Elucidating the Mechanism of Refining the Columnar Dendrites Through Gas Cooling Pressure During the VAR Process

The vacuum arc remelting (VAR) process generates a shrinkage gap between the ingot and the crucible, and introducing gas cooling into the shrinkage gap can improve the solidification quality of the ingot. As the key process parameter of VAR, gas cooling pressure affects the solidification quality of ingot by refining dendrite through improving thermal conditions, but the refinement limit and dominant factor are still unclear. In this study, by using a finite element phase field (FE-PF) multi-scale model, the limit of dendrite refinement via gas cooling pressure and the mechanism by which gas cooling pressure influences dendrite growth were elucidated from the perspectives of temperature gradient and cooling rate. Increasing pressure improves the interfacial heat transfer coefficient, accelerate ingot solidification, and reduce the depth of the molten pool. The temperature gradient and cooling rate both increase with increasing pressure, resulting in a decrease in the dendrite arm spacing. When the pressure increases to 1333.2 Pa, the gap width becomes the limiting factor, and the refinement effect reaches its limit. Increasing the gas cooling pressure, the temperature gradient is the dominant factor affecting the columnar dendrite growth, and more initial dendrites promote the reduction of dendrite arm spacing. According to the simulation data, contour maps of dendrite arm spacing changing with thermal conditions were drawn to predict the distribution of dendrite arm spacing in ingots under different gas cooling pressures. This provides theoretical support and reference for adjusting gas cooling pressure in VAR process.