Study On The Separation Of Primary Silicon In Hypereutectic Al-Si Melt During Electromagnetic Directional Solidification

Al-Si solvent refining to purify metallurgical silicon show advantages: lower process temperature,no complex compound generated and the cheap materials.It’s attracting many researchers.How to separate the purified silicon is still a key step of the whole process and restrain the technology be popular and should be done systemic research.In this dissention,alternating current electromagnetic induction combined with directional solidification technology is used to separate primary silicon from hypereutectic Al-Si melt.It involves electromagnetic,heat transfer,flow,component segregation,phase transition and other complex physical and chemical processes.Therefore,it needs to be deeply studied theoretically and technically.The study provides some useful exp erimental parameters and theoretical support for the industrial application of electromagnetic directional solidification technology in separation of silicon based alloys.By experimental process and theoretical calculation,this dissention used industrial silicon and aluminum to melt to get hypereutectic Al-Si melt.By refining and separating the primary silicon from the industrial silicon and the hypereutectic Al-Si melt,the primary Si-rich area after the electromagnetic directional solidification was compared with the impurity removal efficiency,the effective segregation coefficients of B and P were studied.The advantages of Al-Si electromagnetic induction directional solidification in impurity removal efficiency is confirmed.By controlling the heat preservation mode,directional solidification rate,directions,distance,heat exchange conditions and current frequencies,the separation conditions of primary silicon from hypereutectic Al-Si melt were obtained.The mechanism of separation of primary silicon from hypereutectic Al-Si melt under electromagnetic induction directional solidification is revealed by using quenching experiment to fix the intermediate process of directional solidification.Hypereutectic Al-Si melt was modified by adding tin.The separation of primary silicon is realized.The flow field,temperature field and magnetic field of the melt under different conditions are simulated by software named COMSOL,and the distribution of the flow field,temperature field and electromagnetic fiel d of the melt are obtained and confirmed by experimental phenomenon.XRD、ICP、SEM、TGA、EDS and OM are used to analyze and tested.The main research contents and conclusions are as follows:1.The separation behavior of main impurity elements Fe,Ti,Ca,B and P in industrial silicon,Al-Si refining and Al-Si electromagnetically induced directional solidification samples was studied.The effective partial coagulation coefficients of B and P in Al-Si electromagnetic refining were 0.192 and 0.127.Electromagnetic stirring enhanced the transportation of characteristics of impurities.After using aluminum-silicon system for refining and directional solidification,the effective separation coefficient of P and B was significantly reduced to 0.102 and 0.051,and the removal effect of impurity elements was enhanced.2.The separation of primary silicon from hypereutectic Al-Si melts under different conditions was studied.The effects of lift,heat preservation,temperature gradient,magnetic field frequency,directional solidification rate and direction on the separation of primary silicon were studied.The results show that the directional solidification rate of primary silicon is lower than 1 μm/s in the resistance furnace,while the directional solidification rate is lower than 120 μm/s in the electromagnetic induction furnace.The insulation conditions greatly affect the separation effect of primary silicon from hypereutectic Al-Si melt.The higher the temperature gradient,the better the separation effect of silicon phase from hypereutectic Al-Si melt.Compared with 30 kHz,the melt can be effectively stirred at 3kHz,and the primary silicon can be effectively separated from the hypereutectic Al-Si melt at 10 μm/s directional solidification rate.Finally,a crack penetrated the sample and divided it into two parts: the upper part was the area of Al-Si alloy,the lower part was the area of primary silicon enrichment,and the silicon content was close to 90 wt.%.3.The mechanism of separation of primary silicon from hypereutectic Al-Si melt under electromagnetic directional solidification is revealed.The skin depths of the melt at 50,30 and 3kHz was 1.027,1.876 and 5.930 mm,respectively.The electromagnetic force is mainly concentrated in the surface layer of the melt and cannot penetrate into the aluminum-silicon melt.The electromagnetic force is manifested as the force of oscillation inward.4.The continuous growth model of silicon phase is proposed and verified by experiments.Through experiment and simulation,the growth of hypereutectic aluminum-silicon melt under the condition of alternating current electromagnetic induction was studied.It was found that the maximum velocity of the melt under the conditi on of 3kHz was 0.92cm/s.The formation of silicon enrichment area is mainly caused by ostwal maturation and quasi-ostwald maturation of primary silicon,and the maximum growth speed of primary silicon in the vertical direction is 120μm/s.5.The effects of different directional solidification rates and initial silicon content on the separation of hypereutectic Al-Si melts were studied.Under the condition of 3kHz,when the directional solidification rate increased from 10 μm/s to 100 μm/s,the silicon content in the silicon enriched area decreased from nearly 90 wt.% to 70 wt.%.The effect of different initial silicon content(35,45,55 and 65 wt.%)on the separation efficiency was studied.It was found that the silicon content in the silicon phase enrichment area after separation all reached more than 85 wt.%,and the initial silicon content had no effect on the separation effect.6.The separation efficiency of primary silicon was improved.In order to improve the growth condition of primary silicon by adding tin,the separation of primary silicon was strengthened.With the addition of 30 wt.% tin,the silicon content in the primary silicon enrichment area increased from 70 wt.% to 90.45 wt.% at the directional solidification rate of 100 μm/s.Compared with the directional solidification rate of 10 μm/s,the enrichment efficiency of silicon phase was basically the same,and the time required for directional solidification was reduced by 90%.