Tensile, Fatigue Properties and Their Anisotropies of Al-Mg Alloy Fabricated by Wire-Arc Additive Manufacturing.

The microstructure, mechanical properties (tensile, fatigue, etc.) and the anisotropies of the Al-Mg alloy fabricated by wire arc additive manufacturing are studied in this work. The results show that the microstructure of the deposited alloy is composed of coarse columnar grains in the inner-layer region and fine equiaxed grains in the interlayer region. The tensile and fatigue properties exhibit strong anisotropies. The ultimate tensile strength (258 MPa), yield strength (140 MPa), elongation (21.3%), and fatigue life (2.56 × 105) of the sample along travel direction (0° direction) are the best, whereas those of the sample along the deposited direction (90° direction) are the lowest and those of the sample along 45° direction are the medium. It is found that the lowest strength and elongation of the sample in the deposited direction can be attributed to the large weak bonding areas between the deposition layers, whereas the lowest fatigue property is associated with the fatigue crack propagation along the grain boundaries of the columnar grains.

Cork Powder Residues Processing by Binder Jetting.

Cork-based formulations adapted to binder jetting processes were herein developed and investigated. Two cork powder sets with different particle size distributions were studied to evaluate cork particles' ability to pack. Cork powders exhibiting a coarse distribution revealed a higher packing ability. In addition, owing to cork's lower affinity to water-based binders, the addition of two hydrophilic additives was explored. 3D-printed (3DP) cork parts with a simple geometry were first printed. An innovative technique was evaluated as a postprocessing phase to improve cork particle adhesion after printing. Inspired by the production of expanded cork agglomerates, use of autoclave technique as a postprocessing phase for cork parts was proposed. After the autoclave, 3DP parts exhibited an improved adhesion of cork particles, demonstrated by morphological and mechanical analyses. Fourier transform infra-red analyses demonstrated that the polysaccharide and suberinic fractions were also affected by the autoclave. 3DP cork parts with a complex design solution were successfully printed. This study contributes to new and complex design solutions for cork-based products maintaining cork's natural lightness, warmness, and softness to the touch.

Experimental Investigation on Friction Behavior of Selective Laser Sintering Processed Parts.

Selective laser sintering (SLS) is a powder bed fusion additive manufacturing process that uses polymer powders to produce functional parts directly from digital 3D models. SLS supports small- to medium-batch fabrication of customized products for various end-use applications. These parts can be used as tooling to support conventional manufacturing and inspection where mechanical and tribological behaviors are important. This article evaluates the friction behavior of parts fabricated by SLS polyamide (PA12) and glass bead-filled polyamide (PA-GF) materials. Pin-on-Disk tribometer was used to characterize the behavior in dry sliding conditions with part build orientation, load, and speed as factors. Taguchi's approach and analysis of variance are used to quantify the influence of these factors on the friction coefficient. Both materials show reduction in friction coefficient with the increase in load; on the other hand, an increase in speed affects adversely. However, the influence of part orientation is comparatively lesser than the other two factors considered in this study. The debris, pin, and disk wear surfaces were examined to learn wear mechanisms. It is observed that surface porosity resulting from the fabrication process plays a significant role in friction behavior. The glass beads in PA-GF material function as reinforcement against load and influences the friction behavior. Low friction and wear characteristics make both materials as a prospective contender for tooling application.

Magnetic Stimulation for Programmed Shape Morphing: Review of Four-Dimensional Printing, Challenges and Opportunities.

In the field of Additive Manufacturing, four-dimensional (4D) printing has emerged as a promising technique to fabricate smart structures capable of undergoing shape morphing in response to specific stimuli. Magnetic stimulation offers a safe, remote, and rapid actuation mechanism for magnetically responsive structures. This review provides a comprehensive overview of the various strategies and manufacturing approaches employed in the development of magnetically stimulated shape morphing 4D-printed structures, based on an extensive literature search. The review explores the use of magnetic stimulation either individually or in combination with other stimuli. While most of the literature focuses on single-stimulus responsive structures, a few examples of multi-stimuli responsive structures are also presented. We investigate the influence of the orientation of magnetic particles in smart material composites, which can be either random or programmed during or after printing. Finally, the similarities and differences among the different strategies and their impact on the resulting shape-morphing behavior are analyzed. This systematic overview functions as a guide for readers in selecting a manufacturing approach to achieve a specific magnetically actuated shape-morphing effect.

