Interfacial Heat Transfer In Shot Sleeve And Die Cavity During High Pressure Die Casting Process

High pressure die casting(HPDC) process, as an advanced metal forming method with high efficiency, good casting dimensional accuracy, excellent mechanical properties and good castability for thin-walled complex parts, has been widely used in automotive, aerospace, communications, electronics and other fields. During the HPDC process, the interfacial heat transfer of the molten metal in the shot sleeve and die cavity determines the initial state and solidification mode of castings and plays an important role for the quality of the casting. Therefore, the study on the heat transfer throughout the HPDC process for determining the interfacial heat transfer coefficients(IHTCs) in the shot sleeve and die cavity and establishing models for accurate interfacial heat transfer boundary conditions has a great significance for the process optimization, the prediction and control of the quality of castings, for eliminating casting defects and for the application of numerical simulation technology in the development of HPDC industry.In this work, experimental and numerical simulation methods have been systematically adopted to investigate the interfacial heat transfer problems of molten metal in the shot sleeve and die cavity. The difficulties of temperature measurements in determining heat transfer coefficients by inverse heat conduction problem(IHCP) as in most of the inverse problems were studied. The temperature measurement schemes were designed to study interfacial heat transfer in HPDC process, including general temperature measurement units, special shot sleeve for temperature measurements and dies. Die casting experiments were systematically conducted and precise temperature datum in the shot sleeve and dies were obtained at different die casting process conditions.After comprehensive study on the numerical solution technique of IHCP and the interfacial heat transfer mechanism during the solidification of die castings, a two-dimensional inverse heat transfer model for the molten metal in the shot sleeve considering the dynamic injection conditions were established. The model was coupled with solving the thermal field of molten metal both in the shot sleeve and die cavity. During this work, the future time step has been optimized and the stability conditions for inverse model have also been analyzed. The acceptable domains of selecting inverse process parameters for the inverse calculation of the HPDC process have been determined, and the inverse calculation program of the IHCP has been successfully developed for obtaining the IHTCs during the HPDC process.By using the inverse heat transfer model and program, The temperature distributions in the shot sleeve and the IHTCs between the molten metal and shot sleeve at different locations have been carefully calculated and the results show that the IHTCs have a big difference between static no injection condition and conventional die casting conditions, however,, the interfacial heat transfer coefficients decrease along the injection direction in the shot sleeve and the temperature in the middle part of the shot sleeve is lower than in both ends of shot sleeve. Under conventional die casting conditions, the heat transfer coefficient exhibits bimodal phenomenon at the end of shot sleeve due to the impact of the plunger movement. Finally, the influence of the process parameters, such as the fill ratio, the alloy composition, the injection speeds at slow shot or fast shot stages and the casting pressure on the IHTCs were analyzed, the nucleation of the externally solidified crystals(ESCs) in shot sleeve and their distributions in the castings were predicted.The interfacial heat flux density(IHFD) and the IHTC between the metal and die were successfully determined. The influences of the mold filling process and process parameters on the IHTCs were analyzed. Furthermore, an interfacial heat transfer boundary model between metal and die was established and applied for the temperature distribution simulation and thermal equilibrium analysis of a practical die casting, the calculated results verified the rationality of the inverse calculation model and program.