Heat Treatment of Aluminum and Aluminum Alloys

The application of the term heat treatable to aluminum alloys, both wrought and cast, is restricted to the specific operations employed to increase strength and hardness by precipitation hardening thus the term heat treatable serves to distinguish the heat treatable alloys from those alloys in which no significant strength improvement can be achieved by heating and cooling.

annealing, quenching, quenchant, solution heat treatment, age hardening, The institute of materials engineering australasia, IMEA, ageing, metals,

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The non-heat-treatable alloys depend primarily on cold work to increase strength.


Annealing is applied to both grades to promote softening. Complete and partial annealing heat treatments are the only ones used for non-heat-treatable alloys. The exception is the 5000 series alloys which are sometimes given low-temperature stabilization treatment and this is carried out by the producer.

Annealing is carried out in the range 300-410°C depending on the alloy. Heating times at temperature vary from 0.5 to 3 hours, conditional on the size of the load and the alloy type. Generally, the time need not be longer than that required to stabilize the load at temperature. Rate of cooling after annealing is not critical.

Where parts have been solution heat-treated a maximum cooling rate of 20°C per hour must be maintained until the temperature is reduced to 290°C. Below this temperature, the rate of cooling is not important.

Solution Heat Treatment

This is applicable to the heat-treatable alloys and involves a heat treatment process whereby the alloying constituents are taken into solution and retained by rapid quenching. Subsequent heat treatment at tower temperatures i.e. aging or natural aging at room temperature allows for controlled precipitation of the constituents thereby achieving increased hardness and strength.

Time at temperature for solution treatment depends on the type of alloy and the furnace load. Sufficient time must be allowed to take the alloys into solution if optimum properties are to be obtained.

The solution treatment temperature is critical to the success of the procedure. It is desirable that the solution heat treatment is carried out as close as possible to the liquidus temperature in order to obtain the maximum solution of the constituents. Accurate furnace temperature and special temperature variation must be controlled to within a range of ±5°C for most alloys. Overheating must be avoided i.e. exceeding initial eutectic melting temperatures. Often the early stages of overheating are not apparent but will result in a deterioration of mechanical properties.

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Proper solution heat treatment of the aluminum alloys requires expert knowledge of the alloy being treated plus the correct heat treatment plant.


This is a critical operation and must be carried out to precise limits if optimum results are to be obtained. The objective of the quench is to ensure that the dissolved constituents remain in solution down to room temperature.

The speed of quenching is important and the result can be affected by excessive delay in transferring the work to the quench. The latitude for the delay is dependent on the section and varies from 5 to 15 seconds for items of thickness varying from 0.4mm to 12.7mm. Generally, very rapid precipitation of constituents commences at around 450°C for most alloys and the work must not be allowed to fall below this temperature prior to quenching.

Another factor to be considered in quenching is the work load and the ability of the quenchant to extract the heat at sufficient rate to achieve the desired results.

The usual quenching medium is water at room temperature. In some circumstances, slow quenching is desirable as this improves the resistance to stress corrosion cracking of certain copper-free Al-Zn-Mg alloys.

Parts of complex shapes such as forgings, castings, impact extrusions and components produced from sheet metal may be quenched at slower quenching rates to improve distortion characteristics.

Thus a compromise must be considered to achieve a balance of properties in some instances. Quenchants used in slower quenching applications include water heated to 65-80°C, boiling water, aqueous solutions of polyalkalene glycol or forced air blast.

Age Hardening

After solution treatment and quenching, hardening is achieved either at room temperature (natural aging) or with a precipitation heat treatment (artificial aging). In some alloys, sufficient precipitation occurs in a few days at room temperature to yield stable products with properties that are adequate for many applications. These alloys sometimes are precipitation heat treated to provide increased strength and hardness in wrought and cast alloys. Other alloys with slow precipitation reactions at room temperature are always precipitation heat treated before being used.

In some alloys, notably those of the 2xxx series, cold working of freshly quenched materials greatly increases its response to later precipitation treatment. Mills take advantage of this phenomenon by applying a controlled amount of rolling (sheet and plate) or stretching (extrusion, bar and plate) to produce higher mechanical properties. However, if the higher properties are used in the design, reheat treatment must be avoided.

Where natural aging is carried out the time may vary from around 5 days for the 2xxx series alloys to around 30 days for other alloys. The 6xxx and 7xxx series alloys are considerably less stable at room temperature and continue to exhibit changes in mechanical properties for many years. With some alloys, natural aging may be suppressed or delayed for several days by refrigeration at -18°C or lower. It is common practice to complete forming, straightening and coining before aging changes material properties appreciably. Conventional practice allows for the refrigeration of alloys 2014 - T4 rivets to maintain good driving characteristics.

The artificial aging or precipitation heat treatments are low-temperature long-time processes. Temperatures range from 115-200°C and times from 5-48 hours. As with solution treatment, accurate temperature control and spatial variation temperatures are critical to the process and generally, temperatures should be held to a range of ±7°C.

The change of time-temperature parameters for precipitation treatment should receive careful consideration. Larger particles or precipitates result from longer times and higher temperatures. The objective is to select the cycle that produces the optimum precipitate size and distribution pattern. Unfortunately, the cycle required to maximize one property, such as tensile strength, is usually different from that required to maximize others such as yield strength and corrosion resistance. Consequently, the cycles used represent compromises that provide the best combination of properties.


Source: Materials World, Vol. 12, No. 3, pp. 37-38, March 2004.


For more information on this source please visit The Institute of Materials, Minerals and Mining.



  1. Jean Grandmont Jean Grandmont Canada says:

    ex: u have a piece of 7075 t0 0.050 thick u have cold worked it to the max u coud cose it became to hard to  keep at it ---- coud u heat treated it to reanealead it again  --- 935f  for 35 min then quench in cold water ---- then keep reworked it  then finalised the heat treatment  at 935f for 35 min then quench  then cook at 250f fro 24 hrs  then air coold == coud u also  expedite the artificial aging by cooking at 350f fror 12 hrs

    • Michael Entrekin Michael Entrekin United States says:

      7075 at 920 for 35 min then within 10 sec put it in water for 3 minutes . After that you can put it in a freezer for up to a week if its at 0 degrees ..straighten it n then put it in the oven at 250 for 24 hours then it will be in the t6 condition

  2. Random Blogger Random Blogger New Zealand says:

    I baked an aluminium cookie sheet at 250C and now it has gone soft, will it age-harden naturally?

    • Guher Tan Guher Tan Turkey says:

      No of course not :)
      Ageing can only occur in certain alloys which are already way more harder than the Al cookie sheet.

  3. hershel rollins hershel rollins United States says:

    Dies sanding change the hardness of aluminun

  4. Mahmoud Gomaa Abuzeed Mahmoud Gomaa Abuzeed Egypt says:

    I have a problem of an aluminium extruded profiles entered the aging furnace at 180°c for 6 hours.and not giving the required hardness limit.did any one have a solution please.

    • Rana Ramadan Rana Ramadan Egypt says:

      i have the same problem.. i have annealed and measured hardness for one sample and then for another sample solutionizing is done and also hardness measured and for another sample solutionizing plus aging 1 hr occured and hardness measured.. the hardness measured by aging is the least of them and the annealing is the highest how?

    • Abdeladim Moftah Abdeladim Moftah Libya says:

      I have the same problem.. Please contact me to find out the best way to solve the problem.

  5. NEEL neel NEEL neel India says:

    I need TO Solution heat treat AA 6082 deep drawn cup  at 530'c. How much time I should keep it in muffle furnace to get optimum mechanical properties.
    After that i m going to artificially age it at 180'c.

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