Stretch forming is the basic bending forming method for aluminum profiles commonly used in the metal components of roof racks, which are important for both the appearance and load-bearing of vehicles. To address issues such as wrinkling on the inner side, surface depressions, and springback during the stretch forming process, these defects can be effectively resolved by selecting appropriate materials, optimizing cross-sectional shapes, and adjusting process parameters, thereby improving the forming accuracy.
This study analyzes the formability issues, such as significant rebound, upper flange depressions, surface straight lines, and inner wrinkling, that occur during the stretch forming process of flush-mounted aluminum alloy roof racks. Corresponding solutions are explored.
Establishment of the Roof Rack Stretch Forming Model
To explore the formability during the stretch forming process of roof racks, a finite element model was developed for a specific flush-mounted aluminum alloy roof rack. The material used for stretch forming was 6063-T1 aluminum, and its measured tensile properties are presented in Table 2. The engineering stress-strain curve was converted to the true stress-strain curve for calculations. A friction coefficient of 0.15 was set between the profile and the mold. After stretch forming, the product underwent heat treatment and reached the T5 state.
| Yield strength/MPa | Tensile strength/MPa | Elongation/% | E/GPa | Poisson’s ratio | K/MPa | n | r | |
| 6063-T1 | 84.5 | 148 | 23.8 | 23.2 | 0.3 | 264 | 0.18 | 0.3 |
Schematic diagram of cross-section and structure of stretch-bending workpiece for luggage rack Fig. 2:
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Surface Straight Lines
The initial design section of the roof rack is shown in Figure 3, but during the trial process, surface straight lines appeared, as shown in Figure 4a, affecting the appearance quality. Analysis revealed that the supporting edge in the section caused uneven metal flow and localized stress concentration due to the fast stretch forming speed, leading to the formation of straight lines on the outer surface. Calculating and analyzing the stress distribution validated this conclusion. By optimizing the section and eliminating the supporting edge, the new section in Figure 2a significantly improved the issue without compromising the load-bearing requirements of functional testing, as shown in Figure 4b.
Figure 4 Comparison of surface straight marks before and after improvement
Surface Depressions
During the stretch forming process of the roof rack, the outer material is noticeably thinned, resulting in depressions on the A-side, as shown in Figure 5.
Based on the force and deformation analysis, surface depressions are primarily related to the relative height (h) of the cross-section, relative thickness (t) of the upper flange, pre-stretching amount (δ), and the minimum relative bending radius (R). Researchers such as Zhou Xianbin from Beihang University defined the concept of relative dimensions for rectangular profiles, as depicted in Figure 6a. Here, the relative height is denoted as h’ = HJB, and the relative thickness of the upper flange is represented by t = t/H. Additionally, assuming the minimum bending radius is R, the relative bending radius is R’ = R/t, and the stretching amount δ represents elongation deformation during pre-stretching. Surface relative depression is defined as the ratio of the actual depression amount to the height. The same definition applies in this case, as shown in Figure 6b.
Conclusions from Controlled Variable Calculations and On-Site Trial Production:
Effect of Relative Height on Surface Depressions
Within a certain range (required for the roof rack’s design and extrusion processing), as the cross-section’s relative height increases, the maximum surface relative depression shows a decreasing trend. At this point, overall depressions are hardly noticeable due to the enhanced support provided by the increased relative height of the upper flange, which reduces the occurrence of depressions. However, the relative height should not be excessively large because it would result in greater tensile stress on the outer layer during bending, leading to increased vertical forces perpendicular to the flange and causing larger flange depressions, which would not meet appearance requirements. In this case, the relative height ranged from 0.87 to 1.12, effectively resolving the depression issue.
Effect of Relative Thickness of the Upper Flange on Surface Depressions
Within a certain range, increasing the relative thickness of the upper flange leads to a certain decrease in the maximum surface relative depression. This is because the increased thickness enhances the strength and resistance to collapsing. However, the upper flange thickness should not be excessively thick, as it would affect the forming effect and functional requirements without further improving depressions.
Effect of Pre-Stretching Amount on Surface Depressions
With an increase in the pre-stretching amount, the thinning of the upper flange becomes more significant, and beyond a certain value, the maximum relative surface depression also increases. This is due to the reduction in wall thickness, which leads to decreased strength and resistance to collapsing. Therefore, determining a reasonable pre-stretching amount is an effective measure to avoid surface depressions, and it should be adjusted in correspondence with the bending radius.
Effect of Minimum Relative Bending Radius on Surface Depressions
The most acute bending occurs at the point with the minimum relative bending radius in the bend arc, where the material thinning rate is highest, making it susceptible to depressions. When the thickness of the flange remains constant, the influence of the bending radius is equivalent to that of the relative bending radius. Within the process control range, as the bending radius decreases, depressions gradually appear and increase on the surface. This is because a smaller bending radius accelerates material flow, leading to insufficient material in localized regions and reduced strength. Therefore, setting a minimum bending radius ensures that stretch forming occurs without sudden changes, avoiding surface depressions.
Other Factors Affecting Surface Depressions
If it is inconvenient to change the section or stretch forming parameters during the actual stretch forming process of the roof rack, controlling depressions can be achieved by placing core materials (such as acrylic bars) inside the cavity, as shown in Figure 7. Core materials provide support to the surface, preventing collapse. After stretch forming, the core material is removed and straightened for continuous use.
In conclusion, within a reasonable range, appropriately increasing the relative height, relative thickness of the upper flange, and minimum relative bending radius, while setting an upper limit for stretching, can effectively improve surface depressions in the upper flange. Moreover, using core materials and optimizing the distribution of the cross-section into two chambers can also address the depression issue. Among these factors, the relative height and bending radius have a more significant impact. An initial design of the profile surface can be used as a basis, and variable control methods can be employed to calculate optimal parameters, which can then be validated through practical trials. In this case, through calculations and trial analyses, it was found that a relative height of 1.06 and a relative thickness of the upper flange of 6.5% resulted in smaller depressions.
Works Cited: Discussion on Stretch Bending Process of Aluminum Alloy Luggage Rack 2017. Authors: Duan Jichao, Zhang Yilin; He Liangyong; Zhao Qiang, Wang Yuquan; Liu Deman
