Corrosion Behavior of the Friction Stir Welded AZ31 Magnesium Alloy

Magnesium alloys are considered to be a valuable engineering material since they offer a significant potential to reduce weight due to their low density, good thermal and fatigue properties, among others. However, problems appear when trying to apply conventional welding techniques to these alloys. The typical low weldability of magnesium alloys with traditional joining processes involving melting is expected to be improved with Friction Stir Welding (FSW) since it is a solid state joining process. Magnesium alloys are also very susceptible to the heating effects being an advantage for the FSW process since it works at lower temperatures than the conventional methods. The FSW process is used in this work with 2 mm thick AZ31 plates and the samples were welded at 100 mm/min and 800rpm with a constant applied force of 600kg and 0.50 tilt angle. These alloys are known to be very susceptible to corrosion in saline environments [1] and some authors have found that usually, in magnesium alloys, some particles are not dissolved in the magnesium matrix and will promote pitting corrosion [2]. When analyzing the base material the existence of undiluted particles (white articles) aligned with the material matrix was observed. After welding, there was an increase of these particles in the heat affected zone (HAZ) while the nugget that suffers both mechanical and thermal stresses have promoted the decrease of the grain size and also reduced the number of undiluted particles in this area (Figure 1).