Research Of Crystal Nucleation And Growth Among Pure Aluminium Was Refined By Master Alloy

Grain refinement is widely used in the aluminum processing industry, but in all sorts of grain refinement methods, adding grain refiner is considered to be the easiest and most effective methods; of course it is also an important part of aluminum alloy melt treatment technology. But it is still lack of a kind of grain refiner which can satisfy the refining effect and is no pollution and can be widely available aluminum alloy grain refining agent, In addition the relevant opinions and the mechanism of grain refinement also did not reach a consensus.In terms of the present stage, the theories about the mechanism of grain refinement are mainly the peristaltic reaction, phase diagram theory, theory of boride, carbide and boride, metastable phase theory, TiC core theory, crystal separation theory and so on, But most of these mechanisms can only one-sided explain some phenomenon, and because the alloy solidification process itself is very complex does not rule out in the process of some is the result of several kinds of mechanism combination, that is to say, to a certain extent, these theories are not perfect still need to continue. For now, other grain refinement from the perspective of the classical solidification theory analyses the thermodynamic cause is lack of related research.This paper mainly studied that the pouring temperature and cooling rate on the Al-Ti-B, Al, Ti-C, laboratory self-control among Al-TiC and Al-Ti alloy have effete on the the change of the solidified structure of pure aluminum, And using the thermal analysis technology we analyzed the difference of various thermodynamic parameters added different kinds of master alloy and different content from the perspective of the classical solidification theory, the reasons behind the change of thermodynamics is anglicized and discussed.On the basis of it, author puts forward the possible mechanism of grain refinement. The main conclusions are as follows:(1)Different casting temperature of aluminum melt had great effect on the formation and size of equinoctial crystal of pure aluminum added Al-Ti-C,Al-Ti-B,A1-Ti,Al-TiC alloy aluminum. It can be seen without adding alloy aluminum sample from the surface to the middle of the outer circumferential distributed a large columnar crystals and the central portion surrounded by the columnar crystal area distributed larger size fine equiaxial crystal when casting temperature was heated to T=680℃, T=730℃.However, sample was nearly all of the coarse columnar crystals. When casting temperature was heated to T=800℃, T=900℃.The higher of casting temperature, the coarser columnar crystals. But when adding the mass fraction of the alloy is lesser than certain content, with the rising of casting temperature the size of the grain was coarse.(2) Different solidification cooling rate of aluminum melt had great effect on the formation and size of equinoctial crystal of pure aluminum added A1-Ti-C, Al-Ti-B, Al-Ti, Al-TiC alloy aluminum. It can be seen without adding alloy aluminum sample from the surface to the middle of the outer circumferential distributed large columnar crystals and the central portion surrounded by the columnar crystal area distributed larger size fine equinoctial, sample is nearly all of the coarse columnar crystals. After adding alloy, the aluminum macro organizational changed significantly, all of ingot organization columnar crystal changed to fine equinoctial crystal.At the same solidification cooling conditions, With the increasing of the mass of master alloy, the amount of columnar crystals decreased gradually in aluminum solidified organizational structure, however, the equinoctial crystal increased gradually.(3) With the increasing of the mass of master alloy, the amount of columnar crystals decreased gradually in aluminum solidified organizational structure, however, the equinoctial crystal increased gradually. When the mass of master alloy was added a certain amount, quantity and size of equinoctial crystal were closely related to cooling velocity.Along with Al-5Ti-C alloy ingots added to increase the amount of tissue columnar grain reduction equiaxed grain increased. In the Al-5Ti-C alloy to add the same amount, because of the faster cooling melt, type wall generated a large number of equiaxed primary crystal in the melt, which may be preserved and the more moderate axis crystal ingot number of organizations the more, the more fine grain size. Pouring the melt as the temperature rises, the process of the casting medium free wall of the primary crystal produced by the high temperatures of molten aluminum melted again, causing Ingots coarse equiaxed grain, columnar grain increased. And when the temperature of the liquid inside the mold reduced, can produce a stable grain on wall type, mould temperature rising will reduce the temperature difference between inside it with casting solution, the result reduce the grain free movement of the heat convection, and there is no casting solution mixing vibration, thus avoiding the type wall free of grain and the formation of a stable solidification shell, enabled the columnar crystal to grow easily. So in the hope to get an ingot casting with isometric crystal, as far as possible to reduce the pouring temperature, prevent free crystal to melt crystal again. Because when cooling capacity of the mold is too high, a large number of crystal nucleus is densely formed in the wall, makes it easier for the adjacent grain contact and the formation of a stable solidification shell, so as to prevent free of grain. So the wall cooling capacity prone to grain type floating near the surface of must not be too high.(4)The formation of crystal grain in the solidified structure under the action of master alloy is the result of the crystal grain free and the results of the TiC particles heterogeneous nucleation dual function, when the content of the master alloy is higher, the affections of solidification cooling rate and casting temperature of aluminum melt is smaller.The author thinks that is mainly because the addition amount of master alloy is higher,"TiC particles-rich Ti transition zone" heterogeneous nucleation in alpha Al crystallization process took a leading role.(5) Through the analysis of thermodynamic parameters of the solidified structure of pure aluminum added the master alloy (Al-Ti-C, Al-Ti-B, Al, Ti, Al-TiC), the result showed that adding master alloy can obviously reduce the wetting angle of pure aluminum solidification, increase the rate of the solidification process of nucleation. The solidified structure of pure aluminum without the master alloy (Al-Ti-C, Al-Ti-B, Al, Ti, Al-TiC), its wetting angle is larger and nucleation rate is small. However after adding the middle refining alloy, wetting angle will decreases with the increase of the content of the master alloy and wetting angle will be further reduced to increase the nucleation rate in the solidification process.The innovation point of the article is to:by using the classical solidification theory and using thermal analysis technology, analyzed the change of the thermodynamic parameters in the process of grain refining grain refining and the thermodynamics reasons behind it. And on the basis of this, author advances the possible mechanism of grain refinement.