Porosity defects are currently a major factor that hinders the widespread adoption of laser-based metal additive manufacturing technologies. One common porosity occurs when an unstable vapor depression zone (keyhole) forms because of excess laser energy input. With simultaneous high-speed synchrotron x-ray imaging and thermal imaging, coupled with multiphysics simulations, we discovered two types of keyhole oscillation in laser powder bed fusion of Ti-6Al-4V. Amplifying this understanding with machine learning, we developed an approach for detecting the stochastic keyhole porosity generation events with submillisecond temporal resolution and near-perfect prediction rate. The highly accurate data labeling enabled by operando x-ray imaging allowed us to demonstrate a facile and practical way to adopt our approach in commercial systems. Laser fusion techniques build metal parts through a high-energy melting process that too often creates structural defects in the form of pores. Ren et al. used x-rays to track the formation of these pores while also making observations with a thermal imaging system. This setup allowed the authors to develop a high-accuracy method for detecting pore formation from that thermal signature with the help of a machine learning method. Implementing this sort of tracking of pore formation would help avoid building parts with high porosity that are more likely to fail. ?BG Thermal imaging can detect pore formation during laser powder bed fusion, helping to ensure quality control.

The metal additive manufacturing industry is actively developing instruments and strategies to enable higher productivity, optimal build quality, and controllable as-built microstructure. A beam controlling technique, laser oscillation has shown potential in all these aspects in laser welding; however, few attempts have been made to understand the underlying physics of the oscillating keyholes/melt pools which are the prerequisites for these strategies to become a useful tool for laser-based additive manufacturing processes. Here, to address this gap, we utilized a synchrotron-based X-ray operando technique to image the dynamic keyhole oscillation in Ti-6Al-4V using a miniature powder bed fusion setup. We found good agreement between the experimental observations and simulations performed with a validated Lattice Boltzmann multiphysics model. The study revealed the continuous and periodic fluctuations in the characteristic keyhole parameters that are unique to the oscillating laser beam processing and responsible for the chevron pattern formation at solidification. In particular, despite the intrinsic longer-range fluctuation, the oscillating technique displayed potential for reducing keyhole instability, mitigating porosity formation, and altering surface topology. These insights on the oscillating keyhole dynamics can be useful for the future development and application of this technique.

The metal additive manufacturing industry is actively developing instruments and strategies to enable higher productivity, optimal build quality, and controllable as-built microstructure. A beam controlling technique, laser oscillation has shown potential in all these aspects in laser welding; however, few attempts have been made to understand the underlying physics of the oscillating keyholes/melt pools which are the prerequisites for these strategies to become a useful tool for laser-based additive manufacturing processes. Here, to address this gap, we utilized a synchrotron-based X-ray operando technique to image the dynamic keyhole oscillation in Ti-6Al-4V using a miniature powder bed fusion setup. We found good agreement between the experimental observations and simulations performed with a validated Lattice Boltzmann multiphysics model. The study revealed the continuous and periodic fluctuations in the characteristic keyhole parameters that are unique to the oscillating laser beam processing and responsible for the chevron pattern formation at solidification. In particular, despite the intrinsic longer-range fluctuation, the oscillating technique displayed potential for reducing keyhole instability, mitigating porosity formation, and altering surface topology. These insights on the oscillating keyhole dynamics can be useful for the future development and application of this technique.

The presence of alpha/alpha on prior ,B/ ,B grain boundaries directly impacts the final mechanical properties of the titanium alloys. The ,B/ ,B grain boundary variant selection of titanium alloys has been assumed to be unlikely owing to the high cooling rates in laser powder bed fusion (L-PBF). However, we hypothesize that powder characteristics such as morphology (non-spherical) and particle size (50-120 mu m) could affect the initial variant selection in L-PBF processed Ti-6Al-4V alloy by locally altering the cooling rates. Despite the high cooling rate found in L-PBF, results showed the presence of ,B/,B grain boundary alpha lath growth inside two adjacent prior ,B grains. Electron backscatter diffraction micrographs confirmed the presence of ,B/ ,B grain boundary variant selection, and synchrotron X-ray high-speed imaging observation revealed the role of the shadowing effect on the locally decreased cooling rate because of keyhole depth reduction and the consequent ,B/,B grain boundary alpha lath growth. The self-accommodation mechanism was the main variant selection driving force, and the most abundant alpha/alpha boundary variant was type 4 (63.26 degrees//[ 10 5 5 3 over line ]). The dominance of Category II alpha lath clusters associated with the type 4 alpha/alpha boundary variant was validated using the phenomenological theory of martensite transformations and analytical calculations, from which the stress needed for the ,B->alpha transformation was calculated.(c) 2023 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science \& Technology.

