On the feasibility of joining additively-manufactured 316L stainless steel and poly-ether-ether-ketone by ultrasonic energy

Ultrasonic Joining (U-Joining) process is a friction-based joining technique capable of producing through-the-thickness reinforced (TTR) hybrid joints between surface-structured metals and unreinforced or fiber-reinforced thermoplastics. Previously, the process feasibility has been demonstrated to join injection-molded Ti-6Al-4V and extruded unreinforced and laminated glass-fiber-reinforced polyetherimide structures, resulting in joints with improved out-of-plane strength and fatigue performance. However, there is an unexplored field concerning the joinability of additively manufactured (AM) metal and polymer parts via U-Joining. AM allows the production of structures with complex designs, but joining AM parts can represent a challenge as defects might appear along the process, such as delamination. In this work, the feasibility of joining laser powder bed fusion (LPBF) 316L stainless steel and fused filament fabricated (FFF) poly-ether-ether-ketone (PEEK) via U-Joining is demonstrated. Optical and scanning electron microscopy showed that TTRs were fully inserted into the polymer and that molten PEEK could flow around and penetrate metal surface cavities, thereby improving the macro- and micromechanical interlocking between the parts. Finally, quasi-static lap shear tests were performed, showing an improvement in the ultimate lap shear force up to 2.8 times (from 1.1 ± 0.2 kN to 3.2 ± 0.2 kN) and the displacement at break up to 2.1 times (from 0.7 ± 0.1 mm to 1.45 ± 0.2 mm) when compared to unreinforced (flat pinless) reference joints produced with the same energy input.