Papers by Author: Xue Lei Tian

Abstract: Plasma etching, SEM and quantitative metallographic analysis technology were used to study the influence of Si on nucleation and growth process of spherical graphite in Ni-C alloys solidification microstructure. Pure Si, Ni-Si alloys and Fe-Si alloys were added into Ni-C melts respectively, and the solid microstructures were compared. The density, diameter size and roundness of spherical graphite were studied in this work to reveal the influence of Si on the morphology of graphite. The Ni-C alloy was etched by plasma etch, and the morphologies and composition of graphite nucleus were observed by scanning electron microscope (SEM) and electron energy spectrum analysis (EDS). The results suggested that the high Si melt zone in melt strongly promoted the enrichment and carbide of graphite, which both promoted the nucleation and growth of graphite. The change in Si content had a large effect on the roundness of the graphite, the higher the Si content, the better the roundness of the graphite.

Abstract: A mathematical physical model of microstructure evolution in Al-Si eutectic solidification process based on cellular automaton (CA) model was developed. Before the establishment of the model, the relevant near-eutectic experiments were carried out to analyze the effect of cooling rates measured by temperature curves on the eutectic structure which was observed through optical microscope (OM) and scanning electron microscope (SEM). Then a multiphase nucleation-growth CA model was applied to simulate the Al-Si irregular eutectic structure. The model adopted an alternative nucleation mechanism to investigate the influence of the critical nucleation value associated with solute concentration during solidification process. The growth kinetics took into account the solute and thermal field. According to the crystal structure of nonfaceted eutectic Al and faceted eutectic Si, different capturing rules were employed to calculate the growth of eutectic. In addition, the model was also used to research the irregular eutectic growth under different undercooling conditions. The results revealed that smaller critical nucleation value (absolute value) or higher eutectic undercooling tended to get a more refined eutectic microstructure. By compared with experimental results, it is indicated that the microstructure evolution of Al-Si eutectic growth can be reproduced quantitatively by numerical simulation with this model.

Abstract: This paper presents the relationship between the carbon atom diffusion coefficient in the austenite and the temperature during the nodular cast iron solidification under different cooling rates or with different carbon contents. Pouring the wedge-shaped casting explores the influence of cooling rate on the diffusion coefficient. The other part explores the change of the diffusion coefficient with different carbon contents by water quenching to save the organization in the solidification. Results show that both the cooling rate and the carbon content can affect the diffusion coefficient, and it decreases as the cooling rate increases. More attempts were also done to correlate the diffusion coefficient with the temperature in different carbon content. It has been found that the diffusion coefficient decreases as the temperature increase.

Abstract: The objective of this paper is to introduce a simple experimental apparatus based on the applied forces for quantitative assessment on hot tearing behavior in aluminum alloys. According to the experimental procedure, molten metal is cast in the rod-shaped mold cavity. One side of the casting specimen is hooked by a steel bolt which restrains its free contraction and transfers the tensile forces during solidification. A steel threaded rod connected to a load cell which records the real-time measurement of the tensile forces during every experiment. Thermal history is monitored by K-type thermocouple. The data of the temperature and tensile forces are acquired by a data acquisition system. Test of the experimental apparatus is conducted with A356 and Al-5wt%Cu alloy to investigate the accuracy of the experimental apparatus and modify its operating parameter. The tensile forces curves and the temperature curves of the specimens are obtained by experiment. This data provide useful information about hot tearing formation and solidification characteristics, from which their quantitative relations are derived. In this manner, the hot tearing behavior in aluminum alloys can be studied by this experimental apparatus based on the applied forces.

Abstract: Based on the source code of 3D modeling software called Sinovation, the user interface is developed by Visual Studio 2005, then 3-demensional computer aided design (CAD) module of gating system for iron castings is built by the Script language of SINOVATION. The CAD module has a friendly interface, quick and accurate calculation, and is able to design complicated gating system. As the efficiency and accuracy for gating system designing are improved,

Abstract: The liquid structure of Ni₂Sb₉₈ eutectic alloy is studied with the method of X-ray diffraction and measurements of viscosity in this paper. Covalent bonding structures, which are characterized by the shoulder on the high- Q side of the first peak of structure factors, are observed over the full measuring temperature range. And the split of second peak of the pair correlation function suggests that the chemical short-range order structure exits in the melt. Correspondingly, variation of the viscosity obeys the Arrhenius law except the abrupt point near 1025K and the electrical resistivity changes abnormally at about 1020K. In addition, the coordination numbers prove that the A7-like structure is almost destroyed in liquid Ni₂Sb₉₈ alloy. This research is aim to provide new insight into the structure evolution of liquid alloys during the cooling process.

Abstract: Two S-type thermocouples connected to paperless recorder are used to record the temperature change during solidification of the wedge spheroidal-graphite casing. The cool-down time and eutectic temperature are obtained from the cooling curves which are calculated by Pro CAST software. Aided by the image-pro plus software, graphite nodules’ mean diameter and amount can be obtained. Carbon diffusion coefficient of graphite nodule is calculated according to the spherulite growth model. The result is in good agreement with the reference date which shows the validity of parameters. It could provide the parameters for simulation of casting solidification.

Abstract: The residual Mg content ,spherodize ratio and nuclei of spherodial graphites(abbreviated SG) were studied by casting a series of similar composition ductile iron rods and white samples with different holding time of spheroidizing. The results indicate that the residual Mg content in the ductile iron was less and less as the holding time grows ,which causing the morphology of graphites transfer from spherodial to oval. Spherodize ratio change law is not continuous as increasing the residual Mg content which is divided at the 0.02%. The morphologies of nuclei of SGs core are typical examples of the highly magnified structure of spherical and irregular polygons; the diameters are about 0.5~2μm, consisting of (Mg, Ca)S along with CeO₂ and MgO. Disregistry δ between the nucleus compounds and graphite are low,which indicates that nucleus compounds are good nucleating site for SGs.The Mg element in nuclei of SG just act as nucleation substrates.it is the Mg which maybe simple substance in the metallic matrix that act as the role of spherodizer determines the graphite shape.

Abstract: Structural analysis software for amorphous alloy has been developed. The structure factor, distribution function, the first coordination number, the first coordination radius, the correlation radius, the atomic number of cluster and the atomic average density of amorphous alloy can be gotten with this software. This software’s runtime stability and calculated accuracy was verified by its practical application. Structure of amorphous Zr₄₁Ti₁₄Ni₁₀Cu_12.5Be_22.5 alloy was exactly analyzed using this software, which proved that the design of the software is precise and the calculation is exact.

Abstract: Shrinkage porosity is often found in Spheroidal graphite iron (S. G. Iron) castings because of the mushy zone and special volume change during their solidification. Although the volume expansion is very important to the shrinkage porosity simulation of S.G. Iron castings, conventional methods for predicting the porosity defects do not consider it. A Series of macro-micro models such as macro heat transfer calculation and microstructure formation simulation are proposed to simulate the solidification of S. G. Iron castings. The nucleation and growth models are employed to calculate the accurate latent heat and volume change especially graphite expansion during the solidification. The pressure induced by graphite expansion is introduced as a parameter to predict the shrinkage porosity and a new shrinkage porosity criterion is developed. Cooling curves and solid fraction of each phase are compared with experimental castings. At the same time, the porosity area ratio of castings is compared with the results calculated by several porosity criterions. The results show that the new shrinkage porosity simulation criterion of S. G. Iron castings based on macro-micro models is accurate on shrinkage porosity shape, size and distribution simulation.