A Preliminary Framework for Modelling Polycrystalline Metal Processing Using Ultrafast Pulsed Laser
Lacking fundamental understandings of the non-thermal ablation process and the underlying mechanism that governs the coupled thermal-mechanical generation impacts the broader application of ultrashort lasers. Knowledge pertaining to ablation and electron-lattice energy transport is required to describe the interaction of ultrashort pulses with polycrystalline metallic materials. Fundamental knowledge is gained through considering
the conservation of energy and momentum and by following the characteristic time scale that dictates the complex dynamic process that involves laser absorption, recombination and impact ionization, electron-lattice equilibrium, and plasma plume formation. The paper considers a modified two-temperature model for the electron-lattice energy transport to eliminate the conflict of energy transport by introducing relaxation times. Heat diffusion based on carrier density is also considered. Material removal mechanism is expressed as
evaporation plasma plume formation from both the microscopic and macroscopic perspectives. The lack of dissipation energy in current electron-lattice energy transport is also noted. The issues with classical Taylor-type crystallographic approaches are discussed and an alternative intermediate approach based on a modified Taylor's model is adopted to transform from the local to global scale. A modified crystallographic approach is adopted by fictitiously representing each material point as single crystal and the surrounding grain boundaries with the
crystallographic lattice orientation. The coupled thermo-elasto-plastic constitutive equations are also considered
for describing the stress rate-strain rate relation in polycrystalline thermoplastic response. Specifically, the
equations incorporate the kinematics and flow rules based on the multiplicative decomposition of the deformation gradients and the dislocation evolution on each slip system.