Comparison of microstructure and thermal-fluid coupling heat transfer behavior of strip-cast Nd-Fe-B flakes at different cooling roller speeds

Strip-casting is a key step among processes for manufacturing the Nd-Fe-B permanent magnets. Current research on the process parameters of the strip casting process mostly relies on the trial-and-error method. In this paper, the influence of the cooling roller speed on the strip-cast Nd-Fe-B flakes is investigated by a combination of simulation and experiment. The microstructures of the strip-cast flakes prepared at different roller speeds are observed and analyzed. The simulation is further performed by using the thermal-fluid coupling method. The temperature field and cooling water flow field of the cooling rollers at different roller speeds are obtained to explain the principle of the microstructural evolution. The temperature field distribution affects the uniformity of the columnar grains on both sides of the strip-cast flake. Higher or lower temperature can also lead to the formation of α-Fe, cluster rare earth (RE) rich phases, or ultra-fine equiaxed grains. An increase in cooling water flow rate is helpful to obtain the columnar grain with even thickness. The simulation results are consistent with the microstructural results. Furthermore, the simulated results and microstructural analyses are combined to indirectly evaluate the trend of magnetic properties of final sintered magnets from the tested strip-cast flakes. By verifying the performance of magnets prepared under the condition of only changing the roller speed during the entire process flow, the measurement results are consistent with the evaluation results, proving that the combined analysis proposed in this paper can reasonably predict the trend of magnetic properties of sintered Nd-Fe-B permanent magnet materials from the strip-cast flakes prepared at different roller speeds. This paper can provide a transferable idea and method for using numerical simulation to assist in the design and production of Nd-Fe-B flakes and sintered magnets.