Development of Semi-solid Al-Zn-Mg-Cu Die Cast Alloy with Low Thermal Cracking, High Strength and Toughness

Taking Al-Zn-Mg-Cu alloys as the research object, new semi-solid aluminum alloy materials with low thermal cracking, high strength and high toughness were developed through composition design, thermal cracking evaluation and heat treatment process optimization. Combined with thermodynamic calculations and experiments, the influence of different components on the forming properties, mechanical properties and microstructure evolution of semi-solid aluminum alloys was explored. Three alloys with low thermal cracking were obtained, i.e., Al4Zn6Mg2Cu, Al7Zn6Mg4 Cu and Al10Zn6Mg2 Cu.The results show that the best alloy performance is achieved by an optimized heat treatment system(470 ℃/24 h +120 ℃aging for 24 h). The new alloy employs a two-phase strengthening mode consisting of T' and η' phases. The bigger the Zn/Mg ratio, the higher the content of discoidal η' phase in the alloy, the lower the content of spherical T' phase. The spherical phase is more conducive to improving the alloy's plasticity. Based on the evaluation of strength, hardness and plasticity, it has been determined that the Al4Zn6Mg2 Cu alloy exhibits optimal mechanical properties. The yield strength, tensile strength and elongation of the alloy reach 421 MPa, 473 MPa and 9%, respectively. The mechanical properties of the alloy are significantly superior to those of the semi-solid 7075 alloy.