Heat Treatment of Aluminium
Introduction
Aluminium is a metal which is extremely abundant on Earth. With it's wide range pf properties, mechanical and electrical, it is therefore a very commonly used material in all industries.
Davis Scientific Treatments has been offering many of the vacuum heat treatment processes for aluminium, and it's alloys, for many years.
The use of the term "heat treatable" to aluminium alloys, whether in cast or wrought form, is restricted to certain specific operations.
These processes, if employed correctly, increase strength and hardness through Age or Precipitation Hardening. These distinguish the heat treatable alloys, from those in which no significant improvement in their properties may be obtained.
Those aluminium alloys which do not respond to heat treatment may have strength improvement by cold working.
Annealing
Both types of aluminium alloys can be softened by the Annealing process. It is carried out at a temperature between 300 and 400 deg C. The time at temperature can be anything up to 3 hours, depending of size of components, and the actual alloy of aluminium.
Generally the time is only that needed to stabilise the batch size at the set temperature. The cooling rate, following annealing, is not important.
Solution Heat Treatment
Solution heat treatment, on its own improves, the ductility of the alloy, and so permits cold forming operations [such as press forming and bending].
Heat treatable alloys are processed, where the alloying constituents are taken into solution, then retained in this condition by rapid quenching, often in water at room temperature.
Subsequent treatments may be carried out at lower temperatures, where the alloying consituents are then precipitated throughout the microstructure, giving increased strength and hardness.
Control of the solution treatment temperature is critical. For most alloys, the temperature has to be accurate to + or - 5 deg C. This means that the treatment requires expert knowledge, and good quality furnace plant.
Quenching
Optimal results require careful control of the quenching process. Speed is a vital factor, as delays in the transfer affect the properties of the completed treatment.
Size of components also has to be considered, as the quenchant has to be able to extract the heat at the required rate, to obtain the desired properties.
Precipitation [Age] Hardening
This treatment follows on from Solution treatment, and Quenching. It is performed either at room temperature [ageing naturally], or with a Precipitation treatment.
The specific Precipitation treatments vary widely, dependent on the particular aluminium alloy.
With many alloys, it takes place gradually at room temprature. But other alloys need a low but elevated temperature to cause the precipitation process. Typically a temperature in range 110-220 deg C is used, but for a long time, sometimes 48 hours.
As with Solution treatment, the accurate control of temperatures is critical, usually held to + or - 7 deg C.
Stress Relieving
Stress relieving of aluminium components is necessary, where intensive machining or heavy forming/bending processes are involved. This treatment removes the severe internal stresses, which may have been generated, and which may cause distortion.
This is achieved by heating to a pre-determined temperature, for a certain time, dependent on the particular aluminium alloy. In this way, the internal stresses are largely removed, whilst maintaining the original balance of mechanical properties.
General
Heat treatment of aluminium alloys is thus a tricky process, where the optimal properties blend of strength, hardness, and corrosion resistance [for example] is specific for each aluminium alloy. Care must therefore be taken in the determination of the processes described above.
Davis Scientific Treatments (DST)
We at DST have extensive experience in the vacuum heat treatment of a wide range of aluminium alloys.
Please contact us if you have any application, however challenging it may appear to you.