Strength, size control and productivity are the three main factors, which take into account when deciding to make a competitive aluminum profile.
Aluminum alloys are grouped into two categories - heat and heat reinforcing neuprochnyaemye. By thermally neuprochnyaemym alloys include alloys of the series 1xxx, 3xxx and 5xxx, which reach their full strength during subsequent deformation at room temperature. Thermally reinforcing alloys - alloys 2xxx series, 4xxx, 6xxx and 7xxx - get their full strength due to:
- quenching of profiles from the temperature at the exit from the matrix (state T5);
- hardening of profiles from the temperature of a separate furnace heating in special tanks with water (condition T6).
Aluminium alloy 6063 (AD31)
6xxx series alloys have as a main component of the reinforcing particles of magnesium silicide, Mg2Si. Solubility Particle Mg2Si in solid solution of aluminum increases with temperature - at ambient temperature, it is 0,2 %, and in the vicinity of the melting point - 1,9 %. magnesium and silicon content, which typically is expressed as the Mg content2Si, is 0,5-1,4 % for the popular alloy 6063 (he is also a domestic alloy AD31).
Full strength of the alloy 6063 (AD31) is achieved, when the particles Mg2Si having a size of not more than, than 0,25 m. These particles completely lose their reinforcing effect, when they take the dimensions more 2,5 m. therefore, when we see in the structure of the aluminum profile particles Mg2size of Si, let us say, 30 m, its high strength can not be considered.
The three stages of heat hardening on press
In conventional thermal hardening of aluminum alloy on the press 6063 obtains its full strength - condition T6 - during a three-stage heat hardening process (Figure 1):
1-stage: Dissolution of Mg particles2Si in the workpiece before leaving the matrix;
2-stage: press hardening - fixing of dissolved magnesium and silicon in solid solution;
3-stage: artificial aging - uniform release of the smallest particles of Mg2Si in the profile screen.
Ctadiya 1 – dissolving particles Mg2Si Aluminum
The first thermal bonding step requires such time-temperature pressing conditions, which ensure complete dissolution of all the particles Mg2Si in solid solution of aluminum at the time profile of the output matrix.
This can be achieved on the press in two ways:
1) Heating the workpiece to a temperature of at least 500 ºC and an output profile from the die at a temperature of at least 500 oC.
2) Heating the workpiece to the temperature 430-455 ° C and pressing at high speed, to obtain a temperature profile at the exit from the die is not less than 500 ºС (figure 2).
The second embodiment gives a higher productivity without damaging the molded surface profile. To fully apply this option, you need to have:
- sufficient force on the ram;
- sufficiently high pressing speed;
- a sufficiently high pressing ratio (drawing);
- properly homogenized workpiece;
- possibility to heat the workpiece to a temperature 425-455 ° C in less than 20 minutes.
Prolonged heating of the workpiece, particularly in the temperature range 260-425 ° C, leads to a loss of strength of the future profile of particle coarsening Mg2Si, then if the workpiece is not heated prior to compression, at least to 500 ° C.
Stage 2 - hardening in the press
In a second step is required to cool the hot pressed, coming out of the matrix, fast enough to temperature, close to room. This is necessary to retain all dissolved magnesium silicide (Mg2Si) in a solid solution of a cooled profile. The necessary rate of cooling is proportional to the number of Mg2Si, which is contained in the alloy 6xxx series.
alloy 6063 (AD31), which contains a 0,8 % Mg2Si, the cooling rate in the temperature range of from 450 to 200 ° C should be at least 50-60 ° C per minute. alloy 6061 (AD33) containing 1,4 % Mg2Si cooling rate in this temperature range should be at least 550 ° C.
It means, that the profile of the alloy 6063 It can be cooled to a temperature 200 ° C. 4-5 minutes, while the profile of the alloy 6061 (AD33) this should be done for 25-30 seconds.
The importance of the cooling rate after compression is explained as follows. At the output of the whole matrix silicide magnesium dissolved in aluminum. The critical temperature range of 450 to 200 ° C in this aluminum, a strong tendency to the uncontrolled release of a particulate Mg2Si. If such a selection occurs, it completely eliminates the possibility of complete hardening on press. This tendency to separation commonly referred to as "quench sensitivity" and it mainly depends on the content of Mg2Si.
Figure 3 schematically shows the process of thermal hardening with hardening press for aluminum alloy 6063 (AD31) and, including - sensitive nature of hardening.
FIELD left front allocation Mg2Si shows, that all particles Mg2Si dissolved in the aluminum. FIELD right of this serves to indicate relative size and intensity of precipitated particles Mg2Si.
Extruded, which gets hardened in the press - pass cooling, for example, on the dotted line, It receives state T1 or T4 on the international classification. In this state, the profile is very plastic and can be subject to, for example, the radius of the cattle from gïbke.
Stage 3 - artificial aging
Artificial aging aluminum alloys - a controlled accelerated selection smallest particles Mg2Si with dimensions of about 0,25 um profiles with states T1 and T4. This is achieved delay profiles at an elevated temperature for several hours. Alloy artificial aging 6063 It is typically used at a temperature of exposure 175-180 ° C. 5-6 hours.
Artificially aged condition profiles typically have a T5 or T6 strength and 50 % higher than T1 or T4 states.
The aging oven is set strict temperature control – ± 3 ° C. It's necessary, to provide a uniform strength across the cages profiles in the furnace.
The cooling rate after artificial aging for strength aluminum profiles has no special meaning.
Source: Carl V. Lynch, Aluminum Extrusion Technology Seminar, Chicago, 1969