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An Al–10Ce-8Mn (wt%) alloy was designed and fabricated by laser powder bed fusion additive manufacturing (AM). The rapid cooling rates of the AM process produced a refined microstructure with a large fraction of reinforcing intermetallic phases. The tensile properties of the alloy were characterized in the as-fabricated state and following thermal exposure. The properties of the as-fabricated microstructure showed exceptional high-temperature performance and strength retention at elevated temperatures up to 400 °C relative to benchmark wrought Al and AM Al alloy properties. Characterization of the microstructure and thermodynamic modeling of the ternary Al–Ce–Mn system rationalized the solidification and solid-state phase transformations. Analysis of the relevant strengthening mechanisms for both the as-fabricated and thermally exposed conditions was performed.

Traditional design and qualification methodologies for parts manufactured by traditional methods are being applied to Additive Manufacturing (AM) without understanding the nuances of the machines. While mapping process variables and tracking build data is helpful, some variables such as build geometry, support structure, and part melt order have not been researched in depth. Changing these variables can result in significant variations in material properties and defect structure such that the process appears to be unreliable compared to traditional manufacturing. Therefore, this research focuses on the need to understand the effects of overlooked variables such as melt order and nested geometry on the distribution of defects and bulk material properties in Ti-6Al-4 V alloy builds manufactured using the Arcam AB ® electron beam powder bed fusion process 1 . This study collected and analyzed process log data and near infrared (NIR) images for every layer to correlate trends in porosity formation and mechanical performance. The location of pores, while naturally stochastic, is heavily influenced by the cross-sectional area as detected by NIR images and correlates with the failure sites from uniaxial testing.

The paper presents a detailed study on the microstructure evolution and cracking tendency of a novel Fe-Cr-Ni-Mo-W-B hard facing alloy deposited on a carbon steel substrate using blown powder directed energy deposition technique. CALPHAD approach was used to understand the phase equilibrium in the system and to optimize the volume fraction of the hard facing precipitates. The preliminary deposits showed extensive cracking in the hard facing deposit, which was attributed to solidification cracking. To evaluate the cracking tendency of the alloy solidification sequence of the current alloy was evaluated using a Scheil Gulliver simulation and contrasted with the solidification sequence of the commercially available alloys. The simulations showed that the alloy used in this current study showed a primary FCC mode of solidification as opposed to a primary BCC mode of solidification in the commercial alloys. Crack free deposits were then fabricated by preheating the substrate to 400 °C thereby reducing the thermal strains. The segregation of the elements and phase fractions calculated using the THERMOCALC's® TCFE8 data base were then compared with the experimentally measured phase fractions and compositions. Both the simulations and the experimental work showed that the as solidified microstructure consists of a (Cr,Fe)2B phase in a FCC matrix. However, while CALPHAD is able to capture the phase fractions of the various phases during build fabrication, the partitioning behavior of W is not accurately captured. In this work we have developed a novel alloy and demonstrate that crack free deposits can be obtained with this new alloy with a hardness close to 600 VHN (55 HRC).

Additive manufacturing of aluminum alloys is largely dominated by a near-eutectic Al-Si compositions, which are highly weldable, but have mechanical properties that are not competitive with conventional wrought Al alloys. In addition, there is a need for new Al alloys with improved high temperature properties and thermal stability for applications in the automotive and aerospace fields. In this work, we considered laser powder bed fusion additive manufacturing of two alloys in the Al–Ce–Mg system, designed as near-eutectic (Al–11Ce–7Mg) and hyper-eutectic (Al–15Ce–9Mg) compositions with respect to the binary L → Al + Al11Ce eutectic reaction. The addition of magnesium is used to promote solid solution strengthening. A custom laser scan pattern was used to reduce the formation of keyhole porosity, which was caused by excessive vaporization due to the high vapor pressure of magnesium. The microstructure and tensile mechanical properties of the alloys were characterized in the as-fabricated condition and following hot isostatic pressing. The two alloys exhibit significant variations in solidification structure morphology. These variations in non-equilibrium solidification structure were rationalized using a combination of thermodynamic and thermal modeling. Both alloys showed higher yield strength than AM Al-10Si-Mg for temperatures up to 350 °C and better strength retention at elevated temperatures than additively manufactured Scalmaloy.

Additive manufacturing promises a major transformation of the production of high economic value metallic materials, enabling innovative, geometrically complex designs with minimal material waste. The overarching challenge is to design alloys that are compatible with the unique additive processing conditions while maintaining material properties sufficient for the challenging environments encountered in energy, space, and nuclear applications. Here we describe a class of high strength, defect-resistant 3D printable superalloys containing approximately equal parts of Co and Ni along with Al, Cr, Ta and W that possess strengths in excess of 1.1 GPa in as-printed and post-processed forms and tensile ductilities of greater than 13% at room temperature. These alloys are amenable to crack-free 3D printing via electron beam melting (EBM) with preheat as well as selective laser melting (SLM) with limited preheat. Alloy design principles are described along with the structure and properties of EBM and SLM CoNi-base materials., Additive manufacturing promises a major transformation of the production of high economic value metallic materials. Here, the authors describe a new class of 3D printable superalloys that are amenable to crack-free 3D printing via electron beam melting as well as selective laser melting.

Additive Manufacturing (AM) can create novel and complex engineered material structures. Features such as controlled porosity, micro-fibers and/or nano-particles, transitions in materials and integral robust coatings can be important in developing solutions for fusion subcomponents. A realistic understanding of this capability would be particularly valuable in identifying development paths. Major concerns for using AM processes with lasers or electron beams that melt powder to make refractory parts are the power required and residual stresses arising in fabrication. A related issue is the required combination of lasers or e-beams to continue heating of deposited material (to reduce stresses) and to deposit new material at a reasonable built rate while providing adequate surface finish and resolution for meso-scale features. Some Direct Write processes that can make suitable preforms and be cured to an acceptable density may offer another approach for PFCs.

