Simulation of mold filling and prediction of gas entrapment on practical high pressure die castings

Element parameters including volume filled ratio,surface dimensionless distance,and surface filled ratio for DFDM(direct finite difference method)were proposed to describe shape and location of free surfaces in casting mold filling processes.A mathematical model of the filling process was proposed specially considering the mass,momentum and heat transfer in the vicinity of free surfaces.Furthermore,a method for gas entrapment was established by tracking flow of entrapped gas.The model and method were applied to practical ADC1 high pressure die castings.The gas entrapment prediction was compared with the fraction and maximum size of porosities in the different casting parts.The comparison shows validity of the proposed model and method.The study indicates that final porosities in high pressure die castings are dependent on both gas entrapment during mold filling process and pressure transfer within solidification period.

Mold Filling and Prevention of Gas Entrapment in High-pressure Die-casting

This paper presents some results of direct observation of mold filling in a specially designed die-casting by X-ray diffraction, including comparison with numerical simulation. Based on such work the authors discuss how to prevent gas entrapment and propose new methods.

Underwater drag reduction by gas

Publications on underwater drag reduction by gas have been gathered in the present study.Experimental methods,results and conclusions from the publications have been discussed and analyzed.The stable existence of gas is a requirement for underwater drag reduction induced by slippage at the water-solid interface.A superhydrophobic surface can entrap gas in surface structures at the water-solid interface.However,many experimental results have exhibited that the entrapped gas can disappear,and the drag gradually increases until the loss of drag reduction with immersion time and underwater flow.Although some other surface structures were also experimented to hold the entrapped gas,from the analysis of thermodynamics and mechanics,it is difficult to prohibit the removal of entrapped gas in underwater surface structures.Therefore,it is essential to replenish a new gas supply for continued presence of gas at the interface for continued underwater drag reduction.Active gas supplement is an effective method for underwater drag reduction,however,that needs some specific equipment and additional energy to generate gas,which limits its practical application.Cavitation or supercavitation is a method for passive gas generation,but it is only adaptive to certain vehicles with high speed.Lately,even at low speed,the evaporation induced by liquid-gas-solid interface of a transverse microgrooved surface for continued gas supply has been discovered,which should be a promising method for practical application of underwater drag reduction by gas.