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Showing papers in "Additive manufacturing in 2018"


Journal ArticleDOI
TL;DR: In this article, the authors provide a brief discussion about additive manufacturing and also the most employed additive manufacturing technologies for polymers, specifically, properties under different loading types such as tensile, bending, compressive, fatigue, impact and others.
Abstract: 3D printing, more formally known as Additive Manufacturing (AM), is already being adopted for rapid prototyping and soon rapid manufacturing. This review provides a brief discussion about AM and also the most employed AM technologies for polymers. The commonly-used ASTM and ISO mechanical test standards which have been used by various research groups to test the strength of the 3D-printed parts have been reported. Also, a summary of an exhaustive amount of literature regarding the mechanical properties of 3D-printed parts is included, specifically, properties under different loading types such as tensile, bending, compressive, fatigue, impact and others. Properties at low temperatures have also been discussed. Further, the effects of fillers as well as post-processing on the mechanical properties have also been discussed. Lastly, several important questions to consider in the standardization of mechanical test methods have been raised.

822 citations


Journal ArticleDOI
TL;DR: In this article, the state-of-the-art in composite 3D printing is presented, showing a distinction between short fiber feedstocks and continuous fiber feedstock. But, the state of the art is limited by the brittle continuous carbon fibres cannot be deposited freely through small steering radii and sharp angles.
Abstract: Fused filament fabrication (FFF) is a 3D printing technique which allows layer-by-layer build-up of a part by the deposition of thermoplastic material through a nozzle. The technique allows for complex shapes to be made with a degree of design freedom unachievable with traditional manufacturing methods. However, the mechanical properties of the thermoplastic materials used are low compared to common engineering materials. In this work, composite 3D printing feedstocks for FFF are investigated, wherein carbon fibres are embedded into a thermoplastic matrix to increase strength and stiffness. First, the key processing parameters for FFF are reviewed, showing how fibres alter the printing dynamics by changing the viscosity and the thermal profile of the printed material. The state-of-the-art in composite 3D printing is presented, showing a distinction between short fibre feedstocks versus continuous fibre feedstocks. An experimental study was performed to benchmark these two methods. It is found that printing of continuous carbon fibres using the MarkOne printer gives significant increases in performance over unreinforced thermoplastics, with mechanical properties in the same order of magnitude of typical unidirectional epoxy matrix composites. The method, however, is limited in design freedom as the brittle continuous carbon fibres cannot be deposited freely through small steering radii and sharp angles. Filaments with embedded short carbon microfibres (∼100 μm) show better print capabilities and are suitable for use with standard printing methods, but only offer a slight increase in mechanical properties over the pure thermoplastic properties. It is hypothesized that increasing the fibre length in short fibre filament is expected to lead to increased mechanical properties, potentially approaching those of continuous fibre composites, whilst keeping the high degree of design freedom of the FFF process.

475 citations


Journal ArticleDOI
TL;DR: In this paper, a detailed summary of mechanical properties of printed parts for different composite material systems is presented and discussed, including the flow and resulting fiber orientation, the bond formation between adjacent beads and the thermomechanical solidification behavior of the deposited material.
Abstract: Recent advancements in the Additive Manufacturing (AM) Fused Filament Fabrication (FFF) approach are described with focus on the application to tooling and molds for composite materials and structures. A detailed summary of mechanical properties of printed parts for different composite material systems is presented and discussed. These material systems are comprised of discontinuous fiber-reinforced polymers characterized by fiber orientation dominantly parallel to the direction of the extrudate. An overview of the FFF process and its physical phenomena is given including the flow and resulting fiber orientation, the bond formation between adjacent beads and the thermomechanical solidification behavior of the deposited material. Based on reviewed research in these different phenomena, future research needs are discussed and desirable objectives are formulated.

457 citations


Journal ArticleDOI
TL;DR: In this paper, powder bed fusion technology was employed to fabricate the cellular structures of various relative densities out of Maraging steel, and compressive testing was performed to deduce the mechanical properties of the considered cellular structures.
Abstract: Recent advances in additive manufacturing facilitated the fabrication of parts with great geometrical complexity and relatively small size, and allowed for the fabrication of topologies that could not have been achieved using traditional fabrication techniques. In this work, we explore the topology-property relationship of several classes of periodic cellular materials; the first class is strut-based structures, while the second and third classes are derived from the mathematically created triply periodic minimal surfaces, namely; the skeletal-TPMS and sheet-TPMS cellular structures. Powder bed fusion technology was employed to fabricate the cellular structures of various relative densities out of Maraging steel. Scanning electron microscope (SEM) was also employed to assess the quality of the printed parts. Compressive testing was performed to deduce the mechanical properties of the considered cellular structures. Results showed that the sheet-TPMS based cellular structures exhibited a near stretching-dominated deformation behavior, while skeletal-TPMS showed a bending-dominated behavior. On the other hand, the Kelvin and Gibson-Ashby strut-based topologies exhibited a mixed mode of deformation while the Octet-truss showed a stretching-dominated behavior. Overall the sheet-TPMS based cellular structures showed superior mechanical properties among all the tested structures. The most interesting observation is that sheet-based Diamond TPMS structure showed the best mechanical performance with nearly independence of relative density. It was also observed that at decreased volume fractions the effect of geometry on the mechanical properties is more pronounced.

