The demand for fuel-efficient transportation created the need for lightweight hybrid structures. This often requires the joining of similar and dissimilar materials to obtain complex structures for load bearing and aesthetics. Traditionally, joining is achieved by mechanical fastening and adhesive bonding. However, these methods have limitations, such as the requirement to drill holes and the associated stress concentration. Friction stir welding (FSW) is a new solid-state joining method that proved to be very effective in welding aluminum alloys. This project aims to investigate the joining mechanisms of polymers and metals to polymers by friction stir welding and friction lap welding. Experimental and numerical methods will be used to investigate the material flow and microstructure/crystallinity evolution in the stir zone and the joining mechanisms between metals and polymers. The welding of polymers, such as HDPE, PP, and PA66 to aluminum alloys will be conducted to achieve lightweight hybrid structures.