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Utilizing lightweight Al alloys in various industrial applications requires achieving precise pressure tightness and leak requirements. Vacuum pressure impregnation (VPI) with thermosetting polymers is commonly used to address leakage defects in die-cast Al alloys. In this study, the efficacy of the VPI technique in sealing alloy parts was investigated using a combination of nondestructive micro X-ray computed tomography (micro XCT) and a standard leak test. The results demonstrate that the commonly used water leak test is insufficient for determining the sealing performance. Instead, micro XCT shows distinct advantages by enabling more comprehensive analysis. It reveals the presence of a low atomic number impregnates sealant within casting defects, which has low grey contrast and allows for visualizing primary leakage paths in 3D. The effective atomic number of impregnated resin is 6.75 and that of Al alloy is 13.69 by dual-energy X-ray CT. This research findings will contribute to enhancing the standard VPI process parameters and the properties of impregnating sealants to improve quality assurance for impregnation in industrial metals.

In special relativity, four-vectors, such as space–time, energy–momentum and electromagnetic potential vectors, follow the Lorentz transformation. The transformations of space–time and energy–momentum vectors have been tested by time dilation and rest mass energy experiments, respectively. However, the Lorentz transformation of electromagnetic potentials has never been directly demonstrated in experiments due to the limitations of the diagnostics. Here we show the spatiotemporal electric-field profiles around a highly energetic electron beam with a subpicosecond temporal resolution obtained by a terahertz technique based on electro-optic sampling. We observe the Coulomb field contraction in the propagation direction of the beam under the Lorentz transformation. After passing the beam through a metallic plate, we investigate the birth of the Coulomb field around it and follow the spatiotemporal evolution of the spherical wavefront, which is derived by the Liénard–Wiechert potentials. The measured electric-field profile in the far propagation distance is experimentally confirmed to coincide with the calculation of the Liénard–Wiechert potentials and the Lorentz transformation. Our ultrafast measurement of a spatiotemporal relativistic electric field provides both additional experimental evidence of special relativity and a platform for detailed experimental research of energetic charged particle beams and electromagnetic radiation physics.

Electro-optic (EO) sampling is employed to measure the electric field profiles generated by a relativistic electron bunch along the propagation and in the radial directions. The longitudinal (temporal) profile is investigated by changing the time delay between the electron bunch and the pulsed probe laser, while the transverse (radial) profile is acquired by laterally shifting the path of the electron bunch. Experimental results show good agreement with three-dimensional particle-in-cell calculations. We demonstrated a promising method to simultaneously obtain the longitudinal and transverse beam sizes by utilizing the detected spatio-temporal electric field distribution around the electron bunch.