PSI - Issue 42

V.K. Yadav et al. / Procedia Structural Integrity 42 (2022) 594–601 V.K. Yadav et al., 2022/ Structural Integrity Procedia 00 (2019) 000 – 000

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techniques. Due to the poor weldability with conventional joining techniques, friction stir welding (FSW) technique developed by Thomas (1991) has been proven to have many advantages to fabricate these components. FSW utilizes the frictional heat to plasticize the metal underneath. The extensive plastic deformation between the tool shoulder and metal results in the micro-structural in-homogeneity such as refined and stable micro-structure within the stir zone (SZ), and twisted and elongated grains in the thermo-mechanically affected zone (TMAZ) (Aydin et al. (2010); Hu et al. (2011); Yadav et al. (2020)). This micro-structural in-homogeneity often provides favourable site for the crack initiation under cyclic loading and makes the joints more susceptible to intergranular cracking under corrosive environment, resulting the catastrophic failure of the structure. Therefore, it is necessary to examine the fatigue behavior of the welded components after pre-corrosion and for improved long-term serviceability. Fatigue crack initiation and its growth in the materials often found to be occurs due to the formation of persistent slip bands (Christ (2018)), generated tool marks and heterogeneous microstructures (Yadav et al. (2022)), local stress concentration due to defects (Gaur et al. (2020)). Several research studies focussing the microstructural in homogeneity, developed residual stresses (Sutton et al. (2002); Bussu and Irving (2003); Baillas (2013); Yadav et al. (2022)), and fatigue behavior of welded aluminum alloys were carried out. Few studies also devoted to investigate the corrosion behavior of welded AA2024 due to grain refinement by (Niu et al. (2019); Moreto et al. (2021)). A handful of investigations are also available related to pre-corrosion behavior of welded AA2024 by (Jariyaboon et al. (2007); Kang et al. (2010)). Though several studies are existing on the corrosion behavior of aluminum 2xxx alloys, yet effects on mechanical and fatigue properties of the welded joints are only handful. This study aims to bridge the gap. The corrosion potential and charge transfer resistance of the welded joints were estimated using electrochemical and potentiodynamic polarization tests. Also, the fatigue crack propagation rate for different load-ratios (i.e., R = 0.1 and 0.5) of welded joints after exposure of 100 hrs in the 3.5% NaCl solution was investigated, and the obtained results with underlying The high strength aluminum alloy of grade 2024-T3, 20 mm thick plate, was considered for the present study. The chemical composition (% weight) in as-received form are given as: 4.31% Cu, 1.38% Mg, 0.59 % Mn, 0.31% Fe, 0.16% Si, 0.27% Zn, 0.03%Cr and remaining Al. Plates were partitioned into the desired dimension of 5 mm thick using a band saw. The friction stir weld was performed using a customized 15 HP milling machine. Before the weld, plates edges were cleaned using acetone to ensure that edges are free from any foreign particles. The weld utilizes a cylindrical tool with a rotation speed of 931 RPM and a feed rate of 70 mm/min. The weld was performed along the rolling direction of the plate. To ensure that weld is defect free, weld’s transverse cross -section was polished and examined using stereoscope. Weld’s transverse cross -section was considered to get in-site of microstructural feature. A small cubical specimen was polished using sand papers of grades #320-1200. These samples were then electropolished using the mixture of methanol and perchloric acid in a ratio of 4:1 at an operating voltage of 13 volt for 17s at a maximum operating temperature between -1 to -2 0 C. The metallographic feature was examined using Electron Backscattered Diffraction (EBSD) detector equipped with Field Emission Scanning Electron Microscope (FE-SEM). Weld’s nugget zone was exposed to a freshly prepared salt solution (3.5% NaCl solution). Rectangular samples of fixed sizes were kept in saltwater for definite time i.e., 10, 24, 48, 100, and 200 hours. One sample from each category was taken out and examined using Electrochemical Impedance Spectroscopy (EIS) technique to investigate the polarization resistance. These specimens after immersing in the saltwater for definite time intervals were also examined using SEM. To investigate the corrosion phenomena, electrochemical analysis of the weld nugget zone was carried out in the 3.5% NaCl solution at an operating temperature of 24 0 C using Gamry 1010 potentiostat. A customized (in-house) damage mechanisms are discussed in this work. 2. Materials & Experimental Methodology