Material structure and defects created during additive manufacturing contribute to the variability in mechanical properties of these parts, limiting their use for critical applications. Three-dimensional crystallographic, structural, and chemical information in electron beam melted Inconel 718 was collected from the sub-micron to the millimeter scale using the TriBeam tomography system. The relationship of defects to the surrounding microstructure has been studied in detail. Lack of fusion defects created sharp discontinuities in the grain morphology that extended several build layers beyond the layer where the defect originated. Nitrides and carbides in the size range of 1.25 μm to 10 μm were observed throughout the volume, with pronounced nitride clustering. The similarities between solidification in additive manufacturing and traditional manufacturing processes are demonstrated by the comparable size and distribution of carbides measured for industrial-scale samples of 718. Defects created during additive manufacturing cannot be easily remedied via standard post-processing techniques, and will likely diminish the mechanical performance of additively manufactured parts.

Local microstructure control in electron beam powder bed fusion (EB-PBF) is of great interest to the additive manufacturing community to realize complex part geometry with targeted performance. The local microstructure control relies on having a detailed understanding of local melt pool physics (e.g., 3-D melt pool shape as well as spatial and temporal variations of thermal gradient (G) and solidification rate (R)). In this research, a new scan strategy referred to as ghost beam is numerically evaluated as a candidate to achieve the targeted G and R of IN718 alloy. The boundary conditions for simulations, including the speed (490 mm/s) and spatial locations of the beam within a given layer, are obtained by using series of snapshot images, recorded at 12,000 frames per second, using a high-speed camera. The heat transfer simulations were performed using TRUCHAS an open-source software deployed within a high-performance computational infrastructure. The simulation results showed that reheating at short beam on-time and time delay decreases both G and R. Local variation of R at the center of the melt pool trailing edge showed periodic temporal fluctuations. Finally, the ghost beam scan strategy was compared to other existing raster and spot scan strategies.

Ti-6Al-4V is the most studied alloy via electron beam melting (EBM) with regards to microstructure and mechanical behavior, but there have been no attempts to correlate in situ porosity detection and fatigue behavior. This is required for widespread adoption of EBM because conventional evaluation is often time-consuming and expensive. This paper presents the use of near-infrared imaging to detect and quantify porosity in Ti-6Al-4V builds fabricated from different powder feedstocks and its correlation to their fatigue response. We report that, for identical processing parameters, the variability in powder feedstock causes variations in porosity and scatter in the respective fatigue response.

Creep rupture samples were fabricated by additive manufacturing (AM) via electron beam melting (EBM) to study the effects of grain morphology (equiaxed/columnar grains) and loading direction (longitudinal/transverse) with respect to build direction on creep deformation at 923 K (650 °C) with applied stresses of 580 and 600 MPa. The observed minimum creep rates and creep rupture lives of EBM Inconel 718 after post-processing by hot isostatic pressing (HIP) were found to be comparable to wrought material. The material with equiaxed grains exhibited low creep strain (2 pct) and short creep lifetimes (800 hours), whereas longer times (approximately 4500 hours) and high creep strain (up to 23 pct) were observed for material with columnar grains. The high stress exponent (n > 14) reflected the resistance to dislocation motion by γ” particles during creep. A precipitate-free zone (PFZ) was observed around the grain boundary δ phase. Creep damage occurred as voids and cracks in the PFZ. Optimal post-processing of EBM Inconel 718 material should be explored to prevent δ phase embrittlement.

In the electron beam powder bed fusion (EB-PBF) process, a substantial number of high-gamma prime Ni-based superalloys are considered as non-printable due to a high propensity to form cracks. In this research, we focused on computational modeling framework to predict solidification-related cracking phenomena in EB-PBF processes. The cracking analysis was performed on cylindrical overhang structures where the cracks are observed only on one side of the part. Comprehensive microstructural characterization correlated the cracking tendency to low-melting point liquid-film formation along columnar grain boundaries with high misorientation angles due to partitioning of alloying elements. Uncoupled numerical thermal and mechanical models were used to rationalize the relationship between process parameters, build geometry, and cracking. The simulations showed asymmetric temperature distributions and associated asymmetric tensile thermal stresses over a cross section due to differences in section modulus and periodic changes in beam scanning directions. The results provide a potential pathway based on spatially varying beam scanning strategies to reduce the cracking tendency during additive manufacturing of complex geometries on the overhang structure in high-gamma prime nickel-based superalloys.

Innovative designs for turbines can be achieved by advances in nickel-based superalloys and manufacturing methods, including the adoption of additive manufacturing. In this regard, selective electron beam melting (SEBM) and selective laser melting (SLM) of nickel-based superalloys do provide distinct advantages. Furthermore, the direct energy deposition (DED) processes can be used for repair and reclamation of nickel alloy components. The current paper explores opportunities for innovation and qualification challenges with respect to deployment of AM as a disruptive manufacturing technology. In the first part of the paper, fundamental correlations of processing parameters to defect tendency and microstructure evolution will be explored using DED process. In the second part of the paper, opportunities for innovation in terms of site-specific control of microstructure during processing will be discussed. In the third part of the paper, challenges in qualification of AM parts for service will be discussed and potential methods to alleviate these issues through in situ process monitoring, and big data analytics are proposed.

