Failure modes and fatigue life estimations of spot friction welds in lap-shear specimens of aluminum 6111-T4 sheets. Part 2: Welds made by a flat tool

Abstract

Failure modes of spot friction welds made by a flat tool in lap-shear specimens of aluminum 6111-T4 sheets are investigated based on the approach presented in Part 1. Optical and scanning electron micrographs of the welds before and after failure under quasi-static and cyclic loading conditions are examined. The micrographs show that the microstructure, geometry, and the failure modes of the welds made by the flat tool are quite different from those of the welds made by a concave tool. Under quasi-static loading conditions, the failure mainly starts from cracking near the boundary of the stir zone close to the upper sheet surface inside the weld. Under cyclic loading conditions, the experimental results indicate three types of fatigue cracks. The first type initiates and grows near the boundary of the stir zone close to the upper sheet surface inside the weld as that under quasi-static loading conditions. The second type initiates and grows into the lower sheet outside the stir zone. The third type initiates from the bend surface of the upper sheet outside the weld. Under low-cycle loading conditions, the dominant kinked fatigue cracks are the first type growing near the boundary of the stir zone. Under high-cycle loading conditions, the dominant kinked fatigue cracks are the second type growing in the lower sheet outside the stir zone. Based on the experimental observations of the paths of the dominant kinked fatigue cracks, the fatigue crack growth model presented in Part 1 is then adopted to estimate the fatigue lives of the spot friction welds made by the flat tool. The fatigue life estimations based on the fatigue crack growth model with the global and local stress intensity factors as functions of the kink length and the experimentally determined kink angles agree well with the experimental results. A comparison of the experimental results suggests that the failure strengths of spot friction welds under quasi-static loading condition can cautiously be used as references to select a tool and the corresponding processing parameters. However, fatigue tests must be performed to validate the performance of spot friction welds under cyclic loading conditions.