Surface-segregation mediated Mg2Si and Mg(Zn,Cu,Al)2 formation on primary α-Al phase during solidification of the hot-tear sensitive AA 7xxx aluminum alloys

Computer-aided thermal analysis and phase diagram calculation using the ThermoCalc® software were used to study the solidification path of Al7.8Zn2.6Mg2Cu alloy containing average values of 0.1 wt% Si and 0.1 wt% Fe as impurity elements. The Al6(Fe,Cu), Mg2Si, and Mg(Zn,Cu,Al)2 phases were detected as the main secondary phases precipitated during the solidification of the alloy. A four-step surface segregation-mediated description was proposed to explain the formation mechanism of the Mg-bearing, Mg2Si and Mg(Zn,Cu,Al)2, phases: i) surface segregation of Mg atoms form the bulk toward the surface of the primary α-Al phase, ii) adsorption of liquid Si atoms on the Mg-rich surface, iii) nucleation of Mg2Si, and iv) lateral growth of Mg2Si nuclei on the surface. Interfering with the kinetics of any or all of the first three steps may postpone or preclude the Mg2Si formation, alleviating the hot-tearing susceptibility of the alloy. Mg2Si can decrease the non-equilibrium solidus point of the alloy by depleting the source of Mg atoms on the surface of the primary α-Al phase, thereby postponing the formation of the Mg(Zn,Cu,Al)2 phase to lower temperatures. The freezing range is thus enlarged, increasing the hot-tearing susceptibility of the alloy. Therefore, the surface segregation of the primary α-Al phase appears to be a key feature for understanding the hot tearing phenomena of an alloy. A more accurate calculation of the solidification path to be used with Kou’s index of hot-tearing requires taking into account the possibility of surface segregation.