Discussion on Stretch Forming Process of Aluminum Roof Racks (1)

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).

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