Microhardness and residual stress of dissimilar and thick aluminum plates AA7181-T7651 and AA7475-T7351 using bobbin, top, bottom, and double-sided FSW methods

Abstract

Friction stir welding (FSW) is a relatively new manufacturing process (invented in 1991 at the Welding Institute, UK) and more than 5000 scientific articles have been published in the past 10 years in indexed journals demonstrating the robustness of the research. However, further research is necessary to ensure safe use of the technique for structural components, particularly with reference to the aeronautics industry. Residual stresses and their consequences on the life of welded products must be fully understood, in addition to their correlations with other properties such as hardness, strength, and microstructure. This paper is a part of the research being conducted to evaluate the impact of four FSW techniques (bobbin, top-sided, bottom-sided, and doublesided) on the mechanical properties in dissimilar thick joints (12.7 mm) of aluminum alloys (AA7181-T7651 and AA7475- T7351) used in the aeronautics industry. The residual stresses were measured using the incremental blind hole technique and analyzed using the integral method. The longitudinal residual stresses were all positive, with values between 100 and 200 MPa in the stir zone, whereas the transverse ones were all negative, with values between 0 and − 100 MPa. It was possible to verify that the Bobbin process produced lower values of residual stresses and demonstrated better stability in its distribution compared to all the other FSW methods tested.