There is a growing interest in using recycled materials and economically produced powder in additive manufacturing processes. State-of-the-art powder bed fusion additive manufacturing processes typically use spherical powder that are produced using an atomization process. However, using irregularly shaped Ti-6Al-4V powder from the Hydride-Dehydride (HDH) process is more economical because fewer processing steps are required and it can use recycled material as feedstock. In this work, the use of HDH powder in the electron beam additive manufacturing (EBAM) process is investigated. Deposition parameters for the HDH powder were developed, followed by a detailed study of as-built porosity and microstructure. The powder flow characteristics were also studied, and the as-built part porosity was compared to the parts built using spherical atomized powder. This work demonstrates the successful use of non-spherical HDH powder in the EBAM process.

Accurate detection, characterization, and prediction of defects has great potential for immediate impact in the production of fully-dense and defect free metal additive manufacturing (AM) builds. Accordingly, this paper presents Defect Structure Process Maps (DSPMs) as a means of quantifying the role of porosity as an exemplary defect structure in powder bed printed materials. Synchrotron-based micro-computed tomography (μSXCT) was used to demonstrate that metal AM defects follow predictable trends within processing parameter space for laser powder bed fusion (LPBF) materials. Ti-6Al-4 V test blocks were fabricated on an EOS M290 utilizing variations in laser power, scan velocity, and hatch spacing. In general, characteristic under-melting or lack-of-fusion defects were discovered in the low laser power, high scan velocity region of process space via μSXCT. These defects were associated with insufficient overlap between adjacent melt tracks and can be avoided through the application of a lack-of-fusion criterion using melt pool geometric modeling. Large-scale keyhole defects were also successfully mitigated for estimated melt pool morphologies associated with shallow keyhole front wall angles. Process variable selections resulting in deep keyholes, i.e., high laser power and low scan velocity, exhibit a substantial increase of spherical porosity as compared to the nominal (manufacturer recommended) processing parameters for Ti-6Al-4 V. Defects within fully-dense process space were also discovered, and are associated with gas porosity transfer to the AM test blocks during the laser-powder interaction. Overall, this work points to the fact that large-scale defects in LPBF materials can be successfully predicted and thus mitigated/minimized via appropriate selection of processing parameters.

Modelling the mechanical behavior of polycrystalline materials based on their evolving microstructure and the anisotropic properties of their constituent single crystal grains is nowadays an indispensable tool to establish physically-based relationships between processing, structure and properties of this ubiquitous type of materials. These models have found multiple applications in Material Science, Mechanics of Materials, and Earth Sciences. This article reviews the specialization to polycrystalline materials of a spectral formulation developed in the last two decades to efficiently solve the micromechanical behavior of heterogeneous materials. This review provides a consolidated account, using a unified notation, of the various numerical implementations of the spectral formulation for polycrystalline materials deforming in different constitutive regimes, each of which requires specific numerical strategies. Examples are given that illustrate these implementations for each constitutive behavior, as well as comparisons with other models, and applications to different materials, including the use of experimental data for input of the calculations and validation of the model predictions.

The microstructure of additively manufactured (AM) metals has been shown to be heterogeneous and spatially non-uniform when compared to conventionally manufactured metals. Consequently, the effective mechanical properties of AM-metal parts are expected to vary both within and among builds. Here, we present a framework for simulating process-(micro) structure-property relationships of AM metals produced via direct laser deposition (DLD). The framework predicts grain nucleation and competitive growth as a function of thermal history for a multi-pass, multi-layer DLD process. The resulting three-dimensional microstructure is automatically sub-sampled to perform virtual mechanical testing throughout the build domain using a parallelized elasto-viscoplastic fast Fourier transform code, accounting for grainboundary strengthening. The effective stress-strain response of each subsampled volume is automatically analyzed to extract effective mechanical properties, which are used to generate property maps showing the spatial variability of effective mechanical properties throughout the simulated build volume. As a demonstration, the framework is applied to different DLD stainless steel 316L build volumes having different process-induced microstructures. The multi-physics framework and property maps could provide a path toward qualification of AM-metal parts.

A high-speed synchrotron X-ray imaging technique was used to investigate the binder jetting additive manufacturing (AM) process. A commercial binder jetting printer with droplet-on-demand ink-jet print-head was used to print single lines on powder beds. The printing process was recorded in real time using high-speed X-ray imaging. The ink-jet droplets showed distinct elongated shape with spherical head, long tail, and three to five trailing satellite droplets. Significant drift was observed between the impact points of main droplet and satellite droplets. The impact of the droplet on the powder bed caused movement and ejection of the powder particles. The depth of disturbance in the powder bed from movement and ejection was defined as interaction depth, which is found to be dependent on the size, shape, and material of the powder particles. For smaller powder particles (diameter less than 10 μm), three consecutive binder droplets were observed to coalesce to form large agglomerates. The observations reported here will facilitate the understanding of underlying physics that govern the binder jetting processes, which will then help in improving the quality of parts manufactured using this AM process.