Formation Mechanism of Fluorescent Linear Defects in ZM6 Mg-RE Alloy Castings

Abstract:

Rare-earth magnesium alloys, as a new type of high-strength lightweight structural material, have become increasingly widespread in the aerospace industry. However, issues related to casting defects have grown more prominent. Fluorescent linear defects are a typical defect found in this class of alloys. Nevertheless, their formation mechanism remains unclear, and there is a notable lack of precise control methods to mitigate them. This study systematically characterized the composition, morphology, and microstructure of fluorescent linear defects in a sand-cast low-pressure ZM6 (Mg-Nd-Zr) alloy casing component, and revealed the defect evolution mechanism through numerical simulations of the filling flow field. The results show that fluorescent defects are formed by multi-scale and multi-morphology double-layer oxide films. Meanwhile, the oxide film in ZM6 consists of porous MgO as the matrix, containing Nd₂O₃ particles. This film exhibits poor toughness and is prone to fragmentation, resulting in the formation of small-sized oxide inclusions around the oxide film. Combined with the analysis of the casting structure and filling flow field, it is found that the convergence and backflow of the melt during the filling process cause the oxide film on the melt surface to fold and be incorporated, thereby generating double-layer oxide film that can be displayed by fluorescent inspection.