The Research Of The Corrosion Fatigue Characteristics Of The Aviation Aluminum Alloy

At present,aviation aluminum alloy in civil aircraft structure consumption accounted for70-80%.In the past,a large number of failures were caused by corrosion fatigue coupling damage of aircraft aluminum alloy structures.Corrosion fatigue coupling failure is a common failure mode of high strength aluminum alloy structure.It is easy to cause the damage of aluminum alloy structure because of its weak forewarning,strong destructive and high harmfulness.Therefore,the study of corrosion fatigue coupling of aviation aluminum alloy is very important to ensure the normal use of aviation aluminum alloy and aviation safety.In this thesis,the effect of acidic NaCl solution coupled with tensile alternating load on the corrosion fatigue and other durability behavior of 7075-T651 aluminum alloy was studied.Electrochemical test and low cycle fatigue test were carried out to explore the effect of corrosion time on 7075-T651 aluminum alloy.A simulation model for corrosion fatigue prediction of7075-T651 aluminum alloy was established based on multi-p Hysical field simulation software.The microstructure of fatigue fracture was observed by scanning electron microscope（SEM）,and a set of grap Hic processing methods for corrosion pits was established based on MATLAB programming.Finally,a simulation model for corrosion fatigue prediction is established.The main results obtained are as follows:（1）The average maximum tensile strength of 7075-t651 aluminum alloy specimens is12941N,which provides a reference for the setting of maximum tensile strength parameters in fatigue test.Based on the principle of damage equivalence,it is calculated that the corrosion degree of the specimen corroded in acidic NaCl solution for 5.5h is equivalent to the corrosion degree of the skin of the aircraft parked for 1 year.The corrosion fatigue test shows that the corrosion time and fatigue life are negatively correlated.The average fatigue life of specimens after 30 hours of corrosion is reduced by 35.3%,and that after 55 hours of corrosion is reduced by 44.9%.Electro-chemical analysis shows that the self-corrosion potential and current of7075-T651 aluminum alloy decrease with the decrease of solution p H value,indicating that the corrosion dynamic process slows down,providing basic parameters for corrosion fatigue simulation.（2）According to the fracture morp Hology of 7075-T651 aluminum alloy corrosion fatigue specimen,multiple pitting pits appeared in the specimen after pre-corrosion.Under alternating load,cracks generated,expanded and finally merged.Based on MATLAB software,the corrosion morp Hology image is input,and image processing techniques such as filtering function,histogram transformation and image enhancement are used to establish an image processing method with less noise points and clearer edges of the corrosion image.It can also be used to calculate the pit area.（3）Multi-physical field simulation software is used to simulate the corrosion fatigue of7075-T651 aluminum alloy specimens in acidic NaCl solution environment.It is found that the corrosion fatigue life decreases with the increase of corrosion time.When the corrosion time is from 0H to 20H,the corrosion fatigue life decreases from 4052 to 3105.The simulation data is in good agreement with the test data.When the corrosion time is 25H to 55H,there is a slight deviation between simulation data and test data,but the deviation is within the allowable range.The effects of different concentration of H₂SO₄ and NaCl on the corrosion fatigue performance of 7075-T651 aluminum alloy are studied,and the fatigue strength and corrosion thickness are obtained.It is found that in a certain period of time,when the concentration of Cl-is constant,the corrosion pit depth gradually decreases and the fatigue life increases with the increase of p H value.When p H value is constant,with the increase of Cl-,the corrosion pit depth gradually increases,and the fatigue life gradually decreases.（4）Based on the grey system theory,the grey prediction model GM（1,1）is established to predict the fatigue life of 7075-t651 aluminum alloy samples after different corrosion time.When the corrosion time is 40H,the fatigue life is 2435,and the relative error is only 0.25%.Corrosion fatigue tests from 0 hours to 55 hours were used to verify the accuracy of the gray GM（1,1）prediction model in the prediction of aluminum alloy corrosion fatigue life.