As a light alloy with high strength and toughness, good corrosion resistance, good workability and welding performance, aluminum alloy is widely used in the aerospace field. Due to its excellent comprehensive performance, it has become the skin and frame of aircraft and the main structure of spacecraft. The preferred material [1-3], the main role is to bear and transmit loads . At present, most aircraft in our country have the phenomenon of “flying less and parking more”. Among them, more than 95% of the calendar life of military aircraft is parked on the ground, and the ground corrosion effect is much higher than that of air fatigue, which is the main factor affecting the calendar life of military aircraft. 5-7].
Compared with military aircraft, the ground parking time of civil aircraft can account for up to 60% . Taking the x7 and x8 aircraft as examples, the ground parking time accounts for more than 95% of the service period, and the corrosion problem has a serious impact on the life of the aircraft. Considering the influence of different regions, the corrosion damage is dominant for the aircraft parked in the coastal state for a long time, and the service state is close to the pre-corrosion fatigue process. For military aircraft that often operate at high altitudes and civil aircraft that often start and stop, the influence of the alternating action between high-altitude corrosion fatigue and ground corrosion needs to be considered. The service process can be considered as the result of the alternation of “corrosion + corrosion fatigue”.
When ignoring the tiny corrosion damage caused by high-altitude flight, it is considered that corrosion damage is dominant when the aircraft is parked at the airport, and fatigue damage is dominant when flying at high altitude, and its service process is close to the alternating process of “corrosion+fatigue+corrosion+fatigue+…”. During the one-time service of the spacecraft, the exposure to the corrosive environment during the ground test and assembly process is short, and the aluminum alloy hardly corrodes in the space environment .
In the process of reuse, in addition to the fatigue problem caused by the alternating load, it also faces the corrosion problem during the repeated ground maintenance, test and assembly process. Different from the corrosion fatigue of the aircraft, the reusable spacecraft does not have the influence of high-altitude corrosion during the service process, and is more in line with the “corrosion-fatigue” alternating process. With the continuous development of the aerospace industry, high-strength 2xxx series (2A12, 2024, 2219, 2090, etc.) and ultra-high-strength 7xxx series (7050, 7075, 7A04, 7055, etc.) aluminum alloys are widely used in the aerospace industry. The most widely used and used alloys are mainly used in wings, fuselage skins, upper and lower edge strips of spar, pressure chambers and fairings . At the same time, there are also some 5xxx series (5A06, 5456, 5086, 5B70, etc.) rust-proof aluminum alloys, which are often used as aircraft engines, gearboxes, bracket structures and Spacecraft integral panels, etc. , as well as a small amount of 6xxx series (6A02, 6013, etc.) and 8xxx series (8091, etc.) aluminum alloys, are used to manufacture aircraft engine parts. The corrosion is mainly concentrated in the fuselage skin, the wing front spar web, the upper and lower edge strips of the wing spar, the outer skin rivets of the fuselage truss, the tail frame and other parts, which are also the dangerous parts of corrosion fatigue failure .
Corrosion fatigue failure is a common failure form of components under the synergistic/interaction of corrosive environment and alternating loads. It is not only the superposition of simple corrosion and fatigue effects, but the synergistic effect produced by the interaction of corrosion factors and fatigue stress is greater than either of the two effects alone [13-15]. For decades, domestic and foreign scholars have carried out a large number of corrosion fatigue test research, and made outstanding contributions to the exploration of the corrosion fatigue mechanism of aerospace aluminum alloys, the influencing factors of corrosion fatigue and the change of fatigue life. In recent years, the unique characteristics of the fatigue life of aluminum alloys under the alternating action of corrosion fatigue have been found. Starting from the corrosion fatigue mechanism of aluminum alloys, this paper introduces the current main corrosion fatigue test techniques, summarizes and analyzes the main factors affecting corrosion fatigue and the characteristics of fatigue life under alternating corrosion fatigue, and forecasts the future development trend of aluminum alloy corrosion fatigue.
1 Corrosion Fatigue Mechanism Of Aluminum Alloys
1.1 Corrosion fatigue crack initiation mechanism
In the corrosive environment, the initiation of fatigue cracks is a complex process, which is closely related to the corrosive environment, mechanical factors, material factors, etc. Among them, the corrosive environment has an important influence. The corrosion fatigue mechanism found in the current research is mostly dominated by corrosion factors. There are the following:
- Local corrosion theory. Cracks are usually initiated under the synergistic effect of corrosive environment and cyclic loading, and are caused by corrosion pits formed on the surface of the material and stress concentration caused by local defects [16-17]. At present, it is believed that the corrosion pits formed by pitting corrosion in localized corrosion play an important role in the initiation process of fatigue cracks in aluminum alloys [18-19], while the effect of corrosion on the fatigue life of the high stress cycle region is not obvious, but it is obvious in the low stress region  ]. This theory is applicable to aluminum alloy materials that are sensitive to pitting corrosion and have localized corrosion, and has certain limitations, while aluminum alloy materials that are not sensitive to pitting corrosion can also produce corrosion fatigue processes without localized corrosion .
