On the feasibility of Ultrasonic Joining of 3D-printed PEEK to rolled AISI 304 stainless steel reinforced with cold metal transfer welded pins

One of the main strategies applied by transportation industries to improve products’ energy efficiency is the combination of lightweight alloys and thermoplastics in the so-called hybrid structures. Combining these dissimilar materials allows for unique solutions for different engineering requirements; however, it also demands advanced joining techniques and assembly approaches to adequately combine their main positive properties, such as the high mechanical strength of metals and the low density of polymers. Nonetheless, joining these materials also represents a great challenge due to their physical-chemical dissimilarities. The Ultrasonic Joining (U-Joining) is a recently developed solid-state joining process. The process has been shown suitable to produce hybrid joints between metal-injection molded (MIM) and additively manufactured (AM) surface-structured metals and thermoplastic-based materials. Even though U-Joining is not restrained by any dimensional constraints, surface-structured metallic components produced by current AM and MIM technologies are often limited by the size of their building volumes and mold cavities, respectively. Therefore, ultrasonically joining large parts, such as aircraft and aerospace fuselage panels is currently not viable. In order to overcome these limitations, this work intends to apply Cold Metal Transfer (CMT) to fabricate welded pins as reinforcement in rolled AISI 304 stainless steel sheets and investigate its joinability with poly-ether-ether-ketone (PEEK) 3D-printed by Fused Filament Fabrication (FFF). The joining parameters were optimized through Design of Experiments, to maximize the single-lap joint ultimate lap shear force (ULSF). Furthermore, their influence on the resulting fracture mechanisms was evaluated. The obtained results show that the ULSF strongly depends on the joining energy and pressure. Finally, microstructural analysis was carried out at the metal-polymer interface of the optimized joining condition to evaluate joint formation and bonding mechanisms.