The development of high-performance steels characterized by the synergistic combination of high strength, large ductility and low density is essential for energy conservation and the mitigation of global carbon emissions. Inspired by the high entropy alloy design strategy, the quinary system Fe-Mn-Al-Ni-C steels, as a novel class of compositionally complex steels (CCSs), have emerged as a prominent research frontier. We provide a comprehensive review of the alloy design strategies in this field, delineating the evolution from traditional Fe-Mn-Al-C lightweight steels to advanced CCSs. We systematically examine the microstructural transition toward dual- nanoprecipitation enabled by increased compositional complexity. Furthermore, the underlying strengthening and strain-hardening mechanisms, specifically dual-precipitation strengthening, twinning-induced plasticity (TWIP), and the shearing of brittle B2 intermetallics, are critically reviewed. Finally, we propose critical future research directions aimed at further enhancing the mechanical property and industrial viability of CCSs.

The fabrication of complex hollow structures with variable cross-sections and curvatures remains challenging due to the limited deformability and poor demolding performance of conventional rigid molds. This study proposes a novel strategy that integrates 3D printing of continuous fiber-reinforced smart mold with a dimension-reduction wire-drawing demolding method. A photothermal dual-crosslinked polymer network was developed to enable reversible stiffness modulation, ensuring high rigidity during molding and flexibility during demolding. To optimize fabrication performance, a machine learning framework based on gradient boosting regression was employed to model and analyze the influence of key printing parameters on both mechanical strength and dimensional accuracy. Using this approach, smart mold with a bending strength of 620.71 MPa and a printing error of 2.46 % were successfully fabricated. The method was further validated through the forming and demolding of representative geometries, including dumbbell-shaped and variable-section components. Results confirm the feasibility and robustness of the approach under extreme forming conditions. This digitally driven, material-process-structure integrated solution offers broad application potential for the precision manufacturing of complex hollow composite structures in solid rocket motors, aerospace engineering, and architectural fabrication.

This study investigates the multi-factor coupling effects on the deposition quality of 3D-printed continuous carbon fiber reinforced thermosetting epoxy composites. Utilizing Response Surface Methodology (RSM) combined with a Box-Behnken Design (BBD), the synergistic impacts of printing temperature, speed, spacing, and layer height on flexural properties were systematically evaluated. The established high-precision quadratic regression models (R-2 > 0.98) revealed that internal porosity and fiber damage are predominantly governed by strong "saddle-shaped" coupling effects: the geometric matching between printing spacing and layer height, and the rheological compensation between temperature and speed. By correlating 3D response surfaces with scanning electron microscopy (SEM) and micro-computed tomography (CT) analyses, the physical mechanisms of defect formation under parameter mismatches were systematically elucidated. Multi-objective optimization determined a global optimal process window (printing temperature (T) = 120 degrees C, printing speed (V) = 500 mm/min, printing spacing (S) = 1.3 mm, and layer height (H) = 0.4 mm). Validation experiments demonstrated excellent predictive accuracy for flexural strength (604.97 MPa) with a relative error of only 2.1%. Notably, this optimal configuration minimized the internal void content to an ultra-low level of 2.3% and effectively suppressed premature delamination, resulting in an actual flexural strength increase of 27.7% compared to the unoptimized baseline. Meanwhile, the observed deviation in flexural modulus was theoretically linked to deposition-induced micro-level fiber waviness.

Combining topology optimization with continuous fiber 3D printing offers a promising route to lightweight design of continuous fiber-reinforced polymer (CFRP) composites. However, conventional methods aligning fibers with principal stresses often cause fiber discontinuities and stress mismatches at biaxial junctions, limiting overall structural integrity. In biological junctions such as wood branches and bamboo nodes, fibers provide superior strength and toughness through interwoven and interlocking arrangements. This study proposes a synergistic framework for the design and fabrication of CFRP structures. The framework integrates dual-stressdriven Solid Orthotropic Material with Penalization (SOMP) topology optimization, stress-guided structural decomposition and path reorganization, and a bio-inspired woven junction printing strategy. By matching biaxial stresses, orthogonal fiber orientations are assigned and converted into manufacturable continuous toolpaths. At junctions, the bio-inspired sinusoidal woven fibers create macroscale interweaving with microscale interlocks. These features enhance stress transfer and shear resistance. The effectiveness of the framework was validated using the Messerschmitt-Bo & uml;lkow-Blohm (MBB) beam. Compared with the non-crossing SOMP-optimized MBB baseline, the laminated orthogonal junctions increased the specific load and specific stiffness by 46.18% and 37.14%, respectively. Bio-inspired woven junctions achieved superior enhancements of 96.04% and 68.27%. Strain and failure analyses revealed that the woven bundle interlocks establish stress-transfer pathways. This mechanism reduces stress concentrations and prevents junction delamination and fracture. Overall, this framework provides a practical design-to-manufacture route for high-performance 3D printed CFRP composite structures.

