ABSTRACTWe developed and applied a novel approach for shape agnostic detection of multiscale flaws in laser powder bed fusion (LPBF) additive manufacturing using heterogenous in-situ sensor data. Flaws in LPBF range from porosity at the micro-scale ( 1 mm). Existing data-driven models are primarily focused on detecting a specific type of LPBF flaw using signals from one type of sensor. Such approaches, which are trained on data from simple cuboid and cylindrical-shaped coupons, have met limited success when used for detecting multiscale flaws in complex LPBF parts. The objective of this work is to develop a heterogenous sensor data fusion approach capable of detecting multiscale flaws across different LPBF part geometries and build conditions. Accordingly, data from an infrared camera, spatter imaging camera, and optical powder bed imaging camera were acquired across separate builds with differing part geometries and orientations (Inconel 718). Spectral graph-based process signatures were extracted from this heterogeneous thermo-optical sensor data and used as inputs to simple machine learning models. The approach detected porosity, layer-level distortion, and geometry-related flaws with statistical fidelity exceeding 93% (F-score).

ABSTRACTIn additive manufacturing of metals, numerous techniques have been employed to sense print defects. Among these, acoustic-based sensing has the advantage of low cost and shows the most potential to identify both external and internal defects as an in-situ monitoring system. Using acoustic signals, researchers have broadly investigated non-machine learning and machine learning-based approaches to identify defects like balling, micro defects, lack of fusion pores, keyhole pores, cracks, and porosity. While most of these works have shown promising results for laser-based AM systems, few have explored how acoustic signals can be used effectively for Wire Arc Additive Manufacturing (WAAM) defect detection. This paper proposes a methodology to construct machine learning (ML)-based models on identifying geometrically defective bead segments using acoustic signals during the WAAM process. Geometrically defective bead segment or geometric defect is a defect that causes voids in the final printed part due to incomplete fusion between two non-uniform overlapping bead segments. Such a defect is currently not explored in the literature. The proposed methodology uses a novel dataset labeling approach to identify good and bad bead segments based on an optimal threshold of the range of mean curvature. Furthermore, the methodology targets defective bead segments based on acoustic feature inputs like Principal Components (PC) or Mel Frequency Cepstral Coefficients (MFCC). To understand the resulting performance of the defect identification models constructed based on the proposed methodology, experiments are performed and tested on a variety of ML models (KNN, SVM, RF, NN, and CNN) based on the Inconel 718 material. The results show that the combinatorics of two acoustic input features and five ML models can be able to identify geometrically defective segments accurately with F1 score that ranges from 80% to 85%.

ABSTRACTWith the development of 5G technology, the miniaturised and highly integrated electronic devices urgently require thermal management materials possessing high thermal conductivity and mechanical properties. In this work, isotactic polypropylene (iPP)/high-density polyethylene (HDPE)-based dielectric composites possessing ideal thermal conductivity and balanced mechanical properties were prepared via Fused Filament Fabrication (FFF). The advanced material properties were achieved by the introduction of hybrid fillers and tailored polymer crystalline structure. The highly oriented h-BN, oriented iPP crystalline and iPP/HDPE epitaxy crystalline were observed. Meanwhile, we studied the effect of the ratio of hybrid fillers on various properties of composites. The thermal conductivity of iPP/HDPE/h-BN/Al2O3 composites reach 1.802 W·m−1·K−1. The impact strength and tensile strength reach 13.23 KJ/m2 and 40 MPa, respectively. In addition, the composites maintain ideal dielectric properties. This work offers a feasible strategy to fabricate dielectric and thermal conductive composites with balanced mechanical properties using semicrystalline polymer through FFF process.

ABSTRACTMultiphoton photopolymerisation (MPP), also known as 3D nanoprinting, was studied using a wavelength-tunable femtosecond laser. The possibility of using any colour of the spectrum from 500 to 1200 nm with a fixed pulse width of 100 fs revealed an interplay of photophysical mechanisms more delicate than just two-photon photopolymerisation. An effective order of absorption, i.e. the X-photon absorption, as well as optimal exposure conditions were assessed for photosensitised and pure SZ2080TM pre-polymer. The tunability of wavelength greatly influenced the dynamic fabrication window (DFW), optimised conditions resulting in a 10-fold increase. Furthermore, a non-trivial energy deposition by X-photon absorption was noted with an onset of a strong lateral size increase at longer wavelengths and can be understood as due to reaching epsilon-near-zero conditions. Such a control over the voxel aspect ratio and, consequently, the photopolymerised volume, may boost 3D nanoprinting efficiency. Overall, the results reveal wavelength being an important degree of freedom to tailor the MPP process and, if optimised, benefiting broad applications in areas of micro-optics, nanophotonic devices, metamaterials and tissue engineering.

