Analysis of the mechanical properties and of micrographs of refill friction stir spot welded 7075-T6 aluminium sheets

The main aim of the investigations was to determine the significance of parameters of RFSSW for the strength parameters of the resulting joint. RFSSW joints were made in 0.8-mm-thick 7075-T6 aluminium alloy using different welding parameters (tool rotational speed v, tool plunge depth, and joining time). The load capacity of joints was determined by shear tests. The optimum welding parameters which assure the highest load capacity (4.09 kN) in the tensile/shear test are: tool rotational speed of 3000 rpm, tool plunge depth of 1.55 mm, and joining time of 1.25 s. In the conditions of the static tensile test under pure shear the highest joint capacity (6.48 kN) is found for the joint welded at a tool plunge depth of 1.7 mm. Depending on the tool plunge depth, three types of joint damage were observed during tensile/shear tests: plug type fracture, shear fracture, and plug-shearracture. Incomplete refill is the main weld defect observed which is located on the path along which the sleeve plunges into the sheet. It was also found that alclad between the upper and lower sheets worsens the joint quality between the stir zone and thermo-mechanically affected zone.