**4. Conclusions**

In summary, we studied the effect of a magnetic field on the micro-pore formation in an Al alloy. It is found that TMF treatment can suppress the shrinkage porosity effectively, resulting in a reduction in both volume fraction of the micro-pore and grain size after TMF treatment. The analyses show that TMF can enhance the feeding ability in the solidification progress, resulting in decreased volume fraction of the porosity in the Al alloy. The grain refinement can also be introduced by the TMF. Detailed mechanism investigations revealed that micro-pores can only be suppressed by the TMF when it is applied during nucleation and the early stage of the grain growth. Therefore, it is suggested that the TMF be applied in the nucleation stage instead of the whole solidification progress. The refined grains and reduced micro-pores caused by the TMF in the materials lead to improved mechanical properties.

**Author Contributions:** Y.X. guided the investigation and experiments, analyzed the experiment data and mechanism, wrote the paper, and revised the paper; L.W. optimized the research plan and revised the paper; B.H. made the diagram and graph, and revised the paper; E.G. made the diagram and graph, and revised the paper; M.W. collected the data, and revised the paper; Y.S. searched literature, and optimized the diagram and charts.

**Funding:** This research was funded by the Defense Industrial Technology Development Program (Grant No. JCKY2017205B032).

**Acknowledgments:** The use of X-ray synchrotron beam line BL13W1 at the Shanghai Synchrotron Radiation Facility (SSRF) is acknowledged. The authors wish to express thanks to them. Helpful discussion with James Mertens in the Arizona State University is also greatly appreciated.

**Conflicts of Interest:** The authors declare no conflict of interest.
