Although semi continuous casting has been applied in the industry for nearly a hundred years, surface segregation zones (manifested as segregation nodules, cold shuts, and surface solute rich layers) are still one of the most stubborn defects that plague high-end aluminum production.

The essence of surface segregation tumor is the result of low melting point eutectic enrichment being "squeezed" onto the surface of the ingot. Its formation is a complex process of multi factor coupling:

And the molten material is rapidly cooled in the crystallizer, causing the surface layer to solidify rapidly and form a solidified shell. Due to solidification shrinkage, a gap is formed between the surface of the ingot and the working surface of the crystallizer. The thermal conductivity of the air gap is much lower than that of aluminum, resulting in a sudden drop in heat exchange at that location.

The outward heat dissipation of the solidified shell is blocked, while the liquid phase of the internal liquid cavity still conducts heat inward through the already solidified layer. This process of "heat gain greater than heat loss" causes secondary heating of the solidified shell. When the secondary heating temperature exceeds the melting point of the low melting point composition, the solidified shell undergoes local remelting.

The extruded surface of low melting point eutectic contains a large amount of low melting point eutectic (rich in Mg ₂ Si, Al ₆ Mn and other second phases) melt. Along the gaps between crystals and dendrites, the oxide film formed during the original crystallization is broken, squeezed into the gaps between the ingot and the crystallizer, and condensed into segregation nodules. Metallographic examination shows that the segregation tumor area is usually a low melting point eutectic enrichment zone, where the size of the second phase is several times larger than that of the substrate, the distribution is also dense, and the volume fraction of the second phase is several times larger. Sometimes, primary crystals (such as feather shaped or block shaped Mg ₂ Si, or Al ₆ Mn with pores between phases) can be found at the segregation tumor site.

Segregation tumor in the surface segregation zone is an abnormal tissue that must be removed before deformation processing. If not completely removed, residual segregation tumors will be carried into the surface or interior of the deformed product, which will greatly reduce the mechanical properties of the alloy and easily cause cracking and failure during use; There is a large gap between the aluminum alloy matrix and the segregation tumor, which becomes a source of fatigue cracks; Containing a large amount of low melting point eutectic, the alloy is prone to overburning and surface bubbles during heat treatment. This means that an ingot with segregation nodules, if the segregation layer is not completely removed during the rolling process, will periodically show "black lines" and "peeling" on the surface of the subsequent extruded profiles; During the rolling of sheet metal, rolling bubbles will form; In aerospace forgings, it becomes a fatal defect that fails non-destructive testing.

Based on multiple studies, the external process factors that affect the formation of surface segregation nodules mainly include:

• Crystallizer structure: crystallizer taper, graphite lining state, lubrication conditions.
• Casting temperature: excessive temperature exacerbates the deepening of liquid cavities and remelting.
• Casting speed: too fast speed makes the solidified shell thinner, which is prone to remelting.
• Cooling water intensity: insufficient cooling is the direct cause of segregation nodules.
• Liquid level height control: liquid level fluctuation destroys the integrity of the oxide film.
• Casting funnel position: uneven liquid flow distribution leads to local cooling differences.