Direct Additive Manufacturing Research Articles

A critical review of direct laser additive manufacturing ceramics

A critical review of direct laser additive manufacturing ceramics

4D Printing Hybrid Soft Robots Enabled by Shape-Transformable Liquid Metal Nanoparticles.

In recent years, soft robotics has emerged as a rapidly expanding frontier research field that draws inspiration from the locomotion mechanisms of soft-bodied creatures in nature to achieve smooth and complex motion for diverse applications. However, the fabrication of soft robots with hybrid structures remains challenging due to limitations in material selection and the complex, multi-step processes involved in traditional manufacturing methods. Herein, a novel direct one-step additive manufacturing (3D printing) approach is introduced for the fabrication of hybrid robots composed of soft and rigid components for sophisticated tasks. Inspired by the shape-transformable liquid metal nanoparticles (LMNPs), a functional material toolkit with tuneable mechanical properties and deformability is developed by integrating differently shaped gallium-based nanoparticles (GNPs) into the 3D printing polymers. Then the direct printing of assembled or one-piece hybrid soft-rigid robots is presented through a single recipe of GNPs-integrated inks. This fabrication method enables precise control of the mechanical properties and shape memory properties within the hybrid structures of robot body with a customized structure design. Their capabilities are further demonstrated through the design and fabrication of hybrid robots as high-precision gripper, bioinspired motor, and hand rehabilitation device.

Comparing Degradation Mechanisms, Quality, and Energy Usage for Pellet- and Filament-Based Material Extrusion for Short Carbon Fiber-Reinforced Composites with Recycled Polymer Matrices

Short carbon fiber (sCF)-based polymer composite parts enable one to increase in the material property range for additive manufacturing (AM) applications. However, room for technical and material improvement is still possible, bearing in mind that the commonly used fused filament fabrication (FFF) technique is prone to an extra filament-making step. Here, we compare FFF with direct pellet additive manufacturing (DPAM) for sCF-based composites, taking into account degradation reactions, print quality, and energy usage. On top of that, the matrix is based on industrial waste polymers (recycled polycarbonate blended with acrylonitrile butadiene styrene polymer and recycled propylene), additives are explored, and the printing settings are optimized, benefiting from molecular, rheological, thermal, morphological, and material property analyses. Despite this, DPAM resulted in a rougher surface finish compared to FFF and can be seen as a faster printing technique that reduces energy consumption and molecular degradation. The findings help formulate guidelines for the successful DPAM and FFF of sCF-based composite materials in view of better market appreciation.

Assessment of the trueness of additively manufactured mol3% zirconia crowns at different printing orientations with an industrial and desktop 3D printer compared to subtractive manufacturing

ObjectivesThis study endeavours to investigate the effect of printing orientation on the trueness of additively manufactured molar zirconia crowns. The areal surface roughness and the characteristics of the marginal regions of the crowns were also considered. MethodsTwelve molar crowns were manufactured at 0°, 45°, and, 90° printing orientations in a Lithoz and AON zirconia printer, respectively. Twelve milled crowns were used as a comparison. Samples were scanned and analysed in metrology software to determine the trueness of the groups. Regions of interest were defined as the margins, intaglio surface and contact points. Areal surface roughness and print layer thickness were further analysed using a confocal laser scanning microscope. ResultsThe results indicate that there are clear differences between the investigated desktop (AON) and industrial (Lithoz) 3D printer. The 45° Lithoz group is the only sample group showing no significantly different results in trueness for all regions analysed compared to the milled group. Areal surface roughness analysis indicates that the print layers in the marginal regions are within clinically tolerable limits and surface characteristics. ConclusionsThe printing orientation for zirconia crowns is critical to trueness, and differences are evident between different AM apparatuses. Considerations for design and orientation between different apparatuses should therefore be considered when utilising direct additive manufacturing processes. The areal surface roughness of the marginal regions is within acceptable clinical limits for all manufacturing processes and print orientations considered. Clinical significanceThe materials and apparatuses for additive manufacturing of zirconia crowns are now clinically acceptable from the perspective of the trueness of a final crown for critical functional surfaces and areal surface roughness of the marginal regions.

