Microstructure and mechanical properties of Al–7Si–4Cu–1Ni aluminum alloys with Mn and Mg additions produced by gravity casting and squeeze casting

In this research, gravity casting (GC) and squeeze casting (SC) were applied to investigate the microstructure and mechanical properties of Al–7Si–4Cu–1Ni with Mn and Mg additions. The ingots produced by SC presented higher mechanical properties with Mn and Mg. SC had refined grain size of α-Al matrix, generated micron second phases of eutectic Si, AlCuNi (Al 7 Cu 4 Ni and Al 3 CuNi), Alpha (Al 6 MnSiCuNi), Q (Al 5 Cu 2 Mg 8 Si 6 ) and nano phases of Mg 2 Si and Al 2 Cu(Mg). The multi-sized second phases guaranteed the deformation resistance and mechanical properties. In addition, the nano phases Al 2 Cu(Mg) and Mg 2 Si phases, generated in nano Alpha (Al 6 MnSiCuNi) phase and eutectic Si, had continuously improved the mechanical properties. With the addition of Mn and Mg, the prominent mechanical properties at room temperature were realized, including yield strength of 191.7 MPa, ultimate tensile strength of 270.5 MPa, elongation of 1.21 %, obtained via SC. Meanwhile, mechanical properties at 350 °C involving yield strength of 75.3 MPa, ultimate tensile strength of 101.2 MPa and elongation of 3.27 %. Moreover, the fracture mode transformed to brittle fracture from ductile fracture with the addition of Mn and Mg. Grain refinement guaranteed the basic properties, the dislocations density increased and dislocations were pinned by second phases, additionally, dislocations increased along the second phases and grain boundaries and the mechanical properties were further improved. The research proved the necessity of SC with Mn and Mg addition to achieve guaranteed mechanical properties at both room temperature and high temperature of 350 °C for engine piston alloy.