High strength aluminum alloy 7175 and 7050, the newer variants of the baseline alloy AA7075, has widely been used for the aerospace forging requiring higher strength and damage tolerance. As a thick forging, however, the alloy with conventional chemistries reveal the massive evolution of coarse equilibrium phases and high quench sensitivity.
Futhermore, there has been a strong tendency for 7xxx alloys to be used as large and thick semiproducts. These thick materials, however, often have poor mechanical and fracture properties since they receive less deformation during hot working and slower quench cooling. This is especially due to that thick (open die) forgings are usually made of large diameter ingots, reaching several hundred milimeters, which accompany large dendrite size and concurrent heavy segregations owing to slow casting speed. In addition, these thick materials are commonly slow quenched (in hot water) to reduce residual stress, making the alloy more quench-sensitive. Through the many slow processes, there are frequent evolutions of the coarse equilibrium phases and constituent particles which have an adverse effect on strength, damage tolerance and hot workability. Therefore, they often entail the alloying modifications and the introduction of thermomechanical processes.
In this study, alloy design of commercial aluminum alloy 7175 and 7050 are newly optimized to improve the microstructural homogeneity and the resulting final properties of the very thick forgings. The effects of alloy redesign, principally the change of Zn:Mg ratio, and cooling rate after solutionizing on the microstructural evolutions, the mechanical, fracture and SCC properties are investigated during the large ingot processing to thick forgings. The main concept of alloy design is rather diluted Cu+Mg content with higher Zn:Mg ratio. The 7175 and 7050 alloy large ingots with 530 to 700mm diameter are used as starting materials. Analysis of DSC, TEM and SEM, and measurements of mechanical and fracture properties are used.
In the first part, 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 ($Al_2CuMg$) is effectively inhibited by higher Zn:Mg ratio rather than higher solutionizing temperature. The formation of M-phase ($MgZn_2$) and T-phase ($Al_2Mg_3Zn_3$) is closely related with both constitution of alloying elements and cooling rate. Slow cooling after homogenization promotes the coarse reprecipitation 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. At a given temperature, as Cu+Mg contents are decrease or Zn:Mg ratio increases for constant Cu level, the alloy moves from a two-phase (α+S) to a single phase (α) field, thus minimizing the brittle S-phase. The reprecipitation of coarse M-phase during slow cooling after solutionizing in low Zn:Mg alloy is related with the high instability of supersaturated solid solution due to higher Cu+Mg contents. T-phase reprecipitated during slow cooling is found to be not a ternary phase, but a Cu-bearing quaternarty phase formed at high temperature around 400℃. It would be due to the high Mg and long duration at higher temperature (slow cooling rate). Result of tensile and impact properties also ensure the microstructural homogeneity of the modified alloy. S-phase governs the general fracture behaviors of the conventional 7175 alloys due to its brittleness. The modified alloy shows higher ductility and fracture toughness compared to the conventional alloys.
In the second part, the effects of alloying modification of 7175 alloys, primarily Zn:Mg ratio, on the age hardening response and the quench sensitivity are investigated. As a results, the modified alloy (diluted but with higher Zn:Mg ratio) is fairly less quench-sensitive compared to the conventional alloys (concentrated but with lower Zn:Mg ratio).
In the fast quench, the modified 7175 alloy (higher Zn:Mg ratio) tends to accelerate the decomposition rate to form more stable phases, giving a lower strength with higher electrical conductivity. However, high response to natural aging of the modified alloy, due to higher Zn:Mg ratio, enhances the subsequent precipitation of hardening phases and increases the final strength. This preaging effect is not appeared in low Zn:Mg ratio alloy. Higher hardness and lower electrical conductivity level in the conventional 7175 alloys are due to high Cu+Mg content and low Zn:Mg ratio. High Cu+Mg content induces greater supersaturation and concurrent large amount of η`+η precipitation.
In the slow quench, the higher Zn:Mg ratio promotes homogeneous precipitation, while the lower Zn:Mg ratio leads to the greater amount of heterogeneous precipitation on the Cr-dispersoids. It is due to the concentrated alloying elements (Zn+Mg+Cu) which induce a high instability of the supersaturated solid solution and concurrent rapid decomposition of equilibrium phases during slow quench. Results of the mechanical properties and electrical conductivity tests also ensure the microstructural homogeneity of the modified alloy, which is uniquely applicable in 7175 thick forgings.
In the final section, similar alloy design concept is applied to 7050 alloys. As in the 7175 alloy, the modified 7050 alloy has controlled amount of T- and S-phases, and lowered quench sensitivity compared to the conventional 7050 alloy.
Consequently, the 7XXX alloy redesign proposed in this study is effective for the minimizing the low melting point T and S-phases, providing a flexible heating range, Δ T∼20℃ or more. Together with low quench sensitivity, the redesigned alloy with microstructural homogeneity would ensure higher mechanical and fracture properties, and better hot workability, which is expected to be uniquely applicable in 7175 and 7050 alloy thick forgings.
