Effect of Mn and B Addition on the Microstructure and Properties of Al-Si-Cu-Mg Cast Alloy

An Al-8Si-1Cu-0.35Mg casting alloy with (Mn + B) composite addition was prepared via traditional ingot metallurgy. The effects of (Mn + B) composite addition on the alpha-Al dendrites, AlB2 phase, Fe-rich phase, fluidity, and mechanical properties of the hypoeutectic Al-8Si-1Cu-0.35Mg casting alloy were systematically studied via optical microscopy, scanning electron microscopy, energy-dispersive spectroscopy analysis, and fluidity and mechanical property tests. Results show that (Mn + B) addition refines the alpha-Al dendrites. The alpha-Al phase and AlB2 particles function as heterogeneous nucleation particles to promote alloy nucleation, and the coarse dendrites are transformed into fine equiaxed crystals. The best effect is observed when 0.6 wt% Mn and 0.1 wt% B are added. (Mn + B) composite addition improves the morphology of the Fe-rich phase in the alloy and transforms the needle-like beta-Fe phase into a dendritic Fe-rich phase and alpha-Fe phase. Furthermore, the grain refinement of the alloy and the improvement of the Fe-rich phase enhance the fluidity of the alloy. When 0.6 wt% Mn and 0.3 wt% B are added, the fluidity is the best, which is 77.5% higher than that without addition. (Mn + B) composite addition has minimal effect on the hardness of the alloy. The best mechanical properties are obtained upon the addition of 0.6 wt% Mn and 0.2 wt% B. In particular, the tensile strength and elongation are 235 MPa and 8.6%, respectively. These values are 40.1% and 100% higher than those obtained without addition, respectively. The fracture mode of the alloy also changes from ductile-brittle mixed fracture to ductile fracture.