Influence of Mg/Si ratio on the microstructure and formability of 6061 cast-rolled sheets

Three kinds of 6061 aluminum alloy cast-rolled plates with Mg/Si mass ratios of 1.23, 1.45, and 1.7 are prepared using NF6–300 vertical twin-roll casting and rolling mill. Metallurgical microscope (OM), x-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), room temperature tensile test, and other analysis methods studied the influence law and mechanism of Mg/Si ratio on the microstructure and texture of 6061 cast-rolled sheet. The best Mg/Si ratio cast-rolled sheet is then analyzed for formability. Research indicates that with increasing Mg/Si ratio, the grain size shows first decreasing and then increasing trend. Morphology of precipitated phases at the edge and the central of the sheet are different, however, the texture type has no obvious change, and a strong {112} 〈110〉 texture and weaker rotating cube {001} 〈110〉 texture is always observed. When the Mg/Si ratio is 1.45, the average grain size is the smallest, the edge size is 39.78 μm, the central size is 31.31 μm, and the precipitated phase is evenly distributed at the edge and central of the cast-rolled sheet. In addition, the elongation reaches 8.9%, the n-value is 0.072, and the r-value is 1.161 when the cast-rolled sheet reaches 90° along the cast-rolled direction under T6 state heat treatment.


Introduction
6XXX series aluminum alloy has good aging hardening characteristics and welding performance, and after stamping molding through baking hardening the performance can be further improved.Therefore, 6XXX series aluminum alloy has become a priority choice in automotive lightweight applications [1][2][3][4][5][6].To use 6XXX series aluminum alloy in the automotive field, in addition to mechanical properties such as strength and elongation, it is necessary to consider the deep drawing performance.However, the formability of the 6XXX series aluminum alloy limits its wide range of applications [7,8].Compared with traditional aluminum alloy plate preparation technology, the twin-roll casting process is short, has low energy consumption, low space requirements for equipment, and low cost, gaining wide attention from scholars.However, 6XXX series aluminum alloy castrolling plate research focuses on the microstructure and mechanical properties and not on the roll sheet formability [9][10][11].
The improvement in the formability of 6xxx series aluminum alloys is associated with the refinement of grain size, precise control of solute elements (especially Si), and elimination of coarse second-phase particles [12].In addition, the sheet forming process and texture factors such as heat treatment affect the formability.Some studies [13][14][15][16] show that needle-like β-Al 5 FeSi can be transformed into spherical α -Al 12 (Fe, Mn) 3 Si phase, and spherical α -Al 12 (Fe, Mn) 3 Si is beneficial to the enhanced formability of 6XXX series aluminum alloy.Kuijpers et al [16,17] found that the presence of Si could reduce the chemical potential between β-Al 5 FeSi and α -Al 12 (Fe, Mn) 3 Si, thus inhibiting the conversion rate of β→α.In contrast, Paul Rometsch [18] found that increasing Si content and decreasing Mg/Si ratio could improve the work hardening and strain rate hardening capacity of 6XXX series aluminum alloy, thus improving the tensile plasticity and tensile formability of the alloy.Zang [19] found that the Mg/Si ratio could adjust the grain size, texture and precipitated phase of Al-Mg-Si alloy, and then affected the formability of the sheet.Many studies have shown that an appropriate amount of Mg/Si could improve the formability, the microstructure and the properties of the aluminum sheets [20,21].Therefore, precise control of the Mg/Si ratio is very important to investigate the formability of 6xxx series aluminum alloys.Seong-Guk Son et al [22] used twin-roll strip casting, asymmetric rolling, and T6 state treatment to obtain a 6xxx series aluminum alloy plate with both high strength and high formability.The study showed that asymmetric rolling could achieve refinement.The {115} 〈552〉 texture did not disappear completely due to T6 state treatment in the sheet, and {115} 〈552〉 texture combines with {110} 〈110〉 rotated-Goss texture, effectively increasing the plastic strain ratio r ̅ and reducing the plastic strain anisotropy index Δr, thereby, improving the formability of Al-Mg-Si alloy.By studying the microstructure and texture of cold-rolled sheets after local deformation, Zhou et al [23] found that different texture types (recrystallization texture and deformation texture) lead to different metal deformation.Due to the cube ({100} 〈001〉) texture and deformation texture, plastic deformation increased during cold rolling.The volume fraction of recrystallized texture decreased and that of deformed texture increased.Takayuki et al [24] confirmed the positive correlation between the r-value and deep drawing properties by preparing samples with {111} texture and r ̅ -value greater than 1.4, and the {111} shear texture was formed by warm rolling and differential rolling, which improved r ̅ -value and the formability of Al-Mg-Si alloy.Loganathan et al [25] studied changes in plastic strain ratio R and strain hardening index N by annealing under different soaking time, and results showed that the annealing process, strain hardening index and elongation increased, and the forming property was better when the rolling direction was 0°or 90°rather than 45°, and the sheet anisotropy decreased with decreasing soaking time, which is beneficial to the forming.
To sum up, to optimize the 6XXX series aluminum alloy sheet formability of alloy elements, forming process must take some factors into account such as the texture.In this paper, the effect of the Mg/Si ratio on the 6061 cast-rolling plate microstructure is investigated.Then the best Mg/Si ratio is studied for roll sheet formability, to provide a certain reference for the development and application of aluminum alloy in the automotive field.