Effects of Pulse Interval on Forming Process in Wire Arc Metal Additive Manufacturing Using Pulsed Arc Plasma.

To overcome the material processing challenges induced by high levels of heat input in wire arc additive manufacturing (WAAM), an innovative WAAM method using pulsed arc plasma (PAP-WAAM), was developed by the authors in the previous study. In this method, the PAP generated by the pulsed voltage was used as the heat source. The pulse interval can be defined as the time interval between adjacent pulse voltages, which determines the ignition time and frequency of the arc plasma, thus influencing the forming process. However, the effect of pulse interval on the forming process has not yet been revealed. Here, the effects of pulse interval on forming process during the PAP-WAAM of Ti6Al4V, including thermal behavior, arc plasma characteristics, and metal transfer process, were investigated by experiments and simulation. The results exhibited that the interpass temperature and maximum peak temperature decrease with increasing pulse interval at the same arc plasma power, indicating an alleviation of heat accumulation along the building direction. As the pulse interval increased, the ignition mode of the arc plasma changed from ignition between the tungsten electrode and the previously deposited layer to ignition between the tungsten electrode and filler wire, which increased the proportion of discharge energy allocated to the filler wire, thus reducing the overall heat input required for material deposition. When the pulse interval was 300 and 400 ms, only the uninterrupted bridging transfer mode was observed during the deposition process. The uninterrupted bridging transfer is considered to contribute to forming a smooth and consistent layer appearance. In addition, longer pulse intervals resulted in less surface oxidation, narrower wall thickness, and better macrostructure, attributed to reduced heat input and improved effective heat dissipation. This research reveals the effect of pulse interval on forming process during PAP-WAAM, which benefits the fabrication of desirable metal parts.

Additively Manufactured Zirconia for Dental Applications with In Situ Color Gradation Control.

The current process for creating zirconia reconstructions for teeth is time-consuming, expensive, and results in tool wear and raw material waste. An alternative method, near-net shape additive manufacturing capable of producing samples with color gradient is presented as an innovative and more efficient solution. A low-cost robocasting system, which enabled the co-extrusion of two different ceramic inks, was designed and developed. Moreover, hydrogel-based ceramic inks, with rheological properties suitable for the present system, containing commercially available Yttria-stabilized zirconia (white and yellow) powders, were produced. Parts of different color shades and color gradients, with a high aspect ratio and good green body stability, were printed. In addition, precise color measurements were carried out, and co-extruded parts were compared with ultraviolet-C photofunctionalized parts. High fractions of binder in green bodies caused distortions in the samples during air drying and upon sintering. Debinding and sintering at 1500°C yielded parts of relatively low density (4.90-5.09 g/cm3) and hardness (500 HV10-1100 HV10). A slightly different sintering behavior was observed for parts of different compositions. Density and hardness increased with the fraction of iron oxide-containing ink.

Scalable Hierarchically Structured Materials from a Multiscale Particle System Enabled by Microscaffolds.

Structural hierarchy is the key to manufacturing multiscale particle-based composite materials. A novel manufacturing method was developed to generate scalable hierarchical structures in concrete. The new method used 3D-printed microscaffolds to interact with the multiscale particle packing in concrete, resulting in a structured lightweight composite material. The size of internal members can vary by more than two orders of magnitude, to adapt to different applications. Based on compression tests and microstructural investigation by optical microscope and quantitative nanomechanical mapping, we found that the new material is 63.93% more efficient in energy absorption capacity compared with traditional lightweight concrete. Our experimental trials also showed that introducing structural hierarchy can reduce the consumption of cementitious material in the system by up to 14% and significantly reduce the use of scaffolds. The method could be applied to a board spectrum of multiscale particle-based materials, such as dental cement and bone implant materials, to improve material performance and efficiency in medical and construction applications.

