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Attaining high relative density (RD) is of paramount importance for any new alloy system manufactured through the laser powder bed fusion (L-PBF) process. However, the conventional design of experiment (DOE) methods poses a significant challenge due to the large number of process parameters involved. This study explores the data-driven machine-learning (ML) approach to confine the search for the optimized process parameters to the most significant process parameters. The relevant datasets were obtained from existing literature spanning across the last decade on 11 different alloy systems. The collected datasets were divided into 80:20 for training and testing. In this work, a detailed framework is presented to identify the most appropriate ML model to represent the complexities and nonlinearities in the data accurately. Among the employed models, the gradient boosting–particle swarm optimization (GB-PSO) exhibited the highest predictive performance, with mean absolute error (MAE) and coefficient of determination (R2) values of 0.20 and 0.99 for training and 0.73 and 0.95 for testing, respectively. The Shapley additive explanations (SHAP) analysis was utilized to comprehend the global and local significance of material properties and machine process parameters. The reduced experimental design from the data-driven ML framework is used to validate the predictions from the trained hybrid GB-PSO model. Validation for achieving the highest RD is carried out on the Inconel 718 alloy system deposited in-house. [ABSTRACT FROM AUTHOR]

Copper laser powder bed fusion (Cu-LPBF) is known as a challenging manufacturing process. Since copper is an excellent thermal and electrical conductor, low to medium laser power is insufficient to provide good wettability and fusion between either adjacent tracks or deposited layers due to heat dissipation and high reflectivity. Particle surface modification has therefore been proposed to improve optical absorptivity. A simple method to enhance optical absorptivity is to create a thin oxide film by heating copper powder in an air furnace. The first phase of the current work emphasizes that this technique is detrimental to the part quality if a medium laser power range is used. The second phase presents a novel technique of two stages of in-situ copper oxide reduction during LPBF. It provides an innovative approach for combining laser cleaning and melting in LPBF. The newly suggested technique uses recycled and surface oxidized Cu powders to develop a simple and chemical-free method to recover the contaminated powder. A significant improvement is achieved by applying laser surface cleaning to the powder and solidified layers. The oxygen content is reduced by 70% in the LPBF samples compared to the initial state of the oxidized powder. [ABSTRACT FROM AUTHOR]

Laser powder bed fusion (LPBF) is an additive manufacturing process which can produce complex 3D parts from digital models. The performance of parts fabricated by LPBF is largely dependent on the characteristics of the powder feedstock, in particular, the particle size distribution. The coarsening of powder particles may limit the potential for reusing powder in further builds, as consistency in powder quality is crucial for ensuring consistent parts performance when using reused powder, especially in aerospace applications. However, there is a lack of systematic understanding regarding the causes and nature of powder coarsening in LPBF. In this work, the characteristics of powder samples from different locations in the build chamber during LPBF were studied to understand the particle size evolution and determine the origin of coarsening, which has not been previously reported. Meanwhile, powder coarsening was found to have a detrimental effect on the relative density and surface quality of as-built parts, highlighting the importance of exploring the mechanisms of powder coarsening and finding ways to control it in LPBF. The relationship between powder in key locations in the build chamber and its effect on powder coarsening has been established. Layer thickness is identified as a critical factor in causing powder coarsening due to the fine powder size characteristic in the powder bed. Spatter, in its various forms, plays a direct or indirect role in powder coarsening. Sintered powders resulting from spatter and the laser scanning borders of as-built parts were observed to contribute to the powder coarsening. Therefore, three main mechanisms (layer thickness, spatter, sintered powder) associated with the powder coarsening are therefore proposed. This work provides insight and potential solutions to control powder coarsening and maintain consistent parts performance. [ABSTRACT FROM AUTHOR]

The formation mechanism of pore defects and surface ripples takes a crucial position in quality control during the laser powder bed fusion process. This paper explores the evolution of the melt pool in the LPBF process and elucidates the impact of flow state and temperature field distribution on the melt pool with different process parameters. Through computational fluid dynamics methodology, the study clarifies the formation mechanism of pore defects and surface ripples. The discrete element method was applied to simulate the powder bed using a set of processing parameters. The free surface of the melt pool was then calculated employing the volume of fluid method. It was found that surface ripples on the melt pool and irregular pore defects in the lap joints appeared due to the lack of fusion at a laser energy density of 32 J/mm3 under the pre-set conditions. As the laser scanning speed dropped to 600 mm/s, keyholes were generated inside the melt pool due to the recoil pressure. Similar phenomena of pore defects and surface ripples were observed in the experiments. The measured and simulated average widths of the molten track showed good agreement, with relative errors of less than 6%, confirming the accuracy of the simulated formation. Our work sheds new light on the quality control of LPBF fabricated parts for the future. [ABSTRACT FROM AUTHOR]

The promising routes of metallic additive manufacturing favor the fabrication of refined functional components. Nonetheless, dealing with the Laser Powder Bed Fusion (L-PBF) parameters and costs for making the most of this technology is still an issue. The challenge gets bigger when considering less explored parameters, such as the process atmosphere and build location. So, this study aimed to address the main differences in microstructure, crystallography, mechanical properties, and surface finishing of maraging steel components manufactured under two different atmospheres (N2 and a mixture of 75%Ar-25%He atmospheres) and three build locations arranged along the gas flow direction. Based on experimental characterizations, the porosity fraction, microstructural constituents and metallurgical flaws, phase composition and martensite matrix crystallographic parameters, macrotexture, mechanical properties, roughness, and surface residual stress were investigated and compared for the different manufacturing conditions. The porosity fraction had the most expressive changes with the gas type and the build location, reaching a 53% overall reduction under N2 and a 112 to 302% rise for the front located sample compared to the others. The underlying mechanisms involved in the observed results, mainly related to process by-products behavior, were discussed. Broadly, the N2 atmosphere stands out over the analyzed properties. These findings underscore the role of atmospheric selection in the LPBF process, considering the overall quality and performance of the fabricated material and the possibility of cost reduction. [ABSTRACT FROM AUTHOR]

Laser powder bed fusion (LPBF) is a rapid prototyping technology with high forming accuracy, which can print complex metallic components applied in the aerospace field. The aluminum alloy parts manufactured by LPBF can better achieve lightweight structures than other metallic materials due to the combination of materials and structure. However, many metallurgical defects such as spherification, porosity, coarse grain, and crack are more likely formed inside the printed components during the LPBF process, resulting in the decline of properties of the aluminum alloy. In order to solve the above-mentioned problems, researchers have conducted in-depth exploration to improve the mechanical properties of LPBF fabricated Al parts. In this paper, the methods to improve the forming quality of aluminum alloy are explored from the direction of regulation strategies for microstructure and properties. The current research status of LPBF-processed aluminum alloy is discussed from the aspects of process parameter optimization, alloying element modification, reinforcing phase addition, and post-treatment. Furthermore, the microstructure and properties of LPBF-processed aluminum alloy are systematically reviewed. [ABSTRACT FROM AUTHOR]

