7050 aluminum alloy is a type of Al-Zn-Mg-Cu alloy developed by Alcoa Corporation in the 1970s through component regulation of 7075 aluminum alloy.
Zn and Mg in 7050 aluminum alloy can form a strong aging effect of the MgZn2 phase, which is the main strengthening phase of high-strength aluminum alloy, significantly improving the strength of 7050 aluminum alloy.
Cu can reduce the potential difference between grain boundaries and intracrystalline regions, suppress its susceptibility to intergranular cracking, and expand the stable temperature range of GP zones, making the alloy less prone to excessive aging.
Zr has a good effect on increasing the recrystallization temperature and refining the grain size of the alloy, and can maintain the stability of Zn, Mg and Cu in solid solution, significantly reducing the quenching sensitivity of 7050 aluminum alloy.
At present, it is difficult to achieve a stable technical level of material properties after heat treatment for 7050 aluminum alloy material, and there are often cases of mismatched electrical conductivity in actual production.
Electrical conductivity cannot be equated with stress corrosion resistance and sensitivity factors.
Therefore, it is very significant to find the factors that influence the heat treatment process on electrical conductivity and match the electrical conductivity of forgings with other properties.
Test materials and methods
(1) In this article, 7050 aluminum alloy square material was used, and its standardized chemical composition is shown in Table 1.
Table 1 Chemical composition (mass fraction, %) of aluminum alloy 7050.
| turns on | 7050 |
| Al | remaining |
| Zn | 5.7~6.7 |
| Ass | 2.0~2.6 |
| mg | 1.9~2.6 |
| Yes | <0.12 |
| Zr | 0.08~0.15 |
| You | <0.06 |
| Faith | <0.15 |
| Mn | <0.10 |
| Cr | <0.04 |
| Other | <0.15 |
(2) Forging dimensions. The forging dimensions and effective thickness are shown in Table 2.
Table 2 Forging dimensions and effective thickness.
| Forging size | Effective thickness of forgings |
| 550mm × 295mm × 174mm | 174mm |
(3) The heat treatment system is shown in Table 3. The accuracy of the equipment used in the heat treatment process is ±3°C.
Table 3 Heat treatment system for 7050 T7452 aluminum alloy.
| Heat treatment status | Forged number | Solid solution | Cold deformation | First level opportunity | Secondary Opportunity |
| T7452 | A | 477℃×65h, water cooled |
2%~3% | 121℃×6h, air-cooled |
175℃ × 8h, air-cooled |
| B | 471℃×65h, water cooled |
2%~3% | 121℃×6h, air-cooled |
175℃ × 8h, air-cooled |
|
| W | 471℃×6.5h, water cooled |
2%~3% | 121℃×6h, air-cooled |
175℃ × 10h, air-cooled |
|
| D | 471℃×65h, water cooled |
2%~3% | 121℃×6h, air-cooled |
175℃ × 12h, air-cooled |
To investigate the above questions, based on production, four sets of experimental plans were designed. The heat treatment system of forging A and forging B changes the temperature of the solution, while the others remain unchanged; The heat treatment system of B forging, C forging and D forging increases the secondary aging time by 2 hours each time, while the other parameters remain unchanged.
Experimental Results and Analysis
The influence of four heat treatment systems on the electrical conductivity of forged parts.
The electrical conductivity of 7050 aluminum alloy is mainly affected by the degree of alloying, matrix recrystallization and precipitation of solutes in the solid solution during solution treatment and aging process.
In this study, four heat treatment systems were used to treat the forgings, and electrical conductivity was measured at five points for each forging using an eddy current-type electrical conductivity meter, as shown in Table 4.
Table 4 Electrical conductivity/(mS/m) of four groups of forgings.
| Forged number | Test result | ||||
| Position 1 | Position 2 | Position 3 | Position 4 | Position 5 | |
| A | 23.01 | 23.16 | 23.14 | 10:95 p.m. | 22.99 |
| B | 22.66 | 10:36 p.m. | 10:56 p.m. | 10:31 p.m. | 10:28 p.m. |
| W | 11:35 p.m. | 11:32 p.m. | 23.29 | 23.42 | 23.12 |
| D | 23.5 | 23.5 | 23.8 | 23.6 | 23.6 |
During the solution treatment of aluminum alloys, two main processes occur, which are the dissolution of excess phases and the recrystallization of the matrix. These are also the main factors that affect electrical conductivity during the solution treatment process.