Experimental
Table 1 shows the measured chemical composition of three 6061 aluminum alloy cast-rolled plates studied in this experiment.These three types of 6061 cast-rolled plates were prepared using 6061 aluminum alloy and the master alloy Al-20 wt%Si.First, the 6061aluminum alloy raw material was added to the graphite clay crucible and placed in an intermediate frequency magnetic induction melting furnace, and heated until the 6061aluminum alloy melted.Then Al-20 wt%Si master alloy was added, stirred evenly with a graphite rod for 2 min, stopped heating when the temperature of the molten aluminum alloy reached 800 °C and kept for 15-30 min.After the heat preservation, the alloy was cooled down and cast by an NF6-300 vertical twin-roll thin strip continuous caster.The casting temperature was set at 690 °C, the casting speed was 15 m min −1 , and the roll gap width was 2 mm.The effect of solution and aging treatment on the texture of the material may lead to uneven strain distribution, which will affect the strain and stress distribution of the material during the forming process.This may lead to the instability of the forming performance and the inconsistency of the deformation performance of the treated material.Therefore, as long as the formability of the solution-aged sheet is guaranteed, a high-quality finished sheet can be obtained.The T6 heat treatment of cast-rolled aluminum alloy plates were carried out by NWTX13B high-temperature resistance furnace, that is, after solution heat treatment, artificial aging treatment was carried out.The solution temperature was maintained at 530 °C for 2 h and then quenched to room temperature.The aging temperature was 120 °C, the aging time was 6 h, and finally aircooled to room temperature.ZEISS Axio Vert.A1 optical microscope was used to study and analyze the grain size and morphology of 6061 cast-rolled plates.The grain size was measured by intercept method.The metallographic sampling is shown in figure 1, and the corrosion solution ratio is shown in table 2. Jsm-6480lv scanning electron microscope and EM-30AX COXEM desktop scanning electron microscope were used to observe the morphology and distribution of precipitated phase at the edge and center of cast-rolled 6061 sheets in ND direction.Semi-quantitative phase analysis of cast-rolled sheets with different Mg/Si mass ratios was carried out by Dutch Panako diffractograph.The diffraction angle ranged from 10°to 90°, and the scanning speed was 10°min -1 .The crystal pole diagrams of {111}, {200} and {220} were measured, and the ODF diagram was calculated.XRD test sample size was 12 mm (rolling direction) × 10 mm (transverse).
At room temperature, the yield strength, tensile strength, and elongation of the cast-rolled sheet with the best element ratio were measured by the UTM5305 electronic universal testing machine.The work-hardening index (n-value) and plastic strain ratio (r-value) were calculated from the stress-strain curve.The tensile speed was 2.82 mm min -1 .n and r values were calculated according to GB/T5028-2008 and GB/T5027-2007, respectively.The tensile sample size is shown in figure 2, where the sample thickness is 3.5 mm.

