Hardening of aluminum profiles on a press
The cooling rate of aluminum profiles - hardening - immediately after exiting the press must be fast enough to retain magnesium and silicon in solid solution. This ensures the achievement of maximum mechanical properties of the profile material due to their release during subsequent hardening by aging.
The required cooling rate of a solid solution of alloying elements - magnesium and silicon in aluminum - to ensure the hardening effect depends on the cross-sectional dimensions of the aluminum profile and the methods of its cooling:
- calm air,
- fans,
- water mist,
- water spray cooling or
- in a stream of water.
The figure and table show the minimum permissible cooling rates of aluminum profiles for various alloys of the 6xxx series. For 6060 alloy aluminum profiles (AD31 aluminum alloy), still air or fan cooling is usually sufficient, while for 6061 alloy profiles, spray water cooling or water flow cooling is required.
Aluminum alloys
These materials can be subjected to hardening - it increases strength and makes it possible to strengthen the workpiece by aging in the future.
(Picture 1)
Hardening leads to the formation of a supersaturated solution of alloying substances in a matrix solution of the base metal with a nonequilibrium structure. The hardening technology is as follows:
- The aluminum alloy is heated to the temperature of complete or partial dissolution of excess intermetallic phases in aluminum.
- Maintained at this temperature.
- Quickly cools to a temperature of 18-20 degrees.
The greater the degree of supersaturation with alloying elements, the higher the strength of the alloy.
Through artificial or natural aging, an unstable state is transferred to a stable one, and strength increases while the level of ductility decreases.
Aging of aluminum alloys
Aging of 6xxx series alloys is carried out to improve the mechanical properties of aluminum profiles. The extent of this increase in properties depends on the type of alloy and aging conditions. These conditions vary for:
- natural aging, which occurs “by itself” at room temperature, and
- artificial aging, which is carried out at elevated temperatures, about 160-200 ºС.
The strength of 6xxx series alloys is directly related to their ability to resist the movement of dislocations during deformation. When stress is applied to a material, dislocations form and move within it. As stress increases, the number and density of dislocations moving in the material increases until the material finally fails.
The movement of dislocations is inhibited due to the presence of Mg2I particles and therefore the strength of the aged aluminum alloy increases. The size and density of these particles are controlled by aging parameters. A small number of small particles of β”-Mg2I can do little to stop dislocations moving through a material, but when there are many of them they impede the movement of dislocations and this increases the strength of the material.
If the particles grow too large (β'-Mg2I and β-Mg2I), there are too few of them due to the limited content of Mg and Si in aluminum. In this case, dislocations easily bypass these particles and the strength of the material decreases.
This is demonstrated by the schematic graph of typical artificial aging in the figure below.
heat treatment of aluminum alloy 6060 (AD31)
The peak strength properties are achieved with a large number of small β”-Mg2I particles. For 6xxx alloys, typical artificial aging parameters are a temperature of 170 ºС for 8 hours or 185 ºС for 6 hours.
Heat treatment of aluminum alloys
Heat treatment of aluminum profiles is used to modify the properties of the aluminum alloys from which they are made by changing their microstructure. The main strengthening mechanisms in aluminum alloys are strengthening due to solid solution alloying and strengthening due to precipitation of secondary phases. As a rule, one of these mechanisms is dominant in the alloy.
Solid solution of aluminum alloys
A solid solution is obtained by heating an aluminum alloy, during which all phases present in it dissolve to form one homogeneous phase - aluminum with alloying elements dissolved in it. With increasing temperature, the solubility of elements increases, and with decreasing temperature, it decreases. The mechanism of hardening is that when the aluminum alloy is cooled quickly enough, the dissolved elements remain in the atomic lattice of aluminum and distort and elastically deform it. This distorted atomic lattice hinders the movement of dislocations and, consequently, plastic deformation of the alloy and thereby increases its mechanical strength.
Aging of aluminum alloys
Aluminum alloys that are hardened by age contain a certain amount of soluble alloying elements, such as some combinations of copper, magnesium, silicon, manganese and zinc. When subjected to appropriate heat treatment, these dissolved atoms combine into very small particles that are released within the grains of the aluminum alloy. This process is called aging, as it occurs “by itself” at room temperature. To speed up and achieve greater efficiency in strengthening the aluminum alloy, aging is carried out at elevated temperatures, say 200 °C.
Hardening of aluminum profiles on a press
Press hardening is a very cost-effective technology for heat treatment of aluminum profiles compared to hardening with separate heating. When hardening in a press, aluminum profiles are cooled based on the temperature at which they leave the matrix. A necessary condition for hardening on a press is that the heating temperature range of the aluminum alloy for hardening must coincide with the temperature range of aluminum profiles at the outlet of the press. This, in principle, is carried out only for “soft” and “semi-hard” aluminum alloys - technical aluminum, aluminum alloys of the 3xxx and 6xxx series, as well as low-alloy alloys of the 5xxx series (with magnesium up to 3%) and some aluminum alloys of the 7xxx series without copper alloying (7020, 7005 (our 1915), 7003). The hardening effect for 3xxx and 5xxx aluminum alloys is very small and, as a rule, is not taken into account. The final mechanical properties of aluminum alloys 3xxx and 5xxx are obtained not as a result of thermal hardening, but during subsequent cold hardening, which may include heat treatment operations: one or more annealing. The strengthening phase for alloys of the 6xxx series is the Mg2Si compound.
