We report on the implementation of a high frequency beam oscillation (wobbling) strategy for improving process control during laser powder bed fusion of single weld tracks on Inconel 625. Oscillation frequencies ranging from ~ 600 Hz to 7000 Hz, and different oscillation trajectories (circular, parallel or perpendicular to the direction of scanning) were explored. Highspeed imaging was used to elucidate the dynamics of the melt pool induced by the wobble beams, along with in situ absorptivity measurements to substantiate our hypothesis that the dynamic nature of wobble beams reduces absorptive losses due to laser-vapor interactions and results in improved coupling at the melt pool. Operando X-ray radiography was carried out to visualize sub-surface melt flow dynamics, correlate to spatter mechanisms and optimize the window for improving process stability. Our observations indicate that wobble beams increase the aspect ratio of the melt pool by up to 4×, depending on the oscillation frequency and energy input. Highspeed imaging and X-ray radiography reveal an optimized process parameter window for reducing spatter, improving absorptivity and creating a stable melt pool at high (several kHz) oscillation frequencies.

With a growing demand for high-quality fabrication, the interest in real-time process and defect monitoring of laser powder bed fusion (LPBF) has increased, leading manufacturers to incorporate a variety of online sensing methods including acoustic sensing, photodiode sensing, and high-speed imaging. However, real-time acquisition of high-resolution melt pool images in particular remains computationally demanding in practice due to the high variability of melt pool morphologies and the limitation of data caching and transfer, making it challenging to detect the local lack-of-fusion (LOF) defect occurrences. In this work, we propose a new acoustic and thermal information-based monitoring method that can robustly infer critical LPBF melt pool morphological features in image forms and detect spatially-dependent LOF defects within a short time period. We utilize wavelet scalogram matrices of acoustic and photodiode clip data to identify and predict highly time-resolved (within a 1.0 ms window) visual melt pool characteristics via a well-trained data-driven pipeline. With merely the acoustic and photodiode-collected thermal emission data as the input, the proposed pipeline enables data-driven inference and tracking of

Physics-based materials and process modeling has developed to the point where it is supporting the design and qualification of new components produced by many processes, including additive manufacturing (AM). Combinations of computational models that utilize various material and process parameters for AM processes, and which predict the evolution of microstructure and defects, and location-specific mechanical properties are key elements of what is termed model-based material definition (MBMD). These computational tools, when used with integrated, interdisciplinary modeling workflows, provide a holistic engineering framework for additive manufactured materials and processes. Property prediction of printed legacy materials or emerging alloys requires the understanding and control of input powder characteristics and the manufacturing processing path, starting with the feedstock and including, e.g., the local thermal history at scales from individual melt pools to full-scale component geometries. The use of model-based material definitions is critical to capture the spatial variation of processing paths and consequently spatial variations of microstructure and properties throughout the entire volume of printed components. Statistical descriptions of microstructure, whether measured or computed, enable the establishment of statistically equivalent representative volume elements (SERVEs) as discretized sub-volumes of entire component volumes that can be used to subsequently predict local mechanical properties. MBMD enables establishment of component manufacturing and property key characteristics, which can be used as component and process qualification and certification requirements. The alignment of model-guided component testing and associated key characteristics provides for a path for efficient, smart certification and qualification of new AM materials and components. This article is an analysis and demonstration of the application of MBMD to AM materials and components. The MBMD framework described herein is deemed to be an optimal approach for AM component design, manufacture, and qualification.

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 aim of this manuscript is to give a compact overview of the results that illustrate the applicability of processing-structure-property relationships in the increasingly important context of 3D printing of metals. A process qualification approach based on the physics-based understanding of defect formation in laser powder bed fusion (L-PBF) additive manufacturing (AM) is investigated for an aerospace-grade titanium alloy (Ti-6Al-4V). A physically interpretable qualification approach is critical for enabling L-PBF part certification for structure-critical applications. This approach relies on systematic experimentation, characterization, testing, and data analysis tasks including design of experiments varying power and velocity to generate varying defect populations, process window development based on defect structure, high throughput fatigue testing, and fractography, 2D porosity characterization, and use of extreme value statistics to develop a porosity metric that, in turn, could have predictive power for the variation in fatigue performance. Results from four-point bend fatigue tests demonstrate that a process window can be defined based on this key mechanical property. This relatively high throughput approach can, in turn, support a reduced set of round bar fatigue tests typically used for qualification. Overall, the proposed ecosystem for process qualification of L-PBF AM shows promise and is expected to apply to other materials and powder bed fusion AM technologies.

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).

Design of an additively manufactured molten salt (MS) to supercritical carbon dioxide (sCO2) primary heat exchanger (PHE) for solar thermal power generation is presented. The PHE is designed to handle temperatures up to 720 $\,^\circ$C on the MS side and an internal pressure of 200 bar on the sCO2 side. In the core, MS flows through a three-dimensional periodic lattice network, while sCO2 flows within pin arrays. The design includes integrated sCO2 headers located within the MS flow, allowing for a counterflow design of the PHE. The sCO2 headers are configured to enable uniform flow distribution into each sCO2 plate while withstanding an internal pressure of 200 bar and minimizing obstruction to the flow of MS around it. The structural integrity of the design is verified on additively manufactured (AM) 316 stainless steel sub-scale specimens. An experimentally validated, correlation-based sectional PHE core thermofluidic model is developed to study the impact of flow and geometrical parameters on the PHE performance, with varied parameters including the mass flowrate, surface roughness, and PHE dimensions. A process-based cost model is used to determine the impact of parameter variation on build cost. The model results show that a heat exchanger with a power density of 18.6 MW/m3 (including sCO2 header volume) and effectiveness of 0.88 can be achieved at a heat capacity rate ratio of 0.8. The impact of design and AM machine parameters on the cost of the PHE are assessed.

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.

Hot cracking as the major concern in the manufacturing process of metal alloys is detrimental to part performance and can lead to catastrophic failure. However, current research in this field is restricted to the scarcity of the relevant hot cracking susceptibility data. Here, using the DXR technique provided at 32-ID-B beamline of Advanced Photon Source (APS) at Argonne National Laboratory, we characterized the hot cracking formation in Laser Powder Bed Fusion (L-PBF) process for ten commercial alloys (Al7075, Al6061, Al2024, Al5052, Haynes 230, Haynes 160, Haynes X, Haynes 120, Haynes 214, and Haynes 718). The extracted DXR images captured the post-solidification hot cracking distribution and allow the quantification of the hot cracking susceptibility of those alloys. We further exploited this in our recent effort on hot cracking susceptibility prediction $[$1$]$ and established a hot cracking susceptibility dataset posted on Mendeley Data for the purpose of facilitating the relevant research in this field.