Alloy 400 is a corrosion-resistant, NiCu-based material which is used in numerous industrial applications, especially in marine environments and the high-temperature chemical industry. As conventional manufacturing limits geometrical complexity, additive manufacturing (AM) of the present alloy system promises great potential. For this purpose, a robust process chain, consisting of powder production via gas atomization and a design of experiment (DoE) approach for laser powder bed fusion (LPBF), was developed. With a narrow particle size distribution, powders were found to be spherical, flowable, consistent in chemical composition, and, hence, generally applicable to the LPBF process. Copper segregations at grain boundaries were clearly detected in powders. For printed parts instead, low-intensity micro-segregations at cell walls were discovered, being correlated with the iterative thermal stress applied to solidified melt-pool-near grains during layer-by-layer manufacturing. For the production of nearly defect-free LPBF structures, DoE suggested a single optimum parameter set instead of a broad energy density range. The latter key figure was found to be misleading in terms of part densities, making it an outdated tool in modern, software-based process parameter optimization. On the microscale, printed parts showed an orientation of melt pools along the build direction with a slight crystallographic [101] texture. Micro-dendritic structures were detected on the nanoscale being intersected by a high number of dislocations. Checked against hot-extruded reference material, the LPBF variant performed better in terms of strength while lacking in ductility, being attributed to a finer grain structure and residual porosity, respectively. [ABSTRACT FROM AUTHOR]

The present research aims to improve the surface quality of laser powder bed fusion (LPBF) built cobalt chrome alloy (CoCr) by pulse laser remelting. The laser power, overlap percentage, and spot diameter are varied to determine the best parameters for laser polishing. The microhardness and microstructural changes associated with pulse laser remelting are described. Surface morphology and surface roughness parameters are reported for various laser remelting parameters. Among the three different spot diameters, 1, 2, and 3 mm, the maximum (> 50%) reduction in surface roughness was obtained for 2 mm with 1J laser energy and 50% X and 20% Y as well as 20% X and 50% Y overlaps. A DoE- and RSM-based approach has been utilized for parametric optimization of the surface roughness. The squared laser power is identified as a critical parameter by ANOVA. A Pareto chart confirms the significance of squared laser power across spot diameters. Optimization suggests that a 2-mm spot diameter works best with specific parameters to produce a better surface finish. However, the nanosecond order laser pulses have a limitation on the depth of surface modification without severe ablation and thereby on the reduction in the surface roughness. [ABSTRACT FROM AUTHOR]

Additive manufacturing (AM) technologies have demonstrated a promising material efficiency potential in comparison to traditional material removal processes. A new directed energy deposition (DED) category AM process called wire arc additive manufacturing (WAAM) is evolving due to its benefits which include faster build rates, capacity to build large volumes, and inexpensive feedstock materials and machine tools compared to more technologically mature powder-based AM technologies. However, WAAM products present challenges like poor surface finish and lower dimensional accuracy compared to powder-based processes or machined parts, prevalence of thermal distortions, residual stresses, and defects like porosity, cracks, and humping, often requiring post-processing operations like finish machining and heat treatment. These post-processing operations add to the production cost and environmental footprint of WAAM-built parts. Therefore, considering the opportunities and challenges presented by WAAM, this paper analyses the environmental impact, production costs, and mechanical properties of WAAM parts and compares them with those achieved by laser powder bed fusion (LPBF) and traditional computer numerical control (CNC) milling. A high-strength low-alloy steel (ER70S) mechanical part with medium complexity was fabricated using WAAM. Based on the data collected during this experiment, environmental impact and cost models were built using life cycle assessment and life cycle costing methodologies. WAAM was observed to be the most environmentally friendly option due to its superior material efficacy than CNC milling and has a better energy efficiency than LPBF. Also, WAAM was the most cost-friendly option when adopted in batch production for batch sizes above 3. The environmental and cost potential of WAAM is amplified when used for manufacturing large products, resulting in significant material, emission, and cost savings. The fabricated WAAM part demonstrated good mechanical properties comparable to that of cast/forged material. The methodology and experimental data presented in this study can be used to calculate environmental impacts and costs for other products and can be helpful to manufacturers in selecting the most ecofriendly and cost-efficient manufacturing process. [ABSTRACT FROM AUTHOR]

Laser powder bed fusion (L-PBF) fuses metallic powder using a high-energy laser beam, forming parts layer by layer. This technique offers flexibility and design freedom in metal additive manufacturing (MAM). However, achieving the desired surface quality remains challenging and impacts functionality and reliability. L-PBF process parameters significantly influence surface roughness. Identifying the most critical factors among numerous parameters is essential for improving quality. This study examines the effects of key process parameters on the surface roughness of AlSi10Mg, a widely used aluminum alloy in high-tech industries, fabricated by L-PBF. Part orientation, laser power, scanning speed, and layer thickness were identified as crucial parameters via cause-and-effect analysis. To systematically examine their effects, the Taguchi method was employed within the framework of the design of experiment (DoE). Experimental results and statistical analysis revealed that laser power, scanning speed, and layer thickness significantly influence surface roughness parameters: arithmetic mean (Ra) and root mean square (Rq). Main effect plots and energy density analyses confirmed their impact on surface quality. Microscopic investigations identified surface flaws such as spattering, balling, and porosity contributing to poor quality. Given the complex interplay between parameters and surface quality, accurately predicting their effects is challenging. To address this, machine learning models, specifically random forest regression (RFR) and support vector regression (SVR), were used to predict the effects on surface roughness. The RFR model's R2 values for predicting Ra and Rq are 97% and 85%, while the SVR model's predictions are 85% and 66%, respectively. Evaluation metrics demonstrated that the RFR model outperformed SVR in predicting surface roughness. [ABSTRACT FROM AUTHOR]

Laser powder bed fusion (LPBF) is a metal additive manufacturing (AM) process where a part is created layer-by-layer through the melting and solidification of powder layers. The main challenge in LPBF is quality assurance. An estimation of powder rheology/flowability, as a quality-impacting factor, is therefore necessary prior to printing. To predict powder flowability based on morphological features without cost- and time-inefficient experiments, this study explores using machine learning algorithms (ML). It focuses on a sample set of standard and off-size Ti6Al4V (Ti64) powders, aiming to (1) assess ML regressors and equation fitting for flowability prediction, using the small original dataset and (2) overcome data limitations' impact on regression through data augmentation techniques. Results showed conventional ML regressors like support vector regression (SVR) sufficed for dynamic flowability prediction. Still, more complex targets such as shear flowability and hall flowability required innovative approaches like genetic programming (equation fitting) and synthetic minority over-sampling (SMOTE), respectively, as proposed in this work. [ABSTRACT FROM AUTHOR]

This paper presents a study on the thermal and electrical performance of lattice structures designed for a heat exchanger, where the geometry is optimized for enhanced heat management through a numerical simulation scheme based on the response surface method. The goal was to achieve effective heat conduction and convection. Lattice structures were selected for their inherent porosity, enabling efficient heating and cooling cycles. The optimized lattice geometry was fabricated using Laser powder bed fusion (L-PBF) additive manufacturing, with a "green" laser emission wavelength (λ = 515 nm) to improve energy absorption and final product density. Experimental evaluations assessed the thermal and electrical performance of the L-PBF-produced samples, including both workpieces and lattice structures designed for additive manufacturing. Microscopic and microstructural studies were conducted to identify potential manufacturing defects, such as porosities and cracks, resulting from the L-PBF process. Despite some defects persisting in certain lattice structures, the results showed significant improvements in both thermal and electrical properties compared to those obtained using the traditional infrared laser L-PBF process. An enhanced 3D-printed heat exchanger design is proposed, emphasizing lattice-like internal structures. This advancement offers superior control, increased durability, and improved safety for hydrogen storage devices. This research represents a significant contribution to the field of heat exchanger design and additive manufacturing, especially in the context of hydrogen storage technologies. It demonstrates the potential for lattice structures and innovative manufacturing processes to revolutionize the efficiency and safety of thermal systems. [ABSTRACT FROM AUTHOR]

