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Texturing is an approach to tailoring the properties of materials along specific directions by arranging and growing grains directionally. In this work, a commercially available binder jetting machine equipped with the roller compaction feature was used to fabricate textured ceramics via templated grain growth. Two types of powders were prepared: one with 100 wt% alumina nanoparticles as the baseline, and the other with 75 wt% alumina nanoparticles as the matrix and 25 wt% alumina nanoplatelets as the templates for grain growth. The bulk density of the sintered samples with the alumina nanoplatelets was 4 % lower than that without them. The scanning electron microscopy and X-ray diffraction results showed that morphological and crystallographic textures were achieved in the samples containing the alumina nanoplatelets. The flexural strength results indicated that the textured structure led to an improvement in the flexural strength, from 77 MPa to 124 MPa, despite the lower bulk density of the textured samples. This study demonstrates that commercially available binder jetting machines with the roller compaction feature can effectively produce textured ceramics with customized properties along desired directions.

In binder jetting additive manufacturing, the final part density is closely related to powder bed density and green density. Though powder bed density is considered one of the most critical factors to control and predict green density, their difference has not been fully understood. In this work, experiments and simulations were employed to investigate their difference for a free-flowing powder in a counter-rotating roller powder spreading system. Using an alumina powder with excellent flowability, both experiments and simulations show that powder bed density is higher than green density. Furthermore, simulations provide an in-depth understanding of the density difference. Specifically, in the powder bed density scenario where particles are able to move across different powder layers, a slower powder flow with a smooth velocity transition is observed. In the green density scenario where powder is spread on a printed powder layer, the confined layer space generates a powder flow with higher velocity and more turbulence. As a result, the powder packing structure in the green density scenario has more void space near layer interface than that in the powder bed density scenario. This paper reports a reason for the difference between powder bed density and green density for the first time.

The objective of this experimental investigation is to identify the effect of roller traverse and rotation speeds on ceramic binder jetting additive manufacturing. Green and sintered densities were found decreasing with increasing roller traverse speed, and this decrease was more significant at a low roller rotation speed. A high roller traverse speed led to more cavities on the powder bed surface after spreading a new layer on a printed layer. Furthermore, a high roller traverse speed caused more cracks on the powder bed surface after jetting the binder. These defects observed during printing were inherited by the green and sintered parts, which were confirmed by X-ray computed tomography and scanning electron microscopy, respectively. Finally, the sintered flexural strength of the parts printed at a high roller traverse speed was more than 30% higher than that of the parts printed at a low roller traverse speed.

In powder bed additive manufacturing, feedstock powder can be dispensed onto powder bed through a hopper. The hopper dispensing system assisted by ultrasonic vibration is effective, especially for powder with small particle sizes (e.g., <10 μm). However, the ultrasonic hopper dispensing system does not dispense powder at a constant rate. The current remedy for this issue is to dispense more powder than necessary to ensure there is sufficient powder to cover the powder bed. However, this practice often results in longer dispensing time and larger amount of dispensed powder than necessary, and hence, higher manufacturing cost of printed parts. Therefore, it is desirable to understand powder dispensing behavior of the ultrasonic hopper dispensing system. This paper reports an experimental study on powder dispensing behavior of an ultrasonic hopper dispensing system equipped on a commercial binder jetting 3D printer (Innovent+, ExOne, PA, USA). Dispensing variables investigated are ultrasonic mode, ultrasonic intensity, and initial feedstock amount loaded into the hopper. In addition, the effects of cumulative number of dispensing cycles are also investigated. Results show that dispense rate changes as ultrasonic mode changes; it decreases when vibration intensity decreases, initial feedstock amount increases, and cumulative number of dispensing cycles increases. Based on the findings from this study, several approaches can be proposed to minimize the variation in dispense rate during printing, for example, to increase ultrasonic intensity as cumulative number of dispensing cycles increases.

