Effect of Tool Rotational Speed and Dwell Time on the Joint Strength of Friction Stir Spot Welded AA6061-T6 Aluminum Alloy Sheets.

Friction stir spot welding (FSSW) is a solid state welding technique, was developed by Mazda Motor Corporation and Kawasaki Heavy Industries as a novel derivative of the friction stir welding as an alternative to the traditional fusion welding processes. FSSW successively used to join similar and dissimilar metals. Tool rotational speed and dwell time are the most effective FSSW process parameters. In the present investigation, the role of the tool rotational speed and the dwell time in determining the strength of the FSSW joints was studied using AA6061-T6 aluminum alloy sheets with thickness of 1.8 mm as a workpiece material. A classic milling machine was employed to carry out the welding process. Four different values of tools rotational speed with two dwell time values were taken to produce the FSSW lap-shear specimens. Four specimens were performed for each FSSW process condition. three specimens were averaged to evaluate the tensile-shear fracture load, while the remaining specimen was used for the micro-Vickers hardness examination and the microstructure observation. The investigation reported an increase in the joint strength within a certain range of tool rotational speed and dwell time values corresponding to grain refinement in the weld zone. The variation in mechanical properties was attributed to the corresponding frictional heat generation and material flow during the welding process. Strain hardening and dynamic recrystallization determined the grain refinement and the weld nugget hardness. Lower mechanical properties were observed with the excessive frictional heat generation and material flow with the too high speeds and dwell time values.