Mass-Loss Testing in Magnesium AZ31 Friction Stir Spot Welds

The choice of welding parameters during friction stir welding of AZ31 magnesium alloy can significantly affect the morphology and extent of corrosion attack. Mass-loss testing in combination with optical analysis revealed that both tool rotation speed and dwell time significantly altered the corrosion rate, morphology, and location of attack. With increasing tool rotation rate, the corrosion morphology shifted from general attack of the upper surface near the shoulder to mainly localized pitting within the shoulder surface for a constant dwell time. For a constant tool rotation rate, increased dwell time increased the susceptibility to localized corrosion but did not change the location. Both observations are consistent with the microstructural evolution of the thermo-mechanically affected zone during welding that is attacked preferentially. The scanning reference electrode technique (SRET) has shown the stir zone to be a noble region that increased in size with increasing rotational speed. The stir zone was found to be noble as a result of the reduced particle area fraction associated with β-phase dissolution during the welding process.