The selective orientation of grains during laser powder bed fusion (L-PBF) leads to anisotropy in mechanical properties. The current understanding of the evolution between the different morphologies of the grains is still limited. The present work is aimed at revealing the grain evolution process through the development of a coupled discrete elemental method-computational fluid dynamics-metacellular automata model. The results show that most of the grains observed through characterization experiments are irregularly shaped columnar grains and exhibit anisotropy in mechanical property tests. The grains observed in the simulations can be generalized into four types: V-shaped grain, slanted grain, vertical grain, and equiaxial grain. During the overlapping of interlayer and intertrack, some of the slanted grains are retained and others grow into V-shaped grains. In addition, some of the V-shaped grains coarsen during remelting and resolidification and subsume the slanted grains in the direction in which they grow, eventually developing into coarse vertical grain. Therefore, an appropriate increase in hatch distance is beneficial to reduce anisotropy. This study contributes to an in-depth understanding of grain transformation during remelting and resolidification and guides the design of specific microstructures. [ABSTRACT FROM AUTHOR]

The article presents a study of the tribocorrosion phenomenon and its effects on Inconel 718 alloy produced conventionally by extrusion and additively manufactured using the laser powder bed fusion method. In addition, the samples were subjected to a heat treatment process to change their properties. The research was carried out using the pin-on-disk method in 3.5% NaCl. Based on the study, it was found that the material made with additive technology is more resistant to tribocorrosion phenomenon, and the difference from conventionally made material is about 50%. The synergistic effect between friction and corrosion (ΔZ) occurred. However, heat treatment in the AA-2 variant ensures higher hardness and reduces purely mechanical wear (ZM) and the synergy effect (ΔZ). [ABSTRACT FROM AUTHOR]

The high tensile residual stresses in the laser powder bed fusion (LPBF) manufactured H13 steel lead to severe cracks and low mechanical properties. To solve this problem, the laser shock peening (LSP) was applied to H13 steel manufactured by LPBF to eliminate tensile residual stress and improve mechanical property. The evolution mechanism of defects and residual stress during LPBF-LSP process were accounted for using the finite element method. The numerical results indicate that the compressive residual stress induced by LSP shock was beneficial for pores and cracks closure. A single shock cannot completely eliminated the tensile residual stress generated in LPBF process, and the mechanism of tensile residual stresses elimination by shock wave and rarefaction wave was proposed. The LSP process parameters of LPBF manufactured h13 steel were optimized by single point impact tests, and the effects of residual stress induced by LSP on defect, microstructure, and nano-hardness were experimentally studied. The experimental result shows that the defect closure zone depth increases with the number of laser impacts. The nano-hardness near the surface was increased by 31% than the LPBF sample after double LSP treatment. The strengthening mechanism of LSP-H13 was clarified as dislocation strengthening. This work verifies the feasibility of tensile residual stress elimination and mechanical properties improvement of the LPBF manufactured H13 steel by LSP treatment. [ABSTRACT FROM AUTHOR]

Soft magnetic composite (SMC) products have been widely used in electromagnetic applications due to their unique properties, including extremely low eddy current loss and relatively low total core loss at medium and high frequencies. However, there were few studies that investigated the contradictory of mechanical and magnetic properties to process FeSiCr SMC powders. To ensure that the parts fabricated by selective laser melting (SLM) satisfy the application requirements and to assist manufacturing engineers in choosing optimal process parameters, the relationship of four key process parameters (oxygen concentration, laser power, scanning speed, and hatch distance) and three fabricated part properties (permeability, core loss, and ultimate strength) was established by the machine learning models. Three experiments based on the L9 orthogonal array design were conducted to optimize the material properties and to collect data for modeling. Five machine learning algorithms were investigated, and the better ones were adopted to estimate the part properties with the input SLM parameters, which were outperformed the inference based on the regression method. A manufacturing parameter suggestion system was developed with the selected machine learning models and a multi-objective optimal algorithm, NSGA-II, to assist engineers in determining the optimal process parameters regarding the conflictive mechanical and magnetic property requirements. [ABSTRACT FROM AUTHOR]

In this research, we investigate the dynamic behavior of Inconel 718 fabricated through laser powder bed fusion (L-PBF), addressing a notable knowledge gap regarding the correlation between process parameters and dynamic properties. The process parameters adopted are deducted from an extension of the Rosenthal solution, formulated to increase the process productivity while avoiding the typical production process defects. The dynamic Young modulus and the structural damping of the material are estimated as a function of the process parameters through ping tests reproducing the flexural vibrations of the specimens in as-built, solutioned, and aged conditions. The microstructure and porosity are investigated through metallographic analyses. The results show a substantial influence of the L-PBF process parameters on the dynamic Young modulus, which markedly increases as the energy density is reduced (23%) and progressively becomes more similar to the conventionally produced material. This influence stands in stark contrast to the relatively modest impact of heat treatments, which underlines a negligible effect of the process-induced residual stress. The structural damping remained approximately constant across all test conditions. The elastic response of the material is found to be primarily influenced by the different microstructures produced as the L-PBF process parameters varied, particularly in terms of the dimensions and shape of the solidification structures. The unexpected relationship between the dynamic Young modulus, energy density, and microstructure unveils the potential to fine-tune the material's dynamic behavior by manipulating the process parameters, thereby carrying substantial implications for all the applications of additively manufactured components susceptible to significant vibratory phenomena. [ABSTRACT FROM AUTHOR]

Laser-based powder bed fusion of metals (PBF-LB/M) is an additive manufacturing process enabling the fabrication of parts with highly complex and customizable geometries, enhanced strength-to-weight properties, and minimized material waste. Despite its unique capabilities, PBF-LB/M needs research and innovation efforts to enhance process dynamics and product quality, as well as to broaden its adoption in high-value industrial sectors, such as aerospace and biomedical. In this context, in-process monitoring solutions and post-process part quality evaluations are fundamental to improving the process towards sustainable, first-time-right, and zero-defect production. This paper describes a novel building plate concept for metal laser powder fusion, whose characteristics were specifically designed to enable and improve the performances of in-process monitoring and high-resolution X-ray computed tomography (CT) measurements. In particular, the plate features markers for perspective correction in off-axis optical monitoring and dismountable inserts with machined geometrical elements to be used for the precise alignment between high-resolution CT reconstructions, in-process gathered data, and building volume geometry. The plate capabilities were demonstrated through examples related to in-process monitoring and post-process X-ray CT measurements. [ABSTRACT FROM AUTHOR]

Additive manufacturing (AM) has the potential for improving the sustainability of metal processing through decreased energy and materials usage compared to casting and forging. Laser powder bed fusion (LPBF) of high-temperature alloys such as nickel alloy 718 is one of the key modalities supporting this effort. One of the major drawbacks to LPBF is its slow build speed on the order of 5–10 cubic centimeters per hour print speed. This experimental study investigates how to increase the productivity of the LPBF process by switching from a traditional Gaussian laser shape to a ring laser shape using a nLight multi-modal laser. The objective is to increase productivity, reducing energy consumption and time, without sacrificing mechanical properties by switching to the ring laser thereby improving the sustainability of LPBF. Results include measuring the energy consumption of an Open Additive LPBF system during 718 printing and comparing the microstructure and mechanical properties of the two different lasers. [ABSTRACT FROM AUTHOR]

