Investigations on hot cracking susceptibility of Mg-xGd-1Er-0.5Zr alloys

Hot tearing is a potential problem that deteriorates the ductility of Mg castings. The effects of Gd content on the hot cracking susceptibility (H) of Mg-xGd-1Er-0.5Zr (x=8.0, 10.0, 12.0, mass fraction, %) were predicted based on the Clyne-Davies model, and verified by using the "constraint rod" permanent mold test. The results show that the variation of H value of the Mg-xGd-1Er-0.5Zr alloys can be effectively predicted by the Clyne-Davies model and the theory values are in consistent with the experimental ones. The microstructure and the precipitates in the as-cast Mg alloys were analyzed by scanning electron microscopy (SEM) and X-ray diffractometery (XRD). It is demonstrated that the H value decreases with the increase of Gd content. The higher Gd contents lead to the increase of the volume fraction of Mg5(Gd, Er) eutectic phase, which is facilitated to feed the micro-cracks between the dendrite arms during the terminal solidification stage, so as to decrease the H value. In addition, the eutectic phase also inhibits the nucleation and growth of α-Mg dendrites and thus reduces the solidification temperature range of the alloy. Combined with the fracture surface observation, it is suggested that the liquid film and bridge between the grains during solidification improve the grain-binding force, which is also considered as another important reason responsible for reducing the H value.

Duke Scholars

Author Zhaohui Wang Pathology