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Laser-based directed energy deposition (L-DED) offers significant advantages for repairing metal structures, particularly in the marine and offshore industry where corrosion-resistant materials like Monel K-500 (a Ni-Cu alloy) lack strength and wear resistance. This study addresses this issue by producing Monel K-500 with high-strength Stellite 6 (a Co-Cr alloy). Two types of multi-material samples were created using L-DED: interlayered and mixed powder samples. The interlayered samples experienced cracking at the material interface, while the mixed powder samples were crack-free and exhibited improved mechanical properties, with yield strength increasing from 208.3 MPa to 490.2 MPa and ultimate tensile strength from 429.7 MPa to 887.0 MPa compared to single Monel K-500 samples. This research demonstrated the potential of in-situ alloying during the L-DED process to enhance alloy properties and highlighted the challenges of abrupt compositional changes leading to cracking, suggesting mitigation strategies for successful production.

Multi-material additive manufacturing holds immense potential for performance and functionality enhancement. Past research efforts primarily focused on the horizontal interface (perpendicular to the sample build direction) during the laser-powder bed fusion (LPBF) of multi-materials, whereas a few studies demonstrated that the vertical interface (parallel to the sample build direction) is in fact the main obstacle towards high-integrity multi-material fabrication in a three-dimensional space. In this work, facilitated by our own patented powder spreading device, we explored the mechanisms behind defect formation along both the horizontal and vertical interfaces during the LPBF production of 316L stainless steel and K220 copper multi-materials. High-fidelity fluid dynamics simulations were also conducted to rationalise the experimental observations. A practical process parameter optimisation approach is also proposed at the end, with the aim of mitigating those large defects currently occurring near the vertical interface.

ABSTRACTMetal oxide nanorods exhibit promising potential as reinforcement fillers in various polymer matrices, but their application in the Multi Jet Fusion (MJF) technique is rarely reported. In this work, surface-modified zinc oxide nanorods (SMZnO) were synthesized and incorporated into polyamide 12 (PA12) powder to enhance the mechanical properties of the MJF-printed parts. Compared to ZnO, SMZnO exhibited better dispersion, resulting in markedly enhanced mechanical performances. The ultimate tensile strength and the Young's modulus of the MJF-printed SMZnO/PA12 nanocomposites were 62.02 MPa and 2.28 GPa in the X orientation and 64.07 MPa and 2.34 GPa in the Y orientation, equivalent to 27.85%, 59.44%, 29.12%, and 54.97% increments, respectively. The flexural strength and modulus demonstrated similar improvements in the X and Y orientations, confirming the uniform mechanical enhancement effect of homogenously distributed SMZnO. This work provides a novel and facile approach for the additive manufacturing of polymeric nanocomposites with superior mechanical performance.

Additive manufacturing (AM) offers the advantages of direct near-net-shape production, reduced material waste, and shortened production lead time, showing great potential to revolutionise the manufacturing industry. The flexible movements of the deposition head and the build platform allow directed energy deposition (DED) to conduct the repair process of damaged high-value metallic parts. However, the resulting heterogeneous microstructure and its effect on the mechanical properties of the repaired parts have not been widely realised. In this work, the repair of Monel alloy parts, known for their excellent mechanical properties and high corrosion resistance, was conducted by the laser-DED process with low and high laser power settings, respectively. Different from the fine, equiaxed grains in the original part, the as-deposited Monel alloy consists of large columnar grains. The mechanical performance across the interface between the original and newly deposited material was tested and analysed along horizontal and vertical loading directions. The yield strength and elongation of repaired Monel alloy parts were 409.1 MPa and 35.2% along the horizontal loading direction, which both significantly surpass the corresponding values along the vertical loading direction. This study lays the groundwork for designing a laser-DED process to achieve high performance for repaired metallic parts.

Polymeric materials for powder bed fusion additive manufacturing have been attracting extensive research interest due to their vast potential for fabricating end-use functional parts. Here, a high-fidelity multiphysics approach combining the discrete element model with the computational fluid dynamics model has been developed to simulate the printing process of polymers in powder bed fusion, involving powder recoating, melting, and coalescence. The developed approach considers particle flow dynamics, the reflection, absorption, and transmission of infrared laser radiation, and the viscous flow of polymer melt. The pore formation mechanisms due to lack of fusion and gas entrapment in polyamide 12 parts printed via selective laser sintering are studied. The simulation results reveal that lower polymer viscosity would be beneficial to the densification rate of the printed parts. Excessively small powder particles would degrade powder bed quality due to the agglomeration of polymer powder, thus leading to high porosity in the printed parts.

The 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.

