Highlights • Cold and pulsed-spray metal transfer are compared for DED-Arc of IN718 components. • Infrared thermography showed the effects of thermal fields and thermal cycles during deposition. • Digital image correlation has been used to study strain heterogeneity during tensile tests. • A low travel speed and high inter-pass temperature led to greater yield strength and more consistent ductility. • Build interruptions resulted in strain localisation and formation of solidification cracks. Directed energy deposition via electric arc (DED-Arc) and wire-feed system can offer a low-cost, high deposition rate method to produce large-scale, near-net-shape structural components from Inconel 718 (IN718). In this work, DED-Arc with cold metal transfer (CMT) has been compared with pulsed-spray droplet transfer (GMAW-P) using different build strategies by correlating the local cyclic thermal histories and the position of deposition interruptions to the resulting tensile properties. The measured heat input (0.18 – 0.59 kJ/mm) and inter-pass temperature (80 – 650 °C) affected the thermal field and residence times in the 900 – 600 °C temperature range, where γ’/γ” phases are more likely to form, thereby impacting the yield strength along the build direction (370 – 460 MPa). Furthermore, CMT deposition with a lower travel speed (0.4 m/min vs. 1.0 m/min) prevented the onset of solidification defects, contributing to more consistent tensile ductility. Tensile properties of material surrounding build interruptions developed strain localisation and failed prematurely as compared to regions without, due to localised cracking and transient thermal fields generated once the deposition resumed. Graphical Download : Download high-res image (257KB) Download : Download full-size image

Since its introduction in the 1980s, 3D printing has advanced as a versatile and reliable tool with applications in different fields. Among the available 3D printing techniques, two-photon polymerization is regarded as one of the most promising technologies for microscale printing due to its ability to combine a high printing fidelity down to submicron scale with free-form structure design. Recently, the technology has been enhanced through the implementation of faster laser scanning strategies, as well as the development of new photoresists. This paves the way for a wide range of applications, which has resulted in an increasing number of available commercial systems. This work aims to provide an overview of the technology capability by comparing three commercial systems in a round-robin test. To cover a wide range of applications, six test structures with distinct features were designed, covering various aspects of interest, from single material objects with sub-micron feature sizes up to multi-material millimeter-sized objects. Application-specific structures were printed to evaluate surface roughness and the stitching capability of the printers. Moreover, the ability to generate free-hanging structures and complex surfaces required for cell scaffolds and microfluidic platform fabrication was quantitatively investigated. Finally, the influence of the numerical aperture of the fabrication objective on the printing quality was assessed. All three printers successfully fabricated samples comprising various three-dimensional features and achieved submicron resolution and feature sizes, demonstrating the versatility and precision of two-photon polymerization direct laser writing. Our study will facilitate the understanding of the technology maturity level, while highlighting specific aspects that characterize each of the investigated systems.

The hydrogen embrittlement (HE) is investigated for the titanium alloyTi-6Al-4V, processed by different manufacturing methods. The mechanical characteristics of conventionally cold rolled, as-built laser powder bed fused (L-PBF) and heat treated L-PBF Ti-6Al-4V are compared under in-situ electrochemical hydrogen charging at room temperature. The microstructure determines the susceptibility to HE, which is examined by a combination of in-situ constant extension rate tensile tests, X-ray tomography, X-ray diffraction, hydrogen melt extraction, scanning electron microscopy, and electron backscatter diffraction. The presence of the β phase facilitates an increased hydrogen uptake capacity, resulting in an enhanced HE-susceptibility. In-situ hydrogen assisted failure of a duplex α-β microstructure in conventionally cold rolled Ti-6Al-4V occurs via a strain rate dependent mechanism, i.e. hydrogen enhanced localised plasticity (HELP) at fast strain rates and a combination of HELP and stress induced hydride formation (SIHF) at slow strain rates. Meanwhile, the martensitic α’ microstructure of as-built L-PBF Ti-6Al-4V shows clear evidence of prevalent hydrogen degradation via SIHF after slow straining. However, hydrogen significantly deteriorates the mechanical behaviour in L-PBF material due to a fatal synergetic interaction with porosity related stress fields, dominating failure characteristics after hydrogenation. The same effect applies for the inherent porosities in heat treated L-PBF Ti-6Al-4V with a fine α-β microstructure. Therefore, knowledge on these microstructural interactions with hydrogen will allow the development of better HE resistant Ti-6Al-4V. Graphical Download : Download high-res image (311KB) Download : Download full-size image

