The Effect of Discrete In-Situ Consolidation on the Micro- and Macro-Structure of Additively Manufactured Continuous Fiber-Reinforced Thermoplastic Composites

Additive manufacturing of continuous carbon fiber-reinforced thermoplastics using fused filament fabrication (FFF) presents new opportunities for optimizing the design of thin shell structures, such as antenna reflectors and deployable booms for spacecraft. Despite these advantages, uptake of FFF has been hampered by poor mechanical properties brought on by poor consolidation. Discrete in-situ consolidation (DISC) of thermoplastic composites, a process by which a flat-bottomed heated tool applies localized heat and pressure along the length of each bead after deposition, is a promising strategy for improving the properties of parts made by FFF. However, the morphological changes at the micro- and macro-structural level induced by DISC are not well understood, limiting the accuracy of models used to predict the mechanical and thermomechanical behaviors of parts. Here, the effect of DISC processing parameters on individual beads is explored; DISC is found to cause significant bead spreading and thinning, as well as inducing a repeatable convex warpage in each individual bead, a phenomenon unique to this process. Through mechanical testing and microscopy, the effectiveness of DISC as an in-situ method for improving material quality is confirmed, with samples measured on average to have a void content of 0.016%. Specimens consolidated with DISC achieve tensile stiffness, tensile strength, and interlaminar shear strength on par with established conventional composite processing techniques i.e., compression molding. The changes in filament fiber morphology during DISC are explored with micro computed tomography and finite element modeling, revealing that uneven fiber packing in the filament cross-section after DISC is the cause of bead warpage. Finally, mechanical and thermomechanical models were developed, with the thermomechanical model incorporating the bead-level warpage observed. When applied to beads arranged in quasi-weave patterns, these models accurately predicted the mechanical response to tensile loading within 12% of experimental results. While the thermomechanical model was not as accurate in its predictions, the effect of processing effects was demonstrated to be vital to thermal warpage behavior. This work demonstrates that DISC performs well and resolves some of the shortcomings of FFF, and lays the groundwork for its use in designing and manufacturing load-bearing parts.