The evolution mechanism and strengthening mechanism of the second phase in pulsed laser welded joints of rare earth magnesium alloy

Magnesium alloy welded joints face significant challenges in terms of high tensile properties, and behind these high tensile properties lies fundamental research on microstructure. In this paper, aiming at the problem of the second phase that is difficult to control during the welding process of magnesium alloys, pulsed laser welding was successfully carried out on Mg-5Al-2Gd-0.5Mn magnesium alloy plates. The combined technology of zonal Scanning electron microscope (SEM), transmission electron microscope (TEM) and electron backscatter diffraction (EBSD) was adopted, aiming to study the mechanism and strengthening mechanism of the entire process of the evolution of the second phase from the nanometer scale to the micrometer scale during the welding process. So as to achieve the purpose of regulating the generation of the second phase to improve the performance of the joint. The research results show that within the crystal, there are mainly circular sub-micron-sized Al2Gd phases and rod-shaped Al2Gd phases combined with self-phase. At the same time, there are also a small amount of post-generated ones combined with Al8Mn4Gd phases and Al2Gd phases. At the grain boundary (GB), there are mainly nano-scale Al8Mn4Gd phases, along with a small amount of nano-scale Mg17Al12 and Al12Mn phases. There are also clusters of sub-micron polycrystalline phases newly formed by combining Al8Mn4Gd phases with other phases. The average hardness and ultimate tensile strength (UTS) of the joint are 68.9 HV and 261.7 MPa, respectively. Because during the formation process of the rare earth (RE) phase with similar orientation characteristics to Mg, a large number of dislocations are absorbed, reducing the degree of plastic deformation of the material. In the magnesium alloy, the Mg17Al12 phase is replaced by the RE phase that is stuck within the grains and at the GB. The RE phase forms a conconsistent phase boundary with the Mg matrix, hindering the movement of dislocations. The contribution rate of the second phase strengthening is 47.7 %. At the same time, the formation of the RE phase promotes the formation of GB. The grains were refined, the contribution rate of grain boundary strengthening is 37.6 %, and the RE phase significantly improved the mechanical properties of the joint. The research result provides the necessary basic theory for regulating the complex second phase of magnesium alloys to improve the mechanical properties of magnesium alloy joints.