The thermal-strengthening processes for the three heat-treatment aluminum alloys are solutionize and precipitation-hardening treatments. Both of these procedures (particularly the solutionize process) demand:

• Temperature accuracy
• Temperature uniformity within the process chamber
• Furnace cleanliness

Without the above three conditions being met, it is unlikely that good metallurgical results will be accomplished.

The solutionize process temperature is extremely close to what is known as the eutectic grain boundary (GB) melting point. GB melting can be caused by direct radiation onto the work and by the work being too close to the heating elements. It is an aerospace mandatory requirement that the aluminum alloy being solution treated does “not see” ANY radiant heat from the heating elements. This necessitates that the heating elements for the furnace be located away from the process chamber (heater box) and to blow the heated air into the process chamber. Another method would be to place a louvered shield in front of the heating elements.

If the alloy “sees” radiant heat, it can cause microscopic surface melting at the aluminum-alloy grain boundaries. This phenomenon can only be seen with microscopy (unless, of course, it is blatantly obvious that the aluminum alloy has been exposed to radiant heat for long cycle times). If GB melting has occurred, it would be appropriate to scrap the part, unless there is excess surface stock that can be removed to machine off and to get underneath the surface GB melting. One can ask the question, how deep will the melting go? The simple answer is that it will depend on:

• The furnace heating element design
• The temperature uniformity of the furnace
• The time at the solutionize process temperature

The next blog will discuss the need for temperature uniformity and the processes.