426 citations


Journal ArticleDOI
TL;DR: In this article, the authors identified the materials processing challenges in wire-arc additive manufacturing (WAAM), including high residual stresses, undesirable microstructures, and solute segregation and phase transformations at solidification.
Abstract: Wire Arc Additive Manufacturing (WAAM) is attracting significant attention in industry and academia due to its ability to capture the benefits of additive manufacturing for production of large components of medium geometric complexity. Uniquely, WAAM combines the use of wire and electric arc as a fusion source to build components in a layer-by-layer approach, both of which can offer significant cost savings compared to powder and alternative fusion sources, such as laser and electron beam, respectively. Meanwhile, a high deposition rate, key for producing such components, is provided, whilst also allowing significant material savings compared to conventional manufacturing processes. However, high quality production in a wide range of materials is limited by the elevated levels of heat input which causes a number of materials processing challenges in WAAM. The materials processing challenges are fully identified in this paper to include the development of high residual stresses, undesirable microstructures, and solute segregation and phase transformations at solidification. The thermal profile during the build poses another challenge leading to heterogeneous and anisotropic material properties. This paper outlines how the materials processing challenges may be addressed in WAAM by implementation of quality improving ancillary processes. The primary WAAM process selections and ancillary processes are classified by the authors and a comprehensive review of their application conducted. Strategies by which the ancillary processes can enhance the quality of WAAM parts are presented. The efficacy and suitability of these strategies for versatile and cost effective WAAM production are discussed and a future vision of WAAM process developments provided.

392 citations


Journal ArticleDOI
TL;DR: In this paper, a two-wavelength imaging setup is used to account for changes in emissivity and temperature fields are captured at 100 kHz with a resolution of 20μm during the processing of a simple Ti6Al4V component.
Abstract: In laser powder bed fusion, melt pool dynamics and stability are driven by the temperature field in the melt pool. If the temperature field is unfavourable defects are likely to form. The localised and rapid heating and cooling in the process presents a challenge for the experimental methods used to measure temperature. As a result, understanding of these process fundamentals is limited. In this paper a method is developed that uses coaxial imaging with high-speed cameras to give both the spatial and temporal resolution necessary to resolve the surface temperature of the melt pool. A two wavelength imaging setup is used to account for changes in emissivity. Temperature fields are captured at 100 kHz with a resolution of 20 μm during the processing of a simple Ti6Al4V component. Thermal gradients in the range 5–20 K/μm and cooling rates in range 1–40 K/μs are measured. The results presented give new insight into the effect of parameters, geometry and scan path on the melt pool temperature and cooling rates. The method developed here provides a new tool to assist in optimising scan strategies and parameters, identifying the causes of defect prone locations and controlling cooling rates for local microstructure development.

371 citations


Journal ArticleDOI
TL;DR: The cold spray additive manufacturing (CSAM) has been shown to retain the original properties of the feedstock, to produce oxide-free deposits, and to not adversely influence underlying substrate materials during manufacture.
Abstract: Cold spray is a solid-state coating deposition technology which has recently been applied as an additive manufacturing process to fabricate individual components and to repair damaged components. In comparison with fusion-based high-temperature additive manufacturing processes, cold spray additive manufacturing (CSAM) has been shown to retain the original properties of the feedstock, to produce oxide-free deposits, and to not adversely influence underlying substrate materials during manufacture. Therefore, CSAM is attracting considerable attention from both scientific and industrial communities. Although CSAM is an emerging additive manufacturing technology, a body of work has been carried out by various research groups and the technology has been applied across a range of manufacturing areas. The purpose of this paper is to systematically summarize and review the CSAM-related work to date.

