Thermal skin effect in laser-assisted tape placement of thermoplastic composites
Aachen / Apprimus Verlag (2019) [Book, Dissertation / PhD Thesis]
Page(s): 1 Online-Ressource (X, 203 Seiten) : Illustrationen, Diagramme
The thesis at hand presents thermal models to predict the temperature development in laser-assisted thermoplastic tape placement processes. A focus is placed on the time-dependent through-thickness temperature distribution at increased speeds. The thermal models make it possible to obtain the:1) melt pool depth,2) melt dwell time and length, 3) bond interface cooling rate and 4) through-thickness elastic modulus within the tape and substrate at various process speeds. The formerly predicted development of a thermal skin layer at high speeds is described quantitatively. An analysis of the known bonding mechanisms reveals that the thermal skin effect will at some point inevitably begin to influence the weld strength, by affecting the macroscopic draping, the intimate surface contact development and autohesion. Experiments with a novel test setup are presented, which enable the consolidation at defined temperature gradients and defined tape speeds. The setup was used for an initial investigation on the general effect of temperature gradients at a consolidation speed of 1 m/s. Two conflicting positive effects of high gradients were observed: On the one hand, the backside of the tape stays solid (not molten), remaining flat and smooth, which is expected to be beneficial for subsequent laydowns and for the silicon roller. On the other hand, the high gradients inhibit the melt flow and malleability of the tape, due to a more narrow melt pool and higher stiffness: Surface steps of 60 µm were thick enough to prevent the filling of a 2 mm-wide area at the edge of the step change. This will have a significant influence on the mechanical properties of the final part.In order to control the thermal state within the tape - independently from the process speed - several irradiation strategies are investigated and compared on their usefulness and feasibility to be applied in tape placement systems. Maximization of the surface temperature was found to be easy to implement, but also limited, because the degradation temperature of the respective polymer sets a barrier. Changing the irradiation distribution promises the most benefits and widest field to control T(z,t), but is expected to be more difficult to realize with state-of-the-art laser system technologies and todays (not inverse) heat transfer simulation tools. The most feasible strategy was found to be adaptive irradiation lengths, either created e.g. by VCSEL or by electromechanical zoom homogenizers. The correlation between irradiation length, laser power, surface temperature and thermal penetration is described by use of the analytical solutions. Optimization potentials for the laser technologies are presented. Finally, a novel optimization strategy for laser-assisted tape placement processes is derived with the goal of adjusting the melt dwell time relative to the available pressuring time of the respective consolidation roller, by changing the laser irradiation length properly.