ABSTRACTThe single-material overlapping models are incompatible with multi-material wire arc additive manufacturing (WAAM). A newly developed generalised model considers dissimilar adjoining beads in multi-material WAAM. The geometric model of dissimilar overlapping beads coupled with an algorithm identifies the process conditions for the two materials to maintain the same bead heights. The model, implemented for stainless-steel and creep-resistant-steel pair, yields significant scientific and practical findings. Compared to a fixed overlapping distance in single-material, e.g. 0.66 or 0.738 times the bead width, the multi-material overlapping distance is a complex function of individual bead widths. The bi-metallic interface fusion is affected by the molten metal flow, bead dimensions, and heat input. Contrary to the prevailing notion of a flat-top surface in the intermediate layer ideal for multi-layer deposition, a slight hill ensures a defect-free interface. The repeatable and defect-free bi-metallic walls and matrix is expected to have a breakthrough in multi-material WAAM.

ABSTRACTZirconium (Zr) alloys are widely used in nuclear energy because of their excellent mechanical properties and low thermal neutron absorption cross-section. This work investigated the printability, microstructure, and mechanical properties of Zr-4 alloy additively manufactured by laser powder bed fusion (LPBF) for the first time. The effect of annealing temperature on the microstructural and the mechanical property evolution of the printed Zr-4 alloy was studied. The results exhibited that the Zr-4 alloy with a high relative density of 99.77% was obtained using optimised printing parameters. With an increase in the annealing temperature, the formed α phase of the Zr-4 alloy changed from an acicular shape to a coarse-twisted shape, and finally to an equiaxed shape. Such microstructure change endowed the alloy with a high compressive strength of 2130 MPa and compressive strain of 36%. When the annealing temperature exceeded 700°C, Zrx(Fe2Cr) compounds were precipitated, strengthening the alloy by pinning effect. These findings provide valuable guidance for the manufacture of geometrically complex Zr alloy parts for nuclear power applications.

ABSTRACTAdditive manufacturing (AM) is a key enabler and technological pillar of the fourth industrial revolution (Industry 4.0) as it increases productivity and improves resource efficiency. However, current AM systems present some limitations in terms of fabrication time, versatility, and efficiency. The concept of teams of robots represents a novel approach for AM aiming to address these limitations. This review paper discusses the current state-of-the-art of the use of cooperative AM systems based on gantry systems, robotic arms, and mobile robots. The information flow, path planning and slicing strategies are discussed in detail, and several examples of the use of cooperative AM systems are provided. Finally, major research challenges and future perspectives are discussed.

ABSTRACTIn this work, a new design principle, i.e. doping with refractory metal particles with a low diffusion rate to prevent the formation of cracks and to improve the mechanical properties of bulk metallic glass (BMG) composites, was put forward. It was proven that crack-free and dense Mo(p)/Cu47Zr47Al6 BMG composites with enhanced mechanical properties can be produced via LPBF. The dislocations generated in the Mo particles can release thermal stress, thereby inhibiting the formation of thermal-cracks. The fracture patterns of Mo particles show that they can delay rapid of crack expansion, thereby improving the inherent strength and toughness of the material.

ABSTRACTAmong various bio-inspired structures, sutures are a prominent structure which has evolved independently to optimize their functionalities. The diabolical ironclad beetle suture-inspired structure was fabricated using multi-material additive manufacturing (3D printing) system with TangoBlackPlus (TBP) as the soft suture layer and VeroWhitePlus (VWP) as the hard material. The print quality of the specimen was assessed through the optical microscope images, and a nanoindentation test was performed to investigate the interfacial hardness between TBP and VWP. Flexural properties of the suture structure when changing the thickness of the soft layers were then studied. Experiments were continued to identify the effect of combining different sizes of suture modules to develop the suture structure. A numerical simulation model was then generated and validated using the experimental results to proceed with the parametric study. A design of experiment (DoE) was developed to analyse the effect of changing the suture geometry to optimize performance. The research concluded that gradually decreasing the size of the suture allowed the structure to withstand higher loads. It was also evident that the deformability of the structure could be increased by incorporating smaller interlocking angles and larger a:b ratios, while larger interlocking angles and smaller a:b ratios generate stiff structures.

ABSTRACTThe emergence of metamaterial has provided an unprecedented ability to manipulate electromagnetic waves, especially in the terahertz band where there is a lack of natural response materials. However, most metamaterials are fixed single function due to the fixed structure at the beginning of design. The paper reports a reconfigurable multi-functional terahertz metamaterial with variable structures based on mortise and tenon mechanism. And a hybrid 3D printing method based on FDM and E-jet is proposed to fabricate the metamaterials, which simplifies the processing process, improves the speed, and reduces the cost compared to traditional semiconductor processing methods. Through flexible mortise and tenon connections, the metamaterial can achieve: (1) narrowband transmission and broadband absorption; (2) perfect reflection; (3) narrowband reflection and broadband absorption. Relying on ingenious design and processing, the multi-functional metamaterials are expected to be widely used in fields such as electromagnetic shielding, radar stealth, communication and so on.

