Inhomogeneous Skin Formation and Its Effect on the Tensile Behavior of a High Pressure Die Cast Recycled Secondary AlSi10MnMg(Fe) Alloy

The current study investigated the microstructure evolution, mechanical properties, and fracture behavior of a high pressure die cast (HPDC) novel secondary alloy. The as-cast microstructure comprised (i) Primary α-Al, (ii) α-Al₁₅(FeMn)₃Si₂ intermetallics, and (iii) Al–Si eutectics. The microstructure starting from the surface through the depth of the HPDC casting consisted of (i) fine-grained skin at surface, (ii) increased Al–Si eutectics at intermediate location, and (iii) coarse α-Al dendrites at center. Accordingly, the hardness increased from skin to the intermediate section and then decreased toward the center of the casting. The formation of skin layer was highly discontinuous, which was attributed to the complicated fluid flow pattern inside the die cavity. The skin layer indicated to slightly improve the strength of the HPDC alloy; however, it restricted the ductility of the material with a large variation. Such ductility behavior resulted from a fracture mechanism triggered by the inhomogeneous skin because of its poor bonding with the adjacent matrix. Even though the secondary alloy contained casting defects and α-Al₁₅(FeMn)₃Si₂ intermetallics that are known to be driving factors for the fracture in such materials, the effects from the inhomogeneous skin turned out to be predominant in the current study.