Glass fibre (GF) and glass bead (GB)–reinforced polyamide1212 (PA1212) was additively manufactured by selective laser sintering. The effects of laser power and GF content on the tensile and tribological properties of the printed specimens with a base GB weight fraction of 40 wt.% were investigated. The strengthening mechanism of GFs/GBs was illustrated by analyzing the interfacial adhesion between the fillers and the PA1212 matrix. The specimens with 40 wt.% GBs and 10 wt.% GFs fabricated at a laser power of 30 W exhibited a strength of 52 MPa, a friction coefficient of 0.23, and a wear rate of 0.0011 mm3/N·m. The selected optimal laser power and GF addition contributed to the strong interfacial adhesion, which realised flat surface morphology and an adequate encapsulation of fillers in the specimen. The reinforcement of GBs/GFs in PA1212 can serve as a reference for a deeper understanding of the strengthening mechanisms for other additively manufactured engineering plastics.

Multi jet fusion (MJF), a powder-based additive manufacturing technology, is suitable for fabricating fibre-reinforced polymer composites. However, the types of reinforcement fibres applied in MJF are limited due to the incompatibility between the fibres and polymer. Herein, a simple and cost-effective surface modification method for Fe3O4-coated aramid fibres (Fe3O4@AF hybrids) is proposed to fabricate polyamide 12 (PA12) composites by MJF. The optimal fibre content for Fe3O4@AF/PA12 composites is 6 wt%, which resulted in 10.83% and 16.05% higher ultimate tensile strength and Young’s modulus, respectively, compared with AF/PA12. The addition of Fe3O4@AF hybrids also improved the flame retardancy of AF/PA12 by reducing the peak heat release rate (PHRR) and postponing the temperature at PHRR. This practical surface modification method can be potentially applied to other reinforcement fibres for fabricating functional polymer composites by other powder-based manufacturing technologies.

Multi Jet Fusion (MJF) has attracted extensive attention because of its ability to print support-free complex structures. However, the mechanical properties of MJF-printed polymer parts are still unsatisfactory for certain industrial requirements. Herein, by leveraging the fibre reinforcement effect and high specific strength of polyimide (PI) fibres, this work developed PI/polyamide 12 (PA12) composites with largely enhanced mechanical performance via MJF. Specifically, the tensile strength and modulus were increased by 43% and 42%, and the flexural strength and modulus were improved by 39% and 46%, respectively, compared to those of the neat PA12 parts. Furthermore, the incorporation of lightweight PI fibres endowed the composites with high specific tensile strength (67.60 kN·m/kg) and specific flexural strength (93.70 kN·m/kg), which are superior to those of MJF-printed PA12 composites reinforced with other fibres. This work provides new insights into enhancing the mechanical performance of lightweight parts printed by MJF and other powder-based techniques.

Multi Jet Fusion (MJF) has gradually been utilised for the commercial fabrication of final products. In particular, the mechanical properties of MJF-printed polyamide 12 (MJF PA12) parts have been widely investigated by manipulating the process parameters such as the printing orientation and the cooling rate. In this work, the effects of build positions on the thermal history, crystallization, and mechanical properties of MJF PA12 parts were investigated. The thermal history of printed parts was obtained using the MJF Thermal Prediction Engine software developed by Hewlett-Packard HP Labs (HP Inc.). As compared to the parts printed at the boundaries, those in the centre region experienced a slower cooling rate, which was favourable for crystal growth. Both the crystallite size and crystallinity of the parts in the centre region were larger than those of the parts at the boundaries. The parts printed in the centre region had slightly higher tensile modulus and significantly smaller elongation at break and strain energy density than those printed at the boundaries. The tensile strength of the parts at different locations was comparable. This work serves as a guide to selecting the build position of MJF-printed parts to achieve desirable mechanical properties.

Mechanical anisotropy greatly influences the applications of materials printed by additive manufacturing techniques such as Multi Jet Fusion (MJF) and selective laser sintering. However, the mechanical anisotropy of MJF-printed fibre–reinforced polymer composites has not been well understood. In this work, the effect of the fibre length on the mechanical performance of MJF-printed glass fibre–reinforced polyamide 12 (GF/PA12) composites is systematically investigated. Both experimental and simulation results confirm that longer fibres are in favour of fibre alignment in the powder spreading direction. The composite parts with longer fibres exhibit higher porosity. When GFs with an average length of 226 μm are added, the ultimate tensile strength and tensile modulus of the composites measured in the powder bed spreading direction are remarkably improved by 51% and 326%, respectively, as compared with those of neat PA12 specimens. This work provides guidance for the printing of other high-strength fibre–reinforced polymer composites.

Metal additive manufacturing (AM) has tremendous potential for manufacturing geometrically complex metallic products with freedom of design and desirable performance. Qualification and certification are critical to ensuring the successful adoption of metal AM by various sectors and accelerating the standardisation progress. This article provides a comprehensive review on the status of standardisation and challenges for the qualification and certification of metal AM, which is a timely summary to bridge the readiness gap between metal AM technologies developed at academia and industry. It begins with introducing fundamentals of metal AM qualification and certification. Thereafter, the status of standards, rules, and regulations developed by international organisations and communities is summarised and discussed. The challenges are then addressed, including the qualification and certification process, materials, process reliability and repeatability, post-processing, data, and standards and protocols. Finally, the conclusion and future outlook are drawn to provide guidance for future research and development.