Direct additive manufacturing of transparent fused silica glasses using an integrated paste printing and laser in-situ melting technology

Fused silica glasses are of great importance for applications in optics, photonics, electronics, and biochemistry. However, their harsh processing nature still constrains the design and development of fused silica glasses for advanced applications. Additive manufacturing (AM) is among the promising techniques to improve the fabrication of fused silica glasses. Herein, we develop an integrated paste printing and laser in-situ melting technology to additively manufacture fused silica glasses without post-heat-treatments. For every layer, the fused silica paste was first printed by an air-pumped dispenser based on the predesignated CAD model, and subsequently melted by a CO2 laser to become transparent glasses. To improve the paste printing process, the shear thinning property of the developed fused silica paste was investigated to evaluate the printability of the paste, and the control strategy of the paste printing parameters was developed for precise thickness control of the printed paste layer. By optimizing the laser processing parameters, the surface waviness and roughness of the laser-melted glass layers were effectively minimized, indicating the surface uniformity was improved. To evaluate the optical properties of the laser-melted glasses, the optical transmittance and refractive indices of the fused silica glasses melted by different laser powers were characterized. Through stacking the laser-melted fused silica glasses layer-by-layer, three-dimensional (3D) fused silica glass components with unusual thin-wall structures and embedded microfluidic channels have been directly printed, demonstrating that the developed glass AM technology is promising for advanced applications such as microfluidics, integrated optics, and metamaterials.

An apparatus to study arc-wire direct energy metal deposition additive manufacturing process in a drop tower microgravity platform.

Understanding the mechanisms and dynamics of molten metal droplet transfer within the plasma of a directed energy deposition arc process in microgravity is critical for optimizing the build process with minimal defects. This paper presents a unique experimental setup designed to investigate the transfer of molten metal droplets in the microgravity environment of a drop tower. The primary design of the apparatus revolves around accommodating, within the confines of the drop tower experimental capsule, essential components, including a high-speed camera with necessary filters for capturing molten metal droplets, a consumable electrode wire-arc setup, batteries, a linear traverse stage for single bead deposition, sensors, data acquisition systems, online communication systems, and the control system. These systems are secured to withstand the high deceleration forces at the end of a free fall in the drop tower. The arrangement has demonstrated consistent deposition outcomes, capturing clear images of droplet transfers using a high-speed camera along with voltage, current, and temperature data during the microgravity state induced by free fall. This apparatus will serve as a foundational element in establishing a viable additive manufacturing capability for space applications, as it provides fundamental insights into the transfer of molten metal droplets.

Fabrication and Characterization of Plasma Sprayed TiO2 and Li4Ti5O12 Materials as All Active Material Lithium-Ion Battery Electrodes

Two strategies to increase battery energy density at the cell level are to increase electrode thickness and to reduce the amount of inactive electrode constituents. All active material (AAM) electrodes provide a route to achieve both of those aims toward high areal capacity electrodes. AAM electrodes are often fabricated using hydraulic compression processes followed by thermal treatment; however, additive manufacturing routes could provide opportunities for more time-efficient and geometry-flexible electrode fabrication. One possible route for additive manufacturing of AAM electrodes would be to employ plasma spray as a direct additive manufacturing technology, and AAM electrode fabrication using plasma spray will be the focus of the work herein. TiO2 and Li4Ti5O12 (LTO) powders were deposited onto stainless steel substrates via plasma spray processing to produce AAM battery electrodes, and evaluated with regards to material and electrochemical properties. The TiO2 electrodes delivered low electrochemical capacity, <12 mAh g−1, which was attributed to limitations of the initial feed powder. LTO plasma sprayed AAM electrodes had much higher capacity and were comparable in total capacity at a low rate of discharge to composite electrodes fabricated using the same raw powder feed material. LTO material and electrochemical properties were sensitive to the plasma spray conditions, suggesting that tuning the material microstructure and electrochemical properties is possible by controlling the plasma spray deposition parameters.

