Papers by Author: Hong Yan

Abstract: Compression behavior of a novel type of SiC/Al co-continuous composites was investigated using the finite element method and a new constitutive model were presented in this paper. The results show that the new constitutive model of the co-continuous composites can predicate the compression behavior of the composites accurately according to the simulative data. The fitting curve of the compression behavior achieved from the constitutive formula accords with the simulation results, which improves the reliability of the simulation on composites.

Abstract: A rich variety of two-phase microstructures have been shown to form in the directionally solidified Pb-30wt.%Bi alloy. The selection of microstructure is shown to be depended upon the value of G/V (temperature gradient in the liquid divided by growth rate). The microstructure evolution is also discussed based on the competitive growth of different phase at various G/V ratios in directional solidification. Experimental results indicate that the primary a phase is the leading phase and grows with a dendritic/cellular microstructure while the peritectic b phase grows with cellular/planar microstructure in interdendritic/intercellular region at low G/V value. In contrast, competitive structures but not the discrete banded structure is present at high G/V ratio which makes the primary and secondary phase to grow with planar/cellular interface.

Abstract: A new constitutive model for semi-solid composites forming was derived with analyzing constitutive relationships of semi-solid metal and composites forming. Parameters in the constitutive model were determined by the multiple nonlinear regression method. A constitutive relationship of semi-solid SiCp/AZ61 composites thixotropic plastic forming was proposed. The calculated results were good agreement with the experimental ones. The proposed constitutive model has the higher forecast precision and practical significance. The construction of constitutive model provides references for the thixotropic forming theory, simulation and technology.

Abstract: Semi-solid magnesium alloy slurry was produced by a newly self-developed mechanical stirring machine in order to prepare continuous semi-solid slurry. The changing principle of semi-solid microstructure was studied under different conditions—the changing of temperature, stirring rate, cooling rate and stirring time. It was showed that the lower the temperature of slurry, the higher the stirring rate, the longer the stirring time and the faster the cooling rate, the grain will be smaller and more evenly rounded. When the stirring speed and temperature are the same, better semi-solid spherical grains can be gained by increasing the stirring time. At the same stirring temperature and higher stirring speed, semi-solid spherical particles are smaller, rounder and more evenly; at a low stirring speed, we can't get ideal semi-solid spherical grains even if we increase the stirring time. The ideal technological parameters of semi-solid AZ61 alloy produced with no-dendritic structure were recommended to be 600°C～605°C (stirring end temperature) 、3min～ 5min(stirring time) and 500r/min(stirring rate). Compared with conventional casting alloy, semi-solid alloy mechanical properties were greatly improved. The non-dendrite semi-solid forming mechanism was studied; mechanical stirring semi-solid non-dendritic formation was the result of a variety of factors, the key cause is mechanical shear stress.

Abstract: The semi-solid AZ61 wrought magnesium alloy was fabricated by mechanical stirring method. Thixo-forging set-up were designed and made. The traditional forging and thixo-forging processes of AZ61 magnesium alloy were studied with both computer numerical simulation and experimental methods. The constitutive model of semi-solid AZ61 alloy was established in our prior literature. The differences between traditional forging and thixo-forging processes were analyzed. Results indicated that smaller load and uniform strain & stress were acquired in thixo-extrusion process. Simulation results were good agreement with experimental ones.

Abstract: SiCp/AZ61 composites were fabricated by a semi-solid stirring method. The effects of volume fraction of SiC particles, stirring temperature and stirring time on the mechanical properties of SiCp/AZ61composites were studied with an orthogonal experimental method. The results indicate that volume fraction of SiC particles has a remarkable influence on the mechanical properties of SiCp/AZ61 composites, the second were stirring time and stirring temperature. In this experiment condition, the optimum technological parameters of SiCp/AZ61 composites fabricated by semi-solid stirring method were volume fraction of SiC particles 6%, stirring temperature 595°C and stirring time 5 min. Fractographies show that the fracture morphology of SiCp/AZ61 composites with volume fraction of SiC particles 6% is brittle.

Abstract: The microstructural evolution of SiCp/AZ61 composites during partial remelting was investigated. The results indicated that the suitable technological parameters of SiCp/AZ61 composites were the reheating temperature of 595°C ~ 600°C and isothermal holding time of 30min ~ 60min. The separation of microstructure of AZ61 matrix was faster than that of SiCp/AZ61 composites during the initial stage. In the meantime, SiCp/AZ61composites hold high stability during remelting process and a fine semi-solid thixoforming microstructure can be obtained with increases in temperature and holding time. In addition, the samples of SiCp/AZ61composites were susceptible to serious deformation and running out above 610°C.

Abstract: The perfect semi-solid magnesium alloy had been prepared using ultrasonic stirring method. The effect of technological parameters such as melt temperature, ultrasonic power and ultrasonic treatment time on the microstructure of semi-solid AZ61 alloy had been investigated. The results showed that the better refined and well-distributed globular semi-solid magnesium alloy had been acquired with the increasing of output power of ultrasonic stirring when melt temperature and ultrasonic treatment time were at a certain range.

Abstract: Base on the coupled three-dimensional model of flow temperature and apparent viscosity on semisolid materials, the mould filling processes of liquid and semisolid magnesium alloys have been analyzed. The simulation results showed that the semisolid mould filling with the characteristic of pseudo-plasticity and Bingham type is steady and the air entrapment is much smaller than that of liquid die casting. Therefore, the semisolid die casting can prevent from those defects produced by the traditional die casting, which offer a new technology for lower interior defects and better quality.

Abstract: The traditional extrusion and thixo-extrusion processes of wrought magnesium alloy were studied with both computer numerical simulation and experimental methods. The thixo-extrusion set-up was made. The constitutive model of semi-solid AZ61 alloy was established in our prior literature. The comparison of two forming processes had been done. Results indicated that smaller load and uniform strain & stress were acquired in thixo-extrusion process. Simulation results were in good agreement with experimental ones.