The characteristics of bent parts are large curvature radius, significant springback, and curvature variation along the longitudinal axis. Aluminum profile bending is widely used in aerospace, aviation, automotive, construction, and high-speed rail industries for forming bent components, such as fuselages, wing reinforcements and frames, arched building frames, curved door and window frames, train noses, roof contour supports, and arched door and window skeletons.
In these applications, aluminum alloy extrusion profiles are extensively formed using bending techniques. Among them, the stretch bending process is the most common, popular, and widely applied aluminum profile bending method.
Working Principle and Steps
The basic principle of aluminum profile stretch bending is: the bending die is fixed on the table, and the support arm stretches the profile around the bending die. Under the combined action of tensile force and bending moment, the profile undergoes plastic deformation and gradually conforms to the die shape, producing the desired component.
Stretch bending machine involves first applying axial tensile force until the cross-sectional stress exceeds the profile’s yield limit, then applying a bending load. The axial tensile force is maintained during bending, allowing the profile to gradually conform to the die.
This process improves the stress distribution across the profile cross-section during bending, enhancing forming quality. The axial tensile force also mitigates wrinkling on the inner side and optimizes stress distribution, reducing springback and improving dimensional accuracy.
Three basic steps of stretch bending:
- The stretching cylinder clamps the material and applies a pre-tension force to reach the material yield strength.
- The bending cylinder rotates, while the stretching cylinder applies axial tension according to the program, allowing the profile to conform to the die.
- Additional stretching is applied according to the material’s springback behavior.
During the bending process, the stretching bending machine continuously applies axial force to the profile, compensating for elongation and preventing wrinkling, resulting in smooth curves.
Characteristics of the Stretch Bending Process
Stretch bending, like roll bending, is a commonly used cold bending method for metal profiles. It features wide applicability and stable forming. It is especially suitable for thin-walled, square tube, and special-shaped profiles with a single bending radius.
It is called “single radius” because when forming multi-arc profiles, the corresponding bending dies (dubbed the “mother of industry”) are complex to manufacture, adjust, and operate.
The basic principle is the combined action of tensile force and bending moment, which shapes the profile by plastic deformation conforming to the die. Compared with roll bending, stretch bending usually operates near the material yield limit, providing high precision and good surface quality.
Key characteristics of stretch bending:
- Standard stretch forming machine can generally bend components up to 180°, but cannot achieve 360° or larger angles in a single operation like roll bending.
- Oxidized aluminum surfaces are aesthetically pleasing, dirt-resistant, and easy to clean.
- Components can be assembled using matching aluminum profile fittings without welding, making the process environmentally friendly, lightweight, and easy to install or transport.
- The inner surface of the bent component is the neutral layer, while the remaining areas undergo tensile strain.
- Consequently, all bent profiles are slightly longer than their original length.
- Aluminum density is only 2.7 g/cm³ (~1/3 of steel, copper, or brass) and exhibits excellent corrosion resistance in air, water, saltwater, petroleum, and many chemical environments. Titanium plating or other surface treatments enhance wear resistance and aesthetics.
- Material allowance must be reserved at both ends during bending due to unavoidable clamping damage, a significant difference from roll bending or tube bending.
- Stretch bending cannot effectively form small-radius components. If the radius is too small, stress concentration may cause fractures, related to excessive tensile strain on the outer layer.
Additional advantages of aluminum profiles in stretch bending include good machinability, non-magnetic properties, important for electrical and electronics applications, and non-flammability, suitable for flammable or explosive environments.
Advantages of Stretch Bending
Primary advantages:
- Can bend complex profile cross-sections.
- Allows multi-arc, variable-radius bending.
- High bending precision, stable springback, good dimensional consistency.
- Effectively reduces residual stress, ensuring product stability.
- Cold working improves the material’s mechanical properties.
Compared with other forming methods:
- Can form structurally complex parts.
- Suitable for profiles with high strength-to-yield ratios.
- Combines features of multiple forming techniques.
- High bending accuracy with minimal springback.
- Flexible manufacturing capabilities.
Key Stretch Bending Techniques
Bending radius design must not exceed the material elongation limit.
Stretch bending can result in wall thinning, cracking, wrinkling, or cross-sectional distortion. These defects are closely related to material properties, cross-section shape, and process parameters.
Stress states differ within the material: the neutral layer experiences minimal stress, the inner layer (conforming to the die) is under compression, and the outer layer is under tension.
Critical factors:
- Apply sufficient pre-tension to avoid wrinkling.
- Balance forces to prevent outer layer cracking or excessive cross-sectional distortion.
Calculation of Stretching Force
For technical capability assessment, consider:
- Cylinder clamping distance vs. material elongation.
- Clamp size vs. profile cross-section.
- Maximum tensile force required for the material.
Forming force formula:
Fstretch = 1.25 × Scross-section × σs
Where:
- Fstretch = Required force of the bending cylinder (N)
- Scross-section = Material cross-sectional area (mm²)
- σs = Material yield strength (MPa)
Aluminum Profile Stretch Bending Calculator
Calculate required force and validate bending radius
Profile Dimensions
Material Parameters
Bending Parameters
Calculation Results
The bending process is feasible if: Actual Strain ≤ Material Elongation
– Safe: Actual Strain ≤ 80% of Material Elongation
– Warning: Actual Strain between 80%-95% of Material Elongation
– Danger: Actual Strain ≥ 95% of Material Elongation (risk of cracking)
Calculation Explanation
Force Calculation: Force(tons) = Cross-Section Area(mm²) × Yield Strength(MPa) × Coefficient(0.765) ÷ 10
Bending Validation: Based on material elongation criteria: ε = [(L₂ – L₁) / L₁] × 100% ≤ Material Elongation
Easy Way vs Hard Way: “Easy Way” means bending around the profile’s weak axis, requiring less force and allowing smaller bending radius; “Hard Way” means bending around the strong axis, requiring more force and larger bending radius.
Material Parameters: 6061-T6 yield strength is approximately 240MPa with 10% elongation, 6063-T6 is approximately 170MPa with 10% elongation.
Common Problems and Solutions in Aluminum Profile Stretch Bending
Problems:
- Contour deviations: due to die springback, material inconsistencies, radius variation, or uneven cross-section.
- Surface defects: cracks, wrinkles, concaves, and side indentations.
- Twisting after bending: caused by asymmetric cross-sections or stress imbalance.
- Excessive vertical deviation: misalignment of cross-section relative to reference plane.
- Excess material length: extra length required for dies, clamps, and machine clearance.
- Safety hazards: sudden material breakage can damage equipment or injure personnel.
Solutions:
- Adjust die curvature based on calculations and tests to compensate for springback; add cylinders for better die conformance.
- Prevent surface defects by modifying cross-section, adjusting tension, or filling cavities before bending.
- Reduce twisting by adding vertical push cylinders and maintaining pressure after forming.
- Correct vertical deviations by adjusting die curvature and post-processing.
- Optimize material length to reduce waste while ensuring proper forming.
- Implement safety measures: protective shields, standard operating procedures, and operator training; prohibit standing near active stretch bending machine.
Conclusion
Aluminum alloy profile stretch bending is a key process in modern manufacturing and construction industries. By optimizing bending parameters, improving die design, and strengthening process control, it is possible to enhance forming precision, surface quality, minimize springback, reduce defects, and achieve high-quality, cost-effective production.