Shou Mei Xiong

A study of the interfacial heat transfer between an iron casting and a metallic mould

Abstract Computer simulation of the solidification process is used widely in industry in the design of the mould and the die configuration. The heat-transfer coefficient between the mould and the cast metal is one of the important variables. In this work, the heat-transfer behavior and the time-dependent heat-transfer coefficient between an iron casting and a metallic mould were investigated by inverse heat-conduction analysis. A one-dimensional inverse heat calculation program, which can be used in both rectangular and cylindrical coordinate systems, was developed based on a non-linear estimation method. Temperature distributions along the direction of heat flux from the casting to the mould were measured, the predicted temperature profile being found to compare well with the experimental results. The heat-transfer coefficient was found to drop rapidly during the initial stage of solidification and then increase with the solidification process after a short time period of steady stage. It is concluded that a three stage segmented linear equation of the coefficient can be used to represent the heat-transfer behavior and be implemented in numerical analysis of the casting process.

Numerical methods to improve the computational efficiency of thermal analysis for the die casting process

Abstract The temperature distributions in the die casting and the dies are of great influence on the quality of the die casting and the life cycle of the dies, so that many efforts have been made in the numerical simulation of heat transfer in the die casting process. In order to improve the computational efficiency of thermal analysis of the die casting process, three kinds of numerical methods were employed. First, the component-wise splitting method was adopted to solve the three-dimensional heat transfer problem in the die casting process. The method is unconditionally stable and suitable for simulating fast transient phenomena and for computations on fine meshes. Secondly, an irregular mesh system was developed to reduce the total grid number of the analysis system. The corresponding finite difference algorithm of the component-wise splitting method was also developed. Thirdly, a transient surface layer concept was used to divide the computational area into a transient area and a steady area, where different time steps can be applied to these areas to improve the computational efficiency. A three-dimensional thermal analysis system was developed with the numerical methods and a practical die casting component was simulated with focus on the computational efficiency.

Development of a Para-AMR algorithm for simulating dendrite growth under convection using a phase-field–lattice Boltzmann method

By combining adaptive mesh refinement and parallel computing, a high performance numerical algorithm was developed to simulate dendrite growth against convection using a phase-field–lattice Boltzmann method. Numerical tests on both 2-D and 3-D dendrite growth cases revealed that, by employing moderate amount of computing resources ( 10 1 –10 2 1 0 1 – 1 0 2 parallel processes), this algorithm, without compromising any accuracy, could improve the computational efficiency by 2–3 orders of magnitude, or for most cases shorten the overall elapsed simulation time by 95%, comparing with the normally applied explicit algorithm. Besides, the computational stability or convergence of the algorithm could be maintained even when the local volume fraction of solid approached ∼100% ∼ 100 % , which could not be achieved if other implicit algorithms like SIMPLE was employed.

Effect of Sr and Nd on Microstructure and Mechanical Properties of AZ91-1Si Alloy

In present work, trace elements Sr and Nd were added into AZ91-1wt%Si alloys. The alloys were cast into a permanent mould and then machined into test bars. The microstructure and mechanical properties at room temperature of the specimens were investigated. Results showed that complicated Chinese script Mg2Si phase decreased in size with the increase of Sr addition. When Sr addition increased to 0.1wt%, the Mg2Si phase was changed from Chinese script shape into uniform polygon shape completely. At the same time, mechanical properties improved due to the morphology modification of the Mg2Si phase. An intermetallic compound containing Mg, Al, Nd and Si was found when Nd was added to the alloy. Remarkable modification on the shape and distribution of the Mg2Si phase was observed because of the intermetallic compound, which leads to a great change in mechanical properties. The grain refinement mechanism of Sr and Nd elements on the Mg2Si phase was discussed.

Microstructure Simulation of Magnesium Alloy

A cellular automaton (CA)-based model for two-dimensional simulation of the dendritic morphology of magnesium alloys was developed. The model considers solving the solute and heat conservation equations in the modeling domain, including calculation of the solid fraction, the tip velocity, and the solute diffusion process, all of which have significant influence on the dendrite evolution. The microstructure of a step-shape die cast part of AZ91D magnesium alloys was investigated utilizing SEM-EBSD analysis. The microstructure simulation results were compared with the experimental results and they were in good agreement on grain size.

