Abstract: Binder Jet printing is an additive manufacturing technique that dispenses liquid binding agent on powder to form a two-dimensional pattern on a layer. The layers are stacked to build a physical article. Binder Jetting (BJ) can be adapted to almost any powder with high production rates and the BJ process utilizes a broad range of technologies including printing tehniques, powder deposition, dynamic binder/powder interaction, and post-processing methods. A wide variety of materials including polymers, metals, and ceramics have been processed successfully with Binder Jet. However, developing printing and post-processing methods that maximize part performance is a remaining challenge. This article presents a broad review of technologies and approaches that have been applied in Binder Jet printing and points towards opportunities for future advancement.

Abstract: The application of additive manufacturing (AM) in construction has been increasingly studied in recent years. Large robotic arm- and gantry-systems have been created to print building parts using aggregate-based materials, metals, or polymers. Significant benefits of AM are the automation of the production process, a high degree of design freedom, and the resulting potential for optimization. However, the building components and 3D-printing processes need to be modeled appropriately. In this paper, the current state of AM in construction is reviewed. AM processes and systems as well as their application in research and construction projects are presented. Moreover, digital methods for planning 3D-printed building parts and AM processes are described.

Abstract: Metal additive manufacturing (AM) has garnered tremendous research and industrial interest in recent years; in the field, powder bed fusion (PBF) processing is the most common technique, with selective laser melting (SLM) dominating the landscape followed by electron beam melting (EBM). Through continued process improvements, these methods are now often capable of producing high strength parts with static strengths exceeding their conventionally manufactured counterparts. However, PBF processing also results in large and anisotropic residual stresses (RS) that can severely affect fatigue properties and result in geometric distortion. The dependence of RS formation on processing variables, material properties and part geometry has made it difficult to predict efficiently and has hindered widespread acceptance of AM techniques. Substantial investigations have been conducted with regards to RS in PBF processing, which have illuminated a number of important relationships, yet a review encompassing this information has not been available. In this review, we survey and assemble the knowledge existing in the literature regarding RS in PBF processes. A discussion of background mechanics for RS development in AM is provided along with methods of measurement, highlighting the anisotropic nature of the stress fields. We then review modeling efforts and in-process experimental measurements made to advance process understanding, followed by a thorough analysis and summary of the known relationships of both material properties and processing variables to resulting RS. The current state of knowledge and future research needs for the field are discussed.

Abstract: This review article summarizes the current state-of-the-art for biomimicry in additive manufacturing. Biomimicry is the practice of learning from and emulating nature - which can be increasingly realized in engineering applications due to progress in additive manufacturing (AM). AM has grown tremendously in recent years, with improvements in technology and resulting material properties sometimes exceeding those of equivalent parts produced by traditional production processes. This has led to the industrial use of AM parts even in highly critical applications, most notably in aerospace, automotive and medical applications. The ability to create parts with complex geometries is one of the most important advantages of this technology, allowing the production of complex functional objects from various materials including plastics and metals that cannot be easily produced by any other means. Utilizing the full complexity allowed by AM is the key to unlocking the huge potential of this technology for real world applications – and biomimicry might be pivotal in this regard. Biomimicry may take different forms in AM, including customization of parts for individuals (e.g. medical prosthesis, implants or custom sports equipment), or optimization for specific properties such as stiffness and light-weighting (e.g. lightweight parts in aerospace or automotive applications). The optimization process often uses an iterative simulation-driven process analogous to biological evolution – with an improvement in every iteration. Other forms of biomimicry in AM include the incorporation of real biological inputs into designs (i.e. emulating nature for its unique properties); the use of cellular or lattice structures – for various applications and customized to the application; incorporating multi-functionality into designs; the consolidation of numerous parts into one and the reduction of waste, amongst others. Numerous biomimetic design approaches may be used – broadly categorized into customized/freeform, simulation-driven and lattice designs. All these approaches may be used in combination with one another, and in all cases with or without direct input from nature. The aim of this review is to unravel the different forms of biomimetic engineering that are now possible – focusing mainly on functional mechanical engineering for end-use parts, i.e. not for prototyping. The current limits of each design approach are discussed and the most exciting future opportunities for biomimetic AM applications are highlighted.

