The severe thermal gradients associated with selective laser melting (SLM) additive manufacturing (AM) generate large residual stresses (RS) that geometrically distort and otherwise alter the performance of printed parts. Despite broad research interest in this field, it has remained challenging to measure warpage in general as well as RS distributions in situ, which has obfuscated the mechanisms of stress formation during the printing process. In pursuit of this goal, we have developed a non-destructive framework for RS measurement in SLM parts using three-dimensional digital image correlation (3D-DIC) to capture in situ surface distortion. A two-dimensional analytical model was developed to convert DIC surface curvature measurements to estimates of in-plane residual stresses. Experimental validation using stainless steel 316 L “inverted-cone” parts demonstrated that residual stress varied across the surface of the printed part, and strongly interacted with the component geometry. The 3D-DIC based RS measurements were validated by X-ray diffraction (XRD), with an average error of 6% between measured and analytically derived stresses. Systematic variation in RS was attributed to the sector-based laser raster strategy, which was supported by complementary finite element calculations. Calculations showed that the heterogeneous RS distribution in the parts emerged from the sequential re-heating and cooling of the new surface, and changed dynamically between layers. The unique DIC based RS methodology brings substantial benefits over alternatively proposed in situ AM RS measurements, and should facilitate enhanced process optimization and understanding leading towards AM part qualification. Graphical abstract Download : Download high-res image (122KB) Download : Download full-size image

Polyhydroxyalkanoate (PHA) composites containing siliceous sponge spicules (SSS) were prepared from three-dimensional (3D) printing filaments. Acrylic acid (AA)-grafted polyhydroxyalkanoate (PHA-g-AA) and SSS were used to improve the compatibility of the SSS within the PHA matrix. Mechanical and morphological characterizations indicated that the improved adhesion between the SSS and PHA-g-AA enhanced the tensile strength at failure and Young’s modulus of the composite compared with that of PHA/SSS. The PHA-g-AA/SSS composites were also more water-resistant than the PHA/SSS composites. Human foreskin fibroblasts (FBs) were seeded on two series of these composites to assess cytocompatibility. FB proliferation was greater for the PHA/SSS composites than the PHA-g-AA/SSS composites. Moreover, SSS enhanced the antioxidant, anti-inflammatory and antibacterial properties of PHA-g-AA/SSS and PHA/SSS composites, demonstrating the potential of PHA-g-AA/SSS and PHA/SSS composites for biomedical material applications. Graphical abstract Download : Download high-res image (152KB) Download : Download full-size image

Anisotropy in dielectric properties can have deleterious effects in structures intended for use in high-field environments. We show that dielectric anisotropy is introduced into parts fabricated using additive manufacturing techniques based on the orientation in which the part is printed. Dielectric strength testing data, based on the ASTM D149 standard, are presented for samples fabricated using the polymer jetting (PolyJet), stereolithography (SLA), fused deposition modeling (FDM), and selective laser sintering (SLS) additive manufacturing techniques. Each unique printing direction available, based on the printing method, was examined in turn. Each printing technique was found to introduce anisotropic dielectric properties within the sample coupons that were a function of the original orientation in which the part was printed, and the direction of structural susceptibility was found to be print-method dependent. Differences in dielectric strength for coupons printed in different orientations were found to exceed 70% for some combinations of printing technique and polymer. Overall, test coupons printed with stereolithography (SLA) were found to exhibit the lowest degree of dielectric strength anisotropy between print orientations. Dielectric failure mechanisms are discussed.

Space technology has been an early adopter of additive manufacturing (AM) as a way of quickly producing relatively complex systems and components that would otherwise require expensive and custom design and production. Space as an environment and long-term survivability pose challenges to materials used in AM and these challenges need to be addressed. Atomic layer deposition (ALD) is an effective coating method enabling conformal and precise coating of the complete AM print. This work analyses how an ALD coating of aluminium oxide on acrylonitrile butadiene styrene (ABS) and polyamide PA 2200 plastic AM prints benefits and protects them. This was studied in the context of in-space propulsion fluidics, where propellant flow properties also matter. AM was performed with material extrusion and selective laser sintering methods that are commonly used. Tests were performed with a simple bang-bang controller test setup and a mass spectrometer, and the existence of the coating was confirmed with scanning electron microscope imaging.