- Anodic dissolution-membrane rupture theory. The passivation film on the aluminum alloy surface is broken under the action of external force. The part protected by the oxide film and the partially exposed part constitute a galvanic cell in a corrosive environment. The exposed area is continuously dissolved as an anode until the oxide film is repaired, and then the process of film rupture – anode dissolution – film repair is repeated continuously, thus causing fatigue cracks germination.
- Surface adsorption theory. The surface of the aluminum alloy in the corrosive environment will adsorb active substances, reducing its surface energy, surface strength, and mechanical properties. Under the action of cyclic stress at this time, it is easy to cause fatigue damage. After the hydrogen atoms are adsorbed on the surface of the aluminum alloy, they will diffuse into the interior to cause hydrogen embrittlement damage.
1.2 Corrosion fatigue crack growth mechanism
In the process of corrosion fatigue, taking a smooth specimen as an example, the crack initiation life only accounts for 10% of the total corrosion fatigue life, while the crack propagation life accounts for 90%. Therefore, the service life of corrosion fatigue specimens is dominated by corrosion fatigue extended life. The corrosion fatigue propagation mechanism is partially similar to the initiation mechanism, and there are three main corrosion fatigue crack propagation theories:
- Deformation activation promotes anodic dissolution mechanism;
- Hydrogen embrittlement mechanism;
- Surface energy drop model.
In the early days, it was believed that the corrosion fatigue crack growth of aluminum alloys was dominated by the anodic dissolution theory, and the anodic dissolution mechanism explained the local anodic dissolution of periodically exposed crack tips, which would accelerate the crack growth of corrosion fatigue, as shown in Figure 1. The anodic dissolution mechanism depends on the rupture of the protective film at the crack tip and subsequent repassivation of the newly exposed fresh metal surface. The growth rate of corrosion fatigue cracks will be controlled by the anodic dissolution rate, repassivation rate, oxide film rupture rate, and mass transfer rate of reactants to the dissolved surface of the exposed surface. Further research found that the anodic dissolution of aluminum in the matrix would generate hydrogen. Figure 2 shows the process of hydrogen-induced cracking crack propagation under stress:
- The stress at the crack tip concentrates and causes dislocation movement;
- Under the action of stress, the interaction between hydrogen and dislocations and the diffusion of hydrogen atoms increase the hydrogen concentration at the crack tip;
- When the hydrogen concentration at the crack tip reaches a critical value, hydrogen embrittlement occurs, causing the tip crack to propagate forward, and the process is repeated.
At present, for the propagation of corrosion fatigue cracks, people are more and more inclined to the mechanism of hydrogen embrittlement, but how hydrogen diffuses into the material and causes hydrogen embrittlement failure still needs to be explored from the microscopic mechanism of hydrogen-induced cracking of alloys. The surface energy drop model is similar to the adsorption theory in the crack initiation mechanism. Both this model and the hydrogen embrittlement mechanism belong to the environment that causes the fatigue properties of the material itself to change. However, the surface energy drop model has a small application range and the effect of adsorbed active particles on the material. The mechanism is also unclear .
The corrosion fatigue crack initiation and propagation mechanisms of materials are closely related to the corrosive environment, and the initiation of cracks is greatly affected by the pitting corrosion pits produced in localized corrosion. Both the anodic dissolution mechanism and the hydrogen-induced cracking theory have a great influence on the crack propagation. In corrosion fatigue dominated by anodic dissolution, crack propagation is the process of alternating stress and chemical reaction and dissolution of the material surface in the corrosive medium. In corrosion fatigue dominated by hydrogen-induced cracking, hydrogen embrittlement occurs in the matrix due to the diffusion of hydrogen atoms into the matrix, and brittle cracks are generated and propagated under the action of alternating stress. These two mechanisms do not exist independently, but are in a state of coexistence and competition. At the same time, studies have found that the corrosion fatigue crack growth mechanism does not exist alone, and often a certain mechanism is the main mechanism, and multiple mechanisms exist in combination.
2 Corrosion Fatigue Test Technology
Due to the extremely long failure process of the material in the real service environment, it is difficult to carry out fatigue research in the actual corrosive environment. Therefore, it is necessary to establish an accelerated corrosion fatigue test method under laboratory conditions, including the equivalence of the actual service environment and the laboratory corrosion environment and the simulation of the corrosion fatigue environment.
