New technologies such as artificial intelligence, cloud computing, digital manufacturing, and smart sensing and communication build the foundation for the Fourth Industrial Revolution (or Industry 4.0). The global production and supply chain are taking a fundamental shift, as the Internet of things, self-monitoring, and fully automation enable the machine to detect and analyze issues and make decisions by itself without human intervention. Although Industry 4.0 is not about a single technology, additive manufacturing (AM, also known as 3D printing) is undoubtedly a key component that fits in this new paradigm nicely because of its digital nature. In fact, AM is a suite of manufacturing technologies that build parts in an automated, layer-by-layer fashion, based on sliced computer aided design (CAD) files. The recent decade has seen a major boost in technical innovation, industry adoption, and public attention, although some major AM technologies were invented about 40 years ago. Accompanying the increasing media attention to AM are the dynamic business landscape and vibrant research and development (R&D) activities across academia, government labs, and industry.
AM has found many unique applications in various industries, particularly for producing highly customized parts and those with extreme geometric complexity for weight management and/or performance enhancement. Some companies are now using AM to manufacture end-use products, while many others use it to build tools on-site and on demand. Despite the popularity and promise, in those sectors that have strict product qualification and certification requirements, the adoption of AM techniques has been slow. Indeed, many technical barriers have to be overcome before AM can reach its full potential as a group of transformative technologies. This is particularly true for those fusion-based metal AM processes, in which a metal feedstock in powder or wire form is melted by a scanning laser or electron beam and the molten metal then solidifies rapidly as the energy source moves away. Extreme thermal conditions are involved in these AM processes, which trigger complex energy and mass flow and multi-phase interactions and promote the development of far-from-equilibrium microstructures and phases. Meanwhile, the intricate interplay between many transient phenomena results in structure defects and build anomalies. Therefore, product quality control remains a critical issue for AM.
Industry 4.0 features a blurred boundary between cyber and physical worlds, enabled by the development of digital twin technology. The digital twins associated with manufacturing technologies synergistically integrate physical science and data science and provide numerical tools for simulating and managing the product design, development, manufacturing, and service throughout the entire product life cycle. Equally important to the computational models are the process monitoring and control systems, which necessitate the development and deployment of advanced sensors and innovative data frameworks to bridge the factory product line and the virtual space. However, high-fidelity models and process monitoring systems for AM have not yet been fully developed, particularly those for simulating and detecting defects and build anomalies. This is largely caused by the missing of some critical process and material parameters and the fundamental understanding of defect generation mechanisms.
Operando experiment is one of the most effective approaches for quantitative measurements and mechanistic studies. While most of the optical imaging and sensing methods are capable of providing the morphological and thermal information on or above a sample surface, the sub-surface microstructure and phase evolution in the sample during the AM process cannot be in situ characterized with high resolutions. The synchrotron x-ray source, thanks to its high flux and high penetration power, offers the unique capability for studying real (bulk) materials under real conditions in real time. In recent years, operando systems that can faithfully simulate AM processes have been developed for synchrotron experiments. With the full-field x-ray imaging technique, the real-space information of the sample can be quantitatively characterized, such as melt pool and keyhole morphologies and fluctuations, powder motion and spattering, melt flow, and solidification velocity in laser AM. More importantly, the generation of structure defects (e.g., porosity and cracks) can be directly visualized. With the x-ray diffraction technique, other critical process parameters and material structure change can be measured, such as cooling rate, stress development, phase transformation, precipitation, and texture formation. Through the endeavor of the synchrotron and user community, the research from these operando experiments in the last few years has already provided great insights into the physics underlying some of the most complex AM processes, and quantitative results that were urgently needed for further developing numerical models and process sensing schemes.
In the “Operando systems for synchrotron studies of additive manufacturing processes” Special Issue in Review of Scientific Instruments, we have eight contributions on AM research that were carried out at six synchrotron facilities around the world. These articles introduce the recent development in the operando AM apparatus for x-ray imaging and diffraction experiments.
The article contributed by Zhang et al. reviews the history of applying lab-based and synchrotron x-ray imaging techniques for the in situ study of the laser welding process.1 Modern fusion-based AM techniques can, in fact, find their root in welding. Therefore, early operando AM experiments were largely inspired by and derived from the welding experiments, particularly those performed at the Osaka University in the late 1990s and early 2000s.2–4 The majority of the synchrotron experiments reviewed in the article were performed at the Super Photon Ring – 8 (SPring-8) in Japan.
