Microstructures and Failure Mechanisms of Spot Friction Welds in Lap-Shear Specimens of Aluminum 6111-T4 Sheets

Microstructures and failure mechanisms of spot friction welds in aluminum 6111-T4 lap-shear specimens are investigated based on experimental observations. Two types of tools, a Type I tool with a flat tool shoulder and a Type II tool with a concave tool shoulder, were used to join the aluminum sheets with different processing parameters. Optical micrographs of the cross sections of spot friction welds made by the two types of tools in lap-shear specimens before and after failure are examined. These spot friction welds show the failure mode of nugget pullout under lap-shear loading conditions. However, the micrographs show different microstructures and failure mechanisms for spot friction welds made by the two types of tools with different processing parameters. The experimental observations suggest that under lap-shear loading conditions, the failure is initiated near the stir zone in the middle part of the nugget and the failure propagates along the circumference of the nugget to final fracture.