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Showing papers in "Additive manufacturing in 2020"


Journal ArticleDOI
TL;DR: A comprehensive review on the state-of-the-art of ML applications in a variety of additive manufacturing domains can be found in this paper, where the authors provide a section summarizing the main findings from the literature and provide perspectives on some selected interesting applications.
Abstract: Additive manufacturing (AM) has emerged as a disruptive digital manufacturing technology. However, its broad adoption in industry is still hindered by high entry barriers of design for additive manufacturing (DfAM), limited materials library, various processing defects, and inconsistent product quality. In recent years, machine learning (ML) has gained increasing attention in AM due to its unprecedented performance in data tasks such as classification, regression and clustering. This article provides a comprehensive review on the state-of-the-art of ML applications in a variety of AM domains. In the DfAM, ML can be leveraged to output new high-performance metamaterials and optimized topological designs. In AM processing, contemporary ML algorithms can help to optimize process parameters, and conduct examination of powder spreading and in-process defect monitoring. On the production of AM, ML is able to assist practitioners in pre-manufacturing planning, and product quality assessment and control. Moreover, there has been an increasing concern about data security in AM as data breaches could occur with the aid of ML techniques. Lastly, it concludes with a section summarizing the main findings from the literature and providing perspectives on some selected interesting applications of ML in research and development of AM.

274 citations


Journal ArticleDOI
TL;DR: In this article, the effects of carbon fiber reinforcement on the structure and mechanical properties of 3D printed parts are investigated within the body of literature, and current and potential applications of additively manufactured carbon fiber composites in the context of desktop 3D printing and big area additive manufacturing are discussed.
Abstract: While polymer additive manufacturing (AM) has advanced significantly over the past few decades, the limitations in material properties, speed of manufacture, and part size have relegated this technology to the space of rapid prototyping rather than the legitimate manufacture of end-use parts. Carbon fiber offers a low density, a low coefficient of thermal expansion, and high thermal conductivity and is an ideal material for bringing polymer-based AM from the realm of form and fit to that of form, fit, and function. Use of carbon fiber in AM can improve material properties, reduce the time required to manufacture functional parts compared with traditional subtractive technologies, and reduce warping, thereby enabling a larger possible build envelope. Therefore, the addition of carbon fiber to various AM technologies is of increasing interest in academic and industrial communities. This paper examines the work performed in this fast-growing area to date. Specifically, the effects of fiber reinforcement on the structure and mechanical properties of 3D printed parts are investigated within the body of literature. Upper bounds for tensile properties of carbon fiber composites are theoretically evaluated and compared with experimentally measured values. Moreover, current and potential applications of additively manufactured carbon fiber composites in the context of desktop 3D printing and big area AM are discussed. Recent innovations and industry breakthroughs in this field are also examined. This review is intended to organize and synthesize the present body of work surrounding AM of carbon fiber reinforced plastics, identify the most promising technologies, and prescribe viable research and development paths forward to advance AM from the application space of rapid prototyping to that of functional, load-bearing, end-use parts.

236 citations


Journal ArticleDOI
TL;DR: In this article, the theoretical background and engineering capabilities of CSL with an emphasis on photopolymerization of ceramic resins are discussed and some constraints and characteristics designed to achieve optimal printability and photo-curability goals.
Abstract: In recent years, there have been rapid advances in our understanding of ceramic stereolithography (CSL) as a precise and high-resolution additive manufacturing (AM) technique to fabricate complex ceramic parts. This review highlights the theoretical background and engineering capabilities of CSL with an emphasis on photopolymerization of ceramic resins. We present certain constraints and characteristics designed to achieve optimal printability and photo-curability goals in ceramic resins and discuss in details about the parameters that can affect these properties. We then describe the current market status of CSL as well as its remaining challenges and promising future directions.

178 citations


Journal ArticleDOI
TL;DR: In this paper, the authors provide an overview of the research progress in metallic FGMs fabricated by laser metal deposition (LMD), an AM process that is widely used in metallic materials.
Abstract: Functionally graded materials (FGMs) have attracted much research interest in the industry due to their graded material properties, which result from gradually distributed compositions or structures. In recent years, metallic FGMs have been widely studied, and additive manufacturing (AM) has become an important approach to build metallic FGMs. This paper aims to provide an overview of the research progress in metallic FGMs fabricated by laser metal deposition (LMD), an AM process that is widely used in metallic materials. Firstly, the unique material properties and advantages of FGMs are introduced. Then, typical recent findings in transition path design, fabrication, and characterization for different types of metallic FGMs via LMD are summarized and discussed. Finally, challenges in fabricating metallic FGMs via LMD are discussed, and other related aspects in the area of FGMs such as model representation and numerical simulation are proposed for further investigation.

