Papers by Author: Hai Dong Zhao

Abstract: The mathematical model for microstructure formation and porosity prediction during solidification process of SG iron casting was established and applied to a practical brake housing casting. Quantitative microstructure analysis of specimens machined from the castings was compared with the simulation, and the two results are in acceptable agreement on nodule counts and size, pearlite fractions and hardness. It is indicated that the model can calculate the fraction of ferrite and pearlite more accurately, and specially can reflect the effect of both under-cooling during solidification and the nodules formed in eutectic period on the pearlite content. The present porosity prediction was compared with those of a former method and commercial software, which leads to that the current methods used for porosity prediction should be investigated and improved further.

Abstract: To decrease weight and increase fuel efficiency, an Al-Cu-Mn alloy frame part with vertical indirect squeeze casting process was fabricated to replace original parts with spheroidal graphite iron, which is used in a special vehicle. Mold filling of the frame with pouring temperature of 700°C and injection velocity of 0.03m/s was simulated. According to the results vents and overflows were designed and the dies were produced with H13 steel. Furthermore, the casting trial producing was conducted. Cross sections of the castings were checked. A number of specimens were obtained from different parts of the castings to evaluate microstructure, and both high dense and fine grain microstructures were found in the specimens. Mechanical properties of the tensile samples with T5 heat treatment were measured, and the average tensile strength and elongation were 387 MPa and 8.8%, respectively.

Abstract: Microstructure and tensile properties of as-cast and T6-treated Al-4%Cu-3%Si alloy with different applied solidification pressures were investigated. The pressure has significant effect on the formed microstructure. It is observed that as the pressures increase, the porosities were removed, and the size and the secondary dendrite arm spacing of primary α-Al grains decreased evidently, and the morphology of eutectic Si particles changed from long acicular to small granular type. Hence, the tensile properties of the alloy were improved. The research has shown that the aspect ratio (A.R.) of the Si particles decreased after the solution and aging treatment. The A.R of the Si particles in the alloys with applied solidification pressure decreased more remarkably and this indicated that the Si particles can be more prone to be dissolved into α-Al matrix. Thus, the increase of the elongation was achieved.

Abstract: Microstructure and impact properties of slow injection A356 die castings with local pressurization have been investigated. The microstructure in the casting different regions were analyzed and compared. The casting impact absorbed energy varies from 1.17 to 2.35J, and is higher than that of other pressure die casting process. Furthermore, fracture mechanisms of different regions in castings are discussed. The results show that, the impact absorbed energy decreases with increase in SDAS. Also significant fluctuations of the impact properties in the thick-walled and local pressurization regions are found.

Abstract: Double-stream-pouring continuous casting (DSPCC) is a novel technique to prepare gradient composites. In this paper, a two-dimensional steady mathematical model coupled flow, heat and solution transport in the solidification zone of DSPCC has been established based on the standard k-ε turbulent model. The variation of thermo-physical properties of the alloys against the temperature is also considered. The effects of casting speed on the temperature field, velocity field and composition of the researched composite are analyzed based on the proposed mathematical model. An experimental 2024/3003 aluminum composite was prepared by using a laboratory-made DSPCC facility. Selected results of experiment are in a good agreement with that of numerical simulation.

Abstract: To describe the shape and location of free surfaces in the mold filling simulation, parameters including volume filled ratio, surface dimensionless distance, and surface filled ratio for the DFDM (Direct Finite Difference Method) elements were proposed. A model of mold filling process was established, considering the mass, momentum and heat transfer in the vicinity of free surface. It had been applied to an experimental Al casting. In-situ observation and record of actual mold filling process of the casting with special X-ray apparatus were carried out, and were used to validate the simulation results. The validation showed that the model could predict the mold filling process well. It was found that back pressure of gas in the mould cavity had minor effect on change of liquid flow when using pressurized feeding system and sand mould with good air permeability.

Abstract: The fabrication of a large Al-4.5wt%Cu-0.5wt%Mn wheel with 670mm diameter by indirect squeeze casting process was studied. The casting system including modified hydraulic press and die structure was introduced, and the casting procedures was designed and described. The filling behavior of the casting process was simulated with software Flow3D. It was found that while flowing from spoke to rim, turbulence flow of liquid melt in the vicinity of free surface was found, and that at the end of the filling, the unwanted solidification might occur. To reduce the turbulent flow and the unwanted solidification, a modified injection condition was proposed. The simulation results with the modified injection indicated that the turbulence was prevented and the unwanted solidification during the filling was reduced as well. The squeeze wheel castings were fabricated with the modified injection condition. Cross sections of the castings were checked. A number of specimens were obtained from different parts of the castings to evaluate microstructure of the wheel, and both high density and fine grain microstructures were found in the specimens. Mechanical properties of the tensile samples from the wheels with T5 heat treatment were measured. The average tensile strength and elongation were 390 MPa and 10%, respectively.