Effect of nano TiN/Ti refiner addition content on the microstructure and properties of as-cast Al-Zn-Mg-Cu alloy

A novel grain refiner consisting of nanoparticulate TiN suspended in a Ti metal powder (abbr. TiN/Ti refiner) was prepared by high energy ball milling. The effect of the TiN/Ti refiner addition contents on microstructures and properties of Al-Zn-Mg-Cu alloy was investigated in detail. The results show that the TiN/Ti refiner addition changes the α(Al) matrix morphology of the modified Al-Zn-Mg-Cu alloy from dendrite to equiaxed crystaline. With increasing TiN/Ti refiner addition contents from 0 wt.% to 0.5 wt.%, the average grain size of Al-Zn-Mg-Cu alloy decreases sharply from 400 μm to 78.5 μm. The SEM, EDS, XRD, and DSC results demonstrate that secondary phases of the modified Al-Zn-Mg-Cu alloy contain three kinds of intermetallic phases: σ[Mg(Zn, Cu, Al)2], θ(Al2Cu) and iron containing intermetallic of Al7Cu2Fe. With refined α(Al) grains, secondary phases are dispersed, refined and distributed uniformly. With increasing TiN/Ti addition contents from 0 wt.% to 0.5 wt.%, the yield strength, tensile strength and Vickers hardness of the modified Al-Zn-Mg-Cu alloys enhance from 36.6 MPa to 74.6 MPa, from 175 MPa to 235 MPa and from 118 HV to 152.5 HV, respectively. In addition, the yield strength σy, tensile strength σb and Vickers hardness Hv of the modified Al-Zn-Mg-Cu alloys show Hall-Peteh dependences on the grain size D. A typical interdendritic fracture is observed on the tensile fracture surface of the as-cast Al-Zn-Mg-Cu alloy. Grain boundary and secondary phase regions act as an easy path for crack propagation.