Segregations In Aluminum Alloys Ingots Cast By LFEC Process

Segregation of alloying elements in the ingots of Al alloys, including microsegregation and macrosegregation, is a kind of structural defects which is the main reasons for uneven microstructures and properties of products. In this study the microsegregation and macrosegregation in the aluminum alloy will be improved by application of the low frequency electromagnetic field in DC casting, which will build a technical support and the theoretical base for casting aluminum ingots with high quality.Al-4.5%Cu and the7075aluminum ingots with diameter of200mm were cast by the DC and LFEC (low frequency electromagnetic casting) processes. The grain size, the secondary dendrite arm spacing (SDAS) and the areal fraction and size of eutectics in DC ingots with that in LFEC ingots was measured and compared. The distributions of the solute elements were determined by EAMA, by which the distribution profiles of solute elements were drawn out and the effective distribution coefficients were calculated. Combined with the calculation of the microsegregation, the mathematical model was established, by which the flow field, the temperature field and the distribution of the solute during cast were simulated. At last, the reasons for improvements of macrosegregation were discussed.First, the microsegregation in the two kinds of aluminum alloy ingots was experimental investigated. Compared to the as-cast structure in DC ingots, the grains were well refined and the SDAS were larger in the LFEC ingots, which are resulted from the increase in nucleation rate and a thinner concentration boundary layer in the solid/liquid interface due to forced convection caused by the electromagnetic stirring in LFEC process. Same time, the constitutional under-cooling in the tip of the dendrite became weak which made the difference between the prior growth direction and the other growth directions small, so that the SDAS became larger in the LFEC process.The areal fraction and the size of the nonequilibrium eutectic were measured. It was found that both the areal fraction and the size of the nonequilibrium eutectic became smaller in LFEC ingots because the cooling rate during solidification was accelerated by the low frequency electromagnetic field. More solute elements were solid solved in the α-Al phase, and the areal fraction and the size of the nonequilibrium eutectic in the grain boundary decreased. The effects of magnetic intensity are more remarkable.In the Al-4.5%Cu alloy, the concentration profiles of Cu were plotted using the EPMA data by F-G (area scan) approach. It was found that compared to the DC process, the concentration of Cu in initial solidification zones is higher and the concentration of Cu in final solidification zones is lower because of higher cooling rate in LFEC process. The difference of Cu content in the center and the edge of the dendrites became smaller, which means that the solute became more uniform in the dendrite, that is to say the microsegregation in the Al-4.5%Cu alloy was improved in LFEC process.The distribution profiles of the alloying elements in7075alloy were drown out by F-G approach, by which the effective distribution coefficients of Zn, Mg and Cu were calculated. The results shown that the effective distribution coefficients of Zn, Mg, Cu increased to a unit in different ways before the dendrite network formed because of interactions of the solutes. It was found that the concentrations of the solutes in the grains of the LFEC ingots were higher and the effective distribution coefficient of the solutes were also higher than that of DC ingots, which means that the microsegregation of7075Al alloy was improved in LFEC ingot.Based on the calculations of effective distribution coefficient in the microsegregation, a mathematic model was established to describe the flow fields, the temperature fields and the concentration distributions in the DC and LFEC processes well. The numerical simulation results shown that the temperature filed became uniform and the sump during casting became shallow as a result of the forced convection driven by the electromagnetic field, which made the "floating grains", which were short of solute elements, move along the solidification front difficult, and a few of them deposited in the center of the ingot, thus the negative segregation in the center of the ingot was mitigated. On the other hand, the shrinkage in sub-surface forced the liquid full of solute elements to move between the dendrite networks, and leads to a more severe negative segregation in the sub-surface.At last, it was found that during casting, the phenomena in the microscale and macroscale were interknitted and interacted. The improved microsegregation was help to the alleviation of the macrosegregation. At the same time, the changes of the velocity and the temperature field in macroscale had effects on microsegregation.The results in this paper show that the LFEC process can improve the microsegregation and the macrosegregation obviously in the aluminum ingots. It is a simple and effective method to cast the aluminum ingots with high quality.