Abstract: The emergence of additive manufacturing and 3D printing technologies is introducing industrial skills deficits and opportunities for new teaching practices in a range of subjects and educational settings. In response, research investigating these practices is emerging across a wide range of education disciplines, but often without reference to studies in other disciplines. Responding to this problem, this article synthesizes these dispersed bodies of research to provide a state-of-the-art literature review of where and how 3D printing is being used in the education system. Through investigating the application of 3D printing in schools, universities, libraries and special education settings, six use categories are identified and described: (1) to teach students about 3D printing; (2) to teach educators about 3D printing; (3) as a support technology during teaching; (4) to produce artefacts that aid learning; (5) to create assistive technologies; and (6) to support outreach activities. Although evidence can be found of 3D printing-based teaching practices in each of these six categories, implementation remains immature, and recommendations are made for future research and education policy.

Abstract: Metal additive manufacturing, despite of offering unique capabilities e.g. unlimited design freedom, short manufacturing time, etc., suffers from raft of intrinsic defects. Porosity is of the defects which can badly deteriorate a part’s performance. In this respect, enabling one to observe and predict the porosity during this process is of high importance. To this end, in this work a combined numerical and experimental approach has been used to analyze the formation, evolution and disappearance of keyhole and keyhole-induced porosities along with their initiating mechanisms, during single track L-PBF of a Ti6Al4V alloy. In this respect, a high-fidelity numerical model based on the Finite Volume Method (FVM) and accomplished in the commercial software Flow-3D is developed. The model accounts for the major physics taking place during the laser-scanning step of the L-PBF process. To better simulate the actual laser-material interaction, multiple reflection with the ray-tracing method has been implemented along with the Fresnel absorption function. The results show that during the keyhole regime, the heating rises dramatically compared to the shallow-depth melt pool regime due to the large entrapment of laser rays in the keyhole cavities. Also a detailed parametric study is performed to investigate the effect of input power on thermal absorptivity, heat transfer and melt pool anatomy. Furthermore, an X-ray Computed Tomography (X-CT) analysis is carried out to visualize the pores formed during the L-PBF process. It is shown, that the predicted shape, size and depth of the pores are in very good agreement with those found by either X-CT or optical and 3D digital microscopic images.

Abstract: Because many of the most important defects in Laser Powder Bed Fusion (L-PBF) occur at the size and timescales of the melt pool itself, the development of methodologies for monitoring the melt pool is critical. This works examines the possibility of in-situ detection of keyholing porosity and balling instabilities. Specifically, a visible-light high speed camera with a fixed field of view is used to study the morphology of L-PBF melt pools in the Inconel 718 material system. A scale-invariant description of melt pool morphology is constructed using Computer Vision techniques and unsupervised Machine Learning is used to differentiate between observed melt pools. By observing melt pools produced across process space, in-situ signatures are identified which may indicate flaws such as those observed ex-situ. This linkage of ex-situ and in-situ morphology enabled the use of supervised Machine Learning to classify melt pools observed (with the high speed camera) during fusion of non-bulk geometries such as overhangs.

Abstract: This paper discusses the effects of process parameters in TIG based WAAM for specimens created using Hastelloy X alloy (Haynes International) welding wire and 304 stainless-steel plate as the substrate. The Taguchi method and ANOVA were used to determine the effects of travel speed, wire feed rate, current, and argon flow rate on the responses including bead shape and size, bead roughness, oxidation levels, melt through depth, and the microstructure. Travel speed and current were found to have the largest effect on the responses. Increasing travel speed or decreasing current caused a decrease in melt through depth and an increase in roughness. Printing strategies were tested using specimens of multiple layers and no significant difference was found between printing layers in the same direction and printing layers in alternating directions. No observable interface between the layers was present suggesting a complete fusion between layers with no oxidation. Three distinct zones were identified within the three- and eight-layer samples. The zones were characterized by the size and distribution of the molybdenum carbide formations within the matrix grain formations.

