The grain boundary character distribution of an equiatomic CoCrFeMnNi high-entropy alloy (HEA) was statistically determined using electron backscatter diffraction. The face-centered cubic (fcc) HEA exhibits an exceptionally high fraction of the E3 coherent twin boundary, constituting 94% of the total E3 grain boundary (GB) population, which is nearly double the levels observed in Cu (51%) and Ni (61%). This reflects an increased thermodynamic preference for the lowest-energy state, driven by suppressed GB energy anisotropy and a deeper relative energy minimum at the coherent twin configuration compared to fcc elemental metals (Au, Cu, and Ni). Beyond E3 misorientation, high-entropy effects from configurational entropy and local lattice distortion further stabilize {111} symmetrical twist configurations, leading to significant population enrichment across multiple misorientations, including E7, E13b, and E21a. Ultimately, these findings reinforce the high-entropy grain boundaries (HEGBs) framework, providing a foundation for next-generation multifunctional high-entropy materials.

Non-equilibrium microstructures develop during the rapid solidification in laser directed energy deposition. The inevitable formation of eutectic phases, typically represented by the Laves phase, during the solidification of Nibased superalloys is generally detrimental to the alloy properties and may even induce cracking. Understanding the precipitation of such eutectic phases in relation to dendritic growth and solute segregation is particularly important for reducing cracking susceptibility and suppressing crack initiation. A multi-scale model was employed to simulate melt pool dynamics, dendrite growth, solute segregation, and eutectic phase precipitation during laser directed energy deposition of technological precipitation-strengthened Ni-based superalloys under varying energy inputs, considering low to high segregation alloys. The model accurately captures macro-scale melt pool flow heat transfer and micro-scale competitive dendrite growth, validated against experimental melt pool dimensions, primary dendrite arm spacing (PDAS), and eutectic phase morphology. More significantly, the alloy-specific undercooling parameter quantitatively governs the microstructural response across various Nibased superalloys: higher values of alloy-specific undercooling parameter enhance heterogeneous nucleation, columnar-to-equiaxed transition, and eutectic formation, whereas lower values favour columnar dominance and refined microstructures. These collective insights establish critical process-alloy-microstructure linkages essential for controlling cracking susceptibility.

Fusion-based additive manufacturing of high strength 2024 alloy is frequently limited by coarse columnar grains, microsegregation, and solidification defects, which jointly prevent the balance between strength and toughness. To address these limitations, a hybrid additive manufacturing strategy integrating directed energy deposition (DED) with cyclic interlayer friction stir processing (FSP) was proposed in this work. The hybrid process generated a multizone heterogeneous microstructure consisting of deposition zones, remelt zones, nugget zones, and thermomechanically affected zones, with spatial variations in grain structure, recrystallization behavior, dislocation density, texture, and precipitation state. Severe plastic deformation during FSP transformed the deposited coarse grains into ultrafine equiaxed grains in the nugget zone, where the average grain size was refined to 2.34 mu m, while cyclic reheating during subsequent deposition promoted non-uniform precipitate redistribution and microstructural transitions across adjacent regions. Compared with the as-deposited DED alloy, the hybrid-built material exhibited markedly improved tensile properties, with a yield strength of 272 MPa, an ultimate tensile strength of 349 MPa, and an elongation of 6.3%, along with reduced anisotropy. The property enhancement is associated with the synergistic contributions of grain-boundary strengthening, defect elimination, precipitation regulation, and hetero-deformation-induced back-stress strengthening enabled by the layered heterogeneous architecture. The present study provides mechanistic insight into heterostructure evolution under coupled thermo-mechanical conditions and offers an effective route for improving the strength-ductility balance of additive manufacturing precipitation-strengthened aluminum alloys.

Phase field simulations play a key role in the understanding of microstructure evolution in additive manufacturing. However, they have been found extremely computationally expensive. One of the reasons is the small time step requirement to resolve the complex microstructure evolution during the rapid solidification process. This paper investigates the possibility of using a class of stabilized time integration algorithms to accelerate such phase field simulations by increasing the time steps, based on a phase field model dedicated to simulating the solidification of 316L stainless steel during additive manufacturing, particularly in a regime where the solid-liquid interface is moving fast and there is absolute interfacial stability with negligible composition variations. The specific computational framework, incorporating the finite element method and the stabilized time integration algorithms, was developed. A theoretical analysis on energy stability was conducted, based on a revisited energy law derived for the phase field model. The numerical results confirmed that the proposed framework can effectively enforce the numerical stability and a decreasing energy requirement for the phase field simulations with at least two orders-of-magnitude larger time steps over conventional explicit methods. 2D and 3D phase field simulations have been conducted with relevant physical and kinetic parameters for 316L stainless steel. This computational framework can be easily adapted for different phase field models and open numerous opportunities for efficient phase field simulations.

