Al-Si-Mg-Cu-Ni alloy is widely used in the manufacture of high-performance car engine parts. Coarse, dendritic α-Al and large primary Si are common in Al-Si-Mg-Cu-Ni alloy DC casting billet, which is harmful to the performance of the final product. In this paper, a pulsed magnetic field melt treatment technique was applied to the melt in the launder of a DC casting platform to modify the α-Al and primary Si in the billet. A transient numerical model was established to analyze the electromagnetic field, flow field and temperature field in the melt during the pulsed magnetic field treatment. The effect of the magnetic energy on the clusters in the melt was analyzed. We found that during the pulsed magnetic field melt treatment, the number of clusters close to the critical size was increased due to the cluster formation work being reduced by the magnetic energy, which facilitated nucleation and refined the solidification structure. Furthermore, the flow velocity increased, and temperature homogenized in the melt during the pulsed magnetic field melt treatment, which benefitted the clusters close to the critical size distributed and maintained in the melt uniformly. The experimental results show that the α-Al and primary Si were small and homogeneous following the pulsed magnetic field melt treatment. The size of α-Al and primary Si was reduced by 25.6–44.4% and 32.2–54.1%, respectively, in the billet center compared to the conventional process.

The grains refinement and secondary phase modification mechanism of Al–Mg–Si–La alloy under pulsed magnetic field during solidification were studied in detail. With application of pulsed magnetic field, the microstructure of Al–Mg–Si–La alloy consists of fine and equiaxed grains. Meanwhile, the smaller size of Mg₂Si homogeneously dispersed in α-Al grains, and the number of AlFeSi and LaAlSi phase obviously decreased compared with the ones without pulsed magnetic field treatment. The cluster nucleation at the liquid region during early solidification stage is dominant reason for grains refinement in Al–Mg–Si–La alloy under pulsed magnetic field. Theoretical calculation indicated that pulsed magnetic field provides maximum energy in the alloy melt at about 760°C, which decreased the critical radius and energy barrier for nucleation.

The 6082 alloy is used to produce DC (direct-chilling) billets for subsequent forging or rolling processes. It is necessary to control the solidification structure in casting billets during the casting process. Based on the previous work, the energy of the magnetic field was found to reduce the formation work of the larger cluster in the melt, leading to the formation of larger clusters in the melt, facilitating nucleation and refining the solidification structure. Inspired by this, the pulsed magnetic field melt treatment was applied upon the launder of the casting platform to affect the morphology, size and distribution of α-Al dendrites in the 6082 alloy DC-casting billet with a diameter of 380 mm. The experiment result shows that after the pulsed magnetic field melt treatment, the morphology of α-Al grains in the billet changed from coarse dendritic to small globular and its size was reduced by 18.5–45.4% compared to the existing process. Meanwhile, the difference in morphology of α-Al between the billet centre and edge decreased at the same time. This work may provide a new method for the grain refinement of the 6082 alloy DC-casting billet.