2.1 Simulation of corrosion fatigue environment
At present, corrosion fatigue is mainly simulated in three coupling forms:
- Corrosion fatigue synchronous test technology, that is, the corrosion environment and alternating load act simultaneously until failure occurs, and the obtained test data is the result of the synergistic effect of fatigue factors and environmental factors. This type of experimental operation is relatively complex, as shown in Figure 3. Generally, it is necessary to make a corrosion environment box on the fatigue testing machine to establish the corrosion fatigue environment condition, so as to realize the fatigue durability test and crack state detection of the material under the corrosive medium. Huang Xiaoguang et al.  used the “environmental box” to study the effect of pH change on the corrosion fatigue crack growth of LY12CZ aluminum alloy under the synergy of 3.5% NaCl corrosion fatigue, and found that in a neutral environment, the crack growth was caused by the anodic dissolution mechanism. However, as the pH decreases, this leading role is gradually replaced by the hydrogen evolution reaction. The hydrogen ions in the acidic environment cause hydrogen embrittlement of the crack tip material, and the corrosion fatigue crack growth rate will be greatly increased. Wang Chiquan et al.  studied the fatigue life of two aviation aluminum alloys (2E12-T3, 7050-T7451) under 3.5% NaCl corrosion fatigue synergy, and found that as the stress level decreased, the synergistic effect of corrosion and fatigue load increased, The decrease in fatigue performance is more pronounced.
- Pre-corrosion fatigue test technology, that is, after corrosion and nucleation, the fatigue load is increased until fatigue failure. According to the data [5-7], the fatigue loading time of my country’s military aircraft is less than 5% of the calendar time, while more than 95% of the time is in a grounded state, and the high-altitude corrosive environment has little effect on the fatigue strength. For this type of aircraft, the model of ground corrosion first and air fatigue later is generally used, that is, the pre-corrosion fatigue test technology is used. However, when the aircraft has many coastal missions and the fatigue loading time is long, it is necessary to consider the alternating effect of corrosion and fatigue. Research has found that damage from pre-corrosion accelerates the fatigue failure process. Huang Yanfeng et al.  studied the characteristics of corrosion pits and fatigue crack growth of 7075-T6 aluminum alloy under different pre-corrosion times. With the increase of corrosion time, the surface size of corrosion pits increases, which usually converges with adjacent corrosion pits to form concave pits. Pit clusters, where multiple cracks originate from a single pit or dimple cluster and propagate at higher stress levels. Liu et al.  conducted pre-corrosion fatigue tests on 2024-T62 aluminum alloys and found that pre-corrosion had a significant effect on the fatigue S-N curve and fatigue crack initiation behavior, but had no effect on the crack propagation behavior. Xu Liang et al.  pointed out that due to the existence of pre-corrosion damage, the crack initiation life after pre-corrosion only accounts for less than 20% of the total life, and the fatigue life decreases sharply. Sankaran et al.  conducted pre-corrosion experiments on 7075-T6 aluminum alloy by cyclic salt spray method, and found that pitting corrosion would reduce the fatigue life to 1/6 to 1/8 of the original. In the research on corrosion fatigue of high-strength aluminum alloys for aircraft, there is also a prediction study on corrosion fatigue life based on finite element simulation. Medina-Pérez et al.  combined corrosion tests with finite elements to study the fatigue life of pre-corroded airfoils, and accurately evaluated the fatigue life of pre-corroded aircraft wings. Cerit et al.  studied the stress distribution at the semi-elliptical corrosion point based on finite element analysis, and found that the aspect ratio (a/2c) of the pit is the main parameter affecting the stress concentration factor (SCF). (Pitting) Stress concentration at the location of corrosion and is a potential cause of fatigue crack nucleation. Hu Ping  used the elastic-plastic damage constitutive equation and the elastic-plastic damage evolution model to evaluate the fatigue damage, proposed a continuous damage mechanics method, and improved the pit evolution model, and used ABAQUS numerical simulation to effectively predict the Corrosion fatigue life of aluminum alloys. Lv Shengli et al.  used AFGROW software to simulate the influence of various corrosion damage and corrosion pit depth on the remaining fatigue life of the specimen, predicted the stress and crack nucleation sites caused by corrosion pits, and established an effective and reliable prediction. Engineering methods for fatigue life of corroded specimens. At present, in the research of corrosion fatigue of aircraft aluminum alloy, the application of pre-corrosion fatigue test technology is extensive and comprehensive.
