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Quality of magnesium alloy is the practical application of the lightest metal structure material, at the same time, the magnesium alloy with high strength and specific stiffness, elastic modulus, good biocompatibility, good thermal conductivity and electromagnetic shielding ability and damping shock absorption performance is good wait for an advantage, is widely used in aerospace, transportation, national defense war industry, equipment manufacturing and 3 c electronics and other fields, Known as "the most promising green engineering materials in the 21st century".
Rolling is one of the main methods to produce magnesium alloy sheet, which can produce sheet of different thickness and width flexibly. Rolling is the most economical and effective method to prepare magnesium alloy sheet in plastic forming. The rolling process can refine grain, improve microstructure and significantly improve mechanical properties of alloy. Many kinds of rolling techniques have been developed over the years. However, the current magnesium alloy sheet rolling technology is not mature, the rolled sheet anisotropy is high, the stamping formability is poor, the edge cracking is serious, the material utilization rate is low, therefore, it is necessary to study the magnesium alloy sheet rolling method to promote the development of magnesium alloy.
What are the rolling methods of magnesium alloy?
Rolling mode can be divided into conventional rolling and special rolling. Conventional rolling is divided into traditional rolling, hot extrusion rolling and double roll continuous casting rolling; Special rolling mainly include: asynchronous rolling, cross rolling, large strain rolling, electric pulse rolling and so on.
1. Conventional Rolling (NR)
1.1 Traditional rolling
Traditionally, magnesium alloy plate is prepared by hot rolling and warm rolling of magnesium alloy ingot on a rolling mill, and sometimes by cold rolling at the end.
The traditional rolling method can refine the microstructure grain of magnesium alloy and improve the mechanical properties to a certain extent, but it also has some shortcomings. The cost of magnesium alloy sheet rolling production is always high because of the process of multi-pass and small deformation rolling, which requires intermediate annealing and heating of rolls, which is inefficient and wastes resources.
1.2 Hot extrusion rolling
Generally speaking, the hot extrusion rolling method is a rolling process method that rolls the plate into the required shape by hot extrusion.
Because of the extrusion process is conducted in the container, make plate heat slower, the greatest degree of play its plasticity, to a certain extent, inhibits magnesium alloy cracks, so the method is suitable for most of the production of magnesium alloy sheet, but this method still exists some disadvantages, such as rolling sheet sizes cannot too big, waste is big, the process of rolling material yield is low, Large investment in equipment and instruments.
1.3 Double roll continuous casting and rolling
Magnesium alloy magnesium alloy slabs are prepared by using the two-roll continuous casting (TRC) technology, as shown in Figure 1 and Figure 2, which are horizontal two-roll casting and vertical two-roll casting respectively. It USES two counter-rotating rolls as the mould, after melting magnesium alloy ingredients, through the distribution system (casting mouth) will melt evenly into between casting roller, casting roll casting and plastic processing one (solution at the same time of rapid cooling occurred plastic deformation of magnesium alloy), with high efficiency was the demand of magnesium alloy slab.
This process does not go through hot rolling. Compared with the traditional method, the double-roll continuous casting technology has the advantages of short production cycle, high efficiency, low cost, high energy saving, high material yield, smooth surface and compact structure. However, magnesium alloy sheet prepared by this method has large grain size, difficult to eliminate the oxide inclusion and casting defects, and the size of the sheet is limited. Therefore, many researchers are now adding some metal refiners to refine castings to obtain magnesium alloy cast-rolled plates with fine and uniform grains. In short, this technology has great development potential.
Two, special rolling
2.1 Asynchronous Rolling (DSR)
The principle of asynchronous rolling is shown in Figure 3. By changing the diameter, speed and friction conditions of the upper and lower rolls, the linear velocity of the surface of the two work rolls is not equal, forming a "rolling zone" in the rolling deformation zone. The rolling zone reduces the hindrance effect of the horizontal pressure of external friction on deformation, thus significantly reducing the total pressure of rolling deformation. Therefore, the use of asynchronous rolling makes the sheet grain refinement, and the substrate texture strength is weakened, the yield strength is reduced, the plasticity is improved.
