Study On Refinement Of The Primary Silicon In Hypereutectic Al-si Alloy By Boron

It is well known that the hypereutectic Al-Si alloys have many advantages,such as small thermal expansion coefficient,high wear resistance and corrosion resistance property,better high-temperature performance.However,due to the coarser primary Si phase existing in the structure,the mechanical properties of the alloy is damaged severely.This greatly restricts the use of the hypereutectic A1-Si alloys in a large scale. How to improve the size and the morphology of the primary silicon in hypereutectic Al-Si alloys is becoming the focus of research.In the present study,the refinement of the primary silicon in the hypereutectic Al-20wt.%Si alloy by boron was investigated.1.The refinement of the primary silicon in the hypereutectic Al-20wt.%Si alloy by boron was systematically investigated.The results show that the boron addition can obviously refine the primary silicon in the hypereutectic Al-20wt.%Si alloy.The size of the primary silicon decreases with the increase of the boron content from 0 to 0.065 wt.%,and the minimum size of the primary silicon can be obtained when the boron content is of about 0.065 wt.%.However,the excessive boron is not beneficial to the refinement of the primary silicon.Calcium and magnesium elements can make the refining effect of B on the primary silicon more effective.C₂Cl₆ treatment weakens the refining effect of boron.The compounds formed by the reaction of B with Al,Si and Ca or Mg may which can serve as the nucleation substrates for primary silicon,be responsible for the good refining effect of boron on the primary silicon.2.The variation of the size and the morphology of the primary silicon in the hypereutectic Al-20wt.%Si alloy modified by boron with the melt temperature were investigated.It has been shown that the melt temperature has some influence on the primary silicon modified by boron.The size of the primary silicon decreases with increasing the melt temperature.When the temperature is increased up to 950℃,the size of the primary silicon is refined to 47.7μm from 90.1μm at 710℃.The reason may be that the higher melt temperature can refine the nucleus of primary silicon and optimize the melt structure of the alloy.3.The effect of the cooling rate of the alloy melt on the refinement of the primary silicon in the hypereutectic Al-20wt.%Si alloy by boron was systematically investigated at different melt temperature.It is concluded that the cooling rate obviously affects the refining effect of boron on the primary silicon.The size of the primary silicon decreases with the increase of the cooling rate,but such an action of the cooling rate lowers with the increase of the melt temperature.The reason is that different cooling rates cause the different degree of super-cooling,so affecting crystallization and growth of the primary silicon.Besides,the alloy melt structure is homogenized with the increase of the melt temperature gradually,which can also increase the degree of the super-cooling.The high degree of the super-cooling can promote the nucleation of the primary silicon,suppress the growth of the primary silicon.But,the sensitivity of the primary silicon to the cooling rate decreases with the increase of the melt temperature.