Effect Of Crystallization Pressure On Secondary Dendrite Arm Spacing Of Vacuum Counter-pressure Casting Aluminum Alloy

Vacuum counter-pressure casting (VCPC) is a kind of advanced precision formingmethod of the anti-gravity in which the filling are at low counter-pressure and thesolidification are at high pressure. It’s employed widely in near net shape，highaccuracy，high performance，complicated and thin-wall nonferrous alloy castings foraviation，aerospace，national defense，car and automobile industry. The methods ofcontrol the solidification process is the key of the VCPC to obtain high-quality castings.It is ensured that the casting have excellent feeding conditions, which make the casthave dense organizations and excellent performance. The secondary dendrite armspacing (SDAS) is an important characteristic of the solidification microstructure, andthe SDAS of VCPC aluminum alloy casts under different solidification pressure wasstudy, the relationship between the crystallization pressure and the SDAS of thealuminum alloy was established. The theoretical and technical has important theoreticaland practical significances for the VCPC thin-wall aluminum alloy castings.In this paper, the effect of the solidification pressure and the thickness on theSDAS was study by simulation and experiment. The SDAS of sample indifferentcrystallization pressure (250kPa、300kPa、350kPa、400kPa、450kPa) and thickness(3mm、6mm、9mm、12mm) was simulated by ProCAST software. At the same time, thealuminum alloy samples are obtained by means of VCPC casting equipment in the samecondition. And the SDAS is observed and measured by means of metallographicmicroscope.The results show that the solidification pressure has a great effect on SDAS. Whenthe crystallization pressure increases, the nucleation energy decreased, the critical radiusof nucleus become smaller and the nucleation number increased, and the SDASdecreased and the grain size become finer with the improvement on crystallizationpressure. When the crystallization pressure is less than350kPa, the SDAS decreasessteadily, but when the crystallization pressure is greater than350kPa, the higher thesolidification pressure, the decrease effect in the SDAS is more obvious. When thecrystallization pressure exceeds400kPa, the effect becomes small. On the other hand,the cast solidification rate will decrease when the wall thickness increase and the SDAS will increase. The experimental results show that: when the wall thickness of cast issmaller than6mm, the SDAS have an evident increase, but when the wall thickness ofcasts is bigger than6mm, the increases of the SDAS tendency become steadily. Therelationships between the solidification pressure and the SDAS was obtained and shownas below：And the mathematical relationships between the feeding speed and the SDAS wasestablished and shown as below：In the solidification process of the VCPC, the values of C0、C1、C2、C3are constantswhen the alloy is determined. It can be seen from the above equation, the feedingvelocity during the solidification process becomes bigger when the SDAS decreases.The results of the study provide a basis theoretical basis, and lay a good foundation forthe promotion and application of the VCPC technology.