Aluminum Stretch Forming: Techniques, Considerations, and Analysis

Bending and roll bending, similar to stretch forming, are among the commonly used cold forming techniques for metal profiles. They exhibit characteristics of wide applicability and stable forming. Particularly suitable for bending thin-walled, square tubes, and single-radius profiled parts. Fabrication of multi-segment arc-shaped workpieces for bending can be complex. Let me elaborate on the bending of aluminum profiles!

Characteristics of Aluminum Profile Bending:

Typically, stretching and bending equipment or processes can only bend workpieces equal to or less than 180°, unlike roll bending, which can process bending angles of 360° or more in a single operation. This is generally stated, as there are rotary stretch bending devices, albeit less common.

During bending, the inner surface of the workpiece becomes the neutral layer, while other areas extend, meaning theoretically, all bent parts will be slightly longer post-bending.

When stretching, it’s essential to leave material allowances regardless of the type of workpiece being processed, a significant difference from roll bending or press braking.

Stretching and bending cannot form workpieces with small radii. If the forming radius is too small, the workpiece tends to experience defects like fracture.

Interestingly, stretch forming processes are rarely used abroad, unlike roll bending.

Considerations for Aluminum Stretch Forming

1. Aluminum alloy profiles should only be moved to the stretching rack for stretching when cooled to below 50°C. If the temperature is too high, stretching can not only cause burns to personnel and sealing strips but also, due to incomplete relief of internal stresses in aluminum alloy profiles, result in twisting, distortion, and functional impairments even before and after aging.
2. Stretching should be controlled to about 1%. It’s important to note that excessive stretching may result in errors in scale at the ends, surface distortion (fish scale marks), low elongation, high hardness, and increased brittleness (low plasticity). Insufficient tension may reduce the compressive strength and hardness of the profiles. Even aging (quenching) may not improve hardness, and the profiles are prone to bending.
3. To control stretching deformation and overall dimensional changes in the profiles, appropriate special clamping pads and methods should be selected. Special attention should be paid to the rational and effective use of stretching pads, especially for open-ended, arc-shaped, cantilevered, and serrated profiles.
4. Attention should be paid to the stress conditions of small feet, fine teeth, long legs, curved surfaces, inclined surfaces, openings, and visual points. Profiles with large width-to-thickness ratios, long outer extension walls, large radii, non-uniform wall thicknesses, and peculiar shapes should be avoided to prevent local or point deformations, distortions, spirals, and other defects in the profiles.
5. Decorative aluminum profiles with high ornamental value should be turned over vertically and horizontally to facilitate uniform heat dissipation, reduce the occurrence of transverse bright spots due to uneven heat dissipation and different degrees of crystallinity, especially for wide and thick aluminum profiles.
6. During picking, moving, and stretching, no mutual friction, pulling, stacking, blocking, or entanglement is allowed, and there should be a certain gap between them. Aluminum alloy profiles that are prone to twisting or have large lengths should be handled in a timely manner and maintained as needed.

Equivalent Transformation of Aluminum Profile Bending Process:

Aluminum profiles are generally obtained through extrusion molding. Compared with roll forming, which is relatively simple, extrusion molding has good quality control and stability. Aluminum profile bending involves gradually fitting the profile between two end chucks onto the mold surface. This method requires pre-determination of the motion trajectories of the two chucks. However, for three-dimensional bending, it is challenging to accurately predict the elongation and deformation of the profiles during stretching. Moreover, there is currently no specialized analysis software available for aluminum profile bending. Inevitably, aluminum profiles will collapse and wrinkle during bending. For typical basic analysis software, excessive mesh distortion may occur, leading to calculation failure. Therefore, it is almost impossible to pre-determine the bending trajectory through simulation algorithms. To address this, this paper proposes a bending trajectory acquisition method based on the geometric characteristics of the product.

Comparison of Mechanical Properties between Steel and Aluminum:

By comparing the mechanical properties of steel and aluminum, it can be inferred that the main difficulties in aluminum alloy forming are:

• Low elongation and small deformation range, making it difficult to form parts and prone to cracking;
• Lower elastic modulus than steel plate, resulting in greater post-forming rebound of aluminum alloy parts than steel plate, making it difficult to control the dimensional accuracy of products.

Rebound value is also one of the assessment indicators for aluminum profile bending, with analysis results showing rebound values in the X and Y directions for the product.

1. According to the principle of relative motion, converting the double-head stretch forming of aluminum profiles into single-head stretch forming can effectively simplify the calculation model of aluminum profile stretch forming.
2. When comparing and analyzing the rebound values with different reference bases (fixed end, clamping end, and middle section) of the product, it is found that using the middle section as the reference base for rebound yields results closer to the actual ones.
3. When the number of discrete points is less than 10, the product deviates from the mold cavity and cannot be smoothly fitted to the mold. When the number of discrete points is greater than 10, within a certain range, the rebound value decreases rapidly with the increase of discrete points; thereafter, the rebound value changes little with the further increase of discrete points.