Hong Yang Zhao

Study on Magnesium Alloy Thin Sheet by Symmetric Rolling and Asymmetric Rolling Process

Magnesium alloy is honored as green engineering material for its low density, high specific strength, high specific rigidity, well cutting processing property, well electromagnetic shielding property, heat conduction and easy to recycle. In this paper, AZ31 Magnesium alloy sheet at difference thickness were prepared by symmetric and asymmetric rolling employed with six-roller mill. Microstructure of the two kinds of rolling magnesium alloy thin sheets at 0.5mm thickness were investigated. The grain distribution of AZ31magnesium alloy sheets made by asymmetry rolling at room temperature are more uniform than those made by symmetry process. The grains made by asymmetry are more tiny and the tensile strength and elongation increased obviously and the mechanical properties got better. At room temperature, value of n increased. Large value of n benefit to stamping forming. At room temperature, the value of LDR of asymmetry rolling sheets is 1.26, which was higher than symmetry rolling. So asymmetry rolling benefits to stamping forming.

Study on the Stiffness Compensation Control Method in Twin Roll Strip Casting Process

In this paper, the stiffness compensation is used in the roll-gap controller to improve the reliability of the control system. Simulation research indicates that this method can enhance the control quality than traditional control strategy.

Fuzzy Controller Optimized by Genetic Algorithm for the Molten Metal Level in the Twin Roll Strip Casting Process

In the twin roll strip casting process, the control for the molten metal level is difficult owing to non-linear and time-varying of the control parameters. In the paper, on the basis of the geometry shape of the molten metal pool and the continuity of metal, the level mathematic model is built. Then a fuzzy control strategy is adopted to control the level. Moreover GA(Genetic algorithm) is used to obtain better fuzzy parameters. Simulations show the fuzzy controller optimized by GA greatly improves the control performance.

Thermal Flow-Solidification Simulation and Analysis of Strip Defects in Vertical Twin-Roll Casting of Magnesium Alloy

A quantitative understanding of the twin-roll casting process is required to get high quality as-cast magnesium alloy strips. In this paper, a thermal flow-solidification simulation was carried out to study the behavior of casting zone and its effects on defects generation deeply. Results show that a lower pouring temperature is not suitable for producing defect-free magnesium alloy strips. With increasing of the casting speed, the tendency of cracks formation will getting smaller because of the more uniform temperature distribution. A low pool level leads to a small metal-roll contact area, and a sharp temperature distribution will generates under this situation, which is not good for strips quality.

Modeling of Dynamic Recrystallization Process in AZ31 Magnesium Alloy Using Cellular Automaton Method

A modified 2-D CA model has been developed to simulate dynamic recrystallization behavior of Magnesium (Mg) alloy during hot deformation processing. Based on the fact that Mg has an HCP crystal structure with six-fold symmetry, the model employs the hexagonal CA lattice. The initial microstructure with prescribed grain size was generated by a normal grain growth algorithm. The DRX model consists of dislocation density evolution model, DRX nucleation model and recrystallization grain growth model. DRX grain morphology and size, flow curve were simulated by the present model. The calculated results were compared with the available experimental findings in AZ31 Mg alloy, the predictions show very good agreement with the experimental results.

Study on the Preparation and High Temperature Oxidation Resistance of Mo(Si,Al)2 Layer on the Mo Surface

A layer of Mo (Si,Al)2 was prepared on the sample surface of Mo substrate by powder coating method using Al-Si mixed powder and then diffusion reaction in argon atmosphere at 900°C. The results of oxidation resistance showed that the Mo samples coated with the Mo (Si,Al)2 layer had an excellent oxidation resistance in air even at 1200°C. The reason of oxidation resistance is the formation of a dense alumina film on the sample surface. Therefore, the further oxidization of the sample can be prevented. In addition, The formation of Mo5(Si,Al)3 layer was also observed at the interface between the Mo substrate and the Mo (Si,Al)2 layer.

Optimization of Process Parameters for Unidirectional Solidification of Magnesium Alloy

The unidirectional solidification process of magnesium alloy needs to establish a specific temperature gradient in casting mold, the direction of crystal growth and heat flow are in the opposite direction in the unidirectional solidification. The process can better control the grain orientation, and eliminate the horizontal grain boundary, so to attain columnar grain structure and excellent performance of magnesium alloy. In this paper, Numerical simulation is carried out by orthogonal experiments in order to obtain the optimal process parameters according to the actual experimental device. Different process parameters are taken into account, including pulling speed, cooling time and cooling intensity. FEM (finite element method) is employed to calculate the temperature field and reached a straight shape of temperature gradient distribution which is conductive to achieve unidirectional solidification microstructure. PFM(phase field method) is adopted into the microstructure calculation. The microstructure obtained by PFM is in agreement with the actual pattern by the optical microscope observation.

Electrochemical Behavior of AZ Magnesium Alloy Containing Rare Earth Element

The electrochemical behavior of a new magnesium alloy (AZ61) containing rare earth elements-cerium (Mg-Al-Zn-Mn-Ce alloys) was investigated in 3% NaCl electrolyte using electrochemical methods such as linear sweep voltammetry, Tafel curves and electrochemical impedance spectroscopy. Scanning electron microscopy was used to characterize the surface morphologies of magnesium and its alloys. The results shows that compared with that of the most commonly used Mg alloy–AZ61, the cerium containing magnesium alloy exhibited higher electrochemical activity, and higher corrosion resistance. The electrochemical activity of Mg-Al-Zn-Mn-Ce was higher than that of Mg and Mg-Al-Zn-Mn-Ce alloys in 3% NaCl. The corrosion resistive order decreased in the following sequence: Mg-Al-Zn-Mn-Ce > Mg-Al-Zn-Mn > Mg. The electrolytes favored anodic magnesium oxidation, but the alloying element of Ce facilitated the formation of dense passive films on alloy surfaces.

Roll-Gap Control System of Twin Roll Strip Caster Based on Feed Forward-Feedback

In this paper, the established control system and its control algorism of a new twin roll strip caster developed by authors is presented. It is illustrated the roll-gap control strategy of the twin roll strip caster based on a feed forward-feedback system. From the experimental results, the susceptibility of control convergence time, stable and accurate are shown on a higher level than traditional control strategy.

Microstructure Evolution of AZ31 Magnesium Alloy in Rolling Zone

In order to study the details of deformation and microstructures evolution during rolling and annealing process, magnesium alloy AZ31 blank, cast by unidirectional solidification and then sliced, has been rolled at high temperature with one pass deduction of 50% in a 2-high mill. During the rolling process, the mill was stopped, and the rolled sheet was rapidly cooled by the cold rolls. Samples with full length of rolling bite zone were cut form the semi-rolled sheet. Because twinning deformation can be reserved even the sample was polished and etched. The continuous changes of microstructure, from entrance to exit of roll bite, both rolled and annealed samples, were evaluated by Optical Microscope (OM), Scanning Electron Microscope (SEM). The results revealed that grain refinement mechanism was twinning intersection which segmented large grains into small pieces. The average grain size is 7µm after one pass 50% reduction hot rolling and 350°C, 15min annealing.