Solution annealing / Solutionizing
Solution heat treating temperatures and time at temperature for the heat treatable aluminum casting alloys. Steel baskets or racks which allow free circulation of the air during heat treatment and of water during quenching, should be used in the solution heat treatment of aluminum castings. Proper packing of the castings in these baskets is most important. there should be sufficient space between castings to insure free circulation and to minimize distortion
Owing to their relatively low strength at elevated temperatures, aluminum castings distort rather easily if crowded or jammed together during solution heat treatment. In fact, it may often be necessary to support intricate castings on special fixtures to prevent distortion. Under normal operating conditions, the usual practice is to charge the load into a furnace operating at the specified temperature. However, the load can be charged into a cold furnace and then be brought up to the soaking temperature. In either case, the soaking period at the solution heat treatment temperature should be specified.
Different alloys may be heat treated together in the same load if their heat treatment temperatures are similar. Soaking for periods longer than those
specified has no detrimental effects on the castings. Therefore, if different alloys are treated together, the soaking time should be governed by the alloy requiring the longest period.
At the completion of the soaking period, the castings are rapidly quenched from the solution temperature. Drastic quenching tends to set up stresses which will often result in distortion, especially if the castings are intricately designed. To avoid quench stresses and distortion, boiling water is generally used as the quenching medium. In certain cases, quenching in still air or moving air is employed.
Quenching should be performed as quickly as possible, with a minimum delay in transferring the load from the furnace to the quench tank. In actual practice, transfer intervals of one minute or less should be attained. Immediately after quenching, the alloys have increased ductility, so that warped or distorted castings may be straightened easily. Since as quenched castings begin to age even at room temperatures, the straightening operation should be performed as soon as possible after quenching.