Failure Analysis and Simultaneous Optimization of Flexural Strength and Elastic Modulus in Friction Stir Welded <scp>PP</scp> / <scp>HDPE</scp> – <scp> TiO ₂ </scp> Nanocomposite Joints

ABSTRACT This research examines friction stir welding (FSW) between polypropylene (PP) and high‐density polyethylene (HDPE) to produce hybrid polymer joints. To address the issue of PP's relatively high melting temperature, TiO 2 nanoparticles were added to HDPE using a 3D‐printing process, improving its thermal and mechanical behavior and reducing the melting point gap between the two plastics. The modified HDPE/TiO 2 nanocomposite was subsequently joined to PP using FSW. Tensile and bending tests, along with SEM and DSC analyses, were used to characterize the fracture surfaces of the nanocomposite and the welded interfaces. The response surface methodology (RSM) was employed to model the effect of rotation speed, feed rate, and axial force on the flexural strength and elastic modulus of the welded specimens. The results demonstrated that an increase in rotating speed to 1300 rpm and a decrease of the feed rate to 10 mm/min reduced microstructural defects, which in turn enhanced the mechanical performance of the joints. Fracture surface analysis revealed that increasing the rotating speed from 800 to 1300 rpm induced a transition from brittle cleavage‐dominated failure to ductile dimple‐rich morphology, while excessive feed rates (30 mm/min) reverted the fracture mode to fully brittle behavior, highlighting the critical role of process parameters in controlling the failure mechanism of dissimilar polymer FSW joints. Optimal weld performance, characterized by simultaneous maximization of flexural strength and elastic modulus, was achieved under the following conditions: a rotating speed of 1318 rpm, a feed rate of 16 mm/min, and an axial force of 5 kN.