3.1 Macro morphology
Figure 3 shows the hot rolling process experiment of aluminum/magnesium/aluminum composite plate with and without hard-plate with different reductions (40%, 60%, 80%) and the comparison of the morphology characteristics of the products. Figure 3b) shows the comparison of the macroscopic morphology of different deformation composite plates with and without hard-plate after rolling. It can be seen from the figure that with the increase of the reduction, the length of the composite plate after rolling shows an increasing trend. At the same time, it can be clearly seen that with the increase of the reduction, the forming ability of the composite plate during the hot rolling of the hard-plate less plate has reached the limit. When the reduction amount reaches 80%, the hot-rolled composite plate without hard-plate not only produces nonlinear distortion along the rolling direction, but also has a large number of edge cracks  defects. On the contrary, the composite plate after the hard-plate is rolled has a better shape and no edges. Crack defects occur. It can be seen that the hard-plate can hot-roll the composite plate and improve the deformation uniformity, and inhibit the generation of edge cracks.
Figure 3a) shows a partial enlarged view of a rolled composite plate without hard-plate with a reduction of 40%. It can be seen that the rolled composite plate without hard-plate has obvious war page at both ends of the rolled plate and separation between the layers; combined with Fig. 3b), it can be seen that on the contrary, the hard-plate rolled composite plate achieves close bonding and good quality, No interfacial gap visible to the naked eye. This is due to the small bite force when the plate enters the roll and when it leaves the roll during the rolling process, and cannot provide sufficient interface pressure, so that the composite plate cannot be tightly connected, and delamination and bending appear at the end of the composite plate. With the protection of the hard-plate, most of the tensile force in the rolling direction (RD) can be converted into the compressive stress in the transverse direction (TD), which improves the welding pressure between layers, and significantly improves the uniformity of deformation of each part of the composite board. A partial enlargement of 60% reduction with or without hard-plate is shown in Fig. 3c). On the right side is the composite plate without hard-plate after rolling. Minor edge cracks have begun to appear on both sides, and there is no edge crack defect when there is hard-plate. This is due to the direct contact between the plate and the roll in the traditional hot rolling process of the composite plate, and the shear stress on the edge of the plate is purely shear stress, which is also the internal cause of cracking in this part . When the hard-plate is applied, the rolling force is transmitted through the hard-plate, and the composite plate does not directly contact the roll. Since the size of the hard-plate is larger than the width of the composite plate in the normal direction (ND), even if the ND is slightly extended during the rolling process, the hard-plate can still play a "protective" role, thereby reducing the occurrence of edge cracks.
3.2 Interface structure
In order to further analyze the influence of the presence or absence of hard-plate on the interface structure of the hot-rolled composite plate joints. Figure 4 shows the interface structure and morphology of hot-rolled composite plates with and without hard-plate under different reduction conditions during hot rolling at 350°C. The properties of the two aluminum/magnesium bonding interfaces of the composite plate are approximately the same, so take the Al/Mg/Al side interface of the composite plate after rolling to observe the diffusion layer through SEM as shown in Fig. 4d, h), which can be seen under low magnification The bonding interface of the Al/Mg layer of the composite plate without hard-plate hot-rolled is curved and uneven; and the interface of each layer under the action of the hard-plate is almost
linear and smooth. It can be seen that the addition of the hard-plate can improve the composite plate during the hot rolling process. Uniform deformation ability. When the reduction rate is 40%, there are microcracks and even voids at the Al-Mg interface of rolling without hard-plate, as shown in Fig. 4a), which shows that the composite plate is still in mechanical bonding at this time; Fig. 4e) the hot-rolled composite plate Al-Mg with hard-plate The diffusion layer can be clearly observed at the interface. The gray bands Al17Mg12 appearing through element diffusion in the AZ31B layer are all marked, while in the AA1060 layer there is a dark band Al3Mg2. And the interface between the layers is clear, so as to achieve metallurgical bonding . The research results show that: compared with the traditional hot-rolled composite plate, the hard-plate can be the first to achieve the metallurgical bonding of the composite plate at a smaller reduction rate. Figure 4b) When the reduction is 60%, the Al-Mg interface of the hot-rolled composite plate without hard-plate begins to form an intermetallic compound layer; and Fig. 4c) can see that there is a diffusion layer at the bonding interface when the reduction is 80%, but the metal The layers of the inter-compounds Al17Mg12 and Al3Mg2 are not clear. At this time, it can be seen from Fig. 3a) that the hot-rolled composite plate without hard-plate under this condition has reached the shape limit. Figure 4f, g) the addition of the hard-plate can change the stress state of the composite plate, and change the tensile stress of some rolls to compressive stress. Even if the reduction reaches 60% or 80%, the edge does not appear to be strained, which means that the increase the hard-plate can effectively improve the forming performance of the composite plate rolling.
