The Effect of Post-Weld Heat Treatment on the Corrosion Performance of Friction Stir Welded High Strength Aluminum Alloys

Of all the high strength aluminum alloys, 7000 series alloys have been used most extensively for aircraft structures. Joining 7000 series alloy components using friction stir welding (FSW) results in welds with higher strength than fusion welded components. However, the combination of stresses arising from welding and service conditions (such as marine environment and de-icing fluids) can result in the localised corrosion and particularly stress corrosion cracking (SCC) of high strength aluminum alloys. Welds in 7000 series alloys show different corrosion behaviour to the parent metal due to microstructural differences across the thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ) and stir zone. Stress corrosion cracking (SCC) is known to initiate in the HAZ and / or nugget regions and behaviour varies considerably with alloy composition and welding parameters. Despite the fact that potential applications require an understanding of environmental performance, limited corrosion studies have been conducted to date. Recent research at TWI has been undertaken to determine the corrosion performance of FSWs in high strength Al alloys as a function of heat treatment and develop PWHT solutions for improving the corrosion resistance of high strength Al FSW joints.

Work undertaken included an evaluation of the effects of FSW process parameters and a wide selection of post-weld heat treatments (down-selected from technical literature) on the microstructure and mechanical properties of friction stir welded 7050 and 7475 Al alloys. Intergranular corrosion (IGC) tests and four-point bend beam SCC tests were performed in accordance with ASTM Standard G110-92, ASTM G39 and ASTM G44 standards. Stress corrosion cracking tests were also undertaken on friction stir welded 7050 alloy coupons which had been coated with cold spray deposited aluminum after welding without post-weld heat treatment.