Fatigue failure is one of the important reasons for the failure of aluminum alloy components. At the same time, due to the difficulty in detecting fatigue cracks in aluminum alloy components during use, preventive measures are difficult to implement and can easily cause catastrophic consequences. Therefore, it is necessary to understand the fatigue characteristics of aluminum alloys.

1. Characteristics and classification of fatigue

The process of local permanent structural changes (such as cracks) or complete fracture of materials under sufficient cyclic loading is called fatigue. Fatigue can be divided into stress fatigue and strain fatigue based on the magnitude of cyclic stress and fatigue control methods. Stress fatigue refers to the phenomenon where a material experiences a relatively low maximum cyclic stress and only undergoes elastic deformation during cycling, resulting in a high fatigue life (greater than 105 cycles), also known as high cycle fatigue; Strain fatigue refers to the maximum cyclic stress that a material experiences, which is generally greater than its yield limit. During the cycling process, the material undergoes plastic deformation, resulting in a lower fatigue life (less than 105 cycles), also known as low cycle fatigue.

There are many significant differences between high cycle fatigue damage and traditional static damage:

1) The cyclic stress borne by the alloy is much lower than the yield limit of the material, and fatigue failure may occur.

2) Regardless of whether the alloy material is plastic or brittle, there is no significant plastic deformation during fatigue fracture, and it is a sudden brittle fracture. Therefore, the damage caused by fatigue damage is very serious.

3) The fatigue fracture surface is significantly different from the traditional tensile fracture surface. The fatigue fracture surface can clearly show features such as crack initiation zone, crack propagation zone, crack instantaneous fracture zone, fatigue striation, and secondary cracks; Fatigue fracture often contains a lot of information, and to some extent, the fatigue damage mechanism can be summarized through fracture analysis.

4) Fatigue damage is a process of continuous accumulation of internal damage in materials. Materials will be subjected to cyclic loading for a considerable period of time before fracture and failure occur. Crack initiation and propagation account for the vast majority of fatigue damage time, while fatigue fracture only accounts for a small part of the total fatigue life. Therefore, the research focus on fatigue damage is on the initiation and propagation mechanism of fatigue cracks, and targeted measures are taken to suppress the generation and propagation of cracks, thereby improving the fatigue performance of materials.

The fatigue performance of alloys is influenced by various factors, such as microstructure and external environment, so the failure process is relatively complex. However, according to its development process, it can be roughly divided into the following three stages:

1) Crack initiation stage. Under continuous alternating loads, materials will develop small cracks internally. Generally speaking, fatigue cracks originate from internal defects of the material, such as pore inclusions, coarse second phase particles, and slip bands.

2) Crack propagation stage. Once fatigue cracks occur, under continuous external loads, the microscopic cracks in the material will undergo irreversible propagation. In the initial stage, the fatigue crack propagation rate is slow and spreads through shear. The cracks are also very small, with a size of about 10 μ m. At this stage, cracks undergo transgranular propagation under the obstruction of grain boundaries, and the propagation rate gradually increases.

3) Unstable fracture stage. Due to the continuous propagation of fatigue cracks, the cracks reach a critical size, and the material cannot withstand fatigue loads and rapidly expands, leading to instantaneous fracture.

2. Factors affecting the fatigue performance of aluminum alloys

The factors that affect the fatigue crack propagation of aluminum alloys include two aspects: internal factors and external factors.

2.1 Internal influencing factors

The internal influencing factors that affect the fatigue crack propagation of aluminum alloys mainly include grain size, grain orientation, anisotropy, specimen defects, residual stress, etc.

2.1.1 Grain size

During the crack propagation stage, grain boundaries hinder dislocation slip and slow down the crack propagation rate (FCPR), thereby improving the fatigue life of aluminum alloys. In theory, refining grains can increase the proportion of grain boundaries per unit volume and improve fatigue life, but this contradicts experimental results, which show that as grain size increases, the fatigue crack growth threshold (Δ Kth) gradually increases and the FCPR gradually decreases.

PAOPS et al. believe that this is mainly caused by crack deflection and crack closure effects under low stress ratios. After Wang Bo's research, it is believed that the deflection degree of cracks in the alloy increases significantly with the increase of grain size, the FCPR decreases, and there is a clear phenomenon of premature crack closure in local areas. WANGQG et al. found that the fatigue life of A356/A357 alloy is determined by its microstructure (secondary dendrite arm spacing, SDAS), eutectic structure, and heat treatment state; And for aluminum alloys treated with Sr, when SDAS<60 μ m, the fatigue life of the alloy decreases with the increase of SDAS; When SDAS>60 μ m, the fatigue life of the alloy increases with the increase of SDAS.

