Study On The Divorced Growth Behavior Of Eutectic Solidification In Hypereutectic Al-Si Alloys

The divorced growth behavior of the eutectic solidification of A390 alloy under different casting methods(glass tube suction casting,metal mold casting and sand mold casting)were investigated in this paper.The effect of alloying elements on the divorced growth behavior of the eutectic solidification in hypereutectic Al-Si alloys was also studied.The casting methods have different impacts on the divorced growth behavior in the hypereutectic A390 alloy with different melt treatments(at different temperatures with 10 min holding time from 720? to 560?).The eutectic solidification of all melts(except for that treated at 560?)with different treatments can be completely divorced by the glass tube suction casting,and,in the case of the melt treated at 650? for 10 min,the refinement of the Si particles is the best,with a mean size less than 15?m and an average shape factor larger than 0.8.Only coarse flake eutectic Si phase can be obtained by metal mold casting,the eutectic solidification is completely in the form of the coupled growth,namely completely non-divorced growth,and the obtained mean size of the primary Si particles is more than 50?m.In the case of the sand mold casting,the divorced growth behavior of the eutectic solidification can be observed only in the melts treated at 650? and 600?,however,the obtained mean size of the Si particles is about 30?m,less than that by the metal mold casting but greatly larger than that by the glass tube suction casting.The nucleation and growth of the Si particles during the molding filling process and in the precipitation stage of the primary Si phase and the cooling condition in the stage of eutectic solidification(L(?)a-Al + Si)in the cases of three casting methods are analyzed and discussed.Two necessary conditions for the divorced growth of the eutectic solidification in A390 alloy are obtained: sufficient Si particles produced in the mold filling process and in the precipitation stage of the primary Si phase and a slow cooling rate in the eutectic solidification stage.In the case of the glass tube suction casting,the inner diameter/ thickness of the glass tube,modifier addition and ultrasonic vibration have some influences on the divorced growth of the eutectic solidification of the hypereutectic A390 alloy melt.When the inner diameter of the glass tube is 10 mm and the thickness is 1mm,the eutectic solidification is completely divorced and fine Si particles with the highest shape factor and with the best uniform distribution can be obtained.The addition of P element can refine the Si particles and has little influence on the divorcement of the eutectic solidification.The addition of mischmetal has a similar effect with P addition.But the addition of Sr element will suppress the divorcement of the eutectic solidification and deteriorate it.The ultrasonic vibration has no influence on the divorcement.When the vibration time is 3min,fine Si particles with the highest shape forced can be obtained.In the case of the glass tube suction casting,the effect of the alloying elements of Cu,Mg,Fe,Mn and Zn on the divorced growth of the eutectic solidification in hypereutectic Al-16 Si alloy was studied.Combined with the solute theory,it is found that the constitutional supercooling caused by solute enrichment in the growth front of Si crystals plays a very important role in the divorced growth behavior of the eutectic solidification.The constitutional supercooling caused by the solute enrichment in the growth front of Si crystals promotes the continuous nucleation and growth of Si crystals in the precipitation stage of the primary Si phase,so that the number of Si particles is sufficient to produce the divorced growth of the eutectic solidification when the alloy melt enters the eutectic solidification stage.