Influence of roll-bending deformation on microstructure and texture in an Al-Li alloy with different extrusion aspect ratio

The purpose of this investigation is to study the effect of roll bending deformation on the microstructure and texture in an Al-Li alloy with different extrusion aspect ratios. The results indicated that the substructure of most extruded fiber grains is not completely eliminated on the ND-ED section at position B with a low extrusion aspect ratio. However, the texture type of the ND-TD section changes from a single <114> texture to an adjacent <112> texture after roll-bending deformation, and the texture intensity decreases from 8.3 to 6.1. With the increase of the extrusion aspect ratio, the effect of roll-bending deformation on the microstructure began to be obvious, especially at position C. A large number of elongated recrystallized grains along the ED direction were transformed into approximately equiaxed grains on ND-ED section after the roll-bending deformation. There were also a small number of approximately equiaxed cell structures. The relative frequency of the low-angle grain boundary rose slightly from 42.1% to 46.7%. The single <313> texture moved to the adjacent <112> texture, and the texture strength increased from 4.8 to 6.3.


Introduction
In recent years, the structural materials for foreign cryogenic tanks have developed from Al-Mg alloy and Al-Cu alloy to Al-Li alloy.The reason for using these alloys comes primarily from the low density, high specific strength and specific stiffness, and excellent weldability.For example, the alloy 2195 (Al-4.0Cu-l.0Li-0.4Mg-0.4Ag-0.12Zr)has a 5% lower density than 2219, which is gradually being replaced for large structures such as cryogenic tanks for launch vehicles [1][2][3].A cryogenic tank's formed structural elements primarily consist of a box bottom and a box barrel portion.The box barrel section is generally welded from multiple wallboards with the same curvature.In order to minimize the manufacturing cost and improve the production efficiency, usually by using special-section extrusions or built-up structures [4].However, because of the varied section thicknesses produced by special section extrusions, microstructure and texture will be distributed unevenly.G. Tempus et al. [5] found that <111> and <100> fiber textures are developed with the predominance of the <111> texture in aluminum products shaped with smaller aspect ratios (width/thickness).Hales et al. [6] studied the relationship between microstructure and texture of the 2195Al-Li alloy in a near-net-shape 'T-stiffened' extrusion.A thinner section extrusion had a ribbon-shaped grain morphology and a texture comprised of strong intensities at the Brass and Cube-type orientations and moderate intensities at the S′ 2 orientation.In contrast, the thicker section had a mixture of ribbon-and cigar-shaped grains and a texture consisting of well-developed <111> fiber (dominated by the Ex 1 component) and <100> fiber (dominated by the Cube component).Currently, it is common to perform roll-bending deformation on extruded wallboard, which can also change the uneven plastic strain of the material along the thickness direction.However, the correlation between the evolution of microstructure caused by roll-bending deformation has not been clearly studied.
In this paper, the microstructure characterization was carried out in three different positions on a special-section 2195 alloy extrusion plate.Through the comparative study of three different positions at the extrusion section before and after roll-bending deformation, the effect of roll-bending deformation on grain morphology, grain boundary misorientation angles, and texture was determined.

Materials and experiment
The starting material applied in this work was received as an extruded plate of AA2195 Al-Li alloy.The size of the billet is Ø192×650 mm, and the chemical composition is Al-3.78Cu-0.98Li-0.46Mg-0.36Ag-0.11Zr(wt.%).The calculated extrusion ratio was 18.69.After extrusion, the extrusion plate was subjected to a subsequent T4 heat treatment.Then, the plate was deformed by roll-bending.The bending length and radius are 1680 mm and 1875 mm, respectively.The roller spacing is 520 mm, the bending time is 26.77 s, the maximum reduction of the driving wheel is 19.1 mm, and the speed is 0.69 rad/s.taken from positions A, B, and C in the plate section, and the cross-sectional shape and sampling position are shown in figure .1.The extrusion direction of the profile is denoted as the ED direction, and the transverse direction as the TD direction.
The morphology and orientation of the grains were characterized using the electron backscatter diffraction (EBSD) technique.The sample used for the EBSD test was first mechanically polished to a mirror surface, and then electrolytic polishing was used to remove the residual stress on the surface.The electrolyte is a mixed solution of 10% perchloric acid and 90% ethanol.The prepared samples were tested on the JSM-7800F electron microscope equipped with the EBSD detector, and the collected data were analyzed by HKL CHANNEL 5 software.

