Refill Friction Stir Spot Welding (Refill Fssw) Of Magnesium Lithium Alloys: Effects Of Air And Argon Cooling

Magnesium-lithium alloys are among the lightest commercially-available structural alloys. However, the reactive nature of Mg and Li makes fusion welding of this family of alloys very difficult, if not impossible. The solid-state joining processes are considered suitable alternatives since the joining can be carried out at temperatures well below the melting of these alloys. Refill Friction Stir Spot Welding (Refill FSSW) of 2.2 mm thick Mg-Li plates, using air and argon as the cooling gas, was investigated. Initial examinations revealed formation of adherent oxides-based residues on the tool shoulder. The microstructural examination of the air-cooled samples showed the presence of oxide inclusions within the weldment. As expected, the argon-cooled welding process reduced oxide formation and additionally resulted in flat and smooth welded surfaces. Nevertheless, and somehow unexpectedly, the presence of oxide inclusions in the air-cooled samples had no noticeable effect on the shear strength or microhardness of the joints.