The application of the term heat treatable to aluminium 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.
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 the non-heat treatable alloys. The exception is the 5000 series alloys which are sometimes given low temperature stabilisation 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 stabilise 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. ageing or natural ageing at room temperature allows for a 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 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.
Proper solution heat treatment of the aluminium alloys requires an 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 dependant on 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.
After solution treatment and quenching, hardening is achieved either at room temperature (natural ageing) or with a precipitation heat treatment (artificial ageing). 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 design, reheat treatment must be avoided.
Where natural ageing 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 ageing 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 ageing changes material properties appreciably. Conventional practice allows for refrigeration of alloys 2014 - T4 rivets to maintain good driving characteristics.
The artificial ageing 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 maximise one property, such as tensile strength, is usually different from that required to maximise others such as yield strength and corrosion resistance. Consequently, the cycles used represent compromises that provide the best combination of properties.