The article contributed by Escano et al. reports the development of a powder spreading system for operando synchrotron x-ray imaging experiments.5 Many AM techniques involve spreading the feedstock powder layer by layer, such as laser powder bed fusion (LPBF), binder jetting, and electron beam melting. In these processes, the characteristics of the raw powder affect its spreadability, which will then influence the quality of the powder bed quality and the defect level in the final build. With this miniature spreading system, the team demonstrated that the moving speed of the powder in any location in respect of the recoater can be quantitatively measured using in situ synchrotron x-ray imaging. The experiment was performed at the Advanced Photon Source (APS) in the USA.
The article contributed by Krohmer et al. reports the development of a LPBF simulator for operando synchrotron x-ray scattering experiments, which offers all essential functions of an industrial LPBF system.6 Also detailed in the article are three application cases, which highlight the unique capabilities of x-ray diffraction and small-angle scattering techniques in characterizing the stress development, precipitates, and phase transition in alloy systems that interest the AM community. These experiments were performed at PETRA III in Germany.
The article contributed by Webster et al. reports the development of a blown-powder directed energy deposition (DED) system for operando synchrotron x-ray imaging experiments.7 This system features functions that allow multi-layer deposition and quick sample switching. Therefore, high-throughput experiments can be operated, which is particularly beneficial for a quick survey of a large parameter space. The experiment was performed at the APS.
The article contributed by Yang et al. reports the development of a three-dimensional freeze printing (3DFP) system for operando synchrotron x-ray imaging experiments.8 The 3DFP integrates freeze casting and AM, which is an emerging technique for printing porous structures. X-ray imaging allows the direct observation of the freeze front, the deposited material, and the growing ice simultaneously. Therefore, a fundamental understanding of the physics underlying 3DFP can be gained. The experiments were performed at the APS and the Stanford Synchrotron Radiation Lightsource (SSRL) in the USA.
The article contributed by Martin et al. reports the development of a LPBF testbed for operando synchrotron imaging and diffraction experiments.9 This system is designed to simulate the realistic gaseous environment and laser conditions of a commercial LPBF machine. A series of sensors is integrated in the testbed, which can collect optical, acoustic, and electronic signals during laser processing with x-ray imaging simultaneously. The experiments were performed at the SSRL. Data from these operando experiments can support not only the calibration and validation of numerical models but also the development process monitoring systems.
The article contributed by Dass et al. reports the development of blown-powder DED system for operando synchrotron x-ray diffraction experiments.10 In this system, the powder nozzle and the heating laser are decoupled and controlled independently. This design allows the operator to control the powder feeding and laser melting with flexible separation in space and time. The experiment was performed at the Cornell High Energy Synchrotron Source (CHESS) in the USA.
The article contributed by Lhuissier et al. reports the development of a miniature LPBF replicator for operando synchrotron x-ray imaging experiments.11 Different from other LPBF simulators, this system is designed for x-ray computed tomography (CT) measurements, and therefore, 3D information on the sample can be obtained between each build layer. This unique capability offers additional opportunities for studying the process-structure (defect) relationship in LPBF. The experiment was performed at the European Synchrotron Radiation Facility (ESRF) in France.
The recent surge in operando synchrotron experiments on AM started only about five years ago,12–14 yet there are now more than 20 apparatuses being used at almost all major synchrotron facilities by an expanding research community. Although this Special Issue is not able to cover all the ongoing instrumentation efforts, readers can certainly find other important works in the references cited in the above articles. By now, operando synchrotron experiments have already created a huge impact on the field of AM by providing new knowledge and quantitative data. Undoubtedly, along with the advance of x-ray sources and techniques and the improvement of AM simulators, operando synchrotron experiments will continue to fuel the development of AM technologies and promote their wider adoption in industry.
I thank the Editorial Office of Review of Scientific Instruments for managing and promoting this Special Issue. On behalf of the user community, I would like to extend our gratitude to the beamline scientists and engineers at the synchrotron facilities for their tremendous efforts to develop x-ray techniques, maintain beamline instruments, and support our experiments.
