There are only three groups of aluminum alloys that will respond to strengthening by thermal means. They are: 2xxx, 6xxx and 7xxx.

The strengthening mechanism is by solutionizing at a suitable temperature followed by quenching, and the strengthening comes from the precipitation-hardening treatment.

The purpose of solution treating is to put the maximum practical amounts of solute elements such as copper, silicon, magnesium or zinc into a solid solution in the aluminum matrix. Because the solution treatment can only take place at an elevated temperature, which is slightly below the eutectic melting temperature, great care must be taken with temperature accuracy and temperature uniformity within the process furnace. Care should also be taken that the furnace atmosphere is dry to prevent what is known as high-temperature oxidation (HTO). This condition can be further aggravated if the moist atmosphere is contaminated with gaseous sulfur products, which will result in surface blisters forming on the surface of the aluminum.

Surface blisters can also be formed as a result of temporary overheating. So, it is very apparent that great care must be given to the furnace cleanliness, atmosphere contaminations, process-temperature forming, process-temperature uniformity and time at temperature.

The solutionizing is to be followed by the quenching operation. The secret of the quenching operation is the transfer time of the workpiece from when the furnace door begins to open to the moment that the workpiece is completely submerged in the quench medium. If too much time is taken on the quench transfer, the solute elements will begin to come out of solution. So, time is of the essence for the transfer.

Next week we will finish the discussion of quenching and precipitation hardening.