Additive manufacturing (AM) promises to redesign traditional manufacturing by enabling the ultimate in agility for rapid component design changes in commercial products and for fabricating complex integrated parts. By significantly increasing quality and yield of metallic alloy powders, the pace for design, development, and deployment of the most promising AM approaches can be greatly accelerated, resulting in rapid commercialization of these advanced manufacturing methods. By successful completion of a critical suite of processing research tasks that are intended to greatly enhance gas atomized powder quality and the precision and efficiency of powder production, researchers can help promote continued rapid growth of AM. Other powder-based or spray-based advanced manufacturing methods could also benefit from these research outcomes, promoting the next wave of sustainable manufacturing technologies for conventional and advanced materials.

This study reports the effect of process temperature on microstructure evolution of electron beam melted Inconel 718. Samples fabricated at 915°C had fine grain boundary δ (∼200 nm) along with coar...

The use of manufacturing to generate topology optimized components shows promise for designers. However, designers who assume that additive manufacturing follows traditional manufacturing techniques may be misled due to the nuances in specific techniques. Since commercial topology optimization software tools are neither designed to consider orientation of the parts nor large variations in properties, the goal of this research is to evaluate the limitations of an existing commercial topology optimization software (i.e. Inspire®) using electron beam powder bed fusion (i.e. Arcam®) to produce optimized Ti-6Al-4V alloy components. Emerging qualification tools from Oak Ridge National Laboratory including in-situ near-infrared imaging and log file data analysis were used to rationalize the final performance of components. While the weight savings of each optimized part exceeded the initial criteria, the failure loads and locations proved instrumental in providing insight to additive manufacturing with topology optimization. This research has shown the need for a comprehensive understanding of correlations between geometry, additive manufacturing processing conditions, defect generation, and microstructure for characterization of complex components such as those designed by topology optimization.

Low alloy carbon manganese (C-Mn) steel builds were fabricated using a wire based additive manufacturing system developed at Oak Ridge National Laboratory. Specimens were fabricated in the X,Y and Z direction and detailed mechanical testing was performed. The mechanical testing results showed a significant scatter in tensile ductility and significant variation in Charpy toughness. Further detailed microstructure characterization showed significant microstructural heterogeneity in builds fabricated in each direction. The scatter in mechanical properties was then rationalized based on the microstructural observations and the underlying changes in the local heat transfer conditions. The results indicate that when fabricating parts using C-Mn low alloy steel welds the process parameters and tool path should be chosen such that the cooling rate from 800 °C to 500 °C is greater than 30 s to avoid formation of martensite austenite (MA) phases, which leads to toughness reductions.

High-temperature Al-9Cu-6Ce and Al-9Cu-6Ce-1Zr (wt%) alloys were designed for fabrication with laser powder bed fusion additive manufacturing (AM). An ultra-fine eutectic structure comprising FCC-Al and particles of a previously unidentified Al8Cu3Ce intermetallic phase was obtained with an inter-particle spacing of approximately 280 nm. The inherent hot-tearing resistance of the eutectic alloys resulted in > 99.5% relative density. A thermodynamic model suggested improved hot-tearing resistance of the present alloys relative to the benchmark AM AlSi10Mg alloy. The Al-Cu-Ce alloy exhibited superior thermal stability with approximately 75% of the as-fabricated hardness retained after 200 h exposure at 400 °C, owed to the coarsening resistance of the intermetallic particles. The Al-Cu-Ce-Zr alloy age-hardened through precipitation of nanoscale Al3Zr precipitates. The aged microstructure was stable at 350 °C with a 13% higher hardness after 200 h exposure compared to the as-fabricated condition. The combined influence of ultra-fine spacing and coarsening resistance of the intermetallic particles resulted in the higher yield strength of the Al-Cu-Ce and Al-Cu-Ce-Zr alloys compared to AM AlSi10Mg and Scalmalloy at temperatures greater than 200 °C. This work essentially demonstrates that thermally stable Al alloys with exceptional mechanical properties can be produced by additive manufacturing.

Additive manufacturing (AM) presents a new design paradigm for the manufacture of engineering materials through the layer-by-layer approach combined with welding theory. In the instance of difficult to process materials such as tungsten and other refractory metals, AM offers an opportunity for radical redesign of critical components for next-generation energy technologies including fusion. In this work, electron beam powder bed fusion (EB-PBF) is applied to process pure tungsten to study the influence of process parameters on the defect density of the material. An in-situ image analysis algorithm is applied to pure tungsten for the first time, and is used to visualize the defect structure in AM tungsten. Finally, a cracking mechanism for AM tungsten is proposed, and suggestions for suppression of cracks in pure tungsten are offered.

Spatial control of microstructure within a three-dimensional component has been a dream of materials scientists for centuries. However, limitations in traditional manufacturing processes prevent detailed control over the distribution of microstructures in a single part. Here, we demonstrate the ability to control grain structure and crystallographic texture during metal additive manufacturing for arbitrary cross-sections of a practical size, with profound implications for the design and optimization of next-generation products. The key to this advance is a new geometry agnostic scan path algorithm that manipulates the spatial distribution of solidification conditions. Utilizing a fundamental understanding of solidification dynamics and a model of the heat transfer during processing, we have designed this algorithm to manipulate the natural competition between epitaxial dendrite growth and grain nucleation. With this algorithm, we successfully controlled the grain structure of Ni-based superalloy IN718 in the shape of the Mona Lisa.

