Improving Bendability of 6000-Series Extruded Aluminum Profiles Using Dies through Shear Stress and Flow-Velocity Reduction

Abstract

Extruded profiles of Al–Mg–Si alloys are typically used in automotive applications. In this study, the microstructure of 6000-series alloys was improved in terms of the energy absorption property (bendability) using the shear stress and flow velocity generated during extrusion instead of conventional transition-metal alloying, because the latter presents challenges in reuse and recycling. This was accomplished using dies with different numbers of feeder holes. Furthermore, in a novel step, complementary finite element analysis (FEA) was conducted to investigate the effects of this improvement on microstructure strengthening. FEA results indicated that the profile obtained using the 4-hole die had lower internal shear stress distribution and aluminum flow velocity than using the 5-hole die. Cross-sectional electron backscatter diffraction observations revealed that the microstructure of the profile obtained using the 4-hole die had fine crystal grains and a strong internal cube or Goss preferential orientation, whereas that obtained using the 5-hole die exhibited coarse grains and an increased number of intermediate orientations. Consequently, the profile obtained using the 4-hole die exhibited improved bendability; furthermore, this die maintained the same tensile strength as the other specimen. Reducing the processing shear stress and flow velocity during extrusion allowed grain refinement and improved bendability at the same tensile strength.

1. Introduction

Currently, 6000-series Al–Mg–Si alloys are mainly used in architectural products and manufacturing structural components in automotive applications.^1,2) For automotive applications in particular, energy absorption properties attributable for the strength and ductility of these alloys are essential for maintaining collision safety. As is the standard practice, transition metals such as Mn, Cr, and Zr are added to improve the strength and ductility of the alloys and achieve fine crystal grains or fiber structures.^3–5) However, these additive elements act as impurities that disturb component adjustment in the casting during the reuse and recycling of the extruded aluminum profile.^6,7)

To address this issue, we explored severe plastic deformation (SPD) processes for microstructural improvements, such as equal-channel angular pressing, which produces ultrafine-grain microstructures without relying on additive elements.^8,9) The difference in the cross-sectional microstructure can thus improve ductility via the stress generated during extrusion. In the context of industrial applications, the optimization of the die structure is considered the primary approach to achieve this goal. However, to date, die structure optimization has focused on obtaining the required shape of the product and improving the extrusion process (parameters such as extrusion speed and wastage). Nevertheless, its effect on the microstructure needs to be sufficiently considered. Compared to that of the SPD processes, the effect of stress–strain applied to die structures is relatively weak. However, previous studies have investigated the correlation between die-induced processing stress during extrusion and the microstructure of the product obtained.^10–12)

Accordingly, in this study, profiles with identical geometry were extruded using two dies with different parameters for an objective comparison. Moreover, the effects of shear stress and aluminum flow velocity on the microstructure during extrusion were examined through finite element analysis (FEA). FEA is widely applied to the extrusion process as an effective tool for predicting profile shapes that fulfill product standards and reliably obtaining heat generation and strain values. However, more is needed to sufficiently account for the relationship between these quantities and the evaluation of the internal microstructure.¹³⁾ To the best of our knowledge, no existing work has utilized FEA with a complementary in situ experimental procedure to precisely evaluate the effect of extrusion-originated shear stresses and flow velocity on the alloy microstructure. As a result, this study achieved improved bendability via the microstructure by adjusting the shear stress applied by the die on the extruded aluminum profiles.

2. Experimental Procedure

2.1 Profile shape and die design

The cross-section of the extruded profile used is shown in Fig. 1. It has a square double hollow shape, commonly used for bumper beams in automotive applications, and the shorter side with a thickness of 4 mm is the impact (crash) surface. The energy absorption characteristics of the impact surface were analyzed via bending tests, as is standard. The central beam dividing the double hollow is crucial for resisting the impact force, and feeding aluminum to it during extrusion is important for designing a die that mitigates tearing defects and achieves high extrusion speeds to improve production output.¹⁴⁾

Fig. 1

Profile shape.

