Research On The Microstructure Evolution Of Al-38.5%Cu Hypereutectic Alloy At Constant And Changing Growth Rates Under Directional Solidification

In this paper, directionally solidified microstructures of Al-38.5%Cu hypereutectic alloys were investigated both at constant growth rates and at the change growth rates. The solidification behaviors of Al-38.5%Cu hypereutectic alloys, including coupled growth zone and adjustment mechanisms of lamellar spacing of eutectic were researched by an abruptly changing growth rate in directional solidification. The formation mechanism of primary Al₂Cu phase and the effect of thermosolutal convection on directionally solidified microstructures of Al-Al₂Cu hypereutectic alloys were presented and discussed.At a given temperature gradient of 250K/cm and the growth rate of 1μm/s, coupled growth lamellar eutectic of Al-38.5%Cu alloy was obtained, which was in agreement with the theoretical analysis result that indicated the growth rate should be less than 3.55μm/s. As the growth rate ranged from 5μm/s to 490μm/s, the primary Al₂Cu phase grew ahead of the growth interface of coupled eutectic and its morphology was changed at various growth rates.Thermosolutal convection has a remarkable effect on the directionally solidified microstructures of Al-38.5%Cu hypereutectic alloys. The results show that with the increase of solid fraction, the quantity of primary Al₂Cu phase was decreased gradually. Using the Scheil equation to predict and analyze the solute segregation in the solidified sample both at V=5 and V=10μm/s, the coupled growth lamellar eutectic could be obtained at the solid fraction larger than 0.62 and 0.7 respectively, which fit in with the experimental results of 0.49 and 0.66 in directional solidification of Al-38.5%Cu hypereutectic alloy.When the growth rate increased abruptly, the growth of θ- Al₂Cu phase was suppressed in directionally solidified microstructures of Al-38.5%Cu hypereutectic alloy. The results show that at higher growth rate, the coupled growth eutectic can keep growth without any coarse primary phases that illustrates the production of in-situ composite improved effectively.The eutectic lamellar spacing of solidified microstructure in Al-38.5%Cu alloy was refined by the abrupt increases of the growth rate. Both at constant growth rates and the abrupt increase growth rates, the values of eutectic lamellar spacing are close to the JH model analysis results.