Effect of Ti, Sc and Zr additions on microstructure and mechanical properties of rheo-diecasting Al-6Zn-2Mg-2Cu alloys

The microstructure and mechanical properties of rheo-diecasting Al-6Zn-2Mg-2Cu alloys microalloyed with Ti, Sc and Sc+Zr were studied by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), hardness testing, and tensile testing. The swirled equilibrium enthalpy device (SEED) process was introduced to prepare the semisolid slurry. Results show that the addition of Ti, Sc, and Sc+Zr refines the grain size and improves the uniformity of the semisolid slurry and then suppresses the growth of the alpha-Al grain during solution heat treatment. The microstructure of the four alloys in as-cast state mainly consists of spherical alpha-Al and the Mg(Al, Cu, Zn)(2) (eta) eutectic phase. Moreover, primary Al3Sc, Al-3(Sc, Zr) and Al3Zr are also found in the micro-alloying alloys. After solution and aging heat treatment, most of the Mg(Al, Cu, Zn)(2) phases dissolve into the alpha-Al matrix, while part of Mg(Al, Cu, Zn)(2) phases transform to Al2CuMg (S) phases. However, the coarse primary Al3Sc and Al-3(Sc, Zr) still remain in the matrix, and promote crack initiation and propagation. With the tensile strength of 553 MPa, yield strength of 463 MPa and elongation of 13.4% at T6 state, trace Ti addition generates more attractive mechanical properties than the other three alloys.