NbC-BASED CERMET PRODUCTION COMPARISON: L-PBF ADDITIVE MANUFACTURING VERSUS CONVENTIONAL LPS POWDER METALLURGY

The production of carbide parts (cermet) by additive manufacturing, such as laser powder bed fusion (L-PBF), has been a great challenge due to the complex optimization of process parameters to improve density, porosity, microcracks or abnormal growth of grains and obtain a microstructure as homogeneous as possible. This work aims to compare the evolution of the microstructure when using the conventional route of powder metallurgy, i.e., liquid phase sintering (LPS) with the L-PBF direct additive manufacturing process, considering the NbC-based carbide material. Sample compositions were prepared in w/%, samples were compacted under 50–125 MPa, without polymeric binders, and they were sintered under a vacuum at temperatures of 1330 °C and 1370 °C. For the L-PBF process, a vibrating device made it possible to improve the fluidity of the mixtures of three alloys, NbC–30Co, NbC–30Ni and NbC–30(Co, Ni). The mixtures exhibited low sphericity, low fluidity and compressibility, which were improved with a roller compactor. Thin powder mixture deposition layers were evenly applied and well distributed across the powder bed to avoid defects and cracks during sintering. The L-PBF process parameters varied including a power of 50–125 W and a laser scanning speed of 25–125 mm·s–1. Different microstructures, identified with a light microscope (LM) and a scanning electron microscope (SEM), and properties obtained with the two processes, direct (L–PBF) and indirect sintering (LPS), were compared.

Fabrication of deformable patient-specific AAA models by material casting techniques.

Abdominal Aortic Aneurysm (AAA) is a balloon-like dilatation that can be life-threatening if not treated. Fabricating patient-specific AAA models can be beneficial for in-vitro investigations of hemodynamics, as well as for pre-surgical planning and training, testing the effectiveness of different interventions, or developing new surgical procedures. The current direct additive manufacturing techniques cannot simultaneously ensure the flexibility and transparency of models required by some applications. Therefore, casting techniques are presented to overcome these limitations and make the manufactured models suitable for in-vitro hemodynamic investigations, such as particle image velocimetry (PIV) measurements or medical imaging. Two complex patient-specific AAA geometries were considered, and the related 3D models were fabricated through material casting. In particular, two casting approaches, i.e. lost molds and lost core casting, were investigated and tested to manufacture the deformable AAA models. The manufactured models were acquired by magnetic resonance, computed tomography (CT), ultrasound imaging, and PIV. In particular, CT scans were segmented to generate a volumetric reconstruction for each manufactured model that was compared to a reference model to assess the accuracy of the manufacturing process. Both lost molds and lost core casting techniques were successful in the manufacturing of the models. The lost molds casting allowed a high-level surface finish in the final 3D model. In this first case, the average signed distance between the manufactured model and the reference was () mm. However, this approach was more expensive and time-consuming. On the other hand, the lost core casting was more affordable and allowed the reuse of the external molds to fabricate multiple copies of the same AAA model. In this second case, the average signed distance between the manufactured model and the reference was () mm. However, the final model's surface finish quality was poorer compared to the model obtained by lost molds casting as the sealing of the outer molds was not as firm as the other casting technique. Both lost molds and lost core casting techniques can be used for manufacturing patient-specific deformable AAA models suitable for hemodynamic investigations, including medical imaging and PIV.

Patient-specific cranioplasty, by direct and indirect additive manufacturing of biopolymers and implantable materials.

Autologous bones are traditionally used in surgical reconstruction of skullcap. Since patients' bones are often unavailable or cause of infections, implantable synthetic materials emerged as promising alternative. These can be shaped by different technologies, while 3D printing offers remarkable chances in terms of flexibility, accuracy, cost-saving and customizability. This study aims to evaluate strengths and limitations of the three main strategies that imply additive manufacturing for the implementation of cranial prosthesis: (i) direct printing of PLA (polylactic acid) skullcaps, mould casting of poly(methyl methacrylate) (PMMA) prosthesis using (ii) silicone mould manufactured from a 3D printed master, (iii) 3Dprinted TPU (thermoplastic polyurethane) mould. All solutions achieved good geometric accuracy and excellent mechanical resistance. Direct printing of the PLA resulted in the fastest strategy, followed by PMMA casting in a silicone mould. The use of silicone was overall more advantageous, due to lower costs and the possibility of sterilization by using autoclaving.

