Multiscale Simulation of Droplets In-flight Solidification Behavior During Spray Forming

The solidification behavior and structure of droplets during the in-flight process play an important role in the final quality of the materials in spray forming. In the study, the macroscopic cooling and microscopic structural evolution of droplets during solidification were simultaneously explored from a multi-scale perspective. Numerical simulations of the cooling and solidification behaviors of droplets in the spray forming of GCr15 bearing steel were carried out by using fluid dynamics and the incorporated solidification models. A model appropriate for estimating the droplet cooling rate of ultrasonic gas atomization (USGA) was developed. The atomic structures and dendrite growth of droplet solidification were studied by molecular dynamics and phase field method. The results show that the thermal history and solidification behavior of a droplet during flight largely depend on its size. The cluster structures during nucleation and solidification of pure Fe droplet transform from the initial dominant FCC-like and ICO-like types to the final BCC structures. Disordered amorphous structure forms in the solidified structure at high cooling rate. Under the gradient temperature field, dendrite arms grow fast towards low temperature direction with developed secondary dendrite arms, and linear solute concentration distribution is formed in dendrite arm due to solute retention.