165 citations


Journal ArticleDOI
TL;DR: In this article, three Ti-6Al-4-V powder lots produced by two different techniques, namely, plasma atomization and gas atomization, were selected and characterized.
Abstract: Laser powder bed fusion (LPBF) additive manufacturing technology is sensitive to variations in powder particle morphology and size distribution. However, the absence of a clear link between the powder characteristics and the LPBF performances complicates the development, selection and quality control of LPBF powder feedstock. In this work, three Ti-6Al-4 V powder lots produced by two different techniques, namely, plasma atomization and gas atomization, were selected and characterized. Following the micro-computed tomography analysis of the powder particles’ morphology, size and density, the flowability of these powder lots was concurrently evaluated using Hall and Gustavsson flowmeters and an FT4 powder rheometer. Using established rheology-based criteria, a figure of merit was proposed to quantify the overall powder suitability for the LPBF process. Next, the same three powder lots were used to 3D-print and post-process a series of testing specimens with different layer thicknesses and build orientations, in order to establish a correlation between the powder characteristics and the geometric and mechanical properties of a final product. This study demonstrates that the use of highly spherical powders with a limited amount of fine particles promotes their flowability and yields LPBF components with improved mechanical and geometric characteristics.

162 citations


Journal ArticleDOI
TL;DR: In this article, a literature review is presented which identifies key advances in metamaterials alongside additive manufacturing and proposes new opportunities for researchers to work together through intra/inter disciplinary research to realize structures which exhibit extraordinary behaviour(s).
Abstract: Metamaterials exhibit properties beyond those exhibited by conventional materials in conventional scenarios. These have been investigated both theoretically and experimentally at length. In many cases the underpinning physical understanding of metamaterials has greatly preceded our ability to manufacture constituent structures. However, the development of additive manufacturing techniques gives new possibilities for the fabrication of complex metamaterial structures, many of which cannot be realised through conventional fabrication methods. The literature to date contains contributions from a diverse group of researchers from the physical sciences, mathematics, and manufacturing technology in the creation of metamaterials for electromagnetic, acoustic and mechanical applications. It is proposed that additive manufacturing holds the key to realise the capabilities of this vibrant research community and permit the creation of new paradigms in fundamental structures but also exploitation through application. For this purpose, a literature review is presented which identifies key advances in metamaterials alongside additive manufacturing and proposes new opportunities for researchers to work together through intra/inter disciplinary research to realise structures which exhibit extraordinary behaviour(s). This review represents a comprehensive account of the state-of-the-art in the production of such metamaterials using additive manufacturing methods and highlights areas, which, based on trends observed in the literature, are worthy of further research and require a coordinated effort on behalf of the afore mentioned disciplines in order to advance the state-of-the-art.

158 citations


Journal ArticleDOI
TL;DR: In this paper, a literature review on business models used in the additive manufacturing industry is presented, focusing on incremental and disruptive applications in closed and open market models, and the economic feasibility of these applications is critically discussed on the background of existing literature.
Abstract: This paper is a literature review on business models used in the additive manufacturing industry. We focus the investigation by categorizing the effects additive manufacturing into four classes by looking at incremental and disruptive applications in closed and in open market models. The economic feasibility of these applications is critically discussed on the background of the existing literature. Additive manufacturing business models is an emerging area of research, where tangible, case-based evidence is still rare, and the views on the business potential of additive manufacturing technologies are strongly divided.

131 citations


Journal ArticleDOI
TL;DR: In this paper, the as-built NiTi microstructure changed from collumnar, in the first deposited layers, to equiaxed in the last deposited ones as a result of different thermal cycle conditions.
Abstract: Wire and Arc Additive Manufacturing (WAAM) was used for fabrication of NiTi parts using a commercialy available Ni-rich NiTi wire as the feedstock material. The as-built parts are near fully austenitic at room temperature as confirmed by differential scanning calorimetry, X-ray diffraction and superelastic cycling. The as-built microstructure changed from collumnar, in the first deposited layers, to equiaxed in the last deposited ones as a result of the different thermal cycle conditions. This is the first work where WAAM NiTi parts exhibit superelastic behavior under tensile conditions, highlighting the potential use of the technique for the creation of parts shaped in a complex manner based on this material and process. The potential to use WAAM for deposition of advanced functional materials is demonstrated.

