Joint Formation And Joining Mechanism Of FSW Between CF/PEEK Composite And 2060-T8 Aluminum Alloy

Lightweight design of structural assembles is an effective approach to property improvement,energy saving and environmental pollution reduction.The heterogeneous structures between polymers/polymer matrix composites and aluminum alloys can fulfill the desired requirements in structural properties and lightweight design.In this paper,aiming at the problems of poor welding integrity,serious thermal mismatch due to the disparity in coefficient of thermal expansion,and low mechanical properties,polymer matrix composites and aluminum alloys were joined via friction stir welding(FSW),where welding metallurgical bonding and mechanical joining were taken in consideration synchronously.The interfacial bonding,enhancement of mechanical properties,joining and fracture mechanisms were investigated for CF/PEEK composites and 2060-T8 aluminum alloys,which provided significant guidance on the high-performance,high-quality joining between aluminum alloys and polymer matrix composites.Aiming at the macro-mechanical joining of heterojunctions,a tapered thread pin with triple facets was designed,which can effectively improve the dynamic flow of plasticized materials during welding process.Welding heat input and material transfer behaviors were ameliorated via optimizing welding parameters,in which the structural dimensions of Al anchors,mechanical properties,as well as interfacial morphologies were optimized.The higher length ratio of the Al anchor to the rotating pin reached 0.8 at the lower welding speed,improving interfacial interlocking of heterojunctions.The tensile shear strength reached 20 MPa when the rotating velocity and welding speed were respectively 800 rpm and 50 mm/min.In order to further improve joint formation of heterojunctions,a welding tool containing an outer stationary shoulder and an inner tapered thread pin with triple facets was designed.The inner tapered thread pin with triple facets determined the inner joint formation and structural dimensions of Al anchors,while the outer stationary shoulder controlled the surface integrity.The good surface integrity and joint formation were acquired based on the new welding tool and lap configuration(CF/PEEK composites at the upper and 2060-T8 aluminum alloys at the lower).Dynamic material flow induced by the tapered thread pin with triple facets caused the large Al anchor directly into the composites and self-riveting structures were subsequently formed,attributing to the primary joining mechanism.Plastic flow of the two materials transformed the surface of the aluminum alloy sheet from flat surface into the coarse surface,and then strengthened the micro-mechanical interlocking effects.Meanwhile,load transfer due to the fibers into aluminum alloys and metallurgical bonding between composites and aluminum alloys played a secondary role.The tensile shear properties of joints were related to the structural dimensions of Al anchors,effective sheet thickness,as well as morphology and distribution of fibers.The length of the Al anchors and effective thickness of sheets were respectively 1.8 mm and 1.0 mm at the rotating velocity of 1600 rpm,welding speed of 30 mm/min.The fracture degree of fibers was low and uniformly redistributed fibers were attained,which benefited to the load transfer during tensile shear tests.The tensile shear strength reached 33 MPa.The joint fracture indicated the pull-out of Al anchors,as well as the pull-out and failure of fibers.Combining metallurgical bonding with mechanical joining,the concept of riveting-welding dual-mode joining was put forward.Friction self-riveting welding(FSRW)between aluminum alloys and composites was realized with the assistance of the porous interlayer structures and macro pre-fabricated holes.The porous oxide layers were produced by the plasma electrolytic oxidation(PEO).They improved the metallurgical bonding between composites and aluminum alloys,enhancing the interfacial bonding.In the meantime,the porous oxide structures were filled with the plasticized materials,strengthened micro-mechanical interlocking,and then enhanced the load-bearing capacity of the heterojunctions.Compared with the joints without porous oxide layers,the tensile shear strength of the PEOed joints was increased by 50%.The macro pre-fabricated holes were introduced on the aluminum alloys.Frictional squeezing improved the polymers flowing into the macro pre-fabricated holes,which was enhanced by the effective material flow.This improved the macro/micro mechanical interlocking between aluminum alloys and composites.The composite rivets were formed,composed of matrix polymer and re-distributed fibers.High frictional heat and severe plastic deformation improved the mixing of fibers and the molecular entanglement between the composite rivets and the base composites,which was beneficial to promoting load transfer performances.This delayed the initiation and propagation of cracks,and enhanced the mechanical performances of heterojunctions.The tensile shear strength of FSRW joints reached 27 MPa when the average hole diameter of porous oxide layers,areal porosity,and macro pre-fabricated hole were 5 ?m,5%,and 2 mm,respectively,which was increased 170% compared with conventional joints without PEO and macro pre-fabricated holes.The fracture of FSRW joints belonged to mixed fracture,which included fracture of fibers and tearing of polymers.To solve the problem that the larger pre-fabricated hole cannot be effectively filled,an extra polymer filling stud was added.It not only acted as mechanical rivets,but also formed intimate contact with aluminum alloys,improving mechanical properties and realzing the high-quality joining of heterojuctions,which effectively broadened the application ranges of FSRW technique.Based on the above research,taken metallurgical bonding and mechanical joining into consideration synchronously,the welding tool system containing the stationary shoulder and the tapered thread pin with triple facets was designed,which efficiently improved the welding formation of heterojunctions and ameliorated the interfacial joining characteristics.FSRW technique was proposed to further improve the macro/micro mechanical interlocking and interfacial joining performances.The porous oxide interlayers and macro pre-fabricated holes were introduced.The hybrid strengthening design with dual-mode joining between polymer matrix composites and aluminum alloys was achieved,which provided technical routes for high-reliability joining of polymer matrix composite/aluminum alloy heterojunctions.