High-speed Synchrotron X-ray Imaging Research Articles

High-speed synchrotron X-ray imaging of melt pool dynamics during ultrasonic melt processing of Al6061

Ultrasonic processing of solidifying metals in additive manufacturing can provide grain refinement and advantageous mechanical properties. However, the specific physical mechanisms of microstructural refinement relevant to laser-based additive manufacturing have not been directly observed because of sub-millimeter length scales and rapid solidification rates associated with melt pools. Here, high-speed synchrotron X-ray imaging is used to observe the effect of ultrasonic vibration directly on melt pool dynamics and solidification of Al6061 alloy. The high temporal and spatial resolution enabled direct observation of cavitation effects driven by a 20.2 kHz ultrasonic source. We utilized multiphysics simulations to validate the postulated connection between ultrasonic treatment and solidification. The X-ray results show a decrease in melt pool and keyhole depth fluctuations during melting and promotion of pore migration toward the melt pool surface with applied sonication. Additionally, the simulation results reveal increased localized melt pool flow velocity, cooling rates, and thermal gradients with applied sonication. This work shows how ultrasonic treatment can impact melt pools and its potential for improving part quality.

A counter-gravity casting system for in situ high-speed synchrotron x-ray imaging characterization.

Counter-gravity casting (CGC) aims to eliminate turbulent melt flow and defect formation during filling and subsequent solidification by pushing high-temperature melt into the mold cavity against gravity with regulated pressure. However, limited by the opaqueness of molten metals and the complexity of the CGC apparatus, it is extremely difficult to directly quantify the high-velocity mold filling and pressurized solidification in real-time. Here, we report the design and characterization of a CGC system capable of in situ monitoring of mold filling and subsequent solidification processes in the synchrotron beamlines by deploying a high-energy, high-speed synchrotron x-ray imaging technique. The high-velocity melt flow and dendrite growth during pressurized solidification have been quantified for systematical process parameter analysis by investigating time-resolved x-ray images of an exemplary Al-Cu alloy. The high-speed imaging results demonstrate that the in situ CGC system provides a useful way to better understand the fundamentals of mold filling, pressurized solidification, and experimental inputs for high-fidelity modeling in scientific and industrial applications.

Dynamics of pore formation and evolution during multi-layer directed energy deposition additive manufacturing via in-situ synchrotron X-ray imaging: A case study on high-entropy Cantor alloy

Blown-powder directed energy deposition (DED) additive manufacturing is impeded for novel alloys processing by perceivable and detrimental porosity. During multi-layer depositions, however, mechanisms of pore formation and evolution remain elusive for developing pore mitigation strategies. Here, conduction-mode multi-layer DED process of an exemplary high-entropy Cantor alloy have been investigated in-situ by high-energy high-speed synchrotron X-ray imaging. Three new pore formation mechanisms are unveiled when depositing first layer and successive layers: gas pore induced by high-velocity powder injection into melt pool, pore generated from swirl shear of turbulent melt flow, and pore trapped by surface wave. Three pore formation mechanisms are reconfirmed: pore inheritance from feedstock powder, pore generation when laser remelting defect-sensitive locations of existing pore from previous layer or unmelted powder attached on the melt pool surface, and pore formation as cooling of melt pool. A unique mechanism for pore elimination is proposed: a counter-Marangoni melt flow is experimentally found in the stable melt pool and contributes to the prolonged pore lifetime at tens of milliseconds scale; pores are prone to coalesce into larger sizes in laser interaction zone and the adjacent location with circulation zone; coalesced larger pores driven by combined effect of Marangoni and buoyant forces easily get eliminated from melt pool. The results of pore formation and evolution dynamics revealed in Cantor alloy provide quantified experimental data for high-fidelity computational modeling and in-depth insights of porosity control for high-entropy alloy printing down to melt pool scale.

Temporal and spatial determination of solidification rate during pulsed laser beam welding of hot-crack susceptible aluminum alloys by means of high-speed synchrotron X-ray imaging

Pulsed laser beam welding is primarily used to join thin-walled components. The use of 6xxx group aluminum alloys is characterized by good mechanical properties but these alloys are prone to hot cracking during solidification, i.e., requirements regarding strength and tightness, as increasingly important for electromobility related applications, cannot be fulfilled. The solidification rate has been identified as dominant factor in pulsed conduction welding which can be adjusted by the pulse shape, i.e., by varying the beam power over time for a single pulse.Pulse shapes with different, linear ramp-down slopes were studied to describe the interaction between beam power and resulting solidification rate for spot welds. Based on rotationally symmetric conditions of the spot welds, the solidification rate can be measured in radial and vertical directions. The welding process of EN AW 6082 alloy was examined by in situ high-speed synchrotron X-ray imaging at the European Synchrotron Radiation Facility (ESRF) for this reason. Frame rates up to 120,000 Hz and subsequent image analysis allowed in-depth analysis of the solidification processes, their dependence on different spatial directions, and the resulting effects on hot crack formation.