Abstract: Goal Additive manufacturing, also known as 3D printing, has begun to play a significant role in the field of medical devices. This review aims to provide a comprehensive overview and classification of additively manufactured medical instruments for diagnostics and surgery by identifying medical and technical aspects. Methods A scientific literature search on additively manufactured medical instruments was conducted using the Scopus database. Results We categorized the relevant articles (71) by considering the novelty of each proposed instrument and its clinical application. Then, we analyzed the relevant articles by examining the reasons behind choosing additive manufacturing technology to produce instruments for diagnostics and surgery. Possible customization (27%) and Cost-effectiveness (23%) were the main reasons expressed. Technical specifications of the additive manufacturing technology and the material used were also analyzed, and a tendency of using material extrusion technology (35% of the applications) and polymeric materials (86% of the applications) was shown. Conclusions Additive manufacturing is opening the door to a new approach in the production of medical devices, which allows the complexity of their designs to be pushed to the extreme. However, we found that technical limitations need to be tackled and important aspects such as sterilization or debris contamination are still not considered to be relevant factors during the design and fabrication process. Keeping in mind the challenges of such a new field, additive manufacturing technology can be considered as a great opportunity to provide easy access to healthcare in developing countries as well as an important step toward patient-specific medicine.

Abstract: Previous studies have shown that 3D printed composites exhibit an orthotropic nature with inherently lower interlayer mechanical properties. This research work is an attempt to improve the interlayer tensile strength of extrusion-based 3D printed composites. Annealing was identified as a suitable post-processing method and was the focus of this study. Two distinct thermoplastic polymers, which are common in 3D printing, were selected to study the enhancement of interlayer tensile strength of composites by additive manufacturing: a) an amorphous polyethylene terephthalate-glycol (PETG), and b) a semi-crystalline poly (lactic acid) (PLA). It was determined that short carbon fiber reinforced composites have lower interlayer tensile strength than the corresponding neat polymers in 3D printed parts. This reduction in mechanical performance was attributable to an increase in melt viscosity and the consequential slower interlayer diffusion bonding. However, the reduction in interlayer tensile strength could be recovered by post-processing when the annealing temperature was higher than the glass transition temperature of the amorphous polymer. In the case of the semi-crystalline polymer, the recovery of the interlayer tensile strength was only observed when the annealing temperature was higher than the glass transition temperature but lower than the cold-crystallization temperature. This study utilized rheological and thermal analysis of 3D printed composites to provide a better understanding of the interlayer strength response and, therefore, overcome a mechanical performance limitation of these materials.

Abstract: Wire and arc additive manufacturing (WAAM) is a viable technique for the manufacture of large and complex dedicated parts used in structural applications. High-strength low-alloy (HSLA) steels are well-known for their applications in the tool and die industries and as power-plant components. The microstructure and mechanical properties of the as-built parts are investigated, and are correlated with the thermal cycles involved in the process. The heat input is found to affect the cooling rates, interlayer temperatures, and residence times in the 800–500 °C interval when measured using an infrared camera. The microstructural characterization performed by scanning electron microscopy reveals that the microstructural constituents of the sample remain unchanged. i.e., the same microstructural constituents—ferrite, bainite, martensite, and retained austenite are present for all heat inputs. Electron backscattered diffraction analysis shows that no preferential texture has been developed in the samples. Because of the homogeneity in the microstructural features of the as-built parts, the mechanical properties of the as-built parts are found to be nearly isotropic. Mechanical testing of samples shows excellent ductility and high mechanical strength. This is the first study elucidating on the effect of thermal cycles on the microstructure and mechanical properties during WAAM of HSLA steel.

