A Dynamic Pressure-based Method To Predict Casting Shrinkage Cavities During Solidification Process Under Multi-force Fields And Its Application

Shrinkage cavity defects is the main problem in the production of castings,greatly affect the product quality and performance of a casting.It is helpful to optimize the casting process and improve casting quality.Existing shrinkages prediction research put forward the solution for the formation of shrinkage cavity under each kind of casting process separately.It has certain limitations that they does not have a unified theory and model of shrinkage cavity formation.We analysed the formation mechanism of shrinkage cavity in two important casting technology of gravity casting and vertical centrifugal casting.The research of modeling and numerical simulation of casting shrinkage formation under multi-force field is carried out.The accuracy of the model is verified by the experiment.The model was applied to production process of casting in foundry enterprise and predicted the distribution of shrinkage cavities.It has important value for foundry enterprises producing high quality castings and controling the cost of production.First,we analyzed the characteristics of connectivity and feeding of isolated liquid area,and the relationship between the evolution of the isolated liquid areas and the formation of the shrinkage cavity,a critical pressure criterion that the isolated liquid areas can feed each other was deduced.The atmospheric pressure and gravity field would be extended to the condition of a conservative force field.We built a non-inertial rotating coordinate system on the rotation axis of vertical centrifugal machine that has the same rotation speed and direction with the vertical centrifugal machine to simplify the force analysis in the molten metal.It is proved in theory that after filling process finished in the vertical centrifugal casting process,the molten metal in the vertical centrifugal machine is also under a conservative force including gravity,centrifugal force and Coriolis force.The distribution rule of internal shrinkage cavities in each isolated liquid area is derived in the framework of a conservative force field.We pointed out that dynamic pressure is the key factor to affect the shrinkage cavity morphology.With the calculation method of shrinkage volume generated during the solidification process,the shrinkage cavities prediction model based on dynamic pressure for multi-force field was proposed.As the formation of shrinkage cavity has a unified physical laws during casting solidification process of multi-force field,the gravity casting and vertical centrifugal casting processes using the same shrinkage cavities prediction method.Second,the technical details of shrinkage cavities prediction model based on dynamic pressure distribution for gravity casting and vertical centrifugal casting are discussed in detail.Including the rapid calculation method for the dynamic pressure in the molten metal,the solid fraction and shrinkage volume distribution technology.We developed the shrin kage cavities prediction module using C++ language and joined InteCAST software by secondary development.Reused the mature temperature field and flow field simulation technology,materials database,pre-process and pose-process in Inte CAST software,achieved the goal of casting shrinkage cavities prediction under gravity casting and vertical centrifugal casting technology.Then,based on the characteristics of model under gravity casting,we designed a group of U-type cast steel casting gravity casting experiment,and verified the high accuracy of the model in the shrinkage cavities prediction in the air of the gravity casting process.Based on the characteristics of model under ertical centrifugalcasting,a group of U-type titanium alloy vertical centrifugal castings were developed and tested.The results demonstrated that the model has a nice precision in the vertical centrifugal casting of vacuum under different rotational speeds.Established the crucial role of pressure in the shrinkage distribution of the casting process and clearly pointed out that the pressure has an important effect on the shrinkage cavities distribution.Finally,two groups of cast steel gravity castings and titanium alloy vertical centrifugal casting with mass production and the complex structure were selected and pouring,and simulated according to the on-site pouring conditions using the model.Shrinkage cavities of two gravity castings are mainly distributed in the risers.Cut the castings and measure the size that of the actual castings.The height of shrinkage cavities of simulation is clear to the height of shrinkage cavities iof the actual castings,the error is only within onr grid(relative error less than 5%).It has extremely high shrinkage prediction accuracy.Two centrifugal casting shrinkage cavity is mainly detected by X-ray nondestructive testing.The simulation results is compared with the n X-ray photograph,they are in good agreement with the locations,shapes,and number of shrinkage cavities.The model has great application effect in the casting production of the enterprise and promotion value.