1. Application of mixed rare earth in high temperature creep resistant die casting magnesium alloy
In view of the poor high-temperature creep resistance of commercial die-casting magnesium alloy, a high-temperature creep resistance die-casting magnesium alloy was developed. The purpose of the development is that the high-temperature creep resistance of cylinder cover of high-power diesel engine should be able to ensure its stable operation at 150 ℃. The effects of CE, y, Nd, Ca and Si on the creep behavior, microstructure, surface treatment and corrosion test of die-casting magnesium alloy at room temperature, 150 ℃ were studied, The creep resistance of the new type of magnesium alloy containing nd and Y is basically the same as that of the mri-153 alloy of Volkswagen Company of Germany, and the technological performance of the developed alloy is the same as that of AZ91 alloy, so the same production process can be adopted. It is better to prepare high performance mg RE alloy by using low cost electrolytic mg re master alloy than by using rich RE alloy in terms of burning loss rate, recovery rate and cost. The addition of Nd and y not only can not change the original grain size of AZ91 alloy, but also can produce solution strengthening effect. It has excellent creep resistance and is a potential creep resistant pressure casting magnesium alloy.
2. Effect of mixed rare earth on friction and wear properties of cast magnesium alloy
The effect of rare earth on the friction and wear properties of AZ91 and AM60 cast magnesium alloys was studied. The results showed that the friction and wear properties of the cast magnesium alloys containing rare earth were significantly better than those of the base alloys. The wear rates of the cast magnesium alloys containing rare earth and the cast magnesium alloys without rare earth increased with the increase of load, The wear resistance of AZ91 rare earth cast magnesium alloy is much higher than that of AM60. The addition of rare earth can refine the structure of the alloy, improve the comprehensive properties of the magnesium alloy, enhance the stability of the oxide film on the wear surface, and improve the bearing capacity of the rare earth magnesium alloy. Under dry friction condition, the wear mechanism of AZ91 and AM60 magnesium alloy is the same, both of them change from oxidation wear to peeling wear. The addition of rare earth elements enhances the stability of the oxide film on the worn surface, improves the bearing capacity of the cast magnesium alloy containing rare earth elements, and effectively improves the wear resistance of the alloy.
3. Effect of mixed rare earth on grain size of AZ91D cast magnesium alloy
The relationship between the mechanical properties and the grain size, dendrite size, eutectic structure and solidification curve of AZ91D cast magnesium alloy was studied. The results show that the grain size of AZ91D magnesium alloy can be coarsened by adding (0.02% ~ 1.2%) mixed rare earth elements, which makes the primary grain nearly twice longer than that without adding rare earth elements. The reason for coarsening is that the mixed rare earth elements react with al8 (Fe, Mn) 5 or ε – almn phases in the alloy melt, which reduces the heterogeneous nucleation core, and the addition of mixed rare earth elements also reduces the room temperature strength of AZ91D alloy, The results show that the strength of AZ91D alloy at high temperature is increased from 150 ℃ to 200 ℃; the addition of rare earth mixture also makes the Mg17Al12 phase in AZ91D alloy broken and reduced, reduces the cleavage effect of the compound on the matrix, and improves the elongation of the alloy.
4. Effect of mixed rare earth on AZ31B wrought magnesium alloy
The effect of adding (0.1% ~ 1.2%) mixed rare earth to AZ31B wrought magnesium alloy on its microstructure and mechanical properties was studied. The results showed that the grain coarsened significantly and the mechanical properties at room temperature decreased after adding rare earth to AZ31B wrought magnesium alloy. On the one hand, the grain coarsening is due to the combination of RE and Al to form al11re3 phase, which consumes a part of aluminum and weakens the refining effect of aluminum on α – Mg grain. On the other hand, re reacts with ε – almn phase to form al re Mn phase, which reduces the heterogeneous nucleation core in alloy melt, The grain coarsening of AZ31B deformed magnesium alloy caused by Rare Earth shows that the minimum temperature of primary nucleation decreases from 628.8 ℃ to 626.3 ℃ on the thermal analysis curve.
