Effect Of External Fields On Solidification Microstructures Of Al-20%Si Alloy

Hypereutectic Al-Si cast alloys have been widely applied in aerospace, aviation and automotive industries, because of their low coefficient of thermal expansion, good high-temperature behavior, low density, high wear resistance and erosion resistance, etc. Especially in automotive field, the characteristics of hypereutectic Al-Si alloy are very suited for manufacturing moto pistons and cylinder blocks and other wear resistance parts. These components not only increase the efficiency of engines, but also make an important significance for reducing the weight of the automobile, improving the utilization of fuel oil and cutting exhaust emission. However, in normal casting conditions, due to the very wide crystallization temperature range and high latent heat of crystallization, primary Si trends to transform into coarse polygonal block and plate-like morphology which badly decrease mechanical properties as well as machinability, limiting its wider application. Many domestic and overseas researchers have done a lot of researches on how to diminish the size of primary silicon and improve the morphology and distribution of primary silicon in the alloy. Currently, methods applied to improve the solidification microstructures of hypereutectic Al-Si alloys are classified into two categories:One is modification treatment through adding modification elements; the other is the casting process through influencing solidification process to improve the microstructures of the alloy. Now the method of modification treatment is quite mature by many investigations, but most of casting processes are in the state of experimental study. In these casting processes, ultrasonic treatment and electromagnetic stirring are the current hot topics of research, but most of previous researches focus on exerting ultrasound and rotating magnetic field on the alloy during the solidification process. Ultrasonic treatment and electromagnetic stirring during solidification process have some effect on microstructure refinement but also result in macro-segregation of primary Si, reducing the usability of the alloy. Thus this paper investigated the effect of transition of melt structure before solidification on microstructures of the alloy by exerting ultrasonic treatment and electromagnetic stirring upon liquidus.It was found that after being treated by ultrasound for60S at720℃and water quenching, the average grain size of primary Si reduced from40-50μm that was untreated to10-20μm and primary Si became spheroidized. Heat preservation treatment being carried out after ultrasonic treatment, as heat preservation time extended, the melt structure transition generated by ultrasonic treatment upon liquidus disappeared gradually. When heat preservation time extended to9min. solidification microstructure came close to that of being untreated. Air cooling being carried out after ultrasonic treatment upon liquidus. grain size of the alloy presented obvious refinement, but plate-like primary Si increased a lot in the solidification microstructure.Being treated by electromagnetic stirring for60S at720℃and water quenching, as excitation voltage increased, macro-segregation of primary Si aggravated gradually. When excitation voltage increased to80V, the extent of primary Si macro-segregation reached the limit. An experiment aiming to research the effect of different stirring time on solidification microstructure under80V excitation voltage showed that primary Si segregation became obvious when stirring time reached to30S. Stirring time increasing from30S to60S, the extent of the primary Si segregation accentuated, but it’s not obvious.Al-20%Si alloy with Sr modification being quenched, compared with the alloy without Sr modification, the size of primary Si and a-Al grains had a certain degree of reduction. Ultrasonic treatment and water quenching being imposed on Sr-modified Al-20%Si alloy, primary Si and a-Al didn’t become more refined but grew up to some extent. Electromagnetic stirring before solidification had a little effect on macro-segregation of primary Si in the Sr-modified Al-20%Si alloy.