In the laser powder bed fusion (LPBF) process, fast solidification of molten powders and significant temperature gradients generate substantial residual stresses in the build part. The induced residual stresses can result in excessive deformation and potential crack initiation and propagation. Development of a predictive reliable model is thus of paramount importance to predict the induced residual stresses in LPBF process without cost and complexities associated with experimental tests. High-fidelity thermo-mechanical models to predict the residual stresses and distortion in the macro-scale are generally impractical owing to their high level of computational time. Recently, the modified inherent strain methodology has proven to be a viable alternative for predicting residual stresses and distortions efficiently. In this study, a multi-scale finite element-based model of the LPBF process for Inconel-718 is developed utilizing modified inherent strain method to estimate the accumulated stress in the process. The stress formation and inherent strain values were evaluated in a micro-scale deposition considering the effect of LPBF process parameters using Abaqus/standard. The material constitutive models, including kinematic hardening, elastic-perfectly plastic, isotropic hardening, and Johnson–Cook model, and their effect on the evolution of plastic strain were investigated. The precision of the prediction was examined based on the quantity of equivalent layers, employing the agglomeration approach in part-scale analysis and subsequently confirmed experimentally through the fabrication of a printed cantilever beam. The proposed framework showed that the kinematic hardening model performed better than other material models, with an error of less than 6% when compared to experimental measurements. Moreover, the study highlights the significance of selecting the most appropriate material model for future studies involving the integration of advanced artificial neural networks and process parameter optimization to achieve precise and computationally efficient predictions. [ABSTRACT FROM AUTHOR]

Niobium (Nb) is a transition metal commonly used as an alloying element for increasing strength, toughness, corrosion resistance, and other properties of steel and superalloys. Pure Nb, however, is a very interesting metal for its excellent superconductivity. This makes it suitable for producing superconducting magnets and devices for particle acceleration systems and particle physics research (e.g., superconducting resonant cavities). In this work, the production of Nb by the Laser-Based Powder Bed Fusion (PBF-LB/M, also known as Laser Powder Bed Fusion or LPBF) process was examined. Manufacturing parameters were investigated to achieve additively manufactured parts with a relative density higher than 99.5% and showing a down-skin surface roughness in the range of 20–70 μm, depending on the inclination angle. Studies related to the limiting angle of self-supported Nb parts were also conducted, and innovative non-contact supporting structures were successfully developed. These allowed to creation of parts with very small overhang angles, without compromising the downward-facing surfaces; indeed at the same time, the as-built surface finish was improved. [ABSTRACT FROM AUTHOR]

Having information about the impact of some parameters on the microstructure of parts manufactured by the laser powder bed fusion process is considered among the serious challenges to be achieved. In this regard, a wide range of scan speed and laser power values were chosen to predict how much these parameters affect the microstructure such as grain orientations, shapes, and sizes. These represent the basic factors to monitor the mechanical properties of printed parts. A two-dimensional microstructure model based on the cellular automata (CA) was adopted to track the grain structure features of 316L stainless steel. The microstructure in XY and XZ planes including the grain orientation and the grain size are debated with details in our outcomes. The results clearly show that the microstructure is significantly influenced by modifying the process parameters. The higher the scan speed, the finer the grain size, and vice versa for the laser power, which improves the mechanical properties. Increasing the scan speed leads to an enhanced grain orientation towards the building direction. Result predictions are approved by experimental investigations. [ABSTRACT FROM AUTHOR]

Additive manufacturing is a powerful process to build complex geometry. Besides the numerous process parameters influencing the mechanical part performances, other parameters related to the initial powder feedstock or component machining are of most importance. In this study, the combined effect of a wide particle size distribution, surface machining and stress-relief heat treatment on the microstructure and mechanical properties (tension and fatigue) of a stainless steel AISI 316L, produced by laser powder bed fusion, is investigated. In order to correctly investigate those parameters separately, the netshape/machined character of the sample, alongside with the heat treatment, is studied for two kinds of powder having different particle size distributions, i.e. narrow and widely spread. Results show that a large spread of particle size is only slightly detrimental to the fatigue life, in particular in high-cycle conditions due to a larger porosity related to a weakly more uneven particle spatial distribution in the bed. Nevertheless, this effect is of a second order compared to machining or heat treatments which greatly affect the mechanical behaviour. Surface machining and moderate heat treatment are then the best post-operational steps to increase the fatigue life in high-cycle fatigue conditions independently of the particle size distribution. Results are discussed in terms of defects, microstructural modifications, surface roughness, martensitic transformation and mechanical loading. [ABSTRACT FROM AUTHOR]

Base plate preheating is a relatively new strategy to influence the mechanical properties and microstructure of IN718. Base plate preheating was studied to reduce Niobium (Nb) segregation in the interdendritic region of IN718 superalloy and to improve mechanical properties through the laser powder bed fusion (L-PBF) process. Here, we compared experimental results for high-temperature (650℃) mechanical properties of specimens built with 200℃ (PH200) and 600℃ (PH600) preheated base plates. The as-built PH200 and PH600 specimens had relatively similar grain morphologies and orientations along the building direction. The PH200 specimen showed a cellular structure and higher Nb segregation at the interdendritic region, whereas the PH600 specimen had a dendritic structure and lower Nb segregation. The lower amount of Nb segregation in the PH600 specimen is mainly because of the cooling rate difference between specimens during the L-PBF process. The PH600 specimen exhibited slightly better tensile properties, longer creep life, and elongation than the PH200 specimen in both the as-built and heat-treated conditions. These differences mainly occurred due to differences in Laves and δ-phase fractions between the PH200 and PH600 specimens. The simulation result indicates that the temperature distribution in the block remains relatively uniform during the building process. [ABSTRACT FROM AUTHOR]

With the advancement of metal additive manufacturing (AM), lattice structures become a promising solution to situations that require lightweight design and yet maintain adequate mechanical strength. Limited by the quality of feedstock materials, the layer-wise process characteristic and the dynamic nature of thermal environment, lattice structures made by AM often suffer from process-induced imperfections such as poor surface finish and notable geometric deviation. In this study, systematic quantitative characterisation methods are developed to address surface quality and geometric discrepancy of NiTi lattice struts made by laser powder bed fusion (LPBF), with a special focus on fatigue-related features such as stress concentration factors. The results show that for the examined strut diameters and inclination angles, the strut diameter plays a significant role in geometry inaccuracy and the inclination angle has a greater effect on surface texture and stress concentration factor distribution on the surface. Lattice struts with diameters greater than 0.7 mm and inclination angles over 40° with respect to the platform exhibit superior manufacturing quality among all configurations of the struts. The proposed approach not only opens a new avenue to evaluate μ-CT data in a more quantitative way but also offers opportunities to develop guidelines for lattice structure design. [ABSTRACT FROM AUTHOR]

Spreading powder into thin layers is a fundamental step in the laser powder bed fusion (PBF-LB) additive manufacturing process. This step is called re-coating and it is typically performed using either a hard, soft or brush-type re-coater blade or a rotating roller, depending on the machine brand and model. With such variety in powder spreading approaches, the question arises whether the used re-coater type has a significant effect on the quality of parts produced? In this study, an industrial contact image sensor integrated to the re-coater of a PBF-LB system was used for powder bed quality monitoring. Powder bed images at 21 µm/pixel resolution, 184 mm scanning width and 95 mm/s re-coating speed were acquired. With this, the effect of using either soft (rubber) or hard (steel) re-coater blade on the processability of challenging features such as thin walls and steep overhangs was studied. In addition, porosity and dimensional accuracy of parts produced using either the soft or hard blade was analyzed with X-ray computed tomography. It is shown that when building bulk material without any complex features, both the hard and soft re-coating blade results in extremely low porosity ≤ 0.001% without any issues in the processability. However, when thin walls and overhangs are produced, differences in processability, porosity and dimensional accuracy are observed as a function of re-coater blade and part orientation. This is an important factor in understanding all the significant sources contributing to the variability on quality of parts produced using different PBF-LB machines. [ABSTRACT FROM AUTHOR]

