Mechanical characterization of friction stir spot microwelds

Microstructures and failure mechanisms of friction stir spot microwelds in 300-μm thin sheet of aluminum 1050 alloy were investigated. As an alternative to conventional soldering and welding in joining thin metals for electronic, medical and microdevices, friction stir welding may be utilized in order to limit the excessive heat damage. Transmission electron microscopy micrographs of the cross sections of friction stir spot microwelds in lap-shear specimens were examined. These microwelds showed the failure mode of nugget pullout under lap-shear loading conditions. The experimental observations suggested that under lap-shear loading conditions, the failure was initiated near the possible original notch tip in the stir zone and the failure propagated along the circumference of the nugget to final fracture. Microindentation hardness data of base metal, heat affected zone, thermal-mechanical affected zone and stir zone were obtained. The interface between the heat affected zone and the thermal-mechanical affected zone was the softest region, where the cracks of friction stir spot microwelds in the lap-shear specimens under the loadings initiated and leaded to fracture of the specimens.