ABSTRACTMultifunctional metamaterials with unique electromagnetic and mechanical properties are highly desired in many fields, including space exploration and satellite communication, where broad tunability of the working frequency and controllable mechanical deformation properties are usually necessary. In this study, we propose a metasurface exhibiting simultaneous electromagnetic frequency selection capability and isotropic negative/positive/near-zero thermal expansion. The metasurface is designed based on chiral structures and is fabricated via 4D printing of continuous fibre composites. Both the effective thermal expansion coefficient and electromagnetic transmission band were investigated in different structural parameters based on theoretical calculation, finite element analysis simulations and experiments. The measured results were in good agreement with the theoretical data which reveal the influence law of structural parameters on thermal deformation and electromagnetic frequency control. Thus, the electromagnetic functionality of the metasurface can be thermally controlled and is expected to be useful in extreme situations where the coupling of multiphysical fields is required.

ABSTRACTUnderstanding powder bed system behaviour in powder spreading is a fundamental issue in binder jetting additive manufacturing (BJAM). This work established a discrete element model incorporating a parallel bond model to compatibly depict local cross-links between powder particles. BJAM parameters including layer thickness, gap compensation, recoat speed, rotation speed, and layer number were studied quantitatively for their effects on recoated powder's packing density and microscopic pore size and bonded layer's breakage and layer shift. Evolutions and influence mechanisms on both layer shift and bond breakage were further elucidated. Some practical implications include: gap compensation corresponding to an ideal recoated powder structure is ∼75 μm; rotation speed should be controlled at 40–120 rad/s to avoid low-rotation-speed layer shift surge and high-rotation-speed breakage; layer shift occurring at a certain stage is irreversible and must deserve well-maintained. This research can provide theoretical guidance for developing BJAM and even support-free powder bed – based additive manufacturing.

ABSTRACTA novel type of tubular structure with negative Poisson's ratio based on gyroid-type triply periodic minimal surfaces (TPMSs) is proposed in this study. This work is an attempt to design auxetic tubular structures based on TPMS. A series of auxetic tubular structures were designed and then fabricated using laser power bed fusion. Compressive behaviours of the fabricated auxetic tubular structures were investigated using experimental and numerical methods. To obtain optimal designs of tubular structures with controllable auxetic properties, the influence of several parameters were investigated comprehensively. Subsequently, several graded auxetic tubular structures were designed based on the parametric analysis and studied numerically. The mechanical properties of the tubular structures could be controlled effectively using the proposed approach. The proposed method can be used for guiding the design and optimisation of auxetic tubular structures, showing excellent potential for various applications such as biomedical devices, vehicle crashworthiness, and protective engineering.

ABSTRACTThe packing characteristics of fiber/polymer powder in powder bed fusion additive manufacturing exhibit a high correlation with the mechanical behaviours of printed composite parts such as homogeneity and anisotropy. A discrete element model has been developed to investigate the packing characteristics of glass fiber/polyamide 12 (PA12) powder, which include fiber orientations, fiber homogeneity, and packing density. The predicted probability distributions of fiber orientations in the powder bed are comparable with those measured in glass fiber–reinforced PA12 composites printed via multi jet fusion. Three types of fibers with different length distributions are adopted to study the effects of the fiber length distribution on their packing characteristics. The simulation results reveal that a large average fiber length is beneficial to fiber alignment in the powder spreading direction but lowers the fiber homogeneity and packing density of the powder bed. Furthermore, varying the fiber length can provide an effective way to regulate fiber orientations in the powder packing process, which would help achieve satisfactory anisotropic mechanical properties for composite parts.

ABSTRACTBased on the triply periodic minimal surface (TPMS), 3D auxetic structures are successfully implemented using a dual-period function. A series of shape-controllable, dual-period deformation functions are obtained by summarising the characteristics of periodic deformation functions and applying Bezier curve fitting methods. Then, with the geometry originating from the Schwarz primitive (P) of TPMS, the periodic shape transformation of TPMS is achieved using the dual-period deformation functions. The property (negative Poisson’s ratio) of the auxetic structure is investigated based on the control parameters (the TPMS c value, periodic function η, and deformation index γ). The auxetic structures can exhibit excellent 3D negative Poisson’s ratio properties, and the Poisson’s ratio can be effectively adjusted. Moreover, a heterostructure with positive and negative Poisson’s ratio structures is obtained and applied to a stem in the hip joint. The simulation proves that the heterostructure can effectively prevent the failure of the bone implant.