Discontinuous carbon fibers (d-CFs) with different surface modifications (sizing, carboxyl-grafting, and norbornene-grafting) were separately dispersed in dicyclopentadiene (DCPD) resins, and the resultant d-CF/DCPD mixtures demonstrated shear-thinning characteristics. Subsequently, d-CF/DCPD mixtures were printed and further transformed into crosslinked polymers through self-propagating frontal polymerization. The norbornene groups grafted on the d-CF surfaces also reacted with the DCPD monomers during the fontal polymerization-synchronized printing, resulting in covalent connections between d-CFs and DCPD resins. The tensile strength of as-printed norbornene-grafting d-CFs/DCPD composites was around 43.3 MPa, which was 170% higher than that of as-printed neat DCPD resin and 15% higher than that of the sized d-CFs/DCPD composites. The toughness of as-printed norbornene-grafting d-CFs/DCPD composites was ~14 MPa, ~33.3% higher than that of the sized d-CF/DCPD composites. More importantly, the presence of norbornene-grafted d-CFs significantly strengthened the bonding strength of printed layers since d-CFs bridged two neighbored layers. The bonding strength of printed layers was enhanced by ~255% with addition of 3 wt% norbornene-grafting d-CFs compared to the neat resin. This method provides a new way to improve the interlayer bonding in energy-efficient 3D printing thermosetting composites.

This study demonstrates the feasibility of fabricating copper/diamond (Cu/Dia) composite samples using binder jetting additive manufacturing for the first time. Binder jetting fabricates parts in two steps: forming green parts with a binder at a low temperature and consolidating the green parts by sintering. The low processing temperature makes binder jetting an excellent candidate to create complex-shaped Cu/Dia parts without causing diamond graphitization. In this study, mixtures of copper-coated diamond powder and pure copper powder were used to print Cu/Dia samples on a binder jetting printer. The printed samples were then sintered under a medium vacuum environment with flowing forming gas (95% nitrogen and 5% hydrogen). The effects of diamond volume fraction (10 vol% and 50 vol%) and sintering temperature (800 °C and 900 °C) on density and porosity, phase composition, and microstructure of Cu/Dia samples were investigated. As the diamond volume fraction increased from 10 vol% to 50 vol%, the sinterability of the powder mixture decreased. Increasing sintering temperature from 800 °C to 900 °C significantly improved sintered bulk density for samples from each powder mixture. When sintering temperature increased from 800 °C to 900 °C, the microstructure evolved from a granular structure to a network structure. In all sintered samples, the distribution of diamond was uniform and there was no sign of diamond graphitization.

An experimental study was conducted on the effects of point distance and hatch distance on pore formation in selective laser melting of tungsten. The Archimedes’ method was used to measure the apparent and closed porosities. Apparent (open) porosity and closed porosity were found to follow different trends: the apparent porosity increased but the closed porosity decreased as hatch distance or point distance increased. The open pores are attributed to the lack of fusion at the large hatch distance or point distance while the closed pores are attributed to the possible spattering and keyhole formation at the smaller hatch distance or point distance. The results led to understanding the effects of hatch distance and point distance on the formation of open and closed pores and provided insight on the optimization of process parameters to further increase the relative bulk density.

Powder-bed additive manufacturing, including powder bed fusion and binder jetting, has a wide range of applications in various industries. Powder spreading is a critical step of powder-bed additive manufacturing and has a determinative impact on powder bed quality and thus final part quality. This paper provides a literature review on powder spreading, focusing on the effects of influencing factors on powder bed quality. Three groups of influencing factors are discussed: spreaders, spreading parameters, and feedstock powder properties. Besides the effects of individual factors, the interaction effects between these factors are also discussed where applicable. Powder bed quality is discussed in terms of powder bed density and powder bed surface condition. Furthermore, knowledge gaps and research opportunities are presented as concluding remarks.

Optical K9 glass has been used in a variety of applications, including optics, electronics, thermodynamics, and fluidics. It has been regarded as a difficult-to-cut material because of its superior mechanical properties. Rotary ultrasonic machining using cutting fluid has been used successfully to machine many different types of brittle materials, such as ceramics, sapphire, and optical K9 glass. Dry machining has been successfully employed in the machining of some materials. However, there are no reported investigations on the rotary ultrasonic machining of optical K9 glass using compressed air as a coolant. This article, for the first time, investigates the rotary ultrasonic machining of optical K9 glass using compressed air as a coolant. The experimental investigations focus on the effects of the input variables (ultrasonic power, spindle speed, and feedrate) on the output variables (edge chipping size, cutting force, surface roughness, and ultrasonic power consumption).

PolyJet 3D printing can be used to fabricate colored physical models of anatomical structures such as skull and heart with realistic appearances. These medical models can be used for surgical simulation and planning of complex operations, as well as anatomy teaching. PolyJet is theoretically capable of producing any color by mixing multiple materials. However, the measured color of a sample printed by PolyJet is often different from the specified color in the printer software. Therefore, it is often difficult to predict the measured color of a sample before printing. This paper reports a study on predictive relationships between measured color and four control factors of PolyJet (i.e., three RGB values of specified color and finish type) by design of experiments and application of multilayer perceptron (MLP) neural network model. Experimental data are collected using a full factorial design of experiments. These data are used to train and test the MLP model using 5-fold cross validation. Then, the prediction performances of the MLP model are compared with a linear regression model and a cubic regression model. The results show that the MLP model is capable of predicting measured color with higher accuracy.