Results and discussion
Figure 3 shows the metallographic structure of 6061 cast-rolled sheets with different Mg/Si ratios after anodic coating on edge and central.Table 3 shows the average grain size of Al-Mg-Si cast-rolled plates with different Mg/Si ratios.Results indicate that the grain size first decreases and then increases, attributed to the influence of the Mg/Si ratio on the precipitation of eutectic structures on the grain boundary of the plate.These eutectic structures can inhibit the growth of the α-Al matrix and play an important role in grain refinement.When Mg/ Si ratio is 1.45, the amount of eutectic structure reaches the maximum.When the Mg/Si ratio exceeds 1.45, the amount of eutectic structure decreases, and the grains become coarse.In addition, the excess Si content also leads to the reduction of crystal core, thereby, decreasing the nucleation rate.
Figure 4 shows the metallographic microstructure of the edge and central of the Al-Mg-Si cast-rolled sheet with an Mg/Si ratio.The grain structure at the edge is different from the central, consisting of a thin layer of fine   When the pouring experiment is carried out, the high-temperature metal solution is in close contact with the copper roller.Because the initial temperature of the copper roller is low and has a strong effect on heat absorption and heat dissipation, the metal liquid that first contacts the copper roller produce great supercooling.At this time, the copper roller is used as the 'base' for non-uniform nucleation, greatly reducing the nucleation work and increasing the nucleation rate.A large number of nuclei are produced in the metal solution, and the  growth direction of these nuclei is not uniform.The nuclei meet each other in a short period, and then stop growing.When the 'substrate' is completely covered by the solid metal, the nucleation rate decreases.At this time, a thin thin layer with different growth directions is equiaxed grain.The formation of fine equiaxed grains on the surface releases the latent heat of crystallization, and the temperature of the copper roll heated by the liquid in the molten pool increases rapidly due to thermal conductivity, resulting in the slow cooling of the remaining liquid in the molten pool.In addition, the temperature gradient becomes gentle, and the nucleation rate decreases.The temperature of the solid-liquid interface is almost the same, and the degree of subcooling is small, which is not enough to generate new crystal nuclei.However, the fine crystals near the liquid phase grow preferentially along the opposite direction of the fastest heat dissipation.As the heat dissipation progresses, the direction of heat dissipation continues to change, and the main axis of the rapidly growing dendrites grows rapidly in the direction opposite to the direction of heat dissipation.Under cooling, the composition significantly affects the crystal structure type and growth shape of solid solution alloy.When the composition zone is small, small bumps generated at the front of the solid-liquid interface stop growing soon after entering the undercooling zone, resulting in the formation of cellular structure at the interface.When the subcooling zone of the component is larger, the convex part grows towards the liquid phase and the subcooling degree becomes larger.At this time, the side of the primary axis of dendrites forms a secondary axis, and the tertiary axis grows on the secondary axis, forming dendritic crystals.Because of the continuous rotation of the casting roll, the heat dissipation direction and speed are not always the same, so a region where cellular crystals and dendritic (columnar) crystals coexist, is formed.
As the casting-rolling process progresses, the internal temperature of the molten pool becomes more and more stable.Because the dendritic crystal branches block each other and interfere with each other, it is difficult for the discharged low-concentration solute to diffuse into the distant liquid, thereby, lowering the melting point of the crystal branch.When it encounters a high-temperature liquid, it locally remelts and automatically falls off.A strong vortex recirculation is generated near the solidified Kiss point, strengthening the shedding of crystal branches and bringing the fallen crystal branch fragments into the center of the molten pool.On the other hand, with the growth of dendrites at the edge, the subcooling zone in the liquid at the edge of the solid-liquid interface becomes larger, conducive to the formation of new crystal nuclei.When the non-uniform nucleation condition is satisfied, these crystals grow in the form of free crystals, and when this growth reaches a certain extent, the growth of columnar crystals is prevented.At the same time, the fallen dendrite fragments and some impurities in the molten pool are used as a new crystallization core to promote the formation of equiaxed crystals in the center.These equiaxed crystals grow in different directions and at almost the same growth rate without obvious weak interfaces, and the tiny cracks generated in the casting and rolling process do not expand.However, there are many shrinkage pores in the central and the tissue is not dense.
Figure 5 shows the XRD patterns of Al-Mg-Si cast-rolled slabs with different Mg/Si ratios.Figure 5 indicates that at different Mg/Si ratios, there is mainly α-Al matrix, strengthening phase Mg 2 Si, iron-rich phase Al 0.5 Fe 3 Si 0.5 , excess Si phase, and manganese-rich phase α-Al 9 Fe 0.84 Mn 2.16 Si.At the beginning of the solidification process, alpha Al primary phase precipitates first, because there is a composition of supercooling and roll casting process elements' actual solidification temperature is different, discharging the Mg and Si element to the remaining liquid, with the solidification of Mg and Si elements concentration.Moreover, in the alpha intergranular residual liquid Al increases to a certain degree of eutectic reaction happen.The contents of Mg and Si elements and the Mg/Si ratio in the Al-Mg-Si cast-rolled plate directly affect the type, size, and content of phases generated by the eutectic reaction.As the Mg/Si mass ratio increases, the intensity of the diffraction peak corresponding to the Mg 2 Si phase gradually increases.When Mg/Si = 1.73, the Mg 2 Si phase is completely formed, and the ratio of less than 1.73 causes excess Si.When the Mg/Si ratio is 1.45 and 1.70, the XRD pattern does not show the excess Si phase.And when the Mg/Si ratio is 1.23, the diffraction peaks corresponding to Si appear in the XRD pattern, indicating that there is an excess Si phase in the corresponding cast-rolled sheet.In 6061 cast-rolled sheets, there is impurity element Fe, which forms Fe-rich phase Al 0.5 Fe 3 Si 0.5 with Al and Si elements during solidification.These Fe-rich phases split the matrix during plastic deformation and cause local cracks, detrimental to the properties of the sheet.When the Mn/Fe mass ratio approaches 1.0, the manganese-rich phase α-Al 9 Fe 0.84 Mn 2. 16 Si is also formed due to the presence of the Mn element in the plate.
The results show that during the solidification process of Al-Mg-Si aluminum alloy, the Mg/Si ratio mainly affects the size, morphology and distribution of the precipitated phase [19,20].Figure 6 shows the SEM images and EDS energy spectra of the edges of Al-Mg-Si cast-rolled sheets at different Mg/Si ratios.Figures 6(a) and (b) show that when the Mg/Si ratio is 1.23, most of the precipitated phases of the cast-rolled sheet are distributed between the grain boundaries in the form of short rods and granules.The precipitated phases are composed of an α-Al+ composition of β-Al 0.5 Fe 3 Si 0.5 +β-AlFeMnSi+Mg 2 Si+Si.It can be seen from figure 6(c) that when Mg/Si mass ratio is 1.45, the precipitated phase mainly exists in skeletal, short rod, and granular forms, most of which are dispersed at grain boundaries, and a few are dispersed into the body.At this time, the precipitated phase is relatively small in size and quantity, and the second phase of these dispersed distributions plays a role of 'pin' grain boundaries.The precipitated phase is composed of α-Al+Mg 2 Si+β-Al 0.5 Fe 3 Si 0.5 +Si in combination with the EDS spectrum in figure 6(d).Figures 6(e) and (f) reveal that when the Mg/Si mass ratio is 1.70, the gray needle-like phase and the black short rod-like phase cross along the grain boundary, and the gray needle-like phase β-Al 5 FeSi and the black short rod-like phase Mg 2 Si, of which β-Al 5 FeSi is brittle, these phases are difficult to combine with the matrix during deformation and generate strong stress concentration points and split the matrix to form a source of cracks, and these β-Fe phases hardly are eliminated due to heat treatment and rolling deformation, which is extremely detrimental to the quality of the plate [26,27].
Figure 7 shows the SEM image and EDS energy spectrum of the central of Al-Mg-Si cast-rolled slabs at different Mg/Si ratios.There are many differences between the eutectic phase morphology of the edge of the cast-rolled sheet.During the casting-rolling process, the edge of the sheet solidifies first, and the central solidifies last.Therefore, the central is compared with the edge of the sheet in terms of cooling rate or temperature gradient.The eutectic phase is mostly concentrated at the equiaxed triangular grain boundary in the center.With an increase in Mg/Si ratio, the change rule of the large-size precipitated phase is firstly decreasing and then increasing.Figures 7(a) and (b) indicate that when the Mg/Si ratio is 1.23, the Si excess amount is large, and it is easy to precipitate and segregate at the grain boundary.The presence of the Fe element promotes the precipitation phase β-Fe.The transformation of the coarse eutectic phase leads to a decrease in the uniformity of the material and embrittlement of the alloy, thereby reducing plasticity.It can be seen from figures 7(c) and (d) that when the Mg/Si ratio is 1.45, the excess amount of Si is small, and the trace Si in the structure and the impurity element Fe are dispersed and distributed in the matrix, and it is difficult for the two to form β -Fe coarse eutectic phase.Compared with the Mg/Si ratio of 1.45, the size of the precipitated phase is smaller.It can be seen from figures 7(e) and (f) that when Mg/Si reaches 1.70, the Mg element is in excess, and it plays a major role in increasing the number of Mg 2 Si phases in the matrix.These Mg 2 Si phases are easy to coarsen, resulting in an increase in size and a significant decrease in the number of core eutectic phases.In summary, when the Mg/Si ratio is 1.45, the crystal grain size is the smallest, and the precipitated phases are mostly fine particles and short rods distributed along the grain boundary, and the distribution is uniform.Therefore, it is considered that the cast-rolled sheet metal formability is excellent when the Mg/Si ratio is 1.45.
Figure 9 shows the stress-strain curve of the cast-rolled sheet at an Mg/Si ratio of 1.45 in three directions of 0°, 45°, and 90°along the cast-rolling direction.Table 4 shows the mechanical properties of all the three directions along the casting-rolling direction after solid solution and artificial T6 heat treatment.According to the stress-strain curve shown in figure 9, when the Mg/Si ratio is 1.45, the Al-Mg-Si cast-rolled sheet is 0°, 45°, and 90°in the three directions of deformation strengthening index n and plastic strain ratio r [28], and further calculation of n ¯and r ¯results are shown in tables 5 and 6.Nomenclature is shown in table 7. The datas show that the n value of the Al-Mg-Si cast-rolled sheet in 90°direction is slightly higher than the other two directions   (0°and 45°).However, the value is not large.The value of n in all three directions is close.As can be seen from table 6, the r-value of Al-Mg-Si cast-rolled plate in the direction of 90°is higher than that in the other two directions (0°and 45°), which also reflects the anisotropy of 6061 cast-rolled plates to a certain extent.

