Study Of Fatigue And Fracture Behavior Of High Strength And Toughness7050Aluminum Alloy Thick Plate

The alloy7050shows advertised combination of strength, fracture toughness, stress corrosion resistance and hardenability, which is one of the best comprehensive aluminum alloy.7050is the first choice in the aerospace application with the reqirements of high strength, stress corrosion resistance and fracture toughness.By the tensile mechanical testing, fatigue life test, fatigue growth test, fracture toughness test and with use of Optical Microscopy(OM), Transmission Electron Microscop(TEM), Scanning Electron Microscopy(SEM) and EBSD observation, the microstructure, fatigue characteristic, fatigue crack growth rate and fracture toughness of high strength and high toughness aluminum alloy7050thick plate in various conditions (T6, T74, RRA) and different thickness (35mm and160mm) were studied. The results are as follows:1. Inside the grains the microstructure of RRA-treated sample is similar to that of the T6peaked. The grain boundary precipitates growth continues during the retrogression stage and the final grain boundary microstructure is similar to that of the overaged T74, and the PFZ is much narrower that of the T74-treated sample. The RRA treated sample exhibit significantly higher tensile properties than that of T74overaged, while the similar tensile properties to that of T6peaked.2. The fatigue strength of the7050-T74alloy thick plate is260Mpa. With the increase of stress amplitude, the distance between the crack source and the free surface of the specimen decreases, and the size of the region without striations is smaller. At the same time the striation distance, the growth rate of cracks and the size of the fatigue rupture region increase.3. RRA treated alloy exhibits enhanced fatigue crack growth resistance than that of T74treated alloy. The160mm thicker plate of7050-T74aluminum alloy is largely recrystallized and displays coarsen grain structure, exhibiting higher fatigue crack growth resistance. The fatigue growth rate of7050alloy plate appears anisotropy. The longitudinal (L-T) fatigue growth rate is significantlt slower than that of the latera (T-L). Stress ratio shows pronounced effect on the fatigue growth behavior. The larger ratio has the larger fatigue growth rate, the smaller fatigue threshold value and lower△K value to the rapid rupture period.4. For over-aged7050alloy plate, the dimples fracture caused by coarsen second particles occupies most of the fracture. The second particles can easily act as the crack initiation thus accelerating fracture of alloy leading to the lower fracture toughness. The smaller fracture toughness of160mmplate than the35mm plate is attributed to the large grain boundary of recrystallized grains, thus contributing to the fracture of alloy. During the fracture toughness of this alloy, the fracture toughness mechanism mainly combunes two kinds:transgranular fracture and intergranular fracture.