- Corrosion-fatigue alternate experiment technology refers to the interactive experiment of environmental corrosion and load fatigue of materials under a certain cycle. The interaction between corrosion and fatigue can lead to changes in the fatigue properties of materials. At present, scholars generally believe that when the aircraft is parked at the coastal airport, it is almost not affected by the fatigue load, and corrosion damage dominates; when the aircraft is in service in the air, the damage caused by the high-altitude corrosive environment can also be ignored, and the fatigue damage accounts for leading. Therefore, it is considered that in the coastal environment, military aircraft with many high-altitude flying missions and civil aircraft with frequent starting and stopping are subjected to alternating effects of corrosion and fatigue during the service process, that is, the cycle process of “corrosion + fatigue + corrosion + fatigue + . . . “. The first alternating process can be equivalent to the pre-corrosion fatigue process, but generally has a smaller impact than the corrosion damage caused by pre-corrosion. After the structure is corroded and damaged, it is subjected to fatigue again, and the two alternate in a certain cycle. This alternate form is also in line with the corrosion fatigue effect of reusable spacecraft during multiple air-to-space round trips and ground repairs. Chen Yueliang et al.  compared the fatigue life of LY12CZ aluminum alloy under the “corrosion-fatigue” and “fatigue-corrosion-fatigue” tests in order to study the effect of the alternating sequence of corrosion and fatigue on the life of aluminum alloy, and found that the latter condition The fatigue life under the pre-fatigue life is longer, and the increase decreases with the increase of the pre-fatigue life, but there is no significant difference in the effect of the alternating sequence of corrosion and fatigue on the fatigue life.
At present, in the research on the coupling form of aluminum alloy corrosion fatigue, pre-corrosion fatigue is mostly, especially for the impact of pitting corrosion and corrosion pits generated by pre-corrosion on crack initiation and growth and fatigue life, and for the synergy and alternation of corrosion fatigue. further research. In the complex interactive form, the crack development mechanism under multiple influencing factors is not clear, and there is still no unified conclusion on the influence law on fatigue life.
2.2 Equivalent in corrosive environment
Real, reliable and intuitive corrosion data can be obtained under the natural environment exposure test, but it is limited by the long test period, high cost and difficult realization. In order to obtain accurate and rapid corrosion fatigue test data in the test, the research of laboratory accelerated corrosion method is of great significance. In order to achieve the corrosion effect of several months or even years in the actual service environment in a short time, it is required to compile the accelerated corrosion equivalent environmental spectrum. The specific principles are as follows [38-39]:
- The principle of equal local corrosion damage in key parts of fatigue must be followed;
- It must be able to reproduce the corrosion damage form, characteristics and corrosion product composition of the key parts of the actual structure in the actual service environment;
- Determine the accelerated equivalent relationship between the corrosion time and the actual corrosion time, that is, under the same reliability conditions, the ratio of the actual corrosion time corresponding to the same corrosion depth to the laboratory accelerated corrosion time;
- The accelerated corrosion equivalent environmental spectrum should be simplified as much as possible to ensure that the corrosion test environment is easy to achieve.
The accelerated corrosion equivalent relationship is the “link” between the laboratory environment and the actual environment, but the determination of the equivalent corrosion time is still a technical problem in domestic and foreign research. At present, the methods for establishing the accelerated corrosion equivalent relationship are as follows:
- Equivalent conversion method based on the criterion that the metal corrosion current Ic is equal;
- Based on the same corrosion damage, the fatigue strength (life) is the same fatigue strength comparison method;
- Corrosion degree comparison method with the same corrosion damage as the criterion.
However, the first two methods are difficult to measure the actual structural corrosion current Ic and the fatigue life of the key parts of the structure in engineering, and both of them consume a lot of time and money, so they are difficult to be widely used. Therefore, the corrosion degree comparison method is effective and feasible in the practical application of determining the accelerated corrosion equivalent relationship. Zhang Sheng et al.  conducted exposure tests on 2024-T4 aluminum alloy samples for 7, 12, and 20 years based on the Wanning test site environment, and then conducted atmospheric pre-corrosion fatigue tests, taking the thickness of the sample as the characteristic of corrosion damage.
According to the principle of equal corrosion damage characteristic quantities, the acceleration equivalent relationship is determined, and a laboratory accelerated corrosion test method for simulating coastal atmospheric corrosion is proposed, but the determined accelerated equivalent relationship is only applicable after exfoliation corrosion occurs. Liu Chengchen et al.  conducted natural exposure test in marine environment and laboratory accelerated corrosion test on 2A12 material, and measured the consistency of corrosion degree by the distribution law of corrosion pit depth formed by pitting corrosion, and obtained the corrosion degree at 95% confidence level. Equivalent relationship between accelerated corrosion time and natural exposure time. The equivalent focus of the corrosive environment is to establish the equivalent corrosion environment spectrum. To determine the equivalent corrosion relationship, it is necessary to have a large number of accelerated simulation test data and corrosion damage test data of key parts of the structure, and then carry out theoretical analysis and derivation, and establish a mathematical model. , the experimental period is longer and the difficulty is greater.