Material extrusion additive manufacturing enables the rapid fabrication of complex structures for a wide range of applications, including within the aerospace and healthcare sectors. Optimizing the performance of additively manufactured polymer structures depends on the ability to predict residual stress induced by the sequential solidification of the extruded layers. Self-equilibrated residual stresses are influenced by the thermal history of the part during the manufacturing process. This work introduces a novel numerical framework for the process modeling of material extrusion additive manufacturing. The numerical modeling is based on higher-order finite elements derived from the Carrera Unified Formulation (CUF), which enables accurate prediction of the stress field while reducing the associated computational overheads. Element activation is adopted to model the evolving structure during material extrusion and deposition. Heat exchange mechanisms such as conduction (filament/filament, filament/build platform), convection, and radiation are included in the model. A transient thermal analysis is performed in this new framework to predict the temperature distribution within the part during its fabrication. Multiple numerical assessments are presented for the material extrusion additive manufacturing of parts ranging in size from 30 mm to over 1500 mm. The predicted thermal profiles are in excellent agreement with reference experimental observations, thereby validating the proposed numerical framework and demonstrating its capability to model the printing process accurately. Results from this work are a milestone toward optimizing additively manufactured polymer parts.

Highlights • A novel blast rig to accurately measure energy absorption in shock-wave loading for lattice structures. • Design of Experiments systematically select layered-hybrid lattice topologies for high-strain rate blast resistance. • Investigation of the influence of lattice topology and relative density on the compressive blast response of the structures. • Experimental campaigns validatesthe effectiveness of lattice structures subjected to shockwave loading. Lattice structures have shown potential for efficient protection against dynamic loading events, especially during high-strain rate scenarios such as a blast. Additive manufacturing enables the design of complex geometries to optimise lattice architecture and increase blast resistance. However, the lack of experimental data related to blast-resistant lattice structures poses challenges in developing and validating theoretical and numerical models. This study aims to design blast-resistant lattice structures that can improve protection efficiency at wider applicability in high-strain rate loadings. For that, hybrid-layered Triply Periodic Minimal Surfaces (TPMS) lattice structures were systematically designed using a Design of Experiments (DoE) approach and manufactured using additive manufacturing (AM). The Blast Hopkinson Pressure Bar (BHPB) rig was used to compare the influence of different lattice topologies and relative densities on energy absorption when specimens were subjected to compressive blast loading. High-speed imaging was utilised to measure transient deformation in addition to the load transferred through the specimens. The experimental results indicated that the BHPB rig could appropriately measure the energy absorption of compressive structures subjected to shockwave loading. Additionally, the results demonstrated that TPMS topology and relative density changes substantially affect its performance. The DoE approach was utilised to predict the performance improvements of layered-hybrid lattice structures, providing valuable data for blast protection specialists and engineers designing AM lattice structures to resist blast loading.

Materials manufactured by additive manufacturing processes based on extrusion of polymer are recent and difficult to model due to their complex and irregular mesostructure. In the case of printed material with parallel extruded rasters, the existing models, based on 2D Representative Volume Elements (RVEs), neglect the influence of the variation of the cavity shape along the printing direction due to printing irregularities such as ghosting or rippling. In this paper, the mesostructure of the printed material is investigated based on X-ray tomography, and a new approach to extract a 3D RVE is proposed. The new method provides typical mesostructural features of interest, and, in particular, the defects induced by printing irregularities. This method is used to define RVEs for different mesostructures associated with different printing conditions. A numerical homogenization of the printed material based on these RVEs is performed using the Fast-Fourier Transform (FFT) method to determine its macroscopic properties. Both macroscopic elastic properties of the printed material and stress concentrations in the mesostructure are studied and compared to estimate the influence of the different typical mesostructures on the mechanical behavior of the printed material. The results establish a significant influence of the variation of the mesostructure induced by the printing conditions on the rigidity of the printed material, showing the efficiency of the method to better predict the structure–properties relationships of material extrusion additive manufacturing (MEAM) based material. Furthermore, the method links the mesostructural defects to the induced stress concentrations in the mesostructure, thus to their criticality to the resistance of the material.