Because of its unique properties, potassium dihydrogen phosphate crystals are widely used for electro-optical applications. Slicing is needed to make potassium dihydrogen phosphate crystal lens with desired shapes. However, reported studies on slicing of potassium dihydrogen phosphate crystal are scarce. This article presents an experimental investigation on slicing of potassium dihydrogen phosphate crystals. Output parameters studied include surface roughness, flatness error, parallelism error, and subsurface damage. Effects of wire sawing input variables (feed rate and diamond grit size) on output parameters are investigated. Comparisons of wire sawing and band sawing are also made.

Lithium ion batteries (LIB) is currently the most promising of all battery technologies for efficient storage of electrical energy and powering of electric vehicles. However, the ongoing LIB research faces multiple issues pertaining to materials, cost and safety. One of the major factors that dictate the performance of LIB is its electrode’s Li-storage (anode/cathode) capacity, cycleability and efficiency. In this article, the authors evaluate the recent experimental progress made in LIB anode materials prepared from polymer-derived ceramics (PDC). PDCs have several unique characteristics that differentiate them from other conventional carbon or silicon-based anodes. Most notably PDCs exhibit high chemical and thermodynamic stability under adverse operational conditions. Learnings from the analysis of experimental observations can be carried forward to design advanced PDCs, thereby improving overall performance of the battery, particularly for automotive applications.

Feedstock powders used in binder jetting additive manufacturing include nanopowder, micropowder, and granulated powder. Two important characteristics of the feedstock powders are flowability and sinterability. This paper aims to compare the flowability and sinterability of different feedstock powders. Three powders were compared: nanopowder (with a particle size of ~100 nm), micropowder (with a particle size of 70 µm), and granulated powder (with a granule size of ~70 µm) made from the nanopowder by spray freeze drying. Flowability metrics measured included apparent density, tap density, volumetric flow rate, mass flow rate, Hausner ratio, Carr index, and repose angle. Sinterability metrics employed included sintered bulk density, volumetric shrinkage, and densification ratio. Results show that the granulated powder has a higher flowability than the nanopowder and a higher sinterability than the micropowder. Moreover, different flowability metric values of the granulated powder are either higher or lower than those of the micropowder, indicating these two powers have a comparably high flowability. Similarly, different sinterability metric values of the granulated powder are either higher or lower than those of the nanopowder, indicating these two powders have a comparably high sinterability.

Because of its high sinterability, nanopowder could be beneficial for ceramic binder jetting additive manufacturing to achieve a high density on printed and sintered parts. However, the flowability of nanopowder is poor because of the large interparticle cohesion. This poor flowability prohibits the usage of nanopowder in ceramic binder jetting. In this study, to improve the flowability of nanopowder, alumina nanoparticles were granulated into micron-sized granules through spray freeze drying. The raw nanopowder and granulated powder were compared by characterizing their flowability and printability. Results showed that the granulated powder had a much better flowability than the raw nanopowder. Because of the superior flowability, the granulated powder formed a denser and smoother powder bed than the raw nanopowder and resulted in denser and smoother printed and sintered samples. These improvements indicated that the printability of nanopowder was improved by granulation.

Nanocomposites have been widely used to improve material properties. Nanoscale reinforcement materials in vat photopolymerization resins improve the hardness, tensile strength, impact strength, elongation, and electrical conductivity of the printed products. This paper presents a literature review on the effects of reinforcement materials on nanocomposite properties. Additionally, preprocessing techniques, printing processes, and postprocessing techniques of nanocomposites are discussed. The nanocomposite properties are summarized based on their applications in the mechanical, electrical and magnetic, and biomedical industries. Future research directions are proposed to improve the material properties of printed nanocomposites.

Commercially available dental computer aided design/computer aided manufacture (CAD/CAM) systems usually use abrasive machining processes such as diamond grinding and milling. One of the major disadvantages of abrasive machining processes is subsurface cracks. In the present paper, rotary ultrasonic machining (RUM) technology is introduced into machining of alumina dental ceramics. The subsurface cracks are observed under scanning electron microscope. Effects of ultrasonic vibration on the subsurface cracks are also investigated. Results show that RUM is a very promising process to significantly reduce subsurface cracks in CAD/CAM of dental ceramics.