Ultrasonic metal welding of AA 6022-T4 lap joints: Part II - Fatigue behaviour, failure analysis and modelling

The present paper reports the second part of a research carried out by the authors on the application of ultrasonic metal spot welding to lap joints made of AA 6022-T4 thin sheets. In particular, starting from the findings of Part 1, tensile-shear fatigue experiments are carried out on ultrasonically spot welded lap joints both in single and double spot configurations. The analysis of consequent failure causes and modes and the application of traditional approaches for fatigue assessment of resistance spot welded joints allow then effectively predicting experimental evidences. Furthermore, an optimisation of the fatigue behaviour of double spot joints (longitudinal spots configuration) is achieved in terms of endurable load and distance between spots, with a possible generalisation to multiple spot configurations. Finally, a comparison of the fatigue performance of the ultrasonic technique to other spot joining techniques is presented.