Abstract: Enhancing the corrosion resistance and improving the biological response to 316 L stainless steel is a long-standing and active area of biomedical research. Here, we analyzed the structure and corrosion tendency of selective laser melted-additively manufactured (AM) 316 L stainless steel (AM 316L SS) and its wrought counterpart. SEM analysis showed a fine (500–800 nm) interconnected sub-granular structure for the AM 316L SS, but a polygonal coarse-grained structure for the wrought sample. Relative to the wrought sample, the AM 316L SS also exhibited a higher charge transfer resistance and higher breakdown potential (˜1000 mV vs. SCE) when tested in biological electrolytes, which included human serum, PBS, and 0.9 M NaCl. A higher pitting resistance (extended passive region) and improved stability of the AM 316L SS was attributed to its dense structure of oxide film and refined microstructure. Finally, material compatibility with pre-osteoblasts was analyzed. Large cytoplasmic extension of osteoblast cells and retention of stiller morphology was observed when cells were cultured on the AM 316L SS as compared to its wrought counterpart, suggesting that the AM 316L SS was a better substrate for cell spreading and differentiation. The differentiation of cultured cells was further validated by western blot for Runx2. Runx2, an anti–proliferative marker indicative of differentiation, was equivalent in cells cultured on either samples, but overall more cells were present on the AM 316L SS. Given its higher corrosion resistance and ability to support osteoblast adherence, spreading and differentiation, the AM 316L SS has potential for use in the biomedical industry.

Abstract: This paper describes a deep-learning-based method for porosity monitoring in laser additive manufacturing process. A high-speed digital camera was mounted coaxially to the process laser beam for in-process sensing of melt-pool data, and convolutional neural network models were designed to learn melt-pool features to predict the porosity attributes in deposited specimens during laser additive manufacturing. With the image processing tools developed in this paper, the extraction of porosity information from raw quality inspection data, such as cross-section images and tomography data sets, can be automated. The CNN models with a compact architecture, part of whose hyperparameters were selected through cross-validation analysis, achieved a classification accuracy of 91.2% for porosity occurrence detection in the direct laser deposition of sponge Titanium powders and presented predictive capacity for micro pores below 100 μm. For local volume porosity prediction, the model also achieved a root mean square error of 1.32% and exhibited high fidelity for both high porosity and low porosity specimens.

Abstract: Residual stresses and distortion in Additive Manufactured (AM) parts are two key obstacles which seriously hinder the wide application of this technology. Nowadays, understanding the thermomechanical behavior induced by the AM process is still a complex task which must take into account the effects of both the process and the material parameters, the microstructure evolution as well as the pre-heating strategy. One of the challenges of this work is to increase the complexity of the geometries used to study the thermomechanical behavior induced by the AM process. The reference geometries are a rectangular and a S-shaped structures made of 44-layers each. The samples have been fabricated by Directed Energy Deposition (DED). In-situ thermal and distortion histories of the substrate are measured in order to calibrate the 3D coupled thermo-mechanical model. Once the numerical results showed a good agreement with the temperature measurements, the validated model has been used to predict the residual stresses and distortions. Different process parameters have been analyzed to study their sensitivity to the process assessment. Different preheating strategies have been also analyzed to check their effectiveness on the mitigation of both distortions and residual stresses. Finally, some simplifications of the actual scanning sequence are proposed to reduce the computational cost without loss of the accuracy of the simulation framework.