Large-Scale Hollow-Core 3D Printing: Variable Cross-Section and Printing Features for Lightweight Plastic Elements.

Hollow-core 3D printing (HC3DP) proposes a new method for the production of lightweight, material-efficient thermoplastic 3D printed elements. This new fabrication approach promises material savings of 50-80%, while increasing the extrusion rate significantly (factor 10). This development pushes HC3DP to a similar fabrication speed as high-resolution concrete 3D printing. However, fundamental research on printing features enabled by this novel 3D printing approach is missing. Therefore, this article investigates printing with user-controlled bead dimensions (same nozzle, different size). It is showcased that the size of the extruded cross-section is determined by the positive air pressure used to inflate the beads. Multiple samples are printed, changing the layer height and width significantly without making changes to the hardware setup. Sections of 3DP samples are analyzed and the parameters of 3DP beads are determined. Furthermore, a set of bespoke 3D printed nozzles is introduced to subdivide the HC3DP beads into distinct areas. So far only regular beads, such as hollow tubes, have been used for 3D printing. Samples of those bespoke sections are analyzed to investigate their behavior when used for 3D printing. Finally, large-scale 3D printing experiments are conducted to investigate how printing features like bridging, cantilevering, or nonplanar 3D printing can be manufactured with hollow extrusion beads. In summary, this article provides insights into the fundamental 3D printing behaviors of HC3DP, showcases new design possibilities with bespoke and variable cross-sections, and finally proposes new research trajectories based on the findings presented.

Intelligent Optimization Method of Piezoelectric Ejection System Design Based on Finite Element Simulation and Neural Network.

This study describes an intelligent method for modeling and optimization of piezoelectric ejection system design for additive manufacturing. It is a combination of neural network (NN) techniques and finite element simulation (FES) that allows designing each parameter of a piezoelectric ejection system faster and more reliably than conventional methods. Using experimental and literature data, a FE model of the droplet ejection process was developed and validated to predict two indexes of droplet ejection behavior (DEB): jetting velocity and droplet diameter. Two artificial neural network (ANN) models based on feed-forward back propagation were developed and optimized by genetic algorithm (GA). A database was established by FE calculations, and the models were trained to establish the relationship between the piezoelectric ejection system design input parameters and each DEB indicator. The results show that both NN models can independently predict the droplet jetting velocity and droplet diameter values from the training and testing data with high accuracy to determine the optimal piezoelectric ejection system design. Finally, the accuracy of the prediction results of the FES and ANN-GA models was verified experimentally. It was found that the errors between the predicted and experimental results were 4.48% and 3.18% for the jetting velocity and droplet diameter, respectively, verifying that the optimization method is reliable and robust for piezoelectric ejection system design optimization.

Finite Element Simulation of Residual Stress in Laser Additive Manufactured Functionally Gradient Materials Based on Bessel Heat Source Model.

Laser additive manufacturing (LAM) technology has the advantages of short manufacturing cycles, low material waste rate, and design ability. It is especially suitable for preparing functionally gradient materials (FGM). However, due to the large temperature gradient and the change in material composition, the residual stress is very high, which will seriously affect the mechanical properties and manufacturing accuracy of the structure. In this study, the thermomechanical coupled finite element model based on the Bessel heat source is established, and the residual stress in LAM TC4/TC11 FGM is obtained. The results show that the Bessel heat source can effectively suppress the generation of residual stress in the additive manufacturing process, and the finite element results are consistent with the experimental results. Compared with the traditional Gaussian heat source, the maximal residual tensile stress is reduced by an average of 28.1%. The value of residual stress increases with the increase in the number of printing layers, and it increases with the increase of the laser power and decreases with the increase of the scanning speed. The overall trend is that the two sides are compressive stress and the middle is tensile stress. The research has important reference significance for the reasonable suppression of the residual stress in FGM produced by LAM.

Review: Application of 3D Printing Technology in Soft Robots.