Despite the advancements in laser directed energy deposition, challenges such as micro-segregation, formation of hard and brittle phases, and poor controllability over solidification microstructures persist due to the rapid melting-solidification transience, complexity of multi-physics metallurgical processes, and instability of multiple processing parameters. These issues impair the performance of deposited layers and hinder further development and broad application of the technology. Recently, electromagnetic field-assisted solidification has emerged as a promising method to tailor solidification structures and suppress defects. In this study, a novel pulsed electromagnetic field (PEMF) assisted laser directed energy deposition was investigated to tailor the microstructure of IN718 superalloy. Comprehensive analysis revealed that PEMF treatment significantly refined grain structures, reducing average grain size by 60.3% from 310 mu m to 123 mu m through enhanced columnar-to-equiaxed transition. Simultaneously, chain-like Laves phase was transformed into fine granular particles with reduced volume fraction under optimal parameters. The mechanical properties of the deposits were synergistically enhanced at room temperature, with yield strength and ultimate tensile strength increasing by 21.1% and 17.3%, respectively, due to combined strengthening from grain refinement, solid solution effects, and elevated dislocation density. High-temperature tensile tests also confirmed a significant improvement in strength. The high-fidelity simulation results indicate that PEMF application effectively reduced thermal gradients from 3.5 & times; 105 to 2.1 & times; 105 K/m while amplifying solidification rate fluctuations, thereby promoting equiaxed grain nucleation via enhanced constitutional undercooling. These microstructural alternations is achieved by the PEMF assisted laser directed energy deposition as a transformative approach for the IN718 superalloy additive manufacturing.

Conventional metallic bone implants often suffer from stress-shielding and limited mechanical adaptability, necessitating the development of biomimetic lattice structures with tunable properties. In this study, a novel Ti6Al4V bio-inspired lattice, termed RaLak, was proposed to mimic bone architecture. The corresponding lattices, with relative densities of 20% and 30% and unit cell sizes of 2-4 mm, were fabricated using laser powder bed fusion (L-PBF) technology. Through experimental investigations, a comprehensive analysis was conducted to evaluate the mechanical behaviour of the proposed lattice in comparison with conventional triply periodic minimal surface (TPMS) lattices. The results indicate that the RaLak lattice exhibits enhanced mechanical performance, achieving an improved balance between stiffness and energy absorption relative to gyroid and diamond TPMS lattices. In particular, the energy absorption of RaLak shows an improvement of approximately 50% compared to the gyroid lattice at 30% RD with a 4 mm cell size. This behaviour is attributed to its hybrid architecture, which promotes more uniform stress distribution and progressive deformation. Furthermore, modified Gibson-Ashby based models were developed to establish relationships between lattice geometry and mechanical response. The proposed design strategy enables effective tuning of mechanical properties, demonstrating strong potential for load-bearing biomedical applications.

Laser additively manufactured microscale metallic lattices show great potential for high-performance applications, yet trade-offs among geometric precision, structural integrity, and computational efficiency still persist. Here, we introduce a stereolithography file format-free (STL-free) hybrid toolpath generation method for laser-based powder bed fusion (PBF-LB) that synergizes implicit geometric modeling with optimized laser scanning strategy, overcoming these limitations. By circumventing traditional mesh-based workflows, our method directly translates implicit lattice geometries into laser toolpaths while precisely regulating energy deposition trajectories. This mesh-free process enables the fabrication of complex shell lattices with ultra-thin walls and enhanced surface quality. In addition to reducing memory usage and processing time by up to 90%, the method yields a synergistic enhancement in mechanical performance, notably improving both strength and toughness. By bridging computational design and fabrication, this framework enables the scalable production of high-performance microscale lattices and unlocks their potential for industrial applications.

