Hybrid structures combine dissimilar materials to attain the best performance of both components and enhance the flexibility of the product design. Joining metal and polymer allows the exploitation of the metal's strength and durability and the lightweight and physico-chemical resistance of the polymer [1]. Therefore, the joined materials complement each other, offering a new structural performance different from the independent behavior of the constituent materials [2]. Polymer-metal hybrid structures are gaining more interest due to the requirements to reduce the structure's weight while maintaining the strength of the manufactured part. Hybrid structures are used in many industries, including transportation, infrastructure construction, and healthcare. For example, using hybrid structures in transportation reduces fuel consumption and improves the overall vehicle fuel efficiency [3].
Joining polymers and metals have inherent issues related to manufacturing restrictions caused by incompatible physical and chemical material properties [4]. Conventionally, adhesive bonding and mechanical fastening are the most common methods to produce a reliable joint between the two materials, but it comes with the expense of the additional weight of the third element. Other limitations exhibited by the traditional joining methods include extensive material surface preparation, the use of harmful chemicals, and stress concentration [1]. Ideally, the ultimate strength of the hybrid joint depends on the maximum load the polymer substrate can withstand. The desired failure mode for polymer-metal joints is net tension failure within the polymer; that is, the bonding strength at the joining area has exceeded that for a stand-alone polymer substrate.
Numerous studies in the literature investigated the different advanced joining methods such as mechanical clinching [5], friction stir lap welding [6] [7], direct laser joining [8], injection molding [9], and hot press joining [10]. Lambiase et al. [3] reviewed the advanced joining methods and classified them based on the joining mechanism. These joining processes achieve a permanent connection through modified mechanical fastening, high thermomechanical deformation, or heat and pressure application. These techniques rely more on understanding the joining mechanisms and enhancing the adhesion at the interface. They offer simplicity, short manufacturing time, no additional material, reliable joining, and a viable automation method. One of the most recent joining methods is material extrusion or Fused Deposition Modelling (FDM) [11]. This new method offers an economical and environmental alternative to join polymer and metal, in addition to its intrinsic freedom of fabricating any desired geometry regardless of its complexity. Material extrusion is a direct joining method where the polymer filament is heated and extruded through a nozzle layer by layer above the surface of the metal to attain a permanent connection [11, 12, 13].
Metal surface modification is the first step of the hybrid joining process, and different methods were used in literature, including sandblasting [12], electrochemical treatment [13], shot peening [14], machining [8], laser structuring [6], and chemical etching [15]. The surface preparation works on removing contamination, increasing the surface roughness, enhancing the wetting, altering the chemical composition, and introducing support structures. Hertle et al. [13] subjected the aluminum surface to HCL, which forms microstructures on the surface before extruding polypropylene (PP). The study focused on the effect of temperature to achieve higher polymer fluidity and promote the filling of the microstructures. Wang et al. [16] reported the increase of the polymer-metal joint strength by 600% by introducing surface texture through laser ablation while joining with friction stir lap welding. Ozlati et al. [17] joined PP and aluminum by punching a hole in the metal and used extruded PP filament to fill it. The effect of the interface area and metal substrate temperature on the joint strength was studied.
The second set of important joining parameters are the extrusion conditions of the polymer, such as the extrusion temperature, layer thickness, deposition speed, build plate temperature, and many others. These factors can alter the wetting behavior of the polymer and the mechanical properties of the printed polymer part. However, a limited number of studies had considered joining through material extrusion by FDM. Falck et al. [11] studied the lap shear strength of ABS and CF-PA6 joined to sandblasted aluminum alloy 2024 and compared it to adhesively bonded joints, where the FDM joints showed higher strength. However, the joining relied on an initial step before depositing the polymer; for ABS-AA joint, the metal surface was coated with ABS slurry, while for CF-PA6–AA joint, a polymer layer was preheated externally above the metal. Further investigation by the authors focused on the stand-alone effect of the printing parameters on the strength of ABS-AA joint using one factor at a time approach, and the optimized tensile force was reported as 1682 N [18]. In addition, Bechtel et al. [12] joined PLA and aluminum alloy 6082 and evaluated the joining process using thermographic monitoring to study the interface adhesion, where the bed temperature and layer thickness influence on the joint's wetting behavior and lap shear strength was investigated. In a subsequent study, Bechtel et al. [19] compared the joining of different thermoplastics, namely ABS, PETG, and PLA, to sandblasted aluminum alloy 6082, and their results suggested that PETG is the most suitable thermoplastic based on substrate temperature and aging time. Aluminum alloys were mostly considered for extrusion based joining as the metal substrate, but Belei et al. [20] used sandblasted titanium alloy joined with fiber-reinforced polyamide (PA-CF) and focused on the effect of the primer layer parameters. The performance of the joint was evaluated based on tensile shear test and three-point bending test.
The present study investigates the capabilities of material extrusion additive manufacturing in fabricating reliable single lap joints of ABS-AA5052. The effect of extrusion process parameters, namely the bed temperature, the printing speed, and the nozzle Z-offset on the strength of the lap joint was studied, where the use of nozzle Z-offset in FDM joining to enhance the joint strength is a newly proposed idea in this study. In addition, while most studies on joining polymer and metal focusses only on mechanical interlocking, this investigation aims at understanding the nature of chemical bonding at the interface through the implementation of analytical techniques such as Raman spectroscopy and X-ray photoelectron spectroscopy.