Advances in manufacturing technologies and materials are crucial to the commercial deployment of energy technologies. We present the case of concentrating solar power (CSP) with molten salt (MS) thermal storage, where low-cost, high-efficiency heat exchangers (HXs) are needed to achieve cost competitiveness. The materials required to tolerate the extreme operating conditions in CSP systems make it difficult or infeasible to produce them using conventional manufacturing processes. Although it is technically possible to produce HXs with adequate performance using additive manufacturing, specifically laser powder bed fusion (LPBF), here we assess whether doing so is cost-effective. We describe a process-based cost model (PBCM) to estimate the cost of fabricating a MS-to-supercritical carbon dioxide HX using LPBF. The PBCM is designed to identify modifications to designs, process choices, and manufacturing innovations that have the greatest effect on manufacturing cost. Our PBCM identified HX design and LPBF process modifications that reduced projected HX cost from \$750 per kilo-Watt thermal (kW-th) (\$8/cm3) to \$350/kW-th (\$6/cm3) using currently available LPBF technology, and down to \$220/kW-th (\$4/cm3) with improvements in LPBF technology that are likely to be achieved in the near term. The PBCM also informed a redesign of the HX design that reduced projected costs to \$140?160/kW-th (\$3/cm3).

This study investigates the use of hydride-dehydride non-spherical Ti-6Al-4V powders in laser powder bed fusion process and the effects of post-heat-treatments on additively manufactured parts. As-built parts show anisotropic microstructure with α′martensite and some βphases. Post heat-treated parts exhibit α+ βphases, with characteristics dependent on the heat treatment. Heat treatment below β-transus leads to homogenized grain structures with improved corrosion resistance. Electrochemical analysis reveals a very stable corrosion rate due to faster formation of a protective passive layer aided by the fine-structured βphase. X-ray photoelectron spectroscopy examines corrosion behavior and film growth mechanism in saline water.

Hot cracking is one of the major defects that can occur in laser-based additive manufacturing. During the terminal stage of solidification, hot cracking initiates when the semi-solid matrix builds up excessive negative (tensile) pressure induced by thermal contraction. This study presents a new quantification of the trends in the above process: we estimate the volume change brought by thermal deformation through a perspective of energy conservation and combine it with the intergranular volume change induced by grain growth and liquid backflow to derive a criterion for hot-cracking initiation. Based on this, we propose two modified indexes that build on prior work, namely: (1) vertical bar dT/d root fs vertical bar 1/root 1-beta and (2) vertical bar dT/d root fs vertical bar 1/E. Here, T is temperature, f(s) is the solid fraction of the semi-solid region, beta is the shrinkage factor and (E) over bar is the material toughness near the solidus temperature. Evaluating these indexes against experimental data reveals that hot-cracking susceptibility is strongly correlated with the second index and indeed is a function of material high-temperature toughness. (C) The Minerals, Metals \& Materials Society and ASM International 2022

A major factor in determining the fatigue life of fracture-critical parts is the effect of process-induced porosity. Prediction of critical pore size in different process regimes of a laser powder bed fusion (L-PBF) processed part could provide invaluable information for process development and qualification and certification efforts. However, the amount of data required to accurately populate the pore size distribution to predict the critical pore size is still unknown. To address this gap, the present study utilizes extreme value statistics to determine the data required to characterize porosity in the L-PBF additively manufactured parts fabricated using different processing conditions. 2D cross-sectional porosity data obtained via optical microscopy was used as an example to demonstrate the statistical modeling approach. The statistical modeling described here can also be applied to other manufacturing processes and other types of data such as 3D porosity measurements, grain size, and inclusions.

Plastic deformation of B2 Nickel Titanium is usually attributed to \110\〈001〉slip and \114\〈221〉deformation twinning. The most commonly observed hot-worked texture of these alloys, is defined by a \111\〈uvw〉gamma fiber and \hkl\〈110〉partial alpha fiber. A Visco-Plastic Self Consistent (VPSC) model was used to establish relationships between microscopic slip and twin activity with the observed macroscopic hot-rolling texture. This knowledge will better aid in modeling NiTi austenite plasticity. Since the primary slip modes in NiTi do not have five independent slip systems, and deformation that can be accommodated by twinning is limited, multiple deformation modes must contribute to NiTi ductility. It is shown that \110\〈001〉slip, \100\〈001〉slip and, \114\〈221〉twin deformation modes need to be active simultaneously to explain the observed textures. The relative CRSS ratios and hardening parameters were varied to study the effect of the deformation modes on the various texture components. Textures observed below 723 K and at less than 80\% rolling reduction were simulated with deformation primarily accommodated on the \110\〈001〉slip mode and \114\〈221〉twinning mode. Textures observed at temperatures greater than 903 K and greater than 80\% rolling reductions were captured in the simulations that included all three deformation modes. Activity on the \100\〈001〉slip mode strongly correlated with the \110\〈110〉texture component observed at high temperatures.