Thermal Gradients Govern Impact Dynamics in Thermoplastic Polymer Cold Spray

Abstract

Most reports simulating in-flight particle temperatures in polymer cold spray ignore intra-particle thermal gradients that develop in-flight, and, to date, no research group has simulated the effects of thermal gradients on polymer particle impact dynamics. This report investigates the effects of intra-particle thermal gradients on the impact response of thermoplastics, using properties of polyether-ether-ketone (PEEK) in a simulated like-onlike cold spray deposition process. Particle velocities and internal temperatures are calculated from compressible flow models of the spray process under several heat transfer assumptions. The commonly used lumped thermal capacitance assumption differs in average particle impact temperature by over 70 °C from simulations involving a more physically representative thermal gradient model assuming finite-rate heat conduction. Additionally, the intra-particle temperature differences of over 100 °C in some particles imply significant variation in material properties within a particle. The predicted average temperatures and temperature gradients are used as initial conditions for a finite element-based impact model that shows distinct mechanical responses of isothermal and thermally graded particles. These results suggest that thermal gradients in polymer particles should be considered for further refinements of model-based predictions for the cold spray of polymers and optimized by process design.

Cold Spray Deposition of Nylon-6 on Glass Fiber-Reinforced Composites

“…Particle temperature and velocity upon impact with the substrate were simulated using the one-dimensional (1D) methodology outlined in Bacha et al 39 The nozzle was modeled as a simple converging-diverging conical geometry with a side inlet for the powder feed. The side inlet was assumed to not affect the main gas flow.…”

“…The model development details are beyond the scope of the present study and are therefore excluded for brevity. However, complete details regarding the particle flight model can be found in Bacha et al…”

“…The model development details are beyond the scope of the present study and are therefore excluded for brevity. However, complete details regarding the particle flight model can be found in Bacha et al 39 Note that the particles were assumed to be perfectly spherical in this work. It has been reported that any deviation from perfect sphericity affects the drag coefficient, altering the impact velocity.…”

Thermal gradient in polymeric particles during the cold spray process

“…A thermal gradient exists within the particle and generally leads to a gradient of thermal properties. In the case of polymer particles, several authors [16,17,29,30] highlighted the importance of this thermal gradient for a better understanding of polymer coating by the cold spray process.…”

“…Bacha et al [17] investigated the impact behavior of polyether ether ketone (PEEK) particles considering different thermal gradients for the particle. They showed that important shear stresses occur in the localized high-temperature regions.…”