Electrochemical corrosion of AA6061-AA7075 double sided FSW joints prepared with and without secondary heating

Abstract

The present study aims to evaluate corrosion behavior of different zones in the double-sided dissimilar friction stir weld between precipitation hardening aluminum alloys AA6061-T6 and AA7075-T651 of 12.7 mm thickness obtained without and with employing secondary/additional heating. Each specific zone of the welds is tested for corrosion potential, corrosion current and impedance spectroscopy. The stir zone formed is more corrosion resistive than AA7075 but less in comparison to AA6061. The corrosion resistivity of AA7075 dominated region increases significantly on the application of secondary heating. The increase is attributed to the disintegration of large corrosion susceptible precipitates due to intense mixing and stirring facilitated at elevated temperature with the application of additional heating. Although, AA6061 experienced trivial decrease in corrosion resistance as high working temperature resulted in increased grain boundary along with high misorientation angle. The scanning electron microscopic images and elemental line scan at corroded stir zone identified AA7075 as the sacrificed alloy due to galvanic coupling with AA6061 at macroscopic scale and dissolution of Mg/Si at microscopic scale. The regions with higher percentage of high angle grain boundaries observed to undergo faster corrosion rates. The heat produced in second pass left annealing effect on prior welded region wherein lower microhardness is noted. • Corrosion resistance of SZ in AA6061-AA7075 DS-FSW joints is enhanced compared to AA7075 while reduced compared to AA6061. • HAZ and TMAZ on RS (AA7075) showed significantly increased corrosion resistance employing secondary heating. • Breakdown of corrosion sensitive intermetallic helped in increasing corrosion resistance. • Slight drop in corrosion resistance of AA6061 observed after secondary heating as increased fraction of HAGBs promoted IGC.