In this study, the impact of texture (columnar/equiax grain structure) and influence of material orientation on the low cycle fatigue (LCF) behavior of hot isostatic pressed (HIP) and heat-treated Inconel 718 fabricated through electron beam melting (EBM) is investigated. Material was tested both parallel and perpendicular (transverse) to the build direction. In all instances, the EBM HIP and heat-treated Inconel 718 performed similarly or exceeded the LCF life of wrought Inconel 718 plate and bar stock under fully reversed strain-controlled loading at 650 °C. Amongst the textures, the columnar grains oriented parallel to the build direction exhibited the highest life on average compared to the transverse columnar and equiax EBM material. Further, in relation to the reference wrought material the parallel columnar grain material exhibited a greater life. While a negligible life difference was observed in the equiax grained material between the two orientations, a consistently lower accumulated inelastic strain was measured for the material loaded parallel to the build direction than the transverse orientation. Failure of the parallel columnar material occurred in a transgranular manner with cracks emanating from the surface whereas the transverse columnar material failed in a intergranular manner, with crack growth occurring through repeated rupture of oxide at the crack-tip. In the case of the equiax material, an influence of material orientation was not observed on the failure mechanism with crack propagation occurring through a combination of debonded/cracked carbides and void formation along twin boundaries resulting in a mixture of intergranular and transgranular crack propagation.

In addition to design geometry, surface roughness, and solid-state phase transformation, solidification microstructure plays a crucial role in controlling the performance of additively manufactured components. Crystallographic texture, primary dendrite arm spacing (PDAS), and grain size are directly correlated to local solidification conditions. We have developed a new melt-scan strategy for inducing site specific, on-demand control of solidification microstructure. We were able to induce variations in grain size (30 μm–150 μm) and PDAS (4 μm - 10 μm) in Inconel 718 parts produced by the electron beam additive manufacturing system (Arcam ® ). A conventional raster melt-scan resulted in a grain size of about 600 μm. The observed variations in grain size with different melt-scan strategies are rationalized using a numerical thermal and solidification model which accounts for the transient curvature of the melt pool and associated thermal gradients and liquid-solid interface velocities. The refinement in grain size at high cooling rates (>10 4 K/s) is also attributed to the potential heterogeneous nucleation of grains ahead of the epitaxially growing solidification front. The variation in PDAS is rationalized using a coupled numerical-theoretical model as a function of local solidification conditions (thermal gradient and liquid-solid interface velocity) of the melt pool.

Additive manufacturing (AM) of metals is rapidly emerging as an established manufacturing process for metal components. Unlike traditional metals fabrication processes, metals fabricated via AM undergo localized thermal cycles during fabrication. As a result, AM presents the opportunity to control the liquid-solid phase transformation, i.e. material texture. However, thermal cycling presents challenges from the standpoint of solid-solid phase transformations. To be discussed are the opportunities and challenges in metals AM in the context of texture control and associated solid-solid phase transformations in Ti-6Al-4V and Inconel 718.

A high-resolution neutron tomography system and a grating-based interferometer are used to explore electron beam-melted titanium test objects. The high-resolution neutron tomography system (attenuation-based imaging) has a pixel size of 6.4 µm, appropriate for detecting voids near 25 µm over a (1.5 cm)3 volume. The neutron interferometer provides dark-field (small-angle scattering) images with a pixel size of 30 µm. Moreover, the interferometer can be tuned to a scattering length, in this case, 1.97 µm, with a field-of-view of (6 cm)3. The combination of high-resolution imaging with grating-based interferometry provides a way for nondestructive testing of defective titanium samples. A chimney-like pore structure was discovered in the attenuation and dark-field images along one face of an electron beam-melted (EBM) Ti-6Al-4V cube. Tomographic reconstructions of the titanium samples are utilized as a source for a binary volume and for skeletonization of the pores. The dark-field volume shows features with dimensions near and smaller than the interferometer auto-correlation scattering length.

Electron beam melting (EBM) is a metal powder bed fusion additive manufacturing (AM) technology that fabricates parts by selectively scanning consecutive powder layers with an electron beam. Additive manufacturing technologies are increasing in importance for aerospace and medical applications, where the demand for a fundamental understanding and predictability of static and dynamic material properties are high. Ti-6Al-4V is the most widely used and studied alloy for this technology, and is the focus of this work in its Extra Low Interstitial (ELI) variation. The layered manufacturing of metallic components by EBM creates a unique directional microstructure, and consequently, anisotropic properties. Microstructure evolution, and its influence on mechanical properties of the alloy in the as-fabricated condition, has been documented by various researchers. However, fatigue crack propagation and the effects of the directional structure have not been sufficiently studied, imposing a barrier for this technology’s potential extension to high-integrity applications. In this study, fatigue crack growth (FCG) both parallel and perpendicular to the build directions was studied for different stress ratios and crack growth stages. The interaction between the directional as-fabricated EBM microstructure and FCG was investigated and compared to that of the equiaxed β annealed microstructure obtained by annealing above the β transus temperature. The FCG threshold, ΔKth, was analytically modeled for the two relative crack propagation directions at different stress ratios, and FCG microstructural mechanisms were established for all three regions of crack propagation.

This study focuses on the development of a design methodology for alloys in AM, using a newly developed Al-Ce alloy as an initial case study. To evaluate the candidacy of this system for directed energy deposition processes, single-line laser melts were made on cast Al-12Ce plates using three different beam velocities (100, 200, and 300 mm/min). The microstructure was evaluated in the as-melted and heat treated conditions (24 h at 300 °C). An extremely fine microstructure was observed within the melt pools, evolving from eutectic at the outer solid-liquid boundaries to a primary Al FCC dendritic/cellular structure nearer the melt-pool centerline. The observed microstructures were rationalized through the construction of a microstructure selection map for the Al-Ce binary system, which will be used to enable future alloy design. Interestingly, the heat treated samples exhibited no microstructural coarsening.