Figure 2 shows a typical porthole die geometry for the single-hollow forming profiles to understand each part of a die (which is different from the die of this study). It consists of two parts—the plate and mandrel. The mandrel has one or more cores with bearings that shape the profile’s inner contour(s). The cores are attached to the rest of the mandrel through legs. Aluminum flows around these legs through feeder holes and rejoins in the welding chamber. The final shape is formed where the mandrel and plate bearings combine. Two dies with different numbers of feeder holes were developed (Fig. 3), similar to those commonly used for the extrusion of profiles with central beams. The design of the 4-hole die renders filling the central beam with aluminum difficult, rendering the standardization of wall thickness challenging, while the resulting low extrusion pressure results in high production output. In contrast, the 5-hole die can directly fill the central beam with aluminum, allowing for reliably reproducible dimensions; however, the higher extrusion pressure decreases productive output. Thus, the influence of die design on the internal microstructure and mechanical properties was evaluated by comparing the profiles extruded via these two dies.

Fig. 2

Illustration of a typical porthole die for aluminum extrusion.

Fig. 3

Aluminum feeder hole alignment of the (a) 4- and (b) 5-hole configurations. The gray parts represent feeder holes.

2.2 Finite element analysis

FEA was performed using HyperXtrude and the Arbitrary Lagrangian–Eulerian (ALE) method.¹⁵⁾ The ALE method is an approach for fluid-structural coupled analysis and is effective for addressing problems such as extrusion and forging, where the material is highly deformed.¹⁶⁾ In addition, die deformation was considered to improve accuracy. However, similar to other computational methods, precisely matching the measured values is difficult due to the differences in the given material conditions based on the deformation or surface/interface states; nevertheless, a relative evaluation of certain tendencies can be effectively realized.

An 8-inch A6061 aluminum alloy cast billet model was used in a direct extrusion press to obtain the profiles, and the container and die temperatures were set to 450°C. The billet and profile parts were meshed using tetrahedra (average size = 11.97 mm) and prisms (0.27 mm). The FEA model contained a total of 3,215,715 elements and 904,765 nodes of aluminum, 2,202,507 elements and 425,349 nodes of die with 4 holes using first-order elements, as well as 3,615,590 elements and 974,875 nodes, 2,240,947 elements and 433,411 nodes with 5 holes. A model of the aluminum billet 700 mm in length and 203 mm in diameter was set to evaluate the pressure at the end of the extrusion. The billet temperature was set to 480°C, the ram speed was set to 7.0 mm/s, the friction coefficient between aluminum and the die mold was set to 0.3, and the convective coefficient between aluminum and the tool system (the container, die, and stem) was set to 3,000 W/(m²·K).¹⁵⁾ Table 1 lists the physical, thermal, and mechanical properties of the A6061 alloy. The output values were the extrusion pressure, profile temperature, von Mises equivalent stress, and shear stress observed in the cross-section of the extruded profile.

Table 1 Physical, thermal, and mechanical properties of the A6061 alloy.

The effective flow stress of the A6061 alloy can be expressed using the Sellars–Tegart inverse sine hyperbolic model to yield steady-state effective deviatoric flow stress. This model can be expressed using eqs. (1) and (2) as follows:^15,17)

  

\begin{equation} \sigma = \frac{1}{\alpha}\mathit{sinh}^{-1}\left[\left[\left(\frac{Z}{A}\right)\right]^{\frac{1}{n}}\right] \end{equation} (1)

  

\begin{equation} Z = \dot{\varepsilon} \cdot \mathit{exp}\left(\frac{Q}{RT}\right) \end{equation} (2)

where Z is the Zener–Hollomon parameter, n is the stress exponent (3.55), Q is the activation energy (145,000 J/mol), A is the reciprocal strain factor (2.40926 × 10⁸ s⁻¹), R is the gas constant (8.314 J/(K·mol)), α is a constant (4.5 × 10⁻⁸ m²/N), $\dot{\varepsilon }$ is the strain rate, and T is the temperature (K).