3 Factors Affecting Corrosion Fatigue
3.1 Material factors
The composition, structure and heat treatment state of different materials are different, and their corrosion resistance is also different, which directly leads to different corrosion fatigue resistance. The study found that when the Mg content in the 5xxx Al-Mg alloy is greater than 3.5%, the β phase (Al3Mg2) precipitates along the grain boundary as the anode to dissolve preferentially, which is prone to intergranular corrosion and stress corrosion . At the same time, the presence of Zn, Fe, Cu, Sn and other elements in the aluminum alloy will also reduce the corrosion resistance of the alloy. For the 7xxx series alloys (Al-Zn-Mg) without Cu element, they have better corrosion resistance. sex. In the Al-Mg-Si alloy A6061, when the Sn content is 0.03%, the grain boundaries can be refined and the corrosion resistance can be improved. However, when the Sn content is high (0.4%), the anode Mg2(Si,Sn) phase is formed and severe corrosion occurs . However, with the addition of Sc and Zr elements, it has the promotion effect of grain refinement and recrystallization, and has a good inhibitory effect on intergranular corrosion and exfoliation corrosion of alloys.
The impact needs further study. The non-heat-treatable-strengthened alloys of the 3xxx and 5xxx series generally have higher corrosion resistance compared to the heat-treatable-strengthened alloys of the 2xxx and 7xxx series . The presence of Al2CuMg and Al2Cu metal compounds in 2024 aluminum alloy can improve its mechanical properties well, but increase the possibility of localized corrosion, mainly because of the susceptibility to intergranular corrosion , which usually requires artificial aging Or surface treatment to improve corrosion resistance. Moutarlier et al.  found that the corrosion resistance of 2024 aluminum alloy in NaCl solution was significantly improved after anodization. Gadpale et al.  found that 2014 aluminum alloy has higher corrosion resistance at lower aging temperature and shorter aging time, because its precipitates are uniformly distributed in the matrix. Niu et al.  analyzed the exfoliation corrosion performance of friction stir welding dissimilar welding of 2024 and 7075 aluminum alloys, and found that 2024 aluminum alloy has the best corrosion resistance, and the maximum corrosion depth is about 0.6 mm smaller than that of 7075 base metal.
The discontinuous distribution of grain boundary precipitates and precipitate-free zone in the stirring side of 2024 aluminum alloy can reduce the degree of intergranular corrosion on the advancing side. At the same time, it is pointed out that the susceptibility to intergranular corrosion is also different for different grain sizes. Holtz et al.  found that the corrosion fatigue properties of 5083-H131 aluminum alloys are related to grain boundary precipitates. Mc Mahon et al.  investigated the corrosion susceptibility of two alloys of different compositions (AA5083 and AA5456) in the tempered state (-H131, -H116 and -SHTQ), for constant -H116 high temperature, Mg-rich alloys AA5456 is less sensitive to intergranular stress corrosion than AA5083. Due to the low corrosion resistance of 2xxx and 7xxx series aluminum alloys, the main research on corrosion fatigue at present is concentrated in such alloys, and there is less research on 6xxx and 8xxx aluminum alloys. Kairy et al.  studied the effect of Cu content on the metastable pitting corrosion behavior of 6xxx series aluminum alloys under aging treatment. It is concluded that the over-aging treatment will form larger precipitates, and the pitting corrosion resistance is the greatest. Through electrochemical tests in 0.1 mol/L NaCl solution, it was found that the pitting corrosion rate decreased with the increase of Cu content. Due to the active chemical properties of Li element, aluminum-lithium alloys have high corrosion susceptibility in complex environments.
At present, a lot of research is devoted to improving the corrosion susceptibility of alloys by changing the type and distribution of precipitates through alloying [52-53] and heat treatment [54-55]. The AA8090-T81 alloy is susceptible to pitting corrosion damage in seawater, and the inhomogeneity of the microstructure will increase its pitting corrosion sensitivity, while the AA2090Al-Li alloy will have Al2Cu phase precipitated at the grain boundary during the aging treatment. , the Al2CuLi phase precipitated at the subgrain boundaries and dislocations, and the Cu-poor no-precipitation zone (PFZ) is formed along the grain boundaries and subgrain boundaries. The PFZ structure has a great influence on its corrosion performance . The composition affects the structure, and the structure determines the performance. Materials with different compositions have different corrosion resistance. The difference of added elements will also lead to changes in corrosion resistance, and the structure, precipitation phase, heat treatment, etc. of the material will affect the change of corrosion sensitivity. Different materials The corrosion fatigue properties of each have their own characteristics, and extensive research is still needed.