314 citations


Journal ArticleDOI
TL;DR: In this paper, the authors investigated the mechanical properties and energy absorption abilities of three types of TPMS sheet structures (Primitive, Diamond, and Gyroid) fabricated by selective laser melting (SLM) with 316 L stainless steel under compression loading and classified their failure mechanisms and printing accuracy with the help of numerical analysis.
Abstract: Designing metallic cellular structures with triply periodic minimal surface (TPMS) sheet cores is a novel approach for lightweight and multi-functional structural applications. Different from current honeycombs and lattices, TPMS sheet structures are composed of continuous and smooth shells, allowing for large surface areas and continuous internal channels. In this paper, we investigate the mechanical properties and energy absorption abilities of three types of TPMS sheet structures (Primitive, Diamond, and Gyroid) fabricated by selective laser melting (SLM) with 316 L stainless steel under compression loading and classify their failure mechanisms and printing accuracy with the help of numerical analysis. Experimental results reveal the superior stiffness, plateau stress and energy absorption ability of TPMS sheet structures compared to body-centred cubic lattices, with Diamond-type sheet structures performing best. Nonlinear finite element simulation results also show that Diamond and Gyroid sheet structures display relatively uniform stress distributions across all lattice cells under compression, leading to stable collapse mechanisms and desired energy absorption performance. In contrast, Primitive-type structures display rapid diagonal shear band development followed by localized wall buckling. Lastly, an energy absorption diagram is developed to facilitate a systematic way to select optimal densities of TPMS structures for energy absorbing applications.

309 citations


Journal ArticleDOI
TL;DR: In this article, a number of strategies that enable lattice structures to be derived from topology optimization (TO) results suitable for additive manufacturing (AM) are presented, evaluated for mechanical performance and assessed for AM specific design related manufacturing considerations.
Abstract: A number of strategies that enable lattice structures to be derived from Topology Optimisation (TO) results suitable for Additive Manufacturing (AM) are presented. The proposed strategies are evaluated for mechanical performance and assessed for AM specific design related manufacturing considerations. From a manufacturing stand-point, support structure requirement decreases with increased extent of latticing, whereas the design-to-manufacture discrepancies and the processing efforts, both in terms of memory requirements and time, increase. Results from Finite Element (FE) analysis for the two loading scenarios considered: intended loading, and variability in loading, provide insight into the solution optimality and robustness of the design strategies. Lattice strategies that capitalised on TO results were found to be considerably (∼40-50%) superior in terms of specific stiffness when compared to the structures where this was not the case. The Graded strategy was found to be the most desirable from both the design and manufacturing perspective. The presented pros-and-cons for the various proposed design strategies aim to provide insight into their suitability in meeting the challenges faced by the AM design community.

301 citations


Journal ArticleDOI
TL;DR: A review of the current materials available for 3D printing that enable the emergence of 4D printing, a "smart material" that responds in a programmed way to an external stimuli can be found in this paper.
Abstract: 3D printing will revolutionize the manufacturing industry. Significant advances in computer aided design, additive manufacturing and materials science have opened up the possibilities of self-assembly systems, self-healing and material property alterations. Printing layer by layer allows complex geometries to be built, previously difficult under conventional manufacturing routes. This paper is a review of the current materials available for 3D printing that enable the emergence of 4D printing, a ‘smart material’ that responds in a programmed way to an external stimuli. The outlook is towards potential space applications, in all areas including deployable structures, antennas and medical supplies.

282 citations


Journal ArticleDOI
TL;DR: A computer vision algorithm is used to automatically detect and classify anomalies that occur during the powder spreading stage of the process, which has the potential to become a component of a real-time control system in an LPBF machine.
Abstract: Despite the rapid adoption of laser powder bed fusion (LPBF) Additive Manufacturing by industry, current processes remain largely open-loop, with limited real-time monitoring capabilities. While some machines offer powder bed visualization during builds, they lack automated analysis capability. This work presents an approach for in-situ monitoring and analysis of powder bed images with the potential to become a component of a real-time control system in an LPBF machine. Specifically, a computer vision algorithm is used to automatically detect and classify anomalies that occur during the powder spreading stage of the process. Anomaly detection and classification are implemented using an unsupervised machine learning algorithm, operating on a moderately-sized training database of image patches. The performance of the final algorithm is evaluated, and its usefulness as a standalone software package is demonstrated with several case studies.