Abstract: By using filaments comprising metal or ceramic powders and polymer binders, solid metal and ceramic parts can be created by combining low-cost fused filament fabrication (FFF) with debinding and sintering. In this work, we explored a fabrication route using a FFF filament filled with 316 L steel powder at 55 vol.-%. We investigated the printing, debinding and sintering parameters and optimized them with respect to the mechanical properties of the final part. Special focus was placed on debinding and sintering in order to obtain components of low residual porosity. Solvent debinding of the printed green bodies created an internal network of interconnected pores and was followed by thermal debinding. Thermal debinding allowed for complete removal of the remaining binder and produced mechanically stable brown parts. Sintering at 1360 °C provided densification of the parts and generated nearly isotropic linear shrinkage of about 20%. Using optimized parameters, it was possible to fabricate 316 L steel components with a density greater than 95% via the material extrusion additive manufacturing, debinding and sintering route, with achievable deflections in a 3-point bending test similar to rolled sheet material, albeit at lower strength.

Abstract: Polyolefin thermoplastics like high density polyethylene (HDPE) are the leaders in terms of world-scale plastics’ production, environmentally benign polymerization processes, recycling, and sustainability. However, additive manufacturing of HDPE by means of fused deposition modeling (FDM) also known as fused filament fabrication (FFF) has been problematic owing to its massive shrinkage, voiding and warpage problems accompanied by its poor adhesion to common build plates and to extruded HDPE strands. Herein we overcome these problems and improve Young’s modulus, tensile strength and surface quality of 3D printed HDPE by varying 3D printing parameters like temperature and diameter of the nozzle, extrusion rate, build plate temperature, and build plate material. Both nozzle diameter and printing speed affect surface quality but do not impair mechanical properties. Particularly, an extrusion rate gradient prevents void formation. For the first time additive manufactured HDPE and injection-molded HDPE exhibit similar mechanical properties with exception of elongation at break. Excellent fusion of the extruded polymer strands and the absence of anisotropy are achieved, as verified by microscopic imaging and measuring the tensile strength parallel and perpendicular to the 3D printing direction.

Abstract: In this paper, we printed Ti-6Al-4V SLM parts based on Taguchi design of experiment and related standards to measure and compare hardness with different mechanical properties that were obtained in our previous research such as density, strength, elongation, and average surface. Then the effect of process parameters comprising laser power, scan speed, hatch space, laser pattern angle coupling, along with heat treatment as a post-process, in relation to hardness was analysed. The relation of measured factors with each other was also studied and related mechanisms were discussed in depth. The original contribution in this paper is in producing a large and precise dataset and the comparison with mechanical properties. Another contribution is related to the analysis of process parameters in relation to hardness and explaining them by rheological phenomena. The results showed an interesting similarity between hardness and density which is highly related to the formation of the melting pool and porosities within the process.

Abstract: Residual distortion is a major technical challenge for laser powder bed fusion (LPBF) additive manufacturing (AM), since excessive distortion can cause build failure, cracks and loss in structural integrity. However, residual distortion can hardly be avoided due to the rapid heating and cooling inherent in this AM process. Thus, fast and accurate distortion prediction is an effective way to ensure manufacturability and build quality. This paper proposes a multiscale process modeling framework for efficiently and accurately simulating residual distortion and stress at the part-scale for the direct metal laser sintering (DMLS) process. In this framework, inherent strains are extracted from detailed process simulation of micro-scale model based on the recently proposed modified inherent strain model. The micro-scale detailed process simulation employs the actual parameters of the DMLS process such as laser power, velocity, and scanning path. Uniform but anisotropic strains are then applied to the part in a layer-by-layer fashion in a quasi-static equilibrium finite element analysis, in order to predict residual distortion/stress for the entire AM build. Using this approach, the total computational time can be significantly reduced from potentially days or weeks to a few hours for part-scale prediction. Effectiveness of this proposed framework is demonstrated by simulating a double cantilever beam and a canonical part with varying wall thicknesses and comparing with experimental measurements which show very good agreement.

