Effects of cooling rates on the solidification behavior, microstructural evolution and mechanical properties of Al–Zn–Mg–Cu alloys

Abstract

The effects of different cooling rates (0.3–66.2 K/s) on the solidification behavior, microstructural evolution, and mechanical properties of Al–Zn–Mg–Cu alloys during a short-flow process were investigated in detail. With the increasing cooling rate, the grain size first increased, then decreased, and finally, again increased, whereas the secondary dendrite arm spacing and the crystalline phase size were greatly reduced. Relatively high cooling rates (3.4–66.2 K/s) significantly increased elemental micro-segregation in α-Al due to the transition of the solidification mode from near-equilibrium to the Scheil mode. Moreover, the element trapping capacity at the end of solidification was enhanced at high cooling rates, especially for alloy D (66.2 K/s). Hence, the crystalline phase content first increased and then decreased at high cooling rates. During the short-flow process, the refinement of the crystalline phase and dendrites significantly promoted the re-dissolution of the crystalline phase and the elemental diffusion process, leading to improved mechanical properties. The yield strength, the tensile strength, and the elongation were improved from 420 MPa, 460 MPa, and 4.5% for alloy A to 457 MPa, 524 MPa, and 11.3% for alloy D, respectively.