2.2 Cross rolling
After cross rolling, the grain size of the sheet becomes smaller, the microstructure is uniform and stable, and the substrate texture is improved, so that the plastic toughness of the sheet is improved, but the stamping formability is slightly reduced. At the same time, the cross rolling technology can optimize the anisotropy of magnesium alloy and make the mechanical properties more uniform, and the mechanical properties of the plate in different directions are less on the surface. In addition, the improvement of substrate texture and grain refinement make the strength of magnesium alloy sheet increase, but the bulging property of magnesium alloy sheet decrease. Because the rolling method is limited to the sheet and the cross rolling process is complicated, it is not suitable for large-scale preparation of magnesium alloy sheet.
2.3 Large Strain Rolling (LSR)
Large strain rolling is a highly plastic deformation technology, which can reduce the grain size to a large extent and obtain ultra-fine grain structure materials. Large strain rolling can be divided into cumulative overlap rolling and equal diameter Angle rolling.
1) cumulative rolling
Cumulative rolling is one of the techniques of severe plastic deformation. This technique is to make two metal plates of equal size obtained by work hardening and degreasing surface treatment at a certain temperature, and then use this technique to repeatedly roll and weld the plates with large strain.
Cumulative pack rolling technology is not only low equipment investment, simple process than other methods, to obtain high yield, and sheet rolling limit strain size, not the final size of the material do not change, but can refine the organization to improve the strength of materials, the process method is considered to be the most is expected to the preparation of large size, high economy, high performance industrial production method of fine granular magnesium alloy sheets. This technique has attractive application prospect, but there are still some shortcomings in the preparation of magnesium alloy plate by this method, such as large plastic deformation and cracking of thick plate during processing, easy oxidation phenomenon during heating, and difficult preparation of large size plate, etc. At present, the cumulative rolling is mainly used to make high performance sheet.
2) Equal Angle rolling
Equal diameter Angle rolling is a special rolling technology which combines rolling with large shear deformation based on the basic principle of equal diameter Angle extrusion. The constant Angle rolling process can improve the grain size of magnesium alloy sheet, reduce the grain size and produce a large number of fine twins, thus improving the strength and plastic deformation ability of the sheet at room temperature.
In addition, the grain of the sheet rolled by equal diameter Angle will rotate under the action of shear force, resulting in the evolution of the base plane orientation into non-base plane orientation, so that the texture of the base plane is obviously weakened. This technology has a great application prospect. Although it has the disadvantages of easy instability and wrinkling and unstable rolling process, with the increase of rolling channel, the strength of the plate becomes smaller, and the plasticity and formability are obviously improved.
2.4 Electric pulse rolling
Two rolls are insulated from each other, and pulse current is applied at the same time for rolling process called electric pulse rolling, which perfectly combines the electric plastic effect and plastic effect of current. Under the action of electric plastic, its plasticity has been greatly improved. Electric pulse rolling technology has many advantages, such as reducing the deformation resistance of materials, improving the mechanical properties and forming limit of materials, reducing processing procedures and improving product quality. Therefore, electric pulse rolling has a great prospect in the plastic processing of magnesium alloys.
Three, the research of rolling forming edge crack
The special crystal structure and physical properties of magnesium alloy lead to edge cracking and other defects in the rolling process. Edge crack not only reduces the yield of magnesium alloy sheet, but also expands in subsequent rolling process, which destroys the continuity of production process and seriously restricts the popularization and application of magnesium alloy. Therefore, it has become a hot topic in material field to reduce or eliminate the edge crack defect of magnesium alloy sheet and improve the finished product performance.
Factors affecting the formation of edge cracks
(1) Rolling temperature: temperature is one of the important factors affecting the rolling process of magnesium alloy sheet. The plastic deformation capacity of magnesium alloy is poor at room temperature, so it is generally needed to be heated before rolling.
(2) roll down system: the roll down system includes both the maximum roll down rate of the pass, and the cumulative total roll down rate. First of all, edge crack will occur when the pass reduction rate reaches a certain value. Secondly, the total pressure reduction rate also has great influence on the formation of edge crack defects.
(3) Rolling speed: rolling speed also has an important effect on the generation of edge crack of magnesium alloy sheet. High speed rolling can improve the rolling formability of magnesium alloy sheet.
(4) Stress state: In the process of metal deformation, most cracks are caused by uneven deformation and high stress concentration caused by obstacles (grain boundary or second phase). When the stress concentration reaches the critical fracture strength, cracks will occur, which is also the case for the generation of edge cracks in magnesium alloy plates.
Reference:
Lu Weina, et al. Status and development of magnesium alloy sheet rolling forming. Metallurgical and Materials.2018.
Liu Jianglin, et al. Research progress of edge cracks in magnesium alloy sheet rolling. Materials Guide.2020.