Electrons injected into the build envelope during powder bed electron-beam additive manufacturing can accumulate on the irradiated particles and cause them to repel each other. Under certain conditions, these electrostatic forces can grow so large that they drive the particles out of the build envelope in a process known as “smoking”. In the present work, we investigate the causes of powder bed charging and smoking during electron-beam additive manufacturing. In the first part of the paper, we characterize the surface chemistry of a common feedstock material—gas-atomized Ti-6Al-4V powder—and find that a thick, electrically insulating oxide overlayer encapsulates the particles. Based on these experimental results, we then formulate an analytical model of powder bed charging in which each particle is approximated as a capacitor, where the particle and its substrate are the electrodes and the oxide overlayer is the dielectric. Using this model, we estimate the charge distribution in the powder bed, the electrostatic forces acting on the particles, and the conditions under which the powder bed will smoke. It is found that the electrical resistivity of the oxide overlayer strongly influences the charging behavior of the powder bed and that a high resistivity promotes charge accumulation and consequent smoking. This analysis suggests new quality control and process design measures that can help suppress smoking.

Electron beam melting (EBM) is a metal powder bed fusion additive manufacturing (AM) technology that is used to fabricate three-dimensional near-net-shaped parts directly from computer models. Ti-6Al-4V is the most widely used and studied alloy for this technology and is the focus of this work in its ELI (Extra Low Interstitial) variation. Microstructure evolution and its influence on the mechanical properties of the alloy in the as-fabricated condition have been documented by various researchers. In the present work, different heat treatments were performed based on three approaches in order to study the effects of heat treatments on the unique microstructure formed during the EBM fabrication process. In the first approach, the effect of various cooling rates after the solutionizing process was studied. In the second approach, a correlation between the variation of α lath thickness during aging and the subsequent effect on mechanical properties was established. Lastly, several combined solutionizing and aging experiments were conducted; the results will be systematically discussed in the context of structural performance and design.

Additive manufacturing (AM) technologies have long been recognized for their ability to fabricate complex geometric components directly from models conceptualized through computers, allowing for complicated designs and assemblies to be fabricated at lower costs, with shorter time to market, and improved function. Lacking behind the design complexity aspect is the ability to fully exploit AM processes for control over texture within AM components. Currently, standard heat-fill strategies utilized in AM processes result in largely columnar grain structures. Proposed in this work is a point heat source fill for the electron beam melting (EBM) process through which the texture in AM materials can be controlled. Through this point heat source strategy, the ability to form either columnar or equiaxed grain structures upon solidification through changes in the process parameters associated with the point heat source fill is demonstrated for the nickel-base superalloy, Inconel 718. Mechanically, the material is demonstrated to exhibit either anisotropic properties for the columnar-grained material fabricated through using the standard raster scan of the EBM process or isotropic properties for the equiaxed material fabricated using the point heat source fill.

A novel technique was developed to control the microstructure evolution in Alloy 718 processed using Electron Beam Melting (EBM). In situ solution treatment and aging of Alloy 718 was performed by heating the top surface of the build after build completion scanning an electron beam to act as a planar heat source during the cool down process. Results demonstrate that the measured hardness (478 ± 7 HV) of the material processed using in situ heat treatment similar to that of peak-aged Inconel 718. Large solidification grains and cracks formed, which are identified as the likely mechanism leading to failure of tensile tests of the in situ heat treatment material under loading. Despite poor tensile performance, the technique proposed was shown to successively age Alloy 718 (increase precipitate size and hardness) without removing the sample from the process chamber, which can reduce the number of process steps in producing a part. Tighter controls on processing temperature during layer melting to lower process temperature and selective heating during in situ heat treatment to reduce over-sintering are proposed as methods for improving the process.

The electron beam melting (EBM) process was used to fabricate Inconel 718. The microstructure and tensile properties were characterized in both the as-fabricated and post-processed state transverse (T-orientation) and longitudinal (L-orientation) to the build direction. Post-processing involved both a hot isostatic pressing (HIP) and solution treatment and aging (STA) to homogenize the microstructure. In the as-fabricated state, EBM Inconel 718 exhibits a spatially dependent microstructure that is a function of build height. Spanning the last few layers is a cored dendritic structure comprised of the products (carbides and Laves phase) predicted under equilibrium solidification conditions. With increasing distance from the build's top surface, the cored dendritic structure becomes increasingly homogeneous with complete dissolution of the secondary dendrite arms. Further, temporal phase kinetics are observed to lead to the dissolution of the strengthening γ ″ and precipitation of networks of fine δ needles that span the grains. Microstructurally, post-processing resulted in dissolution of the δ networks and homogeneous precipitation of γ ″ throughout the height of the build. In the as-fabricated state, the monotonic tensile behavior exhibits a height sensitivity within the T-orientation at both 20 and 650 °C. Along the L-orientation, the tensile behavior exhibits strength values comparable to the reference wrought material in the fully heat-treated state. After post-processing, the yield strength, ultimate strength, and elongation at failure for the EBM Inconel 718 were observed to have beneficially increased compared to the as-fabricated material. Further, as a result of post-processing the spatial variance of the ultimate yield strength and elongation at failure within the transverse direction decreased by 4 and 3× respectively.