129 citations


Journal ArticleDOI
TL;DR: In this paper, a multi-layer 3D printed oral dosage form (polyprintlet) incorporating four antihypertensive drugs including irbesartan, atenolol, hydrochlorothiazide and amlodipine was used to reduce pill burden and improve patient adherence.
Abstract: The introduction of three-dimensional (3D) printing in the pharmaceutical arena has caused a major shift towards the advancement of modern medicines, including drug products with different configurations and complex geometries. Otherwise challenging to create via conventional pharmaceutical techniques, 3D printing technologies have been explored for the fabrication of multi-drug loaded dosage forms to reduce pill burden and improve patient adherence. In this study, stereolithography (SLA), a vat polymerisation technique, was used to manufacture a multi-layer 3D printed oral dosage form (polyprintlet) incorporating four antihypertensive drugs including irbesartan, atenolol, hydrochlorothiazide and amlodipine. Although successful in its fabrication, for the first time, we report an unexpected chemical reaction between a photopolymer and drug. Fourier Transform Infrared (FTIR) spectroscopy and Nuclear Magnetic Resonance (NMR) spectroscopy confirmed the occurrence of a Michael addition reaction between the diacrylate group of the photoreactive monomer and the primary amine group of amlodipine. The study herein demonstrates the importance of careful selection of photocurable resins for the manufacture of drug-loaded oral dosage forms via SLA 3D printing technology.

112 citations


Journal ArticleDOI
TL;DR: In this article, Zhao et al. reported in-situ characterization of melt-flow dynamics in every location of the entire melt pool in laser metal additive manufacturing by populous and uniformly dispersed micro-tracers through high-resolution synchrotron x-ray imaging.
Abstract: Melt flow plays a critical role in laser metal additive manufacturing, yet the melt flow behavior within the melt pool has never been explicitly presented. Here, we report in-situ characterization of melt-flow dynamics in every location of the entire melt pool in laser metal additive manufacturing by populous and uniformly dispersed micro-tracers through in-situ high-resolution synchrotron x-ray imaging. The location-specific flow patterns in different regions of the melt pool are revealed and quantified under both conduction-mode and depression-mode melting. The physical processes at different locations in the melt pool are identified. The full-field melt-flow mapping approach reported here opens the way to study the detailed melt-flow dynamics under real additive manufacturing conditions. The results obtained provide crucial insights into laser additive manufacturing processes and are critical for developing reliable high-fidelity computational models.

108 citations


Journal ArticleDOI
TL;DR: A review of the literature concerning melt pool simulation can be found in this article, where the physical theory underlying the current benchmark models is first presented and the main approximations and assumptions discussed.
Abstract: Reliable computational models of metal additive manufacturing will assist in optimising part quality, and are likely to play a role in component qualification. A key component of these models will be a detailed simulation of flow and heat transfer in and around the melt pool formed as the powder bed is melted. This paper reviews the burgeoning literature concerning melt pool simulation. The physical theory underlying the current benchmark models is first presented and the main approximations and assumptions discussed. The individual capabilities of the leading simulation groups around the world are listed in detail. Publications by less prominent research groups are also summarised. Finally, the overall status of melt pool simulation and the implications for model development are discussed.

Journal ArticleDOI
TL;DR: In this article, a coincorporation of submicron Si and TiB2 to an Al-Zn-Mg-Cu alloy is proposed to solve the long-standing problem by reducing solidification shrinkage and, simultaneously, enhancing its fracture toughness.
Abstract: 7xxx Al alloys such as Al-Zn-Mg-Cu are typical lightweight materials of excellent mechanical performance. Their near-net-shape manufacturing by selective laser melting (SLM) additive manufacturing, however, remains challenging due to hot-cracking prone nature of these alloys, when subjected to rapid solidification during the SLM process. In this study, we propose that co-incorporation of submicron Si and TiB2 to an Al-Zn-Mg-Cu alloy is capable to solve the long-standing problem by reducing solidification shrinkage and, simultaneously, enhancing its fracture toughness. Results show that solidification cracks indeed have been eliminated by the co-incorporation, along with much-refined microstructure. The resultant mechanical properties are high in ultimate tensile (556 ± 12 MPa) and yield strengths (455 ± 4.3 MPa). For disclosing the underlying mechanism, analytical means including high-resolution computer tomography, transmission electron microscopy and electron backscatter diffraction, as well as finite element simulation have been employed. It is aspired that the current approach can enable SLM to process critical engineering materials such as the hard-to-weld Al-Zn-Mg-Cu alloys.

