Because of their enormous potential in biofabrication, adsorption, catalysis, and energy conversion applications, there has been substantial attention to fabricating 4D printed hierarchically porous structures from the molecular level to the macroscopic dimensions. To this end, an understanding of the structure-feature relationship of smart materials in 4D printing is necessary to design innovative constructs, which are not limited to any specific degree of freedom. Here, we report on the fabrication of thermo-responsive macroporous polymerized-high internal phase emulsions (poly-HIPEs) through 3D printing of Pickering-type HIPEs. The oil-in-water Pickering-HIPE-based inks contained methylcellulose/kappa-carrageenan blend (non-crosslinked) as a continuous phase, which was colloidally stabilized by a hybrid of cellulose nanocrystals and cellulose nanofibers. The Pickering-HIPE-based inks showed a non-linear and time-dependent oscillatory response with excellent viscoelastic interfacial properties. The poly-Pickering-HIPEs were readily fabricated by in-situ crosslinking of Pickering-HIPEs during hot-melt extrusion-based printing, which produced a series of 3D printed thermo-responsive hierarchical macroporous structures. The 4D printed objects presented a highly interconnected open-cell porous structure, which was thermo-responsive in nature. Moreover, these 4D structures showed high mechanical strength with outstanding self-recovery performance. Our results offer the prospect of developing a thermo-responsive macroporous construct having shape memory features at different temperatures by regulating emulsion formulation. Graphical Download : Download high-res image (248KB) Download : Download full-size image

Highlights • Airbrush 3D printer was proposed for accessible contact-less additive manufacturing. • The printer attained resolutions of 0 . 5 mm (in-plane) and 0 . 63 μ m (thickness). • It proved a robust method for 3D printing materials that are difficult to deposit. • Bending cantilever actuators were printed from P(VDF-TrFE-CTFE) electroactive polymer. • The actuators produced 340 μ m (quasi-static) and 3 . 7 mm (resonant) tip deflections. The additive manufacturing of electroactive polymer (EAP) devices poses significant challenges due to their distinct structure and dissimilar properties of their constituent materials. It requires deposition of multiple functional materials with different properties, achieving μ m -scale resolution in layer thickness, and executing incremental deposition and curing steps while preserving the previously deposited functional material layers. This study introduces an airbrush 3D printer concept and employs it for fabricating EAP transducers. An airbrush 3D printer was constructed by adapting a standard extrusion printer platform and integrating it with a two fluid atomizer (i.e. an airbrush) as the deposition tool. A process was developed for printing of the bending P(VDF-TrFE-CTFE) actuators with carbon black electrodes, and actuators with a single and dual EAP layers were fabricated. The airbrush printer attained in-plane resolution of 0 . 5 mm , thickness resolutions of 0.63 μ m and allowed atomizing up to 7% P(VDF-TrFE-CTFE) solutions. The 18 mm × 4 mm EAP actuators achieved 340 μ m (440 V p p ) and 3.7 mm (400 V p p , 104 Hz) tip deflections respectively in quasi-static and resonant operation. Airbrush printing therefore proved to be a robust method for printing precursor materials with a wide range of properties, and is anticipated to be a versatile approach for printing other passive and stimuli-responsive materials and devices. Graphical abstract Download : Download high-res image (310KB) Download : Download full-size image

Aluminum alloy 6061-T6511 was printed by additive friction stir deposition onto a cast aluminum A206-T4 plate. Metallography, X-ray powder diffraction, X-ray pole figure analysis, and electron backscatter diffraction of the center, advancing side, and retreating side of the 6061 deposit reveals different texture and grain structure characteristics across the transverse direction and through each layer. These distinct microstructural domains are indicative of spatially varied temperature, strain rate, and strain accumulation during deposition, and consequently of regions differing in the fraction of dynamic recovery and dynamic recrystallization during deposition. Hardness mapping over the complete cross section at 0.75 mm resolution shows the alloy continues to age after active deposition, and hardness increases over the 5 mm nearest the final printed layer. Additional mapping at 0.10 mm resolution shows intralayer hardness variations of the deposit on the retreating side. These observations demonstrate that complex and non-uniform thermal-mechanical transients occur during additive friction stir deposition, resulting in spatially non-uniform microstructure and properties.