Surface quality is one of the critical factors that affect the performance of a laser powder bed fusion part. Optimising process parameters in process design is an important way to improve surface quality. So far, a number of optimisation methods have been presented within academia. Each of these methods can work well in its specific context. But they were established on a few special surfaces and may not be capable to produce satisfying results for an arbitrary part. Besides, they do not consider the simultaneous improvement of the quality of multiple critical surfaces of a part. In this paper, an approach for optimising process parameters to improve the surface quality of laser powder bed fusion parts is proposed. Firstly, Taguchi optimisation is performed to generate a small number of alternative combinations of the process parameters to be optimised. Then, actual build and measurement experiments are conducted to obtain the quality indicator values of a certain number of critical surfaces under each alternative combination. After that, a flexible three-way technique for order of preference by similarity to ideal solution is used to determine the optimal combination of process parameters from the generated alternatives. Finally, a case study is presented to demonstrate the proposed approach. The demonstration results show that the proposed approach only needs a small amount of experimental data and takes into account the simultaneous improvement of the quality of multiple critical surfaces of an arbitrary part. [ABSTRACT FROM AUTHOR]

Enhanced levels of alloy print defects such as porosity are associated with the printing of overhang structures by laser powder bed fusion (L-PBF). This study compared the microstructure and porosity of Ti-6Al-4V overhang structures, with that observed for the bulk alloy. It was observed in the region around the overhang structure that the microstructure exhibited larger grain sizes and was less homogenous, compared to the that obtained within the bulk alloy. An increased level of porosity of up to 0.08% was also observed in the overhang print alloy, compared with the corresponding < 0.02% in the alloy bulk. It is hypothesised that these microstructural changes are associated with the excess heat generated in the overhang region, due to the decreased thermal conductivity of the powder immediately below the print layers, compared with solid alloy. During L-PBF alloy printing, in situ process monitoring of the melt pool emissions was obtained in the near-infrared range and correlated with the properties of the printed parts. This in-process data was used to assist in selecting optimal laser processing conditions, in order to help prevent melt pool overheating at the overhang. By systematically controlling the laser energy during the printing of the first fifteen layers over the overhang structure, the level of porosity was reduced, to the < 0.02% level of the bulk alloy. There was also an associated reduction in the roughness (Ra) of the overhang itself, with its Ra decreasing from 62.4 ± 7.3 to 7.5 ± 1.9 µm. [ABSTRACT FROM AUTHOR]

We present a machine learning workflow to discover signatures in acoustic measurements that can be utilized to create a low-dimensional model to accurately predict the location of keyhole pores formed during additive manufacturing processes. Acoustic measurements were sampled at 100 kHz during single-layer laser powder bed fusion (LPBF) experiments, and spatio-temporal registration of pore locations was obtained from post-build radiography. Power spectral density (PSD) estimates of the acoustic data were then decomposed using non-negative matrix factorization with custom k -means clustering (NMF k ) to learn the underlying spectral patterns associated with pore formation. NMF k returned a library of basis signals and matching coefficients to blindly construct a feature space based on the PSD estimates in an optimized fashion. Moreover, the NMF k decomposition led to the development of computationally inexpensive machine learning models which are capable of quickly and accurately identifying pore formation with classification accuracy of supervised and unsupervised label learning greater than 95% and 90%, respectively. The intrinsic data compression of NMFk, the relatively light computational cost of the machine learning workflow, and the high classification accuracy makes the proposed workflow an attractive candidate for edge computing toward in-situ keyhole pore prediction in LPBF. [ABSTRACT FROM AUTHOR]

The demand for manufacturing components with complex geometries, good mechanical properties, and material efficiency has surged across various industries, encompassing aerospace, military, nuclear, and naval sectors. Laser powder bed fusion (LPBF), as an additive manufacturing (AM) process, has emerged as a promising method for producing ultra-high mechanical strength alloys, like maraging 300 steel (18Ni300M). However, in numerous studies in the literature concerning the effects of processing parameters on the properties of 18Ni300M steel parts fabricated through LPBF, limited attention has been given to the influence that powder atomization methods may exert on the final properties of these parts. This article investigated the effect of gas atomization (GA) and water atomization (WA) processes on the microstructure of 18Ni300M steel powders and the mechanical properties, microstructure, and chemical composition of LPBF-produced parts. The results revealed significant distinctions in the morphology, aggregation degree, and particle size distribution between the GA and WA powders, which directly influenced the microstructure and affected the amount of defects in LPBF-produced parts. Despite the similar mechanical response found in the WA and GA specimens in the elastic region, the samples produced with the WA batch presented a brittle behavior with a ductility of only 4.06%, whereas the GA parts had an elastoplastic behavior with an elongation of 11.52%. The bulks from the WA batch produced in the LPBF process were compromised due to powder contamination with oxygen, which increased gas porosity and effected fragile oxide particles visible on the fracture surface. [ABSTRACT FROM AUTHOR]

In this study, the correlation between the microstructure and tribological performance of Sc and Zr-modified Al–Mg alloy (Scalmalloy®) samples produced via laser powder bed fusion process was evaluated via a dry sliding Pin-on-Disc wear test under different planes, directions, and various normal applied loads. The results revealed a remarkable dependency of wear properties on the as-built microstructure so that different behaviors were observed along the scanning and building planes. The microstructural examination indicated the presence of bi-modal grains and finely shaped equiaxed grains observed in the building and scanning planes, respectively. Increasing the applied loads from 20 to 40 N led to a significant increase in the coefficient of friction (COF) while increasing the load from 40 to 60 N, slightly decreasing the COF for the studied samples. No dependency was found between the COF and the corresponding microstructure at the highest applied load. The anisotropic wear resistance and COF values were predominant at the lowest applied load. Due to tailored as-built microstructural features and different microhardness values, lower wear rates were noticed along the scanning plane for all applied loads. Under the 20 N applied load, however, the worn surface of the scanning plane showed a clearer and smoother surface compared to the building plane surfaces. Ultra-fine equiaxed grains along the scanning plane and columnar grains along the building plane were determined as the main factors creating anisotropic tribological behavior. The outcomes of this study can pave the way toward producing more wear-resistant surfaces and developing components for critical wear applications in as-built conditions with no need for expensive and time-consuming surface treatments. [ABSTRACT FROM AUTHOR]

This article aims to highlight the development of an intermittent controller designed to compensate and rectify the lack of fusion (LoF) zones that are induced during the LPBF process. The initial step involved the usage of the self-organizing map (SOM) algorithm to identify the location of LoF defects. Subsequently, the identified defects undergo clustering through the K-means algorithm to form a matrix of cells on the build plate. The center of each cell that encompasses the defective area is then selected as the optimal position for increasing laser power during the subsequence printed layer. To identify the optimum laser power value, various artificial voids, mimicking actual defects, are embedded in the coupons. The capping layer that closes the artificial void is then fabricated with different laser powers to heal the underlying defects. Based on the optimum laser power and defect size, several controlling rules are defined to change the laser power in situ in the targeted cells located within the capping layer of defects. The change in laser power is transferred as a laser correction file (LCF) to the actuator via the Message Queuing Telemetry Transport (MQTT) broker. Finally, the performance of the controller is evaluated by designing and fabricating two new sets of experiments, including artificial and randomized defects. The results are validated by performing a micro CT scan, in which the density of defects is analyzed on parts produced with and without the controller. The results suggest that the use of the controller increased the density of the sample with randomized defects by up to 1%. [ABSTRACT FROM AUTHOR]