2.3 Material and extrusion condition

Table 2 summarizes the composition of the A6061 alloy, which has high tensile stress and high ductility and is often used in automotive applications.

Table 2 Chemical composition of the A6061 alloy.

Aluminum billets for extrusion were cast via hot-top casting at a casting temperature of 700°C at 120 mm/min and homogenized at 570°C for a holding time of 3.7 h. Next, the billets were cut to a length of 700 mm and heated up to 480°C using an induction billet heater. After that, the heated aluminum billets were extruded at 7.0 mm/s using a 2300 metric ton direct extrusion press and then immediately quenched at 50°C/s using a water spray, thereby reducing the profile temperature from 570 to ∼10°C. Finally, the profile was artificially aged at 200°C for 2.5 h (T6 treatment).

2.4 Electron backscatter diffraction (EBSD) characterization of the alloy cross-section

The cross-sectional structure of the extruded profiles was analyzed using scanning electron microscopy (SEM; JSM-7001 FTTLS, JEOL) combined with EBSD (OIM, TSL Solutions), which was performed along the extrusion direction (ED) of each sample over an area of 1.0 mm × 4.0 mm in increments of 5.0 µm.

2.5 Tensile and bending tests

The effects of microstructural changes on the mechanical properties of the extruded profiles were investigated via bending and tensile tests. Tensile tests were conducted using a 10-ton universal testing machine (Instron-5582), on samples of a standard No. 13-B shape, in accordance with the JIS Z 2241 protocol (Fig. 4(a)). Bending tests were conducted using a three-point bending tester in a 3-ton universal testing machine (Instron-5581), in accordance with the JIS Z 2248 protocol (Fig. 4(b)). The length, width, and thickness of the bending samples were 60, 20, and 4.0 mm, respectively. Samples were cut from the profile along the ED. The bending characteristics were examined based on the occurrence of an initial crack in the cross-section after 13–4R bending, where the ‘R’ values represent the indenter tip radii (5R = 5 mm). Assuming no cracks occurred during the test, bendability could be evaluated by reducing the tip diameter.

Fig. 4

Geometry of specimens in accordance with the JIS used for the (a) tensile test and (b) bending test.

3. Results and Discussions

3.1 FEA

The extrusion pressure calculated using FEA was compared to that obtained experimentally to examine the effects of using a different number of feeder holes at the end of the extrusion (billet length of 30 mm) to exclude the effect of container friction. Extrusion pressure usually includes the friction between the container and billet. When considering only the stress on the extrusion die and the deformation resistance of the aluminum alloy, the extrusion pressure at the end of the extrusion should be compared.

Table 3 shows that the extrusion pressure obtained via FEA was 18 MPa higher for the 5-hole die. The experimental results correlate with the FEA results; the extrusion pressure determined through experiments was 30 MPa higher for the 5-hole die. Additionally, the absolute values and the difference between the values were higher for the experimental results. Thus, FEA results were likely to be underestimated. Further testing is required to reconcile the analysis with these results while accounting for parameters such as accurate extrusion speed and billet temperature. Nonetheless, the results indicated that the extrusion pressure increased as the number of holes increased, and the effect of this phenomenon on the extrusion profiles was further investigated.

Table 3 Extrusion pressures obtained via FEA and experimental measurement.