3.2 Environmental factors
3.2.1 Influence of medium and concentration
The corrosive environment of aerospace aluminum alloys has obvious regional characteristics. The influence of humid atmosphere and industrial atmosphere on the material exists in the inland environment; the influence of the marine atmosphere and seawater on the material exists in the coastal environment. Since the marine atmosphere and seawater form a corrosive environment with high oxygen content and high salt spray , the impact on the fatigue performance of materials is particularly strong. In laboratory experiments of simulated marine environment, NaCl solution with a mass fraction of 3% to 3.5% is often used for simulation . Scholars [59-60] found that the effect of different corrosive media on the corrosion fatigue life of 7xxx series aluminum alloys was in the order of “water in the tank > salt water > salt spray > moist air > laboratory air”, and found that the corrosion The presence of Cl- in the medium will aggravate the stress concentration effect of the sample. In addition, the effects of different corrosive media on the fatigue crack growth rate are also different. The crack growth rate of 7xxx series aluminum alloys in 3.5%NaCl corrosion environment is doubled compared with that in air.
In the low ∆K range, the crack growth rate of LY12-CZ in brine is 3 times higher than that in air, and the effect gradually weakens with increasing ∆K. Prabhu et al.  evaluated the corrosion behavior of 6xxx series Al-Mg-Si alloys in H3PO4 and NaOH solutions with different concentrations, respectively, and found that the corrosion rate in NaOH solution was significantly higher than that in H3PO4 solution, and the corrosion rate was significantly higher in NaOH solution. The rate increases with increasing acid, base concentration and temperature. The concentration of corrosive medium has a significant effect on the crack growth rate. Figure 4 shows the fracture morphology of LY12-CZ aluminum alloy in 3.5% NaCl solution and 5.0% NaCl solution. The crack growth rate was compared by the size of the formed stripes. Yang et al.  found that the crack growth rate in 5.0% NaCl solution was The fatigue crack growth rate is about 4 to 6 times faster than the crack growth rate in 3.5% NaCl solution, indicating that the concentration of the corrosive solution increases, the chloride ion damage to the passivation film increases, and the same corrosion medium under different concentrations has a great influence on the crack growth rate. Big. In addition, the study also found that the longer the action time of the corrosive solution, the greater the damage to the surface of the aluminum alloy, the faster the flow rate of the corrosive solution, and the easier the corrosive medium acts on the surface of the material. Peel off, the shorter the fatigue life.
3.2.2 Influence of temperature and humidity
Changes in ambient temperature and humidity will affect the corrosion fatigue life of high-strength aluminum alloys. In a high humidity environment, the content of water vapor and oxygen is high, and it is easy to react on the surface of the material to generate atomic hydrogen, which can cause hydrogen embrittlement under the action of cyclic loading and accelerate the crack growth rate. As the humidity decreases, the crack growth rate decreases, as demonstrated by crack growth experiments in air and humid air (conducted in the laboratory) . Higher temperature will increase the activity of the medium, accelerate the chemical reaction of the corrosive medium on the surface of the sample, accelerate the formation of corrosion pits, and promote crack initiation. At high corrosion temperature, the corrosion fatigue life of high-strength aluminum alloys will decrease. As shown in Figure 5, when the corrosion temperature increases from 25 °C to 75 °C, the average fatigue life of the alloy decreases, and the higher the temperature, the lower the average fatigue life. more .
3.2.3 Influence of pH value
The change of crack growth rate of aerospace aluminum alloy 7075 at different pH values is shown in Figure 6. With the decrease of pH value, the corrosion of the solution increases, the critical stress intensity factor of the alloy decreases, and the crack growth rate increases, which accelerates the corrosion. Fatigue destruction. In an acidic corrosive environment, the decrease of pH value will cause an increase in the concentration of hydrogen ions, which will promote the dissolution of the passive film on the surface of high-strength aluminum alloys, leading to hydrogen embrittlement or promoting crack growth through metal anodic dissolution ; in a neutral corrosive environment , the acceleration of fatigue crack growth is dominated by anodic dissolution; in an alkaline corrosion environment, due to the occurrence of redox reactions in the solution, hydroxides such as Al(OH)3 are precipitated, forming a passivation film and inhibiting the anodic dissolution of the crack tip. , the crack growth rate decreases. Sheng Hai et al.  found that the 2024-T351 aluminum alloy can form a stable oxide film on the surface of the NaCl solution with p H=5～7, but it is difficult to form the surface of the aluminum alloy in the NaCl solution with p H=3. The stable existing oxide film dissolves in direct contact between the base metal and the corrosive solution. The pH value of the laboratory to study the impact of the marine environment is generally 2 to 7.