Journal ArticleDOI
TL;DR: In this article, the role of volumetric energy density on the microstructural evolution, texture and mechanical properties of 304L stainless steel parts additively manufactured via selective laser melting process is investigated.
Abstract: The role of volumetric energy density on the microstructural evolution, texture and mechanical properties of 304L stainless steel parts additively manufactured via selective laser melting process is investigated. 304L is chosen because it is a potential candidate to be used as a matrix in a metal matrix composite with nanoparticles dispersion for energy and high temperature applications. The highest relative density of 99 %±0.5 was achieved using a volumetric energy density of 1400 J/mm3. Both XRD analysis and Scheil simulation revealed the presence of a small trace of the delta ferrite phase, due to rapid solidification within the austenitic matrix of 304L. A fine cellular substructure ranged between 0.4–1.8 μm, was detected across different energy density values. At the highest energy density value, a strong texture in the direction of [100] was identified. At lower energy density values, multicomponent texture was found due to high nucleation rate and the existing defects. Yield strength, ultimate tensile strength, and microhardness of samples with a relative density of 99 % were measured to be 540 ± 15 MPa, 660 ± 20 MPa and 254 ± 7 HV, respectively and higher than mechanical properties of conventionally manufactured 304L stainless steel. Heat treatment of the laser melted 304L at 1200 °C for 2 h, resulted in the nucleation of recrystallized equiaxed grains followed by a decrease in microhardness value from 233 ± 3 HV to 208 ± 8 HV due to disappearance of cellular substructure.

Journal ArticleDOI
TL;DR: In this article, the defect structure process maps (DSPMs) were used to quantify the role of porosity as an exemplary defect structure in powder bed printed materials and demonstrated that large-scale defects in LPBF materials can be successfully predicted and thus mitigated/minimized via appropriate selection of processing parameters.
Abstract: Accurate detection, characterization, and prediction of defects has great potential for immediate impact in the production of fully-dense and defect free metal additive manufacturing (AM) builds. Accordingly, this paper presents Defect Structure Process Maps (DSPMs) as a means of quantifying the role of porosity as an exemplary defect structure in powder bed printed materials. Synchrotron-based micro-computed tomography (μSXCT) was used to demonstrate that metal AM defects follow predictable trends within processing parameter space for laser powder bed fusion (LPBF) materials. Ti-6Al-4 V test blocks were fabricated on an EOS M290 utilizing variations in laser power, scan velocity, and hatch spacing. In general, characteristic under-melting or lack-of-fusion defects were discovered in the low laser power, high scan velocity region of process space via μSXCT. These defects were associated with insufficient overlap between adjacent melt tracks and can be avoided through the application of a lack-of-fusion criterion using melt pool geometric modeling. Large-scale keyhole defects were also successfully mitigated for estimated melt pool morphologies associated with shallow keyhole front wall angles. Process variable selections resulting in deep keyholes, i.e., high laser power and low scan velocity, exhibit a substantial increase of spherical porosity as compared to the nominal (manufacturer recommended) processing parameters for Ti-6Al-4 V. Defects within fully-dense process space were also discovered, and are associated with gas porosity transfer to the AM test blocks during the laser-powder interaction. Overall, this work points to the fact that large-scale defects in LPBF materials can be successfully predicted and thus mitigated/minimized via appropriate selection of processing parameters.

Journal ArticleDOI
TL;DR: In this paper, X-ray tomography was employed to provide insights into pore closure efficiency by HIP for an intentional and artificially-induced cavity as well as for a range of typical process-induced pores (lack of fusion, keyhole, contour pores, etc.) in coupon samples of Ti6Al4V.
Abstract: Hot isostatic pressing (HIP) of additively manufactured metals is a widely adopted and effective method to improve the density and microstructure homogeneity within geometrically-complex metal structures fabricated with laser powder bed fusion (LPBF). The role of pores in the fatigue performance of additively manufactured metal parts is increasingly being recognized as a critical factor and HIP post-processing is now heralded as a method to eliminate pores, especially for high-criticality applications such as in the aerospace industry. Despite the widely reported positive influence on fatigue performance and high efficiency of pore closure, examples have been reported in which pores have not been entirely closed or have subsequently re-opened upon heat treatment. A variety of porosity distributions and types of pores may be present in parts produced by LBPF and the effectiveness of pore closure may differ depending on these pore characteristics. In this work, X-ray tomography was employed to provide insights into pore closure efficiency by HIP for an intentional and artificially-induced cavity as well as for a range of typical process-induced pores (lack of fusion, keyhole, contour pores, etc.) in coupon samples of Ti6Al4V. The same samples were imaged non-destructively before and after HIP and aligned carefully for side-by-side viewing. High pore closure efficiency is demonstrated for all types of cavities and pores investigated, but near-surface pores of all types are shown to be problematic to varying degrees, in some cases perforating the superficial surface and creating new external notches. Subsequent heat treatments (annealing after HIP) in some cases resulted in internal pore reopening for previously closed internal pores as well as a new “blistering” effect observed for some near-surface pores, which the authors believe is reported for the first time. Implications of these results for quality control and HIP processing of LPBF parts are discussed. Finally, the utility of using HIP to consolidate intentionally-unmelted powder in order to improve production rates of powder bed fusion has great potential and is preliminarily demonstrated.

