Microstructural and Mechanical Characteristics of A356 Alloys as a Function of Mn Content and Cast Thickness

In this study, the changes in the microstructural characteristics and mechanical properties of A356 alloys as a function of Mn content and cast thickness were evaluated using structural analysis and tensile tests. Five different A356+x%Mn alloys were prepared by casting in molds of different thicknesses followed by solid solution treatment at 813 K for 195 min and aging treatment at 423 K for 120 min. It was confirmed that the secondary dendrite arm spacing (SDAS) increased with increasing thickness of the cast sample, whilst, for a given thickness, the addition of small amounts of Mn resulted in a decrease of the SDAS. Mn contents of 0.05-0.15% resulted in ~7-9% improvements in the spheroid ratio of the primary Si particles compared to that of the commercial A356 alloy. Further, the spheroid ratio of the primary Si particles obtained in the thin cast samples were higher than that obtained in the thick cast samples. In particular, the addition of small amounts of Mn was also effective in suppressing the formation of the needle-like beta Al-Fe-Si intermetallic compound. The yield and tensile strengths of the thinner cast samples were higher than those of the thicker cast samples. Finally, Mn contents of 0.05-0.15% resulted in enhanced yield and tensile strengths, but Mn content ≥0.1% resulted in decreased elongation.