Aluminum alloys have high plasticity, low resistance, intensified atomic diffusion process at high temperature, and recrystallization accompanied by recovery during thermal deformation, which is beneficial to improve the alloy structure.

Thermal deformation mainly has the following effects on the material: During the thermal deformation process, the grains, impurities, second phases and various defects inside the metal will be elongated along the main deformation direction of the large extension, and the strength of the elongated direction of the structure is generally higher than that of other materials. The strength of the direction, the material exhibits different degrees of anisotropy.

In addition, deformation texture and recrystallized structure may be simultaneously generated during thermal deformation, and they will also cause directionality and inhomogeneity in the material.

The hardening and softening processes occur simultaneously during thermal deformation. The deformation breaks the coarse columnar grains, making the material structure into relatively fine deformed grains, and the mechanism of work hardening and dynamic recovery recrystallization works at the same time.

Due to the enhanced thermal motion of atoms under the action of high temperature, under the action of stress, due to the free diffusion and mutual diffusion of atoms, the inhomogeneity of the chemical composition of the ingot is relatively reduced, and some tiny cracks can be healed.

When the aluminum alloy is deformed at high temperature, the work hardening characteristics are related to the deformation temperature and the deformation speed. The higher the processing temperature, the slower the deformation speed, and the smaller the work hardening value.

Aluminum and aluminum alloys have high stacking fault energy, narrow extended dislocations, and are prone to dynamic recovery to form subgrain structures. When the deformation temperature is high and the deformation speed is fast, the subgrain size formed is small.

If it is cooled quickly after deformation, the recrystallization process may be inhibited, and the subgrain formed during high temperature deformation will remain. The strength of the alloy is related to the size of the subgrain. This strengthening is called substructure strengthening or subgrain strengthening. Forged aluminum alloy heat treatment processing

The possible dynamic recovery mechanisms mainly include: 1) edge dislocation climbing; 2) cross-slip of screw dislocation; 3) pinning dislocation detachment and detachment of 3D dislocation network; 4) sliding screw type Nonconservative motion of the upper edge cuts of dislocations.

Macroscopically, the stress-strain curve of the dynamic recovery material shows that the flow stress reaches a steady state value. The substructure is mainly generated in the dynamic recovery stage during the thermal deformation of the aluminum alloy. With the increase of the deformation degree, the grains are elongated, but the substructure is still an equiaxed subgrain.

The hot working process of aluminum alloy is an extremely complex high temperature, dynamic and instantaneous process. During high temperature deformation, it will experience work hardening, dynamic recovery or dynamic recrystallization. Various deformation mechanisms work together to determine the high temperature deformation characteristics of aluminum alloys. The optimization of process parameters in actual production is very complicated.

Aluminum alloy hot deformation process - aluminum alloy plate and strip hot rolling. General industrial high-strength aluminum alloy rolled sheets and strips (sheets with a thickness of 600mm) are not suitable for extreme cold forming methods such as deep drawing. Due to the limitation of its own ductility, hot rolling is a relatively excellent process method. .

Hot rolling of aluminum alloy sheet and strip refers to rolling above the recrystallization temperature of the rolled alloy.

Hot rolling makes full use of the high temperature plasticity of the alloy, and in a certain high temperature range, the rolled piece is rolled to the required thickness and obtains appropriate mechanical properties.

In the process of hot rolling, the phenomenon of hardening and softening coexist. Due to the influence of the deformation speed, when the recovery and recrystallization softening process is too late, the alloy will undergo work hardening with the increase of the degree of deformation.

Usually, the softening process plays a dominant role in hot rolling, and when the rolling temperature is lowered, the metal work hardening gradually increases.

Hot rolling deformation can significantly reduce energy consumption in production; improve the processability of alloys, and increase production efficiency.

The disadvantages are also obvious: 1) The size of the hot-rolled product is difficult to control and the precision is poor; 2) The performance of the hot-rolled product fluctuates greatly; 3) The surface quality of the hot-rolled aluminum alloy sheet and strip product is poor, because it is easy to produce metal Oxidation, sticking to aluminum, etc.

