Fatigue behaviour of friction stir welded A7075-T6 aluminium alloy in air and 3% NaCl solution

Abstract

Fatigue tests were conducted under fully reversed axial loading (R = − 1) in laboratory air and 3% NaCl solution using friction stir welded (FSW) joints of 7075-T6 aluminium alloy sheets. The FSW joint exhibited lower tensile strength than the parent metal. Heat input during the FSW process redissolved strengthening precipitates, resulting in softening in the weld zone and lower tensile strength. In laboratory air, the fatigue strength of the FSW joint was comparable to that of the parent metal, which could be attributed to grain refinement in the stir zone and dynamic ageing during fatigue loading in the softened weld zone. In 3% NaCl solution, the fatigue strength of the FSW joint was lower than that of the parent metal. Corrosion pits were preferentially formed at the boundary between the thermo-mechanically affected zone and heat-affected zone, which led to premature crack initiation in the FSW joint. Such predominant formation of corrosion pits was due to the sensitization caused by heat history during the FSW process.

Keywords:

• friction stir welding
• fatigue
• corrosion fatigue
• crack initiation
• microstructure

Acknowledgements

We would like to express our appreciation of a subvention for this study from the Fundamental Research Developing Association for Shipbuilding and Offshore.