Aluminium remelt and billet casting plant
This is the final article of the review of the production instruction of the British company Ashford Engineering Services “D.C. Casting Remelt Shop Handbook.” This is a practical guide to the foundry operations of a small extruded aluminum extrusion plant.
This time we will talk about the processing of the aluminum melt before feeding it to the billet casting machine.
Control of melt temperature
Control of melt temperature should be a determining factor. Excessive melt temperature leads to the loss of alloying elements and requires their additional introduction, which increases the cost of the charge.
Temperature control is usually carried out by the temperature of the furnace roof or the temperature of the gases leaving the furnace. However, the predominant factor is the temperature of the melt in the furnace bath.
Thermocouple in the furnace bath
For a thermocouple that is immersed in a furnace melt, the main issue is the service life of its refractory casing. The main problems are the “freezing” of the thermocouple casing in the furnace bath and its mechanical damage when loading the charge into the furnace. Therefore, many casting shops remove the thermocouple from the furnace bath when loading the charge into it, cleaning the furnace and removing slag. At the beginning of the next melting cycle, the thermocouple is placed back in its place in the furnace bath. The hole into which the thermocouple is inserted should be large enough to allow slight adhesion of metal and slag to the thermocouple casing.
Melting furnace tools
The simple tools required for furnace operations are shown in Figure 1. These tools are required for handling scrap in the furnace, removing slag, cleaning the furnace, and collecting samples for spectral analysis. All of these tools have a limited service life and are therefore considered consumables. They are usually made directly at the factory.
Alloy composition and sampling
When all the scrap has been melted and the furnace has already been loaded to its full capacity, a sample of the melt is taken and tested for the content of various alloying elements and impurities. The most effective method for this is spectral analysis. Spectral analysis is performed using a light spectrometer. A light spectrometer is a device in which an electric arc evaporates the metal of a sample, which is made from a sample. Light from the arc passes through a system of prisms and lenses. Photosensitive diodes measure the intensity of each given line of the light spectrum. Each element has its own wavelength and place in the light spectrum. The intensity of each wavelength is proportional to the content of a given chemical element in the test sample. Calibrated photodiodes result in the percentage of all specified chemical elements in the sample.
Based on the results of the spectral analysis, the furnace is recharged – the addition of its various components – in order to achieve the specified chemical composition of the aluminum melt in the furnace.
Removing dross from the melt in the furnace
After adding fluxes to the melt, be sure to remove the slag from the surface of the melt. Slag is a mixture of various predominantly aluminum oxides with other impurities that collect on the surface of the aluminum melt. Before transferring the melt for casting or for subsequent out-of-furnace processing, the slag must be removed.
In small furnaces, slag is removed from the surface of the metal using a long hand tool with a scraper at the end turned 90º (see Figure 1). The foundry worker collects slag from the surface of the melt and directs it through the furnace window into the slag container, which stands directly below the loading window. Figure 2 below shows mechanized deslag devices for large furnaces.
Degassing and fluxing
Fluxing helps make the metal cleaner. Fluxing is carried out using appropriate “powders” or “tablets”, which are thrown into the furnace and the melt is thoroughly mixed. Another method is to blow the melt in a furnace with argon, nitrogen or a mixture of gases, which are fed through silicon tubes immersed in the melt.
The metal requires a certain amount of exposure to “degass” and allow hydrogen to escape from the melt. In addition, during fluxing, gas bubbles floating to the surface of the melt help remove non-metallic inclusions.
Back in the 1980-1990s, 100% chlorine gas was used for degassing. However, this gas is very harmful to human health and, in addition, causes corrosion of metal structures and equipment. Therefore, later they began to use fluxing with a mixture of two gases with a small content of chlorine. Examples of such gas mixtures are:
0.5-2.0% chlorine and more than 90% nitrogen;
5-10% chlorine and more than 90% argon;
5-10% chlorine and more than 90% freon.
In large industries, out-of-furnace degassing is often used in the technological transfer line of the aluminum melt from the melting or holding furnace to the casting machine. These degassing systems are more efficient than oven degassing. For example, they are capable of reducing the hydrogen content in the melt by more than 60%. Figure 3 shows two types of such degassing systems – rotary and with blowing through a porous plug.
Filtration of aluminum melt
The aluminum melt that comes from the melting furnace contains various small and large oxides, as well as other contaminants, such as fragments of the furnace lining or metal wires. To eliminate subsequent problems with the surface quality of extruded aluminum profiles, before supplying the melt for casting pillar ingots, it must be filtered to one degree or another.
Many factories manage to filter the melt only by passing it through a special fiberglass fabric folded in several layers. In this case, small non-metallic inclusions and other contaminants inevitably fall first into the pillar ingots and then into the extruded profiles.
When there are increased demands on the surface of extruded profiles, the melt is filtered using a filtration unit, the main element of which is a ceramic filter. An example of a simple filter installation is shown in Figure 4.
D.C. Casting Remelt Shop Handbook, Ashford Engineering Services, 1997.