Journal ArticleDOI
TL;DR: Functionally Graded Additive Manufacturing (FGAM) is a layer-by-layer fabrication process that involves gradationally varying the material organization within a component to achieve an intended function as mentioned in this paper.
Abstract: Functionally Graded Additive Manufacturing (FGAM) is a layer-by-layer fabrication process that involves gradationally varying the material organisation within a component to achieve an intended function. FGAM establishes a radical shift from contour modelling to performance modelling by having the performance-driven functionality built directly into the material. FGAM can strategically control the density and porosity of the composition or can combine distinct materials to produce a seamless monolithic structure. This paper presents a state-of-art conceptual understanding of FGAM, covering an overview of current techniques that can enable the production of FGAM parts as well as identify current technological limitations and challenges. The possible strategies for overcoming those barriers are presented and recommendations on future design opportunities are discussed.

Journal ArticleDOI
TL;DR: In this article, an in- situ defect detection strategy for powder bed fusion (PBF) AM using supervised machine learning is described, where multiple images were collected at each build layer using a high resolution digital single-lens reflex (DSLR) camera.
Abstract: Process monitoring in additive manufacturing (AM) is a crucial component in the mission of broadening AM industrialization. However, conventional part evaluation and qualification techniques, such as computed tomography (CT), can only be utilized after the build is complete, and thus eliminate any potential to correct defects during the build process. In contrast to post-build CT, in situ defect detection based on in situ sensing, such as layerwise visual inspection, enables the potential for in-process re-melting and correction of detected defects and thus facilitates in-process part qualification. This paper describes the development and implementation of such an in situ defect detection strategy for powder bed fusion (PBF) AM using supervised machine learning. During the build process, multiple images were collected at each build layer using a high resolution digital single-lens reflex (DSLR) camera. For each neighborhood in the resulting layerwise image stack, multi-dimensional visual features were extracted and evaluated using binary classification techniques, i.e. a linear support vector machine (SVM). Through binary classification, neighborhoods are then categorized as either a flaw, i.e. an undesirable interruption in the typical structure of the material, or a nominal build condition. Ground truth labels, i.e. the true location of flaws and nominal build areas, which are needed to train the binary classifiers, were obtained from post-build high-resolution 3D CT scan data. In CT scans, discontinuities, e.g. incomplete fusion, porosity, cracks, or inclusions, were identified using automated analysis tools or manual inspection. The xyz locations of the CT data were transferred into the layerwise image domain using an affine transformation, which was estimated using reference points embedded in the part. After the classifier had been properly trained, in situ defect detection accuracies greater than 80% were demonstrated during cross-validation experiments.

Journal ArticleDOI
TL;DR: In this article, the authors provide an overview of the sustainability of additive manufacturing (SAM), with a focus on energy and environmental impacts, and discuss the opportunities to reduce energy and material consumption through design, material preparation, manufacturing, usage, and end-of-life treatment.
Abstract: Additive Manufacturing (AM) has been rapidly developing over the last decade. It shows great potential in reducing the need for energy- and resource-intensive manufacturing processes, which in turn reduces the amount of material required in the supply chain, and enables more environmentally benign practices. However, the question of how to realize these potential benefits has received little attention. This paper aims to provide an overview of the Sustainability of Additive Manufacturing (SAM). The context of the SAM is introduced, with a focus on energy and environmental impacts. Resource consumption is identified as the most important aspect. Examination from a life cycle perspective is also presented, with explicit discussions on opportunities to reduce energy and material consumption through design, material preparation, manufacturing, usage, and end-of-life treatment. Statistical data analysis provides an overview of impact forecasts, highlighting the importance of and need for thorough research on sustainability. The eco-design concept enabled by AM is identified as the most promising and effective technology, further extending and completing its design capability. This also determines the opportunities for energy and environmental optimization in subsequent processes. Most existing research is in process- and system-specific modeling, and few AM processes and systems have been studied, with generally premature conclusions. General models for each type of AM process are still necessary. Lastly, five research priorities are suggested: improve systematic data integration and management, correlate energy and quality, develop intelligent machinery, focus on material preparation and recycling, and discover innovative applications using AM.