Water-soluble glass patterned by 3D printing is a versatile tool for tissue engineering and microfluidics. Glasses can be patterned layer-by-layer as in conventional fused deposition modeling but also along 3D, “freeform” paths. In the latter approach, extruding heated material through a nozzle translating in 3D space allows for fabrication of sparse, freestanding networks of cylindrical filaments. These freeform structures are suitable for sacrificial molding with a variety of media, leaving complex microchannel networks. However, 3D printing carbohydrate glass in this way presents several unique challenges: 1) the material must resist degradation and crystallization during printing, 2) the glass must be hot enough to flow freely during extrusion and fuse to the printed construct, while cooling rapidly to retain its shape upon exiting the nozzle, 3) the extruder needs to apply high pressure, with rapid stop and start times and 4) the net force that acts on the filament during extrusion must be minimized so that the filament shape is predictable, i.e., coincides with the path taken by the nozzle. First, we review the properties of commercially available carbohydrate glasses and provide a guide for processing isomalt, our material of choice, to achieve the best printing performance. A pressure-controlled, piston-driven extruder is then described which allows for rapid responses and precise control over the material flow rate. We then analyze the heat transfer within the filament and the forces that contribute to the filament’s final shape. We find that the dominant force is due to the radial flow of the molten glass as it exits the nozzle. This analysis is validated on a purpose-built isomalt 3D printer, which we utilize to characterize relationships between extrusion pressure, translation speed, filament diameter, and viscous force. The insights of the physics of the printing process enable fabrication of intricate freeform prints as well as layer-by-layer designs. The practical and theoretical considerations should facilitate adoption of additive manufacturing of carbohydrate glasses with applications to a wide variety of fields, including tissue engineering and microfluidics.

This paper presents an analytical melting model inside the nozzle of a fused filament fabrication process. The model presents the limiting case scenario where the maximum melting rate is controlled by the applied force. Here, instead of having a nozzle filled with polymer melt, the melt is reduced to a melt film at the tip of the filament as it is pushed against the exit of the nozzle. The model uses a mode of melting that is governed by melting with pressure flow melt removal. The model includes effects of initial filament temperatures, heater temperature, applied force, nozzle tip angle, capillary diameter and length as well as rheological and thermal properties. The analytical solution is compared to controlled experiments done on a specially designed set-up. Furthermore, the model is used to assess the effect of nozzle tip angle, heater temperature and initial filament temperature on the melting rate within the nozzle. The comparison between the experiments and the model show that assumptions used for the model development are plausible, and that the model can be used to optimize the melting within a material extrusion additive manufacturing process, as well as predicting the performance of new materials.

3D-printed Ti-6Al-4V components have great potential in the aerospace and biomedical industries. However, their wide application is limited by some inherent disadvantages, such as poor surface finish and high porosity. In this study, an innovative method, electrically-assisted ultrasonic nanocrystal surface modification (EA-UNSM) was introduced to process 3D-printed Ti-6Al-4V samples. The effect of EA-UNSM on surface finish, microstructure, porosity and in-depth hardness was investigated. Compared with the conventional UNSM process, smoother surfaces and lower subsurface porosities were obtained after EA-UNSM. EA-UNSM also led to a thicker plastically-affected depth compared with conventional UNSM. Numerical modelling showed that localized heating occurs near the pores in 3D-printed Ti-6Al-4V subjected to electric current. This localized heating could potentially facilitate pore closure under ultrasonic striking.

This work investigated the utility of three piezoelectric inkjet printers as energetic material deposition systems, focusing on the ability of each system to achieve the seamless integration of energetic material into small-scale electronic devices. Aluminum copper (II) oxide nanothermite was deposited using the three deposition systems. The printers were evaluated based on their robustness to energetic ink solids loading, drop formation reliability, drop quality degradation over time, and the energetic performance of the deposited material. These metrics correlate to the feasibility of a deposition system to successfully achieve high sample throughput while maintaining the energetic performance of the printed material. After initial system testing, the PipeJet P9 500 μm pipe was used to demonstrate the successful deposition of nanothermite in varying geometric patterns with micrometer precision. From these samples, preliminary propagation speed measurements were obtained, which showed a correlation between the printed line widths and burning rates.

To increase the mechanical strength of Zircaloy-4 cladding at high temperatures, partial oxide dispersion-strengthened (ODS) treatment of the cladding tube surface was achieved by using laser processing technology. The microstructural characteristics and stability of the ODS layer formed on the Zircaloy-4 cladding surface were analyzed at temperatures up to 1000 °C. Ring tensile and loss-of-coolant accident (LOCA) simulation tests were performed to evaluate the mechanical properties of the surface ODS treated Zircaloy-4 cladding tube. The formation and uniform distribution of Y2O3 particles formed in the Zr matrix were identified, and the stability of the particles was confirmed up to 1000 °C. When compared to the reference Zircaloy-4 cladding tube, the surface ODS treated Zircaloy-4 cladding tube showed improved mechanical properties at both room temperature and 500 °C, as well as under LOCA simulation conditions.