This study evaluates a series of additively manufactured geometric feature build plates to baseline variations and process capability between multiple Laser Powder Bed Fusion (L-PBF) machine configurations. The geometric feature build plate has dimensions of 140 mm in X-orientation, 140 mm in Y-orientation, and 31.8 mm in Zheight. Geometric features incorporated include but are not limited to X-and Y-distances; varying angle walls; horizontal holes; and vertical features including round holes, concentric hollow cylinders, protruding cylinders, square channels, thin wall thicknesses, freeform surface, and slots. Select features are based on standardized ISO/ ASTM test artifact geometry. Additional features were included that are of interest for various aerospace or industrial applications and to understand practical limitations of various machine configurations. All the features are designed and located to be visually and geometrically accessible for inspection without removal from the base plate and to have high probability for successful printing among L-PBF platforms. Features from each of the 16 build plates were measured using optical and mechanical measurement methods and data compiled for a detailed evaluation of each feature. The systematic error for build accuracy across all features was 23.8 ± 5.5 µm with a 99.9% confidence interval.

The layer-wise deposition of the laser powder bed fusion (L-PBF) process offers a tailorable microstruc-ture with anisotropic properties. This research aimed to investigate the contribution of crystallographic texture on the anisotropic electrochemical/corrosion behavior of a L-BPF processed commercially pure nickel (Ni) cube along surfaces oriented parallel and perpendicular to the building direction. In order to exclusively assess the role of crystallographic texture on the corrosion behavior, the electrochemical measurements were performed on different surfaces of the L-PBF processed cube in alkaline and acidic chloride-free solutions. Orientation microscopy analysis showed that the morphology of grains along the parallel surface to the building direction was columnar, and their crystallographic orientation distribution was uniform. In contrast, the morphology of grains on the surface aligned normal to the building direction was equiaxed and highly oriented toward [110]. This bias in the orientation distribution led to the highest residual affinity for oxidation in comparison to the other planes in the FCC (Face Centered Cubic) crystal structure. In the case of 1 M NaOH solution, the hydroxide layer formed at the exposed surface did not effectively control the cation ejection from the surface. Also, surfaces oriented normal to the building direction, with lower surface atomic density, showed an elevated dissolution rate. Conversely, in the case of 1 M H 2 SO 4 solution, a more integrated and thicker oxide film formed in comparison to the surface aligned parallel to the building direction. This fact led to reduced surface dissolution. This study shows that engineering the topology and microstructure based on the desired properties can remarkably promote the performance of additively manufactured materials in extreme environments.

Selective laser melting (SLM) is an ideal method to directly fabricate products with high geometrical complexity. With low density and good corrosion resistance, aluminum alloys are widely used as important structural materials. Microstructures and mechanical properties of SLMed aluminum alloys have been recently widely studied. Corrosion behavior as a vital concern during the service of SLMed aluminum alloy parts has also drawn many attentions. Previous studies have found that SLM-processed aluminum alloys exhibit better corrosion resistance compared to the casted and wrought counterparts for both Al-Si alloys and high strength 2xxx Al alloys, which is mainly due to the unique microstructure features of SLMed Al alloys. For Al-Si alloys, with different shapes of Si networks, the different building planes show discrepant corrosion behaviors. Owing to the rougher surface with relatively larger numbers of defects, the as-printed surface is vulnerable to corrosion than the polished. Heat treatment has a negative effect on corrosion resistance due to the breakup of Si networks. The microstructure features correlated with the corrosion behaviors were also reviewed in this paper. Some suggestions on the future study of corrosion behaviors of SLMed Al alloys were put forward.

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.

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.

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.

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.

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.

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.

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.