Influences Of Technological Conditions On Hypereutectic Al-Si Alloy Solidification Structure

Hypereutectic Al-Si alloy has the advantages of low coefficient of thermal expansion,high ratio of Strength to density,excellent corrosion resistance and wearing properties.But large-scale Si results in the existing of needle-like eutectic silicon and coarse block primary silicon,which dissevers the bonding of the matrix and aggravates the mechanical properties of the alloy.So hypereutectic Al-Si alloy must be modified to control morphology and size of Si phase.Hypereutectic Al-Si alloy was modified by controlling technological conditions,including the content of complex modifier,cooling rate,melt overheating treatment,which have been studied in order to investigate their effects to the microstructure and performances of hypereutectic Al-Si alloy.Moreover,the interaction mechanism of modification was analyzed preliminarily.The result indicated that:1.P-RE combination modification can modify both the eutectic Si and primary Si of the hypereutectic Al-Si alloy at the same time.P can only modify primary Si while RE can modify eutectic Si and have lightly promote the effect of P modification to primary Si.But it does not influence the influence rule of the phosphorus content of P to the morphology and size of primary Si.The optimal technological condition is adding 0.06wt.%P,0.8wt.%RE and the average Size of primary Si particle is 15μm.2.P-RE combination modification has minor sensitivity to cooling rate.There is a critical cooling rate and below the certain rate P-RE modifier nearly has no effect. Excellent modification effect are obtained under different cooling conditions of metal mold and preheating dry graphite mold,while the effect of colling in metal mold is a little better than the effect of coiling in preheating dry graphite mold.3.Melt overheating treatment can strengthen the effect of P-RE combination modification and can greatly eliminate the influence of colling rate to P-RE combination modification.4.The mechanical properties of alloy are greatly improved by P-RE combination modification,and the room temperature tensile properties rose from 134MPa to 268MPa,and the highest value reached 275 MPa,elongation reached 1.6%.The fracture morphology appears typical brittle and plastic yielding was not obvious.5.From the point of thermodynamics analysis,carborundum is the final product of the reaction of Si atoms of hypereutectic Al-Si alloy and high-temperature activated carbon atoms.While hypereutectic Al-Si alloy in the graphite crucible kept in high-temperature for long time,the activated carbon atoms of the graphite crucible reacted with the melting Si atoms in hypereutectic Al-Si alloy,then carborundums precipitated.Because carborundum is the sphalerite structure which is similar with Si, they made the heterogeneous nucleus of primary Si during Solidification,then primary Si growed on the surface of carborundums firstly and the grains were refined.