ABSTRACTAlthough additive manufacturing (AM) is a net/near-net forming process, significant deviations may occur between the finally manufactured product and the designed object. This study is inspired by the deposition errors in a typical closed-contour structure manufactured via laser-directed energy deposition (DED). A series of experiments were carried out and a multi-physics mechanistic model was developed to reveal the origins and the spatiotemporal variations of the errors in representative processes. The results demonstrate that the irregularity of the deposition can be initiated by the liquid metal flow in the molten pool driven by forces including the recoil pressure and surface tension. The deviations are subsequently aggravated due to the inheritance between layers. Supported by the revealed underlying mechanisms, solution strategies including layer-wise dynamic compensations are thus proposed. The novel scientific findings can be insightful to researches for other metal AM processes and structures considering the universality of the uncovered mechanisms.

ABSTRACTAccording to the material nature, aluminium alloys are widely applied in aerospace, construction and automotive applications due to their characteristics, such as lightweight, good formability and good corrosion resistance. Among the aluminium alloys, scalmalloy (Al-4.49Mg-0.71Sc-0.51Mn-0.27Zr-0.07Fe-0.03Si alloy) was developed to overcome the hot crack issue during the laser powder bed fusion (LPBF) process. Hence, the degree of lightweight can be further improved by introducing this high-specific strength material with a structure of the lightweight design. However, the strengthening mechanism of the heat-treated 3D printed scalmalloy has not been sufficiently explored. In this study, the synergistic effect of the strengthening mechanisms is explored through detailed microstructure analysis. The grain size, size and spacing of the precipitate and coherent phase contribute to the strengthening of scalmalloy. Through the observation of the microstructure feature, the theoretical strength of the heat-treated 3D printed scalmalloy can thus be calculated by three strengthening mechanisms and match the experimental results perfectly.

ABSTRACTThis study presented a comprehensive investigation into the microstructures, strengthening mechanisms and deformation behaviours of a novel N-doped Co-28Cr-6Mo (CCMN) alloy fabricated by laser powder bed fusion (LPBF). In addition to the well-known cellular structures, lattice defects including dislocations and stacking faults (SFs), the near-spherical shaped Cr2N precipitates with two typical size distributions were detected. Tensile test results revealed that the LPBF fabricated CCMN alloy demonstrated superior yield strength of 845 ± 49 MPa and elongation to fracture of 12.7 ± 1.9%. The grain boundaries (∼277 MPa), high density of dislocations (∼176–193 MPa), Cr2N precipitates (∼243 MPa) and SFs (∼131 MPa) are regarded as the dominate strengthening contributors. On the other hand, HCP phase triggered by strain induced martensite transformation (SIMT) and the Lomer-Cottrell locks (L-C locks) associated with the numerous SFs significantly enhanced the alloy strain hardening rate. More importantly, the formed Cr2N nanoprecipitates effectively suppressed the strain localisation and the premature failure along the HCP/FCC interfaces by deflecting the continuous growth of SFs, further contributing to the high ductility of the LPBF processed CCMN alloy. The present study is expected to shed light on the future development of N-doped high-performance cobalt-based alloy for the LPBF process.

ABSTRACTThe emergence of 4D printing from additive manufacturing has opened new frontiers in crashworthiness application. Energy-absorbing structures with fixed geometrical shapes and irreversible deformation stages can be programmed such that after mild or extreme deformation, their initial shapes, properties and functionalities can be recovered with time when actuated by external stimuli. This survey delves into the recently-accelerated progress of shape memory/recovery energy-absorbing metamaterials (EAMM) and energy-absorbing smart/intelligent structures (EASS). First, the introduction gives some fundamental concepts of metamaterials and their application to energy-absorbing structures. Next, some common 3D printing technologies that have led to 4D printed EAMM and EASS are succinctly described. Shape memory materials, their functional properties and recovery process, are then discussed. Finally, various recoverable/reversible energy absorbers with their future challenges and perspectives, are presented. With well-tailored 4D printed EAMM and EASS, reusability with minimal maintenance and higher energy absorption capacity can be retained.

ABSTRACTTantalum (Ta) has excellent prospects in the bone-implant field due to its satisfactory biocompatibility. Two novel Ta lattice structures were designed and printed by selective laser melting (SLM), including the imitation saddle surface (ISS) and the imitation arch bridge telescopic (IABT) structures. Quasi-static compression tests and finite element analysis were adopted to investigate the effects of design parameters on the mechanical properties, deformation modes, and energy absorption of lattice structures. Compared with the typical lattice structure body-centred cubic (BCC) structure, the ISS lattice structure had a higher yield-stress-to-elastic-modulus ratio, and the IABT lattice structure had higher energy absorption. The failure mode of the BCC and ISS lattice structures was shear band formation. The IABT lattice structure showed hierarchical deformation during compression and collapsed with vertical strut buckling. The results indicated that the ISS lattice is the most potential candidate for bone implant applications.