Conclusions
In this paper, three kinds of Mg/Si ratio 6061 aluminum alloy cast-rolled sheets are prepared.The optimal Mg/ Si ratio is determined by optimizing different factors such as grain size, phase composition, and texture, and the formability of the optimal Mg/Si ratio 6061 aluminum alloy cast-rolled sheet is analyzed after T6 state heat treatment.The conclusions are as follows: (1) In the 6061 cast-rolled sheets, there are fine equiaxial grain zone, dendrite (cell) grain zone, and equiaaxial grain zone along the thickness direction.With an increase in Mg/Si ratio, the grain size decreases first and then increases.When the Mg/Si ratio is 1.45, the average grain size is the smallest, the edge size is 39.78 μm, and the central size is 31.31μm.
(2) The Mg/Si ratio has an important influence on the morphology and size of the precipitated phases of the Al-Mg-Si cast-rolled sheet.With an increasing Mg/Si ratio, the morphology and size of the precipitated phases at the edge and central of the sheet change.When the ratio of Mg/Si is 1.45, the precipitated phases at the edge and the central is uniformly distributed.There is no significant change in the texture type of the castrolled sheet compared to Si.The main texture types of the sheet are {112}〈110〉 texture and rotating cube {001}〈110〉 texture.
(3) When the Mg/Si ratio is 1.45, the elongation of the T6 Al-Mg-Si cast-rolled sheet is the highest at 90°along the cast-rolling direction, reaching 8.9%.At this time, the n value of the sheet is 0.072 and the r value is 1.161.