Powder bed fusion is a method of additive manufacturing (AM) where parts are constructed by iteratively melting metal cross-sections to build complex 3D structures. Defects often form during the printing process, where the dynamics of the melt pool can directly contribute to the formation of porous defects in the final part. For instance, insufficient overlap of the produced melt pools can result in unmelted regions of powder, while deep, unstable vapor depression cavities can lead to spherical voids becoming trapped in the substrate. Therefore, in situ of monitoring the melt pool during the melting process can telegraph the formation of defects and assist the creation of fully dense parts. Here, we augment data-driven-based monitoring techniques to enable the 3D visualization of the melt pool underneath the surface, based on the melt pool surface temperature and processing parameters. Specifically, a convolutional neural network (CNN) predicts the topography of the melt pool and keyhole cavity, based on the surface temperature data near the laser focal point and the nominal operating conditions. The data for the laser powder bed fusion process used to train the model is produced by full-field simulations of the meso-scale melting process, with the CFD software FLOW-3D. Data augmentation techniques are implemented to ensure generalizable performance in cases where the temperature data may be obscured and to ensure sharp, accurate predictions of the melt pool boundaries. [ABSTRACT FROM AUTHOR]

Additive manufacturing effective implementations require knowledge about how process planning can influence product outcomes. Tuning non-traditional parameters of laser powder bed fusion (L-PBF) manufacturing, like better part location in the build platform considering the manufacturing atmosphere and projected laser inclination, may have consequences for the final product. However, understanding the material behavior derived from the location selection still needs investigation, which would provide valuable contributions to these process applications. This study investigated how gas flow, spatter ejection, and laser inclination on L-PBF affect maraging steel integrity regarding microstructure and mechanical performance joining experimental and simulation tools. Microscopy techniques, tensile tests, nanoindentations, and the simulation of the gas flow interaction with spatters allowed identifying preferential spatter deposition regions based on the gas flow and its potential consequences on the studied properties of maraging samples manufactured in three build platform locations. For instance, the location less prone to spatter deposition provided 12 to 16% lower microhardness than the other evaluated locations. This observation indicates processing parameters that can be used for managing L-PBF issues. So, these findings show the importance of considering the build location and relative gas flow direction during the L-PBF planning in addition to more trivial parameters. [ABSTRACT FROM AUTHOR]

This study establishes a novel printability criterion for Inconel-718 parts by laser power bed fusion. For this purpose, the regions with D/t ≤ 1.15, L/W > 2.1, and W/D = 2.0 have been identified with lack-of-fusion, balling, and keyhole defects. Regimes within the processing maps related with defects have been regarded as a melt pool geometry function, derived using a FEM model with temperature-dependent thermophysical properties. The data was collected, via design of experiment technique, using validated simulation model. Following that, the acquired data was utilized to train and test a machine learning model based on a backpropagation artificial neural network (ANN). By linking melt pool dimensional ratios to defects, the validated ANN model was used to produce processing maps. The processing maps were validated using experimental analyses, which revealed a consistent correlation between experiments and simulations. The proposed processing maps can be utilized to quickly quantify the Inconel-718 parts generated by the LPBF. [ABSTRACT FROM AUTHOR]

Processability refers to the ease of achieving the required component while maintaining mechanical performance and processing schedules, which are critical for determining the cost and efficiency of using a given material, from the raw condition to the final product of any manufacturing process. Components built using the laser powder bed fusion with metallic alloys (LPBF-M) process show variability in their mechanical performance, which can be attributed to a range of process parameters and characteristics of the powder material employed by each type of machine. These variations are currently hindering the adoption of this technology at the industrial level. This paper presents a processability approach that could be applied in the LPBF-M to evaluate the possibility of speeding up productivity and minimising the effect on the mechanical properties and relative density and is defined based on the process parameters and powder material characteristics that generate the melting pool and meet bonding criteria at a specific build rate. A case study is carried out with stainless steel 316 (SS316), although this processability analysis could be applied to any other alloy. The results show that a wide range of process parameters generates a suitable processability interval with different values of the build rate. It is also found that slow build rates give rise to less variability in the mechanical properties, while faster rates produce more variability; this is caused by a fast-growing melt pool due to the use of high scan velocities for the SS316 alloy under study. [ABSTRACT FROM AUTHOR]

Laser powder bed fusion (LPBF) is a metal additive manufacturing technology that is capable of printing metals and alloys with high quality. This study investigates wear characteristics of LPBF processed 316 L stainless steel and its correlation to the hardness and microstructure of the materials. The wear properties of LPBF specimen and hot rolled 316 L stainless steel were compared. From the analysis of wear characteristics of the samples, it was observed that the results were not consistent with the prediction of Archard's empirical equation. The LBPF processed samples with higher hardness exhibited lower wear resistance compared to the conventionally processed (hot rolled) samples. This study aimed at addressing such phenomenon by understanding the plasticity in those samples. Unconventional plastic deformation in LPBF 316 L samples with negligible work-hardening was observed which was believed to be the main reason for their lower wear resistance compared to conventionally processed samples. [ABSTRACT FROM AUTHOR]

In applications where a combination of good strength and corrosion resistance is required, 17-4 precipitation hardenable (PH) stainless steel is a common material choice. This alloy is traditionally processed through a combination of casting, rolling, and machining. A variety of heat treatments are used to anneal and harden the material via precipitation strengthening. While additive manufacturing (AM) removes many geometric design constraints from these traditional forming processes, until recently, structures fabricated via laser powder bed fusion (L-PBF) were porous and contained undesirable columnar grain structures that contributed to unpredictable and anisotropic mechanical properties. Recent advances in L-PBF processing technology including improved gas flow, powder atomization, and print parameter optimization enable printing of high-quality 17-4 PH with properties that are comparable to traditionally processed material. However, it is yet to be determined whether these properties can be reliably reproduced across various machines and if machine-agnostic material property baselines can be established based on data available thus far. If baselines can be established, their implementation should allow for identification of machines that generate non-conforming material. In this work, we evaluate the consistency of mechanical properties in L-PBF 17-4 PH produced by six vendors with the ultimate goal of establishing mechanical property baselines, which is fundamental to modern qualification paradigms. We find non-conforming data from two vendors by examining anomalies in mechanical response (e.g., transformation induced plasticity effects) and demonstrate that typical sources of variation can be detected using qualification testing protocols. Ultimately, after standard solution annealing and heat treating, the microstructure and mechanical properties across vendors converged with very few, easily explainable exceptions. In particular, powder atomized in nitrogen promoted formation of retained austenite that lead to a yield point phenomenon in as-built conditions, and high surface roughness from as-built surfaces reduced the fatigue strength. However, with conventional post-processing heat treatments and surface polishing, AM 17-4 PH behaved comparably and consistently to conventionally processed material. [ABSTRACT FROM AUTHOR]

Aluminium-based metal matrix composites reinforced with graphene (Gr) and its derivatives have been reported as promising composites due to their superior properties such as strength, damage tolerance, fatigue resistance, and density. However, the crack and porosity susceptibility of Aluminium 2024 Alloy (AA2024) with added Gr when fabricated using additive manufacturing techniques is not sufficiently well understood. The present work addresses this knowledge gap by focusing on the effect of graphene nanoplatelets (GNPs) and scanning speed on the AA2024 composites' wear performance and microstructural and mechanical properties of specimens fabricated using laser powder bed fusion (LPBF). The experimental findings demonstrate that up to 0.5% presence of Gr in the composite improves its crystallite size and microhardness by up to 37.6% and 45%, respectively; however, it increases the porosity and crack formation due to the high laser power requirement. Moreover, the composites' macroscale scratch and nanoscale wear performances showed improvements by up to 50% and 56% with higher Gr concentration (0.5%), suggesting that Gr is distributed uniformly in the structure. The improved understanding of the relationship between microstructure and mechanical characteristics of the GNPs/Al2024 composites fabricated using LPBF in terms of cracking and porosity formation is another significant contribution of this work. [ABSTRACT FROM AUTHOR]

