3D printed objects suffer from intrinsic mechanical weakness due to low interlayer adhesion and defects that result in anisotropy in their mechanical properties. In this report, we examine the ability of a protocol to incorporate covalent bonds between layers to strengthen interlayer adhesion by depositing multi-amines between layers during the fused filament fabrication printing process. The multi-amines may then react with oxygenated functional groups on the deposited filament to form covalent crosslinks between layers. Determination of the interfacial fracture energy and infrared (IR) spectroscopy studies indicate the successful formation of covalent bonds and strengthening of the interlayer interface with a variety of multi-amines. More importantly, IR results also elucidate the relative rate of reaction of the amines with oxygenated functional groups found on the oxidized filament. These results show that the crosslinking reactions primarily occur shortly after deposition of the filament, i.e. at elevated temperatures. The data also show that the reactivity of the amines is not the prevailing factor in determining interfacial strength, as the aromaticity of the crosslinker plays a key role in creating a stronger interface. These results therefore provide important foundational understanding that can be used to apply this protocol to a variety of extrusion based additive processes and materials.

Metamaterials derive their unusual properties from their architected structure, which generally consists of a repeating unit cell designed to perform a particular function. However, existing metamaterials are, with few exceptions, physically continuous throughout their volume, and thus cannot take advantage of multi-body behavior or contact interactions. Here we introduce the concept of multi-body interpenetrating lattices, where two or more lattices interlace through the same volume without any direct connection to each other. This new design freedom allows us to create architected interpenetrating structures where energy transfer is controlled by surface interactions. As a result, multifunctional or composite-like responses can be achieved even with only a single print material. While the geometry defining interpenetrating lattices has been studied since the days of Euclid, additive manufacturing allows us to turn these mathematical concepts into physical objects with programmable interface-dominated properties. In this first study on interpenetrating lattices, we reveal remarkable mechanical properties including improved toughness, multi-stable/negative stiffness behavior, and electromechanical coupling.

A novel method for the additive manufacturing of Ni/WC composite coatings via laser wire deposition is introduced in this work. The processing characteristics of the typical wire transfer modes, including the liquid spreading transfer mode, wire plunging transfer mode, and wire stubbing transfer mode, were analyzed by a high-speed camera system. The phase compositions and microstructural features were clarified by material characterization methods, including X-ray diffraction (XRD), optical microscope (OM), scanning electron microscopy (SEM) in conjunction with energy dispersive spectroscopy (EDS), and transmission electron microscopy (TEM). The dissolution mechanism of the ceramic particles and the microstructural evolution of the coatings obtained under these three wire transfer modes are also discussed. The hardness and wear resistance of the coatings were evaluated using a microhardness tester and a dry sliding wear tester. The results showed that laser deposition with wire plunging transfer mode could effectively avoid the burning and sinking of the ceramic particles in the coatings, resulting a continuous coating with a high volume fraction of retained particles and a low degree of dilution. The coating had a uniform microstructure which was primarily composited of Ni-based solid solution matrix, retained WC/W2C particles, in-situ synthesized WC, M6C, and Ni/Ni3B lamellar eutectic structure. The coating fabricated with wire plunging transfer mode possessed higher hardness and minimum wear loss.

This paper investigates the formation of manufacturing induced fibre misalignment and breakage during fused filament fabrication (FFF) 3D printing of 1 K continuous carbon fibre filament. Single stripes at various turning angles and curvatures are printed by a desktop printer Prusa i3 using a specific brass nozzle and characterised using X-ray computed micro-tomography (µCT) and optical microscopy. A finite element (FE) model of the printing process is also established to support the experimental measurement. It has been found that high porosity and fibre misalignment in the printed straight stripe result from the weak fibre/matrix interface and the uneven pressure executed by the nozzle. Increase of turning angle and/or reducing of curvature radius leads to more aggravated printing defects, including shape inaccuracy, fibre twisting, folding and misalignment, due to the excessive force from the nozzle, debonding with the print bed and the unmatched geometry of nozzle outlet and fibre filament. Severe fibre breakage and significant change of thickness can be seen in the samples with turning angles larger than 120° or curvature radius smaller than 5 mm, while the wrinkles of the stripe in the inner periphery appear more frequently as the curvature radius decreases.

Highlights • The process atmosphere was monitored using a lambda probe and an electrochemical cell. • Increased oxygen and nitrogen pick-up by Ti-6Al-4V under uncontrolled process atmosphere purity were measured. • The surface discoloration observed was correlated to residual oxygen and nitrogen in the atmosphere. • In situ martensite decomposition, oxygen, and nitrogen pick-up occurred due to heat accumulation. • The embrittlement of Ti-6Al-4V due to non-optimum oxygen monitoring is reported. The effect of the residual oxygen concentration in the process atmosphere during laser-powder bed fusion (L-PBF) of Ti-6Al-4V was investigated, using an external oxygen monitoring system equipped with two types of oxygen sensors typically used in L-PBF hardware: a lambda probe and an electrochemical oxygen sensor. The recordings of the oxygen variations during L-PBF highlighted that the electrochemical sensor is more reliable than the lambda probe, whose signal showed a maximum deviation of about 700 ppm O2 after 7 h, attributed to its sensitivity to hydrogen present in the system. The study revealed that proper monitoring of the oxygen in the laboratory scale L-PBF system used is necessary to limit oxygen and nitrogen pick-ups by the built material. Concentrations as high as 2200 ppm O2 and 500 ppm N2 in the Ti-6Al-4V part built under standard conditions were measured, compared to maximum levels of 1800 ppm O2 and 250 ppm N2 with the external oxygen control. In addition, the findings underline the critical effect of the component design, such as the high aspect ratio columns or the lattice structures, on the heat accumulation in case of Ti-6Al-4V, leading to enhanced oxygen and nitrogen pick-up, as high as 600 ppm O2 and 150 ppm N2 difference between the bottom and top of the cylindrical samples of 70 mm height used in this study. The determination of tensile properties of samples built at different heights put in evidence the detrimental effect of the oxygen increase with build height on the ductility, which decreased from 12% to below 6% between the bottom and top positions. This work highlights that the possible presence of impurities in the L-PBF atmosphere can have harmful impact on the properties of Ti-6Al-4V components, which can be mitigated adjusting the oxygen control system.

