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 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 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.
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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