2.1.2 Grain orientation

Cracks will deflect during propagation, and grain orientation is a key factor affecting microcrack deflection. ZHAIT et al. first proposed that the twist angle α and tilt angle β on the crack surface at the grain boundary will affect crack propagation, and established a fatigue crack propagation crystal model. Cracks crossing grains with small twist angles require less energy consumption, so their impact on FCPR is not significant; However, large twist and tilt angles can cause difficulty in crack propagation, delaying or hindering FCPR.

LiuZ et al. studied the fatigue performance of three AA2524 aluminum alloys mainly composed of Goss grains, Cube grains, and Brass grains. They found that the fatigue crack propagation resistance was highest in alloys containing Goss grains, followed by those containing Cube grains, and lowest in alloys containing Brass grains. The reason is that the slip plane {111} of Goss grains and Cube grains is closer to the direction of maximum shear stress than that of Brass grains, and under the same stress conditions, Goss grains and Cube grains are more prone to plastic deformation, reducing stress concentration and damage, while Brass grains are more prone to stress concentration and fatigue damage. Therefore, Brass grains have the lowest resistance to fatigue crack propagation, while Goss grains have the highest resistance to fatigue crack propagation due to their large torsion and tilt angles with surrounding grains, which can easily cause crack deflection. In Cube grains, the fatigue crack propagation resistance is determined by the twist angle and tilt angle between grains. When there is a large twist angle or tilt angle between grains, Cube grains have higher fatigue crack propagation resistance. However, if there is a small tilt angle between grains, Cube grains have almost no fatigue crack propagation resistance.

When studying the crack propagation behavior of aluminum alloys controlled by grain orientation and microstructure topology using uniaxial tensile fatigue tests, WANG KY et al. found that the large twist angle between grains is the main reason for causing crack deflection and improving crack propagation resistance, while the tilt angle has little effect on crack deflection. When studying the fatigue crack propagation phenomenon of 5083 aluminum alloy under different stress ratios, MA MY et al. believed that the torsion angle and Schmidt factor play a key role in the influence of stress ratio R on fatigue crack propagation behavior, and the driving force required for crack propagation increases with the increase of Schmidt factor. At high stress ratios (R=0.5), cracks tend to propagate towards grains with small twist angles and high Schmid factors, while at low stress ratios (R=0.05), cracks tend to propagate towards grains with large twist angles and low Schmid factors.

2.1.3 Anisotropy

The anisotropy of aluminum alloy materials can also affect their fatigue performance. Scholars have found in their research that the fatigue performance of rolled aluminum alloy samples taken parallel to the rolling direction is better than those taken perpendicular to the rolling direction. It was also found that the fatigue performance of samples taken parallel to the rolling direction exhibits isotropy, while samples taken perpendicular to the rolling direction exhibit anisotropy. Xiao Ji et al. believed that it was determined by the effective slip length of grains and the length of inclusions; However, Sun Zhiqiang believes that it is caused by the ordered arrangement of insoluble phases within the grains. When studying the fatigue performance of rolled aluminum alloys, Wang Binwen et al. found that the crack propagation rates were different in the rolling direction, transverse direction, and short transverse direction. The fatigue performance was best in the rolling direction, followed by the transverse direction, and lowest in the short transverse direction. This is mainly the result of the combined effects of factors such as grain size, grain boundaries, inclusions, and precipitation. Meanwhile, the anisotropy of the material also has an impact on crack initiation. JIN et al. found in their study of the anisotropy of fatigue crack nucleation in 7075-T651 aluminum alloy plates that Mg2Si and Al (FeMn) Si phases can lead to various anisotropy of fatigue crack nucleation. Among them, Fe particles in the L-T and L-S planes are prone to crack formation, while Si particles in the T-S plane are prone to crack initiation.

2.1.4 Sample Defects

Due to the susceptibility of defects in aluminum alloys to crack initiation, which seriously affects their fatigue life, there have been many studies on the impact of defects on the fatigue performance of aluminum alloys. It has been found that casting defects significantly reduce the fatigue performance of aluminum alloys. When WANG QG et al. studied the fatigue performance of A356-T6 alloy, they found that casting defects can shorten the time for fatigue crack initiation and propagation, and as the defect size increases, the fatigue life of the alloy decreases. LATTANZIL et al. found that fatigue cracks originate from oxides and cold junctions, and these defects promote FCPR. The type, size, and location of these defects also have a significant impact on the lifespan and dispersion of the specimen. LiuYQ et al. used the rotational bending fatigue test method to study A356-T6 aluminum alloy and found similar conclusions. They quantitatively provided the relationship between the fatigue life of cast aluminum alloy specimens and the size of initial defects and applied stress, namely:

