The heat treatment of aluminum alloys
The heat treatment of aluminum profiles are used to modify the properties of aluminum alloys, from which they are made, by altering their microstructure. The main reinforcing mechanisms in aluminum alloys are hardening due to solid solution alloying and hardening due to the secondary phase precipitates. Usually, One of these mechanisms is dominant in the alloy.
The solid solution of aluminum alloys
The solid solution was prepared by heating aluminum alloy, wherein all phases existing therein are dissolved to form one homogeneous phase - aluminum with dissolved therein alloying elements. With increasing temperature the solubility of the elements increases, with decrease in temperature - is reduced. strengthening mechanism is, that at a sufficiently rapid cooling of the aluminum alloy elements are dissolved in the atomic lattice of aluminum and distort, 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, which are strengthened by aging, contain a certain amount of the soluble alloying elements, for example, Some combinations of copper, magnesium, silicon, manganese and zinc,. With the appropriate heat treatment of these solute atoms are joined in the form of very small particles, which are released inside the grains of aluminum alloy. This process is called aging, so it is "itself" at room temperature. To accelerate and achieve greater efficiency aluminum alloy hardening aging is performed at elevated temperature, let us say, 200 ° C.
Tempering aluminum profiles on the press
Hardening in the press is a very cost-effective technology heat treatment of aluminum profiles compared to quenching with separate heating. When quenching in a press cooling aluminum profiles performed on temperature, with which they come out of the matrix. A necessary condition for hardening on press – aluminum alloy heating temperature range for quenching must coincide with the temperature range of aluminum profiles at the exit of the press. it, basically, It is performed only for the "soft" and "semi-solid" aluminum alloys - technical aluminum, 3xxx series aluminum alloys and 6xxx, as well as low-alloy 5xxx series alloys (with magnesium up to 3 %) and some aluminum alloys of the 7xxx series without copper alloying (7020, 7005 (our 1915), 7003). hardening effect for aluminum alloys and 3xxx 5xxx very insignificant and, usually, not taken into account. The final mechanical properties of aluminum alloys and 3xxx 5xxx not take as a result of thermal bonding, and in the subsequent work hardening, which may include heat treatment operations: one or more annealing. uprochnyayuschey phase alloys 6xxx series is the compound Mg2Si. For details, see. Tempering aluminum profiles on the press
Hardening on the press AD31 alloys, 6060 and 6063
All 6xxx series aluminum alloys can get directly to the hardening press. To fix the solid solution phase in the aluminum must be cooled solution of aluminum profiles at the exit of the press at a speed not less than a certain critical speed. This rate depends on the chemical composition of the aluminum alloy. Usually reinforced cooling fan is enough for the majority of aluminum profiles, however, it is sometimes necessary and cooling them with water or a mixture of air and water. Successful hardening aluminum alloys 6xxx series depends on the thickness profile, as well as on the type of alloy and the chemical composition. In the case of excessively massive aluminum profiles, for example, made of AD33 (6061) alloy and a relatively slow pressing speed, the material at the exit from the matrix may not reach the temperature range, required for hardening of the particles and Mg2Si remains undissolved. Therefore, in the subsequent air, or even the water, cooling profiles of their full hardening will not work. In such cases, use separate heating for quenching in special furnaces - usually vertical, followed by cooling vertical tanks with water. After quenching the aluminum profiles produce their stretching 1,5 – 3 % straightening and stress relief.
Aging of aluminum profiles: artificial and natural
Final heat treatment operation is the aging of aluminum profiles, natural or artificial. Natural aging takes place by itself over time, different for different aluminum alloys - from several weeks to several months. Artificial aging is carried out in special aging furnaces. Typical heat treatment regimes for some aluminum alloys 6xxx are shown in Table 1.
Heat treatment of the aluminum alloys Al-Zn-Mg
Aluminum alloys Al-Zn-Mg without copper alloy (7020, 7005 (1915), 7003) are also referred to as "semi-hard" alloys. They are successfully used in the manufacture of car body, bearing, including, welded, designs. These aluminum alloys successfully undergo age hardening, if the temperature profiles at the exit of the press is at least 400 ° C. Most often they are used at all without forced cooling due to their tendency to stress corrosion. At the same time, for example, Aluminium alloy 1915 It provides even hot-pressed state of the natural aging of the 30 to 35 tensile strength more days 315 MPa.
Tempering aluminum profiles with separate heating
aluminum withPlava Al–Cu–Mg and Al–Zn–Mg–Cu, as well as a series of alloys Al–Mg when a magnesium content 3 % refer to the difficulty pressed. Aluminum-magnesium alloys not subjected to thermal hardening, and the process thermal hardening aluminum alloys Al–Cu–Mg and Al–Zn–Mg–Cu(2xxx and 7xxx) significantly differs from the heat treatment of 6xxx alloys, which is always quenched on the press. The hardening of these alloys, for example, alloys 7075 and 2024 (D16), produce only a single heating, often in vertical furnaces, followed by rapid quenching baths in vertical-water tanks with. The final heat treatment step – aging operation – carried out either at room temperature (natural aging) or at a given elevated temperature for the required time (artificial aging).
Tempering hard aluminum alloy
table 2 strengthening phases represented thermally hardenable hardmetals. When heating furnace for quenching, they are dissolved in solid solution. The process involves heating the extract at a given temperature to achieve a nearly homogeneous solid solution. The cooling rate of the aluminum profiles quenching temperature should exceed a certain critical speed, different for different aluminum alloys, to maximize the strength properties and resistance to intergranular in the aged condition. for instance, alloy 7075 cooling rate should be not less than 300 ° C / c in a temperature range of from 400 to 280 ° C. The hardened aluminum alloys, reinforcing aging, They are unstable. With aging aluminum alloys allocated submicroscopic particles secondary phase, which form an irregular dislocation structure. By forming this structure, and hardening occurs alloy. The size and distribution of precipitates determines the optimum mechanical properties of the aluminum alloy. Typical thermal treatment conditions some solid aluminum alloys are listed in Table 3. The duration of heating depends on the thickness of aluminum profiles.