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1. Chemical Segregation in Ingots The phenomenon of uneven chemical composition distribution in ingots is called segregation. In wrought aluminum alloys, segregation mainly includes intragranular segregation and inverse segregation.

1.1 Intragranular Segregation

The phenomenon of uneven chemical composition within grains in the microstructure is called intragranular segregation.

The microstructural characteristic of intragranular segregation is that after etching, the grains show a ripple-like structure, similar to tree rings. The microhardness varies within the grains, with higher microhardness near the grain boundaries and lower microhardness at the grain centers.

The presence of intragranular segregation leads to extremely uneven chemical composition within the grains and in the ingot's structure, severely deteriorating the ingot's performance, mainly due to:

1) The chemical inhomogeneity and the appearance of unbalanced excess phases caused by intragranular segregation of solid solution lead to a decrease in the alloy's stability against electrochemical corrosion. 2) The appearance of non-equilibrium eutectics or low-melting point constituents lowers the alloy's incipient melting temperature, making the ingot prone to local overheating during subsequent hot deformation or quenching heating processes. 3) Intragranular segregation not only causes the appearance of non-equilibrium phases and increases the quantity of second phases, but these low-melting phases also form a hard and brittle dendritic network around the dendrites, sharply reducing the ingot's plasticity and workability. 4) The chemical inhomogeneity caused by intragranular segregation is inherited into semi-finished products, leading to the formation of coarse grains in processed materials after annealing.

Intragranular segregation is caused by non-equilibrium solidification. Therefore, in the actual production of continuous casting of aluminum alloys, intragranular segregation is unavoidable. An effective method to eliminate intragranular segregation is to subject the ingot to a long period of homogenization treatment.

In continuous casting, methods to mitigate intragranular segregation are:

First, increase the cooling rate and use modification treatment to refine grains and intragranular structure, thereby reducing the extent of intragranular segregation.

Second, adopt a completely opposite method, decrease the cooling rate, and perform deep sump casting similar to ingot mold casting, to reduce the supersaturation of elements like iron and manganese, thereby mitigating the degree of segregation.

Third, select additives that can appropriately alter the solidification properties of the alloy. For example, adding an appropriate amount of iron to 3A21 alloy reduces the concentration of manganese in the solid solution, thereby decreasing the inhomogeneity of manganese distribution within the grains. In actual production, when impurity iron is present, adding titanium is beneficial for reducing segregation in the solid solution grains of 3A21 alloy, because the direction of titanium segregation is exactly opposite to that of manganese segregation. The center of the dendrite contains high titanium, thereby reducing the difference in solid solution concentration between the center and edge portions of the as-cast grain.