Macrosegregation During Direct-chill Casting Of 7050 Alloy

DC casting is a complicated dynamic solidification processing.Many defects is formed during this processing,such as rough microstructures,cracks,uneven structures,macrosegregation and so on.In particular,the macrosegregation is a kind of irreversible defect and cannot be eliminated and weaken by the subsequent heat treatment.Therefore,the control of macrosegregation is very important to improve the quality of aluminum alloy products.In this paper,the subject researched is AA7050 aluminum alloy.Furthermore,a mathematic model is built based on the interaction of multi-physics field during DC casting processing.This model can accurately describe the electromagnetic field,fluid flow,heat transfer,alloy solidification and solute transportation during the conventional DC casting(DC),Air-knife DC casting,low frequency electromagnetic casting(LFEC)and electromagnetic-air knife DC casting processes,and is carried out by the commercial software ANSYS and Fluent.The four processes of the 7050 aluminum alloy billets with diameter of?300mm are calculated by using this mathematic model,and it is a good agreement between the calculated and experimental results.Based on the mathematic model,the effects of the metallurgical parameters(packing solid fraction,diffusion coefficient,initial permeability and floating crystal)on the macrosegregation in the billets during DC casting processing were studied,and the calculated results are as shown:the decrease of the packing solid fraction,the diameter of floating crystal and initial permeability,and the increase of diffusion coefficient can all weaken the macrosegregation in the billets.In addition,the effects of the casting speed,the position of air knife,electromagnetic intensity and combined action of electromagnetic field and air knife on the macrosegregation during the casting processing are studied,and the results are as shown:the macrosegregation is weaken by the decrease of casting speed;the macrosegregation is weaken by applying the low frequency electromagnetic field and with the increase of electromagnetic intensity;the macrosegregation is aggravated by applying air knife and shifting the position of air knife;when it is combined action of electromagnetic field and air knife,the macrosegregation is weaken relative to the air knife DC casting,and with the increase of electromagnetic intensity.Applying electromagnetic field during DC casting can refine the grain effectively and promote the formation of equiaxed grain.Applying air knife can coarsen the grain and promote the formation of dendritic structure,but the electromagnetic field weakens remarkably the role of the air knife.