[China Aluminum Industry Network] The phenomenon of uneven chemical composition in the ingot is called segregation. In the deformed aluminum alloy, segregation mainly includes intragranular segregation and reverse segregation.
1.1 Intragranular segregation The phenomenon of inhomogeneous chemical composition in the grain is called intragranular segregation.
The microstructure of intragranular segregation is characterized by the corrugated, tree-like annual structure in the crystal after etching. The intragranular microhardness is different, the microhardness near the grain boundary is high, and the microhardness of the grain center is low.
The presence of intragranular segregation causes the chemical composition inside the grains and the microstructure of the ingot to be extremely uneven, which seriously deteriorates the performance of the ingot. The main factors are:
1) Inhomogeneities in the chemical composition caused by intragranular segregation of the solid solution and the appearance of an unbalanced excess phase reduce the stability of the alloy against electrochemical corrosion.
2) The appearance of non-equilibrium eutectic or low-melting compositions reduces the initial melting temperature of the alloy, making the ingot more susceptible to local over-heating during subsequent hot deformation or quenching heating.
3) Intragranular segregation not only causes the appearance of non-equilibrium phases and increases the number of second phases. Moreover, these low-melting phases form a hard and brittle dendrite network around the crystallized branches, which drastically reduces the plasticity and processing properties of the ingot.
4) Inhomogeneity of the chemical composition caused by intragranular segregation is inherited into the semifinished product, resulting in formation of coarse grains in the processed material after annealing.
Intragranular segregation is caused by unbalanced crystallization. Therefore, in the actual production of aluminum alloy continuous casting, intragranular segregation is unavoidable. An effective way to eliminate intragranular segregation is to homogenize the ingot for a long period of time. In continuous casting, the method to reduce intragranular segregation is to improve the cooling rate by front-end, use modification treatment to refine the grain and intragranular structure, and narrow the segregation range in the crystal. Secondly, using the completely opposite method, reduce the cooling rate and perform deep liquid cavity casting like ingot casting to reduce the degree of supersaturation of iron, manganese and other elements to reduce the degree of segregation. Third, select some additives that can properly alter the crystalline nature of the alloy. For example, adding an appropriate amount of iron in the 3A21 alloy reduces the concentration of manganese in the solid bath, thereby reducing the non-uniformity of the distribution of manganese within the grains. In practical production, adding titanium in the presence of impurity iron is beneficial to reduce the segregation in the solid solution grains of 3A21 alloy, because the direction of titanium segregation and manganese segregation is exactly the opposite, and the center of the dendritic crystal is high in titanium, thus reducing the cast. The difference in solid-solution concentration between the center of the grain and the edge.
1.2 Reverse segregation The phenomenon that the concentration of solute at the edge of the ingot is higher than the concentration of solute in the center of the ingot is called reverse segregation. The organizational features of reverse segregation are not easily discernible from the microstructure and can only be confirmed by chemical composition analysis.
The reverse segregation in the aluminum alloy ingot is an important reason for the great difference in mechanical properties and physical properties of the ingot and its press-worked products. In regions where the degree of reverse segregation is severe, its chemical composition is even beyond the standard range, causing mechanical properties to be exceeded and scrapped.
Reverse segregation is a kind of mixing phenomenon in the solidification process of aluminum alloy continuous ingots and cannot be completely avoided, nor can it be eliminated by high-temperature homogenization. However, segregation of elements can be controlled within the allowable range according to the formation rules and influencing factors of reverse segregation. Its approach is:
1) Increase the cooling strength of the ingot.
2) Select the appropriate casting speed so that the transition zone does not increase the tilt angle of the exposed liquid level. Generally, it should not exceed 60°.
3) Properly increase the casting temperature.
4) Using a suitable casting funnel, divert the flow evenly and let it flow to the edge of the ingot.
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