Dieless friction stir extrusion joining of aluminum alloy sheets with a pinless stir tool by controlling tool plunge depth

Abstract

Abstract The present work explains dieless friction stir extrusion joining for aluminum alloy sheets, AA5052-H32 and AA6061-T6, of 2 mm thickness each. The joints are strengthened by metallurgical bonding as well as mechanical interlocking. The hook defect and the pinhole formation in friction stir spot welding are successfully eliminated in this new process. Experimental investigation of the effect of tool plunge depth on the formation of the joint and its performance is comprehensively discussed with mechanical performance tests, macro/ microstructuture analysis, hardness measurement, joint morphology and failure mode analysis. The joints possess shear fracture load of 7.7 kN, which is superior than that of friction stir spot welding and conventional friction stir forming. Higher tool plunge depth level ranging from 0.45–0.50 mm is found to be optimum choice for fabricating spot joints with better joint strength and aesthetic appearance. Slight increase in tool plunge depth yields significant improvement in the mechanical interlocking (collar growth). The zones identified through microstructural analysis, which are subjected to severe plastic deformation and frictional heat flux possess significant recrystallization and grain refinement. Reduction in hardness is observed over the joint spot than that of the parent metal. Morphological features of the joint are affected by increase in the plunge depth beyond the optimum range. The critical zones in the joint, which leads to pin failure and bonding delamination failure are also identified.