To address the tedious assembly of traditional beam-rib UAV wings and meet the urgent demand for integrated and efficient manufacturing of complex structures in fixed-wing UAVs, this study, inspired by the longitudinal and transverse vein distribution of dragonfly wings, proposes a novel variable-stiffness integrated UAV wing skeleton structure. Combined with the nonlinear load distribution along the wing span, a three-stage gradient variable-stiffness configuration from wing root to wing tip is designed. The main load-bearing wing beams and cross ribs form an integrated network load-bearing architecture, simulating the multi-path collaborative force transmission mechanism of dragonfly wings to optimize load transfer pathways. Continuous fiber-reinforced composite (CFRP) 3D printing technology is employed to achieve the integrated manufacturing of the bionic wing skeleton with ±45° cross-fiber trajectories. Performance verification is conducted through quasi-static tests and field flight tests. The results show that the measured wingtip deflection is 20.07% lower than the simulated value, and the structural stiffness meets the requirements of reliability design. The study confirms that the bionic variable-stiffness design inspired by dragonfly wing veins, combined with continuous fiber 3D printing technology, can efficiently achieve lightweight, high-performance, and integrated manufacturing of UAV wings, providing important reference and technical support for the engineering application of bionic structural design and additive manufacturing in the aerospace field.

The development of oxide-dispersion strengthened (ODS) alloys for high-temperature applications is often limited by microstructural inhomogeneity of powder feedstock. This work demonstrated a novel ultra-fast melt-quenching process, termed Plasma Spray-Quenching (PS-Q), for the mass production of ODS AlxCoCrFeNi powders. Oxide dispersoids formed in-situ within the powders. The oxide composition and size were governed by the Al content: Al-rich compositions promoted the formation of fine Al₂O₃ dispersoids (∼65 nm), while lower Al levels led to coarse Cr2O3 or (Al, Cr)2O3 particles (>400 nm). A “bait effect” mechanism in multi element systems is proposed to explain this instantaneous formation process involving selective oxidation, inward transport and nanoscale stabilization. An optimal dispersoid distribution density (7.92 /μm3) and average size (83 nm) were achieved at 30 at% Al. At an equivalent Al concentration, significant ODS effect was confirmed, reaching a peak hardness of 5.24 GPa. After 100 h of isothermal oxidation at 900 °C, the optimized powder exhibited a low mass gain of 3.9 mg/cm2 and a TGO growth rate of 6.31 nm/h, an order of magnitude lower than commercial NiCrAlY powder. This superior performance was attributed to the dispersoids effectively impeding ionic diffusion and suppressing interfacial softening. The PS-Q process provides a viable route for producing high-performance ODS alloy feedstock with tailored microstructures.

Welding speed is a critical parameter in CMT+P-based WAAM of 205C aluminum alloy, which is used in aerospace structures such as the XX-BQ missile casing. This study investigates how welding speed affects residual stress and warping in deposited components. Four welding speeds were tested under otherwise constant parameters. Warping was measured via 3D laser scanning, and residual stress was evaluated using X-ray and LCR ultrasonic methods. Results indicate that welding speed governs heat input per unit length and significantly influences the magnitude, symmetry, and variability of residual stress. Warping behavior closely follows the stress evolution. At 10.0 mm/s, both residual stress and warping reach their minimums, with the lowest warping symmetry index (WSI), indicating the most balanced thermo-mechanical response. These findings establish the linkage between welding speed, heat input, and structural distortion, and guide stress mitigation in high-integrity WAAM aluminum alloy components.

