Mathematical modeling and numerical simulation of layer growth and phase transformation during galvannealing process

In the galvannealing process, a galvanized sheet is given an annealing treatment to transform the zinc coating to the Zn–Fe intermetallic coating through diffusional reactions. In the present work, a mathematical model for predicting the growth of ζ (zeta), δ (delta), and Γ (gamma) during the galvannealing process has been developed by using fundamental diffusion data. The primary diffusion mechanism is the movement of zinc through the multiphase of the Zn–Fe intermetallics. The values of iron content in the coating were predicted and compared to the data obtained from experimental studies. With the increase of galvannealing temperature, the growth rate of the δ phase increases, while the growth rate of the ζ phase decreases. At the same iron content in the coating, the thickness of the Γ phase layer is thicker at a higher temperature. The new type of galvannealing process of first high annealing temperature and then low annealing temperature is validated as a better galvannealing process, which should be a range of practical application. The simulated results agree well with experimental observations, and the model can be utilized to optimize the galvannealing parameters.