# Continuous Fiber-Reinforced Material Extrusion with Hybrid Composites of Carbon and Aramid Fibers
## 4 Conclusions and Outlook  
In this work, additively manufactured hybrid composites with carbon and aramid fibers in a PA6 matrix and varying mixing ratios were investigated. The intention behind the use of hybrid composites is to improve or adjust mechanical properties. Tensile, flexural and impact tests were carried out to determine the effects of hybridization. Specimens were prepared with carbon to aramid mixing ratios of 100/0, 70/30, 50/50, 30/70 and 0/100. Based on the mechanical properties of the non-hybrid specimens, a Rule of Hybrid Mixture approach was followed to predict the mechanical properties at different mixing ratios. In  
Figure 9: SEM images of the fracture surfaces of specimens with 50% relative carbon and aramid fiber content respectively: (**a**,**b**) tensile specimen; (**c**,**d**) flexural specimen; (**e**,**f**) impact specimen. addition, the microstructure of the fracture surfaces of the specimens was analyzed with SEM examinations. The investigations have shown that hybrid composites of thermoplastics with continuous fiber reinforcement produced with MEX offer a possibility to influence the mechanical properties of the parts in the desired way. The material mixture of carbon and aramid fibers investigated showed a significant improvement in impact strength with increasing aramid fiber content. The tensile and flexural strengths as well as the stiffnesses, on the other hand, decreased with increasing aramid fiber content. The impact strength of additively manufactured carbon fiber-reinforced parts could thus be effectively improved by adding aramid fibers. However, this is accompanied by low strengths and stiffnesses of the composite. The application of the RoHM equations resulted in a good approximation to the experimentally determined values, with an average deviation of about 5.6% when the material properties were known for only one fiber material. The RoHM equations are therefore well suited to unidirectionally reinforced hybrid composites fabricated by MEX and can be used to predict the mechanical properties of hybrid parts. The damage mechanisms of printed hybrids are similar to those of non-hybrid printed composites. In particular, debonding and delamination occur as main damage mechanisms. Significant differences between the failure modes of the carbon and aramid fibers could not be found. Implementation of the selected interply approach in existing printer systems is possible by adding an additional fiber extruder. It is also conceivable to change the fiber materials during the printing process. This also requires an adaptation of the slicing software. In the future, the use of prefabricated hybrid fiber filaments (intrayarn) should also be investigated. In particular, the improved dispersion and its influence on the mechanical properties are of interest. This approach could be implemented without changes to existing filament-based MEX-setups and the associated slicing software. In addition, further investigations should be carried out with other fiber materials and combinations. In addition to the established fiber materials, such as carbon, glass or aramid, natural fibers can also be implemented, which could reduce costs as well as improving the adjustability of the mechanical properties. In addition to the fiber material or the fiber combination, the fiber orientation and the layer configuration are also crucial. In this context, the high flexibility in strand deposition of MEX offers enormous potentials for local adaption of the mechanical properties. For this purpose, appropriate strategies and methods have to be developed which take into account the combination of different fiber materials. Conceptualization, T.H. ; methodology, T.H. ; software, T.H. ; validation, T.H. and S.G.; formal analysis, T.H., S.K. and S.G.; design and manufacturing of the test specimens T.H. ; investigation, T.H. ; resources, T.H. and S.G.; data curation, T.H. ; writing--original draft preparation, T.H. ; writing--review and editing, S.K., S.G. and N.W. ; visualization, T.H. ; supervision, G.K., N.W.