A DISLOCATION DYNAMICS BASED CONSTITUTIVE MODEL AND EXPERIMENTAL VALIDATIONS BY 3D MICROSCALE LASER DYNAMIC FORMING OF METALLIC THIN FILMS

摘要

Microscale Laser Dynamic Forming (μLDF) shows great potential in fabricating the robust and high-aspect-ratio metallic microcomponents by the high speed plasma Shockwave. Experiments revealed that strain rate and sample size play important roles in determining the final results of μLDF. To further understand the deformation behavior, we develop a constitutive model integrating size effects and ultrahigh strain rate effects to predict the ultimate plastic deformations. To derive this model, 3-D Discrete Dislocation Dynamics (DDD) simulations are first set up to investigate the dislocation evolutions and the dynamic responses during Shockwave propagation. It is observed that there exist three dynamic stages during deformation process, and the initial strain hardening rate in Stage II increases with strain rate. The simulation also reveals that stain softening occurs only for the smaller cell size due to two competing mechanisms. In addition, the simulation predicts that the flow stress and yield strength increase with the strain rate and decrease with cell size. The modified mechanical threshold stress (MTS) model integrating these effects is implemented in Abaqus/Explicit and predicts the deformation depth and thickness variations in good agreement with the experimental results.