Journal ArticleDOI
TL;DR: In this article, a literature review of numerical simulation models of the EBM process is provided, which is mainly classified according to the level of approximation introduced into the modelling methodology, such as mesoscopic or FE approach.
Abstract: The Electron Beam Melting (EBM) process is an additive manufacturing process in which an electron beam melts metallic powders to obtain the geometry of a specific part. The use of an electron beam in the AM field is relatively recent. Numerous applications have already been made in the aerospace and medical fields, in which the EBM process is used to produce complex parts, made of an excellent quality material, for which other technologies would be expensive or difficult to apply. Because of the growing interest of industry in this technology, the research community has been dedicating a great deal of effort to making the EBM process more reliable. The modelling of the EBM process is considered of utmost importance as it could help to reduce the process optimisation time, compared with the trial and error approach, which is currently the most widely used method. From this point of view, the aim of this paper has been to provide a literature review of numerical simulation models of the EBM process. The various studies on numerical modelling are presented in detail. These studies are mainly classified according to the level of approximation introduced into the modelling methodology. The simulations have first been categorised according to the powder modelling approach that has been adopted (i.e. mesoscopic or FE approach). The studies have then been categorised, as far as FE-based simulations are concerned, as either uncoupled or coupled modelling approaches. All the current approaches have been compared, and how the researchers have modelled the EBM process has been highlighted, considering the assumptions that have been made, the modelling of the material properties, the material state change, and the heat source. Moreover, the adopted validation approaches and the results have been described in order to point out any important achievements. Deviations between numerical and experimental results have been discussed as well as the current level of development of the simulation of the EBM process.

Journal ArticleDOI
TL;DR: In this article, the effect of thermal post-processing of AlSi10Mg parts, using recycled powder, with the aim of improving the microstructure homogeneity of the as-built parts was evaluated.
Abstract: The performance enhancement of parts produced using Selective Laser Melting (SLM) is an important goal for various industrial applications. In order to achieve this goal, obtaining a homogeneous microstructure and eliminating material defects within the fabricated parts are important research issues. The objective of this experimental study is to evaluate the effect of thermal post-processing of AlSi10Mg parts, using recycled powder, with the aim of improving the microstructure homogeneity of the as-built parts. This work is essential for the cost-effective additive manufacturing (AM) of metal optics and optomechanical systems. To achieve this goal, a full characterization of fresh and recycled powder was performed, in addition to a microstructure assessment of the as-built fabricated samples. Annealing, solution heat treatment (SHT) and T6 heat treatment (T6 HT) were applied under different processing conditions. The results demonstrated an improvement in microstructure homogeneity after thermal post-processing under specific conditions of SHT and T6 HT. A micro-hardness map was developed to assist in the selection of the optimized post-processing parameters in order to satisfy the design requirements of the part.

Journal ArticleDOI
TL;DR: In this article, the authors measured the tensile strength of interlayer bond lines in ABS coupons printed in two orientations and found that a plateau of 22MPa was observed for a normalized contact length greater than 0.6 independent of print orientation.
Abstract: Interlayer bonds pose regions of weakness in structures produced via melt extrusion based polymer additive manufacturing. Bond strength was assessed both between layers and within layers as a function of print parameters by performing tensile tests on ABS coupons printed in two orientations. Print parameters considered were extruder temperature, print speed, and layer height. An IR camera was used to track thermal history of interlayer bond lines during the printing process. Contact length between roads was measured from mesostructure optical micrographs. Print speed was found to have a large impact on tensile strength with high speeds generally yielding lower strength. A plateau in tensile strength of 22 MPa was observed for a normalized contact length greater than 0.6 independent of print orientation.

Journal ArticleDOI
TL;DR: In this article, a bibliographical study is presented to identify and classify the parameters and phenomena which influence the appearance of defects in aluminum alloy parts produced using the SLM process and hence the final properties of these parts.
Abstract: In recent years, the SLM process has been studied for the production of aluminum alloy parts, as these alloys demonstrate significant potential for the future, notably due to their low density which allows a considerable reduction in mass. The aim of this bibliographical study is to identify and classify the parameters and phenomena which influence the appearance of defects in aluminum alloy parts produced using the SLM process and hence the final properties of these parts. To do this, a cause tree diagram was created. For each defect or consequence identified (porosities, defects linked with hot cracking phenomena, anisotropy in the material and surface quality), we revealed the potential sources of the appearance of this defect, going back to the initial causes.

Journal ArticleDOI
TL;DR: In this paper, a convolutional neural network (CNN) was used for autonomous detection and classification of spreading anomalies in a laser powder bed fusion additive manufacturing (LPDAM) system.
Abstract: In-situ detection of processing defects is a critical challenge for Laser Powder Bed Fusion Additive Manufacturing. Many of these defects are related to interactions between the recoater blade, which spreads the powder, and the powder bed. This work leverages Deep Learning, specifically a Convolutional Neural Network (CNN), for autonomous detection and classification of many of these spreading anomalies. Importantly, the input layer of the CNN is modified to enable the algorithm to learn both the appearance of the powder bed anomalies as well as key contextual information at multiple size scales. These modifications to the CNN architecture are shown to improve the flexibility and overall classification accuracy of the algorithm while mitigating many human biases. A case study is used to demonstrate the utility of the presented methodology and the overall performance is shown to be superior to that of methodologies previously reported by the authors.

