Microstructure, mechanical and corrosion behaviours in friction stir welding of dissimilar magnesium alloys

Using magnesium alloys reduce the weight of a vehicle considerably, but the available fusion welding methods impose restrictions on the fabrication of magnesium alloy components. Friction stir welding is an efficient method to weld magnesium alloys, but due to extreme plasticization and stirring of materials, the microstructures are transformed. Similar to other alloys, friction stir welded magnesium alloys are also susceptible to corrosion. In the present work, dissimilar AZ31 and AZ91 Mg alloys were friction stir welded at varying welding conditions. The variable presence of small cracks with a high amount of oxide, and microstructural and textural differences result in varying joint strength, that is, joint efficiency, which has a maximum value of approximately 90% of a stronger base material. The maximum corrosion potential obtained in welded zones is approximately equal to a base material that conveys similar electrochemical behaviour. The minimum corrosion rate of 0.22 mm/year is observed in the sample welded at 700 rpm tool rotational speed, 30 mm/min welding speed and 18 mm shoulder diameter. Dissimilar friction stir welding (FSW) of AZ31 and AZ91 at variable conditions is characterized. Some small cracks with high concentrations of MgO and a maximum of 90% joint efficiency are achieved. The maximum corrosion potential in the welded region is approximately equal to a base material that showed similar electrochemical behaviour after FSW.