Grain growth mechanism of AlNiCo magnetic alloy under different withdrawal rates

Accurately controlling the solidification process and finely regulating the grain structure of AlNiCo alloy are necessary steps to improve magnetic properties. AlNiCo were directionally solidified through immersion in a Ga-In alloy liquid bath at varying withdrawal rates (1 mm/min, 3 mm/min, 6 mm/min and 9 mm/min) to optimize grain architecture. This paper investigates the effect of withdrawal rate on the grain structure of AlNiCo and explains the mechanism of grain growth through simulation. With the increase of the withdrawal rates, the size of the grain decreases from 1.525 mm to 0.62 mm and the growth direction becomes random. When the withdrawal rate is lower than 3 mm/min, the grains of AlNiCo have larger size and better orientation, but the magnetic properties are optimal when the withdrawal rate is 3 mm/min (M_s=11.20kGs, B_r=10.82kGs, H_c=1.719 kOe, (BH)_max= 11.20MGOe). This is because the large variations in size and the high number of “isolated island crystals” exacerbate the unevenness of the grain structure, in turn, decreasing (BH)_max and H_c. The simulation results show that too fast or too slow withdrawal rate can cause heat exchange imbalanced, resulting in the liquid-solid interface to bend and grain orientation deviating from the solidification direction. Increasing the withdrawal rate will lead to an increase in undercooling, resulting in a higher nucleation density, which is the main reason for grain refinement. In addition, a withdrawal rate of 3 mm/min can achieve a larger temperature gradient, which slows down the lateral growth of grains and thus reduces the occurrence of “isolated island crystals” grains. Through finer control of the solidification process, it is expected to achieve a more uniform grain structure, further enhancing the magnetic properties of AlNiCo.