Application of Cerium in casting magnesium alloy

In general, CE can purify the alloy, improve the mechanical properties of the alloy, refine the primary crystal phase of the alloy, and make the network eutectic smaller. The other role of CE is to strengthen the matrix by solid solution, and improve the comprehensive mechanical properties of the alloy.

1. Application of Cerium in ZK60 series cast magnesium alloy

The rare earth element Ce added to ZK60 casting magnesium alloy has the effect of grain refinement and grain boundary purification. Dynamic recrystallization occurs after hot extrusion of the test alloy. The recrystallized grain size is refined with the increase of Ce content. With the increase of Ce content, the tensile strength and elongation of the extruded alloy are increased, but when the Ce content exceeds 1.0%, the elongation is decreased. The tensile strength and elongation of the alloy increased after T5 treatment. The optimum comprehensive mechanical properties are as follows: σ B = 318.6mpa in extrusion state, δ = 14.4%, σ B = 338.6mpa, δ = 15.6% after 24 h aging.

Adding 0.52%, 0.94%, 1.51% and 1.98% (mass fraction) ce to ZK60 casting magnesium alloy respectively, the effect of adding amount of CE and heat treatment process on the structure and tensile strength of the alloy was discussed. The results show that the grain size of ZK60 alloy without CE is large, and the grain boundary is surrounded by a large number of scattered skeleton structure. After adding CE, the grain size is obviously refined and the grain boundary is purified; the tensile strength increases with the increase of CE addition, and it will be higher after aging at 150 ℃ for 24 hours. Adding 1.51% Ce into the sample has the highest performance, the strength and elongation can reach 318.6mpa and 14.4%, and the strength and elongation after aging can reach 338.6mpa and 15.6%.

2. Application of Cerium in AZ91 series cast magnesium alloy

When rare earth element Ce is added to AZ91D cast magnesium alloy, CE has obvious grain refinement effect, primary crystal structure becomes fine, and the network eutectic structure breaks into particles and evenly distributes at the grain boundary. After CE modification, rod-shaped al10ce2mn7 compound distributed at the grain boundary appeared, which hindered the growth of primary and eutectic phases, refined the as cast structure of the alloy, and its content increased with the increase of CE addition. The modification mechanism of rare earth Ce to cast magnesium alloy is: in magnesium, the limit solid solubility of rare earth Ce is 0.5%, so CE has two functions in magnesium, one is as alloy element, which can improve the heat resistance of magnesium alloy; the other is to form intermediate phase, refine grain and strengthen alloy. When less Ce (0.4%) is added to the cast magnesium alloy, part of CE will be solid dissolved in the magnesium matrix, and the rest of CE will be enriched in the front of the growth interface of α – Mg grains, and a layer of rare earth Ce interface layer will appear. When the divorced eutectic structure appears, the growth of the divorced eutectic structure will be blocked due to the enrichment of rare earth Ce, so that the semi continuous network of divorced eutectic structure will become intermittent or granular. With the increase of CE addition, CE is directly combined with Al and Mn which are on the front of solidification interface to form rod-shaped al10ce2mn 7 compound. The compound has a high melting point (above 1200 ℃), which prevents the grain from growing and makes the grain finer during solidification.

The stress-strain behavior of mg-0.7% zn-0.7% Zr cast magnesium alloy with 2.8% Ce was studied under different temperature and different tensile speed. The results show that the work hardening properties, recovery and recrystallization properties of the alloy with σ 0.2 = 222.4 MPa, σ B = 257.8 MPa, δ = 12.0% at 623k have an important influence on the plasticity of the material in different temperature range.

3. The application of Cerium in AZ31 series cast magnesium alloy

The effect of Ce on the properties of AZ31 cast magnesium alloy shows that the strength of the alloy increases with the increase of Ce content, among which the alloy with 1% CE has the highest strength, σ B is 321 MPa in the processing state and 259 MPa after annealing. The strengthening effect of CE is different in the processing state and annealing state. In the rolling state, the σ B of the alloy containing 1% Ce is 23 MPa higher than that of the alloy without CE, while in the annealing state, σ B is only 6 MPa higher. The addition of CE can increase the δ of the alloy by more than 3%, especially in the processing state, the δ of AZ31 alloy is less than 2%, and the addition of CE can increase the δ to more than 5%. There are two strengthening mechanisms of Ce: solution strengthening and al4ce phase strengthening. CE can play a solution strengthening role in magnesium alloy, but the addition of CE will reduce the aluminum content in the alloy, weaken the solution strengthening effect of aluminum, and the aluminum solution strengthening effect is greater than the solution strengthening effect of cerium, so the solution strengthening of Cerium in the alloy is not the main. The al4ce phase is broken in the process of processing and produces fine al4ce particles. These particles can interact with dislocations and other defects in the process of processing, while the strengthening effect of the strengthened alloy is reduced after annealing. The addition of CE in AZ31 alloy can form the rod-shaped al4ce phase, and the alloy elements are mostly distributed around the phase. During the processing, the phase is broken to form fine al4ce phase particles. The addition of CE can purify the grain boundary, improve the fracture morphology, and improve the hardness of the alloy. The comprehensive mechanical properties of the alloy containing 1.05% Ce are the best, its σ B is 321 MPa, δ is 6.9%; after annealing, it is 259 MPa and 21.8%.

4. Application of Cerium in pure magnesium

The study on the heating and cooling curves of pure magnesium with the addition of CE shows that the temperature of the alloy liquid decreases, while the temperature of the solid phase line decreases more. It can be seen that the trend of component supercooling of rare earth elements in the front of alloy interface is increasing, which is beneficial to the formation of α – Mg equiaxed nucleus. After adding trace rare earth element Ce to pure magnesium, the grains are obviously refined, and the columnar crystals are all transformed into equiaxed crystals. The grain size of mg Al Zn AZ31 alloy decreased from about 300 μ m to about 30 μ m when CE was added. The refining effect of rare earth in magnesium and magnesium alloys is due to the supercooling of the components caused by the enrichment of rare earth elements at the front of solid-liquid interface during the solidification process, and the formation of new nucleation zones in the supercooling zone and the formation of fine equiaxed grains. Therefore, the increase of supercooling at the front of solid-liquid interface caused by solute redistribution during solidification is the main mechanism of refining magnesium and magnesium alloy by rare earth elements.

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