Initial characterisation of mechanical and corrosion properties of stationary shoulder friction stir welded bare and Alclad AA2024-T3 butt joints

Abstract

Aluminium alloy AA2024-T3 is a lightweight and damage tolerant material, and is therefore often used in aerospace applications. However, this alloy is difficult to weld using conventional fusion welding techniques due to defects caused by the meld pool.

Friction stir welding (FSW) was developed by The Welding Institute (TWI) in 1991 in order to overcome welding difficulties associated with the conventional fusion welding of difficult to fusion weld materials such as AA2024-T3. Since then, efforts have been made to improve the process parameters. One such improvement was the development of a stationary shoulder tool, which reduces heat input by 30% and enables stronger welds with smaller heat affected zones (HAZ). However, since AA2024-T3 is susceptible to pitting corrosion, intergranular corrosion (IGC) and stress corrosion cracking (SCC), the welds need to be protected in order for any product to function in the long term. AA1050 is an often applied clad layer, which acts as a sacrificial anode with respect to the cathodic substrate. The microstructure and corresponding corrosion mechanisms are known for unwelded and FSW’d AA2024-T3, however, little research exists on the corrosion behaviour of stationary shoulder friction stir welded (SSFSW’d) bare and Alclad AA2024-T3 butt welds. Therefore, the main focus of this project is to investigate the mechanical and corrosion properties of SSFSW’d bare and Alclad AA2024-T3. This was done by performing optical microscopy, microhardness tests, open circuit potential measurements, linear polarisation resistance tests and potentiodynamic polarisation tests on the cross-sections of three weld configurations and thicknesses: 1.6 mm bare, 1.6 mm Alclad and 3.2 mm Alclad AA2024-T3 sheets. Simultaneously, tensile tests and immersion tests were also performed using dedicated tensile test and immersion specimens.

Analysis shows that the 1.6 mm bare weld is the strongest weld but also the most susceptible to pitting corrosion, compared to the Alclad welds. Cladding provides sufficient corrosion protection, even to an exposed section of the weld and when mixed into the weld. However, cladding lowers the tensile strength overall, and cladding mixed into the weld reduces the ductility of the weld compared to the bare weld. Furthermore, possible material flow issues at the root of the Alclad welds may cause voids, which lowers ductility and enables pitting at these locations. Nevertheless, even though specimens were immersed in 3.5% NaCl solution for 24 hours and small to severe pitting was visible on the specimens, the ultimate tensile strength was not affected compared to uncorroded weld specimens. However, a reduction of the maximum elongation of the bare specimen was observed after immersion. Similar to conventional FSW, the HAZ/TMAZ was the most susceptible to corrosion, due to the most active corrosion potential at this zone. Based on literature, this was deemed to be due the formation of S-phase precipitates along the grain boundaries in this zone. In all, stationary shoulder friction stir welded bare AA2024-T3 provide the best welds regarding mechanical properties, and should be protected against corrosion after welding instead of using preclad sheets, to avoid issues with the macrostructure and corresponding mechanical properties.