Correlative spatter and vapour depression dynamics during laser powder bed fusion of an Al-Fe-Zr alloy

Spatter during laser powder bed fusion (LPBF) can induce surface defects, impacting the fatigue performance of the fabricated components. Here, we reveal and explain the links between vapour depression shape and spatter dynamics during LPBF of an Al-Fe-Zr aluminium alloy using high-speed synchrotron x-ray imaging. We quantify the number, trajectory angle, velocity, and kinetic energy of the spatter as a function of vapour depression zone/keyhole morphology under industry-relevant processing conditions. The depression zone/keyhole morphology was found to influence the spatter ejection angle in keyhole versus conduction melting modes: (i) the vapour-pressure driven plume in conduction mode with a quasi-semi-circular depression zone leads to backward spatter whereas; and (ii) the keyhole rear wall redirects the gas/vapour flow to cause vertical spatter ejection and rear rim droplet spatter. Increasing the opening of the keyhole or vapour depression zone can reduce entrainment of solid spatter. We discover a spatter-induced cavity mechanism in which small spatter particles are accelerated towards the powder bed after laser-spatter interaction, inducing powder denudation and cavities on the printed surface. By quantifying these laser-spatter interactions, we suggest a printing strategy for minimising defects and improving the surface quality of LPBF parts.

High-speed synchrotron x-ray imaging and multi-physics modeling of molten pool and gas dynamics in laser additive manufacturing of a medium-entropy alloy

High-speed synchrotron x-ray imaging and multi-physics modeling of molten pool and gas dynamics in laser additive manufacturing of a medium-entropy alloy

Insight into keyhole and melt pool dynamics in laser welding of zinc-coated steels by means of high-speed synchrotron X-ray imaging

Laser welding zinc-coated steels is of major importance in automotive engineering and other industries to ensure cost-effective corrosion resistance of assemblies without requiring post-weld rework. However, the low evaporation temperature of zinc is causing welding defects, in particular melt ejections and spatter. In this work, in-situ experiments of laser beam welding were performed using high-speed synchrotron X-ray imaging (up to 40,000 images/second) to determine the dynamics in the keyhole and in the weld pool and to provide detailed explanations for the formation of weld defects. A simplified sample geometry made it possible to weld zinc-coated steel sheets (DX51D Z275) in a lap joint (sheet thickness 1.25 mm each) with a fiber laser (COHERENT HighLight FL-ARM 8000) to describe fundamental phenomena. The high spatial and temporal resolution of the radiography allowed describing the acting mechanisms and their effect on keyhole and melt pool.

High-speed synchrotron X-ray imaging of micro-hole formation during the ablation of grooves by ultrashort laser pulses

The formation of slits with very high aspect ratios is critical for the production of additively manufactured parts for advanced electric drives. In this work, high-speed synchrotron X-ray imaging is utilized to observe slit/groove formation in stainless steel using ultrashort laser pulses. The results reveal that, under the pulse conditions employed here, a narrow, deep slit or groove can be created by the eventual consolidation of micro-holes which are generated in the early stages of the laser-material interaction. The success or otherwise of this micro-hole consolidation has a complex relationship to the process parameters.

Describing the effect of local gas flow on keyhole and melt flow dynamics utilizing high-speed synchrotron X-ray imaging and numerical simulation

Laser beam welding with partial gas shielding using local gas flows has been shown to be very effective in reducing spatter, especially when welding high-alloy steels at high processing speeds (≥ 8 m/min). This paper examines the gas flow induced mechanical effect on keyhole geometry and correlating melt flow by means of a computational fluid dynamics analysis. Therefore, keyhole mode laser beam welding of AISI304 was modeled by using the volume of fluid method in FLOW-3D. The mechanical effect of the partial shielding was implemented by considering the gas flow induced dynamic pressure. By varying flow rates, a widening, and a reduction in fluctuation amplitude of the keyhole rear wall was detected. Furthermore, the melt flow dynamics were characterized by less flow velocity and melt movement. The results were validated by a visual comparison with high-speed synchrotron X-ray imaging, showing a high degree of agreement in modeling accuracy of the keyhole dynamics.

