| Aluminum 7075 exhibits a high strength to weight ratio and easy processing, etc., and is widely used for structural components in aerospace, rail transportation, marine engineering and other industries. Welding is one of the main connection modes of structural components. In practical work, all of these structural components would be interact with the surrounding environment, such as aircraft and humid atmosphere environment, ship and sea water environment, engine blades and high temperature environment, etc.. These environmental media can destroy and damage 7075 aluminum alloy and welded structural components, which will affect the structure’s performance and shorten the life, resulting in the failure of the whole product. Most of the damage and destruction are from the surface, so the surface treatment and modification is an important means to improve the performance of 7075 aluminum alloy on anti environmental damage. As a new type of surface modification technology, laser shock peening (LSP) has the advantages of small thermal effect, high efficiency, high controllability, no pollution and so on. But the research of LSP at home and abroad is mainly focused on the mechanical properties, and most of them are about performance tests at room temperature. There are a few studies on the corrosion resistance of 7075 aluminum alloy and its welded joint after laser shock peened, and there is no unified understanding of mechanism improving the corrosion resistance by LSP. Therefore, it is of great value and practical significance to study the process and mechanism of LSP on improving corrosion resistance, especially in the halogen environment, and high temperature resistance of 7075 aluminum alloy.In this paper, the static yield strength of 7075 aluminum alloy was calculated and analyzed, process parameter of LSP were optimized based on the parameter scope of a Q-switched Nd:YAG laser (GAIA-R, THALES) and the comparison result of surface integrity of them without LSP and with LSP.7075 aluminum alloy and its weldments were treated with a diameter of 3mm, energy of 9J, overlapping rate of 60% and pulse duration of about 10ns.Phase and microstructure of them were analyzed with the aid of the modern test analysis means. The mechanism of grain refinement and high density of dislocation induced by laser shock wave were studied. A series of tests on the improvement of anti environmental damage capability of 7075 aluminum alloy after laser shock peened were carried out, including electrochemical corrosion tests, stress corrosion tests, creep tests and high temperature fatigue tests.7075 aluminum alloy sheets were welded by plasma arc, and the welding fixture was made for reducing the deformation. The welded joints were of practical engineering value. Then they were treated with a diameter of 3mm, energy of 6J, overlapping rate of 60% and pulse duration of about 12ns. The residual compressive stress layer with the depth of 0.3mm was obtained by one number LSP. And the residual stress at the centerline of welded zone was changed from 50MPa to -25MPa. For the heat affected zone, the residual compressive stress was increased by one time. Then the microstructure of 7075 aluminum alloy welded joints after laser shock peened was analyzed, and the mechanism of the residual stress induced by LSP was studied. A lot of experiments about the anti environmental damage capability of 7075 aluminum alloy after laser shock peened were tested, including the electrochemical corrosion test, stress corrosion test, creep test.Based on the above experimental research and mechanism analysis of 7075 aluminum alloy and its welded joint, some important conclusions and innovative achievements of this work were listed as follows:(1) The range of the parameters of the twin structure induced by laser shock 7075 aluminum alloy was determined. A mathematical model of grain size, lattice microdistortion and dislocation density during multiple LSP treatment was established. The influence of laser shock processing on microstructure of 7075 aluminum alloy can be accurately described by this model. Theoretical support for optimizing the parameters of LSP was provided.(2) The abundant experimental data and process parameters of LSP on enhancing electrochemical corrosion resistance of 7075 aluminum alloy and its welded joint were obtained. The superiority of LSP in improving the electrochemical corrosion resistance of 7075 aluminum alloy was also found. After LSP, firstly, free-corrosion potential and pitting potential of samples moved towards the positive direction. Secondly, the polarization curves exhibited an anode passivation zone in a wide range of potential with 712.9 mV. Finally, passive resistance was increased by nearly 30 times. So, electrochemical corrosion resistance of 7075 aluminum alloy was greatly increased.(3) A set of correct and feasible parameters and method of LSP were explored, which was very effective on inhibiting stress corrosion crack initiations, preventing the crack propagation and enhancing stress corrosion resistance of 7075 aluminum alloy and its welded joint. The elongation rate, area reduction rate, time to fracture UTS, static toughness were increased by 11.13%,7075 and 20%, and 8.25%, respectively. Stress corrosion sensitive coefficient of 7075 aluminum alloy welded joint was decreased from 13.6% to 8.25%.(4) An experimental method which was different from treatments existed domestic and abroad has been found to improve the high temperature resistance of 7075 aluminum alloy. LSP treatment has been used to improve the creep rupture life of it by about two times, and can effectively inhibit the temperature sensitivity of it. The time, strain and strain rate at the end of steady stage of 7075 aluminum alloy with different impact parameters were tested, which provided an important theoretical reference for engineering application maintenance of aluminum alloy structural components and forming of 7075 aluminum alloy.(5) The high temperature fatigue life of 7075 aluminum alloy was prolonged by laser shock peening and the highest growth rate was up to 76.41%. The model of residual stress relaxation at high temperature was established, which can be used for explaining the reasons that the high temperature fatigue life of 7075 aluminum alloy was effectively prolonged by LSP. And theoretical support for the improvement of high temperature fatigue performance of aluminum alloy by laser shock peening was provided. |
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