Direct additive manufacturing as spring of new tool steels

Additive manufacturing (AM) is now common in production of metallic matrixes with/without reinforcements (nanoprecipitates) to improve functional and structural mechanical properties of 3D objects. The presence of liquid phase in direct processes allows the possibility to change the conventional chemical composition of materials homogeneously. Powder Bed Fusion (PBF), in which the high nonequilibrium solidification nature resembles a localized high cooling rate but allowing, still the formation nanocarbides in-situ. Thus, it will be the suitable technology to tailor novel functionally gradient metallic materials. Nevertheless, this character is only present in the upper layers due to the shaping being made layer by layer. During shaping, the previous layers undergo post heat treatments contributing for growing the carbide dimension. Hence, metallic alloys can be developed simultaneously with the processing stage by changing the chemical composition by addition of fundamental elements to the matrix powder. In tool steel the improvement of hardening carbide content can contribute to a better performance concerning hardness and abrasion wear resistance. The present study concerns the addition of vanadium powder and allotropes of carbon, in correlation with the partition coefficient between vanadium and carbon in the steel selected - AISI H13, processed by selective laser melting (PBF/SLM), without requiring adjusting the processing parameters. The hardness attained is analogous to high speed steels, since this allows layers with a similar matrix to the wrought tool steel but with higher content of hardening carbide (VxCy). The sustainability of the final product is evident.

Optimization of Components with Topology Optimization for Direct Additive Manufacturing by DLMS.

This paper presents a novel design methodology that validates and utilizes the results of topology optimization as the final product shape. The proposed methodology aims to streamline the design process by eliminating the need for remodeling and minimizing printing errors through process simulation. It also eliminates the repeated export and import of data between software tools. The study includes a case study involving the steering column housing of a racing car, where Siemens NX Topology Optimization was used for optimization, and verification analysis was conducted using the NX Nastran solver. The final solution was fabricated using AlSi10Mg via direct metal laser sintering on a 3D printer and successfully validated under real conditions. In conclusion, this paper introduces a comprehensive design methodology for the direct utilization of topology optimization, which was validated through a case study, yielding positive results.

Direct additive manufacturing of metal parts for automotive applications

Additive manufacturing or three-dimensional (3D) printing offers tremendous opportunities for the automotive industry to reduce vehicle weight and improve vehicle performance through consolidating parts, tailoring material properties, enabling multifunctional components, and simplifying manufacturing processes. However, additive manufacturing research for direct manufacturing of automotive components is limited, partially because of the sheer volume of automotive production. In this paper, we explore the potential of direct manufacturing of metallic vehicle components using additive manufacturing coupled with the generative design approach. Using a radar mounting bracket as an example, we show that desired part strength and safety performance can be achieved with 42 % weight reduction and simplified manufacturing and assembly process. A production cost analysis was also conducted based on two mainstream additive manufacturing systems. The result suggests that the cost of metal powder is the limiting factor for wider adoption of additive manufacturing in the automotive industry.

Achieving high-performance pure tungsten by additive manufacturing: Processing, microstructural evolution and mechanical properties

It is always a great challenge to manufacture defect-free and high-performance pure tungsten by laser additive manufacturing until now. In this work, using nanopowder as precursors, the pure tungsten green parts were successfully manufactured by indirect additive manufacturing technology, powder extrusion printing. The subsequent debinding-sintering process of the green parts has been optimized to obtain high-relative-density, fine-grain and defect-free tungsten samples. The relative density of the sample increased with the increase of sintering temperature. The microstructures of pure tungsten prepared by direct and indirect additive manufacturing techniques were studied comparatively, including laser powder bed fusion and powder extrusion printing. The formation of microcracks in pure tungsten samples is mainly due to the high residual stress provided by laser powder bed fusion and the high ductile-to-brittle transition temperature characteristics of tungsten. The sintering temperature of the pure tungsten samples with the highest compressive strength (1290 ± 10 MPa) was determined to be 2000 °C. Compared with other sintered samples, defect-free samples sintered at 2000 °C had finer grains (9.8 μm) while maintaining high relative density (99.1 ± 0.4%). The higher strength of the sintered samples at 2000 °C is related to the fine-grain strengthening and high relative density compared to samples obtained from other direct additive manufacturing technologies and sintered at other temperatures. The indirect additive manufacturing technique based on the use of nanopowder precursors developed in this work provides a new technical approach for the additive manufacturing of high-performance tungsten and/or other refractory metals.

Additive Manufacturing by Heating at a Patterned Photothermal Interface

Direct additive manufacturing (AM) of commercial silicones is an unmet need with high demand. We report a new technology, heating at a patterned photothermal interface (HAPPI), which achieves AM of commercial thermoset resins without any chemical modifications. HAPPI integrates desirable aspects of stereolithography with the thermally driven chemical modalities of commercial silicone formulations. In this way, HAPPI combines the geometric advantages of vat photopolymerization with the materials properties of, for example, injection molded silicones. We describe the realization of the new technology, HAPPI printing using a commercial Sylgard 184 polydimethylsiloxane resin, comparative analyses of material properties, and demonstration of HAPPI in targeted applications.