Effect of partial and global shielding on surface-driven phenomena in keyhole mode laser beam welding

Partial shielding by means of local gas supply has proven to be very effective in reducing spatter. Besides the effect of gas-induced dynamic pressure, the shielding of oxygen is also highly relevant for melt pool dynamics and spatter formation due to the growth of oxides and the influence on surface tension. Therefore, this paper addresses the effect of local supplied argon on oxide growth and seam topography during keyhole mode laser beam welding of high-alloy steel AISI 304. To determine the shielding quality, the results are compared to laser beam welding in a global argon atmosphere. The topography of the upper weld seams was analyzed by scanning electron microscopy (SEM). An X-ray microanalysis (EDX) in line scan modus was performed to determine and to locate the elements which are covering the specimen surface. The chemical state of the found elements was quantified by X-ray photoelectron spectroscopy (XPS). In a last step, high-speed synchrotron X-ray imaging was performed to separate the effect of the gas-induced pressure and the gas-induced shielding on keyhole geometry. The results show that a local supply of argon contributes to a significant difference in oxide growth, affecting melt pool convection and weld seam geometry. It was further shown that the effect of gas flows at low flow rates is primarily because of oxygen shielding, as no significant difference in keyhole geometry was found by high-speed synchrotron X-ray imaging.

Understanding the formation of “false friends” (hidden lack of fusion defects) in laser beam welding by means of high-speed synchrotron X-ray imaging

This paper provides a fundamental understanding of “false friend” formation, i.e., hidden defects associated with lack of fusion, using an experimental setup that allowed an insight into the processing zone based on high-speed synchrotron X-ray imaging. The setup enabled the welding of a lap joint of AISI 304 high-alloy steel sheets (X5CrNi18-10/1.4301), with the ability to adjust different gap heights between top and bottom sheet (up to 0.20 mm) and to acquire high-speed X-ray images at 100 kHz simultaneously with the welding process. On this basis, a time-resolved description of the “false friend” formation can be provided by visualizing the interaction between keyhole and melt pool during laser welding and solidification processes within the gap area. The bridgeability of the gap was limited due to the gap height and insufficient melt supply leading to the solidification of the bridge. The distance between the solidified bridge and the keyhole increased with time, while the keyhole and melt pool dynamics initiated the formation of new melt bridges whose stability was defined by melt flow conditions, surface tension, and gap heights. The alternating formation and solidification of melt bridges resulted in entrapped areas of lacking fusion within the weld, i.e., “false friends.” Finally, based on the results of this study, a model concept is presented that concludes the main mechanisms of “false friend” formation.

Localized keyhole pore prediction during laser powder bed fusion via multimodal process monitoring and X-ray radiography

Systematic fault detection and control during laser powder bed fusion (L-PBF) has been a long-standing objective for system manufacturers and researchers in the additive manufacturing (AM) industry. This manuscript investigates a data fusion approach for detection of keyhole porosity formation during laser irradiation of Ti-6Al-4V substrates by concurrent recording of thermally induced optical emission measured using both off-axis and coaxial photodiode sensors, and acoustic emission. Subsurface defect formation was monitored via high-speed synchrotron X-ray imaging at 20,000 frames per second, enabling temporal registration of keyhole pore formation events to the monitoring signals at a resolution of 50 µs. We developed data fusion machine learning (ML) models for localized prediction of keyhole pore formation at various time scales ranging from 0.5 ms to 2 ms. The signal segments were featurized using two independent approaches: (1) power spectral density (PSD) and (2) highly comparative time series analysis (HCTSA) framework. The extracted features from different sensor modalities were fused together to construct a multimodal feature space and sequential feature selection was used to determine the most informative features for training the ML models. The predictive performance was evaluated for three classifying algorithms: Support Vector Machine (SVM), K-Nearest Neighbor (KNN), and Gaussian Naive Bayes (GNB). As a result, pore formation events were predicted with up to 0.95 F1-score, 1.0 recall and 0.94 accuracy. The most heavily weighted features indicate that model performance is chiefly governed by the acoustic monitoring signal, with a secondary contribution from the optical emission sensors.

