Nonuniform crystallization of PEEK in fused filament fabrication and its influence on subsequent mechanical properties

Abstract

As a typical additive manufacturing process, fused filament fabrication (FFF) commonly utilizes a cooling fan to speed up cooling and solidification of thermoplastic melts, thereby preventing the melts from flowing and improving the manufacturing quality. However, the temperature gradient created by the cooling fan often induces nonuniform crystallization, and further affects the mechanical properties in subsequent service, particularly for the thermoplastics polyether ether ketone (PEEK) with a high processing temperature. Therefore, tracing the dynamic crystallization is the key issue to achieve an integrated simulation suitable for analyzing the material-process-property relationship, and ultimately to improve the manufacturing quality. In this study, we developed a continuous phase-evolution model, suitable in the process simulation of FFF manufacturing of PEEK. Compared with existing phase-evolution models, this developed model considers the potential plastic deformation of continuously formed crystals in subsequent service. Each newly formed crystal phase is modeled by one newly added elastic-plastic branch with an initial stress-free state. Therefore, both the initial configuration at the formation moment and its impacts on the subsequent plastic deformation can be traced. By introducing the effective phase concept, the continuous added phases are equivalent to one effective phase, significantly reducing the computational burden of dynamic crystallization in PEEK. Consequently, the developed model can be implemented into the user defined subroutine for the finite element analysis, and the FFF manufacturing can be modeled by the element activation technology according to the real manufacturing path. To validate the developed model, the FFF manufacturing of a quadrangular prism specimen and the subsequent nanoindentation tests were studied. Both the crystallinity evolution during manufacturing and the mechanical properties in subsequent nanoindentation tests, respectively, at the downwind side and at the upwind side can be well predicted, indicating that the developed method can be used to design the FFF manufacturing process of engineering components.