Control of Equilibrium Phases (M,T,S) in the Modified Aluminum Alloy 7175 for Thick Forging Applications

Abstract

Microstructural evolution, especially for the coarse equilibrium phases, M-, T- and S-phase, are investigated in the modified aluminum alloy 7175 during the primary processing of large ingot for thick forging applications. These phases are evolved depending on the constitutional effect, primarily the change of Zn:Mg ratio, and cooling rate following solutionizing. The formation of the S-phase (Al 2 CuMg) is effectively inhibited by higher Zn:Mg ratio rather than higher solutionizing temperature. The formation of M-phase (MgZn 2 ) and T-phase (Al 2 Mg 3 Zn 3 ) is closely related with both constitution of alloying elements and cooling rate. Slow cooling after homogenization promotes the coarse precipitation of the M- and T-phases, but becomes less effective as the Zn:Mg ratio increases. In any case, the alloy with higher Zn:Mg ratio is basically free of both T and S-phases. The stability of these phases is discussed in terms of ternary and quaternary phase diagrams. In addition, the modified alloy. Al-6Zn-2Mg-1.3%Cu, has greatly reduced quench sensitivity through homogeneous precipitation, which is uniquely applicable in 7175 thick lkirgings.