Contrary to the heat treatment of steels in order to make the steel hard, aluminum heat treatment commences by making the aluminium completely soft. The purpose of the solutionizing procedure is simply (as the name implies) to take the alloying elements into solution, which will eventually strengthen the particular aluminum alloy. There are only three groups of aluminum alloys that can be thermally strengthened: alloys in the 2000 series, alloys in the 6000 series and alloys in the 7000 series.
The purpose of the solutionize procedure is to simply put into solution the maximum amount of alloying elements that are present in that particular alloy. This can be likened to making a saturated solution of salt and water. A saturated solution of saltwater simply means that there is an excessive amount of salt present in the water. The way to take the excess salt into water is to simply heat the water. The salt will now go into solution. If the saltwater is now allowed to cool down from the heated temperature, however, the salt will precipitate out of the solution. To keep the salt in its solute form, you could freeze it (the principle of aluminum solutionizing treatment).
The problem with aluminum solutionizing is that the required solutionize temperature is very close to the liquidus temperature, where grain boundary melting can initially begin. For this reason, temperature uniformity within the furnace process chamber is absolutely critical to the procedure. If one does not have good temperature uniformity, only localized solutionizing will occur. Further, if the part that is being treated has varying cross-sectional thicknesses, one will need to ensure that the part has uniform solutionize temperature throughout its cross section without causing grain-boundary melting.
In order to achieve temperature uniformity, it is necessary to ensure that the air within the furnace process chamber is not static but has movement. Therefore, the furnace must have an air-circulation fan or systems of fans to ensure uniform heated air movement.
We will finish this discussion in part 2 next week.