During laser melting of metals, localized metal evaporation resulting in the formation of a keyhole shaped cavity can occur if processing conditions are chosen with high power density. An unstable keyhole can have deleterious effects in certain applications (e.g., laser powder bed fusion) as it increases the likelihood of producing defects such as porosity. In this work, we propose a pipeline that enables complete segmentation and extraction of various geometric features in keyholing conditions. In situ synchrotron high-speed X-ray visualization at the Advanced Photon Source provides large datasets of experimental images with a high spatio-temporal resolution across a range of laser parameters for Ti-6Al-4V. Computer vision image processing techniques were used to extract time-resolved quantitative geometric features (e.g., depth, width, front wall angle) throughout keyhole evolution which were subsequently analyzed to understand the relationship between the variation of local keyhole geometry and processing conditions. This analysis is the first to employ a data-driven approach to further our understanding of the keyholing process regime.

A brief overview of the state of texture and anisotropy is provided with the motive of inspiring younger readers to engage in this topic. The International Conference on Texture of Materials ICOTOM has been active since 1969 up through the recent 19th meeting in Japan in 2021. The series initially focused on the problem of reconstructing three-dimensional orientation distributions from diffraction data which typically provided two-dimensional projections in the form of pole figures following the pioneering work of Bunge [1] and Roe [2]. In recent years, the advent of automated orientation mapping in the scanning electron microscope [3] and 3D mapping via synchrotron x-rays [4][5] has provided vastly more detailed data on texture and, crucially, has connected texture more closely with microstructure. Alongside this has been the development of simulation tools to predict texture formation and the anisotropic properties of polycrystalline materials. This has mostly been a accomplished via a mix of mesoscale models, e.g. [6], and more detailed methods that include microstructure. The latter are predominantly based on the finite element method complemented by the spectral method [7].

Powder-entrapped gas, which can occur naturally in gas-atomized powder, can induce porosity in parts fabricated with powder-based metal additive manufacturing processes. This study utilized synchrotron-based x-ray computed tomography and an in situ high-speed imaging technique, dynamic x-ray radiography (DXR), to investigate the formation of powder-induced porosity using 17-4 PH stainless steel powders with a controlled size distribution and intentionally varied entrapped gas contents. While powder with a low entrapped gas content showed no net part porosity increase, the results showed a strong correlation between the porosity in the powder and the porosity in the builds made from powder with a high entrapped gas content relative to typical gas-atomized powder. A threshold value was developed to classify porosity induced by powder-entrapped gas based on pore morphology measured using computed tomography. Transfer and coalescence of pores during laser melting was observed directly with DXR.

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.

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.

Habit plane variants (HPVs) are traditionally used as the fundamental microstructure unit in micromechanical models of shape memory alloys. Recently, an approach using Correspondence Variants (CVs) has emerged. Previous research has shown that HPVs cannot completely explain superelastic deformation of Nitinol, especially beyond initiation of the martensitic phase transformation that gives rise to superelasticity. In this work, a statistical comparison of both modeling approaches is presented for the case of superelastic Nitinol subjected to uniaxial tension and uniaxial compression. Specifically, volume fractions of variants that contribute to deformation were calculated using the two models and then compared. The relative differences were found to be about 30\% in tension and 10\% in compression at high strains. Allowing detwinning to occur in the HPV model reduced the relative differences in CV volume fractions to about 20\% in tension. (C) 2020 The Authors. Published by Elsevier Ltd.

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.