Highlights • Combined modeling approach for site-specific solid state phase transformations. • Semi-analytical heat conduction model simulates thermal profile. • MatCalc thermo-kinetic software predicts solid-state phase transformations. • Application to IN738 electron beam powder bed fusion qualitatively matches experiments. • Microstructure-property predictions suitable for other processes & alloys. Metal additive manufacturing (AM) has now become the perhaps most desirable technique for producing complex shaped engineering parts. However, to truly take advantage of its capabilities, advanced control of AM microstructures and properties is required, and this is often enabled via modeling. The current work presents a computational modeling approach to studying the solid-state phase transformation kinetics and the microstructural evolution during AM. Our approach combines thermal and thermo-kinetic modelling. A semi-analytical heat transfer model is employed to simulate the thermal history throughout AM builds. Thermal profiles of individual layers are then used as input for the MatCalc thermo-kinetic software. The microstructural evolution (e.g., fractions, morphology, and composition of individual phases) for any region of interest throughout the build is predicted by MatCalc. The simulation is applied to an IN738 part produced by electron beam powder bed fusion to provide insights into how γ′ precipitates evolve during thermal cycling. Our simulations show qualitative agreement with our experimental results in predicting the size distribution of γ′ along the build height, its multimodal size character, as well as the volume fraction of MC carbides. Our findings indicate that our method is suitable for a range of AM processes and alloys, to predict and engineer their microstructures and properties. Graphical Download : Download high-res image (286KB) Download : Download full-size image

Inspired by the hollow-tube architecture and chambers found in bamboo, we propose a design concept for mechanical metamaterials based on the hybridization of plate lattice and hollow-truss lattice (HPHL) to enhance the mechanical performance of lattice metamaterials in this paper. The design feasibility of HPHL is demonstrated through sample manufacturing using the laser powder bed fusion technique. Quasi-static compressive tests are conducted to investigate the crushing behavior of HPHL. Compared with the typical plate lattice, the HPHL exhibits a more effective crushing region, a higher mean crushing stress and a larger specific energy absorption (SEA). The SEA of HPHL is improved by 53.18% compared with that of the pure plate lattice. Based on the verified numerical simulations, the crushing mechanisms of HPHL are investigated through examining the deformation process and the stress-strain curve. It is found that the incorporation of hollow-truss lattices effectively delays the densification of the plate lattice and provides a more homogeneous and efficient stress distribution in HPHL, leading to higher crushing stress and higher energy absorption efficiency. Moreover, the effects of various design parameters, such as the hollow-truss diameter dT, hollow-truss thickness tT and plate thickness tP, on the mechanical behavior of HPHL are further explored to identify the optimal HPHL and realize the mechanical customization. The bionic hybrid design strategy proposed in this paper paves a new promising route to improve the mechanical performance of lattice metamaterials. Graphical Download : Download high-res image (308KB) Download : Download full-size image

This paper presents in-situ X-ray computed tomography (CT) experiments used to study the flow behavior within a conventional hot-end of a fused filament fabrication printer. Three types of experiments were performed to better understand the melt and flow behavior. In one experiment, 360° CT scans were conducted, focusing on the air gap between the filament and the nozzle wall. In a second experiment, the flow profile inside the nozzle was studied using radiography. To provide a good contrast to the surrounding nozzle material, filament was prepared containing small amounts of tungsten powder as a contrast agent. During a third test, the extruder forces were measured and compared with the X-ray results and the predictions of a numerical simulation. The CT scans showed that at higher filament speeds, less area of the nozzle wall is in contact with the melt. This means that a larger part of the barrel section is occupied by an air gap between the solid filament and the nozzle wall. In contrast to the filament speed, the influence of the heater temperature shows no discernible effect on the part of the nozzle filled with melt. Radiographic evaluation of the velocity profile revealed a parabolic distribution under the studied conditions, closely matching numerical simulations modeling the flow as isothermal and non-Newtonian. The study’s findings offer potential for improving nozzle design. Furthermore, the presented experimental method can serve as a valuable tool for future validation of more complex numerical simulations.

The defect-free fabrication of complex geometries with homogeneous properties using line-based hatching strategies remains a major challenge in electron beam powder bed fusion (PBF-EB). Open PBF-EB machines enable the development of new scanning-strategy-based compensation schemes to solve this challenge. This study introduces a new return time compensation scheme for line-based hatching strategies in PBF-EB, that enables the fabrication of complex geometries with a homogeneous melt pool geometry over constant cross-sections in build direction. Using thermal simulations and melt surface analysis, it is shown that the proposed compensation scheme prevents geometry-induced defect formation in two complex model geometries, in comparison with basic line-based hatching strategies. It is illustrated that the developed compensation scheme has the potential to transfer arbitrary melt pool geometries to complex parts. Ultimately, the universality of the return time compensation scheme is demonstrated and compared to existing process-parameter based compensation schemes.