Study On Numerical Simulation Of Gas Entrapment And External Solidified Crystals During Mold Filling Of High Pressure Die Casting Process

The gas porosity caused by the air entrapment and the coarse microstructure caused by the external solidified crystals(ESCs)are the most common defects in high pressure die casting(HPDC)process.The generation and the evolution of these defects are closely related to the mold filling process of the liquid metal flowing in the die cavity with high speed.Therefore,prediction of the mold filling process and its related defects using computer simulation technology is of great importance for optimizing the design and improving the quality of castings in HPDC process.A three-dimensional free surface reconstruction algorithm based on the Young's interface assumption and a liquid transportation algorithm based on the operator non-split principle were developed.A new solution algorithm to the control equations and its post-processing technology were developed according to the cut-cell mesh system.Fluid flow simulation and water analog comparison were conducted to verify the validity and the superiority of the developed algorithms.The normal vectors of different interfaces were calculated and the surface tension forces were transformed to the N-S equations along with the continuity equations.In this way,a mathematical model was proposed to simulate the mold filling process in HPDC and the governing equations were solved by Finite Difference Method.A water analog system for the HPDC process was built and experiments were carried out with two sets of moulds.The experimental results were compared with the simulation results qualitatively and quantatively to verify the accuracy of the new model.Meanwhile,the improvement in accuracy of the modeling results after consideration of the surface tension was discussed.In order to predict the gas porosities,the air entrapment process and its continuing evolution during the mold filling process were analyzed.Then,the bubble searching method,the bubble breaking criterion and the bubble movement equation were developed to build the model for the prediction ofentrapped air bubbles.The locations of marcro and micro bubbles were found in the experiments and were compared with the simulation results to verify the ability of defect prediction of the model.By combining the mold filling model with the particle motion equation,a fluid-particle model was proposed to simulate the motion and the distribution of ESCs in HPDC process.The ESCs' force analysis was conducted and the initial conditions were set according to the practical parameters.Simulation results in shot sleeve and castings were compared with the experimental results to verify the model's validity.After that,ESCs' flow in the tensile bar casting under different conditions were observed and the normalized distance were measured periodically to reveal ESCs' distribution patterns.Finally,a hypothesis connecting the distribution patterns and the generation of defect bands were put forward to predict the solidification defects.Experimental results were displayed and compared to support the hypothesis.The mold filling processes of practical die castings were simulated and the gas porosities and ESCs defect were detected by applying all those models.Experimental results using X-ray inspection,CT tomographic scanning and metallography counting were compared with the simulation results.Good agreements obtained in comparison show that the models and the algorithms presented in this study have the ability to simulate the mold filling process and to predict the locations of defects at a higher accuracy.Moreover,by improving the die design of castings,the simulation methods can provide a reliable technical reference for optimizing the HPDC process and improving the quality of die castings.