Sliver is a casting defect formed during solidification of nickel-based single-crystal superalloys, which degrades the high temperature properties of superalloys. However, the formation mechanism of sliver cannot be fully deduced from the solidified microstructure alone and thus requires clarification. In this work, a multi-scale model of thermo-mechanical deformation was developed to investigate the formation of sliver. Results reveal that casting contraction-driven dendrite deformation induces significant plastic strain localisation, which is inferred as a key step to dendrite fragmentation and formation of sliver defects. Two distinct dendrite deformation mechanisms during sliver formation are identified: The first mechanism is contraction-driven plastic deformation of dendritic trunks along 45 degrees, and the second mechanism arises from constraint from mould protuberances causing large-angle bending of dendrites. This work sheds light on the cross-scale mechanisms of dendrite deformation and fragmentation for sliver defect formation, providing a basis for defect mitigation.

In-situ alloying additive manufacturing (AM) has emerged as a versatile and efficient strategy for the rapid development of new alloys. Notably, when refractory particles are introduced during the in-situ alloying AM process, the resulting partially-melted particles can significantly alter the solidification behaviour and ultimately reshape the microstructural features of the fabricated alloy. However, the mechanisms by which these partially-melted particles influence microstructural evolution remain insufficiently understood. In this study, a representative binary titanium alloy, Ti–30Ta, characterised by a pronounced mismatch in physical properties, was fabricated via in-situ alloying AM. The evolution of microstructure and mechanical properties under conduction, transition, and keyholing melt-pool modes was systematically investigated. In the transition mode, partially-melted Ta particles suppressed α″-martensite formation in Ta-rich regions (~42wt.% Ta) while promoting brittle α′-martensite, leading to limited ductility. In contrast, under the keyholing mode, partially-melted Ta particles induced dislocation pinning and facilitated the formation of a high α″ phase fraction, thereby enhancing strength–ductility synergy. Thermal fluid flow simulations reconstructed melt-pool evolution under varying processing conditions, capturing the coupled effects of elemental diffusion, melt-pool morphology, and particle–matrix thermal interactions. This integrated experimental–computational study unveils the mechanistic role of partially-melted particles as active modulators of local thermal and mechanical fields, providing a new understanding of their beneficial effects under optimised conditions. The findings challenge the conventional goal of achieving full compositional homogeneity in in-situ alloying AM and offer a new design paradigm for heterogeneity-engineered, high-performance Ti-based alloys.

Triply periodic minimal surface (TPMS) graded lattices have attracted significant interest for biomedical applications owing to their structural efficiency and bone-mimicking mechanical properties. This study introduces a novel design methodology for spatially grading Ti-6Al-4V TPMS lattices, with grading orientations classified as one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D), and fabricated by laser powder-bed fusion (PBF-LB). The primary aim was to evaluate the influence of gradient direction on mechanical response and energy absorption through quasi-static compression testing. The experimental results highlight that the grading orientation plays a crucial role in tailoring lattice performance. At 30 % RD, the 3-D graded gyroid exhibited superior properties with an elastic modulus of 3.36 f 0.14 GPa and yield strength of 107.93 f 2.8 MPa, outperforming the 1-D graded gyroid (2.48 f 0.11 GPa modulus; 78.94 f 2.2 MPa yield strength). Enhanced energy absorption was also observed for the 3-D grading technique, confirming the advantage of multidirectional density variations. An exception was noted for the 3-D graded diamond at 20 % RD. Overall, graded TPMS lattices enable structural flexibility and mechanical performance comparable to natural bone. The findings provide important insights for designing orthopaedic implants. The developed grading technique has great potential in bone replacement biomaterials.

Understanding vaporization phenomena in laser powder bed fusion (LPBF) additive manufacturing has proven challenging; the links between laser‐induced metal vaporization, rate of elemental loss, and composition irregularities remain unclear. Here, the vapor plume composition and preferential vaporization effect is quantified during LPBF, using in situ 1 kHz laser‐induced breakdown spectroscopy with correlative X‐ray synchrotron radiography, multi‐physics simulations, and energy dispersive X‐ray spectroscopy. It is demonstrated that vaporization increases under keyhole mode, and preferential vaporization causes elemental loss rates of Ni ≈ Fe > Cr > Mo in a Ni‐based superalloy, IN625. It is found that the melt pool temperature (T ≈2300 K) can be approximated by cross‐referencing vapor pressures, and Raoult's law inadequately describes preferential vaporization. Three simulation approaches are compared to show that introducing temperature‐dependent thermophysical properties improves model predictions. The insights into the vapor dynamics of laser‐processed IN625 enhance the understanding of compositional changes and elucidate methods to optimize simulations.