In situ monitoring the effects of Ti6Al4V powder oxidation during laser powder bed fusion additive manufacturing

Making laser powder bed fusion (L-PBF) additive manufacturing process sustainable requires effective powder recycling. Recycling of Ti6Al4V powder in L-PBF can lead to powder oxidation, however, such impact on laser-matter interactions, process, and defect dynamics during L-PBF are not well understood. This study reveals and quantifies the effects of processing Ti6Al4V powders with low (0.12 wt%) and high (0.40 wt%) oxygen content during multilayer thin-wall L-PBF using in situ high speed synchrotron X-ray imaging. Our results reveal that high oxygen content Ti6Al4V powder can reduce melt ejections, surface roughness, and defect population in the built parts. With increasing oxygen content in the part, there is an increase in microhardness due to solid solution strengthening and no significant change in the microstructure is evident.

Machine learning-aided real-time detection of keyhole pore generation in laser powder bed fusion.

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.

Investigation of pore formation mechanisms induced by spherical-powder delivery in directed energy deposition using in situ high-speed X-ray imaging

• Pore formation mechanisms unique to the spherical-powder delivery in laser DED AM were revealed. • The pore formation dynamics were directly observed by high-speed synchrotron X-ray imaging. • A powder delivered to a front laser-beam region or melt pool interacted with melt flow and caused pores. • The delivered particle would change the pore size on its trajectory. • A space induced by the delivered spherical particle led to pore formation. In the laser-based directed energy deposition (DED) process, blown powder additive manufacturing (AM), either spherical or irregular metal particles can be used as the feedstock material to be delivered into a melt pool for fabrication. The delivered particles interact with the melt flow in different regions of melt pool, and the interactions between the liquid melt pool and the spherical particles are different from those between the melt pool and irregular particles. The objective of this investigation is to reveal the mechanisms and dynamics of pore formation unique to spherical-powder delivery in the laser DED AM process. In situ high-speed and high-resolution X-ray imaging showed that delivered spherical particles could induce pore formation mechanisms through the interactions between the melt pool and the particle front surface, the particle side surface, or the particle back surface. These results indicate that spherical particles induce different pore formation mechanisms from irregular particles. This fundamental understanding will benefit further investigations into reducing porosity and improving DED-fabricated part quality.

Vorticity-induced flow-focusing leads to bubble entrainment in an inkjet printhead: Synchrotron x-ray and volume-of-fluid visualizations

The baroclinic torque, generated at the gas-liquid interface due to the misalignment of density and pressure gradients, results in a flow-focusing effect that drives bubble pinch-off from the meniscus. High-speed synchrotron X-ray imaging was used to visualize the entire gas-liquid interface. Volume-of-Fluid numerical simulations were employed to unravel the intricate mechanism.

Deep learning-based monitoring of laser powder bed fusion process on variable time-scales using heterogeneous sensing and operando X-ray radiography guidance

Harnessing the full potential of the metal-based Laser Powder Bed Fusion process (LPBF) relies heavily on how effectively the overall reliability and stability of the manufactured part can be ensured. To this aim, the recent advances in sensorization and processing of the associated signals using Machine Learning (ML) techniques have made in situ monitoring a viable alternative to post-mortem techniques such as X-ray tomography or ultrasounds for the assessment of parts. Indeed, the primary advantage of in situ monitoring over post-mortem analysis is that the process can be stopped in case of discrepancies, saving resources. Additionally, mitigations to repair the discrepancies can also be performed. However, the in situ monitoring strategies based on classifying processing regimes reported in the literature so far operate on signals of fixed length in time, constraining the generalization of the trained ML model by not allowing monitoring processes with heterogeneous laser scanning strategies. As a part of this work, we try to bridge this gap by developing a hybrid Deep Learning (DL) model by combining Convolutional Neural Networks (CNNs) with Long-Short Term Memory (LSTM) that can operate over variable time-scales. The proposed hybrid DL model was trained on signals obtained from a heterogeneous time-synced sensing system consisting of four sensors, namely back reflection (BR), Visible, Infra-Red (IR), and structure-borne Acoustic Emission (AE). The signals captured different phenomena related to the LPBF process zone and were used to classify three regimes: Lack of Fusion (LoF), conduction mode and Keyhole. Specifically, these three regimes were induced by printing cubes out of austenitic Stainless steel (316 L) on a mini-LPBF device with operando high-speed synchrotron X-ray imaging and signal acquisition with the developed heterogeneous sensing system. The operando X-ray imaging analysis ensured that the regimes correlated with the defined process parameters. During the validation procedure of the trained hybrid DL model, the model predicted three regimes with an accuracy of about 98% across various time scales, ranging from 0.5 ms to 4 ms. In addition to tracking the model performance, a sensitivity analysis of the trained hybrid model was conducted, which showed that the BR and AE sensors carried more relevant information to guide the decision-making process than the other two sensors used in this work.

