Microstructural evolutions, 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 forgings.
An effort has been made to present optimum alloy designs of commercial 7175 and 7050 type alloys to be used in thick forgings with proper microstructures and properties. The effects of changing alloy constitutions, primarily [Zn+Mg+Cu] and Zn:Mg ratio, on the evolutions of the coarse equilibrium phases and age hardening response are investigated. As a result, equilibrium phases (M,T,S) are evolved depending on alloy constitutions and cooling rate following solutionizing. The formation of the T- and S-phases is effectively controlled in the novel alloys (dilute and with high Zn:Mg ratio). In the slow quench, the redesign promotes homogeneous precipitation of η-phases, leading to higher mechanical properties than conventional alloys. The novel alloys provide microstructural homogeneity and extended heating range, ensuring 7xxx thick forging applications.
Recent 7xxx aluminum alloys have been designed for the finite use of thick semiproduct
with contolled amount of constituent phases which mostly evolve during ingot preheat. In this
study, the effects of constitutional change and preheat conditions of 7175 and 7050 type alloys on the
evolution of constituent phases [M-, T-, S-phase and dispersoid] are presented. The constiuents
evolve depending on the constitutional effect, primarily the change of Zn:Mg ratio, preheat condition
comprising temperature and cooling rate following preheat. T- and M-phase are reprecipitated during
cooling after preheat, depending on the alloy constitutions. S-phase is evolved depending on the
constitution and preheat temperature, rather than preheat cooling rate. Prominent precipitation
temperature interval of constituents are discussed in view of quaternary phase evolutions. In addition,
evolutions of dispersoids together with M-phase are discussed. Specific alloy designs and preheat
conditions could provide controlled microstructures for the thick 7xxx semiproducts.
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