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.

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.

The present study investigated the sensitivity of material constitutive models on thermomechanical responses in laser powder bed fusion additive manufacturing of Ti-6Al-4V. Uniform scan strategies with scan lengths of 0.5, 1, and 2 mm were applied so that wide ranges of thermal histories could be generated. The Johnson-Cook (JC) and Mechanical Threshold Stress (MTS) material plasticity models were chosen to capture the influence of strain, strain rate, and temperature. The JC model is a phenomenological model which is known for its easy implementation and excellent agreement with material testing results. On the other hand, the MTS model is a more complex physics-based internal state variable plasticity model that is expected to provide more accurate estimation, particularly for cases involving changes in strain rate and temperature. Numerical results revealed that both JC and MTS models provided a similar stress evolution, however, the plastic strain evolution was more realistic using the MTS model. Moreover, it was found that the maximum strain and the strain rate in the LPBF process are high compared to typical quasi-static testing, i.e., ~ 2% and ~ 4 s−1, respectively. Accordingly, the material models should be calibrated with data obtained under similar deformation conditions. The choice of scan length also strongly affects in-plane stress anisotropy. Ultimately, we show both qualitatively and quantitatively the dependency of mechanical behavior prediction in LPBF on the choice of material models.

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.

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.

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.

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.

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.