Journal ArticleDOI
TL;DR: In this paper, the authors proposed a numerical model to simulate the extrusion of a strand of semi-molten material on a moving substrate, within the computation fluid dynamics paradigm, and quantified the effect of the gap distance and the velocity ratio on the size and the shape of the strand.
Abstract: We propose a numerical model to simulate the extrusion of a strand of semi-molten material on a moving substrate, within the computation fluid dynamics paradigm. According to the literature, the deposition flow of the strands has an impact on the inter-layer bond formation in extrusion-based additive manufacturing, as well as the surface roughness of the fabricated part. Under the assumptions of an isothermal Newtonian fluid and a creeping laminar flow, the deposition flow is controlled by two parameters: the gap distance between the extrusion nozzle and the substrate, and the velocity ratio of the substrate to the average velocity of the flow inside the nozzle. The numerical simulation fully resolves the deposition flow and provides the cross-section of the printed strand. For the first time, we have quantified the effect of the gap distance and the velocity ratio on the size and the shape of the strand. The cross-section of the strand ranges from being almost cylindrical (for a fast printing and with a large gap) to a flat cuboid with rounded edges (for a slow printing and with a small gap), which substantially differs from the idealized cross-section typically assumed in the literature. Finally, we found that the printing force applied by the extruded material on the substrate has a negative linear relationship with the velocity ratio, for a constant gap.

Journal ArticleDOI
TL;DR: In this article, the effect of shot-peening on fatigue resistance of additively manufactured (AM) AlSi10Mg specimens fabricated by selective laser melting (SLM) following surface treatment by shot peening was investigated.
Abstract: The effect on fatigue resistance of additively manufactured (AM) AlSi10Mg specimens fabricated by selective laser melting (SLM) following surface treatment by shot-peening was investigated. Specimen surface was shot-peened with either steel or ceramic balls. Nano-indentation measurements revealed that shot-peening caused surface hardening, with the hardness profile from the surface to the interior of the bulk disappearing 50 μm below the surface. Surfaces polished before shot-peening or following removal of about 25–30 μm from the surface after shot-peening by either mechanical or electrolytic polishing showed improved fatigue resistance and fatigue limit. Fractography of broken specimens demonstrated that for shot-peened specimens, the site of fatigue crack initiation was deeper than that for specimens that had not undergone shot-peening. The fracture area of AM-SLM AlSi10Mg specimens before and after shot-peening displayed a ductile fracture with relatively deep dimples. In contrast to AM specimens, the final fracture area of die-cast samples exhibited a brittle fracture surface, containing numerous cleavage facets and micro-cracks.

Journal ArticleDOI
TL;DR: In this paper, the potential environmental implications of additive manufacturing related to key issues including energy use, occupational health, waste, lifecycle impact, and cross-cutting and policy issues, in terms of their current state-of-the-art, research needs, and recommendations, respectively.
Abstract: Additive manufacturing (AM), commonly known as “three-dimensional (3D) printing,” is the process of joining materials to make objects from 3D model data, usually layer by layer. AM provides a cost-effective and time-efficient way to fabricate products with complicated geometries and advanced material properties and functionality. Based on the 2014 National Science Foundation (NSF) Workshop on Environmental Implications of Additive Manufacturing, this paper outlines potential environmental implications of AM related to key issues including energy use, occupational health, waste, lifecycle impact, and cross-cutting and policy issues, in terms of their current state-of-the-art, research needs, and recommendations, respectively.

Journal ArticleDOI
TL;DR: In this article, bimetallic structures were fabricated using laser engineering net shaping (LENS™), a commercially available additive manufacturing technique to understand processing ability and measure resultant interfacial and thermal properties of Inconel 718 and copper alloy GRCop-84.
Abstract: To understand processing ability and measure resultant interfacial and thermal properties of Inconel 718 and copper alloy GRCop-84, bimetallic structures were fabricated using laser engineering net shaping (LENS™), a commercially available additive manufacturing technique. It was hypothesized that additively combining the two aerospace alloys would form a unique bimetallic structure with improved thermophysical properties compared to the Inconel 718 alloy. Two approaches were used: the direct deposition of GRCop-84 on Inconel 718 and the compositional gradation of the two alloys. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray Diffraction (XRD), Vickers microhardness and flash thermal diffusivity were used to characterize these bimetallic structures to validate our hypothesis. The compositional gradation approach showed a gradual transition of Inconel 718 and GRCop-84 elements at the interface, which was also reflected in the cross-sectional hardness profile across the bimetallic interface. SEM images showed columnar grain structures at the interfaces with Cr2Nb precipitate accumulation along grain boundaries and the substrate-deposit interface. The average thermal diffusivity of the bimetallic structure was measured at 11.33 mm2/s for the temperature range of 50 °C–300 °C; a 250% increase in diffusivity when compared to the pure Inconel 718 alloy at 3.20 mm2/s. Conductivity of the bimetallic structures increased by almost 300% compared to Inconel 718 as well. Such structures with designed compositional gradation and tailored thermal properties opens up the possibilities of multi-material metal additive manufacturing for next generation of aerospace structures.

