Basic principles of oil air lubrication casting

Limitations of Traditional Direct Cooling Casting

In traditional direct cooling (DC) semi continuous casting, molten aluminum directly contacts the inner wall of the crystallizer to form an initial solidification shell. Due to the solidification shrinkage of the metal, an air gap gradually forms between the ingot and the crystallizer wall, resulting in a decrease in heat transfer efficiency, a rise in liquid cavity temperature, and the flow of low melting point phases along the dendrite and grain boundaries to the surface, forming a reverse segregation layer. This segregation layer cannot be eliminated through subsequent homogenization annealing, and may even further diffuse during the annealing process, becoming the root cause of surface defects in extruded products. In addition, the friction between the initial solidification shell and the crystallizer wall in traditional casting can also lead to surface roughness, tensile marks, cold shuts, and other defects.

The core principle of oil air lubrication casting

The core principle of oil air lubrication casting can be summarized as the guideline of "using gas as the medium, oil as the lubricant, gas film insulation, and extremely thin segregation". Specifically, casting oil and compressed air are respectively transported through their respective oil and gas supply pipelines to the corresponding holes opened on the crystallizer body, and then enter the pre designed small gaps inside the graphite ring for mixing, forming a uniform and stable oil and gas film between the inner wall of the graphite ring and the melt.

This layer of oil and gas film has multiple functions: firstly, the casting oil forms a lubricating film on the inner surface of the graphite ring, greatly reducing the frictional resistance between the solidification shell and the crystallizer wall, making the surface of the ingot smoother; Secondly, the air cushion layer formed by compressed air plays a role in insulation, greatly reducing the heat dissipation of the melt through the crystallizer wall, minimizing the first cooling (crystallizer cooling), and thus delaying the premature formation of the initial solidification shell; Finally, due to the extremely thin initial setting shell and low contact pressure, the thickness of the reverse segregation layer is compressed to a fraction or even one tenth of that of traditional casting.

The molten aluminum completes a cooling process at the graphite ring and forms an extremely thin solidified shell. As the casting platform drives the ingot head to move downward at a constant speed, the liquid aluminum continuously replenishes and fills the inner cavity of the solidified shell; When the ingot leaves the lower outlet of the crystallizer, the cooling water sprayed from the crystallizer directly impacts the surface of the ingot, providing secondary cooling (direct water cooling) to the solidified shell, gradually solidifying the molten metal that has not yet fully solidified inside, and ultimately forming aluminum alloy round bars with excellent surface quality and uniform internal structure.

The national standard "Terminology for Aluminum and Aluminum Alloys" (GB/T 8005.1) defines oil air sliding casting as "a casting method that uses compressed air and lubricating oil to form a layer of oil air lubrication insulation film on the inner wall of the crystallizer through porous graphite rings, in order to improve the surface quality and internal structure of the ingot, and directly sprays water-cooled ingots at the outlet of the crystallizer.