Journal ArticleDOI
TL;DR: In this paper, a convolutional neural network architecture for pixel-wise localization of layer-wise powder bed imaging data is presented, which can return segmentation results at the native resolution of the imaging sensor, seamlessly transfer learned knowledge between different additive manufacturing machines, and provide real-time performance.
Abstract: Increasing industry acceptance of powder bed metal Additive Manufacturing requires improved real-time detection and classification of anomalies. Many of these anomalies, such as recoater blade impacts, binder deposition issues, spatter generation, and some porosities, are surface-visible at each layer of the building process. In this work, the authors present a novel Convolutional Neural Network architecture for pixel-wise localization (semantic segmentation) of layer-wise powder bed imaging data. Key advantages of the algorithm include its ability to return segmentation results at the native resolution of the imaging sensor, seamlessly transfer learned knowledge between different Additive Manufacturing machines, and provide real-time performance. The algorithm is demonstrated on six different machines spanning three technologies: laser fusion, binder jetting, and electron beam fusion. Finally, the performance of the algorithm is shown to be superior to that of previous algorithms presented by the authors with respect to localization, accuracy, computation time, and generalizability.

Journal ArticleDOI
TL;DR: In this article, the influence of the B4C content on the printability, microstructure and mechanical properties of SLM-printed (TiB + TiC)/Ti composites was studied.
Abstract: Titanium matrix composites reinforced with titanium boride (TiB) and titanium carbide (TiC) were fabricated in situ via selective laser melting (SLM). Varying contents of boron carbide (B4C) from 0 to 5 wt% were added to pure Ti to prepare blended powders for SLM. The influence of the B4C content on the printability, microstructure and mechanical properties of SLM-printed (TiB + TiC)/Ti composites was studied. The relative densities of all the fabricated composites were greater than 97.8 %; an increase in the B4C content resulted in a decrease in their relative density. The microstructure of the composites varied from a lath-shaped structure (0 wt%) to a dendritic structure (1 wt%) and to a cellular + dendritic structure (2–5 wt%), which was determined by the change of thermal convection within melt pools. Both the dendritic and cellular structures were arranged by coalescent clusters composed of TiB whiskers and TiC particles, while the clusters were formed through a self-joining behavior of the TiB whiskers that were mechanically locked with the TiC particles. The composites with the addition of 1 wt% B4C exhibited the highest ultimate tensile strength of 946 MPa, yield strength of 762 MPa and elastic modulus of 128 GPa, which were 62.4 %, 49.2 % and 15.3 % higher than those of the Ti matrix, respectively. The remarkable enhancement in the mechanical strengths was attributed to the synergistic effect of dispersion strengthening and grain refinement strengthening. The (TiB + TiC)/Ti composites fabricated in situ by SLM with an optimized material formulation not only ensured comparable mechanical properties to those of their counterparts manufactured by conventional processes but also demonstrated the potential in manufacturing complex and/or customized parts with design freedom for industrial applications.

Journal ArticleDOI
TL;DR: In this article, the authors show that selective laser melting (SLM) can also act as a metallurgical method to modify the Ni/Ti ratio of NiTi shape memory alloys.
Abstract: In this work, we show that selective laser melting (SLM), apart from producing complex structures, can also act as a metallurgical method to modify the Ni/Ti ratio of NiTi shape memory alloys, and thus providing a feasible approach to tailor the transformation temperatures and to modify the mechanical performance of NiTi alloys. NiTi samples were fabricated by SLM with a large variation of process parameters, e.g. scanning speed (v) from 400 to 1200 mm s−1, hatch spacing (h) from 40 to 110 μm, and laser power (P) from 60 to 200 W. It is found that the martensite transformation temperature changes monotonously with the respective change of v, h or P. The composition analysis suggests that the different amount of Ni-loss under various SLM process conditions is the main reason for the evolution of transformation temperatures. Most importantly, good mechanical (total elongation >10 %) and functional properties under tensile mode have been obtained despite of the large variation of SLM process parameters and the presence of extensive defects. The good tensile properties and tailorable phase transformation temperatures will provide great potential to make novel NiTi smart structures.

Journal ArticleDOI
TL;DR: In this paper, a novel and efficient mechanical agitation process is used to inoculate AlSi10Mg powder with LaB6 nanoparticles which resulted in a homogenous, crack-free, equiaxed, very fine-grained as built microstructures.
Abstract: Metal additive manufacturing offers a tool to bring formerly unmanufacturable, geometrically complex, engineered structures into existence. However, considerable challenges remain in controlling the unique microstructures, defects and properties that are created through this process. For the first time this work demonstrates how LaB6 nanoparticles can be used to control such features in Al alloys produced by Selective Laser Melting (SLM). A novel and efficient mechanical agitation process is used to inoculate AlSi10Mg powder with LaB6 nanoparticles which resulted in a homogenous, crack-free, equiaxed, very fine-grained as built microstructures. The substantial grain refinement is attributed to the good crystallographic atomic matching across the Al/LaB6 interfaces which facilitated Al nucleation on the LaB6 nanoparticles. The LaB6-inoculated AlSi10Mg exhibited near-isotropic mechanical properties with an improved plasticity compared with un modified AlSi10Mg.

