Study On Density And Solidification Feeding Mechanism Of Al-Cu Alloy Casting By The Grade-controlled Solidification Of Vacuum Counter-pressure Casting

Al-Cu alloy has been widely used in aerospace industry for its low-density，excellent mechanical properties at room and high temperature. But it has a broad rangeof crystallizing temperature, leading to some foundry defects such as shrinkageporosity. Vacuum Counter-pressure Casting (short for VCPC) is an advanced andprecision counter-gravity forming method which is conducive to the production oflarge and complex thin-wall casting. The constant pressure was employed during thewhole solidification process of traditional VCPC, which is unfavorable to thesolidification feeding of liquid metal at the middle or late solidification. Thegrade-controlled solidification of VCPC is adopted in this paper to prepare the ZL205Asamples whose density and solidification feeding mechanism were studied. The resuiltswill offer theoretical and technical support for Al-Cu alloy casting production by thegrade-controlled solidification technique of VCPC.In this paper, the major parameters, first-order pressure, grade-controlled pressureand grade-controlled time, were studied through testing and analyzing the effects onthe desity of Al-Cu alloy samples whose wall thicknesses were different. Addtionally,the feeding mechanism of liquid metal was investigated between all solidificationstages of the grade-controlled solidification of VCPC. The results showed that: Thesolidification pressure has made a great effect on the samples desity while thegrade-controlled pressure was more dramatic. Although the increase velocity becamemore and more slower, the density improved with the increase of first-order andgrade-controlled pressure; The density climbed as the casting wall thickened under thecondition of the same first-order pressure; As the grade-controlled pressure becamehigher, the castings with thicker walls had more evident density velocity when thegrade-controlled pressure under300KPa, while the desity was less affected by thecasting wall thickness when the pressure above300KPa; The changing trend of densitydecreased first and then rose just like V shape from gate to top, moreover, the densitywere much more even with the increase of solidification pressure; The maxium valueof density were different when castings wall thickness variated, and maxium density values with the samples wall thickness of6mm,9mm,12mm, corresponding with thegrade-controlled time of240s,230s,220s.Besides, the relationship among the feeding velocity, first-order pressure and thecasting wall thickness was established at the early stage of the grade-controlledsolidification of VCPC. It was pointed that the feeding velocity increased as thefirst-order pressure went up. Moreover, the thicker the casting wall was, the higher thefeeding velocity was.The relationship between the feeding force and the grade-controlled pressure atthe middle or late stage of the grade-controlled solidification of VCPC was alsoestablished. It was pointed that the increase of the grade-controlled pressure was infavor of adding feeding force. Finally, the relation was found between the feedingforce and the solidification time. The exponential function relation showed that thefeeding force lost remarkably as the solidification time went by once the interdendriticfeeding stage began, so the grade-controlled pressure should be imposed immediatelyto compensate the loss and ensure the dendrite feeding.