Figures 5 and 6 show the shear stress and von Mises stress distribution of the extruded profiles in the cross-section of the ED, 2 mm and 3 mm away from the die exit (Y = 2, 3 mm), examined in the shorter beam of the extruded profile. This is also the evaluation surface for the bending tests. Both stresses were more highly distributed in the 5-hole die ((c) and (d)) than the 4-hole die ((a) and (b)) and were observed to be highly distributed near the outer surface, which is the contact area of the die bearing surface. In particular, for the 5-hole die Fig. 5(c), the distribution was enlarged over the 24-MPa region (colored red) at a depth of 1 mm from the surface of the posterior side. The high shear stress near the bearing surface area was due to friction between the bearing surface and the internal aluminum plastic flow stress.¹⁸⁾ The difference between 4-hole and 5-hole dies, as well as the extrusion pressure, are induced by the die structure. Figure 7 shows the velocity-vector and strain distributions of aluminum from inside the die to the bearing and profile exit. As shown in Fig. 7(d) and (e), a large feeder hole existed at the center of the mandrel of the 5-hole die. The aluminum supply route to the outer part of the profile took a large detouring path, and the angle of the deformation region was considerably large. Therefore, a large velocity-vector distribution existed around the exit, and the strain distribution was slightly expanded in the high-strain region. Figure 8 shows the view from the front of the billet side 1 mm before the bearing edge; the figure demonstrates that the area having a larger velocity vector expanded more, especially the outer side of the 5-hole die. Hence, the magnitude and depth of the shear stress near the surface significantly influence the formation of the internal microstructure.

Fig. 5

Shear stress distribution of the extruded profiles obtained via the (a) and (b) 4-hole die (c) and (d) 5-hole die. Cross sections of (a) and (c) Y = 2 mm (b) and (d) Y = 3 mm.

Fig. 6

von Mises stress distribution of the extruded profiles obtained via the (a) and (b) 4-hole die, (c) and (d) 5-hole die. Cross sections of (a) and (c) Y = 2 mm (b) and (d) Y = 3 mm.

Fig. 7

Flow velocity and vector plot obtained via (a) and (b) 4-hole die, (d) and (e) 5-hole die. (a) and (d) show the overall views including inside of the die, (b) and (e) show the enlarged views of the die exit. Strain distribution of the enlarged view of the die exit obtained via the (c) 4-hole die and (f) 5-hole die.

Fig. 8

Flow velocity and vector plot of the (a) 4-hole die and (b) 5-hole die. The indicating area was billet side 1 mm before bearing edge. High-velocity area widely expanded in the outer part of the profile of the (b) 5-hole die.

Figure 9 shows the temperature distribution of the same section of the samples as those shown in Figs. 5 and 6. The temperature of the two profiles did not change significantly and remained in the 573–587°C range. The extruded profiles obtained experimentally were also found at approximately 570°C before water quenching, demonstrating no significant change due to different die configurations. Thus, these results are in good agreement with those of FEA. The extrusion temperature is an important factor affecting microstructure formation during recrystallization, atomic diffusion, and stress–strain release. However, these results indicate that shear stress dominated the formation of microstructures in this system. The A6061 extruded profile in this system has a fully recrystallized microstructure, and temperature and strain induced by shear stress are important factors for the formation of the recrystallized microstructure.

Fig. 9

Temperature distribution of the extruded profiles obtained via (a) 4- and (b) 5-hole die configurations.

3.2 EBSD

Figures 10 and 11 show the inverse pole figure (IPF) map obtained from a cross-sectional view along the ED. Figure 10 shows a larger area of 18 mm width, almost the same as the specimen width used for the mechanical property test. In this area, microstructures were almost uniform for each specimen. Figure 11 shows the details of the microstructure for clarity. The internal structure of the extruded profile exhibited a surface layer and an inner bulk structure separated clearly by a boundary. The bulk structure primarily consisted of cube and Goss orientations, in which {100} was aligned along the ED, and the surface layer was deformed primarily in the {110} orientation.^19,20) The crystal grains were fine in the profile obtained via the 4-hole die, and a strong preferential orientation (cube and Goss) was observed in the internal structure. In contrast, the grains coarsened in the sample obtained via the 5-hole die, especially near the bearing surface. The center of the internal structure remained unaffected.

Fig. 10

IPF maps of the extruded profiles obtained from wider areas using (a) 4- and (b) 5-hole die configurations.

Fig. 11

IPF maps of the extruded profiles obtained using (a) 4- and (b) 5-hole die configurations.

