2, Casting speed: the "command stick" for the depth of the liquid cavity

Casting speed (usually measured in mm/min) is the core process parameter of semi continuous casting, which directly determines the shape and depth of the liquid cavity.

2.1 The double-edged sword effect of acceleration

The numerical simulation of 7A60 aluminum alloy ϕ 360mm ingot based on ProCAST software revealed a clear pattern: increasing the casting speed enhances the flow of the melt, extending the length of the liquid cavity from 205.2mm to 638.1mm; however, excessively high casting speed increases the risk of remelting and leakage, and at the same time, the cross-sectional equiaxed grain area of the ingot decreases from 60567.8mm ² to 50511.3mm ².

Another study on 2024 aluminum alloy ϕ 300mm ingots also confirmed that as the casting speed increases, the depth of the liquid cavity increases, the paste like zone widens, and the strength and volume of solidification shrinkage increase, thereby exacerbating the negative segregation at the center of the ingot; At the same time, the thickness of the solidified shell and feather crystal region decreases, the thickness of the equiaxed crystal region increases, and the average grain size of equiaxed crystals decreases.

These two seemingly contradictory sets of data reveal the true role of casting speed:

2.2 The critical relationship between feather crystals and cold insulation

When the casting speed is too high, the thickness of the initial solidified shell in the crystallizer is insufficient, and the surface of the ingot is prone to cold shut (cold shut is a transverse concave defect on the surface). More seriously, the deepening of the liquid cavity shortens the residence time of the solidified shell in the crystallizer. If the secondary cooling is not compensated in time, the thickness of the outer shell of the ingot when it exits the crystallizer will be insufficient, which can easily lead to leakage accidents.

Therefore, the selection of casting speed in industrial practice must be comprehensively considered: alloy grade (high alloying such as 7055 and 7075 require slower speed); Ingot diameter (large diameter requires reduced speed); Cooling intensity (if the cooling capacity is strong, it can be moderately accelerated)