The selection principle of holding time is to make the strengthening phase dissolve satisfactorily under normal heat treatment temperature, to make the solid solution fully and evenly, and to refine the grains.
The properties of hardening time of aluminum alloy are mainly based on quenching temperature, alloy, type of products and previous solid solution state structure (strengthening stage characteristics and size distribution), product shape (including the size of section thickness), heating mode (salt bath furnace and air circulation furnace, continuous or discontinuous heating), heating medium, cooling mode and horizontal charging, And organizational factors to determine performance requirements.
For the alloy of the same brand, the following factors should be taken into account when determining the holding time
(1) The shape of the product. The holding time of quenching heating is closely related to the shape of the product (including the thickness of the section). The greater the section thickness, the longer the holding time. For the semi-finished products with large cross-section and the workpieces with small shape change, the quenching time should be appropriately extended to make the quenching stage fully dissolved. The holding time of large forgings and die forgings is several times that of thin ones.
(2) Heating temperature. The holding time of quenching is closely related to the heating temperature. The higher the heating temperature is, the faster the strengthening phase melts into solid solution and the shorter the holding time.
(3) Degree of plastic deformation and product type. Pressure processing before heat treatment can accelerate the dissolution of strengthening phase. The larger the deformation degree, the smaller the strengthening phase size and the shorter the holding time. If recrystallization occurs in the process of heating after cold deformation, care should be taken to prevent excessive recrystallization grains. The critical deformation degree should not be treated before solution treatment. In order to keep extrusion effect, the heat preservation time of extruded products should be shortened. If the time of quenching and heat preservation is too long, some or all of the extrusion effect will disappear due to the recrystallization process, and the longitudinal strength of the product will be reduced. The greater the degree of extrusion deformation is, the shorter the time of heat preservation is.
(4) Original organization. After the product is quenched in advance and reheated, the heat preservation time can be shortened significantly. The dissolution rate in the hardening stage before annealing is significantly slower than that in the cold working stage.
(5) Evenness of blank. For products with insufficient homogeneity, the residual strengthening stage is more and larger, so the holding time should be extended. The common purpose of solution treatment and homogenization is to completely dissolve and harden, but generally speaking, the accuracy of homogenization annealing furnace is low, so it is easy to increase the homogenization temperature too enthusiastically, in order to fully eliminate the non-equilibrium crystalline phase. In addition, the homogeneous annealing time is long and the economic benefit is low, so the link homogenization and quenching process can be considered as the full solution strengthening stage according to the properties of the product alloy and the problems solved by the treatment technology, because in the strengthening stage, after the large deformation of the structure is seriously broken, the size is smaller and the solution quenching is easier to be solved.
(6) Organization and performance requirements. When product granularity is needed, the reduction of residence time should be considered. In addition, double quenching and step quenching processes were developed to obtain fine grain structure. Secondary quenching refers to the use of two short-term quenching at the same high temperature, but the sum of the two quenching holding times is the same as the original holding time, the principle of which is not to give the grain growth time and opportunity; the first stage of graded quenching uses low temperature to complete the development of sub grains in the structure, reducing the driving force of recrystallization, so that the grains are not easy to grow in the high temperature solution of the second stage. When the products have high requirements of corrosion, fracture toughness and fatigue properties, such as aviation aluminum alloy, the quenching and holding time should be extended by at least one time.
Other factors, such as alloy properties, heating conditions, heating medium and furnace capacity, must also be considered. The dissolution rate of each strengthening phase is different. For example, the dissolution rate of Mg2Si is faster than that of Mg2Al3. Quenching and holding time must ensure that the strengthening phase can be fully dissolved, so that the alloy can get the maximum strengthening effect. But the heating time should not be too long. In some cases, heating for a long time will reduce the properties of the alloy. For some alloys with coarse grains (such as 6063, 2A50, etc.) at heating temperature, the holding time should be shortened to avoid grain growth under the condition of ensuring hardening. For large quantity and large size parts, the heat preservation time is longer. Small furnace capacity, large distance between parts and short holding time.
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