Metal additive manufacturing (MAM) processes can directly produce fully dense near net shape components, which are widely used in aerospace, medical, and defense applications. Due to its unique fabrication benefits, additive manufacturing has become one of the fastest-growing and most-active research directions worldwide. However, MAM is carried out under extreme thermodynamic conditions that involve metal melting and solidification, interactions among different elements, and generation of thermal stresses. Hence, various internal defects, such as lack of fusion porosity, pores, cracks, and internal stresses, will inevitably be generated during the MAM process. Normally, defects and residual stresses will significantly affect the quality and mechanical properties of the MAMed components. To eliminate the defects and control the residual stresses, researchers have been focusing on the kinetic behavior of the molten pool, formation mechanism of defects and unstable solid-state phase transformations, and the evolution of the residual stresses. It is wildly recognized that in situ characterization of the defect formation mechanisms in the molten pool and monitoring of the residual stress changes during the MAM process is very challenging. Because the traditional measurement techniques, such as X-ray detection, X-ray diffraction, and ultrasonic detection, can only analyze defects and residual stresses after the components have been manufactured, it is necessary to find a technique capable of performing in situ analysis during the MAM process.
The rapidly developing synchrotron radiation and neutron diffraction-based characterization technologies have proven to be some of the most effective methods for in situ analysis of defect formation mechanisms, crack initiation, phase transformation, and stress evolution during the AM process. This paper reviews the principles of synchrotron radiation and neutron diffraction technologies and their advantages and practical applications in AM, and summarizes the recent progress and future prospects of their applications in AM.
Over the past decade, with the rapid development of characterization techniques based on synchrotron radiation and neutron diffraction, a large volume of research has been carried out to investigate the formation mechanisms and distribution of internal stresses during the AM process (Tables 1 and 2). The synchrotron radiation-based characterization methods can broadly be divided into the following three different types: synchrotron X-ray imaging, synchrotron X-ray diffraction, and synchrotron computed tomography. The synchrotron X-ray imaging can characterize in situ the formation process of internal three-dimensional defects in materials and analyze in situ the molten pool dynamics. The synchrotron X-ray diffraction can be used to analyze the internal stress states and phase transformation processes in materials, and works with the tensile test to dynamically analyze in situ the internal dislocation density of parts. The synchrotron computed tomography can reconstruct three-dimensional models of additively manufactured components to analyze surface defects, and can assess the impact of internal defects during the service process of components using in situ mechanical tests. The neutron diffraction technologies can be divided into non-in-situ neutron diffraction, in situ neuron diffraction, and electrically neutral nuclear scattering techniques. In addition to examining the macroscopic residual stresses in additive components, the characterization based on the neutron diffraction technologies can also measure the metal texture, crystal lattice parameter changes, strain, grain size, density of dislocations, and other parameters. It can also detect the concentration and the location of light elements, such as hydrogen and lithium, in the crystalline structure. Using the synchrotron X-ray imaging, Qu of the University of Wisconsin-Madison, has discovered that nanoparticles can be adopted to eliminate all types of large spatters by simultaneously stabilizing molten pool fluctuations and controlling liquid droplet coalescence. They have also demonstrated that the control of laser powder bed interaction instabilities by TiC nanoparticles is feasible, which has led to the elimination of large spatters and printing of lean-defect samples with good consistency and enhanced properties (Figure 2). Beese of the Pennsylvania State University and Oak Ridge National Laboratory performed in situ neutron diffraction studies of lattice strain evolution and offered a new perspective on the understanding of dislocation-solute interactions and their impact on work-hardening behavior in high-temperature alloys. These observations can pave the way for a fundamental understanding of the abnormal increase in strength at elevated temperatures commonly observed in a wide range of high-temperature structural alloys and may have important implications for tailoring thermomechanical properties using microstructure control in MAM.
Although the characterization techniques based on synchrotron radiation and neutron diffraction have been widely used in the AM process, further development is still needed to expand their applications in AM along the following directions. In situ detection techniques still need improvements, and the temperature field, velocity field, cooling rate, and solidification parameters must be considered in real-time models to reduce the internal defects and improve the quality of additively manufactured components. In addition, in situ inspection of the deposition processes should combine high-resolution and ultrafast synchrotron X-ray imaging, high-speed light photography, and infrared thermometry to develop in situ AM characterization techniques with higher resolution and contrast sensitivity. Furthermore, molten pool dynamics models should be established, which can then guide the design and optimization of AM process parameters. Meanwhile, taking the advantage of the synchrotron radiation and neutron-based X-ray diffraction techniques, the internal stress distribution and microstructure evolution during metal forming and servicing processes can be analyzed. Moreover, the advantages of adopting the synchrotron radiation and neutron-based techniques for measuring the three-dimensional stress field at the crack tip can help to establish the elastoplastic nonlinear micromechanical models of additively manufactured components, which can be used to analyze the fatigue and fractures caused by the multiscale stress field variations.
