A non-equiatomic AlCoCr0.75Cu0.5FeNi alloy has been identified as a potential high strength alloy, whose microstructure and consequently properties can be widely varied. In this research, the phase structure, hardness, and magnetic properties of AlCoCr0.75Cu0.5FeNi alloy fabricated by laser powder bed fusion (LPBF) are investigated. The results demonstrate that laser power, scanning speed, and volumetric energy density (VED) contribute to different aspects in the formation of microstructure thus introducing alterations in the properties. Despite the different input parameters studied, all the as-built specimens exhibit the body-centered cubic (BCC) phase structure, with the homogeneous elemental distribution at the micron scale. A microhardness of up to 604.6 ± 6.8 HV0.05 is achieved owing to the rapidly solidified microstructure. Soft magnetic behavior is determined in all as-printed samples. The saturation magnetization (Ms) is dependent on the degree of spinodal decomposition, i.e., the higher degree of decomposition into A2 and B2 structure results in a larger Ms. The results introduce the possibility to control the degree of spinodal decomposition and thus the degree of magnetization by altering the input parameters of the LPBF process. The disclosed application potentiality of LPBF could benefit the development of new functional materials.

The powder bed fusion (PBF) process is widely
adopted in many manufacturing industries because of its
capability to 3D print complex parts with micro-scale precision.
In PBF process, a thermal energy source is used to selectively
fuse powder particles layer by layer to build a part. The build
quality in the PBF process primarily depends on the thermal
energy deposition and properties of the powder bed. Powder
flowability, powder spreading, and packing fraction are key
factors that determine the properties of a powder bed.
Therefore, the study of these process parameters is essential to
better understand the PBF process. In our study, we developed
a two-dimensional powder bed model using the granular
package of the LAMMPS molecular dynamics simulator.
Cloud-based deposition was adopted for pouring powder
particles on the powder bed. The spreading of particles over the
substrate was mimicked like a powder bed system. The powder
flowability in the proposed study was analyzed by varying the
particle size distribution. The simulation results showed that a
greater number of larger particles in a power sample results in
an increase in the Angle of Repose (AOR) which ultimately
affects the flowability. Two different kinds of recoater
geometry were considered in this study: circular and
rectangular blades. Simulation results showed that depending
on the recoater shape there is a change in the packing fraction
in the powder bed. Cross-sectional analysis of the power bed showed a significant presence of voids when a greater number
of larger particles existed in the powder batch. The packing
fraction of the powder bed was found to be a strong function of
particle size distribution. These analyses help understand the
influence of particle size and recoater shape on the powder bed
properties. Findings from this study help to provide a guideline
for choosing particle size distribution if the spherical particles
are considered. While the present study focuses on the spherical
powder particles, the proposed system can also be adapted to
the study of powder bed with aspherical particles.

The Inconel 625 (IN625) superalloy has a high strength, excellent fatigue, and creep resistance under high-temperature and high-pressure conditions, and is one of the critical materials used for manufacturing high-temperature bearing parts of aeroengines. However, the poor workability of IN625 alloy prevents IN625 superalloy to be used in wider applications, especially in applications requiring high geometrical complexity. Laser powder bed fusion (LPBF) is a powerful additive manufacturing process which can produce metal parts with high geometrical complexity and freedom. This paper reviews the studies that have been done on LPBF of IN625 focusing on the microstructure, mechanical properties, the development of residual stresses, and the mechanism of defect formation. Mechanical properties such as microhardness, tensile properties, and fatigue properties reported by different researchers are systematically summarized and analyzed. Finally, the remaining issues and suggestions on future research on LPBF of IN625 alloy parts are put forward.

