Analysis and treatment of deterioration of aluminum alloys

The inhomogeneity of the ingot structure affects the performance of the ingot, and ingots for forging, rolling and extruding are particularly undesirable in reducing the columnar structure of the alloy process. In general, alloys with fine grain structure, fine grain internal structure, and uniform distribution of surplus phases have the best as-cast properties and the highest pressure processing plasticity. The use of increased cooling rate, low-temperature casting (provided by the Encyclopedia of Industry), ultrasonic oscillating casting, electromagnetic casting and other measures are conducive to obtaining the above-mentioned ideal organization, but these methods have limitations, only the use of alloy deterioration Treatment is the fundamental means of adjusting the structure of the ingot.

First, the deterioration process overview

The so-called metamorphic treatment is the process of changing the microstructure of the as-cast alloy under the action of a small amount of special additives (modifiers) to increase the degree of dispersion of the metal or alloy. At present, the technical terms of this treatment method are not uniform, some are called detailed processing, and others are called gestation processing. The classification of deterioration processing is also different. Some people classify the deterioration process into three categories according to the final structure change characteristics of metals and alloys. The treatment that changes the size of primary dendrites and other primary crystals is called the first type of metamorphic treatment, and the treatment that changes the internal structure of primary dendrites is called the second type. Metamorphic treatment, to change the treatment of eutectic tissue called the third type of metamorphic treatment. Some people also classified the deterioration process into three types of four groups according to the properties of the modifying agents (see Table 2—5—3). It is also classified according to the physical effects and metallurgical effects on the crystallized alloy. Obviously, the boundaries between these concepts are indistinguishable. In this manual, metamorphism is understood as the increase in the dispersion of metal and alloy ingots.

At present, there are various theories illustrating the deterioration process, among which there are well-known nucleation theory, carbide theory, peritectic reaction theory, atomic structure theory, etc., but none of them can fully explain this. process. This is because, first, the metamorphic treatment process is very complicated, not only related to the smelting conditions, but also related to the casting conditions. Second, uncontrollable impurities have an effect, and the interaction of certain elements in the aluminum alloy also has an influence. They are enhanced. Or weaken the effect of grain refinement.

According to the nucleation theory, the grain refinement is due to the presence of nuclei, and the melt begins to crystallize on the nuclei. These particles may be transition metal carbides, borides, and aluminides, and their lattice constants are similar to those of aluminum solid solutions (4.04 Å) (see Table 2—5—4). According to this theory, additives added as modificators should meet the following requirements: 1 Chemical composition at high temperature does not change, and there is sufficient stability in aluminum melt; 2 The melting point of the additive should be higher than that of aluminum; 3 Additives and aluminum The crystal lattice should adapt to the structure and size; 4 forms a strong and strong adsorption bond with the melt atoms being processed.

At present, in addition to aluminum-titanium additives, aluminum-titanium-boron additives are generally used in aluminum-titanium-boron additives (wires or cakes). In addition, newly developed aluminum-titanium-boron-rare earth modifers and aluminum-titanium-carbon modificators have also been used.