When solutionizing aluminum, one also needs to be extremely careful of a condition known as high-temperature oxidation on the immediate surface of the aluminum alloy. The general cause of high-temperature oxidation (HTO) is high humidity levels and high moisture levels within the furnace process chamber. The mechanism of HTO comes from the decomposition of the moisture into oxygen and hydrogen. The condition is aggravated when atomic hydrogen begins to diffuse into the aluminum alloy surface and can cause blistering of the surface.
It is mandatory (particularly for aerospace components) that the furnace selected for solutionizing is only used for the processing of aluminum alloys. The furnace needs to be clean of all debris that might be present on the furnace hearth.
In order to lock the solute elements into the aluminum matrix, it is necessary to cool the aluminum alloy very rapidly. This necessitates quenching. If one follows the AMS aluminum heat-treatment specifications, it will be seen that the transfer time from the furnace into the quench medium is very short. The time for transfer of the solutionizing aluminum alloy begins from the moment that the furnace door begins to open. This time for transfer can range anywhere (depending on cross-sectional thickness) from 5 seconds up to 30 seconds from the moment the door begins to open to the point at which the aluminum alloy is completely immersed and submerged below the quench-medium level. If the transfer time exceeds the prescribed time limit, incomplete solutionizing will occur, which means nonuniform metallurgical and mechanical conditions of the particular alloy.
If that occurs, it will be necessary to re-solutionize the alloy and ensure that the transfer time is met from commencement of door opening to the complete immersion of the component. This has led to the development of some spectacular load handling and transfer from the furnace to the quench tank, especially drop-bottom-type furnaces where the load simply relies on a gravity drop from the furnace to the quench medium.
Distortion is also a significant factor in the solutionizing of aluminum alloys. The simplest method to reduce distortion is to heat the quench-medium water. Air also can be used as a quench medium, but its use is limited and confined to thin cross-section materials and less quench-sensitive alloys. Major developments to reduce distortion have occurred in the field of polyalkylene glycol (PAG) solutions mixed with water. Once again, one needs to take care of the maximum solution strength of the PAG mixed with water.
Next time in part 3, we will continue with the next phase of the heat treatment of aluminum alloys – precipitation treatments.