Soft robots, inspired by living organisms in nature, are primarily made of soft materials, and can be used to perform delicate tasks due to their high flexibility, such as grasping and locomotion. However, it is a challenge to efficiently manufacture soft robots with complex functions. In recent years, 3D printing technology has greatly improved the efficiency and flexibility of manufacturing soft robots. Unlike traditional subtractive manufacturing technologies, 3D printing, as an additive manufacturing method, can directly produce parts of high quality and complex geometry for soft robots without manual errors or costly post-processing. In this review, we investigate the basic concepts and working principles of current 3D printing technologies, including stereolithography, selective laser sintering, material extrusion, and material jetting. The advantages and disadvantages of fabricating soft robots are discussed. Various 3D printing materials for soft robots are introduced, including elastomers, shape memory polymers, hydrogels, composites, and other materials. Their functions and limitations in soft robots are illustrated. The existing 3D-printed soft robots, including soft grippers, soft locomotion robots, and wearable soft robots, are demonstrated. Their application in industrial, manufacturing, service, and assistive medical fields is discussed. We summarize the challenges of 3D printing at the technical level, material level, and application level. The prospects of 3D printing technology in the field of soft robots are explored.

Physics-Informed Machine Learning for Accurate Prediction of Temperature and Melt Pool Dimension in Metal Additive Manufacturing.

The temperature distribution and melt pool size have a great influence on the microstructure and mechanical behavior of metal additive manufacturing process. The numerical method can give relatively accurate results but is time-consuming and, therefore, unsuitable for in-process prediction. Owing to its remarkable capabilities, machine learning methods have been applied to predict melt pool size and temperature distribution. However, the success of traditional data-driven machine learning methods is highly dependent on the amount and quality of the training data, which is not always convenient to access. This article proposes a physics-informed machine learning (PIML) method, which integrates data and physics laws in the training parts, overcoming the problems of low speed and data availability. An artificial neural network constrained by the heat transfer equation and a small amount of labeled data is developed to predict the melt pool size and temperature distribution. Besides, the locally adaptive activation function is utilized to improve the prediction performance. The result shows that the developed PIML model can accurately predict the temperature and melt pool dimension under different scanning speeds with a small amount of labeled data, which shows significant potential in practical application.

Defects in Metal Additive Manufacturing: Formation, Process Parameters, Postprocessing, Challenges, Economic Aspects, and Future Research Directions.

Metal additive manufacturing (AM) is a revolutionary technological advancement that has made significant inroads in a wide range of sectors, including aerospace, defense, automotive, health care, and engineering applications. It offers unprecedented design freedom, reduced material waste, and enhanced performance, in addition to significant enhancements to fabrication processes. Microstructural defects and internal stresses formed during fabrication directly affect the fabricated product's surface integrity, quality, and service life. Identification, characterization, and prediction of these defects help significant and direct production of defect-free structures with high density. This article provides detailed insights concerning the common defects, mitigation techniques, and challenges reported in both powder bed fusion-based and wire arc AM methods. Defects such as porosity may develop due to the powder sphericity, roughness of the powder, preheating, process parameters, build environment, postprocessing techniques, and environmental factors. Therefore, a critical study of the techniques, alloys, process parameter optimization, and different postprocessing techniques to tone down the defects is made from their formations.

Review on Multiscale Transport Phenomena in the Metal-Based Additive Manufacturing Process.

Metal-based additive manufacturing (MAM) method with high freedom and special fabricate technology has presented great universality in the aerospace and biomedicine field. However, a wide range of process parameters in the MAM method challenge the experimental study on the formation and evolution of defects. The numerical simulation presents its excellent accuracy and economy in predicting the evolution of multiphysics phenomena and was hence widely applied. In the current review, the available MAM methods with the fundamental phenomena were reviewed. Based on scales, numerical approaches divided into three categories were discussed and focused on their main prediction objectives and strengths or weaknesses of all the scales. To display the prediction results closer to real physical phenomena, advanced multiscale models coupled with various single-scale models are provided. The high prediction accuracy and computational efficiency enable better parameter control and defect avoidance. As a supplement and development to the physical-driven model, the data-driven model provides a new perspective on MAM methods. Based on the database generated by the physical-driven model and experiment, the data-driven models without calibration of input parameters are shown. In addition, this review discussed the development direction of numerical simulation, aiming to provide a reference for technical research in this field.

