Characterization of friction stir-based linear continuous joining of aluminium alloy to structural polymer

Abstract Dissimilar continuous joining of lightweight metal alloys to structural polymer is a major manufacturing challenge, demanding feasible and reliable solutions. Through-slot extrusion joining (TSEJ) is a friction stir-based processing technique investigated in the manufacturing of continuous linear joints between aluminium alloy AA5754 overlapping structural polymer polyether ether ketone (PEEK). An intermediate rigid and thin titanium extrusion die protects locally the polymer from thermal degradation and promotes the formation along the joint path of a continuous double hook-like feature of extruded aluminium into the polymer component, providing macro-mechanical interlocking. A set of four tools with different probe geometries at three positions relative to the extrusion die were investigated for process stability, tensile-shear strength, and microstructure. Four tensile-shear specimens are extracted from each joint, and the cross-section from the best TSEJ condition is chosen for microstructural analysis via optical microscopy and scanning electron microscope. Three distinct failure modes were identified. The best condition provided an average tensile-shear load of 110 kN/m. For the best joint condition, 84 % of the test population presents an extension at fracture load over 1.9 mm, mostly concentrated at the joint thermomechanical affected zone. The tool position with bias toward the flow side of the extrusion slot shows to improve the strength of joints. The microstructural analysis along the interface of AA5754 to PEEK shows micro-mechanical interlocking, intercalated layers of these materials and adhesion as joining mechanisms.