This study deals with additive manufacturing (AM) products. Employing AM technology, complex-shaped, and lightweight parts can be fabricated using the aluminum alloy, AlSi10Mg. Typically, AM of this alloy is performed by laser powder bed fusion (LPBF). The mechanical properties of LPBF products are crucial for many engineering applications. Therefore, there have been efforts to measure the dynamic and quasi-static properties of such products. However, both the dynamic and quasi-static shear behaviors of this alloy are yet to be investigated. The present study is focused on experimentally investigating the shear behavior of LPBF AlSi10Mg. Quasi-static shear tests were performed according to the ASTM B565 protocol, whereas dynamic shear tests were conducted using a standard split Hopkinson pressure bar equipped with an innovative sample holder that generates pure shear in a sample. The results of the performed tests showed that as the shear load rate increased, the shear strength considerably increased. In addition, in the quasi-static regime, the shear strength was practically independent of the product build direction. In contrast, under the dynamic shear conditions, samples built horizontally failed at a shear strength approximately 10% lower than that of those manufactured vertically. To investigate the build orientation effect on the shear behavior, this study conducted extensive microstructural characterization along with fracture analysis. Crack nucleation and propagation were analyzed in view of the effects of the unique microstructural characteristics of the LPBF AlSi10Mg, including the morphologies of the melt pool boundaries. The results obtained in this study suggested that for engineering applications in which dynamic loads are expected, the build orientation of AlSi10Mg parts produced using LPBF technology must be considered.

NASA has been developing and advancing regeneratively-cooled channel wall nozzle technology for liquid rocket engines to reduce cost and schedules associated with fabrication. One of the primary methods being advanced is Laser Wire Direct Closeout (LWDC). LWDC was developed to provide an additively manufactured laser deposited closeout of the coolant channels that also forms the structural jacket in-situ. This technique has been previously demonstrated through process development and hot-fire testing on a series of subscale nozzles at NASA Marshall Space Flight Center. The hot-fire test articles were fabricated using monolithic alloys to simplify the fabrication process. Ongoing research is being conducted to further expand use of this process for increased scale and bimetallic or multi-alloy options. The use of multi-alloys is desired to fully optimize the combination of materials in the radial and axial directions to reduce overall weight of the nozzle and allow for higher thermal and structural margins on the channel wall nozzle. NASA recently completed process development and hot-fire testing of a series of channel wall nozzles that incorporate a copper-alloy as the hotwall liner material and a superalloy and combination thereof for the structural jacket using the LWDC technique. The fabrication process was further advanced by using a multi-alloy axial joint using explosive bonding integrating a copper-alloy at the forward end of the nozzle hotwall and a stainlessalloy for the remaining length. A third alloy was then used for the channel closeout using the LWDC process. This paper will describe the process development using the LWDC process for channel closeout utilizing the multi-alloys, hardware design and results from hot-fire testing on subscale multi-alloy LWDC channel cooled nozzles.

Residual stresses and deflections are two major issues in laser-based powder bed fusion (L-PBF) parts. One of the most efficient and reliable ways for predicting residual stresses and final distortions is via a calibrated numerical model. In this work, a part-scale finite element thermo-mechanical model for Ti6Al4V is developed in the commercial software Abaqus/CAE 2018. The flash heating (FH) method is used as the initial multi-scaling law to avoid time-consuming meso-scale simulations. The model has been verified by doing a mesh-independency analysis. To check the validity of the model, dedicated experiments involving samples with specific scanning strategies were performed. Experimental measurements were made by optical 3D scanning with the fringe projection technique. An in-house made Python script was written for the stripe-wise and layer-wise partitioning of the numerical model, along with material and boundary condition attributions. As expected, the results show that layer-wise FH is insensitive to the scanning pattern and will lead to an isotropic stress field. It is shown that the FH method overestimates the minimum deflection magnitude compared to the experiments by 46.2 %. Sequential FH (SFH) is then proposed to resolve this problem. Results show that by refining the stripe widths in SFH from 15 mm to 1.5 mm, the deviation between the predicted and measured deflection reduces from 35.7 % to 1.19 %. However, the required computational time increases from 9.3 h to 65 h.

