Effect of Process Parameters on the Tensile Strength of Hybrid Friction Stir Lap Joints of Polypropylene and AA5052

Hybrid lap joints of polypropylene (PP) and aluminum alloy AA5052 were fabricated using friction stir lap welding under different conditions of tool rotation speed, traverse speed and tilt angle, while placing PP on top. A full factorial design of experiments with three levels of each process parameter was performed, and the lap shear strength was evaluated for each run. The temperature at the interface between PP and AA5052 was measured by thermocouples, and an inverse heat conduction technique was utilized to estimate the approximate temperature distribution in PP. Cross sections of the joint were also examined using optical and scanning electron microscopy. It was found that decreasing the traverse speed, increasing the rotational speed and increasing the tilt angle resulted in an increase in the joint strength. These conditions also resulted in higher process temperatures, which lead to a more uniform weld nugget with less voids, due to the enhanced material flow. Moreover, morphology analysis revealed that mechanical interlocking caused by the anchoring of metal hooks in PP mainly contributed to the strength and performance of the joint. A direct relationship was found between the hook length on the advancing side and the lap shear strength.