This paper presents a novel path generation method designed to reduce conflicts between excessive gaps or overlaps and laying defects on complex surfaces. First, a defect model is established for complex surfaces. Then, a mathematical model is formulated to calculate laying gaps based on path laying angles. By integrating these two models and introducing gap and angle optimization variables, a path optimization method is developed, constrained by both laying gaps and defects. The accuracy of the mathematical model is validated across various types of surfaces. Results show that, with appropriate parameter selection, the model’s maximum prediction error is 0.04 mm. A comparative study is conducted with a conventional path generation method on a complex surface. The laying gap range is reduced from −4.9 mm to 12.3 mm to an engineering-acceptable range of 0 mm to 2.5 mm, with no observed laying defects, thereby demonstrating the effectiveness of the proposed method.

The effects of biochar on Bermuda grass growth and mechanical properties of vegetated soil were investigated in this study. Six groups of soil column tests were conducted, including two degrees of compaction (DOC) (70% and 90%) and two types of biochar content (5% and 10% by soil dry weight), with two groups of bare soil serving as a reference (soil used in the test was classified as silty sand with gravel, i.e., SM). It was found that biochar increased the effective cohesion by up to 70% and slightly enhanced the effective internal friction angle while mitigating the detrimental effects of wetting–drying cycles, with the effective cohesion and friction angle retaining up to 73% and 99% of their initial values, respectively. Root biomass initially increased and then decreased as biochar content increased, particularly at a low degree of compaction of soil (i.e., 70% DOC was two times that of 90% DOC). The effective cohesion of intact biochar–root–soil initially increased up to 23% (at the biochar content of 5%, 90% DOC) and then decreased as biochar content increased, regardless of DOC. At the optimal biochar content (5%), the effective cohesion and internal friction angle of rooted soil were 1.4 and 1.1 times greater at low DOC (70%). For the remolded biochar–root–soil composite, at a high degree of compaction (90% DOC), the effective cohesion increased with the increase in root and biochar content. For a given root content, the shear strength of the remolded biochar–root–soil mixture was higher than that of intact biochar–root–soil (i.e., the shear strength of intact soil at 5% of biochar content was 87% of remolded soil), suggesting that the remolded soil mixture overestimated the biochar–root–soil strength. Generally, the present study demonstrates that a 5% biochar addition is optimal for enhancing plant root growth and soil strength, particularly under low compaction. Biochar significantly improves the mechanical performance of root–soil composites and mitigates the degradation of soil strength under wetting–drying cycles.

The low solubility of interstitial carbon in traditional metal matrices restricts the in-depth exploration of interstitial solid solutions. However, supersaturated solid solution films could be obtained by the rapid quenching effect of magnetron sputtering. In this study, (Al0.5CoCrFeNi)Cx films were deposited by co-sputtering Al0.5CoCrFeNi and carbon targets. The evolution of microstructure and properties under different carbon target powers were investigated. Remarkably, carbon-supersaturated films exhibited a maximum carbon content of 22 at% without forming carbides. Carbon perturbed the random distribution of HEA elements, leading to local chemical ordering and the formation of FCC2 phase (Al-based solid solution). The crystallinity of FCC1 and BCC increased first and then decreased. Various elements displayed localized clustering behavior. Appropriate supersaturated carbon increased the hardness and modulus to 7.97 GPa and 173.5 GPa, respectively. Also, the supersaturated carbon promoted amorphization and densification, resulting in excellent corrosion resistance, with a maximum corrosion potential of -0.022 V. This study updates the understanding of the influence of carbon on the microstructure and properties of HEA supersaturated solid solutions.

Continuous fiber 3D printing enables complex thermoset composite structure fabrication by monofilament deposition and layer-by-layer stacking. However, this process produces weak interfaces between layers and filaments, increasing materials' anisotropy. This study proposed an interlayer and inter-line simultaneous enhancing method through printing a core-shell structured filament. This filament was prepared using a shell consisting of polyetherketone-cardo (PEK-C) particle-toughened matrix and a core containing dissolved PEK-C toughened matrix with continuous fibers. Microstructural observations demonstrated that the toughened particles were regionally distributed in the interlayer and inter-line, simultaneously strengthening dual interfaces. Experimental results show that the interlaminar and the inter-line shear strengths were increased by 21.44 % (i. e., 87.63 MPa) and 58.56 % (i.e., 65.61 MPa), respectively. The difference between the interlayer and inter-line shear strength was reduced by 28.46 %, indicating an improvement in anisotropy. This simultaneous improvement further enhanced the impact resistance of the 3D-printed composites, reducing the damaged area by 25.2 %. Fracture studies showed that the regionally distributed microparticles and the nanoscale thermoplastic phases formed by dissolved PEK-C form a multiscale toughening system, enhancing the interlayer and inter-line interfaces. This study's core-shell filament printing strategy and the regionalized enhancement method offer a pragmatic solution to the multi-interface problem of 3D-printed composites.

