Friction Stir Welding of Al Alloy 2219-T8: Part I-Evolution of Precipitates and Formation of Abnormal Al 2 Cu Agglomerat
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AL alloy (AA) 2219 exhibits excellent mechanical properties in a wide temperature range from 23 K to 523 K ( 250 C to 250 C), which gives it great potential for use in aerospace structures, especially in the construction of liquid cryogenic rocket fuel tanks.[1,2] However, the tensile strength of a fusionwelded sample is only 50 to 60 pct that of the base metal using processes such as electron beam welding or gas tungsten arc welding due to the formation of porosity and solidification cracking.[3–5] Friction stir welding (FSW) is a solid-state joining technique that does not form the defects caused by fusion welding.[6–9] Malarvizhi and Balasubramanian compared the mechanical properties of AA2219-T8 joints made by tungsten arc welding and FSW, showing that the JU KANG, Ph.D. Student, is with the Department of Mechanical Engineering, Tsinghua University, Beijing, China, and with the Fontana Corrosion Center, The Ohio State University, Columbus, OH 43210-1185, and also with North China Electric Power Research Institute Co. Ltd., Beijing 100045, China. Contact e-mail: kangjusin@ 163.com ZHI-CAO FENG and I. WEN HUANG, 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, 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 3, 2016. Article published online July 12, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
ultimate tensile strength of joints was 242 and 342 MPa, respectively.[3] AA2219 is a typical age-hardened Al-Cu alloy in which precipitation aging follows the sequence: supersaturated solution (SSS) fi a + Guinier–Preston (GP) zones fi a + h¢¢ fi a + h¢ fi a + h, where the SSS represents the trapped Cu atoms in Al matrix due to solutionizing and quenching.[10] Heat treatments such as artificial aging can cause the Cu in solution to precipitate from the matrix and form GP zones, h¢¢, h¢, and h phases. The GP zones, h¢¢, and h¢ are metastable phases, while h is a stable equilibrium phase.[10–16] The solubility of Cu in Al matrix has the highest value about 5.65 wt pct at the eutectic temperature, 821 K (548 C). It decreases with temperature to about 0.5 wt pct at room temperature.[10] Since AA2219 contains a high Cu concentration of 5.8 to 6.8 wt pct, it contains a large amount of Al-Cu phases at room temperature including h¢¢, h¢, and h, which strengthen the alloy by hindering dislocation motion.[9–12] However, h phase contributes little to the strengthening because of its larger size.[10] Furthermore, h phase is cathodic to the Al matrix, so it forms a local galvanic cell. When the alloy is exposed to a corrosive environment, corrosion initiates and propagates at the h/matrix interfaces, resulting in pitting or intergranular corrosion attack.[17–21] Therefore, the nature of Al-Cu phase plays an impor
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