Journal ArticleDOI
TL;DR: This review is envisioned to provide an essential framework on modeling techniques to supplement the experimental optimization process and highlight fundamental modeling strategies, considerations, and results, as well as validation techniques using experimental data.
Abstract: Next generation, additively-manufactured metallic parts will be designed with application-optimized geometry, composition, and functionality. Manufacturers and researchers have investigated various techniques for increasing the reliability of the metal-AM process to create these components, however, understanding and manipulating the complex phenomena that occurs within the printed component during processing remains a formidable challenge-limiting the use of these unique design capabilities. Among various approaches, thermomechanical modeling has emerged as a technique for increasing the reliability of metal-AM processes, however, most literature is specialized and challenging to interpret for users unfamiliar with numerical modeling techniques. This review article highlights fundamental modeling strategies, considerations, and results, as well as validation techniques using experimental data. A discussion of emerging research areas where simulation will enhance the metal-AM optimization process is presented, as well as a potential modeling workflow for process optimization. This review is envisioned to provide an essential framework on modeling techniques to supplement the experimental optimization process.

Journal ArticleDOI
TL;DR: Rod shaped samples of AlSi10Mg additively manufactured using recycled powder through direct metal laser sintering (DMLS) process showed higher quasi-static uniaxial tensile strength in both horizontal and vertical build directions than those of cast counterpart alloy as discussed by the authors.
Abstract: Rod shaped samples of AlSi10Mg additively manufactured using recycled powder through direct metal laser sintering (DMLS) process showed higher quasi-static uniaxial tensile strength in both horizontal and vertical build directions than those of cast counterpart alloy. In addition, they offered mechanical properties within the range of other additively manufactured counterparts. TEM showed that the microstructure of the as-built samples comprised of cell-like structures featured by dislocation networks and Si precipitates. HRTEM studies revealed the semi-coherency characteristics of the Si precipitates. After deformation, the dislocation density increased as a result of generation of new dislocations due to dislocation motion. The dislocations bypassed the precipitates by bowing around them and penetrating the semi-coherent precipitates. Strengthening of recycled DMLS-AlSi10Mg alloys manufactured in both directions was attributed to Orowan mechanism (due to existence of Si precipitates), Hall-Petch effect (due to eutectic cell walls), and dislocation hardening (due to pre-existing dislocation networks). Due to the slightly different microstructure, the contribution of each strengthening mechanism was slightly different in identical samples made with virgin powder.

Journal ArticleDOI
TL;DR: In this paper, a review on the application of finite element method to optimize process parameters and improve the mechanical performance of a part fabricated by powder-bed-fusion additive manufacturing process is provided.
Abstract: This survey aims to provide a review on the application of finite element method to optimize process parameters and improve the mechanical performance of a part fabricated by powder-bed-fusion Additive Manufacturing process. The state-of-the-art finite element models in the simulation of powder bed fusion process are reviewed. Numerical modeling methodologies of the laser beam melting or electron beam melting process at the macro-level are summarized in detail. Specifically, the importance of pre-processing of the part model, process parameters, mesh scheme, and temperature-dependent material properties are clarified. Simulation techniques used to reduce the computational cost are also discussed. Then the existing finite element models in the simulation of powder-bed fusion processes are reviewed and discussed. Simulation results are classified based on the characteristics of the melt pool and the printed part. Then the simulation results are validated by the experiment results. Finally, the significance of finite element method in the connection of other Additive Manufacturing issues such as material design, in-process monitoring and control, and process optimization are explained. The drawbacks of existing finite element models are summarized. And potential new methods to optimize process parameters of PBF process are proposed.

