Quality of DC cast aluminium billets
Aluminum ingots for aluminum extrusion are produced by the method direct-chill (DC) casting.
Control of the chemical composition of the aluminum alloy
For popular extruded aluminum alloys 6060 and 6063 magnesium and silicon content, and the ratio of their content, have the greatest influence on their moldability - ease of pressing, than any other factor.
To control the chemical composition of an alloy generally used optical emission spectrometer. Metal samples are taken from the smelting furnace:
- to razlivki, to make an adjustment of the chemical composition and, if necessary,
- again after making alloying additions and mixing, To make sure, that the adjustment has been performed correctly.
Metallurgy of aluminum alloys is very complex and therefore small amounts of some undesirable elements, such as iron and sodium, may have a disproportionate impact. Therefore, the spectrometer must have a sufficiently wide range of elements to be determined, to allow to carry out a detailed analysis of the chemical composition of the produced aluminum alloy.
Filtration of molten aluminum
Oxide inclusions are removed from the melt through a ceramic filter, which is placed downstream of the melt. Oxide inclusions fall into the melt from the furnace lining, of lead gutters, as well as the contaminated charge. The charge - it is all the raw materials, which are loaded into the melting furnace: technological scrap, purchase scrap, primary ingots, secondary in pigs, ligature alloys. Oxides can cause excessive wear and damage to the extrusion dies during pressing time.
To remove coarse dirt, for example, oblomkov linings, frequently used filters made of special refractory grid, usually in multiple layers. Ceramic filters are quite expensive and, Moreover, require constant heating. Therefore, small enterprises often costing only a fiberglass filter, which is installed directly at the outlet of the melting (holding) furnace.
The hydrogen content in the aluminum melt
Excessive hydrogen content can be controlled by degassing the melt in the furnace itself or in a separate degasser, which is installed in the line casting. A large hydrogen content can cause porosity ingots, blistering on the profiles and further profiles material embrittlement.
The level of hydrogen content is measured by the number of cubic centimeters of hydrogen per 100 g of metal. In the production of aluminum profiles usually considered limiting content 0,20 look3 hydrogen 100 g of metal.
The approximate level of hydrogen content can be measured at the factory using simple equipment. Accurate measurements require sophisticated equipment, and they are usually performed in specialized laboratories.
Aluminum Melt Temperature Control
Poor control of temperature of molten aluminum, and particularly a too high temperature in the casting of ingots results in problems with the quality of metal ingots and surface quality. With increasing aluminum melt temperature increases the solubility of hydrogen in it.
Tight control of the melt temperature increases the probability of successful start casting pillars in the casting machine and ensures a successful casting of pillars.
Methods of casting ingots
Apply various technologies casting pillars. Casting technology determines the casting table design, which are arranged crystallizers.
A modern and convenient technology is the Hot Top casting method.. This method is used as a constant supply of lubricant to the molds, and with a single grease mold before casting. On the very best machines, permanent lubrication is supplied mixed with air or argon (for example, Air Slip technology).
Even in the 1990s, it was widely used technology of casting pillars with the float control system supplying a melt crystallizers. Although this technology, basically, It provides good casting results, it is almost universally supplanted by casting systems with heat nadsadkami.
An advantage of casting systems Hot Top, especially with a constant supply of lubricant, It is, that posts, which are produced by this method, have a reduced thickness so-called inversion of the surface layer. This layer comprises a high content of alloying elements and impurities, which can produce surface defects profiles. This may be of particular importance, for example, when pressing the profiles for subsequent anodizing.
homogenization of ingots
Homogenization of ingots - holding at a given temperature (above 500 ºС) for several hours followed by rapid cooling. Proper homogenization ingots increases its compressibility and thermal efficiency of subsequent processing profiles.
Cooling of bars after homogenization
Increased rate and uniformity of cooling after heating pillars cages pillars in homogenization furnace are very important. With slow and non-uniform cooling cages pillars may be formed in the metal structure unfavorable.
experience shows, that own foundry plant for extrusion of aluminum profiles capable of producing ingots of high-quality poles. Despite, that the primary feedstock such plants are own technological waste aluminum and recycled aluminum, they are capable of producing ingots, which are not inferior quality ingots largest producers of primary aluminum ingots. The key to this is a well-chosen equipment, as well as a motivated and qualified staff.
Source:
- D.C. Casting remelt shop handbook, Ashford Engineerig Services, Great Britain, 1997
