Friction Stir Welding of Al Alloy 2219-T8: Part II-Mechanical and Corrosion
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INTRODUCTION
In Part I of this study, two types of abnormal agglomerations of h particles were found in the weld nugget zone (WNZ) of a friction stir welded (FSW) Al alloy (AA) 2219-T8 joint.[1] h particle agglomerations with compact and scattered morphologies were labeled as Type I, and ring-shaped agglomerations of normal size h particles were labeled as Type II. h (Al2Cu) phase is a typical precipitate that forms in Al-Cu alloys with a size less than 25 lm. [2] Precipitated phases can provide considerable strength, but are not beneficial to the properties of Al alloys if the heat treatment is done improperly.[3] The h phase, which is generated during overaging, does not improve the strength because their size is too large to hinder dislocation motion.[3,4] The h particle also possesses high hardness and low plasticity, which can induce microvoids due to the mismatch deformation between h and the adjacent matrix. These microvoids can merge together to form a crack and eventually cause failure. As JU KANG, Ph.D. Student, is with the Department of Mechanical Engineering, Tsinghua University, Beijing, 100084, China, and with the Fontana Corrosion Center, The Ohio State University, Columbus, Ohio, 43210-1185, and also with the North China Electric Power Research Institute Co. Ltd., Beijing 100045, China. Contact e-mail: [email protected] ZHI-CAO FENG and JI-CHAO LI, Ph.D. Students, and G.S. FRANKEL, Professor, are with the Fontana Corrosion Center, The Ohio State University. GUO-QING WANG, Professor, is with the China Academy of Launch Vehicle Technology, Beijing, 100076, China. AI-PING WU, Professor, is with the Department of Mechanical Engineering, Tsinghua University, and also with the State Key Laboratory of Tribology, Tsinghua University, Beijing, 100084, China. Manuscript submitted March 13, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
a result, h particles are often found at dimple rupture fracture surfaces of Al alloys.[5–7] Regarding corrosion properties, h phase can deteriorate the corrosion resistance of Al alloys. Although h itself has a high corrosion resistance, the heterogeneous structure of alloys containing h phase can lead them to be susceptible to localized corrosion.[8–11] A localized galvanic couple forms with h phase as the cathode and the matrix as the anode.[12] Cathodic reactions including oxygen reduction reaction and hydrogen evolution occur on the h phase, which increase the local pH, leading to the breakdown of Al oxides on the matrix and fresh matrix exposed to the electrolyte. Meanwhile, cathodic reactions consume electrons provided by Al dissolution, driving trenching attack at the h/matrix interface.[12–17] The effects of abnormal agglomerations of h particles on the mechanical properties of welded joints were studied in two previous works. Tensile tests were conducted by Li and Shen on a repaired FSW AA2219-T6 joint containing abnormal agglomerated h particles, indicating that the joint fracture was away from the abnormal h particles.[18] Based on this, they proposed that abnormal
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