.2 E12. Aluminum Alloy Fatigue Damage In The Complex Environment Behavior Research

Aircrafts' life and reliability require aeronautical aluminum alloy has excellent anti-fatigue and corrosion properties. The microstructure of alloy and its complex service environment have close relationship with its fatigue crack growth (FCG) performance. The influence of complex environment on the FCG properties of 2E12 aluminum alloy was investigated by means of fatigue tests in hot-humid environment, thermal impact, room temperature fatigue crack growth and salt spray. The OM, SEM, TEM and EDS were used to analyze the microstructure evolution. The experimental results of the fatigue crack propagation behavior of 2E12 alloy under complex environment were analyzed and discussed. The main conclusions are as follows:(1) The influence of sequential exposure to thermal impact and fatigue on the FCG and microstructure of 2E12 was studied. The results show although the thermal impact promote the growth of GPB and precipitation of S" increasing the resistance of dislocation movement, while reducing the number of dislocation brought by fatigue, thus reducing the concentration of stress caused by piled-up of dislocations, which will improve the fatigue properties of alloy. However the disadvantage is that more oxygen will easily diffuse into the crack tip along the grain boundary resulting in oxidation and embrittlement of grain boundary, and the Al_xO_y film formed on the surface of crack tip will reduce the degree of recovery of slip bands. These factors result in the acceleration of FCG;(2) The FCG of 2E12 is accelerated markedly due to sequential exposure to salt spray and fatigue. In low stress intensity factor range, the acceleration is the most which is 1.5 times faster than that under lab environment at room temperature. With the further propagation of crack, the influence of corrosive environment on FCG decreases, under the same△K, the FCG is 30% faster than that under the lab environment;(3) The 2E12 and 2024 alloys mainly present corrosion fatigue feature under the accelerated corrosion environment, among the factors affecting FCG, the effect of salt spray and hot-humid is greater than that of the thermal impact. The increased crack growth rate is mainly due to a combination of hydrogen embrittlement and anodic dissolution at the crack tip under the salt spray and hot-humid environment;(4) The FCG rates under corrosive environments fit well by using modified Paris formula, which well describes the FCG rate of stable propagation range under a corrosive environment. Based on the formula, the crack growth life of the alloys under the corrosive environments was assessed that was in good agreement with experimental results.