This work presents a new technique to generate uniform and micron-sized metal powders for additive manufacturing. By collecting discrete chips resulting from ultrasonic vibration machining, we demonstrate the feasibility of all solid-state production consistent powders with tight dimensional tolerance, the ability to control powder geometry, and good efficiency. The technique offers a new route for sustainable and low-cost manufacturing of high-quality metal powders. The powder generation mechanism is analyzed with a special tool path design to ensure consistent dimensions over multiple cuts. An analytical model to predict the dimensions of produced powders under different cutting parameters is introduced. Aluminum and brass powders of different dimensions are produced, and the overall shear ratio that governs the deformation during the machining process is calibrated with the experimental results. The morphology consistency of produced powders is investigated over multiple hours of production, illuminating the role of tool wear on final powder shape. A high-efficiency powder collection system and a scalable solution for parallel production are proposed for the introduced technique. Additive manufacturing experiments (laser powder bed fusion) are conducted using produced A356 aluminum powders, demonstrating the printability of produced powders in additive manufacturing. The microhardness of the printed parts for five different process parameters is measured to be 45% higher than the raw material on average. Graphical abstract Download : Download high-res image (387KB) Download : Download full-size image

The ability to modulate microstructures at different locations within a part (i.e., site-specific microstructures) can lead to tailored properties and enhanced performance. However, it remains challenging to efficiently model the mechanical response of such materials due to the need for extensive multiscale analysis. To address this issue, we demonstrate through microstructure-based finite element simulations, that the mechanical properties of a randomly distributed two-phase microstructure are primarily dependent on the phase fraction. Therefore, by establishing a mathematical relationship between the phase fraction and mechanical properties, the local material properties can be efficiently calculated from the local microstructure. Later, these site-specific properties were incorporated into a finite element structure to predict the global mechanical properties of the sample part. To validate the effectiveness of the proposed method, low alloy steel with site-specific microstructures was fabricated using binder jet additive manufacturing by depositing varying concentrations of carbon binder at different locations inside the part. We observed a strong agreement between the experimental and simulation stress-strain responses of the steel samples. This method can potentially pave the way for the development of a non-destructive qualification method for additively manufactured parts, as well as provide an efficient approach to multiscale modeling of material behavior, which may help accelerate the design and innovation of novel material systems. Graphical Download : Download high-res image (272KB) Download : Download full-size image

Polydimethylsiloxane (PDMS) with excellent stretchability and biocompatibility, has been widely used as a flexible substrate for wearable and stretchable electronics. However, the drawback of poor tear resistance of PDMS greatly limits its applications, such as flexible electronics. Herein, a gradient structure Ecoflex-PDMS-Ecoflex (E-P-E) combining PDMS and Ecoflex was proposed by combining multi-material 3D printing with functional gradient material design to improve the tear resistance of flexible substrates. The E-P-E gradient structure substrate has a 73 times increase in fracture energy and a 7 times increase in tear strength compared with conventional PDMS substrates. Liquid metal-based strain sensors made with our designed E-P-E gradient structure substrate can be directly worn on the finger with low hysteresis (0.47%), good cyclic stability (0.04% change in sensitivity after 1000 cycles of 100% strain), and high strain range (>180%), which showed a broad application prospect in the fields of human motion monitoring and human-computer interaction. Graphical Download : Download high-res image (121KB) Download : Download full-size image A gradient structure Ecoflex-PDMS-Ecoflex (E-P-E) combining PDMS and Ecoflex was proposed by combining multi-material 3D printing with functional gradient material design to improve the tear resistance of flexible substrates. The E-P-E structure synergistically integrated the benefits of PDMS and Ecoflex while exhibiting exceptional rebound elasticity and crack insensitivity, showcasing a robust overall mechanical performance. Compared with the conventional PDMS substrate, the fracture energy, tear strength, fracture stress, and fracture strain were enhanced by 73 times, 7 times, 12 times, and 6.5 times, respectively.

In this paper, high-fidelity direct numerical simulations (DNSs) are carried out to explore the behavior of viscoelastic polymer flow in direct ink writing (DIW) additive manufacturing. The impact of ink elasticity on the stress for the strand deposition process has been examined. The exponential Phan-Thien Tanner (PTT) model is used to characterize the shear-thinning viscoelastic rheological behavior of polymeric fluids, and the gas-polymer interface is captured by the volume of fluid (VOF) method. Focusing on the extrusion and deposition of polymer, the effects of ink elasticity represented by the Weissenberg number (Wi), normalized printing speed and gap height on the formation of the printed strand have been studied. The results indicate that the polymer flow out of the nozzle is quite stable with time, and the polymeric stress balance is disturbed in the middle of the printed strand. For high Wi, the strand has significant deformation and an obvious large head (the earliest printed bulge in the deposited strand), whereas a larger normalized printing speed leads to a large stress region. In particular, a small normalized gap height has a squeezing effect on the deposited strand, and a visible depression in the middle of the strand can be obtained for large heights. Additionally, the degree of polymer deformation exhibits a major monotonic dependence on the normalized printing speed and gap height. The nonlinear relationships between the maximum stress components and process parameters show that the maximum stress decreases with Wi, but different printing speeds and gap heights have almost no effect on the stress components in the build platform-normal direction. The above analysis contributes to the inference of the deformation effects in different flow states, which can provide guidance to DIW additive manufacturing.