The effect and mechanism of hot cracking resistance of mg Al Cast Magnesium alloy were studied by adding mixed rare earth elements (0.1% ~ 1.2%). The results show that after adding mixed rare earth elements to mg Al alloy, the hot cracking resistance of the alloy decreases significantly. The reason is that rare earth causes grain coarsening of mg Al alloy and reduces the fracture strain required for hot crack initiation, Rare earth reduces the eutectic structure in mg Al alloy, shortens the opening time of dendrite channel, and is not conducive to alloy feeding. Moreover, rare earth also increases the temperature of eutectic reaction, which makes the strength of liquid film between dendrites decrease. When α – Mg dendrites are connected into framework, al11re3 phase distributed between dendrites is easy to block dendrite channel, which increases the difficulty of alloy feeding, The solidification shrinkage of al11re3 phase is different from that of α – Mg matrix, which is easy to produce solidification shrinkage stress and promote the initiation of hot cracks. The influence mechanism of rare earth on the hot cracking resistance of mg Al alloy can be classified as follows: (1) rare earth causes the grain coarsening of mg Al alloy, which reduces the fracture strain required for hot crack initiation; (2) rare earth reduces the eutectic structure in mg Al alloy, shortens the opening time of filling channel, which is not conducive to the alloy’s feeding, and rare earth also increases the temperature of eutectic reaction, The results show that the strength of the liquid film between the dendrites decreases in the later stage of solidification; (3) when α – Mg dendrites join into a framework, the al11re3 phase distributed between the dendrites is easy to block the dendrite channel, which increases the difficulty of alloy feeding; (4) the different cooling shrinkage of al11re3 phase and α – Mg matrix is easy to produce the solidification shrinkage stress and promote the initiation of hot cracks.
5. Effect of mixed rare earth on fatigue properties of die casting magnesium alloy
The effect of mixed rare earth on the high cycle fatigue properties of AZ91D die casting magnesium alloy was analyzed. The results showed that the microstructure of the alloy was refined obviously by adding a proper amount of mixed rare earth. However, with the increase of rare earth addition, al11ce3 phase increased, coarsened and cut the structure. At the same time, β – Mg17Al12 phase decreased, which reduced the strength and plasticity of the alloy. Under the condition of stress ratio r = 0.1 and cyclic base number 107, the fatigue limit increased with the addition of rare earth, but when the addition of rare earth further increased, the fatigue performance showed a downward trend. The results of SEM analysis show that the fatigue fracture of magnesium alloy with rare earth is quasi cleavage fracture.
The microstructure and phase composition of three kinds of magnesium rare earth alloys were analyzed by OM, SEM and XRD. The effect of re on the as cast microstructure, mechanical properties, especially the mechanical properties at high temperature was studied. The magnesium rare earth phases formed by rare earth elements and magnesium are mg12ce, mg17ce2, mg12nd, mg24y5, mg41nd, which are mainly distributed in the grain boundary of as cast structure. The mechanical properties at room temperature and high temperature were tested on the as cast samples of the alloy. The results show that the strengthening effect of Nd is better than that of CE. At high temperature, the joint strengthening effect of Nd and Y is better than that of Nd, while the effect of Ce on high temperature strength is not obvious.
6. Mechanism of flame retardant effect of mixed rare earth on cast magnesium alloy
According to the rule of Hume Rosselli experience, when the relative value of the atomic radius difference is more than 15%, only the solid solution with small solid solubility can be formed. Some rare earths (such as y, La, CE, etc.) have a relative atomic radius close to 15% of that of magnesium. Therefore, the solubility of rare earths in magnesium alloy is very small, which is called “surface active element”. Therefore, when the crystal grows up, the rare earth concentrates in the phase boundary, and has the tendency of surface aggregation in the liquid state. The following reactions will take place in the magnesium alloy with rare earth addition:
2Mg (l) +O2 (g) =2MgO (s)
3MgO (s) +2RE =RE2O3 (s) +3Mg (l)
4RE (l) +3O2 (g) =2RE2O3 (s)
The MgO in magnesium alloy was removed by the reaction. The flame retardant process of rare earth is actually a process of continuous reaction with MgO. The concentration of rare earth on the surface of the melt and the rate of aggregation have a significant effect on the flame retardancy of magnesium alloy. The melt surface is mainly covered by a composite oxide film composed of MgO, RE2O3, etc. The oxidation behavior of metal melts is usually controlled by the performance of oxide surface film. According to Pilling-Bedworth, the oxide density aMgO film aMgO<1 is loose and unprotected. However, the a value of rare earth elements such as CE is more than 1, and it can form a dense oxide film, which is protective and can reduce the oxidation speed of alloy melt, so it has the effect of preventing the combustion of magnesium alloy during melting.