The aluminum extrusion process is a high-temperature dynamic process, and we observed dynamic recrystallization.²⁰⁾ During the process, strain is continuously introduced even after the formation of new recrystallized grains. Because the new grains are constantly deformed while recrystallizing, the time for recovery is insufficient; moreover, and the recrystallized grains are expected to contain dislocations and subgrain boundaries. In addition, the amount of dislocations may vary for each grain. The introduced dislocations remain even after the plastic deformation is complete. The boundary between adjacent new grains with different strain states may move to the side having higher dislocation density, resulting in grain coarsening. The tendency is expected to be facilitated in the case of a 5-hole die.

As can be observed from the entire crystallographic structure in Fig. 11(b), the coarsening was prominent in the surface structure, grain growth was non-uniform, and coarsening did not occur in the cross-sectional center area of the profile, which was also consistent with the results of grain coarsening by grain boundary migration.^21–23)

The differences in the equivalent strain distribution were small, as shown Fig. 7. However, according to Ikeda et al., the effects of the rapid increase in strain rate (strain-rate gradient) are highly effective.²⁴⁾ Clear difference in velocity distributions shown in Fig. 8 imply that the strain-velocity gradient was likely to be larger in the 5-hole die. However, detailed observation of the grain growth process and quantitative evaluation of the amount of dislocations are required to support these considerations, and these are important issues for the future work. Generally, the coarsening of grain size leads to stress concentration at grain boundaries and decreased ductility.²⁵⁾ Furthermore, the bearing surface area, where the shear stress was highest, had grains at {110} aligned parallel to the ED aggregates. The {110} orientation is inferior to the bending.²⁶⁾

A crystal direction map (Fig. 12) was constructed to examine the area fraction in specific crystal directions for investigating the relationship between the preferred orientation and the shear stress distribution determined via FEA. The preferred crystal orientations were focused on three orientations parallel to the ED (⟨001⟩//ED and ⟨110⟩//ED) and normal direction; ND (⟨111⟩//ND), wherein ⟨110⟩//ED and ⟨111⟩//ND develop notably with increasing shear stress.^18,20) The orientations not included in the specified orientation (⟨001⟩//ED, ⟨110⟩//ED, and ⟨111⟩//ND) were defined as ‘Others’.

Fig. 12

Crystal direction map of the extruded profiles obtained using the (a) 4- and (b) 5-hole die configurations and (c) total area fraction of each crystal direction.

Considering the total area fraction of the different orientations (c), the ⟨110⟩//ED comprising cube and Goss orientations decreased by 39% in the 5-hole profile. In contrast, the ⟨110⟩//ED and ⟨111⟩//ND orientations increased by 64%. Thus, it is inferred that the cube and Goss orientations were disturbed by the increase in shear stress, while the other orientations, including ‘Others’, were favored with increasing shear stress. In particular, ⟨110⟩//ED is a typical fcc shear texture.

Hence, the surface microstructure was in a three-dimensional complex stress–strain state, forming a deformed texture. The internal microstructure formed the texture upon horizontal stress application owing to the plastic flow of aluminum and was aligned in the ED. The surface microstructure was developed by increasing the shear stress as aluminum flows through the die wall area.

Shear stress distribution inside the die through FEA was included in Fig. 13. The 5-hole dies have a more complex internal die structure with a hole in the center, which increases the wall surface area and shear deformation area, resulting in an overall increase in shear stress. A previous study verified that an increase in shear-stress-induced die structure caused the development of the surface microstructure using a simplified die model.²⁷⁾

Fig. 13

Shear stress distribution of aluminum from inside the die to the extruded profiles obtained via the (a) and (b) 4-hole die, (c) and (d) 5-hole die. Cross sections of (a) and (c) X = 0 mm (b) and (d) X = 16 mm.

