Research On Microstructure And High-cycle Fatigue Property Of High Strength Aeronautic Aluminum Alloy

Due to the advantages of small density, high specific strength, high corrosion resistance, good formability, rich resource and low cost, the aluminum alloys are widely used in aviation, spaceflight, ship, nuclear and weapon industries, in which it can not be replaced by other materials. Acting as main structural materials of airframe, the aluminum alloys’ development and application in aviation industry is closely related with the development of the aircraft. Therefore, it is of great importance of S-N curves of aeronautic aluminum alloys in designing structural materials of aircraft; furthermore, it is also of great scientific significance and wide application to avoid fatigue by improving the fatigue strength of aluminum alloys in designing aircraft.In this paper, GPS-100high-frequency fatigue testing machine was used to test the different parameters of S-N curves of2024and1161aeronautic aluminum alloys, and different stress ratios and stress concentration factors were designed to study the law of aluminum alloys’ fatigue failure. The stresses, fatigue fracture surfaces, and microstructures of2024aluminum alloy samples were deeply studied by TEM, SEM, EDS, XRD, OM and Abaqus.The results show that the fatigue strength of2024aluminum alloy will obviously decrease as the stress concentration factor increases, which means that the samples of2024aluminum alloy has notch effect. When the stress ratio is0.06, the fatigue strength of smooth samples (Kt=1) is227.85MPa and the fatigue strength of notch samples (Kt=3) is92.00MPa; when the stress ratio is-1, the fatigue strength of smooth samples (Kt=1) is135.40MPa and the fatigue strength of notch samples (Kt=3) is62.79MPa. The stress ratio also has distinct influence on fatigue strength; the lower the stress ratio, the lower fatigue strength will get. When the stress concentration factor is3, and the stress ratio is respectively0.5,0.06,-1, the fatigue strength is respectively141.07MPa,92.00MPa,62.79MPa.Fatigue crack initiation of2024alloy mainly occurred on the sample surface, with initiation from the surface defects, the second phase particles and the interface between the second phase particles and matrix. Fracture showed a tortuous fatigue crack path at the early stage. Crack prioritily went along the favorable slip plane, and it did not deflect if favorable slip planes in the adjacent grains are the same, but deflected if not.As a result, crystallography feature was formed on the fracture surface. In addition, the fatigue fracture surface of2024aluminum alloy mainly showed transgranular fracture.The strength of2024aluminum alloy was reinforced by strain hardening, dispersion strengthening and solid solution strengthening. The main reinforcing phase of2024aeronautic aluminum alloy in this thesis was Cu2Mn3Al20phase which was tiny and dispersed. The tiny dispersed phase could obstruct dislocation motion, which leaded dislocations’ tangling and dislocation density’s increasing. Obviously, the tangling dislocations and high density of dislocations could improve resistance of dislocation motion, which effectively prevent the micro-cracks’nucleating and propagating, therefore, the fatigue strength was improved. There were some CuAl3or (Mn,Fe)Al6phases which were bigger than Cu2MnaAl20in2024aeronautic aluminum alloy as well as normal2024aluminum alloy. These bigger phases could obstruct the cracks’ propagating; once the particles were broken or separated from matrix they would promote the cracks’ propagating and dimples’ forming which was harmful for fatigue strength.