Fabrication of nuclear reactor components using additive manufacturing (AM) methods is now a practical option since the AM technologies have advanced to allow for building of complex parts with high quality materials. To assess the mechanical performance of printed components in reactor-relevant conditions and to build a property database for the AM 316L stainless steel (SS), mechanical testing and characterization were performed before and after neutron irradiation. Miniature tensile specimens were irradiated at the High Flux Isotope Reactor (HFIR) to 0.2 and 2 displacements per atom (dpa) at 300 and 600°C. The AM 316L SS was tested in the as-built, stress-relieved, and solution-annealed conditions, and the wrought (WT) 316L SS in solution-annealed condition as a reference alloy. The baseline test result showed that the AM 316L SS, regardless of the post-build heat treatment, had higher strength than the WT 316L SS, but similar ductility. Post-irradiation tensile testing was conducted at RT, 300°C, and 500°C for selected irradiation conditions. Neutron irradiation induced significant changes in the mechanical behavior of the AM stainless steels, including both hardening and softening. Although the as-built 316L steel after 300°C irradiation showed necking just after yielding, the overall property changes of the as-printed alloy became less significant after 600°C irradiation. Irradiation-induced ductilization was also observed after the higher temperature irradiation. In general, the strength change was smaller in the relatively stronger as-built and stress-relieved AM SSs than in the solution-annealed AM and WT SSs. These relatively lower strength 316L SSs overall retained higher ductility in the irradiation conditions tested, but the stronger 316L SSs demonstrated a similar level of ductility after the higher temperature (600°C) irradiation. It is a positive assessment for the AM 316L materials that no embrittlement was observed within the test and irradiation conditions of the experiment.

As in welding, directed energy deposition (DED) additive manufacturing (AM) generates complex residual stresses and distortions commensurate with the complexity of the scan pattern used for deposition. To date, measuring DED distortions on complex geometries has only been achieved post process, discarding the complex thermomechanical history that leads to that final material state. In this work, surround stereo digital image correlation (DIC) is used to 3D map surfaces and strain tensors in-situ in a powder-blown laser DED system. Infrared thermography is then projected onto these surfaces to record the full thermomechanical history of printed parts. DIC presents a unique challenge to DED AM, as no part exists at the beginning of deposition, which (a) prevents application of an appropriate speckle pattern and (b) denies the user a zero strain reference frame. Solutions to these problems are proposed and their limitations explored herein. In sum, this work presents a relatively low-cost solution to monitoring and optimizing the unique temporal artifacts induced by complex scan strategies that was previously unobtainable.

This work focuses on the use of x-ray computed tomography for non-destructive characterization of additively manufactured parts. Ti-6Al-4V parts manufactured using electron beam melting were used. Within this context a comparative study between laboratory and synchrotron-based x-ray computed tomography (LXCT and SXCT) is presented showing the advantages of both techniques. Additionally, the interplay between field-of-view, resolution and sub-sample extraction is systematically presented both qualitatively and quantitatively for LXCT. Overall, it was concluded that laboratory based μXCT offered a compelling alternative to SXCT for static measurements involving defect characterization in AM parts while synchrotron-based techniques offered unmatched performance for dynamic and sub-micron studies.

The nucleation and growth of chimney pores during powder-bed electron-beam additive manufacturing is investigated using in situ infrared thermography and micro-computed tomography. The chimney pores are found to nucleate heterogeneously at dimples on the side surfaces of additively manufactured components, and to grow through a molten-film rupture process. Further, these nucleation and growth processes are found to be strongly influenced by the beam diameter. Several strategies for suppressing the formation of chimney pores are discussed in light of these results.

In additive manufacturing, microstructural control is feasible via processing parameter alteration. However, the window for parameter variation for certain materials, such as Ti-6Al-4V, is limited, and alternative methods must be employed to customize microstructures. Grain refinement and homogenization in cast titanium alloys has been demonstrated through the addition of hypoeutectic concentrations of boron. This work explores the influence of 0.00 wt.%, 0.25 wt.%, 0.50 wt.%, and 1.0 wt.% boron additions on the microstructure and bulk mechanical properties of Ti-6Al-4V samples fabricated in an Arcam A2 electron beam melting (EBM) system with commercial processing parameters for Ti-6Al-4V. Analyses of EBM fabricated Ti-6Al-4V + B indicate that the addition of 0.25–1.0 wt.% boron progressively refines the grain structure, and it improves hardness and elastic modulus. Despite a reduction in size, the β grain structure remained columnar as a result of directional heat transfer during EBM fabrication.

The fabrication of 3-D parts from CAD models by additive manufacturing (AM) is a disruptive technology that is transforming the metal manufacturing industry. The correlation between solidification microstructure and mechanical properties has been well understood in the casting and welding processes over the years. This paper focuses on extending these principles to additive manufacturing to understand the transient phenomena of repeated melting and solidification during electron beam powder melting process to achieve site-specific microstructure control within a fabricated component. In this paper, we have developed a novel melt scan strategy for electron beam melting of nickel-base superalloy (Inconel 718) and also analyzed 3-D heat transfer conditions using a parallel numerical solidification code (Truchas) developed at Los Alamos National Laboratory. The spatial and temporal variations of temperature gradient (G) and growth velocity (R) at the liquid-solid interface of the melt pool were calculated as a function of electron beam parameters. By manipulating the relative number of voxels that lie in the columnar or equiaxed region, the crystallographic texture of the components can be controlled to an extent. The analysis of the parameters provided optimum processing conditions that will result in columnar to equiaxed transition (CET) during the solidification. The results from the numerical simulations were validated by experimental processing and characterization thereby proving the potential of additive manufacturing process to achieve site-specific crystallographic texture control within a fabricated component.

Electron beam melting (EBM) is a metal powder bed fusion additive manufacturing (AM) technology that makes possible the fabrication of three-dimensional near-net-shaped parts directly from computer models. EBM technology has been continuously evolving, optimizing the properties and the microstructure of the as-fabricated alloys. Ti-6Al-4V ELI (Extra Low Interstitials) titanium alloy is the most widely used and studied alloy for this technology and is the focus of this work. Several research works have been completed to study the mechanisms of microstructure formation, evolution, and its subsequent influence on mechanical properties of the alloy. However, the relationship is not completely understood, and more systematic research work is necessary in order to attain a better understanding of these features. In this work, samples fabricated at different locations, orientations, and distances from the build platform have been characterized, studying the relationship of these variables with the resulting material intrinsic characteristics and properties (surface topography, microstructure, porosity, micro-hardness and static mechanical properties). This study has revealed that porosity is the main factor controlling mechanical properties relative to the other studied variables. Therefore, in future process development, decreasing the porosity should be considered the primary goal in order to improve mechanical properties.

