME mechanism, experimental quantification is challenging due to resolution limitations. This study characterizes Zn diffusion in TWIP steel during RSW by conducting energy dispersive X-ray spectroscopy (EDS) line scans ahead of LME cracks in both the rolling direction (RD) and normal direction (ND) over weld times from 700 to 1700 ms. Results reveal that Zn diffusion distance increases with weld time, with consistently higher diffusion in the ND. To compare experimental measurements with diffusion theory, an FEA simulation based on Fick’s law was employed to approximate bulk Zn diffusion under varying temperatures. The model predicts Zn diffusion trends consistent with experimental observations. Although the diffusion distance predicted in the simulation exceeds measured values, directional trends are accurately captured. A theoretical framework to compare GB and bulk diffusion was proposed. GB diffusion distance of Zn is estimated to be approximately 30 times greater than bulk diffusion, establishing a quantitative link between weld time and Zn diffusion during RSW of TWIP steel.

This study explores Wire Arc Additive Manufacturing (WAAM) as a repair method for forging tools, comparing it to conventional H11 tool steel. Laboratory tests were conducted on four samples obtained from different WAAM layers, which are representative of the respective layer positions for hardness and wear tests at room temperature and 500 °C. Chemical composition analysis was performed to verify compliance with tool steel standards, and evaluations included hardness testing at both temperatures, tensile strength assessment, wear resistance measurements (ball-on-disc mode), and hardness mapping to verify layer consistency. The WAAM material exhibited wear factors on the order of 10⁻⁶ mm³/Nm, below typical literature values reported for H11 tool steel, and showed a clear correlation between layer hardness and wear response. Industrial validation through hot forging trials demonstrated that WAAM-repaired tools achieved 3000 cycles compared to 2500 cycles for conventionally repaired tools, with approximately 60% shorter repair time and 21% lower filler material consumption. These results confirm WAAM as a viable and superior method for cladding and repairing forging tools, offering improved durability, higher efficiency, and reduced use of expensive alloying elements in industrial applications.

This study investigates the microstructural evolution, crystallographic texture, and mechanical properties of a nickel-aluminium bronze (NAB)/410 NiMo martensitic stainless steel bimetallic structure fabricated via wire arc additive manufacturing (WAAM). The work provides a detailed assessment of the interfacial characteristics, phase transformations, and texture development across both alloys, as well as their impact on mechanical behaviour. Microstructural characterisation using optical microscopy, SEM, EBSD, and TEM revealed that the stainless steel region predominantly consists of martensite with minor fractions of δ-ferrite, α-ferrite, and retained austenite. A distinct Fe-rich interfacial layer was identified, from which columnar dendrites grow epitaxially into the NAB side. The NAB region exhibits α-phase grains with various k-phase precipitates whose distribution and morphology evolve with build height due to thermal cycling and dilution effects. Evidence of liquid metal embrittlement (LME) was observed at the interface. Texture analysis indicated a weak crystallographic texture in the stainless steel due to solid-state transformations, whereas the NAB exhibited a more textured structure, especially <101>//BD. Despite the presence of LME cracks at the interface, tensile testing consistently resulted in fracture within the NAB region, away from the interface, demonstrating effective load transfer across the interface under uniaxial tensile loading parallel to the building direction. The bimetallic structure achieved an ultimate tensile strength of 587 MPa and total elongation of ∼12%.

Liquid metal embrittlement (LME) during resistance spot welding (RSW) of Zn-coated twinning-induced plasticity (TWIP) steel results in intergranular cracking driven by the interaction of liquid Zn, tensile stress, and the grain boundary (GB) network. This study investigates how GB misorientation and orientation relative to the electrode force loading axis influence the LME crack path across six weld times from 700 ms to 1700 ms. A comparative framework was developed in which, at each triple junction along the LME crack, the misorientation and angle relative to the loading axis of the chosen LME grain boundary are evaluated against those of the unchosen LME-free grain boundary. Σ3 coherent twin boundaries were LME-free at all weld times regardless of their orientation to the stress axis. Non-twin coincident site lattice (CSL) boundaries (Σ5 to Σ41) were predominantly LME-free at low weld times but progressively became LME GBs at high weld times, when their stress normalisation factor exceeded that of the competing boundary. The fraction of triple junctions where the LME grain boundary had the higher stress normalisation factor increased from 58% at 700 ms to 91% at 1700 ms, while the preference for the higher-misorientation-angle boundary declined from 73% to 56% over the same range. An LME susceptibility index incorporating grain boundary energy, temperature-dependent Zn diffusivity, and stress alignment is proposed as a framework for predicting crack path selection and guiding grain boundary engineering strategies (GBE) to reduce LME in resistance spot welded TWIP steel.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.