Calculation of welding tool pin width for friction stir welding of thin overlapping sheets

Abstract

The fabrication friction stir welded lap joints with Al 7075-T6 sheets is examined with a wide range of tool pin geometries and dimensions while comparing microstructures, bonded area geometry, hardness, and fracture load during overlap shear testing. The tool pin geometry and features significantly affected the hook geometry and thus weld strength and failure mode. A hook feature is prone to forming when a larger diameter or helical threaded tool pins are used and is shown to deteriorate overlap shear fracture load by reducing the bonded ligament in the upper sheet. This hook feature can be controlled by removing the helical threads on the tool pin and replacing these with concentric grooves. The overlap shear fracture load is controlled by a combination of the sheet thickness, the width of the bonded area, and extent of the hook feature. These factors are captured in a model which was shown to provide good predictions of the fracture load and can be used to select tool pin sizes for varying overlapping sheet thicknesses.