Review on Engineering of Bone Scaffolds Using Conventional and Additive Manufacturing Technologies.

Bone is a complex connective tissue that serves as mechanical and structural support for the human body. Bones' fractures are common, and the healing process is physiologically complex and involves both mechanical and biological aspects. Tissue engineering of bone scaffolds holds great promise for the future treatment of bone injuries. However, conventional technologies to prepare bone scaffolds cannot provide the required properties of human bones. Over the past decade, three-dimensional (3D) printing or additive manufacturing technologies have enabled control over the creation of bone scaffolds with personalized geometries, appropriate materials, and tailored pores. This article aims to review recent advances in the fabrication of bone scaffolds for bone repair and regeneration. A detailed review of bone fracture repair and an in-depth discussion on conventional manufacturing and 3D printing techniques are introduced with an emphasis on novel studies concepts, potentials, and limitations.

Smoothing the Rough Edges: Evaluating Automatically Generated Multi-Lattice Transitions.

Additive manufacturing is advantageous for producing lightweight components while addressing complex design requirements. This capability has been bolstered by the introduction of unit lattice cells and the gradation of those cells. In cases where loading varies throughout a part, it may be beneficial to use multiple, distinct lattice cell types, resulting in multi-lattice structures. In such structures, abrupt transitions between unit cell topologies may cause stress concentrations, making the boundary between unit cell types a primary failure point. Thus, these regions require careful design to ensure the overall functionality of the part. Although computational design approaches have been proposed, smooth transition regions are still difficult to achieve, especially between lattices of drastically different topologies. This work demonstrates and assesses a method for using variational autoencoders to automate the creation of transitional lattice cells, examining the factors that contribute to smooth transitions. Through computational experimentation, it was found that the smoothness of transition regions was strongly predicted by how closely the endpoints were in the latent space, whereas the number of transition intervals was not a sole predictor.

Mechanical Properties of 3D Printed Parts and Their Injection Molded Alternatives Subjected to Environmental Aging.

Additive manufacturing is the technology used in medical, industrial, or lifestyle applications. The scientific literature include works reporting various manufacturing parameters' influence on changes in additive manufacturing components' mechanical behavior, especially with fused filament fabrication (FFF). The changes in mechanical strength and toughness of FFF compared to injection molding parts were studied. In the study, the FFF and injection molded parts were aged in buffered saline solution in temperature of 37°C. The results show that by differentiating the orientation of the fibers during fabricating, it is possible to reach strength values similar to injection molded parts. Therefore, it was reported that the mechanical strength and toughness changed significantly after aging, and the FFF components lost strength more quickly than their injected alternatives. The research results can be useful during the fabrication of mechanically stable and biodegradable components, which can be more easily recycled than their injected alternatives when used with warmer temperatures and humidity. This article completes the present state of the art on the problem of environmental aging of parts produced from biodegradable materials. Especially, the research was related to the multilayer laminate structure.

A Review on Residual Stress in Metal Additive Manufacturing.

A new method for producing parts in the expanding sector is additive manufacturing. The appropriate name for three-dimensional (3D) printing is additive manufacturing because it produces the part layer by layer. Plastics and metals can be 3D printed in large quantities with the precise surface finish and feature quality needed in additive manufacturing. More specifically, direct metal sintering, direct energy deposition, and metal binder jetting are used in 3D printing. The computer-aided design model is completed when the powder bed has been successively scanned and lowered. The metal sintering process uses a powder bed with powder metal, and laser selectively melts a flattened bed of powder, which is done with roller with successive rolling of new layer on previous into desired shape before a new layer is pushed on top of the previous layer. As a result, the new layer has solidified on top of the earlier layer, causing the prior layer to melt back again. Because of the unique thermal cycle, this results in residual stress (RS). The unique thermal cycle of metal additive manufacturing is characterized by (1) rapid heating rate caused by high energy intensity and steep temperature gradients; (2) rapid solidification with high cooling rates because of the small volume of melt pool; and (3) melt back, which involves simultaneous melting of the top powder layer and re-melting of underlying previously solidified layers. The presence of RS in metal additive manufacturing (AM) creates difficulties that restrict the process's ability to produce parts at an industrial scale. During and after manufacturing, these forces may cause parts to distort and crack. This can be solved by heating the powder bed, which will lessen this type of issue. The causes, traits, and reduction of RS are the main topics of this review article. A number of conceptual approaches to reducing RS are addressed to provide some useful inspiration for creating a methodical RS balancing procedure for AM. These approaches are based on the state and future of the relevant techniques.