The influence of the hard-plate on the interface structure of the hot-rolled composite plate was analyzed by EDS. Figure 5 shows the comparison of the thickness of the intermetallic compound diffusion layer under different conditions. Figure 5a) among them, shows the thickness of the intermetallic compound diffusion layer with and without the hard-plate at a reduction of 40%. It can be seen that the addition of the hard-plate increases the diffusion layer from 2.1µm to 23µm, which is 9.9 times. Compared with the traditional hot-rolled composite plate, the increase in the thickness of the diffusion layer is very significant. Compared with the former, the 60% reduction in Fig. 5b) that shows that the thickness of the interface diffusion layer of the hot-rolled composite plate with or without the hard-plate has increased to varying degrees. The thickness of the interface diffusion layer reaches 38µm under the condition of the hard-plate. Reach the maximum value; Fig. 5c) the thickness of the diffusion layer is 5.5 µm
under the condition of 80% reduction, and it can be seen that the Al element content drop to 0 periodically. It also proves that the 80% reduction is too large, resulting in discontinuous distribution of the Al17 Mg12 layer. Figure 5d) shows the variation of the diffusion layer thickness with different reductions under the action of the hard-plate or not. It can be seen from the figure that, compared with the rolling without hard-plate, the addition of hard-plate can increase the thickness of the diffusion layer between the composite plates. Under the three reduction conditions, the thickness of the diffusion layer increases to different degrees, up to 20.9µm. The thickness of the diffusion layer reaches its peak at a reduction rate of 60%. Figure 5e) shows the growth rate of the diffusion layer thickness of the hot-rolled composite sheet with the same reduction amount compared with the composite sheet without the hard-plate effect. It can be seen that at a reduction rate of 40%, the thickness of the diffusion layer increases by 90.8% and reaches its peak. The 60% reduction rate is the smallest, still reaching 42.1%
3.3. XRD phase analysis
From the foregoing analysis, it can be seen that the three-layer composite board has two interface bonding layers, and since they are both Al/Mg composite interfaces, one side is taken for XRD phase analysis. Figure 6 shows the XRD pattern of the interface of the hot-rolled composite plate with and without hard-plate. Figure 6a ~ c) are the X-ray diffraction patterns of different reductions with or without the hard-plate. It can be seen that with the increase of the reduction, the number and intensity of diffraction peaks increase to different degrees, among which the diffraction peaks of Al and Mg. The quantity and intensity are the highest. The Al3Mg2 and Al17Mg12 intermetallic compounds also exist only in trace amounts. Figure 6a) can be seen that the number of diffraction peaks of intermetallic compounds without hard-plate is small and the intensity is low. There is no Al3Mg2 diffraction peak on the diffraction pattern without hard-plate reduction of 40%, and Al3Mg2 diffraction peaks begin to appear under the effect of hard-plate. Figure 6b, c) It can be seen that the number of diffraction peaks of Al3Mg2 and Al17Mg12 intermetallic compounds under the condition of 60% reduction is significantly increased, and the peak value is the highest. Especially the hot-rolled composite plate with hard-plate has the largest number of diffraction peaks. The appearance of the diffraction peak of the corresponding substance can indicate that this substance is generated at the interface of the composite plate after the rolling process, XRD observations proved the existence of the intermetallic. In the experiment, several XRD tests were performed on different positions of multiple sets of rolled plates under different process parameters, in order to more intuitively express the diffraction of intermetallic compounds Al3Mg2, Al17Mg12, and Al and Mg elements. The number of peaks and the degree of strength clarify the action mechanism of the hard-plate on the hot rolling of the composite plate. Figure 6d) After statistical analysis of a large number of XRD data of the rolled sheet under the three sets of reductions with the effect of the hard-plate, the maximum and minimum values of the diffraction peak ratio of each substance are shown in the figure. The ratio of diffraction peaks of Mg element at the interface is the largest. Although the diffraction peaks of Al3Mg2 and Al17Mg12 are relatively few, they still exist. (Because other elements are too small to be ignored, only the total content of Al, Mg, Al3Mg2, Al17Mg12 is regarded as 100%). A preliminary conclusion can be drawn that intermetallic compounds are formed under the action of the lining board without reduction, and the various layers of the composite board achieve different degrees of metallurgical bonding.