Abstract Overhangs are one of the most critical features in the design of parts for Powder Bed Fusion. In particular, accuracy and roughness of these geometrical elements are the main concerns while designing for this process. Support structures are the most common solutions adopted to preserve the quality of overhangs and ensure the appropriate completion of the whole process. This work aims to correlate geometrical parameters of overhangs and supports to the distortion and roughness of parts. For this purpose, an experimental campaign is carried out on specimens manufactured via a powder bed fusion of stainless steel. Appropriate process parameters have been chosen thanks to preliminary experimental tests. Both displacement and roughness are measured using optical and contact systems. The two methods show considerable differences that are examined in the light of process features. The analysis of results highlights that the design of support structures has to be carried out in accordance with the thickness and inclination of the overhangs in order to minimise distortions. Local defects occurring on the edges are also highlighted by comparing contact with optical measurements. An upward warping of all the geometries is observed after annealing. Roughness underlines the fact that the inclination of surfaces is the only influential factor, since the support design can efficiently dissipate the thermal energy from the overhangs. Comparative analysis of contact and optical measurements also highlights the limitations of the first type of measurements in detecting small surface defects.

Tungsten (W) as a structural component has grown roots in many special applications owing to its radiation-shielding capabilities and its properties at elevated temperatures. The high ductile-brittle transition temperature (DBTT) and the very high melting point of tungsten however have limited its processability to certain technologies such as powder metallurgy. Laser powder bed fusion (LPBF) has been introduced in recent years as an alternative for manufacturing tungsten parts to overcome the design limitations posed by powder metallurgy technology. A review of the literature shows significant improvements in the quality of tungsten components produced by LPBF, implying a strong potential for manufacturing tungsten with this technology and a need for further research on this subject. This review paper presents the current state-of-the-art in LPBF of unalloyed tungsten, with a focus on the effect of process parameters on the developed structure/properties and identifies current knowledge gaps.

Mechanical properties of additively manufactured parts are sensitive to the presence of pores form during the manufacturing process. The impact of pores on the mechanical performance has been investigated extensively with respect to different parameters, such as pore volume fraction, shape, and size. However, statistical investigations focusing on the relationships between the spatial distribution of pores and process parameters; and consequently, the performance of the manufactured parts are scattered and limited. Also, these sparse investigations usually suffer from an ambiguous definition of terminologies. For instance, the required criteria to consider a point pattern as complete spatial randomness (CSR) are generally not clarified. Moreover, no numerical formalism is yet developed to show how much the observed spatial results are statistically significant. To address these shortcomings, the statistical definition of CSR in a point pattern and the procedure to quantitatively determine the deviation of a pattern from CSR were discussed. The explained statistical approach was used to investigate the effect of scanning speed parameter on the spatial distribution of gas pores in laser powder bed fusion manufactured stainless steel parts. Furthermore, and to highlight the impact of the spatial distribution of pores on mechanical properties, fatigue performances of parts with clustered and randomly distributed pores were simulated by finite element analysis. It is shown that by reducing the scanning speed, the spatial distribution of gas pores deviated more from CSR, and correspondingly fatigue performance deteriorated.

GRCop-42 is a high conductivity, high-strength dispersion strengthened copper-alloy for use in high heat flux applications such as liquid rocket engine combustion devices. This alloy is part of the family of NASA-developed GRCop, copper-chrome-niobium alloys. GRCop alloys were developed for harsh environments specific to regeneratively-cooled combustion chambers and nozzles with good oxidation resistance. Significant development was completed on the GRCop-84 and GRCop-42 alloys in the extruded wrought form demonstrating feasibility for combustion chambers. NASA has recently developed a process for additive manufacturing, specifically Powder Bed Fusion (PBF) or Selective Laser Melting (SLM), of GRCop-42 to establish parameters, characterize the material, and complete testing of components with complex internal features. This evolution of the GRCop-42 was based on the successful predecessor development work using GRCop-84 with the motivation of establishing a new copper-alloy option for use in NASA, government, and industry programs with SLM. A few advantages have been shown with the GRCop-42 that include higher conductivity and faster build speeds over the GRCop-84, and a simplified powder supply chain. Initial property development has shown that it is possible to produce high density builds with strengths equivalent to wrought GRCop-42 and a conductivity greater than GRCop-84. The GRCop-42 has completed process development and initial properties have been established. Several demonstrator combustion chambers have also been fabricated with the SLM GRCop-42 that include integral channels and closeouts. Additional test units have been fabricated and are completing substantial hot-fire testing to demonstrate performance of the material, process, and design.