The cube and Goss orientations are known to exhibit good bending properties. For orientations with small Taylor factors, the deformation is observed mainly by uniform slip deformation during bending without shear zone deformation assistance.²⁶⁾ This type of orientation and fine grains of the microstructure is expected to improve the mechanical properties (especially bending, in the case of a 4-hole die).²⁶⁾

3.3 Tensile and bending tests

The effects of internal microstructural changes on mechanical properties were investigated via bending and tensile tests. Table 4 shows that the tensile strength, 0.2% yield stress, and elongation (which listed an average of n = 3) were identical for the two profiles, whereas the 4-hole profile exhibited superior bendability (4-hole: 5R and 5-hole: 9R). In addition, the TS and YS standard deviations were greater for the 5-hole profile, which is attributed to uneven loading and fracture behavior due to the coarse-grained microstructure.

Table 4 Tensile and bending properties of the extruded profiles.

Figure 14 shows the appearance of each specimen after the bending test from top and side views. The specimen obtained via the 4-hole die exhibited no cracks even after 5R, thus demonstrating a significant improvement over the 9R result of the 5-hole profile bending test. These results indicated that despite demonstrating a similar level of strength and elongation, the bending properties of the two profiles varied considerably due to differences in the inner structure. Generally, tensile strength and bendability are correlated.²⁸⁾ However, for the same level of tensile strength, bendability is considerably affected by local elongation (area reduction), which varies with the precipitate-free zone (PFZ) width, precipitates, and texture state.^29,30) Materials with high local elongation have good bendability, while those with brittle fractures have low local elongation and poor bendability. This phenomenon is supported by extruded profiles exhibiting anisotropy in bendability depending on the test direction (ED or TD).³¹⁾ The grain size near the surface, far from the neutral plane, significantly affects bendability. As discussed, the inner structure with a 5-hole die with coarse grains at the outer side (near the surface) periphery deteriorated bendability. Stress concentration on grain-boundary precipitates is proportional to the length and width of the slip zone. Considering the slip-zone length equal to the grain size, coarse grains tend to cause grain-boundary cracking, resulting in reduced ductility.³²⁾ In contrast, the 4-hole profile exhibited a fine, relatively uniform grain size and a uniform preferential orientation in the ED, which is attributable for the improved bendability. In addition, previous research showed that stress is concentrated in areas where the differences between the surface and bulk microstructures and misorientation are large and that cracking occurs from these areas.^29,30) Typically, the PFZ is also an important factor in the bendability of an alloy; however, in the previous study, the PFZ level of A6061 extruded profile was predicted to be almost identical, approximately 50 microns, for the same profile geometry and cooling rate soon after extrusion.³³⁾

Fig. 14

Sample appearance after the bending test.

4. Conclusion

In this study, microstructural changes in the extruded profiles were observed under different shear stresses, strain distributions, and flow velocity gradients (strain rate gradient) generated by changing the inner structure of the die owing to the number of feeder holes of the dies during extrusion. The results supported the aim of reducing transition metals for microstructural enhancement of the alloys as such transition-metal-based alloys cannot be easily reused and recycled. The main conclusions are summarized as follows.

1. (1)    FEA results indicated that the extrusion pressure, shear stress, and flow velocity in the cross-section of the profile obtained using the 4-hole die decreased. However, the cross-sectional temperatures of the extruded profiles were almost identical and obtained experimentally were also found to be same temperature.
2. (2)    EBSD cross-sectional analysis showed that the profile obtained from the 4-hole die had fine crystal grains and a strong internal cube and Goss preferential orientation, whereas that obtained via the 5-hole die had coarser crystal grains. This result suggested that the development of the cube and Goss orientations were disturbed by the increase in shear stress. In contrast, the other orientations (⟨110⟩//ED and ⟨111⟩//ND) were favored.
3. (3)    The 4-hole die exhibited improved bendability although there was no significant difference between the obtained tensile strengths of the two dies.
4. (4)    Reducing the processing shear stress and flow velocity during extrusion resulted in deformation texture reduction, grain refinement, and improved bendability, while retaining the same strength.

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