3.3 Mechanical factors
3.3.1 Effect of stress ratio
The stress ratio has a significant effect on the corrosion fatigue crack growth rate. Generally speaking, with the increase of the stress ratio, the crack growth rate gradually increases.  measured the da/d N-∆K curves under different stress ratios (R=0.1, 0.3, 0.5), and found that with the increase of R value, the fatigue crack threshold value gradually decreased. Sabelkin et al.  studied the transformation process of corrosion pits to cracks in 7075-T6 aluminum alloy in 3.5% saline environment, and found that in the same corrosion environment, the crack initiation period decreased with the increase of applied fatigue stress. Liu et al.  studied the pre-corrosion experiments of 2024-T62 aluminum alloy under different stress ratios, and found that in laboratory air, there is a small crack effect when R=-1, but not when R=0.06; In 3.5% NaCl solution, when R=-1, 0.06, no small crack effect appears, indicating that the small crack effect under pre-corrosion is not obvious. However, when R=0.06, the effects of different corrosive media on crack propagation are basically the same. The corrosion fatigue crack growth rate has obvious threshold characteristics. It can be seen from the Paris formula that it increases with the increase of the stress intensity factor ∆K, but when ∆K is close to the threshold value, the crack growth rate increases continuously with the increase of the stress ratio. rise. When in the region of high ΔK level, the crack growth rates under different stress ratios are not much different. It can be considered that the stress ratio mainly affects the corrosion fatigue crack growth in the region near the threshold value.
3.3.2 Influence of loading frequency
The loading frequency has an important influence on the crack growth rate, but its influence mechanism needs to be further verified. It is generally accepted that lower frequencies are more favorable for the synergy between the corrosive environment and cyclic loading, which tend to promote crack growth [73-74]. At 1-10 Hz, the lower the loading frequency, the higher the corrosion fatigue crack growth rate . The explanation is that the reduction of the loading frequency makes the corrosion time relatively prolonged, and the hydrogen generated by corrosion diffuses sufficiently from the crack tip to the interior, causing hydrogen embrittlement. , to accelerate crack growth. But the experiment found that when the frequency was lower than 1 Hz, its crack growth rate was significantly lower than that of higher frequencies, and almost no difference in crack growth was found between 0.1 Hz and 1 Hz. Menan et al.  found that there is also a negative frequency dependence, that is, the crack growth rate of Al-Cu-Mg alloy in salt solution decreases with decreasing frequency, and a similar effect is also observed in Al-Zn-Mg alloy . From this, two hypotheses are proposed: one is the crack closure effect formed at low frequency, and the closure effect caused by corrosion reaches saturation in this frequency range; the other is the generation of passivation film and the mechanism of anodic dissolution and/or hydrogen embrittlement competition between them . Li Xudong et al.  proposed a model for the effect of loading frequency on the corrosion fatigue crack growth rate of aerospace aluminum alloys. The experimental verification is only applicable to the steady-state growth region, and the evaluation structure for crack growth close to the transient region is low. Shafiq et al.  also demonstrated that at lower loading frequencies, the corrosion fatigue crack growth rate decreases with the increase of loading frequency, and the effect of frequency on crack growth is closely related to the corrosion resistance of materials. Therefore, frequency is a key factor, and its effect on the corrosion fatigue crack growth rate has not been clarified, and research on a variety of different materials is needed to reveal its mechanism of action on corrosion fatigue crack behavior.
3.3.3 Influence of loading waveform
The waveform, as a function of frequency and stress intensity factor range (ΔKi), mainly affects the corrosion fatigue properties of aluminum alloys from frequency and dwell time. Loading waveforms include sine wave, triangle wave, square wave, sawtooth wave, etc. Regardless of the waveform, the longer the holding time in the high stress region, the higher the crack growth rate (FCGRs), and the stress corrosion is easily induced in the corrosive environment to accelerate the failure process. Menan et al.  studied the variation of crack growth rates (FCGRs) of 2024 aluminum alloy under sine wave and sawtooth wave, as shown in Fig. 7, and found that at a frequency of 1 Hz, the FCGRs under negative sawtooth wave were significantly different from those under high sawtooth wave. The FCGRs measured at the same frequency were similar and higher than those measured at the same frequency under the sinusoidal signal. Regardless of cycle duration, with a positive sawtooth wave, the propagation at the same frequency is slower than with a sinusoidal waveform. Kuang et al.  conducted fatigue tests on uncorroded specimens in the high-low loading sequence, and found that the critical cumulative damage was less than 1; while under the low-high loading sequence, the critical cumulative damage was greater than 1. However, the accumulation law of fatigue damage of the specimens after pre-corrosion shows the opposite law.