Initial microstructure of the special-shaped extrusion plate sections
Figure .2 shows the inverse pole figure (IPF) maps of EBSD data on the samples at positions A, B, and C of the plate section before roll bending deformation.On the observation longitudinal ED-ND section, the low angle grain boundaries (LAGBs with a misorientation angle of 2°~15°) were represented by red lines, and the high angle grain boundaries (HAGBs with a misorientation greater than 15°) by blue lines.It can be seen that partial recrystallization occurs at different sections, and the recrystallized grains are arranged along the extrusion direction.Position A reveals more recrystallized grains, and the morphology is an approximately equiaxed fine grain structure.The equiaxed grain size is unevenly distributed, with an average equivalent grain diameter of 7.44 μm.The ratio of the long axis to the short axis is 2.07.The grain morphology at positions B and C is almost a layered microstructure, consisting of highly elongated grains.This is a typical extruded fiber structure, and many fine equiaxed grains are formed along the grain boundaries of the extruded fiber.The deformed grains contain a large amount of substructure inside.However, the extruded fiber structure at position B is thicker than that at section C, and the layered structure with a thickness of about 20 μm is the majority.The equivalent average diameter of the recrystallized grains is 1.61 μm, and the long and short axes of the grains are 1.85.The width of the extruded fiber at position C is small, and the fine equiaxed grains formed along the grain boundary of the extruded fiber structure increase.This is due to the crushing of the extruded fiber at section C under large compression deformation, resulting in many small-sized, equiaxed grains inside.The average grain equivalent diameter is 2.25 μm, and the aspect ratio is 2.50.
Figure .3 shows the relative frequency of misorientation angles in positions A, B, and C of the plate section before roll bending deformation, where ƒ represents the fraction of LAGBs.It can be seen that position A retains more equiaxed grain microstructures and elongated grains along the extrusion direction, with LAGBs proportions of 50.4%.The original equiaxed grains are elongated at position B, and the morphology presents slender grains.Many LAGBs appear around HAGBs, and the relative frequency of LAGBs increases to 54.5%.The deformation mechanism of the alloy in this area is mainly dynamic recovery (DRV).With the increase in the extrusion aspect ratio, the alloy shows more severe plastic deformation.The grains in section C are distributed in stripes, in which LAGBs are transformed into HAGBs.A small amount of fine dynamic recrystallization (DRX) grains appear around the large-sized slender grains, and the relative frequency of LAGBs is significantly reduced to 42.1%, which indicates the improvement of recrystallization.The proportion of LAGBs gradually decreases with the increase in aspect ratio; that is, a greater degree of DRX occurs in the areas with a larger aspect ratio.This phenomenon is mainly related to the different deformation degrees of each area.The material at the position with a larger aspect ratio undergoes a greater deformation, thereby storing more energy in the deformed grains.The stored energy promotes the DRV and DRX, thus reducing the number of LAGBs [7,8].

Microstructure after roll-bending deformation
In order to further study the effect of roll bending deformation on the microstructure of the extruded profiles, the sections A, B, and C were characterized by EBSD along the ED-ND section, as shown in figure .4. As shown in the IPF maps, position B shows long-axis grains along the extrusion direction, and the grains have a certain preferred orientation.Compared with the extruded plate, the orientation types at different positions obviously change.Especially for position C, the grain orientation no longer has a certain preferred orientation along the extrusion direction, and the grain orientation tends to be random.It is found that the textures of the positions were mainly along <101> and <111>, and the texture intensity was obviously higher in the extruded plate.Observing the grain morphology and grain boundary distribution of different positions, many substructures are formed inside the elongated grains at position B, which indicates that position B bears less extrusion deformation when the aspect ratio is small and only a small part of the deformed grains recrystallize, while most substructures are not completely eliminated.After roll-bending deformation, the positions A and C are superimposed with the larger deformation of the previous alloy cross section.The distortion energy near the two positions is higher, and a large number of dislocations are formed and accumulated, which accelerates the occurrence of dynamic recrystallization and changes the grain morphology at position C.The internal substructure of the long axis grain recrystallizes and transforms into finely equiaxed grains.Compared with the roll-bending deformation state, it can be seen that in position B with a smaller extrusion aspect ratio, the proportion of LAGBs is reduced to 48.2%, which indicates that the roll-bending deformation introduces more stored deformation energy into the deformed grains and the stored deformation property promotes the occurrence of dynamic recrystallization of the alloy, thereby reducing the proportion of LAGBs.In addition, after roll-bending deformation, the microstructure distribution at different sections tends to be more consistent.