Traditional end effectors for collaborative robots are generally either highly specialized to a specific shape of part or are unable to deal with high temperatures, indicating the existence of a niche for an end effector designed for both, leveraged in the automation of post-processing metal additive manufacturing parts. The design described and qualified in this manuscript is composed of a custom-fabricated 4140 alloy steel finger and an aluminum oxide–casted ceramic insulator. Characterization tests were performed to quantify the thermal and mechanical properties of the casted material, and thermal simulations were performed to optimize design. The simulated temperature difference across the body of the insulator is 767.58°C when placed in contact with a 1000°C steel plate, modeled from a conducted thermal shock test. With an expected maximum operating temperature of 800 to 1000°C, the insulator will withstand the conditions for grabbing. Additionally, a pair of thermocouples are incorporated in the gripper to monitor the temperature of the workpiece to ensure safe handling. The designed high-temperature gripper will be a useful tool in enabling capabilities of metal additive manufacturing as well as the safe handling of high-temperature objects. [ABSTRACT FROM AUTHOR]

This study investigated the microstructures and tensile properties of as-build and heat-treated (including T6, T2, and T6-T2 treatments) AlSi10Mg samples fabricated by laser powder bed fusion (LPBF). The microstructure analysis revealed that the trend of eutectic Al-Si phase distribution along grain boundaries for heat-treated samples was significantly weakened, and the eutectic Al-Si phase was transformed into Si particles which were uniformly distributed in α-Al matrix. For the T6-T2-treated specimens, the Si content in particles was the highest. Meanwhile, the T6 treatment increased the proportion of equiaxed grains on vertical section most significantly. However, in the wake of heat treatments, the average grain size on horizontal section had little change while the average grain size on vertical section was increased. The properties analysis demonstrated that the T6-T2-treated specimens obtained the best elongation and worst strength. Besides, the T6 treatment eliminated the anisotropy of the LPBFed specimens basically and synergistically obtained the best combination of strength and plasticity. The T6 treatment weakened the direction difference to the greatest extent, whereas the T2 treatment minimized the stress concentration. According to the comparative finding, the Si content in particles played an important part in properties. Besides, the relationship among inverse pole figure (IPF) maps, pole figure (PF) maps, the mass fraction and distribution of Si in Al matrix and Si particle, the grain size, kernel average misorientation (KAM), and mechanical properties under different heat treatments was established, which provided a new idea for the selection of heat treatments on LPBFed parts. [ABSTRACT FROM AUTHOR]

Intelligent additive manufacturing is the direction of development, and changing parameters during the manufacturing process is an effective measure to adjust the quality of the part. The purpose of this paper is to investigate the stability of single-track and multilayer fabricated Ti-6Al-4 V designed and fabricated by laser powder bed fusion with high degrees of freedom. The effects of single-track, relative density, surface quality, and microstructure and mechanical properties were investigated by designing a process strategy based on the interval powder layer thickness and regulating the process parameters. The survey shows that the point distance of less than 35 μm, the exposure time of longer than 120 μs, and the hatch spacing of greater than 70 μm can ensure the relative density of specimens with high layer thickness. The microstructure consists mainly of acicular martensite αʹ filled in columnar β grains, and acicular martensite αʹ grains are relatively sparse. The tensile strength, yield strength, and elongation of the specimens were 1109 MPa, 1053 MPa, and 5.6%, respectively. The fracture is a combination of ductile fracture and brittle fracture. The building rate was 9 mm3/s, which is higher than the results obtained in previous studies. The survey can provide theoretical support for researchers and data support for engineers. [ABSTRACT FROM AUTHOR]

In laser powder bed fusion (LPBF), the effects of operating conditions on thermal gradients and residual stresses are the utmost challenges that require significant attention. The magnitudes of residual stress in the printed layers, as well as the distribution along the printed components, have not been well explained for LPBF parts. In this study, a 3D finite element thermo-mechanical model has been established to investigate the effect of operating conditions on thermal distribution, melt pool evolution, residual stress distribution, and part distortion. The printed AISI 316L stainless steel cubes have been characterized experimentally. The results showed a proportional correlation among the number of layers, thermal distribution, and melt pool dimensions. A combination of compressive and tensile stresses was recorded in the LPBF-ed parts. The Cauchy stresses were maximum in magnitude at the bottom and top surfaces along the xx- and yy-orientations, while these stresses increased in magnitude along with the part-build orientation (zz) within the whole printed cube except the top surface. The Von Mises stresses were minimal than Cauchy stresses. A maximum displacement was identified at the printed components' contours, gradually decreasing from top to side walls and top surface. An inverse correlation was identified among average Von Mises stresses (AVMS), laser power (LP), and hatch distance (HD); however, a proportional relationship is presented between laser scanning speed (LSS) and AVMS. The average displacement (AD) presented an inverse relationship with LSS and HD, while a proportional correlation has been presented between LP and AD. Average thermal distribution (ATD) revealed an inverse effect on AVMS and a proportional effect on AD. In the printed parts, only austenite-gamma phase was identified along (111), (200), and (220) orientations, with a lack-of-fusion defect in the morphology. [ABSTRACT FROM AUTHOR]

Metals and alloys in amorphous state are promising for the use as structural and functional materials due to their superior properties related to the absence of such defects as grain boundaries and dislocations. Obtaining the amorphous state requires quenching from liquid state with a high cooling rate. In conventional technologies, this is the reason for a considerable size limitation hindering application of amorphous alloys. Additive manufacturing (AM) is free of the size limitation. The so-called bulk metallic glass (BMG) alloys have extremely low critical cooling rates and can be used in AM. Recent studies on AM from Zr-based BMGs by selective laser melting (SLM) has proved the possibility of attaining the amorphous state and revealed partial crystallization to be the principal drawback of this process. The present work aims to analyze the conditions for partial crystallization and attempts to control it by optimizing the process parameters. A comprehensive parametric analysis is accomplished in single-track SLM experiments with Zr-based BMG alloy Vit 106. The observed microstructures are related to the temperature fields and thermal cycles estimated by an analytic heat-transfer model in the laser-impact zone. In the cross section of a laser track, a central bright domain is identified as the remelted zone. An annular darker crystallization zone encircles the remelted zone. The model fits the experimentally obtained dependencies of the remelted zone size versus laser power and scanning speed and indicates that 43 ± 2% of the incident laser energy is transferred into the substrate thermal energy. The principal energy losses are reflection of the incident laser radiation and material evaporation. Primary crystalline inclusions existing in the substrate before laser processing dissolve at the laser melting. The mean cooling rate in the remelted zone is up to 4 orders of magnitude greater than the critical cooling rate. Therefore, homogeneous nucleation is not expected. Nevertheless, the theoretically estimated crystallization times are sufficient for a considerable crystal growth in the heat-affected zone where primary crystalline inclusions and nuclei are not completely dissolved. [ABSTRACT FROM AUTHOR]

