[Techniques] Grain Refinement and Modification of Magnesium Alloy Melted by Magnesium Alloys
Grain refinement is an important way to improve the performance of magnesium alloy castings. The finer the magnesium alloy grains, the better the mechanical properties and plastic processing properties. The grain refinement operation during the smelting of the magnesium alloy is handled properly, which can reduce the hot cracking tendency during the solidification of the casting. In addition, after the magnesium alloy is processed through the grain refining operation, the intermetallic compound phases in the casting are finer and more evenly distributed, thereby shortening the homogenization treatment time or at least improving the homogenization treatment efficiency. Therefore, the grain refinement of magnesium alloys is particularly important.
Magnesium alloys have two types of methods for grain refinement in the smelting process, ie, modification and strong external field effects. The former mechanism is to add high-melting point substances in the alloy liquid to form a large number of nucleation sites to promote the nucleation and crystallization of the melt and obtain a fine grain structure.
The basic principle of the latter is to apply an external field electric field, magnetic field, ultrasonic waves, mechanical vibration and stirring to the alloy melt to promote the nucleation of the melt, and to destroy the dendrite that has formed to become a free crystal, so that the number of crystal nuclei increases. It can also strengthen the conduction process in the melt and eliminate segregation of components. In addition, rapid solidification techniques, semi-solid forming techniques, and ingot deformation can also increase the nucleation rate of magnesium alloys, suppress the growth of nuclei, and significantly refine grains.
In the smelting process, the magnesium alloy solution is required to be subjected to a modification treatment. The purpose of the modification treatment is to refine the crystal grains, improve the mechanical properties of the alloy, and to purify the alloy solution to some extent. Therefore, this is a critical process. Whether the modifier can refine the grains of the magnesium alloy depends on whether the crystal structure of the particles in the liquid phase is similar to the crystal structure of magnesium and whether the lattice constant of the particles is similar to the lattice constant of magnesium. The fault is a modificator that satisfies the above conditions. The peritectic reaction occurs wrongly to refine the magnesium crystal grains. Generally, the wrong amount of the magnesium alloy is 0.7% of the solution mass percentage. Increasing the wrong content does not make the magnesium. The grains are more refined, but the bulky mis-phases appear, which affects the properties of the alloy.
However, if the magnesium alloy contains aluminum, Zr and Al form an intermetallic compound ZrA16. The surface structure of this compound is different from that of magnesium and cannot play a role of refinement. Therefore, the aluminum-containing magnesium alloy is not refined with the wrong element. ZM5 uses a carbon-containing modificator to chemically produce A14C3 or Mg-Al-Mn-C compounds in the alloy to become crystalline cores and achieve refinement. Metamorphic temperature is generally better at 710-740°C. Cast magnesium alloy commonly used effect better mutator as shown in Table 12-1
In the early days, people used a superheating treatment method to superheat the molten magnesium alloy to 875-9250C after being refined, rapidly cooled to the pouring temperature for 10-15 minutes, and then cast it, which has the effect of grain refinement. Studies have shown that the superheating and metamorphic treatment can significantly refine the Mg17Al 12 phase in the ZM5 alloy, but this process has major drawbacks. In the process of superheating and metamorphic treatment, the overheating temperature of the magnesium alloy melt is very high, thereby significantly increasing the burning of magnesium. The loss reduces the service life and production efficiency of the site A and increases the iron content and energy consumption in the melt. Therefore, the application of overheat deterioration treatment in production practice is not universal and has basically been eliminated. At present, the metamorphic agents often used in smelting magnesium alloys include carbonaceous materials, hexachloroethane (C2CQ and high melting point additives such as Zr.Ti.B.V), and the like. The following briefly introduces the mechanism and effect of grain refinement of several commonly used modificators.
1 Carbonaceous modifier. C does not solid-solubilize in Mg, but reacts with Mg to form Mg2C, and MgC2 compounds. Carbon has a significant grain refinement effect on Mg-Al-based or Mg-Zn-based alloys, while the refinement effect on Mg-Mn-based alloys is very limited. Many assumptions have been made on the mechanism by which carbon-containing modifiers refine grains of magnesium alloys. The Battelle Memorial Institute believes that after C is added to the melt of Mg-Al alloy, C reacts with Al to form a large number of fine, disperse A-ring 3 particles. The lattice constant is very close to that of Mg and can be used as a nucleation particle so that it can be fine. Magnesium alloy grains. At present, this assumption is widely accepted, but it still lacks experimental evidence.