Abstract: The columnar to equiaxed transition (CET) of grain structures associated with processing conditions has been observed during metallic additive manufacturing (AM). However, the formation mechanisms of these grain structures have not been well understood under rapid solidification conditions, especially for AM of superalloys. This paper aims to uncover the underlying mechanisms that govern the CET of AM metals, using a well-tested multiscale phase-field model where heterogeneous nucleation, grain selection and grain epitaxial growth are considered. Using In718 as an example, the simulated results show that the CET is critically controlled by the undercooling, involving constitutional supercooling, thermal and curvature undercoolings in the melt pool, which dictates the extent of heterogeneous nucleation with respect to the grain epitaxial growth during rapid solidification.

Abstract: Consumer-grade plastics can be considered a low-cost and sustainable feedstock for fused filament fabrication (FFF) additive manufacturing processes. Such materials are excellent candidates for distributed manufacturing, in which parts are printed from local materials at the point of need. Most plastic waste streams contain a mixture of polymers, such as water bottles and caps comprised of polyethylene terephthalate (PET) and polypropylene (PP), and complete separation is rarely implemented. In this work, blends of waste PET, PP and polystyrene (PS) were processed into filaments for 3D printing. The effect of blend composition and styrene ethylene butylene styrene (SEBS) compatibilizer on the resulting mechanical and thermal properties were probed. Recycled PET had the highest tensile strength at 35 ± 8 MPa. Blends of PP/PET compatibilized with SEBS and maleic anhydride functionalized SEBS had tensile strengths of 23 ± 1 MPa and 24 ± 1 MPa, respectively. The non-compatibilized PP/PS blend had a tensile strength of 22 ± 1 MPa. PP/PS blends exhibited reduced tensile strength to ca. 19 ± 1–3 MPa with the addition of SEBS. Elongation to failure was generally improved for the blended materials compared to neat recycled PET and PS. The glass transition was shifted to higher temperatures for all of the blends except the 50–50 wt. % PP/PET blend. Crystallinity was decreased for the blends, but was highest in the 75–25 wt. % PP/PS and the 50-50 wt. % PP/PET blend with SEBS-maleic anhydride. Solvent extraction of the dispersed phase revealed polypropylene was the matrix phase in both the 50–50 wt. % PP/PET and PP/PS blends.

Abstract: This paper reports on X-ray tomography of a series of coupon samples (5 mm cubes) produced under different process parameters, for laser powder bed fusion of Ti6Al4V. Different process parameters result in different pore formation mechanisms, each with characteristic pore sizes, shapes and locations within the 5 mm cube samples. While keyhole pores, lack of fusion pores and metallurgical pores have been previously identified and illustrated using X-ray tomography, this work extends beyond prior work to show how each of these not only exist in extreme situations but how they vary in size and shape in the transition regimes. It is shown how keyhole mode porosity increases gradually with increasing power, and how this depends on the scan speed. Similarly, lack of fusion pores are shown to occur following scan tracks in situations of poor hatch overlap, or a similar but different distribution of lack of fusion porosity due to large layer height spacing, showing respectively vertical and horizontal lack of fusion pore morphologies. Increased spacing between hatch scan tracks and contour scan tracks is demonstrated to form a near-surface porosity similar to that previously reported for slowing at the end of scan tracks which can cause keyhole mode porosity. Insights from 3D images allow improvements in parameter choices for optimized density of parts produced by laser powder bed fusion, and generally allow a better understanding of the porosity present in additively manufactured parts.

Abstract: Laser powder bed fusion (LPBF) is a proven additive manufacturing (AM) technology for producing metallic components with complex shapes using layer-by-layer manufacture principle. However, the fabrication of crack-free high-performance Ni-based superalloys such as Hastelloy X (HX) using LPBF technology remains a challenge because of these materials’ susceptibility to hot cracking. This paper addresses the above problem by proposing a novel method of introducing 1 wt.% titanium carbide (TiC) nanoparticles. The findings reveal that the addition of TiC nanoparticles results in the elimination of microcracks in the LPBF-fabricated enhanced HX samples; i.e. the 0.65% microcracks that were formed in the as-fabricated original HX were eliminated in the as-fabricated enhanced HX, despite the 0.14% residual pores formed. It also contributes to a 21.8% increase in low-angle grain boundaries (LAGBs) and a 98 MPa increase in yield strength. The study revealed that segregated carbides were unable to trigger hot cracking without sufficient thermal residual stresses; the significantly increased subgrains and low-angle grain boundaries played a key role in the hot cracking elimination. These findings offer a new perspective on the elimination of hot cracking of nickel-based superalloys and other industrially relevant crack-susceptible alloys. The findings also have significant implications for the design of new alloys, particularly for high-temperature industrial applications.