Journal ArticleDOI
TL;DR: In this article, SiC particles were added to the molten pool during WAAM of a high strength low alloy steel, and the introduction of these high melting point particles promoted grain refinement and precipitation of Fe3C due to SiC dissociation.
Abstract: In this work, SiC particles were added to the molten pool during WAAM of a high strength low alloy steel. The introduction of these high melting point particles promoted grain refinement, and the precipitation of Fe3C due to SiC dissociation. The microstructural evolution was studied by optical and electron microscopy techniques and high energy synchrotron X-ray diffraction. Additionally, mechanical testing and hardness profiles were obtained for the SiC-containing and SiC-free parts. An improvement in the mechanical strength of the SiC-added WAAM parts was observed, which was attributed to the refined grain structure and finely dispersed Fe3C.

Journal ArticleDOI
TL;DR: In this article, the features and formation mechanisms of five unique types of spatter during the LPBF process were revealed by in-situ high-speed, high-energy x-ray imaging.
Abstract: Spatter causes defect formation, powder redistribution and contamination in laser powder bed fusion (LPBF) additive manufacturing process It is critical to distinguish different types of spatter and understand their features and formation mechanisms This work reveals the features and formation mechanisms of five unique types of spatter during the LPBF process by in-situ high-speed, high-energy x-ray imaging Spatters observed during LPBF testing are quantified by their speed, size, and direction Distinct quantifiable characteristics for each type of spatter are identified Effects of the laser power, scan speed, and ambient pressure on spatter formation and features are unraveled A spatter formation map for AlSi10Mg alloy is constructed

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the relationship between laser powder bed fusion (LPBF) parameters and defect formation with a focus on hot cracking and discussed the process window for the LPBF manufacturing of defect-free components of AA7075 alloy.
Abstract: Laser powder bed fusion (LPBF) is an attractive technology of manufacturing high-strength aluminium alloy parts for the aircraft and automobile industries, limited by poor processability of these alloys. This work was aimed at finding the process window for the LPBF manufacturing of defect-free components of AA7075 alloy. Optimization of the parameters was performed at each stage of the multi-stage research, i.e. for single tracks, thin walls and volumetric specimens. At each stage, the relation between LPBF parameters and defect formation with a focus on hot cracking was investigated and discussed. Due to the optimization of process parameters, the density of volumetric specimens above 99 % was reached and vaporization losses of the alloying elements were significantly reduced, but solidification cracks could not be eliminated. It was found that solidification cracks were formed by the liquid film rupture mode, mainly along columnar grain boundaries. The EDS microanalysis showed intergranular microsegregation, not only of the main alloying elements (Zn, Mg, Cu) but also of minor elements such as Si. Silicon may play a significant role in increasing susceptibility to cracking by increasing the stability of the liquid film. Reduction in the silicon impurity content in the AA7075 powder gives a chance to reduce susceptibility to cracking with no change of the alloy specification.

Journal ArticleDOI
TL;DR: In this article, the state of the art for processing high-temperature (i.e., traditional processing temperatures exceeding 250°C) thermoplastic polymers by the melt-based, AM processes of material extrusion (MatEx) and laser powder bed fusion (PBF).
Abstract: The strengths of additive manufacturing (AM), especially the tool-less manufacturing paradigm and rapid production of low-volume products, are well-aligned with the needs of manufacturing of expensive, high-temperature resistant, engineering thermoplastic polymers. High temperature polymer parts made with AM for either tooling or end-use applications have been implemented in the aerospace, automotive, and biomedical fields. However, parts made from these polymers using traditional manufacturing processes are generally high-value parts in low-quantity production runs. Moreover, AM processing of these polymers present significant challenges due to limitations associated with large thermal gradients, residual stress buildup, and interlayer adhesion as well as the inability of the printers to consistently maintain required high processing temperatures. This review highlights the current state of the art for processing high-temperature (i.e., traditional processing temperatures exceeding 250°C) thermoplastic polymers by the melt-based, AM processes of material extrusion (MatEx) and laser powder bed fusion (PBF). The authors address common challenges to AM of high-temperature polymers and gaps in fundamental understanding of the process-structure-property relationships needed to identify the machine design, process parameter selection, and synthetic modifications to enable processing.