The carbon-containing modifiers commonly used in industry include magnesite (MgCO), marble (CaCO), sacred, stone coal, coke, C02, carbon black, natural gas, etc. Among them, MgC03' CaC03 is the most common. Take MgC03 as an example. After MgC03 is added to the Mg-Al alloy melt, the following reaction occurs: MgCO3=MgO+C02↑C02+2Mg=2MgO+C3C+4A1=A14C3 A large number of fine and infusible A14C3 particles are produced in the magnesium alloy melt. It is suspended and acts as a nucleation base during the solidification process.The addition amount of MgCO is generally 0.5%-0.6% of the melt quality of the alloy, the melt temperature is 760-7800C, and the modification treatment time is 5-8 min.
2 Hexachloroethane (C2C1) G C2CL is one of the most commonly used metamorphic agents in magnesium alloy smelting, and can simultaneously achieve the dual effects of degassing and grain refinement. QLJin et al. conducted a study on the effect of C2C16 on the grain refinement of AZ31 alloys. Research shows that Al-C-0 compound particles are formed in the casting to act as the core of the nuclei. After AZ31 was modified by C2C16, the grain size decreased from 2805m to 1205m, and the tensile strength increased significantly.
For ZM5 alloys, the effect of CA modification is much better than that of MgCO. In addition, C plant 16 and other modificators can also be used for compound modification, and the effect is better.
Place C2C1 at the bottom of the Mg-Al alloy melt. Or cyclochlorobenzene sheet can also achieve the dual purpose of grain refinement and degassing.
3 Other modificators o Zr has obvious grain refining effect on Mg-Zn, Mg-RE and Mg-Ca alloys. It is the most commonly used grain refiner in magnesium alloy smelting, but its crystals The mechanism of grain refinement is not yet clear.
In the case of adding the same amount of error, there are significant differences in the grain refinement effect due to different alloying conditions. Sauerwald believes that only the part of the error that dissolves into the magnesium during casting is grain refining. This view was widely recognized before the mid-1960s. Based on the observation of optical microscopy, some studies have suggested that Zr particles are separated from the melt at the peritectic temperature and react with the magnesium solution to form an error-rich Mg-based solid solution until the residual content in the remaining melt drops to a lower value. At the same time, he pointed out that the formation of rich-ring particles near the peritectic temperature has the effect of promoting the nucleation of the melt. Since a-Zr is a nucleation point for magnesium alloys, Tamura et al. studied the formation of Mg- by adding 1% mass fraction of Zr to 7200C magnesium under different process conditions (including stirring time and melt standing time zone). Influence of the grain size of Zr alloy They found that the grain refinement effect was more significant when the magnesium melt was re-mixed prior to pouring, and that the solid solution error before and after re-stirring did not change, indicating that part of the insoluble Mg solution also has the wrong Grain refinement.M.Qian et al. believe that the grain refinement effect of Mg-Zr alloys mainly comes from the solid-solution dislocations in magnesium, while the part without the solid solution has only about 30% grain refinement. .
In general, the misfed amount in the Mg-Zr alloy melt is slightly higher than the theoretical value. The Mg-Zr alloy achieves the best grain refinement effect only when the melt is soluble in the acid and misses saturation. Due to the fact that various contaminants may also be present in the melt, resulting in the formation of acid-insoluble siliceous compounds, aluminum and silicon should not be included in the melt as much as possible. In addition, it is necessary to retain the bell-containing residues (including acid-insoluble cesium) in the bottom of the pot. In order to prevent the liquid residue from being poured into the castings, a sufficient amount of molten alloy volatility should be reserved in the crucible after casting. The quality of clever%). When pouring, try to avoid excessive turbulence and overflow of the melt, and ensure enough time for standing in the smelting process.
Table 12-2 shows the modifiers and their amounts and treatment temperatures of Mg-Al alloys.
Mg-Al f alloys also need to be refined after modification. ZM 1 . ZM2. ZM3 "The ZM4 and ZM6 alloys use Zr for grain refinement of the alloy and do not require the above modification. For Mg-Zn alloys, adding 0.5% mass fraction of Zr can be very good. Effect: The grain size of the Mg-Mn alloy can be refined by 0.5% Sc+(0.3-0.5)%Sm, and (0.20.8)% La can also refine the grain of the Mg-Mn alloy.
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