Comparative study of polygonal pins with knurling shoulder on friction stir processed composites of AA7075-T651

The knurling shoulder with four polygonal pins, for example, square (TKS), triangular (TKT), pentagon (TKP) and hexagon (TKH), was innovatively designed and used to fabricate composites of AA7075-T651/SiC/Graphene. The impact of various polygonal pins with knurling shoulder on microstructure, mechanical, and tribological properties was thoroughly investigated and analyzed. Material flow, particle distribution, and various region developments were examined using optical microscopy and scanning electron microscopy with energy dispersive X-ray spectroscopy. The microhardness and wear properties of the made-up composites were measured and compared with each tool design. Based on the findings, a triangular with knurling shoulder distributes SiC and graphene particles more uniformly than other tool design combinations. Among the investigated tool configurations, the triangular pin with knurled shoulder (TKT) demonstrated superior performance in terms of uniform dispersion of SiC and graphene particles. The enhanced material shearing and stirring action associated with this geometry promoted severe plastic deformation and facilitated accelerated dynamic recrystallization, leading to significantly refined grains with minimal clustering. As a result, the TKT-processed composites exhibited enhanced surface integrity and notable improvements in functional performance. Compared to the base alloy, the grain size, volume loss and wear rate were reduced by 98%, 64% and 58%, respectively, while the microhardness, and coefficient of friction increased by 39%, and 62%, respectively. These improvements highlight the effectiveness of the TKT tool in optimizing reinforcement distribution and improving mechanical and wear characteristics.