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.

The elastoplastic transition of a metastable 13-Ti alloy, Timetal-18, is studied using in-situ high energy synchrotron X-ray diffraction microscopy (HEDM). The measured evolutions of the complete elastic strain (and stress) tensor(s), resolved shear stress, lattice rotation and rotation of the stress state of the grains are compared with the predictions of the elasto-viscoplastic Micromechanical Analysis of Stress-Strain Inhomogeneities with fast Fourier transform (MASSIF) code instantiated with an experimentally measured microstructure which matched that of the sample. The preferred glide plane of dislocations with 1/2 111 Burgers vectors of the BCC alloy was explored. It was found that the polycrystalline stress-strain response could be equally well described by any of the candidate glide planes or combinations thereof (i.e., pencil glide). However, simulations involving slip on \112\ planes yielded a marginally better description of the individual grain-level responses, as compared to the simulation involving only the \110\ planes. The small (typically <1?) crystallographic reorientations that the grains undergo during the elastoplastic transition, are insufficient to permit discrimination between candidate slip modes. The resolved shear stress (RSS) distributions showed a sharp increase in skewness around macroscopic yield and it was found that the hardening during the elastoplastic transition is primarily due to intergranular interactions. Analysis of ?hard? and ?soft? grains suggests non-Schmid effects may be present, even in these low strain rate, room temperature experiments. Finally, examination of the individual responses revealed ?strain softening? in some of the grains. Intragranular heterogeneity in the orientation and stress state are highlighted as important areas for future investigations, which may reveal answers to unresolved questions in this research.

Solidification or hot cracks are commonly observed defects in a number of metal alloys and may lead to deterioration of additively manufactured parts quality. In this study, ultra-high-speed x-ray radiography experiments enable the observation and characterization of bundles of hot-cracks that form in monobloc AA6061 substrate. The crack bundles are related to meltpool characteristics and pore formation. Crack propagation rate is also presented for the case of a crack that initiates from a pore. Two types of relevant pore formation are also described, namely keyhole porosity and crack-remelting porosity. The results of this study are expected to facilitate the validation of theoretical and numerical models of solidification cracking.

Bragg coherent diffraction imaging has the potential to provide significant insight into the structure-properties relationship for crystalline materials by imaging, with nanoscale resolution, three-dimensional strain fields within individual grains and nanoparticles. The capability of present-day synchrotrons to locate and measure a multiplicity of Bragg reflections from a single grain makes it possible to recover the full strain tensor with nanometer resolution. Recent methods for coupling reconstructions from several peaks to determine the strain tensor have been developed and applied to synthetic data, but have not been applied to experimental data. Here, using a coupled genetic reconstruction algorithm, we reconstruct an experimental data set and demonstrate improvements in the ability to resolve vector-valued displacement fields internal to the particle as compared to what is achieved with a noncoupled approach. The coupled approach developed in this work was also validated on simulated data sets. In both simulated and experimental data, reconstructions from our coupled Bragg peak algorithm show improvements over the noncoupled independent reconstruction method of 5\% in terms of accuracy and 53\% in terms of consistency.

Bragg coherent diffraction imaging has the potential to provide significant insight into the structure-properties relationship for crystalline materials by imaging, with nanoscale resolution, three-dimensional strain fields within individual grains and nanoparticles. The capability of present-day synchrotrons to locate and measure a multiplicity of Bragg reflections from a single grain makes it possible to recover the full strain tensor with nanometer resolution. Recent methods for coupling reconstructions from several peaks to determine the strain tensor have been developed and applied to synthetic data, but have not been applied to experimental data. Here, using a coupled genetic reconstruction algorithm, we reconstruct an experimental data set and demonstrate improvements in the ability to resolve vector-valued displacement fields internal to the particle as compared to what is achieved with a noncoupled approach. The coupled approach developed in this work was also validated on simulated data sets. In both simulated and experimental data, reconstructions from our coupled Bragg peak algorithm show improvements over the noncoupled independent reconstruction method of 5\% in terms of accuracy and 53\% in terms of consistency.

Abstract Parts produced via laser powder-bed fusion additive manufacturing exhibit complex microstructures that depend on processing variables and often vary widely in crystallographic texture and grain morphology. The need to understand, predict, and control these microstructural variations motivates the development of modeling tools capable of accurately predicting LPBF microstructures. Monte Carlo (MC) Potts models have been employed to successfully model the formation of grain structures in additively manufactured parts but have lacked the ability to simulate crystallographic texture. We present an extension of the MC Potts model that assigns an orientation to each grain and penalizes growth of solid into the fusion zone based on proximity of the nearest 〈100〉 crystal direction to the local temperature gradient direction. This allows for crystallographically selective growth to drive texture formation during the development of the solidification microstructure in each melt track. LPBF builds of alloy 718 with a unidirectional scan pattern provided microstructures with substantial variations in grain size, grain morphology, and texture. These distinctive albeit atypical microstructures were used to validate the simulation method, i.e. good agreement was obtained between the simulated and experimental grain shapes and textures.

The influence of thermomechanical processing (TMP) of a Ti-6Al-4V alloy on the transformation texture and intervariant boundary network were investigated by conventional EBSD mapping along with the five-parameter boundary analysis approach. The texture characteristics of Ti-6Al-4V subjected to deformation in the beta regime followed by the beta-+alpha martensitic transformation were examined using visco-plastic self-consistent simulation and forward calculation of the transformation texture. Comparison of the simulated and experimental texture characterisitcs revealed that the transformed alpha texture was dominated by the variant selection associated with substructure development in the beta parent phase and the occurrence of specific self-accommodating alpha variants in the microstructure, promoting the quadrilateral and/or V shape variant arrangement. This resulted in a progressive increase in the 63.26 degrees/[10553] intervariant boundaries with the strain increment, at the expense of 60 degrees/ [1 1 20]. Moreover, the grain boundary network for all conditions was dominated by the triple junctions (grain boundary network) terminating on 63.26 degrees/[10553] and 60 degrees/[1 1 20] intervariants. It is shown that the elastic interactions among the variants during the martensitic transformation is the dominant parameter affecting the grain boundary network, despite the presence of dislocation based variant selection.

Metal additive manufacturing is a fabrication method that forms a part by fusing layers of powder to one another. An energy source, such as a laser, is commonly used to heat the metal powder sufficiently to cause a molten pool to form, which is known as the melt pool. The melt pool can exist in the conduction or the keyhole mode where the material begins to rapidly evaporate. The interaction between the laser and the material is physically complex and difficult to predict or measure. In this article, high-speed X-ray imaging was combined with immersion ultrasound to obtain synchronized measurements of stationary laser-generated melt pools. Furthermore, two-dimensional and three-dimensional finite-element simulations were conducted to help explain the ultrasonic response in the experiments. In particular, the time-of-flight and amplitude in pulse-echo configuration were observed to have a linear relationship to the depth of the melt pool. These results are promising for the use of ultrasound to characterize the melt pool behavior and for finite-element simulations to aid in interpretation.