Three groups of the same 7050 Al-alloy plates were all aged firstly at 121 ℃ for 6 h and then aged subsequently at 163 ℃ for 0, 12 and 24 h respectively. The corrosion performance of the aging treated alloy plates was characterized by means of mass loss measurement, potentiodynamic polarization curve measurement and corrosion cracking propagation measurement, as well as optical microscope (OM). The results show that the corrosion type of the aged alloy plates changed from localized corrosion to general corrosion gradually with increasing ageing time and the corrosion rate decreased constantly. Meanwhile, the manner of corrosion cracking propagation for the aged alloys transformed from intergranular to transgranular gradually with the increasing ageing time. This makes the rate of corrosion cracking propagation become lower continuously. The corrosion current density for the different aged alloy plates were assessed by potentiodynamic polarization technique, which may be ranked as the following order: peak-aged state >under-aged state > over-aged state.
实验选用80 mm厚的7050铝合金热轧板为原料。其实际化学成分 (质量分数,%) 为：Zn 6.06,Mg 2.20,Cu 2.12,Zr 0.11,Fe 0.08,Si 0.04,Al余量。
从热轧板的1/4厚度处截取3 mm厚的样品,在空气电阻炉中进行固溶处理 (473 ℃,1 h),室温水淬。然后立即进行双级时效处理,时效制度为121 ℃/6 h+163 ℃/0 h,121 ℃/6 h+163 ℃/12 h和121 ℃/6 h+163 ℃/24 h。对应的样品分别标记为SA0,SA12和SA24,分别对应于7050铝合金板材的欠时效、峰值时效和过时效状态。
失重法测量腐蚀速率是按照ASTM-G31进行的。矩形试样尺寸为30 mm×20 mm×3 mm,顶端打一圆形小孔,通过尼龙绳将样品悬挂于溶液中。试样依次经400#,800#,1200#和1600#的SiC砂纸打磨至表面无明显划痕,然后在3.5% (质量分数) NaCl溶液中室温浸泡7 d,化学法去除腐蚀产物 (在80 gL-1铬酐+200 mLL-1磷酸+800 mL去离子水中浸泡10 min,并用软毛刷轻拭样品表面),经水洗后,再用30% (体积分数) HNO3溶液出光3~5 s,水洗,热风充分干燥后称重。每组实验选用3个平行试样,取其平均值计算失重速率。
极化曲线测试采用Pt作为辅助电极,饱和甘汞电极为参比电极的三电极体系,利用Chi660c型电化学工作站在25 ℃下测量。试样暴露面积1 cm×1 cm。极化曲线测试的动电位扫描速率为1 mVs-1,扫描范围为开路电位±0.2 V。
将式 (9) 带入式 (2),可得
当二级时效时间超过12 h后,合金暴露表面 (
关于电化学测试评价合金耐蚀性能有效性的问题目前仍存在较大争论,广大研究者也还在努力寻找更有效的预测合金耐蚀性的电化学测试方法。就本研究而言,极化法所得材料腐蚀性能的变化规律与腐蚀裂纹观察的结论一致,说明极化法可以对合金腐蚀性能作出有效的判断,但不能说明极化测试所得电化学参数值与合金晶间腐蚀行为相对应,因为根据腐蚀电流密度计算所得合金腐蚀裂纹扩展速率 (约0.5 mm/a) 远低于观察所得合金纵截面裂纹扩展速率 (12 h即可达到约0.1 mm)。原因可能是电化学测试是对合金表面各种腐蚀的综合、快速的响应,并不针对某一具体的腐蚀类型,因此用其来预测裂纹扩展速率肯定会出现较大偏差。然而当合金整体腐蚀越发严重时,裂纹快速扩展的几率也将增加,所以电化学测试可在一定程度上用来表征合金的腐蚀倾向。
The authors have declared that no competing interests exist.
