Pore Formation and Shape Control Simulation of Lotus Aluminum by the Phase-Field Method

We have simulated pore formation and shape control of lotus aluminum by the phase-field method. The simulated material, lotus aluminum, contains anisotropic internal pores, and it is produced by the continuous casting method in a hydrogen atmosphere. Since it is known experimentally that the pore shape of lotus aluminum changes with the pull-out speed, the simulation varied the movement speed of the temperature gradient zone (equivalent to the pull-out speed in the continuous casting method) by proportional differential (PD) control with the pore width as the target value. As a result, a simple PD control ensured the pores closed during the growth process. To keep the pore growth linear, we found that a lower limit of the interface temperature should be set and the temperature gradient zone should be stopped below this lower limit. However, a problem occurred in the pore shape. To mitigate necking of the pore, PD control was done only when the pore width became larger than the target value under the conditions such that the pore expanded easily (i.e., the pull-out movement was stopped for a certain time immediately after nucleation and the initial speed of the temperature gradient zone was decreased). Then, we found the best condition to achieve linear pore growth without necking. Under the same condition, we simulated multiple pore growths by allowing multiple nucleations. As a result, we observed that although the shape control was applied only to a certain single pore, the other pores also grew linearly if the timing of their nucleation was close to that of the target pore.