13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Effect of different micro-arc oxidation coating layer types on fatigue life of 2024-T4 alloy Xi-Shu Wang1,*, Xing-Wu Guo2, Xu-Dong Li3, Norio Kawagoishi4 1 Department of Engineering Mechanics, Tsinghua University, Beijing 100084, P.R. China 2 National Engineering Research Center for Light Alloy Net Forming School of Material Science & Engineering, Shanghai Jiao Tong University, Shanghai, 200030, P.R. China 3 Department of Mechanical Engineering, Qingdao Campus of Naval Aeronautical Academy, Qingdao, 266041, P.R. China 4 Department of Mechanical System Engineering, Daiichi Institute of Technology, Kirishima, 899-4395, Japan * Corresponding author: xshwang@tsinghua.edu.cn Abstract Due the fatigue life of aluminum alloys is strongly sensitive to the environmental effect in the most engineering applications, one of effectively improved methods on the strength and fatigue life of aluminum alloys is generally to take a coating process on the surface of these aluminum alloys. And the micro-arc oxidation ceramics coating method is not only to enhance the fatigue strength but also to avoid the environment corrosion. In this work, a dual-spindle rotating bending environment fatigue tests for two types aluminum alloys were carried out. The effects of differently relative humidity and oxidation ceramics coating types on fatigue life of aluminum alloys were discussed. These results indicated that the fatigue life (Nf>10 6) of 7074-T6 alloy began obviously to degrade when the relative humidity is over than 75% and there is a threshold. At the same time, the fatigue lives of 2024-T4 alloy by using the different oxidation ceramics coating treatments present the different changed tendencies. For example, the effect of micro-arc oxidation ceramics coating with sealing hole technology on fatigue life is positive and the effect of other two oxidation ceramics coating technologies on fatigue life is negative, such as micro-arc oxidation coating and hard oxidation coating. Keywords Fatigue life, Aluminum alloy, Micro-arc oxidation technology, Coating layer, Relative humidity 1. Introduction Aluminum alloys have been widely used for making structural parts in transport vehicles such as ships and aircraft for the purpose of reducing their weight. However, these structures are usually exposed to the natural environment throughout their lifetime. In these fields, very often the main requirements for the components include high-cycle fatigue (HCF) and corrosion resistance [1], especially the corrosion fatigue (CF) is defined as the sequential stages of aluminum alloys damage that develops during accumulated load cycling in an aggressive environment [2-4]. In addition, the effect of the relative humidity of over 60% on the HCF life of aluminum alloys can not be ignored. This is because the decrease in fatigue life was mainly the acceleration of crack growth caused by brittle fracture under rotating bending and the transition to shear mode crack accompanied with glide plane decohesion and void formation under ultrasonic loading respectively [5,6]. The first requirement is achieved usually by a proper material selection to enhance the fatigue resistance of aluminum alloy but not to avoid the higher cost. The second requirement for aluminum alloys is achieved by using corrosion resistance coatings [7,8]. Micro-arc oxidation ceramics process is a classical approach to enhance the fatigue properties [9] and to provide one of countermeasures against the fatigue corrosion damage of aluminum alloys. However, the enhanced behavior and effective mechanism to fatigue property of aluminum alloys exists still some arguments and/or there are not enough experimental evidences. In this work, we sum up the effect of the relative humidity on the fatigue life of aluminum alloys then investigate the effects of different micro-arc oxidation ceramics methods on the fatigue life of Al 2024-T4 alloy. The results indicated that the fatigue life (Nf>10 6) decreased obviously when the relative humidity scopes from 50% to 75% and the effect of micro-arc oxidation ceramics coating with sealing hole on the fatigue life of Al 2024-T4 alloy is
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