Thermo-mechanical characterization of friction stir spot welded AA6060 sheets: Experimental and FEM analysis

• Mechanical properties of the joints strictly related to rotational speed: optimal conditions for intermediate values.
• Mechanical properties decrease for high feed rate and increase for low plunging depth and dwell time.
• Optimal mechanical properties for intermediate conditions of welding temperature and forces.
• The proposed FEM model well predict welding forces and temperature.
• Prediction of the joint resistance using FEM simulations, basing on mean stress and temperature.

A study was carried out to evaluate how the friction stir spot welding process parameters affect the temperature distribution in the welding region, the welding forces and the mechanical properties of the joints. An experimental campaign was performed by means of a CNC machine tool and FSSW lap joints on AA6060-T6 aluminum alloy plates were obtained. Five thermocouples were inserted into the samples to measure the temperatures during the tool plunging. A set of tests was carried out by varying the process parameters, namely rotational speed, axial feed rate, plunging depth and dwell time. Axial welding forces were measured during the execution of the experiments by means of a piezoelectric load cell. The mechanical properties of the joints were assessed by executing shear tests on the specimens. A correlation between process parameters and joints properties was found.The collected experimental data were also used to set up and to validate a simulative model of the process. The peculiarity of the developed FEM model is a 2D approach used for the simulation of a 3D problem, in order to guarantee a very simple and practical model able to achieve results in a very short time. The 2D FEM model, based on a specific external routine for the calculation of the developed thermal energy due to the friction between tool and workpiece, was set up using the commercial code Deform 2D. An index for the prediction of the joint shear resistance using FEM simulations was finally proposed and validated.