Stereolithography (SLA) and digital light processing (DLP) are powerful additive manufacturing techniques that address a wide range of applications including regenerative medicine, prototyping, and manufacturing. Unfortunately, these printing processes introduce micrometer-scale anisotropic inhomogeneities due to the resin absorptivity, diffusivity, reaction kinetics, and swelling during the requisite photoexposure. Previously, it has not been possible to characterize high-resolution mechanical heterogeneity as it develops during the printing process. By combining DLP 3D printing with atomic force microscopy in a hybrid instrument, heterogeneity of a single, in situ printed voxel is characterized. Here, we describe the instrument and demonstrate three modalities for characterizing voxels during and after printing. Sensing Modality I maps the mechanical properties of just-printed, resin-immersed voxels, providing the framework to study the relationships between voxel sizes, print exposure parameters, and voxel-voxel interactions. Modality II captures the nanometric, in situ working curve and is the first demonstration of in situ cure depth measurement. Modality III dynamically senses local rheological changes in the resin by monitoring the viscoelastic damping coefficient of the resin during patterning. Overall, this instrument equips researchers with a tool to develop rich insight into resin development, process optimization, and fundamental printing limits.

Damping mechanisms are a crucial factor for influencing the vibration behavior of dynamic systems. In many applications vibrations are undesirable and need to be reduced by appropriate measures. For instance, vibrations in vehicles can reduce driving comfort or in civil engineering resonance damage can occur in constructions. An interesting and cost-effective way of increasing damping is particle damping. In modern processes of additive manufacturing, like laser powder bed fusion (LPBF), unmelted powder can be left inside a structure on purpose after making and thus producing integrated particle dampers already. Additively manufactured particle damping has not yet reached the industrial level because there are no detailed specifications for the design process. This includes the modeling of (non-linear) dynamic properties, based on numerous design parameters. The state of the art reveals that the effect of particle damping has been convincingly demonstrated, but transferability of the obtained information is still limited. In this paper the effect of particle damping is investigated experimentally with LPBF manufactured beam structures made of AlSi10Mg. Particle damping is evaluated in terms of performance curves for different beam parameter sets. The aim is to help the designer, who needs to keep amplitudes in certain range to estimate the damping of the potential particle damper via the given performance curves. Damping is determined via experimental modal analysis by impulse excitation. The response is evaluated in the frequency domain using the Circle-Fit method with a focus on the beams first bending mode of vibration. Beyond that, a significantly increased damping could be verified up to the seventh bending mode covering a frequency range between 600 Hz and 18k Hz. Damping through particle-filled cavities shows up to 20 times higher damping compared to the same component with fused powder.

Photopolymerization-based Additive Manufacturing (AM), a technique in which a product is built in a layerwise fashion by local curing of a liquid monomer, is increasingly being adopted by the high-tech sector. Nevertheless, industry still faces several challenges to improve the repeatability of product quality, as recognized by several authorities on AM standardization. It is commonly recognized that there is a need for an in-depth understanding, in-situ monitoring and real-time control of the curing process to work towards end-products of higher quality. This motivates the investigation on closed-loop control of the curing process and the build-up of material properties. This pioneering research contributes to the development of a control-oriented model in the form of a state-space description that describes the multiphysical photopolymerization process and connects curing kinetics, heat flow, strain and stress evolution. This work focuses on one spatial dimension and is extendable to higher dimensions. Moreover, an extension to existing control systems theory is proposed to anticipatively control the process through the quadratic tracking framework. The control strategy is based on sequential linearization of the nonlinear model obtained from multiphysical modeling. This theoretical-numerical approach demonstrates the potential of model-based control of the material property build-up during vat photopolymerization processes such as stereolithography and serves as a proof of principle.

This paper demonstrates the utilisation of in-situ synthesised novel metal organic framework (MOF)-polymer nanocomposite laser-sintered parts with enhanced CO2 adsorption properties. Making use of polyamide PA12, one of the most common materials in powder bed fusion process as the base polymer, an in-situ synthesis of nanofiller ZIF-67 crystals on the surface of polyamide polymer particles was proposed to allow the fabrication of a nanocomposite powder with a good dispersion, reducing any health and safety handling issue arising from use of loose nanoparticles. This in-situ synthesis method allowed a maximum exposure of the ZIF-67 nano-porous sites. Laser sintering was used to fabricate porous structures with additional macro-pores and controlled cavities to increase the surface area. The laser-sintered ZIF67-PA12 part at only 2.6% wt ZIF-67 concentration exhibited a CO2 capacity of 3.02 and 4.89 cm3/g at 298 K and 273 K at 1 bar. This in-situ synthesis method of making ZIF67-PA12 powders combined with the design freedom and the ease of fabrication of parts opens opportunities in a wider range of applications for MOFs such as energy storage and conversion. Graphical Download : Download high-res image (287KB) Download : Download full-size image