Papers by Author: Hua Hou

Abstract: The Phase Field model of solidification processes was carried out coupled with temperature field model. The influence of interface atomic time on dendrite growth morphology in undercooled melt was simulated with pure nickel. The experimental results show that when the interface atomic motion time parameter is minor, the liquid-solid interfaces were unstable, disturbance can be amplified easily so the complicated side branches will grow, and the disturbance speed up the dendrite growth. With the increase of , the liquid-solid interfaces become more stable and finally the smooth dendrite morphology can be obtained.

Abstract: The Explicit Finite Difference (EFD) method is used for calculating the energy conservation equation during solidification. In order to improve the computational efficiency, the equivalent specific heat method is adopted to calculate the latent heat and the high order Alternating Direction Implicit (ADI) method is also applied, which is fourth order in space and second order in time. The degree of similarity between the simulation results and experimental results is analyzed quantitatively by the Hamming Distance (HD) for the first time, and results show that this high order mathematical model based on the equivalent specific heat method and the high order ADI method is faster and more accurate than the EFD method.

Abstract: In this paper, according to the characteristics of squeeze casting solidification process, the calculation model (FDM format) of the partial differential equations with high thermal conductivity is used to the numerical simulation of temperature field. Dynamic isolated multi-molten pool judgment method is used to determine the position of the pool and FEM is used to calculate the pressure of pool center. If the pressure of molten pool center has been down to 0, the liquid metal closed in the scale will be solidification under the condition of no pressure, and will shrinkage based on the way of gravity shrinkage. The equivalent liquid surface descending method of isolated molten pool is used to predict the formation of shrinkage defect; the simulation result is coinciding with experimental data.

Abstract: A new FDM/ FEM model is developed to simulate the temperature field during the solidification process of squeeze casting. So we can transform the FDM mesh into FEM mesh directly ,then established the relationship of pressure change and melting temperature change and correct the size of melting temperature value and other thermal physical parameters (such as the thermal conductivity)which is related to the temperature ,and establish the temperature and thermal physical parameter relationship to get a data base. The solidification process of AM50A magnesium alloy is simulated. Squeeze casting experiments are also incited for validating the new FDM/FEM model. It is shown that the results of numerical simulation are in agreement with the experimental results.

Abstract: The dendrite growth process was simulated with the Phase Field Model coupled with the fluctuation. The effect of fluctuation intensity on the dendrite morphology and the thermal fluctuation together with the phase field fluctuation on the forming of side branches were investigated. Result shows that with the decrease of thermal fluctuation amplitude, the furcation of dendrites tip also decreased, transverse dendrites become stronger and lengthways dendrites becomes degenerate, Doublon structure disappeared, finally a quite symmetrical dendrites structure formed. Thermal fluctuation can result in the unsteadiness of dendrites side branches, it is also the main reason of forming side branches, yet phase field fluctuation has little contribution to the side branches, it is usually ignored in the calculation; when the value of F_u is appropriate, the thermal noise can lead the side branches, but cannot change the steady behavior of the dendrites’ tip.

Abstract: The microscopic phase field approach was applied for modeling the early precipitation process of Ni₇₅Al_xV_25-x alloy. Without any a prior assumption, this model can be used to simulate the temporal evolution of arbitrary morphologies and microstructures on atomic scale. Through the simulated atomic pictures, calculated order parameters and volume fraction of the θ (Ni₃V) and γ′ (Ni₃Al) ordered phases, Ni₇₅Al_xV_25-x alloys with Al composition of 0.05, 0.053 and 0.055 (atom fraction) were studied. Results show: For these alloys, θ and γ′ precipitated at the same time. With the increase of Al content, the amount of γ′ phase is increasing and that of θ phase is decreasing; the precipitation characteristic of γ′ phase transforms from Non-Classical Nucleation and Growth (NCNG) to Congruent Ordering + Spinodal Decomposition (CO+SD) gradually, otherwise, the precipitation characteristic of θ phase transforms from Congruent Ordering + Spinodal Decomposition (CO+SD) to Non-Classical Nucleation and Growth (NCNG) mechanism gradually. Both θ and γ′ has undergone the transition process of mixture precipitation mechanism with the characteristic of both NCNG and CO+SD mechanism. No incontinuous transition of precipitation mechanism has been found.

Abstract: A research program was simulated to study the effects of pouring temperature, squeeze pressure and die temperature on the tensile, elongation and hardness properties of AZ91D magnesium alloys using anycasting software. The curves with different processing parameters on mechanical properties have been painted. The results indicated that mechanical properties increased firstly, then decreased when the pouring temperature increased to 670°C, and gradually increased with the increasing of squeeze pressure. The affect laws of mould temperature are similar as ones of pouring temperature. Eventually found that the squeeze casting got better mechanical properties(σb= 225MPa, δ= 3.6%, Vickers hardness=62) on the pouring temperature 670°C, mold temperature 180°C, holding pressure 120Mpa, pressure duration 25s.

Abstract: Recently, the surface mounting devices (SMD) have been rapidly developed for miniaturisation of electronic applications such as cellular phones, cameras, computers, etc. Low temperature sintering NiCuZn and MnCuZn ferrite was employed at most cases due to its co-firability with Ag (below 960OC).The purpose of this study is to fabricate NiCuZn and MnCuZn ferrite sintered body with high-strength and high-frequency magnetic properties. Following is the procedure: firstly, NiCuZn and MnCuZn ferrite powder were synthesized under CO2 atmosphere at 500 OC from the mixed doxalate synthesized by liquid phase deposition method; then a small amount of boric acid [H3BO3] was added to the powder, and the NiCuZn and MnCuZn ferrite powder compact were prepared with Newton press and CIP methods; finally, NiCuZn and MnCuZn ferrite sintered body was fabricated by sintering at 900 OC under CO2 atmosphere. The effect of boric acid additives on growth of particle and sintering temperature were discussed. The high frequency magnetic properties and density as well as bending strength of the NiCuZn and MnCuZn sintered magnet with various adding of H3BO3 as well as at various sintered temperatures were evaluated by using Impedance analyzer, TMA (Thermo Mechanical Analyzer), TEM (Transmission Electronic Microscopy) and 3-point bending test. From the evaluated results, the most suitable producing conditions were determined and NiCuZn ferrite with wider scope of high frequency and high bending strength were obtained.

Abstract: Based on the solidification features of ductile iron and affecting factors for ductile iron shrinkage defect, the model of the ductile iron solidification is built and put forward a new defect predictive method EIECAM (Enclosed-Isolated area Expansion and Contraction Accumulation Method) model to predict defect. in DECAM, the liquid shrinkage, solidified shrinkage and graphitizing expansion during solidification are computed dynamically in the enclosed-isolated area , and the effect of graphite expansion on the wall movement is also accounted. Based on this method end cover of QT500 ductile iron casting is simulated and made the defect predictive, study its solidification process and the defect generation position, and make the experimental identification on the defect. It is resulted that the method can be able to predict the casting defect authentically.

Abstract: With the microscopic phase-field model, the early precipitation mechanisms of the ternary Ni75AlxV25-x alloys with middle Al composition were explored by computer simulation in this paper. Through the simulated atomic pictures and composition order parameters of precipitates, we can explain the complex precipitation mechanisms of θ (Ni3V) and γ′ (Ni3Al) ordered phases. Simulated results also show that the precipitation characteristic of γ′ phase transforms from non-classical nucleation and growth to congruent ordering + Spinodal decomposition gradually, otherwise, the precipitation characteristic of θ phase transforms from congruent ordering + Spinodal decomposition to non-classical nucleation and growth mechanism gradually.