A ply surface modeling and path planning approach is presented to reduce placement defects in complex composite structures with large and variable thickness. The mechanisms of thickness-induced defects are analyzed through mathematical modeling. An algorithm based on the fast marching method (FMM) is developed to generate ply surfaces for complex-geometry components with large and variable thickness. This ply surface modeling is integrated with a mesh-based path planning algorithm designed to minimize defects on thick and variable-thickness structures. Simulation and experimental results confirm significant improvements: automated fiber placement (AFP) trials on a spar show that 45 degrees ply gaps decrease from 3.21 to 1.45 mm, satisfying tolerance requirements, and all wrinkle defects are eliminated. This work provides an effective solution for low-defect AFP manufacturing of complex, thick composite components.

Complex curved composite components often rely on multiple reference curve algorithms for path planning in automated fiber placement. However, the reference curves are typically manually drawn. Moreover, designing the reference curves follows an iterative planning-analysis-improvement process, which can be inefficient. A new approach for the automatic pre-analysis and optimized generation of reference curves for fiber placement is proposed in this paper to enhance the efficiency of reference curve analysis and generation. Firstly, a pre-analysis algorithm for reference curves based on triangular meshes is proposed. This algorithm analyzes the theoretical geodesic curvature and angular deviation of the path before its planning. Subsequently, a comprehensive evaluation index for reference curve generation is formulated based on the pre-analysis algorithm, and the reference curve is optimized using genetic algorithms. The results demonstrate that the pre-analysis algorithm accurately computes the steering radius distribution of the path. Areas with over-limit steering radius can be eliminated while maintaining angular deviations within 10° by utilizing optimized reference curves for path planning.

4D printing of continuous fiber-reinforced shape memory polymer (SMP) composites is a promising technology that has emerged in recent years. Nonetheless, combining superior load-bearing capacity with fast-response shape reconstruction in intelligently-driven structures is still a challenge due to the inferior thermal conductivity and mechanical properties of the composites along the transverse and thickness directions of the continuous fibers. In this study, a multi-scale 4D printing strategy for 3D carbon medium-reinforced thermosetting SMP (Ts-SMP) is proposed. This 3D reinforcement structure is realized by graphene synergized with continuous carbon fibers (GR-CCF) to provide multidirectional thermal conductivity and load-bearing paths for the composites. The reinforcement mechanism of GR-CCF for Ts-SMP is systematically investigated. The GR-CCF reinforced Ts-SMP composites exhibited superior performance, with a bending strength of 676.99 MPa, shape fixation ratio, and shape recovery ratio of 98.62% and 99.07%, respectively, and the local shape response reconfiguration within 6 s under near-infrared light excitation. This study provides a reference for the integrated fabrication of high-performance Ts-SMP composites for complex structures. The composites offer prospective applications in space deployable antenna locking hoops, flexible solar sail support ribs, and optically driven robot actuators. In this study, a multi-scale 4D printing strategy for graphene-continuous carbon fiber synergistically reinforced thermosetting shape memory polymer composites is proposed. This material demonstrates multifunctional integration, such as superior load bearing, and fast response shape reconfiguration. Their possible applications in locking loops and energy locking of spatially scalable structures are explored. image

The microstructural evolutions and variations in mechanical performance of electron beam welded (EBW) Ti-6Al-4V (Ti64) alloy have been investigated. The effects of heat treatment on the microstructure of welded samples have been studied after post-welding solution treatment and ageing. The martensitic phase α' has been confirmed using transmission electron microscopy (TEM). Electron backscatter diffraction (EBSD) has been used to investigate the phase and grain morphology. Results showed that the martensitic α′ phase coarsened, the size of heat-affected zone (HAZ) changed and grains in the base materials (BMs) had grown after the post-weld heat treatments (PWHT). The tensile behaviour of electron beam welded Ti64 has been investigated using in situ tensile testing monitored by optical microscopy. The deformation and failure were directly revealed during the in situ tensile process. Results showed that the EBW Ti64 samples have different failure locations after receiving different post-weld heat treatments. The relationship between the post-weld heat treatments, microstructural evolution and mechanical properties of EBW Ti64 were investigated. Thermodynamic databases were used to predict mechanical properties—including the yield strengths—of the titanium alloy for different grain sizes, representing different post-weld heat treatment operations, and these were embedded into a finite element modelling framework to simulate the tensile testing specimens to understand the mechanical fields experienced such as stresses and strains, just prior to failure.