Multi Jet Fusion (MJF) is a fast-growing powder bed fusion(PBF) additive manufacturing technique which features low production costs and high production speeds. However, MJF currently suffers from limited choice of commercially available composite powders. Here, a new type of composite powder, aramid fibre (AF)–filled polyamide 12 (PA12), was developed for MJF to enhance the mechanical properties of the printed parts. The process–structure–property relationship was established by analysing the fibre arrangement in the composite and systematically investigating the effects of the fibre fraction, fibre length, layer thickness, build orientation, and post-annealing process on the structures and mechanical properties of the printed parts. The results showed significant enhancement in the mechanical performance of the AF/PA12 composites parts in the roller recoating direction along which the fibres preferred to align. The ultimate tensile strength and Young’s modulus of the optimised composite parts were increased by 27% and 179% and further improved by 40% and 216% through a post-annealing process, respectively, compared with those of the neat PA12 part. The manufacturing methodology of these high-strength light-weight composites can be further extended to other PBF techniques for applications in a broad spectrum of fields.

Micro-supercapacitors (MSCs) with excellent electrochemical behaviours and flexibility possess great promise for portable and wearable electronic devices. A novel type of hybrid-dimensional Fe2O3/graphene/Ag ink is developed and extruded into MSC electrodes through the direct ink writing-based three-dimensional (3D) printing. The optimal solid-state MSC device exhibits a maximum areal capacitance of 412.3 mF cm−2 at 2 mA cm−2, a correspondingly high energy density of 65.4 μWh cm−2 and 89% capacitance retention for over 5000 charge and discharge cycles. The superior electrochemical performance is profited by the high electron transport synergistically boosted by two-dimensional graphene nanosheets and one-dimensional Ag nanowires, and the high pseudocapacitive behaviours of Fe2O3 nanoparticles. The 3D printed MSC exhibits reliable flexibility with remarkable retention of 90.2% of its original capacitance after 500 bending cycles. The current 3D printing fabrication demonstrates an efficient route for advanced miniaturised electrochemical energy storage.

Polymeric cellular structures based on triply periodic minimal surfaces (TPMS) have been widely studied for applications in multiple disciplines due to their multifunctionality. However, there is limited acoustic application by TPMS-based structures as their acoustic properties remain largely unknown. In this paper, TPMS-based structures are fabricated by additive manufacturing and investigated as a novel solution to sound absorption in the upper midrange frequency. Structures based on three typical surface types (Primitive, Gyroid and Diamond) with three geometry-related parameters (volume fraction, unit cell size and height) are manufactured by stereolithography and tested by two-microphone impedance method in the frequency range of 2000-6000 Hz. The results show that the structures based on Diamond surfaces exhibit excellent absorption abilities among the three types in a wide bandwidth. High absorption coefficients can be achieved by a large volume fraction or a small unit cell size while the effective frequency ranges can be adjusted by the height. This study extends the multifunctionality of TPMS-based cellular structures to include acoustic absorption and will facilitate the development of guidelines on designing the optimal acoustic absorbers by cellular structures in future.

A discrete element model has been developed to simulate the packing process of fibre/polymer composite powder for powder bed fusion additive manufacturing. The geometric shapes of polymer powder particles and fibres are represented by multi-sphere particles and individual cylinders with round ends, respectively. The numerical model can help to understand the flow dynamics of composite powder particles and the formation mechanisms of voids in powder packing processes. The numerical model has been utilised to analyse the effects of packing parameters on the packing quality of the powder bed. The simulation results suggest that the increase of the powder layer thickness is beneficial to the increase in the packing density and the decrease in the surface roughness of the powder bed. A high roller spreading velocity degrades the packing quality of the powder bed. Furthermore, a small number of fibres in the composite powder particles are in favour of the packing quality, but a further increase in the fibre number reduces it.

It has been reported that the addition of nanofillers/nanoparticles into the thermoplastic polymers could enhance the toughness of the polymer matrix. In this work, the mechanical and thermal properties of a multi-walled carbon nanotubes (CNT)/polyamide 11 nanocomposite for additive manufacturing was evaluated. Well-dispersed PA11/CNT nanocomposite powders were processed successfully by laser sintering. Compared to the pristine PA11, the fracture toughness of the PA11/CNT nanocomposite was enhanced by ∼54% by incorporating of only 0.2 wt% CNTs. With differential scanning calorimetry, X-ray diffraction and scanning electron microscope fractography analysis, the nanostructure and the toughening mechanism which lead to the toughness improvement was well identified and understood.