In the formula, σ a represents the applied stress; Di is the initial size of the defect on the sample; C and m are constants of the material. By comparing theoretical and experimental data, it was found that the model has good applicability and can guide the engineering application of aluminum alloys to a certain extent. At the same time, studies have found that under high stress levels, defects have a relatively small impact on the dispersion of fatigue life; At low stress levels, defects have the greatest impact on the dispersion of fatigue life, and different crack sources also have varying effects on the fatigue life of aluminum alloys. When Zhang Lan et al. studied the effect of crack source location on the high cycle fatigue performance of 6005A aluminum alloy, they found that the fatigue life of the sample with crack source located at the void defect was the longest, followed by the second phase particle, and the lowest at the oxide inclusion. Moreover, the fatigue life of the sample at the void defect was one order of magnitude higher than that at the oxide inclusion.

2.1.5 Residual stress

Residual stress is caused by inconsistent deformation in different areas within the material. In reality, almost all materials have residual stresses inside. The residual stress has an increasing or delaying effect on FCPR, mainly reflected in the correction of the average stress intensity factor (Kmean), stress intensity factor range (Δ K), or stress ratio. The deformed aluminum alloy produced by rolling will generate residual stress inside the material due to severe deformation. The residual stress inside the material varies with different rolling methods.

2.2 External influencing factors

2.2.1 External Load

By studying the propagation of fatigue cracks under constant tensile load, constant compressive load, tensile compressive load, and alternating load, it was found that the maximum circumferential force of the material under load showed a trend of first increasing and then decreasing, and eventually tended to be constant, reflecting the process of rapid, slow, and stopped crack propagation. At the same time, cracks are prone to initiate and propagate under alternating loads or tensile and compressive loads. There are also studies showing that the fatigue life of specimens under rotational bending is about 5 times higher than that under axial loading, mainly determined by stress gradient effects and smaller stress bodies under rotational bending; And under the same stress amplitude, the fatigue life of the specimen under tensile tensile fatigue test is lower than that under tensile compressive fatigue test.

At room temperature and in a non corrosive environment, the load frequency has little effect on FCPR, but in high temperature or corrosive environments, the load frequency will play an important role. Research has found that the FCPR of 2024 aluminum alloy in salt solution decreases with decreasing frequency, and a similar phenomenon has been observed in Al Zn Mg alloy. Therefore, in high temperature or corrosive environments, when the frequency is low, prolonged contact, oxides, or corrosion products can cause crack closure and delay crack propagation; When the frequency is high, the strain rate of the material increases, and the internal atomic motion intensifies, causing an increase in local temperature and stress, creating conditions for an increase in FCPR. Li Xudong et al. established FCPR models for corrosion of LC9 aluminum alloy under different loading frequencies, but found during verification that the model could only predict crack propagation in the stable expansion zone well, and had a large deviation in predicting FCPR in the transient fracture zone.

After the maximum stress is determined, as the stress ratio increases, the stress amplitude decreases, and the crack tip will remain in the same stress state for a longer period of time, delaying FCPR and improving fatigue life. This conclusion also exists in high vacuum environments. During the research process, models related to the influence of stress ratio R on the FCPR of aluminum alloys were gradually established, such as the Forman model, Pearson model, Walker model, Raju model, and other modified models. These predictive models can all predict the FCPR of aluminum alloys and have a high degree of consistency with experimental results, which can provide certain guidance for engineering practice. Among numerous models, the Forman model and Walker model are widely used due to their simple form and accurate prediction of results.

2.2.2 Service environment

The service environment seriously affects the fatigue life of aluminum alloys, with humidity, temperature, and corrosive environment being the main factors.

Temperature: Research has shown that temperature is one of the key factors affecting the fatigue damage behavior of high-strength aluminum alloys, and the fatigue damage mechanisms of high-strength aluminum alloys are also different in high and low temperature environments. Compared to room temperature, the fatigue process of high-strength aluminum alloys in low-temperature environments is often slower, manifested as longer crack initiation time and lower crack propagation rate.

Numerous studies have confirmed that high-strength aluminum alloys have longer fatigue life in low-temperature environments. However, only when the ambient temperature is below -30 ℃, will there be a significant impact on the fatigue performance of high-strength aluminum alloys, that is, there exists a critical temperature. Correspondingly, high temperature will increase the fatigue crack propagation rate and shorten the fatigue life of high-strength aluminum alloys.

