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.
Basics 1#: Roof racks
Roof racks are essential accessories mounted on the top of vehicles, serving both as a secure and convenient way to carry luggage and as an element for styling and decoration. They are commonly used on hatchback cars, SUVs, and MPVs, among others. The components of the roof rack are usually made of closed-section aluminum profiles, which can be categorized into integrated and standing types based on their relationship with the roof. The standing type can be further divided into two-legged and multi-legged standing types.
For flush-mounted roof racks, an integrated design is usually adopted, which boasts an aesthetically pleasing contour, blending seamlessly with the vehicle’s body, making it an essential developmental form for the structure of roof racks. Due to the need for compatibility with the vehicle’s roof shape, this type of roof rack generally features large curvature and is primarily produced using the stretch forming process. The forming processes for the components of the roof rack include bending, stretch forming, and hydraulic forming, as shown in the table below.
Basics 2#: Comparison of Roof Rack Forming Processes
| Forming process | Press Forming | Stretch Forming | Hydroforming |
| Advantage | The equipment cost is relatively lower, and the process is more mature | Can achieve large arc | Hydroforming can realize non-equal section, improve strength and rigidity, and the product block structure is simple |
| Disadvantage | Extrusion molding requires equal cross-section and can only achieve small arcs | Extrusion molding requires equal cross-section, and the precision of the front and rear ends is high and difficult to guarantee | High technical difficulty and high cost |
| Cost | Generally | Higher | High |
This article introduces the stretch forming process and formability concept of aluminum alloy roof racks, analyzes the influencing factors of stretch forming formability, establishes a stretch forming model for the roof rack profiles, and explores the problems and solutions encountered during the stretch forming process. The information presented here serves as a reference for material selection, design, and process planning for roof racks.
Basics 3#: Stretch Forming Process
Stretch forming is a cold working bending process in which the workpiece is bent while being subjected to tangential tension. The bending occurs under the combined action of bending moment and tensile force. During stretch forming, the stress inside the workpiece’s cross-section is mainly tensile stress, with minimal or no compressive stress. As a result, the formed part experiences low springback, good conformity to the mold, reduced wrinkling, and high forming accuracy. Depending on the forming principles and equipment, stretch forming can be classified as rotary stretch forming and draw bending, with the latter being generally used for roof racks.
Basics 4#: Force control or displacement control
The stretch forming process for profiled sections can be achieved through force control or displacement control. Roof racks typically employ displacement control, and to improve accuracy, a three-step method is often used, involving pre-stretching, bending, and final tightening. As shown in Figure 1, a certain pre-tension is applied to both ends of the profile, inducing a specific amount of pre-deformation with the pre-tension usually around the yield point. Then, while maintaining the pre-tension, the profile is bent through the movement of the jaws. Finally, additional tightening force is applied to further reduce springback and enhance conformity to the mold. In the displacement control method, the profile’s ends are fixed, and continuous bending against the mold is applied to achieve the desired shape while internal tensile stress within the profile increases.
Compared to bending methods like press bending and roll bending, the internal stress distribution during stretch forming differs. The former exhibits a stress state where the outer layer experiences tension, the neutral layer balances tensile and compressive forces, and the inner layer is under compression, separated by the neutral layer. In contrast, the pre-stretching in the stretch forming process increases tension on the outer layer, causing the neutral layer to move towards the inner layer, partially offsetting the compressive stress. When the neutral layer reaches the inner layer, the entire cross-section experiences tensile stress, resulting in a change in stress state. If the innermost point experiences significant tensile stress up to the yield point, removing the tension results in the workpiece maintaining the bent shape without springback. Stretch forming has the advantages of high bending angles and minimal springback, but it may also lead to thinning, depressions, or wrinkling on the bottom surface as defects.
Basics 5#: Formability of Stretch Forming
The formability of profiled sections during stretch forming refers to their ability to be smoothly shaped and meet the required accuracy. It includes three formability indicators: cross-section retention, shape retention, and fracture resistance. Cross-section retention refers to the ability to maintain the original cross-sectional shape during the stretch forming process, resisting cross-section distortion. Shape retention relates to the capability of the profiled section to maintain its final shape and dimensions after unloading, resisting elastic recovery. Fracture resistance refers to the ability of the profiled section to withstand necking and fracture during the stretch forming process.
The formability and forming accuracy of profiled sections during stretch forming are closely related to material mechanical properties, the state of the workpiece’s cross-section (shape and dimensions), and the stretch forming process (type, sequences, bending states, and core materials).
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:
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.
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.