This study investigates the influence of sintering temperature and hot isostatic pressing (HIP) on the microstructure, mechanical properties, and electrochemical behavior of equiatomic FeNiCoCr high-entropy alloy (HEA) samples produced via binder jet 3D printing (BJ3DP). Green samples with a relative density of approximately 59.3 ± 0.4% were initially printed using pre-alloyed powders. Subsequently, sintering was performed at temperatures ranging from 1410 to 1440 °C for 4 h with an interval of 10 °C. The sintered samples exhibited a single face-centered cubic phase, and their relative densities increased with higher sintering temperatures, reaching values of approximately 78.1%, 88.3%, 91.0%, and 98.1% for temperatures of 1410, 1420, 1430, and 1440 °C, respectively. The effect of hot isostatic pressing (HIP) at 1200 °C under a pressure of 150 MPa for 2 h in an argon atmosphere was found to be minimal for samples with relative densities below 91.0%. However, HIP substantially enhanced both the densities and mechanical properties for samples with relative densities of 91.0% or higher. The samples sintered at 1440 °C (#S1440) displayed the optimal mechanical properties, i.e., the values of yield strength (σy), ultimate tensile strength (σUTS), and total elongation (εt) are ∼173 MPa, ∼476 MPa, and ∼41%, respectively. Further processing #S1440 sample using HIP at 1200 °C (noted as #S1440H) led to an improved relative density to ∼99.4%, along with enhanced strength and ductility. Electrochemical testing in a 3.5 wt% NaCl solution demonstrated superior performance of the #S1440H sample compared to wrought 316 L stainless steel (SS), although it exhibited slightly lower corrosion resistance compared to #S1440 due to coarser grains. X-ray photoelectron spectroscopy (XPS) analyses identified the formation of Co(Fe,Cr)2O4 in the passive film, contributing to the improved corrosion resistance of FeCoNiCr.

The oxidation behaviors of directed energy deposit (DED) 316L SS with various post-DED heat treatments (HTs) as well as commercial 316L SS were investigated in a 650 ℃/20 MPa sCO2 environment. In addition to the detailed microstructure analyses, ultraviolet photoelectron spectroscopy (UPS) analysis was performed to measure the valence electron configuration, and the results were considered in understanding the observed oxidation behavior. Compared to commercial 316L SS, the unique large columnar grains and decrease in valence electron density of DED 316L SS benefit the oxidation resistance by retarding cation and anion migrations, even though they have similar oxide layer consisting of a Fe-rich outer oxide layer and a Cr- and Fe-rich internal oxidation zone. After stress-relieving HT at 850 ℃, the oxidation resistance is further enhanced owing to the HT-induced dislocation density decrease, resulting in a thinner sandwich-like oxide layer with chromia-type outer and inner sublayers and a silicon oxide middle sublayer. Additional improvement in the oxidation resistance is observed after homogenization HT at 1150 °C, with a thinner oxide layer and less inward intergranular oxidation, which could be attributed to an additional decrease in valence electron density.

This work aims to examine the mechanical and dielectric properties of continuous glass fiber (CGF) reinforced poly-ether-ether-ketone (PEEK) composite (CGFRPCs) with different fiber content prepared by additive manufacturing. To address the problems of fiber breakage during the preparation of continuous fiber composite filaments and the low fiber content of 3D printed components, a stepped filament preparation die is proposed and designed, and the influences of die form on the micromorphology and mechanical properties of filaments, as well as the influence of die diameter on the micromorphology, fiber content, mechanical properties and roundness of filaments during the preparation of CGFRPCs filaments are examined. In addition, the influence of dies with different diameter on the mechanical strength of the specimens printed by their prepared filaments is examined, and finite element analysis is utilized to analyze the mechanical strength and interlayer failure process during the flexural damage process of the printed specimens via Hashin fracture criterion and cohesive contact model. Finally, the influence of fiber content on the dielectric properties of the 3D printed specimens are examined. The results show that the stepped die can significantly reduce the degree of breakage of the filament and improve the mechanical strength of its printed specimens. The reduction of the die diameter improves the fiber content and roundness of the filaments. The 0.8 mm die formed filament and its printed specimen reach the highest tensile strength of 1114.63 MPa and 730.21 MPa, respectively. In addition, the flexural strength, interlaminar shear strength, and impact strength of the printed specimens reach the maximum values of 562.54 MPa, 43.88 MPa, and 180 kJ/m2 at die diameter of 0.9 mm, 0.85 mm, and 1.2 mm, respectively. The results of the dielectric properties reveal that the dielectric constant and dielectric loss tend to increase with the increase of fiber content.