Journal
MATERIALS TRANSACTIONS
Volume 44, Issue 1, Pages 181-187Publisher
JAPAN INST METALS
DOI: 10.2320/matertrans.44.181
Keywords
aluminum alloy 7175; large ingot; Zn : Mg ratio; S-phase; T-phase; homogenization; thick forging; quench sensitivity
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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 sotutionizing. The formation of the S-phase (Al2CuMg) is effectively inhibited by higher Zn:Mg ratio rather than higher solutionizing temperature. The formation of M-phase (MgZn2) and T-phase (Al2Mg3Zn3) 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 us 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.
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