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

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 in situ/operando 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 in situ/operando 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 in situ/operando 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.