Hardened aluminum alloy refers to heating an alloy to a suitable temperature for a sufficient time to completely transfer one or more soluble components of the alloy into solid solution and then cooling the material quickly enough to keep these components in solid solution.
Furnace solution heat treatment
Classical solution heat treatment of aluminium alloys occurs by separately heating the product in an furnace and then rapidly cooling it in a tank with water, as well as other cooling methods. Solution heat treatment is included as a necessary operation when processing aluminum alloys in the T3 condition, T4, T6, T7, T8 and T9, depending on the subsequent regime of aging.
Incomplete quenching aluminum alloys
At the same time, in practice for a long time for some of the profiles of aluminum alloys hardened produce not a single heating, and heating treatment with hot articles, often with the pressing temperature directly at the exit of the press.
It should be noted, that usually in the standards this issue is stated very vaguely and ambiguously. Quenching hot deformation temperature called incomplete quenching and T5 denote. Under full hardening only understand tempering from the separate heating designation T6. About any possibility to reach a state of complete hardening without a separate heating furnace is not mentioned anywhere.
Without proper control of heating and cooling profiles of the press hardening can really be incomplete. Incomplete hardening can be caused by:
- insufficient complete dissolution of the hardening components of the alloy during heating of the workpiece and / or subsequent deformation heating of the metal during pressing;
- insufficiently rapid cooling of the profiles after leaving the matrix and therefore premature precipitation of the hardening components of their solid solution (cm. Fig. 1).
Aluminum alloys for full hardening on press
Of all the standards only American Standard ASTM B 807 regulates the procedure hardening aluminum alloys directly on the press. At the same time it limits the possibility of quenching at the press rather narrow circle of alloys of the series 6xxx and 7xxx. It is understandable, because such heat treatment is possible only for those alloys, whose chemical composition makes it possible to make it virtually.
Aluminum alloy, which can be "officially" to harden the press, the standard includes the following:
- 6005, 6005A, 6105;
- 6061, 6262, 6351;
- 6060, 6063, 6101, 6463;
- 7004, 7005;
- 7029, 7046, 7116, 7129, 7146.
For these alloys, in press hardening it is a cheaper alternative, instead of expensive quenching process with a separate heating furnace. Thus it is possible to achieve the status T3 aluminum alloys, T4, T6, T7, T8 and T9.
Some of these "American" alloys are purely American and have no analogues, for example, in European Standard EN 573-3. It belongs, for example, to alloys 7004, 7046, 7146.
On the other hand, This list does not contain the "European" alloy 6082, which in Europe is also quenched in a press. Judging from the contents of magnesium and silicon, and Mn, hardening conditions for the alloy 6082 close to the conditions of hardening alloy 6351.
What ASTM B Says 807
Standard ASTM B 807 requires when applying hardening of aluminum profiles on a press, it is necessary to strictly control the following three technological parameters:
1) the temperature of the workpiece when loading it into the press:
- different for different groups of alloys, from 510 to 555 ºС;
2) the maximum time interval between the exit of the profile from the matrix and its entry into the hardening zone:
- from 45 to 90 seconds depending on the thickness of the profile;
3) the minimum cooling rate of the profile in the quenching zone until it reaches the temperature 200 ºS:
- from 70 ° C / min for the alloy 6060;
- to 500 ° C / min for the alloy 6061 .
The temperature of the workpiece while loading it into a press
The temperature of the workpiece at the beginning of compression should not be below or above, than indicated in the table 1 for the respective alloy. In doing so, it must be borne in mind, that the surface temperature of the preform or column can be significantly different from their central part temperature. It is necessary to have data on this visible under temperature and the resulting metallurgical structure, and strength properties. After exiting the billet heating furnace must be loaded into the press as soon as possible, podstuzhivaniya avoid it to a temperature below a minimum value, specified for this alloy in Table 1.
The pause between the metal matrix and the release of quenching
The maximum time interval between the output of the molded profile of the matrix and its entrance into the quench zone should not exceed the value, indicated in table 2. The maximum permissible time between the release of the metal matrix and its entry into the quench zone is usually determined by the thinnest portion of the profile.
This condition depends on the pressing speed, as well as the duration of idle press process at restart preform. As can be seen from the table 2, minimum pause between the output of the matrix metal and the beginning of the quenching is 45 seconds. This time is quite enough to restart the workpiece in the press and passing 1-1,5 meters from the matrix to the entrance of the cooling unit on the press.
Minimum permissible quenching rate
Quenching profile must achieve a temperature below the 200 ° C within a specified short period of time, which depends on the quantity of alloy. This requirement depends on the cooling rate of the quenching profile in the press zone. The minimum cooling rate for various alloy groups should meet the table 3. In this case it should be excluded repeated thawing metal.
Adjustment of process parameters
The above requirements for temperature and time parameters can be narrowed in the above range in view of the specifics of production equipment, as well as the pressing conditions, for example, the ratio of pressing (drawing) and the shape of the cross-section of the profile. These changes should have statistical evidence of strength and other properties of profiles.
Organization of control on the press hardening technology
Profiles can be considered hardened on the press, if this ensures proper control of process parameters needed, as well as their registration and storage.
Control of preform temperature prior to compression
control technologies should ensure the exclusion of metal overheating or damaging pollution furnace atmosphere. for instance, induction heating requires measurements of temperature gradients along the workpiece. When applying a direct gas heating is necessary to correctly position the thermocouple due to the potential inhomogeneity of the surface temperature.
Temperature control before pressing should include, least:
- billet temperature in the heating furnace, and
- workpiece temperature at the outlet of the furnace immediately before loading into the press.
metal temperature control after the release from the press
After leaving the press the control measuring points of the temperature profile is, least:
- metal temperature at the inlet to the hardening device;
- metal temperature at the outlet from the quenching device.
Control of timing
Control and register the following timings:
- time from the exit of the billet from the heating furnace and the beginning of pressing;
- the time between the exit of the metal from the press and its entry into the quenching device;
- residence time profile in the quenching device.
cooling rate control
The following cooling methods are used:
- the passage of the profile through water or a mixture of water and glycol in a standing wave;
- immersion profile quenching tank;
- water supply from sprayers to the profile;
- supply of air-water mixture to the profile;
- supply of compressed air to the profile;
- a combination of the above methods;
- other methods.
hardening alloys 6060 and 6061
To understand the essence of quenching at the press it is useful to compare the conditions for alloys 6060 and 6061. The figure schematically shows the differences required of cooling rates of these alloys for quenching, i.e. creation of a solid solution of magnesium and silicon falling by preventing particles Mg2Si.
Recommended methods for cooling and the minimum cooling rate are indicated in Table 4. As seen, for profiles of alloy 6060 with thin walls, for example, most construction system profiles, enough cooling fans on the output table press. For profiles of alloy 6061 until thick 10 mm is sufficient cooling the water-air mixture, and it must have spreyernoe water cooling for profiles with a greater thickness.
- ASTM B807/B807M-20 Standard Practice for Extrusion Press Solution Heat Treatment for Aluminum Alloys