Wrinkling
The stretch forming process of the roof rack is prone to the defect known as wrinkling, which can affect the contact with the mold and surface quality. Wrinkling is an internal instability-related defect, which can be improved by using supplementary stretching measures.
When the pre-stretching amount is insufficient to move the neutral layer to the inner layer material, the outer surface experiences significant tensile stress, while the inner surface experiences certain compressive stress. The presence of compressive stress may cause wrinkling on the inner surface. In cases of mild wrinkling, it can be improved through supplementary stretching, but this may require a significant amount of additional stretching and could cause section deformation. In more severe cases of wrinkling, supplementary stretching measures may not fully correct the issue, resulting in the rejection of the workpiece. Excessively small minimum bending radii can exacerbate the compressive stress on the inner side and lead to unstable wrinkling.
How to prevent wrinkling
Therefore, ensuring sufficient pre-stretching and avoiding excessively small bending radii are fundamental measures to prevent wrinkling. The supplementary stretching process and the use of core molds can significantly improve mild wrinkling. However, excessively increasing the stretching amount, while preventing non-contact with the mold and unstable wrinkling, may cause excessive thinning and result in surface depressions. Hence, the actual stretching amount should be determined within the intermediate range that ensures neither wrinkling nor surface depressions.
Springback
Springback is a common bending defect that can lead to reduced mold contact and forming precision. It can be effectively compensated or improved through proper cross-section optimization and process control. Parameters used for evaluating bending springback include springback radius, springback gap, and springback angle, which are chosen based on specific models.
With an increase in the stretching amount, the springback gap and springback angle decrease until a certain value is reached, beyond which further increase in the stretching amount no longer has a significant impact on springback. The physical significance of this upper limit is that at this point, the neutral layer moves to the inner layer of the profile, and there is no compressive stress on the cross-section. When the pre-stretching is not sufficient to move the neutral layer to the inner layer of the profile, springback can be controlled to a certain extent through subsequent supplementary stretching, promoting mold contact.
Cross-section optimization can also control springback
For example, within a certain range, increasing the relative height and moving the interval layer of a double-chamber cross-section downward to make the two layers closer can reduce the impact of springback. Increasing the minimum relative bending radius, making the bending deformation zone smoother, can also control springback, but the bending radius must correspond to the stretching amount. An increase in the coefficient of friction can lead to larger springback because frictional forces hinder the transmission of tension throughout the profile, causing uneven distribution of tensile stress along the cross-section and length direction. To reduce the springback of the stretch-formed part, lubricants should be added between the mold and the profile to decrease the coefficient of friction. Additionally, compensating for the springback of different parts of the profile can be achieved by modifying the mold’s arc surface or structure to produce overbending during the forming process, ensuring that the shape of the profile after unloading matches the required shape of the part.
(c) New bend radius
Figure 8 Comparison of the new section section with the new bending radius
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.
- The stretch forming process enables the realization of the large curvature requirements for the Roof Rack made of aluminum alloy. The formability of the stretch forming process mainly includes section retention, final shaping, and fracture resistance.
- During the stretch forming process, certain defects such as surface straight marks, surface depressions, inner layer wrinkling due to instability, and springback may occur. However, under the premise of reasonable material selection, these defects can be effectively avoided or improved through cross-section optimization and process optimization.
- The elimination of support edges perpendicular to the outer surface of the cross-section can eliminate surface straight marks.
- Within a reasonable range, increasing the relative height, relative thickness of the upper panel, and minimum relative bending radius while setting an upper limit for the stretching amount can effectively improve surface depressions on the upper panel. Additionally, using core molds can also help to improve depressions.
- Ensuring sufficient pre-stretching and avoiding excessively small bending radii are fundamental measures to prevent wrinkling. The supplementary stretching process and the use of core molds can significantly improve mild wrinkling.
- Within a certain range, setting a reasonable stretching amount, increasing the relative height and minimum relative bending radius, reducing the coefficient of friction, and modifying the mold’s arc surface can effectively control springback.
Stretch forming is a complex pr
Summary
To eliminate inner wrinkling, significantly improve surface depressions and workpiece springback, the cross-section shape and dimensions of the profile were optimized, and reasonable minimum bending radii and pre-stretching amounts were chosen based on calculation results and verified through trials. Comprehensive improvements were made to the product structure and process, and the new cross-section design and comparison with the original, as well as the new bending radius, are shown in Figure 8. The new design and process achieved the intended goals, and the roof rack’s shape and functionality met the requirements of the test items, satisfying the product’s specifications.
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