This study aims to determine how different parameters affect the tensile strength of dissimilar materials joined using Friction Stir Welding (FSW), specifically the AA 8090 aluminum alloy and four polymers: PEEK, Nylon 6, HDPE, and PP. The predominant strengthening mechanism is mechanical interlocking, in which plasticized aluminum permeates the softened polymer phase. However, a crucial secondary mechanism, interfacial chemical bonding, is responsible for the stronger bonds seen in polar polymers (PEEK and Nylon 6), which form covalent and hydrogen bonds. This mechanism is absent in non-polar polymers like PP and HDPE, resulting in less efficient joints. The research found that controlling process parameters specifically tool rotation and traverse speed is crucial, as they directly influence the heat generation and material flow that determine the joint's tensile strength. The results indicate that each polymer requires a specific "processing window." This is necessary to balance the heat and material flow to improve the joint's integrity while minimizing defects like voids or excessive melting.