Mg-Nd-Zn alloy is one of the most widely used magnesium rare earth alloys. In this paper, Mg nd Zn alloy semi-solid slurry was prepared by electromagnetic stirring. The formation mechanism of non dendritic primary particles in the slurry was studied, and the effect of electromagnetic field on the thermodynamics of alloy melt was discussed. The effects of squeeze casting temperature on the rheological properties of the alloy were investigated The strengthening mechanism of the alloy with different forming methods was discussed. In addition, in order to improve the high-temperature mechanical properties of Mg nd Zn alloy, a small amount of Y element was added to the alloy for alloying, and the effect of Y element on the microstructure and mechanical properties at room temperature and high temperature was explored. Finally, the alloy with higher comprehensive mechanical properties was optimized and rheo squeeze casting was carried out. The main conclusions are as follows
1. The formation mechanism and evolution mechanism of non dendritic primary particles in mg nd Zn alloy melt during electromagnetic stirring (under the action of rotating magnetic field) were investigated, and the effect of Zr particles on the microstructure of semi-solid non dendritic slurry was clarified. The effects of different stirring parameters on the semi-solid microstructure of mg-3nd-0.2zn (nz30) and mg-3nd-0.2zn-0.4zr (nz30k) alloys were investigated by electromagnetic stirring. The results show that homogeneous solute solubility field and temperature field are formed in the alloy melt after electromagnetic stirring The primary phase of alloy slurry changes from dendrite to three kinds of particles with different morphologies: spherical, rosette and fine dendrite. The evolution process is nucleation, dendrite fusing and subsequent ripening. Combined with the criterion of dendrite fusing, the formation mechanism of primary phase in the process of electromagnetic stirring can be well explained. The results show that the electromagnetic stirring can not only improve the distribution of Zr particles, but also increase the content of active Zr particles and promote the nucleation of nz30k alloy slurry. Finally, the ideal semi-solid structure of fine and round primary particles uniformly suspended in liquid phase can be obtained. The evolution process is nucleation and subsequent ripening. By comparison, it can be found that the main reason for the formation of primary particles in mg nd Zn alloy is the fusing of dendrites and the direct formation of fine and round primary particles in the melt.
2.The influence of electromagnetic stirring parameters on the alloy melt was revealed. Based on the thermodynamic differential equation, the mathematical relationship between the process parameters (voltage, frequency) and the melt undercooling, melt temperature was established
When other parameters remain unchanged, increasing the voltage or decreasing the frequency will increase the undercooling of the alloy melt. With the increase of undercooling, the critical nucleation radius and nucleation work decrease, and the nucleation rate and nucleation quantity increase.
3.The process parameters of electromagnetic stirring for Mg nd Zn alloy melt were optimized. The variation of solid rate with stirring time can be described by a linear equation, which can be used to control the solid rate in rheological forming process. The relationship between primary particle size and stirring time can be described by aging equation. With the increase of voltage and frequency, the particle size and shape coefficient first decrease and then increase. Based on this, a new electromagnetic stirring rheo squeezefor Mg nd Zn alloy was developed, and the optimum process parameters were obtained. For nz30 alloy, the optimal process parameters are 120-180s, 300-350v, 20Hz. The optimum process parameters are 180v-30hz and zn30s.
4.The effect of die temperature on the casting and mechanical properties of rheo squeeze casting nz30k alloy was investigated. With the increase of die temperature, the grain size of nz30k alloy increases gradually, the dislocation density decreases, the eutectic phase is finer and the secondary solidification structure decreases. In addition, as the density of the alloy increases, the microstructure becomes more and more compact. The mechanical properties of the alloy increase gradually from 200 ℃ to 300 ℃. However, with the further increase of die temperature, the particle size increases and the mechanical properties decrease. The strengthening contribution of yield strength of as cast alloy mainly comes from grain boundary strengthening and primary second phase strengthening. After T6 treatment, the mechanical properties of the alloy first increase and then decrease with the increase of die temperature. The strengthening contribution of yield strength mainly comes from grain boundary strengthening and precipitation strengthening.
5.The effect of pressure on the casting and mechanical properties of rheo squeeze casting nz30k alloy was investigated. Based on Clausius Clapeyron equation, the mathematical expressions of pressure, undercooling and nucleation rate are derived
It is found that the nucleation rate of nz30k alloy increases with the increase of pressure when the undercooling is less than 13.8 ℃. Under the experimental conditions, with the increase of pressure, the nucleation rate of alloy melt increases, the solid solubility of Nd and Zn in α – Mg increases, and the dislocation density increases. The results show that the mechanical properties of as cast alloy first increase and then decrease with the increase of pressure. The main reasons for the increase of mechanical properties are grain refinement and microstructure densification. The main reasons for the decrease of mechanical properties are the plastic deformation between primary particles and the large stress concentration. The tensile strength and yield strength of the alloy can reach 309 MPa and 165.5%, respectively.
6.The strengthening mechanism of Mg nd Zn alloy formed by different processes (metal mold casting, squeeze casting and rheo squeeze casting) was revealed. Compared with the metal mold casting alloy and the conventional squeeze casting alloy, the grain size of rheo squeeze casting alloy is smaller, the eutectic structure is smaller and less, and it is evenly distributed among the particles, so the comprehensive mechanical properties of rheo squeeze casting alloy are higher. After T6 treatment, the precipitates in rheo squeeze casting nz30k alloy are mainly β″ phase and β′ phase, and the precipitates in metal mold casting alloy and conventional squeeze casting alloy are mainly β′ phase. Due to the grain refinement effect, the mechanical properties of the alloy prepared by rheo squeeze casting are higher than those prepared by metal mold casting; due to the large precipitation strengthening effect, the mechanical properties of the alloy prepared by rheo squeeze casting are higher than those prepared by conventional squeeze casting.
7.The effect of trace y element on the microstructure and mechanical properties of Mg nd Zn Alloy by rheo squeeze casting was investigated. With the increase of Y content, the average grain size of as cast alloy obtained by two forming methods decreases. In addition, a small amount of mg24y5 phase is formed in the alloy containing y, which is uniformly distributed at the grain boundary of the alloy containing 0.39 wt.% Y (0.12 at.% y). The aging strengthening effect of the alloy increases with the increase of Y content. After aging treatment, the number of long rod like zn2zr3 phase in the alloy with 0.39 wt.% y content increases significantly, and the number of long rod like zn2zr3 phase in the alloy with 0.39 wt.% y content is more than that in the alloy without y content due to the segregation of Zn atoms.
8.Under all experimental conditions, the alloy with 0.39 wt.% y content has the best comprehensive mechanical properties, especially in the range of 200-300 ° C, the metal mold casting alloy has higher yield strength (about 150 MPa). The improvement of mechanical properties is mainly due to grain refinement, solution strengthening, zn2zr3 phase and β′ phase strengthening. In addition, Arrhenius model was established to describe the effect of deformation temperature and strain rate on the flow stress of 0.39 wt.% y alloy in metal mold casting: σ = (169.69 ̇ (223045 )) 0.098. The strain index and activation energy of the alloy are higher than those of the alloy without y element.
9.Compared with the metal mold casting alloy, the 0.39 wt.% y alloy has finer grains and better mechanical properties. After T6 treatment, the yield strength, tensile strength and elongation of the alloy can reach 168 MPa, 329 MPa and 8.7% respectively.