Study On Melting Casting Process And Properties Of Al-50wt.%Si Composite Material

Al-Si composites with high volume fraction have the characteristics of low density,low thermal expansion coefficient and high thermal conductivity,these properties make it a kind of excellent electronic packaging material.The composites prepared by melting and casting method have the advantages of low porosity,high interfacial bonding strength and low cost.However,there are a lot of plate-like primary silicon in the solidified structure of the Al-Si composites with high volume fraction of Si prepared by melting and casting method,which seriously deteriorates the mechanical and physical properties of this composites.Therefore,technological measures must be taken to change the size and morphology of primary silicon,so as to improve its performance.In this paper,Al-50 wt.%Si composites are prepared by melting and casting method,the effects of melt superheat treatment,rare earth and phosphorus modification on solidified structure of Al-50 wt.%Si composites are studied.And effects of P addition on the bending properties,thermal conductivity and thermal expansion coefficient of Al-50 wt.%Si composites are researched also.The results and conclusions are as follows:(1)Effect of melt superheat treatment on the solidified structure of Al-50 wt.%Si composites is studied.The melts of Al-50 wt.%Si composites are overheated at the temperature range of 1150~1450 °C.The results show that the size and morphology of primary and eutectic silicon in the Al-50 wt.%Si composites are not obviously changed,primary silicon is still the coarse plate shape and eutectic silicon is still slender needle shape,The results indicate that the melt superheat treatment in this temperature range has little effect on the solidified structure of Al-50 wt.%Si composites.(2)Effect of mixed rare earth modification on solidified structure of Al-50 wt.%Si composites is studied.The melts of Al-50 wt.%Si composites are modified by adding 0.4~1.2wt.% mixed rare earth.The results show that rare earth elements have obvious modification effect on the eutectic silicon in solidified structure of Al-50 wt.%Si composites,the shape of eutectic silicon is changed from needle to granular,and when amount of rare earth is 1.2%,the modification effect is best.However,the rare earth elements have little effect on the primary silicon in Al-50 wt.%Si composites.(3)Effect of P modification on the solidification morphology of primary silicon in Al-50 wt.%Si composites is researched.Al-3P master alloy is used to modify the Al-50 wt.%Si composites melt.The results show that P elements have significant modification on the primary silicon in Al-50 wt.%Si composites,make the size and shape of primary silicon changed obviously.and when amount of P is 0.5wt.%,the modification effect is best,the morphology of the primary silicon is transformed from coarse strips to small and dispersed blocks,and average size of primary silicon reduced to 40?m.Moreover,P modification can make eutectic silicon in Al-50 wt.%Si composites change from needle to short rod.(4)The mechanism of P modification on primary silicon was researched through XRD,SEM-EDS and thermal analysis.The results show that The modification mechanism of P on primary silicon is AlP particles generated in melt become heterogeneous cores of primary silicon,nucleation rate of primary silicon is greatly improved,that makes primary silicon refined and distributed,and after 0.5%P modification,the critical super-cooling degree of primary silicon crystallization in Al-50 wt.%Si composite is reduced by 9.2 °C compared with that without modification.(5)P modification significantly refine the solidified structure of Al-50 wt.%Si composite,which made the bending strength and thermal conductivity of the material improved significantly.When amount of P was 0.5%,the bending strength and thermal conductivity of Al-50 wt.%Si composites reached to the highest level,of which the bending strength reached 206.27 MPa,which is The bending strength is 206.27 MPa,which is 3.12 times that of the unmodified material.And thermal conductivity reached 131.25 W/(m·K),which is 1.57 times that of unmodified material.However,the thermal expansion coefficient of Al-50 wt.%Si composites did not change significantly by P modification.