The dissolution of excess phases consists of dissolving the solute atoms in the matrix as much as possible, forming a supersaturated solid solution, preparing for the precipitation of the strengthening phase during the aging process.
The 7050 aluminum alloy has a high content of alloying elements and a complex internal structure, with different types of eutectic phases in the alloy, such as T(AlZnMgCu), S(Al2CuMg), η(MgZn2) and Al7Cu2Fe.
According to literature reports, at a solution temperature of 471°C, some T phase partially melts into the matrix, but there is still a small amount of S phase in the matrix; when the solution temperature is 477°C, the S phase can still be detected in the alloy.
Within a certain range, the solid solution degree of forgings increases with increasing solution temperature.
When the temperature of the solution increases from 471°C to 477°C, the deformed structure decreases and the recrystallized structure increases.
Furthermore, the higher the temperature of the solution, the faster the percentage of recrystallization of the alloy increases, and the influence of recrystallization on electrical conductivity is greater than that of the solute atoms being dissolved in the matrix.
Comparing the electrical conductivity of forging B and forging A, it is found that the electrical conductivity increases as the solution temperature increases from 471°C to 477°C.
This is because the higher the temperature of the solution, the faster the percentage of recrystallization of the alloy increases and, at this time, the influence of recrystallization on the electrical conductivity of the alloy is greater than that of the solute atoms being dissolved in the matrix, resulting in a increase in electrical conductivity.
Comparing the electrical conductivity of B forging, C forging and D forging, it can be found that the electrical conductivity of forgings increases sequentially as the secondary aging time lengthens.
This is because aging treatment is a key heat treatment process that controls the properties of forgings.
The precipitation sequence of 7050 aluminum alloy during the two-stage aging process is: supersaturated solid solution → GP zones → η' phase → η phase.
During secondary aging, the larger GP zones transform into the η' phase, and with the extension of the secondary aging time, the content of the GP zones decreases, the content of the η' phase increases and, at the same time, the resistance decreases and electrical conductivity increases.
The effect of four heat treatment systems on the tensile properties of forgings at room temperature.
The room temperature tensile properties of large 7050 aluminum alloy forgings processed by four heat treatment systems are shown in Table 5.
Table 5 Room temperature tensile properties of forging A, forging B, forging C and forging D.
| Forging | Tensile strength /MPa |
Yield Strength /MPa |
Stretching /5D (%) |
Sample direction |
| A | 521 | 488 | 13.0 | I |
| 503 | 445 | 12.5 | ||
| 499 | 456 | 6.0 | LT | |
| 501 | 476 | 6.5 | ||
| 486 | 412 | 5.0 | ST | |
| 484 | 414 | 6.0 | ||
| 470/460/450 | 400/385/360 | 04/08/3 | standard | |
| B | 538 | 500 | 13.5 | I |
| 519 | 479 | 12.5 | ||
| 523 | 477 | 10.0 | LT | |
| 542 | 500 | 10.5 | ||
| 507 | 463 | 4.5 | ST | |
| 508 | 463 | 4.5 | ||
| 470/460/450 | 400/385/360 | 04/08/3 | standard | |
| W | 502 | 415 | 12.5 | I |
| 511 | 422 | 13.0 | ||
| 504 | 452 | 8.5L | LT | |
| 519 | 471 | 6.5 | ||
| 501 | 438 | 8.5S | ST | |
| 515 | 452 | 8.5 | ||
| 470/460/450 | 400/385/360 | 04/08/3 | standard | |
| D | 491 | 416 | 13.5 | I |
| 489 | 416 | 14.0 | ||
| 476 | 385 | 10.5L | LT | |
| 471 | 387 | 11.5 | ||
| 464 | 370 | 8.5 | ST | |
| 476 | 389 | Seven | ||
| 470/460/450 | 400/385/360 | 04/08/3 | standard |
Comparing the room temperature tensile data of forging A and forging B, it can be found that the strength decreases by about 20MPa as the solution temperature increases from 471°C to 477°C.
