Damping properties of magnesium surface layers developed by friction stir processing

Commercial pure cast magnesium (Mg) was subjected to friction stir processing (FSP) at a rotational speed of 600 rpm and tool traverse speed of 60 mm/min. The samples extracted from cast Mg and friction stir processed magnesium have been analysed for the microstructure, hardness, and temperature-dependent damping capacity. The average grain size of as cast pure Mg is reduced from 780 to 26 μm after FSP. There is about a 15% increase in hardness after FSP. The dynamic mechanical analyser (DMA) measured the damping capacity using a three-point bending mode. The FSPed Mg sample exhibited 50% lower damping capacity than the cast Mg at room temperature. However, as the temperature increased, significantly above 80°C, the damping capacity of the FSPed magnesium was greater than that of the cast magnesium. At room temperature, the damping capacity of FSPed Mg was decreased because of the formation of tangled dislocations. In contrast, the increase in damping capacity above 80°C was due to the increased grain boundary area.