The selection principle of holding time is to make the strengthening phase dissolve satisfactorily at normal solution heat treatment temperature, and make the solid solution fully uniform and the grain fine.
The quenching holding time of aluminum alloy mainly depends on the quenching heating temperature, the nature of the alloy, the type of products, the microstructure before solution (the distribution characteristics and size of strengthening phase), the shape of the product (including the size of section thickness), the heating mode (salt bath furnace and air circulation furnace, continuous or discontinuous heating), the heating medium, the cooling mode and the amount of charging, and so on It is determined by the requirements of organization and performance.
For the same brand of alloy, the following factors should be considered in determining the holding time
(1) The shape of the product. The holding time during quenching is closely related to the shape of the product (including the size of the section thickness). The larger the section thickness is, the longer the holding time is. For semi-finished products with large cross-section and workpieces with small deformation, the strengthening phase is coarse, and the holding time should be appropriately extended to make the strengthening phase fully dissolve. The holding time of heavy forgings and die forgings is several times longer than that of thin ones.
(2) Heating temperature. The higher the heating temperature is, the faster the strengthening phase dissolves into the solid solution, and the shorter the holding time is.
(3) Plastic deformation degree and product type. Pressure processing before heat treatment can accelerate the dissolution of strengthening phase. The larger the deformation, the smaller the size of strengthening phase and the shorter the holding time. The recrystallization of cold deformed workpieces during heating should be avoided. The critical deformation should not be processed before solution treatment. The holding time of extruded products should be shortened to maintain the extrusion effect. If the holding time of quenching heating is too long, the partial or total extrusion effect will disappear due to the recrystallization process, and the longitudinal strength of products will be reduced. The greater the degree of deformation, the shorter the holding time.
(4) The original organization. The heat preservation time of pre quenched products can be significantly shortened when they are heated again. However, the dissolution rate of strengthening phase of pre annealed products is significantly slower than that of cold processed products.
(5) The homogenization degree of the blank. If the homogenization is not enough, the residual strengthening phase is large, so the heat preservation time should be longer. The common purpose of solution treatment and homogenization is to make the strengthening phase fully dissolved, but in general, the accuracy of homogenization annealing furnace is low, so it is easy to overburnt in order to eliminate the non-equilibrium crystalline phase and increase the homogenization temperature. In addition, the homogenization annealing time is long and the economic benefit is low. Therefore, the linkage process of homogenization and quenching can be considered according to the nature of the product alloy and the processing technology to solve the problem of full solid solution of strengthening phase. Because the strengthening phase in the structure is seriously broken after large deformation and the size becomes smaller, it is easier to solid solution during quenching.