Aluminum alloy heat treatment

The preparation of ingots before hot rolling is also very important, including the following points:

Homogenization treatment: Due to the high cooling rate of semi-continuous casting, the diffusion process in the solid phase is difficult, and it is easy to cause uneven crystallization. As a result, the composition and structure of the ingot will be uneven, that is, intragranular segregation, which will greatly reduce the plasticity of the ingot. Therefore, the ingot must be homogenized before rolling;

Ingot milling: The purpose is to remove defects such as precipitation, slag inclusion, scarring and surface cracks on the surface of aluminum alloy ingots, reduce metal and non-metallic indentation defects of plates, improve the surface quality of plates, and enhance the performance of aluminum-clad plates. Weldability and corrosion resistance. During the experiment, the samples were ground with coarse sandpaper, and the oxides were removed before rolling;

Etching: This chemical method is used in industrial production to remove dirt and oil stains on ingots. Except for high magnesium and high zinc aluminum alloy ingots, all other ingots need to be etched and washed;

Ingot heating: Due to the low deformation resistance and good plasticity of aluminum alloy at high temperature, heating the ingot before rolling can eliminate the residual stress generated by a part of the ingot, so that the alloy has good processing performance.

The above treatment is to eliminate the stress effect during processing and maintain the smooth movement of dislocations. Macroscopically, the fluidity of the metal is guaranteed and the workability of the product is improved.

The most important thing for the hot rolling process is the design of the process system. We will explain the influence of each process parameter and some relatively mature design arrangements later.

For the 5000 series alloy, a low Mg (Mg content between 2.2% and 2.8%), non-hardening by heat treatment, has the characteristics of medium strength, good weldability and corrosion resistance.

In addition to optimizing the alloy composition based on the microalloying mechanism and the microstructure uniformity mechanism, the exploration and optimization of the processing technology, deformation mechanism, heat treatment, plasticity and other aspects of this series of aluminum alloys are the most important progress.

For example, the optimization of hot rolling technology on 5000 series alloys, 5052 aluminum alloy is one of the most widely used alloys in 5000 series alloys. The gap heats the billet to maintain the uniformity of the hot rolling temperature. Compared with the traditional rolling technology, the mechanical properties of the finished product are improved by 20%.

So what process parameters should be paid attention to when hot rolling? The hot rolling process parameters mainly include the starting rolling temperature, the final rolling temperature, the rolling speed, the total processing rate, the processing rate of the passes, etc.

Rolling temperature: The final rolling temperature of hot rolling should ensure the required performance and grain size of the product. If the temperature is too high, the grain size will be too coarse and cannot meet the performance requirements. If the temperature is too low, it will cause work hardening and increase energy consumption. Lead to uneven grain size and deterioration of alloy properties;

Hot rolling speed: The rolling speed is an important factor affecting the plasticity of the metal, so the determination of the rolling speed should not only consider the production efficiency, but also consider the improvement of the plasticity of the alloy;

Determination of total processing rate: The total processing rate of hot rolling of most aluminum and aluminum alloys can reach more than 90%. The determination principle is: the high temperature plasticity range is wide and the hot brittleness is small and the total processing rate of the aluminum material is large; for the billet used for cold rolling, when determining the total processing rate of hot rolling, there should be enough cold deformation for cold rolling to pass through. Cold rolling controls product performance and surface quality; for hot-rolled products, the casting structure should be transformed into a processing structure; when the rolling mill capacity is large and the equipment conditions allow, if the quality of the ingot is good and the heating is uniform, the total processing of hot rolling can be increased accordingly. Rate;

Determination of pass processing rate: At the beginning of the rolling stage, the pass processing rate is relatively small; in the intermediate rolling stage, with the improvement of the processing performance, the pass processing rate should be increased as much as possible; at the end of the rolling stage, the general pass processing rate If it is reduced, the temperature of the two passes after hot rolling is lower, and the deformation resistance is larger, and the reduction should be controlled within the range that can ensure good shape conditions and thickness deviation. The pass processing rate is determined by the pass reduction, which is an important indicator of the rolling process. It determines the quality of the product and the productivity of the rolling mill together with the rolling temperature and rolling speed. The cast structure of most aluminum alloys, especially the deformed structure, can allow a large reduction. Increasing the reduction can improve the non-uniformity of alloy deformation, make the hot-rolled sheet and strip structure uniform and stable, greatly reduce the possibility of ingot cracking, and greatly improve production efficiency. When rolling thick plates, in order to obtain good quality, in each rolling pass, the deformation in the height direction must penetrate the entire thickness of the rolling stock.

The improvement of the process and the optimization of the source production will also return to the production, and the accumulation of technology still needs the precipitation of years. We talked about the process parameters of hot rolling, have you realized it?