Papers by Author: Shou Mei Xiong

Abstract: The microstructural characteristics of externally solidified crystals (ESCs) and porosities in a non-heat-treated high-pressure die-cast AlSi9MnVZr alloy are investigated under two distinct process conditions: one with the application of lower intensification pressure and the other with higher intensification pressure. Optical microscopy (OM), scanning electron microscopy (SEM), and computed tomography (CT) are employed to analyze the ESCs and porosity distribution. The alloy's microstructure primarily consists of primary α-Al, ESCs, Al-Si eutectic, and iron-rich phases. ESCs nucleate in the shot sleeve, while α-Al forms within the die cavity. When the lower intensification pressure is applied, larger dendritic ESCs are observed, along with significant gas porosity, shrinkage pores, and numerous smaller dispersed pores, resulting in a high porosity fraction. Conversely, the application of higher intensification pressure results in a notable refinement in ESCs morphology, with a significant reduction in their diameter and area fraction. Additionally, the size and fraction of porosity decrease substantially, indicating a marked improvement in casting quality.

Abstract: 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.

Abstract: 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.

Abstract: 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.

Abstract: 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.

Abstract: 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.

Abstract: High pressure die casting is the most common method in making magnesium alloys for both auto parts and 3C products. Pressure variations in the mold during mold filling and solidification process have direct influences on the quality and properties of die castings. In this paper, a cylinder head cover was produced to experimentally study pressure variations in the mold during magnesium alloy die-casting process in real time for the first time. Pressure varies at different positions in the mold during die casting process. This study indicates that mold filling and solidification process of magnesium alloy die castings can be described by pressure curves obtained by pressure measurement at different test positions in the cavity in real time.

Abstract: In present work, Si and Sr elements were added into AZ91 alloy and cast directly into test samples using permanent mold. Mechanical properties of the samples at room temperature were evaluated by tensile test and the microstructure was analyzed. The results show that β-phase (Mg17Al12) of AZ91 alloy decreases with the addition of Si element and Mg2Si phase forms at the same time. Irregular Mg2Si phase precipitates preferentially at the grain boundaries at a low silicon content level. With the increase of the Si content, Mg2Si phase shows a complicated “Chinese- script” shape distributed at the grain boundary which leads to a lower ultimate tensile strength. Subsequently, the addition of Sr element has a remarkable effect on the form and distribution of Mg2Si phase of AZ91-Si alloys.

Abstract: 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.

Abstract: 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.