3.4 Alternate factors
The most complicated effect of corrosion fatigue is the interaction between corrosion environment and cyclic stress. This interaction refers to the interaction between corrosion environment and alternating load fatigue, that is, alternating within a certain period. In the case of multiple alternations, the cumulative characteristics of corrosion fatigue damage need to be considered. Fatigue damage increases cumulatively with applied load cycles, resulting in material failure with corrosion time. Yao Weixing et al.  proposed a cumulative fatigue damage rule based on LY12-CZ aluminum alloy under alternating corrosion or cyclic loading, and obtained a good agreement with the test results under different combinations of corrosion time, load level and cycle times. Remaining fatigue life. Li Xiaohong et al.  studied the damage behavior of 2A12 aluminum alloy in alternate corrosion fatigue tests, and the coupling effect of corrosion and fatigue will aggravate the reduction of fatigue life. The more frequent the alternating process of corrosion fatigue, the more serious the damage to the specimen. Among many influencing factors, the influence of unit corrosion time and unit fatigue cycle on the alternate process of corrosion fatigue is particularly important.
The corrosion alternating frequency affects the single corrosion time of the sample. Under the condition of a certain total corrosion time, the higher the corrosion fatigue alternating frequency, the shorter the relative corrosion time, the fatigue is dominant in the alternating process, and the fatigue life is relatively high. Cui Tengfei et al.  conducted equivalent experiments on 7B04-T6 aluminum alloy, and the laboratory cycle accelerated corrosion for 192 h (equivalent to 12 months of external field exposure) to simulate 2, 3, 6, and 12 external field corrosion. 1, 2, 4 and 6 alternating frequency tests of corrosion fatigue were carried out respectively, as shown in Figure 8 (n represents the alternating frequency). The study proved that the alternating frequency of corrosion fatigue increased, and the fatigue life was relatively increased. . This is because the alternating frequency determines the corrosion time. The longer the corrosion time is, the easier it is to cause stress concentration, thereby promoting the initiation and expansion of micro-cracks in the fatigue loading stage, providing more corrosion channels for the next corrosion, resulting in more serious damage. Chen Yueliang et al.  found that the alternate life of corrosion fatigue may exceed the total fatigue life, which means that the alternate test can improve the fatigue life. Chen Yajun et al.  studied the effect of alternating corrosion multiaxial fatigue behavior of 7075-T651 aviation aluminum alloy under constant unit fatigue cycles n and different unit corrosion times t.
The increase in the frequency of alternation, the longer the total corrosion time. Figure 9 shows the change of the surface morphology of the sample with the alternation frequency. When t=6 h, under the constant alternation of corrosion and multiaxial fatigue loads, the damage accumulation intensifies, and the previous corrosion damage will accelerate the initiation and propagation of fatigue cracks. After the corrosion time increases, the corrosion damage dominates, which greatly reduces the fatigue life, until the cumulative damage reaches the maximum value and fracture failure occurs. When the unit corrosion time and the fatigue cycles change at the same time in the alternating process, Zhang Haiwei et al.  proposed to use the flight strength=fatigue cycle cycles/corrosion days to define the loading mode for the corrosion fatigue alternate life of LY12CZ aluminum alloy. As shown in Figure 10, the greater the flight strength, the greater the influence of fatigue factors and the smaller the corrosion effect. Under the same flight strength, the larger the single fatigue cycle, the higher the corrosion fatigue life. At the same time, the fatigue life under alternating corrosion fatigue is greater than the pre-corrosion fatigue life. However, when the single corrosion time and the fatigue cycles change proportionally, it is impossible to simply judge the damage degree of corrosion or fatigue, and it is necessary to compare the influence degree and dominance of the two. At present, there is no certain law about the effect of corrosion fatigue on the life under the alternating action.
4 Conclusion And Outlook
This paper mainly reviews the research status of corrosion fatigue of aerospace aluminum alloys in recent years. Corrosion fatigue is an extremely complex problem, and the influencing factors are complex and changeable. The current research focuses on pre-corrosion fatigue, and the research on corrosion fatigue under the synergistic/alternative action of corrosion-fatigue is still in its infancy. , fatigue life and other aspects of research still need to be further explored, there are still the following aspects to be further studied:
- Change laws of fatigue behavior and related mechanisms under the synergistic/alternating action of corrosion-fatigue. The research of corrosion fatigue is to accurately obtain the fatigue life prediction under the determination of many influencing factors, different corrosion fatigue mechanisms, and different crack forms, and to establish a model that can effectively predict the corrosion fatigue life.
- The equivalent of corrosion time is equivalent. In the corrosion fatigue experimental research, the test is usually carried out in an accelerated corrosion environment, but there is no mature equivalent relationship between the actual corrosion time in the natural environment and the corrosion time established in the laboratory, and it is difficult to ensure the test results in the laboratory accelerated corrosion research. reliability.
- Corrosion fatigue damage evolution. The damage evolution of corrosion fatigue is an extremely complex problem, which is highly dependent on materials and environments. A large number of experiments are needed to study the corrosion fatigue damage evolution model.
- Numerical simulation and experimental research are organically combined. Using simulation technology, combining theory with practice, that is, organically combining numerical simulation and experimental research, is of great significance to the study of aluminum alloy corrosion fatigue problems.
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