Effects Of Physical Fields On Iron-phases And Solidification Structure Of Direct Chill Casting Ingots In Al-Si Alloy

Al and Al-alloys are extensively used in automotive, aerospace applications and foundry industry due to their low density, high specific strength, and good wear resistance, etc.. It is well known that grain refinement and elimination of iron-containing intermetallic copounds (IMCs) are greatly useful for Al-Si alloy castings. At present, the Al-Ti-B alloy, Sr and Mn are commonly used in industry to refine grains, modify acicular Si and iron-containing IMC in Al-alloys, respectively. However, they are always accompanied by agglomeration of refining phases, poisoning failure and composition variation. By contrast, the application of physical fields can be used to achieve grain refinement and elimination of the harmful phases in Al-Si alloys without the defects above-mentioned.In order to refine the solidification structure, modify the morphology of the iron-containing IMCs and improve the mechanical properties of Al-Si ingot, ultrasonic field (USF), electromagnetic field (EMF) and the compound fields were applied during the direct chill (DC) casting process of Al-8%Si alloy. Then USF was carried out in Al-12%Si-2%Fe and Al-2%Fe melt to modify the AlSiFe IMCs and Al3Fe phase. Then the effects of physical fields on solidification microstructure and IMCs were studied; the interaction mechanism of EMF and USF on solidification was also discussed.The results are showed below:1. The application of power ultrasound in different temperature region is useful to the refinement of primary and eutectic Al3Fe phases. Under USF, the average size of primary Al3Fe reduced from160μm to20μm, length-width ratio reduced from15to2; for the eutectic Al3Fe, the average size reduced from30μm to8μm and length-width ratio from16to4. With the combination of rapid solidification, eutectic Al-Al6Fe replaced Al-Al3Fe, and there was no primary Al3Fe in the microstructure. USF can bring about undercooling to the treated melts, which is beneficial to the nucleation and then the refinement of Al3Fe phases. 2. USF had a big influence on the morphology and composition of AlFeSi IMCs formed in the Al-12%Si-2%Fe alloy. When USF was applied over660℃, the metastable a-AlSiFe phase was obtained, instead of acicular β-AlSiFe. The average size of iron phase was highly reduced from400μm to90μm. When USF was applied at640℃, the acicular β-AlSiFe phase was replaced by the rod-like ones without any changes of the composition. The ultrasonic cavitation-induced nucleation mechanism and cavitation failure mechanism should be responsible for the phase transformation and morphological evolution of AlFeSi IMCs.3. Under the combination of rotating magnetic field (RMF) and USF, an ingot with uniform and refined equiaxed grain was obtained. When they were applied simultaneously, RMF can enlarge the ultrasonic treated region, while USF can enhance the electromagnetic effect on refinement, resulting in the distinct grain refinement in the whole ingot.4. When USF was applied during the DC casting process of Al-8%Si alloy, limited by the effective region of cavitation, the structure of the ingot was much finer (with the average size of70μm) in the center but coarser at the edge (1200μm). When intermediate frequency electromagnetic (IFEM) field was applied solely, the grain refinement (the grain size reduced from1200μm to600μm) and improvement of mechanical properties (the ultimate tensile strength enhanced from180MPa to200MPa) were obtained, when the current was100A. Based on the numerical simulation, the IFEM field brings about fluid flow, mass and heat transfer in the melt, and thus influences the nucleation as well as growth process during the solidification process.5. When IFEM field and USF were applied during DC casting process, obvious refinement (with the grain size of150μm) and improved mechanical properties (with the ultimate tensile strength of215MPa) were obtained. Under the effects of ultrasonic cavitation-induced nucleation and forced convection by IFEM field, uniform and refined equiaxed grains were substituted for the developed dendrites, resulting in the remarkable improvement of mechanical properties.