The stages of this study, in which conformal cooling channels (CCCs) integrated mould core produced by LPBF (laser powder bed fusion) and CCC integrated mould core produced by SM (subtractive manufacturing), are used in a hybrid primary mould, consisting of design, numerical study, mould core production with AM (additive manufacturing) and SM, product production and mould testing with PI (plastic injection), instant thermal measurement with IRT (infrared thermography), and OM (optical microscope) examination. The findings of the transient simulation reveal that the additively manufactured CCC had a cooling time reduction of 234% when compared to the conventional straight-drilled CCC. In addition, the flow rate of the coolant in the mould core produced with AM increased approximately three times. Overall, the findings of the steady-state simulation show that the total cooling capacity of the AM-CCC arrangement is 5.9% higher than that of the traditional straight-drilled cooling channel (SM-CCC). [ABSTRACT FROM AUTHOR]

The main objective of this research is to investigate the capability of laser powder bed fusion (LPBF) to manufacture complex heat exchangers with gyroid-shaped channels. First, the gyroid's geometric features are investigated including the network type, thickness, unit cell size, and aspect ratio. Computational fluid dynamics (CFD) were used to screen these designs based on their thermal and fluid dynamics performance. Then, the manufacturability of various AlSi10Mg gyroid designs is tested using the LPBF, and the microstructure was investigated for defects. Finally, five heat exchanger prototypes were manufactured using LPBF; four of them were based on gyroid designs, and the fifth one was a conventional design for comparison. A heat transfer analysis is performed to compare the performance of the heat exchangers using conjugate heat transfer (CHT) methods. It was found that manipulating the sheet gyroid thickness and aspect ratio can result in a reasonable tradeoff between air side and coolant side pressure drops while maintaining the same heat rejection value. The suggested process parameters were able to successfully print the gyroid heat exchanger cores with thicknesses higher than 0.2 mm successfully. The LPBF-fabricated gyroid heat exchangers outperformed the conventional design. This study paves the road to a new generation of crossflow heat exchanger designs. [ABSTRACT FROM AUTHOR]

Intelligent additive manufacturing is the future direction, and changing parameters during manufacturing process is an effective measure to adjust the quality of parts. In this study, the stability behavior of the manufacturing process under three process strategies, low layer thickness fabrication (LLTF), high layer thickness fabrication based on the interval of the powder layer thickness (IPLT), and changing of layer thickness strategy (CLTS), was comparatively investigated. Multilayer experiments and mechanical properties experiments were performed during laser powder bed fusion, and the correlation between manufacturing quality and powder layer thickness was compared. By optimizing the process parameters, the surface quality of CLTS is similar to that of LLTF, and the value of minimum surface roughness can be between 22 and 23 μm. Only balling effect due to spattering and a small amount of porosity were found in the cross-section of CLTS. The relative density of most of the fabricated parts is higher than 99%, and the highest relative density is up to 99.99%. IPLT has longer and thicker martensite than LLTF, and the β-grain of CLTS is also coarser than LLTF. The tensile properties of CLTS are similar to those of LLTF, and the ultimate tensile strength, yield strength and elongation of CLTS are 1205 MPa, 1107 MPa and 6.8%, respectively. Due to the anisotropy of the LPBF, the horizontally constructed Ti–6Al–4 V specimens yielded higher strengths, while the vertically constructed specimens obtained better elongation. The fracture of the part is characterized by a mixture of brittle fracture, ductile fracture and quasi-dissociative fracture. The surface quality, relative density and mechanical properties of CLTS are similar to those of LLTF, while the forming efficiency is much higher than that of LLTF, which can reach 7.35 mm3/s. [ABSTRACT FROM AUTHOR]

The development of laser powder bed fusion (LPBF) has made it possible to produce complex three-dimensional components for high-temperature applications. The LPBF process needs to be refined to address several key factors, such as high-temperature elongation, microstructure heterogeneity, and mechanical anisotropy. Hastelloy-X Ni-based superalloy was used to illustrate these issues in this study. First, 0.046 wt.% of yttrium (Y) was added to Hastelloy-X (HX-y) to prevent grain boundary embrittlement. The second step involves two kinds of post-heat treatments; i) at 850 °C for 2 h (DA) for carbides and ii) solution heat treatment at 1240 °C for 8 h upon aging at 850 °C for 2 h (HTA). The creep properties of the samples were compared at 900 °C/ 80 MPa to understand the effect of Y addition and post heat treatments. The HX-y specimen was strengthened by solid solution and dispersion of Y-rich oxides, together with stabilization of oxygen-based contamination at the grain boundaries. The DA and HTA HX-y specimens had better creep properties than the HX specimens. The HX-y specimen showed superior creep properties to the HX specimen due to the presence of carbides M6C and Cr23C6 inside grains and at grain boundaries. However, carbides remained stable even at high temperatures within grains and at grain boundaries. Nevertheless, the HTA HX-y specimen exhibited superior isotropic creep properties. As a result of grain boundary pinning, serrated grain boundaries prevented grain boundary sliding. In contrast, HX specimens exhibited poor creep properties. This study confirmed that the optimal addition of Y together with the optimization of post-heat treatment drastically enhances the creep properties of materials fabricated by the LPBF process. [ABSTRACT FROM AUTHOR]

Laser powder bed fusion (LPBF) includes a few printing techniques widely used, in recent years, concerning the additive manufacturing of Ti6Al4V alloys. These produced parts, typically utilized in sectors such as aerospace and biomedical, are characterized by very high added value. It is therefore fundamental to identify the influence of process parameters typical of LPBF technology on the occurrence of warping leading to process failure. This study deals with the characterization of single-track and "micro-scale" level warping phenomena which may lead to protrusion of material over the powder bed and process failure before normal termination. This phenomenon was investigated as a function of process parameters, referring also to the strength and ductility characteristics of the manufactured samples. With this purpose, several samples were printed using variable process parameters both in terms of line energy density (LED) values and in terms of laser power and speed combinations such as to guarantee constant LED values. For the samples that did not show significant micro-warping phenomena, in addition to the transversal and longitudinal geometric characterization of the single track, tensile tests were performed to determine both the resistance of the material and the ductility characteristics. The single tracks, for given process parameters, were printed on a homogeneous material substrate. For every single track, a microstructural and morphological transverse and longitudinal characterization has been carried out and the measured geometrical features were correlated to the process parameters. The obtained results allowed the identification of a new threshold parameter, indicating the limit operating conditions beyond which significant warping phenomena and process failure occur. [ABSTRACT FROM AUTHOR]

In the last decade, additive manufacturing (AM) technologies have had an extraordinary research interest in each of the relevant topics: essential manufacturing method development, raw materials, optimization of process parameters, characterization of the final product, residual stresses, material microstructure, and modeling. After these developments, additive manufacturing processes started to be rapidly included in industrial production. The needs and problems encountered in practice bring new research topics. An industrial product generally combines many parts produced by different manufacturing methods — machining results in higher geometric accuracy and surface finish than the additive manufacturing process. Therefore, additively manufactured parts will require post-process drilling of products to achieve necessary tolerances. This study discusses the drilling performance of additively produced AlSi10Mg parts with different drill bits post-process. This study investigates thrust force, surface roughness, temperature, and chip formation in drilling AM AlSi10Mg parts with HSS-G, TiN, and TiAlN drills. The thrust force values obtained in the building direction are higher than in the transverse direction, albeit by a small margin. A significant difference of 23% in drilling tool torque values and up to 35% in thrust force values was measured. The lowest thrust force and torque were obtained with the TiN-coated drill bit, while the highest was with the TiAlN. TiAlN cutting tool exhibits the highest temperature value of 80.6 °C. The laser traces form a layered structure in the Z direction, and the cutting edge coincides with more grain boundaries, making it difficult to drill in this direction. [ABSTRACT FROM AUTHOR]