Abstract: The effects of layer orientation and surface roughness on the mechanical properties and fatigue life of 316L stainless steel (SS) fabricated via a laser beam powder bed fusion (LB-PBF) additive manufacturing process were investigated. Quasi-static tensile and uniaxial fatigue tests were conducted on LB-PBF 316L SS specimens fabricated in vertical and diagonal directions in their as-built surface condition, as well as in horizontal, vertical, and diagonal directions where the surface had been machined to remove any effects of surface roughness. In the machined condition, horizontally built LB-PBF specimens possessed higher fatigue resistance, followed by vertically built specimens, while the lowest fatigue resistance was obtained for diagonal specimens. Similarly, in the as-built condition, vertical specimens demonstrated better fatigue resistance when compared to diagonal specimens. Furthermore, the detrimental effects of surface roughness on fatigue life of LB-PBF 316L SS specimens was not significant, which may be due to the presence of large internal defects in the specimens. Anisotropy of LB-PBF 316L SS specimens was attributed to the variation in layer orientation, affecting defects’ directionality with respect to the loading direction. These defect characteristics can significantly influence the stress concentration and, consequently, fatigue behavior of additive manufactured parts. Therefore, the elastic-plastic energy release rates, a fracture mechanics-based concept that incorporates size, location, and projected area of defects on the loading plane, were determined to correlate the fatigue data and acceptable results were achieved.

Abstract: Size and shape of a melt pool play a critical role in determining the microstructure in additively manufactured metals. However, it is very challenging to directly characterize the size and shape of the melt pool beneath the surface of the melt pool during the additive manufacturing process. Here, we report the direct observation and quantification of melt pool variation during the laser powder bed fusion (LPBF) additive manufacturing process under constant input energy density by in-situ high-speed high-energy x-ray imaging. We show that the melt pool can undergo different melting regimes and both the melt pool dimension and melt pool volume can have orders-of-magnitude change under a constant input energy density. Our analysis shows that the significant melt pool variation cannot be solely explained by the energy dissipation rate. We found that energy absorption changes significantly under a constant input energy density, which is another important cause of melt pool variation. Our further analysis reveals that the significant change in energy absorption originates from the separate roles of laser power and scan speed in depression zone development. The results reported here are important for understanding the laser powder bed fusion additive manufacturing process and guiding the development of better metrics for processing parameter design.

Abstract: Risk-averse areas such as the medical, aerospace and energy sectors have been somewhat slow towards accepting and applying Additive Manufacturing (AM) in many of their value chains. This is partly because there are still significant uncertainties concerning the quality of AM builds. This paper introduces a machine learning algorithm for the automatic detection of faults in AM products. The approach is semi-supervised in that, during training, it is able to use data from both builds where the resulting components were certified and builds where the quality of the resulting components is unknown. This makes the approach cost efficient, particularly in scenarios where part certification is costly and time consuming. The study specifically analyses Laser Powder-Bed Fusion (L-PBF) builds. Key features are extracted from large sets of photodiode data, obtained during the building of 49 tensile test bars. Ultimate tensile strength (UTS) tests were then used to categorise each bar as ‘faulty’ or ‘acceptable’. Using a variety of approaches (Receiver Operating Characteristic (ROC) curves and 2-fold cross-validation), it is shown that, despite utilising a fraction of the available certification data, the semi-supervised approach can achieve results comparable to a benchmark case where all data points are labelled. The results show that semi-supervised learning is a promising approach for the automatic certification of AM builds that can be implemented at a fraction of the cost currently required.

