1. Segregation of chemical components in ingots The phenomenon of uneven distribution of chemical components in ingots is called segregation. In the wrought aluminum alloy, the segregation mainly includes intragranular segregation and reverse segregation.
1.1 Intracrystalline segregation
The phenomenon of uneven chemical composition in grains in the microstructure is called intragranular segregation.
The microstructure characteristic of intragranular segregation is that the corrugated intragranular structure resembles a tree ring-like structure after etched. The microhardness within the grains is different, the microhardness near the grain boundary is high, and the microhardness of the grain center is low.
The presence of intragranular segregation makes the chemical composition inside the crystal grains and the structure of the ingot extremely non-uniform, and seriously deteriorates the performance of the ingot, mainly:
1) The inhomogeneity of the chemical composition and the unbalanced excess phase caused by the intracrystalline segregation of the solid solution reduce the stability of the alloy against electrochemical corrosion. 2) The appearance of non-equilibrium eutectic or low-melting composition lowers the starting melting temperature of the alloy, which makes the ingot prone to local overburning during the subsequent thermal deformation or quenching heating process. 3) Intragranular segregation not only causes the appearance of non-equilibrium phases and increases the number of second phases, but also these low melting phases form a hard and brittle dendrite network around the crystal branches, which sharply reduces the plasticity and processing properties of the ingot. 4) The inhomogeneity of the chemical composition caused by intragranular segregation is inherited into the semi-finished product, resulting in the formation of coarse grains in the processed material after annealing.
Intracrystalline segregation is caused by unbalanced crystallization. Therefore, in the actual production of aluminum alloy continuous casting, intragranular segregation is inevitable. An effective way to eliminate intragranular segregation is to homogenize the ingot for a long time.
In continuous casting, the method to reduce intragranular segregation is:
First, increase the cooling rate and adopt modification treatment to refine the grains and intra-granular structure, and reduce the scope of intra-granular segregation.
Second, use the completely opposite method to reduce the cooling rate and perform deep-cavity casting similar to ingot casting to reduce the degree of supersaturation of elements such as iron and manganese, thereby reducing the degree of segregation.
Third, select some additives that can appropriately change the crystalline properties of the alloy. For example, adding an appropriate amount of iron to the 3A21 alloy reduces the concentration of manganese in the solid bath, thereby reducing the uneven distribution of manganese within the crystal grains. In actual production, in the presence of impurity iron, adding titanium is beneficial to reduce the segregation in 3A21 alloy solid solution grains, because the directions of titanium segregation and manganese segregation are opposite, and the center of the dendritic crystal contains high titanium, thereby reducing casting The difference between the solid solution concentration in the center and the edge of the crystal grain.