Phase-field model for mesoscale simulation of crystallization morphology evolution in 3D printing of thermoplastic composite

Abstract

3D printing has enabled the production of continuous fiber-reinforced thermoplastic composites (CFRTPCs) with exceptional mechanical properties and design flexibility. The crystalline morphological evolution during 3D printing of CFRTPCs, particularly the transcrystalline layer (TCL) that is critical for the mechanical performance, remains poorly modeled and understood. Herein, we develop a non-isothermal phase-field model (PFM) for mesoscale simulating the crystallization process and morphology evolution during the 3D printing of thermoplastic composites.The PFM is derived by constructing a free energy density functional incorporating order parameters to characterize the crystallization process, with the phase-field evolution equations being intrinsically coupled to heat transfer. In particular, the fiber-crystalline melt contact angle-dependent sporadic nucleation processes are integrated into the PFM using an explicit nucleation algorithm. Phase-field simulations of crystallization behavior in isotactic polypropylene (iPP) under conditions emulating 3D printing are demonstrated to verify the PFM capability. The influence of contact angle (θ), convective coefficient (h), and fiber spacing (D) on non-isothermal crystallization is comprehensively examined. It is found that TCL thickness increases with decreasing θ, smaller h, and larger D, until it reaches saturation. Slower cooling during 3D printing could significantly amplify the effects of both contact angle and fiber spacing on the TCL evolution. The proposed PFM could enable mesoscale simulation of 3D-printed thermoplastics and help optimization of crystallization and printing parameters.