Study on the Strengthening Mechanism of Rare-Earth Element Ce on the Laser Welded Joints of Magnesium Alloys

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 Mg17Al12 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.