In the field of aluminum alloy processing, direct cold casting (DC casting) is known as the "magic hand of metal forming". When the aluminum alloy melt at around 700 ℃ is injected into the crystallizer, in just a few tens of minutes, the liquid metal transforms into a cylindrical ingot with a regular outer diameter and dense internal structure - the core driving force behind this is water. More precisely, it is a "water cooling magic" that combines primary and secondary cooling. The success or failure of this magic is mainly reflected in the game of three key variables: the speed of casting, the strength of cooling intensity, and the thickness of the surface segregation zone. Any loss of control of a variable will leave irreversible defects such as segregation nodules, cold shuts, and tensile cracks on the surface of the ingot, which will be amplified into batch scrap in subsequent extrusion and rolling processes.

1. The two-stage (actually three-stage) mechanism of water cooling

Phase 1: One cooling (crystallizer wall cooling)

After the high-temperature aluminum liquid is injected into the crystallizer, it comes into direct contact with the inner wall (which is lined with graphite), and the surface layer rapidly cools down within seconds and forms a solidified shell. The heat in this stage is conducted to the water through the crystallizer wall, which is called primary cooling. The heat carried away by a single cooling process accounts for approximately 10% -15% of the total heat content of the ingot. Although it does not take away much heat, it is the key to "forming" - the solidified shell must have sufficient thickness to withstand the static pressure of the internal liquid cavity without aluminum leakage.

Phase 2: Secondary cooling (spray forced water cooling)

At the moment when the ingot separates from the lower edge of the crystallizer, high-pressure cooling water is directly sprayed from the spray hole onto the hot surface of the ingot. At this point, heat exchange occurs between the solidified ingot shell and water, known as secondary cooling. The heat carried away by secondary cooling accounts for about 70% -80% of the total heat content of the ingot, and is the dominant force for the complete solidification of the ingot. It is the strong effect of secondary cooling that causes the liquid phase inside the ingot to solidify layer by layer from the outside to the inside, forming a typical "V" - shaped liquid cavity.

Phase 3: Three cooling cycles (natural slow cooling)
After the ingot is completely removed from the spray zone, it relies on the accumulation of water and air in the foundation pit to slowly dissipate heat, gradually releasing the internal stress of the casting and reducing the risk of cracking. The cooling rate in this stage is relatively slow and has little impact on the crystallization process of the ingot, but it is of great significance for stress relaxation and preventing low-temperature cracking.