3.4 Metal flow behavior
It can be seen from the foregoing analysis that the addition of the hard-plate changes the stress state of the composite plate rolling, which is bound to have an important influence on the deformation flow behavior and uniformity. In order to further study the hard-plate, the composite plate can be uniformly formed under the roll and solve the problem of edge cracks. In view of the above problems and the more vivid expression of the location of the picture taken during simulation, the upper cuboid in Fig. 7 shows two areas a and b, which correspond to the two parts taken respectively-the central part of the composite board and the edge of the single-layer aluminum plate section. Figure 7a) is a partial enlarged view of the metal flow behavior of the middle part of the hot-rolled composite plate with a hard-plate and the corresponding aluminum plate edge; Fig. 7b) is a simulation diagram of the corresponding non-lined plate. From the simulation, it can be seen that the metal flow in the middle part of the hot-rolled composite plate with the hard-plate function tends to be biased toward the normal direction; the metal flow of the composite plate without the hard-plate function is completely horizontal, which is consistent with the rolling direction. From the comparison of the partial enlarged view, the metal flow at the edge of the aluminum plate in the composite panel can be seen. In the rolling direction, we can see that the metal flow direction of the aluminum plate with the hard-plate is the lower right, while the metal flow of the aluminum plate without the hard-plate is only slightly biased to the normal direction (ND). This is because the aluminum plate is in direct contact with the hard-plate and the hard-plate is larger than the aluminum plate in the edge part, which is affected. This also proves that under the action of the hard-plate, the rolled plate is more compressed and less stretched when the edge is formed. Under the combined action of the shear stress of the roller and the compressive stress of the hard-plate, the metal flow behavior of the traditional rolled clad plate is changed into the RD unidirectional force direction under the combined action of the two forces. The two-way force causes the metal of the composite plate under the action of the hard-plate to flow into a non-horizontal direction. Therefore, the occurrence of edge cracks is reduced.
3.5 Mechanical properties
Since the hard-plate changes the stress state and deformation behavior of the composite plate during hot rolling, the effect of the hard-plate plate has a related influence on the mechanical properties of the composite plate. Figure 1b) shows the sampling method for mechanical performance testing. Through the foregoing analysis, it can be seen that the hard-plate has a significant effect on the composite plate in the RD direction. Therefore, the tensile samples in this experiment are all taken in the RD direction. Figure 8a) is the engineering stress-strain curve of the composite plate with different reductions under the action of the hard-plate. Figure 8b) shows the comparison of the yield strength and elongation of the three sets of hot-rolled composite plates with reduction ratios under the action of the hard-plate. It can be seen that with the increase of the reduction, the strength value and elongation first increase and then decrease. The yield strength and tensile strength of the composite plate are higher at 60% reduction, and the yield strength reaches the maximum. The value is 172.3MPa, and its elongation is also the largest, reaching 21.5%. It can be seen that the comprehensive mechanical properties of the hard-plate hot-rolled composite plate are the best when the reduction is 60%.