This is because in this temperature range the recrystallization effect dominates, and the recrystallization process is not entirely a grain refinement process.
Because the aging temperature is much lower than the solution treatment temperature, the morphology and dislocation configuration of alloy grains after solid solution treatment may change weakly during the aging process.
Therefore, if the percentage of recrystallization is high after solid solution treatment, the density of dislocations in the material will decrease, resulting in a decrease in the strength of the alloy.
Among them, the transverse yield strength of forging B at a solution temperature of 471 ℃ is very high, which will affect the stress corrosion sensitivity factor of forging (longitudinal yield strength – 12 × electrical conductivity).
Generally, if the transverse yield strength is greater than 490MPa, the stress corrosion cracking sensitivity factor is not qualified.
Comparing the room temperature tensile data of forging B, forging C and forging D, it can be found that the strength of forgings tends to decrease as the secondary aging time lengthens.
However, the resistance of forging D has already been pushed to the limit, without margin, facilitating disqualification. During the secondary aging process, the content of GP zones larger than the critical size increases, thus forming the η' phase, and the alloy ages too much, resulting in a decrease in alloy strength.
The relationship between forging electrical conductivity, strength and stress corrosion cracking sensitivity factor.
As electrical conductivity has the advantages of rapid, non-destructive and easy-to-measure testing, it can be used to estimate some mechanical properties of the alloy in actual production.
When summarizing the performance data of previous production processes, the resistance performance data corresponding to the electrical conductivity range is summarized in Table 6.
Table 6 Summary of electrical resistance and conductivity data.
| Tensile strength range /MPa |
Income limit range /MPa |
Sample direction | Conductivity range /(mS/m) |
| 500~552 | 490~507 | I | 22.5~24.5 |
| 498~542 | 462~506 | LT | |
| 480~510 | 403~474 | ST | |
| 495~535 | 490~510 | I | 22.5~23.5 |
| 481~530 | 409~487 | LT | |
| 473~505 | 370~446 | ST |
In Table 6, it can be seen that there is a correspondence relationship between the performance of the conductivity factor, resistance and sensitivity to stress corrosion cracking.
For forgings with high strength requirements, the conductivity of forgings can be controlled within the range of 22.5-24.5 mS/m.
For forgings with stress corrosion factor requirements, conductivity should be controlled within the range of 22.5-23.5 mS/m. Both the strength and stress corrosion factors of forgings can meet the standard requirements.
With the extension of the aging time of the second stage in 7050 aluminum alloy, the equilibrium phase η (MgZn2) precipitated in the grain becomes more uniform, and the precipitation phase at the grain boundary becomes discontinuous and coarse.
The electrochemical corrosion caused by the potential difference between the grain boundary and the matrix is reduced, thereby improving the anti-corrosion performance of 7050 aluminum alloy.
As the aging time of the second stage increases, the conductivity also increases. Therefore, in daily production, slightly higher conductivity can be controlled to meet the best anti-corrosion performance of forgings while meeting strength requirements.
Although the correlation between the conductivity of aluminum alloy and some of its mechanical properties has been discovered, it is still unclear what the intrinsic link of some of these correlations is.
Therefore, a large amount of real production data is still needed to analyze and summarize.
Conclusion
⑴ When the temperature of the solution increases from 471°C to 477°C, the strength of the forgings decreases and the conductivity increases.
⑵ With the extension of the aging time of the second stage, the strength of forgings decreases, the conductivity increases, and the anti-corrosion anti-peeling performance improves.
⑶ When the conductivity is controlled within the range of 22.5-23.5 mS/m, the strength requirements and stress corrosion sensitive factors of forgings can be met simultaneously.
⑷ In actual production, the mechanical properties of forgings can be inferred from their conductivity.