Abstract: This study investigates the feasibility of achieving high deposition rate using wire + arc additive manufacturing in stainless steel to reduce lead time and cost of manufacturing. The pulse MIG welding technique with a tandem torch was used for depositing martensitic stainless steel 17-4 pH. The mechanical and metallurgical properties of the manufactured component were analysed to evaluate the limitations and the extent to which the rate of deposition reaches a maximum without any failure or defect being evident in the manufactured component. Deposition rate of 9.5 kg/h was achieved. The hardness was matched for the as deposited condition.

Abstract: The build-up of residual stresses in a part during laser powder bed fusion provides a significant limitation to the adoption of this process. These residuals stresses may cause a part to fail during a build or fall outside the specified tolerances after fabrication. In the present work a thermomechanical model is used to simulate the build process and calculate the residual stress state for Ti–6Al–4V specimens built with continuous and island scan strategies. A layer agglomeration, or lumping, approach is used to speed up the computations. A material model is developed to naturally capture the strain-rate dependence and annealing behavior of Ti–6Al–4V at elevated temperatures. Results from the thermomechanical simulations showed good agreement with synchrotron X-ray diffraction measurements used to determine the residual elastic strains in these parts. However, the experimental measurements showed higher residual strains for the specimen built with an island scan strategy; a trend not fully captured by the simulations. Parameter studies were performed to fully understand the advantages and limitations of the current simulation methodology. Reasons for both the computational and experimental findings are discussed.

Abstract: Additive manufacturing of soft magnetic materials and components based on laser powder bed fusion (L-PBF) offers new opportunities for soft magnetic core materials in efficient energy converters. For more favorable material compositions like FeSi6.7 (strategy 1) with larger electrical resistivity and close-to-zero magnetostriction a maximum permeability of μmax = 31,000, minimum coercivity of Hc = 16 A/m and hysteresis losses of 0.7 W/kg at 1 T and 50 Hz have been realized. To further reduce eddy current losses significantly, novel topological structures like inner slits (strategy 2) and multilayered structures of alternating layers of electrically insulating material and soft magnetic material (strategy 3) are suggested. Feasibility, functionality and potential of the different strategies (and combinations thereof) are discussed based on first prototypes and supporting simulations. The results are compared to conventional electrical steel and SMC (soft magnetic composites).

Abstract: Mechanisms underlying the evolution of texture and microstructure during selective laser melting (SLM) and their combined effects on the mechanical response of 316L stainless steel are presented. Long columnar grains with a fiber texture || build direction (BD) evolved in the SLM printed material. Fiber texture was stronger in the horizontal build compared to the vertical build. Use of bidirectional scanning strategy enforced epitaxial growth of grains across melt pools present within a single printed layer. || BD texture evolved as a consequence of maintaining the balance between epitaxy and growth of [100] along maximum thermal gradient. High dislocation density and not grain size effect of the ultra-fine cellular structure, imparted high strength to 316L. Lower average Schmid factor and smaller effective grain size in the horizontal build by virtues of crystallographic and morphological textures, respectively, imparted higher yield strength than the vertical build. The horizontal build demonstrated higher strain hardening rate in the early stages of deformation compared to the vertical build due to higher crystallographic texture dependent twinning. However, the higher rate of dislocation annihilation led to a continuous decline in the strain hardening rate of the horizontal build. In contrast, a stable strain hardening rate was maintained in the vertical build, which led to higher ductility than the horizontal build. In summary, the roles of non-equilibrium microstructure and texture (crystallographic and morphological) in regulating mechanical properties elucidated here, can be utilized in designing additively manufactured structural components of 316L stainless steel.