Discussion
The extrusion parameters greatly influence the microstructures of aluminum alloys and further determine the mechanical properties of the final products, including aspect ratio, extrusion temperature, and extrusion speed.The change in aspect ratio will cause different stress-strain states and lead to the formation of different grain structures.As shown in figure.2, position B has an almost lamellar microstructure, consisting of a mixture of highly elongated grains and a few finely equiaxed grains.The plastic deformation of position C is more severe because of the increased aspect ratio.On the alloy's surface, there is a lot of deformation activation energy.The highly elongated, deformed grains without recrystallization gradually start to reshape into finely equiaxed grains as the degree of recrystallization rises.The relative frequency of LAGBs in position B is greater than that in position C, which has an impact on the entire occurrence of dynamic recovery and dynamic recrystallization at position B. This finding suggests that the large aspect ratio can promote the recovery of deformed grains.This change is consistent with the study results of Wang et al. [9].The grain structure and texture differences between positions B and C reflect the geometric dependence of material flow during extrusion.The change in aspect ratio also leads to the formation of different texture types.As shown in figure.6, position B with a low aspect ratio mainly undergoes axisymmetric deformation, and the <114> texture component is formed.The intensity of the texture component is 8.3, which is close to the <001> fiber texture.It has been demonstrated that the fiber textures of <111> and <100>can be formed during the axisymmetric deformation process [10,11].With the increase of the extrusion aspect ratio, the deformation state of the material at position C changes towards plane strain deformation.The peak position of the texture moves along the α-fiber, the <114> texture component disappears, and a new <313> texture component is formed.The texture intensity is 4.8.This is probably because that dynamic recrystallization occurred more completely, which destroyed the original deformation texture and led to a weaker texture in position C.The change in stress and strain states of the material is affected by the aspect ratio.As the aspect ratio increases, the stress and strain state of the deformed material will change, which leads to the rotation of the grains to a more stable orientation.After the roll-bending deformation, the morphology of the grains will change during the deformation process, depending on the deformation level and mode.It can be seen from figure.4 that the deformed grains in position B are elongated along the ED direction as a layered structure, regardless of whether the roll-bending deformation is used.However, more finely equiaxed grains are found in position C, indicating that the microstructure is affected by compression and additional shear stress.According to the relative frequency of misorientation angles before and after deformation, it can be seen that the proportion of LAGBs after roll-bending deformation is significantly higher than that of the undeformed.The proportion of LAGBs is 42.1% and 46.7% before and after deformation, respectively.The lower degree of recrystallization after roll bending deformation is attributed to the stored deformation energy consumed during the deformation process through dynamic recovery and recrystallization.Roll-bending deformation changes the deformation mode of the material, which means that the texture type also undergoes a corresponding transformation.As shown in the figure, the crystal orientation of position B moves down from <114> to <112>, and the crystal orientation of position C moves up from <313> to <112>, forming a new adjacent double texture.By comparing the influence of roll-bending deformation on the microstructure of positions B and C, it is shown that roll-bending deformation has a relatively serious impact on the microstructure of sections with higher aspect ratios.

Conclusions
In order to obtain a better understanding of the effect of roll-bending deformation on the microstructure evolution of Al-Li alloy extruded plates with different extrusion aspect ratios, characterization of their microstructure and texture was performed.The main conclusions of this work can be summarized as follows: 1、 The extrusion aspect ratio strongly affects the microstructure of the Al-Li alloy.The positions B and C have a layered microstructure, consisting of a highly elongated grain structure and a large amount of substructure.With the extrusion aspect ratio increasing, the thickness of the layered structure and the relative frequency of LAGBs significantly decreased to 20μm and 42.1%, respectively.This indicates that the larger aspect ratio promotes the occurrence of dynamic recrystallization.2、 The rolling-bending deformation has a great influence on the large aspect ratio of position C.The grain morphology changes from highly elongated grain structure to finely equiaxed grain in position C, and the relative frequency of the LAGBs increases to 46.7%.The rolling-bending deformation changes the type of texture and forms a new <112> texture component on position C.The texture intensities are 6.3.

Figure 1 .
Figure 1.section shape and of the sampling at different positions of the extrusion plate.

Figure 3 .
Figure 3. (a)-(c) The relative frequency of misorientation angles in positions A, B, and C, where ƒ represents the fraction of LAGBs.

Figure 4 .
Figure 4. (a)-(c) IPF maps in positions A, B, and C after rollbending deformation; (d)-(f) the corresponding distribution of

Figure 5 .
Figure 5. (a)-(c) The relative frequency of misorientation angles in positions A, B, and C after roll-bending deformation, where ƒ represents the fraction of LAGBs.

Figure 6 .
Figure 6.The pole figures in positions A, B, and C: (a-c) as-extruded state, (d-f) after roll-bending deformation.