Theta-shaped specimens were additively manufactured out of Inconel 718 powders using an electron beam melting technique, as a model complex load bearing structure. Two different build strategies were employed; producing two sets of specimens. Microstructural and micro-mechanical characterizations were performed using electron back-scatter, synchrotron x-ray and in-situ neutron diffraction techniques. In particular, the cross-members of the specimens were the focus of the synchrotron x-ray and in-situ neutron diffraction measurements. The build strategies employed resulted in the formation of distinct microstructures and crystallographic textures, signifying the importance of build-parameter manipulation for microstructural optimization. Large strain anisotropy of the different lattice planes was observed during in-situ loading. Texture was concluded to have a distinct effect upon both the axial and transverse strain responses of the cross-members. In particular, the (200), (220) and (420) transverse lattice strains all showed unexpected overlapping trends in both builds. This was related to the strong {200} textures along the build/loading direction, providing agreement between the experimental and calculated results.

Additive manufacturing (AM), widely known as 3D printing, is a method of manufacturing that forms parts from powder, wire or sheets in a process that proceeds layer by layer. Many techniques (using many different names) have been developed to accomplish this via melting or solid-state joining. In this review, these techniques for producing metal parts are explored, with a focus on the science of metal AM: processing defects, heat transfer, solidification, solid-state precipitation, mechanical properties and post-processing metallurgy. The various metal AM techniques are compared, with analysis of the strengths and limitations of each. Only a few alloys have been developed for commercial production, but recent efforts are presented as a path for the ongoing development of new materials for AM processes.

During the electron beam melting (EBM) process, builds occur at temperatures in excess of 800°C for nickel-base superalloys such as Inconel 718. When coupled with the temporal differences between the start and end of a build, a top-to-bottom microstructure gradient forms. Characterized in this study is a microstructure gradient and associated tensile property gradient common to all EBM Inconel 718 builds, the extent of which is dependent on build geometry and the specifics of a build’s processing history. From the characteristic microstructure elements observed in EBM Inconel 718 material, the microstructure gradient can be classified into three distinct regions. Region 1 (top of a build) is comprised of a cored dendritic structure that includes carbides and Laves phase within the interdendritic regions. Region 2 is an intermediate transition zone characterized by a diffuse dendritic structure, dissolution of the Laves phase, and precipitation of $$\delta $$ needle networks within the interdendritic regions. The bulk structure (Region 3) is comprised of a columnar grain structure lacking dendritic characteristics with $$\delta $$ networks having precipitated within the grain interiors. Mechanically, at both 20°C and 650°C, the yield strength, ultimate tensile strength, and elongation at failure exhibit the general trend of increasing with increasing build height.

A low-cost additive manufacturing technology, electron beam melting (EBM), is employed for the first time to fabricate titanium liquid/gas diffusion media with high-corrosion resistances and well-controlled multifunctional parameters, including two-phase transport and high electric/thermal conductivities. Its application in proton exchange membrane electrolyzer cells (PEMECs) has been investigated in-situ with modular galvano (MG) and galvano electrochemical impedance spectroscopy (GEIS) and characterized ex-situ with SEM and XRD. Compared with conventional woven and sintered liquid/gas diffusion layers (LGDLs), much better performance is obtained with EBM-fabricated LGDLs due to a significant reduction of ohmic losses. The EBM technology components exhibited several distinct advantages in fabricating LGDLs: well-controllable pore morphology and structure, rapid prototyping, fast manufacturing, highly customizable design, and economic. In addition, by taking advantage of additive manufacturing, it is possible to fabricate complicated three-dimensional designs of virtually any shape from a digital model into one single solid object faster, cheaper, and easier, especially for titanium components. More importantly, this development will provide LGDLs with well-controllable pore morphologies, which will be valuable to develop sophisticated models of PEMECs with optimal and repeatable performance. Furthermore, it could lead to a manufacturing solution that greatly simplifies the PEMEC/fuel cell components.

Primary solidification of ternary compounds Al20Mn2Ce and Al10Mn2Ce were analyzed through the coupling of the thermodynamic modeling and classic nucleation theory. Thermodynamic models of Al20Mn2Ce and Al10Mn2Ce were developed using the CALPHAD approach based on first-principles calculated enthalpy of formation and experimental data obtained from this work and the literature. The analysis suggested that despite the larger thermodynamic driving force for nucleation of Al10Mn2Ce, nucleation is preferred for the Al20Mn2Ce phase in the highly undercooled liquid due to its smaller interfacial energy. Therefore, manufacturing methods with rapid cooling rates will favor primary solidification of Al20Mn2Ce for Al-rich Al–Ce–Mn alloys.

Metal additive manufacturing (AM) has evolved from scientific curiosity to a potential paradigm shift in materials manufacturing owing to its ability to control microstructure and fabricate complex geometries. However, geometry, often ignored as a variable, can have a significant impact on microstructure evolution, thereby indirectly limiting the design flexibility offered by AM. Currently, the process of retaining microstructure with changing geometry relies on trial and error-based processing followed by microstructural evaluation that places significant demands on time and costs associated with experimentation and characterization. We demonstrate that lightweight thermal models can be effectively used to predict microstructural changes with geometry. These models can in turn be used as precursors to developing geometry independent scan strategies and achieve microstructure control in various alloy systems. Even if the thermal signatures predicted by the model are not accurate, scan strategies can be developed to mimic the thermal conditions during initial process development. We present a case study on one of the most studied AM alloys: Ti-6Al-4V.