Prevention, Mitigation and Correlative Acoustic Spectroscopy of Lithium-Ion Battery Thermal Runaway

Scrutiny of lithium-ion battery failure has seen heavy regulations and restrictions with well-reported headlines of catastrophic thermal runaway of commercial electronic devices and electric vehicles. Thermal runaway occurs at a critical temperature when the rate of heat generation exceeds that of dissipation, highly exothermic decomposition reactions occur and temperatures in excess of 600 ᵒC can be measured 1. These concerns continue to grow with applications of greater energy density and capacity, such as the proliferation of electric vehicles.Isolation of electronically conductive components within the electrode assembly to prevent short circuit has shown promise to prevent the onset of thermal runaway within lithium-ion cells. Here 2, the authors present a metal-coated polymer current collector (PCC) and thermally stable separator (TSS) to prevent and mitigate thermal runaway. 2.10 Ah capacity 18650-geometry cells were subject to thermal, internal short circuit and nail penetration abuse techniques with these materials and their commercial counterparts for individual comparison of the PCCs and TSS. Reductions in cell mass were observed on the order of 5%, due to the majority of the PCC consisting of the polymer substrate rather than pure aluminium and copper in commercial current collectors. High speed synchrotron facilities provided insights into the function of the polymer current collectors with post- mortem X-ray computed tomography. During mitigation testing, cells were taken to failure with thermal and with an internal short circuit device (ISCD) within a Fraction Thermal Runaway Calorimeter (FTRC) to measure the reduction in calorific output. This highlighted the difficulty that exists with detecting gas-induced and internal structural degradation within lithium-ion batteries, especially outside specialized diagnostic laboratories to characterize these rapid changes with elapse within seconds. Attenuation and wave velocity determined by the propagative properties of ultrasound through different phases, prominently highlights structural, temperature and formation of different phases which occur during thermal runaway. Such analysis provided by acoustic spectroscopy are characterised by variations in the attenuation and signal peak shifts. Further information which can be derived from spatial mapping using acoustic spectroscopy 3,4 include: state of charge, state of health, electrochemical performance.Cells made with the aluminium-coated PCC demonstrated 100% success in prevention of thermal runaway during nail penetration, retaining a voltage above 4.00 V. Cells manufactured with the PCC, both PCC and TSS, exhibited a reduction in total energy output on average of 19.4% and 41.3% respectively during thermal abuse. ISCD triggered thermal runaways also observed reductions of 24.6% and 30.3% with these respective cell material configurations. Prevention of thermal runaway is a transformative advancement in improving lithium-ion battery safety, especially in the cylindrical 18650-format cell. Additionally, significant reductions in the calorific output during failure were measured. These safety innovations provides efficacy of these materials independent of cell chemistry (provided they are stable in the operating environment) while also increasing the energy density and be assimilated into existing manufacturing technology.Furthermore, the authors demonstrate correlative high-speed acoustic spectroscopy during routine cycling of a commercial 210 mAh lithium-ion pouch cell and during thermal abuse 5. Mechanical deformation such as gas-induced delaminations, which have been linked to degradation in electrochemical performance, capacity retention, cell state of health and impending thermal runaway, were detected. This degradation was observed with characteristic acoustic spectroscopy signal peaks changes during cycling due to cell manufacturing defects and during induced thermal abuse, were corroborated with high speed synchrotron X-ray imaging. This analysis can be observed immediately after application of the acoustic probe which emphasises the rapid diagnostics insights provided by acoustic spectroscopy as a robust field deployable technique. Thus, integration into battery management systems, second-life evaluation/recycling or as in-line cell metrology during manufacturing would provide real time direct insight into these metrics. Finegan DP, Darcy E, Keyser M, et al. Identifying the Cause of Rupture of Li-Ion Batteries during Thermal Runaway. Adv Sci. 2018. doi:10.1002/advs.201700369Pham MTM, Darst JJ, Walker WQ, et al. Prevention of lithium-ion battery thermal runaway using polymer-substrate current collectors. Cell Reports Phys Sci. 2021. doi:10.1016/j.xcrp.2021.100360Robinson JB, Pham M, Kok MDR, Heenan TMM, Brett DJL, Shearing PR. Examining the Cycling Behaviour of Li-Ion Batteries Using Ultrasonic Time-of-Flight Measurements. J Power Sources. 2019;444.Robinson JB, Maier M, Compton T, Alster G, Brett DJL, Shearing PR. Spatially resolved ultrasound diagnostics of Li-ion battery electrodes. Phys Chem Chem Phys. 2018. doi:10.1039/c8cp07098aPham MTM, Darst JJ, Finegan DP, et al. Correlative acoustic time-of-flight spectroscopy and X-ray imaging to investigate gas-induced delamination in lithium-ion pouch cells during thermal runaway. J Power Sources. 2020;470:228039. doi:10.1016/j.jpowsour.2020.228039

