The as-cast structure of 7055 aluminum alloy is composed of matrix grains and uniformly distributed second-phase particles, among which the second-phase is mainly distributed in a network pattern at the grain boundaries (Figure 1). After high-temperature (350-500℃) and long-term (6-24 hours) homogenization treatment, the α(Al) grains increase in size.
2. The second phase integrates a bone-like strengthening phase, mainly consisting of T(Al2Zn3Mg5) and S(Cu-MgAl2) phases; There are still small amounts of phases such as MgZn2, Mg2Si, AlMnFeSi and (FeMn)Al6 in the alloy. Among them, there are also some precipitated phases (such as Al3Zr), whose sizes range from 20 to 50nm. These precipitated phases have a pinning effect on the grain boundaries and hinder the growth of grains during deformation and solution treatment. This keeps the degree of recrystallization and the size of recrystallized grains in an appropriate balance, thereby achieving an ideal combination of strength, fatigue resistance and corrosion resistance.
The relatively high content of alloying elements in 7055 aluminum alloy affects the precipitation kinetics process of the alloy, but does not change the basic aging precipitation process. The typical precipitation sequence of enhanced precipitation phases is:
SSSS(supersaturated solid solution)-GP region -η' transition phase (MgZn2)-η equilibrium phase (MgZn2)
The size, quantity, distribution of the GP zone, η' phase and η phase in the alloy, as well as the characteristic of no precipitation zone (PFZ) at the grain boundaries, basically determine the performance of the alloy.
4. The first phase to precipitate is the metastable phase that is coherent with the matrix, which becomes the core for the formation of the thermally stable phase. The GP region is coherent with the matrix, the η' phase is semi-coherent with the matrix, and the η phase is non-coherent with the matrix.
5. The η' and η phases are directly formed during quenching and aging. After long-term aging, non-coherent phases precipitate and are relatively large in size.
The macroscopic properties of 7055 aluminum alloy (such as tensile strength, electrical conductivity, fracture toughness, corrosion resistance, etc.) mainly depend on the shape and characteristics of the final microstructure (matrix precipitated phase (MPt), grain boundary precipitated phase (GBP), and intergranular non-precipitated zone (PFZ)).
7. During single-stage aging, the stress sensitivity index decreases with the extension of time. The reason for this is that the particles of the grain boundary precipitated phase grow larger, the spacing increases, and the non-precipitated zone widens. After the alloy was quenched in water at room temperature and quenched in boiling water for aging, its precipitation characteristics were as follows:
① The size difference of the precipitated phases at the grain boundaries is small, and they are continuously distributed in a chain-like pattern. The grain boundaries have no precipitation bands and are relatively narrow, with average widths of approximately 33nm and 43nm respectively.
After air quenching and aging, the size of the precipitated phases at the grain boundaries varies greatly and shows a discontinuous distribution.
③ The grain boundaries have no precipitation bands and are very wide, approximately 102nm.
④ High-temperature pre-precipitation can form discontinuous precipitated phases at grain boundaries. The 465℃ pre-precipitation solution treatment not only ensures the alloy's strength but also enhances its resistance to stress corrosion. Moreover, as the pre-precipitation time after solution treatment increases, the precipitated phases at grain boundaries become coarser and more discrete, the stress intensity factor of alloy cracks increases, and the rate of stress corrosion crack propagation decreases, with the tensile strength increasing by 6%. The conductivity decreased by 8% (Srivatsan T S, Anand S, Sriram S). et aL The high-cyclefafigue and fracture behavior of aluminum alloy 7055U3. Matcr Sci, 2000,281: Under the three states of T6, T73 and RRA, the fractures of the specimens are all intergranular fractures and intragranular fractures, which are mixed fractures with both forms coexisting. However, under different heat treatment conditions, the proportions of intergranular fractures and intragranular fractures are different, and the corresponding elongation is also different, that is, T73 > T6 > RRA.
8. In the T6 state, the precipitated phases within the alloy grains are fine and uniformly dispersed, while the precipitated phases at the grain boundaries are strip-shaped and continuously distributed, and the PFZ at the grain boundaries is very narrow. After RRA treatment, the precipitated phases within the grains grew larger, the precipitated phases at the grain boundaries became significantly coarser and discontinuous, the PFZ at the grain boundaries widened, and the parallel banded structure still existed. After T73 treatment, the coarsening of the precipitated phases within and at the grain boundaries was more obvious, and the PFZ at the grain boundaries was further widened (Li Hai, Zheng Ziqiao, Wang Zhixiu). Research on the Secondary Aging Characteristics of 7055 Aluminum Alloy -(II) Microstructure and Fracture Morphology Characteristics. Rare Metal Materials and Engineering, 2005,34:1230
9. The matrix precipitate phase (MPt) determines the strength of the alloy. There is still no definite conclusion on which MPt can achieve the best combination of strength, toughness and stress corrosion resistance. P.n. Alder et al. hold that when the precipitated phase in the matrix is mainly the GP region, the alloy has the highest strength. At this time, the strengthening mainly stems from the internal stress caused by the GP region in the matrix and the chemical effect of dislocations passing through them, which hinders the dislocations. J.k. Park et al. believe that the strengthening effect is the best when the matrix structure is mainly the η' phase.
No matter what kind of particle is used as the strengthening phase, the larger its volume fraction and the more dispersed it is, the better the strengthening effect will be. If the precipitated phase particles have high rigidity and are evenly distributed, it will be beneficial for the resistance to stress corrosion and toughness.
11. The grain boundary precipitated phase (GBP) largely depends on the grain boundary structure. The size and morphology of GBP can vary significantly due to different grain boundaries. The continuous reticular distribution of GBP reduces the performance of the alloy. This is because:
After aging of the alloy, the grain boundary precipitates are mostly η' phase or η phase, which have a certain degree of mobility relative to the matrix. During the deformation process, they will hinder the relative movement of grains, thereby reducing the toughness and plasticity of the material.
② According to the anodic dissolution model and hydrogen embrittlement mechanism, a continuous chain-like grain boundary structure will enhance the alloy's SCC sensitivity. Therefore, to improve toughness and SCC resistance, the grain boundary precipitate phase is preferably discontinuously distributed coarse semi-coherent (non-coherent) particles. Liu Shengdan, Zhang Xinming, You Jianghai. Wait. The influence of trace zirconium on the quenching sensitivity of 7055 aluminum alloy [J]. Rare Metal Materials and Engineering, 2007, (4) : 607
Another parameter that affects the performance of the alloy is the grain boundary no-precipitation zone (PFZ), which appears near the grain boundaries during quenching. These areas are relatively weak and prone to stress corrosion fracture. Liu Shengdan, Zhang Xinming, You Jianghai. Wait. The influence of trace zirconium on the quenching sensitivity of 7055 aluminum alloy [J]. Rare Metal Materials and Engineering, 2007, (4) : 607
In conclusion, optimizing the heat treatment process of high-solute alloys to control the microstructure means achieving the best combination of matrix precipitated phase (MPt), grain boundary precipitated phase (GBP), and grain boundary non-precipitated zone (PFZ), thereby optimizing the matching of high strength, high toughness, and corrosion resistance of the alloy.