Journal ArticleDOI
TL;DR: In this article, the microstructure, residual stress, and mechanical properties of the as-printed specimen and specimens annealed at 773-1573 K for 2'h were compared.
Abstract: To widen the applications of FeCoCrNi high-entropy alloys (HEAs) fabricated via selective laser melting, their mechanical properties must be improved, and annealing plays an important role in this regard. In this study, the microstructure, residual stress, and mechanical properties of the as-printed specimen and specimens annealed at 773–1573 K for 2 h were compared. As the annealing temperature increased, the specimen structure recrystallized from all columnar grains to equiaxial grains containing numerous annealing twins. The dislocation network, which formed during the solidification process under considerable shrinkage strain, decomposed into dislocations. The residual stress, yield strength, and hardness decreased, while the plasticity and impact toughness increased. During the deformation of as-printed and low-temperature-annealed specimens, the dislocation network remained unchanged and provided resistance to the dislocations moving within it, thus strengthening the specimen. The tensile strength remained largely unchanged owing to the reduction in the residual stress during low-temperature annealing, as well as the formation of the twinning network and dislocation wall under large deformation upon high-temperature annealing. Meanwhile, the ductility greatly increased, thus increasing the potential for industrial application of HEAs.

Journal ArticleDOI
TL;DR: In this article, the severity of density and unit cell size grading as well as the building direction affects the stiffness, energy absorption and structural response of additively manufactured (AM) short fiber-reinforced lattices with same relative density.
Abstract: Architectured structures, particularly functionally graded lattices, are receiving much attention in both industry and academia as they facilitate the customization of the structural response and harness the potential for multi-functional applications. This work experimentally investigates how the severity of density and unit cell size grading as well as the building direction affects the stiffness, energy absorption and structural response of additively manufactured (AM) short fibre-reinforced lattices with same relative density. Specimens composed of tessellated body-centred cubic (BCC), Schwarz-P (SP) and Gyroid (GY) unit cells were tested under compression. Compared to the uniform lattices of equal density, it was found, that modest density grading has a positive and no effect on the total compressive stiffness of SP and BCC lattices, respectively. More severe grading gradually reduces the total stiffness, with the modulus of the SP lattices never dropping below that of the uniform counterparts. Unit cell size grading had no significant influence on the stiffness and revealed an elastomer-like performance as opposed to the density graded lattices of the same relative density, suggesting a foam-like behaviour. Density grading of bending-dominated unit cell lattices showcased better energy absorption capability for small displacements, whereas grading of the stretching-dominated counterparts is advantageous for large displacements when compared to the ungraded lattice. The severity of unit cell size graded lattices does not affect the energy absorption capability. Finally, a power-law approach was used to semi-empirically derive a formula that predicts the cumulative energy absorption as a function of the density gradient and relative density. Overall, these findings will provide engineers with valuable knowledge that will ease the design choices for lightweight multi-functional AM-parts.

Journal ArticleDOI
TL;DR: In this article, Li et al. investigated the correlation between the ex situ melt track properties and the in situ high-speed, high-resolution characterization of laser powder bed fusion (LPBF) components.
Abstract: Laser powder bed fusion (LPBF) has broad application prospects due to its high fabrication accuracy and excellent performance, but the dynamic mechanical properties of LPBF components are relatively low due to defects of the melt track such as protrusions and depressions, whose generation mechanisms remain unclear. In this work, we investigate the correlation between the ex situ melt track properties and the in situ high-speed, high-resolution characterization. We correlate the protrusion at the starting position of the melt track with the droplet ejection behaviour and backward surging melt. We also reveal that the inclination angles of the depression walls are consistent with the ejection angles of the backward-ejected spatter. Furthermore, we quantify the vapour recoil pressure by in situ characterization of the deflection of the typical forward-ejected spatter. Our results clarify the intrinsic correlation of the melt track properties, which is important for the stable LPBF formation with few defects.

Journal ArticleDOI
TL;DR: In this paper, the effect of shielding gas flow velocity on porosity and melt pool geometry in laser powder bed fusion additive manufacturing is studied, and it is shown that decreasing the gas flow leads to a drastic loss of penetration of single scan tracks, leading to increased lack-of-fusion porosity at the part level.
Abstract: Metal additive manufacturing is moving from rapid prototyping to on-demand manufacturing and even to serial production. Consistent part quality and development of a wider range of available materials are key for wider adoption. This requires control and optimization of various laser and scanning parameters. Therefore, process modeling has been extensively pursued to reduce experimental runs in the search for parameters that produce dense, high-quality parts for the given alloy. However, these optimal parameters remain machine-specific if conditions defined by the machine architecture are not considered. Previous studies have shown that shielding gas flow is one such parameter that affects porosity and mechanical properties of parts produced with laser powder bed fusion. However, a lack of consensus remains regarding which phenomena are responsible for the observed decrease in quality. In this study, the effect of shielding gas flow velocity on porosity and melt pool geometry in laser powder bed fusion additive manufacturing is studied. It is shown that decreasing the gas flow velocity leads to a drastic loss of penetration of single scan tracks, leading to increased lack-of-fusion porosity at the part level. This is attributed to the obstruction of the laser beam by the process-induced vapor plume emissions of the individual tracks being scanned. As the vapor plume, and how effectively it is removed by the shielding gas flow, have a significant effect on the melt pool geometry in laser powder bed fusion, models aiming at predicting the melt pool geometry and attempts to transfer process parameters from one machine to another should consider the effect of the shielding gas flow.