Effect of Si, Ca and Sr on the Creep-Resistance of AZ91D Alloy

In present work, Sr, Ca and Si elements were added into AZ91D and cast into a metallic mold. The influence of these elements on AZ91D magnesium alloy were studied systematically with the purpose of developing a new magnesium alloy with good mechanical properties at both room and high (150°C~200°C) temperature. Additionally, a simple method to qualitatively analyze the high-temperature creep performances of magnesium alloys was designed and implemented.

Numerical Simulation of Die-Casting Mold Filling Process by Using Fractional Step Method

In the numerical simulation of mold filling process, the calculation efficiency has been a key point for practical applications due to the complexity and thin-section of die castings. In current research work, a fractional step method was applied in the solution of the unsteady Navier-Stokes equations, which can be implemented with a single solution to the momentum/pressure equations at each time step. This method may avoid the decrease in efficiency induced by iteration. A water analog system was designed and developed to simulate the die casting process. The flow patterns were recorded by a high speed camera with a capturing rate of 500 frames per second. The simulation results were consistent with the experimental ones. Besides, the fluid flow patterns of several components were simulated by the fractional step and VOF algorithm, and the SOLA-VOF algorithm respectively. The simulation results showed that the combination of the fractional step method and VOF method can improve the computational efficiency to some extent in numerical simulation of mold filling process.

Microstructure Simulation of Die Casting Magnesium Alloy

A cellular automaton (CA) based model for two-dimensional simulation of the grain morphology of high pressure die casting magnesium alloy was developed. The heterogeneous nucleation, the solute redistribution both in liquid and solid, the interface curvature and the growth anisotropy during solidification were also considered in the model. By fitting the curve of grain density distribution, parameters of continuous nucleation equation based on Gaussian distribution were calculated. The microstructure simulation of step-shape die castings of AM50 magnesium alloy was studied. The metallographic microstructure of the castings at the surface and center of three steps with different initial die temperature was investigated. The simulation results were compared with the experimental results and they were in good agreement on average grain size.

Influence of Processing Parameters on Interfacial Heat Transfer Coefficient at the Metal-Die Interface of Die Cast AM50 Alloy

This paper focuses on the determination of the heat flow density (HFD) and interfacial heat transfer coefficient (IHTC) during the high pressure die casting (HPDC) process of AM50 alloy. A specially designed “step shape” casting is used during the die casting experiment. Based on the temperature measurements inside the die, HFD and IHTC are successfully determined. Calculation results indicate that HFD and IHTC at the metal-die interface increases sharply right after the fast injection process until approaching their maximum values, and after that their values decrease to a much lower level until the dies are opened. Casting thickness has a great influence on both of the HFD and IHTC. Process parameters, such as the intensification pressure, the piston velocity, have little influences on HFD while on the other hand the die temperature has a great influence on the HFD. The IHTC seems to be independent upon all those process parameters so the IHTC peak values maintain at a particular level when the casting thickness is fixed.

Numerical Simulation on the Liquid Metal Flow of the Acceleration Phase in the Shot Sleeve of Cold Chamber Die Casting Process

Slow shot velocity and its acceleration phase in the shot sleeve have great influence on the flow pattern of the liquid metal in the shot sleeve. In this paper, a three-dimensional model based on the SOLA-VOF algorithm was developed and used to simulate the flow of melt in the shot sleeve. The mathematical model was verified by water analog experiments with constant plunger velocities. Based on numerical simulation results, the influences of the plunger acceleration on the wave profile of the liquid metal in the shot sleeve under different fill ratios and sleeve diameters were investigated. The results indicated that in order to avoid air entrapment in the shot sleeve, the optimal acceleration value to the critical slow shot velocity increases with the increase of the fill ratio, and the range of suitable acceleration becomes wider as well. With the same fill ratio, the value of suitable acceleration rises as the plunger diameter increases.