Effect Mechanism Of Mg-RE On Eutectic Microstructure And Solidification Behavior Of Al-Si Alloy

With the rapid development of the automotive and aerospace fields,the demand for light weight is also increasing,because the aluminum-silicon alloy has better mechanical properties,corrosion resistance and wear resistance.Therefore,it has been widely used in the field of rail transportation and aerospace.However,there are a large number of coarse strips of eutectic silicon in the microstructure of the conventional as-cast aluminum-silicon alloy.This results in a significant decrease in the mechanical properties of the casting.It limits the development and application of aluminum-silicon alloys.Therefore,improving the morphology and size of eutectic silicon has become a key issue for improving the performance of as-cast aluminum-silicon alloys.The alloying and modification treatment are effective means to solve this problem.In this paper,Mg-La and Mg-Gd magnesium rare earths are used to modify Al-Si-Cu casting alloys respectively.The effects of the amount of magnesium and rare earth added and the pressure on the microstructure and mechanical properties of the alloy were studied using optical microscope,scanning electron microscope,transmission electron microscope and X-ray diffraction.The nucleation and growth mechanism of eutectic silicon before and after metamorphic treatment of Mg-La and Mg-Gd master alloys are clarified.The effects of two modifiers on the long-term and remelting properties of eutectic silicon were studied.The influence rules of holding time and remelting times on alloy composition,melt gettering,microstructure and mechanical properties were obtained.The results show that Mg-La and Mg-Gd two intermediate alloys have a significant modification effect on coarse eutectic silicon in Al-Si-Cu alloy.When the addition amount of the two magnesium rare earths is 1.0wt.%,The complete modification effect is achieved.At this time,the tensile strength distribution of the alloy is 254.7MPa and 262.6MPa,and the elongation is 2.7%and 2.86%,respectively.Compared with the performance of unmodified alloys,they are improved by more than 20%.There is not observed twin bands inside the unmodified eutectic silicon,and the growth of eutectic silicon is mainly a step growth mechanism.After modification of 1.0wt.%Mg-La master alloy,a large number of unidirectional twin bands and nano-cluster particles appeared in the eutectic silicon,and some nano-cluster particles segregated and adsorbed on the twin bands.As a result,the<112>growth rate of eutectic silicon is reduced,and the effect of refining eutectic silicon is achieved.The 1.0wt.%Mg-Gd master alloy metamorphic eutectic silicon has high-density alternating twinning bands,and the alternating twins exhibit an angle of 70.5°.At this time,the eutectic silicon grows in two directions and exhibits branching.The eutectic silicon morphology is fibrous.When the two magnesium rare earths are added in the same amount of 1.0wt.%,The absorption and deterioration of the alloy melt are studied.The research shows that the alloy melt will increase the absorption after a long time of heat preservation and multiple remelting.And reduce the deterioration effect.When the holding time is 0.5h,the minimum inspiratory volume of the melt is 0.15ml/100g,and the minimum inspiratory volume is 0.18ml/100g in 1h after adding Mg-Gd.As the number of remelting increases,the melt gettering shows an upward trend.The reason is that the active rare earth element combines with oxygen to form a rare earth oxide,which reduces the tendency of aluminum to combine with oxygen,hinders the formation of gas parasitic windows,and achieves the effect of reducing the amount of melt getter.However,due to the combination of rare earth and oxygen during the heat preservation and remelting process,the content of effective metamorphic elements in the melt is reduced,and it cannot become nucleation particles of eutectic silicon and inhibit its growth.Therefore,the melt loses its metamorphic effect after long-term heat preservation and repeated remelting.The squeeze casting process was used to study the influence of the interaction between magnesium rare earth and pressure on the morphology and mechanical properties of eutectic silicon.The results show that pressure can change the morphology and size of eutectic silicon,and the size of eutectic silicon decreases with the increase of pressure;when the pressure is 180MPa,the average size of eutectic silicon after adding Mg-La alloy is only 1.8?m The tensile strength and elongation were 287MPa and 3.1%,respectively.At the same time,when the molten metal solidifies under the pressure,as the pressure increases,the eutectic silicon in the microstructure tends to be distributed discretely,and the distance between the two eutectic silicon increases.According to the eutectic J-H classic growth model,pressure parameters are introduced,and the growth model of eutectic silicon under pressure is established based on the classic solidification theory.