The coarsening behavior and strengthening effect of L12-Ni3(Ti,Al) precipitates in a face-centered-cubic (FCC) (FeCoNi)92Al2.5Ti5.5 high entropy alloy have been systematically investigated. The coherent L12 precipitates, uniformly distributed throughout the FCC matrix, consistently retain a spherical shape. The coarsening rate coefficient of precipitate is determined by employing the Philippe-Voorhees (PV) model, suggesting excellent thermal stability. Furthermore, the elemental partitioning and compositional evolution of the L12 precipitates is analyzed by atom probe tomography, which identify aluminum (Al) as the slowest diffusion species during the coarsening process. In addition, the precipitation strengthening effect is quantified to ascertain the optimal size of the precipitates. Our study enhances the understanding of precipitate coarsening in high entropy alloys, presenting valuable insights into their thermal stability and mechanical properties.

Precipitates are crucial for crafting mechanically strong metallic materials. In this work, we report the dislocation cutting of B2 (ordered body-centered cubic) nanoprecipitates, typically considered nonshearable intermetallics, in a lightweight compositionally complex steel during cryogenic tensile loading. Shearing is enabled by the high strength level for dislocation glide within the austenitic matrix, attributed to the substantial strengthening from subnanoscale local chemical ordering zones and the pronounced solid solution strengthening from the multiprincipal elements in the matrix. This mechanism not only harnesses the intense strengthening and strain hardening provided by otherwise impenetrable brittle nanoprecipitates but also introduces ductility through their sequential shearing with ongoing deformation. Our steel thus showcases ultrahigh cryogenic tensile strength up to 2 gigapascal at a remarkable tensile elongation of 34%. This study reveals a new strategy for designing high-performance structural materials.

To improve the bending and interlayer properties of hybrid-fiber-reinforced composites (HFRCs), a functionally graded hybrid (FGH) strategy is proposed inspired by the graded structure of bamboo. Specimens were prepared by utilizing the continuous-fiber 3D-printing manufacturing process, achieving graded changes in the fiber content and properties of the composites between and within layers. The differences in the mechanical properties and failure mechanisms of traditional interlayer HFRCs (IHFRCs) and functionally graded HFRCs (FGHFRCs) were comparatively analyzed. The mechanical-property test results demonstrated that the FGH strategy can further improve the mechanical properties of HFRCs. Compared with those of the IHFRCs, the bending strength and interlayer shear strength of the FGHFRCs increased by a maximum of 25.95 % and 41.20 %, respectively. Macro-micro fracture morphology analysis revealed that the interlayer hybrid led to a risk of delamination failure. However, the FGH effectively reduced the interlayer performance differences, changed the direction of crack propagation along the interlayer, and effectively suppressed the generation of delamination damage, which was conducive to further improving the properties of HFRCs. Owing to their enhanced properties and positive hybrid effect, the manufacturing process and the FGH strategy have considerable potential in engineering applications.

Continuous fiber-reinforced thermoset composites (CFRTCs) 3D printing offers a promising solution to fabricate lightweight, high-strength sophisticated composite structures. However, the delamination resistance of 3D printed CFRTCs is decreased by the weak fiber-matrix and interlayer adhesion caused by the process principle. To increase the interlayer toughness of 3D printed CFRTCs, this study developed a printing matrix toughened by various polyetherketone-cardo (PEK-C) forms by modulating its dissolution state. The results showed that the interlaminar toughening effects of the particle dispersion and dissolved dual form of PEK-C were superior to the insoluble particles or the dissolved PEK-C. As a result, the mode I and mode II interlaminar fracture toughness increased by 112.38 % and 189.01 %, respectively. And the synergistic effect of dual-form PEK-C was determined. Fractographic investigation revealed that the dissolved PEK-C experienced the reaction-induced phase separation initiating a nanoscale thermoplastic phase and developing a multi-scale PEK-C toughening system with the microscale PEK-C particles. Moreover, morphological observation of the particle and PEK-C phases demonstrate multi-scale synergistic toughening mechanisms of mono-component PEK-C. This study presents an innovative technique for interlayer toughening applicable to the CFRTCs 3D printing, illustrates the toughening principle, and shows its promise as a general strategy.

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