Physical Simulation of Medium and Low Temperature Rolling of Mg-Li Alloy Sheet

In this paper, LZ91 magnesium-lithium alloy was taken as the research object. At the rolling temperature of room temperature and 453 K, the plane strain compression physical simulation experiment was carried out on the sample with one pass reduction of 15 % and three passes total reduction of 45 %. After each pass of rolling, the sample was annealed at 503 K for 30 min. Then the effects of different treatment processes on the microstructure of LZ91 magnesium-lithium alloy were studied by metallographic microscope, X-ray diffractometer and scanning electron microscope. The results show that under the same strain, the stress of room temperature rolling is larger than that of 453 K rolling; with the increase of annealing passes, the effect of work hardening gradually weakened. With the increase of rolling amount, the original mechanically mixed ' α ' phase and ' β ' phase will deform along the deformation direction, showing a streamline shape. Rolling at 453 K, the diffraction peak of the sample with 45 % total reduction of three passes changes the most.


Introduction
Magnesium-lithium alloy, as an alternative metal to traditional structural materials, has attracted much attention.The density of magnesium alloy can be lower than 1 g/cm3 by adding appropriate amount of lithium.It has excellent mechanical properties, strong penetration resistance to high-energy particles, electromagnetic shielding performance and other advantages [1].In the field of communication electronics, biomedical materials, especially in the field of aerospace, it has a very large application prospect [2].Although the specific stiffness of magnesium-lithium alloy is high, the absolute strength of magnesium-lithium alloy is low.The tensile strength of magnesium-lithium alloy is generally about 100 MPa, and the tensile strength of deformed magnesium-lithium alloy is about 200 MPa.Therefore, as a structural part, it cannot meet the actual engineering needs [3][4].
Since the 1940s, foreign countries began to research and develop magnesia lithium alloy, but China started relatively late [3].The processing of magnesium alloy is roughly divided into two kinds, one is cast magnesium alloy, the other is deformed magnesium alloy, the latter grain is finer [5]. in comparison.At present, the research of AZ magnesium alloys mainly focuses on the simulation deformation of ascast magnesium alloys such as rolling and hot compression [6].Tang Di et al. [7] studied the effect of rolling process on magnesium alloys and found that under the same temperature, the larger the pass down rate, the finer the grains.Wei Songbo et al [8].studied AZ31B magnesium alloy through single pass rolling experiment and concluded that temperature is an important factor affecting the rolling effect, but the higher the temperature is, the better.Liu Wenhui et al. [9] and Xu Chenyang et al. [10] found that the microstructure was refined when the deformation amount was relatively large through different passes of pressure reduction of magnesium lithium alloy sheet.In addition to temperature and pass compression rate, annealing process parameters will also affect the properties of the final magnesium alloy.Plane strain compression method is a method to compress the sample with a constant compression area.The true stress of the flake sample can be easily calculated by placing it between two die plates for compression.In the plane strain compression experiment, many scholars have done a lot of work.
Therefore, in this paper, LZ91 magnesia-lithium alloy is taken as the research object, and the effects of rolling temperature, reduction amount and annealing on LZ91 magnesia-lithium alloy are studied by plane strain compression physical simulation experiment, so as to explore the appropriate experimental parameters for rolling magnesia-lithium alloy.

Experimental Materials
The composition of as-cast LZ91 is shown in table 1

Experimental Methods
The experimental route is shown in figure 1.The physical simulation experiment of plane strain compression of LZ91 magnesium-lithium alloy was carried out on the Gleeble-3500 thermal simulation machine at room temperature and 453 K, respectively, with a single cut of 15% and a total cut of 45% in three passes.The annealing process was 30min annealing at 503 K after each pass of rolling.After that, the microstructure morphology of the sample was observed and analyzed by metallographic microscope, scanning electron microscope and X-ray diffractometer, and the conclusion was drawn.With the increase of strain, the stress becomes slow with the increase of strain.For the sample rolled at 453 K, a platform appears.By comparing the amount of a single order of reduction, as shown in figure 3 (a), after rolling at room temperature, the strain of a single order reduction of 15% after annealing at 503 K for 30min is 193 MPa; After rolling at 453 K, the strain of annealing at 503 K for 30min after a single cut of 15% is 101 MPa; After rolling at room temperature, the strain is 203 MPa after annealing at 503 K for 30 min after rolling at a total reduction of 45% for three passes.When rolling at 453 K, the strain of annealing at 503 K for 30 min after rolling at 45% after three passes total reduction is 107 MPa.It can be seen that the stress of rolling at 503 k is higher than that of rolling at room temperature under the same amount of pressure and the same strain.As shown in figure 3, the slope of the curve with a total reduction of 45% between three passes at room temperature decreases relatively in the second and third stages except for the approximate slope of the curve with a 15% reduction in the first and one pass.This is due to the dynamic recrystallization in the annealing process which softens the effect of work hardening and the reduction of dislocation density.This is caused by the increase of the slip surface.The strain is 193 MPa after annealing at 503 K for 30min at room temperature after one step reduction of 15%, and 203 MPa after annealing at 503 K for 30min at room temperature after three steps reduction of 45%.However, the work hardening rate decreased significantly in the process of the second and third passes rolling.It can be seen that the dynamic recrystallization effect brought by the multi-pass annealing softened the material, and the stress did not rise sharply with the strain.