<italic>Operando</italic> monitoring microstructures and defects in laser fusion additive manufacturing of metals

<p indent="0mm">Laser fusion additive manufacturing of metals has been growing tremendously over the past decade. Despite its unrivaled capability to directly fabricate complex geometries, the accuracy, repeatability, and reliability of the build remain a major concern. From the micro perspective, it is the unfavorable microstructures and defects/anomalies that deteriorate the service performances such as fatigue life and corrosion resistance. The microstructures here involve not only melt pool and keyhole but also grain morphology and size, crystal orientation, interface structure, composition homogeneity, phase stability, etc. The defects/anomalies mainly include porosity, balling, lack of fusion, spattering, element loss, oxidation, micro residual stress and deformation, microcracking, and rough surface. Accurate detection and characterization of these features are critical for their understanding and control. Traditionally, we use optical microscope, electron microscope, X-ray tomography and diffraction, and so forth, to perform postmortem analysis and track the thermal traces the local material has experienced. However, such information and evidence are typically static and limited. This poses difficulties in recreating and interpreting the microstructural formation. In this review, we summarize recent progress on<italic> in situ</italic>/<italic>operando</italic> monitoring the laser fusion additive manufacturing of metals. It mainly covers high-speed visible light and thermal imaging, laboratory X-ray imaging, ultrasonic measurement, and high-speed synchrotron X-ray imaging and diffraction. Beyond traditional postmortem characterizations, the <italic>in situ</italic>/<italic>operando</italic> techniques have demonstrated the capability to capture the transient and dynamic formation and evolution processes of various microstructures and defects. They are revolutionizing the research mode in the field of metal additive manufacturing, from trial and error to mechanism oriented. In the former, we continue to optimize the objective function (e.g., grain morphology and size) until it arrives at a satisfactory solution, largely based on limited experience and knowledge and costly iterations. In the latter, we quest for the ultimate solution to manufacturing defect-free and perfect structures through direct exploration of the physical origins of the aforementioned features. Among the various techniques, we address high-speed synchrotron X-ray imaging and diffraction. It is an emerging approach in this field and is more suitable to monitor the internal microstructural evolution of metals, e.g., melt pool and keyhole dynamics, pore formation, cracking, rapid solidification, and phase transformation, because of the micrometer spatial resolution, sub-nanosecond temporal resolution, megahertz frame rate, and millimeter penetration capacity. Nevertheless, other techniques like high-speed visible light and thermal imaging have irreplaceable advantages and application scenarios. They can provide complementary information (e.g., thermal distribution on the melt pool surface, vapor plume dynamics, and three-dimensional spatter morphologies) for comprehensive understanding of the fundamental mechanisms in laser melting. Looking ahead to the next decade, the integration of multiple <italic>in situ</italic>/<italic>operando</italic> techniques and the translation among the resultant multi-dimensional signals, as well as big data computing and high precision modeling, will bridge understanding of mechanisms on the microscale and tailoring of properties on the macroscale. This will promote sustainable growth of the additive manufacturing industry.

Direct observation of crack formation mechanisms with operando Laser Powder Bed Fusion X-ray imaging

Laser powder bed fusion ( L -PBF) is a versatile additive manufacturing process that can print geometrically complex metal parts for a variety of applications. However, poor control of defect formation during processing hampers its widespread industrial adoption. Many materials suffer from a high crack susceptibility during L -PBF, which results in degraded mechanical properties, and is an obstacle to the certification of critical parts. In order to unveil the mechanisms of crack formation in a prone-to-cracking nickel-based superalloy, we employ high-speed synchrotron X-ray imaging in combination with a miniaturized L -PBF set-up that reproduces real processing conditions. This unique set-up provides operando imaging of crack formation during L -PBF. Complementary post-mortem inspection of crack morphology and thermal simulations supported by operando X-ray diffraction-based measurements of the temperature evolution allow to identify the cracking mechanism and to differentiate solidification cracking from liquation.