Selective laser melting (SLM) was used to fabricate an AlCuMnZr alloy. The microstructural features that resulted from additive manufacturing (AM) were significantly refined compared to the corresponding cast alloy features. A combination of fine equiaxed and columnar grains along with in-situ formation of θ′ precipitates during AM leads to enhanced yield strength (up to 300 °C) in the as-fabricated AM alloy. The refinement of brittle intermetallics and a bimodal grain size distribution leads to improved tensile elongation in the AM alloy. The results illustrate the microstructural advantages that can result from additive processing over conventionally processed microstructures.

Recycling metal powders in the Additive Manufacturing (AM) process is an important consideration in affordability with reference to traditional manufacturing. Metal powder recyclability has been studied before with respect to change in chemical composition of powders, effect on mechanical properties of produced parts, effect on flowability of powders and powder morphology. However, these studies involve ex situ characterization of powders after many use cycles. In this paper, we propose a data-driven method to understand in situ behavior of recycled powder on the build platform. Our method is based on comprehensive analysis of log file data from various sensors used in the process of printing metal parts in the Arcam Electron Beam Melting (EBM) ® system. Using rake position data and rake sensor pulse data collected during Arcam builds, we found that Inconel 718 powders exhibit additional powder spreading operations with increased reuse cycles compared to Ti-6Al-4V powders. We substantiate differences found in in situ behavior of Ti-6Al-4V and Inconel 718 powders using known sintering behavior of the two powders. The novelty of this work lies in the new approach to understanding powder behavior especially spreadability using in situ log file data that is regularly collected in Arcam EBM® builds rather than physical testing of parts and powders post build. In addition to studying powder recyclability, the proposed methodology has potential to be extended generically to monitor powder behavior in AM processes.

This paper reviews currently available information on the processing and understanding of Alloy 718 fabricated through powder bed additive manufacturing processes, specifically selective laser melting, electron beam melting, and binder jet additive manufacturing. In each instance, the microstructures formed exhibit attributes unique to the process used. Through post-processing, these materials are capable of achieving property behaviors similar to that of the long utilized wrought material. While AM processes are complex, computational modeling has been successfully applied to capture the heat and mass transfer, microstructure evolution, and constitutive response of the material.

Two sets of “theta”-shaped specimens were additively manufactured with Inconel 718 powders using an electron beam melting technique with two distinct scan strategies. Light optical microscopy, mechanical testing coupled with a digital image correlation (DIC) technique, finite element modeling, and neutron diffraction with in situ loading characterizations were conducted. The cross-members of the specimens were the focus. Light optical micrographs revealed that different microstructures were formed with different scan strategies. Ex situ mechanical testing revealed each build to be stable under load until ductility was observed on the cross-members before failure. The elastic moduli were determined by forming a correlation between the elastic tensile stresses determined from FEM, and the elastic strains obtained from DIC. The lattice strains were mapped with neutron diffraction during in situ elastic loading; and a good correlation between the average axial lattice strains on the cross-member and those determined from the DIC analysis was found. The spatially resolved stresses in the elastic deformation regime are derived from the lattice strains and increased with applied load, showing a consistent distribution along the cross-member.

Powder bed-based additive manufacturing technologies offer a big advantage in terms of reusability of the powders over multiple cycles that result in cost savings. However, currently there are no standards to determine the factors that govern the powder reuse times. This work presents the results from a recyclability study conducted on Inconel 718 and Ti-6Al-4V powders. It has been found that the Inconel 718 powders are chemically stable over a large number of cycles and their reuse time is limited by physical characteristics of powders such as flowability. Ti-6Al-4V, on the other hand, finds its reuse time governed by the oxygen pick up that occurs during and in between build cycles. The detailed results have been presented.

Additive Manufacturing provides the opportunity to fabricate components of nearly limitless complexity compared to that of traditional manufacturing techniques. However, thermal gyrations imparted into the material from the passing of the heat source cause challenges in fabricating complex structures with the proper process parameters. While the thermal history of the material can be simulated, validating the simulations requires access to thermal data generated through in-situ process monitoring. While generation of in-situ thermal data seems trivial, acquiring and developing reliable calibrations for metallic materials is difficult due to the physical state of the material transitioning from powder to liquid to a solid. To be discussed is the methodology taken to integrate IR in-situ process monitoring within the electron beam melting process and the approach developed to accurately correlate a materials emissivity to temperature during the build process. Further the wealth of information contained within the thermal data will be discussed in the context of understanding of microstructural evolutions within the material during the build process, identification of material defects, and ability to determining the similarity/repeatability of builds fabricated with identical processing parameters as based only on the thermal signature of the build.

To reduce the uncertainty of build performance in metal additive manufacturing, robust process monitoring systems that can detect imperfections and improve repeatability are desired. One of the most promising methods for in situ monitoring is thermographic imaging. However, there is a challenge in using this technology due to the difference in surface emittance between the metal powder and solidified part being observed that affects the accuracy of the temperature data collected. The purpose of the present study was to develop a method for properly calibrating temperature profiles from thermographic data to account for this emittance change and to determine important characteristics of the build through additional processing. The thermographic data was analyzed to identify the transition of material from metal powder to a solid as-printed part. A corrected temperature profile was then assembled for each point using calibrations for these surface conditions. Using this data, the thermal gradient and solid-liquid interface velocity were approximated and correlated to experimentally observed microstructural variation within the part. This work shows that by using a method of process monitoring, repeatability of a build could be monitored specifically in relation to microstructure control.