Additive manufacturing and in vitro study of biological characteristics of sulfonated polyetheretherketone-bioactive glass porous bone scaffolds.

Polyetheretherketone (PEEK), a high-performance special engineering plastic, has gradually been used in bone substitutes due to its wear resistance, acid and alkali resistance, non-toxicity, radiolucency, and modulus close to that of human bone. However, its stable biphenyl structure determines strong biological inertness, thus artificial interventions are required to improve the biological activity of fabricated PEEK parts for better clinical applications. This study developed a novel strategy for grafting bioactive glass (BAG) onto the surface of PEEK through sulfonation reaction with concentrated sulfuric acid (H2SO4), aiming to improve the bioactivity of printed porous bone scaffolds manufactured by fused deposition modeling (FDM) to meet clinical individual needs. In vitro biological study was conducted on sulfonated polyetheretherketone-bioactive glass (SPEEK-BAG) scaffolds obtained by this strategy. The results demonstrated that the optimal modification condition was a 4-hour sulfonation reaction with 1 mol/L concentrated H2SO4 at high temperature and high pressure. The scaffold obtained under this condition showed minimal cytotoxicity, and the Ca/P molar ratio, yield compressive strength, and compressive modulus of this scaffold were 2.94 ± 0.02, 62.78 MPa, and 0.186 GPa respectively. The hydrophilicity and the biomineralization ability of PEEK modified by the proposed strategy were substantially improved. The SPEEK-BAG bone scaffolds exhibited excellent biocompatible properties, suggesting that the sulfonation reaction and BAG effectively enhanced the proliferation and differentiation of osteoblasts. The presented method provides an innovative, highly effective, and customized strategy to improve the biocompatibility and bone repair ability of printed PEEK bone scaffolds for virous biomedical applications.

Zirconia crowns manufactured using digital light processing: Effects of printing angle and layer thickness on the accuracy.

OBJECTIVES: This study investigated the effects of build angle and layer thickness on the trueness and precision of zirconia crowns manufactured using digital light processing (DLP) technology. MATERIALS AND METHODS: Single crowns were fabricated from zirconia using DLP technology. The crowns were manufactured with three different representative build angles (0°, 45°, and 90°) and two different layer thicknesses (30 µm and 50 µm). After printing, the specimens were non-contact-scanned, and their accuracy was assessed using a 3D analysis software. Root mean square (RMS) values were used to determine trueness and precision. Color maps were generated to detect deviations within the specimens. Statistical analyses were conducted using two-way ANOVA. RESULTS: Build angle and layer thickness significantly affected trueness and precision (p < 0.05). At a 30-µm layer thickness, the crowns printed at angles of 0° (32.2 ± 3.2 µm) and 45° (33.9 ± 2.4 µm) demonstrated the best marginal trueness compared to those in other groups (p < 0.05). Notably, those printed at an angle of 90° exhibited the best intaglio surface trueness (37.4 ± 4.0 µm). At a 50-µm layer thickness, the crowns printed at an angle of 90° exhibited the lowest accuracy concerning marginal and intaglio surface aspects (27.7 ± 8.2 µm). CONCLUSIONS: Both the build angle and layer thickness significantly affected the dimensional accuracy of DLP-printed zirconia crowns, with the 30-µm layer thickness offering superior trueness. Optimal results were achieved using build angles of 0° and 45° in conjunction with thinner layers, minimizing marginal defects. CLINICAL SIGNIFICANCE: All zirconia crowns produced at different build angles and layer thicknesses satisfied clinical requirements. Specific combinations of these factors realized the fabrication of single crowns that possessed the highest accuracy.