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The Influence of Metallurgical Pores on Fatigue Behaviors of Fusion Welded AA7020 Joints |
Zhe SONG1, Shengchuan WU1(), Yanan HU1, Guozheng KANG1, Yanan FU2, Tiqiao XIAO2 |
1 State Key Laboratory of Traction Power, Southwest Jiaotong University, Chengdu 610031, China 2 Shanghai Synchrotron Radiation Facility (SSRF), Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China |
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Cite this article:
Zhe SONG, Shengchuan WU, Yanan HU, Guozheng KANG, Yanan FU, Tiqiao XIAO. The Influence of Metallurgical Pores on Fatigue Behaviors of Fusion Welded AA7020 Joints. Acta Metall Sin, 2018, 54(8): 1131-1140.
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Abstract With the rapid development of Chinese high-speed railway system, the urgent demand for lighter weight structures is increasing, and aluminum alloys are widely applied into manufacturing the railway train and critical safety components. As a medium strength aluminum alloy, the 7020 aluminum alloy shows a great potential. Hybrid laser welding has currently become one of the most important welding techniques for medium and high strength aluminum alloys. Nevertheless, intrinsic defects such as pores and shrinkages physically determine the fatigue resistance of the welded joint. Based on in situ synchrotron radiation X-ray computed microtomography (SR-μCT), the population, location and size of gas pores within AA7020 hybrid welded joints are firstly identified and counted. The critical size of gas pores, affecting the fatigue properties of welded joints, is acquired by combining the statistical results of the pores and the average grain size of the hybrid weld. Meanwhile, the qualitative relationship between pore size, effective stress and fatigue life is discussed through in situ fatigue life data via SR-μCT and fracture morphology. By using the finite element analysis, detailed works have been performed on the stress state near the pores of different positions inside the joint. Through the simulation analysis, the stress concentration coefficient around the pores firstly increases, then decreases, and finally tends to a stable trend as the location of the pore-like defect is transferred from the surface to the inside. Besides, the influence of porosity on fatigue crack initiation, fatigue crack growth and sudden breaking process is also analyzed using fatigue crack growth experiment. In conclusion, the results show that the critical pore size of hybrid laser welded joint can be qualitatively identified as 30 μm; the SR-μCT and fracture analysis show that larger surface and sub-surface pores are more likely to initiate fatigue cracks, and the fatigue crack propagation experiment further shows that the porosity has very little effect on the long crack growth but significant influence on the crack front.
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Received: 26 October 2017
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Fund: Supported by National Natural Science Foundation of China (No.11572267), Science and Technology Research and Development Project of Sichuan Province (No.2017JY0216), Open Research Project of State Key Laboratory for Strength and Vibration of Mechanical Structures of Xi'an Jiaotong University (No.SV2016-KF-21) |
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