Influence of Ni and Y Plus Ni Complex Additions on Microstructure and Mechanical Properties of Al–7Si Alloy

Grain refinement of Al–Si alloys is one of the effective methods for improving the elongation (El) and ultimate tensile strength (UTS). The influences of alone adding Ni (0.5, 1.0, 1.5, and 2.0%) and complex adding Ni (0.5, 1.0, 1.5, and 2.0%) plus 0.4%Y on the evolution of α-Al, eutectic Si, as well as mechanical performance of Al–7Si alloy, were investigated. All specimens were characterized by electron probe microanalysis, transmission electron microscopy, optical microscope, and scanning electron microscope. The results indicated that the secondary dendrite arm spacing (SDAS) of primary α-Al was markedly decreased with increasing Ni content in Al–7Si alloy. When the Ni addition concentration increased to 1.5%, the developed dendritic α-Al were effectively refined into fine equiaxed dendrites. Rare earth Y (0.4%) plus Ni were together added to Al–7Si alloys. The results showed that complex addition of Ni plus Y had a more outstanding refinement effect of α-Al and eutectic Si than that of Ni alone addition in Al–7Si alloy. The coarse and needlelike eutectic Si was modified into fine fibrous shape, and the SDAS of α-Al was significantly reduced when 1.5%Ni plus 0.4%Y was simultaneously added to Al–7Si alloy. Meanwhile, the mechanical performance test illustrated that the El and UTS were improved to 10.8% and 217.8 MPa, respectively.