The Effect of Ingot Processing on Fatigue Properties of 7475-T6 Plates

The hot-rolled material exhibited the best overall combination of fatigue properties for the maximum strength temper. The predominantly unrecrystallized microstructure of hot-rolled 7475-T6 promoted a high-energy-absorbing transgranular fracture mode and led to superior resistance to fatigue crack propagation and unstable fracture. The more recrystallized intermediate thermo mechanical treated (ITMT) materials experienced a higher degree of intergranular fracture which contributed to higher fatigue crack growth rates and lower fracture toughness values.

The low cycle fatigue and high cycle fatigue lives of the experimental materials were relatively unaffected by changes in microstructure produced by ingot processing. However, quantitative metallography showed that the crack initiation resistance of ITMT-7475-T6 was somewhat improved over that of the hot-rolled material. Crack initiation at slip bands, which occurred extensively in the unrecrystallized hot-rolled microstructure, was severely limited by the fine grain size and random texture produced by ITMT processing.

High strength-density ratios have made the 7XXX aluminum alloys extremely attractive for use in aircraft structures. Despite this superior property, limited corrosion, fatigue, and fracture resistance has restricted their commercial usage in maximum-strength tempers. The problem of stress corrosion cracking (SCC) and exfoliation corrosion resistance has been greatly alleviated by the introduction of the T73 and T76 tempers. Improvements in fracture toughness at high strength levels have been shown by recently developed alloys such as 7175, 7475, and 7050, largely as a result of increased alloy purity and specialized processing controls.

The application of intermediate thermo mechanical treatments (ITMT) to some 7XXX alloys has also shown promise for increasing fracture toughness levels. These specialized ingot processing techniques utilize low working temperatures to introduce a high degree of strain energy into the ingot and subsequent recrystallization results. In a relatively fine equlaxed grain structure, ITMT materials can also be produced by hot-rolling as-recrystallized structures to produce fine, elongated grain structures. These ITMT microstructures have been shown to provide higher levels of ductility, toughness, and short transverse SCC resistance in 7075-T6 compared to the largely unrecrystallized lamellar structure of conventionally processed hot-rolled plate.

Based on increased levels of toughness and SCC resistance, the new 7XXX alloys have made possible improvements in the overall reliability of structural aircraft components. However, further increases in component performance will necessarily be based on higher levels of fatigue resistance.

Optical metallography disclosed the presence of Al-Fe-Si constituent particles aligned roughly in the rolling direction of the plate. These particles ranged in size from approximately 5 to 20 μm and were probably Al₂Cu, Mg₂Si, or FeAl₆, resulting from the presence of the iron and silicon.

Precipitation of Al₂CuMg occurred in 7050 during slow cooling to the temperature used for ITMT processing. However, the lower copper content of 7475 compared to 7050 and a higher homogenization treatment effectively prevented the occurrence of Al₂CuMg as a major constituent phase.

Monotonic Properties and Fracture Toughness

The monotonic fracture surfaces are helpful in explaining the effect of microstructure on the ductility and fracture properties of 7475. The ductility of 7475-T6 AR is improved over that of 7475-T6 HR despite the occurrence of the marked intergranular fracture observed for 7475-T6AR. By contrast, the HR variant exhibits a predominantly ductile fracture appearance.

Brittle fracture would be expected to cause a serious loss of ductility in the fine-grained AR material. However, it appears that the reduced grain size of 7475-T6 AR has overcome the embrittling effect of intergranular fracture, probably by increasing the macroscopic homogeneity of deformation. Thus, homogeneous deformation compensates for the detrimental effect of intergranular fracture and contributes to the superior-ductility of the 7475-T6 ITMT materials. It should be noted that the positive effects of ITMT on ductility are probably dependent on the type.

From the study on tensile deformation and fracture in these age-hardened materials, it is evident that two opposing factors must be balanced in order to attain the optimum combination of properties. The macroscopic homogeneity of deformation (on a grain-size scale) must be maintained so that localized fracture is delayed and ductility enhanced. This is favored by fine, recrystallized structures, e.g., 7475-T6 AR.

Neglecting the residual stress effects observed during early portions of the LCF tests, 7475-T6 samples showed extensive cyclic hardening during strain-controlled fatigue. In order to obtain a quantitative assessment of low cycle fatigue (LCF) crack initiation resistance for the 7475-T6 materials, a detailed analysis was made of the surfaces of the fatigued specimens. Consequently, it is concluded that the AR microstructure offers superior resistance to LCF crack initiation compared to the unrecrystallized grain structure of 7475-T6 HR.

Fatigue Crack Propagation

7475-T6 UK exhibited the best resistance to fatigue crack growth, i.e., slowest fatigue crack propagation (FCP) rates, over the range of AK studied. 7475-T6 AK+HR showed slightly superior FCP resistance compared to 7475-T6 AR. It is apparent from a correlation of KCP fracture appearance and observed da/dN value, that the ductile, featureless fracture of 7475-T6 HR promotes the best resistance to fatigue crack growth.

Recently, several workers have sought to develop predictive relationships for fatigue crack growth resistance based on measurable LCF and microstructural parameters. Due to the constraint of surrounding material, a finite volume of material at the crack tip undergoes a condition of strain-controlled cycling. Damage is accumulating in this region.

This relationship has been applied to the 7475-T6 fatigue data obtained in the present investigation. Since the microstructure of 7475-T6 contains predominantly shearable precipitates, app value of one-half the average grain length in the rolling direction was chosen. It may have seemed possible to use the subgrain size for this parameter. However, it was shown previously that the texture of the hot-rolled materials made the subgrain boundaries ineffectual barriers to fatigue deformation. Other research has supported this contention for aluminum alloys containing shareable precipitates.

Parameters such as dispersion spacing and distribution, subgrain size, degree of recrystallization and texture could have led to some variability in this respect. It is important to note that, even though quantitative agreement with observed FCP rates was not always obtained, the relative positions of the data were correctly predicted from LCF and microstructural parameters.

Conclusions

• The unrecrystallized microstructure of hot-rolled 7475-T6 exhibited the best overall combination of properties of the materials studied. For applications where total fatigue performance must be considered (i.e., resistance to crack initiation, crack growth, and unstable fracture), the HR structure should be chosen in preference to that of more fully recrystallized ITHT materials for the T6 temper.
• Total LCF and HCF lives of the experimental materials were relatively unaffected by changes in microstructure produced by ingot processing. Some slight improvement in LCF life of 7475-T6 AR at low strain amplitudes was indicated.
• 7475-T6 HR exhibited the best FCL resistance of the materials studied. This improvement was attributed to the promotion of a high-energy absorbing transgranular type of fracture by subgrains in the unrecrystallized material. High-angle grain boundaries in 7475-T6 AR provided for a lower-energy intergranular fracture mode.
• Improved homogeneity of deformation due to the fine grain size led to ductility improvements in 7475-T6 AR compared to the hot-rolled materials.
• The occurrence of low-energy intergranular fracture in 7475-T6 AR produced low fracture toughness values compared to the unrecrystallized or partially recrystallized variants of the alloy.
• An equation based on LCF and microstructural parameters proposed by Chakrabortty correctly predicted the relative order of FCP resistance for the three variants of the 7475-T6 alloy.