Manufacturing Constraints in Topology Optimization for the Direct Manufacturing of Extrusion-Based Additively Manufactured Parts

Additive manufacturing is a potentially disruptive technology with a high impact on supply chains and part design. While generally allowing much higher degrees of freedom in design than formative and subtractive manufacturing techniques, the necessity of support structures can diminish the impact of additive manufacturing. This article presents a methodology based on finite spheres to integrate knowledge about process limitations into topology optimization for the direct extrusion-based additive manufacturing of parts with maximized stiffness and strength and minimized support structures. This methodology has been included within our self-developed Freeware Z88 Arion® V3. We investigated the impact of the manufacturing constraints on the additive manufacturing process regarding effective material usage on application test examples. The test results showed that the design proposals created while applying the finite spheres and two-step smoothing needed significantly less or no support material for all application examples.

Direct additive manufacturing of TiCp reinforced Al2O3-ZrO2 eutectic functionally graded ceramics by laser directed energy deposition

Laser directed energy deposition (LDED) provides an ideal manufacturing technique to produce ceramic matrix composites, which are endows with superior and customizable mechanical properties by the reinforcement of gradient distribution of rigid particles. In this paper, we for the first time manufactured TiCp reinforced Al2O3-ZrO2 eutectic functionally graded ceramics with two different transition modes using LDED. The results show that the gradient transition realizes gradually increase of TiCp particles in Al2O3-ZrO2 eutectic matrix. With the increase of TiCp content, the morphology of Al2O3-ZrO2 eutectic matrix changes from lamellar or rod-shaped to irregular shape. Meanwhile, LDED realizes controllable fabrication of properties in different positions of gradient materials, and the wear resistance of TiCp rich regions has increased by 43.4% compared with pure Al2O3-ZrO2 region.

Thermoelectric Printhead Cooler for a Stable Process and Curing Control in RTV-2 Silicone Additive Manufacturing by Direct Ink Writing

The direct additive manufacturing, such as Direct Ink Writing (DIW), of silicone parts holds cost and complexity advantages. However, for extruding strands of low viscosity heat curing silicone rubbers, a build plate temperature of up to 90°C is used, which comes with the challenge of early vulcanization inside the printhead. Thus, this paper presents a thermoelectric cooler for universal printheads. At a build plate temperature of 90°C it prolongates vulcanization by 198.5 % (∆tpot life = 508.8 s), which allows a reduction of print speed for higher quality. Additionally, the influence of external temperature is reduced and the viscosity can be controlled.

Direct Additive Manufacturing of Al2O3-TiCp Composite Ceramics by Laser Directed Energy Deposition

Direct Additive Manufacturing of Al₂O₃-TiC_p Composite Ceramics by Laser Directed Energy Deposition

One-step additive manufacturing of TiCp reinforced Al2O3–ZrO2 eutectic ceramics composites by laser directed energy deposition

With the rapid and adaptable process properties, laser directed energy deposition (LDED) is increasingly becoming popular for the direct additive manufacturing of melt growth eutectic composite ceramics. In this research, LDED is used to fabricate TiCp-reinforced Al2O3–ZrO2 eutectic ceramics. The inhibitory mechanism of various ratios (0 wt % - 50 wt %) of TiCp on crack and porosity defects is investigated. In addition, a summary of the influence of TiCp on the microstructure and mechanical properties is provided. The results indicate that TiCp particles are uniformly distributed throughout the matrix, and the eutectic microstructure around TiCp exhibits an irregular eutectic transition as a result of a decrease in solidification rate. The mismatch of thermal expansion coefficient and elastic modulus between TiCp and Al2O3–ZrO2 eutectic matrix causes the cracks to be pinned and transgranular, which suppresses the cracks effectively. Meanwhile, doping TiCp particles enhances the molten pool impact and increases the molten pool temperature, which accelerates the gas escape rate, leading to the porosity decreasing from 6.37% to 0.29%. In comparison to Al2O3–ZrO2 eutectic ceramics, 50 wt% TiCp content results in the greatest flexural strength and fracture toughness, with increases of 34.18 and 13.42%, respectively. The greatest compressive strength is attained at 30 wt% TiCp doping, which is approximately 83.75% greater than eutectic ceramics.