Study On Melt Mixing Process And Nucleation Rate During Controlled Diffusion Solidification Based On Numerical Simulation

In the castings obtained by conventional casting process,the primary phase is always the large dendritic crystal,and the defects such as shrinkage cavity,shrinkage porosity and hot crack are often taken place.Therefore,in order to obtain high-quality products with expected microstructures,many solidification techniques have been developed.However,its application is restricted to varying degrees due to its inability to mass production,the need for expensive equipment or harsh external conditions.Therefore,this paper proposes a new type of controlled diffusion solidification technology to solve the challenges of preparing high-quality castings.Controlled diffusion solidification is a technology in which two precursor alloys selected according to specific composition and temperature are quickly mixed to obtain the desired target alloy.This method can produce the castings with fine and non-dendritic structure.The mixing method use d in this work is not to pour the high-temperature precursor alloy melt into the low-temperature ones and then mechanically stir to accelerate the mixing,but to pour the two precursor alloys into the same container at the same time.However,due to the co mplexity of the mixing process,it is difficult to clarify the mixing process of the precursor alloy,grain refinement mechanism and the effect of process parameters on it through experiments.Therefore,this work takes hypoeutectic Al-8%Si alloy as the target alloy and the flow field,temperature field and concentration field distribution during the forced convection mixing process of controlled diffusion solidification are firstly simulated by Fluent software.Following that,the nucleation rate during su bsequent solidification is predicted.Finally,the simulation results and experimental results are compared.The main conclusions of this work are as follows:1.In the mixing process of controlled diffusion and solidification,due to the effect of turbulence,the macroscopic mixing dominated by eddy diffusion and main convective diffusion can mix the two master alloys and form a target alloy melt with uniform temperature and composition on the macroscopic level without mechanical stirring.This mixing process is better than that of the conventional controlled diffusion solidification.2.The mixing process of controlled diffusion and solidification improves the heat and mass transfer capabilities of the melt,so that the mixed melt forms a large number of microscopically different areas of composition and temperature,while the temperature of the mixed melt is only a few degrees overheated.Therefore,in the subsequent solidification process,the supercooled areas with high solvent content in the mixed melt nucleate,which increases the nucleation rate and thereby refines the crystal grains.3.With the decrease of Si content in the precursor alloy,the temperature field and concentration field of the mixed melt are more uniform,but because the overall temperature of the temperature field is higher,resulting in a reduction in the nucleation rate from 52% to 11%.4.With the increase of precursor alloy temperature,the temperature field and concentration field reach uniformity faster,but the equilibrium temper ature of the mixed melt will be further higher than the liquidus temperature of the target alloy,so that the volume of supercooled areas in the melt is decreased,and the nucleation rate decreased from 52% to 0.5.With the shortening of the mixing time,t he faster flow rate of the two precursor alloy melts increases the turbulence of the mixed melt and makes the temperature field and the concentration field more uniform,and the nucleation area increases,and the nucleation rate increases from 38% to 52%.6.The experimental results show that the variation trend of primary ?-Al grain size in Al-8%Si alloy with the composition,temperature of precursor alloy and mixing time is consistent with the above simulation results,which indicates that it is feasible to simulate the mixing process of controlled diffusion solidification and predict the nucleation rate qualitatively using FLUENT software.