Microstructure Analysis
After the physical simulation experiment of plane strain compression, metallographic microstructure of the sample was observed at 200x magnification, as shown in figure 4. The phase microstructure of LZ91 magnesium-lithium alloy is composed of -Mg and -Li.αβThe white part is the phase, which is closely packed hexagonal crystal structure (hcp), and the gray part is the phase, which is body-centered cubic crystal structure (bcc).αβIt can be seen from figure 4 that the phases in the original as-cast LZ91 sample have no obvious direction, and the white α phase and the gray β phase present a mechanical mixing state.As shown in figure 5, by comparison, at the same rolling temperature, with the increase of pass reduction, the original chaotic structure distribution began to be arranged in the direction of deformation, and the α phase began to show a certain orientation.Because the phase has a close-packed hexagonal crystal structure and a cubic crystal structure, it is harder than the β phase.Therefore, the β phases coordinate their deformation during the deformation process.When the rolling amount is 45%, the orientation of α phase and βphase is more obvious, and the two-phase structure is elongated along the rolling direction into thin strips, showing the streamline characteristics.
(a) single step of the reduction of 15%.
(b) three times total pressure of 45%.As shown in figure 6, under the premise of the same pass reduction, it is found that with the increase of rolling temperature, the phase size becomes smaller.This is because there is no obvious dynamic recrystallization in the process of rolling at room temperature.The rolling process and annealing can only refine the grain of the phase, but cannot change the phase dispersion.Therefore, the size of the phase rolled at room temperature is larger.

Display the Experimental Results of x-Ray Diffraction
The relative strength was calculated according to the diffraction peak intensity and the strongest diffraction peak corresponding to other crystal faces, as shown in table 2. It can be seen that for the ascast original sample, the strongest diffraction peaks are (110) and (101).When rolling at 453 K, only (110) maintains the strongest diffraction peak in the 15% one-pass reduction.However, with the increase of rolling amount, that is, when rolling at 453 K, the total reduction of three passes is 45%, and the strongest diffraction peak is (200).Therefore, with the increase of rolling deformation, the preferred orientation may change.When rolling at room temperature, the maximum diffraction peak of 15% sample is (100) and (101).Compared with the original sample in as-cast state, the maximum diffraction peak is still (101).For rolling at room temperature, the sample with three passes total pressure of 45% has the highest diffraction peak (110), which also maintains the highest diffraction peak under the original sample in the as-cast state.While rolling at 453 K, the sample with three passes total pressure of 45% has the highest diffraction peak of (200).It can be seen that the rolling temperature also has an effect on the preferred orientation, which may be because the non-base slip actuation, which is not easy to actuate, causes the change of the preferred orientation under the medium temperature rolling.

Analysis of SEM Results
As shown in figure 7, it can be seen that there are a small amount of white fine granular precipitates on the grain boundaries.This is because there are a lot of impurities at the grain boundaries, which is conducive to non-uniform nucleation, so it is conducive to the generation of second phase at the grain boundaries.In addition, it can be seen that there are some pits.This is because the loading time is too short and the load is too large during the plane-strain compression experiment, which destroys the grain and leaves pits.

Conclusion
(1) By comparing the true stress-strain curves of the same pass reduction and different rolling temperatures, it is found that because rolling at room temperature will cause greater work hardening effect, the stress of rolling at room temperature is larger than that of rolling at 453K under the same strain variable.By comparing the true stressstrain curves of the same rolling temperature and different passes, it is found that the effect of work hardening is gradually weakened with the increase of annealing passes.
(2) Through the analysis of the metallographic microstructure treated with different process parameters, it is found that with the increase of rolling amount, the original mechanical mixed phase and phase will deform along the deformation direction, showing a streamline.αβIn addition, the rolling temperature will also affect the size of the phase.
(3) According to the results of X-ray diffraction experiment, at the same rolling and different passes, it is found that the strongest diffraction peak (110) in the original cast state, (101) becomes (200) after 453K passes 45%, so the preferred orientation may be changed, and the temperature will also affect the preferred orientation.According to the results of SEM, it is found that the content of each phase conforms to the law of phase diagram.

See figure 2 .
Under the experimental conditions of this experiment, the true stressstrain curves of LZ91 magnesium-lithium alloy samples are generally similar in trend.When the deformation begins, the stress changes very fast with the strain, the stressstrain curve is almost linear, and the work hardening rate is

Figure 2 .
Figure 2. True stress-strain curves at different rolling temperatures with the same pass a one pass reduction of 15%; b three times total pressure of 45%.

Figure 3 .
Figure 3. True stress-strain curves at the same rolling temperature and different reduction rates: a. the rolling temperature is room temperature; b. rolling temperature 453 K.

Figure 4 .
Figure 4. Microstructure of LZ91 in its original cast state.

Figure 5 .
Figure 5. Rolling at 453 K, different amount of down tissue.
(a) Rolling at room temperature.(a)Rolling at 453 K.

Figure 6 .
Figure 6.Microstructure of 45% total reduction in three passes at different rolling temperatures.

Table 2 .
Peak intensity ratio between each peak and the strongest diffraction peak.