Influence of the kissing bond on the mechanical properties and fracture behaviour of AA5083-H112 friction stir welds

The kissing bond phenomenon in AA5083-H112 friction stir butt welds was investigated in joints welded using a matrix of welding parameters, with tool rotation speeds of 800, 1000, and 1200 rpm and feed speeds 100, 200, and 300 mm/min. The length of the kissing bond along the cross-section normal to the welding direction was measured to quantify its influence on the mechanical properties. A combination of optical microscopy, scanning electron microscopy, transmission electron microscopy, tensile and fatigue testing were used to elucidate the impact of the kissing bond on the microstructural and mechanical properties development. The fracture type, location, and morphology were studied for the various conditions. The results showed that the welding parameters had a substantial effect on the length of the kissing bond, which was found to decrease with the increase in the welding heat input, as estimated based on the rotation and feed speeds. Moreover, the length and morphology of the kissing bond had a significant influence on the tensile and fatigue fracture type. A shear fracture type was characteristic for welds showing high tensile properties and long fatigue life, whereas fracture along the kissing bond was characteristic for poor tensile properties and short fatigue life.