AbstractSubstitution of conventional metals by lightweight materials is inevitable. However, employment of multi-materials in a structure presents a challenge and choosing the right materials, design, and manufacturing technique is essential in the development of any engineering structural application. New joining and additive manufacturing techniques complement the successful integration of materials, design, and production. Conventional joining methods, such as adhesive bonding and mechanical fastening, face technological limitations when used to join dissimilar materials, such as metals and composites. The relatively long curing time of an adhesive is also a significant drawback for adhesive bonding. In recent years, new joining techniques have been investigated to develop hybrid joints that overcome the limitations of traditional joining methods.
There is an increased interest in recent advances in the field of additive manufacturing (AM). These offer the flexibility to produce complex geometric parts, such as sandwich structures with AM honeycomb cores. Combining the principles of joining and polymeric AM is the main aim of the present doctoral thesis, which introduces AddJoining, a new technique that was co-invented by the author (patent application by HZG, DE 102016121267A1, 2018) to contribute to the manufacturing options for joining multi-material parts. The process was inspired by additive manufacturing and joining technology principles, and this new technique uses polymer 3D printing, e.g., fused filament fabrication, FFF (also known as fused deposition modeling, FDM), to add layers of polymer or composite to a metal substrate. The AddJoining process has a potential to produce structures with geometric flexibility, such as honeycomb cores. As an early phase of this technology this PhD work was devised to understand and develop the fundamentals of the AddJoining process by joining transportation grade lightweight aluminum 2024-T3 with a combination of unreinforced polyamide 6 (PA6) and carbon-fiber reinforced polyamide 6 (CF-PA6).
To understand the process-induced changes in the selected material combination (aluminum 2024-T3 and the composite [PA6/CF-PA6]4S), three combinations of process parameters were selected. The optimized process parameters with coating temperature (CT) at 229 °C and heating bed temperature (HBT) at 77 °C were kept constant. Moreover, the optimized HBT was fixed and the minimum and maximum values of CT selected at 20 °C and 150 °C, respectively. Intimate contact of polymer-to-polymer was promoted purely by temperature, which led to strong adhesion and influenced polymer bond formation. From a microstructural aspect, it was shown that strengthening occurred close to the interface of the aluminum 2024-T3 by reprecipitation of particles once solubilized, due to exposure to high processing temperatures. Within the composite [PA6/CF-PA6]4S the process induced a global modification of the polymer, the effect of HBT slightly changed the thermal properties of the composite part. However, to better understand the local changes on the polymer, a stepwise approach was carried out to combine the phase-identification of the polymer and nanohardness investigations. The results offered a more comprehensive understanding of the local variations in material properties by means of the phase-changes in each layer while it indicates the presence of two polymorphs of α-PA6 and γ-PA6. Thus, it is associated with the presence of a thermal history of AddJoining, which can affect the crystallinity locally and decrease the hardness. From the hybrid joint mechanical performance, the influence of heating bed temperature on secondary bending could be an indication of low stiffness in the composite part caused by a variation in its crystal structure. This indicates that changes to PA6 phases could have led to a variation in hybrid joint’s ductility and strength.
The mechanical performance of AddJoining hybrid joints was assessed by a wide range of mechanical tests. An interfacial intralaminar failure mode was observed with quasi-static loading. Compared to adhesively bonded joints there was a significant increase in ultimate lap shear force (ULSF). In addition, the S-N curves obtained from fatigue testing indicated outstanding results and the hybrid joints produced reached their fatigue limits (106 cycles) with loading levels corresponding to 30 % of ULSF. By monitoring stiffness degradation, it was seen that damage evolution was dominated mostly by fiber rupture throughout fatigue life due to the high stiffness stability of hybrid joints.
To summarize, this PhD work has been successful in fulfilling its objectives, namely describe the AddJoining process fundamentals and mechanical behavior. Moreover, this work shows the potential for the AddJoining technique in the manufacturing of future complex multi-material parts for structural applications with tailored design aspects to improve damage tolerance and load-bearing carrying.