Figure 1 .
Figure 1.Schematic diagram of experimental sampling of the cast and rolled plate.

Figure 5 .
Figure 5. XRD pattern of 6061 cast rolled plate with different Mg/Si ratios.

Figure 8
Figure8shows the ODF diagram of 6061 cast-rolled plates at different Mg/Si ratios.At different Mg/Si ratios, there are always strong {112} 〈110〉 textures and weaker textures on j2 = 45°section.In rotating cube {001} 〈110〉 texture, the change in Si content has no obvious effect on the change of texture type.In summary, when the Mg/Si ratio is 1.45, the crystal grain size is the smallest, and the precipitated phases are mostly fine particles and short rods distributed along the grain boundary, and the distribution is uniform.Therefore, it is considered that the cast-rolled sheet metal formability is excellent when the Mg/Si ratio is 1.45.Figure9shows the stress-strain curve of the cast-rolled sheet at an Mg/Si ratio of 1.45 in three directions of 0°, 45°, and 90°along the cast-rolling direction.Table4shows the mechanical properties of all the three directions along the casting-rolling direction after solid solution and artificial T6 heat treatment.According to the stress-strain curve shown in figure9, when the Mg/Si ratio is 1.45, the Al-Mg-Si cast-rolled sheet is 0°, 45°, and 90°in the three directions of deformation strengthening index n and plastic strain ratio r[28], and further calculation of n ¯and r ¯results are shown in tables 5 and 6.Nomenclature is shown in table7.The datas show that the n value of the Al-Mg-Si cast-rolled sheet in 90°direction is slightly higher than the other two directions

Figure 8 .
Figure 8. ODF diagram of 6061 cast rolled plate at different Mg/Si ratios.
equiaxed crystals with different growth directions, larger cellular crystals, and clustered dendritic (columnar) crystals.

Table 3 .
Average grain size of Al-Mg-Si cast-rolled plates with different Mg/Si ratios.

Table 4 .
Mechanical performance index.

Table 5 .
Deformation strengthening index n value.

Table 6 .
r-value of plastic strain ratio.