Aging furnace for aluminum
It is known, that approximately 60-70 % pressed aluminum profiles all over the world are made from alloys of the 6xxx series, primarily, alloys 6060, 6063, 6061 and 6005A. These aluminum alloys are thermally hardening. It means, that they have the ability to retain a solid magnesium aluminum silicide solution by quenching in a press, then to allocate it in controlled conditions of elevated temperature aging furnace. This mechanism has the naming of "age hardening", and the process step itself - artificial aging. During artificial aging of aluminum profiles 6xxx series alloys obtained predetermined strength properties - tensile strength, limit ductility and elongation.
natural aging
Magnesium silicide allocation process of a solid solution of aluminum is very complex. It involves the isolation and growth of clusters, that hinder the movement of dislocations, that provides a hardening material and. 6xxx series alloys, which after hardening leave be aged at room temperature, gradually gain strength for a period of 100 to 500 hours. This process is called natural aging. alloy 6060 with a minimum content of magnesium and silicon, this process may take several months. Therefore, the natural aging process is to some extent impractical due to delay supply of products. At the same time, profiles in the T4 state, have increased plasticity. Therefore, this condition applies, for example, for profiles, which will be subjected to bending.
Artificial aging
The aging of aluminum alloys is significantly faster at elevated temperatures. This is usually achieved by a higher level of strength, than with natural aging.
Artificial aging profiles of the 6xxx series alloys comprises heating to a temperature of about 180 ° C and holding at it for several hours, usually, 4-5.
Such treatment of aluminum profiles provide enhanced diffusion of dissolved aluminum and silicon atoms to magnesium direction portions, which starts selection of intermetallic particles Mg2Si. At the same time, the crystal lattice surrounding them is subjected to elastic deformation, since these secretions do not fit well into it. All this creates obstacles to dislocation motion, who are trying to get through these areas. The result is an increase in yield strength and tensile strength of the alloy.
If an aluminum alloy is maintained at an elevated temperature for too long, then there is a growth of these particles, and some of them disappear, and the yield strength of the alloy will decrease. In this case we say, that the aluminum alloy has an over aged condition (Figure 1).
Figure 1 - The effect of the duration of artificial aging
on the microstructure and strength of aluminum alloy [1]
The figure 2 shows the dependence of the strength of the alloy profiles 6063 artificial aging at different temperatures from 170 to 245 oC. strength level depends on the temperature, and on the exposure time with her. A typical process for aging profiles of alloy 6063 is aging in the aging furnace at a temperature 185 ° C within 4-5 hours.
Figure 2 - Influence of temperature and heating time
on the level of strength of the alloy profiles 6063 [1]
As can be seen from the figures 1 and 2 artificial aging process requires accurate machining of profiles with temperature for a predetermined time for each aluminum alloy. If the temperature profiles in the aging ovens will vary significantly - the length, width and height of the furnace, and then the strength properties of the profiles in the load will be different. Typically, the tolerance of cages temperature profiles in the furnace is ± 3 oC.
Aging oven device
aging furnace may have different design according to the specific production requirements.
Most ovens have the following basic structural elements:
- working chamber
- air circulating system
- One or two fan air circulation.
- The combustion chamber and the gas heating air.
- Gas burner and its control system.
Direct or indirect heating
Most aging furnace with direct heating. It means, that the gas combustion products are mixed with circulating air.
When indirect – indirect – heating the gas burning in special radiant tube and exits to the chimney outwardly. The circulating air flows along the outer surface of the radiant tubes and is heated by radiation and convection,. Indirect heating surface excludes contact profiles with combustion products, but it requires increased gas flow.
Longitudinal or transverse air flow
In most designs of aging furnaces, hot air is blown along the length of the cage of the profiles (Figure 3). At the same time, there are design options for furnaces with cross-flow air (Figure 4).
Figure 3 - Aging furnace with longitudinal air flow [2]
Figure 4 - Aging furnace with cross air flow [2]
Theoretically furnace with transverse blowing air can provide better uniformity of temperature profiles on cage, but have a lower thermal efficiency [2].
Uniformity of temperature profiles along the cage is highly dependent on the type of cages and density profiles styling. Heat transfer from the hot air to the profiles occurs primarily by convection. Therefore important maximum contact between the surface sections and circulating hot air.
Furnace length: efficiency and uniformity
The thermal efficiency of the oven aging increases with its length. However, Unfortunately, with increasing length of the furnace deteriorates uniformity cages temperature. Therefore, the optimum length of the furnace is a compromise between these two factors. Normally furnace accommodate the length of 1 or 2 basket, Four ovens are baskets length. The length of each basket about 7 m. For long kilns provide multiple zones of heating and air circulation.
Modern oven aging
An example of modern aging furnace aging furnace can serve twice the length of Tecalex [3] (figure 5).
Figure 5 - Oven-aging company Tecalex [3]
Circulating air is heated by the principle of direct heating. This ensures maximum thermal efficiency of the furnace. Gas burner installed in the combustion chamber, which is separated from the working chamber. Well-thought-out design and high quality thermal insulating materials ensure that the furnace is very low energy consumption. The principle of operation of the furnace based on hot air circulation in the longitudinal direction.
Baskets with profiles are moved by means of drive rollers in the floor of the shop, and the furnace. The temperature in the oven chamber is controlled by six thermocouples, are installed in different zones.
In the furnace two centrifugal turbines installed, which provide good heat out of the hot air flow over the entire cross section profiles cages. This gives a good uniformity of temperature and cage, Consequently, the same strength (hardness) for all cage profiles.
oven doors are equipped with a series of baffles, that help to evenly distribute the hot air over the cross section cages and effectively returning it to heat.
proper loading aging furnace
If the hot air in the aging furnace is any path bypassing cages profiles, then the heat transfer from the air is reduced, heating cycle becomes longer, gas flow rate increases, a cage for temperature uniformity deteriorates. Therefore, the main purpose of loading the aging furnace is to provide maximum airflow through your batch profiles. EXAMPLE normal dense cages in an oven aging is shown in Figure 6.
Figure 6 - Normal dense loading of profiles into the aging oven [3]
When the incomplete loading furnace - partial or incomplete number of baskets baskets - a furnace section forming a free passage for the hot air. In this case, hot air almost does not get between the profiles and the efficiency of their heating is significantly reduced (figure 7). For efficient processing of Partial tank system typically provide special deflectors, which direct air on partial cage.
Figure 7 - Circulation of hot air with incomplete loading of the aging furnace [1]
Moreover, When the oven is aging most massive profiles cages should be placed so, that they "get" the most hot air. The massive profiles may require longer heating, so this method of loading contributes to equalization of temperature profiles in the furnace working chamber.
Sources:
- R.W. Hains Precipation Aging – Aluminum Extrusion technology Seminar, Chicago, 1977
- Al Kennedy Age Ovens //The Extrusion Press Maintenance Manual, 2004
- Company Tecalex (Spain).
