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Since the commercial applications of rare earth magnesium alloys are increasing gradually, there are considerable advantages to developing lower cost and higher performance magnesium alloys with high abundance rare earth (RE) elements. However, the alloying order of a matrix magnesium alloy is completely changed with the addition of RE elements. Therefore, further study of the strengthening mechanism of Ce element in magnesium alloys is required. In this work, the thermodynamic stability of the possible second phases in a Mg-Al-Mn-Ce multicomponent magnesium alloy were analyzed, based on first-principle calculations, and the precipitation sequence of the key RE phases was deduced as a consequence. Combined with Scanning Electron Microscope (SEM), X-ray Diffractometer (XRD), Energy Dispersive Spectrometer (EDS), and other experimental methods, it was investigated whether the preferentially precipitated second phases were the nucleation core of primary α-Mg. The complex alloying problem and strengthening mechanism in a multi-elemental magnesium alloy system were simplified with the aid of electronegativity theory. The results showed that the preferentially precipitated Al11Ce3 and Al10Ce2Mn7 phases could not be the nucleation core of primary α-Mg, and the grain refinement mechanism was such that the second phases at the grain boundary prevented the growth of magnesium grains. Moreover, the tensile test results showed that the reinforced structure, in which the Al-Ce phase was mixed with Mg-Al phase, was beneficial for improving the mechanical properties of magnesium alloys, at both ambient temperature and high temperature.

The thermomechanical stability of majority precipitates formed by conventional alloying elements in magnesium alloys is generally poor. Hence the morphology and structure of these precipitates are highly susceptible to the welding thermal cycle, which results in the softening of the heat-affected zone (HAZ). Rare-earth (RE) precipitates are generally thermodynamically stable. Therefore, it is necessary to conduct an in-depth discussion on whether RE precipitates reduce the softening of the HAZ. In this paper, Ce-containing magnesium alloy was successfully welded by fiber laser welding. Scanning electron microscope (SEM), X-ray diffraction (XRD), energy dispersive spectrometer (EDS), and micro-hardness tester were employed to analyze the welded joints. Consequently, the distribution characteristics of RE precipitates in both fusion zone (FZ) and HAZ were revealed. Moreover, based on the solution experiments of the welded joints, the evolution mechanism of the precipitates in welded joints during the thermal cycle was deduced, and the softening mechanism of the HAZ was clarified. Thereafter, the relative intrinsic mechanism of RE precipitates in reducing the softening of the HAZ and improving the mechanical properties of FZ was explored. The results showed that the HAZ was narrow, with a width of only 100-200 μm. The morphology and distribution of the less thermally stable Mg 17 Al 12 precipitated in HAZ changed significantly after the thermal cycle. In contrast, RE precipitates remained stable, which is extremely important for reducing the softening of the HAZ. In addition, the precipitates in FZ were transformed into micron-sized particles and precipitated at the edge of dendrites, resulting in a hardness improvement of the FZ.

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