Mechanical and electromagnetic shielding properties of ultrafine-grained Mg98.5Zn0.5Y alloy prepared by underwater friction stir processing

• Mg98.5Zn0.5Y alloy's low density, high strength, and broad-spectrum EMI shielding give it great potential. • UFSP enhances both mechanical and EMI shielding properties of Mg98.5Zn0.5Y. • Grain refinement and more 14H LPSO phase formation greatly improve EMI shielding. The coarse grains and the difficulty in refining the second phase of the traditionally cast LPSO phase magnesium alloy have limited the further improvement of its mechanical and electromagnetic shielding properties. In this paper, the extruded Mg 98.5 Zn 0.5 Y alloy was used as the base material, and the underwater friction stir processing (UFSP) technique was adopted to successfully prepare an ultrafine-grained structure by suppressing the thermal accumulation effect. The research results show that compared with the conventional friction stir processing (FSP), UFSP further refined the average grain size from 10.1 μm to 6.0 μm and increased the volume fraction of the LPSO phase by 6.63%. Under the combined effect of fine grain strengthening and second phase strengthening, the ultimate tensile strength of the alloy increased from 253.3 MPa to 276.4 MPa, and the elongation increased from 14.2 to 17.3%. Meanwhile, the electromagnetic interference shielding effectiveness of the UFSP samples remained between 87.4 and 108.3 dB in the 30–6000 MHz frequency band, which was optimized compared to the FSP samples (89.3–105.5 dB). This was mainly attributed to the interface polarization effect caused by a large number of grain boundaries and the multiple reflection losses of electromagnetic waves by the layered LPSO structure. The study indicates that UFSP can effectively prepare Mg 98.5 Zn 0.5 Y alloys with better comprehensive properties, providing a new approach for their application in electromagnetic protection fields.