Modelling of flow phenomena during DC casting

Abstract

Modelling of Flow Phenomena during DC Casting Jan Zuidema The production of aluminium ingots, by semi-continuous casting, is a complex process. DC Casting stands for direct chill casting. During this process liquid aluminium transforms to solid aluminium while cooling down. This is not an instantaneous transformation, but occurs in temperature interval. In the casting process the latent heat is moved away by convection and conduction. A number of problems may occur during solidification, because the solidification rate is rather high. The difference in density between liquid and solid aluminium is 7% and because of that solidification porosities may form during solidification when transport of liquid metal is insufficient. If besides this also high enough thermal stresses are present, cracks may be induced from these pores. The cracks, that originate during the solidification of the metal are called hot cracks. This in contradiction with cold cracks. These are formed due to high stress levels in the already solidified aluminium. What happens from the moment that the casting process is started? This is one of the questions that is treated in this thesis. To be able to describe the DC Casting process, it is necessary to have a good model that describes the phenomena at hand adequately and that also enables to do predictions on process changes. The model, that is used , is based on the differential equations that describe the heat- and fluid dynamics transport. In chapter 2 a description of the model is given. Numerical methods for solving these equations are also given there. Without validation, a model is of little use. Model validation can be performed using various methods during DC Casting experiments. Temperature recordings enable to follow the evolution of the temperature during the casting- and solidification process in some important positions. Because part of the heat transport in the DC Casting process is convection driven, it is also important to find information on the flow pattern and velocities in the liquid aluminium. Various methods to evaluate velocities in liquid metals are described in chapter 2. Based on their use for measuring the molten aluminium velocity during DC Casting, a ranking of the methods is made. Good boundary conditions are essential for reliable predictions of the behaviour of aluminium during the mould filling and subsequent solidification. In chapter 3 the determination of the most important boundary conditions is described. Those boundary conditions determine the amount of heat transport in that area of the system, where the heat transfer to the surroundings is the largest. Through sophisticated experiments the heat transfer from a plate of aluminium to (boiling) water could be predicted. This heat transfer model enables the prediction of temperatures close to the surface of the solidifying aluminium. By instrumentation of the experimental casting facility with thermocouples, the temperature close to the surface of the ingot could be measured as function of time. Calculations using the casting- and solidification model with the constant heat transfer coefficients, and calculations with the new model for water cooling were performed. This showed, that in the first centimetres from the surface of the ingot the new model gives a better description of the experiment. Further away from the surface towards the centre of the billet, the constant heat transfer coefficient model is just as good as the new model. The best method to measure liquid metal velocities during DC Casting is a method that is based on magnetic induction. The sensor to measure velocities is based on a ferro magnet, surrounded by a conduction tube with spot-welded thermocouples. Between two opposing thermocouples an inductive voltage is measured, which is proportional with the velocity of the surrounding medium. To test this method, a prototype of this sensor was constructed. In chapter 4 this sensor is described. The sensor was firstly tested in a water setup. Because the electrical conductivity of the water was several orders of magnitude less than that of liquid metals, this gave an unsatisfactory result. It was decided to perform fluid flow experiments in liquid tin. From these experiments results were obtained which gave a velocity dependent signal for the probe output. Unfortunately, electro-magnetical shielding from the surroundings was not good enough. This prevented to quantitatively test the sensor. In chapter 5 the start-up phase of the casting is covered. In the first part of this chapter, a calculation described in the literature was repeated, using the casting- and solidification model. This showed that, due to lack of published data, more validation was necessary to fully validate the model. With the aid of the experimental casting facility, a number of casting experiments were conducted. The experiments were recorded with a video camera and the temperature during casting was recorded by thermocouples at a number of locations in the ingot. The experiments served as the basis for a number of calculations with the casting- and solidification model. A conclusion, that could be drawn from the combination of the results of the casting experiments and the calculations , was that the description of the inlet of the mould in the model as an jet of aluminium was a good description of reality during the first tenths of seconds of the process. From the moment that the aluminium melt level is overflowing the inlet level, the jet description is not accurate anymore and should be replaced with a filling over the complete width of the mould. During casting, unwanted inclusions are filtered from the liquid aluminium by degassers and filters, before the casting table is reached. In order to comply with the increasing demands for the quality of cast products, an additional method to filter out particles from the launder system was evaluated. This is the subject of chapter 6. Manipulation of the flow pattern enables to create wakes, where particles can settle. Two dimensional calculations of the flow pattern in the mould with special flow modifiers, show that this settling is possible in theory. However, it is more likely, that transient velocity fluctuations prevent the settling of particles. To create a better method for settling of unwanted inclusions, a cyclone was designed, which enables the separation of particles 20 micrometers in diameter from a stream of liquid aluminium. The cyclone was used in a series of calculations with the fluid-flow and solidification model. The outcome of these calculations has resulted in a patent for this type of cyclone. Validation measurements using a water model of the cyclone have confirmed the working of the cyclone in separation of denser particles out of a slow flowing medium. The velocity of the particles was therefore tracked using a laser and camera set-up. The laser exposed a two-dimensional area of the water model. By auto-correlation of two subsequent images, it was possible to extract the particulate rate in this area. Velocity patterns indicated, that particles had a tendency of settling down in the lower part of the cyclone. Because there was no possibility to count all particles going in and out of the system, no quantitative comparison with the model was possible.