In-situ alloying is a promising material processing method for the direct fabrication of alloys with arbitrary elemental content. In-situ alloying of biomedical titanium-niobium alloys using laser powder bed fusion (LPBF) necessitates a fundamental understanding of the underlying physics governing solute transport, which remains a challenge in addressing the 'porosity-segregation dilemma'. This work developed a thermal-fluid-solute model integrated with actual solidification paths for the LPBF process to advance the in-depth understanding of melt pool dynamics, solute mixing, and elemental segregation mechanisms during in-situ alloying of Ti-Nb alloys. It reveals the influence of solute content and energy density on the melt pool dynamics. Higher Nb fraction up to 41 wt.% results in a deeper and shorter melt pool with a stronger flow, and higher energy density leads to a larger melt pool with a liquid metal flow circuit. Moreover, Nb-rich bands parallel to the melt pool boundaries are formed due to solute partition and the strong flow through the mushy zone. Additionally, Nb-rich ridges are formed between tracks due to insufficient heat at the track edges and the fluid buildup effect. During the LPBF of the second layer, a 'wide-narrow-wide' melt pool is formed with vertical stratification of Nb-rich and Nb-lean.

316L stainless steel (SS316L) is widely utilised due to its excellent corrosion resistance. However, its yield strength is relatively low, ranging from 170 to 200 MPa, which limits its engineering applications as a structural material. Reinforcing the metal with ceramic nanoparticles has been considered an effective approach to improve the strength of SS316L. However, the synthesis of such nanocomposites has been a long-term challenge with the conventional casting process due to the high tendency of agglomeration of nanoparticles. Laser powder bed fusion (L-PBF) additive manufacturing (AM) provides an opportunity to overcome this issue for fabricating stainless nanocomposites by moderating the powder constitution. This study demonstrates the preparation of a dense SS316L matrix TiN-WC nanoparticles reinforced nanocomposite with enhanced strength via L-PBF. Materials characterisation indicates the distribution and dispersion homogeneities of the TiN-WC nanoparticles in the SS316L matrix. A thermal computational fluid dynamics model explains the melt pool dynamics and temperature distribution of the composite powder bed. The yield strength, ultimate tensile strength, and elongation to fracture of the nanocomposites are over 700 MPa, 1000 MPa, and 30

Directed energy deposition (DED) laser additive manufacturing (AM) is a promising technique for building complex components and performing repair applications. However, large defects can form through coalescence of argon bubbles from the feedstock powder, potentially reducing end-component mechanical performance. Here, we used correlative high-speed synchrotron X-ray and infrared imaging, coupled with multiphysics modelling to develop a strategy to control defect formation. We demonstrate that the bubble dynamics can be controlled by appropriately modulating the laser power, temporarily disrupting the Marangoni flow, enabling bubble release. The bubble control mechanisms discovered here provide a way to achieve defect-lean AM.

In the laser powder-bed fusion (L-PBF) of Ti-6Al-4 V (Ti64) alloy, cracking defects often caused by lack-of-fusion (LoF) defects cannot be avoided. These defects strongly impact the structural stability of Ti64 implants. In this work, the influence of LoF defects on the tensile and fatigue behaviours of Ti64 implants at different energy densities is evaluated. Three primary approaches were used to characterise the LoF defects in the 10 mm gauge volume of near-net-shaped produced hourglass samples: Image-J software, scanning electron microscopy (SEM), and X-ray computed tomography (mu -CT). A low-cycle fatigue (LCF) test at a stress ratio of -1 and an extensive fractography study of the fractured Ti64 samples were used to investigate the LoF porosity-induced cracking and fatigue effect. The tensile properties of Ti64 samples processed by the L-PBF are generally more affected by microstructural changes resulting from variations in energy density than by LoF defects. A fine microstructure enhances tensile strength, while a coarser microstructure favours ductility. This is evident in samples LPBF-AD and LPBF-BD1, which demonstrate finer and coarser microstructures but show the lowest (8.31 %) and highest ductility (17.58 %), respectively. The best and worst fatigue performance samples were LPBF-AD (600 MPa) and LPBF-BD2 (200 MPa), respectively, and LoF defects significantly impacted this finding. Moreover, it has been demonstrated that cracks originate and propagate faster from the

