Sub-zero Temperature Dependence of Tensile Response of Friction Stir Welded Al-Cu-Li (AA2198) Alloy
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INTRODUCTION
THIRD-GENERATION aluminum-lithium alloys based on ternary systems of Al-Cu-Li have been the center of significant research activity in last couple of decades. These alloys have low density, high strength, and high stiffness in combination with good weldability as compared to other aluminum alloys commonly used in the aerospace industry.[1] Al-Cu-Li-based alloys are generally strengthened by a combination of GP zones, T1, h¢, and d¢ precipitates in peak-aged condition.[2] Joining of sheets or plates made of Al-Cu-Li alloys is an inevitable step in designing structural units. Being a
NIRAJ NAYAN is with the Materials and Mechanical Entity, Vikram Sarabhai Space Centre, Indian Space Research Organization, Trivandrum 695 022, India and also with the Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Mumbai 400 076, India. MANASIJ YADAVA and NILESH P. GURAO are with the Department of Materials Science and Engineering, Indian Institute of Technology Kanpur, Kalyanpur, Kanpur 208016, India. S.V.S. NARAYANA MURTY is with the Materials and Mechanical Entity, Vikram Sarabhai Space Centre, Indian Space Research Organization, Mumbai 400 076, India contact e-mail: [email protected] SIVASAMBU MAHESH is with the Department of Aerospace Engineering, Indian Institute of Technology Madras, Chennai, 600 036, India. M.J.N.V. PRASAD and I. SAMAJDAR are with the Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay Manuscript submitted August 11, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
solid-state joining process, friction stir welding (FSW) has been a process of choice for joining Al-Li alloys. This is majorly because of poor weldability under fusion welding owing to the oxidation of Li and its consequent losses in addition to the solidification-related issues pertaining to Al. Further, it has been reported that FSW eliminates certain weld defects such as weld porosity commonly associated with fusion welding.[3,4] The microstructure obtained during FSW consists of fine equiaxed and dynamically recrystallized grains [5] as compared to cast grain structure obtained during fusion welding. The weld nugget at the seam is surrounded by a thin layer of thermo-mechanically affected zone which may be enveloped by slightly heat-affected zone on the two sides of the weld. The frictional heat generated at the tool–base metal interface and the associated plastic deformation destabilize the precipitate microstructure of the parent metal which undergoes coarsening–reversion–reprecipitation reaction sequence.[6–11] Thus, even though an FSW weld nugget has a wrought microstructure, FSW on a peak-aged aluminum alloy leaves a region of low strength in the weld zone which significantly affects the load-carrying capacity of the welded structure.[7,12–14] Tensile properties of FSW of Al-Cu-Li-based alloys have been studied by several researchers[7,12,13] which report the joint strength of 50 to 70 pct of the peak-aged base alloy strength.
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