Heat treatment of aluminum alloy
Aluminum alloy heat treatment those common processes
The product is cooled to room temperature at a certain cooling rate after heating to a certain temperature and holding it for a certain period of time. Through atom diffusion and migration, the microstructure is more uniform, stable and the internal stress is eliminated. The plasticity of the material can be greatly improved, but the strength will be reduced.
(1) Homogenization annealing of ingot: holding for a long time at high temperature, then cooling at a certain speed (high, medium, low, slow), so as to homogenize the chemical composition, microstructure and properties of ingot, which can increase the plasticity of material by about 20%, reduce the extrusion force by about 20%, increase the extrusion speed by about 15%, and improve the surface treatment quality of material.
Intermediate annealing: also known as local annealing or inter-process annealing, is to improve the plasticity of materials, eliminate the internal processing stress of materials, at a lower temperature holding a shorter time, in order to facilitate the continued processing or to obtain a certain combination of properties.
(3) Complete annealing: also known as finished product annealing, is at a higher temperature, holding a certain time, to obtain a fully recrystallized state of the soft structure, with the best plasticity and lower strength.
Two. Solution quenching treatment
The supersaturated solid solution is formed by fully dissolving the second phase or other soluble components in the material into the aluminum matrix and then keeping the supersaturated solid solution at room temperature by a rapid cooling method. It is an unstable state. At high energy level, solute atoms can be precipitated at any time. But at this time, the material is of high plasticity and can be cold worked or straightened.
(1) On-line quenching: For some alloys with low quenching sensitivity, solid solution can be obtained at high temperature during extrusion, and then quenched by air cooling (T5) or water mist cooling (T6) to obtain certain microstructure and properties.
(2) Off-line quenching: For some alloy materials with high quenching sensitivity, they must be re-heated to a higher temperature and kept for a certain time in a special heat treatment furnace, and then quenched into water or oil in a transfer time of no more than 15 seconds to obtain certain microstructure and properties, which can be divided into salt bath quenching and air quenching according to different equipment. Fire, vertical quenching, horizontal quenching.
Three, time limitation
After solution quenching, the unstable supersaturated solid solution will decompose and the second phase particles will precipitate (or precipitate) from the supersaturated solid solution and distribute around the grain of alpha (AL). The strengthening effect is called precipitation strengthening.
Some alloys (such as 2024, etc.) can produce precipitation strengthening effect at room temperature, called natural aging, some alloys (such as 7075, etc.) at room temperature precipitation strengthening is not obvious, but at a higher temperature precipitation strengthening effect is obvious, known as artificial aging.
Artificial aging can be divided into under aging and over aging.
Underageing: In order to obtain some properties, lower ageing temperature and shorter ageing time should be controlled.
Overaging: In order to obtain some special properties and better comprehensive properties, aging is carried out at higher temperatures or at longer holding time.
Multi-stage aging: In order to obtain some special properties and good comprehensive properties, the aging process is divided into several stages. It can be divided into two stages and three stages.
Four, regression processing
In order to improve plasticity, facilitate cold bending or rectify shape and position tolerance, the quenched and aged products can be restored to the new quenching state in a short time at high temperature called regression treatment.