Abstract
This research mainly focused on the effects of solution treatment on high-cycle fatigue properties, microstructure evolution, and fatigue fracture morphology of the high strength aluminum alloy (7075 aluminum alloy). The S-N curves and fatigue performance parameters of the alloy were obtained. We found that longer solution treatment time significantly influences the high-cycle (N ≥ 105) fatigue properties of the Al-Zn-Mg-Cu alloy. Under the loading stress of 240 MPa, and the solution treatment of 2 h compared to 1.5 h, 1 h, and 0.5 h, the fatigue life was respectively improved by about 95.7%, 149%, and 359%. The microstructure observations conducted with a scanning electron microscope (SEM) and transmission electron microscope (TEM) are as follows: recrystallization occurs in the grains of the 7075 aluminum alloy under solution treatment, and the grains become large with the length of the solution treatment time. Cracks mainly initiate from the undissolved large phases, and prolonging the solution time can effectively promote the dissolution of the T phase and S phase, decrease the number of dislocations, and lower the rate of the initiation of fatigue cracks at the undissolved large phases due to dislocation glide and dislocation pile-up. In the second stage of crack propagation, the secondary cracks reduce the driving force and the rate of crack propagation, promoting the fatigue properties of the 7075 aluminum alloy, which can be verified by the observation result that fatigue striation widths become narrower with longer solution treatment times.
Highlights
Solution treatment refers to the process of dissolving the coarse second phase in the alloy at a certain temperature and time, and using rapid cooling to obtain a supersaturated solid solution [1].In solution treatment, the time and the temperature are the core parameters [2], which have direct influences on the degree of the solid solution and the quality of the subsequent precipitated phase during the ageing and strengthening stage.The effect of solution treatment on the microstructure and the performance of the 7050 aluminum alloy has been discussed in the literature [3,4,5,6]
The results show that solution treatment can have a significant effect on the grain size and the morphology of the second phase
Samples were cut from the solution treatment specimens, characteristics were observed under a TecnaiG220 transmission electron microscope (TEM)
Summary
Solution treatment refers to the process of dissolving the coarse second phase in the alloy at a certain temperature and time, and using rapid cooling to obtain a supersaturated solid solution [1]. While the second phases are dissolving into the base, the proportion of recrystallization and the size of the sub-grain increase, causing the weakening of the mechanical properties of the alloy [3]. In these studies, Li [4] found. The second phase of the aluminum alloy, the recrystallization fraction, and the change of the grain morphology have a great influence on the material’s fatigue property [11,12,13,14,15]. According to the rules of alloy microstructure evolution and the analysis of fracture morphology, the influence mechanism is discussed in order to provide a theoretical guidance for the optimization of parameters in solution treatment and for the enhancement of the fatigue properties of aluminum alloys
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