Cooling intensity: the "master switch" for organizational refinement

Cooling intensity usually refers to the combined effect of the flow rate, pressure, and water temperature of the secondary cooling water. It is the most effective lever for regulating the solidification structure.

1. Quantitative effect of cooling intensity on microstructure Research data shows that within a certain range, as the cooling intensity increases, the crystallization rate of the melt increases, and solute elements in the melt do not have time to diffuse, resulting in an increase in undercooling, an increase in crystal nuclei, and the formation of fine grains. At the same time, the size of the transition zone decreases, the density of the ingot increases, the tendency to loosen decreases, and the degree of regional segregation decreases. The study on the semi continuous casting of 6061 aluminum alloy near the liquidus line further points out that a decrease in the primary cooling intensity and an increase in the secondary cooling intensity are both beneficial for the formation of uniform and fine near spherical structures; When the casting speed reaches 150mm/min and the secondary cooling intensity increases, a small, uniform, and nearly spherical microstructure of 6061 semi-solid billet can be obtained.

2. The marginal effect of cooling intensity should be noted that stronger cooling intensity is not necessarily better. On the one hand, excessive secondary cooling can create a significant temperature gradient between the surface and core of the ingot, resulting in: rapid solidification of the surface → developed columnar crystals → and even feather crystals; Sudden increase in thermal stress → surface cracks and internal thermal cracking tendency of the ingot increase; For certain alloys (such as 5052), excessive cooling can exacerbate unbalanced crystallization and promote the formation of segregation nodules; On the other hand, when the cooling water volume increases to a certain limit, the heat transfer coefficient tends to saturate. Continuing to increase the water flow rate has little effect on improving the solidification rate, but instead causes water resource waste and ingot turbulence.

3. Process window for cooling water pressure: Industrial tests using 5052 aluminum alloy as an example show that increasing the cooling water pressure intensity to 0.20 MPa can significantly suppress the formation of surface segregation nodules on ingots. This data provides an important engineering reference: there is a clear optimal window for cooling water pressure, below which segregation tumors may spiral out of control, and above which microcracks may be induced on the surface of the ingot.