Ultrasonic joining of additively manufactured metal-polymer lightweight hybrid structures

Ultrasonic joining is a novel friction-based joining technique to produce through-the-thickness reinforced hybrid joints between surface-structured metals and unreinforced or fiber-reinforced thermoplastics. The reinforcements' presence is responsible for improving the out-of-plane strength of the parts, enhancing their damage tolerance. The process feasibility has been successfully demonstrated to join additively manufactured (AM) metal and polymer parts. However, further investigation of its main advantages and the joining process of subcomponents to support the technique's further development is still missing. This paper aims to demonstrate the application of U-Joining to fabricate AM 316L and PEEK hybrid structures produced via laser powder bed fusion and fused filament fabrication, respectively. Firstly, the quasi-static single lap shear performance of coupon specimens produced with optimized joining parameters was assessed. The results indicate an improvement of 2.7 times in the ultimate lap shear force and 5.9 times in the displacement " when compared to non-reinforced flat samples. Fracture surface analyses of tested samples exhibited a mixture of cohesive and adhesive failure. Further microstructural analyses at the metal-polymer interface showed micromechanical interlocking between the parts. As observed, the PEEK was able to flow and penetrate the cavities at the metallic specimen's rough surface due to the joining friction heat input. Finally, a selected skin-stringer-bracket case study was analyzed, showing the potential of AM and U-Joining to drastically reduce the structure's weight by about 64%. To validate this idea, a scaled-down skin-stringer-bracket technology demonstrator was successfully fabricated.