Research On Microstructure And Properties Of Aluminum Filter-cavity Processed By Vacuum Die Casting

High pressure die casting was extensively used in mass production of automotive,telecommunication and electronic industries for its high efficiency,near net forming,etc.But in conventional die casting process,alloy melts was injected into mould cavity at a high velocity.Gas in the cavity can not be pushed out by melt in time,thus being left in the casting,causing porosities.Vacuum die casting was an advanced die casting technology.In vacuum die casting,the gas was extracted before filling process.So gas entrapped in casting could be largely reduced by this method.Relative researches show that gas entrapment in die castings could contaminate their mechanical properties,and porosities in materials would also reduce its thermal conductivity.So vacuum die casting can improve both mechanical properties and thermal conductivity.It is very suitable for fabricating the 5^th generation communication filter-cavity,which needs higher strength and thermal conductivity.Firstly,by mathematical modelling and experimental verification,a new model considering die leakage was carried out to calculate the cavity gas pressure in developed vacuum system,in which a tank was added between the vacuum pump and mould cavity.The practical cavity gas pressure was obtained in cold shot experiment on vacuum die casting mold of filter-cavity casting.The results show that the new model has a high correlation with practical cavity gas pressure.By applying this model,the factors influencing gas pressure in die cavity during evacuation process was studied.It was found that in the beginning of evacuation,die pressure declined quickly,then the declining of the pressure gradually slowed down,and the air leaked into die cavity would equal with evacuation at last,the cavity pressure can not be reduced hereafter;air leakage could be reduced by improving mould clamping force;the gap between shot sleeve and plunger was partly enveloped while the shot sleeve was filled by melt,thus reducing air leakage;and prolonging evacuation time could result in lower cavity pressure and pressure fluctuation.Secondly,a vacuum die casting experiment was carried out to produce filter-cavity castings with good appearance.We studied the influence of cavity gas pressure on mold filling with FLOW-3D Cast commercial software.In the fabrication of filter-cavity castings,AlSi12Fe and AlSi8 aluminum alloys was used.The forming problems of castings was solved by parameter optimization.During the process,we studied the influence of die casting parameters on casting forming.It is worth noting that in the fabrication of AlSi12Fe filter-cavity castings,an abnormal phenomenon was found that the forming of castings was more difficult while cavity gas pressure was lower.The reason is analyzed by combining the simulation and practical die casting experiment.Lastly,the porosity,microstructure,tensile properties and thermal conductivity of castings with different initial cavity pressures and different alloys was studied.Including the porosity,microstructure,tensile properties and thermal conductivity of casting with AlSi12Fe alloy and initial cavity gas pressure of 275 mbar;and the porosity,microstructure,tensile properties and thermal conductivity of casting with AlSi8 alloy and initial cavity gas pressure of 217mbar and 114 mbar.The results show that by reducing cavity pressure,porosities in casting can be largely reduced;and its yield strength increased with the decrease of initial cavity pressure;while the initial cavity gas pressure was about the same,the tensile properties of AlSi8 was better than and AlSi12Fe;the thermal conductivity decrease with the increase of initial gas pressure and the increase of porosity;the thermal conductivity of AlSi8 was much higher than AlSi12Fe.