Laser powder bed fusion (LPBF) of Polyamide 12 (PA12) using a near-infra-red (NIR) beam is largely unexplored; therefore, the beam-matter interaction, evolution mechanisms of the melt pool and defects remain unclear. Here, we employed a combination of in situ synchrotron X-ray imaging, ex situ materials characterisation techniques, and high-fidelity process simulations to study these behaviours during LPBF of PA12. Our results demonstrate that the NIR absorption of PA12 can be improved by 600 times through powder surface modification with C, P and Al species. In situ X-ray images reveal that the PA12 powders undergo melting, viscous merging, volume expansion, warping, solidification, and shrinkage before forming a solid track. Our results uncover the bubble evolution mechanisms during LPBF of PA12. During laser scanning, the high-energy laser beam produces organic substances/vapours which are trapped inside bubbles during viscous merging. These bubbles continue to shrink due to vapour condensation as the polymer cools under a cooling rate range of 200 - 600 K s-1. Using the collected data, we have developed a data-driven bubble shrinkage criterion to predict the bubble shrinkage coefficient using the bubble half-life, improving the build quality of LPBF polymeric parts.

Limited work exists on the potential for microstructural control during Laser Beam Powder Bed Fusion (LB-PBF) in alpha+ R Ti-alloys, particularly the impact of the process parameters on the prior-R grain size and its subsequent influence on the strength-ductility balance. Through controlling the solidification conditions, this work explores the role of the heat input parameters and laser scanning strategies in controlling the prior-R grain size and consequently the mechanical performance in LB-PBF-processed Ti-6Al-4V. Electron backscattered diffraction (EBSD) was used in combination with R-phase reconstruction to elucidate the effect of the heat input and scanning strategies on the microstructure, combined with coupled thermal microstructural modelling. The study highlights the possibility for controlling the prior-R grain size through manipulating the solidification conditions, ultimately resulting in enhanced ductility in refined structures. Moreover, whilst the majority of LB-PBF literature relies on the energy density concept to express the heat input, its significance when it comes to the microstructural characteristics was found to be minimal. Conditions with similar energy densities yielded different prior-R grain sizes, and to a lesser extent variations in the alpha '/alpha-lath size following a 650 & ring;C/2hrs stress-relief treatment. Similarly, the use of large island scanning strategies resulted in finer prior-R grain sizes, and ultimately higher ductility following stress-relief treatment. The study highlights the possibility for controlling the microstructure in LB-PBF of Ti-alloys, both to improve the mechanical performance and design tailored microstructures.

NiTi alloys fabricated via additive manufacturing (AM) often suffer from coarse grains, brittle intermetallic phase accumulation, and limited control over phase transformation behavior, resulting in compromised performance and impeded functional applications. To address this challenge, a generalisable strategy for intermetallic modulation and functional gradient design has been proposed and validated through directed energy deposition (DED). By employing multiple deposition modes (Mixed NiTi, Graded Ti/Ni, and Graded Ti/NiTi), tailored microstructure gradients were achieved. This approach enabled spatial control over the formation of key intermetallics, resulting in simultaneous enhancement of martensitic transformation behavior and mechanical performance (nanohardness, compressive strength). A coupled simulation-experimental analysis revealed universal mechanisms of temperature evolution and solute transport in melt pools, which underlie intermetallic development during AM. The findings contribute a broadly applicable methodology for designing gradient architectures in metallic systems, offering new avenues for tailoring functional and structural performance.

Phase field simulations play a key role in the understanding of microstructure evolution in additive manufacturing. However, they have been found extremely computationally expensive. One of the reasons is the small time step requirement to resolve the complex microstructure evolution during the rapid solidification process. This paper investigates the possibility of using a class of stabilized time integration algorithms to accelerate such phase field simulations by increasing the time steps, based on a phase field model dedicated to simulating the solidification of 316L stainless steel during additive manufacturing, particularly in a regime where the solid-liquid interface is moving fast and there is absolute interfacial stability with negligible composition variations. The specific computational framework, incorporating the finite element method and the stabilized time integration algorithms, was developed. A theoretical analysis on energy stability was conducted, based on a revisited energy law derived for the phase field model. The numerical results confirmed that the proposed framework can effectively enforce the numerical stability and a decreasing energy requirement for the phase field simulations with at least two orders-of-magnitude larger time steps over conventional explicit methods. 2D and 3D phase field simulations have been conducted with relevant physical and kinetic parameters for 316L stainless steel. This computational framework can be easily adapted for different phase field models and open numerous opportunities for efficient phase field simulations.