Humidity: Studies have shown that as the environmental humidity increases, the fatigue resistance of high-strength aluminum alloys decreases. Scholars believe that the fresh crack surface formed during crack fatigue propagation will react with water in the air to generate a large amount of reduced hydrogen, which will then migrate to the crack tip through diffusion and dislocation pipeline transportation, thereby reducing the stress required for aluminum alloy cracking, increasing the crack propagation rate and shortening the fatigue life of aluminum alloys. In addition, some scholars believe that humid environments not only shorten the fatigue life of high-strength aluminum alloys, but also increase the variability of their fatigue performance.

Corrosive environment: Corrosive environment poses great risks to the use of aluminum alloys, and corrosion mainly affects the initiation and early expansion stages of cracks. By studying the fatigue performance of different types of aluminum alloys under corrosive media, it was found that the influence of corrosive media on high-strength aluminum alloys is greater than that on high plasticity aluminum alloys, and the crack initiation and propagation life under stress corrosion mode is much lower than that under mechanical fatigue mode after pre corrosion at the same stress level. It is believed that this is mainly due to the slow crack propagation during the initial stage of crack initiation and propagation, which allows sufficient time for the corrosive medium to react with the matrix and crack tip, resulting in intergranular corrosion or hydrogen embrittlement, affecting the fatigue life of the specimen; In addition, corrosive environments can cause corrosion damage to the surface of aluminum alloys, accelerate the initiation and propagation of fatigue cracks, and severely reduce the fatigue life of aluminum alloys.

2.2.3 Surface condition of the specimen

Fatigue crack initiation often occurs on the surface or subsurface of the specimen, so the surface state of the material will have an impact on its fatigue performance. Roughness is the main factor affecting the fatigue performance of specimens in different surface states, and the larger the surface roughness of the specimen, the shorter the fatigue life. The fatigue life of machined specimens is better than that of as cast specimens, and cracks in as cast specimens originate from roughness depressions near casting defects, while cracks in machined specimens originate from holes near the surface. Regarding this phenomenon, researchers believe that it is due to the smooth surface of the specimen, which is in a plane stress state when loaded, and the stress concentration phenomenon is not obvious, reducing the occurrence of crack initiation and propagation, thereby improving the fatigue life of the specimen; The rougher the surface of the specimen, the more severe the stress concentration phenomenon, which is conducive to the initiation and propagation of fatigue cracks and seriously damages the fatigue life of the specimen.

3. How to improve the fatigue performance of aluminum alloy?

To improve the fatigue performance of aluminum alloys, the following aspects can be taken into consideration:

Alloying treatment: Alloying treatment can not only optimize the matrix structure, but also improve its properties and broaden the range of material applications. The mechanism by which adding different microalloying elements affects the fatigue performance of alloys varies.

Heat treatment: The internal microstructure of materials can be improved through heat treatment processes, such as the distribution, quantity, and size of precipitated phases. The state of these precipitated phases is the fundamental factor affecting alloy properties. Therefore, the rational use of heat treatment processes to control precipitated phases is one of the methods to promote material use.

Surface strengthening technology: The fatigue performance of aluminum alloys is highly sensitive to surface or near surface defects, and the residual stress field on the surface is the main factor in improving the fatigue performance of the specimen. By introducing residual compressive stress on the surface of aluminum alloy through mechanical deformation, a higher dislocation density is generated in its near surface area, hindering dislocation movement and improving its surface hardness and strength. Under external loading, residual compressive stress on the surface will consume a portion of tensile stress, reduce the driving force for fatigue crack propagation, and thus improve fatigue performance. Research has found that shot peening enhances the fatigue performance of aluminum alloys by creating high residual compressive stress near the surface of the material, which tends to move towards the interior of the material's surface layer; Laser shock peening is the ability to generate high residual compressive stress while maintaining the depth of stress influence, thereby affecting fatigue performance; However, compared with mechanical strengthening and laser shock, deep rolling can introduce a more reasonable and stable residual stress distribution, resulting in the best comprehensive performance of the material.

4. Summary

This article introduces the characteristics and classification of fatigue, the influencing factors of fatigue performance of aluminum alloys, and measures to change fatigue. Defects and damages in aluminum alloys have a significant impact on their fatigue performance. Therefore, by improving production and processing technology, defects and damages in aluminum alloys can be reduced, the initiation and propagation of fatigue cracks can be delayed, and the fatigue life of aluminum alloy products can be improved.

After more than a hundred years of development, metal fatigue has formed a relatively complete disciplinary system. However, there are still some issues to be addressed regarding the fatigue behavior of materials, such as the complexity and sometimes non universality of crack initiation and propagation models, and the gap between predicting the fatigue life of materials and their practical engineering applications. With the R&D of more advanced equipment and testing technology, as well as the development of Internet technology, there will be greater breakthrough space for crack initiation and growth mechanism and fatigue life prediction in material fatigue.