Journal ArticleDOI
TL;DR: In this paper, laser powder bed fusion of aluminum alloy (AA) 6061 used powder bed heating at 500°C in combination with other experimentally determined processing parameters to produce crack-free components.
Abstract: During solidification of many so-called high-performance engineering alloys, such as 6000 and 7000 series aluminum alloys, which are also unweldable autogenously, volumetric solidification shrinkage and thermal contraction produces voids and cracks. During additive manufacturing processing, these defects can span the length of columnar grains, as well as intergranular regions. In this research, laser powder bed fusion (LPBF) of aluminum alloy (AA) 6061 used powder bed heating at 500 °C in combination with other experimentally determined processing parameters to produce crack-free components. In addition, melt-pool banding, which is a normal solidification feature in LPBF, was eliminated, illustrating solidification process modification as a consequence of powder bed heating. Corresponding microindentation hardness and tensile testing of the as-fabricated AA6061 components indicated an average Vickers hardness of HV 54, and tensile yield, ultimate strength, and elongation values of 60 MPa, 130 MPa, and 15%, respectively. These mechanical properties and those of heat treated parts showed values comparable to annealed and T6 heat treated wrought products, respectively. X-ray diffraction and optical microscopy revealed columnar grain growth in the build direction with the as-fabricated, powder-bed heated product microstructure characterized by [100] textured, elongated grains (∼ 25 μm wide by 400 μm in length), and both intragranular and intergranular, noncoherent Al-Si-O precipitates which did not contribute significantly to the mechanical properties. The results of this study are indicative that powder bed heating may be used to assist with successful fabrication of AA6061 and other alloy systems susceptible to additive manufacturing solidification cracking.

Journal ArticleDOI
TL;DR: In this paper, the authors investigated laser beam focus shift, or "defocus" using a dynamic focusing unit, in order to increase the laser spot size, which can lead to a potential productivity increase by 840%.
Abstract: Despite its many benefits, Selective Laser Melting's (SLM) relatively low productivity compared to deposition-based additive manufacturing techniques is a major drawback. Increasing the laser beam diameter improves SLM's build rate, but causes loss of precision. The aim of this study is to investigate laser beam focus shift, or “defocus”, using a dynamic focusing unit, in order to increase the laser spot size. When applied to the SLM process, focus shift can be integrated into a “hull-core” strategy. This involves scanning the core with a high productivity parameter set using defocus while enabling return to the focused smaller spot size position for hull scanning. To assess the process stability, single line scans were made from 316L stainless steel powder. The consolidated melt pool morphology was analyzed and correlated with the process parameters comprising laser power, scanning speed and defocus distance. In order to link the melt pool morphology with the heat input, Volumetric Energy Density, Normalized Enthalpy and Rosenthal equation were considered. The suitability of using the Normalized Enthalpy as a design parameter to predict the melt pool depth and Rosenthal equation to predict its width was highlighted. This study shows that within a single laser setup, implementing defocus can lead to a potential productivity increase by 840%, i.e. to 18.8 mm3/s.

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TL;DR: In this article, a review of experimental deposition procedures for the cold spray additive manufacturing is presented, which is classified into various baseline working conditions, specific processing including deposition of nanotechnological components, composites-based structures and hybrid coating with substrate deposition.
Abstract: Today, cold gas dynamic spray (CGDS) technology has thrived with considerable capabilities for manufacturing various technological depositions. The deposition conditions have been developed through many years and that have led to produce ample experimental data which is available in the literature. But, recent research and development activities also reveal innovative findings regarding various deposition conditions. This paper contains a review of experimental deposition procedures for the cold spray additive manufacturing. Details of processing conditions are reported and classified into various categories of baseline working conditions, specific processing including deposition of nanotechnological components, composites-based structures and hybrid coating with substrate deposition. Available substrate treatments and their contributions on the deposition capability were also included. A large collection of experimental data from the literature is addressed in the Section 7.

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TL;DR: In this paper, the main objective of this paper is to review the main NDT techniques and assess the capability of detecting WAAM defects, for inspection either in a monitoring, in-process or post-process scenario.
Abstract: The present work addressed the challenges of identifying applicable Non-Destructive Testing (NDT) techniques suitable for inspection and materials characterization techniques for Wire and Arc Additive Manufacturing (WAAM) parts. With the view of transferring WAAM to the industry and qualifying the manufacturing process for applications such as structural components, the quality of the produced parts needs to be assured. Thus, the main objective of this paper is to review the main NDT techniques and assess the capability of detecting WAAM defects, for inspection either in a monitoring, in-process or post-process scenario. Radiography and ultrasonic testing were experimentally tested on reference specimens in order to compare the techniques capabilities. Metallographic, hardness and electrical conductivity analysis were also applied to the same specimens for material characterization. Experimental outcomes prove that typical WAAM defects can be detected by the referred techniques. The electrical conductivity measurement may complement or substitute some destructive methods used in AM processing.