Study On The Technologies Of Morphologic Control Of Primary Silicon In Al-30% Si Alloy

The hypereutectic Al-Si alloy is an important casting alloy.With the increase of silicon content,the wear resistance of the hypereutectic Al-Si alloy improved greatly,the thermal expansion coefficient reduced greatly,the thermal stability improved,the casting performance developed,the density decreased and the performance of high temperature enhanced.However,in the untreated cast hypereutectic Al-Si alloys,there are a lot of coarse laths and five-stellate primary silicon,which seriously damaged the microstructure and greatly reduced the mechanical performance of Al-30%Si alloys.At present,the research on the morphologic control of primary silicon in hypereutectic Al-Si alloys has made some progress in modification treatment and rapid solidification.However,these methods have not obvious effect,or too complex and costly.to be used with industrial production and application.In this paper,the research has studied systematically on the influences of mechanical stirring process and liquid-solid two-phase reheating on the primary Si micro structure of Al-30%Si alloy.So as to obtain appropriate process parameters,prepared Al-30%Si alloy with fine primary silicon structure.Several phenomenon can be observed in this research.The micros tructure of primary silicon can be improved obviously by mechanical stirring under constant temperature.The primary silicon particles are spheroidized and the edges and corners are passivated.However,when the stirring hold too long time,the primary silicon structure will be worsened.The effect of mechanical stirring,cooling rate and stirring time on the morphology of primary silicon was significant under continuous cooling.In the appropriate process conditions,the equivalent diameter of the primary silicon was reduced to 25?m and the shape factor K reached 0.81.But if the stirring time was too long,the content of impurity in the melt will increase and the microstructure will be deteriorated.The liquid-solid two-phase reheating can effectively improve the morphology of primary silicon and get the rounded globular primary silicon.The equivalent diameter of primary silicon about 28?m and the shape factor K rose to 0.87.The two-stage reheating can improve the uniformity of organization.However,if the process parameters not appropriate,the morphology of primary silicon will deterioration.The primary silicon structure after liquid-solid two-phase reheating is spherical and ellipsoidal in the three-dimensional space.In this paper,the microstructure of primary silicon was improved by modification,mechanical stirring and liquid-so lid two-phase reheating.The structure of primary silicon morphology was small,round and uniform distribution.This technology was suitable for practical production due to simple equipment,easy operate,effective and low cost.It is an effective way to control primary silicon microstructure morphology in Al-30%Si alloy.