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

Abstract

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.

The Role of cobalt and nickel intermetallic phases on the mechanical properties and microstructure evolution of Al-Zn-Mg-Cu alloys

“…These are mainly; T-AlMg 4 Zn 11 , S-Al 2 CuMg, and η-MgZn 2 phases. Generally the accepted precipitates' formation sequence for the 7000 series alloys are as reported in 27,28 . Metastable the h′phase stands as the main strengthening precipitate for these alloys.…”

Predicting the quench sensitivity of Al–Zn–Mg–Cu alloys: A model for linear cooling and strengthening

“…shown that S-Al2CuMgphase formation also occurs in this high temperature range for a range of other Cu containing 7xxx alloys [19,20]. AA7020 contains no Cu and hence no S-Al2CuMg phase forms in these samples.…”

“…In addition to the 4 phases considered in the model, it is known [19,20] that on very slow cooling (<0.01 K/s) also T (Al2Mg3Zn3) phase can form. As these very slow cooling rates have no relevance to the quench sensitivity of these alloys, we will not seek to incorporate this phase in the predictions.…”

Microstructural Evaluation and Mechanical Properties of an Al-Zn-Mg-Cu-Alloy after Addition of Nickel under RRA Conditions

“…Typically, the eutectic structures of aluminium-zinc-magnesiumcopper alloys consist of α-Al and MgZn 2 . Several phases such as S-Al 2 CuMg, T-Al 2 Mg 3 Zn 3 , and Al 2 Cu are created during solidification process aluminum alloys (Al-ZnMg-Cu) [3,4]. The zinc-magnesium ratio significantly affects the formation of the MgZn 2 phase.…”

“…Compton et al [7] found that adding nickel into pure aluminium forms Al 3 Ni through the eutectic reaction and increases alloy hardness. Yang [8] and Boyuk [9] claimed that blending nickel into aluminium-silicon alloys forms Al 3 Ni in addition to the Al 7 Cu 4 Ni and Al 9 FeNi phases during solidification. For aluminium-silicon-nickel al-loys, a nickel-rich dispersed phase insoluble at high temperature improves mechanical properties for applications in high temperatures [10][11][12].…”