Prediction Technique Of Shrinkage Cavity And Shrinkage Porosity Of Magnesium Alloy In Low Pressure Casing

With many advantages, such as lower specific gravity, better casting properties and so on, Magnesium alloy is increasingly widely used in many fields including automobile industry, electronic communication , aircraft and spacecraft etc. Low pressure casting is used as one of important casting process for producing quality magnesium alloy casting to utilize its characters of smooth filling and progressive solidification, but its application is impacted by defects of shrinkage cavity and shrinkage porosity. Therefore predicting techniques of it become one of important problems solved urgently at present.Alternating Direction Implicit method (abb. ADI) is first applied to establish the mathematical model of 3D temperature field, which could largely improve arithmetic speed and numerical precision, and its value is absolutely stable. But its arithmetic speed and numerical precision are highly impacted by its time step. A method of variable time step that is more simply is established, which insure improving the arithmetic speed and the numerical precision of temperature fields. Through a casting experiment and a numerical simulation, it is shown that ADI method and variable time step could better calculate temperature fields in solidification process.And then according to the character of wide solidification range of magnesium alloy and energy conservation principle, equivalent specific heat method is applied for latent heat treatment. It is indicated that this model could exactly reflect the release of latent heat with alloys of wide solidification range as well as alloys of narrow solidification range.Subsequently, a new predicting method of shrinkage cavity and shrinkage porosity based on low pressure casting is presented and its mathematical model is established. The dual rules with gravity feeding and pressure feeding and the relationship of the pressure and critical solid fraction all are considered, which could much more agree with solid laws of low pressure casting of magnesium alloy. This method is integrated into EasyCast software to simulate solidification process, and then its results are compared with experiment results. It is indicated that this method could better display positions and sizes of shrinkage cavity and shrinkage porosity. Finally a series of orthogonal simulating experiments are introduced. It is shown that shrinkage cavities and shrinkage porosities the are highly impacted by pressure holding time and holing pressure, and the new solidification simulation software established through the above models is viable, and these models and the revealed laws is so correct that they play an important role to instruct the scientific research and production.