Aluminum alloys have been the primary material of choice for structural components of aircraft since about 1930. Although polymer matrix composites are being used extensively in high-performance military aircraft and are being specified for some applications in modern commercial aircraft, aluminum alloys are the overwhelming choice for the fuselage, wing, and supporting structure of commercial airliners and military cargo and transport. Well known performance characteristics, known fabrication costs, design experience, and established manufacturing methods and facilities, are just a few of the reasons for the continued confidence in aluminum alloys that will ensure their use in significant quantities for the rest of this century and likely well into the next one. But most significantly, there have been major advances in aluminum aircraft alloys that continue to keep them in a competitive position. In the early years aluminum alloys were developed by trial and error, but over the past thirty years there have been significant advances in our understanding of the relationships among composition, processing, microstructural characteristics and properties. This knowledge base has led to improvements in properties that are important to aircraft applications. This review covers the performance and property requirements for airframe components in current aircraft and describes aluminum alloys and product forms which meet these requirements. It also discusses the structure/property relationships of aluminum aircraft alloys and describes the background and drivers for the development of modern aluminum alloys to improve performance. Finally, technologies under development for future aircraft are discussed.
Driven by the increasing requirements from aircraft producers, Hoogovens Aluminium Rolled Products GmbH, together with Hoogovens Research & Development, has enhanced the property combinations of their aircraft materials. For these types of material, optimised processing routes as well as new alloy chemistries have been investigated. Whilst retaining the strength levels required by the aerospace industry, new processing routes offer major improvements in ductility, toughness, fatigue performance and in reduction of residual stress in large dimension plate and sheet products. A further goal of investigating new alloy chemistries is the trend towards new joining techniques such as welding and brazing for aircraft structures. These new joining techniques require different property combinations compared to the conventional aerospace alloys. In parallel to these improved processing routes and new alloy developments, new ultrasonic inspection techniques have been developed, which are able to predict fatigue performance and residual stress in thick plate products.
The corrosion behaviour of AA7050 on different quench rates and ageing conditions has been studied using immersion tests and complementary techniques like SEM, TEM-EDS and Scanning Kelvin probe force microscopy. The results reveal that overageing temper can effectively reduce corrosion sensitivity caused by peak-ageing treatment. The comparison of corrosion resistance in the over-aged and severely over-aged alloys shows significant dependence of the changes in corrosion resistance with ageing time on quench rate. The results are discussed in terms of the changes in the composition and distribution of precipitates, width of precipitate-free zone and diffusivity of alloying elements.
Atmospheric corrosion of alclad and extruded 2024 and 7075 were investigated by weight loss, loss in mechanical properties and depth of pitting over 20 years. The results demonstrated the inner cladding layer on alclad ones had higher corrosion resistance. After 20 years exposure, the cladding had not been penetrated by pitting and those alclads retained their mechanical properties well. Exfoliation occurred on extruded ones in coastal and industrial atmospheres. Especially in coastal atmosphere extruded 2024 suffered severe exfoliation and experienced rapid deterioration of mechanical properties. Furthermore, morphology and chemical compositions of corrosion products were analysed by SEM, XRD and EDS.
Nine test sites were established in the Eastern coast (Gulf Sea) and Western coast (Red Sea) of Saudi Arabia. In each environment, sulphur dioxide and chloride deposition rates were measured and the corrosion rates of rectangular samples (aluminium) were determined. In two typical test stations (a marine and a marine-industrial), the time of wetness was registered, then the influence on metallic corrosion rates discussed. The obtained data were used for classification of atmospheric aggressivity, according to ISO 9223. It was observed that the aggressivity categories of marine zones are , , and (the highest one), whereas the marine-industrial sites classify as , , , and . Corrosion products and contaminants have been analysed by X-ray diffraction. By using the equation = , where is corrosion rate after years, is corrosion rate after the first year of exposition, and is constant, based on values, the corrosion rate values of aluminium samples are predicted.
There is little or no correlation between grain-boundary, or matrix, microstructures and stress corrosion cracking (SCC) susceptibility. Grain-boundary microchemistry, especially the copper content of grain-boundary precipitates (GBP) is the most important factor. Further evidence for this is a correlation between the initial open-circuit potential of ‘fresh’ brittle intergranular fracture surfaces, the copper content of GBP, and the plateau SCC velocities of overaged 7079 and 7075 alloys. Preliminary comparisons of overaged (T7) and peak-aged (T651) material have also been made, and support the view that beneficial effects of overaging on SCC resistance is associated with increasing copper content of GBPs.