**1. Introduction**

The shape casting process offers an effective way to produce complex components in one single production step [1]. However, the use of this technique is also limited by the formation of defects such as micro-pore, hot tearing [2,3], and so forth. Porosity/micro-pore is one of the major defects in castings, which is usually induced by gas segregation and solidification shrinkage in the mushy zone. According to the research of Campbell [4], the presence of micro-pore reduces the mechanical properties of the cast component, including fatigue life, tensile strength, ductility, and surface quality.

No porosity could be found in the castings if the gas is absent and the feeding is adequate. However, many regions of the castings are not fed, and then the micro-pore may form in a number of ways [4]. The problem of micro-pore formation in casting alloys continues to be of interest despite the many computational models that have been proposed. The problem of the micro-pore formation is very complex, as it involves many materials (that is, initial gas content, melt purity) and processing (that is, temperature, temperature gradient, cooling rate, applied pressure) parameter interactions in complex physics [5]. It has been shown that the nucleation of the microporosity is influenced by foreign impurities [6]. According to the previous work performed by Felberbaum [7], we know that the porosity which is constrained to grow between narrow inter-dendritic liquid channels has a higher curvature, and thus a higher internal pressure than that of a free-growth spherical one, and the fraction

of the porosity, hence, decreases with an increasing curvature. Therefore, increasing the curvature of the porosity and decreasing the grain size are effective methods to decrease the fraction of the porosity.

The introduction of the electromagnetic technique as a new method for tailoring the microstructure and micro defection of alloys has attracted increasing attention. The application of the high magnetic field suggested a possibility of adjusting the morphology of solid-liquid phase during the solidification of Al-Cu alloys [8], which shows a potential way to control the distribution and amount of porosities. Previous research [1,2] found that the application of the magnetic field can cause grain refinement, in which some of the resulting microstructures are much better than those used by other solidification technology, for example, supergravity solidification [9]. Zuo et al. [10] pointed out that the increasing external magnetic field can tilt the growth direction of the lamellar eutectic and decrease the coarse eutectic lamellar spacing during solidification, leading to a higher strength. The work by Erb et al. [11] indicated that even a low magnetic field (1 to 10 millitesl as) has a significant effect on the orientation and distribution of the reinforced particles in artificial composites. Therefore, the application of a magnetic field during the solidification process is an effective method to reduce defects and optimize microstructure and properties. However, despite the successful applications of the magnetic field in the material fabrication, the mechanisms of the effect of the electromagnetism on the melt are not yet well understood. Therefore, more work is needed to understand the role of the magnetic field in each stage of the solidification progress.

The recent development of high-resolution X-ray tomography imaging techniques proposed a useful method to explore the structure evolution in the solidification process. Holm et al. [12] have enabled three-dimensional (3D) observations of the microporosity morphology. Lee and Hunt [13] first applied this technique to visualize the formation of the porosity in Al-Cu alloys with a micro-focus X-ray source. However, the resolution was about 25 μm and the intensity of the beam was also a limiting factor. The synchrotron X-ray tomography provides higher resolution and higher flux capabilities at beamlines, and, thus, better characterization can be performed, which results in an improved understanding of the mechanisms involved in the material processing [14].

In this work, the effect of TMF on the micro-pore formation and on the morphology of micro-pores and grain size are compared in the solidification process of Al-Cu alloys under a traveling magnetic field (TMF) with various magnetic flux densities. X-ray tomography was performed to characterize the 3D morphology of the micro-pores in Al-Cu alloys after the alloy was solidified. Meanwhile, different strategies of magnetic treatments were performed in different stages of solidification to evaluate the effect of this treatment, and its mechanism was discussed and revealed.