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TL;DR: In this paper, the authors present a summary of research that has been conducted to produce bulk cermets by additive manufacturing, including selective laser sintering/melting (SLS/SLM), laser engineering net shaping (LENS), direct laser fabrication (DLF), binder jet 3D printing, and 3D gel/direct-ink-write/robocasting printing.
Abstract: Cermets are a category of materials including ceramic and metallic phases, which possess the combined properties of both phases. Over the last few decades, numerous conventional processes such as powder metallurgy techinques and casting have been proposed for the fabrication of cermet components. In recent years, additive manufacturing (AM) has emerged as a promising method that can eliminate most of the limitations of conventional production methods. Among AM processes, selective laser sintering/melting (SLS/SLM), laser engineering net shaping (LENS), direct laser fabrication (DLF), binder jet 3D printing, and 3D gel/direct-ink-write/robocasting printing have been investigated for manufacturing bulk cermet parts. This study presents a summary of research that has been conducted to produce bulk cermets by additive manufacturing.

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Naoki Takata1, Mulin Liu1, Hirohisa Kodaira1, Asuka Suzuki1, Makoto Kobashi1 
TL;DR: In this article, microstructural characteristics of a SLM-built Al-Si-10Si-0.3Mg alloy and their changes upon annealing at elevated temperatures were investigated.
Abstract: To identify the dominant contributing factor in the anomalously high strength of Al–Si-based alloys fabricated by selective laser melting (SLM), microstructural characteristics of a SLM-built Al–10Si–0.3 Mg alloy (AlSi10Mg) and their changes upon annealing at elevated temperatures were investigated. The as-built AlSi10Mg alloy exhibits a peculiar microstructure comprising of a number of columnar α-Al (fcc) phase with concentrated Si in solution. Numerous nano-sized particles were observed within the α-Al matrix. At elevated temperatures, a number of Si phase (diamond structure) precipitates consumed the solute Si in the columnar α-Al phase, but the microstructure of the α-Al matrix changed slightly. After annealing at elevated temperatures, the tensile strength of the as-built AlSi10Mg alloy substantially decreased accompanied by a reduction in the strain hardening rate. The supersaturated solid solution of the α-Al phase containing numerous nano-sized particles enhanced the strain hardening, resulting in the anomalous strengthening of the SLM-built AlSi10Mg alloy. The microstructural features were formed due to rapid solidification at an extremely high cooling rate in the SLM process, which provides important insights into controlling the strength of Al–Si-based alloys fabricated by SLM.

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TL;DR: In this article, a Sc/Zr modified Al-Mg alloy was processed by both selective laser melting (SLM) and directed energy deposition (DED) to obtain a heterogeneous grain structure, which consisted of ultrafine equiaxed grains bands and columnar grains domains.
Abstract: In this work, a Sc/Zr modified Al-Mg alloy was processed by both selective laser melting (SLM) and directed energy deposition (DED). Due to different precipitation behavior of primary Al3(Sc,Zr)-L12 nucleation sites, a heterogeneous grain structure was formed in SLMed sample, which consisted of ultrafine equiaxed grains bands and columnar grains domains, while a fully equiaxed grain structure was obtained in DEDed sample. Tensile results showed that the as built SLMed sample had a good combination of strength and ductility. The yield strength of SLMed sample (335 ± 4 MPa) was about 2.8 times that of DEDed sample (118 ± 3 MPa), however, the ductility in uniform elongation (23.6 ± 1.9%) was still comparable to that of DEDed sample (23.8 ± 2.6%). Based on the relationship between the heterogeneous grain structure and strain hardening behavior, the strength-ductility synergy mechanism of the SLMed Al-Mg-Sc-Zr alloy was discussed. Stress partitioning tests showed that the contribution of back stress hardening to flow stress was higher in SLMed sample than DEDed sample, while effective stress hardening showed an opposite trend. Despite the overall strain hardening ability of SLMed sample was limited by the high dynamic recovery rate of ultrafine equiaxed grains, additional back stress hardening, which was caused by strain partitioning between equiaxed grains bands and columnar grains domains, improved its strain hardening ability and resulted in the good combination of strength and ductility.