Aluminum (Al) alloy is the second most widely used engineering alloy after steel. Compared with steel, its density is 1/3 of steel, and it is non-magnetic and excellent in corrosion resistance. Precipitation-strengthened aluminum alloys can also be processed into relatively hard materials. Therefore, their special mechanical properties (high specific strength) play an important role in lightweight applications, and are increasingly used in transportation industries such as airplanes, trains, and automobiles. The transportation structure needs to withstand alternating forces, and the stress that the material bears is periodic in nature, so the fatigue resistance of the material is very important. The fatigue performance of high-strength aluminum alloy is poor, and the fatigue strength is about 1/3 of its tensile strength. This is one of the fatal weaknesses of aluminum alloy, which greatly limits the application range of aluminum alloy. Although materials scientists work hard to control the microstructure of aluminum alloys to make them harder, the increase in fatigue strength is far less than that of steel.
Researchers from Monash University in Australia proposed an alternative conceptual method that can greatly improve the high cycle fatigue (HCF) performance of precipitation-strengthened aluminum alloys. The fatigue life of aluminum alloys has increased by an order of magnitude or more, and the fatigue strength has increased to 1/2 of the tensile strength (basically the same as steel). Related papers were published in Nature Communications with the title "Training high-strength aluminum alloys to with stand fatigue".
Paper link:
https://www.nature.com/articles/s41467-020-19071-7
The commercial aluminum alloys AA2024, AA6061 and AA7075 produced by Kaiser Aluminum were used in this study. Alloys are available in rod and plate shapes. Bars are used for tensile and fatigue tests, and plates are used for fatigue crack growth (FCG) tests. The alloy is solid-solution treated in a salt bath and air-cooled to imitate the industrial cooling process. It is aged in an oil bath, partly under-aged (UA), and the other part is peak-aged (PA). After aging, the alloy is stored in a refrigerator at -35℃. Minimize natural aging.
Studies have found that the yield strength and tensile strength of the UA alloy are smaller than the PA alloy, but have a higher fatigue life. The key feature of precipitation-strengthened aluminum alloys is the presence of precipitation-free zones (PFZ) near the grain boundaries. Although both UA and PA precipitation-strengthened aluminum alloys contain PFZ, their behavior is different under high-cycle fatigue loading. In the UA state, the PFZ contains solutes. In the early fatigue process, the microplasticity is located in the soft PFZ, but the movement of dislocations will produce vacancies, which promotes dynamic precipitation and strengthens the PFZ. The size of the dynamic precipitates is on the order of 1nm; in the cycle process The PFZ of the PA sample also has a certain degree of plasticity, but because they are all vacancies and the solute has been exhausted, the vacancies generated by the dislocation movement have no solute to promote dynamic precipitation, so they are not strengthened.
This article proposes a method to improve the fatigue performance of aluminum alloys, applying a specific cycle training program to repair PFZ through dynamic precipitation to reduce the strength difference between the inside of the grain and the PFZ. Perform complete reverse cycle (R=-1) training at 0.2Hz, AA2024 performs 450 cycles; AA6061 performs 700 cycles; AA7075 performs 450 cycles. After training, the fatigue life of AA2024 alloy is increased by an order of magnitude; compared with the PA state, the life of the trained AA7050 is increased by 25 times, and the fatigue strength is close to 1/2 of the tensile strength; the improvement of AA6061 is less.