Experimental Research On Mechanical Properties Of AA 7055 Aluminum Alloys At Different Temeratrues And Strain Rates

7055 aluminum alloy is the most advanced aluminum alloy with extremely high strength, preferable tenacity and favorable resistance to stress corrosion, which has broad application prospects. Materials will be subjected by various loads in complicated application environment; so, studying the mechanical properties of the materials under different loading conditions is the basis for application and design of the materials. At present, the research on 7055 aluminum alloy is just at the starting stage in China, and the research on the mechanical behaviors of 7055 aluminum alloy under different loading conditions is still very scarce. Meanwhile, at present, the split Hopkinson pressure bar (SHPB) and the split Hopkinson tensile bar (SHTB) are the most commonly used test equipments of dynamic mechanics. But there are no unified and consummation standard for these experimental methods and equipments. There are still some problems need to be concerned: just as how to accurately gain the preset strain rates in SHPB test; and how to connect specimen to the bars of SHTB, etc.Based on the above background, in this thesis, we researched and developed SHPB and SHTB tests at first. Then, we systematically studied the mechanical properties and behaviors of the AA 7055-T77 aluminum alloy (produced by Alcoa) under different temperatures and strain rates; moreover, combined with micro-structure analysis, we preliminarily analyzed its mechanism. At the same time, as a comparison, part of the tests were performed on three reference materials: 45 vol.% SiCp/2024Al composite, 2024 aluminum alloy and 2A12 aluminum alloy. Finally, to evaluate the resistance of hypervelocity impact, we studied and contrastive analyzed the crater behaviors of AA 7055 aluminum alloy and the reference materials thick plates under hypervelocity impacted. The major research content in the thesis shows as follows:First, a strain rate prediction formula of SHPB test is derived from one-dimensional stress wave theory and two-wave method calculation formula. On the basis of this prediction formula, we present a SHPB test design method that can easily verify scheduled strain rates, and demonstrate and verifiy this method through numerical simulation and experiment test.Second, we've designed a kind of wedge-shaped bayonet specimen connective mode and corresponding specimen form for SHTB device, and proved it's effective and feasible through numerical simulation and experiment test.Third, low strain rate uniaxial compressive tests of AA 7055 aluminum alloy under different temperatures was performed by Gleeble thermo-simulator machine, the temperature range was 300~750K and the strain rate was 0.001s-1,0.01s-1 and 1s-1 respectively. Dynamic compressive tests and dynamic tensile tests of AA7055 aluminum alloy at the room temperature was performed by SHPB and improved SHTB device; the test range of strain rate is: dynamic compression at 900~5000s-1, while dynamic tension at 500~1600s-1. The stress-strain relationships of AA 7055 aluminum alloy at the above loading conditions were obtained.Forth, according to the experimental results, we suggest a temperature effect additive function containing critical transformation temperature. Based on the experimental results of AA 7055 aluminum alloy, we propose a strain rate effect function coupling with temperature and a segmented strain-hardening function with effective hardening strain. Synthesizing the above result, we construct a modified Johnson-Cook model. The flow stress of a 7050-T7451 aluminum alloy over a wide temperature and strain rate rage were predicted by the modified Johnson-Cook model, the result is consistent with the experimental results (get from literature [42]) and much better than the result of primary Johnson-Cook model. Compared with the primary Johnson-Cook model, the predicting results of the flow stress of AA 7055 aluminum alloy under different loading conditions by the modified Johnson-Cook model is consistent with the experimental results much better. It indicates that the modified Johnson-Cook model proposed in this thesis has preferable applicability for aluminum alloys.Fifth, at the same time, similar tests were performed on three reference materials: 45 vol.% SiCp/2024Al composite, 2024 aluminum alloy and 2A12 aluminum alloy; and the mechanical properties and the materials parameters of these three materials were obtained. Combined with the experimental dates from literatures, the results show that the suggested temperature softening function is also suitable for these reference materials and some other materials.Sixth, on the basis of single dynamic compressive test, we performed cyclic dynamic compressive tests on AA 7055 aluminum alloy and 2024 aluminum alloy (contrast material) by SHPB, combining the macro-mechanical characters and microstructure changes analysis, studied the evolution of shearing localization problem of AA 7055 aluminum alloy during dynamic compression. We reveal the forming mechanism and some developmental rules of the shearing localization in the specimen of aluminum alloys during dynamic compression.At last, we studied the crater behavior of AA 7055 aluminum alloy and two reference materials thick plates under hypervelocity impact by two-stage light gas gun system, the impact conditions were: 2017 aluminum alloy sphere pellet with a diameter of 4.76mm, velocity range was 1.40~4.47km/s; GCr15 steel sphere pellet with a diameter of 5mm, velocity range was 1.56km/s~2.36km/s. We observed and analyzed the microstructure near the craters on the target plates of two aluminum alloy. The results indicate that the mechanical properties of target material have significant effects on the size and damage morphology of the crater and the intensity of target material is inversely to depth of carter.From this study, we get more convenient and efficient SHPB and SHTB testing methods; systematically obtain the mechanical properties and material parameters of AA 7055 aluminum alloy; and establish a constitutive model characterizing the mechanical properties of aluminum alloys at different temperatures and strain rates. The results have some guiding significance and reference value for the design, development and engineering application of the ultrahigh strength aluminum alloys as 7055 aluminum alloy and dynamic mechanical testing methods.