Hardening on a press of aluminum profiles from alloys AD31, 6060 and 6063
All aluminum alloys of the 6xxx series can be hardened directly on the press. To fix the dissolved phases in the aluminum solid solution, it is necessary to cool the aluminum profiles at the outlet of the press at a speed not lower than a certain critical speed. This speed depends on the chemical composition of the aluminum alloy. Typically, enhanced cooling by fans is sufficient for most aluminum profiles, but sometimes it is necessary to cool them with water or a mixture of air and water. Successful hardening of 6xxx series aluminum alloys depends on the thickness of the profile, as well as the type of alloy and its chemical composition. In the case of overly massive aluminum profiles, for example, from alloy AD33 (6061) and a relatively slow pressing speed, the material exiting the matrix may not reach the temperature range required for hardening and some of the Mg2Si particles will remain undissolved. Therefore, with subsequent air, or even water, cooling of the profiles, their complete hardening will not be possible. In such cases, separate heating is used for hardening in special furnaces - usually vertical, followed by cooling in vertical tanks with water. After hardening, aluminum profiles are stretched by 1.5 - 3% to straighten and relieve residual stresses.
Aging of aluminum profiles: artificial and natural
The final operation of heat treatment of aluminum profiles is aging, natural or artificial. Natural aging occurs naturally over a period of time, which varies for different aluminum alloys - from several weeks to several months. Artificial aging is carried out in special aging ovens.
T-states of aluminum alloys
Different options for aging parameters correspond to different designations of the state of aluminum alloys:
- T1 – cooled after pressing to room temperature and naturally aged;
- T4 – after pressing, hardened with separate heating and naturally aged;
- T5 – cooled after pressing to room temperature and artificially aged to maximum strength properties;
- T6 – after pressing, hardened with separate heating and artificially aged to maximum strength properties.
To refer to other aging treatments that are specifically designed to produce mechanical properties that differ from the maximum strength properties. For example, states T52 and T591 are used for aluminum profiles that are subject to bending, and state T7 is used for profiles that are used at elevated temperatures.
Annealing methods for aluminum sheets
Annealing of aluminum alloys is not mandatory. But in some cases, without this heat treatment method it is impossible to achieve the desired characteristics of the material.
The reason for the use of annealing may be the special state of the alloy, which can be expressed in a decrease in the ductility of the material.
The use of annealing is recommended when observing three types of conditions:
- The non-equilibrium state characteristic of cast products is associated with the difference in temperature conditions. The cooling rate of cast products significantly exceeds the recommended one, at which the effect of equilibrium crystallization is achieved.
- Plastic deformation. This condition may be caused by technological requirements for the characteristics and shape of the finished product.
- Heterogeneous structure of the material caused by other heat treatment methods, including hardening and aging. In this case, one of the alloying components separates into the intermetallic phase, accompanied by supersaturation of the components.
The above problems can be eliminated by annealing. Normalization of the structure and condition of the aluminum alloy is accompanied by an increase in ductility. Depending on the type of nonequilibrium state, various annealing methods are selected.
Today there are three annealing modes:
- Homogenization. Designed for processing cast ingots. During the heat treatment of ingots at high temperatures, a uniform structure is achieved. This simplifies the rental process while reducing production costs. In some cases it can be used to improve the quality of deformed products. The annealing temperature is maintained within 500 degrees, followed by holding. Cooling can be done in several ways.
- Recrystallization. Used to restore deformed parts. This requires pre-treatment with a press. The annealing temperature varies in the range from 350 to 500 degrees. The holding time does not exceed 2 hours. The speed and method of cooling has no special limits.
- Heterogenization. Additional annealing after other heat treatment methods. This method is necessary for softening aluminum alloys. This processing method makes it possible to reduce the degree of strength while simultaneously increasing the level of ductility. Annealing is carried out at approximately 400 degrees Celsius. Exposure is usually 1-2 hours. This type of annealing significantly improves the performance characteristics of the metal and increases the degree of corrosion resistance.
Stretching and curing profiles
The usual practice for the manufacture of extruded aluminum profiles includes stretching them from 0.5% to 3% and then curing them with a delay of a day of artificial aging for profiles made of low-alloy 6xxx alloys (no more than 0.9% Mg2I), for example, aluminum alloys AD31, 6060 and 6063. This helps to achieve optimal mechanical properties of the profiles after aging.
However, such a delay for higher strength aluminum alloys (Mg2I content more than 0.9%), for example, 6061, can lead to reduced mechanical properties of the aluminum profile material. These alloys contain copper in an amount of at least 0.1%, which counteracts the effect of delayed artificial aging on the final mechanical properties of thermally strengthened aluminum profiles.
