Effect Of Standing Time After Mixing And Precursor Alloy Composition On The Microstructure Of Controlled Diffusion Solidification Hypoeutectic Al-Si Alloy

In order to overcome the defects such as shrinkage and thermal cracking in the castings obtained from conventional casting,researchers have developed the controlled diffusion solidification（CDS）technology,but the existing CDS process leads to unavoidable abnormally coarse grain defects in the castings due to poor mixing effect;and there are strict requirements on the quality ratio of the precursor alloy,which makes it impossible to perform CDS casting for some compositions of the alloy.For this reason,this group proposes the CDS casting technology with simultaneous mixing,which can solve the above problems well.However,the time interval between mixing and pouring is described（standing time）in vague terms such as"rapidly","immediately",etc.Furthermore,the existing studies do not provide detailed explanations about the correlation between the influence of the precursor alloy composition on the CDS process.Therefore,in this paper,using hypoeutectic Al-Si alloy as the target alloy,pure Al and Al-Si alloys with different Si contents as the precursor alloy,the effects of the standing time after mixing and the composition of the precursor alloy on the microstructure of the hypoeutectic Al-Si alloy were investigated based on a combination of numerical simulations,theoretical calculations and experiments.The following conclusions were drawn:（1）During the standing process,the air involved in the mixed melt due to the mixing of the two precursor alloys will overflow from the melt in the form of bubbles within 1.2 s;In the early stage of standing（0-0.5 s）,there is still some convection effect in the mixed melt,which makes the solute field get further homogenized,and because the thermal conductivity is much higher than the solute diffusivity,the temperature field in the mixed melt is more homogeneous than the solute field,which is very beneficial to enhance the nucleation rate.（2）It is due to the thermal conductivity being much higher than the solute diffusivity that the melt in the pure Al pockets close to the interface of the pure Al/Al-12Si melts is first rapidly chilled to a supercooling state by the surrounding low-temperature Al-12Si melt,which then causes nuclei to promptly form in this pure Al melt.During the standing process,part of the nuclei will remelt,while another part of the nuclei grows towards the pure Al melt side with a stable solid/liquid interface,forming spherical non-dendritic grains,which increase in size and decrease in number as the standing time increases.But eventually,all the non-dendrite grains formed will be remelted completely due to the superheating property of the mixed melt.Therefore,due to the obvious grain remelting during the standing process,the CDS process should cast the mixed melt within 10 s after mixing in order to obtain non-dendritic microstructure castings.（3）In the CDS process of mixing pure Al（Alloy 1）with Al-Si alloys of different Si content（Alloy 2）,the mixing temperature of both precursor alloys is 5 K above the respective liquid phase line temperature.Since the mixing temperature of alloy 1 is constant,when alloy 2 is a hypoeutectic Al-Si alloy,its liquid phase line temperature（mixing temperature）decreases with the increase of Si content,so the chilling effect of alloy 2 on alloy 1 is enhanced,leading to the increase of the nucleation supercooling region,i.e.,the nucleation rate increases.At the same time,theΔG(ΔG=|G_liquidus|-|G_max|)value is gradually increasing,the mixed melt superheat is gradually decreasing,the grain remelting phenomenon is weakening,which makes the microstructure of the casting better;while when alloy 2 is a hypereutectic Al-Si alloy,the liquid-phase line temperature increases with the increase of Si content,so the cooling effect of alloy 2on alloy 1 is weakening,which leads to the gradual decrease of the nucleation rate.At the same time,theΔG value is gradually decreasing,the mixed melt superheat gradually increases,and the grain remelting phenomenon is enhanced,which makes the microstructure of the casting worse.（4）In addition,to obtain hypoeutectic Al-Si alloy castings with spherical non-dendritic microstructure,the Gibbs free energy condition（ΔG>0）should be satisfied when selecting the two precursor alloys for CDS,and the larger theΔG value,the better the microstructure formed in the casting.In this work,when pure Al and Al-12Si are used as two precursor alloys,theΔG value is the largest（205.19 J）,so the microstructure morphology of its castings is the finest and rounded,with an average grain size of 41.3μm and a shape factor of 1.34,thus overcoming the casting defects such as shrinkage and thermal cracking in conventional castings.