In recent times, smart manufacturing systems and materials are being utilized increasingly for producing parts with high strength-to-weight ratios. Consolidation of several assembly parts into a single and lightweight component plays a vital role in enhancing the mechanical properties of systems as well as effective energy conservation in several sectors, including aviation, maritime, and automotive industries. Additive Manufacturing (AM) has enabled the manufacturing of lightweight components by depositing materials only when needed and significantly reduce required assemblies. From the different AM technologies, Selective Laser Melting (SLM) is a prominent process used to fabricate near-net-shape and good surface quality parts using metal, ceramic, and polymer materials. The potential of realizing customized complex metallic structures with applications in the biomedical, transportation, and energy industries has piqued the interest of the scientific society in SLM. Although substantial progress has been achieved in comprehending the SLM process and fabricating different materials with this process, however, the industrial application is still restricted. Limitations related to printing of multi-materials, inadequate information on the efficient process parameters for different materials, and porosities on the printed parts are some of the main hurdles buried in this method that prevent it from manufacturing functional parts. Therefore, this review article provides a comprehensive and state-of-the-art study on the SLM process regarding the types of materials used, the printing of reflective material and multi-materials, the effect of several process parameters on thermo-mechanical properties of parts, printing of lattice structure, a novel support structure technique, types of post-processing methods, and basic information on simulation software used for SLM processes. Moreover, the article describes future prospects and suggests research directions for the SLM process.

NASA has been developing and advancing regeneratively-cooled channel wall nozzle technology for liquid rocket engines to reduce cost and schedules associated with fabrication. One of the primary methods being advanced is Laser Wire Direct Closeout (LWDC). LWDC was developed to provide an additively manufactured laser deposited closeout of the coolant channels that also forms the structural jacket in-situ. This technique has been previously demonstrated through process development and hot-fire testing on a series of subscale nozzles at NASA Marshall Space Flight Center. The hot-fire test articles were fabricated using monolithic alloys to simplify the fabrication process. Ongoing research is being conducted to further expand use of this process for increased scale and bimetallic or multi-alloy options. The use of multi-alloys is desired to fully optimize the combination of materials in the radial and axial directions to reduce overall weight of the nozzle and allow for higher thermal and structural margins on the channel wall nozzle. NASA recently completed process development and hot-fire testing of a series of channel wall nozzles that incorporate a copper-alloy as the hotwall liner material and a superalloy and combination thereof for the structural jacket using the LWDC technique. The fabrication process was further advanced by using a multi-alloy axial joint using explosive bonding integrating a copper-alloy at the forward end of the nozzle hotwall and a stainless-alloy for the remaining length. A third alloy was then used for the channel closeout using the LWDC process. This paper will describe the process development using the LWDC process for channel closeout utilizing the multi-alloys, hardware design and results from hot-fire testing on subscale multi-alloy LWDC channel cooled nozzles.

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The microstructure and hardness of the melted area in a titanium grade 2 sample processed by Laser Spot Welding were compared to those processed by Selective Laser Melting. The results show that the materials obtained with the two processes have very similar characteristics. On the basis of what had been observed, it can be inferred that the Laser Spot Welding technique could be a low-cost way to verify the possibility of obtaining the desired properties with new alloys processed by Selective Laser Melting.