Effect of Internal Electromagnetic Stirrings DC Casting on Grain Refinement and Segregation of Large-Sized Billet of 2219 Alloy

An advanced method called internal electromagnetic stirring (I-EMS) was investigated to resolve the engineering problems like coarse-grain, inhomogeneous structure and macrosegregation. The electromagnetic stirrer functioned with internal-cooling was inserted in the melt during DC casting. In this study, a round billet of 2219 alloy DC cast with a diameter of 880mm under I-EMS process condition was produced, and its structure and composition distribution were comparatively characterized. The results show that the mean grain size decreased from the range of 872, 1023, 332 μm to the range of 317, 438, 271 μm at different billet positions with I-EMS. I-EMS consequently produce superior grain refinement and homogeneity. The effect of I-EMS on the grain-refinement and macrosegregation was also discussed.

Contactless Ultrasonic Treatment in Direct Chill Casting

Uniformity of composition and grain refinement are desirable traits in the direct chill (DC) casting of non-ferrous alloy ingots. Ultrasonic treatment is a proven method for achieving grain refinement, with uniformity of composition achieved by additional melt stirring. The immersed sonotrode technique has been employed for this purpose to treat alloys both within the launder prior to DC casting and directly in the sump. In both cases, mixing is weak, relying on buoyancy-driven flow or in the latter case on acoustic streaming. In this work, we consider an alternative electromagnetic technique used directly in the caster, inducing ultrasonic vibrations coupled to strong melt stirring. This ‘contactless sonotrode’ technique relies on a kilohertz-frequency induction coil lowered towards the melt, with the frequency tuned to reach acoustic resonance within the melt pool. The technique developed with a combination of numerical models and physical experiments has been successfully used in batch to refine the microstructure and to degas aluminum in a crucible. In this work, we extend the numerical model, coupling electromagnetics, fluid flow, gas cavitation, heat transfer, and solidification to examine the feasibility of use in the DC process. Simulations show that a consistent resonant mode is obtainable within a vigorously mixed melt pool, with high-pressure regions at the Blake threshold required for cavitation localized to the liquidus temperature. It is assumed that extreme conditions in the mushy zone due to cavitation would promote dendrite fragmentation and coupled with strong stirring, would lead to fine equiaxed grains.

Study on Solidification Structure Evolution of Direct-Chill Casting High Purity Copper Billet Using Cellular Automaton-Finite Element Method

A heat transfer model and a cellular Automation-Finite Element (CAFE) coupling model were established to analyze the solid/liquid (S/L) interface and solidification structure evolution of high purity copper Direct-chill (DC) casting billet under different casting conditions. The simulation and actual experimental results of liquid sump shape and solidification structure were first compared to verify the accuracy of the model. It is proved that the model is effective for simulating the solidification structure of the actual DC casting high purity copper billet. After that, the model was used to predict the solidification structure under different casting temperatures, casting speeds, and heat transfer coefficients. It is shown that, with the increase of casting temperature, the grain size decreases first and then increases. There is a compromise between grain size and its uniformity, and the grain size is more uniform at higher casting temperature. With the increase of casting speed, the depth of liquid sump and the height of the S/L interface increase, but the total grain number of the billet cross-section decreases gradually. As the heat transfer coefficient increases, the depth of the casting liquid sump becomes shallow, but the height of the solid-liquid interface increases and the grain size increases gradually. For the preparation of high purity copper billets with large cross-sectional dimensions by DC casting, a fine solidified structure could be obtained by appropriately reducing the casting speed and cooling intensity.