Papers by Author: Zi Dong Wang

Abstract: The general mathematical formulation under unidirectional solidification with mushy layer is developed. For the steady state of solidification, concentration and temperature fields in the mushy region are given as functions of a solid volume fraction. The nonlinear model of solidification in mushy zone is changed into system of the second order ordinary differential equation with free boundary conditions, which is solved using numerical calculating method. Solid volume fraction at the solid-mush interface is also obtained. For fixed control conditions, mushy layer width of Fe-Ni alloy in unidirectional solidification is predicted.

Abstract: This paper investigates the interaction between the convective flow induced by the straining flow and the interface morphology of a particle growing in the convective undercooled melt. The resulting approximate solution of the particle shape shows that the convection makes the growing particle enhance its growth velocity and protrude near the surface where the flow is incoming, but decreases its growth near the surface where the flow is outgoing, and both the incoming flow and the anisotropic surface tension make the velocity of the particle have a superposition effect along the favorite crystalline direction. The convection makes the particles growing in the undercooled melt evolve into various shapes of the interface morphology which have high strength/weight ratio and specific surface fraction and then help to form the final material of excellent mechanical and physical properties.

Abstract: A new kind of bainite steel with ultra-low carbon content and Nb, Ti alloys has been developed. By applying thermomechanical control process, water quenching and tempering at different temperature, excellent properties have been obtained when tempered at 450°C, with the yield strength of 813MPa and elongation of 16.2%. The morphology observed by SEM shows that the microstructure consists of fine lath-shaped bainite, polygonal ferrite, quasi-polygonal ferrite and a small fraction of residual austenite or martensite-austenite constituents. In a TEM study plenty of precipitates with the size about 5-10nm were observed interacting with the dislocations, which is very significant for the optimization of strength and ductility.

Abstract: Pure carbon, pure iron and single crystall silicon were firstly loaded into a pure graphite crucible to obtain a hypereutectic Fe-C melt by carburizing at high temperatures in this paper. The powder of oxide La₂O₃ and CeO₂ were added respectively to the melt under the same process conditions and cast iron samples were obtained. The graphite morphology of the samples was observed under optical microscope and SEM. Matrix of the samples were etched by electrolysis way. At last non-ferrous residue on the surface of cast iron samples was gathered, calcined,observed under SEI and BEI and analysed with EDS to find the trace of oxide La₂O₃ or CeO₂. The results showed that oxide La₂O₃ or CeO₂ was really added into the melt but the morphology of graphite did not change. The only effect of the oxides was making the amount of graphiteto increase slightly. All the results of this paper indicat that the oxide La₂O₃ or CeO₂ is possibly the base for graphite flake to precipitate but is not a sufficient condition for graphite to nodularize.

Abstract: The vibration noise control is critical in engineering fields and the phononic crystal provides a new mean to control the vibration noise. The band gap structure of one dimensional phononic crystal is studied in this paper. By using the equivalent masses method we obtain the band gap structure which depends on M, m and β, the influences of initial frequency of vibration band gap via the material density, the diameter of rod and the lattice constant are analyzed. The results show that the low-frequency broadband gap characteristic is obtained by increasing M and reducing m, β. The initial frequency of vibration band gap decreases as the metal material density, the diameter of metal rod or the lattice constant increase. Some conclusions from numerical calculation are examined by the vibration experiment.

Abstract: Thin walled copper tube is prepared in vacuum melting and argon protecting continuous unidirectional solidification equipment by a short process method. The size of the thin walled copper tube is ф39×1.8 mm. The effect of process parameters on the solidification structure and the surface quality of the thin walled copper tube is investigated. The mechanical properties and electrical properties are tested and analyzed. The results show that the thin walled tube with columnar crystal structure and good quality surface can be obtained continuously and stably under the most appropriate process of the melting temperature is between 1423K and 1453K, the cooling water volume is 0.2～0.5m3•h-1, the cooling distance is 50mm, the drawing speed is not higher 20m•h-1. The elongation can reach 60%, and conductivity can reach 102.6%IACS.

Abstract: The liquid-conveying elbow is an very important part in the piping system, in this paper, the internal flow field in the pipeline elbow is studied, its structure is optimized, and the guide characteristic is improved. The results show that the internal total pressure difference in the original elbow is 29506.27 Pa, the Max-velocity is 7.12 m/s, and the internal flow field is very uneven；The internal total pressure difference can be greatly reduced through setting up the guide plates appropriately, its internal total pressure difference decreases as the number of guide plates increases, the Max-velocity decreases first and then increases as the number increases; When the number is a constant, the optimal result of guide plate according to the gold ratio arrangement is superior to the equidistance ratio arrangement; When the export length of guide plate is 50 mm, the total pressure difference is 14942 Pa in the elbow, comparing it to the result of the original elbow, it decreases 49.26%, the optimal design has an very obvious effect.

Abstract: The Mg-Zr alloys with the Zr contents of 0.2%, 0.4%, 0.6% and 0.8% were prepared using induction melting, metal mold centrifugal casting process. The microstructures of the alloys were analyzed by scanning electron microscope, energy spectrum analysis, transmission electron microscope and their mechanical properties were also measured with tensile test. The results show that there are many micro- and nano-sized Zr-rich or Zr particles dispersed in Mg-Zr alloys. The nano-sized Zr-rich particle consists of a rectangle shaped Zr core and its surrounding Mg (Zr) solid solution shell with round exterior; a single Zr particle appears a coherent lattice relationship with matrix.

Abstract: The Screw Down System (SDS) is a typical hydraulic servo control system. It is the key actuator of cold rolling mill. The dynamic performance of SDS has a significant effect on the accuracy of thickness. Hydraulic servo control system is not always optimal and stable due to highly nonlinear and parameter uncertainties. In this paper, the most important nonlinear relating flow and pressure is analyzed. A nonlinear self-adaptive compensation method is designed. Modeling and simulating the hydraulic servo control system of SDS using this method, the numerical simulation results show that the maximum overshoot and static error of the compensated system are more less than that of without nonlinear compensation. The overshoot of the compensated system is less than 10% when the cold strip is rolling. So the thickness tolerance of the cold strip can be guaranteed easily. This self- adaptive compensation method can be used to other hydraulic servo control systems.

Abstract: In the crystal growth process, the temperature distribution and concentration distribution at the solid-liquid interface edge are always the hot problems. In this paper, we study the concentration distribution at the solid-liquid interface edge under the natural convection conditions, we find that the concentration field is oscillating exponential decline or rose along the crystal growth direction. We also study the dendrite morphology of Al-La alloys using the experimental method, the results show that the microstructure of Al-35%La alloys is different from the common microstructure of hypereutectic alloy during the conventional casting process, the first crystalline phase is Al11La3, which composition is discontinuous along the growth direction, the main dendrite is composed of α-Al alternating with Al11La3, the results of SEM and XRD show that the chemical composition along the main dendrite exhibits periodic behavior, therefore, this microstructure is named as periodic diphase dendrite structure.