Tempering to press the profiles of alloy 6063

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

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.

Full strength of the alloy 6063 reached, 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.

Figure 1 – Three stages of thermal hardening of aluminum profiles on the press [1]

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).

Figure 2 - alloy pressing conditions 6063 for an optimum combination of performance, strength and surface quality [1]

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 required cooling rate is proportional to the amount of Mg₂Si, which is contained in the alloy 6xxx series.

alloy 6063, which contains a 0,8 % Mg₂Si, the cooling rate in the temperature range of from 450 to 200 ° C should be at least 55 °C (100F) per minute. alloy 6061 with the content 1,4 % Mg₂Si cooling rate in this temperature range should be at least 650 °C (1200F) per minute.

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 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 and, including - sensitive nature of hardening.

Figure 3 - Scheme thermal hardening the alloy at a quenching press 6063 [1]

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 of aluminum alloys:

• Additional exposure of profiles in conditions T1 and T4 at elevated temperature for several hours. It is called “artificial aging”. Alloy artificial aging 6063 It is typically used at a temperature of exposure 175-180 ° C. 5-6 hours.
• Artificially aged profiles receive the state of T5 or T6 and their strength to 50 % higher than T1 or T4 states.
• Profiles in states T1 and T4 are called “naturally aged”. They gain their predetermined strength through 2-3 weeks.

Strict temperature control is set in the artificial aging furnace: ±3 °С. 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:

1. Carl V. Lynch, Aluminum Extrusion Technology Seminar, Chicago, 1969