Effect of retrogression and reaging (RRA) on pitting and stress corrosion cracking (SCC) resistance of stir zone of high strength AA7075-T651 alloy joined by friction stir welding

Abstract

The main purpose of this study is to investigate the influence of retrogression and re-aging post weld heat treatment (RRA-PWHT) on potentiodynamic corrosion (PC) and stress corrosion cracking resistance (SCC) of stir zone (SZ) of AA7075-T651 alloy butt joints developed using friction stir welding (FSW). FSW was employed to overcome the problems in fusion welding of AA7075-T651 alloy such as solidification cracking, heat affected zone (HAZ) softening, grain coarsening, dissolution of strengthening precipitates and inferior mechanical performance. The SZ of joints were subjected to potentiodynamic corrosion test in 3.56 wt% NaCl solution. The circumferential notch tensile (CNT) specimens were subjected to SCC test at various conditions of axial stress level in 3.56 wt% NaCl solution. The microstructural features of butt joints SZ were characterized using optical (OM), scanning electron (SEM) and transmission electron microscope (TEM). The results disclosed that RRA-PWHT joints exhibited greater PC and SCC resistance compared to as-welded joints. The threshold stress level for the failure of CNT specimens of parent metal, as-welded joints and RRA-PWHT joints was 242 MPa, 175 MPa and 198 MPa, respectively. The threshold stress level of RRA-PWHT joints and as-welded joints is 81.81% and 72.31% of parent metal threshold stress level. It is correlated to increase in vol% and coarsening of grain boundary precipitates with enrichment of copper at the grain boundaries.