Analysis and treatment of deterioration of aluminum alloys

[ALUMINIUM NETWORK] 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 better as-cast properties and higher pressure processing ductility. The use of increased cooling rate, low-temperature casting (provided by the Encyclopedia of Industry), ultrasonic oscillating casting, electromagnetic casting and other measures are all 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 refinement 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 characteristics of the final structure changes of metals and alloys: the treatment of changing primary dendrites and other primary crystal sizes is called the former metamorphic treatment, and the treatment that changes the internal structure of primary dendrites is called second 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 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.

Table 2-5-3 Categories of Modified Agents and Their Properties

At present, there are various theories that describe the process of metamorphism. Among them, there are well known nucleation theory, carbide theory, peritectic reaction theory, atomic structure theory, etc. However, no one theory can fully explain this process. . This is because, on the front, the process of modification is complex, 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 effect. They are enhanced. Or weaken the effect of grain refinement.

According to the theory of nucleation theory, 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 temperatures does not change, there is sufficient stability in the 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 modifiers have also been used.

Table 2-4-5 lattice constants of carbides, borides and aluminides

Note: 1 Aluminum is refined and sodium is an aluminum-silicon alloy modifier. For ease of comparison, the data are listed together.

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